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.