MR imaging-guided focused ultrasound surgery of uterine leiomyomas: a feasibility study

Focused Ultrasound: Noninvasive Image-Guided Therapy

ABSTRACT

Invasive open surgery used to be compulsory to access tumor mass to perform excision or resection. Development of minimally invasive laparoscopic procedures followed, as well as catheter-based approaches, such as stenting, endovascular surgery, chemoembolization, brachytherapy, which minimize side effects and reduce the risks to patients. Completely noninvasive procedures bring further benefits in terms of reducing risk, procedure time, recovery time, potential of infection, or other side effects. Focusing ultrasound waves from the outside of the body specifically at the disease site has proven to be a safe noninvasive approach to localized ablative hyperthermia, mechanical ablation, and targeted drug delivery. Focused ultrasound as a medical intervention was proposed decades ago, but it only became feasible to plan, guide, monitor, and control the treatment procedures with advanced radiological imaging capabilities. The purpose of this review is to describe the imaging capabilities and approaches to perform these tasks, with the emphasis on magnetic resonance imaging and ultrasound. Some procedures already are in clinical practice, with more at the clinical trial stage. Imaging is fully integrated in the workflow and includes the following: (1) planning, with definition of the target regions and adjacent organs at risk; (2) real-time treatment monitoring via thermometry imaging, cavitation feedback, and motion control, to assure targeting and safety to adjacent normal tissues; and (3) evaluation of treatment efficacy, via assessment of ablation and physiological parameters, such as blood supply. This review also focuses on sonosensitive microparticles and nanoparticles, such as microbubbles injected in the bloodstream. They enable ultrasound energy deposition down to the microvascular level, induce vascular inflammation and shutdown, accelerate clot dissolution, and perform targeted drug delivery interventions, including focal gene delivery. Especially exciting is the ability to perform noninvasive drug delivery via opening of the blood-brain barrier at the desired areas within the brain. Overall, focused ultrasound under image guidance is rapidly developing, to become a choice noninvasive interventional radiology tool to treat disease and cure patients.

Transducer module apodization to reduce bone heating during focused ultrasound uterine fibroid ablation with phased arrays: A numerical study

BACKGROUND

During magnetic resonance-guided focused ultrasound (MRgFUS) surgery for uterine fibroids, ablation of fibrous tissues in proximity to the hips and spine is challenging due to heating within the bone that can cause patients to experience pain and potentially damage nerves. This far-field bone heating limits the volume of fibroid tissue that is treatable via MRgFUS.

PURPOSE

To investigate transducer module apodization for improving the ratio of focal-to-bone heating (Δ T ratio $\Delta T_{\mathrm{ratio}}$ ) when targeting fibroid tissue close to the hips and spine, to enable MRgFUS treatments closer to the bone.

METHODS

Acoustic and thermal simulations were performed using 3D magnetic resonance imaging (MRI)-derived anatomies of ten patients who underwent MRgFUS ablation for uterine fibroids using a low-frequency (0.5 MHz $0.5 \ \text{MHz}$ ) 6144-element flat fully-populated modular phased array system (Arrayus Technologies Inc., Burlington, Canada) at our institution as part of a larger clinical trial (NCT03323905). Transducer modules (64 elements $64 \ \text{elements}$ per module) whose beams intersected with no-pass zones delineated within the field were identified, their output power levels were reduced by varying blocking percentage levels, and the resulting temperature field distributions were evaluated across multiple sonications near the hip and spine bones in each patient. Acoustic and thermal simulations took approximately20 min $20 \ \text{min}$ (7 min $7 \ \text{min}$ ) and1 min $1 \ \text{min}$ (30 s $30 \ \text{s}$ ) to run for a single near-spine (near-hip) target, respectively.

RESULTS

For all simulated sonications, transducer module blocking improvedΔ T ratio $\Delta T_{\mathrm{ratio}}$ compared to the no blocking case. In just over half of sonications, full module blocking maximizedΔ T ratio $\Delta T_{\mathrm{ratio}}$ (increase of 82% ± $\pm$ 38% in 50% of hip targets and 49% ± $\pm$ 30% in 62% of spine targets vs. no blocking; mean ± SD), at the cost of more diffuse focusing (focal heating volumes increased by 13% ± 13% for hip targets and 39% ± 27% for spine targets) and thus requiring elevated total (hip: 6% ± 17%, spine: 37% ± 17%) and peak module-wise (hip: 65% ± 36%, spine: 101% ± 56%) acoustic power levels to achieve equivalent focal heating as the no blocking control case. In the remaining sonications, partial module blocking provided further improvements in bothΔ T ratio $\Delta T_{\mathrm{ratio}}$ (increased by 29% ± 25% in the hip and 15% ± 12% in the spine) and focal heating volume (decrease of 20% ± 10% in the hip and 34% ± 17% in the spine) relative to the full blocking case. The optimal blocking percentage value was dependent on the specific patient geometry and target location of interest. Although not all individual target locations saw the benefit, element-wise phase aberration corrections improved the averageΔ T ratio $\Delta T_{\mathrm{ratio}}$ compared to the no correction case (increase of 52% ± 47% in the hip, 35% ± 24% in the spine) and impacted the optimal blocking percentage value. Transducer module blocking enabled ablative treatments to be carried out closer to both hip and spine without overheating or damaging the bone (no blocking:42 ± 1 mm $42\pm 1 \ \text{mm}$ /17 ± 2 mm $17 \pm 2 \ \text{mm}$ , full blocking:38 ± 1 mm $38\pm 1 \ \text{mm}$ /8 ± 1 mm $8\pm 1 \ \text{mm}$ , optimal partial blocking:36 ± 1 mm $36\pm 1 \ \text{mm}$ /7 ± 1 mm $7\pm 1 \ \text{mm}$ for hip/spine).

CONCLUSION

The proposed transducer apodization scheme shows promise for improving MRgFUS treatments of uterine fibroids, and may ultimately increase the effective treatment envelope of MRgFUS surgery in the body by enabling tissue ablation closer to bony structures.

Experimental Evaluation of an Ultrasound-Guided High-Intensity-Focused Ultrasound Probe for Sonication of Artery

OBJECTIVES

This study aimed to develop an ultrasound-guided high-intensity-focused ultrasound (USgHIFU) probe for arterial sonication and to evaluate vascular contraction.

METHODS

The USgHIFU probe comprised two confocal spherical transducers for sonication and a US color Doppler flow imaging probe for guidance. A vessel-mimicking phantom was sonicated in two directions. In the vascular radial direction, an isolated rabbit aorta embedded in ex vivo pork liver was sonicated at different acoustic powers (245 and 519 W), flow rates (25, 30, and 50 mL/minute), and sonication energies (519, 980, and 1038 J). Changes in the postsonication vessels were evaluated using US imaging, microscopic observation, and histopathological analysis.

RESULTS

Beam focusing along the vascular radial direction caused significant deformation of both tube walls (n = 4), whereas focusing along the axial direction only affected the contraction of the anterior wall (n = 4). The contraction index (Dc) of the vessel sonicated at 245 W and 980 J was 56.2 ± 9.7% (n = 12) with 25 mL/minute. The Dc of the vessel sonicated at 519 W and 1038 J was 56.5 ± 7.8% (n = 17). The Dc of the vessel sonicated at 519 J total energy was 18.3 ± 5.1% (n = 12).

CONCLUSION

The developed USgHIFU probe induced greater vascular contractions by covering a larger area of the vessel wall in the radial direction. Sonication energy affects vascular contraction through temperature elevation of the vessel wall. When the acoustic power was high, an increase in acoustic power, even with comparable sonication energy, did not result in greater vessel contraction.

Ultrasound-guided high-intensity focused ultrasound for symptomatic uterine fibroids: clinical outcome of two European centers

OBJECTIVES

The aim of this study is to assess the clinical outcome and mid-term efficacy of ultrasound-guided high-intensity focused ultrasound (USgHIFU) as a treatment for symptomatic uterine fibroids at two major European HIFU centers.

MATERIALS AND METHODS

This bi-center longitudinal clinical study involved the treatment of 100 patients with symptomatic uterine fibroids using USgHIFU (n = 59 in Germany, n = 41 in Bulgaria). Clinical outcomes were evaluated at 6 weeks, 6 months, and 1 year follow-up utilizing the uterine fibroid symptoms-quality of life questionnaire for fibroid-related symptoms and health-related quality of life as well as MRI imaging for determining the fibroid volume.

RESULTS

The mean fibroid volume reduction rate was 33.2 ± 22.9%, 51.3 ± 24.2%, and 59.1 ± 28.0% at 6 weeks, 6 months, and 1 year, respectively (each p < 0.001). The mean symptom severity score decreased from 43.9 ± 18.8 at baseline to 35.4 ± 18.2 at 6 weeks, 31.1 ± 20.0 at 6 months, and 23.1 ± 14.0 at 1 year (each p < 0.001). The mean QOL score improved from 56.5 ± 23.4 at baseline to 65.4 ± 22.2 at 6 weeks, 72.5 ± 19.5 at 6 months, and 79.4 ± 15.3 at 1 year (each p < 0.001). No major complications were observed, though two patients experienced temporary sciatic nerve irritation following the procedure. Four patients had pregnancies and deliveries without any complications after USgHIFU therapy.

