Treatment envelope evaluation in transcranial magnetic resonance-guided focused ultrasound utilizing 3D MR thermometry.

Current clinical targets for transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) are all located close to the geometric center of the skull convexity, which minimizes challenges related to focusing the ultrasound through the skull bone. Non-central targets will have to be reached to treat a wider variety of neurological disorders and solid tumors. Treatment envelope studies utilizing two-dimensional (2D) magnetic resonance (MR) thermometry have previously been performed to determine the regions in which therapeutic levels of FUS can currently be delivered. Since 2D MR thermometry was used, very limited information about unintended heating in near-field tissue/bone interfaces could be deduced. In this paper, we present a proof-of-concept treatment envelope study with three-dimensional (3D) MR thermometry monitoring of FUS heatings performed in a phantom and a lamb model. While the moderate-sized transducer used was not designed for transcranial geometries, the 3D temperature maps enable monitoring of the entire sonication field of view, including both the focal spot and near-field tissue/bone interfaces, for full characterization of all heating that may occur. 3D MR thermometry is achieved by a combination of k-space subsampling and a previously described temporally constrained reconstruction method. We present two different types of treatment envelopes. The first is based only on the focal spot heating-the type that can be derived from 2D MR thermometry. The second type is based on the relative near-field heating and is calculated as the ratio between the focal spot heating and the near-field heating. This utilizes the full 3D MR thermometry data achieved in this study. It is shown that 3D MR thermometry can be used to improve the safety assessment in treatment envelope evaluations. Using a non-optimal transducer, it is shown that some regions where therapeutic levels of FUS can be delivered, as suggested by the first type of envelope, are not necessarily safely treated due to the amount of unintended near-field heating occurring. The results presented in this study highlight the need for 3D MR thermometry in tcMRgFUS.

Introduction

Purpose Thermal ablation

Study Objective To demonstrate that three-dimensional MR thermometry can be used to evaluate treatment envelopes and detect unintended near-field heating during transcranial MR-guided focused ultrasound, improving safety assessment.

Animal model / Human subject Lamb (Ovis aries); strain: not specified; age: not specified (described as a recently euthanized lamb); sex: not specified

Disease model Healthy

MRI or image guidance method MR image guidance — 3D MRI with MR thermometry monitoring using an MR-compatible phased-array transducer; targeting via mechanical positioning and electronic beam steering (Image Guided Therapy system). No phase-aberration correction applied.

Outcomes and Safety

Summary of Outcomes Focused ultrasound with a 1‑MHz transducer produced therapeutic focal heating (≥10°C) in large intracranial regions per 2D MRTI, but 3D MRTI revealed substantial unintended near‑field heating—minimal in the phantom (near‑field rise ~2–4°C; focal:near‑field ratio up to ~3) and severe in the lamb skull (near‑field rise ~20–30°C; ratio ≤1 over most of the brain) making many targets unsafe. Successful conditions were achieved when the phantom was positioned closer to the transducer (larger acoustic window) with electronic steering (up to ~1.5 cm between planes and 5 mm lateral steering tested), yielding acceptable focal‑to‑near‑field ratios; the lamb configuration produced excessive near‑field heating and was not considered safe.

Safety-related matter The study identified significant unintended near-field heating—up to ~20–30°C in the lamb model and in many regions the near-field heated more than the focal spot—indicating potential safety risks and that areas appearing treatable by focal temperatures alone may not be safely treated. No adverse clinical events were reported in these phantom and ex vivo lamb experiments, but the authors emphasize the need for 3D MR thermometry for safety assessment and further studies in clinical systems/human skulls.

Brain Region

Visualization unavailable

Ultrasound Parameters

Ultrasound instrument MR-compatible phased-array ultrasound transducer (256 elements, 1-MHz, 13-cm radius of curvature), Imasonic (Voray-sur-l’Ognon, France); accompanying positioning/electronic beam steering hardware/software by Image Guided Therapy (Pessac, France)

FUS Frequency 1 MHz

FUS Mode continuous

Pulse duration 19.8 s (phantom); 17.3 s (lamb)

Duration of a single FUS session 19.8 s (phantom sonication); 17.3 s (lamb sonication)

Focal Characteristics focal depth: 8 mm; focal length: 13 cm (radius of curvature); aperture size: None

Treatment frequency Multiple sessions

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