Signal strength on a 0.15-T magnetic resonance imager

Signal strength in magnetic resonance (MR) images made on a 0.15-T imager was calculated for spin-echo (SE) and inversion-recovery with echo (ISE) pulse sequences and was compared to measurements of signal strength for aqueous MnCl2 solutions whose T1 and T2 relaxation times encompassed the range of values commonly found for tissues. Although measured signal strength generally agreed with calculated signal strength (correlation coefficient = 0.996 for both SE and ISE), significant reductions in measured strength (greater than 15%) were sometimes observed. Diffusion alone could not account for this discrepancy. Further experiments showed that imaging gradients reduced the effective T2 estimated from the imager. In addition, SE pulse sequences with short repetition times exhibited a reduction in signal strength.