Improved volumetric MR-HIFU ablation by robust binary feedback control

Volumetric MR-guided high-intensity focused ultrasound versus uterine artery embolisation for treatment of symptomatic uterine fibroids: comparison of symptom improvement and reintervention rates

PURPOSE

To compare the effectiveness of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) with that of uterine artery embolisation (UAE) for treatment of uterine fibroids.

METHODS

Between January 2010 and January 2013, 51 women with symptomatic uterine fibroids underwent MR-HIFU. Follow-up and MR imaging were compared to 68 women treated with UAE, who fulfilled eligibility criteria for MR-HIFU - e.g., size (≤ 12 cm) and number (≤ 5) of fibroids. We compared median symptom severity (tSSS), total health-realted quality of life (HRQoL) scores, and reintervention rates. The adjusted effect on symptom relief and HRQoL improvement was calculated using multivariable linear regression. Cox regression was applied to calculate the adjusted risk of reintervention between both treatments.

RESULTS

Median tSSS improved significantly from baseline to three-month follow-up (P < 0.001) for both MR-HIFU (53.1 (IQR [40.6-68.8]) to 34.4 (IQR [21.9-46.9]) and UAE (65.3 (IQR [56.3-74.2]) to 21.9 (IQR [9.4-34.4]). In addition, significantly better HRQoL scores were observed after three months (P < 0.001). However, in multivariate analysis, UAE had a stronger effect on symptom relief and HRQoL improvement than MR-HIFU (P < 0.001). Patients treated with MR-HIFU had a 7.1 (95 % CI [2.00-25.3]; P = 0.002) times higher risk of reintervention within 12 months (18/51 vs. 3/68).

CONCLUSION

Both MR-HIFU and UAE result in significant symptom relief related to uterine fibroids. However, MR-HIFU is associated with a higher risk of reintervention.

KEY POINTS

• This study compared outcomes between volumetric MR-HIFU and UAE for uterine fibroids. • Both MR-HIFU and UAE result in significant symptom relief and quality of life improvement. • UAE had a stronger positive effect on the clinical outcomes. • Reintervention rate after MR-HIFU ablation was significantly higher than after UAE.

Integrated ultrasound and magnetic resonance imaging for simultaneous temperature and cavitation monitoring during focused ultrasound therapies

PURPOSE

Ultrasound can be used to noninvasively produce different bioeffects via viscous heating, acoustic cavitation, or their combination, and these effects can be exploited to develop a wide range of therapies for cancer and other disorders. In order to accurately localize and control these different effects, imaging methods are desired that can map both temperature changes and cavitation activity. To address these needs, the authors integrated an ultrasound imaging array into an MRI-guided focused ultrasound (MRgFUS) system to simultaneously visualize thermal and mechanical effects via passive acoustic mapping (PAM) and MR temperature imaging (MRTI), respectively.

METHODS

The system was tested with an MRgFUS system developed for transcranial sonication for brain tumor ablation in experiments with a tissue mimicking phantom and a phantom-filled ex vivo macaque skull. In experiments on cavitation-enhanced heating, 10 s continuous wave sonications were applied at increasing power levels (30-110 W) until broadband acoustic emissions (a signature for inertial cavitation) were evident. The presence or lack of signal in the PAM, as well as its magnitude and location, were compared to the focal heating in the MRTI. Additional experiments compared PAM with standard B-mode ultrasound imaging and tested the feasibility of the system to map cavitation activity produced during low-power (5 W) burst sonications in a channel filled with a microbubble ultrasound contrast agent.

RESULTS

When inertial cavitation was evident, localized activity was present in PAM and a marked increase in heating was observed in MRTI. The location of the cavitation activity and heating agreed on average after registration of the two imaging modalities; the distance between the maximum cavitation activity and focal heating was -3.4 ± 2.1 mm and -0.1 ± 3.3 mm in the axial and transverse ultrasound array directions, respectively. Distortions and other MRI issues introduced small uncertainties in the PAM∕MRTI registration. Although there was substantial variation, a nonlinear relationship between the average intensity of the cavitation maps, which was relatively constant during sonication, and the peak temperature rise was evident. A fit to the data to an exponential had a correlation coefficient (R(2)) of 0.62. The system was also found to be capable of visualizing cavitation activity with B-mode imaging and of passively mapping cavitation activity transcranially during cavitation-enhanced heating and during low-power sonication with an ultrasound contrast agent.

CONCLUSIONS

The authors have demonstrated the feasibility of integrating an ultrasound imaging array into an MRgFUS system to simultaneously map localized cavitation activity and temperature. The authors anticipate that this integrated approach can be utilized to develop controllers for cavitation-enhanced ablation and facilitate the optimization and development of this and other ultrasound therapies. The integrated system may also provide a useful tool to study the bioeffects of acoustic cavitation.

Thermal fixation of swine liver tissue after magnetic resonance-guided high-intensity focused ultrasound ablation

The aim of this study was to investigate experimental conditions for efficient and controlled in vivo liver tissue ablation by magnetic resonance (MR)-guided high-intensity focused ultrasound (HIFU) in a swine model, with the ultimate goal of improving clinical treatment outcome. Histological changes were examined both acutely (four animals) and 1 wk after treatment (five animals). Effects of acoustic power and multiple sonication cycles were investigated. There was good correlation between target size and observed ablation size by thermal dose calculation, post-procedural MR imaging and histopathology, when temperature at the focal point was kept below 90°C. Structural histopathology investigations revealed tissue thermal fixation in ablated regions. In the presence of cavitation, mechanical tissue destruction occurred, resulting in an ablation larger than the target. Complete extra-corporeal MR-guided HIFU ablation in the liver is feasible using high acoustic power. Nearby large vessels were preserved, which makes MR-guided HIFU promising for the ablation of liver tumors adjacent to large veins.

Volume transfer constant (K(trans)) maps from dynamic contrast enhanced MRI as potential guidance for MR-guided high intensity focused ultrasound treatment of hypervascular uterine fibroids

Higher perfusion of uterine fibroids at baseline is recognized as cause for poor efficacy of MR-guided high intensity focused ultrasound (HIFU) ablation, and higher acoustic power has been suggested for the treatment of high-perfused areas inside uterine fibroids. However, considering the heterogeneously vascular distribution inside the uterine fibroids especially with hyper vascularity, it is not easy to choose the correct therapy acoustic power for every part inside fibroids. In our study, we presented two cases of fibroids with hyper vascularity, to show the differences between them with different outcomes. Selecting higher therapy acoustic powers to ablate high-perfused areas efficiently inside fibroids might help achieving good ablation results. Volume transfer constant (K(trans)) maps from dynamic contrast-enhanced (DCE) imaging at baseline helps visualizing perfusion state inside the fibroids and locating areas with higher-perfusion. In addition, with the help of K(trans) maps, appropriate therapy acoustic power could be selected by the result of initial test and therapy sonications at different areas with significantly different perfusion state inside fibroids.

