The role of diffusion and perivascular spaces in dynamic susceptibility contrast MRI

We investigated the effects of brain tissue orientation, diffusion, and perivascular spaces on dynamic susceptibility contrast MRI. A 3D numerical model of a white matter voxel was created that consists of an isotropic capillary bed and anisotropic vessels that run in parallel with white matter tracts and are surrounded by perivascular spaces. The signal within the voxel was simulated by solving the Bloch-Torrey equation. Experimental perfusion data were acquired with a gradient echo dynamic susceptibility contrast scan. White matter fibre orientation was mapped with diffusion tensor imaging. Our numerical model of the contrast agent induced increase in R2*, as a function of tissue orientation, was fit to dynamic susceptibility contrast MRI data from thirteen subjects by minimizing the bias-corrected Akaike information criterion. White matter blood volume fraction in both the isotropic and the anisotropic vessels was determined as a free parameter, and results were analyzed as a function of diffusivity and perivascular space size. Total white matter blood volume was found to be 2.57%, with one third of the blood residing in blood vessels that run parallel with white matter tracts. Gradient echo dynamic susceptibility contrast MRI strongly depended on white matter tissue orientation and, according to the numerical simulations, this effect is amplified by diffusion and perivascular spaces.

Anisotropic cerebral vascular architecture causes orientation dependency in cerebral blood flow and volume measured with dynamic susceptibility contrast magnetic resonance imaging

Measurements of cerebral perfusion using dynamic susceptibility contrast magnetic resonance imaging rely on the assumption of isotropic vascular architecture. However, a considerable fraction of vessels runs in parallel with white matter tracts. Here, we investigate the effects of tissue orientation on dynamic susceptibility contrast magnetic resonance imaging. Tissue orientation was measured using diffusion tensor imaging and dynamic susceptibility contrast was performed with gradient echo planar imaging. Perfusion parameters and the raw dynamic susceptibility contrast signals were correlated with tissue orientation. Additionally, numerical simulations were performed for a range of vascular volumes of both the isotropic vascular bed and anisotropic vessel components, as well as for a range of contrast agent concentrations. The effect of the contrast agent was much larger in white matter tissue perpendicular to the main magnetic field compared to white matter parallel to the main magnetic field. In addition, cerebral blood flow and cerebral blood volume were affected in the same way with angle-dependent variations of up to 130%. Mean transit time and time to maximum of the residual curve exhibited weak orientation dependency of 10%. Numerical simulations agreed with the measured data, showing that one-third of the white matter vascular volume is comprised of vessels running in parallel with the fibre tracts.

Abstract 14957: Investigation of Myocardial Birefringence using Polarization-Sensitive Optical Coherence Tomography in a Mouse Model of Myocardial Infarction

Introduction: Although several imaging modalities have been utilized to characterize myocardium, the birefringence of myocardium has not been spotlighted. The polarization-sensitive optical coherence tomography (PS-OCT) is a novel imaging method for the characterization of tissue birefringence. Hypothesis: The birefringence of infarcted myocardium would be different from non-infarcted myocardium. We examined the myocardial birefringence in a mouse coronary artery ligation model using PS-OCT. Methods: Male C57B/6 mice (20-30g) underwent thoracotomy and the ligation of left anterior descending artery. At post-myocardial infarction (post-MI) 3 hours or on post-MI day 28, the left anterior wall of the mice was imaged with PS-OCT in vivo and ex vivo. The local phase retardation for tissue birefringence, the local principal axis directional uniformity for tissue orientation, and the degree of polarization uniformity were demonstrated and compared with collagen fiber staining (picrosirius red). For PS-OCT system, wavelength-swept laser with 140 nm bandwidth (center wavelength = 1,280 nm) and 120 kHz A-line rate was used. Results: The endocardial border could be detected by high birefringence of myocardium. There was no difference in the birefringence between systole and diastole. The infarcted anterior wall showed dyskinesia or hypokinesia. The systolic wall thickening was lower in the infarcted wall compared to the normal anterior wall. At post-MI 3 hours, there was no significant change in myocardial birefringence. On day 28, the infarcted myocardium with scar tissue showed low level of local birefringence with irregular tissue orientation compared to the normal myocardium. The 3D reconstructed images were used to differentiate transmural or subendocardial MI. Conclusions: Our study demonstrated that the PS-OCT would be helpful for the evaluation of myocardial function and tissue characterization in myocardial infarction.

FC12-03 - DTI-based in vivo mapping of subregions within the human hypothalamus

IntroductionThe hypothalamus is involved in many aspects of behavioral responses but parcellations of hypothalamic subnuclei have only been feasible in post-mortem brains. Thus it would be-from a clinical point of view-highly desirable if hypothalamic subnuclei could be delineated also noninvasively in living subjects. This study is a first step in this direction: We exploited the directionality information inherent in high-resolution DTI data to map subregions of the hypothalamus in healthy volunteers.directionality information using DTI data to map subregions of the hypothalamus.MethodsWe scanned 10 subjects with a Siemens 3 T scanner, acquired DTI and T1 scans. We computed the similarity of fiber orientations between all voxels and subjects, and clustered the similarity matrix in 3 regions using a k-means algorithm.ResultsThe diffusion images showed anisotropic tissue orientation within the hypothalamus which was consistent across subjects. The clustering in 3 regions resulted in an anatomically coherent arrangement of clusters across hemispheres and subjects. In each ROI, we found an anterior, a posteromedial, and a lateral subdivision with consistent microscopic tissue orientations across subjects.ConclusionThis is to our best knowledge the first study that demonstrates the fine-grained microstructural organization within the human hypothalamus noninvasively in living subjects.

On the Bidirectional Viscoelastic Behavior of the Human Achilles Tendon

Soft tissues such as tendons and ligaments are made up of groups of collagen fascicles surrounded by a sheath of epitenon. The friction between these structural fibers and their surrounding ground substance has been suggested to be the main contributor to the observed viscoelastic response of the tissue[1]. During normal daily activities such as walking, climbing stairs or jumping, these tissues are subjected to alternating loading and unloading conditions. Depending on the load and tissue orientation, this alternating loading condition may not be uniformly applied over the entire volume of the tissue. In some instances, certain fiber bundles are in tension (loaded) while others might be unloading. Hence, the development of accurate predictive models requires characterization of not only the loading behavior, but also the unloading behavior. To our knowledge, there are few models that specifically address the unloading behavior of the tissue.