Real-time implementation of a dual-mode ultrasound array system: in vivo results

Focused Ultrasound, an Emerging Tool for Atherosclerosis Treatment: A Comprehensive Review

Focused ultrasound (FUS) has emerged as a promising noninvasive therapeutic modality for treating atherosclerotic arterial disease. High-intensity focused ultrasound (HIFU), a noninvasive and precise modality that generates high temperatures at specific target sites within tissues, has shown promising results in reducing plaque burden and improving vascular function. While low-intensity focused ultrasound (LIFU) operates at lower energy levels, promoting mild hyperthermia and stimulating tissue repair processes. This review article provides an overview of the current state of HIFU and LIFU in treating atherosclerosis. It focuses primarily on the therapeutic potential of HIFU due to its higher penetration and ability to achieve atheroma disruption. The review summarizes findings from animal models and human trials, covering the effects of FUS on arterial plaque and arterial wall thrombolysis in carotid, coronary and peripheral arteries. This review also highlights the potential benefits of focused ultrasound, including its noninvasiveness, precise targeting, and real-time monitoring capabilities, making it an attractive approach for the treatment of atherosclerosis and emphasizes the need for further investigations to optimize FUS parameters and advance its clinical application in managing atherosclerotic arterial disease.

Image-Guided Measurement of Radiation Force Induced by Focused Ultrasound Beams

The radiation force balance (RFB) is a widely used method for measuring acoustic power output of ultrasonic transducers. The reflecting cone target is attractive due to its simplicity and long-term stability, at a reasonable cost. However, accurate measurements using this method depend on the alignment between the ultrasound beam and cone axes, especially for highly focused beams utilized in therapeutic applications. With the advent of dual-mode ultrasound arrays (DMUAs) for imaging and therapy, image-guided measurements of acoustic output using the RFB method can be used to improve measurement accuracy. In this article, we describe an image-guided RFB measurement of focused DMUA beams using a widely used commercial instrument. DMUA imaging is used to optimize the alignment between the acoustic beam and reflecting cone axes. In addition to image-guided alignment, DMUA echo data is used to track the displacement of the cone, which provides an auxiliary measurement of acoustic power. Experimental results using a DMUA prototype with [Formula: see text] shows that 1-2 mm of misalignment can result in 5%-14% error in the measured acoustic power. In addition to the use of B-mode image guidance for improving measurement accuracy, we present preliminary results demonstrating the benefit of displacement tracking using real-time DMUA imaging during the application of (sub)therapeutic focused beams. Displacement tracking provides a direct measurement of the radiation force with high sensitivity and follows the expected dependence on changes in amplitude and duty cycle (DC) of the focused ultrasound (FUS) beam. This could lead to simpler, more reliable methods for measuring acoustic power based on the radiation force principle. Combined with appropriate computational modeling, the direct measurement of acoustic radiation force could lead to reliable dosimetry in situ in emerging applications such as transcranial FUS (tFUS) therapies.

A narrative review on the application of high-intensity focused ultrasound for the treatment of occlusive and thrombotic arterial disease

Objectives

High-intensity focused ultrasound (HIFU) is a noninvasive therapeutic modality with a variety of applications. It is approved for the treatment of essential tremors, ablation of prostate, hepatic, breast, and uterine tumors. Although not approved for use in the treatment of atherosclerotic arterial disease, there is a growing body of evidence investigating applications of HIFU. Currently, percutaneous endovascular techniques are predominant for the treatment of arterial pathology; however, there are no endovascular techniques of HIFU available. This study aims to review the state of current evidence for the application of HIFU in atherosclerotic arterial disease.

Methods

All English-language articles evaluating the effect of HIFU on arterial occlusive and thrombotic disease until 2021 were reviewed. Both preclinical and human clinical studies were included. Study parameters such as animal or clinical model and outcomes were reviewed. In addition, details pertaining to settings on the HIFU device used were also reviewed.

Results

In preclinical models, atherosclerotic plaque progression was inhibited by HIFU, through decreases in oxidized low-density lipoprotein cholesterol and increases in macrophage apoptosis. Additionally, HIFU promotes angiogenesis in hindlimb ischemic models by the upregulation of angiogenic and antiapoptotic factors, with increased angiogenesis at higher line densities of HIFU. HIFU also promotes thrombolysis and conversely induces platelet activation at low frequencies and higher intensities. Various clinical studies have attempted to translate some of these properties and demonstrated positive clinical outcomes for arterial recanalization after thrombotic stroke, decreased atherosclerotic plaque burden in carotid arteries, increase in tissue perfusion and a decrease in diameter stenosis in patients with atherosclerotic arterial disease.

