Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

its samples ofI-125 and Co-57 to activities less than about 0.2!lCi. However, largeractivities of some radionuclides may be counted without significant losses; forexample, activities of Cr-51 as large as 5 !lCi may be counted, because only aboutone out of every ten decays yields a gamma ray.Quality Assurance for the Sodium Iodide Thyroid Probeand Well CounterBoth of these instruments should have energy calibrations (as discussed earlierfor an SCA system) performed daily, with the results recorded. A backgroundcount and a constancy test, using a source with a long half-life such as Cs-137,also should be performed daily for both the well counter and the thyroid probeto test for radioactive contamination or instrument malfunction. On the day th~econstancy test is begun, a counting window is set to tightly encompass the photopeakand a count is taken and corrected for background. Limits called actionlevels are established which, if exceeded, cause the individual performing the testto notify the chief technologist, physicist, or physician. On each subsequent day,a count is taken using the same source, window setting, and counting time; correctedfor background; recorded; and compared with the action levels. (If eachday's count were mistakenly compared with the previous day's count instead ofthe first day's count, slow changes in the instrument would not be discovered.)Periodically, the constancy test count will exceed the action levels because ofsource decay. When this happens, the first-day count and action levels should becorrected for decay.Also, spectra should be plotted annually for commonly measured radionuelides,usually 1-123 and 1-131 for the thyroid probe and perhaps Co-57 (Schillingtests), 1-125 (radioimmunoassays), and Cr-51 (red cell volumes and survival studies)for the well counter, to verify that the SCA windows fit the photopeaks. Thistesting is greatly simplified if the department has an MCA.A dose calibrator, shown in Fig. 20-27, is used to measure the activities of doses ofradiopharmaceuticals to be administered to patients. The U.S. Nuclear RegulatoryCommission (NRC) and state regulatory agencies require that doses of x-ray- andgamma ray-emitting radiopharmaceuticals be measured with a dose calibrator.Most dose calibrators are well-type ionization chambers that are filled with argon(Z = 18) and pressurized to maximize sensitivity. Some less expensive dose calibratorsinstead use GM tubes near a chamber for the insertion of the dose. Most dosecalibrators have shielding around their chambers to protect users from the radioactivematerial being assayed and to prevent nearby sources of radiation from affectingthe measurements.A dose calibrator cannot directly measure activity. Instead, it measures theintensity of the radiation emitted by a dose of a radiopharmaceutical. The manufacturerof a dose calibrator determines calibration factors relating the intensity ofthe signal from the detector to activity for specific radionuelides commonly used innuelear medicine. The user pushes a button or turns a dial on the dose calibrator todesignate the radionuelide being measured, thereby specifying a calibration factor,and the dose calibrator displays the measured activity.