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During the installation, a new system is calibrated to satisfy gradient distortions according to the ACR standards. Deviations of more than 2 mm over the 20 cm phantom are considered unacceptable. However, it is not uncommon for a system to drift away from initial calibration. Service engineers do not routinely check the linearity of the gradients and I've seen some systems that deviated as much as 4-6 mm on a given axis. I think it is indeed very prudent for every scientific site to have a quality control program in place. It involves a very short ~5 min daily scan with an inexpensive ACR phantom. A faculty of our program, Geoff Clarke has a very nice power point presentation on what involves http://www.docstoc.com/docs/525161/Quality-Control-and-the-ACRs-MRI-Accreditation-Program cheers pk ________________________________ From: FSL - FMRIB's Software Library on behalf of Scott Kolbe Sent: Tue 4/7/2009 5:35 PM To: [log in to unmask] Subject: Re: [FSL] T1 Structural Distortion I'm sure I heard that the Siemens Trio TIM has very close to linear gradients. can anyone confirm this? scott Lars Tjelta Westlye wrote: > Hi Matt, > > I'm not sure what kind of distortions you are thinking about, but you may > want to check out this tool for correcting geometrical distortions due to > gradient non-linearity, which at least works great with our avanto mprages > (though the distortions are small). See ref below. > > http://www.nbirn.net/tools/gradient_non_linearity/index.shtm > > Cheers > Lars > > > Jorge Jovicich, Silvester Czanner; Douglas Greve; Elizabeth Haley; Andre > van der Kouwe; Randy Gollub; David Kennedy; Franz Schmitt; Gregory Brown; > James MacFall; Bruce Fischl; Anders Dale. Reliability in Multi-Site > Structural MRI Studies: Effects of Gradient Non-linearity Correction on > Phantom and Human Data, Neuroimage. 2006 Apr 1;30(2):436-43. > > > > >>> On Mon, Apr 6, 2009 at 8:26 PM, Matt Glasser <[log in to unmask]> wrote: >>> >>>> Hi, >>>> >>>> I was recently told that typical T1 images (e.g. MPRAGE) have some >>>> distortion. Having previously thought that they are essentially >>>> undistorted, >>>> I am curious of the magnitude of any distortion in typical structural >>>> images, >>>> and the mechanism behind it if it does indeed exist. >>>> >>>> Thanks, >>>> >>>> Matt. >>>> -- ======================== Scott Kolbe Neuroimaging Group Florey Neuroscience Institutes and Centre for Neuroscience University of Melbourne VIC, Australia, 3010. ph: +61 3 8344 1887 email: [log in to unmask] website: www.neuroimaging.org.au/index.php?id=383