We have scanned a cuboid phantom several times using spin echo and fast spin echo sequences in different planes and with flipped encoding directions, and we always find that the dimension of the phantom across frequency encoding/readout is larger than phase encoding (1-2%). When we scanned it using another scanner (different vendor) and the same parameters we didn't see this difference. I know it's an issue with our sequences, but what exactly?
Dear Xen,
The most likely cause of the problem is not your sequences, but one or more miscalibrated gradients. Gradients commonly drift over time and need to be recalibrated periodically. A service engineer will know exactly how to do this.
A less likely but possible cause is that your Bo field contains significant inhomogeneities due to improper adjustment of shims, gradient offsets, or presence of a ferromagnetic foreign object. Once again, a service engineer should be consulted who can identify and fix this problem.
ADE
Thanks for your reply.
Actually we did these scans to calibrate the gradients. It's an animal scanner thus QA is pretty scarce (once a year, and not that effective!). Even after we calibrated the coils this difference remained; each of the three gradient coils seems to do the same thing, make the phantom look larger in any of the two directions in the imaging plane where the frequency encoding is set. You might think we have a small bandwidth, but we set a large one (100kHz) to avoid artifacts in frequency encoding but the problem persists.
We will have a look to the other possible causes you mentioned and see how we can get through this.
Xen