Radiotherapy planning of the pelvis using distortion corrected MR images: the removal of system distortions

Image distortion is an important consideration in the use of magnetic reson ance (MR) images for radiotherapy planning. The distortion is a consequence of system distortion (arising from main magnetic field inhomogeneity and n onlinearities in the applied magnetic held gradients) and of effects arisin g From the object/patient being imaged. A two-stage protocol has been devel oped to correct both system- and object-induced distortion in pelvic images which incorporates measures to maintain the quality, accuracy and consiste ncy of the imaging and correction procedures. The first stage of the correc tion procedure is described here and involves the removal of system distort ion. Object- (patient-) induced effects will be described in a subsequent w ork. Images are acquired with the patient lying on a fiat rigid bed, which reproduces treatment conditions. A frame of marker tubes surrounding the pa tient and attached to the bed provides quality assurance data in each image . System distortions in the three orthogonal planes are mapped using a sepa rate phantom, which fits closely within the quality control frame. Software has been written which automates the measurement and checking of the many marker positions which the lest objects generate and which ensures that pat ient data are acquired using a consistent imaging protocol. Results are pre sented which show that the scanner and the phantoms used in measuring disto rtion give highly reproducible results with mean changes of the order of 0. 1 mm between repeated measurements of marker positions in the same imaging session. Effective correction for in-plane components of system distortion is demonstrated.