Comparison of diffusion tensor imaging by cardiovascular magnetic resonance and gadolinium enhanced 3D image intensity approaches to investigation of structural anisotropy in explanted rat hearts.
BACKGROUND
Cardiovascular magnetic resonance (CMR) can through the two methods 3D FLASH and diffusion tensor imaging (DTI) give complementary information on the local orientations of cardiomyocytes and their laminar arrays.


METHODS
Eight explanted rat hearts were perfused with Gd-DTPA contrast agent and fixative and imaged in a 9.4T magnet by two types of acquisition: 3D fast low angle shot (FLASH) imaging, voxels 50 × 50 × 50 μm, and 3D spin echo DTI with monopolar diffusion gradients of 3.6 ms duration at 11.5 ms separation, voxels 200 × 200 × 200 μm. The sensitivity of each approach to imaging parameters was explored.


RESULTS
The FLASH data showed laminar alignments of voxels with high signal, in keeping with the presumed predominance of contrast in the interstices between sheetlets. It was analysed, using structure-tensor (ST) analysis, to determine the most (v1(ST)), intermediate (v2(ST)) and least (v3(ST)) extended orthogonal directions of signal continuity. The DTI data was analysed to determine the most (e1(DTI)), intermediate (e2(DTI)) and least (e3(DTI)) orthogonal eigenvectors of extent of diffusion. The correspondence between the FLASH and DTI methods was measured and appraised. The most extended direction of FLASH signal (v1(ST)) agreed well with that of diffusion (e1(DTI)) throughout the left ventricle (representative discrepancy in the septum of 13.3 ± 6.7°: median ± absolute deviation) and both were in keeping with the expected local orientations of the long-axis of cardiomyocytes. However, the orientation of the least directions of FLASH signal continuity (v3(ST)) and diffusion (e3(ST)) showed greater discrepancies of up to 27.9 ± 17.4°. Both FLASH (v3(ST)) and DTI (e3(DTI)) where compared to directly measured laminar arrays in the FLASH images. For FLASH the discrepancy between the structure-tensor calculated v3(ST) and the directly measured FLASH laminar array normal was of 9 ± 7° for the lateral wall and 7 ± 9° for the septum (median ± inter quartile range), and for DTI the discrepancy between the calculated v3(DTI) and the directly measured FLASH laminar array normal was 22 ± 14° and 61 ± 53.4°. DTI was relatively insensitive to the number of diffusion directions and to time up to 72 hours post fixation, but was moderately affected by b-value (which was scaled by modifying diffusion gradient pulse strength with fixed gradient pulse separation). Optimal DTI parameters were b = 1000 mm/s(2) and 12 diffusion directions. FLASH acquisitions were relatively insensitive to the image processing parameters explored.


CONCLUSIONS
We show that ST analysis of FLASH is a useful and accurate tool in the measurement of cardiac microstructure. While both FLASH and the DTI approaches appear promising for mapping of the alignments of myocytes throughout myocardium, marked discrepancies between the cross myocyte anisotropies deduced from each method call for consideration of their respective limitations.
Inserm U1045 - Centre de Recherche Cardio-Thoracique, L'Institut de rythmologie et modélisation cardiaque LIRYC, Université de Bordeaux, PTIB - campus Xavier Arnozan, Avenue du Haut Leveque, 33604, Pessac, France. olivier.bernus@u-bordeaux.fr.
Olivier
O
Bernus
Institute of Cardiovascular and Medical Sciences, University of Glasgow, BHF Glasgow Cardiovascular Research Centre, 126 University Place, Glasgow, G12 8TA, UK. aleksandra.radjenovic@glasgow.ac.uk.
Aleksandra
A
Radjenovic
Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand. m.trew@auckland.ac.nz.
Mark L
ML
Trew
Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand. i.legrice@auckland.ac.nz.|Department of Physiology, University of Auckland, Auckland, New Zealand. i.legrice@auckland.ac.nz.
Ian J
IJ
LeGrice
Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand. g.sands@auckland.ac.nz.
Gregory B
GB
Sands
School of Computing, The University of Leeds, Leeds, LS2 9JT, UK. d.r.magee@leeds.ac.uk.
Derek R
DR
Magee
Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand. b.smaill@auckland.ac.nz.|Department of Physiology, University of Auckland, Auckland, New Zealand. b.smaill@auckland.ac.nz.
Bruce H
BH
Smaill
Mathematical Cell Physiology, Max-Delbrück-Center for Molecular Medicine (MDC), Robert-Rössle-Straße 10, 13125, Berlin, Germany. stephen.gilbert@mdc-berlin.de.
Stephen H
SH
Gilbert
D000818:Animals; D003287:Contrast Media / Q000008:administration & dosage*; D056324:Diffusion Tensor Imaging / Q000379:methods*; D019786:Gadolinium DTPA / Q000008:administration & dosage*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods*; D000067268:Isolated Heart Preparation; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000166:cytology*; D011237:Predictive Value of Tests; D017208:Rats, Wistar; D016277:Ventricular Function, Left
D003287:Contrast Media; D019786:Gadolinium DTPA