Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

nected to the contactors that dose the circuit on the low-voltage side of the transformer.After the selected time has elapsed, the electromagnet is de-energized to allowthe contactors to open and turn off the applied voltage to the x-ray tube. When thecircuit has significant power (high positive or negative voltage), the switch contactswill remain in the dosed position. Only when the power in the circuit passes throughzero, at intervals of 1/120 second (8 msec), can the contactors release to terminate theexposure. The accuracy and reproducibility of these timers are poor, especially forshort exposure times, because the increment of time occurs in 8-msec intervals.For three-phase and constant-potential generators, high-voltage triode, tetrode,or pentode switches initiate and terminate the exposure on the secondary side of thecircuit under the control of the electronic timer or phototimer. The operating principlesof these devices were discussed earlier (see Diodes, Triodes, Tetrodes). Becausethe ability to open and dose the switch is independent of the power on the circuit,exposure times of 1 msec or less are possible.The high-frequency inverter generator typically uses electronic switching onthe primary side of the high-voltage transformer to initiate and stop the exposure.A relatively rapid response resulting from the high-frequency waveform characteristicsof the generator circuit allows exposure times as short as 2 msec.Alternatively, a grid-controlled x-ray tube can be used with any type of generatorto switch the exposure on and off by applying a bias voltage (about ~2000 V)to the focusing cup.Phototimers are often used instead of manual exposure time settings in radiography.Phototimers operate by measuring the actual amount of radiation incident on theimage receptor (e.g., screen-film detector) and terminating x-ray production whenthe proper amount is obtained. Thus the phototimer helps provide a consistentexposure to the image receptor by compensating for thickness and other variationsin attenuation in a particular patient and from patient to patient.The phototimer system consists of one or more radiation detectors, an amplifier,a density selector, a comparator circuit, a termination switch, and a backuptimer. X-rays transmitted through the patient instantaneously generate a smallsignal in an ionization chamber (or other radiation-sensitive device such as asolid-state detector). An amplifier boosts the signal, which is fed to a voltage comparatorand integration circuit. When the accumulated signal equals a preselectedreference value, an output pulse terminates the exposure. A user-selectable "density"switch on the generator control panel adjusts the reference voltage to modifythe exposure.For most general diagnostic radiography, the phototimer sensors are placed infront of the image receptor to measure the transmitted x-ray flux through thepatient (see Figure 5-34). Positioning in front of the image receptor is possiblebecause of the high transparency of the ionization sensor at higher kVp values(although this is not the case for mammography). In the event of a phototimerdetector or circuit failure, a "backup timer" safety device terminates the x-ray exposureafter a preset time.In order to compensate for different x-ray penetration through distinctly differentanatomy, three sensors are typically used in a chest or table cassette standwith phototiming capability as shown in Fig. 5-34. The technologist can select