Evaluation of magnetic field homogeneity using in-out signal cycle mapping in gradient recalled echo images of a mixed water/oil phantom as a rough indication for daily quality control

Abstract

Objective: Magnetic field (B₀) homogeneity is important for the performance of a magnetic resonance imaging (MRI) system. Traditionally, B₀ homogeneity was measured using the spectral peak or phase-mapping methods. However, these procedures are not generally accessible to the MRI operator and are rarely performed routinely. This study proposes a novel method for measuring B₀ homogeneity that can be implemented in daily quality control (QC).

Methods: When a uniformly mixed water/oil phantom was imaged using a gradient recalled echo (GRE) pulse sequence, the signal intensity dynamically changed with echo time (TE). From this, the resonant frequency was calculated with a simplex curve-fitting algorithm on a pixel-by-pixel basis. The standard deviation of resonant frequency (SD) was used as the index of B₀ homogeneity. The appropriate TE pattern and feasibility of B₀ homogeneity evaluation were examined.

Results: Over seven TEs (choosing nominal in-phase, out-phase, and the midpoints of both) were required to measure stable SD in a 1.5-T scanner. As B₀ homogeneity worsened, the SD became larger at the off-center position. Although a positive correlation was observed with the width of the spectral peak obtained by the phase-difference method, the SD value was about 5 × 10⁴ times greater. Therefore, SD can be used only as an index of B₀ homogeneity. Similar results were obtained using a 0.3-T scanner. A  map and SD can be obtained by acquiring several GRE images of a water/oil mixed phantom within a few minutes.

Conclusions: In-out signal cycle mapping can be easily implemented for daily QC in all MRI scanners.