Magnetic Resonance Imaging and Positron Emission Tomography in the Evaluation of Cerebrovascular Disease

Andres Fajardo and Michael C. Dalsing

The evolution of diagnostic tools available to study cerebrovascular disease has advanced in recent years. Although some of the major tests aid in identifying occlusive or aneurysmal disease, especially in patients with contraindications to standard contrast studies, the newest techniques have the goal of identifying patients at higher risk for cerebrovascular events possibly related to plaque instability or inflammatory activity within the plaque.

Among these diagnostic modalities, magnetic resonance imaging (MRI) and positron emission tomography (PET) have generated high expectations. However, the high cost and need for specialized equipment have prevented these modalities from being adopted for routine evaluation of most patients.

Magnetic Resonance Imaging

MR angiography (MRA) was introduced in the late 1980s. Multiple technical modifications over the subsequent years have improved image quality and reproducibility. The introduction of contrast-enhanced MRA in 1992 greatly improved the image quality. Gadolinium chelated contrast agents led to a substantial decrease in the T1 relaxation time, resulting in high intravascular signal intensity. This technique has a diagnostic accuracy of 90% and specificity greater than 95% in quantifying the degree of stenosis at the carotid artery bifurcation. It also has sensitivity greater than 95% for the diagnosis of intracranial aneurysms.

However, the discovery of nephrogenic systemic fibrosis (NSF) and its relationship to gadolinium administration in patients with renal impairment required caution in the routine use of this agent. Nephrogenic systemic fibrosis is a fibrosing disease primarily identified in the skin and subcutaneous tissues, but it is also known to involve other internal organs. Signs and symptoms can develop and progress rapidly, and some affected patients develop contractures and joint immobility. Death ensues in some patients, presumably as a result of visceral organ involvement. Cardiac muscle, skeletal muscle, and the visceral muscle of the gastrointestinal tract (esophagus, stomach) in addition to the eyes and meninges have all been affected.

Other methods have been proposed to replace gadolinium as a contrast agent. Electrocardiogram (ECG)-gated steady-state free precession (SSFP) MRA (NATIVE TrueFISP: true fast imaging with steady-state precession) non–contrast-enhanced angiography of arteries and veins is one such technique. It allows visualization of blood flow within a vascular bed after preparation of the imaging volume with an inversion recovery pulse, and to avoid substantial flow artifacts, the technique is performed using ECG triggering, with the data being acquired during diastole. This technique has been successful in measuring cross-sectional vessel area. The results obtained do not appear to significantly differ from the results obtained when using contrast-enhanced MRA. However, further studies comparing these techniques are required to confirm these findings.

In addition to determining reduction of cross-sectional diameter, gadolinium-based contrast agents can enhance visualization of the fibrous cap of atherosclerotic plaque, and dynamic contrast enhancement can give an indication of plaque inflammation and vascularity. MRI of atherosclerosis plaque can provide information on plaque volume and composition. Most research has been centered on imaging of the aorta and carotid artery. Compared with other modalities, MRI appears to have the greatest potential for characterizing plaque. This is as a result of the ability of MRI to use multiple contrast weightings (T1, T2, and proton density). It has been shown that MRI of the carotid artery can differentiate four main plaque components: fibrous cap, lipid-rich necrotic core, intraplaque hemorrhage, and calcification. There have even been studies showing a correlation between particular changes in the plaque components in patients with recent neurologic events. Some authors have shown that there is a reduction in atherosclerotic plague burden of the carotid arteries and the aorta after 12 months of statin treatment as determined by MRI. They showed that lower levels of low-density lipoprotein (LDL) following appropriate therapy is associated with a decrease in plaque size.

Vascular MRI continues to evolve in terms of sequence development, hardware evolution, and contrast-agent production. Also, the development of a range of targeted molecular and cellular contrast agents could allow identification of the molecular constituents of individual plaques. These molecular agents include nanoparticles, liposomes, and lipoproteins. Some early animal studies have shown encouraging results.

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