A recent FDA warning highlights the potential for increased radiation exposure in patients who undergo PET scans of the heart. By Michael Lim

ARE heart scans using radioactive isotopes safe?

Last month, there were news reports of contamination of beef in Japan by radioactive isotopes, notably caesium-137. More recently, the US Food and Drug Administration (FDA) issued a safety alert to the public about the potential for inadvertent increased radiation exposure in patients who undergo positron emission tomography (PET) scans of the heart. This was followed by an FDA alert to healthcare professionals to stop performing heart PET scans using radioactive rubidium-82 (Rb-82) injection from CardioGen-82 (a radioactive isotope generator).

A radioactive isotope is an atom with extra particles (neutrons) in its nucleus which makes it unstable. It will decay and split, releasing positrons (positively- charged electrons) which then collide with electrons in neighbouring atoms, generating gamma rays which are then detected by the PET scanner.

What is of concern here is that whether it is through ingestion of radioactively contaminated beef or injection of radioactive isotopes into your body for medical diagnostic testing, the body will continue to be exposed to radiation from within as long as the radioactive isotope remains in the body and continues to emit radiation.

Types of radiation

To understand this better, it is useful to distinguish the different categories of radiation. Scientists use the penetrating abilities of each of these radiations to differentiate and name them. The weakest form of radiation is called alpha radiation and it can be stopped by a sheet of paper. The next strongest form of radiation – beta radiation – can be stopped by 6mm of aluminium. The most powerful form – gamma radiation – is high-energy radiation that can be stopped only by several millimetres of lead.

The difference between X-rays and gamma rays is not just the difference in their energy, but also the source of the radiation. X-rays overlap only with the lower spectrum of the entire range of gamma radiation. In addition, X-rays are generated by energetic electron processes in an X-ray machine and only affect the patient when he or she is exposed to the X-ray machine, whereas gamma radiation is generated by decay of the isotope and hence the exposure time depends on the time the isotope takes to stop emitting radiation.

The FDA safety alert arose after two patients were found to have detectable levels of radiation a few months after their PET scans. Interestingly, the excessive radiation from the PET scans was detected only when the patients were crossing a US border checkpoint where radiation detectors identified radiation originating from the patients.

The radiation was found to originate from radioactive strontium isotopes. A PET scan had been performed on each patient a few months earlier with Rb-82, which has a short half-life (75 seconds). Rb-82 is derived from a radioactive strontium isotope which has a half-life of 25 days. It is believed that these patients were injected with strontium isotopes in addition to Rb-82.

Excessive radiation exposure

What was also startling about the FDA safety alert was that it said the estimated excessive radiation exposure of about 90 mSv (Sievert or Sv is a unit of measurement of the effect of radiation on the body; mSv = millisievert) received by the patients as a result of strontium exposure, based upon modelling performed by the Los Alamos National Laboratory, “appears similar to the amount of cumulative radiation exposure some patients receive during cardiac diagnostic evaluations with other radionuclides”.

This is very significant as the radiation dose is equivalent to more than 30 years of annual background radiation exposure of a human being.

The United Nations Scientific Committee on the Effects of Atomic Radiation estimates that the worldwide average background dose for a human being is about 2.4 mSv per year.

The Fukushima nuclear accident in Japan resulted in the release of radioactive caesium-137 (Cs-137) which has a half-life of 30 years and is a strong emitter of gamma radiation. Cs-137 has found its way into the food chain and ingestion of tainted food products will mean that the person will be subjected to radiation as long as the radioactive isotope remains in the body, and this isotope will continue to emit radiation beyond the duration of a human lifespan.

Making informed choices

PET and Single Photon Emission Computerised Tomography (SPECT) scans are diagnostic tests for the heart which involve the injection of radioactive isotopes into the body. Both tests are used to determine if any segment of the heart muscle is not getting enough blood flow as a result of blockage of heart arteries or previous damage to the heart.

As the isotope traverses the entire circulation of the body, the whole body is exposed to radiation. In particular, for rubidium PET scans, the three sites with the highest accumulation of isotopes are the kidneys followed by the heart and the lungs. This is unlike diagnostic tests that utilise X-ray tests, such as the Computerised Tomography (CT) scan of the heart or angiogram of the heart arteries, where usually only the heart and chest are exposed to radiation.

When choosing a test to assess whether there is sufficient blood flow to segments of the heart muscle, it is good to be aware that there are alternatives that do not involve any X-rays or gamma radiation.

The use of ultrasound techniques combined with exercise or drugs to increase the heart rate (stress echocardiography) is one option that is commonly available.

Another technique involves the use of magnetic resonance imaging (MRI) to assess blood flow to the heart muscles and it has been reported that this technique has 100 per cent sensitivity and more than 90 per cent specificity.

Another advantage of the heart MRI scan is that it is able to assess the function of the heart, viability of the heart muscle and the presence of scar tissue in the heart. More recently, it has been reported that imaging of heart arteries with MRI is comparable to CT imaging.

Although the medical community has long been assured that radiation from the rubidium PET scan is low, the recent FDA safety alert is a wake-up call as the estimated radiation dosage from the current exposure is more than 30 times the previous estimate.

Furthermore, what is worrying is that the FDA states that this high radiation dose was also seen in other diagnostic heart tests using radioactive isotopes.

If not for the fact that the two patients had set off the alarms of radiation detectors at the border controls, we would have never known of the danger as patients do not have their radiation status assessed after a radioisotope scan.

At this point, it is difficult to determine the extent of the problem of patients who have undergone similar scans and are exposed to excessive radiation. Hence, in this era of rapid medical advances, it is important to have a good understanding of the choices available before making a decision.