Patient radiation exposure during medical procedures is a growing concern among health care providers, professional organizations as well as the general public. Medical radiation (of all subtypes) has increased by over 700% since 1980. Because of its value in diagnosis and prognosis in patients with known or suspected obstructive coronary disease, radionuclide myocardial perfusion imaging use has also increased over the last 25 years. Nuclear imaging accounts for approximately 25% of medical radiation. Cardiac imaging represents ~50% of all nuclear imaging procedures but is responsible for nearly 85% of all nuclear radiation doses.1–4
Optimizing radiation exposure for patients is of considerable importance for patient safety and should be taken into account when ordering tests. For the nuclear cardiologist, this impacts choices in testing protocols, equipment, and even tracers. Radiation-reduction strategies should also take into account the value of the testing procedure and should not be performed at the expense of image quality, and thus the value of the examination itself. This chapter will present concepts of the consequences of radiation exposure, methods of measuring radiation exposure in medical imaging and in particular, nuclear cardiology, describe current radiation exposure in common cardiovascular single-photon emission computed tomography (SPECT) and positron emission tomography (PET) myocardial perfusion imaging (MPI) procedures, and discuss methods of reducing radiation exposure through instrumentation changes, protocol changes, and tracer choice.
The exact consequences of radiation exposure are uncertain. The deterministic effects of direct radiation to an organ system, such as epidermal reactions, are well studied. However, understanding the consequences of radiation exposure to an individual is more obscure and difficult to assess. These stochastic effects of radiation exposure acquired during medical imaging are indeed difficult to apply to an individual’s lifetime risk of developing cancers. This is in part due to variations in radiation types, exposure rates and quantities, and tissue susceptibilities as well as timing of procedures. In addition, malignancy generated by radiation exposure is often indistinguishable from those occurring from other causes.2 Data estimated from the coronary computed tomography (CT) literature suggest that a 10-mSv radiation exposure increases lifetime risk of developing a fatal malignancy by 0.0005%.4 This represents a small, but measureable increase in lifetime risk, but is difficult to quantify when considering an individual patient. For perspective, Table 7-1 illustrates comparative risks of death from both radiation sources and other causes.4
| Exposure | Estimated Risk of Fatal Malignancy or Lifetime Odds of Dying (per 1000 Individuals) |
|---|---|
| Effective radiation dose | |
| 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 (1–3%) | 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 |
Evaluating lifetime risk due to radiation exposure, one must consider age and life expectancy at time of exposure. Certainly, as demonstrated in Figure 7-1, augmenting a patient’s lifetime malignancy risk is weighed more heavily at younger decades when many tissues are more rapidly replicating than when older and cell lines tend to be more senescent. Gender differences exist as well, and should also be acknowledged when considering testing in nuclear cardiology.4–6
Measures of radiation and subsequent exposure are complex and more thoroughly discussed elsewhere (see Chapters 3 and 4). Typically, the Sievert unit (or milliSievert) is the most commonly used stochastic variable to compare radiation exposure levels from different sources. A Sievert (Sv), or milliSievert (mSv), is a unit which estimates the biological effect that 1 joule of radiation energy has on 1 kilogram of body tissue. Background radiation (from sources such as radon) produces exposure on the order of 3 mSv/year. The estimated radiation exposure from a typical rest/stress SPECT MPI is approximately 12 to 15 mSv of radiation. This classifies nuclear cardiac imaging as a high-dose procedure by regulatory bodies.1,2,4
Table 7-2 represents the recommended radiotracer doses and estimated radiation exposure from recently published American Society of Nuclear Cardiology (ASNC) Guidelines.3 The number of radiopharmaceuticals that are available, combined with a variety of the protocols, create a heterogeneous potential of exposure to US patients undergoing cardiac nuclear testing.
