PRECISION, ACCURACY, AND IMAGE PLANE UNIFORMITY IN NMR RELAXATION-TIME IMAGING ON A 1.5-T WHOLE-BODY MR-IMAGING SYSTEM

In order to evaluate the accuracy, precision and image plane uniformit y a phantom study was undertaken on a 1.5 T whole-body MR imaging syst em (Philips Gyroscan S15 HP). The manufacturers algorithm for obtainin g relaxation time images was used. This method uses Ratios and Least S quares (RLSQ-algorithm) on data obtained from a combination of a multi -echo (CPMG) and an inversion recovery pulse sequence, yielding a calc ulated T-1-, T-2- and a proton density image simultaneously. T-1 and T -2 accuracy was measured in phantoms consisting of CuSO4 and MnCl2 in aqueous solution in different concentrations. The results were compare d to a reference T-1 and T-2 obtained by spectroscopy. The accuracy er ror over the entire relaxation time range was expressed as the root me an square of the modulus of the difference between the reference and t he image measurement and was 2.3% for T-1 and 4.1% for T-2. Precision or reproducibility was measured by II double estimations on 11 samples . The precision error was 0.35% for both T-1 and T-2. Image plane unif ormity, i.e., the homogeneity of pixel values throughout the image pla ne, was measured by 3 consecutive image readings from a phantom of 19 cm in diam. in 8 directions 45 degrees apart and 1, 3, 5, 7, 9 cm from the center. Errors in the imaging plane were expressed as the deviati on from the center value. For both T-1 and T-2 the maximum deviation w as less than 5% over the entire image in the transverse plane. It is c oncluded that when the pulse sequence timings are carefully optimized, the mixed imaging sequence in combination with the RLSQ algorithm use d in this MRI system is a reliable and precise means of obtaining rela xation time data.