Real-time visualization of high-intensity focused ultrasound treatment using ultrasound imaging

Acoustic Cavitation Emissions Predict Near-complete/complete Histotripsy Treatment in Soft Tissues

OBJECTIVE

Histotripsy is a non-invasive acoustic ablation technique that leverages cavitation to impart mechanical damage to a viscoelastic medium, such as tissue. Although histotripsy bubbles and lesions can be imaged with a variety of modalities, reliable methods to predict tissue disruption across different tissue-types remain to be determined.

APPROACH

Several ex-vivo bovine tissues were ablated by intrinsic threshold histotripsy over a range of pulse-per-location acoustic doses. Acoustic Cavitation Emission (ACE) signals were captured following every other therapeutic pulse using transmit-receive capable histotripsy arrays. Final bubble lifespan, lifespan-slope, and percent-reduction were calculated and correlated against histologic necrosis score (0-5: 0=0% necrosis, 5=>95% necrosis) and residual structure score (0-4: 0=none present, 4=intact) to evaluate the ability of features from ACE-signals to predict histotripsy-induced damage. Further, optimal ACE-feature thresholds were determined for binary evaluation of whether a necrosis score equal or greater than 4 had been reached.

RESULTS

Measured lifespans increased and lifespan-slopes decreased with pulses per location (ppl) and eventually plateaued in all tissue types, in similar trends to those previously observed in tissue phantoms. Necrosis score increased and residual structure decreased with increasing acoustic dose. Bubble lifespan-slope and percent-reduction correlated well with necrosis score. Thresholds able to predict the necrosis score of 4 or greater in brain, liver, and kidney were calculated with high sensitivity/specificity (>80%). The necrosis score of 4 and 5 is expected to correspond to near-complete/complete ablation by histological evaluation.

CONCLUSION

Features measured from ACE-signals, particularly the lifespan-slope and percent reduction, were used to predict near-complete/complete ablation of large-volume histotripsy treatments in ex vivo bovine liver, kidney, and brain tissues with good accuracy. Tissue heterogeneities were observed to impact the histotripsy damage and corresponding ACE-signals, and thus the predication accuracy.

Passive cavitation mapping for biomedical applications using higher order delay multiply and sum beamformer with linear complexity

Ultrasound-induced cavitation can be used in various biomedical therapies, including localized drug delivery, sonoporation, gene transfer, noninvasive sonothrombolysis, lithotripsy, and histotripsy. It can also enhance thermal ablation of tumors and facilitate trans-blood-brain-barrier treatments. Accurate monitoring of cavitation activity, including dose and location, is essential for the safe and effective application of these therapies. Passive cavitation mapping (PCM) is a key technique used to achieve this. However, conventional Delay and Sum (DAS) beamforming methods suffer from low resolution and high side-lobe levels in standard diagnostic ultrasound transducer, limiting their effectiveness or are computationally expensive, in the case of robust capon beamformer (RCB). To address these challenges, we propose a higher-order nonlinear Delay Multiply and Sum (DMAS) beamformer for improved passive cavitation mapping. Our approach utilizes a novel implementation with linear complexity, using a determinant from symmetrical polynomials. Simulation and experimental results demonstrate that the proposed method enhances both axial and lateral point spread function, resolution and increasing image quality, while exhibiting linear complexity. These improvements suggest that higher-order nonlinear beamforming is a promising advancement for more accurate and reliable cavitation monitoring in biomedical applications.

Monitoring focused ultrasound ablation surgery (FUAS) using echo amplitudes of the therapeutic focused transducer

OBJECTIVE

B-mode sonography is commonly used to monitor focused ultrasound ablation surgery (FUAS), but has limitations in sensitivity. More accurate and reliable prediction of coagulation is required.

METHODS

The focused ultrasound (FUS) transducer was adapted for echo reception. Numerical simulations compared the normalized echo amplitudes from the FUS transducer and imaging probe at varying tissue depths and frequencies with a 3 mm necrosis at focus. An ex vivo experiment then evaluated echo changes from the FUS transducer and ultrasound imaging probe under different settings. Finally, coagulation prediction using FUS echo data was compared to sonography in a clinical ex vivo context.

RESULTS

The echo amplitudes from the FUS transducer exhibit a less pronounced decline with increasing tissue penetration depth compared to the ultrasound imaging probe. In ex vivo bovine liver experiments at depths of 2 cm and 4 cm, the FUS transducer detected normalized echo amplitudes that were significantly larger (i.e., 2∼3 folds) than those received by the ultrasound imaging probe. Moreover, multi-layered ex vivo tissue experiments that replicate clinical conditions revealed that coagulation prediction utilizing the FUS transducer's echo amplitudes achieved superior accuracy (91.2% vs. 60.3 %), sensitivity (92.1% vs. 54.5 %), and negative prediction (78.9% vs. 30.6 %), but similar specificity (88.2% vs. 84.6 %) and positive prediction (95.9% vs. 93.8 %) in comparison to sonography.

CONCLUSION

The echo amplitude of the FUS transducer serves as a sensitive and dependable metric for monitoring the FUAS outcomes. Its utilization may augment the procedure's safety and efficacy.

2D ultrasound thermometry during thermal ablation with high-intensity focused ultrasound

The clinical use of high intensity focused ultrasound (HIFU) therapy for noninvasive tissue ablation has recently gained momentum. Guidance is provided by either magnetic resonance imaging (MRI) or conventional B-mode ultrasound imaging, each with its own advantages and disadvantages. The main limitation of ultrasound imaging is its inability to provide temperature measurements over the ranges corresponding to the target temperatures during ablative thermal therapies (between 55 °C and 70 °C). Here, variations in ultrasound backscattered energy (ΔBSE) were used to monitor temperature increases in liver tissue up to an absolute value of 90 °C during and after HIFU treatment. In vitro experimental measurements were performed in 47 bovine liver samples using a toroidal HIFU transducer operating at 2.5 MHz to increase the temperature of tissues. An ultrasound imaging probe working at 7.5 MHz was placed in the center of the HIFU transducer to monitor the backscattered signals. The free-field acoustic power was set to 9 W, 12 W or 16 W in the different experiments. HIFU sonications were performed for 240 s using a duty cycle of 83 % to allow ultrasound imaging and raw radiofrequency data acquisition during exposures. Measurements showed a linear relationship between ΔBSE (in dB) and temperature (r = 0.94, p < 0.001) over a temperature range from 37 °C to 90 °C, with a high reliability of temperature measurements below 75 °C. Monitoring can be performed at the frame rate of ultrasound imaging scanners with an accuracy within an acceptable threshold of 5 °C, given the temperatures targeted during thermal ablations. If the maximum temperature reached is below 70 °C, ΔBSE is also a reliable approach for estimating the temperature during cooling. Histological analysis shown the impact of the treatment on the spatial arrangement of cells that can explain the observed variation of ΔBSE. These results demonstrate the ability of ΔBSE measurements to estimate temperature in ultrasound images within an effective therapeutic range. This method can be implemented clinically and potentially applied to other thermal-based therapies.

Agar-based Phantom for Evaluating Targeting of High-intensity Focused Ultrasound Systems for Breast Ablation

Aim

Phantoms are often utilized for the preclinical evaluation of novel high-intensity focused ultrasound (HIFU) systems, serving as valuable tools for validating efficacy. In the present study, the feasibility of a homogeneous agar-based breast-shaped phantom as a tool for the preclinical evaluation of HIFU systems dedicated to breast cancer was assessed. Specifically, the effect of the increased phantom curvature on temperature increase was examined through sonications executed on two sides having varied curvatures.

