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BackgroundThe 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).MethodsSix 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.ResultsThe 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.ConclusionsWe 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.

Gradient coils generate a magnetic field with a linear spatial variation that superimposes over the main magnetic field of a magnetic resonance imaging (MRI) system; such superimposition of the magnetic fields enables the encoding of the spatial position in MRI. A rapid change in the gradient field induces eddy currents in the conducting structures of an MRI system, resulting in the production of image artifacts. An objective of the gradient coil design phase is to predict both the coil's performance with respect to eddy currents and the image quality (IQ) before the coil is manufactured. In this paper, an integrated simulation environment is presented that combines the gradient coil design with an image formation simulation to predict the IQ. Here, an unshielded, uni-planar gradient set was simulated. Further, a study was conducted to determine the effect of frequency on the eddy currents induced in the conducting structures of the main magnet coil while exciting the uni-planar gradient set. The knowledge acquired from this study was applied to the IQ simulation, and a time-dependent simulation of a gradient echo pulse sequence was performed. The IQ of the uni-planar gradient set was predicted, and the input and reference images as well the images distorted by the eddy currents are shown.

IV successfully reduced the scan time to a single breath-hold feasible for many patients and remarkably facilitated the scan prescription, because there was no image aliasing concern. Care should be taken in using IV for T2 mapping because of T2 relaxation time overestimation.