Future Trends in Coronary CT Angiography

Introduction

Conventional coronary angiography is currently the gold standard for assessing coronary artery patency. Both spatial and temporal resolutions are excellent and permits treatment with percutaneous coronary intervention at the same session (Roberts, Bax, & Davies, 2008). The procedure usually takes about 30 minutes and demands the use of radiation and radiopaque contrast media. (Scanlon, 1999) Conventional coronary angiography, however, fails to visualize the artery wall and so is incapable to reveal an outward remodeling, as it creates a 2D coronary “lumenogram” (Naghavi, 2003). More precise depiction of coronary arteries is achieved combining intravascular ultrasound (IVUS) and conventional coronary angiography. (Papadogiorgaki et al., 2008), (Giannoglou et al., 2007). Both conventional coronary angiography and IVUS are invasive methods and their application has the risk of major complications for the patient i.e. arrhythmia, stroke, coronary artery dissection and access site bleeding (1.8%, mortality rate 0.1%) (Hoffmann, 2005) which can raise in the case of a patient being in an unstable condition. Thus, the need for non-invasive methods for 3D coronary imaging have emerged, and modalities such as multislice computed tomography (MSCT), electron beam computed tomography and magnetic resonance imaging are now clinically available (Escolar, 2006).

MSCT is the basis for coronary computed tomography angiography (CCTA). MSCT is a spiral CT, where an increased number of detector rows (channels) allow multiple channels of data to be acquired simultaneously. At the beginning, the number of channels was 4 and subsequently has increased to 16-, 64-, 128- and lately to 256- and 320-channels. Multiple heart beats are being scanned during MSCT performing, while heart is synchronized with simultaneous ECG recording and data acquisition during all cardiac phases allows the use of data from certain parts of each consecutive cardiac cycle. Last generation MSCTs tend to be dual band and dual energy. Dual source means they contain 2 X-ray sources and 2 sets of detectors offset 90° from each other in the CT gantry increasing this way temporal resolution, while dual energy refers to the area of interest being scanned using x rays generated at different voltages, improving thus tissue differentiation and spatial resolution. Post-processing of axial images is performed on an independent workstation where retrospective reconstruction of the coronary arteries takes place. Reconstruction is achieved using special software, where maximum intensity projection [MIP] and three-dimensional multiplanar (MPR) reformats are mainly used (Roberts, Bax, & Davies, 2008), (Mark et al.), (Hassan, Nazir, & Alkadhi, 2010).

CCTA surmounts the complications that may arise from undergoing conventional coronary angiography. It is a high speed method (about 5 sec), takes place during a single breath and there is no need for hospitalization. CCTA offers the possibility to distinguish the arterial wall and indicate an outward remodeling. In addition fewer amounts of contrast media are needed. The mean radiation dose for 64-channel CT compared to conventional coronary angiography is 15 mSv (range 12-18 mSv) vs. 7mSv (range 2-16mSv). However, with tube current modulation the mean radiation dose for 64-channel CT is 9 mSv (range 8-18mSv) and with prospectively triggered CCTA is 3 mSv (range 2-4mSv) (Mark et al.). Prospective ECG-gating that is applied on latest MSCTs decreases radiation dose further compared to retrospective ECG-gating being present in most MSCTs studies: in phases of maximal cardiac movement there is tube-current modulation and less beam emission (Wijesekera, Duncan, & Padley). The entry of 320-MSCT has diminished more the radiation dose needed (4.2 vs. 8.5 mSv; p<0.05), as well as the volume of contrast media (80 vs. 111 ml; p<0.001) (Dewey et al., 2009).