While scanning the heart allows immediate detection of heart disease, some technologies used may pose greater risks to the patient than others. By Michael Lim
ADVANCES in medicine have enabled physicians to detect heart disease earlier and to reduce the number of deaths arising from heart attacks. This has been made possible in part by earlier detection of heart disease through the use of various technologies to scan the heart.
Scans of the heart can broadly be divided into those with and those without X-ray radiation. If the heart scan is done for a good reason and the centre has a proven track record for accuracy and uses low dose X-ray radiation, the potential benefits from the medical information obtained from the scan will far outweigh any theoretical risk resulting from the exposure to X-ray radiation.
X-ray radiation is a weak cancer-causing agent; cancers arising from excessive exposure to X-ray radiation will generally take at least a decade to manifest.
But a heart scan can provide immediate detection of heart disease. To maximise the potential benefits of a scan, one should ask the right questions about heart scans and know what to look for, especially if the scan involves the use of X-ray radiation.
Will I be exposed to radiation?
Heart scans that use ultrasound (echocardiograms) or magnetic resonance imaging (MRI) do not use X-rays. Computerised Tomography (CT) scans and nuclear heart scans do.
The measurement unit for the health impact of radiation on the body is the sievert (Sv).
An echocardiogram of the heart while the heart rate is elevated by physical exercise or drugs (stress echocardiography) is a relatively safe option. Its major limitation is that the accuracy is highly operator-dependent, and generally inferior to other heart scans.
Nuclear scans of the heart involve raising the heart rate by exercise or drugs and injecting radioactive isotopes into the body, hence exposing the body to X-ray radiation.
Both these tests are limited by their inability to visualise the heart arteries.
Based on International Commission on Radiological Protection (ICRP) data, the radiation doses for nuclear heart scans vary from 11 to 14 millisieverts (mSv) for a technetium setamibi scan, 22 to 31 mSv for a thallium scan, and 13 mSv for a rubidium scan.
While CT scans of the heart arteries enable direct three-dimensional visualisation of the heart arteries, they involve X-rays to the chest and the injection of iodine-based contrast media. The radiation impact is more significant for the young and for pre-menopausal women, hence X-ray radiation should generally be avoided.
In a 2007 Circulation Journal publication by Einstein, it was reported that with a 64-slice CT scanner, the radiation dosage from a heart scan was eight to 15 mSv for men, but between 10 and 21 mSv for women. This was reduced to between seven and 14 mSv when using dose-reduction scanning techniques.
If a CT heart scan has to be performed in a premenopausal woman, it should not be more than 3 mSv, based on the latest guideline by the American Heart Association.
Safe alternatives to the CT scan are MRI of the heart arteries or stress MRI scans of the heart, which can be done without X-ray radiation exposure. Stress MRI is highly accurate and safe. The MRI can be performed without the need for an injection.
In our experience, the use of MRI of the heart arteries has reduced the need for exposure to X-ray radiation for those who are assessed for suspected heart artery disease, and it is especially useful for the young and for pre-menopausal women.
The major limitation is that there are very few centres in the world with experience in doing MRI of the heart arteries. In principle, when considering the choice of tests for women, the current guidelines from the American Heart Association advocate the use of heart tests without X-ray radiation as the first choice.
Are test results accurate?
Last week, an overseas patient came for a consultation. His relative had accompanied him for a CT scan of his heart arteries at a large overseas regional centre, which reported no blockages in his heart arteries. But being a poorly controlled diabetic, he decided to seek a second opinion.
As his CT scan images were unavailable, a treadmill test was performed and it was significantly abnormal. Contrary to the CT report, the test showed him to have 80 per cent narrowing in two of the three major heart arteries.
Therefore, before your physician refers you to a centre for heart scans, you may want to find out more about the accuracy of the scans. Most centres will quote accuracy data from internationally published studies, not from their own centres. A 2010 New England Journal of Medicine publication by Patel from Duke University said it had been shown that real-world accuracy of heart scans is substantially lower than the results obtained by highly-experienced centres in medical journals.
For scans that involve X-ray radiation, centres that are able to demonstrate high sensitivity and specificity in excess of 90 per cent will be preferred.
If you are going for a CT scan
In the July 2014 issue of Circulation Journal, Einstein wrote an article entitled “Radiation from Cardiac Imaging Test: Questions You Should Ask”. It focused on CT scans and nuclear scans, both of which produce X-ray radiation.
For heart artery CT scan, firstly, you should ask about the estimated radiation dose you may be exposed to.
While some centres can perform the CT scan of the heart arteries with less than 3 mSv, others do it with radiation doses exceeding 20 mSv.
In a study that examined the radiation doses from 50 centres, the radiation dose for a CT heart scan showed a seven-fold difference between the highest and the lowest radiation dose, the highest dose being about 30 mSv.
This highlights the importance of asking for the radiation dose from the centre rather than accepting data published by other centres or using data provided by manufacturers.
Secondly, check if the centre reduces the voltage of the X-ray tube for appropriate patients; it may be possible to do this for thinner individuals, thus limiting exposure to radiation.
Thirdly, does the centre routinely use a radiation-reduction scanning protocol, such as prospectively ECG-triggered scanning with dose modulation?
This may not be possible for those with irregular heart rhythm.
Fourthly, find out what is the likelihood of having to repeat a scan as it means a double dose of X-ray radiation.
Patients may need to be put through a second scan because they did not follow instructions or moved during the scans, compromising the images captured.
This rarely happens in experienced centres using CT scanners which have at least 128 slices, as patients are required to hold their breath for only a few seconds during the scan.
If you are going for a nuclear heart scan
Unlike a CT scan, which means that a specific localised region of the body is exposed to X-rays for a few seconds, a nuclear scan entails radiation exposure to the whole body from the circulating radioactive isotopes injected into the bloodstream, and which will take a relatively longer time to be removed by the body.
Firstly, check if the centre uses isotopes other than thallium for your scan, as thallium scans typically result in higher radiation dosages. If a nuclear scan is necessary, avoid using thallium.
Secondly, check if the centre can perform stress scans only. Conventional scans involve scanning the heart at rest and when under peak stress, and comparing both sets of images which have been acquired after radioactive isotope injections.
Going for only a stress scan will reduce the radiation exposure, though this may not be appropriate for all patients.
Thirdly, check if the centre performs attenuation correction or prone imaging, which are techniques to reduce incorrect results.
In summary, wherever possible, avoid heart tests that involve X-ray radiation, especially if you are young or are pre-menopausal.
However, if a heart scan is necessary, check on the accuracy, estimated radiation dose specific to that centre, the routine application of measures to reduce radiation exposure and the likelihood of having to repeat the test.
Finally, remember, that despite the radiation exposure, a heart scan can potentially prevent death from heart disease.