MRI Phenotyping of the Mouse Heart: Novel Methods and Applications for Perfusion and Calcium Flux.

摘要

Magnetic resonance imaging (MRI) is a powerful and widely used medical imaging modality in clinical practice. When MRI is used to study mice, the roles of individual genes involved in human diseases, ranging from cancers to diabetes and heart disease, can be elucidated using MRI. Typically, when studying the heart with MRI, cardiac MRI (CMR) is used to phenotype genetically altered mice in terms of left ventricular (LV) structure and global function, contractile function, and infarct size and LV remodeling after myocardial infarction (MI). However, development of novel CMR methods could also enable measurement of L-type Ca2+ channel (LTCC) function and myocardial perfusion in the healthy and diseased mouse heart. In this dissertation, the development and applications of a dynamic manganese (Mn)-enhanced CMR method for assessing an index of LTCC function (LTCCI), and an improved arterial spin labeling (ASL) method for measurement of myocardial perfusion are presented.;Genetically altered mice have been widely used to study the roles of nitric oxide synthase (NOS) isoforms in the heart. However, the roles of neuronal NOS (nNOS) and endothelial NOS (eNOS) in regulating LTCC and contractile functions remain controversial. In addition, the role of nNOS in modulating myocardial perfusion remains uninvestigated. We used CMR methods to phenotype WT, eNOS -/-, and nNOS-/- mice regarding LV structure, baseline function, beta-adrenergic and muscarinic cholinergic responsiveness, and perfusion reserve. In order to assess in vivo modulation of LTCC function by nNOS and eNOS, we developed a dynamic Mn-enhanced CMR method to quantify LTCCI in the mouse heart under similar physiological stimulation. At baseline, LTCCI was significantly higher in nNOS-/- mice compared to both eNOS-/- and WT mice. In contrast, basal contractile function was similar in all groups. With dobutamine, nNOS-/- mice demonstrated attenuated inotropic and lusitropic reserves and a flat LTCCI response, while both WT and eNOS-/- mice demonstrated positive LTCCI and contractile responses. In response to beta-adrenergic and muscarinic cholinergie stimulation, LTCCI and contractile function decreased from dobutamine levels in all groups. Perfusion and perfusion reserve with either dobutamine or an adenosine receptor agonist are both normal in nNOS -/- mice compared to WT mice. Our results indicate that nNOS, and not eNOS, plays a dominant role in modulating LTCC and contractile functions at baseline, and during beta-adrenergic stimulation.;Experimental myocardial infarction (MI) in mice is an important disease model in part due to the ability to study genetic manipulations. MRI has been used to assess cardiac structural and functional changes after MI in mice, but changes in myocardial perfusion after acute MI have not previously been examined. ASL non-invasively measures perfusion, but is sensitive to respiratory motion and heart rate variability, and is difficult to apply after acute MI in mice. To account for these factors, a cardio-respiratory gated (CRG) ASL sequence using a fuzzy C-means algorithm to retrospectively reconstruct images was developed. Using this method, myocardial perfusion was measured in remote and infarcted regions at 1, 7, 14, and 28 days post-MI. Baseline perfusion was 4.9 +/- 0.5 (ml/g·min) and one day post-MI decreased to 0.9 +/- 0.8 (ml/g·min) in infarcted myocardium (P0.05 vs. baseline) while remaining at 5.2 +/- 0.8 (ml/g·min) in remote myocardium. During the subsequent 28 days, perfusion in the remote zone remained unchanged, while a partial recovery of perfusion in the infarct zone was seen. This technique, when applied to genetically-engineered mice, will allow for the investigation of the roles of specific genes in myocardial perfusion during infarct healing.