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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., Competing Interests: There are no conflicts of interest., (Copyright: © 2024 Journal of Medical Physics.)

Purpose: The current study provides insights into the challenges of safely operating a magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS) robotic system in a high-field MRI scanner in terms of robotic motion accuracy., Materials and Methods: Grid sonications were carried out in phantoms and excised porcine tissue in a 3T MRI scanner using an existing MRgFUS robotic system. Fast low-angle shot-based magnetic resonance thermometry was employed for the intraprocedural monitoring of thermal distribution., Results: Strong shifting of the heated spots from the intended points was observed owing to electromagnetic interference (EMI)-induced malfunctions in system's operation. Increasing the slice thickness of the thermometry sequence to at least 8 mm was proven an efficient method for preserving the robotic motion accuracy., Conclusions: These findings raise awareness about EMI effects on the motion accuracy of MRgFUS robotic devices and how they can be mitigated by employing suitable thermometry parameters., Competing Interests: There are no conflicts of interest., (Copyright: © 2024 Journal of Medical Physics.)

Aim: Agar-based phantoms are popular in high intensity focused ultrasound (HIFU) studies, with magnetic resonance imaging (MRI) preferred for guidance since it provides temperature monitoring by proton resonance frequency (PRF) shift magnetic resonance (MR) thermometry. MR thermometry monitoring depends on several factors, thus, herein, the PRF coefficient of agar phantoms was estimated., Materials and Methods: Seven phantoms were developed with varied agar (2, 4, or 6% w/v) or constant agar (6% w/v) and varied silica concentrations (2, 4, 6, or 8% w/v) to assess the effect of the concentration on the PRF coefficient. Each phantom was sonicated using varied acoustical power for a 30 s duration in both a laboratory setting and inside a 3T MRI scanner. PRF coefficients were estimated through linear trends between phase shift acquired using gradient sequences and thermocouple-based temperatures changes., Results: Linear regression ( R = 0.9707-0.9991) demonstrated a proportional dependency of phase shift with temperature change, resulting in PRF coefficients between -0.00336 ± 0.00029 and -0.00934 ± 0.00050 ppm/°C for the various phantom recipes. Weak negative linear correlations of the PRF coefficient were observed with increased agar. With silica concentrations, the negative linear correlation was strong. For all phantoms, calibrated PRF coefficients resulted in 1.01-3.01-fold higher temperature changes compared to the values calculated using a literature PRF coefficient.2 = 0.9707-0.9991) demonstrated a proportional dependency of phase shift with temperature change, resulting in PRF coefficients between -0.00336 ± 0.00029 and -0.00934 ± 0.00050 ppm/°C for the various phantom recipes. Weak negative linear correlations of the PRF coefficient were observed with increased agar. With silica concentrations, the negative linear correlation was strong. For all phantoms, calibrated PRF coefficients resulted in 1.01-3.01-fold higher temperature changes compared to the values calculated using a literature PRF coefficient., Conclusions: Phantoms developed with a 6% w/v agar concentration and doped with 0%-8% w/v silica best resemble tissue PRF coefficients and should be preferred in HIFU studies. The estimated PRF coefficients can result in enhanced MR thermometry monitoring and evaluation of HIFU protocols., Competing Interests: There are no conflicts of interest., (Copyright: © 2024 Journal of Medical Physics.)