Conclusions

In current preclinical and clinical data, the safety and efficacy of HIFU shows great promise in the treatment of atherosclerotic arterial disease. Future focused studies are warranted to guide the refinement of HIFU settings for more widespread adoption of this technology.

Optimization of a random linear ultrasonic therapeutic array based on a genetic algorithm

Given their advantage of suppressing grating lobes, randomly arranged linear arrays have potential for use in ultrasonic treatment. The current work proposes a method based on genetic algorithm to optimize the random arrangement of array elements, so that the suppression effect of grating lobes can be significantly improved with reduced calculating time. The maximum and average kerfs of array elements are used as genes, and the ratio of the maximum to the secondary maximum sound pressure at the focal plane is used as the optimized target. Typically, the calculation requirements of the current method can be reduced to ∼ 25% of the traversing method. We further discuss how the kerf width, the effective ratio of element areas and the ratio of focal distance to array aperture affect the suppression of grating lobes. For a typical linear array with 32 elements (1-MHz operating frequency, 1.5-mm element width and 150-mm focal distance), the results suggest that the grating lobes are suppressed well when (1) the ratio of maximum width to average width of the element is between 5 and 8, (2) the ratio of the effective element area to the area of the whole array is between 0.5 and 0.9, and (3) the ratio of the effective emission aperture to the actual emission aperture of the array is as large as possible. Based on optimized parameters, an experimental array was fabricated and the measured results of corresponding sound field were entirely consistent with the simulated results (Given her role as an Associate Editor of this journal, Juan Tu had no involvement in the peer-review of articles for which she was an author and had no access to information regarding the peer-review. Full responsibility for the peer-review process for this article was delegated to another Editor of this journal.).

Safety and feasibility study of non-invasive robot-assisted high-intensity focused ultrasound therapy for the treatment of atherosclerotic plaques in the femoral artery: protocol for a pilot study

INTRODUCTION

Peripheral arterial disease (PAD) is an atherosclerotic disease leading to stenosis and/or occlusion of the arterial circulation of the lower extremities. The currently available revascularisation methods have an acceptable initial success rate, but the long-term patency is limited, while surgical revascularisation is associated with a relatively high perioperative risk. This urges the need for development of less invasive and more effective treatment modalities. This protocol article describes a study investigating a new non-invasive technique that uses robot assisted high-intensity focused ultrasound (HIFU) to treat atherosclerosis in the femoral artery.

METHODS AND ANALYSIS

A pilot study is currently performed in 15 symptomatic patients with PAD with a significant stenosis in the common femoral and/or proximal superficial femoral artery. All patients will be treated with the dual-mode ultrasound array system to deliver imaging-guided HIFU to the atherosclerotic plaque. Safety and feasibility are the primary objectives assessed by the technical feasibility of this therapy and the 30-day major complication rate as primary endpoints. Secondary endpoints are angiographic and clinical success and quality of life.

ETHICS AND DISSEMINATION

Ethical approval for this study was obtained in 2019 from the Medical Ethics Committee of the University Medical Center Utrecht, the Netherlands. Data will be presented at national and international conferences and published in a peer-reviewed journal.

TRIAL REGISTRATION NUMBER

NL7564.

Spatio-Spectral Ultrasound Characterization of Reflection and Transmission Through Bone With Temperature Dependence

Transcranial focused ultrasound (tFUS) is a promising approach for the treatment of neurological disorders. It has proven useful in several clinical applications, with promising outcomes reported in the recent literature. Furthermore, it is currently being investigated in a range of neuromodulation (NM) and ablative applications, including epilepsy. In this application, tFUS access through the temporal window is the key to optimizing the treatment safety and efficacy. Traditional approaches have utilized transducers with low operating frequencies for tFUS applications. Modern array transducers and driving systems allow for more intelligent use of the temporal window by exploiting the spatio-spectral transmission bandwidth to a specified target or targets within the brain. To demonstrate the feasibility of this approach, we have investigated the ultrasound reflection and transmission characteristics for different access points within the temporal window of human skull samples ex vivo. Different transmit-receive (Rx) configurations are used for characterization of the spatio-spectral variability in reflection and transmission through the temporal window. In this article, we show results from a dual-piston transducer set up in the frequency range of 2-7 MHz. Broadband pulses as well as synthesized orthogonal frequency division multiplexed (OFDM) waveforms were used. The latter was used to improve the magnitude and phase measurements in 100-kHz subbands within the 2-7 MHz spectral window. A temperature-controlled water bath was used to characterize the change in reflection and transmission characteristics with temperature in the 25°C-43°C range. The measured values of the complex reflection and transmission coefficients exhibited significant variations with space, frequency, and temperature. On the other hand, the measured transmission phase varied more with location and frequency, with smaller sensitivity to temperature. A measurement-based hybrid angular spectrum (HAS) simulation through the human temporal bone was used to demonstrate the dependence of focusing gain on the skull profile and spatial distribution of change of speed of sound (SOS) at different skull temperatures.