Magnetic resonance guided high-intensity focused ultrasound for image-guided temperature-induced drug delivery

Magnetic resonance guided high-intensity focused ultrasound (MR-HIFU) is a versatile technology platform for noninvasive thermal therapies in oncology. Since MR-HIFU allows heating of deep-seated tissue to well-defined temperatures under MR image guidance, this novel technology has great potential for local heat-mediated drug delivery from temperature-sensitive liposomes (TSLs). In particular, MR provides the ability for image guidance of the drug delivery when an MRI contrast agent is co-encapsulated with the drug in the aqueous lumen of the liposomes. Monitoring of the tumor drug coverage offers possibilities for a personalized thermal treatment in oncology. This review focuses on MR-HIFU as a noninvasive technology platform, temperature-sensitive liposomal formulations for drug delivery and image-guided drug delivery, and the effect of HIFU-induced hyperthermia on the TSL and drug distribution. Finally, the opportunities and challenges of localized MR-HIFU-mediated drug delivery from temperature-sensitive liposomes in oncology are discussed.

MR-guided focused ultrasound surgery, present and future

MR-guided focused ultrasound surgery (MRgFUS) is a quickly developing technology with potential applications across a spectrum of indications traditionally within the domain of radiation oncology. Especially for applications where focal treatment is the preferred technique (for example, radiosurgery), MRgFUS has the potential to be a disruptive technology that could shift traditional patterns of care. While currently cleared in the United States for the noninvasive treatment of uterine fibroids and bone metastases, a wide range of clinical trials are currently underway, and the number of publications describing advances in MRgFUS is increasing. However, for MRgFUS to make the transition from a research curiosity to a clinical standard of care, a variety of challenges, technical, financial, clinical, and practical, must be overcome. This installment of the Vision 20∕20 series examines the current status of MRgFUS, focusing on the hurdles the technology faces before it can cross over from a research technique to a standard fixture in the clinic. It then reviews current and near-term technical developments which may overcome these hurdles and allow MRgFUS to break through into clinical practice.

Technique to displace bowel loops in MRI-guided high-intensity focused ultrasound ablation of fibroids in the anteverted or anteflexed uterus

OBJECTIVE

In MRI-guided high-intensity focused ultrasound ablation of uterine fibroids, bowel interposition in the sonication path is often problematic. The purpose of this article is to discuss a bowel-manipulation technique to displace the bowel loop, which consisted of sequential applications of urinary bladder filling, rectal filling, and urinary bladder emptying.

CONCLUSION

This technique contributed to a decreased screening failure rate and succeeded in consistently displacing the bowel loop, thus allowing safe treatment of fibroids in the anteverted or anteflexed uterus.

Evolution of the ablation region after magnetic resonance-guided high-intensity focused ultrasound ablation in a Vx2 tumor model

OBJECTIVES

Volumetric magnetic resonance (MR)-guided high-intensity focused ultrasound (HIFU) is a completely noninvasive image-guided thermal ablation technique. Recently, there has been growing interest in the use of MR-HIFU for noninvasive ablation of malignant tumors. Of particular interest for noninvasive ablation of malignant tumors is reliable treatment monitoring and evaluation of response. At this point, there is limited evidence on the evolution of the ablation region after MR-HIFU treatment. The purpose of the present study was to comprehensively characterize the evolution of the ablation region after volumetric MR-HIFU ablation in a Vx2 tumor model using MR imaging, MR temperature data, and histological data.

MATERIALS AND METHODS

Vx2 tumors in the hind limb muscle of New Zealand White rabbits (n = 30) were ablated using a clinical MR-HIFU system. Twenty-four animals were available for analyses. Magnetic resonance imaging was performed before and immediately after ablation; MR temperature mapping was performed during the ablation. The animals were distributed over 7 groups with different follow-up lengths. Depending on the group, animals were reimaged and then killed on day 0, 1, 3, 7, 14, 21, or 28 after ablation. For all time points, the size of nonperfused areas (NPAs) on contrast-enhanced T1-weighted (CE-T1-w) images was compared with lethal thermal dose areas (ie, the tissue area that received a thermal dose of 240 equivalent minutes or greater [EM] at 43°C) and with the necrotic tissue areas on histology sections.

RESULTS

The NPA on CE-T1-w imaging showed an increase in median size from 266 ± 148 to 392 ± 178 mm(2) during the first day and to 343 ± 170 mm(2) on day 3, followed by a gradual decrease to 113 ± 103 mm(2) on day 28. Immediately after ablation, the NPA was 1.6 ± 1.4 times larger than the area that received a thermal dose of 240 EM or greater in all animals. The median size of the necrotic area on histology was 1.7 ± 0.4 times larger than the NPA immediately after ablation. After 7 days, the size of the NPA was in agreement with the necrotic tissue area on histology (ratio, 1.0 ± 0.2).

CONCLUSIONS

During the first 3 days after MR-HIFU ablation, the ablation region increases in size, after which it gradually decreases in size. The NPA on CE-T1-w imaging underestimates the extent of tissue necrosis on histology in the initial few days, but after 1 week, the NPA is reliable in delineating the necrotic tissue area. The 240-EM thermal dose limit underestimates the necrotic tissue area immediately after MR-HIFU ablation. Reliable treatment evaluation techniques are particularly important for noninvasive, image-guided tumor ablation. Our results indicate that CE-T1-w imaging is reliable for MR-HIFU treatment evaluation after 1 week.

In vitro localized release of thermosensitive liposomes with ultrasound-induced hyperthermia

Localized drug delivery with ultrasound-induced hyperthermia can enhance the therapeutic index of chemotherapeutic drugs by improving efficacy and reducing systemic toxicity. A novel in vitro method for the activation of drug-loaded thermosensitive liposomes is described. In particular, a dual-compartment, acoustically transparent container is used in which thermosensitive liposomes suspended in cell culture medium are immersed in a thermally absorptive medium, glycerol. Hyperthermia is induced with ultrasound in the glycerol, which in turn heats the culture medium by thermal conduction. The method approximately mimics the in vivo scenario of thermosensitive liposomes collected in the interstitial spaces of tumors, where ultrasound induces hyperthermia in the tumor tissue, which in turn heats the thermosensitive liposomes by conduction and induces release of the encapsulated drug. The acoustic conditions for the desired hyperthermia are derived theoretically and validated experimentally. Eighty percent release of doxorubicin from thermosensitive liposomes is achieved.

Reduction of peak acoustic pressure and shaping of heated region by use of multifoci sonications in MR-guided high-intensity focused ultrasound mediated mild hyperthermia

PURPOSE

Ablative hyperthermia (>55 °C) has been used as a definitive treatment for accessible solid tumors not amenable to surgery, whereas mild hyperthermia (40-45 °C) has been shown effective as an adjuvant for both radiotherapy and chemotherapy. An optimal mild hyperthermia treatment is spatially accurate, with precise and homogeneous heating limited to the target region while also limiting the likelihood of unwanted thermal or mechanical bioeffects (tissue damage, vascular shutoff). Magnetic resonance imaging-guided high-intensity focused ultrasound (MR-HIFU) can noninvasively heat solid tumors under image-guidance. In a mild hyperthermia setting, a sonication approach utilizing multiple concurrent foci may provide the benefit of reducing acoustic pressure in the focal region (leading to reduced or no mechanical effects), while providing better control over the heating. The objective of this study was to design, implement, and characterize a multifoci sonication approach in combination with a mild hyperthermia heating algorithm, and compare it to the more conventional method of electronically sweeping a single focus.