Background: The main goal of the study was to find the magnetic resonance imaging (MRI) parameters that optimize contrast between tissue and thermal lesions produced by focused ultrasound (FUS) using T1-weighted (T1-W) and T2-weighted (T2-W) fast spin echo (FSE) sequences., Methods: FUS sonications were performed in ex vivo porcine tissue using a single-element FUS transducer of 2.6 MHz in 1.5 and 3 T MRI scanners. The difference in relaxation times as well as the impact of critical MRI parameters on the resultant contrast-to-noise ratio (CNR) between coagulated and normal tissues were assessed. Discrete and overlapping lesions were inflicted in tissue with simultaneous acquisition of T2-W FSE images., Results: FUS lesions are characterized by lower relaxation times than intact porcine tissue. CNR values above 80 were sufficient for proper lesion visualization. For T1-W imaging, repetition time values close to 1500 ms were considered optimum for obtaining sufficiently high CNR at the minimum time cost. Echo time values close to 50 ms offered the maximum lesion contrast in T2-W FSE imaging. Monitoring of acute FUS lesions during grid sonications was performed successfully. Lesions appeared as hypointense spots with excellent contrast from surrounding tissue., Conclusion: MRI monitoring of signal intensity changes during FUS sonication in grid patterns using optimized sequence parameters can provide useful information about lesion progression and the success of ablation. This preliminary study demonstrated the feasibility of the proposed monitoring method in ex vivo porcine tissue and should be supported by in vivo studies to assess its clinical potential., Competing Interests: There are no conflicts of interest., (Copyright: © 2024 Journal of Medical Ultrasound.)

Background: Tissue-mimicking phantoms (TMPs) have been used extensively in clinical and nonclinical settings to simulate the thermal effects of focus ultrasound (FUS) technology in real tissue or organs. With recent technological developments in the FUS technology and its monitoring/guided techniques such as ultrasound-guided FUS and magnetic resonance-guided FUS (MRgFUS) the need for TMPs are more important than ever to ensure the safety of the patients before being treated with FUS for a variety of diseases (e.g., cancer or neurological). The purpose of this study was to prepare a tumor-mimicking phantom (TUMP) model that can simulate competently a tumor that is surrounded by healthy tissue., Methods: The TUMP models were prepared using polyacrylamide (PAA) and agar solutions enriched with MR contrast agents (silicon dioxide and glycerol), and the thermosensitive component bovine serum albumin (BSA) that can alter its physical properties once thermal change is detected, therefore offering real-time visualization of the applied FUS ablation in the TUMPs models. To establish if these TUMPs are good candidates to be used in thermoablation, their thermal properties were characterized with a custom-made FUS system in the laboratory and a magnetic resonance imaging (MRI) setup with MR-thermometry. The BSA protein's coagulation temperature was adjusted at 55°C by setting the pH of the PAA solution to 4.5, therefore simulating the necrosis temperature of the tissue., Results: The experiments carried out showed that the TUMP models prepared by PAA can change color from transparent to cream-white due to the BSA protein coagulation caused by the thermal stress applied. The TUMP models offered a good MRI contrast between the TMPs and the TUMPs including real-time visualization of the ablation area due to the BSA protein coagulation. Furthermore, the T2 -weighted MR images obtained showed a significant change in T2 when the BSA protein is thermally coagulated. MR thermometry maps demonstrated that the suggested TUMP models may successfully imitate a tumor that is present in soft tissue., Conclusion: The TUMP models developed in this study have numerous uses in the testing and calibration of FUS equipment including the simulation and validation of thermal therapy treatment plans with FUS or MRgFUS in oncology applications., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright: © 2023 Journal of Medical Ultrasound.)

Background: Noninvasive neurosurgery has become possible through the use of transcranial focused ultrasound (FUS). This study assessed the heating ability of single element spherically focused transducers operating at 0.4 and 1.1 MHz through three-dimensional (3D) printed thermoplastic skull phantoms., Methods: Phantoms with precise skull bone geometry of a male patient were 3D printed using common thermoplastic materials following segmentation on a computed tomography head scan image. The brain tissue was mimicked by an agar-based gel phantom developed in-house. The selection of phantom materials was mainly based on transmission-through attenuation measurements. Phantom sonications were performed through water, and then, with the skull phantoms intervening the beam path. In each case, thermometry was performed at the focal spot using thermocouples., Results: The focal temperature change in the presence of the skull phantoms was reduced to less than 20 % of that recorded in free field when using the 0.4 MHz transducer, whereas the 1.1 MHz trans-skull sonication produced minimal or no change in focal temperature. The 0.4 MHz transducer showed better performance in trans-skull transmission but still not efficient., Conclusion: The inability of both tested single element transducers to steer the beam through the high attenuating skull phantoms and raise the temperature at the focus was confirmed, underlying the necessity to use a correction technique to compensate for energy losses, such those provided by phased arrays. The proposed phantom could be used as a cost-effective and ergonomic tool for trans-skull FUS preclinical studies., Competing Interests: There are no conflicts of interest., (Copyright: © 2023 Journal of Medical Ultrasound.)

