  We investigated the repeatability of image quality metrics such as SNR, image
uniformity, and geometrical distortion at 0.05T over ten days and three
sessions per day. The measurements included temperature, humidity, transmit
frequency, off-resonance maps, and 3D turbo spin echo (TSE) images of an in
vitro phantom. This resulted in a protocol with nine pulse sequences. We also
acquired a 3T data set for reference. The image quality metrics included
computing SNR, image non-uniformity, and eccentricity (to assess geometrical
distortion) to investigate the repeatability of 0.05T image quality. The image
reconstruction included drift correction, k-space filtering, and off-resonance
correction. We computed the coefficient of variation (CV) of the experimental
parameters and the resulting image quality metrics to assess repeatability. The
range of temperature measured during the study was within 1.50C. The
off-resonance maps acquired before and after the 3D TSE showed similar hotspots
and changed mainly by a global constant. The SNR measurements were highly
repeatable across sessions and over the ten days, quantified by a CV of 4.9%.
The magnetic field inhomogeneity effects quantified by eccentricity showed a CV
of 13.7% but less than 5.1% in two of the three sessions over ten days. The use
of conjugate phase reconstruction mitigated geometrical distortion artifacts.
The repeatability of image uniformity was moderate at 10.6%, with two of three
sessions resulting in a CV of less than 7.8%. Temperature and humidity did not
significantly affect SNR and mean frequency drift within the ranges of these
environmental factors investigated. We found that humidity and temperature in
the range investigated did not impact SNR and frequency. Our findings indicate
high repeatability for SNR and magnetic field homogeneity; and moderate
repeatability for image uniformity.