METHODS

Simulations (acoustic and thermal) and measurements (acoustic, with needle hydrophone) were performed. In addition, heating performance of multifoci and single focus sonications was compared using a clinical MR-HIFU platform in a phantom (target = 4-16 mm), in normal rabbit thigh muscle (target = 8 mm), and in a Vx2 tumor (target = 8 mm). A binary control algorithm was used for real-time mild hyperthermia feedback control (target range = 40.5-41 °C). Data were analyzed for peak acoustic pressure and intensity, heating energy efficiency, temperature accuracy (mean), homogeneity of heating (standard deviation [SD], T10 and T90), diameter and length of the heated region, and thermal dose (CEM(43)).

RESULTS

Compared to the single focus approach, multifoci sonications showed significantly lower (67% reduction) peak acoustic pressures in simulations and hydrophone measurements. In a rabbit Vx2 tumor, both single focus and multifoci heating approaches were accurate (mean = 40.82±0.12 °C [single] and 40.70±0.09 °C [multi]) and precise (standard deviation = 0.65±0.05 °C [single] and 0.64±0.04 °C [multi]), producing homogeneous heating (T(10-90) = 1.62 °C [single] and 1.41 °C [multi]). Heated regions were significantly shorter in the beam path direction (35% reduction, p < 0.05, Tukey) for multifoci sonications, i.e., resulting in an aspect ratio closer to one. Energy efficiency was lower for the multifoci approach. Similar results were achieved in phantom and rabbit muscle heating experiments.

CONCLUSIONS

A multifoci sonication approach was combined with a mild hyperthermia heating algorithm, and implemented on a clinical MR-HIFU platform. This approach resulted in accurate and precise heating within the targeted region with significantly lower acoustic pressures and spatially more confined heating in the beam path direction compared to the single focus sonication method.The reduction in acoustic pressure and improvement in spatial control suggest that multifoci heating is a useful tool in mild hyperthermia applications for clinical oncology.

Ultrasonic enhancement of drug penetration in solid tumors

Increasing the penetration of drugs within solid tumors can be accomplished through multiple ultrasound-mediated mechanisms. The application of ultrasound can directly change the structure or physiology of tissues or can induce changes in a drug or vehicle in order to enhance delivery and efficacy. With each ultrasonic pulse, a fraction of the energy in the propagating wave is absorbed by tissue and results in local heating. When ultrasound is applied to achieve mild hyperthermia, the thermal effects are associated with an increase in perfusion or the release of a drug from a temperature-sensitive vehicle. Higher ultrasound intensities locally ablate tissue and result in increased drug accumulation surrounding the ablated region of interest. Further, the mechanical displacement induced by the ultrasound pulse can result in the nucleation, growth and collapse of gas bubbles. As a result of such cavitation, the permeability of a vessel wall or cell membrane can be increased. Finally, the radiation pressure of the propagating pulse can translate particles or tissues. In this perspective, we will review recent progress in ultrasound-mediated tumor delivery and the opportunities for clinical translation.

Toward real-time availability of 3D temperature maps created with temporally constrained reconstruction

PURPOSE

To extend the previously developed temporally constrained reconstruction (TCR) algorithm to allow for real-time availability of three-dimensional (3D) temperature maps capable of monitoring MR-guided high intensity focused ultrasound applications.

METHODS

A real-time TCR (RT-TCR) algorithm is developed that only uses current and previously acquired undersampled k-space data from a 3D segmented EPI pulse sequence, with the image reconstruction done in a graphics processing unit implementation to overcome computation burden. Simulated and experimental data sets of HIFU heating are used to evaluate the performance of the RT-TCR algorithm.

RESULTS

The simulation studies demonstrate that the RT-TCR algorithm has subsecond reconstruction time and can accurately measure HIFU-induced temperature rises of 20°C in 15 s for 3D volumes of 16 slices (RMSE = 0.1°C), 24 slices (RMSE = 0.2°C), and 32 slices (RMSE = 0.3°C). Experimental results in ex vivo porcine muscle demonstrate that the RT-TCR approach can reconstruct temperature maps with 192 × 162 × 66 mm 3D volume coverage, 1.5 × 1.5 × 3.0 mm resolution, and 1.2-s scan time with an accuracy of ±0.5°C.

CONCLUSION

The RT-TCR algorithm offers an approach to obtaining large coverage 3D temperature maps in real-time for monitoring MR-guided high intensity focused ultrasound treatments.

Enhanced drug delivery in rabbit VX2 tumours using thermosensitive liposomes and MRI-controlled focused ultrasound hyperthermia

PURPOSE

The efficacy of anticancer drugs in solid tumours is impaired by their inability to reach all cancer cells in sufficient concentration to cause cytotoxicity. Hyperthermia-triggered release of drugs from thermosensitive liposomes can increase tumour drug concentration, but tumour-specific drug delivery requires precise temperature control, and effects on microregional distribution of anticancer drugs in tumours are unknown. Here we evaluate thermally triggered release of doxorubicin in a rabbit tumour model by comparing free versus thermosensitive liposomal doxorubicin administered systemically during magnetic resonance imaging (MRI)-controlled focused ultrasound hyperthermia.

MATERIALS AND METHODS

Twelve rabbits with a transplanted VX2 tumour in each thigh had a 10 mm diameter region in one tumour heated to 43°C using focused ultrasound with temperature control by MRI thermometry. Delivery of doxorubicin to tumours and normal tissues was quantified by fluorescence in tissue homogenates, and by fluorescence microscopy.

RESULTS

Using thermosensitive liposomal doxorubicin (2.5 mg/kg), doxorubicin concentrations in heated tumours were 26.7 times higher than in unheated tumours (n = 7, p = 0.017, two-sided Wilcoxon signed-rank test). There was no significant enhancement with free doxorubicin in heated versus unheated tumours (n = 3, p = 0.5). With thermosensitive liposomes (8.3 mg/kg), fluorescence microscopy demonstrated increased doxorubicin fluorescence in heated versus unheated tumours, co-localised with nuclear staining throughout the tumour.

CONCLUSIONS

Localised image-guided delivery of high concentrations of doxorubicin to cancer cells was achieved non-invasively in implanted tumours with temperature-sensitive drug carriers and a preclinical MRI-controlled focused ultrasound hyperthermia system.

SPECT/CT imaging of temperature-sensitive liposomes for MR-image guided drug delivery with high intensity focused ultrasound

The goal of this study was to investigate the blood kinetics and biodistribution of temperature-sensitive liposomes (TSLs) for MR image-guided drug delivery. The co-encapsulated doxorubicin and [Gd(HPDO3A)(H₂O)] as well as the ¹¹¹In-labeled liposomal carrier were quantified in blood and organs of tumor bearing rats. After TSL injection, mild hyperthermia (T=42 °C) was induced in the tumor using high intensity focused ultrasound under MR image-guidance (MR-HIFU). The biodistribution of the radiolabeled TSLs was investigated using SPECT/CT imaging, where the highest uptake of ¹¹¹In-labeled TSLs was observed in the spleen and liver. The MR-HIFU-treated tumors showed 4.4 times higher liposome uptake after 48 h in comparison with controls, while the doxorubicin concentration was increased by a factor of 7.9. These effects of HIFU-treatment are promising for applications in liposomal drug delivery to tumors.