| First Injection | Second Injection | Total Dose (mSv) | Total Dose if Stress Only (mSv) | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Given at | Activity (mCi) | Activity (MBq) | Dose (mSv) | Given at | Activity (mCi) | Activity (MBq) | Dose (mSv) | |||
| Tc-99m protocols | ||||||||||
| Tc-99m one-day stress first/stress only | Stress | 8–12 | 296–444 | 2.0–3.0 | Rest | 24–36 | 888–1332 | 7.0–10.5 | 9.0–13.5 | 2.0–3.0 |
| Tc-99m one-day rest/stress | Rest | 8–12 | 296–444 | 2.3–3.5 | Stress | 24–36 | 888–1332 | 6.1–9.1 | 8.4–12.6 | n/a |
| Tc-99m two-day stress/rest | Stress | 8–12 | 296–444 | 2.0–3.0 | Rest | 8–12 | 888–1332 | 2.3–3.5 | 4.3–6.5 | 2.0–3.0 |
| Tc-99m two-day stress/rest—large patient | Stress | 18–30 | 666–1110 | 4.5–7.6 | Rest | 18–30 | 666–1110 | 5.2–8.7 | 9.8–16.3 | 4.5–7.6 |
| Tc-99m two-day rest/stress | Rest | 8–12 | 296–444 | 2.3–3.5 | Stress | 8–12 | 296–444 | 2.0–3.0 | 4.3–6.5 | n/a |
| Tc-99m two-day rest/stress large patient | Rest | 18–30 | 666–1110 | 5.2–8.7 | Stress | 18–30 | 666–1110 | 4.5–7.6 | 9.8–16.3 | n/a |
| Tl-201 protocols | ||||||||||
| Tl-201 stress/redistribution rest | Stress | 2.5–3.5 | 92.5–129.5 | 10.9–15.3 | n/a | n/a | n/a | n/a | 10.9–15.3 | 10.9–15.3 |
| Tl-201 stress/redistribution rest/reinjection | Stress | 2.5–3.5 | 92.5–129.5 | 10.9–15.3 | Rest | 1–2 | 37–74 | 4.4–8.8 | 15.3–24.1 | n/a |
| Tl-201 rest/redistribution | Rest | 2.5–3.5 | 92.5–129.5 | 10.9–15.3 | n/a | n/a | n/a | n/a | 10.9–15.3 | n/a |
| Dual-isotope Tl-201 rest/Tc-99m stress | Rest | 2.5–3.5 | 92.5–129.5 | 10.9–15.3 | Stress | 8–12 | 296–444 | 2.0–3.0 | 13.0–18.3 | n/a |
| Dual-isotope Tl-201 rest/Tc-99m stress—large patient | Rest | 3.0–3.5 | 111–129.5 | 13.1–15.3 | Stress | 18–30 | 666–1110 | 4.5–7.6 | 17.7–22.9 | n/a |
| I-123 protocol | ||||||||||
| MIBG | Rest | 10 | 370 | 4.6 | n/a | n/a | n/a | n/a | 4.6 | n/a |
| Newer technology reduced-dose protocols | ||||||||||
| Tc-99m one-day stress first/stress only | Stress | 4–6 | 148–222 | 1.0–1.5 | Rest | 12–18 | 444–666 | 3.5–5.2 | 4.5–6.7 | 1.0–1.5 |
| Tc-99m one-day rest/stress | Rest | 4–6 | 148–222 | 1.2–1.7 | Stress | 12–18 | 444–666 | 3.0–4.5 | 4.2–6.3 | n/a |
| Tc-99m two-day stress/rest | Stress | 4–6 | 148–222 | 1.0–1.5 | Rest | 4–6 | 148–222 | 1.2–1.7 | 2.2–3.3 | 1.0–1.5 |
| Tc-99m two-day stress/rest—large patient | Stress | 9–15 | 333–555 | 2.3–3.8 | Rest | 9–15 | 333–555 | 2.6–4.4 | 4.9–8.1 | 2.3–3.8 |
| Tc-99m two-day rest/stress | Rest | 4–6 | 148–222 | 1.2–1.7 | Stress | 4–6 | 148–222 | 1.0–1.5 | 2.2–3.3 | n/a |
| Tc-99m two-day rest/stress—large patient | Rest | 9–15 | 333–555 | 2.6–4.4 | Stress | 9–15 | 333–555 | 2.3–3.8 | 4.9–8.1 | n/a |
Radiation exposure has been a concern for professional societies for several years. As a result, ASNC published an “Information Statement” in 2010 intended to address radiation exposure.1 The recommended reduction in nuclear cardiac imaging was to utilize less than 9 mSv per patient in at least 50% of nuclear cardiology studies. The writing committee also recommended methods of reducing radiation to an individual patient, illustrated in Figure 7-2, such as the use of technetium-based tracers rather than thallium-201, stress-only imaging and cardiac PET over SPECT, if available.
Figure 7-2
Recommended means of reducing radiation exposure in patients. (Reproduced with permission from Cerqueira MD, Allman KC, Ficaro EP, et al. Recommendations for reducing radiation exposure in myocardial perfusion imaging. J Nucl Cardiol. 2010;17(4):709–718.)
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