- Coronary Artery Imaging
Coronary artery related heart disease is the leading cause of deaths in the United States. The clinical standard of diagnosis is x-ray arteriography, which is invasive and expensive, and involves certain complications and even fatalities. In addition, it provides little information on the functional significance of the disease. MR imaging is a non-invasive and widely available procedure without the use of iodinated contrast media and ionizing radiation. More importantly, MRI has the ability to provide multiple tests of the heart, including coronary artery and heart morphology, blood flow to the myocardium, cardiac function, and myocardial tissue characterization. These tests could potentially be performed in the same imaging session to provide a comprehensive assessment of the coronary disease for improved patient care and cost savings.
The visualization of coronary arteries is one of the most challenging tasks of MR imaging because of the small sizes, the constant motion, and the tortuous courses of coronary arteries. We have developed various techniques for faster data acquisition, higher spatial resolution, higher signal to noise and contrast to noise ratios, while minimizing image artifacts. These techniques have been tested in patients with promising sensitivity, specificity, and accuracy in detecting clinically significant coronary artery disease. Further technical developments are required for routine clinical applications.
- Myocardial Perfusion Assessment
Myocardial perfusion and perfusion reserve are crucial physiological parameters in assessing ischemic heart disease. The decision to enhance myocardial blood flow either by angioplasty or surgery is often made on the assumption that regional flow is compromised by a coronary stenosis and can be partially or wholly reversed by treatment. The decision to revascularize is frequently based on invasive coronary angiography with left ventriculography or on the basis of nuclear techniques and evaluation of contractile reserve by dobutamine echocardiography. Recently, first-pass perfusion MRI using gadolinium-based contrast has demonstrated utility for direct visualization of myocardial blood flow in animal models and in patients. Because first-pass techniques require high temporal resolution, image artifacts may occur, mimicking perfusion defects.
Our research focuses on solving these problems by assessing myocardial perfusion using the blood oxygenation level dependent (BOLD) effect, whereas deoxygenated hemoglobin is paramagnetic and causes signal loss in T2*- or T2-weighted images, without the administration of MR contrast media and the associated risks in patients with renal insufficiency. The ultimate goal is to be able to evaluate myocardial oxygen consumption, which will have important applications in cardiology research and clinics. We overcame technical challenges in T2*- and T2-weighted imaging of the heart and demonstrated the direct correlation between changes in MR BOLD image signal intensity and those in regional myocardial perfusion reserve measured using fluorescent microspheres. Our work shows in animals that regional myocardial flow deficit corresponding to significant coronary artery stenosis can be clearly visualized in unprocessed MR BOLD images, representing a major improvement in image quality and signal contrast over previous work.
In the meantime, we have dramatically improved spatial resolution, spatial coverage, signal to noise ratio, and reduced image artifacts with the development of 3D whole-heart contrast-enhanced myocardial perfusion approach in recent years. This and other new developments may play an important role in the adoption of MRI myocardial perfusion evaluation as a routine clinical test.
- MRI-Guided Endovascular Procedures
Compared to conventional x-ray guidance, MRI avoids ionizing and iodinated contrast media. More importantly, MRI has the ability to measure changes in end-organ function at the time of the procedure. Our research on MRI-guided vascular interventions has focused on the development of techniques for monitoring guidewire and catheters, and on developing MR angiographic techniques with intravascular contrast injection. Specifically, we have developed fast imaging techniques for real-time catheter and guidewire tracking and vascular roadmap acquisitions with a frame rate of 7-9 /s. We demonstrated, for the first time, the capability to perform coronary artery catheterizations with MRI guidance using a femoral artery approach and the feasibility to obtain coronary MRA images with intravascular contrast injection.
- 3D MRI of Atherosclerosis
Acute ischemic coronary syndromes often result from the rupture of a mildly to moderately stenotic coronary artery plaque, leading to thrombus formation. It is therefore important to detect atherosclerotic plaque formation in the arterial wall in an early stage, before significant coronary artery stenoses reduce blood flow. We have developed MR techniques to enable visualization of arterial wall and atherosclerosis in 3D with isotropic resolution, efficient coverage of the entire region, and adequate contrast between lumen and vessel wall. The major technical challenge is to suppress blood signal in the lumen to create adequate contrast between vessel wall and blood. Recently, We developed a flow sensitive dephasing method to effectively suppress blood signal even in regions with slow flow. This may allow efficient evaluation of the plaque burden of the entire body.
- MR Angiography Without Contrast Media
Contrast-enhanced imaging is the clinically accepted conventional method for MR angiography. However, patients with renal insufficiency who receive gadolinium-based agents are at risk for developing a debilitating and a potentially fatal disease known as nephrogenic systemic fibrosis. Therefore, MR angiographic methods that don’t require contrast agents will have vital importance for patients with renal insufficiency. We developed a novel non-contrast MR angiography method that overcomes many challenges in conventional methods and is particularly effective in areas with slow or complex flow such as in peripheral arteries or distal to stenosis. This method is currently undergoing preliminary clinical evaluation in several centers.
- Myocardial Tissue Characterization
Our research focuses on developing MRI methods for studying ischemic heart disease. Our efforts span the spectrum of ischemic heart disease: (a) critical coronary narrowing; (b) myocardial infarction; and (c) heart failure and sudden cardiac death. The overriding goal of our work is to develop the scientific basis for rapid translation of pre-clinical findings to clinical patient care
Ongoing research efforts include the development of MRI methods:
- To detect, stage, and develop new insights into coronary artery disease on the basis of homeostatic changes in myocardial oxygenation and/or perfusion;
- To assess chest pain in the acute-care setting;
- To evaluate short- and long-term effects of reperfusion therapies;
- To understand and evaluate the mechanisms governing ischemic heart failure and sudden cardiac death;
- To deliver and characterize cellular therapeutics into the heart muscle; and
- To develop image-processing approaches to accurately characterize myocardial tissue on the basis of MR signals.
Contributions to date have included: oxygen-sensitive approaches for evaluating coronary artery disease, development of off-resonance imaging methods for interventional and cellular MRI, development of imaging methods for characterizing acute myocardial infarctions, and development methods to assess intravascular blood pressure.