Kenneth L. Matthews II, PhD Louisiana State University, Baton Rouge, LA
Dacian Bonta, MD, PhD Emory University, Atlanta, GA

Any efforts at dose reduction for combined nuclear medicine and X-ray CT imaging must account for the contribution to radiation dose of each imaging method. The two radioisotope imaging methods commonly incorporated with CT scanning are PET and SPECT. The CT data provides anatomical images to aid localization and attenuation correction data for PET and SPECT image reconstruction.

Provided below are links and resources related to the design of PET-CT and SPECT-CT systems, radiation dose from these imaging studies, and comments on applications relevant to the goal of dose reduction in PET-CT and SPECT-CT imaging. Comparative background information about typical radiological and nuclear medical imaging procedures is also provided, as follows:

• Mettler F.A. Jr., Bhargavan M., Faulkner K., et al., “Radiologic and Nuclear Medicine Studies in the United States and Worldwide: Frequency, Radiation Dose, and Comparison With Other Radiation Sources — 1950–2007” Radiology, 2009: 253(2). Available at: http://radiology.rsna.org/content/253/2/520.abstract.

  Summarizes and compares reports of the United Nations Scientific Committee on Effects of Atomic Radiation and the U.S. National Council on Radiation Protection and Measurements.

• National Council on Radiation Protection & Measurements, “Report No. 160 - Ionizing Radiation Exposure of the Population of the United States (2009)”. Available at http://www.ncrppublications.org/Reports/160

  Documents 2006 levels of exposure of individuals in the United States to ionizing radiation sources; an update of the NCRP 93 report from the 1980s.

• United Nations Scientific Committee on the Effects of Atomic Radiation, “UNSCEAR 2008 Report Vol. I: Sources of Ionizing Radiation (2008).” Available at: http://www.unscear.org/unscear/en/publications/2008_1.html

  Estimates radiation exposure (world averages) from ionizing radiation sources. In particular, Annex A reports on medical radiation exposure.

• Society of Nuclear Medicine, “SNM and Safe/Beneficial Medical Uses of Radiation.” Available at: http://interactive.snm.org/index.cfm?PageID=6995

  This quick reference page provides general information about nuclear medicine and links to additional resources and information about radiation.

  Radiation dose in PET-CT and SPECT-CT results from internal dose due to the PET or SPECT radiopharmaceutical and external dose from the CT X-ray beam. Radiation dose from PET and SPECT depends on the radioisotope, the biokinetics of the pharmaceutical, and the injected activity. Radiation dose and the effective dose from internally distributed radioisotopes are typically determined using the Medical Internal Radiation Dose (MIRD) schema.

• Medical Internal Radiation Dose Committee of the Society of Nuclear Medicine, “MIRD Pamphlet No. 21: A Generalized Schema for Radiopharmaceutical Dosimetry —Standardization of Nomenclature.” Available at: http://interactive.snm.org/docs/MIRD_Pamphlet_No21.pdf

  This pamphlet covers the basic concept and nomenclature for dose calculations from internally distributed radionuclides. The reference list, as well as other publications on the Society of Nuclear Medicine website, leads the reader to further details.

Dose from the CT unit of a dual-modality system is characterized in the same way as a stand-alone CT, typically using the CT dose index (CTDIvol), the dose length product (DLP), or similar methods. CT dose depends on mAs, kVp, gantry rotation speed, and other factors. The reader is referred to the Image Wisely article by Dixon and Boone (insert link on final website) for detailed information about CT dose characterization and the impact of acquisition parameters on radiation dose. Depending on the area of body imaged and the patient’s age group, the effective dose can also be calculated from the CTDIvol or the DLP values by use of linear conversion factors.

The radiation risks of concern for diagnostic procedures are the stochastic effects (radiation-induced cancers and genetic mutations). These risks are assessed by the effective dose, measured in Sieverts. The effective dose associated with a diagnostic PET/CT or SPECT/CT procedure is the sum of the effective dose due to the nuclear component and the effective dose due to the CT component.

Dose reduction for PET or SPECT potentially can be achieved by using tracers with shorter physical and biological half-lives, by scaling injected activity by patient weight (compared to use the same injected activity for all patients), or by using high sensitivity 3-D-mode PET scanners or high sensitivity SPECT collimators.