Reducing Radiation Dose in Nuclear Cardiology

Guest Blogger: Thomas H. Hauser, MD, MPH, MMSc, @ThomasHauser_MD, member of the ASNC Social Media Task Force---
Nuclear cardiology studies play a key role in the diagnosis of heart disease, the determination of prognosis, and the appropriate management of patients.  Although these benefits are clear, we must also be mindful of the potential downsides associated with these studies, including the use of ionizing radiation.
The average annual radiation exposure to the general population has been increasing, with exposure from medical sources increasing dramatically (a 7 fold increase from 1980 to 2006) and accounting for half of the total exposure.1  Nuclear medicine, and nuclear cardiology in particular, is responsible for a large portion of medical radiation exposure.  While there is much uncertainty about the true risk of the radiation exposure involved in diagnostic imaging, it is generally presumed that there is some small risk.  As nuclear cardiologists, our patients trust us to use our imaging studies wisely so that the benefits of imaging can be maximized and the risks minimized.

The American Heart Association (AHA) has issued statements and advisories regarding the use of ionizing radiation in cardiac imaging studies,2, 3 and the American Society of Nuclear Cardiology (ASNC) has incorporated radiation considerations into its guidelines.4 These are excellent resources for nuclear cardiologists to refer to when considering the radiation exposure of their patients.  The AHA has recommended focusing on three main areas with regard to radiation exposure (paraphrasing from Fazel, et al)2:
  1. Education: ensuring that patients and clinicians understand the potential benefits and risks of medical imaging studies;
  2. Justification: ensuring that the imaging procedure is clinically necessary and appropriate; and
  3. Optimization: ensuring that radiation exposure from imaging is kept as low as reasonably achievable. 
This can be generally summed up in the concept of patient centered imaging, where the physician tailors the imaging to the needs of the individual patient, with particular focus on justification and optimization.4 
We have a responsibility as nuclear cardiologists to be educated about the risks of radiation exposure to our patients, and a responsibility to educate our patients about these risks.  A symposium sponsored by the National Institutes of Health proposed shared decision making as a model for interacting with patients, with recommendations that any use of radiation be disclosed to the patient and consideration of a more formal informed consent procedure for any study that has a radiation dose of >3 mSv.5  This proposed limit would include the vast majority of nuclear cardiology studies.  While written informed consent is probably not necessary, it is certainly worthwhile to have a discussion with the patient about the various options and come up with an optimal imaging strategy together.
Appropriate use criteria are an excellent way to provide justification for using an imaging study.  These guidelines are created using a formal process that provides an expert synthesis of the best data available regarding the use of imaging studies in heart disease.  ASNC has participated in the generation of the most recent multi-modality appropriate use criteria, which cover a wide array of clinical scenarios and provide guidance about what tests are appropriate for any particular situation.6  One of the best ways to limit radiation exposure is to not perform a test when it is not indicated.  A study evaluated the appropriateness of nuclear myocardial perfusion studies using a prior version of the appropriate use criteria and found that 66% were appropriate and 13% were inappropriate, with remainder either uncertain or unclassified.7  Simply eliminating the inappropriate studies would have a significant effect in reducing radiation exposure in the population. 
How can we optimize our imaging procedures?  The latest ASNC guidance document for SPECT myocardial perfusion imaging goes into great detail about how we can perform high quality imaging while using the least amount of radiation possible.4  Some of the key recommendations from this document are: 
  1. Use the lowest amount of activity possible for the patient and imaging system.  This also includes the careful consideration of which tracer to use.  PET tracers result in less radiation exposure than SPECT tracers, so it is preferred to use PET imaging when available and appropriate.  Among the SPECT tracers, Tc99m is preferable to Tl201, and dual isotope protocols should be avoided if at all possible (but may be necessary during Tc99m shortages).
  2. Perform stress-first or stress-only imaging when suitable, so that rest imaging can be eliminated in many cases.
  3. Use solid state CZT detectors, which have a higher sensitivity for the detection of photons, and advanced software solutions, such as iterative reconstruction, resolution recovery, and noise reduction.  These innovations can allow for reduction in the administered activity while maintaining image quality. 
While not all of these recommendations can be implemented by every nuclear laboratory, they certainly provide a starting point in planning imaging protocols and when considering a new imaging system.
How are we doing?
This advice has now been available for several years.  So how are we doing in reducing radiation exposure to our patients?  There is good evidence that incorporating this advice into practice can have a dramatic effect in reducing that amount of radiation used in nuclear cardiology imaging studies, reducing radiation exposure to patients by more than 50%.8  However, recent analyses using data from the Intersocietal Accreditation Commission showed high variability across nuclear laboratories and continued high average radiation exposure (Figure 2).9  Clearly we have more to do.