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.

An Optimized Control Approach for HIFU Tissue Ablation Using PDE Constrained Optimization Method

High-intensity focused ultrasound (HIFU) is a widely used technique capable of providing noninvasive heating and ablation for a wide range of applications. However, the major challenges lie in the determination of the position and the amount of heat deposition over a target area. In order to assure that the thermal area is confined to tumor locations, an optimization method should be employed. Sequential quadratic programming and steepest gradient method with closed-form solution have been previously used to solve this kind of problem. However, these methods are complex and computationally inefficient. The goal of this article is to solve and control the solution of inverse problems with partial differential equation (PDE) constraints. Therefore, a distinguishing challenge of this technique is the handling of large numbers of optimization variables in combination with the complexities of discretized PDEs. In our method, the objective function is formulated as the square difference between the actual thermal dose and the desired one. At each iteration of the optimization procedure, we need to develop and solve the variation problem, the adjoint problem, and the gradient of the objective function. The analytical formula for the gradient is derived and calculated based on the solution of the adjoint problem. Several factors have been taken into consideration to demonstrate the robustness and efficiency of the proposed algorithm. The simulation results for all cases indicate the robustness and the computational efficiency of our proposed method compared to the steepest gradient descent method with the closed-form solution.

A Dual-mode 2D Matrix Array for Ultrasound Image-guided Noninvasive Therapy

Focused ultrasound (FUS) lacks reliable real-time image guidance, which hinders the development of non-invasive ultrasound treatment in many important clinical applications. A dual-mode ultrasound array, capable of both imaging and therapy offers a new and reliable strategy for image-guided ultrasound therapy applications. The strategy has the advantages of real-time use, low cost, portability and inherent registration between imaging and therapeutic coordinate systems. In this work, a dual-mode two-dimensional (2D) matrix array with 1 MHz center frequency and 256 elements for ultrasound image-guided non-invasive therapy is reported. The array can provide three-dimensional (3D) volumetric ultrasound imaging and 3D focus control. Ultrasound imaging and therapeutic applications for the brain of small animals demonstrated the multi-functional capability of the dual-mode 2D matrix array. A method of rat brain positioning based on ultrasound imaging was proposed and verified. Transcranial ultrasound image-guided bloodbrain barrier (BBB) opening of multiple-targets was achieved in vivo, using the proposed dual-mode 2D array. The obtained results indicate that the dual-mode 2D matrix array is a promising method for practical use in ultrasound image-guided non-invasive therapy applications.

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.

3D synthetic aperture imaging with a therapeutic spherical random phased array for transcostal applications

Experimental validation of a synthetic aperture imaging technique using a therapeutic random phased array is described, demonstrating the dual nature of imaging and therapy of such an array. The transducer is capable of generating both continuous wave high intensity beams for ablating the tumor and low intensity ultrasound pulses to image the target area. Pulse-echo data is collected from the elements of the phased array to obtain B-mode images of the targets. Since therapeutic arrays are optimized for therapy only with concave apertures having low f-number and large directive elements often coarsely sampled, imaging can not be performed using conventional beamforming. We show that synthetic aperture imaging is capable of processing the acquired RF data to obtain images of the field of interest. Simulations were performed to compare different synthetic aperture imaging techniques to identify the best algorithm in terms of spatial resolution. Experimental validation was performed using a 1 MHz, 256-elements, spherical random phased array with 130 mm radius of curvature. The array was integrated with a research ultrasound scanner via custom connectors to acquire raw RF data for variety of targets. Imaging was implemented using synthetic aperture beamforming to produce images of a rib phantom and ex vivo ribs. The array was shown to resolve spherical targets within ±15 mm of either side of the axis in the focal plane and obtain 3D images of the rib phantom up to ±40 mm of either side of the central axis and at a depth of 3-9 cm from the array surface. The lateral and axial full width half maximum was 1.15 mm and 2.75 mm, respectively. This study was undertaken to emphasize that both therapy and image guidance with a therapeutic random phased array is possible and such a system has the potential to address some major limitations in the existing high intensity focused ultrasound (HIFU) systems. The 3D images obtained with a therapeutic array can be used to identify and locate strong scattering objects aiding to image guidance and treatment planning of the HIFU procedure.

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.

