Development of MRI Methods to Assess Coronary Microvascular Function in Mice

Each year, over eight million patients in the USA visit the emergency department for chest pain or angina. The most common cause of the anginal symptom is myocardial ischemia. Traditionally, diagnosis of ischemia and the cause of angina has focused on the evaluation of the obstruction of coronary arteries by angiography for coronary artery disease (CAD). Nevertheless, a substantial percentage of patients with anginal symptoms and who show ischemia on stress testing have a normal coronary angiogram.
With the recognition of ischemia without obstructive CAD, the emerging concept in cardiology is that multiple factors including microvascular disease may be significant contributors to myocardial ischemia, and that obstructive CAD is one of the multiple potential causes. Indeed, studies have shown that a significant portion of ischemic patients has coronary microvascular disease (CMD). Patients with CMD experience recurrent chest pain and have high rates of cardiovascular events. However, the disease mechanism for CMD is not fully understood, and there is no established treatment for CMD.
Mouse models of human heart disease are widely used to investigate molecular and cellular mechanisms of disease and to investigate potential new therapies. Magnetic resonance imaging (MRI) in mice enables noninvasive and serial assessment of cardiovascular physiology and pathophysiology. In this dissertation, we aim to develop MRI methods to assess coronary microvascular function in mice. The new imaging tools combined with gene-modified mice and diet-induced obese mice would help study molecular mechanisms behind CMD and establish models of CMD to test traditional or novel treatments.
Coronary microvascular endothelial dysfunction is a biomarker and subtype of CMD. In Specific Aim 1 and 2, we developed a minimally invasive MRI method to probe coronary microvascular endothelial function or, more specifically, of coronary microvascular endothelial nitric oxide synthase (eNOS) function. Using this method, we demonstrated that coronary microvascular eNOS dysfunction precedes impairment of myocardial perfusion reserve (MPR) in a mouse model of impaired MPR without obstructive CAD. The results from these two aims are summarized in Chapter 2.
Ischemia can be assessed by perfusion MRI. In Specific Aim 3, we developed an improved perfusion MRI pulse sequence for mice, called self-gated steady-state pulsed arterial spin labeling, that quantitatively assess mouse myocardial perfusion in under 5 minutes and without contrast agents. The development and evaluation of the pulse sequence are summarized in Chapter 3.
Finally, in an additional project, we tested the use of nonmetallic compound nitroxides as an alternative to gadolinium for first-pass contrast-enhanced myocardial perfusion imaging in mice. The results of nitroxides-enhance perfusion imaging are summarized in Chapter 4.