AAPM Task Group 241: A medical physicist’s guide to MRI‐guided focused ultrasound body systems

Abstract

Magnetic resonance‐guided focused ultrasound (MRgFUS) is a completely non‐invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

Robotic system for magnetic resonance imaging‐guided focused ultrasound treatment of thyroid nodules

“…Notably, the most acclaimed commercially available MRgFUS systems employ phased-array transducers, with successful treatment of large nodules highly dependent on electronic steering of the millimetre-sized focal point. 93 In contrast, the MRgFUS system proposed herein employs mechanical steering of the ultrasonic beam, with navigation of the single-element transducer to the target area achieved via the developed robotic positioning mechanisms. Consequently, considering that independent elements of phased array transducers require individual RF amplifiers for activation, 93 the mechanical steering of the single-element transducer employed by the proposed system results in significantly reduced costs compared to commercially available MRgFUS systems.…”

“…92 Furthermore, it is worth stating that the marketed MRgFUS systems are embedded into dedicated MRgFUS patient tables that are integrated with the MRI scanner only upon execution of treatment. 93 Consequently, such systems are only compatible with MRI scanners from certain manufacturing companies, while significant space is also required for proper storage of the MRgFUS table. 93 In contrast, the configuration of the herein system with the C-arm attached on the MRI table via the 2 brass screws enables easy and effortless attachment of the system within the MRI scanner for targeted treatment.…”

“…93 Consequently, such systems are only compatible with MRI scanners from certain manufacturing companies, while significant space is also required for proper storage of the MRgFUS table. 93 In contrast, the configuration of the herein system with the C-arm attached on the MRI table via the 2 brass screws enables easy and effortless attachment of the system within the MRI scanner for targeted treatment. Furthermore, this specific design configuration allows the integration of the proposed system within all clinical MRI scanners independent of the manufacturing company, while the highly compact dimensions of the developed system require only a small space for storage purposes.…”

Robotic system for top to bottom MRgFUS therapy of multiple cancer types

“…Upon operation, this cone is filled with degassed water enabling proper transmission of the ultrasonic beam through an opening at its bottom that is sealed with a membrane. It is noted that acoustic coupling membranes are commonly used in MRgFUS systems usually being part of the transducer assembly 18 . In the current study, the membrane was made of silicone with a mean acoustic attenuation of 1.58 ± 0.20 dB/cm‐MHz (at 1.1 MHz).…”

“…It is noted that acoustic coupling membranes are commonly used in MRgFUS systems usually being part of the transducer assembly. 18 In the current study, the membrane was made of silicone with a mean acoustic attenuation of 1.58 � 0.20 dB/cm-MHz (at 1.1 MHz). Given its very small thickness of 0.2 mm, the induced attenuation of the ultrasonic beam is considered minimal.…”