Novel Views of Vulnerable Plaque
The early indications suggested vulnerable plaque imaging should have become the natural next step in coronary artery disease (CAD) diagnosis. Cardiologist James E. Muller, MD, was the first person to recognize in 1989 that there was something wrong about the commonly held belief about arterial narrowing and MI.   

He offered a new view with vulnerable plaque as the cause. Researchers would soon appreciate that an infarction is actually a cascade of catastrophic events beginning with the rupture of the plaque’s thin fibrous cap and the release of its lipid core into the coronary blood stream. Clotting blood produces a thrombus blocking the artery. Within minutes, myocardial muscle cells become ischemic and start to die.

Revelations about the actual cause of MI led researchers to seek out new ways to diagnose, evaluate, treat and prevent coronary heart disease, stroke and other lethal forms of atherosclerosis. The effort capitalized on contemporary discoveries in immunology, angiogenesis, genomics and proteomics. It became an important part of the new scientific disciplines of molecular medicine and molecular imaging. New intravascular ultrasound (IVUS), optical coherence tomography (OCT) and noninvasive PET, SPECT, MRI, multidetector CT technologies generated images and data elucidating plaque behavior.

But bridging the gap between bench science and clinical practice  has not been easy, according to Zahi A. Fayad, MD, director of cardiovascular imaging research at the Mount Sinai Medical Center in New York City. “We first had to understand the biology, which was not simple,” he says. “Finding the right tool is not obvious. It’s a long road, but we are getting closer.”

In the meantime, the diagnosis of CAD has not changed appreciably. Invasive x-ray coronary angiography remains the gold standard. Cardiac catheterization reliably assists to diagnose the location and severity of CAD, but it is an anatomic test, largely divorced from the actual cause of infarction. It rates the severity of disease and the likelihood of infarction on the old notion of arterial narrowing.

Fayad noted the imperfections of cardiac catheterization at the 2011 Radiological Society of North America meeting. “Non-significant luminal stenosis of less than 50 percent is really the main culprit in acute MI,” he said. “And there’s more and more evidence that it’s the non-stenotic lesions that lead to stroke.”

Vulnerable plaque imaging has not yet proven to be an effective alternative to cardiac catheterization, either. The prospective, multicenter PROSPECT trial demonstrated in 2011 that high-resolution virtual-histology IVUS (VH-IVUS) was not up to the task. The study, involving 697 patients with acute coronary syndrome (ACS), aimed at identifying the clinical and lesion-related factors that place patients at risk for MI and other adverse cardiac events. Both three-vessel coronary angiography and VH-IVUS for plaque characterization were performed after a percutaneous coronary intervention.

During three-year follow-up, 11.6 percent of patients had major adverse cardiovascular events that were associated with untreated coronary segments. Most showed no evidence of severe stenosis on conventional angiography, but a small luminal area, a large plaque burden, and the presence of a thin-cap fibroatheroma (TCFA) were consistently observed with VH-IVUS.

However, lead author Gregg W. Stone, MD, concluded the VH-IVUS was not ready to play a diagnostic role. The presence of TCFA correlated well with the subsequent risk of a major event, but only 26 of 595 plaques with TCFA were the culprit sites of a major CV event in the three years following initial assessment.

The trial demonstrated the impracticality of using imaging to predict which plaque will rupture, says Marco Costa, MD, PhD, director of the Vascular Research Institute at Case Western Medical School in Cleveland. “It is very difficult to study the topic because of the incidence of rupture is very small. Most patients develop stable progression,” he adds.

The low specificity of VH-IVUS also raised questions about its value for plaque assessment, says David Vancraeynest, MD, PhD, a professor of cardiovascular imaging science at the Catholic University of Louvain in Brussels. “Virtual histology also has limited spatial resolution, which precludes assessment of the most prominent feature of TCFAs: the fibrous cap thickness,” he says.

Moving along molecular pathways

Due to molecular imaging research, direct anatomical observation is not the only way to detect and characterize vulnerable plaques. Its presence does not arise from a single event, but is a part of a long pathophysiological process involving several phases and specific molecular markers signaling its progression.

The relevant molecular pathways have been described in detail, though researchers still do not agree which ones offer the best hope for characterizing plaque risk, Fayad says. “I’m not convinced that we have enough evidence to say which markers we need to focus on,” he says. “We know inflammation is important, but we still need to learn more about it. Nobody has done a study where they are combining information from different tests. I would like to see research directed in that area.”

Inflammation plays a key role in acute destabilization of atherosclerotic plaque, according to Francesca Pugliese, MD, PhD, senior clinical lecturer in advanced cardiovascular imaging at William Harvey Research Institute, London. Dense inflammatory infiltrates usually are found at the site of plaque rupture in patients dying from acute MI or stroke.

Studies have established FDG as a useful tool for assessing intraplaque inflammatory activity in atherosclerotic disease, says Pugliese. FDG uptake in such studies is highly reproducible, paving the way for the first clinical drug trials using FDG uptake as a surrogate marker for plaque inflammation in the mid-2000s.

