Cardiac CT: reducing radiation dosage possible

Published in The Hindu on June 11, 2009

CT scan
Reducing scan voltage from 120 kVp to 100 kVp and lowering heart rates are most effective in bringing down the radiation dosage.

One of the best non-invasive diagnostic tools used for studying people with coronary heart disease symptoms is the multi-slice computed tomography (CT) scan. The CT angiography was first introduced in 2000, and has become popular in India in the last few years.

While it has certain limitations in correctly diagnosing blocks in the artery, the biggest criticism levelled against it has been that a patient is exposed to high X-radiation. One CT heart scan is equivalent to 400-500 chest X-rays.

It becomes all the more bad when it is used as a screening tool. The number of people undergoing CT heart scans has increased many times in the developed countries.

The stress has therefore been to reduce the X-ray dosage without compromising the final image quality. CT heart scans are particularly amenable to radiation dose reduction. But the techniques required to implement this has been a challenge to doctors.

No compromising

A study published in the latest issue of the Journal of the American Medical Association (JAMA) has shown that it is indeed possible to reduce the X-ray dosage without compromising image quality. And the amount of reduction achieved has been 53.3 per cent.

About 4900 patients from 15 participating hospitals in Michigan State, U.S., were studied by the researchers.

The study demonstrated that though several options are available to reduce the radiation dosage, the most effective ones are a reduction in scan voltage from 120 kVp to 100 kVp and lowering the heart rates.

Lowering of heart rates using beta blockers allowed for maximum use of tube current modulation and narrowing of the scan acquisition window to a great extent. “Our results confirm that heart rate reduction was strongly associated with decreased radiation dose,” notes the paper.

One interesting observation was that the centres which achieved the maximum dosage reduction were the ones that had lower scan volumes.

Using these two techniques — reduction of scan voltage and lowering the heart rates — resulted in a median radiation dose reduction of 53.3 per cent. They were able to achieve a 48 per cent reduction within the first three months of the one-year period of investigation.

Not misplaced

Based on this, the authors strongly agree that the concern about CT heart scans resulting in relatively high radiation doses is not misplaced. “The present study tends to support those concerns with respect to imaging sites that fail to monitor their doses or implement effective dose-reduction methods,” they noted.

According to the authors, it is indeed possible to reduce the radiation dosage by using existing technology and technical methods without any impairment of diagnostic image quality.

Heart rate not a factor

Most of the CT heart scanners used in the country are the 64-slice ones that scan the heart within five heartbeats. The authors note that the use of a single heartbeat 320 slice scan will result in greater reduction in radiation dosage. And this reduction will be regardless of the heart rate and whether the person is obese or not.

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