Purpose: Tissue-mimicking materials (TMMs) have a prominent role in validating new high intensity focused ultrasound (HIFU) therapies. Agar-based TMMs are often developed mimicking the thermal properties of muscle tissue, while TMMs simulating fat tissue properties are rarely developed. Herein, twelve agar-based TMMs were iteratively developed with varied concentrations of agar, water, glycerol and propan-2-ol, and characterized for their suitability in emulating the thermal conductivity of human fat tissue., Methods: Varied agar concentrations (2%, 4%, 6%, 8%, 12%, 16% and 20% w/v) were utilized for developing seven water-based TMMs, while a 20% w/v agar concentration was utilized for developing two water/alcohol-based TMMs (50% v/v water and 50% v/v either glycerol or propan-2-ol) and three alcohol-based TMMs (varied glycerol and propan-2-ol concentrations). Thermal conductivity was measured for all TMMs, and the tissue mimicking material (TMM) exhibiting thermal conductivity closest to human fat was considered the optimum fat TMM and was further characterized using ultrasound (US) and Magnetic Resonance Imaging (MRI)., Results: For the seven water-based TMMs an inverse linear trend was observed between thermal conductivity and increased agar concentration, being between 0.524 and 0.445 W/m K. Alcohol addition decreased thermal conductivity of the two water/alcohol-based TMMs to about 0.33 W/m K, while in the alcohol-based TMMs, increased concentrations of propan-2-ol emerged as a modifier of thermal conductivity. The optimum fat TMM (33.3% v/v glycerol and 66.7% v/v propan-2-ol) exhibited a 0.231 W/m K thermal conductivity, and appeared hypoechoic on US images and with increased brightness on T1-Weighted MRI images., Conclusion: The optimum fat TMM emulates the thermal conductivity of human fat tissue and exhibits a fat-like appearance on US and MRI images. The TMM is cost-effective and has a long lifespan and possesses great potential for use in HIFU applications as a fat TMM., (© 2022. Società Italiana di Ultrasonologia in Medicina e Biologia (SIUMB).)

Purpose: The popularity of Magnetic Resonance guided Focused Ultrasound (MRgFUS) as a beneficial therapeutic solution for many diseases is increasing rapidly, thus raising the need for reliable quality assurance (QA) phantoms for routine testing of MRgFUS systems. In this study, we propose a thin acrylic film as the cheapest and most easily accessible phantom for assessing the functionality of MRgFUS hardware and software., Methods: Through the paper, specific QA tests are detailed in the framework of evaluating an MRgFUS preclinical robotic device comprising a single element spherically focused transducer with a nominal frequency of 2.75 MHz. These tests take advantage of the reflection of ultrasonic waves at a plastic-air interface, which results in almost immediate lesion formation on the film at a threshold of applied acoustic energy., Results: The phantom offered qualitative information on the power field distribution of the FUS transducer and the ability to visualize different FUS protocols. It also enabled quick and reliable assessment of various navigation algorithms as they are used in real treatments, and also allowed for the assessment of the accuracy of robotic motion., Conclusion: Therefore, it could serve as a useful tool for detecting defects in system's performance over its lifetime after establishing a baseline while concurrently contributing to establish QA and calibration guidelines for clinical routine controls., (© 2022. Società Italiana di Ultrasonologia in Medicina e Biologia (SIUMB).)

