Shielding In The Nuclear Medicine

Radiation exposure rates in the nuclear medicine laboratory can range from over 100 R/hr [e.g., contact exposure from an unshielded ~37 GBq (l Ci) Tc 99m generator eluate or a therapeutic dose ~11 GBq (300 mCi) of I-131] to natural background. The exposure rate at any distance from a particular radionuclide can be calculated using the specific exposure rate constant (r).

The specific exposure rate constant (expressed in units of R-cm^2 /mCi.hr) is the exposure rate in R/hr at 1 cm from 1 mCi of the specified radionuclide:

Exposure rate (R/hr) = rA/d^2

where r = specific exposure rate constant R.cm2 /mCi·hr, A = activity in mCi, and d = distance in centimeters from a point source of radioactivity. Because very low energy photons are significantly attenuated in air and other intervening materials,

specific exposure rate constants usually ignore photons below a particular energy. For example, r20 represents the specific exposure rate constant for photons >20 keV

Tungsten, lead, or leaded glass shields are used in nuclear medicine to reduce the radiation exposure from vials and syringes containing radioactive material.

Table shows specific exposure rate constants and lead HVLs for radio nuclides commonly used in nuclear medicine.

Syringe shields are used to reduce personnel exposure from syringes containing radioactivity during dose preparation and administration to patients. Syringe shields can reduce hand exposure from T c-99m by as much as 100-fold. Leaded glass shields are used in conjunction with solid lead shields in radiopharmaceutical preparation areas. Radiopharmaceuticals are withdrawn from vials surrounded by thick lead containers (called "lead pigs") into shielded syringes behind the leaded glass shield in the dose preparation area

Dose preparation workstation. The technologist is drawing the radiopharmaceutical from a vial shielded by a "lead pig" into a syringe contained in a syringe shield. Further protection from radiation exposure is afforded by working behind the lead "L-shield" with a leaded glass window. The technologist is wearing a lab coat and disposable gloves to prevent contamination. A film badge is worn on the lab coat to record whole-body exposure and a thermoluminescent dosimeters (TLD) finger ring dosimeter is worn inside the glove to record the extremity exposure.

Persons handling radionuclides should wear laboratory coats, disposable gloves, finger ring TLD dosimeters, and body dosimeters. The lead aprons utilized in diagnostic radiology are of limited value in nuclear medicine because, in contrast to their effectiveness in reducing exposure from low-energy scattered x-rays, they do not attenuate enough of the medium-energy photons emitted by Tc-99m (140 keV) to be practical

Radioactive waste storage areas are shielded to minimize exposure rates. Mirrors mounted on the back wall of highlevel radioactive material storage areas are often used to allow retrieval and manipulation of sources without direct exposure to the head and neck. Beta radiation is best shielded by low atomic number (Z) material (e.g., plastic or glass), which provides significant attenuation while minimizing bremsstrahlung x-ray production. For high-energy beta emitters (e.g., P-32), the low Z shield can be further shielded by lead to attenuate bremsstrahlung. With the exception of positron emission tomography (PET) facilities, which often shield surrounding areas from the 511-ke V annihilation radiation, most nuclear medicine laboratories do not find it necessary to shield the walls within or surrounding the department.

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