CONCLUSION

To our knowledge, this is the first longitudinal study conducted in two major European HIFU centers that reveals the clinical efficacy of USgHIFU ablation on symptomatic uterine fibroids. Our results confirm that USgHIFU is a non-invasive approach with a low risk of complications, offering an innovative treatment option for affected women.

KEY POINTS

Question To evaluate mid-term clinical efficacy and safety of US-guided high-intensity focused ultrasound (HIFU) for treating symptomatic uterine fibroids and patient outcomes across two European centers. Findings US-guided HIFU treatment resulted in significant fibroid volume reduction (up to 59.1% after 1 year) improving symptoms and quality of life with no major complications. Clinical relevance This prospective longitudinal study provides preliminary data assessing mid-term efficacy and clinical outcomes of ultrasound-guided HIFU. It is shown to be a low-risk, non-invasive treatment option for symptomatic uterine fibroids that reduces fibroid size and improves patients' quality of life.

Predictive value of contrast-enhanced MRI for the regrowth of residual uterine fibroids after high-intensity focused ultrasound treatment

OBJECTIVES

To investigate whether the signal intensity (SI) ratio of residual fibroid (RF) to myometrium using Contrast-Enhanced Magnetic Resonance Imaging (CE-MRI) could predict fibroid regrowth after high-intensity focused ultrasound (HIFU) treatment.

MATERIALS AND METHODS

A retrospective analysis was conducted among 164 patients with uterine fibroids who underwent HIFU. To predict the RF regrowth, the SI perfusion parameters were quantified using the RF-myometrium SI ratio on CE-MRI on day 1 post-HIFU and then compared with the fibroid-myometrium SI ratio on the T2-weighted image (T2WI) and Funaki classification 1 year later. Thirty cases from another center were used as an external validation set to evaluate the performance of RF-myometrium SI ratio.

RESULTS

The predictive performance of the RF-myometrium SI ratio on CE-MRI on day 1 post-HIFU (Area Under Curve, AUC: 0.869) was superior to that of the preoperative and postoperative fibroid-myometrium SI ratios on the T2WI (AUC: 0.724, 0.696) and Funaki classification (AUC: 0.663, 0.623). Multivariate analysis showed that the RF- myometrium SI ratio and RF thickness were independent factors. The RF-myometrium SI ratio reflects the long-term rate of re-intervention (r = 0.455, p < 0.001).

CONCLUSION

The RF-myometrium SI ratio on CE-MRI exhibits greater accuracy in predicting RF regrowth compared to the SI classification and the SI ratio on T2WI.

CRITICAL RELEVANCE STATEMENT

The ratio of residual uterine fibroid to myometrial signal intensity on contrast-enhanced (CE)-MRI can reflect residual blood supply, predict regrowth of fibroids, and thus reflect long-term re-intervention rate and recovery situation of clinical high-intensity focused ultrasound (HIFU) treatment.

KEY POINTS

Contrast-enhanced (CE)-MRI can indicate the blood supply of residual uterine fibroids after high-intensity focused ultrasound (HIFU) treatment. The predictive capability of CE-MRI ratio surpasses T2WI ratio and the Funaki. Residual fibroids can serve as a measure of the long-term efficacy of HIFU.

Effect of "T2-rim sign" related parameters on high-intensity focused ultrasound ablation of uterine fibroids

PURPOSE

To investigate the effect of "high-signal-intensity peripheral rim on T2-weighted MR images (T2-rim sign)" related parameters on non-perfused volume ratio (NPVR) after high-intensity focused ultrasound (HIFU) ablation of uterine fibroids.

METHODS

Data from 616 patients with uterine fibroids treated with HIFU were retrospectively analyzed. Univariate and multivariate logistic regression was used to analyze the factors influencing the ablation effect. The effect of T2-rim sign on ablation parameters and results was also analyzed. Spearman correlation analysis was used to compare the correlation between coverage ratio, average thickness of T2-rim sign and NPVR in 207 cases of fibroids with T2-rim sign.

RESULTS

The presence of T2-rim sign was an independent risk factor affecting the ablation effect. The coverage ratio of T2-rim sign was negatively correlated with treatment efficiency (r = -0.174, p = 0.012) and NPVR (r = -0.186, p = 0.007), and positively correlated with energy efficiency factor (EEF) (r = 0.156, p = 0.024). The average thickness of T2-rim sign was positively correlated with treatment intensity (r = 0.203, p = 0.003) and negatively correlated with NPVR (r = -0.363, p < 0.001). There was a negative correlation between the average thickness of the T2-rim sign and NPVR in isointense fibroids (r = -0.484, p < 0.001).

CONCLUSION

The presence of T2-rim sign increases the difficulty of ablation and reduces the ablation effect. In clinical practice, the presence and related parameters of T2-rim sign should be fully considered when screening for HIFU indications and formulating treatment plans.

Therapeutic ultrasound transducer technology and monitoring techniques: a review with clinical examples

The exponential growth of therapeutic ultrasound applications demonstrates the power of the technology to leverage the combinations of transducer technology and treatment monitoring techniques to effectively control the preferred bioeffect to elicit the desired clinical effect.Objective: This review provides an overview of the most commonly used bioeffects in therapeutic ultrasound and describes existing transducer technologies and monitoring techniques to ensure treatment safety and efficacy.Methods and materials: Literature reviews were conducted to identify key choices that essential in terms of transducer design, treatment parameters and procedure monitoring for therapeutic ultrasound applications. Effective combinations of these options are illustrated through descriptions of several clinical indications, including uterine fibroids, prostate disease, liver cancer, and brain cancer, that have been successful in leveraging therapeutic ultrasound to provide effective patient treatments.Results: Despite technological constraints, there are multiple ways to achieve a desired bioeffect with therapeutic ultrasound in a target tissue. Visualizations of the interplay of monitoring modality, bioeffect, and applied acoustic parameters are presented that demonstrate the interconnectedness of the field of therapeutic ultrasound. While the clinical indications explored in this review are at different points in the clinical evaluation path, based on the ever expanding research being conducted in preclinical realms, it is clear that additional clinical applications of therapeutic ultrasound that utilize a myriad of bioeffects will continue to grow and improve in the coming years.Conclusions: Therapeutic ultrasound will continue to improve in the next decades as the combination of transducer technology and treatment monitoring techniques will continue to evolve and be translated in clinical settings, leading to more personalized and efficient therapeutic ultrasound mediated therapies.

A novel scoring system based on magnetic resonance imaging for the prediction of the difficulty of ultrasound-guided high-intensity focused ultrasound ablation for uterine fibroids

OBJECTIVE

To develop a novel scoring system based on magnetic resonance imaging (MRI) for predicting the difficulty of ultrasound-guided high-intensity focused ultrasound (USgHIFU) ablation for uterine fibroids.

MATERIALS AND METHODS

A total of 637 patients with uterine fibroids were enrolled. Sonication time, non-perfused volume ratio (NPVR), and ultrasound energy delivered for ablating 1 mm3 of fibroid tissue volume (E/V) were each classified as three levels and assigned scores from 0 to 2, respectively. Treatment difficulty level was then assessed by adding up the scores of sonication time, NPVR and E/V for each patient. The patients with score lower than 3 were categorized into low difficulty group, with score equal to or greater than 3 were categorized into high difficulty group. The potential predictors for treatment difficulty were compared between the two groups. Multifactorial logistic regression analysis model was created by analyzing the variables. The difficulty score system was developed using the beta coefficients of the logistic model.

RESULTS

Signal intensity on T2WI, fibroid location index, largest diameter of fibroids, abdominal wall thickness, homogeneity of the signal of fibroids, and uterine position were independent influencing factors for the difficulty of USgHIFU for uterine fibroids. A prediction equation was obtained: difficulty score = 17 × uterine position (anteverted =0, retroverted =1)+71 × signal intensity (hypointense = 0, isointense/hyperintense = 1) +8 × enhancement (homogenous = 0, heterogeneous = 1)+25×(largest diameter of fibroids-20) +35 × (fibroid location index -0.2) +1×(abdominal wall thickness -5).

CONCLUSIONS

This scoring system established based on MRI findings can be used to reliably predict the difficulty level of USgHIFU treatment of uterine fibroids.

The value of susceptibility weighted imaging for immediate assessing the hyperacute outcome of MRgFUS ablation for uterine fibroids: a preliminary study

PURPOSE

To investigate the value of susceptibility weighted imaging (SWI) for assessing the hyperacute outcome of ablation of uterine fibroids immediately after magnetic resonance-guided focused ultrasound (MRgFUS) treatment.

METHODS

This retrospective imaging study included patients who underwent SWI and contrast-enhanced (CE) MR within 15 min of MRgFUS ablation for uterine fibroids. Two readers independently assessed the SWI features of ablative lesions and their association with the non-perfused volume (NPV) ratio. The intraclass correlation coefficient (ICC) and diagnostic value of SWI findings were calculated.

RESULTS

A total of 27 uterine fibroids from 21 participants (mean age 40.1 ± 7.2 years) were analyzed. 51.9% (14/27) leiomyomas had NPV ratio ≥90%. In post-ablation SWI images, the interobserver ICC for the relative signal intensity and hypointense peripheral rim were 0.613 and 0.843, respectively (both p < .001). There was a significant difference in the prevalence of hypointense peripheral rim in leiomyomas with NPV ratio ≥90% and < 90% (p < .01), while the prevalence of relative signal intensity showed no significant difference (p > .05). When using the complete hypointense peripheral rim as a diagnostic criterion to identify NPV ratio ≥ 90%, readers 1 and 2 showed diagnostic sensitivity, specificity, and accuracy of 85.7%, 76.9%, 81.5%, and 78.6%, 76.9%, 77.8%, respectively.

