Introduction

MRI at 7T B₀-field is a developing methodology capable to increase the physical sensitivity and spatial resolution of cardiac MR-images. Compared to clinical field strength, T₂*-quantification becomes highly sensitive to the structural changes in cardiac tissue at 7T B0-field strength. It may provide valuable information on tissue remodeling in both acute and chronic cardiac diseases. In this work, we present the results of a pilot study correlating 7T cMRI T₂*-measurements data with the results of (i) late-gadolinium-enhancement (LGE) imaging and (ii) first-pass myocardial perfusion measurements in a pig model of chronic myocardial infarction.

Materials and Methods

In total n=8 pigs (German Landrace) were used with the approval of the local Animal Welfare Committee. Myocardial infarction (MI) was induced by occluding the left anterior descending coronary artery with a balloon catheter, followed by subsequent reperfusion after 90 minutes. All measurements were done using a Magnetom™ “Terra” 7T MR-scanner (Siemens, Erlangen, Germany) using three custom-designed 8Tx/16Rx cardiac array coils each adopted for specific animal weight range [1]. Each animal was scanned 7 days before induction of MI and at days 3, 14, and 58 after the operation (referred to as “pre-MI”, and MI-1,2,3 scans respectively). T2*-measurements were performed using a cardiac triggered by an acoustic system (EasyACT, MRI.Tools) mGRE-sequence with 9 TE-times distributed in the range [1.1..14.6]ms, and using an in-plane pixel size of 2.2x2.5mm. A contiguous stack of 10-13 slices with 6mm thickness was acquired in short-axis heart view orientation. The volume for B0-shimming volume was set to fit the visible heart dimensions on each measured slice [2]. Myocardial perfusion was visualized using the first-pass methodology and both low dose (~0.016mmol/kg bw) and high dose (~0.1mmol/kg bw) gadolinium DOTA (“Gadovist”) injection. Two slices per heartbeat were measured within healthy and infarcted myocardium zones using a spatial resolution of 0.95x1.4x6mm. The LGE stack was acquired using an IR-GRE sequence with TE/TI=1.9/400ms and voxel size=1.2x2x6mm. The previously acquired CINE stack was used for both T₂* and LGE stack planning. The analysis of images was done using in-house developed Matlab scripts (Mathworks, Natick, USA) and MEDIS software (Medis Imaging, Leiden). Two methods of T₂* analysis were compared: (i) manual ROI selection and (ii) “blind” evaluation based on the automatic splitting of LV myocardium on segments according to the AHA-scheme.

Results

The mean weights of the animals were 34±4kg and 76±8kg in the first and last MR-sessions respectively. Figure 1 shows examples of T₂*-weighted images in healthy and infarcted myocardium. A strong negative contrast of the infarction zone compared to healthy tissue is observed. Signal reduction is by a factor of 1.5-2.0 (at TE=5ms). Figure 2 demonstrates the example of ROI and segment based T₂*-curves fitting for pre-MI and MI-1 scans. The coefficient of determination (R²) of fitting ranges within [0.95..0.99] for manually selected ROIs and is reduced to [0.9..0.97] for segment-based evaluation because of the intrinsically broader distribution of T2* within automatically distributed segments. Figure 3 shows the correlation observed between LGE and T₂*-weighted imaging. In the acute stage (MI-1), T₂*-weighted images show areas with a significant signal reduction which is localized inside the larger LGE-contrasted regions. T₂*-contrast persists but becomes less prominent in the scans performed later (3-8 weeks after MI) when severe tissue remodeling and formation of scar zone occurs. Figure 4 depicts the correlation of the visualization of infarction areas by T₂*-contrast with those in first-pass myocardial perfusion MRI. Figure 5 shows that without physiologic motion within a remodeled infarcted myocardial wall (highlighted by LGE) a tiny (~2mm) T₂*-contrasting zone in the endocardium is revealed.

Discussion

Our results allow suggesting that at 7T T₂*-contrast may visualize tissue alterations after myocardial infarction at early stages (3-20 days). T₂* reduction can be reliably quantified on both manually selected ROI data fitting as well as on segment-based analysis. The latter allows for better standardization of the T₂*-quantification process using multi-slice data and finding correlations with parameters measured by the other MR-protocols. Regions with negative T2*-contrast correlate with scar regions highlighted by LGE. However, the area of T₂*-reduction is systematically smaller when compared to the area with positive LGE-contrast. Moreover, it was observed to be smaller when tissue remodeling progressing. An interesting and probably essential observation is the substantial correlation of both localization and size of the T₂*-contrasted regions with the regions of perfusion deficit detected by the DCE-MRI. To our best knowledge, such observation concerning T₂* contrast in the myocardium tissue was not reported so far. The subsequent measurements with increased spatial resolution and followed by post-mortem, histology may help in a better understanding of the link between post-infarction tissue remodeling and obstruction of the microcirculation.

Conclusion

Conclusion The obtained result allows supports the hypothesis that 7T B₀-field T₂* becomes essentially sensitive to the changes in micro-perfusion caused by the rupture of the cell membranes due to the ischemia. In the post-infarction tissue the shortening of T₂* is related to both heterogeneity of a scar collagen matrix density and the structural disarray compared to the organized sheetlets of cardiomyocytes [2]. This combination of factors can make the T₂*-contrast-based imaging at 7T a potentially valuable complementary methodology to the traditional cardiac MRI-techniques for assessment of the post-infarction tissue remodeling.

Acknowledgements

Financial support: German Ministry of Education and Research (BMBF, grants: 01EO1004, 01E1O1504). Steven Nguen, Dr. Oleg Poznansky, and Alena Kollmann are acknowledged for the help in the animal experiments.