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KCE reports 82 C Multislice CT in Coronary Heart Disease iv
of beta-blockers prior to the examination. Artifacts induced by coronary calcium remain a major
limitation for using MSCT. The calcium burden of a given patient can be assessed radiologically, prior
to multislice scanning, and is expressed as the Agatston score. In patients with an Agatston score
above 400, MSCT scanning is not performed because unreliable images are to be expected. The
radiation hazards of CT have only recently been fully recognized. Newer generation scanners and
newer scanning protocols induce less radiation in selected patients, but there is a trade-off between
dose reduction and diagnostic quality of the images.
SAFETY
The high radiation dose remains the most undesirable safety disadvantage of 64-SCT. The estimated
mean effective radiation dose per patient in clinical trials was 15 and 20 mSv and with modulated
protocols 7 and 14 mSv for males and females, respectively. This corresponds to the dose delivered
by 500 chest X-rays and it is markedly higher compared with the dose associated with a CCA which
is about 2–7 mSv. Lifetime cancer risk estimates for a standard MSCT depends on age and gender and
in a simulation study varied from 1 in 143 for a 20-year-old woman to 1 in 3261 for an 80-year-old
man.
As for CCA, MSCT necessitates the intravenous administration of a contrast medium. This can give
rise to allergic reactions and to renal failure. Currently, most patients are given a beta-blocker prior
to the MSCT examination to improve image quality, although this seems to be less compelling when
using dual-source 64-SCT devices. The administration of beta-blockers in the radiology department
may impose an additional risk to patients.
DIAGNOSTIC PERFORMANCE
Most published clinical trials are dealing with the diagnostic accuracy of 64-SCT as an imaging tool,
referring to CCA as the gold standard. A coronary stenosis that reduces the internal diameter of the
vessel by at least 50% on CCA is considered as being obstructive in most trials. In all published 64-
SCT studies, in populations at intermediate or high pre-test probability of obstructive CAD, test
sensitivity is good and ranges between 95 and 100%, indicating a very good negative predictive value.
Test specificity on the other hand performs less well. In a meta-analysis of trial results published
between 2005 and 2007, pooled specificity was 91% (87.5-94) and in our meta-analysis of recent
studies it was 83.5% (79.8-86.8). In one large trial, test performance was compared in women vs. men.
Whereas sensitivity was excellent in both sexes (93-100), specificity was acceptable in men (90%; 81-
95) but poor in women (75%; 95% CI: 62-85).
Virtually all patients enrolled in trials were already scheduled for invasive CCA. This questions the
external validity of the findings. Whether the performance of MSCT can be reproduced in less
selected patients at lower prevalence of CAD remains to be assessed. Good quality images require
patients to be in a stable sinus rhythm, they shoud be not too obese, and they should have noncalcified
coronary arteries.
So far, only one small randomized trial has been published that studied the effect of MSCT on patient
outcomes. In this trial, patients initially referred to MSCT underwent more radiotoxic procedures
than those randomised to nuclear imaging, and had an increase in revascularisations without an
effect on 6-month outcomes, incorporating death, myocardial infarction, readmissions and late
office visits.
PATIENT ISSUES
Apart from the technical implications of MSCT, i.e. the exposure to ionising radiation and the
administration of intravenous contrast media, MSCT can also affect patients by nature of the
uncertainties associated with its diagnostic performance. Not only false positive and false negative
results can be undesirable, but correctly identifying a significant narrowing of coronary artery or the
incidental finding of an extracardiac abnormality can result in unwanted effects, e.g. by promoting
more downstream investigations and treatments.
Positive and negative predictive values of 64-SCT for the diagnosis of obstructive CAD in everyday
clinical practice are unknown. So far, there is no evidence from clinical trials on a beneficial effect of
MSCT on patient outcomes such as symptom control, prevention of myocardial infarction or the
prolongation of life.