Purpose: The present study was stimulated by the continuous growth of commercially available high intensity focused ultrasound (HIFU) systems for fat reduction. Herein, HIFU was utilised for fat ablation using a single-element ultrasonic transducer operating in thermal mode., Methods: The custom-made concave transducer that operates at 1.1 MHz was assessed on excised porcine adipose tissue for its ability to reduce fat. Ultrasonic sonications were executed on the adipose tissue utilising acoustical power between 14 and 75 W and sonication time in the range of 1-10 min. The mass of the adipose tissue sample was weighed afore and after ultrasonic sonications and the effect of the sonication on the mass change was recorded., Results: Mass change was linearly dependent with either increasing acoustical power or sonication time and was in the range of 0.46-1.9 g. High acoustical power of 62.5 W for a sonication time of 1 min and a power of 75 W for a sonication time of 5 min, respectively resulted in the formation of a lesion or possible cavitation on the piece of excised adipose tissue., Conclusion: The study demonstrated the efficacy of the proposed transducer in achieving a reduction of excised fat tissue. The present findings indicate the potential use of the transducer in a HIFU system indicated for the reduction of subcutaneous adipose tissue where increased values of acoustical power can result in increased amounts of fat reduction., (© 2022. Società Italiana di Ultrasonologia in Medicina e Biologia (SIUMB).)

Purpose: In recent years, veterinary medicine has enhanced its applications beyond traditional approaches, progressively incorporating the Focused Ultrasound (FUS) technology. This study investigated the ability of FUS to precisely ablate naturally occurring canine and feline soft tissue sarcomas (STS)., Methods: Six dogs and four cats with superficial tumours were enrolled in the study. The tumours were treated with a Magnetic Resonance guided FUS (MRgFUS) robotic system featuring a single element spherically focused transducer of 2.6 MHz. The tumours were then removed by surgery and sent for hematoxylin and eosin (H&E) staining., Results: The MRgFUS system was capable of inflicting well-defined overlapping lesions in the tumours. The anatomical sites of the treated tumours were the neck, leg, face, back and belly. Coagulative necrosis was evidenced by histopathology assessment in 80% of cases., Conclusion: Therefore, this technology can be a therapeutic solution for veterinary cancer and a model for advancing the knowledge on human STS., (© 2022. Società Italiana di Ultrasonologia in Medicina e Biologia (SIUMB).)

Background: The mechanical properties of therapeutic ultrasound (US) have attracted scientific interest for thrombolysis enhancement in combination with thrombolytic agents and microbubbles (MBs). The aim of the study was to develop an in vitro model to observe how the effects of sonothrombolysis change in the case where a tissue-mimicking material (TMM) is placed in the path of the US beam before the clot., Methods: Fully retracted blood clots were prepared and pulse sonicated for 1 h under various conditions. The system was in a state of real circulating flow with a branch of an open bypass and an occluded tube containing a blood clot, thus mimicking the case of ischemic stroke. The effectiveness of thrombolysis was quantified in milligrams of clots removed. An agar-based TMM was developed around the occluded tube., Results: The clot breakdown in a TMM was found to be more pronounced than in water, presumably due to the retention of the acoustic field. A higher level of acoustic power was required to initiate clot lysis (>76 W acoustic power) using only focused US (FUS). The greatest thrombolysis enhancement was observed with the largest chosen pulse duration (PD) and the use of MBs (150 mg clot mass lysis). The synergistic effect of FUS in combination with MBs on the enzymatic fibrinolysis enhanced thrombolysis efficacy by 260% compared to thrombolysis induced using only FUS. A reduction in the degree of clot lysis was detected due to the attenuation factor of the intervening material (30 mg at 1 and 4 ms PD)., Conclusion: In vitro thrombolytic models including a TMM can provide a more realistic evaluation of new thrombolytic protocols. However, higher acoustic power should be considered to compensate for the attenuation factor. The rate of clot lysis is slow and the clinical use of this method will be challenging., Competing Interests: There are no conflicts of interest., (Copyright: © 2022 Journal of Medical Ultrasound.)