Materials and Methods

Assessment was performed utilizing a 1.1 MHz focused transducer. Sonications on the two phantom sides were executed at varied acoustical power in both a laboratory setting and inside a 1.5 T magnetic resonance imaging scanner. Sonications were independently performed on two identical phantoms for repeatability purposes.

Results

Temperature changes between 7.1°C-34.3°C and 5.1°C-21.5°C were recorded within the decreased and increased curvature sides, respectively, for acoustical power of 3.75-10 W. High-power sonications created lesions which were approximately symmetrically formed around the focal point at the decreased curvature side, while they were shifted away from the focal point at the increased curvature side.

Conclusions

The present findings indicate that increased curvature of the breast phantom results in deformed focal shapes and decreased temperatures induced at the focal area, thus suggesting treatment correction requirements in the form of focus control or accurate robotic movement. The developed breast-shaped phantom can be utilized as an evaluation tool of HIFU systems dedicated to breast cancer since it can visually verify the efficacy of any system.

Passive Elastography for Clinical HIFU Lesion Detection

High-intensity Focused Ultrasound (HIFU) is a promising treatment modality for a wide range of pathologies including prostate cancer. However, the lack of a reliable ultrasound-based monitoring technique limits its clinical use. Ultrasound currently provides real-time HIFU planning, but its use for monitoring is usually limited to detecting the backscatter increase resulting from chaotic bubble appearance. HIFU has been shown to generate stiffening in various tissues, so elastography is an interesting lead for ablation monitoring. However, the standard techniques usually require the generation of a controlled push which can be problematic in deeper organs. Passive elastography offers a potential alternative as it uses the physiological wave field to estimate the elasticity in tissues and not an external perturbation. This technique was adapted to process B-mode images acquired with a clinical system. It was first shown to faithfully assess elasticity in calibrated phantoms. The technique was then implemented on the Focal One® clinical system to evaluate its capacity to detect HIFU lesions in vitro (CNR = 9.2 dB) showing its independence regarding the bubbles resulting from HIFU and in vivo where the physiological wave field was successfully used to detect and delineate lesions of different sizes in porcine liver. Finally, the technique was performed for the very first time in four prostate cancer patients showing strong variation in elasticity before and after HIFU treatment (average variation of 33.0 ± 16.0 % ). Passive elastography has shown evidence of its potential to monitor HIFU treatment and thus help spread its use.

Suppressing HIFU interference in ultrasound images using 1D U-Net-based neural networks

Objective.One big challenge with high-intensity focused ultrasound (HIFU) is that the intense acoustic interference generated by HIFU irradiation overwhelms the B-mode monitoring images, compromising monitoring effectiveness. This study aims to overcome this problem using a one-dimensional (1D) deep convolutional neural network.Approach. U-Net-based networks have been proven to be effective in image reconstruction and denoising, and the two-dimensional (2D) U-Net has already been investigated for suppressing HIFU interference in ultrasound monitoring images. In this study, we propose that the one-dimensional (1D) convolution in U-Net-based networks is more suitable for removing HIFU artifacts and can better recover the contaminated B-mode images compared to 2D convolution.Ex vivoandinvivoHIFU experiments were performed on a clinically equivalent ultrasound-guided HIFU platform to collect image data, and the 1D convolution in U-Net, Attention U-Net, U-Net++, and FUS-Net was applied to verify our proposal.Main results.All 1D U-Net-based networks were more effective in suppressing HIFU interference than their 2D counterparts, with over 30% improvement in terms of structural similarity (SSIM) to the uncontaminated B-mode images. Additionally, 1D U-Nets trained usingex vivodatasets demonstrated better generalization performance ininvivoexperiments.Significance.These findings indicate that the utilization of 1D convolution in U-Net-based networks offers great potential in addressing the challenges of monitoring in ultrasound-guided HIFU systems.

In vivo evaluation of focused ultrasound ablation surgery (FUAS)-induced coagulation using echo amplitudes of the therapeutic focused ultrasound transducer

OBJECTIVE

Monitoring sensitivity of sonography in focused ultrasound ablation surgery (FUAS) is limited (no hyperechoes in ∼50% of successful coagulation in uterine fibroids). A more accurate and sensitive approach is required.

METHOD

The echo amplitudes of the focused ultrasound (FUS) transducer in a testing mode (short pulse duration and low power) were found to correlate with the ex vivo coagulation. To further evaluate its coagulation prediction capabilities, in vivo experiments were carried out. The liver, kidney, and leg muscles of three adult goats were treated using clinical FUAS settings, and the echo amplitude of the FUS transducer and grayscale in sonography before and after FUAS were collected. On day 7, animals were sacrificed humanely, and the treated tissues were dissected to expose the lesion. Echo amplitude changes and lesion areas were analyzed statistically, as were the coagulation prediction metrics.

RESULTS

The echo amplitude changes of the FUS transducer correlate well with the lesion areas in the liver (R = 0.682). Its prediction in accuracy (94.4% vs. 50%), sensitivity (92.9% vs. 35.7%), and negative prediction (80% vs. 30.8%) is better than sonography, but similar in specificity (80% vs. 100%) and positive prediction (100% vs. 100%). In addition, the correlation between tissue depth and the lesion area is not good (|R| < 0.2). Prediction performances in kidney and leg muscles are similar.

CONCLUSION

The FUS echo amplitudes are sensitive to the tissue properties and their changes after FUAS. They are sensitive and reliable in evaluating and predicting FUAS outcomes.

Ultrasound Monitoring of Simultaneous high-intensity focused ultrasound (HIFU) therapy using minimum-peak-sidelobe coded excitation

Bipolar sequences can be readily transmitted by ultrasound (US) pulser hardware with the full driving voltage to boost the echo magnitude in B-mode monitoring of HIFU treatment. In this study, a novel single-transmit bipolar sequence with minimum-peak-sidelobe (MPS) level is developed not only to restore the image quality of US monitoring but also remove acoustic interference from simultaneous HIFU transmission. The proposed MPS code is designed with an equal number of positive and negative bits and the bit duration should be an integer multiple of the period of the HIFU waveform. In addition, different permutations of code sequence are searched in order to obtain the optimal encoding. The received imaging echo is firstly decoded by matched filtering to cancel HIFU interference and to enhance the echo magnitude of US monitoring. Then, Wiener filtering is applied as the second-stage pulse compression to improve the final image quality. Simulations and phantom experiments are performed to compare the single-transmit MPS decoding with conventional two-transmit methods such as pulse-inversion subtraction (PIS) and Golay decoding for their performance in simultaneous US monitoring of HIFU treatment. Results show that the MPS decoding effectively removes HIFU interference even in the presence of tissue motion. The image quality of PIS and Golay decoding, on the other hand, is compromised by the uncancelled HIFU components due to tissue motion. Simultaneous US monitoring of tissue ablation using the proposed MPS decoding has also demonstrated to be feasible in ex-vivo experiments. Compared to the notch filtering that also allows single-transmit HIFU elimination, the MPS decoding is preferrable because it does not suffer from the tradeoff between residual HIFU and speckle deterioration in US monitoring images.

