Introduction
Cardiac MRI (cMRI) using T2*-contrast has shown the potential to detect pathophysiological changes after ischemia-reperfusion-induced injury following myocardial infarction (MI). The use of 7T MRI allows to increase the resolution and sensitivity of T2* measurements as a potential surrogate marker of myocardial injury, e.g. regarding hemorrhage and iron deposition. We present the results of the analysis of 7T T2*-contrast MRI obtained in a large animal study before and after MI. The aim of this work is to compare the accessibility of post-MI injury markers from 7T T2*-images with two alternative approaches: T2*-maps and grayscale analysis in T2*-weighted images.
Methods
After approval by the Animal Welfare Committee, MI was induced in 7 German Landrace Pigs by occlusion of the LAD followed by reperfusion after 90min. Three in-house-developed weight-matched 8Tx/16Rx cardiac arrays were used for measurements at the Magnetom™ "Terra" 7T MR scanner (Siemens, Erlangen). Scans were done 7 days prior to MI-induction (MRI0) and after 3-4 (MRI1) and 7-10 days (MRI2), respectively. A short-axis view contiguous stack (10-13 slices, 6 mm thickness) was acquired with a multi-gradient echo (mGRE) sequence (TE distributed within 1.1 to 14.6ms) with in-plane pixel size 2.2x2.5mm. An acoustic trigger system (EasyACT, MRI.Tools) for cardiac gating was used.
Slices above and below the suspected infarction area were considered (Fig. 1a). T2*-maps were generated after manual segmentation of the left ventricle (Fig. 1b) and subsequent segment definition according to the AHA scheme. Grayscale analysis of T2*-weighted images was performed using the contrast differences by selecting the infarcted and two reference areas (Fig. 2a). In two echoes (TE=1.09/TE=4.83ms) the mean value of the grayscale Gm and the standard deviation Gstd in the marked areas were determined to calculate the relative contrast as GR=Gm(TE=1.09ms)/Gm(TE=4.83ms) and the coefficient of variation as CoV=Gstd/Gm.
Results
The development of T2* in two exemplary slices is shown in Fig. 1. In non-infarcted tissue (S1 position: basal to MI) typical T2* values range within 10-20ms. The native reduction of T2* in the posterior and lateral region is explained by the susceptibility influence of the lung. The S2 position shows a clear contrast mark in both the T2*-weighted image and the T2*-maps (Fig. 1a and c) within presumable infarcted area. This effect is evident in both measurements (MRI1 and MRI2) after MI.
A significant quantitative difference between non-infarcted and infarcted tissue (S1 and S2 position yellow label) and between remote and infarcted regions (blue and yellow) is evident using relative T2* grayscale contrast (Fig. 2b) and heterogeneity represented by CoV (Fig. 2c).
Discussion and Conclusion,
The T2* contrast strength of 7T MRI allows reliable detection of infarcted tissue with both traditional T2*-maps and grayscale intensity statistics of T2*-weighted images. The reduced range of T2* values in myocardium at B0=7T compared with B0=1.5/3T may make the use of statistical metrics from two T2*-weighted images preferable for detecting and quantifying infarct areas by contrast loss and heterogeneity. This may allow better exploration of the potential of 7T to increase the spatial resolution of cMRI and more detailed characterization of tissue changes after MI.
Acknowledgments
Financial support: German Ministry of Education and Research (BMBF, grants: 01EO1004, 01E1O1504)
https://dgk.org/kongress_programme/jt2022/aP1234.html