Marinos Yiannakou

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 gels 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 phantoms functionality.

Evaluation of focused ultrasound algorithms: Issues for reducing pre-focal heating and treatment time

BACKGROUND Due to the heating in the pre-focal field the delay between successive movements in high intensity focused ultrasound (HIFU) are sometimes as long as 60s, resulting to treatment time in the order of 2-3h. Because there is generally a requirement to reduce treatment time, we were motivated to explore alternative transducer motion algorithms in order to reduce pre-focal heating and treatment time. MATERIALS AND METHODS A 1 MHz single element transducer with 4 cm diameter and 10 cm focal length was used. A simulation model was developed that estimates the temperature, thermal dose and lesion development in the pre-focal field. The simulated temperature history that was combined with the motion algorithms produced thermal maps in the pre-focal region. Polyacrylimde gel phantom was used to evaluate the induced pre-focal heating for each motion algorithm used, and also was used to assess the accuracy of the simulation model. RESULTS Three out of the six algorithms having successive steps close to each other, exhibited severe heating in the pre-focal field. Minimal heating was produced with the algorithms having successive steps apart from each other (square, square spiral and random). The last three algorithms were improved further (with small cost in time), thus eliminating completely the pre-focal heating and reducing substantially the treatment time as compared to traditional algorithms. CONCLUSIONS Out of the six algorithms, 3 were successful in eliminating the pre-focal heating completely. Because these 3 algorithms required no delay between successive movements (except in the last part of the motion), the treatment time was reduced by 93%. Therefore, it will be possible in the future, to achieve treatment time of focused ultrasound therapies shorter than 30 min. The rate of ablated volume achieved with one of the proposed algorithms was 71 cm(3)/h. The intention of this pilot study was to demonstrate that the navigation algorithms play the most important role in reducing pre-focal heating. By evaluating in the future, all commercially available geometries, it will be possible to reduce the treatment time, for thermal ablation protocols intended for oncological targets.

MRI‐compatible breast/rib phantom for evaluating ultrasonic thermal exposures

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.

Evaluation of a small flat rectangular therapeutic ultrasonic transducer intended for intravascular use.

HighlightsIntravascular transducer for atherosclerosis.Flat rectangular unfocused transducer.Intravascular transducer for thrombosis. Background: The aim of the proposed study was to evaluate the performance of a flat rectangular (2 × 10 mm2) transducer operating at 4 MHz. The intended application of this transducer is intravascular treatment of thrombosis and atherosclerosis. Methods: The transducers thermal capabilities were tested in two different gel phantoms. MR thermometry was used to demonstrate the thermal capabilities of this type of transducer. Results: Temperature measurements demonstrated that this simple and small transducer adequately produced high temperatures, which can be utilized for therapeutic purposes. These high temperatures were confirmed using thermocouple and MR measurements. Pulsed ultrasound in combination with thrombolytic drugs and microbubbles was utilized to eliminate porcine thrombi. Conclusions: The proposed transducer has the potentials to treat atherosclerotic lesions using the thermal properties of ultrasound, since high temperatures can be achieved in less than 5 s. The results revealed that the destruction of thrombi using pulsed ultrasound requires long exposure time and high microbubble dosage.

MRI-guided coupling for a focused ultrasound system using a top-to-bottom propagation

BackgroundA 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.MethodsThe 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.ResultsThe 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.ConclusionsAn 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.

MRI guided focused ultrasound robotic system for animal experiments

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.

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

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.

Software that controls a magnetic resonance imaging compatible robotic system for guiding high-intensity focused ultrasound therapy

Background and Objectives: This study describes a software application for controlling a focused ultrasound system that was guided by magnetic resonance imaging (MRI). Materials and Methods: The softwares functionalities were tested using a custom-made electronic system, MRI compatible robotic systems, and a high-intensity focused ultrasound (HIFU) system. The experiments were conducted in gel phantoms to test the motion accuracy and functionality of the system. Results: The software includes the following functionalities: (a) patient database (patient identification number, age, weight, gender, etc.); (b) acquisition of MRI images; (c) transducer movement; (d) transducer coordinates; (e) ultrasound control; (f) MRI thermometry; (h) temperature measurement with thermocouple; (i) command history (command name, starting time, and remaining time); and (j) MRI compatible camera. Evaluation experiments were conducted to test the software for accuracy, functionality, and communication with MRI. Conclusions: User-friendly software was developed to control an MRI-guided HIFU system. The software was evaluated in phantom experiments and it was found to accomplish all the intended functions.

A multipurpose positioning device for magnetic resonance imaging-guided focused ultrasound surgery

Background and Objectives: An magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS) positioning device was developed with 3 identical Cartesian stages. The robotic system can be utilized to move a focused ultrasound transducer for performing various MR-guided applications. Materials and Methods: A single element spherically focused transducer of 4 cm diameter, focusing at 10 cm, and operating at 1.14 MHz was used during the evaluation of the robotic system. The propagation of ultrasound was either lateral or superior to inferior. MRI thermometry algorithms were developed to assess the thermal effects of MRgFUS. The proposed robotic system was developed using a three-dimensional printer. Results: The system was tested successfully in a gel phantom for various tasks (robot motion, functionality, and MR compatibility). Controlled thermal lesions were created in the gel phantom. The lesion creation was monitored successfully using MRI thermometry. Conclusions: The system was tested successfully for its functionality and its MR compatibility. This system has the potential to be used for focused ultrasound applications in the brain, breast, abdominal, and thyroid.

Amyloid beta plaque reduction with antibodies crossing the blood brain barrier opened with focused ultrasound in a rabbit model

The main objective of the study was to treat Alzheimers disease (AD) plaques using focused ultrasound (FUS) induced blood brain barrier (BBB) opening with and without delivery of antibodies in a rabbit model for AD. The animal model was achieved by feeding a high cholesterol diet to rabbits for 4 months. A single spherically focused MRI compatible transducer was used which operated at 1 MHz, had a focal length of 10 cm and diameter of 4 cm. By increasing the number of sessions, the number of plaques decreased (both for antibodies, and without antibodies). This study demonstrated that by opening the BBB, it will be possible to deliver exogenous antibodies to the brain, which eliminates Amyloid β plaques.