Improved source localization in passive acoustic mapping using delay-multiply-and-sum beamforming with virtually augmented aperture

High-intensity focused ultrasound (HIFU) is a promising non-invasive treatment method whose applications include tissue ablation, hemostasis, thrombolysis and blood-brain barrier opening etc. Its therapeutic effects come from the thermal necrosis and the mechanical destruction associated with acoustic cavitation. Passive acoustic mapping (PAM) is capable of simultaneous monitoring of HIFU-induced cavitation events using only receive beamforming. Nonetheless, conventional time exposure acoustics (TEA) algorithm has poor spatial resolution and suffers from the X-shaped artifacts. These factors lead to difficulties in precise localization of cavitation source. In this study, we proposed a novel adaptive PAM method which combines Delay-Multiply-and-Sum (DMAS) beamforming with virtual augmented aperture (VA) to overcome the problem. In DMAS-VA beamforming, the magnitude of each channel waveform is scaled by p-th root while the phase is multiplied by L. The p and L correspond respectively to the degree of signal coherence in DMAS beamforming and the augmentation factor of aperture size. After channel sum, p-th power is applied to restore the dimensionality of source strength and then the PAM image is reconstructed by accumulating the signal power over the observation time. Based on simulation and experimental results, the proposed DMAS-VA has better image resolution and image contrast compared with the conventional TEA. Moreover, since the VA method may introduce grating lobes into PAM because of the virtually augmented pitch size, DMAS coherent factor (DCF) is further developed to alleviate these image artifacts. Results indicate that, with DCF weighting, the PAM image of DMAS-VA beamforming could be constructed without detectable image artifacts from grating lobes and false main lobes.

Artificial intelligence-assisted ultrasound-guided focused ultrasound therapy: a feasibility study

OBJECTIVES

Focused ultrasound (FUS) therapy has emerged as a promising noninvasive solution for tumor ablation. Accurate monitoring and guidance of ultrasound energy is crucial for effective FUS treatment. Although ultrasound (US) imaging is a well-suited modality for FUS monitoring, US-guided FUS (USgFUS) faces challenges in achieving precise monitoring, leading to unpredictable ablation shapes and a lack of quantitative monitoring. The demand for precise FUS monitoring heightens when complete tumor ablation involves controlling multiple sonication procedures.

METHODS

To address these challenges, we propose an artificial intelligence (AI)-assisted USgFUS framework, incorporating an AI segmentation model with B-mode ultrasound imaging. This method labels the ablated regions distinguished by the hyperechogenicity effect, potentially bolstering FUS guidance. We evaluated our proposed method using the Swin-Unet AI architecture, conducting experiments with a USgFUS setup on chicken breast tissue.

RESULTS

Our results showed a 93% accuracy in identifying ablated areas marked by the hyperechogenicity effect in B-mode imaging.

CONCLUSION

Our findings suggest that AI-assisted ultrasound monitoring can significantly improve the precision and control of FUS treatments, suggesting a crucial advancement toward the development of more effective FUS treatment strategies.

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.

In Situ Measurement of Acoustic Attenuation for Focused Ultrasound Ablation Surgery Using a Boiling Bubble at the Focus

OBJECTIVE

Acoustic attenuation in the propagation path of focused ultrasound ablation surgery determines the energy loss toward the focal region and is critical to the consequent treatment outcomes. In situ non-invasive, reliable, and accurate measurement is challenging for multi-layered heterogeneous tissues within the focusing angle.

METHODS

A novel measurement approach is proposed and its performance is evaluated using ex vivo porcine tenderloin and bovine heart. A big boiling bubble (i.e., larger than a few millimeters in size) was produced at the focus as a strong reflector inside the tissue, and the echo amplitudes were used to determine the acoustic attenuation. Two models, acoustic ray and energy loss, were developed to derive the equivalent acoustic attenuation coefficient for a focused beam.

RESULTS

The measured acoustic attenuation coefficients of ex vivo porcine tenderloin and bovine heart at 0.97 MHz and a thickness of 3 cm are 0.159 ± 0.002 and 0.250 ± 0.005 Np/cm, respectively, which are all within the scope of measured values in the literature. In addition, the echo amplitude is sensitive to the conditions of the propagation path, and the inverse acoustic attenuation coefficient of the silicone gel pad placed in front of the tissue sample was 0.807 ± 0.002 Np/cm, which is comparable to the measurement using the insertion substitution method, 0.766 ± 0.003 Np/cm.

CONCLUSION

Our proposed approach could determine the tissue acoustic attenuation for focused ultrasound ablation surgery reliably and accurately in situ. The easy operating protocol may allow clinical translation and adoption for improved safety and efficacy.

Real-time monitoring of focused ultrasound therapy using intelligence-based thermography: A feasibility study

Focused ultrasound (FUS) therapy has been widely studied for breast cancer treatment due to its potential as a fully non-invasive method to improve cosmetic and oncologic results. However, real-time imaging and monitoring of the therapeutic ultrasound delivered to the target area remain challenges for precision breast cancer therapy. The main objective of this study is to propose and evaluate a novel intelligence-based thermography (IT) method that can monitor and control FUS treatment using thermal imaging with the fusion of artificial intelligence (AI) and advanced heat transfer modeling. In the proposed method, a thermal camera is integrated into FUS system for thermal imaging of the breast surface, and an AI model is employed for the inverse analysis of the surface thermal monitoring, thereby estimating the features of the focal region. This paper presents experimental and computational studies conducted to assess the feasibility and efficiency of IT-guided FUS (ITgFUS). Tissue phantoms, designed to mimic the properties of breast tissue, were used in the experiments to investigate detectability and the impact of temperature rise at the focal region on the tissue surface. Additionally, an AI computational analysis employing an artificial neural network (ANN) and FUS simulation was carried out to provide a quantitative estimation of the temperature rise at the focal region. This estimation was based on the observed temperature profile on the breast model's surface. The results proved that the effects of temperature rise at the focused area could be detected by the thermal images acquired with thermography. Moreover, it was demonstrated that the AI analysis of the surface temperature measurement could result in near real-time monitoring of FUS by quantitative estimation of the temporal and spatial temperature rise profiles at the focal region.

Real-time assessment of high-intensity focused ultrasound heating and cavitation with hybrid optoacoustic ultrasound imaging

High-intensity focused ultrasound (HIFU) enables localized ablation of biological tissues by capitalizing on the synergistic effects of heating and cavitation. Monitoring of those effects is essential for improving the efficacy and safety of HIFU interventions. Herein, we suggest a hybrid optoacoustic-ultrasound (OPUS) approach for real-time assessment of heating and cavitation processes while providing an essential anatomical reference for accurate localization of the HIFU-induced lesion. Both effects could clearly be observed by exploiting the temperature dependence of optoacoustic (OA) signals and the strong contrast of gas bubbles in pulse-echo ultrasound (US) images. The differences in temperature increase and its rate, as recorded with a thermal camera for different HIFU pressures, evinced the onset of cavitation at the expected pressure threshold. The estimated temperatures based on OA signal variations were also within 10-20 % agreement with the camera readings for temperatures below the coagulation threshold (∼50 °C). Experiments performed in excised tissues as well as in a post-mortem mouse demonstrate that both heating and cavitation effects can be effectively visualized and tracked using the OPUS approach. The good sensitivity of the suggested method for HIFU monitoring purposes was manifested by a significant increase in contrast-to-noise ratio within the ablated region by > 10 dB and > 5 dB for the OA and US images, respectively. The hybrid OPUS-based monitoring approach offers the ease of handheld operation thus can readily be implemented in a bedside setting to benefit several types of HIFU treatments used in the clinics.

Modeling for Quantitative Analysis of Nakagami Imaging in Accurate Detection and Monitoring of Therapeutic Lesions by High-Intensity Focused Ultrasound

OBJECTIVE

Nakagami imaging is an appealing monitoring and evaluation technique for high-intensity focused ultrasound treatment when bubbles are present in ultrasound images. This study aimed to investigate the accuracy of thermal lesion detection using Nakagami imaging.

METHODS

Simulations were conducted to explore and quantify the influence of the bubbles and the subresolvable effect at the boundary of the thermal lesion on thermal lesion detection. The thermal ablation experiments were conducted in phantom and porcine liver ex vivo.

