No abstract

Image distortion is an important consideration in the use of magnetic resonance (MR) images for radiotherapy planning. The distortion is a consequence of system distortion (arising from main magnetic field inhomogeneity and nonlinearities in the applied magnetic field gradients) and of effects arising from the object/patient being imaged. A two stage protocol has been developed to correct both system and object-induced distortion in pelvic images which incorporates measures to maintain the quality, accuracy and consistency of the imaging and correction procedures. The first stage of the correction procedure is described here and involves the removal of system distortion. Object- (patient-) induced effects will be described in a subsequent work. Images are acquired with the patient lying on a flat rigid bed, which reproduces treatment conditions. A frame of marker tubes surrounding the patient and attached to the bed provides quality assurance data in each image. System distortions in the three orthogonal planes are mapped using a separate 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 test objects generate and which ensures that patient data are acquired using a consistent imaging protocol. Results are presented which show that the scanner and the phantoms used in measuring distortion 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.

This feasibility study was performed to evaluate the suitability of MRI in defining appropriate pelvic radiotherapy treatment volumes, and to compare MRI sequences with CT for prostate cancer radiotherapy. Five patients with localized prostate cancer, imaged with four MRI sequences (spin echo (SE) T1, turbo SE (TSE) T2, high resolution TSE (HR) T2, and FLASH 3D (F3D)), compared with their corresponding CT planning scans. Segmentation ability of the following pelvic structures: prostatic apex (PA), prostate, rectum, bladder and seminal vesicles (SV), were evaluated by three independent observers. They used a five point grading scale based on the anatomical definition of the organ boundary, tissue contrast and multiplanar display. Results were averaged for the group and for each sequence. There was no significant interobserver variation in the assessed scores (p > 0.1). The average scores (+/- 1 SD) for all pelvic structures assessed by each imaging sequence were CT 1.3 +/- 0.6; SE T1 2.4 +/- 0.9; TSE T2 2.4 +/- 0.7; HR T2 2.2 +/- 0.7 and F3D 3.4 +/- 0.6. Compared with CT, the average MR score for each assessed pelvic structure was higher with a trend for all transaxial MR sequences to provide improved segmentation of the PA and rectum. The F3D sequence scored highest as it provided multiplanar views and avoided the problem of partial volume averaging. MRI, compared with CT, appears to provide improved definition of pelvic treatment volumes but further work is required to confirm this and to address the issues of MRI associated distortion and dosimetry before MRI can be used routinely for pelvic radiotherapy planning.

Microwave spectra for eight isotopic species of cis and trans nitroils acids are reported. The planarity of both isomers has been accurately established on the basis of the inertial defects. Accurate structures, electric dipole moments and quadrupole coupling constants at nitrogen have been determined for both isomers. The structure of cis nitrous acid is found to be :104.0 and L O N O = 113.6", and for trans nitrous acid: 0-H = 0.958, N--O(H) = 1.432, N-0 = 1.170& LNOH = 102.1 and L O N O = 110.7". Stark effect measurements yield the following values for the components of the dipole moment for cis nitrous acid : pu = 0,306, IJ-6 = 1.389 and the total dipole moment p = 1.423 rf0.005 D. Previous investigations of the Stark effect in the trans isomer have been refined to give vu = 1.378, pb= 1.242 and p = 1.855f0.016D. Orientation of the dipole moment has been established for both isomers from isotopic measurements, despite a moderately large vibrational change in pu for trans DN02. Quadrupole coupling constants in the principal inertial axis system have been determined for cis nitrous acid to be : xaU = 2.05, Xbb = -5.83 and xcc = 3.78 MHz, and for trans nitrous acid: Xaa = 1.73, Xbb = -5.28 and xcc = 3.55 MHz. hypothetical Av *

Centrifugal distortion shifts in the microwave spectra of cis and trans nitrous acids and their deuterated species have been analyzed using the first-order treatments of Hill and Edwards, and Watson. The two methods are in close agreement for the ground vibrational state of planar molecules. Centrifugal distortion constants thus derived have been used, together with the infra-red fundamentals and their isotopic shifts, to determine the quadratic potential function for each isomer.Eleven out of a possible sixteen force constants have been determined as follows (in mdyn A-') : for cis nitrous acid, fi ,(OH) = 6.55, fz2(N=O) = 11.18, fss(N-0) = 2.73, f44(NOH) = 0.88, f55(ONO) = 2.46, f 2 3 = 1.80, fZ4 = 0.22, f34 = 0.53, f 3 5 = 0.35, f45 = 0.18 and fs6(torsion) = 0.167; and for trans nitrous acid, f l l = 7.18, f z 2 = 12.28, f 3 3 = 2.22, f44 = 0.76, fS5 = 2.45,