Cardiac CT Scan
What Is CT Coronary Angiogram?
In the past, non-invasive functional tests of the heart were used, such as treadmill and nuclear studies, to indirectly assess if there were blockages in the coronary arteries. The only way to directly look at the coronary arteries was via cardiac catheterization.
Cardiac CT is a procedure using multi-slice CT to produce coronary artery images. Speed is extremely important in our ability to “freeze” the heart. Since the heart is a rapidly moving structure, the only way to image structures within it, is, if we can scan as fast as the heart beats or close enough.
The heart is continuously in motion and early CT scanners take 1-8 slices a minute (slower than the rate of the heart) resulting in blurry images. Newer generation CT machines now can take 64, 320 or 640 slices per minute.
How Is CT Coronary Angiogram Performed And What Are The Risks Of The Procedure?
Intravenous dye containing iodine is used. In people with kidney disease or diabetes, kidney function may temporarily be affected.
In the newer machines, radiation exposure is much less than that of a MIBI scan or a Cardiac PET scan.
The entire CT scan can be performed in a single breathhold. While, in contrast, MIBI scan take up to 4 hours to perform and Cardiac Pet can take up to half an hour.
Who Should Get A CT Coronary Angiogram?
To Rule Out Coronary Artery Disease
The ability of cardiac CT to rule out coronary artery disease exceeds a predictive value of 99%. This means that when the study is reported to be normal, it will be normal. This makes it a more accurate test than stress-testing, stress-echocardiography, stress-thallium and stress-perfusion MRI. More importantly, the heart does not have to be stressed to get this information.
To Rule Out Coronary Artery Disease
“To Rule Out Coronary Artery Disease” is an issue that often arises in the following situations:
The Patient is Asymptomatic, i.e. there are no problems, such as chest pain or breathlessness, but coronary artery disease needs to be ruled out:
- Family history of coronary artery disease
- Persistent high triglyceride levels and other high risk factors such as smoking, diabetes, etc.
- ECG abnormalities on a routine health check-up
- Equivocal (plus/minus) abnormalities on a routine stress test, done prior to employment, insurance or as part of a routine health check-up
- Moderate to severe hypertension
- Prior to non-coronary surgery in the adult population, etc
- Pre-ASD repair
- Pre-valvular repair
- Pre-tumor surgery
The Patient is Symptomatic, i.e. there is chest pain or breathlessness, but the physician / cardiologist is not convinced that there is coronary artery disease:
- Atypical chest pain (right side, shoulder tip, etc.)
- Suspected syndrome X in a pre-menopausal lady
- Suspected dilated cardiomyopathy
- Anomalous coronary arteries, ectasia or aneurysms
- Cardiac CT is an excellent tool for assessing the status of bypass grafts.
- For stents larger than 3mm in size, cardiac CT is an excellent tool for assessing in-stent lumen. For stents smaller than 2.5mm, the results are still equivocal.
- Cardiac CT is a good tool for assessing cardiac neoplasms
More About Calcium Score
Calcium scoring is a technique where the extent of calcification in the coronary arteries is measured and scored. It has been around for many years and was initially performed using EBCT (electron beam CT), but is now increasingly performed using mult-slice CT.
There is a direct correlation between the extent of calcium in the coronary arteries and the risk of a future cardiac event. For example, a calcium score of more than 400 is considered severe and it would be necessary to take steps to prevent further advancement of atherosclerosis and plaque formation. The higher the calcium score, more likely is there a chance of severe stenosis as well.
A calcium score of 0 does not rule out soft plaques, but statistically rules out significant coronary artery disease. Though the calcium score does not show soft plaques (which however are best seen on a CT angiogram), the higher the calcium score, the more is it likely that there are soft plaques as well. Rupture of a soft plaque is the commonest cause of an acute coronary syndrome (i.e. a heart attack).
Calcium scoring can be offered as a separate stand-alone test, but more and more, since we are reliably able to assess the coronary arteries themselves, it is being performed an an initial part of the entire cardiac CT examination.
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