Magnetic Resonance Imaging-Guided Focused Ultrasound Positioning System for Preclinical Studies in Small Animals.

OBJECTIVES: A positioning device compatible with magnetic resonance imaging (MRI) used for preclinical studies in small animals was developed that fits in MRI scanners up to 7 T. The positioning device was designed with two computer-controlled linear stages. METHODS: The positioning device was evaluated in an agar-based phantom, which mimics soft tissues, and in a rabbit. Experiments with this positioning device were performed in an MRI system using the agar-based phantom. The transducer used had a diameter of 50 mm, operated at 0.5 MHz, and focused energy at 60 mm. RESULTS: Magnetic resonance thermometry was used to assess the functionality of the device, which showed adequate deposition of thermal energy and sufficient positional accuracy in all axes. CONCLUSIONS: The proposed system fits in MRI scanners up to 7 T. Because of the size of the positioning device, at the moment, it can be used to perform preclinical studies on small animals such as mice, rats, and rabbits.

Ultrasonic Attenuation of an Agar, Silicon Dioxide, and Evaporated Milk Gel Phantom.

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.

Characterization of a soft tissue-mimicking agar/wood powder material for MRgFUS applications.

This study describes the development and characterization of an agar-based soft tissue-mimicking material (TMM) doped with wood powder destined for fabricating MRgFUS applications. The main objective of the following work was to investigate the suitability of wood powder as an inexpensive alternative in replacing other added materials that have been suggested in previous studies for controlling the ultrasonic properties of TMMs. The characterization procedure involved a series of experiments designed to estimate the acoustic (attenuation coefficient, absorption coefficient, propagation speed, and impedance), thermal (conductivity, diffusivity, specific heat capacity), and MR properties (T1 and T2 relaxation times) of the wood-powder doped material. The developed TMM (2% w/v agar and 4% w/v wood powder) as expected demonstrated compatibility with MRI scanner following images artifacts evaluation. The acoustic attenuation coefficient of the proposed material was measured over the frequency range of 1.1-3 MHz and found to be nearly proportional to frequency. The measured attenuation coefficient was 0.48 dB/cm at 1 MHz which was well within the range of soft tissue. Temperatures over 37 °C proved to increase marginally the attenuation coefficient. Following the transient thermoelectric method, the acoustic absorption coefficient was estimated at 0.34 dB/cm-MHz. The estimated propagation speed (1487 m/s) was within the range of soft tissue at room temperature, while it significantly increased with higher temperature. The material possessed an acoustic impedance of 1.58 MRayl which was found to be comparable to the corresponding value of muscle tissue. The thermal conductivity of the material was estimated at 0.51 W/m K. The measured relaxation times T1 (844 ms) and T2 (66 ms) were within the range of values found in the literature for soft tissue. The phantom was tested for its suitability for evaluating MRgFUS thermal protocols. High acoustic energy was applied, and temperature change was recorded using thermocouples and MR thermometry. MR thermal maps were acquired using single-shot Echo Planar Imaging (EPI) gradient echo sequence. The TMM matched adequately the acoustic and thermal properties of human tissues and through a series of experiments, it was proven that wood concentration enhances acoustic absorption. Experiments using MR thermometry demonstrated the usefulness of this phantom to evaluate ultrasonic thermal protocols by monitoring peak temperatures in real-time. Thermal lesions formed above a thermal dose were observed in high-resolution MR images and visually in dissections of the proposed TMM.

Robotic system for magnetic resonance guided focused ultrasound ablation of abdominal cancer.

BACKGROUND: A prototype robotic system that uses magnetic resonance guided focused ultrasound (MRgFUS) technology is presented. It features three degrees of freedom (DOF) and is intended for thermal ablation of abdominal cancer. METHODS: The device is equipped with three identical transducers being offset between them, thus focussing at different depths in tissue. The efficacy and safety of the system in ablating rabbit liver and kidney was assessed, both in laboratory and magnetic resonance imaging (MRI) conditions. RESULTS: Despite these organs' challenging location, in situ coagulative necrosis of a tissue area was achieved. Heating of abdominal organs in rabbit was successfully monitored with MR thermometry. CONCLUSIONS: The MRgFUS system was proven successful in creating lesions in the abdominal area of rabbits. The outcomes of the study are promising for future translation of the technology to the clinic.

