Equilibrium signal intensity mapping, an MRI method for fast mapping of longitudinal relaxation rates and for image enhancement

Abstract Introduction Inhomogeneity of magnetic fields, both B0 and B1 , has been a major challenge in magnetic resonance imaging (MRI). Field inhomogeneity leads to image artifacts and unreliability of signal intensity (SI) measurements. This work proposes and shows the feasibility of generating equilibrium signal intensity (SIEq ) maps that can be utilized either to speed up relaxation-rate measurement or to enhance image quality and relaxation-rate-based weighting in various applications. Methods A 1.5-T MRI scanner was used. In canines ( n =4), myocardial infarction was induced, and 48 h after the administration of 0.05 mmol kg−1 Gd(ABE-DTTA), a contrast agent with slow tissue kinetics, in vivo R1 mapping was carried out using an inversion recovery (IR)-prepared, fast gradient-echo sequence with varying inversion times (TIs). To test the SIEq mapping method without the confounding effects of motion and blood flow, we carried out ex vivo R1 mapping after the administration of 0.2 mmol kg−1 Gd(DTPA) using an IR-prepared, fast spin-echo sequence in another group of dogs ( n =2). R1,full maps and SIEq maps were generated from the data from both sequences by three-parameter nonlinear curve fitting of the SI versus TI dependence. R1,full maps served as the reference standard. Raw IR images were then divided by the SIEq maps, yielding corrected SI maps (COSIMs). Additionally, R1 values were calculated from each single-TI image separately, using the SIEq value and a one-parameter curve-fitting procedure ( R1,single ). Voxelwise correlation analysis was carried out for the COSIMs and the R1,single maps, both versus the standard R1,full maps. Deviations of R1,single from R1,full were statistically evaluated. Results In vivo, COSIM versus R1,full showed significantly ( P