Nicos Mylonas

MR compatible positioning device for guiding a focused ultrasound system for the treatment of brain deseases

A prototype magnetic resonance imaging (MRI)‐compatible positioning device that navigates a high intensity focused ultrasound (HIFU) transducer is presented. The positioning device has three user‐controlled degrees of freedom that allow access to brain targets using a lateral coupling approach. The positioning device can be used for the treatment of brain cancer (thermal mode ultrasound) or ischemic stroke (mechanical mode ultrasound).

In vitro and in vivo brain ablation created by high-intensity focused ultrasound and monitored by MRI

In this paper, magnetic resonance imaging (MRI) is investigated for monitoring small and large lesions created by high-intensity focused ultrasound (HIFU) in freshly excised lamb brain and in rabbit brain in vivo. A single-element spherically focused transducer of 5 cm diameter, focusing at 10 cm and operating at 1 MHz was used. A prototype MRI-compatible positioning device that is used to navigate the transducer is described. The effects of HIFU were investigated using T1-W and T2-W fast spin echo (FSE) and fluid-attenuated inversion recovery (FLAIR). T2-W FSE and FLAIR show better anatomical details within the brain than T1-W FSE, but with T1-W FSE, the contrast between lesion and brain is higher for both thermal and bubbly lesions. The best contrast between lesion and brain with T1-W FSE is obtained with TR above 500 ms, whereas with T2-W FSE, the best contrast is observed between 40 and 60 ms. The maximum contrast to noise ratio (CNR) measured with T1-W FSE was approximately 20. With T2-W FSE, the corresponding CNR was approximately 12. With this system, we were able to create large lesions (by producing overlapping lesions), and it was possible to monitor these lesions with MRI with excellent contrast. The length of the lesions in vivo brain was much higher than the length in vitro, indicating that the penetration in the in vitro brain is limited, possibly by reflection due to trapped bubbles in the blood vessels. This paper demonstrates that HIFU has the potential to treat brain tumors in humans. This could be done either using a single-element transducer with a frequency around 1 MHZ or using a multi-element transducer.

MRI-guided Sonothrombolysis of Rabbit Carotid Artery

The potential of magnetic resonance imaging-guided focused ultrasound (MRgFUS) combined with the thrombolytic drug recombinant tissue plasminogen activator (rt-PA) to dissolve clots in the carotid of a New Zealand rabbit in vivo is evaluated. A spherically focused transducer of 5-cm diameter, focusing at 10 cm and operating at 1 MHz, was used. A pulsed ultrasound protocol was used that maintains a tissue temperature increase of less than 1 °C in the clot (called safe temperature). MRgFUS has the potentials to dissolve clots that are injected in the carotid of rabbits in vivo. It was found that the time needed for opening the carotid artery using ultrasound and rt-PA was decreased compared with just using rt-PA. The time needed for opening the artery decreases with increasing acoustic intensity. With an intensity of 20 W/cm2 (spatial average temporal average), which is not causing artery heating, the time needed to completely open the artery was 70 minutes. The proposed protocol was monitored using magnetic resonance angiography every 1 minute.

Destruction of atherosclerotic plaque using pulse ultrasound with a planar rectangular transducer

The aim of this paper is to present a feasibility study for using mechanical mode ultrasound for destroying atherosclerotic plaque with MRI monitoring.

A Prototype MR Compatible Positioning Device for Guiding a Focused Ultrasound System for the Treatment of Abdominal and Thyroid Cancer

A prototype magnetic resonance imaging MRI-compatible positioning device that navigates a high intensity focused ultrasound HIFU transducer is presented. The intended application is to treat eventually tumours in the abdominal and thyroid. The positioning device has 3 user-controlled stages that allow access to various targets using a top to bottom coupling approach. Materials and Methods. The positioning device incorporates only MRI compatible materials such as piezoelectric motors, ABS plastic, brass screws, and brass rack and pinion. Results The MRI compatibility and the accuracy of the system were successfully demonstrated in an open MRI scanner. The robot has the ability to accurately move the transducer thus creating discrete and overlapping lesions in rabbit liver in vivo. This simple, cost effective positioning device can be placed mostly on the structure of an open MRI gantry. Due to the size of this positioning device, the proposed prototype in its current form cannot be used in any closed MRI system. The novelty of this positioning device is the MRI compatible design and its intended application which is the treatment of tumors in the abdominal area using focused ultrasound. This system can be utilized in the future to treat patients with cancer in the liver, kidney, pancreas and thyroid provided that the accuracy of the positioning device is greatly improved.

Liver ablation using a high intensity focused ultrasound system and MRI guidance

The effectiveness of magnetic resonance imaging (MRI) to monitor thermal lesions created by High Intensity Focused Ultrasound (HIFU) in rabbit liver in vivo is investigated. The MRI sequences of T1-weighted, and T2-weighted fast spin echo (FSE) were evaluated. The main goal in this paper was to find the range of repetition time (TR) and range of echo time (TE) which maximizes the contrast to noise ratio (CNR). An ultrasonic transducer operating at 2 MHz was used, which is navigated using a positioning device. With T1W FSE the range of TR under which CNR is maximized ranges from 400 to 900 ms. The maximum contrast measured is approximately 25. With T2W FSE the range of TE that establishes maximum contrast is between 40 ms and 80 ms, with CNR of approximately 14. T1W FSE is much better than T2W FSE in detecting thermal lesions in liver. Both T1W and T2 W FSE were proven successful to image thermal lesions created by HIFU in rabbit liver in vivo.

Penetration of high intensity focused ultrasound ex vivo and in vivo rabbit brain using MR imaging

In this paper magnetic resonance imaging (MRI) is investigated for monitoring the penetration of high intensity focused ultrasound (HIFU) ex vivo and in vivo rabbit brain. A single element spherically focused transducer of 5 cm diameter, focusing at 10 cm and operating at 2 MHz was used. A prototype MRI- compatible positioning device is described. MRI images were taken using fast spin echo (FSE). The length of the lesions in vivo rabbit brain was much higher than the length ex vivo, proving that the penetration in the ex vivo brain is limited by reflection due to trapped bubbles in the blood vessels.