The Optimization of Transcostal Phased Array Refocusing Using the Semidefinite Relaxation Method

Tumors in organs partially obscured by the rib cage represent a challenge for high-intensity focused ultrasound (HIFU) therapy. The ribs distort the HIFU beams in a manner that reduces the focusing gain at the target, which could result in treatment-limiting collateral damage. In fact, skin burns are a common complication during the ablation of hepatic tumors. This problem can be addressed by employing optimal refocusing algorithms that are designed to achieve a specified focusing gain at the target while controlling the exposure to the ribs in the path of the HIFU beam. However, previously proposed optimal refocusing algorithms did not allow for the controlled transmission through the ribs. In this article, we introduce a new approach for refocusing that can more efficiently steer power toward the target while limiting the power deposition on the ribs. The approach utilizes the semidefinite relaxation (SDR) technique to approximate the original (nonconvex) optimization problem. An important advantage of the SDR-based method over previously proposed optimization methods is the control of the side lobes in the focal plane. The method also allows for specifying an acceptable level of exposure to the ribs. Simulation results using a 1-MHz spherical concave phased array focused on an inhomogeneous medium are presented to demonstrate the performance of the SDR refocusing approach. A finite-difference time-domain propagation model was used to model the propagation in the inhomogeneous tissues, including the ribs. Temperature simulations based on the inhomogeneous transient bioheat transfer equation (tBHTE) demonstrate the significance of the improvements in the focusing gain when using the limited power deposition (LPD) method. The results also demonstrate that the LPD method yields well-behaved array excitation vectors, realizable by currently existing drivers.

Reversible neuroinhibition by focused ultrasound is mediated by a thermal mechanism

BACKGROUND

Transcranial focused ultrasound (tFUS) at low intensities has been reported to directly evoke responses and reversibly inhibit function in the central nervous system. While some doubt has been cast on the ability of ultrasound to directly evoke neuronal responses, spatially-restricted transcranial ultrasound has demonstrated consistent, inhibitory effects, but the underlying mechanism of reversible suppression in the central nervous system is not well understood.

OBJECTIVE/HYPOTHESIS

In this study, we sought to characterize the effect of transcranial, low-intensity, focused ultrasound on the thalamus during somatosensory evoked potentials (SSEP) and investigate the mechanism by modulating the parameters of ultrasound.

METHODS

TFUS was applied to the ventral posterolateral nucleus of the thalamus of a rodent while electrically stimulating the tibial nerve to induce an SSEP. Thermal changes were also induced through an optical fiber that was image-guided to the same target.

RESULTS

Focused ultrasound reversibly suppressed SSEPs in a spatially and intensity-dependent manner while remaining independent of duty cycle, peak pressure, or modulation frequency. Suppression was highly correlated and temporally consistent with in vivo temperature changes while producing no pathological changes on histology. Furthermore, stereotactically-guided delivery of thermal energy through an optical fiber produced similar thermal effects and suppression.

CONCLUSION

We confirm that tFUS predominantly causes neuroinhibition and conclude that the most primary biophysical mechanism is the thermal effect of focused ultrasound.

Visualization of the Intensity Field of a Focused Ultrasound Source In Situ

In an increasing number of applications of focused ultrasound (FUS) therapy, such as opening of the blood-brain barrier or collapsing microbubbles in a tumor, elevation of tissue temperature is not involved. In these cases, real-time visualization of the field distribution of the FUS source would allow localization of the FUS beam within the targeted tissue and allow repositioning of the FUS beam during tissue motion. In this paper, in order to visualize the FUS beam in situ, a 6-MHz single-element transducer ( f /2) was used as the FUS source and aligned perpendicular to a linear array which passively received scattered ultrasound from the sample. An image of the reconstructed intensity field pattern of the FUS source using bistatic beamforming was then superimposed on a registered B-mode image of the sample acquired using the same linear array. The superimposed image is used to provide anatomical context of the FUS beam in the sample being treated. The intensity field pattern reconstructed from a homogeneous scattering phantom was compared with the field characteristics of the FUS source characterized by the wire technique. The beamwidth estimates at the FUS focus using the in situ reconstruction technique and the wire technique were 1.5 and 1.2 mm, respectively. The depth-of-field estimates for the in situ reconstruction technique and the wire technique were 11.8 and 16.8 mm, respectively. The FUS beams were also visualized in a two-layer phantom and a chicken breast. The novel reconstruction technique was able to accurately visualize the field of an FUS source in the context of the interrogated medium.

