Sources and correction of higher order geometrical distortion for serial MR brain imaging


from: Proc Society of Photo-Optical Instrument Engineers (SPIE), Bellingham, WA, vol 4322, (February, 2001)

M. Holden, M. Breeuwer, K. McLeish, D. J. Hawkes S. F. Keevil and D. L. G. Hill

Radiological Sciences, GKT, Guy's Hospital, London, SE1 9RT, UK.

e-mail: mark.holden@kcl.ac.uk

Abstract

A specially designed phantom consisting of a 3D array of 427 accurately manufactured spheres together with a point-based registration algorithm was used to detect distortion described by polynomial orders 1-4. More than thirty 3D gradient echo (GRE) and multi-slice spin echo (SE) phantom scans were acquired with a Philips 1.5T Gyroscan ACS2. Distortion was measured as a function of: readout gradient strength (0.72 <= G_{r} <=1.7 mT/m), TR/TE/flip angle, shim settings, and temporal distortion change for 11 weekly scans for the FFE sequence and TR/TE/slice gap for SE. Precision measurements for linear distortion were: scale <= 0.03%, shear <=0.04 degrees. Linear distortion in the readout dependent directions increased with decreased readout strength ( r>0.93 ). There was a significantly higher ( p<0.01 ) sagittal shear for 5 SE scans compared with 5 FFE ones with the same G_{r} - possibly because of slice selection. Different shim settings produced only linear distortion change: up to 2 % scale and 1 degree shear. There was negligible distortion change over time: scale < 0.1 % , shear <= 0.05 degrees. There was a decrease in distortion as a function of polynomial order (r < 0.9, n=33), 75% of the distortion was either first or second order.


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    Mark Holden | Radiological Sciences.
    Email: mark.holden@kcl.ac.uk Last modified: 4 May, 2001.