The quickest way to a patient’s heart is through a CT scanner gantry

Dr Ruchira Marwah

Coronary artery disease (CAD) is a common cause of death in adults. The individuals have no prior symptoms in fifty per cent of the cases. CAD is secondary to narrowing of the coronary vessels from atherosclerosis. Injury to the endothelium leads to an inflammatory reaction. This is due to accumulation of inflammatory cells, smooth muscle cells and fat deposits in the vessel wall leading to the formation of an atherosclerotic plaque that narrows the lumen.

The imaging of CAD is performed with a variety of methods. A nuclear medicine thallium stress test demonstrates areas of myocardial ischaemia or infarction. Catheter Coronary Angiography (CCA) also allows definition of vascular stenosis.

CT scanners have been in use for detecting calcium plaques in the coronary arteries for some time now. But with rapid developments in Computed Tomography (CT) technology, the use of CT has fast graduated from merely being used for calcium scoring to actually performing coronary angiographies.

Those, for whom cardiac CT was “a joke” till a few years back, is today an impressive reality. The rapid progress of CT technology from electron beam to multi-detector, from single slice to four slice, 16 slice and now 64 slices has led to improvement in the quality of images that can be acquired with a resultant increase in the sensitivity and specificity of the diagnosis.

It’s non-invasive nature and quick examination time are a boon to the patients and the medical fraternity.

With increase in the number of detectors and faster speeds, the entire heart can be imaged in a single breathold within a few seconds without any significant motion artifacts. The high speed scanners literally freeze the cardiac motion giving highly diagnostic and clinically useful images. Rubin etc al, Stanford University, California, found that four-channel CT of the aorta was 2.6 times as fast and 4.1 times as efficient as single channel helical CT and requires less than half as much contrast medium. So, if the improvement from single to four slice has given such impressive results, what would a shift from four to 64 slices be?

Non contrast enhanced CT scan is used for coronary calcium scoring which is a marker for coronary atherosclerosis. Higher is the calcium, higher are the chances of a hard cardiac event like myocardial infarction or death. Hence, CT helps in the detection of calcium load in asymptomatic patients and assesses the future possibility of developing a hard cardiac event.

However, it bears no relationship to the extent and localisation of the disease and actually underestimates the true plaque burden. Because a lot of plaques may be non-calcified, and in fact it is these “soft plaques” which are unstable and are more likely to rupture. Since soft plaques do not show up on catheter angiography but can be seen on CT angiography, it is of particular interest to cardiologists and radiologists.

Contrast enhanced CT examination is used to assess the vascular status of the heart after injecting intravenous iodinated contrast medium. Betablockers may be given before the examination to slow down the heart rate.

The stenosis and occlusions in the coronary tree can be assessed to a high degree of accuracy. The number, topography length and degree of stenosis can be assessed. It is specially useful to assess “blind areas” for catheter studies such as the left main trunk.

Because of its high negative predicting value of the order of 96 per cent to 99 per cent, it can be used as a screening examination. Since, if it is normal, we can confidently say that a patient does not need a catheter angiogram because nothing significant will be found and nothing would be done.

In fact, multislice CT scanning of the heart has numerous other uses like post-CABG evaluation, in stent patency, pulmonary vein evaluation after radio-frequency ablation, assessment and treatment planning for patients with hypertrophic cardiomyopathies, cardiac tumours, pericardial disease, congenital heart disease and aneurysm.

Also, clots within the heart can be seen and hence otherwise almost mandatory transesophageal echocardiogram can be avoided. Since, the proximal coronary arteries can almost always be well seen, multi-detector CT is especially useful for assessing cases of anomalous coronary arteries. The anomalous course of the coronary arteries is also easy to diagnose on axial or 3D images. The 3D images acquired after post processing the acquired data are extremely useful for the surgeons to see how things relate to each other before taking up the patient for surgery.

In post-CABG patients not only performing a simple CT examination can assess the status of the graft. In fact, if a patient undergoes surgery and the chest pain persists, there is always a question as to whether this is chest wall pain from surgery, a pleural effusion, sternal infection or is it a coronary artery problem? Multidetector CT is the one solution to all these multiple questions.

In addition CT cardiac perfusion imaging is also making its way from the confines of research towards routine clinical use. These scanners may obviate same traditional cardiac imaging studies and enable a hospital to make better use of its cardiac catheterisation laboratory. The revolutionary changes in CT, especially with the 64-slice scanners, will do away with a lot of invasive diagnostic angiography studies. At the same time, it is more likely to increase the interventional procedures with balloons and stents. CT scanner is not an interventional tool, but is there only to prepare us for treating the patient). Multidetector CT scanners permit a more comprehensive assessment of many cardiac conditions and helps to improve the post intervention follow up care of the patients. As it has been rightly said that “the quickest way to a patients heart is through a CT scanner gantry”.

The writer is radiologist, department of CT and MRI, Bombay hospital. Email: drruchira@rediffmail.com