Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Most film badges can record doses from about 100 ~Gy to 15 Gy (10 mrad to1,500 rad) for photons and from 500 ~Gy to 10 Gy (50 mrad to 1,000 rad) for betaradiation. The film in the badge is usually replaced monthly and sent to the commercialsupplier for processing. An exposure report is received from the vendor inapproximately 2 weeks. The developed film is usually kept by the vendor, providinga permanent record of radiation exposure. The dosimetry report lists the "shallow"dose, corresponding to the skin dose, and the "deep" dose, corresponding to.penetrating radiations.Film badges are small, lightweight, inexpensive, and easy to use. However,exposure to excessive moisture or heat will damage the film emulsion, making doseestimates difficult or impossible. The film badge is typically worn on the part of thebody that is expected to receive the largest radiation exposure or is most sensitive toradiation damage. Most radiologists, x-ray technologists, and nuclear medicinetechnologists wear the film badge at waist or shirt-pocket level. During fluoroscopy,film badges are typically placed at collar level in front of the lead apron to measurethe dose to the thyroid and lens of the eye because the majority of the body isshielded from exposure. Pregnant radiation workers typically wear a second badgeat waist level (behind the lead apron, if used) to assess the fetal dose.Thermoluminescent and Optically StimulatedLuminescent DosimetersSome dosimeters contain storage phosphors, in which a fraction of the electrons,raised to excited states by ionizing radiation, become trapped in excited states.When these trapped electrons are released, either by heating or by exposure to light,they fall to lower energy states with the emission of light.Thermoluminescent dosimeters (TLDs), discussed in Chapter 20, are excellentpersonnel and environmental dosimeters; however, they are somewhat more expensiveand are not as widely used in diagnostic radiology as film badges. The mostcommonly used TLD material for personnel dosimetry is lithium fluoride (LiF).LiF TLDs have a wide dose response range of 10 ~Sv to 10 3 Sv (l mrem to 10 5 rem)and are reusable. Another advantage of LiF TLDs is that the effective atomic numberis close to that of tissue; therefore, the dose to the dosimeter is close to the tissuedose over a wide energy range. TLDs do not provide a permanent record,because heating the chip to read the exposure removes the deposited energy. TLDsare routinely used in nuclear medicine as extremity dosimeters; a finger ring containinga chip of LiF is worn on the hand expected to receive the highest exposureduring radiopharmaceutical preparation and administration. Figure 23-3 shows afinger ring dosimeter and LiF chip. TLDs are also the dosimeter of choice whenlonger dose assessment intervals (e.g., quarterly) are required.Dosimeters using optically stimulated luminance (OSL) have recently becomecommercially available as an alternative to TLDs. The principle of OSL is similar tothat ofTLDs, except that the light emission is stimulated by laser light instead ofby heat. Crystalline aluminum oxide activated with carbon (Alz0 3 :C) is commonlyused. Like TLDs, OSL dosimeters have a broad dose response capability, and arecapable of detecting doses as low as 10 ~Sv (1 mrem). However, OSL dosimetershave certain advantages over TLDs in that they can be reread several times and animage of the filter pattern can be produced to differentiate between static (i.e.,instantaneous) and dynamic (i.e., normal) exposure.