Generation of uniform lesions in high intensity focused ultrasound ablation

High intensity focused ultrasound (HIFU) is emerging as an effective oncology treatment modality according to the clinical experience in the last decade. The temperature at the focus can reach over 65°C within seconds, denaturing cellular proteins and resulting in coagulative necrosis. HIFU parameters are usually kept the same for each treatment spot in tumor ablation. Because of the thermal diffusion from nearby spots, the lesion size will gradually increase as the HIFU therapy progresses, which leads to insufficient treatment of initial spots and over exposure of later ones. From the viewpoint of the physician, uniform lesions with the least energy exposure and the least energy are preferred in tumor ablation. In this study, an algorithm was developed to determine the number of HIFU pulses delivered to each spot in order to generate uniform lesions that fill the region-of-interest completely. The exposure energies required using different scanning pathways (raster scanning, spiral scanning from the center to the outside, and spiral scanning from the outside to the center), spot spacing (1mm, 2mm, 4mm, and 6mm) and motion time (from 0s to 400s) were compared with each other. It is found that spiral scanning from the outside to the center with spot spacing of 2mm and motion time less than 10s needs the least numbers of pulses or HIFU energy in uniform lesion production with the minimal temperature elevation. In addition, the effects of thermal properties of tissue (i.e., specific heat capacity, convective heat transfer coefficient, and thermal conductivity) on HIFU ablation were investigated in order to determine the HIFU treatment planning for various targets. Uniform lesion production in the transparent gel phantom and ex vivo bovine liver samples using the proposed algorithm proved effective and accord with the simulation for different scanning pathways by an extracorporeal clinical HIFU system. Therefore, dynamically adjusting ultrasound exposure energy can improve the efficacy and safety of HIFU ablation, and the treatment planning depends on the scanning protocol and thermal properties of the target.

Volumetric MRI-guided high-intensity focused ultrasound for noninvasive, in vivo determination of tissue thermal conductivity: initial experience in a pig model

PURPOSE

To estimate the local thermal conductivity of porcine thigh muscle at temperatures required for magnetic resonance imaging (MRI)-guided high-intensity focused ultrasound (MRgHIFU) surgery (60-90°C).

MATERIALS AND METHODS

Using MRgHIFU, we performed 40 volumetric ablations in the thigh muscles of four pigs. Thirty-five of the sonications were successful. We used MRI to monitor the resulting temperature increase. We then determined local thermal conductivity by analyzing the spatiotemporal spread of temperature during the cooling period.

RESULTS

The thermal conductivity of MRgHIFU-treated porcine thigh muscle fell within a narrow range (0.52 ± 0.05 W/[m*K]), which is within the range reported for porcine thigh muscle at temperatures of <40°C (0.52 to 0.62 W/[m*K]). Thus, there was little change in the thermal conductivity of porcine thigh muscle at temperatures required for MRgHIFU surgery compared to lower temperatures.

CONCLUSION

Our MRgHIFU-based approach allowed us to estimate, with good reproducibility, the local thermal conductivity of in vivo deep tissue in real time at temperatures of 60°C to 90°C. Therefore, our method provides a valuable tool for quantifying the influence of thermal conductivity on temperature distribution in tissues and for optimizing thermal dose delivery during thermal ablation with clinical MRgHIFU.

MRIgHIFU: a tool for image-guided therapeutics

High-intensity focused ultrasound (HIFU) provides focal delivery of mechanical energy deep into the body. This energy can be used to elevate the tissue temperature to such a degree that ablation is achieved. The elevated temperature can also be used to release drugs from temperature-sensitive carriers or activate therapeutic molecules using mechanical or thermal energy. Lower dose exposures modify the vasculature to allow large molecules to diffuse from blood in the surrounding tissue for local drug delivery. The energy delivery can be targeted and monitored using magnetic resonance imaging (MRI). The online image guidance and monitoring provides treatment delivery that is customized to each patient such that optimal, effective treatment can be achieved. This ability to localize and customize treatment delivery may further enhance the future potential of targeted drugs that are personalized for each patient. This review examines the rapid development of MRI-guided HIFU (MRIgHIFU) methods over the past few years and discuss their future potential.

Volumetric MR-guided high-intensity focused ultrasound ablation of uterine fibroids: treatment speed and factors influencing speed

OBJECTIVES

To assess the treatment speed of volumetric magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) ablation of symptomatic uterine fibroids, with a focus on factors affecting treatment speed.

METHODS

We received Institutional Review Board approval, and informed consent was obtained from all participants. Among 109 patients referred, 43 women (39.4 %) (mean age, 43.7 years), with 112 fibroids passed the screening. We treated 53 symptomatic uterine fibroids (47.3 %, 53/112) (volume, 341.2 ± 256.5 ml) using volumetric MR-HIFU ablation. We assessed procedure times, non-perfused volume (NPV) and treatment speed (NPV/treatment time). We statistically analysed the factors affecting treatment speed using multiple logistic regression tests.

RESULTS

Technical success was achieved in 42 of 43 cases. MR room time (from entrance to exit) and treatment time (from first to last sonication) were 216.0 ± 40.6 min and 131.5 ± 55.9 min, respectively. Immediate NPV was 178.9 ± 147.3 ml, which was 57.4 ± 25.5 % of the fibroid volume. Treatment speed was 81.8 ± 48.0 ml/h. Multivariate analysis showed that a large fibroid volume (P < 0.001), a low signal intensity ratio of fibroid to skeletal muscle on T2-weighted images (P = 0.009) and timing after completion of the learning curve (P < 0.001) significantly increased treatment speed.

CONCLUSION

Volumetric MR-HIFU ablation can effectively treat symptomatic uterine fibroids. The treatment speed appeared to improve when treating large and/or dark fibroids as well as upon completion of the learning curve.

Mild hyperthermia with magnetic resonance-guided high-intensity focused ultrasound for applications in drug delivery

PURPOSE

Mild hyperthermia (40-45 °C) is a proven adjuvant for radiotherapy and chemotherapy. Magnetic resonance guided high intensity focused ultrasound (MR-HIFU) can non-invasively heat solid tumours under image guidance. Low temperature-sensitive liposomes (LTSLs) release their drug cargo in response to heat (>40 °C) and may improve drug delivery to solid tumours when combined with mild hyperthermia. The objective of this study was to develop and implement a clinically relevant MR-HIFU mild hyperthermia heating algorithm for combination with LTSLs.

MATERIALS AND METHODS

Sonications were performed with a clinical MR-HIFU platform in a phantom and rabbits bearing VX2 tumours (target = 4-16 mm). A binary control algorithm was used for real-time mild hyperthermia feedback control (target = 40-41 °C). Drug delivery with LTSLs was measured with HPLC. Data were compared to simulation results and analysed for spatial targeting accuracy (offset), temperature accuracy (mean), homogeneity of heating (standard deviation (SD), T10 and T90), and thermal dose (CEM43).

RESULTS

Sonications in a phantom resulted in better temperature control than in vivo. Sonications in VX2 tumours resulted in mean temperatures between 40.4 °C and 41.3 °C with a SD of 1.0-1.5 °C (T10 = 41.7-43.7 °C, T90 = 39.0-39.6 °C), in agreement with simulations. 3D spatial offset was 0.1-3.2 mm in vitro and 0.6-4.8 mm in vivo. Combination of MR-HIFU hyperthermia and LTSLs demonstrated heterogeneous delivery to a partially heated VX2 tumour, as expected.

