PE, and vice-versa. Indeed, these pathologies can occur together. However, if a ‘triple rule- out’ service is offered, arguably, it will be requested more often than strictly warranted. This is in accordance with the unspoken and doubtful premise that equates more testing with better care, which is used as an alternative to proper clinical assessment18
.
CTCA has an undisputed role in the imaging of patients who have anomalous coronary arteries (figure 5). It has very high sensitivity and specificity for graft stenosis or occlusion in patients who have a coronary artery bypass graft2,19,20
(figure 6). It is less
accurate for evaluation of the native coronary arteries distal to the anastomosis and for evaluation of the anastomosis itself21
. It can be used for assessment of the coronary arteries prior to non-coronary cardiac surgery.
Cardiac CT is good for the investigation of complex adult congenital heart disease when other investigations such as echocardiography and MRI are contraindicated or inadequate (figure 7). Not only does it demonstrate clearly the complex anatomy in these patients, but can also provide information on their ventricular function22
.
There are multiple other uses and potential uses for cardiac CT including: the assessment of valve abnormalities23 ablation24 plaques25
, evaluation of cardiac masses, monitoring the evolution of atheromatous , and myocardial perfusion and delayed enhancement imaging26
.
Figure 7. Oblique MPR from a CTCA demonstrating a membrane (arrow) projecting into the left ventricular outflow tract, which was causing significant stenosis.
RADIATION DOSE The radiation burden associated with CTCA has been the subject of intense interest over the past five years. In part because of raised awareness of the large and increasing radiation dose which CT scanners expose the population to - and the attendant risk of this radiation - and, in part, because the radiation dose delivered by CTCA can be very high: up to 25mSv27
. Unmodulated retrospective ECG-gated CTCA in particular, is a very
high dose examination. Modulated retrospective ECG-gated CTCA, where the maximal radiation dose is only used over a small portion of the cardiac cycle, reducing to 20 per cent or less for the rest, gives significantly lower doses in the region of 7 to 14mSv.
what role Can CtCa play in suspeCted Cad?
Prospective ECG-gating leads to yet lower dose examinations. Most commonly, multiple non-helical scans are obtained, the table moving between each ‘slice’ (usually a 4cm volume) to cover the desired z-axis range, usually three or four slightly overlapped volumes for a typical heart. This yields doses in the region of 1mSv to 4.5mSv27,28
,
which compare favourably with the dose from conventional coronary angiography (mean 7mSv) and nuclear medicine stress tests (eg 99
. mTc-sestamibi mean 9mSv) 29,30 .
For selected patients, a dual-source 128-slice scanner can also scan the entire heart in a single heartbeat with doses in the region of just 1mSv31
It is anticipated that new iterative reconstruction algorithms may produce diagnostic images using less radiation than filtered back projection algorithms32
. Multiphase , left atrial and pulmonary venous imaging to plan radiofrequency
69 2011
IMAGING & ONCOLOGY
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