Fransson A, Ericsson A, Sperber G O
Department of Radiation Physics, Karolinska Institutet, Stockholm, Sweden.
Magn Reson Imaging. 1993;11(2):197-205. doi: 10.1016/0730-725x(93)90024-8.
Transverse relaxation times were estimated from numerical simulations on spin systems using multi-echo spin-echo MRI protocols. The influence of T1 on the echo amplitudes via stimulated echo components was studied. The resulting effects on T2 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 T1 of the T2 estimates was notable for T1 values below approximately 800 msec for all protocols. The PHAPS scheme provided rather accurate, but underestimated, T2 values when selective refocusing was used. With nonselective refocusing, PHAPS T2 values were overestimated and demonstrated a pronounced dependence on magnetic field inhomogeneity. In general, long T2 values were erroneous with the PHAPS protocol. The results indicate that a CPMG protocol structure provides a more robust method for T2 estimations than the PHAPS protocol.
横向弛豫时间是通过使用多回波自旋回波MRI协议对自旋系统进行数值模拟来估计的。研究了T1通过受激回波成分对回波幅度的影响。研究了其对Carr-Purcell（CP）、Carr-Purcell-Meiboom-Gill（CPMG）和相位交替相移（PHAPS；CP和CPMG的组合）等多回波方案的T2估计值的影响。研究了使用空间选择性或非选择性重聚焦脉冲的协议。模拟了体素内0.1、1和10 ppm的静磁场不均匀性。对于所有协议，当T1值低于约800毫秒时，T2估计值对T1的依赖性很明显。当使用选择性重聚焦时，PHAPS方案提供了相当准确但被低估的T2值。使用非选择性重聚焦时，PHAPS的T2值被高估，并且对磁场不均匀性有明显的依赖性。一般来说，PHAPS协议的长T2值是错误的。结果表明，与PHAPS协议相比，CPMG协议结构为T2估计提供了一种更可靠的方法。
