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Measurement of Spin‐Lattice Relaxation Times in Double Spin‐Echo Imaging

Calculation of tissue T1 in double spin‐echo imaging (90°–T‐180°–2T‐180°) using two repetition times (b1 and b2) has entailed an approximation that ignores the two 180° pulses. The theoretical consequence of the simplification is to overestimate T1 with a fractional error that increases with increasing T1 faster than linearly in a manner dependent on the sequence parameters. With b1, b2, and T as parameters, the theory gives a family of curves relating T1 to the predicted ratio of signals acquired at b2 versus b1 intervals. Tissue T1 can be determined from the observed signal ratio with no approximation. Consideration of noise effect on the precision of T1 so determined favors the use, within limits, of higher b2/b1 ratio for the same b1 + b2 time. At constant b2/b1, using longer b1 + b2 permits wider range of T1 to be measurable with a specified minimum precision.