Cardiac CT

CT has been utilized for a) coronary angiography (coronary CTA) and b) coronary calcium scoring. The coronary arteries had conventionally been visualized using invasive coronary angiography that requires inserting a very small tube (catheter) into a blood vessel in the groin or arm, injecting a contrast agent when the catheter tip is at a desired location, and taking pictures under X ray guidance. The coronary arteries can alternatively be visualized using modern CT scanning. This is usually done with multi-detector CT (MDCT) but was earlier also done with electron beam CT (EBCT). Similarly coronary calcification scoring is done either using MDCT (earlier also with EBCT). Coronary calcification is usually defined as a plaque of at least 3 consecutive pixels (typical area of about 1 mm2 ) with a density of more than or equal to 130 Hounsfield units. For many current CT angiographic applications, 16 slice MDCT scanners are the minimum level of technology needed and 64 slice scanners are needed for good visualization of lesions. Current studies indicate that 64 slice CT angiography is highly accurate for exclusion of significant coronary artery stenosis (> 50% luminal narrowing) with negative predictive values in excess of 95% unless there is heavy arterial calcification.

Cardiac CT showing calcified plaques.

  1. What guidelines can help health professionals justify a CT examination and avoid unnecessary radiation dose?
  2. How do radiation doses to patients undergoing cardiac CT procedures compare to doses from other radiographic procedures?
  3. Are cardiac CT examinations valid screening methods for coronary artery disease?
  4. What are the new recommendations for both symptomatic and asymptomatic patients for coronary artery disease?
  5. Is it possible to reduce the radiation dose to patients undergoing CT cardiac procedures?
  6. Is there a risk of cancer being caused by a CT angiographic procedure?

1. What guidelines can help health professionals justify a CT examination and avoid unnecessary radiation dose?

  • Benefits clearly outweigh the risks
  • Risk increases with increasing number of CT examinations on the same patient - justify each CT examination
  • Perform CT examinations with appropriate optimization of technique so as to impart the lowest possible dose without compromising on image quality needed for medical purpose
  • Radiation protection should not cause needed examinations to be refused or not performed.

The justification is best made by a physician who is familiar with the patient’s medical history. Then the CT scan must be optimized for that specific individual. Can this examination be done in a way that reduces dose, for that particular patient?

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2. How do radiation doses to patients undergoing cardiac CT procedures compare to doses from other radiographic procedures?

The radiation dose to the patient is an important issue as doses are not small as compared to other radiological procedures. The radiation dose from a clinically justified single coronary CTA procedure is not something that should be considered as significant risk, but when coronary CTA is to be repeated number of times or when there is lack of clinical justification, the radiation risk is an important issue to consider as benefit-risk ratio may not be weighted towards benefit to the patient. The Table 1 below gives figures of effective doses in CT coronary angiography as compared to other procedures.

Table 1 : APPROXIMATE EFFECTIVE DOSE FROM CT CORONARY ANGIOGRAPHY COMPARED TO OTHER COMMON PROCEDURES

Procedure Effective dose (mSv)
CT coronary angiography (using helical MDCT) 8-30
Low dose axial MDCT with prospective gating 2-3
CT coronary angiography (using EBCT) 1 - 2
CT calcium scoring using MDCT 1-5
CT calcium scoring using EBCT 1
CT scan of thorax 10
Conventional invasive coronary 2-22
angiography but typically 3 - 10
Chest X ray (one posterioanterior film) 0.02
Annual natural background (worldwide average) 2.4


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3. Are cardiac CT examinations valid screening methods for coronary artery disease?

With the current level of knowledge, coronary CTA is not a screening procedure. Standard invasive coronary angiography remains the gold standard for evaluation of coronary anatomy. Some position statements on CT coronary artery calcium score screening are available from professional societies. In 2006 a consensus statement of the American College of Cardiology and the American Heart Association recommended against CT calcium scoring in asymptomatic individuals [AHA 2006].More recently a modest role for this still controversial test in adding incremental risk prediction for asymptomatic patients who have an intermediate risk profile or for patients with atypical cardiac symptoms was suggested (“rule-out examination”) [AHA 2007].