Other Considerations
One last thing to consider is the radiation exposure to our staff.  In our nuclear laboratory, we measured the potential radiation exposure to staff from being in close proximity to patients after being injected with a nuclear radiotracer.  We found that there is the potential for radiation exposure in close proximity to the patient for a short time after injection (Figure 3)10.  Many of our transport staff and echocardiographers may have close interactions with our patients where this exposure might result in a non-trivial radiation dose.  Simple steps such as scheduling echocardiograms prior to nuclear studies and educating transport staff can help to minimize this potential exposure. 
1.           Ionizing radiation exposure of the population of the united states, ncrp report no. 160. Bathesda, MD: National Council on Radiation Protection & Measurements; 2009.
2.           Fazel R, Gerber TC, Balter S, Brenner DJ, Carr JJ, Cerqueira MD, Chen J, Einstein AJ, Krumholz HM, Mahesh M, McCollough CH, Min JK, Morin RL, Nallamothu BK, Nasir K, Redberg RF, Shaw LJ. Approaches to enhancing radiation safety in cardiovascular imaging: A scientific statement from the american heart association. Circulation. 2014;130:1730-1748
3.           Gerber TC, Carr JJ, Arai AE, Dixon RL, Ferrari VA, Gomes AS, Heller GV, McCollough CH, McNitt-Gray MF, Mettler FA, Mieres JH, Morin RL, Yester MV. Ionizing radiation in cardiac imaging: A science advisory from the american heart association committee on cardiac imaging of the council on clinical cardiology and committee on cardiovascular imaging and intervention of the council on cardiovascular radiology and intervention. Circulation. 2009;119:1056-1065
4.           Dorbala S, Ananthasubramaniam K, Armstrong IS, Chareonthaitawee P, DePuey EG, Einstein AJ, Gropler RJ, Holly TA, Mahmarian JJ, Park MA, Polk DM, Russell R, 3rd, Slomka PJ, Thompson RC, Wells RG. Single photon emission computed tomography (spect) myocardial perfusion imaging guidelines: Instrumentation, acquisition, processing, and interpretation. J Nucl Cardiol. 2018;25:1784-1846
5.           Einstein AJ, Berman DS, Min JK, Hendel RC, Gerber TC, Carr JJ, Cerqueira MD, Cullom SJ, DeKemp R, Dickert NW, Dorbala S, Fazel R, Garcia EV, Gibbons RJ, Halliburton SS, Hausleiter J, Heller GV, Jerome S, Lesser JR, Raff GL, Tilkemeier P, Williams KA, Shaw LJ. Patient-centered imaging: Shared decision making for cardiac imaging procedures with exposure to ionizing radiation. J Am Coll Cardiol. 2014;63:1480-1489
6.           Wolk MJ, Bailey SR, Doherty JU, Douglas PS, Hendel RC, Kramer CM, Min JK, Patel MR, Rosenbaum L, Shaw LJ, Stainback RF, Allen JM. Accf/aha/ase/asnc/hfsa/hrs/scai/scct/scmr/sts 2013 multimodality appropriate use criteria for the detection and risk assessment of stable ischemic heart disease: A report of the american college of cardiology foundation appropriate use criteria task force, american heart association, american society of echocardiography, american society of nuclear cardiology, heart failure society of america, heart rhythm society, society for cardiovascular angiography and interventions, society of cardiovascular computed tomography, society for cardiovascular magnetic resonance, and society of thoracic surgeons. J Am Coll Cardiol. 2014;63:380-406
7.           Hendel RC, Cerqueira M, Douglas PS, Caruth KC, Allen JM, Jensen NC, Pan W, Brindis R, Wolk M. A multicenter assessment of the use of single-photon emission computed tomography myocardial perfusion imaging with appropriateness criteria. J Am Coll Cardiol. 2010;55:156-162
8.           Thompson RC, O'Keefe JH, McGhie AI, Bybee KA, Thompson EC, Bateman TM. Reduction of spect mpi radiation dose using contemporary protocols and technology. JACC Cardiovasc Imaging. 2018;11:282-283
9.           Desiderio MC, Lundbye JB, Baker WL, Farrell MB, Jerome SD, Heller GV. Current status of patient radiation exposure of cardiac positron emission tomography and single-photon emission computed tomographic myocardial perfusion imaging. Circ Cardiovasc Imaging. 2018;11:e007565
10.         Tsao CW, Frost LE, Fanning K, Manning WJ, Hauser TH. Radiation dose in close proximity to patients after myocardial perfusion imaging: Potential implications for hospital personnel and the public. J Am Coll Cardiol. 2013;62:351-352