Three‐dimensional diffusion tensor microscopy of fixed mouse hearts

Abstract

The relative utility of 3D, microscopic resolution assessments of fixed mouse myocardial structure via diffusion tensor imaging is demonstrated in this study. Isotropic 100-m resolution fiber orientation mapping within 5.5°accuracy was achieved in 9.1 hr scan time. Preliminary characterization of the diffusion tensor primary eigenvector reveals a smooth and largely linear angular rotation across the left ventricular wall. Moreover, a higher level of structural hierarchy is evident from the organized secondary and tertiary eigenvector fields. These findings are consistent with the known myocardial fiber and laminar structures reported in the literature and suggest an essential role of diffusion tensor microscopy in developing quantitative atlases for studying the structure-function relationships of mouse hearts. The fiber anatomy of the myocardium is known to have a profound impact on the electrical and mechanical properties of both normal and diseased hearts (1,2). Precise knowledge of the tissue fiber structure will contribute significantly in elucidating the complex structure-function relationships of the organ, especially in biomedical engineering studies based on the so-called "morphologically accurate" modeling. Advances in molecular biology have provided unprecedented opportunities to use versatile small animal (e.g., mouse) platforms (3-6) for studying genotypes and phenotypes of myocardial development, damage, or repair associated with specific pathologies. Due to the small size of the mouse heart (with overall volume typically less than 250 mm 3 ), an efficient means to accurately and quantitatively characterize the tissue fiber structure at high or microscopic-resolution would be of great benefit.Most early studies of myocardial fiber structures were based on conventional histological techniques (7-11), which are not only destructive, but also labor-intensive. By probing the tissue microstructure via its influence on the diffusion of water, MR diffusion tensor imaging (DTI) (12) has been advanced as a promising nondestructive alternative to characterize the structure of ordered tissues such as the brain white matter (13-15), myocardium (16 -22), other musculature (23), and cartilage (24). The underlying hypothesis in DTI is that the direction of fastest water diffusion coincides with the local tissue fiber orientation. Reports show evidence that validates a direct correlation between fiber orientations measured by DTI and conventional histology for freshly excised (16), perfused (17), and fixed myocardium (18).In 3D space, the generalized diffusion tensor is a symmetric, second-order 3 ϫ 3 matrix. A complete solution to the six independent parameters, plus an extra term for the diffusion-independent magnetization, requires that the DTI dataset consists of a minimum of seven images. The tradeoffs between scan time and the image signal-to-noise ratio (SNR), aggravated by the nature of diffusion sensitization (i.e., via signal attenuation) and the inadvertent T 2 weighting during the diffusion encoding gr...

Reconstruction and Quantification of Diffusion Tensor Imaging-Derived Cardiac Fibre and Sheet Structure in Ventricular Regions used in Studies of Excitation Propagation

“…Our mean value for the three combined hearts of FA = 0.25 ± 0.09 is similar to measurements from other studies, with values of 0.36 in canine [29], 0.35 in goat [23], 0.27 in mouse [41], and 0.36, 0.32 and 0.3 in diastolic, early and late systolic rat respectively [11] having been reported. Thus, fibre direction in the three hearts was well determined.…”

“…Our results compare well with previous DTI studies examining the transmural course of the fibre inclination angle [11,12,23,28,29,30,33,36,41,53,54]. In the equatorial slice, the mean range of the fibre inclination angle was 135 • in the anterior sector, 87 • in the lateral sector, 113 • in the posterior sector, 131 • in the septal sector and 80 • in the LVFW sector.…”

“…Thus all secondary eigenvectors pointed away from the centroid, approximately in an endocardial to epicardial orientation. Secondary eigenvectors in the heart point principally in the radial direction [20,41,53,54] and so we calculate the sheet angle as the angle between the transverse plane and the projection of the secondary eigenvector onto the radial plane (see Figure 2b). This also allows comparisons to data from histological studies [11,45,46] which are generally taken from slices made in the radial plane.…”

Co‐registration of in vivo human MRI brain images to postmortem histological microscopic images

“…Previous work to co-register in-vivo MRIs to microscopically examined samples has been reported mostly for small animal studies [Jacobs et al 1999;Jiang et al 2004;Meyer et al 2006;Anderson et al 2006], a limited number of primates [Malandain et al 2004;Kaufman et al 2005;Dauguet et al 2007] and relatively few human studies [Schormann et al 1995;Mega et al 1997;Toga et al 1999;Kenwright et al 2003]. Most studies involve brain imaging.…”

“…One of the earlier animal studies was based on a two-step approach where a surface-matching initial co-registration was followed by a nonlinear thin plate splines based warping [Jacobs et al, 1999]. Subsequent studies have relied mostly on maximizing mutual information [Jiang et al, 2004;Meyer et al, 2006] or an image intensity based non-rigid registration [Anderson et al, 2006]. Animal studies have an advantage over human studies in that animals can be sacrificed immediately after an in-vivo scan, and the postmortem brain can be extracted rapidly and scanned in dedicated animal scanners without any significant scheduling delays.…”