1357587 - Tracking Lipid changes in an Ex-vivo Myocardial Infarction using NOE-MTR at 7T

Cardiovascular disease (CVD) is the leading cause of death worldwide [1,2]. CVD encompasses stroke, cardiomyopathy, coronary artery disease (CAD), and other disorders that can lead to myocardial infarctions and heart failure. The pathophysiological processes in each of these diseases can differ, but lipids play a significant role in every model of CVD and have a direct role in cardiovascular function  [1,2,3]. Nuclear Overhauser Effect (NOE) magnetization transfer ratio (MTR) MRI is an emerging technique to study mobile macromolecules such as lipids and proteins [4]. In the ¹H homonuclear NOE imaging of brain, the system is first perturbed using off-resonance low power saturation pulse of long saturation duration lasting a few (3-6s) seconds. Subsequently, the NOE weighted signal is generated via the cross-relaxation of aliphatic protons such as methyl and/or methylene protons of macromolecules (lipids and proteins) with bulk water. This effect appears at 3.5 ppm up-field from bulk water. Since lipids have very short T_2s, standard MRI is not suitable for measuring them. NOE_MTR is a highly sensitive technique to track the lipid changes with good specificity. In this study, we plan to use NOE_MTR as a technique to track the lipid degeneration in infarcted tissue.



Methods:

An anteroseptal infarct was induced in a healthy Yorkshire swine by occlusion of the left anterior descending (LAD) coronary artery for 90 minutes using a balloon catheter, followed by restoration of flow. Sac occurred 9 days post-infarct and ex vivo imaging was done 1-day post-sac. A swine heart with an infarcted tissue was scanned on a 7T whole body scanner (MAGNETOM Terra, Siemens Healthcare, Erlangen, Germany) with a Nova Medical volume coil transmit/32-channel receive proton phased-array head coil. The 2D NOE acquisition parameters are: number of slices = 1, slice thickness = 8 mm, in-plane resolution = 1 x 1 mm², matrix size = 240 x 180, gradient-echo readout TR = 3.5 s, TE = 1.79 ms, read-out flip angle = 4 , averages = 1, SHOT TR = 6000 ms, and a saturation pulse of B_1,rms = 0.72 µT with saturation length of 3s.The z-spectra was acquired at varying saturation offset frequencies from -7.5 to 7.5 ppm (relative to the water resonance) with a step size of 0.1 ppm.

 



Results:

The z-spectra illustrates a decrease in contrast in the aliphatic range (-3.5 ppm) of the infarcted region compared to the normal tissue (Figure 1b).There is a decrease in the NOE_MTR signal in the infarcted region (~15%) compared to the normal tissue (~18%) in the swine heart (Figure 2b).



Conclusion:

The z-spectra and NOE_MTR map both show that there is a drop in NOE signal in the infarcted region compared to the healthy tissue in the pig heart. This is most likely due to a degradation of lipids during the myocardial infarction. This is the first time this has been demonstrated using NOE_MTR. Therefore, NOE_MTR may serve as a biomarker to track lipidomic changes in CVD.