The first attempts to view the heart was done in the 1970s using Computrised Tomography(CT).But since the heart was an organ in constant motion and due to the defects in the equipment at that time,the result doctors got was sufficient only for viewing tumors and other major pathological lesions on the surface of the heart.Eversince researches were made to develop a more competitive imaging of the organ.As a result came slip-ring CT-scanners which used only a single row of x-ray detectors,dual row,four row,16-row, 64-row,and now the latest in trend is the 256-slice CT-scanners.
Cardiac CT or Computerised Tomgraphy is a non-invasive process by which an x-ray machine moves in a circular motion around the heart and takes pictures which are processed by a computer to give three dimentional picture of the heart.Sometimes a contrast material (iodine based) is injected in to one of the veins to give a detailed picture of the vessels in the developed film.The 64-slice CT-scanner allows visualisation of the coronary arteries and the beating heart with high precision.In 2006 Toshiba introduced the 256-slice scanner which promised to improve the image resolution still further.These are by far the latest scanners to give the fastest temporal resolution.
Different types of CT are used for different purposes.MDCT(multidetector computed tomography)is a fast type of CT used since the the heart is an organ in constant motion.This imaging gives superior quality pictures of the heart.MDCT can also detect calcium deposits in the coronary arteries.
Another type of CT scanner called EBCT(electron-beam computed tomography) can also detect the calcium deposits in the heart vessels.
The procedure involves administation of medication to slow down the heart(this improves the diagnostic value of the image),placement of the patient on the table facing the equipment and the actual image aquisition.The image aquisition takes only a few seconds.The patient is monitored for somemore time for his heart beat to get normal.
Clinical utility of Cardiac CT
- To detect Coronary Artery Disease(CAD) or Coronary Heart Disease(CHD).Cardiac CTA or coronary angiography using CT plays a role in the detection of calcium plaques or other atherosclerotic deposits in the coronary vessels which often ends up in severe CHD or CAD.
- If contrast material is used,the coronary arteries and heart valves can be viewed,and can visualise if there is any narrowing or block.Chest pains and heart attacks can be diagnosed and prevented by this technique.
- Cardiac CT can be used to detect calcium buildup in the coronary arteries which can be an early sign of CHD.this type of CT scans thereby is called a Calcium-score screening heart scan.
- To find problems with the aorta which is the main artery that supplies oxygen rich blood from the heart to the organs throughout the body.Two major disorders that can be detected are aneurysm(diseased area of blood vessel that bulges out) and dissection(when the layers of a blood vessel peel away from each other).Both conditions can be life threatening.
- Blood clots in the lungs called pulmonary embolism(PE).
- Problems in the veins carrying oxygenated blood from the lungs to the heart(pulmonary veins).Any irregularities in these veins can cause atrial fibrillation(AF), a kind of irregular heart rhythm.CT can detect these changes in the veins and help o diagnose any disoders early.
- Pericardial diseases can be diagnosed as cardiac CT gives a clear image of the peicarial layers which form the protective sac around our heart.
Limitations:
As CT involves x-rays,the radiation hazards cannot be overruled,in fact the dose of radiation amounts to the natural radiation that we are exposed to over a period of three years.
The risk of cancer cannot be neglected as the radiation involved is relatively high .Younger children and persons below 40 years of age are more under the risk group.
Among the sexes younger women are more prone than men due to the risk of breast cancer in females.In addition numerous fertility issues also can arise after exposure to these procedures.Newer CT procedures which reduce the radiation involved has been developed.Studies have been conducted and proved that MDCT involves more radiation exposure than EBCT.
CT involves iodinated contrasts and medications to slow down the heart such as beta-blockers.The risk of these drugs is minimal and close monitoring of patients are done in the hospitals.
In patients with high heart beat and arrythmias(irregular heart beats),the image quality can get blurred or create artifacts.Also the degree of narrowing of vessels cannot be assessed in narrow or heavily calcified vessels.
Cardiac CT cannot completely replace diagnostic coronary angiography even though the former has numerous advantages over the latter.However for patients with a lesser chance of coronary artery disease,cardiac CT can be a non-invasive alternative to catheter angiography.So at present invasive angiography combined with stress testing will remain the test of choice for patients with a high risk of coronary artery disease.
