Cardiac computed tomography (cardiac CT)
Cardiac computed tomography is becoming increasingly important as a non-invasive diagnostic technique in the diagnosis of cardiovascular diseases. In contrast to cardiac MRI, computed tomography uses iodine-based contrast dyes. Although these can cause adverse reactions in patients, the predictive value of this technology means that its use is usually justifiable.
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What is meant by cardiac computed tomography?
Computed tomography is a computer-based imaging technique that uses a series of x-ray images taken from a variety of different angles in order to produce 2D/3D images of an object. CT scanners, which contain x-ray tubes that rotate around the patient's body at high speeds, can produce cross-sectional images of the heart which are initially used to measure the degree of calcification present inside the coronary arteries. These measurements can help to estimate a patient's risk of having a heart attack. Once a contrast medium has been administered, a modern CT scanner can be used to provide images of the coronary arteries with high spatial resolution. However, this technology is not currently suitable for use in diagnostic screening.
History and development
After physician Allan M. Cormack had laid the foundations for computed tomography, the electrical engineer Godfrey Hounsfield developed several prototypes of CT scanners, with the first patient being scanned in 1971. Hounsfield was awarded the Nobel Prize for Medicine in 1979.
Clinical applications
Cardiac CT has a whole range of clinical applications, of which the main ones include:
Quantification of coronary artery calcification
A coronary artery calcium score can be obtained using a quick CT scan that does not require the use of contrast dyes and is associated with minimal radiation exposure. The calcified deposits inside the coronary arteries are then graded according to their density, and a calcium score (Agatston score) is assigned, with the level of calcium indicating the patient's risk of developing a heart attack within the next few years. A calcium score between 0-10 means that the patient's risk is minimal, whereas a calcium score of >400 means that the patient has a high risk of having a heart attack in the next five years.
| Coronary calcium score (Agatston scoring) | Coronary sclerosis rating | Risk assessment | |
|---|---|---|---|
| 0-10 | None/minimal | Minimal risk | |
| 11-100 | Mild | Mild risk | |
| 101-400 | Moderate | Moderate risk | |
| 401-1000 | Severe | High risk | |
Quantification of coronary artery stenosis (CAS)
- Elimination of coronary heart disease in patients with low to medium pre-test probability of CAS
- Visualization of coronary artery abnormalities
- Detection of coronary artery stenosis
- Evaluation of coronary artery bypass grafts
Cardiac CT scanning is ideal for use in patients with atypical symptoms and low to moderate pre-test probability of cardiovascular disease. Factors that limit the usefulness of this technology include motion artifacts and severely calcified blood vessels and stents.
Additional applications
- Visualization of the thoracic aorta and the great vessels around the heart, combined with an assessment of the pulmonary blood vessels
- Assessment of atria/pulmonary veins in patients with atrial fibrillation prior to scheduled catheter ablation
- Congenital heart defects, particularly those affecting the great vessels
- Neoplasms (tumors) of the heart and pericardium, if images obtained by echocardiography and MRI not sufficient
This technology can generally be used to assess the morphology and function of heart valves, as well as being used for tissue vitality testing and stress testing. However, due to the fact that this technology is associated with radiation exposure, preference should be given to cardiac MRI scanning.
Contraindications
- Severely reduced renal function
- Untreated hyperthyroidism (over-active thyroid)
- Known allergy to iodine-based contrast agents
- Pregnancy
- Patients with diabetes who have not temporarily stopped metformin
- Image quality can be severely impaired in patients with atrial fibrillation and flutter. As a result, CT angiography should be avoided in these patients.
Preparation
Cardiac CT does not require any special preparation. Blood tests will need to be carried out prior to testing, particularly in relation to thyroid (TSH) and kidney function (Crea, GFR). Patients should refrain from eating and drinking 6 hours prior to the start of their cardiac CT. In patients with impaired renal function, intravenous fluids should be administered to ensure that a good level of hydration is maintained. Patients may continue to take blood pressure medication.
During the Cardiac CT
In contrast to normal CT, cardiac CT uses ECG triggering. 60-90ml of a contrast dye are injected through a venous access site. The table will move through the CT machine during scanning. At certain points during the scan the patient will be asked to hold their breath, which will help to minimize motion artifacts. A slow and regular heart rate is also crucial during scanning. If required, and if there are no contraindications, the patient may be given a beta blocker immediately before the start of scanning. This will help to slow their heart rate. The entire procedure will take approximately 15 minutes.
Specialist publications
2010 ACCF/AHA Guideline for Assessment of Cardiovascular Risk in Asymptomatic Adults
J Am Coll Cardiol 2010: 56: 2182‐2199
Konsensusempfehlungen von DRG/DGK/DGPK zum Einsatz der Herzbildgebung mit Computertomografie und Magnetresonanztomografie [DRG/DGK/DGPK consensus recommendations on the use of cardiac imaging using computed tomography and magnetic resonance imaging]
Deutschen Gesellschaft für Kardiologie – Herz- und Kreislaufforschung e.V. (German Cardiac Society)
dgk.org