CONCLUSIONS

An MR-HIFU mild hyperthermia heating algorithm was developed, resulting in accurate and homogeneous heating within the targeted region in vitro and in vivo, which is suitable for applications in drug delivery.

Effects of different parameters in the fast scanning method for HIFU treatment

PURPOSE

High-intensity focused ultrasound is a promising method for the noninvasive treatment of benign and malignant tumors. This study analyzes the effects of scanning path, applied power, and geometric characteristics of the transducer on ablation using fast scanning method, a new scanning method that uses high-intensity focused ultrasound at different blood perfusion levels.

METHODS

Two transducers, six scanning paths, and three focal patterns were used to examine the ablation results of the fast scanning method using power densities from 1.35 × 10(7) W∕m(3) to 4.5 × 10(7) W∕m(3) and blood perfusion rates from 2 × 10(-3) ml∕ml∕s to 16 × 10(-3) ml∕ml∕s. The Pennes equation was solved using the finite-difference time-domain method to simulate the heating procedure.

RESULTS

Based on the results of the fast-scanning method, the different scanning paths exhibited small effect on the total treatment time supported by both simulation and least-square fit. Similar-sized lesions can result from the five different repeated paths, whereas a random path may lead to relative large fluctuations in ablation volume because of asymmetry of the lesions. Higher power levels increase the lesion volume and decrease the treatment time required for ablating a target area using the fast scanning method, whereas increased blood perfusion has the opposite effect on ablation volume and treatment time. A symmetric lesion can be produced through fast scanning method using a 65-element and a 90-element transducer. However, lesion production using the same operation scheme differs between the two transducers.

CONCLUSIONS

Unlike traditional scanning methods, fast scanning method produces a planned lesion regardless of scanning path, as long as the path consists of repeated subsequences. This attribute makes fast scanning method an easy-operation scheme that produces relatively large symmetric lesions in homogeneous tissues. Applied power is the most important factor; however, high blood perfusion levels can limit or even hinder the full ablation of the target area. Therefore, tissue perfusion and transducer type should be given special attention to ensure the success and safety of ablation treatment.

MR thermometry analysis of sonication accuracy and safety margin of volumetric MR imaging-guided high-intensity focused ultrasound ablation of symptomatic uterine fibroids

PURPOSE

To evaluate the accuracy of the size and location of the ablation zone produced by volumetric magnetic resonance (MR) imaging-guided high-intensity focused ultrasound ablation of uterine fibroids on the basis of MR thermometric analysis and to assess the effects of a feedback control technique.

MATERIALS AND METHODS

This prospective study was approved by the institutional review board, and written informed consent was obtained. Thirty-three women with 38 uterine fibroids were treated with an MR imaging-guided high-intensity focused ultrasound system capable of volumetric feedback ablation. Size (diameter times length) and location (three-dimensional displacements) of each ablation zone induced by 527 sonications (with [n=471] and without [n=56] feedback) were analyzed according to the thermal dose obtained with MR thermometry. Prospectively defined acceptance ranges of targeting accuracy were ±5 mm in left-right (LR) and craniocaudal (CC) directions and ±12 mm in anteroposterior (AP) direction. Effects of feedback control in 8- and 12-mm treatment cells were evaluated by using a mixed model with repeated observations within patients.

RESULTS

Overall mean sizes of ablation zones produced by 4-, 8-, 12-, and 16-mm treatment cells (with and without feedback) were 4.6 mm±1.4 (standard deviation)×4.4 mm±4.8 (n=13), 8.9 mm±1.9×20.2 mm±6.5 (n=248), 13.0 mm±1.2×29.1 mm±5.6 (n=234), and 18.1 mm±1.4×38.2 mm±7.6 (n=32), respectively. Targeting accuracy values (displacements in absolute values) were 0.9 mm±0.7, 1.2 mm±0.9, and 2.8 mm±2.2 in LR, CC, and AP directions, respectively. Of 527 sonications, 99.8% (526 of 527) were within acceptance ranges. Feedback control had no statistically significant effect on targeting accuracy or ablation zone size. However, variations in ablation zone size were smaller in the feedback control group.

CONCLUSION

Sonication accuracy of volumetric MR imaging-guided high-intensity focused ultrasound ablation of uterine fibroids appears clinically acceptable and may be further improved by feedback control to produce more consistent ablation zones.

Magnetic resonance imaging-guided volumetric ablation of symptomatic leiomyomata: correlation of imaging with histology

PURPOSE

To describe the preliminary safety and accuracy of a magnetic resonance (MR) imaging-guided high-intensity-focused ultrasound (HIFU) system employing new technical developments, including ablation control via volumetric thermal feedback, for the treatment of uterine leiomyomata with histopathologic correlation.

MATERIALS AND METHODS

In this phase I clinical trial, 11 women underwent MR-guided HIFU ablation (Sonalleve 1.5T; Philips Medical Systems, Vantaa, Finland), followed by hysterectomy within 30 days. Adverse events, imaging findings, and pathologic confirmation of ablation were assessed. The relationship between MR imaging findings, thermal dose estimates, and pathology and HIFU spatial accuracy were assessed using Bland-Altman analyses and intraclass correlations.

RESULTS

There were 12 leiomyomata treated. No serious adverse events were observed. Two subjects decided against having hysterectomy and withdrew from the study before surgery. Of 11 women, 9 underwent hysterectomy; all leiomyomata demonstrated treatment in the expected location. A mean ablation volume of 6.92 cm(3) ± 10.7 was observed at histopathologic examination. No significant differences between MR imaging nonperfused volumes, thermal dose estimates, and histopathology ablation volumes were observed (P > .05). Mean misregistration values perpendicular to the ultrasound beam axis were 0.8 mm ± 1.2 in feet-head direction and 0.1 mm ± 1.0 in and left-right direction and -0.7 mm ± 3.1 along the axis.

CONCLUSIONS

Safe, accurate ablation of uterine leiomyomata was achieved with an MR-guided HIFU system with novel treatment monitoring capabilities, including ablation control via volumetric thermal feedback.

Hyperthermia in bone generated with MR imaging-controlled focused ultrasound: control strategies and drug delivery

PURPOSE

To evaluate the feasibility of achieving image-guided drug delivery in bone by using magnetic resonance (MR) imaging-controlled focused ultrasound hyperthermia and temperature-sensitive liposomes.

MATERIALS AND METHODS

Experiments were approved by the institutional animal care committee. Hyperthermia (43°C, 20 minutes) was generated in 10-mm-diameter regions at a muscle-bone interface in nine rabbit thighs by using focused ultrasound under closed-loop temperature control with MR thermometry. Thermosensitive liposomal doxorubicin was administered systemically during heating. Heating uniformity and drug delivery were evaluated for control strategies with the temperature control image centered 10 mm (four rabbits) or 0 mm (five rabbits) from the bone. Simulations estimated temperature elevations in bone. Drug delivery was quantified by using the fluorescence of doxorubicin extracted from bone marrow and muscle and was compared between treated and untreated thighs by using the one-sided Wilcoxon signed rank test.

RESULTS

With ultrasound focus and MR temperature control plane 0 mm and 10 mm from the bone interface, average target region temperatures were 43.1°C and 43.3°C, respectively; numerically estimated bone temperatures were 46.8°C and 78.1°C. The 10-mm offset resulted in thermal ablation; numerically estimated muscle temperature was 66.1°C at the bone interface. Significant increases in doxorubicin concentration occurred in heated versus unheated marrow (8.2-fold, P = .002) and muscle (16.8-fold, P = .002). Enhancement occurred for 0- and 10-mm offsets, which suggests localized drug delivery in bone is possible with both hyperthermia and thermal ablation.