RESULTS

In the simulation, the estimated lateral and axial size of the thermal lesion in the Nakagami image was 4.91 and 4.79 mm, close to the actual size (5 × 5 mm). The simulation results indicated that the subresolvable region in high-intensity focused ultrasound treatment thermal ablation mainly happened at the boundary between bubbles and the untreated region and does not affect the accuracy of thermal lesion detection. The accurate detection of the thermal lesion using Nakagami imaging mainly depends on bubbles and thermal lesion characterization. Our thermal ablation experiments confirmed that Nakagami imaging has the ability to accurately identify thermal lesions from bubbles.

CONCLUSION

The subresolvable effect is helpful for thermal lesion identification, and precision is related to the Nakagami values chosen for boundary division in Nakagami imaging. Therefore, Nakagami imaging is a promising method for accurately evaluating thermal lesions. Further studies in vivo and in clinical settings will be needed to explore its potential applications.

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.

Monitoring cavitation dynamics evolution in tissue mimicking hydrogels for repeated exposures via acoustic cavitation emissions

A 700 kHz histotripsy array is used to generate repeated cavitation events in agarose, gelatin, and polyacrylamide hydrogels. High-speed optical imaging, a broadband hydrophone, and the narrow-band receive elements of the histotripsy array are used to capture bubble dynamics and acoustic cavitation emissions. Bubble radii, lifespan, shockwave amplitudes are noted to be measured in close agreement between the different observation methods. These features also decrease with increasing hydrogel stiffness for all of the tested materials. However, the evolutions of these properties during the repeated irradiations vary significantly across the different material subjects. Bubble maximum radius initially increases, then plateaus, and finally decreases in agarose, but remains constant across exposures in gelatin and polyacrylamide. The bubble lifespan increases monotonically in agarose and gelatin but decreases in polyacrylamide. Collapse shockwave amplitudes were measured to have different-shaped evolutions between all three of the tested materials. Bubble maximum radii, lifespans, and collapse shockwave amplitudes were observed to express evolutions that are dependent on the structure and stiffness of the nucleation medium.

Development of an ultrasound guided focused ultrasound system for 3D volumetric low energy nanodroplet-mediated histotripsy

Low pressure histotripsy is likely to facilitate current treatments that require extremely high pressures. An ultrasound guided focused ultrasound system was designed to accommodate a rotating imaging transducer within a low frequency therapeutic transducer that operates at a center frequency of 105 kHz. The implementation of this integrated system provides real-time therapeutic and volumetric imaging functions, that are used here for low-cost, low-energy 3D volumetric ultrasound histotripsy using nanodroplets. A two-step approach for low pressure histotripsy is implemented with this dual-array. Vaporization of nanodroplets into gaseous microbubbles was performed via the 1D rotating imaging probe. The therapeutic transducer is then used to detonate the vaporized nanodroplets and trigger potent mechanical effects in the surrounding tissue. Rotating the imaging transducer creates a circular vaporized nanodroplet shape which generates a round lesion upon detonation. This contrasts with the elongated lesion formed when using a standard 1D imaging transducer for nanodroplet activation. Optimization experiments show that maximal nanodroplet activation can be achieved with a 2-cycle excitation pulse at a center frequency of 3.5 MHz, and a peak negative pressure of 3.4 MPa (a mechanical index of 1.84). Vaporized nanodroplet detonation was achieved by applying a low frequency treatment at a center frequency of 105 kHz and mechanical index of 0.9. In ex-vivo samples, the rotated nanodroplet activation method yielded the largest lesion area, with a mean of 4.7 ± 0.5 mm², and a rounded shape. In comparison, standard fixed transducer nanodroplet activation resulted in an average lesion area of 2.6 ± 0.4 mm², and an elongated shape. This hybrid system enables to achieve volumetric low energy histotripsy, and thus facilitates the creation of precise, large-volume mechanical lesions in tissues, while reducing the pressure threshold required for standard histotripsy by over an order of magnitude.

A neuropsin-based optogenetic tool for precise control of Gq signaling

G_q-coupled receptors regulate numerous physiological processes by activating enzymes and inducing intracellular Ca²⁺ signals. There is a strong need for an optogenetic tool that enables powerful experimental control over G_q signaling. Here, we present chicken opsin 5 (cOpn5) as the long sought-after, single-component optogenetic tool that mediates ultra-sensitive optical control of intracellular G_q signaling with high temporal and spatial resolution. Expressing cOpn5 in HEK 293T cells and primary mouse astrocytes enables blue light-triggered, G_q-dependent Ca²⁺ release from intracellular stores and protein kinase C activation. Strong Ca²⁺ transients were evoked by brief light pulses of merely 10 ms duration and at 3 orders lower light intensity of that for common optogenetic tools. Photostimulation of cOpn5-expressing cells at the subcellular and single-cell levels generated fast intracellular Ca²⁺ transition, thus demonstrating the high spatial precision of cOpn5 optogenetics. The cOpn5-mediated optogenetics could also be applied to activate neurons and control animal behavior in a circuit-dependent manner. cOpn5 optogenetics may find broad applications in studying the mechanisms and functional relevance of G_q signaling in both non-excitable cells and excitable cells in all major organ systems.

Feasibility of ultrasound tomography-guided localized mild hyperthermia using a ring transducer: Ex vivo and in silico studies

BACKGROUND

As of 2022, breast cancer continues to be the most diagnosed cancer worldwide. This problem persists within the United States as well, as the American Cancer Society has reported that ∼12.5% of women will be diagnosed with invasive breast cancer over the course of their lifetime. Therefore, a clinical need continues to exist to address this disease from a treatment and therapeutic perspective. Current treatments for breast cancer and cancers more broadly include surgery, radiation, and chemotherapy. Adjuncts to these methods have been developed to improve the clinical outcomes for patients. One such adjunctive treatment is mild hyperthermia therapy (MHTh), which has been shown to be successful in the treatment of cancers by increasing effectiveness and reduced dosage requirements for radiation and chemotherapies. MHTh-assisted treatments can be performed with invasive thermal devices, noninvasive microwave induction, heating and recirculation of extracted patient blood, or whole-body hyperthermia with hot blankets.

PURPOSE

One common method for inducing MHTh is by using microwave for heat induction and magnetic resonance imaging for temperature monitoring. However, this leads to a complex, expensive, and inaccessible therapy platform. Therefore, in this work we aim to show the feasibility of a novel all-acoustic MHTh system that uses focused ultrasound (US) to induce heating while also using US tomography (UST) to provide temperature estimates. Changes in sound speed (SS) have been shown to be strongly correlated with temperature changes and can therefore be used to indirectly monitor heating throughout the therapy. Additionally, these SS estimates allow for heterogeneous SS-corrected phase delays when heating complex and heterogeneous tissue structures.

METHODS

Feasibility to induce localized heat in tissue was investigated in silico with a simulated breast model, including an embedded tumor using continuous wave US. Here, both heterogenous acoustic and thermal properties were modeled in addition to blood perfusion. We further demonstrate, with ex vivo tissue phantoms, the feasibility of using ring-based UST to monitor temperature by tracking changes in SS. Two phantoms (lamb tissue and human abdominal fat) with latex tubes containing varied temperature flowing water were imaged. The measured SS of the water at each temperature were compared against values that are reported in literature.

RESULTS

Results from ex vivo tissue studies indicate successful tracking of temperature under various phantom configurations and ranges of water temperature. The results of in silico studies show that the proposed system can heat an acoustically and thermally heterogenous breast model to the clinically relevant temperature of 42°C while accounting for a reasonable time needed to image the current cross section (200 ms). Further, we have performed an initial in silico study demonstrating the feasibility of adjusting the transmit waveform frequency to modify the effective heating height at the focused region. Lastly, we have shown in a simpler 2D breast model that MHTh level temperatures can be maintained by adjusting the transmit pressure intensity of the US ring.

