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Comparison of ECV measurements during equilibrium between IR- and SR-based Cardiac T1Mapping
Journal of Cardiovascular Magnetic Resonancevolume 16, Article number: P53 (2014)
Cardiac T1 and extracellular volume fraction (ECV), derived from pre- and post-contrast cardiac and blood T1 measurements, are emerging imaging biomarkers of diffuse cardiac fibrosis. The most frequently used cardiac T1 mapping pulse sequence is MOLLI . However, MOLLI is known to be sensitive to rapid heart rate and irregular rhythm, because it is based on inversion-recovery (IR) of magnetization preparation. In response, we developed an arrhythmia-insensitive-rapid (AIR) cardiac T1 mapping pulse sequence based on B1-insensitive saturation-recovery (SR) of magnetization preparation . Our prior study  showed that AIR (scan time = 2-3 heart beats) is faster and yields more accurate cardiac T1 measurements than MOLLI (scan time = 17 heart beats). We sought to compare ECV measurements between SR-based AIR and IR-based MOLLI cardiac T1 mapping at 3T.
Sixteen mongrel dogs with normal myocardium were imaged at 3T (Verio, Siemens). Cardiac T1 maps were acquired in a mid-ventricular short-axis plane using both AIR and MOLLI cardiac T1 mapping at baseline and during equilibrium of Gd-BOPTA (Multihance; 30 min after slow infusion at 0.002 mmol/kg/min). Note that equilibrium ensures identical concentration of Gd-BOPTA for a fair comparison of cardiac and blood T1 measured by two different pulse sequences. Both AIR and MOLLI acquisitions with b-SSFP readout were performed with the following relevant imaging parameters: spatial resolution = 1.4 × 1.4 × 7.0 mm, temporal resolution = 217 ms, flip angle = 35°, and SR time = 600 ms. The AIR acquisition was performed with "paired" consecutive phase-encoding steps in centric k-space ordering to minimize image artifacts due to eddy currents. Blood samples were drawn during MRI for hematocrit calculation. AIR and MOLLI cardiac T1 maps were manually segmented to calculate the myocardial and blood T1 values and subsequently ECV=(1-hematocrit)x(ΔR1, myocardium/ΔR1, blood), where R1 is T1-1, and Δ is the difference between post- and pre-contrast. Paired-wise t-test and Bland-Altman analyses were performed to compare the results.
Figure 1 shows representative AIR and MOLLI cardiac T1 maps which exhibit similarly high image quality. In the 16 dogs studied (mean heart rate = 100 ± 19 BPM), compared with MOLLI, AIR yielded higher T1 measurements (mean difference = 185 ms; p < 0.0001) and lower ECV measurements (mean difference = -0.018; p < 0.0001).
Our study suggests that MOLLI and AIR cardiac T1 mapping pulse sequences yield significantly different T1 and ECV measurements. ECV measurements derived from SR-based AIR and IR-based MOLLI cardiac T1 mapping pulse sequences may need to be adjusted for comparison
Ben B. and Iris M. Margolis Foundation.
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