Heart Vision

Radiation Dose delivered in CTCA

In the early days of performing CT scans of the coronary arteries, the radiation doses delivered were high. This was because scans were performed at full power throughout the cardiac cycle with overlapped spiral technique. This was a reflection of the technology available at the time.

There have been several major developments over the last 5 years which have lowered the dose delivered at CTCA considerably. Doses for most patients now are no higher than for other diagnostic CT scans and comparable with diagnostic coronary angiography.

The techniques for dose reduction include:

  • Tube current modulation (lowering the dose delivered during systole when the coronary arteries are moving and therefore unlikely to yield diagnostic information)
  • Patient attenuation tube current modulation (altering the dose based on the size/density of tissues in the scan field)
  • Prospective gating (switching the tube off during systole and acquiring data only for the short phase of diastole during which the coronary arteries will not be moving)
  • Low kV technique (in slender patients, less powerful x-rays can be used to obtain diagnostic images)
  • Iterative reconstruction techniques (using more powerful computers to reconstruct CT datasets in a way which means that more information can be extracted from a lower amount of radiation)

Another way to reduce the dose delivered is to not perform a calcium score at the same time as the CTCA although this has to be weighed against the utility of the extra information obtained from a calcium score. Calcium scores are not routinely performed unless specifically requested.

HeartVision audits doses delivered during CT studies of the coronary arteries. The "Dose Length Product" (a measure of the total radiation delivered to the patient during the scan) is recorded for every patient scanned. The DLP is multiplied by a conversion factor (based on the part of the body being irradiated) to give an estimated dose (in the case of CTCA: DLP x 0.017 = dose (mSv).)

Our audits have shown that the 2 most important factors in determining dose are:

  • Patient weight & size (heavy patients and those with dense tissue around the chest will receive higher doses). This reflects the amount of radiation required to penetrate the tissues and provide diagnostic images.
  • Patient heart rate (patients with a heart rate below 65 bpm and suitable body weight/build can have a prospectively gated study which significantly reduces the radiation dose).

In patients of suitable build, beta blockers are administered if the heart rate is above 65 bpm (or is variable). This is to allow the scan to be performed with prospective gating (with the associated reduction in dose). Sometimes sedation (premedication with oral lorazepam or intravenous midazolam) is also used if the patient is particularly anxious to reduce the heart rate.

Where a prospectively gated study is not possible (patients with a high heart rate who can't take beta blockers for example) tube current modulation is used with retrospective gating. This means that the x-rays are reduced during parts of the cardiac cycle which won't yield diagnostic information.

The upshot of all these techniques is that the radiation delivered to a patient is now similar to (or lower than) a CTPA, CT abdomen or similar diagnostic study. For a prospectively gated study in a patient of average weight, the dose is less than a person receives from natural background sources in a year.

Whilst CT should not be performed unnecessarily, the small increase in risk associated with the radiation delivered during scanning has to be weighed against the value of the information obtained in evaluation of suspected disease. Coronary artery disease remains a major cause of mortality and morbidity. CTCA provides a minimally invasive test with an excellent negative predictive value (ie. able to rule out disease).

To provide some perspective on the risk of dying from a radiation induced cancer it is sometimes helpful to compare the risk with other activities that we encounter in our every-day lives. For example a dose of 10mSv of radiation (at the high-end of our doses in CTCA) represents a much smaller risk than passive smoking or being married to a smoker, of dying in a motor vehicle accident, of being killed crossing the road or drinking water which falls within US guidelines for arsenic levels:

Table 3. Estimated Risks of Fatal Malignancy or Death Resulting From Radiation Exposure and the Lifetime Odds of Dying as a Result of Selected Activities of Everyday Life

Exposure

Effective radiation dose

Estimated Risk of Fatal Malignancy or Lifetime Odds of Dying (per 1000 Individuals)

   1 mSv (calcium score/lung screen)

0.05

   10 mSv (coronary CTA/abdomen CT, invasive coronary angiography, radionuclide myocardial perfusion study)

0.5

   50 mSv (yearly radiation worker allowance)

2.5

   100 mSv (definition of low exposure)

5

Natural fatal cancer

212

Passive smoking
    Low exposure
4

   High exposure, married to a smoker

10

Radon in home
    US average

3

   High exposure (1% to 3%)

21

Arsenic in drinking water
    2.5 µg/L (US estimated average)

1

   50 µg/L (acceptable limit before 2006)

13

Motor vehicle accident

11.9

Pedestrian accident

1.6

Drowning

0.9

Bicycling

0.2

Lightning strike

0.013

CTA indicates CT angiogram.

National Safety Council estimates are based on data from National Center for Health Statistics and US Census Bureau. Deaths are classified on the basis of the Tenth Revision of the World Health Organization's International Classification of Diseases. Lifetime odds are approximated by dividing the 1-year odds by the life expectancy of a person born in 2005 (77.8 years).


From Circulation. 2009;119:1056-1065

It is also important to weigh the small hypothetical risk of inducing malignancies against the risks of not performing an imaging study, which may include misdiagnoses and failure to administer treatments that could improve medical outcomes. However, the latter argument is currently difficult to support with appropriate statistics, because there are no prospective, randomized trials that demonstrate that cardiac imaging with ionizing radiation can convey survival benefit.

Because radiation-induced malignancies have a biological latency of approximately 10 to 40 years, they are less likely to manifest in older individuals. Recent publications endorsed by the American Heart Association have emphasized that cardiac CT and radionuclide studies are most appropriate in symptomatic patients with anintermediate likelihood of having coronary artery disease. This patient cohort is predominantly older than 50 years of age. Many of these patients may not live long enough for a radiation-induced malignancy to become clinically apparent. Conversely, if an imaging study uncovers a condition for which tailored management can improve patient outcomes, the imagingstudy may result in survival benefit without which the patient might not have lived long enough for a potential malignancy to develop.

For example, for 50-year-old asymptomatic individuals, the lifetime risk of developing coronary artery disease is 52% for men and 39% for women. An argument has been made that if the entire US population of 50- to 55-year-old individuals (18.8 million people) were screened for coronary artery disease with coronary CT angiography every 5 years until the age of 70, the estimated total increase in the number of fatal malignancies over the period of screening would be {approx} 42 900. If such screening could be translated into management strategies that prevented only 10% of sudden cardiac deaths, {approx} 35 500 fewer cardiac deaths might occur per year. However, such potential benefits remain unproven. Rigorous studies are needed to establish that, for example, the rapidly expanding use of cardiac CT conveys individual and societal benefits. The present Writing Group does not endorse screening for heart disease in asymptomatic low-risk patients with imaging modalities that expose asymptomatic individuals to ionizing radiation.

Dr. Latham Berry MBChB FRANZCR