EPI distortion and the N/2 ghost on DWI
Diffusion images stretch near air and can show a half-FOV ghost. These are the causes and the fixes.
EPI distortion and N/2 ghost
Stretched or compressed anatomy and signal pile-up near the skull base, sinuses and rectal gas on DWI; or a copy of the image shifted by half the FOV.
EPI has a very low bandwidth in the phase direction, so small field errors cause large shifts. The N/2 ghost comes from timing differences between odd and even echoes.
- Parallel imaging to shorten the echo train
- Multi-shot or readout-segmented DWI (MUSE / IRIS / RESOLVE)
- Reduced-FOV DWI (FOCUS / Zoom DWI / ZOOMit)
- Good shim; empty the rectum before prostate DWI
- For N/2 ghost: repeat the prescan; if persistent, call service
Multi-shot methods take two to four times longer. Parallel imaging lowers SNR.
This page is one entry from the MRI Technologist Reference
The full reference has 31 protocols, 9 dynamic study timelines, 19 artifacts, 16 scanner problems and 15 safety topics, as a PDF in English and Arabic plus an interactive tool.
Educational reference. It does not replace your site protocols, the radiologist's instructions, the MR safety officer, or the manufacturer's instructions for any implant, device or contrast agent. Parameter values are typical starting points at 1.5 T and must be adapted to the scanner, coil and patient.