A simplified model relating signal intensity in an MR image to spin-lattice relaxation time (T1), repetition time (TR), number of signal averages and the average tip angle (-alpha) 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 (delta T1) in T1 images calculated from two measured spin-echo images is in good agreement with the theoretically predicted values of delta T1/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 TR of 400-500 ms. The longer the TR of the other spin-echo image, the lower the T1 image noise, but past a TR 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 TR spin-echo image rather than increasing the TR of the second spin-echo image. (b) The error is reduced and the optimum TR1 is reduced as -alpha is increased from 63 to 90 degrees. (c) For a range of T1, optimal selection of TR1 and TR2 based on an intermediate value of T1, results in relatively little increase over optimal values in delta T1/T1 for the entire T1 range.
Normally, cardiac triggering in MR spin-echo imaging restricts repetition times (TR) to integral values of the cardiac period (TC), and introduces irregularities in TR due to variations in TC. We have investigated how much these restrictions decrease the accuracy and precision of spin-lattice relaxation (T1) values of the myocardium calculated from two cardiac-triggered spin-echo images. By introducing additional excitation pulses, TR can effectively be reduced to a fractional value of TC and considerable improvement in T1 precision is possible. For TC = 800 ms, the improvement in T1 precision is 30% when two spin-echo images of TR = 1/2 X TC and TR = 2 X TC are used to calculate T1 instead of two images with TR = TC and TR = 2 X TC. The irregularities in TR decrease both T1 precision and accuracy. Irregularities of the order of 15% in a mean TR of 800 ms produce a fourfold decrease in precision. Since irregularities in TC easily exceed 15%, MRI data should be acquired when individual TR values are approximately within +/- 15% of the subject's mean TC.
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