CONCLUSIONS

This work has demonstrated the feasibility of using a 256-element ring array transducer for temperature monitoring; however, future work will investigate minimizing the difference between measured SS and the values shown in literature. A hypothesis attributes this bias to potential volumetric average artifacts from the ray-based SS inversion algorithm that was used, and that moving to a waveform-based SS inversion algorithm will greatly improve the SS estimates. Additionally, we have shown that an all-acoustic MHTh system is feasible via in silico studies. These studies have indicated that the proposed system can heat a tumor within a heterogenous breast model to 42°C within a narrow time frame. This holds great promise for increasing the accessibility and reducing the complexity of a future all-acoustic MHTh system.

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.

A Novel Approach for the Detection of Every Significant Collapsing Bubble in Passive Cavitation Imaging

Passive cavitation image (PCI) shows the power distribution of the acoustic emissions resulting from cavitation bubble collapses. The conventional PCI convolves the emitted cavitation signals with the point spread function of an imaging system, and it suffers from a low spatial resolution and contrast due to the increased sidelobe artifacts accumulated during the temporal integral process. To overcome the problems, the present study considers a 3-D time history of instantaneous PCIs where cavitation occurs at the local maxima of the main lobes of the beamformed cavitation field surrounded by the sidelobes largely spreading out in a time-space domain. A spatial and temporal gating technique was employed to detect the local maxima so that cavitation bubbles can be identified with their collapsing strength. The proposed approach was verified by the simulation for single and multiple cavitation bubbles, proving that it accurately detects the location and strength of the collapsing bubbles. An experimental test was also carried out for the cavitation bubbles produced by a clinical extracorporeal shock wave therapeutic device, which underpins that the proposed method successfully identifies every individual cavitation bubble.

Golay-Encoded Ultrasound Monitoring of Simultaneous High-Intensity Focused Ultrasound Treatment: A Phantom Study

Ultrasound (US) imaging has high potential in monitoring high-intensity focused US (HIFU) treatment due to its superior temporal resolution. However, US monitoring is often hindered by strong HIFU interference, which overwhelms the echoes received by the imaging array. In this study, a method of Golay-encoded US monitoring is proposed to visualize the imaged object for simultaneous HIFU treatment. It effectively removes HIFU interference patterns in real-time B-mode imaging and improves the metrics of image quality, such as peak signal-to-noise ratio (PSNR), structural similarity (SSIM), and contrast ratio (CR). Compared to the pulse-inversion sequence, the N -bit Golay sequence can boost the echo magnitude of US monitoring by another N times and, thus, exhibits higher robustness. Simulations show that a sinusoidal HIFU waveform can be fully eliminated using Golay decoding when the bit duration of the N -bit Golay sequence ( N is the power of 4) coincides with either odd (Case I) or even (Case II) integer multiples of the HIFU quarter period. Experimental results also show that the Golay decoding with Case II can increase the PSNR of US monitoring images by more than 30 dB for both pulse- and continuous-wave HIFU transmissions. The SSIM index also effectively improves to about unity, indicating that the B-mode image with HIFU transmission is visually indistinguishable from that acquired without HIFU transmission. Though Case I is inferior to Case II in the elimination of even-order HIFU harmonic, they together enable a more flexible selection of imaging frequencies to meet the required image resolution and penetration for Golay-encoded US monitoring.

Localized motion imaging for monitoring HIFU therapy: Comparison of modulating frequencies and utilization of square modulating wave

High-intensity focused ultrasound (HIFU) has been successfully used as a minimally invasive cancer therapy method. For monitoring the therapy, the amplitude-modulated (AM) localized motion imaging (LMI) method had been proposed. This paper compares the performance of AM-LMI while using different sine modulating wave frequencies and proposes the utilization of square modulating waves to gain the advantages of both high and low modulating frequencies. A single element therapy transducer with a 2 MHz central frequency was driven by sine modulating waves with different frequencies (approximate 34, 67, 102, 168, and 201 Hz) and by square modulating waves with two frequencies (34 and 67 Hz). An imaging probe with a 5 MHz central frequency and a 20 MHz sampling frequency was mounted in the center hole of the therapy transducer to acquire pulse-echo data, which were used to estimate the tissue oscillation amplitude induced by the acoustic radiation force of the HIFU beam. The decrease ratio of the oscillation amount was then utilized to estimate the coagulated lesion length during the therapy. The comparison of modulating frequencies demonstrated that a higher frequency could bring higher sensitivity to small lesions, while a lower frequency not only gives greater noise robustness but also promotes the ability to estimate lengths of larger lesions. The utilization of a square modulating wave demonstrated its utility to produce tissue oscillation with multiple frequencies and gain the advantages of both high and low modulating frequencies.

Acoustic beam mapping for guiding HIFU therapy in vivo using sub-therapeutic sound pulse and passive beamforming

OBJECTIVE

Although HIFU has been successfully applied in various clinical applications in the past two decades for the ablation of many types of tumors, one bottleneck in its wider applications is the lack of a reliable and affordable strategy to guide the therapy. This study aims at estimating the therapeutic beam path at the pre-treatment stage to guide the therapeutic procedure.

METHODS

An incident beam mapping technique using passive beamforming was proposed based on a clinical HIFU system and an ultrasound imaging research system. An optimization model was created to map the cross-like beam pattern by maximizing the total energy within the mapped area. This beam mapping technique was validated by comparing the estimated focal region with the HIFU-induced actual focal region (damaged region) through simulation, in-vitro, ex-vivo and in-vivo experiments.

RESULTS

The results of this study showed that the proposed technique was, to a large extent, tolerant of sound speed inhomogeneities, being able to estimate the focal location with errors of 0.15 mm and 0.93 mm under in-vitro and ex-vivo situations respectively, and slightly over 1 mm under the in-vivo situation. It should be noted that the corresponding errors were 6.8 mm, 3.2 mm, and 9.9 mm respectively when the conventional geometrical method was used.

CONCLUSION

This beam mapping technique can be very helpful in guiding the HIFU therapy and can be easily applied in clinical environments with an ultrasound-guided HIFU system.

SIGNIFICANCE

The technique is non-invasive and can potentially be adapted to other ultrasound-related beam manipulating applications.

Photoacoustic computed tomography of mechanical HIFU-induced vascular injury

Mechanical high-intensity focused ultrasound (HIFU) has been used for cancer treatment and drug delivery. Existing monitoring methods for mechanical HIFU therapies such as MRI and ultrasound imaging often suffer from high cost, poor spatial-temporal resolution, and/or low sensitivity to tissue's hemodynamic changes. Evaluating vascular injury during mechanical HIFU treatment, therefore, remains challenging. Photoacoustic computed tomography (PACT) is a promising tool to meet this need. Intrinsically sensitive to optical absorption, PACT provides high-resolution imaging of blood vessels using hemoglobin as the endogenous contrast. In this study, we have developed an integrated HIFU-PACT system for detecting vascular rupture in mechanical HIFU treatment. We have demonstrated singular value decomposition for enhancing hemorrhage detection. We have validated the HIFU-PACT performance on phantoms and in vivo animal tumor models. We expect that PACT-HIFU will find practical applications in oncology research using small animal models.

Safety and feasibility of arterial wall targeting with robot-assisted high intensity focused ultrasound: a preclinical study

PURPOSE

High-intensity focused ultrasound (HIFU) is a potential noninvasive thermal ablation method for the treatment of peripheral artery disease. Dual-mode ultrasound arrays (DMUA) offer the possibility of simultaneous imaging and treatment. In this study, safety and feasibility of femoral artery robot-assisted HIFU/DMUA therapy was assessed.

