In order to quantify the degree of image distortion due to gradient non-linearity, the Morphometry BIRN has constructed a phantom that is available for use at other sites. The phantom is cylindrical (250 mm length x 220 mm diameter) and consists of 25, 10-mm thick, regularly spaced, plastic plates. Each plate had 3 mm diameter holes drilled into it which were arranged into a rectangular grid. The holes were spaced at 10 +/- 0.05 mm intervals.
In order to form larger cavities, the holes were enlarged with a half-spherical depression of 7 mm in diameter on each side of the plate. Therefore, when the plates were glued together (with the 3 mm holes aligned), the phantom formed a three dimensional grid of 7 mm diameter spheres that could be filled with a fluid (water doped with salt). More information can be found in the paper:
Reliability in multi-site structural MRI studies: effects of gradient non-linearity correction on phantom and human data.Jovicich J, Czanner S, Greve D, Haley E, van der Kouwe A, Gollub R, Kennedy D, Schmitt F, Brown G, Macfall J, Fischl B, Dale A. Neuroimage. 2006 Apr 1;30(2):436-43.
Geometric Distortion Phantom
Notes about the phantom scan: Phantom Dimensions (cylindrical):
- External Diameter = 26cm
- Length = 33cm
- This phantom should be scanned after each gradient-hardware-related upgrade or change.
- If the phantom does not fit in your bird-cage RF coil you may still get the necessary scans by using the system’s whole-body RF coil.
- If the phantom does not fit in your non-standard gradient-insert, then this phantom cannot be used to calibrate your system. Please check the dimensions of your head-coil or head rest tray in advance so that we don’t ship you the phantom if it will not fit!
- For the distortion correction MGH is currently using the magnetic gradient’s true-field spherical harmonics as provided by the vendor. We’ll need to know the model of your gradient (maybe we already have the corresponding harmonics . . .), or your help for providing us with the needed information.
- Please suggest a contact person from your site for such issues.
Phantom Acquisition Protocol
- Sequence: 3D spoiled gradient echo (FLASH type)
- Slice orientation: Sagittal
- No partial Fourier
- FOV= 45 cm
- Phase matrix=192
- Readout matrix=256
- Number of Slices: 124 (no skip, whole phantom coverage, avoid wrap around)
- Slice Thickness: 2.0 mm
- TE= minimum allowed with centered (symmetric) echo.
- TR=20 ms
- Slab Selection: No (i.e., hard RF pulses)
- scan 1: flip angle 30 degrees without vendor’s distortion correction
- scan 2: flip angle 30 degrees with vendor’s distortion correction (if available)
- Acquisition time per scan: about 8 min 12 sec
Or, use your standard MP-RAGE / SPGR sequence normally used for structural scans with a larger (40 – 45 cm) FOV.
For further information on the phantom please contact:
Karl Helmer, mBIRN Project Manager
helmer at nmr.mgh.harvard.edu