CONCLUSION

Identifying a complete hypointense peripheral rim on SWI may be a potential imaging marker for assessing the hyperacute outcome of uterine fibroids ablation by MRgFUS, specifically in determining whether the NPV ratio is ≥90%.

A systematic review of preclinical and clinical transcranial ultrasound neuromodulation and opportunities for functional connectomics

BACKGROUND

Low-intensity transcranial ultrasound has surged forward as a non-invasive and disruptive tool for neuromodulation with applications in basic neuroscience research and the treatment of neurological and psychiatric conditions.

OBJECTIVE

To provide a comprehensive overview and update of preclinical and clinical transcranial low intensity ultrasound for neuromodulation and emphasize the emerging role of functional brain mapping to guide, better understand, and predict responses.

METHODS

A systematic review was conducted by searching the Web of Science and Scopus databases for studies on transcranial ultrasound neuromodulation, both in humans and animals.

RESULTS

187 relevant studies were identified and reviewed, including 116 preclinical and 71 clinical reports with subjects belonging to diverse cohorts. Milestones of ultrasound neuromodulation are described within an overview of the broader landscape. General neural readouts and outcome measures are discussed, potential confounds are noted, and the emerging use of functional magnetic resonance imaging is highlighted.

CONCLUSION

Ultrasound neuromodulation has emerged as a powerful tool to study and treat a range of conditions and its combination with various neural readouts has significantly advanced this platform. In particular, the use of functional magnetic resonance imaging has yielded exciting inferences into ultrasound neuromodulation and has the potential to advance our understanding of brain function, neuromodulatory mechanisms, and ultimately clinical outcomes. It is anticipated that these preclinical and clinical trials are the first of many; that transcranial low intensity focused ultrasound, particularly in combination with functional magnetic resonance imaging, has the potential to enhance treatment for a spectrum of neurological conditions.

Diffusion-weighted imaging as a potential non-gadolinium alternative for immediate assessing the hyperacute outcome of MRgFUS ablation for uterine fibroids

The aim of this study was to investigate the value of diffusion-weighted imaging (DWI) as a potential non-gadolinium alternative for promptly assessing the hyperacute outcome of magnetic resonance-guided focused ultrasound (MRgFUS) treatment for uterine fibroids. In this retrospective study we included 65 uterine fibroids from 44 women, who underwent axial DWI (b-value: 800 s/mm2) and contrast-enhanced (CE) MR within 15 min post-ablation. Two blinded observers independently reviewed the DWI findings of ablated necrotic lesions and measured their volumes on DWI and CE images. The post-ablation DWI images revealed clear depiction of ablative necrotic lesions in all fibroids, which were classified into two types: the bull's eye sign (type 1) and the bright patch sign (type 2). The inter-observer intraclass correlation coefficient for classifying DWI signal types was 0.804 (p < 0.001). Volumetric analysis of ablated necrosis using DWI and CE T1-weighted imaging showed no significant variance, nor did the non-perfused volume ratios (all p > 0.05). Bland-Altman analysis revealed a mean difference of 2.38% and 1.71% in non-perfused volume ratios between DWI and CE, with 95% limits of agreement from - 19.06 to 23.82% and - 18.40 to 21.82%, respectively. The findings of this study support the potential of DWI as a viable non-gadolinium alternative for evaluating the hyperacute outcomes of MRgFUS ablation in uterine fibroids.

Transcutaneous Ablation of Lung Tissue in a Porcine Model Using Magnetic-Resonance-Guided Focused Ultrasound (MRgFUS)

Focused ultrasound (FUS) is a minimally invasive treatment that utilizes high-energy ultrasound waves to thermally ablate tissue. Magnetic resonance imaging (MRI) guidance may be combined with FUS (MRgFUS) to increase its accuracy and has been proposed for lung tumor ablation/debulking. However, the lungs are predominantly filled with air, which attenuates the strength of the FUS beam. This investigation aimed to test the feasibility of a new approach using an intentional lung collapse to reduce the amount of air inside the lung and a controlled hydrothorax to create an acoustic window for transcutaneous MRgFUS lung ablation. Eleven pigs had one lung mechanically ventilated while the other lung underwent a controlled collapse and subsequent hydrothorax of that hemisphere. The MRgFUS lung ablations were then conducted via the intercostal space. All the animals recovered well and remained healthy in the week following the FUS treatment. The location and size of the ablations were confirmed one week post-treatment via MRI, necropsy, and histological analysis. The animals had almost no side effects and the skin burns were completely eliminated after the first two animal studies, following technique refinement. This study introduces a novel methodology of MRgFUS that can be used to treat deep lung parenchyma in a safe and viable manner.

A Fully Electronically Steerable Therapeutic Ultrasound Phased Array With MR-Guidance

Recently, MRI-guided focused ultrasound (FUS) has shown great promise in treating various conditions non-invasively.

OBJECTIVE

The focus of this article is to introduce an MRI-guided FUS device, which can provide full electronic steering range without mechanical movement and with low near-field heating. A pilot study was conducted in order to investigate the feasibility, and safety of the device in a large animal model and a pilot clinical trial.

METHODS

A flat, fully steerable FUS phased array with 4096 elements was designed and manufactured to be compatible with an MR scanner. Pre-clinical experiments were carried out for testing the accuracy of the focus at different steering angles as well as evaluating the ablation efficiency using MR thermometry. Eleven patients with uterine fibroids were treated in the pilot clinical trial.

RESULTS

Pre-clinical results showed successful ablation at various steering angles with reasonable targeting accuracy and no off-target heating. During the pilot clinical study, effective fibroid ablation was achieved with significant symptom reduction observed over time. In general, the treatment results showed the system to be effective in ablating deep tissue volumes. The device was successful at efficiently ablating large volumes with minimal near-field heating and eliminating the need for mechanical translation.

CONCLUSIONS

Being capable of providing high acoustic power, full electronic steering range in 3D for large volume ablations, this device can provide a safe and efficient treatment option as an outpatient procedure for uterine fibroids and other pelvic and abdominal tumors.

Deep-Learning-Based High-Intensity Focused Ultrasound Lesion Segmentation in Multi-Wavelength Photoacoustic Imaging

Photoacoustic (PA) imaging can be used to monitor high-intensity focused ultrasound (HIFU) therapies because ablation changes the optical absorption spectrum of the tissue, and this change can be detected with PA imaging. Multi-wavelength photoacoustic (MWPA) imaging makes this change easier to detect by repeating PA imaging at multiple optical wavelengths and sampling the optical absorption spectrum more thoroughly. Real-time pixel-wise classification in MWPA imaging can assist clinicians in monitoring HIFU lesion formation and will be a crucial milestone towards full HIFU therapy automation based on artificial intelligence. In this paper, we present a deep-learning-based approach to segment HIFU lesions in MWPA images. Ex vivo bovine tissue is ablated with HIFU and imaged via MWPA imaging. The acquired MWPA images are then used to train and test a convolutional neural network (CNN) for lesion segmentation. Traditional machine learning algorithms are also trained and tested to compare with the CNN, and the results show that the performance of the CNN significantly exceeds traditional machine learning algorithms. Feature selection is conducted to reduce the number of wavelengths to facilitate real-time implementation while retaining good segmentation performance. This study demonstrates the feasibility and high performance of the deep-learning-based lesion segmentation method in MWPA imaging to monitor HIFU lesion formation and the potential to implement this method in real time.

High-Intensity Focused Ultrasound Ablation of Uterine Fibroids: A Review

Leiomyomas, or uterine fibroids, are growths consisting of muscle and tissue that develop in or on the uterine wall. The most frequent benign uterine tumours in women of reproductive age are thought to be fibroids. Dysmenorrhea, spotting, hypermenorrhoea, abdominal pain, pressure on surrounding organs, and issues with micturition and defecation are among the symptoms that are often present. Fibroids can form as a single nodule or as a cluster. Uterine fibroids, especially large submucosal and intramural uterine fibroids, can cause obstacles to implantation and lead to pregnancy loss. Uterine fibroids can be treated without surgery and with little downtime using focused ultrasound. There is published research showing that women can conceive and have healthy children after therapy, thus protecting fertility. The ablation of uterine fibroids by high-intensity focused ultrasound (HIFU) is successful since the volume of the fibroids is significantly reduced.

Ultrasound image segmentation using Gamma combined with Bayesian model for focused-ultrasound-surgery lesion recognition

This study aims to investigate the feasibility of combined segmentation for the separation of lesions from non-ablated regions, which allows surgeons to easily distinguish, measure, and evaluate the lesion area, thereby improving the quality of high-intensity focused-ultrasound (HIFU) surgery used for the non-invasive tumor treatment. Given that the flexible shape of the Gamma mixture model (GΓMM) fits the complex statistical distribution of samples, a method combining the GΓMM and Bayes framework is constructed for the classification of samples to obtain the segmentation result. An appropriate normalization range and parameters can be used to rapidly obtain a good performance of GΓMM segmentation. The performance values of the proposed method under four metrics (Dice score: 85%, Jaccard coefficient: 75%, recall: 86%, and accuracy: 96%) are better than those of conventional approaches including Otsu and Region growing. Furthermore, the statistical result of sample intensity indicates that the finding of the GΓMM is similar to that obtained by the manual method. These results indicate the stability and reliability of the GΓMM combined with the Bayes framework for the segmentation of HIFU lesions in ultrasound images. The experimental results show the possibility of combining the GΓMM with the Bayes framework to segment lesion areas and evaluate the effect of therapeutic ultrasound.

