WE-G-217A-09: Phase Imaging Measurement of Static Magnetic Field Homogeneity

The ACR MRI accreditation program requires measurement of the magnetic field homogeneity (MFH) in the annual QA. Full implementation of vendor methods can be very time consuming and incorporates techniques not available to clinical physicists. Conversely, many of the less involved techniques proposed tend to be less inaccurate and/or precise. Here, we propose a robust approach to MFH analysis using a simple phase mapping acquisition which is a time efficient compromise.

The root mean square (r.m.s.) for MFH measurement is commonly used over multiple slices (∼24). In regions with high SNR, the MR signal can be reasonably assumed Gaussian distributed. Therefore, the standard deviation of phase values in ROI is equivalent to the r.m.s. of those. The standard deviations of phases in x, y and z are assumed uncorrelated. Thus, only axial, sagittal and coronal planes need to be acquired to approximate the MFH as opposed to a full 3D acquisition. To investigate this concept, phase images were acquired on four 1.5T clinical scanners and one 3.0 clinical scanner (MAGNETOM Espree, Siemens Medical Systems, Signa Excite HD 1.5T, and 3.0T GE Healthcare Technologies). The manufacturer phantoms were scanned using two echo times (delta_TE=TE2-TE1<5ms) using a simple 2D gradient echo acquisition to produce phase images. After acquiring phase difference images in three orthogonal planes, the standard deviation was calculated in three circular ROIs (Diameter=10, 20 and 30cm) in each plane, respectively, to estimate the MFH for the effective DSV.

The MFH values in five scanners using this method were within vendor specifications for the DSV. Additionally, the measured MFH values compared favorably with vendor planned maintenance records with <0.1 ppm discrepancy.

This proposed method may be a reliable and practical for regular MFH measurement in QA programs and providing an independent check of the vendor measurement.