CONCLUSION

MR imaging-controlled focused ultrasound can achieve localized hyperthermia in bone for image-guided drug delivery in bone with temperature-sensitive drug carriers.

Considerations for theoretical modelling of thermal ablation with catheter-based ultrasonic sources: implications for treatment planning, monitoring and control

PURPOSE

To determine the impact of including dynamic changes in tissue physical properties during heating on feedback controlled thermal ablation with catheter-based ultrasound. Additionally, we compared the impact of several indicators of thermal damage on predicted extents of ablation zones for planning and monitoring ablations with this modality.

METHODS

A 3D model of ultrasound ablation with interstitial and transurethral applicators incorporating temperature-based feedback control was used to simulate thermal ablations in prostate and liver tissue. We investigated five coupled models of heat dependent changes in tissue acoustic attenuation/absorption and blood perfusion of varying degrees of complexity. Dimensions of the ablation zone were computed using temperature, thermal dose, and Arrhenius thermal damage indicators of coagulative necrosis. A comparison of the predictions by each of these models was illustrated on a patient-specific anatomy in the treatment planning setting.

RESULTS

Models including dynamic changes in blood perfusion and acoustic attenuation as a function of thermal dose/damage predicted near-identical ablation zone volumes (maximum variation < 2.5%). Accounting for dynamic acoustic attenuation appeared to play a critical role in estimating ablation zone size, as models using constant values for acoustic attenuation predicted ablation zone volumes up to 50% larger or 47% smaller in liver and prostate tissue, respectively. Thermal dose (t(43) ≥ 240 min) and thermal damage (Ω ≥ 4.6) thresholds for coagulative necrosis are in good agreement for all heating durations, temperature thresholds in the range of 54°C for short (<5 min) duration ablations and 50°C for long (15 min) ablations may serve as surrogates for determination of the outer treatment boundary.

CONCLUSIONS

Accounting for dynamic changes in acoustic attenuation/absorption appeared to play a critical role in predicted extents of ablation zones. For typical 5-15 min ablations with this modality, thermal dose and Arrhenius damage measures of ablation zone dimensions are in good agreement, while appropriately selected temperature thresholds provide a computationally cheaper surrogate.

Hyperthermia-triggered drug delivery from temperature-sensitive liposomes using MRI-guided high intensity focused ultrasound

In the continuous search for cancer therapies with a higher therapeutic window, localized temperature-induced drug delivery may offer a minimal invasive treatment option. Here, a chemotherapeutic drug is encapsulated into a temperature-sensitive liposome (TSL) that is released at elevated temperatures, for example, when passing through a locally heated tumor. Consequently, high drug levels in the tumor tissue can be achieved, while reducing drug exposure to healthy tissue. Although the concept of temperature-triggered drug delivery was suggested more than thirty years ago, several chemical and technological challenges had to be addressed to advance this approach towards clinical translation. In particular, non-invasive focal heating of tissue in a controlled fashion remained a challenge. For the latter, high intensity focused ultrasound (HIFU) allows non-invasive heating to establish hyperthermia (40-45 °C) of tumor tissue over time. Magnetic resonance imaging (MRI) plays a pivotal role in this procedure thanks to its superb spatial resolution for soft tissue as well as the possibility to acquire 3D temperature information. Consequently, MRI systems emerged with an HIFU ultrasound transducer embedded in the patient bed (MR-HIFU), where the MRI is utilized for treatment planning, and to provide spatial and temperature feedback to the HIFU. For tumor treatment, the lesion is heated to 42 °C using HIFU. At this temperature, the drug-loaded TSLs release their payload in a quantitative fashion. The concept of temperature-triggered drug delivery has been extended to MR image-guided drug delivery by the co-encapsulation of a paramagnetic MRI contrast agent in the lumen of TSLs. This review will give an overview of recent developments in temperature-induced drug delivery using HIFU under MRI guidance.

Magnetic resonance thermometry at 7T for real-time monitoring and correction of ultrasound induced mild hyperthermia

While Magnetic Resonance Thermometry (MRT) has been extensively utilized for non-invasive temperature measurement, there is limited data on the use of high field (≥7T) scanners for this purpose. MR-guided Focused Ultrasound (MRgFUS) is a promising non-invasive method for localized hyperthermia and drug delivery. MRT based on the temperature sensitivity of the proton resonance frequency (PRF) has been implemented in both a tissue phantom and in vivo in a mouse Met-1 tumor model, using partial parallel imaging (PPI) to speed acquisition. An MRgFUS system capable of delivering a controlled 3D acoustic dose during real time MRT with proportional, integral, and derivative (PID) feedback control was developed and validated. Real-time MRT was validated in a tofu phantom with fluoroptic temperature measurements, and acoustic heating simulations were in good agreement with MR temperature maps. In an in vivo Met-1 mouse tumor, the real-time PID feedback control is capable of maintaining the desired temperature with high accuracy. We found that real time MR control of hyperthermia is feasible at high field, and k-space based PPI techniques may be implemented for increasing temporal resolution while maintaining temperature accuracy on the order of 1°C.

Volumetric MR-guided high-intensity focused ultrasound ablation with a one-layer strategy to treat large uterine fibroids: initial clinical outcomes

PURPOSE

To evaluate initial clinical outcomes of volumetric magnetic resonance (MR)-guided high-intensity focused ultrasound (HIFU) ablation by using a one-layer strategy to treat large (>10 cm in diameter) uterine fibroids, with investigation of the correlation between effectiveness of the one-layer strategy and dynamic contrast material-enhanced (DCE) MR parameters.

MATERIALS AND METHODS

Institutional review board approval and informed consent were obtained. Twenty-seven women (mean age, 44.5 years) with 27 large uterine fibroids (mean diameter, 11.3 cm ± 1.4 [standard deviation] [range, 10.1-16.0 cm]; fibroid volume, 502.5 mL ± 214.3 [range, 253.8-1184.0 mL]) underwent volumetric MR-guided HIFU ablation with a one-layer strategy. (All treatment cells were placed in one coronal plane at a depth of half to anterior two-thirds of the anteroposterior dimension of fibroids.) Treatment time, immediate nonperfused volume (NPV), and effectiveness of a one-layer strategy (ratio of immediate NPV to total volume of treatment cells planned) correlating with baseline DCE MR parameters (volume transfer constant [K(trans)], fractional extravascular extracellular space, and fractional blood plasma volume [Pearson correlation test]), complications, 3-month follow-up volumes, and symptom severity score (SSS) changes (paired t test) were assessed retrospectively.

RESULTS

All treatments showed technical success in one session (mean treatment time, 166.2 minutes ± 38.9). NPV was 301.3 mL ± 119.1, which was 64.2% ± 19.9 (<50%, n = 4; ≥ 50%, n = 23) of fibroid volume. Ratio of immediate NPV to total volume of treatment cells (1.79 ± 0.61) negatively correlated with DCE MR imaging K(trans) values (r = -0.426, P = .017). Minor complications occurred in five patients (18.5% [thermal injury of abdominal wall, n = 3; 30-day leg numbness, n = 1; cystitis, n = 1]). At 3-month follow-up (n = 18), mean SSS had decreased from 37.4 at baseline to 24.0 (P < .001), and volume reduction ratio was 0.64 ± 0.15 (P < .001).