Purpose: The aim of the proposed study was to conduct a feasibility study using a flat rectangular (2 × 10 mm 2 ) transducer operating at 4.0 MHz for creating thermal lesions in an arterial atherosclerotic plaque phantom. The proposed method can be used in the future for treating atherosclerotic plaques in human arteries., Materials and Methods: The flat rectangular transducer was firstly assessed in agar/silica evaporated milk phantom, polyacrylamide phantom and freshly excised turkeytissue phantom. Then, the same transducer was assessed in an arterial atherosclerotic plaque phantom which was created in the laboratory with a very low cost. The recipe of the atherosclerotic plaque phantom was 4% w/v agar, 1% w/v gypsum, 2% w/v butter and 93% water. The amount of plaque removal was evaluated visually and using an X-Ray system., Results: It was shown that the flat rectangular transducer can create thermal lesions on the agar/silica evaporated milk phantom, polyacrylamide phantom and in excised tissue. The size of the lesions matches the geometry of the transducer. Moreover, this transducer destroyed 27.1% of the atherosclerotic plaque phantom with 8 W acoustical power and 30 s duration., Conclusions: This feasibility study demonstrated that atherosclerotic plaque can be destroyed using a very small flat rectangular (2 × 10 mm 2 ) transducer in a very small time interval of 30 s. In future clinical trials the transducer will be incorporated in a catheter which will be inserted intravascular (1-3 mm) wide and can be used to treat atherosclerotic plaques in the coronary arteries., (© 2022. Società Italiana di Ultrasonologia in Medicina e Biologia (SIUMB).)

Purpose: Acoustic characterization of tissue mimicking materials in terms of attenuation, absorption, scattering and propagation velocity is essential for their utilisation in experiments, thus sparing the need for living tissues or cadavers. Although there is a vast literature regarding the acoustic characterization of such materials in terms of attenuation or propagation velocity, there is limited data regarding the quantification of the scattering coefficient. Herein stimulated the utilisation of four agar-based phantoms featuring different sizes of scattering agar-structures on one of their surfaces so as to provide experimental evaluation of the magnitude of scattering., Methods: The agar-based phantoms were developed with 6% w/v agar and 4% w/v silica and featured scatterers of sizes of 0-1 mm. The acoustic properties of propagation speed, impedance, insertion loss and attenuation were evaluated utilising the pulse-echo and through-transmission techniques. Scattering was deduced from the data., Results: The propagation speed measured at 2.7 MHz was in the range of 1531.23-1542.97 m/s. Respectively the attenuation as measured at 1.1 MHz was in the range of 1.216-1.546 dB/cm increasing with increased scatterer size. Respectively the scattering coefficient was in the range of 0.078-0.324 dB/cm. Moreover, the scattering coefficient was linearly dependent on frequency in the range of 0.8-2.1 MHz indicating a 6-23% effect of the total attenuation., Conclusions: The experimental results demonstrate the utilisation of the procedure for quantification of the scattering coefficient of tissue mimicking materials thus improving the diagnostic and therapeutic uses of ultrasound., (© 2021. Società Italiana di Ultrasonologia in Medicina e Biologia (SIUMB).)

Agar gels were previously proven capable of accurately replicating the acoustical and thermal properties of real tissue and widely used for the construction of tissue-mimicking phantoms (TMPs) for focused ultrasound (FUS) applications. Given the current popularity of magnetic resonance-guided FUS (MRgFUS), we have investigated the MR relaxation times T1 and T2 of different mixtures of agar-based phantoms. Nine TMPs were constructed containing agar as the gelling agent and various concentrations of silicon dioxide and evaporated milk. An agar-based phantom doped with wood powder was also evaluated. A series of MR images were acquired in a 1.5 T scanner for T1 and T2 mapping. T2 was predominantly affected by varying agar concentrations. A trend toward decreasing T1 with an increasing concentration of evaporated milk was observed. The addition of silicon dioxide decreased both relaxation times of pure agar gels. The proposed phantoms have great potential for use with the continuously emerging MRgFUS technology. The MR relaxation times of several body tissues can be mimicked by adjusting the concentration of ingredients, thus enabling more accurate and realistic MRgFUS studies., (© 2022 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, LLC on behalf of The American Association of Physicists in Medicine.)