Design and Implementation of a Transmit/Receive Ultrasound Phased Array for Brain Applications

Focused ultrasound phased array systems have attracted increased attention for brain therapy applications. However, such systems currently lack a direct and real-time method to intraoperatively monitor ultrasound pressure distribution for securing treatment. This study proposes a dual-mode ultrasound phased array system design to support transmit/receive operations for concurrent ultrasound exposure and backscattered focal beam reconstruction through a spherically focused ultrasound array. A 256-channel ultrasound transmission system was used to transmit focused ultrasonic energy (full 256 channels), with an extended implementation of multiple-channel receiving function (up to 64 channels) using the same 256-channel ultrasound array. A coherent backscatter-received beam formation algorithm was implemented to map the point spread function (PSF) and focal beam distribution under a free-field/transcranial environment setup, with the backscattering generated from a strong scatterer (a point reflector or a microbubble-perfused tube) or a weakly scattered tissue-mimicking graphite phantom. Our results showed that PSF and focal beam can be successfully reconstructed and visualized in free-field conditions and can also be transcranially reconstructed following skull-induced aberration correction. In vivo experiments were conducted to demonstrate its capability to preoperatively and semiquantitatively map a focal beam to guide blood-brain barrier opening. The proposed system may have potential for real-time guidance of ultrasound brain intervention, and may facilitate the design of a dual-mode ultrasound phased array for brain therapeutic applications.

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.

Dual-Frequency Piezoelectric Endoscopic Transducer for Imaging Vascular Invasion in Pancreatic Cancer

Cancers of the pancreas have the poorest prognosis among all cancers, as many tumors are not detected until surgery is no longer a viable option. Surgical viability is typically determined via endoscopic ultrasound imaging. However, many patients who may be eligible for resection are not offered surgery due to diagnostic challenges in determining vascular or lymphatic invasion. In this paper, we describe the development of a dual-frequency piezoelectric transducer for rotational endoscopic imaging designed to transmit at 4 MHz and receive at 20 MHz in order to image microbubble-specific superharmonic signals. Imaging performance is assessed in a tissue-mimicking phantom at depths from 1 cm [contrast-to-tissue ratio (CTR) = 21.6 dB] to 2.5 cm (CTR = 11.4 dB), in ex vivo porcine vessels, and in vivo in a rodent. The prototyped 1.1-mm aperture transducer demonstrates contrast-specific imaging of microbubbles in a 200- [Formula: see text]-diameter tube through the wall of a 1-cm-diameter porcine artery, suggesting such a device may enable direct visualization of small vessels from within the lumen of larger vessels such as the portal vein or superior mesenteric vein.

Multi-Focus Beamforming for Thermal Strain Imaging Using a Single Ultrasound Linear Array Transducer

Ultrasound-induced thermal strain imaging (TSI) has been used successfully to identify lipid- and water-based tissues in atherosclerotic plaques in some research settings. However, TSI faces several challenges to be realized in clinics. These challenges include motion artifacts and displacement tracking accuracy, as well as limited heating capability, which contributes to low thermal strain signal-to-noise ratio, and a limited field of view. Our goal was to address the challenge in heating tissue in TSI. Current TSI systems use separate heating and imaging transducers, which require physical alignment of the heating and imaging beams and result in a bulky setup that limits in vivo operation. We evaluated a new design for heating beams that can be implemented on a linear array imaging transducer and can provide improved heating area and efficiency as compared with previous implementations. The heating beams designed were implemented with a clinical linear array imaging transducer connected to a research ultrasound platform. In vitro experiments using tissue-mimicking phantoms with no blood flow revealed that the new design resulted in an effective heating area of approximately 0.85 cm2 and a 0.3°C temperature rise in 2 s of heating, which compared well with in silico finite-element simulations. With the new heating beams, TSI was found to be able to detect a lipid-mimicking rubber inclusion with a diameter of 1 cm from the water-based gelatin background, with a strain contrast of 2.3 (+0.14% strain in the rubber inclusion and -0.06% strain in the gelatin background). Lastly, lipid-based tissue in a 1-cm-diameter human carotid endarterectomy (CEA) sample was identified in good agreement with histology.

Development of a spherically focused phased array transducer for ultrasonic image-guided hyperthermia

A 1.5 MHz prolate spheroidal therapeutic array with 128 circular elements was designed to accommodate standard imaging arrays for ultrasonic image-guided hyperthermia. The implementation of this dual-array system integrates real-time therapeutic and imaging functions with a single ultrasound system (Vantage 256, Verasonics). To facilitate applications involving small animal imaging and therapy the array was designed to have a beam depth of field smaller than 3.5 mm and to electronically steer over distances greater than 1 cm in both the axial and lateral directions. In order to achieve the required f number of 0.69, 1-3 piezocomposite modules were mated within the transducer housing. The performance of the prototype array was experimentally evaluated with excellent agreement with numerical simulation. A focal volume (2.70 mm (axial)  ×  0.65 mm (transverse)  ×  0.35 mm (transverse)) defined by the  -6 dB focal intensity was obtained to address the dimensions needed for small animal therapy. An electronic beam steering range defined by the  -3 dB focal peak intensity (17 mm (axial)  ×  14 mm (transverse)  ×  12 mm (transverse)) and  -8 dB lateral grating lobes (24 mm (axial)  ×  18 mm (transverse)  ×  16 mm (transverse)) was achieved. The combined testing of imaging and therapeutic functions confirmed well-controlled local heating generation and imaging in a tissue mimicking phantom. This dual-array implementation offers a practical means to achieve hyperthermia and ablation in small animal models and can be incorporated within protocols for ultrasound-mediated drug delivery.

