Multidetector-row Computed Tomography in the Assessment of Coronary Artery Disease
prospective trigger technique. As the length of data acquisition can vary, this approach allows the reconstruction of several image data sets at different points of the cardiac cycle, depending on the length of the trigger window. Although this technique is not as dose-efficient as pure prospective triggering, it appears to be a virtually ideal mixture to reduce radiation exposure while ensuring sufficient safety for identifying the ideal phase for image reconstruction.
Scanner Technology – Dual-source Computed Tomography versus 320-detector Rows
Starting with four-slice CT scanners in 1998, a ‘slice race’ was started with constantly increasing numbers of detector rows. This slice race did not stop with the introduction of 64-row CT scanners. Recently, 128-, 256- and 320-row single-source systems, as well as 128-slice DSCT scanners, have been introduced into clinical routine practice. While there are great differences in detector widths, ranging from 3.8 to 16cm, all of these scanners have broader detectors compared with the preceding scanner generation. Although this development appears to be quite predictable, there are different philosophies at work. The basic idea is that complete volume coverage of the heart within a single heartbeat may reduce susceptibility to arrhythmia and bring down radiation exposure to the patient. Both approaches use prospective ECG triggering utilising data from a single RR interval. However, with a 16cm detector, no table movement is needed; by contrast, with comparably small detectors such as those mounted in current DSCT scanners, rapid table movement is needed to acquire all projection data within a single heartbeat. Both techniques have specific advantages. While the first approach provides truly isophasic data, it suffers from worse temporal resolution unless data from several rotations and therefore different heartbeats are acquired. It thereby loses the advantage of isophasic data acquisition and limits dose efficiency. The second approach is limited, as data are obtained from different points within the same RR interval. With an average cardiac scan taking about 270ms, the data come from different phases of the diastole. However, high temporal resolution and very low radiation exposure are achieved, and this technique appears to be well-suited for patients with heart rates
exposure down in the range of approximately 1mSv.22
while the second technique brings Therefore, both
techniques are comparable or even superior to catheter coronary angiography in terms of radiation exposure.
Multidetector Computed Tomography in Coronary Artery Disease – Imaging Beyond the Coronary Arteries
The most important limitation of CT coronary angiography in the work-up of suspected or true CAD is the fact that it does not provide information on the haemodynamic relevance of a coronary artery lesion. Previous studies comparing MDCT and myocardial perfusion imaging revealed substantial differences between the presence of stenotic lesions on cardiac CT and signs of ischaemia on myocardial perfusion imaging.23
Figure 1: Forty-six-year-old Female Patient with an Intermediate Risk of Coronary Artery Disease
A B
C D
At a heart rate of 59 beats per minute (bpm), prospective electrocardiogram (ECG) triggering was used for computed tomography (CT) angiography, resulting in a radiation exposure of 2.1msv. While a 3D volume-rendered image demonstrates coronary anatomy (A), coronary artery disease is excluded by curved multiplanar reformats for the left anterior descending artery (B), left circumflex artery (C) and right coronary artery (D).
Perfusion-weighted Imaging
Assessment of myocardial perfusion using CT is based on changes in myocardial tissue attenuation after contrast medium administration. Perfusion defects may be identified as areas of reduced contrast enhancement during rest and/or stress imaging. The first applications of this technique were described as early as 1976.24 Since then, the technique has markedly improved, but was limited to rest imaging. The introduction of isotropic voxels enabled computation of high-quality multiplanar reformats and thereby presentation of CT images along the short and long axes of the heart, as performed in MRI or echocardiography. The development of perfusion-weighted colour maps improved the sensitivity for detecting perfusion abnormalities.25
However, the value of arterial-
phase imaging at rest is limited as it provides neither quantitative data on myocardial perfusion nor direct information on myocyte injury. This is because hypoattenuating myocardium represents not only perfusion deficits but also myocardial infarction. Therefore, other techniques for assessing the effects of CAD on myocardial perfusion were sought.
Herein lies the greatest challenge for cardiac CT imaging in CAD. The introduction of recent CT scanner technology stirred up this field by introducing a number of revolutionary changes. Accordingly, the potential of cardiac CT to evaluate coronary morphology in combination with different approaches towards myocardial perfusion imaging in a comprehensive examination are under investigation.
EUROPEAN CARDIOLOGY
To obtain information on myocardial perfusion reserve, myocardial perfusion imaging needs to be performed under stress (and rest) conditions. An initial study combining arterial-phase rest and stress MDCT including coronary CT angiography was performed in 2005 using 16-slice CT.26
This technique was refined with 64-slice CT27
and an
initial patient study showed promising results with a sensitivity of 93% for detecting vessels with >50% stenosis and a corresponding
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