Review of Robot-Assisted HIFU Therapy

This paper provides an overview of current robot-assisted high-intensity focused ultrasound (HIFU) systems for image-guided therapies. HIFU is a minimally invasive technique that relies on the thermo-mechanical effects of focused ultrasound waves to perform clinical treatments, such as tumor ablation, mild hyperthermia adjuvant to radiation or chemotherapy, vein occlusion, and many others. HIFU is typically performed under ultrasound (USgHIFU) or magnetic resonance imaging guidance (MRgHIFU), which provide intra-operative monitoring of treatment outcomes. Robot-assisted HIFU probe manipulation provides precise HIFU focal control to avoid damage to surrounding sensitive anatomy, such as blood vessels, nerve bundles, or adjacent organs. These clinical and technical benefits have promoted the rapid adoption of robot-assisted HIFU in the past several decades. This paper aims to present the recent developments of robot-assisted HIFU by summarizing the key features and clinical applications of each system. The paper concludes with a comparison and discussion of future perspectives on robot-assisted HIFU.

Combination of Focused Ultrasound, Immunotherapy, and Chemotherapy: New Perspectives in Breast Cancer Therapy

Focused ultrasound is a treatment modality increasingly used for diverse therapeutic applications, and currently approved for several indications, including prostate cancers and uterine fibroids. But what about breast cancer? Breast cancer is the most common and deadliest cancer in women worldwide. While there are different treatment strategies available, there is a need for development of more effective and personalized modalities, with fewer side effects. Therapeutic ultrasound is such an option, and this review summarizes the state of the art in their use for the treatment of breast cancer and evaluate potentials of novel treatment approaches combining therapeutic ultrasound, immuno- and chemo-therapies.

Histological findings in resected leiomyomas following MR-HIFU treatment, single-institution data from seven patients with unfavorable focal therapy

PURPOSE

Magnetic resonance - high-intensity focused ultrasound (MR-HIFU) is a noninvasive treatment option for symptomatic uterine leiomyomas. Currently, pretreatment MRI is used to assess tissue characteristics and predict the most likely therapeutic response for individual patients. However, these predictions still entail significant uncertainties. The impact of tissue properties on therapeutic outcomes remains poorly understood and detailed knowledge of the histological effects of ultrasound ablation is lacking. Investigating these aspects could aid in optimizing patient selection, enhancing treatment effects and improving treatment outcomes.

METHODS AND MATERIALS

We present seven patients who underwent MR-HIFU treatment for leiomyoma followed by second-line surgical treatment. Tissue samples obtained during the surgery were stained with hematoxylin and eosin, Masson's trichrome and Herovici to evaluate general morphology, fibrosis and collagen deposition of leiomyomas. Immunohistochemical CD31, Ki-67 and MMP-2 stainings were performed to study vascularization, proliferation and matrix metalloproteinase-2 protein expression in leiomyomas, respectively.

RESULTS

The clinical characteristics and radiological findings of the leiomyomas prior to treatment as well as qualitative histological findings after the treatment are presented and discussed in the context of current literature. A tentative model for volume reduction is presented.

CONCLUSION

These findings provide insights into potential factors contributing to suboptimal therapeutic outcomes and the variability in histological changes following treatment.

A feasibility study of wireless inductively coupled surface coil for MR-guided high-intensity focused ultrasound ablation of rodents on clinical MRI systems

Recently, to conduct preclinical imaging research on clinical MRI systems has become an attractive alternative to researchers due to its wide availability, cost, and translational application to clinical human studies when compared to dedicated small animal, high-field preclinical MRI. However, insufficient signal-to-noise ratio (SNR) significantly degrades the applicability of those applications which require high SNR, e.g. magnetic resonance guided high-intensity focused ultrasound (MRgHIFU) treatment. This study introduces a wireless inductively coupled surface (WICS) coil design used on a clinical 3 T MRI system for MRgHIFU ablation. To evaluate the SNR improvement and temperature accuracy of WICS coil, the ex vivo experiments were performed on the pork tenderloins (n = 7) and the hind legs of deceased Sprague-Dawley rats (n = 5). To demonstrate the feasibility, the in vivo experiments were performed on the hind leg of Sprague-Dawley rat (n = 1). For all experiments, temperature measurements were performed before and during HIFU ablation. Temperature curves with and without WICS coil were compared to evaluate the temperature precision in ex vivo experiments. The use of WICS coil improves the temperature accuracy from 0.85 to 0.14 °C, demonstrating the feasibility of performing small animal MRgHIFU experiments using clinical 3 T MRI system with WICS coil.

Optimised polymer trapped-air lenses for ultrasound focusing in water exploiting Fabry-Pérot resonance

The concept of employing air volumes trapped inside polymer shells to make a lens for ultrasound focusing in water is investigated. The proposed lenses use evenly-spaced concentric rings, each having an air-filled polymer shell construction, defining concentric water-filled channels. Numerical simulations and experiments have shown that a plane wave can be focused, and that the amplification can be boosted by Fabry-Pérot resonances within the water channels with an appropriate choice of the lens thickness. The effect of the polymer shell thickness and the depth of the channels is discussed, as these factors can affect the geometry and hence the frequency of operation. The result was a lens with a Full Width at Half Maximum value of 0.65 of a wavelength at the focus. Results obtained on a metal-based counterpart are also shown for comparison. An advantage of this polymeric design is that it is easily constructed via additive manufacturing. This study shows that trapped-air lenses made of polymer are suitable for ultrasound focusing in water near 500 kHz.

Magnetic resonance imaging parameter-based machine learning for prognosis prediction of high-intensity focused ultrasound ablation of uterine fibroids

Objectives: To develop and apply magnetic resonance imaging (MRI) parameter-based machine learning (ML) models to predict non-perfused volume (NPV) reduction and residual regrowth of uterine fibroids after high-intensity focused ultrasound (HIFU) ablation.Methods: MRI data of 573 uterine fibroids in 410 women who underwent HIFU ablation from the Chongqing Haifu Hospital (training set, N = 405) and the First Affiliated Hospital of Chongqing Medical University (testing set, N = 168) were retrospectively analyzed. Fourteen MRI parameters were screened for important predictors using the Boruta algorithm. Multiple ML models were constructed to predict NPV reduction and residual fibroid regrowth in a median of 203.0 (interquartile range: 122.5-367.5) days. Furthermore, optimal models were used to plot prognostic prediction curves.Results: Fourteen features, including postoperative NPV, indicated predictive ability for NPV reduction. Based on the 10-fold cross-validation, the best average performance of multilayer perceptron achieved with R² was 0.907. In the testing set, the best model was linear regression (R² =0.851). Ten features, including the maximum thickness of residual fibroids, revealed predictive power for residual fibroid regrowth. Random forest model achieved the best performance with an average area under the curve (AUC) of 0.904 (95% confidence interval (CI), 0.869-0.939), which was maintained in the testing set with an AUC of 0.891 (95% CI, 0.850-0.929).Conclusions: ML models based on MRI parameters can be used for prognostic prediction of uterine fibroids after HIFU ablation. They can potentially serve as a new method for learning more about ablated fibroids.

Combined Therapy Planning, Real-Time Monitoring, and Low Intensity Focused Ultrasound Treatment Using a Diagnostic Imaging Array

Low intensity focused ultrasound (FUS) therapies use low intensity focused ultrasound waves, typically in combination with microbubbles, to non-invasively induce a variety of therapeutic effects. FUS therapies require pre-therapy planning and real-time monitoring during treatment to ensure the FUS beam is correctly targeted to the desired tissue region. To facilitate more streamlined FUS treatments, we present a system for pre-therapy planning, real-time FUS beam visualization, and low intensity FUS treatment using a single diagnostic imaging array. Therapy planning was accomplished by manually segmenting a B-mode image captured by the imaging array and calculating a sonication pattern for the treatment based on the user-input region of interest. For real-time monitoring, the imaging array transmitted a visualization pulse which was focused to the same location as the FUS therapy beam and ultrasonic backscatter from this pulse was used to reconstruct the intensity field of the FUS beam. The therapy planning and beam monitoring techniques were demonstrated in a tissue-mimicking phantom and in a rat tumor in vivo while a mock FUS treatment was carried out. The FUS pulse from the imaging array was excited with an MI of 0.78, which suggests that the array could be used to administer select low intensity FUS treatments involving microbubble activation.

Comparison of computer simulations and clinical treatment results of magnetic resonance-guided focused ultrasound surgery (MRgFUS) of uterine fibroids

PURPOSE

Magnetic resonance-guided focused ultrasound surgery (MRgFUS) can be used to noninvasively treat symptomatic uterine fibroids by heating with focused ultrasound sonications while monitoring the temperature with magnetic resonance (MR) thermometry. While prior studies have compared focused ultrasound simulations to clinical results, studies involving uterine fibroids remain scarce. In our study, we perform such a comparison to assess the suitability of simulations for treatment planning.

