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Myocardial T1 mapping using SMART1Map: initial in vivo experience
Journal of Cardiovascular Magnetic Resonancevolume 15, Article number: P13 (2013)
Recently a single-point, saturation-recovery myocardial T1 mapping sequence (SMART1Map = Saturation Method using Adaptive Recovery Times for cardiac T1 Mapping) was presented . Compared to common methods like MOLLI, SMART1Map measures true T1 instead of apparent T1 relaxation, is more time efficient and can track exact TI times instead of estimated ones based on the heart rate during prescription. In this work we compare SMART1Map to MOLLI in an initial cohort of volunteers.
Versions of SMART1Map and MOLLI were implemented on a GE MR450w scanner. Four healthy subjects were imaged without contrast agent and with informed consent. Single shot images were acquired at the TI times. SMART1Map acquisitions collected 5 TIs in 9 heart beats at 1,1,1,2 and 4 heart beats after saturation. MOLLI acquisitions collected 8 TIs (2,2 and 4 heartbeats after each inversion) in 14 heart beats. A total of 12 data sets for each sequence were collected. T1 values in an ROI in the mid-septum were calculated. Values were computed with both estimated TI times based on the heart rate at the time of scan prescription and true TI times based on pulse sequence measurements of each cardiac interval during scanning. MOLLI values were computed with and without Look-Locker (LL) correction . SMART1Map values represent true, not apparent, T1 values and thus no correction was applied.
Measured T1 values are summarized in Table 1. The mean T1 from SMART1Map (1193 ms) is consistent with T1 values reported in the literature using single-point methods: T1=1219 ms,  and T1=1175 ms . SMART1Map acquires its two long (>1RR) TI samples in two separate blocks of multiple (2 and 4) heartbeats so it is important to account for the effect of heart rate variations on the actual TI times to achieve robust T1 measurements. This is demonstrated by the difference in T1s when using estimated and actual TIs for SMART1Map as in Table 1. Figure 1 plots heart rate variability over all series in these subjects relative to the fixed rate at scan prescription, illustrating 20% variations in heart rate over a breath hold were seen. MOLLI demonstrates less benefit from the use of actual TI times, resulting in T1 values that remain underestimated relative to the expected T1s. This may be due to a variety of confounding factors including the interleaving of TIs, the influence of differential signal recovery, and the behavior of the LL correction, which is not clearly understood and may obscure the effects of heart rate variation.
SMART1Map is a new T1 mapping method that provides accurate, breath held, true myocardial T1 values for normal myocardium. Compared to literature values, it provides more accurate T1 measurements compared to MOLLI which studies have shown underestimates long T1s.
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