Jiaxin Shao^{1}, Kim-Lien Nguyen^{2}, and Peng Hu^{1}

Good T1 estimation accuracy and precision is important for clinical application of the myocardial T1 mapping technique. Although the widely used T1 mapping technique, modified Look-Locker

The MOLLI pulse sequence was implemented on a 1.5T MRI scanner (Avanto Fit Siemens Healthcare; Erlangen, Germany). The 50° flip angle MOLLI with BLESSPC fit was compared to the 35° flip angle MOLLI with the original fit. The original fit refers to the conventional MOLLI T1 estimation algorithm with inversion factor correction (9). Researchers have proposed to use BLESSPC T1 estimation for the FLASH-MOLLI sequence (10). In the current work, we extend the BLESSPC algorithm to calculate T1 values based on the bSSFP- MOLLI sequence to achieve higher SNR. The signal evolution of bSSFP readouts are simulated using the Bloch equation. By assuming a fixed T2 = 45 ms, the modified BLESSPC remains a suitable fit with three parameters M0, T1 and FA.

Nine gel phantoms were used. Reference T1 for each gel phantom was determined by a standard inversion-recovery spin echo technique. MOLLI sequences using FA=35° and 50° were performed at a simulated heart rate of 60 bpm. The two sequences were repeated five times to evaluate T1 estimation precision. In eight healthy volunteers, images of the mid-left ventricular (mid-LV) short-axis were acquired using the same MOLLI sequences at FA=35° and 50°, which were repeated once to evaluate the intra-scan reproducibility. For each MOLLI scan, two T1 maps were generated using the original fit and the BLESSPC fit. The same inversion factor δ = 0.96 (9) was used in both phantom and in vivo studies. While the main focus is to compare the 50° MOLLI with BLESSPC fit to the 35° MOLLI with the original fit. The other two T1 maps (FA=35° using BLESSPC fit and FA=50° using original fit) were generated to demonstrate that the BLESSPC fit is more robust than FA variations when compared to the original fit. In vivo reproducibility was calculated as the absolute difference in the ROI-averaged T1 divided by the average of the repeated scans.

**Results**

Figure 1 and 2 shows comparative phantom examples of T1 estimation accuracy and precision using the proposed technique (50° MOLLI with BLESSPC fit) vs. the standard MOLLI (35°, with the original fit). Compared to the standard MOLLI, the proposed technique provided better accuracy (0.5%±1.6% vs. -3.2%±1.4%) and precision (0.58%±0.07% vs. 0.77%±0.33%) for T1 values ranging from 218 ms - 1884 ms. Figure 1 also shows that the original fit produced larger T1 underestimation at higher flip angle while the BLESSPC fit with MOLLI generated similar T1 estimation errors at FA = 35° and 50°.

Based on native myocardial T1 values from the interventricular septal region of eight volunteers, the proposed technique generated significantly higher T1 values compared to standard MOLLI (1052±26 ms vs. 1005±21 ms, relative 4.7%±1.3%, p<0.001). Native myocardial T1 maps obtained in one healthy volunteer using the two techniques and their corresponding selected ROIs are shown in Figure 3. In this volunteer, the proposed technique generated higher native myocardial T1 values than the standard MOLLI (1040±24ms vs 1007±34 ms). The intra-scan reproducibility of the proposed technique trended to be better than the standard MOLLI technique (0.52%±0.36% vs. 0.96%±0.68%, p = 0.06). In all volunteers, the myocardial T1 value was lower using MOLLI at a higher flip angle and with the original fit (971±23 ms for FA=50°, p < 0.001), whereas the BLESSPC fit was insensitive to flip angle changes (1046±20 ms for FA=35°, p= 0.2).

**Conclusion**

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Figure 1: Phantom studies to assess the accuracy of T1 estimation using the original fit (with inversion factor correction) and BLESSPC fit for MOLLI 5(3)3 acquisitions at FA = 35° and 50°. The original fit produced larger T1 underestimation at higher flip angle while BLESSPC generated similar T1 estimation error at two different flip angles. The proposed technique (FA = 50° with BLESSPC fit) was more accurate than standard MOLLI (FA=35° with the original fit).

Figure
2: Phantom studies to compare the precision of T1 estimation between the proposed
technique (FA=50° MOLLI with BLESSPC fit) and the standard MOLLI technique
(FA=35° with the original fit,
including inversion factor correction). The proposed technique generated more precise T1
estimation for T1 values ranging from 218 ms to 1441 ms.

Figure
3: Myocardial T1 maps from
a healthy volunteer, which
were generated using the standard MOLLI sequence (FA=35 with the original fit,
including inversion factor correction) and the proposed technique (FA=50° MOLLI with BLESSPC fit). Regions
of interest (ROI) was manually selected in the septal region to quantify the
average myocardial T1 value. The proposed technique generated higher native
myocardial T1 values than the standard MOLLI (1040±24ms vs 1007±34 ms), which correlated
well with observations from phantom studies.