**4. T1 mapping with Modified Look-Locker Inversion "MOLLI" recovery**

Currently, the most evaluated sequence for myocardium T1 mapping is a modified Look‐ Locker inversion recovery (MOLLI) sequence [13, 14]. The T1 mapping identifies a significant variation between normal and abnormal myocardium. It demonstrates the myocardial fibrosis among different myocardial disorders include ischemia [15], acute/chronic infraction [16], amyloidosis [17], diabetic [18], dilated and hypertrophic cardiomyopathy [19], and heart failure [8].

MOLLI is a CMR pulse sequence that is used for accurate T1 mapping of myocardium with high spatial resolution. MOLLI is an ECG‐gated pulse sequence scheme and uses three prepared Look–Locker experiments consecutively within one breath‐hold over 17 heartbeats to reconstruct 11 images with different inversion times. Three successive ECG‐triggered LL experiments (LL**1**, LL**2**, and LL**3**) are carried out with three, three, and five single‐shot readouts, respectively, at end diastole of consecutive heartbeats to sample the recovery of longitudinal magnetization after the inversion pulse. MOLLI pulse sequence scheme is illustrated in **Figure 1**. T1 maps can be generated any time before or after contrast agent (e.g., gadolinium) administration [12].

**Figure 1. T1 map of a healthy volunteer**: Using 17 heartbeats to reconstruct 11 images with different inversion times at end of diastole phase. By merging these images into one data set, T1 values are computed for every pixel with three parameters curve fitting. A reconstructed T1 map with parametric color scale is produced for these pixel values and the segmental and global T1 times can be estimated.

The MOLLI sequence has been described, optimized, tested, and retested in phantoms and in large cohorts of healthy volunteers [12, 14] as well as being applied in cardiomyopathies [8, 15, 17, 19, 20]. In addition, the T1 mapping with MOLLI has been validated against histopa‐ thology for assessment of myocardial fibrosis. It demonstrated that the precontrast "native T1" has a linear correlation with the percentage of myocardial fibrosis as measured histologically on invasive myocardial biopsy. T1 times postcontrast administration (10–15 min) had an inverse linear relationship with collagen content in myocardial fibrosis subjects [8, 21, 22].


T1 mapping can be generated for different segments of the myocardium (base, mid‐cavity, and apex) within a single breath‐hold of about 15–20 s. However, the apex T1 values with MOLLI are slightly higher than basal and mid‐cavity. The increasing in T1 values may be caused by partial volume effect and some degree of overestimation effect in apical level of left ventricle [23–25].

T1 mapping with MOLLI has a greater reproducibility, accuracy, and an excellent overall inter‐ and intra‐observer agreement over a wide range of TIs as compared with the traditional LL sequence [13, 14].

However, the T1 mapping with MOLLI sequence is sensitive to extremes of heart rate (brady‐ cardia or tachycardia) [14] leading to a slight underestimation of T1 values. This may be corrected though *heart rate correction* by changing the timing of the readouts with respect to the inversion pulses at different heart rates.

Moreover, MOLLI is also limited by long breath‐hold for about 15–20 s (17 heartbeats to acquire the final T1 maps). This may be difficult for elderly and pulmonary compromised patients and generates respiratory and motion artifacts [26]. Modern in‐line processing provides registration tools to reduce motion artifacts before the computation of final T1 maps (motion‐corrected or "MoCo MOLLI") [27]. A shortened Modified Look‐Locker inversion recovery (*shMOLLI*) with shorter breath‐holds has been validated and recently applied for cardiomyopathies [28, 29].
