Mechanisms of Contrast in NMR Imaging

TLDR

NMR pixel intensity and contrast‑to‑noise have been plotted for CNS tissues, assuming signal proportional to macroscopic transverse spin magnetization. The study aims to present image intensity that removes dependence on pulse‑timing and sequence parameters. T1, T2, and spin density were measured by chi‑square minimization and partial‑saturation scans, using pulse sequences such as partial saturation, saturation recovery, spin echo, and CPMG. T2 images can be obtained from CPMG scans quickly, but T1 and spin density require longer acquisition times.

Abstract

Nuclear magnetic resonance pixel intensity and contrast-to-noise has been computed and presented in graphical form for various tissues in the normal central nervous system, on the assumption that the signal intensity is proportional to the macroscopic transverse spin magnetization at the time of detection. T1, T2, and spin density values were experimentally determined using chi-square minimization techniques. Additionally, spin density was derived from partial saturation scans obtained with a long repetition time compared with the spin-lattice relaxation time. Pulse sequences discussed comprise partial saturation, saturation recovery, spin echo, and Carr- Purcell - Meiboom -Gill ( CPMG ). The complicated dependence of signal and contrast-to-noise on the pulse timing parameters and the specific pulse sequence makes it appear desirable to display image intensity so that the dependence on the extrinsic (operator-selectable parameter) is eliminated. Whereas T2 images can be derived from CPMG scans without excessive time penalty, this is not the case for T1 and spin density.