Spin echo based cardiac diffusion imaging at 7T: An ex vivo study of the porcine heart at 7T and 3T

Abstract

Purpose of this work was to assess feasibility of cardiac diffusion tensor imaging (cDTI) at 7 T in a set of healthy, unfixed, porcine hearts using various parallel imaging acceleration factors and to compare SNR and derived cDTI metrics to a reference measured at 3 T. Magnetic resonance imaging was performed on 7T and 3T whole body systems using a spin echo diffusion encoding sequence with echo planar imaging readout. Five reference (b = 0 s/mm2) images and 30 diffusion directions (b = 700 s/mm2) were acquired at both 7 T and 3 T using a GRAPPA acceleration factor R = 1. Scans at 7 T were repeated using R = 2, R = 3, and R = 4. SNR evaluation was based on 30 reference (b = 0 s/mm2) images of 30 slices of the left ventricle and cardiac DTI metrics were compared within AHA segmentation. The number of hearts scanned at 7 T and 3 T was n = 11. No statistically significant differences were found for evaluated helix angle, secondary eigenvector angle, fractional anisotropy and apparent diffusion coefficient at the different field strengths, given sufficiently high SNR and geometrically undistorted images. R≥3 was needed to reduce susceptibility induced geometric distortions to an acceptable amount. On average SNR in myocardium of the left ventricle was increased from 29±3 to 44±6 in the reference image (b = 0 s/mm2) when switching from 3 T to 7 T. Our study demonstrates that high resolution, ex vivo cDTI is feasible at 7 T using commercial hardware.

Longitudinal assessment of tissue properties and cardiac diffusion metrics of the ex vivo porcine heart at 7 T: Impact of continuous tissue fixation using formalin

“…With 0.62 ± 0.1 (10 −3 mm 2 /s), our ADC values in this study are in good agreement with our previous work at 7 T 32 (0.51‐0.66 [10 −3 mm 2 /s]) using fresh hearts from animals of the same breed and comparable weight. ADC values of fresh hearts reported by Mazumder et al 20 (0.52 ± 0.026 [10 −3 mm 2 /s], Yorkshire, 41‐50 kg) and Agger et al 18 (0.54 ± 0.026 [10 −3 mm 2 /s], Danish Landrace, 50‐60 kg) are slightly lower and ADC values of fixed hearts reported by Wu et al 41 (0.671 ± 0.106 [10 −3 mm 2 /s], minipigs, 45‐50 kg) and Pashakhanloo et al 42 (0.633 ± 0.04 [10 −3 mm 2 /s], Yorkshire, 50 ± 18 kg) are slightly higher.…”

“…We found no systematic difference in diffusivity between hearts measured first and second. This is in agreement with prior experiments analyzing sample stability of fresh hearts following excision 32 . However, variations in heart size and times for transport, tissue preparation and sample fixation may have resulted in different tissue temperatures.…”

“…Aldehyde fixation penetration occurs with a rate of 0.5 to 1 mm/hour at room temperature and we observed the pale yellow color change, which indicates penetration, shortly after immersion. Multiple studies have shown that autolytic effects on cardiac tissue are very slow 14,32,39 . It is therefore unlikely that the gradual increase in T 2 at later stages of fixation is related to latent effects of tissue decomposition.…”

“…All experiments were performed according to the German Animal Welfare Act and the EU Directive 2010/63/EU. Euthanasia was achieved as described in 32 and hearts were collected, rinsed and stored in physiological saline solution. The eight hearts were harvested in four experiments, where two hearts each were excised with ~20 minutes temporal difference.…”

Characterization of Myocardial Microstructure and Function in an Experimental Model of Isolated Subendocardial Damage

“…Our understanding of structural-functional relationships in the heart is incomplete. 28 DT-MRI is a feasible tool to study the 3-dimensional myofiber arrangement that has been applied in various cardiovascular pathologies in both experimental models 9,15,16,19 and patients. 10,14,29 Despite the growing wealth of cardiac DT-MRI studies, however, there is a lack of data regarding changes that occur during early disease states and investigation of layer-specific alterations.…”