Combined Therapeutic and Monitoring Ultrasonic Catheter for Cardiac Ablation Therapies

This study evaluated the feasibility of a combined therapeutic and diagnostic ultrasonic catheter for cardiac ablation therapies. Ultrasound can be used to determine when diseased cardiac tissues have become fully coagulated through a method known as local harmonic motion imaging (LHMI). LHMI is an imaging modality for treatment monitoring that uses acoustic radiation force, displacement tracking and the different mechanical properties of viable and ablated tissues. In this study, we developed catheters that are capable of LHMI measurements. Experiments were conducted in phantoms, ex vivo cardiac samples and the in vivo beating hearts of healthy porcine subjects. In vivo experiments revealed that four of four epicardial sonications revealed a decrease in measured displacements from LHMI experiments and that when lower power was used, no lesions formed and there was no corresponding decrease in measured displacement amplitudes. In addition, two of three endocardial lesions were confirmed and corresponded to a decrease in the measured displacement amplitude.

HIFU Tissue Ablation: Concept and Devices

High intensity focused ultrasound (HIFU) is rapidly gaining clinical acceptance as a technique capable of providing non-invasive heating and ablation for a wide range of applications. Usually requiring only a single session, treatments are often conducted as day case procedures, with the patient either fully conscious, lightly sedated or under light general anesthesia. HIFU scores over other thermal ablation techniques because of the lack of necessity for the transcutaneous insertion of probes into the target tissue. Sources placed either outside the body (for treatment of tumors or abnormalities of the liver, kidney, breast, uterus, pancreas brain and bone), or in the rectum (for treatment of the prostate), provide rapid heating of a target tissue volume, the highly focused nature of the field leaving tissue in the ultrasound propagation path relatively unaffected. Numerous extra-corporeal, transrectal and interstitial devices have been designed to optimize application-specific treatment delivery for the wide-ranging areas of application that are now being explored with HIFU. Their principle of operation is described here, and an overview of their design principles is given.

Automatic Segmentation of Mechanically Inhomogeneous Tissues Based on Deformation Gradient Jump

Variations in properties, active behavior, injury, scarring, and/or disease can all cause a tissue's mechanical behavior to be heterogeneous. Advances in imaging technology allow for accurate full-field displacement tracking of both in vitro and in vivo deformation from an applied load. While detailed strain fields provide some insight into tissue behavior, material properties are usually determined by fitting stress-strain behavior with a constitutive equation. However, the determination of the mechanical behavior of heterogeneous soft tissue requires a spatially varying constitutive equation (i.e., one in which the material parameters vary with position). We present an approach that computationally dissects the sample domain into many homogeneous subdomains, wherein subdomain boundaries are formed by applying a betweenness based graphical analysis to the deformation gradient field to identify locations with large discontinuities. This novel partitioning technique successfully determined the shape, size and location of regions with locally similar material properties for: (1) a series of simulated soft tissue samples prescribed with both abrupt and gradual changes in anisotropy strength, prescribed fiber alignment, stiffness, and nonlinearity, (2) tissue analogs (PDMS and collagen gels) which were tested biaxially and speckle tracked (3) and soft tissues which exhibited a natural variation in properties (cadaveric supraspinatus tendon), a pathologic variation in properties (thoracic aorta containing transmural plaque), and active behavior (contracting cardiac sheet). The routine enables the dissection of samples computationally rather than physically, allowing for the study of small tissues specimens with unknown and irregular inhomogeneity.

