The modified Look-Locker inversion-recovery (MOLLI) sequence is a widely used myocardial T1 mapping technique for tissue characterization. It is known that the MOLLI sequence underestimates myocardial T1 values (>10%) due to magnetization transfer (MT) effects and short myocardial tissue T2 values. In this work, we demonstrate that in vivo reduction of the inversion factor is predominantly responsible for the MT effect that reduces MOLLI T1 values. Thus, using an alternative T1 estimation algorithm with a measured inversion factor for the MOLLI sequence can reduce the MT effect on T1 estimation error.
Methods
The MOLLI pulse sequence was studied on a 1.5T scanner (Avanto Fit, Siemens Healthcare, Erlangen, Germany). In this work, we applied a previous technique, BLESSPC (Bloch equation simulation with slice profile correction) (4), which was originally proposed for spoiled gradient echo readout, to bSSFP-based MOLLI acquisitions. By assuming a fixed T2 = 45 ms, the modified BLESSPC remains a suitable fit with three free parameters (T1, M0, flip angle FA) or four free parameters (T1, M0, FA and inversion factor δ). The “MOLLI+M0” sequence, which acquires an additional proton density weighted image without an inversion pulse 3s following the MOLLI 3(3)3(3)5 acquisition, was implemented for measurement of δ.
T1 estimation using the MOLLI 5(3)3 sequence and the inversion factor estimation using the “MOLLI+M0” sequence was evaluated using simulation, phantoms and in vivo studies. To evaluate the influence of MT on measured inversion factor and T1 estimations, the “MOLLI+M0” sequence and the MOLLI 5(3)3 sequence were simulated at heart rate (HR) = 60 bpm without and with MT. The simulated tissue parameters were the same as that of cardiac muscle used in (3). The “MOLLI+M0” and MOLLI 5(3)3 images were acquired on nine gel phantoms (2%-4% agar) at simulated HR = 60 bpm and on eight healthy volunteers.
BLESSPC four-parameter fitting was applied to the “MOLLI+M0” data to estimate inversion factors. BLESSPC three-parameter fitting was applied to MOLLI 5(3)3 to calculate T1 values using a fixed δ from 0.88 – 1.00 (0.01 increment) in simulations. The measured average δ was used in phantoms and in vivo studies, respectively. For comparison, the T1 values were also calculated using the original MOLLI fitting based on the same data.
The average inversion factor δ measured on 9 phantoms was 0.96±0.01, which is close to the estimated δ value (0.97) based on the simulated non-MT “MOLLI+M0” data. The average δ measured on native myocardial tissues in vivo was 0.88±0.01, which is close to the estimated value (0.89) when MT effects were simulated. Therefore, MT is the major reason for the reduced inversion factor in vivo compared to observations in phantom experiments. As shown in Figure 1, the original fitting will result in >10% additional T1 estimation error due to MT effects. In comparison, using BLESSPC fitting with a measured δ reduced the T1 estimation error to less than 1% in both scenarios similar to phantom (no MT) and in vivo (with MT).
In phantom studies, using BLESSPC fitting with δ = 0.96 reduced the absolute T1 estimation error to 1.3%±0.6% from 7.1%±1.3% (original fitting) for the MOLLI sequence (Figure 2). The average native myocardial T1 value using BLESSPC fitting with δ = 0.88 was 1151.3±22.7 ms, which is close to the previously reported values (1170 ± 9 ms) using the SASHA technique, a method that is less sensitive to MT effects (3). In comparison, the original fit resulted in significantly lower myocardial T1 values (964.4±19.7 ms, relative T1 difference = -16.2%±0.5%, p < 0.001). The T1 estimation difference between the original and BLESSPC fitting in phantoms and in vivo studies agreed well with the simulation results (without MT and with MT effects). Figure 3 shows an example of the myocardial T1 maps generated using the original fitting and BLESSPC fitting for the MOLLI data acquired in a healthy volunteer.
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