Characterization of a Low-Profile, Flexible, and Acoustically Transparent Receive-Only MRI Coil Array for High Sensitivity MR-Guided Focused Ultrasound.

Magnetic resonance guided focused ultrasound (MRgFUS) is a non-invasive therapeutic modality for neurodegenerative diseases that employs real-time imaging and thermometry monitoring of targeted regions. MRI is used in guidance of ultrasound treatment; however, the MR image quality in current clinical applications is poor when using the vendor built-in body coil. We present an 8-channel, ultra-thin, flexible, and acoustically transparent receive-only head coil design (FUS-Flex) to improve the signal-to-noise ratio (SNR) and thus the quality of MR images during MRgFUS procedures. Acoustic simulations/experiments exhibit transparency of the FUS-Flex coil as high as 97% at 650 kHz. Electromagnetic simulations show a SNR increase of 13× over the body coil. In vivo results show an increase of the SNR over the body coil by a factor of 7.3 with 2× acceleration (equivalent to 11× without acceleration) in the brain of a healthy volunteer, which agrees well with simulation. These preliminary results show that the use of a FUS-Flex coil in MRgFUS surgery can increase MR image quality, which could yield improved focal precision, real-time intraprocedural anatomical imaging, and real-time 3D thermometry mapping.

Introduction

Purpose Thermal Ablation

Study Objective To design and evaluate an 8-channel ultra-thin, flexible, acoustically transparent receive-only head coil (FUS-Flex) to improve MRI signal-to-noise ratio and image quality during MR-guided focused ultrasound procedures.

Animal model / Human subject Homo sapiens (human healthy volunteers); strain: N/A; age: not stated; sex: not stated

Disease model healthy

MRI or image guidance method MRI (magnetic resonance guided focused ultrasound, MRgFUS)

Targeted brain region(s) Thalamus

Outcomes and Safety

Summary of Outcomes An 8‑channel acoustically transparent FUS‑Flex receive coil markedly improved MR image SNR (in vivo 7.3× with 2× acceleration ≈11× without acceleration; simulations up to ~13–15×) while producing negligible effects on focused ultrasound (1–5% attenuation, focal shifts <0.4–1.6 mm), with successful acoustic transparency demonstrated across tested frequencies (220 kHz, 650 kHz and generally 100–700 kHz), best at 650 kHz (~97% transparency).

Safety-related matter The authors state that electrical safety for in vivo subjects and MRI systems is a major concern and is outside the scope of this feasibility study, requiring future work; practical safety issues to address include creating a sealed/waterproof design, preventing air bubbles/water permeability, and possible degassing of the coil fabric. They also note volunteer comfort (avoiding mounting screws/frames) and describe current calibration as potentially tedious, risky, and uncomfortable, but report no adverse events in the presented feasibility tests.

Brain Region

Ultrasound Parameters

Ultrasound instrument INSIGHTEC ExAblate Neuro (INSIGHTEC); water-filled transducer, 30 cm diameter (focal length 232 mm, radius 150 mm)

FUS Frequency 220 kHz, 500 kHz, 650 kHz

Focal Characteristics focal depth: None; focal length: 232 mm; aperture size: 300 mm diameter (radius 150 mm)

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