Multivariable Assessment of Coronary Artery Disease Using Cardiac CT Imaging

Background

Multidetector computed tomography (MDCT) is the most recent addition to the arsenal of cardiac imaging modalities. With its unparalleled spatial resolution and well established techniques for contrast enhancement using conventional iodine-based agents, it allows visualization of coronary arteries and is thus increasingly used as an alternative to invasive coronary angiography (ICA) (de Roos A. et al. 07,Deetjen et al. 07,Schroeder et al. 08). The diagnostic value of noninvasive coronary angiography (CTCA) has been established against conventional techniques used for the diagnosis and evaluation of coronary artery disease (CAD), including ICA (Budoff et al. 07,Leber et al. 05,Raff et al. 05,Rubinshtein et al. 07) and SPECT myocardial perfusion imaging (MPI) (Hacker et al. 07,Rubinshtein et al. 07,Schuijf et al. 06). Nevertheless, the physiological significance of intermediate grade stenosis detected by CTCA in individual patients is unknown and such patients are routinely referred for stress testing in order to define individual therapeutic strategy. It has been suggested that intramyocardial distribution of contrast during the arterial phase of enhancement may be related to myocardial perfusion (Cury et al. 07). Several studies have demonstrated hypo-enhanced areas corresponding to myocardial scar tissue in a small number of patients post myocardial infarction (MI) (Gerber et al. 06,Henneman et al. 06,Mahnken et al. 05,Nieman et al. 06,Nikolaou et al. 05), and in animal models of acute MI (George et al. 07,Gerber et al. 06,Hoffmann et al. 04,Lardo et al. 06). Our hypothesis is that perfusion information, which can be extracted from images acquired for CTCA without additional radiation exposure or contrast load, could be a useful addition to the MDCT evaluation of ischemic heart disease (IHD).

Accordingly, we recently completed a study designed to determine the value of MDCT assessment of resting myocardial perfusion in consecutive patients referred to CTCA. In this study, we developed and tested a new technique for quantitative assessment of myocardial perfusion based on analysis of MDCT images acquired for CTCA. The accuracy of resting MDCT perfusion was tested against ICA as well as MPI. Both protocols included a detailed investigation of the sources of inter-technique discordance.

Comparisons against ICA revealed that the majority of perfusion abnormalities detected on MDCT images at rest were associated with either prior MI, as previously reported (Gerber et al. 06,Henneman et al. 06,Mahnken et al. 05,Nieman et al. 06,Nikolaou et al. 05), or reduced blood supply secondary to significant stenosis. This previously unknown finding may have important clinical implications in the context of detection of myocardial ischemia. Although comparisons against resting MPI data showed high levels of agreement, we noted a large number of perfusion defects that were not confirmed by resting MPI. These apparent "false positive" findings were found to be either directly related to suboptimal image quality or were true positives when compared to stress MPI. This latter surprising finding may probably be explained by the effects of nitroglycerin used during MDCT imaging, as well as possible vasodilating effects of the iodine-based contrast media (Limbruno et al. 00), which may to some extent mimic those of vasodilator stress agents used during MPI, namely adenosine or dipyridamole.

The main conclusion of these recent studies was that future studies are needed to explore the full diagnostic potential of MDCT perfusion when used in combination with vasodilator stress.