METHODS

Sonications (N = 67) were simulated retrospectively using acoustic and thermal models based on the Rayleigh integral and Pennes bioheat equation followed by MR-thermometry simulation in seven patients who underwent MRgFUS treatment for uterine fibroids. The spatial accuracy of simulated focus location was assessed by evaluating displacements of the centers of mass of the thermal dose distributions between simulated and treatment MR thermometry slices. Temperature-time curves and sizes of 240 equivalent minutes at 43°C (240EM43 ) volumes between treatment and simulation were compared.

RESULTS

The simulated focus location showed errors of 2.7 ± 4.1, -0.7 ± 2.0, and 1.3 ± 1.2 mm (mean ± SD) in the anterior-posterior, foot-head, and right-left directions for a fibroid absorption coefficient of 4.9 Np m-1  MHz-1 and perfusion parameter of 1.89 kg m-3 s-1 . Linear regression of 240EM43 volumes of 67 sonications of patient treatments and simulations utilizing these parameters yielded a slope of 1.04 and a correlation coefficient of 0.54. The temperature rise ratio of simulation to treatment near the end of sonication was 0.47 ± 0.22, 1.28 ± 0.60, and 1.49 ± 0.71 for 66 sonications simulated utilizing fibroid absorption coefficient of 1.2, 4.9, and 8.6 Np m-1  MHz-1 , respectively, and the aforementioned perfusion value. The impact of perfusion on peak temperature rise is minimal between 1.89 and 10 kg m-3 s-1 , but became more substantial when utilizing a value of 100 kg m-3 s-1 .

CONCLUSIONS

The results of this study suggest that perfusion, while in some cases having a substantial impact on thermal dose volumes, has less impact than ultrasound absorption for predicting peak temperature elevation at least when using perfusion parameter values up to 10 kg m-3 s-1 for this particular array geometry, frequencies, and tissue target which is good for clinicians to be aware of. The results suggest that simulations show promise in treatment planning, particularly in terms of spatial accuracy. However, in order to use simulations to predict temperature rise due to a sonication, knowledge of the patient-specific tissue parameters, in particular the absorption coefficient is important. Currently, spatially varying patient-specific tissue parameter values are not available during treatment, so simulations can only be used for planning purposes to estimate sonication performance on average.

High-intensity focused ultrasound for noninvasive fetal therapy

High-intensity focused ultrasound (HIFU) consists of an ultrasonic beam that is focused within the body to induce tissue necrosis through both heat energy and as a result of cavitation, which occurs without damaging any intervening tissues. Therefore, it is possible to cauterize and treat tumors without surgical invasion by administering HIFU irradiation from outside the body. This approach has been clinically applied in various fields in recent years, and fetal therapy is no exception, with several clinical applications reported, mainly in basic experiments. This review summarizes the recent basic and clinical findings focusing on fetal treatment with HIFU.

Ultrasounds in cancer therapy: A summary of their use and unexplored potential

Ultrasounds (US) are a non-ionizing mechanical wave, with less adverse effects than conventional pharmacological or surgical treatments. Different biological effects are induced in tissues and cells by ultrasound actuation depending on acoustic parameters, such as the wave intensity, frequency and treatment dose. This non-ionizing radiation has considerable applications in biomedicine including surgery, medical imaging, physical therapy and cancer therapy. Depending on the wave intensity, US are applied as high-intensity ultrasounds (HIUS) and low-intensity pulsed ultrasounds (LIPUS), with different effects on cells and tissues. HIUS produce thermal and mechanical effects, resulting in a large localized temperature increase, leading to tissue ablation and even tumor necrosis. This can be achieved by focusing low intensity waves emitted from different electrically shifted transducers, known as high-intensity focused ultrasounds (HIFU). LIPUS have been used extensively as a therapeutic, surgical and diagnostic tool, with diverse biological effects observed in tissues and cultured cells. US represent a non-invasive treatment strategy that can be applied to selected areas of the body, with limited adverse effects. In fact, tumor ablation using HIFU has been used as a curative treatment in patients with an early-stage pancreatic tumor and is an effective palliative treatment in patients with advanced stage disease. However, the biological effects, dose standardization, benefit-risk ratio and safety are not fully understood. Thus, it is an emerging field that requires further research in order to reach its full potential.

Multiple-Focus Patterns of Sparse Random Array Using Particle Swarm Optimization for Ultrasound Surgery

This study aims to investigate the feasibility and potential of sparse random arrays driven by the particle swarm optimization (PSO) algorithm to generate multiple-focus patterns and a large scanning range without grating lobes, which extends the scanning range of focused ultrasound in the treatment of brain tumors, opening the blood-brain barrier, and neuromodulation. Operating at 1.1 MHz, a random spherical array with 200 square elements (sparseness 58%) and a sparse random array with 660 square elements (sparseness 41%) driven by PSO are employed to simulate different focus patterns. With the same radius of curvature and diameter of transducer and element size, the scanning range of the off-axis single focus of a random 200-element array is two times that of an ordinary array using symmetric arrangement. The focal volume of multiple-focus patterns of the random array is 18 times that of the single focus. The single focus of the sparse random array with 660 elements could steer up to ±23 mm in the radial direction, without grating lobes. The maximum distance between two foci in a multiple-focus "S"-shaped deflection is approximately 25 mm. Simulation results illustrate the capability of a focused beam steered in 3-D space. Multiple-focus patterns could significantly increase the focal volume and shorten the treatment time for large target volumes. Simulation results show the feasibility and potential of the method combining PSO with a sparse random array to generate flexible focus patterns that can adapt to different needs in different tissue treatments.

Control of inflammation using non-invasive neuromodulation: past, present and promise

The nervous system has been increasingly recognized as a novel and accessible target in the regulation of inflammation. The use of implantable and invasive devices targeting neural circuits has yielded successful results in clinical settings but does have some risk or adverse effects. Recent advances in technology and understanding of mechanistic pathways have opened new avenues of non-invasive neuromodulation. Through this review we discuss the novel research and outcomes of major modalities of non-invasive neuromodulation in the context of inflammation including transcutaneous electrical, magnetic and ultrasound neuromodulation. In addition to highlighting the scientific observations and breakthroughs, we discuss the underlying mechanisms and pathways for neural regulation of inflammation.

Comparison of Dose and Effectiveness of a Single-Session Ultrasound-Guided High-Intensity Focused Ultrasound Ablation of Uterine Fibroids With Different Sizes

PURPOSE

This study aimed to compare the dose and effectiveness of ultrasound-guided high-intensity focused ultrasound (USgHIFU) ablation of uterine fibroids with different sizes and explore the effect of uterine fibroid size on dose, which provided dose evaluation for clinicians in accordance with the size of uterine fibroids.

MATERIALS AND METHODS

A total of 1,000 patients with symptomatic uterine fibroids who received a single-session USgHIFU treatment were enrolled in this study. The size of fibroids was divided into seven groups: 3-4 cm, 4-5 cm, 5-6 cm, 6-7 cm, 7-8 cm, 8-9 cm, and 9-11 cm. The dose was expressed on the basis of the energy efficiency factor (EEF) as the energy required for ablation per unit volume of tissue, and the non-perfused volume ratio (NPVR) was used to assess the effect of HIFU ablation.

RESULTS

The median NPVR of 88.3% (IQR: 80.3%-94.8%) was obtained, and no significant difference was observed among the seven groups. The classification of T2-weighted image signal intensity fibroids in the 4-5 cm group was compared with that in the 6-7 cm and 8-9 cm groups, and the difference was significant (p < 0.05). However, the proportion of T2WI hyperintense signal fibroids had no significant difference among the seven groups (p > 0.05). The median EEF was 3.88 J/mm³, and a significant difference was observed among the seven groups of EEF (p < 0.05). The EEF of groups with a fibroid size less than 6 cm was more than double the EEF of groups with a fibroid size above 6 cm. In addition, the EEF of groups with a fibroid size of 4-5 cm and 3-4 cm was 3-4 times higher than those with a fibroid size above 7 cm (p < 0.05).

CONCLUSIONS

A single-session HIFU ablation for uterine fibroids of 3-11 cm can obtain an NPVR of more than 80%. The EEF decreased with the increase of the size of uterine fibroids. A fibroid size of 6.5 cm was considered as a clinical meaningful point affecting EEF.

Overcoming delivery barriers in immunotherapy for glioblastoma

Glioblastoma is one of the deadliest forms of primary adult tumors, with median survival of 14.6 months post-diagnosis despite aggressive standard of care treatment. This grim prognosis for glioblastoma patients has changed little in the past two decades, necessitating novel treatment modalities. One potential treatment modality is cancer immunotherapy, which has shown remarkable progress in slowing disease progression or even potentially curing certain solid tumors. However, the transport barriers posed by the blood-brain barrier and the immune privileged status of the central nervous system pose drug delivery obstacles that are unique to brain tumors. In this review, we provide an overview of the various physiological, immunological, and drug delivery barriers that must be overcome for effective glioblastoma treatment. We discuss chemical modification strategies to enable nanomedicines to bypass the blood-brain barrier and reach intracranial tumors. Finally, we highlight recent advances in biomaterial-based strategies for cancer immunotherapy that can be adapted to glioblastoma treatment.