CONCLUSION

Volumetric MR-guided HIFU ablation with a one-layer strategy is safe and effective for treatment of large uterine fibroids. Effectiveness of this strategy showed a significant negative correlation with K(trans) values at baseline DCE MR imaging.

MR-guided high-intensity focused ultrasound for noninvasive cancer treatment

Magnetic resonance (MR)-high-intensity focused ultrasound (HIFU) is an innovative, noninvasive tumour ablation technique. MR imaging and focused ultrasound are combined allowing real-time anatomic guidance and temperature mapping during treatment. Recently, the volumetric ablation approach has been introduced in order to reduce treatment length and provide more homogeneous tumour ablation. After successful treatment of uterine fibroids, MR-HIFU is currently being investigated for the treatment of malignant tumours. Palliative treatment of painful bone metastases is already applied in clinical practice. Several issues need to be further investigated for successful cancer treatment with MR-HIFU, including patient selection criteria, definition of treatment margins and optimal transducer technology.

Volumetric MR-HIFU ablation of uterine fibroids: role of treatment cell size in the improvement of energy efficiency

PURPOSE

To evaluate the energy efficiency of differently sized volumetric ablations in MR-guided high-intensity focused ultrasound (MR-HIFU) treatment of uterine fibroids.

MATERIALS AND METHODS

This study was approved by the institutional review board and informed consent was obtained from all participants. Ten symptomatic uterine fibroids (mean diameter 8.9 cm) in 10 women (mean age 42.2) were treated by volumetric MR-HIFU ablation under binary feedback control. The energy efficiency (mm3/J) of each sonication was calculated as the volume of lethal thermal dose (240 equivalent minutes at 43 °C) per unit acoustic energy applied. Operator-controllable parameters and signal intensity ratio of uterine fibroid to skeletal muscle on T2-weighted MR images were tested with univariate and multivariate analyses to discern which parameters significantly correlated with the ablation energy efficiency.

RESULTS

We analyzed a total of 236 sonications. The energy efficiency of the ablations was 0.42±0.25 mm3/J (range 0.004-1.18) with energy efficiency improving with the treatment cell size (4 mm, 0.06±0.06 mm3/J; 8 mm, 0.29±0.12 mm3/J; 12 mm, 0.58±0.18 mm3/J; 16 mm, 0.91±0.17 mm3/J). Treatment cell size (r=0.814, p<0.001), distance of ultrasound propagation (r=-0.151, p=0.020), sonication frequency (1.2 or 1.45 MHz; p<0.001), and signal intensity ratio (r=-0.205, p=0.002) proved to be significant by univariate analysis, while multivariate analysis revealed treatment cell size (B=0.075, p<0.001), US propagation distance (B=-6.928, p<0.001), and signal intensity ratio (B=-0.024, p=0.001) to be independently significant.

CONCLUSION

Energy efficiency in volumetric MR-HIFU ablation of uterine fibroids improves with increased treatment cell size, independent of other significant contributors such as distance of ultrasound propagation or signal intensity of the tumor on T2-weighted MR imaging.

Volumetric feedback ablation of uterine fibroids using magnetic resonance-guided high intensity focused ultrasound therapy

Objective

The purpose of this prospective multicenter study was to assess the safety and technical feasibility of volumetric Magnetic Resonance-guided High Intensity Focused Ultrasound (MR-HIFU) ablation for treatment of patients with symptomatic uterine fibroids.

Methods

Thirty-three patients with 36 fibroids were treated with volumetric MR-HIFU ablation. Treatment capability and technical feasibility were assessed by comparison of the Non-Perfused Volumes (NPVs) with MR thermal dose predicted treatment volumes. Safety was determined by evaluation of complications or adverse events and unintended lesions. Secondary endpoints were pain and discomfort scores, recovery time and length of hospital stay.

Results

The mean NPV calculated as a percentage of the total fibroid volume was 21.7%. Correlation between the predicted treatment volumes and NPVs was found to be very strong, with a correlation coefficient r of 0.87. All patients tolerated the treatment well and were treated on an outpatient basis. No serious adverse events were reported and recovery time to normal activities was 2.3 ± 1.8 days.

Conclusion

This prospective multicenter study proved that volumetric MR-HIFU is safe and technically feasible for the treatment of symptomatic uterine fibroids.

Key Points

• Magnetic-resonance-guided high intensity focused ultrasound allows non-invasive treatment of uterine fibroids.

• Volumetric feedback ablation is a novel technology that allows larger treatment volumes

• MR-guided ultrasound ablation of uterine fibroids appears safe using volumetric feedback

Realtime control of multiple-focus phased array heating patterns based on noninvasive ultrasound thermography

A system for the realtime generation and control of multiple-focus ultrasound phased-array heating patterns is presented. The system employs a 1-MHz, 64-element array and driving electronics capable of fine spatial and temporal control of the heating pattern. The driver is integrated with a realtime 2-D temperature imaging system implemented on a commercial scanner. The coordinates of the temperature control points are defined on B-mode guidance images from the scanner, together with the temperature set points and controller parameters. The temperature at each point is controlled by an independent proportional, integral, and derivative controller that determines the focal intensity at that point. Optimal multiple-focus synthesis is applied to generate the desired heating pattern at the control points. The controller dynamically reallocates the power available among the foci from the shared power supply upon reaching the desired temperature at each control point. Furthermore, anti-windup compensation is implemented at each control point to improve the system dynamics. In vitro experiments in tissue-mimicking phantom demonstrate the robustness of the controllers for short (2-5 s) and longer multiple-focus high-intensity focused ultrasound exposures. Thermocouple measurements in the vicinity of the control points confirm the dynamics of the temperature variations obtained through noninvasive feedback.

See, reach, treat: ultrasound-triggered image-guided drug delivery

The integration of therapeutic interventions with diagnostic imaging has been recognized as one of the next technological developments that will have a major impact on medical treatments. Therapeutic applications using ultrasound, for example thermal ablation, hyperthermia or ultrasound-induced drug delivery, are examples for image-guided interventions that are currently being investigated. While thermal ablation using magnetic resonance-guided high-intensity focused ultrasound is entering the clinic, ultrasound-mediated drug delivery is still in a research phase, but holds promise to enable new applications in localized treatments. The use of ultrasound for the delivery of drugs has been demonstrated, particularly in the field of cardiology and oncology for a variety of therapeutics ranging from small-molecule drugs to biologics and nucleic acids exploiting temperature- or pressure-mediated delivery schemes.

Reconstruction of fully three-dimensional high spatial and temporal resolution MR temperature maps for retrospective applications

Many areas of MR-guided thermal therapy research would benefit from temperature maps with high spatial and temporal resolution that cover a large three-dimensional volume. This article describes an approach to achieve these goals, which is suitable for research applications where retrospective reconstruction of the temperature maps is acceptable. The method acquires undersampled data from a modified three-dimensional segmented echo-planar imaging sequence and creates images using a temporally constrained reconstruction algorithm. The three-dimensional images can be zero-filled to arbitrarily small voxel spacing in all directions and then converted into temperature maps using the standard proton resonance frequency shift technique. During high intensity focused ultrasound heating experiments, the proposed method was used to obtain temperature maps with 1.5 mm × 1.5 mm × 3.0 mm resolution, 288 mm × 162 mm × 78 mm field of view, and 1.7 s temporal resolution. The approach is validated to demonstrate that it can accurately capture the spatial characteristics and time dynamics of rapidly changing high intensity focused ultrasound-induced temperature distributions. Example applications from MR-guided high intensity focused ultrasound research are shown to demonstrate the benefits of the large coverage fully three-dimensional temperature maps, including characterization of volumetric heating trajectories and near- and far-field heating.

