Gradient-recalled echo (GRE) echo-planar imaging (EPI) is an efficient MRI pulse sequence that is commonly used for several enticing applications, including functional MRI (fMRI), susceptibility-weighted imaging (SWI), and proton resonance frequency (PRF) thermometry. These applications are typically not performed in the mid-field (<1 T) as longer T₂* and lower polarization present significant challenges. However, recent developments of mid-field scanners equipped with high-performance gradient sets offer the possibility to re-evaluate the feasibility of these applications. The paper introduces a metric “T₂* contrast efficiency” for this evaluation, which minimizes dead time in the EPI sequence while maximizing T₂* contrast so that the temporal and pseudo signal-to-noise ratios (SNRs) can be attained, which could be used to quantify experimental parameters for future fMRI experiments in the mid-field. To guide the optimization, T₂* measurements of the cortical gray matter are conducted, focusing on specific regions of interest (ROIs). Temporal and pseudo SNR are calculated with the measured time-series EPI data to observe the echo times at which the maximum T₂* contrast efficiency is achieved. T₂* for a specific cortical ROI is reported at 0.5 T. The results suggest the optimized echo time for the EPI protocols is shorter than the effective T₂* of that region. The effective reduction of dead time prior to the echo train is feasible with an optimized EPI protocol, which will increase the overall scan efficiency for several EPI-based applications at 0.5 T.

Background: Magnetic resonance imaging (MRI) offers superior soft tissue contrast and essential imaging capabilities for modern medicine. MRI is increasingly being used in applications that require a high degree of spatial fidelity; however, distortions are a well‐known limitation of the modality. The mid‐field (0.3 T ≤ B ₀ < 1 T) has advantages in this respect due to being less susceptible to patient‐induced distortions. Purpose: The purpose of this work was to characterize the geometric fidelity of a short‐bore, head‐specific, 0.5T MRI system. Methods: Assessment of spatial fidelity was performed using a 3D gradient recalled echo (GRE) acquisition on a commercial distortion phantom using the validated distortion analysis software provided. B ₀ ‐induced distortions were measured using a 3D field map. Inter‐scanner reproducibility was assessed across four distinct systems of identical make and model, while intra‐scanner repeatability was assessed at one site over six repeat measurements. Results: Inter‐scanner reproducibility measured an average 95th percentile distortion over 100 and 180 mm DSV of 0.15 ± 0.03 and 0.33 ± 0.05 mm. Average 95th percentile distortions due to B ₀ field inhomogeneities over 100 and 180 mm DSV were 0.02 ± 0.01 and 0.07 ± 0.02 mm. Intra‐scanner repeatability measured the uncertainty in distortion values to be 0.020 ± 0.005 mm. Conclusion: The total residual distortions measured in this phantom study were less than half the recommended value required for radiosurgery and significantly better than data published from other MR systems. This demonstrates that in addition to the compact footprint of the Synaptive 0.5T scanner, it exceeds current standards for geometric accuracy.