A feasibility analysis of the ArcBlate MR-guided high-intensity focused ultrasound system for the ablation of uterine fibroids

Purpose

Uterine fibroids are benign gynecologic tumors and commonly occur in women by the age of 50. Women with symptomatic uterine fibroids generally receive surgical intervention, while they do not favor the invasive therapies. To evaluate the feasibility and safety of a novel magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU) modality, ArcBlate, in the treatment of uterine fibroids.

Methods

Nine patients with uterine fibroids and one patient with adenomyosis were treated with ArcBlate MRgHIFU. Tumor size and quality of life were evaluated postoperatively at 1 and 3 months by magnetic resonance imaging (MRI) and the 36-Item Short Form Survey (SF-36), respectively.

Results

All patients completed the ArcBlate MRgHIFU procedure and there were no treatment-related adverse effects either during the procedure or during the 3 months of follow-up. Despite limiting the ablation volume to under 50% of the treated fibroid volume as a safety precaution, tumor volumes were markedly reduced in four patients by 15.78–58.87% at 3-month post-treatment. Moreover, SF-36 scale scores had improved at 3 months from baseline by 2–8 points in six patients, indicating relief of symptoms and improved quality of life.

Conclusion

This study evidence demonstrates the safety and feasibility of ArcBlate MRgHIFU and suggests its potential for treating uterine fibroids.

Development and application of ultrasound contrast agents in biomedicine

With the rapid development of molecular imaging, ultrasound (US) medicine has evolved from traditional imaging diagnosis to integrated diagnosis and treatment at the molecular level. Ultrasound contrast agents (UCAs) play a crucial role in the integration of US diagnosis and treatment. As the micro-bubbles (MBs) in UCAs can enhance the cavitation effect and promote the biological effect of US, UCAs have also been studied in the fields of US thrombolysis, mediated gene transfer, drug delivery, and high intensity focused US. The application range of UCAs is expanding, and the value of their applications is improving. This paper reviews the development and application of UCAs in biomedicine in recent years, and the existing problems and prospects are pointed out.

Ultrasound-guided high intensity focused ultrasound ablation for uterine fibroids: long-term outcomes and factors affecting local recurrence

OBJECTIVE

To assess the long-term outcomes and the factors affecting local recurrence of uterine fibroids after ultrasound-guided high-intensity focused ultrasound (USgHIFU) ablation.

MATERIALS AND METHODS

629 patients with a solitary uterine fibroid smaller than 10 cm in diameter treated with USgHIFU at our institutes between January 2011 and December 2016 were retrospectively analyzed. The patients were requested to take pre-HIFU and one day post-HIFU MRI. The patients were asked to return to the hospital every 3 months until January 2020, for imaging evaluation and to check on improvement in symptoms.

RESULTS

Five hundred and thirty-six patients completed follow-up according to our protocol. The median follow-up time was 69 (interquartile range: 48 to 89) months. Among them, local recurrence was detected in 110 patients. 18 (16.4%) patients required additional treatment between 12 and 24 months after USgHIFU treatment, 59 (53.6%) patients required additional treatment 24 months after USgHIFU. Therefore, in total, 77 patients required additional treatment, of which 32 received USgHIFU and 45 underwent myomectomy. The median non-perfused volume (NPV) ratio in patients with recurrence was 73%, compared to 89% among patients without recurrence. Multivariate analysis showed that NPV ratio, maximum fibroid diameter and fibroid enhancement type were the independent factors affecting the recurrence of fibroids after USgHIFU treatment.

CONCLUSIONS

Achievement of NPV ratio higher than 70% has led to acceptable re-intervention rate during the follow-up period after USgHIFU. NPV ratio, maximum fibroid diameter, and fibroid enhancement type were the independent factors affecting the recurrence of fibroids after USgHIFU treatment.

Anatomical Phase Extraction (APE) Method: A Novel Method to Correct Detrimental Effects of Tissue-Inhomogeneity in Referenceless MR Thermometry-Preliminary Ex Vivo Investigation

Purpose

We present a novel background tissue phase removing method, called anatomical phase extraction (APE), and to investigate the accuracy of temperature estimation and capability of reducing background artifacts compared with the conventional referenceless methods.

Methods

Susceptibility variance was acquired by subtracting pretreatment baseline images taken at different locations (nine pretreatment baselines are acquired and called φ₁ to φ₉). The susceptibility phase data φ_S was obtained using the Wiener deconvolution algorithm. The background phase data φ_T was isolated by subtracting φ_S from the whole phase data. Finally, φ_T was subtracted from the whole phase data before applying the referenceless method. As a proof of concept, the proposed APE method was performed on ex vivo pork tenderloin and compared with other two referenceless temperature estimation approaches, including reweighted ℓ1 referenceless (RW- ℓ1) and ℓ2 referenceless methods. The proposed APE method was performed with four different baselines combination, namely, (φ₁, φ₅, φ₂, φ₄), (φ₃, φ₅, φ₂, φ₆), (φ₇, φ₅, φ₈, φ₄), and (φ₉, φ₅, φ₈, φ₆), and called APE experiment 1 to 4, respectively. The multibaseline method was used as a standard reference. The mean absolute error (MAE) and two-sample t-test analysis in temperature estimation of three regions of interest (ROI) between the multibaseline method and the other three methods, i.e., APE, RW- ℓ1, and ℓ2, were calculated and compared.

Results

Our results show that the mean temperature errors of the APE method-experiment 1, APE method-experiment 2, APE method-experiment 3, APE method-experiment 4, and RW- ℓ1 and ℓ2 referenceless method are 1.02°C, 1.04°C, 1.00°C, 1.00°C, 4.75°C, and 13.65°C, respectively. The MAEs of the RW- ℓ1 and ℓ2 referenceless methods were higher than that of APE method. The APE method showed no significant difference (p > 0.05), compared with the multibaseline method.

Conclusion

The present work demonstrates the use of the APE method on referenceless MR thermometry to improve the accuracy of temperature estimation during MRI guided high-intensity focused ultrasound for ablation treatment.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

Long-term follow-up outcome and reintervention analysis of ultrasound-guided high intensity focused ultrasound treatment for uterine fibroids

OBJECTIVE

To investigate the long-term reintervention of ultrasound-guided high intensity focused ultrasound (USgHIFU) treatment for uterine fibroids and analyze the factors affecting reintervention rate after USgHIFU.

MATERIALS AND METHODS

Three hundred and eight-one patients with uterine fibroids treated by USgHIFU at the third Xiangya Hospital of Central South University from April 2012 to December 2014 were retrospectively reviewed. The factors that affect the reintervention rate were analyzed.

RESULTS

The mean follow-up time was 70.0 ± 9.0 months. During the follow-up period, 86.4% (329/381) of the patients reported symptomatic relief and the fibroids shrank after USgHIFU treatment. Seventy-nine patients received reintervention included myomectomy, a second session of HIFU, and hysterectomy. The overall reintervention rate was 20.7% (79/381). The reasons for reintervention included symptomatic recurrence in 50 (50/79, 63.3%) patients, psychological factors in 14 (14/79, 17.7%) patients, fertility requirement in three (3/79, 3.8%) patients, suspected uterine sarcoma in two (2/79, 2.5%) patients and others in 10 (10/79, 12.7%) patients. The reintervention rate has significant correlation with some factors including age, size, type and the signal intensity on T2 weighted image (T2WI) of the uterine fibroids.

CONCLUSION

USgHIFU for uterine fibroids is effective due to low reintervention rate in a long-term follow-up.

Foundations of Magnetic Resonance-Guided Focused Ultrasonography

The ability of ultrasonography to safely penetrate deeply into the brain has made it an attractive technology for neurological applications for almost 1 century. Having recognized that converging ultrasound waves could deliver high levels of energy to a target and spare the overlying and surrounding brain, early applications used craniotomies to allow transducers to contact the brain or dural surface. The development of transducer arrays that could permit the transit of sufficient numbers of ultrasound waves to deliver high energies to a target, even with the loss of energy from the skull, has now resulted in clinical systems that can permit noninvasive focused ultrasound procedures that leave the skull intact. Another major milestone in the field was the marriage of focused ultrasonography with magnetic resonance thermometry. This provides real-time feedback regarding the level and location of brain tissue heating, allowing for precise elevation of temperatures within a desired target to lead to focal therapeutic lesions. The major clinical use of this technology, at present, has been limited to treatment of refractory essential tremor and parkinsonian tremor, although the first study of this approach had targeted sensory thalamus for refractory pain, and new targets and disease indications are under study. Finally, focused ultrasonography can also be used at a lower frequency and energy level when combined with intravenous microbubbles to create cavitations, which will open the blood-brain barrier rather than ablate tissue. In the present review, we have discussed the historical and scientific foundations and current clinical applications of magnetic resonance-guided focused ultrasonography and the genesis and background that led to the use of this technique for focal blood-brain barrier disruption.

A Prediction of NPVR ≥ 80% of Ultrasound-Guided High-Intensity Focused Ultrasound Ablation for Uterine Fibroids

Objective: To evaluate factors in predicting the treatment outcome of ultrasound-guided high-intensity focused ultrasound (USgHIFU) ablation for uterine fibroids with a non-perfused volume ratio (NPVR) of at least 80%.