Assisted hepatic resection using a toroidal HIFU device: an in vivo comparative study in pig

PURPOSE

Bleeding is the main cause of postoperative complications during hepatic surgery. Blood loss and transfusions increase tumor recurrence in liver metastases from colorectal cancer. A high intensity focused ultrasound (HIFU) device with an integrated ultrasound imaging probe was developed for the treatment of colorectal liver metastasis.

METHODS

The HIFU toroidal-shaped transducer contains 256 elements (working frequency: 3 MHz) and can create a single conical lesion of 7 cm3 in 40 s. Then, the volume of treatment can be significantly increased by juxtaposing single lesions. Presented here is the use of this device in an animal model as a complementary tool to improve surgical resection in the liver. Before transecting the liver, a wall of coagulative necrosis was performed using this device in order to minimize blood loss and dissection time during hepatectomy. Resection assisted by HIFU was compared to classical dissections with clamping [intermittent Pringle maneuver (IPM) group] and without clamping (control group). For each technique, 14 partial liver resections were performed in seven pigs. Blood loss per dissection surface area and resection time were the main outcome parameters.

RESULTS

Conserving liver blood inflow during hepatic resection assisted by HIFU did not increase total blood loss (7.4 +/- 3.3 ml cm(-2)) compared to hepatic resection performed during IPM and controlled blood inflow (11.2 +/- 2.2 ml cm(-2)). Lower blood loss was measured on average when using HIFU, even though difference with clamping (IPM) was not statistically significant (p = 0.09). Resection assisted by HIFU reduced blood loss by 50% compared to control group (14.0 +/- 3.4 ml cm(-2), p = 0.03). The duration of transection when using HIFU (13 +/- 3 min) was significantly lower compared to clamping (23 +/- 4 min, p < 0.01) and control (18 +/- 3 min, p = 0.02). Precoagulation also resulted in sealing blood vessels with a diameter of less than 5 mm, and therefore the number of clips needed in the HIFU group was significantly lower (0.8 +/- 0.2 cm(-2)) when compared to clamping (1.6 +/- 0.2 cm(-2), p < 0.01) and control (1.8 +/- 0.4 cm(-2), p < 0.01).

CONCLUSIONS

This method holds promise for future clinical applications in resection of liver metastases.

Proton resonance frequency shift-weighted imaging for monitoring MR-guided high-intensity focused ultrasound transmissions

PURPOSE

To combine temperature-related information of phase images and magnitude images acquired from an MR spoiled gradient echo sequence using a postprocessing method referred to as PRF-shift-weighted imaging (PRFSWI).

MATERIALS AND METHODS

Phase images are capable of detecting shifts in proton resonance frequency (PRF) caused by local changes in temperature. Magnitude images provide anatomical information for treatment planning and positioning as well as temperature-related contrast. We used PRFSWI to produce a phase-mask and performed multiplication on the magnitude image to increase temperature-related contrast.

RESULTS

Through MRI-guided focused ultrasound (MRIgFUS) experiments (both ex vivo and in vivo), we determined that PRFSWI is capable of enhancing the contrast of a heated area even in the initial stages of transmitting high-intensity focused ultrasound energy.

CONCLUSION

The PRFSWI images are sensitive to changes in temperature and display the heated spot directly in the magnitude images. Although the images do not provide quantitative data related to temperature, this method could be used as a complement to the phase temperature mapping method in the real-time monitoring of MRIgFUS experiments.

Formulation and characterisation of magnetic resonance imageable thermally sensitive liposomes for use with magnetic resonance-guided high intensity focused ultrasound

PURPOSE

Objectives of this study were to: 1) develop iLTSL, a low temperature sensitive liposome co-loaded with an MRI contrast agent (ProHance® Gd-HP-DO3A) and doxorubicin, 2) characterise doxorubicin and Gd-HP-DO3A release from iLTSL and 3) investigate the ability of magnetic resonance-guided high intensity focused ultrasound (MR-HIFU) to induce and monitor iLTSL content release in phantoms and in vivo.

METHODS

iLTSL was passively loaded with Gd-HP-DO3A and actively loaded with doxorubicin. Doxorubicin and Gd-HP-DO3A release was quantified by fluorescence and spectroscopic techniques, respectively. Release with MR-HIFU was examined in tissue-mimicking phantoms containing iLTSL and in a VX2 rabbit tumour model.

RESULTS

iLTSL demonstrated consistent size and doxorubicin release kinetics after storage at 4°C for 7 days. Release of doxorubicin and Gd-HP-DO3A from iLTSL was minimal at 37°C but fast when heated to 41.3°C. The magnitude of release was not significantly different between doxorubicin and Gd-HP-DO3A over 10 min in HEPES buffer and plasma at 37°, 40° and 41.3°C (p > 0.05). Relaxivity of iLTSL increased significantly (p < 0.0001) from 1.95 ± 0.05 to 4.01 ± 0.1 mMs⁻¹ when heated above the transition temperature. Signal increase corresponded spatially and temporally to MR-HIFU-heated locations in phantoms. Signal increase was also observed in vivo after iLTSL injection and after each 10-min heating (41°C), with greatest increase in the heated tumour region.

CONCLUSION

An MR imageable liposome formulation co-loaded with doxorubicin and an MR contrast agent was developed. Stability, imageability, and MR-HIFU monitoring and control of content release suggest that MR-HIFU combined with iLTSL may enable real-time monitoring and spatial control of content release.

A novel approach to identify non-palpable breast lesions combining fluorescent liposomes and magnetic resonance-guided high intensity focused ultrasound-triggered release

The combination of fluorescein-containing liposomes (FCL) and magnetic resonance-guided high intensity focused ultrasound (MR-HIFU)-triggered release is a promising approach for lesion demarcation and more efficient removal of non-palpable breast lesions. Exposure of FCL to ablation temperatures (60 °C) using MR-HIFU would result in palpable, stained tumors, which are more easy to identify during surgical resection. In this study, proof-of-concept concerning fluorescent FCL for MR-HIFU-triggered release and tumor demarcation of non-palpable breast lesions is presented. Ex vivo experiments in human blood and porcine muscle tissue showed increased label release from the liposomes, clear fluorescence enhancement and diffusion of the released compound after heating to 60 °C. Next, fluorescein release of FCL was observed after MR-HIFU-mediated mild hyperthermia (42 °C) and ablation temperature (60 °C) for a short period (30s), which is in line with the clinically relevant MR-HIFU treatment parameters. These results indicate the potential of the FCL as a tool to improve tumor demarcation in patients by MR-HIFU-triggered release. Therefore, this method may offer a new tool for efficient surgical resection of non-palpable breast tumor lesions by enabling proper discrimination between tumor tissue and adjacent healthy tissue.