**7. Radiation exposure**

The doses of 30 MBq and 120 MBq for ventilation and perfusion, respectively, allows excellent V/P SPECT quality at an effective radiation dose of 1.8 mSv (ICRP, 1998). The absorbed dose to the breasts with V/P SPECT is 0.8 mGy (ICRP, 1998). Estimation of exposure is relatively easy when short-lived radio-isotopes are used in nuclear medicine. In contrast, for X-ray technologies this is much more difficult because of a number of factors, such as differences in equipment and the size of the exposed fields. According to ICRP, the average effective dose for 4–16-detector MDCT is 5.4 mSv (Valentin, 2007). Notably, this information was based on computed rather than measured dose data. Hurwitz et al. reported for a current adult PE protocol with 64-detector MDCT a measured effective dose of 19.9±1.38 mSv (Hurwitz et al., 2007). These authors point out that the actual measured dose is about 50% higher than the computed dose. The absorbed dose to the breasts was 35–42 mGy. Absorbed radiation dose to the breast for a single-slice CT study was 20–50 mGy and 30– 50% greater with a four-slice CT. In a very recent study, Hurwitz et al. (Hurwitz et al., 2009) studied radiation dose-saving protocols. Phantoms of women were exposed to MDCT protocols with automatic current modulation, lower tube voltage and bismuth shields over the chest. In the case of a medium sized female patient when automatic current modulation was applied at 140 kVp, breast doses were estimated at 62 mGy and this reduced to 33 mGy when bismuth shields were added. At 120 kVp the doses were 44 mGy without shields and 20 mGy with shields. Some limitations of the study were discussed. No phantom with significant subcutaneous fat was studied. The authors were not able to directly assess the effect of increased noise for the diagnosis of PE. Dose-saving protocols are promising.
