Putting the Face to a NameConcurrent Assessment of Vascular Morphology and Biology
Vasken Dilsizian, MD*,
Jagat Narula, MD, PhD ,*
* University of Maryland School of Medicine, Baltimore, Maryland
University of California-Irvine, Irvine, California
With the hybrid positron emission tomography (PET) and computed tomography (CT) systems, coronary artery anatomy and physiology can be realized during the same imaging session. Whereas the coronary CT angiography provides information on the presence and extent of the luminal stenosis in coronary artery disease (CAD), PET blood flow tracers provide information on the downstream functional consequence of these lesions. Current clinical applications of these imaging studies are focused predominantly on the identification of coronary artery lesions, which can be treated by coronary interventions. With the evolving technology, CT angiography may allow interrogation of the plaque morphology beyond the assessment of the luminal stenosis and the new radio-labeled ligands may expand the application of PET to assess plaque biology and instability. This is particularly relevant in asymptomatic subjects with coronary risk factors in whom the diagnosis often remains elusive before the actual occurrence of an acute coronary event. Accordingly, an important driver for the hybrid cardiac PET-CT imaging will likely come from a paradigm shift in the health care delivery from diagnosis and treatment to prediction and prevention of disease.
The cost-effectiveness of cardiac prevention is perceived to be so large that the number of individuals to be considered for lipid lowering, even based on the well-accepted guidelines, exceeds 50 million in the U.S. alone (1). It is expected that with imaging of the atherosclerotic plaque for risk assessment, it would become possible to adjust medical therapy more closely to the subjects' actual needs and prevent unnecessary health care costs. The clinical usefulness of such a strategy can be better appreciated because at least 50% of acute coronary events originate in coronary arteries that do not reveal significant angiographic stenosis. Regardless of the severity of luminal obstruction, the culprit plaques in the victims of acute coronary events histopathologically demonstrate larger plaque and necrotic core volumes, greater necrotic core-to-plaque volume ratio, and usually significant positive remodeling at the site of vascular involvement; these plaques are covered by thin fibrous caps and are substantially inflamed (2). The characteristic structural features of such plaques have been noninvasively described by CT angiographic imaging in patients presenting with acute coronary events. The most important determinants of plaque instability included positive remodeling and low attenuation plaques (suggestive of necrotic cores) (3). Identification of such coronary lesions in the absence of an event predicts almost a 25-fold higher likelihood for development of an acute coronary event over the follow-up of 2 years as compared with a not positively remodeled plaque of intermediate or higher attenuation (4). The larger the plaque volume or the necrotic core area, the earlier an event occurred. Although the tools that identify the structural features of the plaques predict a greater vulnerability, they fail to identify the critical component of the plaque instability (i.e. inflammation).
Radionuclide tracers, by their nature, reflect pathophysiologic processes at the subcellular level and have been employed for the detection of immunoinflammatory infiltrates. F18-labeled 2-fluorodeoxyglucose (FDG) has been most commonly used for this purpose. The FDG uptake as assessed by PET correlated closely with the extent of inflammation in the carotid endarterectomy specimens and can be assessed serially to monitor the efficacy of treatment (5). The FDG imaging was not easily feasible in the coronary vessels because of a lower resolution of the PET-based imaging techniques. A recent report has vividly demonstrated that a hybrid PET-CT imaging allowed the localization of coronary inflammatory activity (6). The increased FDG uptake is expected to occur in metabolically active cells including the cells of monocyte–macrophage origin in the unstable plaques. A study in this issue of iJACC, targets a more specific process of expression of adhesion molecules on the vascular endothelial cells. This study employs the combined virtues of structural and functional imaging (7).
Integrating the synergistic information that exists between PET and CT to characterize coronary plaques would allow better disease definition, more effective choice of therapeutic intervention, and individualized monitoring of response to treatment. Arguably, this is one of the most exciting fields in cardiovascular medicine, and the trajectory for future growth of targeted molecular imaging with PET-CT for characterizing inflammatory atherosclerosis seems boundless. Better definition and localization of the functional activity is similar to the proverbial expression of putting the face to a name!
* Address for correspondence: Jagat Narula, MD, PhD, Editor-in-Chief, JACC: Cardiovascular Imaging, 3655 Nobel Drive, Suite 630, San Diego, California 92112 (Email: narula{at}uci.edu).
 |
REFERENCES
|
|---|
- National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) Final Report Circulation 2002;106:3143-3421.[Free Full Text]
- Narula J, Garg P, Achenbach S, Motoyama S, Virmani R, Strauss HW. Arithmetic of vulnerable plaques for noninvasive imaging Nat Clin PractCardiovasc Med 2008;5(Suppl 2):S2-S10.[CrossRef]
- Motoyama S, Kondo T, Sarai M, et al. Multislice computed tomographic characteristics of coronary lesions in acute coronary syndromes J Am Coll Cardiol 2007;50:319-326.[Abstract/Free Full Text]
- Motoyama S, Sarai M, Harigaya H, et al. Computed tomographic angiography characteristics of atherosclerotic plaques subsequently resulting in acute coronary syndrome J Am Coll Cardiol 2009;54:49-57.[Abstract/Free Full Text]
- Tahara N, Imaizumi T, Virmani R, Narula J. Clinical feasibility of molecular imaging of plaque inflammation in atherosclerosis J Nucl Med 2009;50:331-334.[Abstract/Free Full Text]
- Rogers IS, Figueroa AL, Nasir K, et al. Assessment of coronary segment inflammation with combined 18F-FDG positron emission tomography and 64-slice multidetector computed tomography[abstract] Circulation 2007;116II–410.
- Nahrendorf M, Keliher E, Panizzi P, et al. 18F-4V for PET-CT imaging of VCAM-1 expression in atherosclerosis J Am Coll Cardiol Img 2009;2:1213-1222.[Abstract/Free Full Text]
|
Top
REFERENCES