The Effect of Heart Rate in Look-Locker Cardiac T1 Mapping

Introduction: Look-Locker inversion recovery produces images at multiple inversion times (TI) after a 180° inversion pulse (1). In non-cardiac and phantom experiments, the sequence timing can be carefully controlled. This allows the user to exactly specify both the TIs and the magnetization recovery time between inversions. In cardiac imaging, however, these timing variables are measured in units of the R-R interval (TRR), which in routine practice is determined by the patient’s heart rate as recorded only at the beginning of the scan. Because the heart rate can vary considerably during imaging, this heart rate dependency can result in significant errors in T1 measurements. For example, when using inversion pulses with segmented imaging, there must be a delay time Trec of up to 5 × T1 before each subsequent inversion to allow the magnetization to fully recover. If the heart rate increases, the shortened Trec will not allow full and consistent magnetization recovery. Furthermore, if there is any variation in TRR during a scan, the TIs will not occur at the nominally expected times of n × TRR, where n is heartbeat number. Both of these situations will produce errors in fitting the relaxation curve and thus in calculating T1. The goal of this work was to develop a Look-Locker sequence that will reduce the errors introduced by intrascan heart rate variations.