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Science to Practice |
1 Cardiovascular Imaging Laboratory, Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 S Kingshighway Blvd, St Louis, MO 63110 woodardp{at}mir.wustl.edu
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Coronary artery disease affects 13.2 million Americans (1) and is also the principal cause of mortality among Europeans who are 65 years of age and older (2).Fig 1 In the United States, 1.5 million individuals undergo invasive x-ray coronary angiography each year (1), and a large number of these individuals also undergo echocardiography within days prior to angiography to assess left ventricular function.
The progression from single- and dual-detector systems to current 16– to 64–detector row systems has allowed improved coronary computed tomographic (CT) angiography, along with better coronary lesion visibility (3–5). With retrospective electrocardiographic gating, data acquired for CT angiography can also be used to create volumetric cine images of the heart and, thus, assess cardiac function. There is no need for repeat scanning, for increase in radiation dose, or for administration of additional contrast material. Two-dimensional multiplanar reconstruction cine images can be created from the volumetric data at increments throughout the R-R interval in any plane desired. In this issue of Radiology, Mahnken et al (6) use data acquired during coronary CT angiography in pigs to create short- and long-axis cine images of the left ventricle suitable for analysis of left ventricular function.
Data acquired for CT angiography can also be used to create volumetric cine images of the heart and, thus, assess cardiac function.
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The Science
Mahnken et al (6) demonstrate excellent correlation of left ventricular ejection fraction, calculated by using analysis of short-axis multiplanar reconstructions of cine CT images obtained through the heart, with left ventricular ejection fraction calculated by using analysis of multiple short-axis cine magnetic resonance (MR) images. For the past several years, MR imaging has been considered the reference standard for the assessment of left ventricular function (7). Mahnken et al (6) also demonstrate excellent correlation between CT and MR imaging for end-diastolic volume, end-systolic volume, stroke volume, and myocardial mass. They, however, do not demonstrate good correlation between CT and MR imaging for peak ejection rate, peak filling rate, time to peak ejection rate, and time from end systole to peak filling rate. This is so, principally because these parameters are time-dependent calculations, and the temporal resolution of the coronary CT scan obtained with electrocardiographically gated 16–detector row CT is poorer than that of the images obtained with cine cardiac MR imaging. CT imaging, therefore, misses the true peak ejection rate and peak filling rate. Advances in CT imaging that increase temporal resolution would be expected to correct this problem. Some advances have already occurred. Gantry rotation time used by Mahnken et al (6) with the 16–detector row system was 420 msec per rotation, thus providing an average temporal resolution of 152 msec. Newer 64–detector row scanners have a gantry rotation time of 330 msec per rotation and could provide temporal resolution approaching 100 msec.
The Practice
Clinical use.—As spiral CT technology continues to improve, information acquired by using both conventional x-ray angiography and echocardiography could be obtained in a single noninvasive examination performed during a single breath hold of 10 seconds. Thus, information required not only for anatomic assessment but also for left ventricular function could be obtained very quickly.
Future opportunities and challenges.—Important for the future are the following: capability of assessing the presence of coronary atherosclerosis and the percent diameter stenosis within the affected coronary artery, determination of whether or not the plaque is soft tissue or calcified tissue (implying plaque stability) and of the effect of the lesion on myocardial function. There is the theoretical possibility of performing a CT examination with pharmacologically induced stress or of using the cine images to assess valve function. While 64–detector row systems have not yet been assessed in clinical trials, 16–detector row systems have demonstrated a sensitivity of 82%–95%, specificity of 98%, and positive and negative predictive values of 87% and 97%–99%, respectively, for coronary atherosclerotic disease detection (4,5). A challenge for industry is improvement of temporal resolution so as to decrease the effects of cardiac motion. This needs to be achieved without an increase in radiation dose. In addition, software that provides the capability to quickly segment and quantitatively analyze data will also be necessary, and some vendors are already responding to this challenge. Methods to decrease the radiation dose also are needed. Currently, a decrease in radiation dose can be achieved by using techniques such as pulsed electrocardiographic gating (a method in which the tube current–time product is decreased during acquisition of data in selected phases of the cardiac cycle that are not required for construction of coronary CT angiographic images). Techniques such as these will be useful in preserving image quality, yet they can decrease by nearly 50% a radiation dose that may otherwise approach 8–10 mSv with 16–detector row systems (8).
Summary
Multi–detector row CT has great potential to provide not only anatomic but also functional cardiac information if the challenges of improved temporal resolution, ease of image analysis, and radiation dose reduction can be met. In this issue of Radiology, Mahnken et al (6) demonstrate with an animal model some indication of what the future holds in cardiac imaging with multi–detector row CT.
References
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