METHODS

In 18 pigs (∼50kg), angiography and diagnostic ultrasound were used to visualize diameter and blood flow of the external femoral arteries (EFA). HIFU/DMUA-therapy was unilaterally applied to the EFA dorsal wall using a 3.5 MHz, 64-element transducer, closed-loop-control was used to automatically adjust energy delivery to control thermal lesion formation. A continuous lesion of at least 25 mm was created by delivering 6-8 HIFU shots per imaging plane perpendicular to the artery spaced 1 mm apart. Directly after HIFU/DMUA-therapy and after 0, 3 or 14 days follow up, diameter and blood flow were measured and the skin was macroscopically examined for thermal damage. The tissue was removed for histological analysis.

RESULTS

No complications were observed. The most frequently observed treatment effect was formation of scar tissue, predominantly in the adventitia and the surrounding tissue. No damage to the endothelium or excessive damage of the surrounding tissue was observed. There was no significant decrease in the mean arterial diameter after HIFU/DMUA-therapy.

CONCLUSION

HIFU/DMUA therapy successfully targeted the vessel walls of healthy porcine arteries, without causing endothelial damage or other vascular complications. Therefore, this therapy can be safely applied to healthy arterial walls in animals. Future studies should focus on safety and dose-finding in atherosclerotic diseased arteries.

Design and Challenges of Sonodynamic Therapy System for Cancer Theranostics: From Equipment to Sensitizers

As a novel noninvasive therapeutic modality combining low-intensity ultrasound and sonosensitizers, sonodynamic therapy (SDT) is promising for clinical translation due to its high tissue-penetrating capability to treat deeper lesions intractable by photodynamic therapy (PDT), which suffers from the major limitation of low tissue penetration depth of light. The effectiveness and feasibility of SDT are regarded to rely on not only the development of stable and flexible SDT apparatus, but also the screening of sonosensitizers with good specificity and safety. To give an outlook of the development of SDT equipment, the key technologies are discussed according to five aspects including ultrasonic dose settings, sonosensitizer screening, tumor positioning, temperature monitoring, and reactive oxygen species (ROS) detection. In addition, some state-of-the-art SDT multifunctional equipment integrating diagnosis and treatment for accurate SDT are introduced. Further, an overview of the development of sonosensitizers is provided from small molecular sensitizers to nano/microenhanced sensitizers. Several types of nanomaterial-augmented SDT are in discussion, including porphyrin-based nanomaterials, porphyrin-like nanomaterials, inorganic nanomaterials, and organic-inorganic hybrid nanomaterials with different strategies to improve SDT therapeutic efficacy. There is no doubt that the rapid development and clinical translation of sonodynamic therapy will be promoted by advanced equipment, smart nanomaterial-based sonosensitizer, and multidisciplinary collaboration.

Opening doors with ultrasound and microbubbles: Beating biological barriers to promote drug delivery

Apart from its clinical use in imaging, ultrasound has been thoroughly investigated as a tool to enhance drug delivery in a wide variety of applications. Therapeutic ultrasound, as such or combined with cavitating nuclei or microbubbles, has been explored to cross or permeabilize different biological barriers. This ability to access otherwise impermeable tissues in the body makes the combination of ultrasound and therapeutics very appealing to enhance drug delivery in situ. This review gives an overview of the most important biological barriers that can be tackled using ultrasound and aims to provide insight on how ultrasound has shown to improve accessibility as well as the biggest hurdles. In addition, we discuss the clinical applicability of therapeutic ultrasound with respect to the main challenges that must be addressed to enable the further progression of therapeutic ultrasound towards an effective, safe and easy-to-use treatment tailored for drug delivery in patients.

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.

High contrast ultrasonic method with multi-spatiotemporal compounding for monitoring catheter-based ultrasound thermal therapy: Development and Ex Vivo Evaluations

OBJECTIVE

Changes in ultrasound backscatter energy (CBE) imaging can monitor thermal therapy. Catheter-based ultrasound (CBUS) can treat deep tumors with precise spatial control of energy deposition and ablation zones, of which CBE estimation can be limited by low contrast and robustness due to small or inconsistent changes in ultrasound data. This study develops a multi-spatiotemporal compounding CBE (MST-CBE) imaging approach for monitoring specific to CBUS thermal therapy.

METHODS

Ex vivo thermal ablations were performed with stereotactic positioning of a 180 directional CBUS applicator, temperature monitoring probes, endorectal US probe, and subsequent lesion sectioning and measurement. Five frames of raw radiofrequency data were acquired throughout in 15s intervals. Using window-by-window estimation methods, absolute and positive components of MST-CBE images at each point were obtained by the compounding ratio of squared envelope data within an increasing spatial size in each short-time window.

RESULTS

Compared with conventional US, Nakagami, and CBE imaging, the detection contrast and robustness quantified by tissue-modification-ratio improved by 37.24.7 (p<0.001), 37.55.2 (p<0.001), and 6.44.0 dB (p<0.05) in the MST-CBE imaging, respectively. Correlation coefficient and bias between cross-sectional dimensions of the ablation zones measured in tissue sections and estimated from MST-CBE were up to 0.91 (p<0.001) and -0.02 mm2, respectively.

CONCLUSION

The MST-CBE approach can monitor the detailed changes within target tissues and effectively characterize the dimensions of the ablation zone during CBUS energy deposition.

SIGNIFICANCE

The MST-CBE approach could be practical for improved accuracy and contrast of monitoring and evaluation for CBUS thermal therapy.

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.

Proton Resonance Frequency Shift Thermometry: A Review of Modern Clinical Practices

Magnetic resonance imaging (MRI) has become a popular modality in guiding minimally invasive thermal therapies, due to its advanced, nonionizing, imaging capabilities and its ability to record changes in temperature. A variety of MR thermometry techniques have been developed over the years, and proton resonance frequency (PRF) shift thermometry is the current clinical gold standard to treat a variety of cancers. It is used extensively to guide hyperthermic thermal ablation techniques such as high-intensity focused ultrasound (HIFU) and laser-induced thermal therapy (LITT). Essential attributes of PRF shift thermometry include excellent linearity with temperature, good sensitivity, and independence from tissue type. This noninvasive temperature mapping method gives accurate quantitative measures of the temperature evolution inside biological tissues. In this review, the current status and new developments in the fields of MR-guided HIFU and LITT are presented with an emphasis on breast, prostate, bone, uterine, and brain treatments. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 3.

Development and characterization of a tissue mimicking psyllium husk gelatin phantom for ultrasound and magnetic resonance imaging

Purpose: To develop and characterize a tissue-mimicking phantom that enables the direct comparison of magnetic resonance (MR) and ultrasound (US) imaging techniques useful for monitoring high-intensity focused ultrasound (HIFU) treatments. With no additions, gelatin phantoms produce little if any scattering required for US imaging. This study characterizes the MR and US image characteristics as a function of psyllium husk concentration, which was added to increase US scattering.Methods: Gelatin phantoms were constructed with varying concentrations of psyllium husk. The effects of psyllium husk concentration on US B-mode and MR imaging were evaluated at nine different concentrations. T1, T2, and T2* MR maps were acquired. Acoustic properties (attenuation and speed of sound) were measured at frequencies of 0.6, 1.0, 1.8, and 3.0 MHz using a through-transmission technique. Phantom elastic properties were evaluated for both time and temperature dependence.Results: Ultrasound image echogenicity increased with increasing psyllium husk concentration while quality of gradient-recalled echo MR images decreased with increasing concentration. For all phantoms, the measured speed of sound ranged between 1567-1569 m/s and the attenuation ranged between 0.42-0.44 dB/(cm·MHz). Measured T1 ranged from 974-1051 ms. The T2 and T2* values ranged from 97-108 ms and 48-88 ms, respectively, with both showing a decreasing trend with increased psyllium husk concentration. Phantom stiffness, measured using US shear-wave speed measurements, increased with age and decreased with increasing temperature.Conclusions: The presented dual-use tissue-mimicking phantom is easy to manufacture and can be used to compare and evaluate US-guided and MR-guided HIFU imaging protocols.

