Because of its 3D capabilities, MDCT has a great potential to be used to guide interventions. In elec-trophysiology, MDCT has already been adopted, because it provides an anatomic roadmap for complex electrophysiologic procedures such as radiofrequency ablation of atrial fibrillation.
Certain properties need to be understood when comparing MDCT and invasive coronary angiography:
1. In MDCT, 3D images of the coronary arteries include atherosclerotic wall plaques. Therefore, in most cases, it is difficult to quantify luminal narrowing from the 3D images. Quantification of luminal stenosis is accomplished by analysis of cross-sectional or curved multiplanar reconstructed images, but these do not provide accurate 3D representation.
2. The contrast agent is not injected as a short bolus; neither is it selectively injected in a coronary vessel. Therefore, one cannot determine the presence of reduced flow or whether opacification of a vessel occurs from anterograde filling or from collaterals. However, this may represent an advantage, such as in visualization of chronic occlusions.
3. The anatomic information obtained in 3D space includes other anatomic structures in the thorax.
Tools exist that allow the display of the coronary images alone, but they often require some user manipulation.
Perhaps one of the most exciting potential applications of MDCT is in the anatomic definition of chronically occluded vessels. MDCT may define patency, anatomic course, caliber of the vessel, length of the stenotic segment, and extent of calcification. In addition, MDCT images may be projected side-by-side with the fluoroscopy images in the catheteriza-tion laboratory.
Until the advent of coronary CT angiography, noninvasive imaging for the detection of CAD had relied mainly on functional imaging techniques to assess perfusion or wall motion abnormalities as indirect evidence of CAD. Functional imaging proved to be very valuable in determining prognosis and establishing the need for revascularization. However, neither echocardiographic, SPECT, nor MRI stress testing can establish the presence of mild-to-moderate CAD. Moreover, decisions regarding revas-cularization cannot rely solely on functional imaging without knowledge of the coronary anatomy.
MDCT is now capable of providing detailed information about the coronary anatomy, including luminal stenosis and wall plaque. Because of the latter, it may establish the presence of atherosclerosis even earlier than invasive coronary angiography. However, the technique is limited in spatial and temporal resolution, making difficult the differentiation between moderate and severe luminal stenosis in most cases.
The rationale for the development of PET-CT or SPECT-CT hybrid systems is that, in most symptomatic patients, knowledge of both coronary anatomy and functional data is required. Hybrid systems consist of an MDCT and either a SPECT or a PET camera mounted next to each other, sharing the same patient table. This facilitates the registration of functional and anatomic data in 3D space (Fig. 3-22). In theory, other hybrid combinations, such as PET-
MRI, are also possible. In other medical fields such as oncology, the use of hybrid PET-CT systems has replaced the use of either modality alone. In onco-logic applications, the benefit of integrating anatomic and functional data is clear, given the small size and large possible volume of distribution of metastatic tumors. However, in cardiology, the development of the technology has advanced before a clinical need has been clearly established. Several questions need to be answered before the clinical community in cardiology universally adopts hybrid systems: Is the dual information necessary in all patients? Will the cost of implementation be feasible if only a fraction of the patients having studies are using both capabilities? Could both MDCT angiography and nuclear perfusion imaging be performed with limited radiation exposure? Is the information obtained in a hybrid system superior to the information obtained from separate MDCT and PET or SPECT systems reviewed together? We certainly wait for all these questions to be answered. In the meantime, hybrid systems will likely prove to be powerful research tools in basic and clinical studies.
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