In Vivo application and localization of transcranial focused ultrasound using dual-mode ultrasound arrays

Focused ultrasound (FUS) has been proposed for a variety of transcranial applications, including neuromodulation, tumor ablation, and blood-brain barrier opening. A flurry of activity in recent years has generated encouraging results demonstrating its feasibility in these and other applications. To date, monitoring of FUS beams has been primarily accomplished using MR guidance, where both MR thermography and elastography have been used. The recent introduction of real-time dual-mode ultrasound array (DMUA) systems offers a new paradigm in transcranial focusing. In this paper, we present first experimental results of ultrasound-guided transcranial FUS (tFUS) application in a rodent brain, both ex vivo and in vivo. DMUA imaging is used for visualization of the treatment region for placement of the focal spot within the brain. This includes the detection and localization of pulsating blood vessels at or near the target point(s). In addition, DMUA imaging is used to monitor and localize the FUS-tissue interactions in real time. In particular, a concave (40 mm radius of curvature), 32-element, 3.5-MHz DMUA prototype was used for imaging and tFUS application in ex vivo and in vivo rat models. The ex vivo experiments were used to evaluate the point spread function of the transcranial DMUA imaging at various points within the brain. In addition, DMUA-based transcranial ultrasound thermography measurements were compared with thermocouple measurements of subtherapeutic tFUS heating in rat brain ex vivo. The ex vivo setting was also used to demonstrate the capability of DMUA to produce localized thermal lesions. The in vivo experiments were designed to demonstrate the ability of the DMUA to apply, monitor, and localize subtherapeutic tFUS patterns that could be beneficial in transient blood-brain barrier opening. The results show that although the DMUA focus is degraded due to the propagation through the skull, it still produces localized heating effects within a sub-millimeter volume. In addition, DMUA transcranial echo data from brain tissue allow for reliable estimation of temperature change.

Ultrasound-guided therapeutic focused ultrasound: current status and future directions

This paper reviews ultrasound imaging methods for the guidance of therapeutic focused ultrasound (USgFUS), with emphasis on real-time preclinical methods. Guidance is interpreted in the broadest sense to include pretreatment planning, siting of the FUS focus, real-time monitoring of FUS-tissue interactions, and real-time control of exposure and damage assessment. The paper begins with an overview and brief historical background of the early methods used for monitoring FUS-tissue interactions. Current imaging methods are described, and discussed in terms of sensitivity and specificity of the localisation of the FUS effects in both therapeutic and sub-therapeutic modes. Thermal and non-thermal effects are considered. These include cavitation-enhanced heating, tissue water boiling and cavitation. Where appropriate, USgFUS methods are compared with similar methods implemented using other guidance modalities, e.g. magnetic resonance imaging. Conclusions are drawn regarding the clinical potential of the various guidance methods, and the feasibility and current status of real-time implementation.

Thermometry and ablation monitoring with ultrasound

In this review we present the current status of ultrasound thermometry and ablation monitoring, with emphasis on the diverse approaches published in the literature and with an eye on which methods are closest to clinical reality. It is hoped that this review will serve as a guide to the expansion of sonographic methods for treatment monitoring and thermometry since the last brief review in 2007.

Evaluation of template matching for tumor motion management with cine-MR images in lung cancer patients

PURPOSE

Accurate determination of tumor position is crucial for successful application of motion compensated radiotherapy in lung cancer patients. This study tested the performance of an automated template matching algorithm in tracking the tumor position on cine-MR images by examining the tracking error and further comparing the tracking error to the interoperator variability of three human reviewers.

METHODS

Cine-MR images of 12 lung cancer patients were analyzed. Tumor positions were determined both automatically with template matching and manually by a radiation oncologist and two additional reviewers trained by the radiation oncologist. Performance of the automated template matching was compared against the ground truth established by the radiation oncologist. Additionally, the tracking error of template matching, defined as the difference in the tumor positions determined with template matching and the ground truth, was investigated and compared to the interoperator variability for all patients in the anterior-posterior (AP) and superior-inferior (SI) directions, respectively.

RESULTS

The median tracking error for ten out of the 12 patients studied in both the AP and SI directions was less than 1 pixel (= 1.95 mm). Furthermore, the median tracking error for seven patients in the AP direction and nine patients in the SI direction was less than half a pixel (= 0.975 mm). The median tracking error was positively correlated with the tumor motion magnitude in both the AP (R = 0.55, p = 0.06) and SI (R = 0.67, p = 0.02) directions. Also, a strong correlation was observed between tracking error and interoperator variability (y = 0.26 + 1.25x, R = 0.84, p < 0.001) with the latter larger.

CONCLUSIONS

Results from this study indicate that the performance of template matching is comparable with or better than that of manual tumor localization. This study serves as preliminary investigations towards developing online motion tracking techniques for hybrid MRI-Linac systems. Accuracy of template matching makes it a suitable candidate to replace the labor intensive manual tumor localization for obtaining the ground truth when testing other motion management techniques.

