Optimization of signal‐to‐noise ratio in calculated T1 images derived from two spin‐echo images

Abstract

A simplified model relating signal intensity in an MR image to spin‐lattice relaxation time (T₁), repetition time (T_R), number of signal averages and the average tip angle (α) of the protons within the slice has been developed. This model has been used to select the optimal repetition times of two spin‐echo images for a fixed total imaging time to maximize signal to noise in calculated T₁ images. Theoretical predictions of T₁ are virtually identical to spectroscopically measured values, and the relative noise (δT₁) in T₁ images calculated from two measured spin‐echo images is in good agreement with the theoretically predicted values of δT₁/T₁. This model predicts that: (a) for a T₁ of approximately 500 ms, the least T₁ image noise is obtained with one of the spin‐echo images collected with a T_R of 400–500 ms. The longer the T_R of the other spin‐echo image, the lower the T₁ image noise, but past a T_R of approximately 1400 ms, T₁ image signal/noise is optimized for the same total imaging time by increasing the number of averages in the shorter T_R spin‐echo image rather than increasing the T_R of the second spin‐echo image. (b) The error is reduced and the optimum T_R1 is reduced as α is increased from 63 to 90°. (c) For a range of T₁, optimal selection of T_R1 and T_R2 based on an intermediate value of T₁, results in relatively little increase over optimal values in δT₁/T₁ for the entire T₁ range. © 1986 Academic Press, Inc.