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Focus on Imaging |
1 From the Department of Radiology, Brigham and Women’s Hospital, 75 Francis St, Boston, MA 02115. Received March 15, 2000; accepted March 20. Address correspondence to the author (e-mail: jfpolak@partners.org).
Index terms: Heart, CT, 51.1211 • Heart, MR, 51.12142 • Heart, PET, 51.12163 • Heart, SPECT, 51.12162 • Heart, US, 51.1298
EDITOR’S NOTE: Imaging is an indispensable component of modern-day medical care. It is a topic of discussion not only at imaging meetings but also at meetings where imaging is not the primary focus. With this issue of Radiology, we introduce a new feature entitled "Focus on Imaging." Its intent is to summarize for our readers the directions of both clinical imaging and research imaging applications as presented at nonimaging meetings. The summaries will be prepared either by Journal reviewers or by current or former members of our Editorial Board.
The first article for this new section has been prepared by Joseph F. Polak, MD, MPH, from the Department of Radiology, Brigham and Women’s Hospital and Harvard Medical School, Boston, Mass. Dr Polak has developed a format we hope to follow for future articles of this type, as information is available to the author—general meeting description, major topics of imaging focus, and a summary of each topic. We hope our readers find this new feature informative, and we welcome your feedback.Anthony V. Proto, MD, Editor
Background
The American Heart Association held its 72nd annual meeting in Atlanta, Georgia, from November 7 through November 10, 1999. Total attendance was 31,359, which comprised 20,213 health care professionals with the remainder representing exhibitors. The American Heart Association, although having a strong cardiology representation, is not a specialty association restricted to cardiologists. The Association is made up of 12 councils, only one of which is clinical cardiology. Cardiovascular radiology has representation as one of these 13 councils, although it is far from having the size of some of the other councils. At the meeting, 17% of the registrants were members of the Clinical Cardiology Council; 11% were members of the Arteriosclerosis, Thrombosis, and Vascular Biology Council; while 1% were members of the Cardiovascular Radiology Council. The 72nd annual meeting had over 4,500 listed abstracts. The major emphasis was given to basic cardiovascular research, cardiac physiology, molecular biology, and genetics.
The main focus on imaging at this conference was on echocardiography, stress echocardiography, cardiac nuclear medicine imaging including single photon emission computed tomography (SPECT) and positron emission tomography (PET), and intravascular ultrasonography (US), mainly for intracoronary imaging; imaging topics outside of these specialized topics included the use of electron-beam computed tomography (CT) for the evaluation of coronary calcification and cardiac function, cardiac magnetic resonance (MR) imaging, and US of the carotid and peripheral vessels. Approximately one-half of the imaging presentations dealing with these newer aspects of imaging were made outside of the sessions supported by the Cardiovascular Radiology Council.
Abstracts with a main emphasis on imaging either were presented during sessions with an imaging theme or were interspersed among varied sessions dealing with themes such as mechanical and myocardial alterations in left ventricular function, blood vessels in hypertension, or predicting the presence of disease and patient outcome. Four sessions were dedicated to MR imaging, six to SPECT and PET, and two to vascular US, as compared to 21 on echocardiography. Of 465 sessions, 42 were focused on imaging, while 18 incorporated imaging either as an outcome or as a key predictor variable.
PET and SPECT
Abstracts dealing with PET and SPECT focused mainly on measures of myocardial viability. Approximately one-third of the abstracts presented under this category relied on SPECT perfusion imaging as the standard of reference. For example, the results of US contrast agents for evaluating myocardial perfusion were correlated with the results of SPECT perfusion imaging, or the results of stress echocardiography were compared to those of stress technetium 99m sestamibi imaging. Observations on myocardial fatty acid metabolism measured with PET were contrasted to the results of myocardial perfusion with SPECT. Many of the abstracts dealing with SPECT focused on technical issues such as strategies for attenuation compensation or methodologies available to facilitate image analysis with edge detectors.
Electron-Beam CT
Detection of coronary artery calcifications has been a topic of active research. The presence of calcium in the coronary arteries has been recognized as a marker of coronary artery disease in the asymptomatic individual and has been used widely in the community as a screening tool. A few abstracts were presented that described the distribution of calcium in patient cohorts and the association of coronary calcium with cardiovascular risk factors. The new findings at the meeting were observations on the fact that coronary calcium scores did not seem to predict myocardial perfusion deficits, plaque burden, or cardiovascular events.