Background: It has been demonstrated that agar-based gel phantoms can emulate the acoustic parameters of real tissues and are the most commonly used tissue-mimicking materials for high-intensity focused ultrasound applications. The following study presents ultrasonic attenuation measurements of agar-based phantoms with different concentrations of additives (percent of agar, silicon dioxide and evaporated milk) in an effort of matching the material's acoustic property as close as possible to human tissues., Methods: Nine different agar-based phantoms with various amounts of agar, silicon dioxide, and evaporated milk were prepared. Attenuation measurements of the samples were conducted using the through-transmission immersion techniques., Results: The ultrasonic attenuation coefficient of the agar-based phantoms varied in the range of 0.30-1.49 dB/cm-MHz. The attenuation was found to increase in proportion to the concentration of agar and evaporated milk. Silicon dioxide was found to significantly contribute to the attenuation coefficient up to 4% weight to volume (w/v) concentration., Conclusion: The acoustic attenuation coefficient of agar-based phantoms can be adjusted according to the tissue of interest in the range of animal and human tissues by the proper selection of agar, silicon dioxide, and evaporated milk., Competing Interests: There are no conflicts of interest., (Copyright: © 2021 Journal of Medical Ultrasound.)

Background: Magnetic resonance-guided focused ultrasound surgery is an incisionless energy-based thermal method that is used for ablating tumors in the veterinary clinic., Aims and Objectives: In this article we describe a prototype of a veterinary system compatible with magnetic resonance imaging intended for small-to-medium-sized companion animals that was developed and tested in vivo in adult rabbits., Methods: Real-time monitoring of the ablation during the experiment was possible with MR thermometry. Experiments involved thermal monitoring of sonications applied in the thigh of the rabbits. A 38-mm diameter transducer operating at 2.6 MHz was used with a 60-mm-focal length. The robotic system employed 3 linear axes and one angular axis. For this study, only X and Y axis were enabled. Due to the target size limitations, motion in Z and Θ was not needed. The functionality of the positioning device was evaluated by means of MR thermometry, demonstrating sufficient heating and accurate motion in both axes of operation., Results: The postmortem findings confirm the ability of the system to induce thermal ablations in vivo in the absence of adverse effects., Conclusions: The device is a reliable and affordable solution for companion animal hospitals, offering and additional tool for the veterinary oncology society., Competing Interests: There are no conflicts of interest., (Copyright: © 2021 Journal of Medical Ultrasound.)

With focused ultrasound (FUS) gaining popularity as a therapeutic modality for brain diseases, the need for skull phantoms that are suitable for evaluating FUS protocols is increasing. In the current study, the acoustical properties of several three-dimensional (3D) printed thermoplastic samples were evaluated to assess their suitability to mimic human skull and bone accurately. Samples were 3D printed using eight commercially available thermoplastic materials. The acoustic properties of the printed samples, including attenuation coefficient, speed of sound, and acoustic impedance, were investigated using transmission-through and pulse-echo techniques. The ultrasonic attenuation, estimated at a frequency of 1.1 MHz, varied from approximately 7 to 32 dB/cm. The frequency dependence of attenuation was described by a power law in the frequency range of 0.2-3.5 MHz, and the exponential index of frequency was found to vary from 1.30 to 2.24. The longitudinal velocity of 2.7 MHz sound waves was in the range of 1700-3050 m/s. The results demonstrate that thermoplastics could potentially be used for the 3D construction of high-quality skull phantoms.

