Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Because gas multiplication does not occur at the relatively low voltages applied toionization chambers, the amount of electrical charge collected from a single interactionis very small and would require a huge amplification to be detected. For thisreason, ionization chambers are seldom used in pulse mode. The advantage to operatingthem in current mode is the freedom from dead-time effects, even in veryintense radiation fields. In addition, as shown in Fig. 20-5, the voltage applied toan ion chamber can vary significantly without appreciably changing the amount ofcharge collected.Almost any gas can be used to fill the chamber. If the gas is air and the walls ofthe chamber are of a material whose effective atomic number is similar to air, theamount of current produced is proportional to the exposure rate (exposure is theamount of electrical charge produced per mass of air). Air-filled ion chambers areused in portable survey meters and can accurately indicate exposure rates from lessthan 1 mR/hr to hundreds of roentgens per hour (Fig. 20-6). Air-filled ion chambersare also used for performing quality-assurance testing of diagnostic and therapeuticx-ray machines, and they are the detectors in most x-ray machine phototimers.Gas-filled detectors tend to have low intrinsic efficiencies for detecting x-raysand gamma rays because of the low densities of gases and the low atomic numbersof most common gases. The sensitivity of ion chambers to x-rays and gamma rayscan be enhanced by filling them with a gas that has a high atomic number, such asargon (Z = 18) or xenon (Z = 54), and pressurizing the gas to increase its density.Well-type ion chambers called dose calibrators are used in nuclear medicine to assaythe activities of dosages of radiopharmaceuticals to be administered to patients;many are filled with pressurized argon. Xenon-filled pressurized ion chambers areused as detectors in some CT machines.FIGURE 20-6. Portable air-filledionization chamber survey meter.This particular instrument measuresexposure rates ranging fromabout 0.1 to 20 Rlhr. (Photographcourtesy Inovision Radiation Measurements,Solon, Ohio.)