Comparison of spoiled gradient echo and steady-state free-precession imaging for native myocardial T1 mapping using the slice-interleaved T1 mapping (STONE) sequence

Abstract

Cardiac T1 mapping allows non-invasive imaging of interstitial diffuse fibrosis. Myocardial T1 is commonly calculated by voxel-wise fitting of the images acquired using balanced steady-state free precession (SSFP) after an inversion pulse. However, SSFP imaging is sensitive to B1 and B0 imperfection, which may result in additional artifacts. A gradient echo (GRE) imaging sequence has been used for myocardial T1 mapping; however, its use has been limited to higher magnetic field to compensate for the lower signal-to-noise ratio (SNR) of GRE versus SSFP imaging. A slice-interleaved T1 mapping (STONE) sequence with SSFP readout (STONE-SSFP) has been recently proposed for native myocardial T1 mapping, which allows longer recovery of magnetization (>8 R-R) after each inversion pulse. In this study, we hypothesize that a longer recovery allows higher SNR and enables native myocardial T1 mapping using STONE with GRE imaging readout (STONE-GRE) at 1.5T. Numerical simulations and phantom and in vivo imaging were performed to compare the performance of STONE-GRE and STONE-SSFP for native myocardial T1 mapping at 1.5T. In numerical simulations, STONE-SSFP shows sensitivity to both T2 and off resonance. Despite the insensitivity of GRE imaging to T2 , STONE-GRE remains sensitive to T2 due to the dependence of the inversion pulse performance on T2 . In the phantom study, STONE-GRE had inferior accuracy and precision and similar repeatability as compared with STONE-SSFP. In in vivo studies, STONE-GRE and STONE-SSFP had similar myocardial native T1 times, precisions, repeatabilities and subjective T1 map qualities. Despite the lower SNR of the GRE imaging readout compared with SSFP, STONE-GRE provides similar native myocardial T1 measurements, precision, repeatability, and subjective image quality when compared with STONE-SSFP at 1.5T.

Keywords: balanced steady-state free precession; cardiovascular MR (CMR) methods; myocardial T1 mapping; relaxometry; slice-interleaved T1 mapping; spoiled gradient echo.

Figure 1

Accuracy of T₁ measurements (i.e. differences between actual and estimated T₁ in ms) obtained in numerical simulations as a function of T₂ times (A) and off-resonance (B) for STONE-SSFP (left column) and STONE-GRE (right column).

Figure 2

Accuracy, precision and repeatability of STONE-GRE and STONE-SSFP obtained in the phantom with 13 vials with different T₁/T₂ values. Accuracy was defined as the difference between the averaged T₁ times over all 15 repetitions and the spin echo T₁ measurements. Precision was defined as the average over all 15 repetitions of the T₁ standard deviation within each vial. Repeatability was defined as the standard deviation over all 15 repetitions of the mean T₁ times within each vial.

Figure 3

Example native T₁ map and corresponding T₁-weighted images obtained in one subject using STONE-GRE (top) and STONE-SSFP (bottom) sequence. Inversion time (TI) of selected T₁-weighted images were 350ms, 1 RR + 350ms, 2 RR + 350ms, 3 RR + 350ms, and Infinity from left to right (RR: the interval time between two R-waves).

Figure 4

Example of native T₁ maps obtained in one subject using the STONE-GRE (top) and the STONE-SSFP (bottom) sequence. T₁ maps are shown for the three mid-ventricular slices and five repetitions of each sequence. All T₁ maps show homogeneous T₁ signal over the entire myocardium, slices, and repetitions.

Figure 5

Measurement, precision, and repeatability of STONE-GRE and STONE-SSFP native myocardial T₁ over all nine healthy subjects using a myocardial segment model based analysis. Precision was defined as the average over the five repeated scans of the standard deviation of T₁ times over each segment. Repeatability was defined as the standard deviation over the five repeated scans of the average T₁ times in each segment. STONE-GRE and STONE-SSFP provided similar measurements (STONE-GRE: 1096±15 ms, STONE-SSFP: 1090±16 ms), precision (STONE-GRE: 61±10 ms, STONE-SSFP: 54±7 ms) and repeatability (STONE-GRE: 24±6 ms, STONE-SSFP: 20±4 ms).

Figure 6

Global myocardial T₁ measurements (top panel), precision (middle panel), and repeatability (bottom panel) of STONE-GRE and STONE-SSFP for each individual subject, measured over the entire five slices. There were no statistical differences between the STONE-GRE and STONE-SSFP sequences in term of measurements (STONE-GRE: 1099±24 ms, STONE-SSFP: 1095±14 ms, p>0.05), precision (STONE-GRE: 69±11 ms, STONE-SSFP: 64±9 ms, p>0.05) and repeatability (STONE-GRE: 22±7 ms, STONE-SSFP: 15±6 ms, p>0.05).

Figure 7

Measurement and repeatability of STONE-GRE and STONE-SSFP native blood T₁ in the right and left ventricle (RV/LV). Repeatability was defined as the standard deviation (over the five repetition scans) of the average T₁ measurements of blood in each slice (5 slices: 1-basal; 2-mid-basal; 3-mid-cavity; 4-mid-apical; 5-apical). STONE-GRE provided similar native blood T₁ measurement (RV: 1537±106 ms vs. 1535±80 ms, p>0.05, LV: 1572±104 ms vs. 1578±85 ms, p>0.05) and inferior repeatability (RV: 64.11±7.78 vs. 26.64±7.04, p=0.008, LV: 55.18±9.15 vs. 23.23±7.08, p=0.004) compared to STONE-SSFP.

Figure 8

Example native T₁ maps obtained with STONE-GRE and STONE-SSFP in patients (patient #1: Non-ischemic cardiomyopathy (NICMP), patient #2: hypertrophic cardiomyopathy (HCM), patient #3: aortic regurgitation (AR)). Both sequences resulted in similar native T₁ time (STONE-GRE: 1107±34 ms vs. STONE-SSFP: 1110±31 ms) and were well correlated (r=0.86, p<0.001).