A feasibility study of noninvasive ablation of ventricular tachycardia using high-intensity focused ultrasound

Advanced software for MRgFUS treatment planning

BACKGROUND AND OBJECTIVES

Herein, a user-friendly software platform for 3-dimensional Focused Ultrasound treatment planning based on Magnetic Resonance Imaging (MRI) images is presented.

METHODS

The software directly retrieves and loads MRI images. Various design tools can be used on the MRI images to define the treatment area and the sonication parameters. Based on the treatment plan, the software controls the robotic motion and motion pattern of Magnetic Resonance guided Focused Ultrasound (MRgFUS) robotic systems to execute the treatment procedure. Real-time treatment monitoring is achieved through MRI images and thermometry. The software's functionality and performance were evaluated in both laboratory and MRI environments. Different treatment plans were designed on MRI images and sonications were executed on agar-based phantoms and polymer films.

RESULTS

Magnetic Resonance (MR) thermometry maps were acquired in the agar-based phantoms. An exceptional agreement was observed between the software-planned treatment area and the lesions produced on the polymer films.

CONCLUSIONS

The developed software was successfully integrated with the MRI and robotic system controls for performing accurate treatment planning and real-time monitoring during sonications. The software provides an extremely user-friendly interface, while in the future it could be enhanced by providing dynamic modulation of the ultrasonic parameters during the treatment process.

Tissue specific considerations in implementing high intensity focussed ultrasound under magnetic resonance imaging guidance

High-intensity focused ultrasound can ablate a target permanently, leaving tissues through which it passes thermally unaffected. When delivered under magnetic resonance (MR) imaging guidance, the change in tissue relaxivity on heating is used to monitor the temperatures achieved. Different tissue types in the pre-focal beam path result in energy loss defined by their individual attenuation coefficients. Furthermore, at interfaces with different acoustic impedances the beam will be both reflected and refracted, changing the position of the focus. For complex interfaces this effect is exacerbated. Moreover, blood vessels proximal to the focal region can dissipate heat, altering the expected region of damage. In the target volume, the temperature distribution depends on the thermal conductivity (or diffusivity) of the tissue and its heat capacity. These are different for vascular tissues, water and fat containing tissues and bone. Therefore, documenting the characteristics of the pre-focal and target tissues is critical for effective delivery of HIFU. MR imaging provides excellent anatomic detail and characterization of soft tissue components. It is an ideal modality for real-time planning and monitoring of HIFU ablation, and provides non-invasive temperature maps. Clinical applications involve soft-tissue (abdomino-pelvic applications) or bone (brain applications) pre-focally and at the target (soft-tissue tumors and bone metastases respectively). This article addresses the technical difficulties of delivering HIFU effectively when vascular tissues, densely cellular tissues, fat or bone are traversed pre-focally, and the clinical applications that target these tissues. The strengths and limitations of MR techniques used for monitoring ablation in these tissues are also discussed.

Computer modeling of radiofrequency cardiac ablation: 30 years of bioengineering research

This review begins with a rationale of the importance of theoretical, mathematical and computational models for radiofrequency (RF) catheter ablation (RFCA). We then describe the historical context in which each model was developed, its contribution to the knowledge of the physics of RFCA and its implications for clinical practice. Next, we review the computer modeling studies intended to improve our knowledge of the biophysics of RFCA and those intended to explore new technologies. We describe the most important technical details of the implementation of mathematical models, including governing equations, tissue properties, boundary conditions, etc. We discuss the utility of lumped element models, which despite their simplicity are widely used by clinical researchers to provide a physical explanation of how RF power is absorbed in different tissues. Computer model verification and validation are also discussed in the context of RFCA. The article ends with a section on the current limitations, i.e. aspects not yet included in state-of-the-art RFCA computer modeling and on future work aimed at covering the current gaps.

A High Intensity Focused Ultrasound System for Veterinary Oncology Applications

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.