66 2011
IMAGING & ONCOLOGY
of partial volume artefact and blooming artefact, it overestimates the size of calcified plaques and thus the degree of stenosis caused by such plaques8
.
Given these limitations, why should we use CT at all for imaging of the heart and coronary vessels? The answer is that a test which is less than perfect may yet be good enough to answer a particular question in a particular clinical context. CTCA can be definitively diagnostic in certain situations. It has the general advantages over conventional angiography of being less costly, of demonstrating plaque density and plaques not visible to conventional angiography. Because the contrast is given intravenously, arterial puncture and intra-arterial catheters are not required.
ECG-GATING AND HEART RATE CONTROL Successful cardiac CT requires ECG-gating and a slow steady heart rate. At a pulse rate of 60bpm, each cardiac cycle lasts one second. A 320 slice CT scanner, or a dual-source 128-slice CT scanner, can image the entire heart in about 0.3 seconds, with a temporal resolution of 150ms and 75ms respectively. This sounds impressively fast, but it constitutes a large fraction of even this rather leisurely cardiac cycle. The right coronary artery, for example, can move up to 4cm during this one second9
.
The heart, however, does not move at a uniform steady pace over the cardiac cycle; it moves rapidly in systole and is relatively static during diastole. So, for example, to obtain motion-free images of the coronary arteries with a 320-slice scanner, it is generally best to acquire the images during diastole rather than systole. In fact, most CT scanners cannot scan through the heart so quickly, instead acquiring the data in aliquots over multiple heartbeats. In either case, we have to have co-ordinate the data we acquire to the cardiac cycle. For this we connect ECG leads to the patient which feed the ECG trace to the CT scanner, enabling the scanner to synchronise the acquisition with the patient’s cardiac cycle. This is ECG-gating: an absolute necessity for cardiac CT.
Cardiac CT can also provide functional information. Here, data are acquired over the whole cardiac cycle. The resulting slices are ordered according to the synchronous ECG data, each of which corresponds to a snapshot of the heart at a particular point in the cardiac cycle. These snapshots can then be viewed as a video loop to look for outflow tract obstruction or wall-motion abnormalities, for example, or to measure ventricular volumes and ejection fractions. Another aspect of function that can be investigated with CT is myocardial perfusion and delayed enhancement in suspected ischaemic heart disease, though research in this area is in its infancy.
ECG-gating can be of two basic types: prospective, in which the x-ray beam is activated during a short predetermined portion of the cardiac cycle, for example between 65 and 75 per cent of the time from the start of one cardiac cycle to the start of the next; and retrospective, when the patient is irradiated continually and afterwards the data collected are matched to the ECG-trace over the period of the acquisition. Both types of
ECG-gating can also be mixed. For example, we can modulate the intensity of the x-ray beam in a retrospective acquisition so that it is maximal in diastole and very low for the remainder of the cardiac cycle (figure 2).
Good quality motion-free images require a slow steady heart rate, so that the length of diastole is long enough for the relatively sluggish CT scanner to collect the data while the heart is at rest. For single source scanners, the heart rate should ideally be less than 65bpm. For dual source scanners, a heart rate up to 75bpm is acceptable. In order to achieve these heart rates, some patients require beta-blocking drugs, either orally an hour before the scan, or intravenously in the scan room10
. Nitroglycerin is used commonly to dilate the coronary
arteries, increasing the diameter of the proximal coronary arteries by a mean of 0.5mm, compensating in part for the limited spatial resolution of CT for such small vessels10,11
.
THE CURSE OF CHOICE Politicians from the major parties have, of late, been convinced that when it comes to health care what patients/clients/customers desperately want is ‘choice’. Whether they will succeed in persuading us that we crave this nebulous concept remains to be seen. Choice, we know, is not an unalloyed good, as it is wont to create anxiety and confusion.
Consider the cardiologist in her outpatient clinic faced with a patient who has, no doubt, appeared there by an unfathomable synergy of freewill and government enforced choice, with recent onset stable chest pain, possibly cardiac in origin. She has a bewildering array of investigations at her disposal: exercise ECG, catheter angiography, echocardiography, stress echocardiography, stress-MRI, delayed enhancement MRI, CTCA, MIBI scan, intravascular ultrasound, PET and, soon perhaps, stress CT. Happily, she can simplify this list into two broad categories: ‘anatomical’ tests which show the coronary artery lumen, and ‘functional’ tests which demonstrate myocardial ischaemia.
A powerful argument in favour of using a functional test first is that a coronary artery stenosis, no matter how severe, is probably not worth treating unless it is causing myocardial ischaemia12
. The genie of choice, however, is not so easily grasped and
bottled because there are other factors to consider. These include sensitivity and specificity of the tests over the whole range of the pre-test probability of disease, as well as the costs, risks, and ancillary benefits of each test.
When should she choose CT?
INDICATIONS FOR CARDIAC CT Most of the research on cardiac CT relates to its use in suspected coronary artery disease (CAD). Like catheter angiography, CTCA is an anatomical rather than a functional test, demonstrating the lumina of the coronary arteries. It is more sensitive for the detection of small plaques and, unlike catheter angiography, it also provides information on the wall of the arteries and the density of any atheromatous plaques (figures 3 and 4). Pragmatically,
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