MR Imaging
A "hot topic" of the meeting was coronary MR angiography. Impressive images of coronary artery plaques were obtained in vivo. The goal of this research has been the detection of plaques at high risk for causing myocardial infarction. Detecting and characterizing these vulnerable plaques has been the emphasis of major research efforts in cardiology. Until now, intracoronary vascular US and angioscopy were the only ways of visualizing these plaques in vivo. The high lipid content of these plaques would make them vulnerable to acute rupture and occlusion of the coronary artery. Impressive MR images of coronary artery plaques from an in vivo porcine model and in humans were presented. These reports suggested that performing in vivo detection of vulnerable plaques in asymptomatic patients would soon become a reality.
The applications of coronary MR angiography to the detection of hemodynamically significant coronary artery stenoses were described in various abstracts emphasizing the use of MR contrast agents, respiratory gating, or breath-hold techniques.
Other presentations on MR imaging dealt with evaluating myocardial viability and function. Topics included the analysis of local myocardial mechanics or perfusion measurements with contrast material–enhanced MR imaging of the myocardium. MR imaging was also shown to offer promise as a technique for evaluating the changes in cardiac function associated with various forms of stress testing: following exercise or following the administration of pharmacologic agents.
A few abstracts concerned the capability of MR angiography to help characterize atherosclerotic plaques in the carotid arteries or score the severity of atherosclerotic changes in other arterial beds. These presentations were made in various sessions sponsored by other councils than the Cardiovascular Radiology Council.
US Imaging
The largest number of abstracts in the topic of noncardiac imaging dealt with US imaging of the arteries. "High-resolution" US imaging has been used to evaluate overall atherosclerotic burden (carotid artery US) or alterations in vascular dynamics (carotid and brachial artery US) and endothelial function (brachial artery US).
Carotid artery US.—Measurements of carotid artery plaque and intima-media wall thickness have been obtained from "high-resolution" US images. These measurements have been accepted as markers of atherosclerosis and have served as a surrogate end point for many epidemiologic and drug studies.
Data presented at the meeting suggested that the interaction between carotid artery plaque, blood pressure, and arterial stiffness is a complex one. Most abstracts dealt with the association between intima-media wall thickness and traditional cardiovascular risk factors such as blood pressure, menopause, and homocysteine levels. Some abstracts described intima-media wall thickness as a measurement against which to determine the value of newer risk factors such as plasma C-reactive protein (a marker of inflammation) or exposure to infectious agents such as Chlamydia pneumoniae.
Brachial artery US.—The past few years have seen the emergence of data showing that the endothelial cells lining the arteries are more metabolically and functionally active than originally believed. Basic research has continued to focus on molecular interactions that distinguish healthy endothelium from various forms of endothelial dysfunction. Endothelial dysfunction has been suspected of contributing to the development of atherosclerosis.
Reports at the meeting showed that "high-resolution" US imaging of brachial artery diameter can be used to assess changes associated with risk factors for coronary artery disease. Brachial artery function was measured as a response of brachial artery diameters to various stimuli. Data from various groups showed the following: (a) Interventions that decrease cholesterol levels improve brachial artery function, and (b) oxidative stress attenuates brachial artery function, and this impairment can be reversed with antioxidants.
New Imaging Approaches
A few abstracts dealt with novel approaches to imaging. Electromechanical mapping can be used to study the electric potential of the ventricular muscle and determine viability. Data on correlative imaging against PET (perfusion and metabolism) were presented at the meeting. One abstract described the possibility of fusion of this type of information with that of fast CT imaging. Another investigation focused on in vivo measurement of the temperature of atherosclerotic plaque. Increases in temperature measured in vivo were predictive of acute coronary events. Preliminary data on a laser-based fluorescent technology to measure atherosclerotic plaque were also presented.
Summary
Imaging topics were not the main focus of the American Heart Association annual meeting. Abstracts focused mainly on the biology of cardiovascular disease, and most of the imaging topics focused on coronary arteriography, echocardiography, nuclear cardiology, and intravascular US. A small minority of abstracts, less than 5%, dealt with other aspects of cardiovascular imaging.
Investigators showed data on the utility of MR imaging and rapid CT as clinical tools for the evaluation of coronary artery disease and myocardial function.
Noninvasive imaging of other arterial beds was strongly emphasized. This effort was mostly focused on US imaging and aimed at direct measurement of arterial wall thickness and atherosclerotic plaque and at measurement of arterial wall function.
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