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.

Precision Targeted Ablation of Fine Neurovascular Structures In Vivo Using Dual-mode Ultrasound Arrays

Carotid bodies (CBs) are chemoreceptors that monitor and register changes in the blood, including the levels of oxygen, carbon dioxide, and pH, and regulate breathing. Enhanced activity of CBs was shown to correlate with a significant elevation in the blood pressure of patients with hypertension. CB removal or denervation were previously shown to reduce hypertension. Here we demonstrate the feasibility of a dual-mode ultrasound array (DMUA) system to safely ablate the CB in vivo in a spontaneously hypertensive rat (SHR) model of hypertension. DMUA imaging was used for guiding and monitoring focused ultrasound (FUS) energy delivered to the target region. In particular, 3D imaging was used to identify the carotid bifurcation for targeting the CBs. Intermittent, high frame rate imaging during image-guided FUS (IgFUS) delivery was used for monitoring the lesion formation. DMUA imaging provided feedback for closed-loop control (CLC) of the lesion formation process to avoid overexposure. The procedure was tolerated well in over 100 SHR and normotensive rats that received unilateral and bilateral treatments. The measured mean arterial pressure (MAP) exhibited measurable deviation from baseline 2–4 weeks post IgFUS treatment. The results suggest that the direct unilateral FUS treatment of the CB might be sufficient to reduce the blood pressure in hypertensive rats and justify further investigation in large animals and eventually in human patients.

Emerging clinical applications of high-intensity focused ultrasound

High-intensity focused ultrasound (HIFU) is a minimally-invasive and non-ionizing promising technology and has been assessed for its role in the treatment of not only primary tumors but also metastatic lesions under the guidance of ultrasound or magnetic resonance imaging. Its performance is notably effective in neurologic, genitourinary, hepato-pancreato-biliary, musculoskeletal, oncologic, and other miscellaneous applications. In this article, we reviewed the emerging technology of HIFU and its clinical applications.

Comparative Study of Pulsed Versus Continuous High-Intensity Focused Ultrasound Ablation Using In Vitro and In Vivo Models

OBJECTIVES

To compare the efficacy of pulsed high-intensity focused ultrasound (PHIFU) versus continuous high-intensity focused ultrasound (CHIFU) ablation at identical doses.

METHODS

Continuous and pulsed HIFU (1200 J) at duty cycles (DCs) of 60% and 20% were examined for their capacity to ablate bovine liver tissue in vitro and rabbit liver tissue in vivo. After ablation, grayscale changes and pathologic characteristics were observed or measured, and the tissue necrosis volume, energy efficiency factor, and average grayscale density were calculated.

RESULTS

The pulsed mode generated greater liquefaction necrosis. An inconspicuous grayscale change was observed for PHIFU at a DC of 20% in some samples, which appeared as an elliptical cavity. The energy efficiency factor of PHIFU at a DC of 60% was significantly lower than that of CHIFU, as observed both in vitro and in vivo (P < .05). The grayscale value and average grayscale density in response to CHIFU were significantly greater than those in response to PHIFU (60% or 20%; P < .05). Histopathologic analysis revealed liquefaction necrosis in all PHIFU groups.

CONCLUSIONS

At identical doses, compared with CHIFU, a single session of PHIFU can generate liquefaction necrosis and at a higher DC can improve ablation efficiency. This increased efficacy of PHIFU may involve enhancement of tissue destruction by cavitation effects and a reduction in the obstruction effect of endogenous microbubbles through cavitation effects or a more effective diffusion of heat.

High-intensity focused ultrasound combined with transcatheter arterial chemoembolization and radiotherapy for advanced hepatocellular carcinoma: A case report

INTRODUCTION

Primary hepatocellular carcinoma (HCC) is one of the most common malignancies, only 10% to 20% of HCC patients are surgically resectable as most of the patients are diagnosed at advanced stages at presentation. The efficiencies of transcatheter arterial chemoembolization (TACE), high-intensity focused ultrasound (HIFU), and three-dimensional conformal radiation therapy (3D-CRT) in patients with advanced HCC have been clinically confirmed. We here report a patient with HCC accompanied by venous tumor thrombus, who was treated with the combination of these 3 therapies. The patient survived for 16 months with good quality of life.

PATIENT CONCERNS

The patient was a 72-year-old male with a primary multicentric HCC accompanied by tumor thrombus in the right hepatic vein. The patient had the symptoms of abdominal distention and liver pain. He refused sorafenib treatment because of personal reason.

DIAGNOSIS

Primary multicentric HCC stage IIIB cT4N0M0, accompanied by tumor thrombus in the right hepatic vein; chronic viral hepatitis B; and hepatitis B virus-related decompensated liver cirrhosis.

INTERVENTIONS

TACE + HIFU + 3D-CRT.

OUTCOMES

The patient had an overall survival of 16 months with good quality of life. Compared with monotherapy, the combined therapy significantly prolonged patient survival time with improved clinical benefits.

CONCLUSION

The combination of TACE, HIFU, and 3D-CRT is safe and effective in the treatment of advanced HCC, which provides a possible comprehensive treatment strategy for advanced HCC.

Thermal Memory Based Photoacoustic Imaging of Temperature

Temperature mapping is essential in many biomedical studies and interventions to precisely control the tissue's thermal conditions for optimal treatment efficiency and minimal side effects. Based on the Grüneisen parameter's temperature dependence, photoacoustic (PA) imaging can provide relative temperature measurement, but it has been traditionally challenging to measure absolute temperatures without knowing the baseline temperature, especially in deep tissues with unknown optical and acoustic properties. Here, we report a new thermal-energy-memory-based photoacoustic thermometry (TEMPT). By illuminating the tissue with a burst of nanosecond laser pulses, TEMPT exploits the temperature dependence of the thermal energy lingering, which is probed by the corresponding PA signals acquired within the thermal confinement. A self-normalized ratiometric measurement cancels out temperature-irrelevant quantities and estimates the Grüneisen parameter. The temperature can then be evaluated, given the tissue's temperature-dependent Grüneisen parameter, mass density, and specific heat capacity. Unlike the conventional PA thermometry, TEMPT does not require the knowledge of tissue's baseline temperature, nor the optical properties. We have developed a mathematical model to describe the temperature dependence in TEMPT. We have demonstrated the feasibility of the temperature evaluation on tissue phantoms at 1.5 cm depth within a clinically relevant temperature range. Finally, as proof-of-concept, we applied TEMPT for temperature mapping during focused ultrasound treatment in mice in vivo at 2 mm depth. As a generic temperature mapping method, TEMPT is expected to find applications in thermotherapy of cancers on small animal models.

Weighting the Passive Acoustic Mapping Technique With the Phase Coherence Factor for Passive Ultrasound Imaging of Ultrasound-Induced Cavitation

Ultrasound (US) cavitation is currently being explored for low-invasive therapy techniques applied to a wide panel of pathologies. Because of the random behavior of cavitation, a real-time spatial monitoring system may be required. For this purpose, the US passive imaging techniques have been recently investigated. In particular, the passive acoustic mapping (PAM) beamforming method enables the reconstruction of cavitation activity maps by beamforming acoustic signals passively recorded by an array transducer. In this paper, an optimized version of PAM, PAM weighted with a phase coherence factor (PAM-PCF), is considered. A general validation process is developed including simulations on a point source and experiments on a wire. Furthermore, using a focused regulated US-induced cavitation generator, reproducible cavitation experiments are conducted in water and in agar gel. The spatial behavior of a bubble cavitation cloud is determined using the PAM-PCF beamforming method to localize the focal cavitation point in two perpendicular imaging planes.

