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KCE Reports 82 Multislice CT in Coronary Heart Disease 61
8 GENERAL DISCUSSION
For the general discussion of the performance of 64-SCT in the diagnosis of CAD, we
follow the hierarchy of diagnostic efficacy as described by Fryback and Thornbury
(appendix). 153, 154 Once a new diagnostic modality has been developed, its diagnostic
accuracy has to be established as the next step. However, making diagnoses is not what
patients are expecting from their physician: they want their symptoms to be alleviated
and/or their life to be prolonged. In order to be worthwile in clinical practice, a
diagnostic modality should alter diagnostic thinking of the attending physician, leading to
a better patient management and improving patient outcomes. From the perspective of
society, these goals should be achieved at a reasonable cost.
8.1 TECHNICAL EFFICACY
The feasibility of anatomic imaging of the coronary arteries by computed tomography
became possible by the introduction of spiral scanning and multislice CT scanning, which
provide smaller pieces of information and cover a larger area faster than conventional
CT. Especially, 64-SCT and dual-source 64-SCT enable imaging of coronary arteries
with acceptable quality, at least in selected patient populations. Some technical
shortcomings remain a matter of concern. Image quality is less adequate in patients with
fast or irregular heart rates, a problem that might be partly overcome by the
administation of a beta-blocker prior to the examination, but nevertheless; patients with
atrial fibrillation have been excluded from most clinical trials. The most bothersome
problem is the presence of coronary artery calcifications that may preclude imaging of
the calcified segments of the coronary tree, due to image blurring (the so-called
“blooming”) that leads to an overestimation of the underlying stenosis or makes
stenosis appraisal impossible altogether. Older age, diabetes and a high Agatston score
(>400) are among the main predictors of poor diagnostic quality, all parameters related
to severe coronary artery calcification. 150, 155 Therefore, prior to contrast enhanced
MSCT, patients are first evaluated for their calcification burden, which is quantified as
the Agatston score. For patients with a score of more than 400, most authors agree
that MSCT is futile. 36, 150 Some authors have suggested that novel technical
developments, such as subtraction techniques, requiring many years will be needed to
resolve the problems associated with coronary calcification. 45
In less selected populations, the number of inconclusive MSCTs can be rather high. For
example in the RCT by Goldstein, 24/99 (24%) of the MSCTs were considered
intermediate or non-diagnostic, necessitating additional testing by MPS and/or CCA. 67
Inconclusive results mostly are due to coronary calcifications or motion artifacts
although morbid obesity in some patients precludes CT scanning. Because of inherent
spatial resolution limits, small calibre vessels (1.8 mg/dl), heart rate greater than 70 b.p.m. refractory to heart-ratelowering
agents, metallic interference (e.g., surgical clips, pacemaker, and/or defibrillator
wires). Moreover, patients must be able to hold still, to follow breathing instruction
(breath holding during 10 to 20 sec), take nitroglycerin, take iodine in spite of steroid
preparation for contrast allergy and lift both arms above the shoulders.
There is a non-negligible cancer risk associated with CT. It is estimated that currently
1.5 to 2.0% of all cancers in the United States may be attributable to the radiation from
(all) CT studies. 156 The 10 to 20 mSv exposure dose used in MSCT reportedly
corresponds on average to 1 new (fatal or nonfatal) cancer for every 1000-2000
scans. 157