Feature | Radiation Dose Management | May 31, 2018 | Dave Fornell, Editor

Dose-Lowering Practices for Nuclear Cardiology

Nuclear cardiology SPECT scan showing myocardial perfusion. A new consensus document issues by ACC and other societies provides guidelines on managing radiation dose.

May 31, 2018 — Here is a checklist of dose-sparing practices for nuclear cardiology that was included in a new 2018 consensus document to guide the optimal use of ionizing radiation in cardiovascular imaging.[1]
 
The consensus document was issued in May 2018 jointly by the American College of Cardiology (ACC), Heart Rhythm Society (HRS), North American Society for Cardiovascular Imaging (NASCI), Society for Cardiovascular Angiography and Interventions (SCAI), Society of Nuclear Medicine and Molecular Imaging (SNMMI) and the Society of Cardiovascular Computed Tomography (SCCT). It includes input from experts from a total of nine cardiology societies and includes best practices for safety and effectiveness when using computed tomography (CT), nuclear imaging and angiographic/fluoroscopic imaging.
 
One section includes a the following checklist for nuclear imaging:
 
Case Selection:
• Consider patient age, co-morbidities, natural life expectancy
• Consider appropriateness and utility of nonradiation-based imaging techniques
 
Modality Selection:
• Select the appropriate technique and radionuclide that provides diagnostic quality information at the least patient radiation dose
• Use scanners with cadmium zinc telluride (CZT) detectors, which can cut dose in half
 
Understand Dose Relationships for the Radiotracers Used:
• N-13 H3 PET
• Rb-82 PET
• Tc-99m SPECT
• Tl-201 SPECT
 
Procedure Planning:
• Use stress-rest protocol in preference to rest-stress when the overall clinical situation (clinical scenario and patient imaging characteristics) is appropriate
• Use the smallest radionuclide dose compatible with adequate count acquisition rates
• Position camera head as close to the patient as possible
• Use iterative reconstruction software to reconstruct clinical images. This improves the image quality of lower dose scans
• Minimize radiation exposure for attenuation correction
 

Other Nuclear Imaging Radiation Considerations:

The consensus document also lists the following safety considerations for the nuclear imaging staff:

• The photons emitted from the subject from radiopharmaceuticals are generally of higher energy than the X-rays emitted from fluoroscopy or computed tomography (CT). High-energy CT uses an energy of 140 keV. In comparison, the vast majority of Tc-99m gamma rays have an energy of around 140 keV. The photons emitted in PET from positron annihilation have an energy of 511 keV. Photons in this energy range readily penetrate the 0.5-mm lead equivalent of conventional diagnostic X-ray protective materials. Therefore, personal shielding devices, such as lead aprons or leaded glasses, are less effective and consequently are rarely used. Instead, nuclear cardiology personnel rely on the principles of time and distance. Because the total photon flux from nuclear studies is far lower than from x-ray tubes, limiting the duration spent near a radioactive subject is generally sufficient limitation of exposure. Therefore, personnel should limit the duration they spend in close proximity to either the dose syringe or the injected subject as much as reasonably possible.

• The X-ray tubes used in CT and fluoroscopy can be turned off so they do not generate X-rays, but radiopharmaceuticals are a continuous source of X-ray activity. The radioactivity also can be excreted via body fluids or spread during administration. Patient blood and excreted body fluids are radioactive and are a potential source of radiation exposure to personnel, particularly if an accident or an error causes a healthcare worker to become contaminated. Careful and routine monitoring for contamination is required. If contamination occurs, a medical physicist often needs to be involved to estimate the dose received by the worker. The reason for involving a medical physicist in cases of contamination is that monitoring devices (body dosimeters or ring badges) assume a relatively uniform dose to the person that can be accurately represented by the dose to the small dosimeter. However, a spill of radiopharmaceutical on a technologist’s shoe could result in a meaningful dose to the foot, but barely register on a dosimeter worn on a coat lapel.

 
 
 
Reference:

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