Dual-frequency piezoelectric transducers for contrast enhanced ultrasound imaging

For many years, ultrasound has provided clinicians with an affordable and effective imaging tool for applications ranging from cardiology to obstetrics. Development of microbubble contrast agents over the past several decades has enabled ultrasound to distinguish between blood flow and surrounding tissue. Current clinical practices using microbubble contrast agents rely heavily on user training to evaluate degree of localized perfusion. Advances in separating the signals produced from contrast agents versus surrounding tissue backscatter provide unique opportunities for specialized sensors designed to image microbubbles with higher signal to noise and resolution than previously possible. In this review article, we describe the background principles and recent developments of ultrasound transducer technology for receiving signals produced by contrast agents while rejecting signals arising from soft tissue. This approach relies on transmitting at a low-frequency and receiving microbubble harmonic signals at frequencies many times higher than the transmitted frequency. Design and fabrication of dual-frequency transducers and the extension of recent developments in transducer technology for dual-frequency harmonic imaging are discussed.

In vivo ultrasound thermography in presence of temperature heterogeneity and natural motions

Real-time ultrasound thermography has been recently demonstrated on commercially available diagnostic imaging probes. In vitro experimental results demonstrate high sensitivity to small, localized temperature changes induced by subtherapeutic focused ultrasound. Most of the published results, however, are based on a thermally induced echo strain model that assumes infinitesimal change in temperature between imaging frames. Under this assumption, the echo strain is computed using a low-pass axial differentiator, which is implemented by a finite-impulse response digital filter. In this paper, we introduce a new model for temperature estimation, which employs a recursive axial filter that acts as a spatial differentiator-integrator of echo shifts. The filter is derived from first principles and it accounts for a nonuniform temperature baseline, when computing the spatial temperature change between two frames. This is a major difference from the previously proposed infinitesimal echo strain filter ( δ-ESF) approach. We show that the new approach can be implemented by a first-order infinite-impulse response digital filter with depth-dependent spatial frequency response. Experimental results in vitro demonstrate the advantages over the δ-ESF approach in terms of suppressing the spatial variations in the estimated temperature without resorting to ad hoc low-pass filtering of echo strains. The performance of the new recursive echo strain filter (RESF) is also illustrated using echo data obtained during subtherapeutic localized heating in the hind limb of Copenhagen rat in vivo. In addition to the RESF, we have used an adaptive spatial filter to remove motion and deformation artifacts during real-time data collection. The adaptive filtering algorithm is described and comparisons with uncompensated estimated spatio-temporal temperature profiles are given. The results demonstrate the feasibility of in vivo ultrasound thermography with high sensitivity and specificity.

Three-dimensional transcranial ultrasound imaging of microbubble clouds using a sparse hemispherical array

There is an increasing interest in bubble-mediated focused ultrasound (FUS) interventions in the brain. However, current technology lacks the ability to spatially monitor the interaction of the microbubbles with the applied acoustic field, something which is critical for safe clinical translation of these treatments. Passive acoustic mapping could offer a means for spatially monitoring microbubble emissions that relate to bubble activity and associated bioeffects. In this study, a hemispherical receiver array was integrated within an existing transcranial therapy array to create a device capable of both delivering therapy and monitoring the process via passive imaging of bubble clouds. A 128-element receiver array was constructed and characterized for varying bubble concentrations and source spacings. Initial in vivo feasibility testing was performed. The system was found to be capable of monitoring bubble emissions down to single bubble events through an ex vivo human skull. The lateral resolution of the system was found to be between 1.25 and 2 mm and the axial resolution between 2 and 3.5 mm, comparable to the resolution of MRI-based temperature monitoring during thermal FUS treatments in the brain. The results of initial in vivo experiments show that bubble activity can be mapped starting at pressure levels below the threshold for blood-brain barrier disruption. This study presents a feasible solution for imaging bubble activity during cavitation-mediated FUS treatments in the brain.

High-intensity focused ultrasound for potential treatment of polycystic ovary syndrome: toward a noninvasive surgery

OBJECTIVE

To investigate the feasibility of using high-intensity focused ultrasound (HIFU), under dual-mode ultrasound arrays (DMUAs) guidance, to induce localized thermal damage inside ovaries without damage to the ovarian surface.

DESIGN

Laboratory feasibility study.

SETTING

University-based laboratory.

ANIMAL(S)

Ex vivo canine and bovine ovaries.

INTERVENTION(S)

DMUA-guided HIFU.

MAIN OUTCOME MEASURE(S)

Detection of ovarian damage by ultrasound imaging, gross pathology, and histology.

RESULT(S)

It is feasible to induce localized thermal damage inside ovaries without damage to the ovarian surface. DMUA provided sensitive imaging feedback regarding the anatomy of the treated ovaries and the ablation process. Different ablation protocols were tested, and thermal damage within the treated ovaries was histologically characterized.

CONCLUSION(S)

The absence of damage to the ovarian surface may eliminate many of the complications linked to current laparoscopic ovarian drilling (LOD) techniques. HIFU may be used as a less traumatic tool to perform LOD.