Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

some x-ray generators, preprogrammed techniques can be selected for various examinations(i.e., chest; kidneys, ureter, and bladder; cervical spine). All console circuitshave relatively low voltage and current levels that minimize electrical hazards.Several x-ray generator circuit designs are in common use, including single-phase,three-phase, constant potential, and medium/high-frequency inverter generators.All use step-up transformers to generate high voltage, step-down transformers toenergize the filament, and rectifier circuits to ensure proper electrical polarity at thex-ray tube.A rectifier is an electrical apparatus that changes alternating current into direct current.It is composed of one or more diodes. In the x-ray generator, rectifier circuitsdivert the flow of electrons in the high-voltage circuit so that a direct current isestablished from the cathode to the anode in the x-ray tube, despite the alternatingcurrent and voltage produced by the transformer. Conversion to direct current isimportant. If an alternating voltage were applied directly to the x-ray tube, electronback-propagation could occur during the portion of the cycle when the cathode ispositive with respect to the anode. If the anode is very hot, electrons can be releasedby thermionic emission, and such electron bombardment could rapidly destroy thefilament of the x-ray tube.To avoid back-propagation, the placement of a diode of correct orientation in thehigh-voltage circuit allows electron flow during only .one half of the AC cycle (whenthe anode polarity is positive and cathode polarity is negative) and halts the currentwhen the polarity is reversed. As a result, a "single-pulse" waveform is produced fromthe full AC cycle (Fig. 5-26A), and this is called a half-wave rectified system.Full-wave rectified systems use several diodes (a minimum of four in a bridgerectifier) arranged in a specific orientation to allow the flow of electrons from thecathode to the anode of the x-ray tube throughout the AC cycle (see Fig. 5-26B).During the first half-cycle, electrons are routed by two conducting diodes throughthe bridge rectifier in the high-voltage circuit and from the cathode to the anode inthe x-ray tube. During the second half-cycle, the voltage polarity of the circuit isreversed; electrons flow in the opposite direction and are routed by the other twodiodes in the bridge rectifier, again from the cathode to the anode in the x-ray tube.The polarity across the x-ray tube is thus maintained with the cathode negative andanode positive throughout the cycle. X-rays are produced in two pulses per cycle,•FIGURE 5-26. (a): A single-diode rectifier allows electron flow through the tube during one half ofthe alternating current (AC) cycle but does not allow flow during the other half of the cycle; it thereforeproduces x-rays with one pulse per AC cycle. (b): The bridge rectifier consists of diodes thatreroute the electron flow in the x-ray circuit as the electrical polarity changes. The electrons flow fromnegative to positive polarity through two of the four diodes in the bridge rectifier circuit for the firsthalf-cycle, and through the alternate diodes during the second half-cycle. This ensures electron flowfrom the cathode to the anode of the x-ray tube, producing x-rays with two pulses per AC cycle.