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4. What are the most recent recommendations for both symptomatic and asymptomatic patients for coronary artey disease?

Recent statements have been issued by American Heart Association (AHA) about coronary CTA in recognition of its wider availability, higher spatial resolution, shorter examination time and better patient adherence. The statements also speak about Magnetic Resonance Angiography (MRA) as a modality with  lack of ionizing radiation and lack of administration of iodinated contrast material [AHA 2008]. The summary points are:

  • CTA or MRA should not be used to screen for coronary artery disease in asymptomatic patients.
  • No multivendor data are available for coronary MDCT CTA or for whole-heart coronary MRA. Multivendor and multicenter validations should be performed.
  • The symptomatic patient at “intermediate risk” for coronary artery disease is more likely to benefit from noninvasive coronary angiography, after initial risk stratification, including patients with equivocal stress-test results.
  • CTA is not recommended for high-risk patients because of radiation dose concerns and usefulness of CTA is uncertain in patients with pronounced coronary calcifications because of probable reduced accuracy.
  • Anomalous coronary arteries can be assessed by both methods (MRA being favoured if available).
  • Reports must include technical quality assessment as well as coronary and significant non-cardiac findings
  • More research is encouraged in order to determine the potential of these techniques to detect, characterize and measure atherosclerotic plaque burden and changes.

These recommendations are subject to changes with the advances in equipment hardware and software.

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5. Is it possible to reduce the radiation dose to patients undergoing CT cardiac procedures?

There are clearly opportunities for dose reduction with almost any type of CT scan [ICRP Publications 87 & 102]. For cardiac CT specifically, use of body weight-adapted MDCT protocols has been shown to reduce the effective dose by about 12% in males and 25% in females [MAHNKEN, et al., 2003]. Careful consideration of technical factors like kVp, mAs or scan length is effective. In addition, studies with mA modulation during the cardiac cycle and new prospective gating techniques like ECG pulsing protocols and whole-organ scanning offer prospects of significant extra reductions. Another way would be to use MRA when available in order to avoid all ionizing radiation.

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6. Is there a risk of cancer being caused by a CT angiographic procedure?

Excess cancer risk at doses below 100 mSv is a debatable issue as there is paucity of data demonstrating this effect in epidemiological studies. Since doses from cardiac CT scans are lower than this, the potential risk can only be estimated by assuming a dose response relationship, but multiple CT scans performed on the same patient may result in 100 mSv or more. The ICRP has estimated that the radiogenic fatal cancer risk for an adult population is about 5%/Sv or (by using the linear non-threshold dose-response hypothesis) 0.005% /mSv. The U.S. National Academy of Sciences BEIR VII committee has recently provided radiogenic cancer estimates of risk by age. Potential radiation risks can be compared to the spontaneous fatal cancer risk (about 20%) and the spontaneous cancer incidence (about 40%) (Table 2). With current focus on breast tissue radio-sensitivity, special attention needs to be given when these procedures are performed in young women. Every possible effort should be made to use low dose protocols.

Table 2 : ESTIMATION OF AVERAGE DOSE AND RISK (age and gender not taken into account) FROM SEVERAL TYPES OF CARDIAC IMAGING PROCEDURES PERFORMED ON AN ADULT POPULATION (N.B. The risk estimates are not to be used for an individual and are valid only for a group)

Approximate effective dose(mSv) Approximate risk per scan of fatal radiogenic cancera Approximate spontaneous risk of fatal cancers
1 mSv 0.005% 20%
2 mSv 0.01 20%
3-5 mSv 0.015-0.025 20%
10 mSv 0.05% 20%
25 mSv 0. 125% 20%

aRadiogenic and spontaneous cancer incidence is approximately twice the fatal risk.

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References
 
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