Assessment of patient exposure to X-radiation from SPECT/CT scanners

In the operation of any SPECT/CT system, in addition to internal radiation exposure (gamma-ray) resulting from administration of radiopharmaceuticals, external radiation exposure (x-ray) from the CT device has to be taken into consideration in the light of recommendations from the International Commission on Radiological Protection. These recommendations include justification of practices (the use of radiation produces sufficient benefit to offset any risks caused by the use of radiation), optimization (the incurred exposure by the use of radiation should be kept as low as reasonably achievable), and dose limitation. The internal radiation exposures of each organ after administration of radiopharmaceuticals are calculated by the MIRD Committee method. For example, the internal radiation exposure index for brain perfusion scintigraphy is 0.8 mGy/37 MBq for N-isopropyl-4-iodoamphetamine((123)I) hydrochloride or 0.19 mGy/37 MBq for ethyl cysteinate dimer. On the other hand, the external radiation exposure from a CT device is calculated using the CT dose index volume (CTDIvol)--a measured and calculated value unique to the CT scanner and scan parameters used--and a dose-length product, which is obtained from CT conditions and generally used as a reference value for CT radiation exposure. However, CTDIvol and dose-length product are calculated values unique to each device, not the value of external radiation exposures of each organ. Therefore, we believe that it is necessary to measure the total (internal plus external) radiation exposure dose from CT. In the present study, using an anthropomorphic phantom for deep-body total absorbed dose measurement, we evaluated the radiation exposure doses (organ-absorbed doses) of each organ under various CT conditions.

The radiation exposure doses of each organ were measured by inserting thermoluminescent dosimeter elements into the phantom under various CT conditions.

The following were brain radiation exposure doses in the head region. For 90 kVp and 25 mAs, 1.39 mGy (CTDIvol, 1.8 mGy), for 90 kVp and 300 mAs, 17.00 mGy (CTDIvol, 21.2 mGy), for 120 kVp and 25 mAs, 3.21 mGy (CTDIvol, 3.8 mGy), for 120 kVp and 300 mAs, 37.79 mGy (CTDIvol, 47.7 mGy), for 140 kVp and 25 mAs, 5.08 mGy (CTDIvol, 5.5 mGy), and for 140 kVp and 300 mAs, 65.07 mGy (CTDIvol, 65.6 mGy). The eye radiation exposure doses were as follows. For 90 kVp and 25 mAs, 1.94 mGy (CTDIvol, 1.8 mGy), for 90 kVp and 300 mAs, 20.31 mGy (CTDIvol, 21.2 mGy), for 120 kVp and 25 mAs, 3.71 mGy (CTDIvol, 3.8 mGy), for 120 kVp and 300 mAs, 49.72 mGy (CTDIvol, 47.7 mGy), for 140 kVp and 25 mAs, 5.44 mGy (CTDIvol, 5.5 mGy), and for 140 kVp and 300 mAs, 69.76 mGy (CTDIvol, 65.6 mGy). In addition, the radiation exposure doses of the cervical, thoracic, abdominal, and pelvic regions were measured in detail.

Our estimated external radiation exposure doses (x-ray) of each organ under various CT conditions, along with the internal radiation exposure doses (gamma-ray) resulting from the administration of radiopharmaceuticals, seem to be useful as reference values in understanding the radiation exposure doses for performing various nuclear medicine examinations.