Methods: One thousand patients with uterine fibroids who received USgHIFU were enrolled. Thirty-two independent variables of four dimensions of data set, including general information of patients, clinical symptoms, laboratory tests, and fibroid imaging characteristics, were used to investigate the potential predictors of the NPVR of at least 80% by multivariate logistic regression. NPVR was the gold standard for evaluating the efficiency of HIFU ablation, and a NPVR of at least 80% was considered sufficient ablation, while partial ablation was defined as having an NPVR of <80%.

Results: Out of 1,000 fibroids, 758 obtained sufficient ablation and 242 obtained partial ablation, and the median NPVR was 88.3% (interquartile range: 80.3–94.8%). The probability of NPVR reaching 80% fibroids with a signal intensity of T2WI of hypointense, isointense, and hyperintense was 86.4, 76.5, and 62.6%, respectively; fibroids with an enhancement type of T1WI of slight, irregular, and regular was 81.5, 73.6, and 63.7%, respectively; and fibroids with uterine anteroposterior of 30–130 mm was 57.7–78.3%, respectively. In patients with a platelet count of 50 × 10⁹/L−550 × 10⁹/L, the probability of NPVR reaching 80% is from 53.4 to 80.1%, respectively.

Conclusions: In predicting NPVR ≥ 80%, the signal intensity on T2WI was the most important factor affecting ablative efficiency, followed by enhancement type on T1WI, uterine anteroposterior, and platelet count.

Learning Multiparametric Biomarkers for Assessing MR-Guided Focused Ultrasound Treatment of Malignant Tumors

Noninvasive MR-guided focused ultrasound (MRgFUS) treatments are promising alternatives to the surgical removal of malignant tumors. A significant challenge is assessing the viability of treated tissue during and immediately after MRgFUS procedures. Current clinical assessment uses the nonperfused volume (NPV) biomarker immediately after treatment from contrast-enhanced MRI. The NPV has variable accuracy, and the use of contrast agent prevents continuing MRgFUS treatment if tumor coverage is inadequate. This work presents a novel, noncontrast, learned multiparametric MR biomarker that can be used during treatment for intratreatment assessment, validated in a VX2 rabbit tumor model. A deep convolutional neural network was trained on noncontrast multiparametric MR images using the NPV biomarker from follow-up MR imaging (3-5 days after MRgFUS treatment) as the accurate label of nonviable tissue. A novel volume-conserving registration algorithm yielded a voxel-wise correlation between treatment and follow-up NPV, providing a rigorous validation of the biomarker. The learned noncontrast multiparametric MR biomarker predicted the follow-up NPV with an average DICE coefficient of 0.71, substantially outperforming the current clinical standard (DICE coefficient = 0.53). Noncontrast multiparametric MR imaging integrated with a deep convolutional neural network provides a more accurate prediction of MRgFUS treatment outcome than current contrast-based techniques.

The feasibility of a noise elimination method using continuous wave response of therapeutic ultrasound signals for ultrasonic monitoring of high-intensity focused ultrasound treatment

Purpose

In this study, the robustness and feasibility of a noise elimination method using continuous wave response of therapeutic ultrasound signals were investigated when tissue samples were moved to simulate the respiration-induced movements of the different organs during actual high-intensity focused ultrasound (HIFU) treatment. In addition to that, the failure conditions of the proposed algorithm were also investigated.

Methods

The proposed method was applied to cases where tissue samples were moved along both the lateral and axial directions of the HIFU transducer to simulate respiration-induced motions during HIFU treatment, and the noise reduction level was investigated. In this experiment, the speed of movement was increased from 10 to 40 mm/s to simulate the actual movement of the tissue during HIFU exposure, with the intensity and driving frequency of HIFU set to 1.0–5.0 kW/cm² and 1.67 MHz, respectively. To investigate the failure conditions of the proposed algorithm, the proposed method was applied with the HIFU focus located at the boundary between the phantom and water to easily cause cavitation bubbles. The intensity of HIFU was set to 10 kW/cm².

Results

Almost all HIFU noise was constantly able to be eliminated using the proposed method when the phantom was moved along the lateral and axial directions during HIFU exposure. The noise reduction level (PRL in this study) at an intensity of 1.0, 3.0, and 5.0 kW/cm² was in the range of 28–32, 38–40, and 42–45 dB, respectively. On the other hand, HIFU noise was not basically eliminated during HIFU exposure after applying the proposed method in the case of cavitation generation at the HIFU focus.

Conclusions

The proposed method can be applicable even if homogeneous tissues or organs move axially or laterally to the direction of HIFU exposure because of breathing. A condition under which the proposed algorithm failed was when instantaneous tissue changes such as cavitation bubble generation occurred in the tissue, at which time the reflected continuous wave response became less steady.

Real-Time Visualization of a Focused Ultrasound Beam Using Ultrasonic Backscatter

Focused ultrasound (FUS) therapies induce therapeutic effects in localized tissues using either temperature elevations or mechanical stresses caused by an ultrasound wave. During an FUS therapy, it is crucial to continuously monitor the position of the FUS beam in order to correct for tissue motion and keep the focus within the target region. Toward the goal of achieving real-time monitoring for FUS therapies, we have developed a method for the real-time visualization of an FUS beam using ultrasonic backscatter. The intensity field of an FUS beam was reconstructed using backscatter from an FUS pulse received by an imaging array and then overlaid onto a B-mode image captured using the same imaging array. The FUS beam visualization allows one to monitor the position and extent of the FUS beam in the context of the surrounding medium. Variations in the scattering properties of the medium were corrected in the FUS beam reconstruction by normalizing based on the echogenicity of the coaligned B-mode image. On average, normalizing by echogenicity reduced the mean square error between FUS beam reconstructions in nonhomogeneous regions of a phantom and baseline homogeneous regions by 21.61. FUS beam visualizations were achieved, using a single diagnostic imaging array as both an FUS source and an imaging probe, in a tissue-mimicking phantom and a rat tumor in vivo with a frame rate of 25-30 frames/s.

Techniques for Temperature Monitoring of Myocardial Tissue Undergoing Radiofrequency Ablation Treatments: An Overview

Cardiac radiofrequency ablation (RFA) has received substantial attention for the treatment of multiple arrhythmias. In this scenario, there is an ever-growing demand for monitoring the temperature trend inside the tissue as it may allow an accurate control of the treatment effects, with a consequent improvement of the clinical outcomes. There are many methods for monitoring temperature in tissues undergoing RFA, which can be divided into invasive and non-invasive. This paper aims to provide an overview of the currently available techniques for temperature detection in this clinical scenario. Firstly, we describe the heat generation during RFA, then we report the principle of work of the most popular thermometric techniques and their features. Finally, we introduce their main applications in the field of cardiac RFA to explore the applicability in clinical settings of each method.

Steering single-element lead zirconate titanate ultrasound transducers using biaxial driving

Previous work has shown that biaxial driving using two phase-offset orthogonal electric fields (propagation and lateral) improves the efficiency of ferroelectric materials by reducing coercivity and, hence, energy dissipation. In the current investigation, we demonstrated the capability of the biaxial method to steer ultrasound waves in single-element piezoceramic transducers made of prismatic lead zirconate titanate (PZT). We conducted finite element analysis simulations for 133 kHz (model 1) and 470 kHz biaxial (model 2) transducers models. We performed experimental validation with biaxially driven single-element transducers (n = 3) operating at an average frequency of 131 kHz with the same characteristics as model 1. For both models, we found non-symmetric steering that was a function of both the phase and power of the second electric field. At a constant electrical power (1 W) on the propagation electrodes, simulations for the 133 kHz model predicted maximal steering of 10.3°, 22.6°, and 30.9° for lateral electrode powers of 0.1 W, 0.5 W, and 1.0 W, respectively. Experimentally, for model 1, the maximal steering was 11.7° ± 1.9°, 23.5° ± 3.5°, and 30.2° ± 4.4° for the lateral electrode powers of 0.1 W, 0.5 W, and 1.0 W, respectively. Simulations for the 470 kHz model predicted maximal steering of 8.8°, 16.1°, and 27° for lateral electrode powers of 0.1 W, 0.5 W, and 1.0 W, respectively. Simulations showed that the cause of the steering asymmetry was a non-uniform shear deformation associated with the slightly off-resonance lateral electric field driving frequency. This is the first demonstration of ultrasound steering using a single-element transducer, which can have important applications for ultrasound focusing with phased arrays.

Synergistic agents for tumor-specific therapy mediated by focused ultrasound treatment

This minireview highlights the recent advances in the therapeutic agents that aim to provide synergistic enhancements of focused ultrasound treatment of tumors. Even though focused ultrasound therapy itself can bring therapeutic effects in cancers, many biochemical agents have been reported in the literature to enhance the treatment efficacy significantly. Until now, many mechanisms have been researched to advance the therapy, such as sonodynamic-plus-chemo-therapy, microbubble-aided therapy, localized release or delivery of nanomaterials, and multimodal image-guided therapy. Here, the novel materials adopted in each mechanism are briefly reviewed to provide a trend in the field and encourage future research towards the successful deployment of focused ultrasound therapy in real clinical environments.