Real-Time HIFU Treatment Monitoring Using Pulse Inversion Ultrasonic Imaging

Real-time monitoring of high-intensity focused ultrasound (HIFU) surgery is essential for safe and accurate treatment. However, ultrasound imaging is difficult to use for treatment monitoring during HIFU surgery because of the high intensity of the HIFU echoes that are received by an imaging transducer. Here, we propose a real-time HIFU treatment monitoring method based on pulse inversion of imaging ultrasound; an imaging transducer fires ultrasound twice in 0° and 180° phases for one scanline while HIFUs of the same phase are transmitted in synchronization with the ultrasound transmission for imaging. By doing so, HIFU interferences can be eliminated after subtracting the two sets of the signals received by the imaging transducer. This function was implemented in a commercial research ultrasound scanner, and its performance was evaluated using the excised bovine liver. The experimental results demonstrated that the proposed method allowed ultrasound images to clearly show the echogenicity change induced by HIFU in the excised bovine liver. Additionally, it was confirmed that the moving velocity of the organs in the abdomen due to respiration does not affect the performance of the proposed method. Based on the experimental results, we believe that the proposed method can be used for real-time HIFU surgery monitoring that is a pivotal function for maximized treatment efficacy.

Ultrasound-assisted photothermal therapy and real-time treatment monitoring

Photothermal therapy (PTT) has the capability for selective treatment, in which light delivered to the target is converted into heat and subsequently causes coagulative necrosis. However, optical scattering in biological media limits light penetration, thus reducing therapeutic efficacy. Here, we demonstrate that the temperatures generated by light and ultrasound energies can be added constructively in resected melanoma cancers, which causes an increase in treatment depth. This method is called dual thermal therapy (DTT). It is also shown that combined ultrasound and photoacoustic images acquired using the pulse sequence proposed in this paper can be used for real-time monitoring of DTT.

Safety and feasibility of ultrasound-triggered targeted drug delivery of doxorubicin from thermosensitive liposomes in liver tumours (TARDOX): a single-centre, open-label, phase 1 trial

BACKGROUND

Previous preclinical research has shown that extracorporeal devices can be used to enhance the delivery and distribution of systemically administered anticancer drugs, resulting in increased intratumoural concentrations. We aimed to assess the safety and feasibility of targeted release and enhanced delivery of doxorubicin to solid tumours from thermosensitive liposomes triggered by mild hyperthermia, induced non-invasively by focused ultrasound.

METHODS

We did an open-label, single-centre, phase 1 trial in a single UK hospital. Adult patients (aged ≥18 years) with unresectable and non-ablatable primary or secondary liver tumours of any histological subtype were considered for the study. Patients received a single intravenous infusion (50 mg/m²) of lyso-thermosensitive liposomal doxorubicin (LTLD), followed by extracorporeal focused ultrasound exposure of a single target liver tumour. The trial had two parts: in part I, patients had a real-time thermometry device implanted intratumourally, whereas patients in part II proceeded without thermometry and we used a patient-specific model to predict optimal exposure parameters. We assessed tumour biopsies obtained before and after focused ultrasound exposure for doxorubicin concentration and distribution. The primary endpoint was at least a doubling of total intratumoural doxorubicin concentration in at least half of the patients treated, on an intention-to-treat basis. This study is registered with ClinicalTrials.gov, number NCT02181075, and is now closed to recruitment.

FINDINGS

Between March 13, 2015, and March 27, 2017, ten patients were enrolled in the study (six patients in part I and four in part II), and received a dose of LTLD followed by focused ultrasound exposure. The treatment resulted in an average increase of 3·7 times in intratumoural biopsy doxorubicin concentrations, from an estimate of 2·34 μg/g (SD 0·93) immediately after drug infusion to 8·56 μg/g (5·69) after focused ultrasound. Increases of two to ten times were observed in seven (70%) of ten patients, satisfying the primary endpoint. Serious adverse events registered were expected grade 4 transient neutropenia in five patients and prolonged hospital stay due to unexpected grade 1 confusion in one patient. Grade 3-4 adverse events recorded were neutropenia (grade 3 in one patient and grade 4 in five patients), and grade 3 anaemia in one patient. No treatment-related deaths occurred.

INTERPRETATION

The combined treatment of LTLD and non-invasive focused ultrasound hyperthermia in this study seemed to be clinically feasible, safe, and able to enhance intratumoural drug delivery, providing targeted chemo-ablative response in human liver tumours that were refractory to standard chemotherapy.

FUNDING

Oxford Biomedical Research Centre, National Institute for Health Research.

Passive acoustic mapping of cavitation using eigenspace-based robust Capon beamformer in ultrasound therapy

Pulse-echo imaging technique can only play a role when high intensity focused ultrasound (HIFU) is turned off due to the interference between the primary HIFU signal and the transmission pulse. Passive acoustic mapping (PAM) has been proposed as a tool for true real-time monitoring of HIFU therapy. However, the most-used PAM algorithm based on time exposure acoustic (TEA) limits the quality of cavitation image. Recently, robust Capon beamformer (RCB) has been used in PAM to provide improved resolution and reduced artifacts over TEA-based PAM, but the presented results have not been satisfactory. In the present study, we applied an eigenspace-based RCB (EISRCB) method to further improve the PAM image quality. The optimal weighting vector of the proposed method was found by projecting the RCB weighting vector onto the desired vector subspace constructed from the eigenstructure of the covariance matrix. The performance of the proposed PAM was validated by both simulations and in vitro histotripsy experiments. The results suggested that the proposed PAM significantly outperformed the conventionally used TEA and RCB-based PAM. The comparison results between pulse-echo images of the residual bubbles and cavitation images showed the potential of our proposed PAM in accurate localization of cavitation activity during HIFU therapy.

Focused ultrasound: tumour ablation and its potential to enhance immunological therapy to cancer

Various kinds of image-guided techniques have been successfully applied in the last years for the treatment of tumours, as alternative to surgical resection. High intensity focused ultrasound (HIFU) is a novel, totally non-invasive, image-guided technique that allows for achieving tissue destruction with the application of focused ultrasound at high intensity. This technique has been successfully applied for the treatment of a large variety of diseases, including oncological and non-oncological diseases. One of the most fascinating aspects of image-guided ablations, and particularly of HIFU, is the reported possibility of determining a sort of stimulation of the immune system, with an unexpected "systemic" response to treatments designed to be "local". In the present article the mechanisms of action of HIFU are described, and the main clinical applications of this technique are reported, with a particular focus on the immune-stimulation process that might originate from tumour ablations.

Real-Time Spatiotemporal Control of High-Intensity Focused Ultrasound Thermal Ablation Using Echo Decorrelation Imaging in ex Vivo Bovine Liver

The ability to control high-intensity focused ultrasound (HIFU) thermal ablation using echo decorrelation imaging feedback was evaluated in ex vivo bovine liver. Sonications were automatically ceased when the minimum cumulative echo decorrelation within the region of interest exceeded an ablation control threshold, determined from preliminary experiments as -2.7 (log-scaled decorrelation per millisecond), corresponding to 90% specificity for local ablation prediction. Controlled HIFU thermal ablation experiments were compared with uncontrolled experiments employing two, five or nine sonication cycles. Means and standard errors of the lesion width, area and depth, as well as receiver operating characteristic curves testing ablation prediction performance, were computed for each group. Controlled trials exhibited significantly smaller average lesion area, width and treatment time than five-cycle or nine-cycle uncontrolled trials and also had significantly greater prediction capability than two-cycle uncontrolled trials. These results suggest echo decorrelation imaging is an effective approach to real-time HIFU ablation control.