Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 23-13. Dose preparation workstation. The technologist is drawing theradiopharmaceutical from a vial shielded by a "lead pig" into a syringe containedin a syringe shield. Further protection from radiation exposure is afforded byworking behind the lead "L-shield" with a leaded glass window. The technologistis wearing a lab coat and disposable gloves to prevent contamination. A filmbadge is worn on the lab coat to record whole-body exposure and a thermoluminescentdosimeters (TLD) finger ring dosimeter is worn inside the glove to recordthe extremity exposure.TABLE 23-15. EXPOSURE REDUCTION ACHIEVED BY A O.5-mm LEADEQUIVALENT APRON FOR VARIOUS RADIATION SOURCESNo. of apronsExposureto reduceEnergy reduction exposure by 90% WeightSource (keV) with 1 apron (i.e., 1 TVL) (Ibs)Scattered x-rays 10-30 >90% -1 -15Tc-99m 140 -70% -2 -30(s-137 662 -6% -36 -540Protection of the Patient in Medical X-Ray ImagingTube Voltage and Beam FiltrationAn important goal in diagnostic imaging is to achieve an optimal balance betweenimage quality and dose to the patient. Increasing the k ltp will result in a greater transmission(and therefore less absorption) of x-rays through the patient. Even though theexposure per mAs increases as the kVp is increased (Fig. 23-9A), an accompanyingreduction in the mAs will decrease the incident exposure to the patient. Unfortunately,there is a concomitant reduction in image contrast due to the higher effectiveenergy of the x-ray beam. Within limits, this compromise is acceptable. Therefore, thepatient exposure can be reduced by using a higher kVp and lower mAs.