Now, FDG PET/CT is overcoming barriers to its implementation to become an option for imaging coronary artery inflammation associated with atherosclerotic plaques.

A high target-to-background ratio is a key to targeting conspicuity for any type of imaging. Physicians cannot see the target if the background is shining bright, as can be the case for hyperintense myocardium from FDG uptake that overwhelms its illumination of intra-arterial inflammation associated with coronary plaques.

Joanna Wykrzykowska, MD, an interventional cardiologist at the Academic Medical Center in Amsterdam, suggests a solution. A low-carbohydrate, high-fat meal the night before and a vegetable oil drink on the morning of an FDG-PET could suppress the myocardial FDG uptake. Also, Ian Schirra Rogers, MD, a cardiovascular fellow at Stanford University Medical Center in California, showed in a study of 25 patients in 2011 how FDG-PET can depict intra-arterial inflammation associated with stented lesions in ACS and stable angina.

FDG-PET & arterial inflammation

The pattern of arterial inflammation mapped with FDG was significantly more evident in the aorta and left main and left anterior descending coronary arteries in ACS patients than patients with stable angina. The observed difference was consistent with the hypothesis that ACS induces a systemic inflammatory state.  In other studies, F18-FDG uptake was correlated with the response of carotid and aortic atherosclerotic plaques to simvastatin therapy.

In the study’s accompanying commentary, Farouc A. Jaffer, MD, PhD, of the cardiovascular research center at Massachusetts General Hospital (MGH) in Boston, noted how FDG-PET would still have to overcome the limitations of poor spatial resolution to play a role. The average volume of a plaque is about 0.1 mL and the volume resolution of F18-FDG PET is about 0.125 mL, which means PET detection of coronary plaques typically spans a single voxel.

Plaque inflammation also can be noninvasively studied with an MRI study enhanced with ultra-small paramagnetic iron oxide. Dysfunctional endothelium initiates an inflammatory reaction within the arterial wall, which allows the accumulation of low-density lipoprotein (LDL), Vancraeynest says.

Oxidized LDL particles are then phagocytised by macrophages. The particles penetrate the plaques and accumulate in plaques with high macrophage content. When the particles are present in a sufficient quantity, they cause a detectable signal, he notes.

Tjun Y. Tang, MD, and colleagues in the vascular unit at Cambridge University in England, demonstrated the feasibility of serial USPIO-MRI to monitor the effect of atorvastatin on plaque inflammation. Reduced ultra-small-super-paramagnetic-iron-oxide (USPIO) uptake was detected as early as six weeks after high-dose atorvastatin therapy.

If one molecular modality is good, two can be better, as Fayad found in a recent phase II randomized, multicenter trial. FDG-PET and MRI measured carotid artery plaque morphology and change in response to dalcetrapib. The drug modulated cholesteryl ester transfer protein to raise high-density lipoprotein cholesterol.

MRI uncovered a significant reduction in lesion size for patients treated with the drug for 24 months, compared with patients who were administered a placebo. FDG PET/CT measured a 7 percent reduction in target-to-background ratio, which indicated reduced inflammation, for the most-diseased segment of artery in the treatment group, compared with the control arm, says Fayad.

Ultimately, multimodality approaches involving noninvasive imaging, a blood test, such as a marker for inflammation, and a genetic test for identifying the patient’s predisposition for atherosclerotic disease may be the ideal solution, Fayad proposes.

With a resolution of 10 to 15 ?m, OCT is an intravascular imaging modality capable of measuring fibrous cap thickness, says Costa. OCT has high sensitivity for thrombus and can examine erosion.

“If there is a possibility to identify plaque with an invasive technique, then OCT would definitely be the technique of choice,” Costa says.

OCT already can play a clinical role by assessing ACS patients without ST-elevation. A recent cohort study, conducted by Costa et al, found OCT changed the management of 80 percent of interventional coronary procedures.

“When they come with an electrocardiogram that is not specific, it doesn’t tell me in the cath lab which vessels to go to,” Costa says. “Some of these patients may have multiple blockages in multiple vessels. That is when an image modality with high resolution can make a big difference.”

3D OCT promises even higher resolution. Costa’s group is developing automated methods for identifying lipid plaques, without direct user involvement. The entire extent of the fibrous cap will be automatically segmented and converted into volume renderings mapping the topography of the fibrous cap.

An intra-arterial catheter, developed in Jaffer’s laboratory at MGH, combines optical frequency-domain imaging and near-infrared fluorescence to simultaneously obtain structural and molecular images. The investigational device, thus far tested in rabbits, involves a fiber optic probe with a constantly rotating laser tip to produce detailed molecular images of arterial walls. Early experience with the device has established its ability to identify the presence of atherosclerotic plaques and enzymatic activity associated with inflammation and plaque rupture.

The use of imaging for detecting and understanding vulnerable plaque continues to evolve as it journeys from bench to bedside. But many physicians see its potential as a valuable tool in both the lab and clinic.