Noninvasive Methods to Assess Atherosclerosis: Part 4

We are going to finish up our series discussing two modalities which most people are not familiar with as way to assess for atherosclerosis.

Cardiac Magnetic Resonance Imaging

MRI uses radio waves and magnets to create images of your organs and tissues. Unlike computed tomography scans (also called CT scans) or conventional x-rays, MRI imaging doesn't use ionizing radiation or carry any risk of causing cancer.

Cardiac MRI is a sophisticated powerful imaging system tool that provides superb anatomic, functional, and tissue images. Since MRI is done for nearly everything, many centers now perform cardiovascular MRI and it is thought to be a cardiac "all-in-one" tool. We will be limiting our discussion to the use of MRI in evaluating atherosclerosis.

Based on the current level of technology available, MR-based evaluation of coronary anatomy remains challenging. The sensitivity, specificity, and accuracy for diagnosing any coronary disease is 100, 85, and 87%, respectively, in patients with either left main coronary artery disease or three vessel disease. Sensitivity refers to the actual positive studies that are correctly identified and specificity refers to the actual negatives that were correctly identified. Atherosclerotic plaque has been studied using cardiac MRI in both the heart and peripheral arteries. In 2002, one study suggested that cardiac MRI might be useful in detecting subclinical atherosclerotic vascular disease. While this technology is very exciting and promising as a screening tool to detect atherosclerosis, it cannot be recommended routinely at this time.

Ankle-Brachial Index

ABI was initially used to identify the presence and extent of peripheral vascular disease (PVD). In my residency, every patient with symptoms of PVD underwent an ABI. While some think this is difficult to do, every medical student learns to do this on their surgery rotation. The only requirements are a blood pressure cuff and a hand held Doppler probe. ABI is measured as the ratio of the systolic blood pressure in the foot/ankle arteries (posterior tibial or dorsalis pedis arteries) over the systolic blood pressure in the arm artery (brachial artery). Both the arm pressures are measured and both the arteries in each leg. The higher leg pressure in each leg is then divided by the higher pressure recorded in the arm. If 1 is a normal value, then incremental reductions indicate worsening vascular disease. In a definitive paper, after adjustments were made for age, LDL cholesterol, and carotid intimal medial thickness (CIMT), an ABI <0.9 was shown to be an independent predictor of cardiovascular events. An ABI <0.9 yielded a 90% sensitivity and 98% specificity for moderate-to-severe obstructive peripheral artery disease as determined by a confirmatory conventional angiogram. While ABI alone is not ideal to screen for mild disease, it is estimated that 40% of patients with positive ABI were asymptomatic, which would mean that it is an effective way to identify vascular disease before it is clinically apparent. A patient with symptoms and an abnormal ABI should then undergo an angiogram to identify the correct anatomic location of vascular disease.

Part 1: CIMT
Part 2: CT Angiogram of the Heart
Part 3: Coronary Artery Calcium Scoring