Magnetic resonance image-guided focused ultrasound robotic system for transrectal prostate cancer therapy.

BACKGROUND: A magnetic resonance image (MRI) guided robotic device for focussed ultrasound therapy of prostate cancer (PC) was developed. The device offers movement in 5 degrees of freedom (DOF) and uses a single-element transducer that operates at 3.2 MHz, has a diameter of 25 mm and focuses at 45 mm. METHODS: The MRI compatibility of the system was evaluated in a 1.5 T scanner. The ability of the transducer to create lesions was evaluated in laboratory and MRI settings, on ex vivo pork tissue and in vivo rabbit thigh tissue. RESULTS: Cavitational and thermal lesions were created on the excised pork tissue. In vivo experiments proved the efficacy of the system in ablating muscle tissue without damaging intervening areas. CONCLUSIONS: The MRI compatible robotic system can be placed on the table of any commercial MRI scanner up to 7 T. The device has the ability of future use for transrectal focal therapy of PC with the patient in supine position.

MRI-guided frameless biopsy robotic system with the inclusion of unfocused ultrasound transducer for brain cancer ablation.

BACKGROUND: A magnetic resonance image (MRI) guided robotic system dedicated for brain biopsy was developed. The robotic system carries a biopsy needle and a small rectangular unfocused, single element, planar ultrasonic transducer which can be potentially utilized to ablate small and localized brain cancer. MATERIALS AND METHODS: The robotic device includes six computer-controlled axes. An agar-based phantom was developed which included an olive that mimics brain target. A rectangular ultrasonic transducer operated at 4 MHz was used. RESULTS: The functionality of the robotic system was assessed by means of ultrasound imaging, MRI imaging, and MR thermometry, demonstrating effective targeting. The heating capabilities of the ultrasonic transducer were also evaluated. CONCLUSIONS: A functional MRI-guided robotic system was produced which can perform frameless brain biopsy. In the future, if a tumour is proven malignant, the needle can be pulled-out and a small ultrasonic transducer can be inserted to ablate the tumour.

MRI-compatible breast/rib phantom for evaluating ultrasonic thermal exposures.

INTRODUCTION: The target of this study was the development of a magnetic resonance imaging (MRI) compatible breast phantom for focused ultrasound which includes plastic (ABS) ribs. The objective of the current study was the evaluation of a focused ultrasound procedure using the proposed phantom that eliminates rib heating. MATERIAL AND METHODS: The proposed phantom was evaluated using two different focused ultrasound exposures. The surrounding breast tissue was mimicked using an agar-silica-evaporation milk gel (2% w/v - 2% w/v - 40% v/v). RESULTS: The attenuation of the ABS was similar to that of ribs. MR thermometry of focused ultrasound exposures were acquired using the breast/rib phantom. In one exposure focused ultrasound was applied with far-field targeting of the ribs. In the other exposure, the transducer was positioned laterally, thus avoiding exposure of the rib to focused ultrasound. CONCLUSIONS: Due to growing interest in using MRI guided focused ultrasound (MRgFUS) for patients with breast cancer, the proposed breast/rib phantom can be utilized as a very useful tool for evaluating ultrasonic protocols.

Acoustic and thermal characterization of agar based phantoms used for evaluating focused ultrasound exposures.

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.

MRI-guided focused ultrasound robotic system for the treatment of bone cancer.

BACKGROUND: A novel MRI-conditional robot was developed that navigates a focused ultrasound (FUS) transducer. With this robotic system the transducer can access bones. The intended application is pain palliation from bone cancer using thermal ablation using FUS. METHODS: The robotic system has four computer-controlled axes (three linear and one angular). The robotic system was manufactured using a digital manufacturing 3D printer, using acrylonitrile butadiene styrene (ABS) plastic. MRI-conditional optical encoders were used to accurately control the robotic system. RESULTS: The robotic system was successfully tested for MRI safety and compatibility, using fast-gradient pulse sequences and a liquid phantom. The robotic system has been tested for its functionality for creating discrete and multiple (overlapping) lesions in a gel phantom. CONCLUSIONS: An MRI-conditional FUS robotic system was developed that has the potential to create thermal lesions with the intention of treating bone cancer for the purpose of pain palliation. Copyright © 2016 John Wiley & Sons, Ltd.

