Abstract
Transverse relaxation times were estimated from numerical simulations on spin systems using multi-echo spinecho MRI protocols. The influence of T 1 on the echo amplitudes via stimulated echo components was studied. The resulting effects on T 2 estimates from the Carr-Purcell (CP), Carr-Purcell-Meiboom-Gill (CPMG), and Phase-Alternating-Phase-Shift (PHAPS; combination of CP and CPMG), multiple echo schemes were examined. Protocols with either spatially selective or nonselective refocusing pulses were studied. An intravoxel static field inhomogeneity of 0.1, 1, and 10 ppm was stimulated. The dependence on T 1 of the T 2 estimates was notable for T 1 values below approximately 800 msec for all protocols. The PHAPS scheme provided rather accurate, but underestimated, T 2 values when selective refocusing was used. With nonselective refocusing, PHAPS T 2 values were overestimated and demonstrated a pronounced dependence on magnetic field inhomogeneity. In general, long T 2 values were erroneous with the PHAPS protocol. The results indicate that a CPMG protocol structure provides a more robust method for T 2 estimations than the PHAPS protocol.
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