Hi Chiara,
ETL could be the guilty part here, but it depends a bit on your acquisition. I'm using topup on data from a GE scanner as well, and have no problems with it so it should work. The thing is, it's not specifically the ETL that's influencing the magnitude of distortions, because the ETL can be shortened by doing partial fourier acquisition which does not influence distortions. The best way I found to calculate the relevant ETL is by taking the phase resolution and divide by the parallel imaging factor along that axis, so for instance 128/2, and then multiply that by the echo spacing. All these things are in the dicom header.
HTH,
Sjoerd
|