Optimization and comparison of myocardial T1 techniques at 3T in patients with aortic stenosis

Abstract

Aims: To determine the optimal T1 mapping approach to assess myocardial fibrosis at 3T.

Methods and results: T1 mapping was performed at 3T using the modified look-locker-inversion sequence in 20 healthy volunteers and 20 patients with aortic stenosis (AS). Pre- and post-contrast myocardial T1, the partition coefficient (λ; ΔRmyocardium/ΔRblood, where ΔR = 1/post-contrast T1 - 1/pre-contrast T1), and extracellular volume fraction [ECV; λ (1 - haematocrit)] were assessed. After establishing the optimal time point and myocardial region for analysis, we compared the reproducibility of these T1 measures and their ability to differentiate asymptomatic patients with AS from healthy volunteers. There was no segmental variation across the ventricle in any of the T1 measures evaluated. λ and ECV did not vary with time, while post-contrast T1 was relatively constant between 15 and 30 min. Thus, mid-cavity myocardium at 20 min was used for subsequent analyses. ECV displayed excellent intra-, inter-observer, and scan-rescan reproducibility [intra-class correlation coefficients (ICC) 1.00, 0.97, and 0.96, respectively], as did λ (ICC 0.99, 0.94, 0.93, respectively). Moreover, ECV and λ were both higher in patients with AS compared with controls (ECV 28.3 ± 1.7 vs. 26.0 ± 1.6%, P < 0.001; λ 0.46 ± 0.03 vs. 0.44 ± 0.03, P = 0.02), with the former offering improved differentiation. In comparison, scan-rescan reproducibilities for pre- and post-contrast myocardial T1 were only modest (ICC 0.72 and 0.56) with no differences in values observed between cases and controls (both P> 0.05).

Conclusions: ECV appears to be the most promising measure of diffuse myocardial fibrosis at 3T based upon its superior reproducibility and ability to differentiate disease from health.

Keywords: aortic stenosis; cardiac magnetic resonance imaging; diffuse myocardial fibrosis; t1 mapping.

Figure 1

Methodology for measuring myocardial T1 at multiple time points and in multiple segments of the left ventricle (A) Measurement of myocardial T1 at multiple time points. ROI were drawn within the borders on the pre-contrast myocardial T1 maps and then copied onto the corresponding post-contrast images at all time points. Minor adjustments were made to avoid artefact and blood pool. An ROI was also drawn in the left ventricular blood pool in order to calculate the partition coefficient (λ) and extracellular volume fraction (ECV) at each time point. This approach demonstrated excellent intra- and inter-observer reproducibility. (B) Assessment of regional variation in T1 measures. Using the anterior and inferior ventricular insertion points as well as the mid-point of the ventricular cavity as reference points, three intersecting lines were drawn to divide the left ventricle into 16 segments. ROI were drawn onto the basal (six segments), mid-cavity (six segments), and apical (four segments) pre-contrast T1 maps with the standardized approach described above. Subsequently, the ROI were copied onto the 20-min post-contrast T1 maps. Pre- and post-contrast T1, λ, and ECV values were assessed in each segment

Figure 2

Variation of different T1 measures with time. (A) Myocardium and blood-pool T1. Post-contrast T1 values are dramatically reduced with the administration of contrast, followed by an exponential increase in values towards baseline. Significant changes in T1 were observed in the first 15 min following contrast administration, while a plateau phase was observed between 15 and 30 min where values remained relatively unchanged. (B) Change in T1 relaxation rates (ΔR1) in the myocardium and blood pool. During the first 15 min, significant changes in ΔR1 values were observed in the myocardium and blood pool. This was followed by values that remained relatively stable between 15 and 30 min. (C and D) Partition coefficient and ECV. Both measures were constant at all the time points examined, suggesting contrast equilibrium between the blood pool and myocardium occurs as early as 2 min. *Denotes significant difference in values (P < 0.05) between the two adjacent time points

Figure 3

Variation in the different T1 measures across the left ventricular myocardium in patients with aortic stenosis. There were no differences in the pre- (A) or post-contrast (B) myocardial T1, partition coefficient (C), and the ECV values (D) across the 16 segments. ICC values were >0.90 for each myocardial segment

Figure 4

Ability of the T1 measures to differentiate patients with aortic stenosis from healthy volunteers. (A) There was no significant difference in pre- and post-contrast myocardial T1 values between healthy volunteers and patients with aortic stenosis. (B) Partition coefficient and ECV values were significantly higher in patients with aortic stenosis compared with healthy volunteers. Results in mean ± SEM