Background: This study describes a series of experimental work completed towards characterizing candidate materials for fabricating brain and muscle tissue mimicking phantoms., Methods: The acoustic speed, attenuation, impedance, thermal diffusivity, specific heat and thermal conductivity were measured., Results: The resulting brain (2% w/v agar-1.2% w/v Silica Dioxide-25%v/v evaporated milk) and muscle tissue recipe (2% w/v agar-2% w/v Silica Dioxide-40%v/v evaporated milk) introduced a total attenuation coefficient of 0.59 dB/cm-MHz and 0.99 dB/cm-MHz respectively. Acrylonitrile Butadiene Styrene (ABS) possessed an attenuation coefficient of 16 dB/cm at 1 MHz which was found within the very wide range of attenuation coefficient values of human bones in literature. The thermal conductivity of the brain tissue phantom was estimated at 0.52 W/m°C and at 0.57 W/m.°Cfor the muscle. These values demonstrated that the proposed recipes conducted heat similar to the majority of most soft tissues found from bibliography. The soft tissue phantoms were also evaluated for their thermal repeatability after treating them repeatedly at different locations with the same sonication protocol and configuration. The average coefficient of variation of the maximum temperature at focus between the different locations was 2.6% for the brain phantom and 2.8% for the muscle phantom., Conclusions: The proposed phantom closely matched the acoustic and thermal properties of tissues. Experiments using MR thermometry demonstrated the usefulness of this phantom to evaluate ultrasonic exposures.

Background: A novel magnetic resonance imaging (MRI)-conditional coupling system was developed that accommodates a focused ultrasound (FUS) transducer. With this coupling system, the transducer can access targets from top to bottom. The intended clinical application is treatment of fibroids using FUS with the patient placed in supine position., Methods: The coupling system was manufactured using a rapid prototyping device using acrylonitrile butadiene styrene (ABS) plastic. Coupling to a gel phantom was achieved using a water bag filled with degassed water. The FUS transducer was immersed in the water bag., Results: The coupling system was successfully tested for MRI compatibility using fast-gradient pulse sequences in a gel phantom. The robotic system with its new coupling system was evaluated for its functionality for creating discrete and multiple (overlapping) lesions in the gel phantom., Conclusions: An MRI-conditional FUS coupling system integrated with an existing robotic system was developed that has the potential to create thermal lesions in targets using a top-to-bottom approach. This system has the potential to treat fibroid tumors with the patient lying in supine position.

Background: The aim of the proposed study was to conduct a feasibility study using a flat rectangular (3 mm × 10 mm) MRI compatible transducer operating at 5.3 MHz for destroying calcified material in an in vitro model. The proposed method can be used in the future for treating atherosclerosis plaques of the coronary, carotid or peripheral arteries., Methods: The system was tested initially on calcium rods. Another test was performed in a hydroxyapatite-polylactide model., Results: A parametric study was performed where the mass of calcified material removed was studied as a function of intensity, pulse repetition frequency (PRF), duty factor (DF) and presence of bubbles., Conclusions: The amount of calcified material removed is directly related to the intensity, PRF and DF. It was found that the presence of bubbles accelerates the removal of calcified material. In order to ensure that pure mechanical mode ultrasound was used, the protocols were designed so that the temperature does not exceed 1 °C.

Background: A prototype magnetic resonance image (MRI)-conditional robot was developed for navigating a high-intensity focused ultrasound (HIFU) system in order to treat prostate cancer transrectally., Materials and Methods: The developed robotic device utilizes three PC-controlled axes: a linear axis for motion along the rectum, an angular axis for rotation in the rectum, and a linear axis to lift the robot up and down. Experiments with the system were performed in a 1.5-T MRI system using gel phantoms., Result: The robot was successfully operated in a 1.5-T clinical MRI system. The effect of piezoelectric motors and optical encoders was quantified based on the reduction of signal to noise ratio. Discrete and overlapping lesions were created accurately by moving the HIFU transducer with the robotic device., Conclusion: An MRI-conditional HIFU robot was developed which can create controlled thermal lesions under MRI guidance. The intention is to use this robot transrectally in the future for the treatment of prostate cancer.

Background: The aim of the proposed study was to conduct a feasibility study using a flat rectangular (3 × 10 mm(2)) transducer operating at 5 MHz for removing atherosclerotic plaque in an in vivo model. The proposed method can be used in the future for treating atherosclerotic plaques in humans., Methods and Results: The plaque in the rabbits was created using high cholesterol diet for 4 months. The amount of plaque removed was studied as a function of intensity, with a fixed pulse repetition frequency (PRF), and duty factor (DF)., Conclusions: The amount of plaque removed is directly related to the acoustic intensity. It was found that the presence of bubbles accelerates the removal of plaque. In order to ensure that pure mechanical mode ultrasound was used, the intensity used does not produce temperatures that exceed 1°C.

