of harm from annual cumulative exposure “to reasonable levels of risk compared with risks from other common sources, i.e., about 1024 to 1026 annually.”4 The 5 mSv an­nual limit for infrequent exposure is made because “annual exposures in excess of the 1 mSv recommendation, usually

to a small number of people, need not be regarded as espe­cially significant to the group as a whole provided it does not often occur to the same groups and that the average exposure to individuals in these groups does not exceed an average annual EfD of about 1 mSv.”

Health care institutions work to ensure that personnel dose limits do not approach EfD limits. Radiologic tech­nologists’ personnel dosimeter readings should be signifi­cantly below a tenth of the maximum EfD limits, even for those technologists who receive the most exposure. Hos­pitals typically establish internal action limits. These lim- its are typically 1/10th of the annual dose limit. The pur­pose of action limits is to trigger an investigation when these limits are exceeded that could uncover the reason for any abnormal exposure. A frequent cause for an excessive reading is due to a personnel dosimeter remaining in an x-ray room while exposures are subsequently made be­cause the dosimeter had unknowingly fallen off the uni­form of a technologist, or was forgotten on a lead apron. Also, sometimes work habits, such as where the technolo­gist usually stands during interventional radiography, can lead to personnel readings that exceed the action limit. The RSO may then initiate corrective action. In general, the RSO must be an active participant along with the im­aging department manager in an ongoing program that is designed to prevent all occupationally exposed personnel from receiving the maximum allowed exposures.

range is of the same order as the amount of natural radiation that average US residents receive in their life- times. What is curious is that the LSS data seems to indicate that Japanese atomic bomb survivors with moderate radiation EqD totals of 5 mSv to 50 mSv (0.5 rem to 5 rem), the equivalent of 1.5 to 15 years of natu­ral radiation, have a reduced cancer death rate com­pared with a normally exposed control population. Additional recent reanalysis of the atomic bomb survi­vor data have shown that effects on survivors are more consistent with a threshold model than the LNT model.14 All of this contradicts the predictions of the BEIR V report and, if fully substantiated, casts doubt on the BEIR V conclusion that any amount of radiation is potentially harmful. The reverse could be true, at least for very moderate amounts of radiation exposure! More specifically, in seven Western states with background radiation levels higher than other states by approxi­mately 1 mSv (100 mrem) per year, residents experience about 15% fewer cancer deaths per 1000 individuals than the US average. There is also much evidence from animal studies that supports the view that low doses of radiation reduce the chances of both cancer and non­malignant disease occurrence.

system is subjected over a period of time to low doses of radiation, it initiates adaptive protection responses17 that can lessen potential DNA damage and conse­quently the malignant transformation of cells. However, until the radiation hormesis theory is sufficiently proven, the medical radiation industry will continue to adhere to a rigid principle of ALARA and the no­threshold concept for radiation protection purposes.

For the protection of radiation workers and the popula­tion as a whole, EfD limits have been established as guidelines (Table 10.3). All medical imaging personnel should be familiar with current NCRP recommenda­tions. For this group the most critical item is the 50 mSv/year (5 rem/year) whole-body occupational dose limit. This annual upper boundary is designed to limit the stochastic (probabilistic) effects of radiation. It takes into account the EqD in all radiation-sensitive organs found in the body.

• Radiation hormesis is the hypothesis that a posi­tive effect exists for specific populations that are continuously exposed to moderately higher levels of radiation.

• The ALARA concept (optimization) states that radi­ation exposure should be kept “as low as reasonably achievable.”

• Explain the significance of exposure reproducibility and exposure linearity.

• Discuss the use of fluoroscopic equipment by non-radiologist physicians who perform interven­tional procedures and identify the responsibilities of the radiographer during such procedures.

When imaging a patient, radiographers must have a means to accurately measure the distance from the anode focal spot to the image receptor to ensure that the quality control recommended* source-to-image recep-