MRI guided focused ultrasound robotic system for animal experiments.

BACKGROUND: In this paper an MRI-guided focused ultrasound (MRgFUS) robotic system was developed that can be used for conducting experiments in small animals.The target for this robotic system regarding motion was to move a therapeutic ultrasound transducer in two Cartesian axes. METHODS: A single element spherically focused transducer of 3 cm diameter, focusing at 7 cm and operating at 0.4 MHz was used. The positioning device incorporates only MRI compatible materials. The propagation of ultrasound is a bottom to top approach. The 2-D positioning device is controlled by custom-made software and a custom-made electronic system which controls the two piezoelectric motors. RESULTS: The system was tested successfully in agar/silica/evaporated milk phantom for various tasks (robot motion, MR compatibility, and MR thermometry). The robotic system is capable of moving the focused ultrasound transducer to perform MR-guided focused ultrasound experiments in small animals. CONCLUSIONS: This system has the potential to be deployed as a cost effective solution for performing experiments in small animals.

MRI-compatible bone phantom for evaluating ultrasonic thermal exposures.

OBJECTIVE: The goal of the proposed study was the development of a magnetic resonance imaging (MRI) compatible bone phantom suitable for evaluating focused ultrasound protocols. MATERIALS AND METHODS: High resolution CT images were used to segment femur bone. The segmented model was manufactured with (Acrylonitrile Butadiene Styrene) ABS plastic using a 3-D printer. The surrounding skeletal muscle tissue was mimicked using an agar-silica-evaporated milk gel (2% w/v-2% w/v-40% v/v). MR thermometry was used to evaluate the exposures of the bone phantom to focused ultrasound. RESULTS: The estimated agar-silica-evaporated milk gel's T1 and T2 relaxation times in a 1.5T magnetic field were 776ms and 66ms respectively. MR thermometry maps indicated increased temperature adjacent to the bone, which was also shown in situations of real bone/tissue interfaces. CONCLUSION: Due to growing interest of using MRI guided Focused Ultrasound Surgery (MRgFUS) in palliating bone cancer patients at terminal stages of the disease, the proposed bone phantom can be utilized as a very useful tool for evaluating ultrasonic protocols, thus minimizing the need for animal models. The estimated temperature measured and its distribution near the bone phantom/agar interface which was similar to temperatures recorded in real bone ablation with FUS, confirmed the phantom's functionality.

MRI compatible head phantom for ultrasound surgery.

OBJECTIVE: Develop a magnetic resonance imaging (MRI) compatible head phantom with acoustic attenuation closely matched to the human attenuation, and suitable for testing focused ultrasound surgery protocols. MATERIALS AND METHODS: Images from an adult brain CT scan were used to segment the skull bone from adjacent cerebral tissue. The segmented model was manufactured in a 3-D printer using (Acrylonitrile Butadiene Styrene) ABS plastic. The cerebral tissue was mimicked by an agar-evaporated milk-silica gel (2% w/v-25% v/v-1.2% w/v) which was molded inside a skull model. RESULTS: The measured attenuation of the ABS skull was 16 dB/cm MHz. The estimated attenuation coefficient of the gel replicating brain tissue was 0.6 dB/cm MHz. The estimated agar-silica gel's T1 and T2 relaxation times in a 1.5 Tesla magnetic field were 852 ms and 66 ms respectively. The effectiveness of the skull to reduce ultrasonic heating was demonstrated using MRI thermometry. CONCLUSION: Due to growing interest in using MRI guided focused ultrasound (MRgFUS) for treating brain cancer and its application in sonothrombolysis, the proposed head phantom can be utilized as a very useful tool for evaluating ultrasonic protocols, thus minimizing the need for animal models and cadavers.