2021-01-19 MRRC: IMPORTANT: Phase Encoding Direction

January 19, 2021

Dear MR users,

For those who use EPI sequences for BOLD fMRI or DWI/DTI studies, we are making you aware of a Siemens’ intrinsic software problem for a Trio/Prisma 3T glitch where sometimes the phase encoding (PE) direction can switch inadvertently between runs as the anatomy sequence is erroneously copied to the EPI sequence. PE direction (typically, Posterior > Anterior) is important for correction of spatial distortion due to susceptibility artifact in EPI sequences. Unfortunately, there is no solution in the scanner yet, so manual checking of PE direction for all EPI acquisitions is required at this time. MR techs have been made aware of this glitch and they will ensure that this does not happen going forward. As an added assurance, we recommend that your research associate double-check the PE direction parameter with the tech before fMRI or DWI sequences. EPI images can be displayed on the console screen in real time, and the distortion patterns, e.g., stretched or compressed in PE direction can be examined. If the wrong PE direction is evident, rescanning of the EPI after PE correction is warranted. Your past data may have been affected by this problem and we encourage you to keep this in mind when reviewing your data. EPI spatial distortion can be somewhat corrected by open software (e.g., B0 field map or topup) by indicating the PE direction that was acquired in the EPI. Siemens has informed us that they are working on a patch to correct this problem.

Chan Hong Moon, PhD
Technical Director of MRRC
MRRC, Radiology, UPMC
Phone: (412) 647-8459
Email: moonc@upmc.edu