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

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) <= 1.7mT/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, possibly because of slice selection. Different shim settings produced only linear distortion change: up to 2 % scale kand 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 = 3), 75% of the distortion was either first or second order.