The effect of temperature and thermal dose (equivalent minutes at 43 degrees C) on ultrasonic attenuation in fresh dog muscle, liver, and kidney in vitro, was studied over a temperature range from room temperature to 70 degrees C. The effect of temperature on ultrasonic absorption in muscle was also studied. The attenuation experiments were performed at 4.32 MHz, and the absorption experiments at 4 MHz. Attenuation and absorption increased at temperatures higher than 50 degrees C, and eventually reached a maximum at 65 degrees C. The rate of change of tissue attenuation as a function of temperature was between 0.239 and 0.291 Np m-1 MHz-1 degree C-1 over the temperature range 50-65 degrees C. A change in attenuation and absorption was observed at thermal doses of 100-1000 min, where a doubling of these loss coefficients was observed over that measured at 37 degrees C, presumably the result of changes in tissue composition. The maximum attenuation or absorption was reached at thermal dosages on the order of 10(7) min. It was found that the rate at which the thermal dose was applied (i.e., thermal dose per min) plays a very important role in the total attenuation absorption. Lower thermal dose rates resulted in larger attenuation coefficients. Estimation of temperature-dependent absorption using a bioheat equation based thermal model predicted the experimental temperature within 2 degrees C.

Time shifts in echo signals returning from a heated volume of tissue correlate well with the temperature changes. In this study the relationship between these time shifts (or delays) and the tissue temperature was investigated in excised muscle tissue (turkey breast) as a possible dosimetric method. Heat was induced by the repeated activation of a sharply focused high-intensity ultrasound beam. Pulse echoes were sent and received with a confocal diagnostic transducer during the brief periods when the high-intensity ultrasonic beam was inactive. The change in transit time between echoes collected at different temperatures was estimated using cross-correlation techniques. With spatial-peak temporal-peak intensities (ISPTP) of less than 950W/cm2, the delay versus temperature relationship was fit to a linear equation with highly reproducible coefficients. The results confirmed that for spatial-peak temperature increases of approximately 10 degrees C, temperature-dependent changes in velocity were the single most important factor determining the observed delay, and a linear approximation could produce accurate temperature estimations. Nonlinear phenomena that occurred during the high-intensity irradiation had no significant effect on the measured delay. At ISPTP of 1115-2698 W/cm2, the delay-temperature relationship showed a similar monotonically decreasing pattern, but as the temperature peaked its slope gradually increased. This may reflect the curvilinear nature of the velocity-temperature relationship, but it may also be related to irreversible tissue modifications and to the use of the spatial-peak temperature to experimentally characterize the temperature changes. Overall, the results were consistent with theoretical predictions and encourage further experimental work to validate other aspects of the technique.

Temperature changes in tissue, caused by high-intensity focused ultrasound, cause time shifts in the echoes that traverse the heated tissue. These time shifts are caused by thermally induced changes in the distribution of the velocity of sound and by thermal expansion within the tissue. Our analytical model relates these shifts to changes in temperature distribution. It is proposed that these relationships can be used as a method for the noninvasive estimation of temperature within the tissue. The model shows that the echo shifts depend mostly on changes in the mean velocity along the acoustical path of the echoes and that no explicit information about the shape of the velocity distribution is required. The effects of the tissue thermal expansion are small in comparison, but may be significant under certain conditions. The theory, as well as numerical simulations, also predicts that the time shifts have an approximately linear behavior as a function of temperature. This suggests that an empirical linear delay-temperature relationship can be determined for temperature prediction. It is also shown that, alternatively, the distribution of temperature in the tissue can be estimated from the distribution of echo delays along the acoustical path. In the proposed system, low-level pulse echoes are sampled during brief periods when the high-intensity ultrasonic irradiation is off, and thus linear acoustic behavior is assumed. The possibility of nonlinear aftereffects and other disturbances limiting this approach is discussed.