Design and evaluation of an open-source, conformable skin-cooling system for body magnetic resonance guided focused ultrasound treatments

PURPOSE

Magnetic resonance guided focused ultrasound (MRgFUS) treatment of tumors uses inter-sonication delays to allow heat to dissipate from the skin and other near-field tissues. Despite inter-sonication delays, treatment of tumors close to the skin risks skin burns. This work has designed and evaluated an open-source, conformable, skin-cooling system for body MRgFUS treatments to reduce skin burns and enable ablation closer to the skin.

METHODS

A MR-compatible skin cooling system is described that features a conformable skin-cooling pad assembly with feedback control allowing continuous flow and pressure maintenance during the procedure. System performance was evaluated with hydrophone, phantom and in vivo porcine studies. Sonications were performed 10 and 5 mm from the skin surface under both control and forced convective skin-cooling conditions. 3D MR temperature imaging was acquired in real time and the accumulated thermal dose volume was measured. Gross analysis of the skin post-sonication was further performed. Device conformability was demonstrated at several body locations.

RESULTS

Hydrophone studies demonstrated no beam aberration, but a 5-12% reduction of the peak pressure due to the presence of the skin-cooling pad assembly in the acoustic near field. Phantom evaluation demonstrated there is no MR temperature imaging precision reduction or any other artifacts present due to the coolant flow during MRgFUS sonication. The porcine studies demonstrated skin burns were reduced in size or eliminated when compared to the control condition.

CONCLUSION

An open-source design of an MRgFUS active skin cooling system demonstrates device conformability with a reduction of skin burns while ablating superficial tissues.

Analysis of magnetic resonance signal intensity changes in the sacrococcygeal region of patients with uterine fibroids treated with high intensity focused ultrasound ablation

Objective: To evaluate the magnetic resonance (MR) signal intensity changes in the sacrococcygeal region of patients with uterine fibroids treated with high intensity focused ultrasound (HIFU).Materials and Methods: Two hundred and sixty-seven patients with uterine fibroids treated with HIFU between January and December 2016 were retrospectively reviewed. All patients underwent enhanced pre- and post-HIFU MRI. Multivariate analysis was used to assess the relationship between the factors and the signal intensity changes in the sacrum and the soft tissue adjacent to the sacrum.Results: Among the 267 patients, 122 (46%) had MR signal intensity changes in the sacrum and/or the soft tissue adjacent to the sacrum after HIFU. Multivariate analysis showed that the position of the uterus, the distance from the dorsal side of the fibroid to the sacrum, and the ablation efficiency were significantly correlated with MR signal intensity changes in the sacrum and the soft tissue adjacent to the sacrum. Further analysis showed a significant relationship between the location of the MR signal intensity changes and uterine size, the enhancement degree of the uterus. Leg pain was only seen in patients with MR signal intensity changes both in the sacrum and the soft tissue adjacent to the sacrum.Conclusions: The location of the uterus, the distance between the dorsal side of the fibroids to the sacrum, and ablation efficiency have a significant relationship with the MR signal intensity changes. The size of the uterus and the degree of enhancement are related to the locations of MR signal changes.

From Anatomy to Functional and Molecular Biomarker Imaging and Therapy: Ultrasound Is Safe, Ultrafast, Portable, and Inexpensive

Ultrasound is the most widely used medical imaging modality worldwide. It is abundant, extremely safe, portable, and inexpensive. In this review, we consider some of the current development trends for ultrasound imaging, which build upon its current strength and the popularity it experiences among medical imaging professional users.Ultrasound has rapidly expanded beyond traditional radiology departments and cardiology practices. Computing power and data processing capabilities of commonly available electronics put ultrasound systems in a lab coat pocket or on a user's mobile phone. Taking advantage of new contributions and discoveries in ultrasound physics, signal processing algorithms, and electronics, the performance of ultrasound systems and transducers have progressed in terms of them becoming smaller, with higher imaging performance, and having lower cost. Ultrasound operates in real time, now at ultrafast speeds; kilohertz frame rates are already achieved by many systems.Ultrasound has progressed beyond anatomical imaging and monitoring blood flow in large vessels. With clinical approval of ultrasound contrast agents (gas-filled microbubbles) that are administered in the bloodstream, tissue perfusion studies are now routine. Through the use of modern ultrasound pulse sequences, individual microbubbles, with subpicogram mass, can be detected and observed in real time, many centimeters deep in the body. Ultrasound imaging has broken the wavelength barrier; by tracking positions of microbubbles within the vasculature, superresolution imaging has been made possible. Ultrasound can now trace the smallest vessels and capillaries, and obtain blood velocity data in those vessels.Molecular ultrasound imaging has now moved closer to clinic; the use of microbubbles with a specific affinity to endothelial biomarkers allows selective accumulation and retention of ultrasound contrast in the areas of ischemic injury, inflammation, or neoangiogenesis. This will aid in noninvasive molecular imaging and may provide additional help with real-time guidance of biopsy, surgery, and ablation procedures.The ultrasound field can be tightly focused inside the body, many centimeters deep, with millimeter precision, and ablate lesions by energy deposition, with thermal or mechanical bioeffects. Some of such treatments are already in clinical use, with more indications progressing through the clinical trial stage. In conjunction with intravascular microbubbles, focused ultrasound can be used for tissue-specific drug delivery; localized triggered release of sequestered drugs from particles in the bloodstream may take time to get to clinic. A combination of intravascular microbubbles with circulating drug and low-power ultrasound allows transient opening of vascular endothelial barriers, including blood-brain barrier; this approach has reached clinical trial stage. Therefore, the drugs that normally would not be getting to the target tissue in the brain will now have an opportunity to produce therapeutic efficacy.Overall, medical ultrasound is developing at a brisk rate, even in an environment where other imaging modalities are also advancing rapidly and may be considered more lucrative. With all the current advances that we discuss, and many more to come, ultrasound may help solve many problems that modern medicine is facing.

Robot-assisted ultrasound navigation platform for 3D HIFU treatment planning: Initial evaluation for conformal interstitial ablation

Interstitial Ultrasound-guided High Intensity Focused Ultrasound (USgHIFU) therapy has the potential to deliver ablative treatments which conform to the target tumor. In this study, a robot-assisted US-navigation platform has been developed for 3D US guidance and planning of conformal HIFU ablations. The platform was used to evaluate a conformal therapeutic strategy associated with an interstitial dual-mode USgHIFU catheter prototype (64 elements linear-array, measured central frequency f = 6.5 MHz), developed for the treatment of HepatoCellular Carcinoma (HCC). The platform included a 3D navigation environment communicating in real-time with an open research dual-mode US scanner/HIFU generator and a robotic arm, on which the USgHIFU catheter was mounted. 3D US-navigation was evaluated in vitro for guiding and planning conformal HIFU ablations using a tumor-mimic model in porcine liver. Tumor-mimic volumes were then used as targets for evaluating conformal HIFU treatment planning in simulation. Height tumor-mimics (ovoid- or disc-shaped, sizes: 3-29 cm³) were created and visualized in liver using interstitial 2D US imaging. Robot-assisted spatial manipulation of these images and real-time 3D navigation allowed reconstructions of 3D B-mode US images for accurate tumor-mimic volume estimation (relative error: 4 ± 5%). Sectorial and full-revolution HIFU scanning (angular sectors: 88-360°) could both result in conformal ablations of the tumor volumes, as soon as their radii remained ≤ 24 mm. The presented US navigation-guided HIFU procedure demonstrated advantages for developing conformal interstitial therapies in standard operative rooms. Moreover, the modularity of the developed platform makes it potentially useful for developing other HIFU approaches.

Magnetic resonance-guided high-intensity focused ultrasound of uterine fibroids: whole-tumor quantitative perfusion for prediction of immediate ablation response

Background

In magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) treatment of uterine fibroids, the immediate ablation response is significantly affected by blood perfusion. The variability of measurement for blood perfusion is critical due to the inherent non-uniformity of tumor perfusion and its dependence on reproducible region of interest (ROI) placement.

Purpose

To investigate the value of whole-tumor ROI (ROIwt) analysis for quantitative perfusion in predicting immediate ablation response of uterine fibroids in MR-HIFU.

Material and Methods

Thirty-one fibroids in 28 eligible patients were treated with MR-HIFU. Quantitative perfusion parameters (K^trans, K_ep, and V_p) derived from dynamic contrast-enhanced MRI were obtained before MR-HIFU treatment. The ROIwt and single-layer ROI (ROIsl) were used for quantitative perfusion analysis. T1 contrast-enhanced MRI immediately after MR-HIFU treatment was conducted to determine the non-perfused volume ratio (NPVR). Intraclass correlation coefficient (ICC) was used for consistency test. Spearman’s correlation and multivariate linear regression were used to investigate the predictors of the NPVR. Received operating characteristic (ROC) curve was used to test the predictive efficacy of quantitative perfusion parameter.

Results

The intra- and inter-observer ICC of the quantitative perfusion parameters from ROIwt were higher than those from ROIsl. Multivariate analysis showed that the K^trans of ROIwt was a predictor of the immediate ablation response. ROC analysis displayed that the AUC of K^trans of ROIwt is 0.817 in predicting the ablation response.

Conclusion

Pretreatment K^trans of ROIwt is more reliable and stable than that of ROIsl. It could be a predictor for the immediate ablation response of uterine fibroids in MR-HIFU.