Minimizing eddy currents induced in the ground plane of a large phased-array ultrasound applicator for echo-planar imaging-based MR thermometry.

BACKGROUND: The study aims to investigate different ground plane segmentation designs of an ultrasound transducer to reduce gradient field induced eddy currents and the associated geometric distortion and temperature map errors in echo-planar imaging (EPI)-based MR thermometry in transcranial magnetic resonance (MR)-guided focused ultrasound (tcMRgFUS). METHODS: Six different ground plane segmentations were considered and the efficacy of each in suppressing eddy currents was investigated in silico and in operando. For the latter case, the segmented ground planes were implemented in a transducer mockup model for validation. Robust spoiled gradient (SPGR) echo sequences and multi-shot EPI sequences were acquired. For each sequence and pattern, geometric distortions were quantified in the magnitude images and expressed in millimeters. Phase images were used for extracting the temperature maps on the basis of the temperature-dependent proton resonance frequency shift phenomenon. The means, standard deviations, and signal-to-noise ratios (SNRs) were extracted and contrasted with the geometric distortions of all patterns. RESULTS: The geometric distortion analysis and temperature map evaluations showed that more than one pattern could be considered the best-performing transducer. In the sagittal plane, the star (d) (3.46 ± 2.33 mm) and star-ring patterns (f) (2.72 ± 2.8 mm) showed smaller geometric distortions than the currently available seven-segment sheet (c) (5.54 ± 4.21 mm) and were both comparable to the reference scenario (a) (2.77 ± 2.24 mm). Contrasting these results with the temperature maps revealed that (d) performs as well as (a) in SPGR and EPI. CONCLUSIONS: We demonstrated that segmenting the transducer ground plane into a star pattern reduces eddy currents to a level wherein multi-plane EPI for accurate MR thermometry in tcMRgFUS is feasible.

Quiet and distortion-free, whole brain BOLD fMRI using T2 -prepared RUFIS.

PURPOSE: To develop and evaluate a novel MR method that addresses some of the most eminent technical challenges of current BOLD-based fMRI in terms of 1) acoustic noise and 2) geometric distortions and signal dropouts. METHODS: A BOLD-sensitive fMRI pulse sequence was designed that first generates T2-weighted magnetization (using a T2 preparation module) and subsequently undergoes three-dimensional (3D) radial encoding using a rotating ultrafast imaging sequence (RUFIS). The method was tested on healthy volunteers at 3T with motor, visual, and auditory tasks, and compared relative to standard gradient and spin echo planar imaging (EPI) methods. RESULTS: In combination with parallel imaging the method achieves efficient and robust 3D whole brain coverage (3 mm isotropic resolution in 2.65 s scan time). Compared with standard EPI-based fMRI, the method demonstrated 1) T2-weighted imaging clean of geometrical distortions and signal dropout, 2) an acoustic noise reduction of ∼40 dB(A), and 3) a consistent BOLD response that is less sensitive (∼1.3% BOLD change) but spatially more specific. CONCLUSION: T2-prepared RUFIS provides quiet and distortion-free whole brain BOLD fMRI with minimal demands on the gradient performance. In particular, auditory fMRI and/or studies involving brain regions near air-tissue interfaces are expected to greatly benefit from the proposed method, especially if performed at ultrahigh field strengths.