Ex vivo imaging of subacute myocardial ischemia with ultra-short echo time 3-D quantitative T1-mapping MRI in mice
Authors: Raty I , Paajanen A , Nissi M J , Ruotsalainen A K , Laidinen S , Yla-Herttuala S , Yla-Herttuala E 
Published: November 2023

Abstract

Introduction
Cardiac magnetic resonance (CMR) is important tool for assessment of structure and function of the myocardium in various cardiac conditions, such as after myocardial infarction (MI). CMR methods rely solely on multi-slice 2-D imaging, which tends to be slow, since every slice is imaged separately with the &gt;3 min scan time. Furthermore, multi-slice methods are somewhat approximate, due to the slice thickness, which is usually around 1 mm for mice and 8 mm for humans. Therefore, 3-D CMR imaging methods with isotropic resolution are urgently needed. In this study, we present quantitative, ultra-short echo time 3-D Look-Locker T1[1] Multi-Band SWeep Imaging with Fourier Transform[2] and Compressed Sensing[3] (LL MB-SWIFT-CS) technique for imaging in MI mice hearts ex vivo.

Purpose
To develop an ultra-short echo time 3-D CMR method for imaging subacute myocardial ischemia quantitatively and in an accelerated way.

Materials and Methods
Hearts were collected 7 days after Left Anterior Descending (LAD) Coronary Artery was ligated in C57BL mice, fixated in 4% PFA in PBS and imaged with LL MB-SWIFT-CS MRI. The heart was scanned in 3-D with a total acquisition time of 3.25 min with resolution of 256³. For T1 determination, saturation recovery (SR) of the longitudinal magnetization was measured with following parameters: TR=3 ms, FA=4º, FOV=25 mm³, total number of unique spokes acquired=16384 and spokes per LL curve=1024, filling the 3-D data space homogeneously 16 times during the recovery. Total scan time was 52 min. After imaging, the SR LL curve was reconstructed into 32 images with CS reconstruction. For comparison, we imaged the hearts with 2-D MRI TRAFF2-mapping method[4] and histology.
For regions of interests (ROIs) analysis, the damaged and remote ROI areas were observed from 2-D MR images and drawn to corresponding anatomical 3-D images. The same ROIs were then used to collect the T1 relaxation time data. For histology, standard paraffin embedding, and tissue processing protocols were used. 4 μm thick sections were cut and stained with Hematoxylin Eosin and Sirius Red staining.

Results and Discussion
The T1 relaxation time values were elevated in the damaged areas (p&lt;0.005) (Fig. 1), as shown earlier[5,6]. The T1 values in the ischemic and remote myocardium were 0.76±0.03s and 0.70±0.02s, respectively. Ischemic areas can also be observed visually in the T1- and 2D TRAFF2-maps (Fig. 2). Sirius Red staining showed increased collagen formation in the ischemic myocardium, supporting the CMR findings. Moreover, hematoxylin-eosin staining showed loss of cardiomyocytes and coronary capillaries, and infiltration of inflammatory cells, like macrophages and lymphocytes.

Conclusion
With this method, we were able to detect the statistically significant differences between damaged and remote tissue based on the T1-relaxation times within the myocardium, allowing isotropic quantitative 3-D assessment of the entire myocardium.T1 values for MB-SWIFT-CSVisual validation of MB-SWIFT-CS