Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

greater overall input power). Next, an inverter circuit creates the high-frequency ACwaveform. This AC current supplies the high-voltage transformer and creates awaveform of fixed high voltage and corresponding low cutrent. Mter rectificationand smoothing, two high-voltage capacitors on the secondary circuit (shaded in Fig.5-32) accumulate the electron charges. These capacitors produce a voltage acrossthe x-ray tube that depends on the accumulated charge, described by the relationshipV = QlC, where V is the voltage (volts), Q is the charge (coulombs), and C isthe capacitance (farads). During the x-ray exposure, feedback circuits monitor thetube voltage and tube current and correct for fluctuations.For kVp adjustment, a voltage comparator/oscillator measures the differencebetween the reference voltage (a calibrated value proportional to the requested kVp)and the actual kVp measured across the tube by a voltage divider (the kV sense circuit).Trigger pulses generated by the comparator circuit produce a frequency thatis proportional to the voltage difference between the reference signal and the measuredsignal. A large discrepancy in the compared signals results in a high triggerpulsefrequency, whereas no difference produces few or no triggc;r pulses. For eachtrigger pulse, the DC/AC inverter circuit produces a corresponding output pulse,which is subsequently converted to a high-voltage output pulse by the transformer.The high-voltage capacitors store the charge and increase the potential differenceacross the x-ray tube. When the x-ray tube potential reaches the desired value, theoutput pulse rate of the comparator circuit settles down to an approximately constantvalue, only recharging the high-voltage capacitors when the actual tube voltagedrops below a predetermined limit. The feedback pulse rate (generator frequency)strongly depends on the tube current (mA), since the high-voltagecapacitors discharge more rapidly with higher mA, thus actuating the kVp comparatorcircuit. Because of the closed-loop voltage regulation, autotransformers forkVp selection and input line voltage compensation are not necessary, unlike othergenerator designs.The mA is regulated in an analogous manner to the kVp, with a resistor circuitsensing the actual mA (the voltage across a resistor is proportional to the current)and comparing it with a reference voltage. If the mA is too low, the voltage comparator/oscillatorincreases the trigger frequency, which boosts the power to the filamentto raise its temperature and increase the thermionic emission of electrons.The closed-loop feedback circuit eliminates the need for space charge compensationcircuits and automatically corrects for filament aging effects.The high-frequency inverter generator is the preferred system for all but a fewapplications (e.g., those requiring extremely high power, extremely fast kVp switching,or submillisecond exposure times). In only rare instances is the constant-potentialgenerator a better choice.Voltage RippleThe voltage ripple of a DC waveform is defined as the difference between the peakvoltage and the minimum voltage, divided by the peak voltage and multiplied by 100:01. I . I70 vo tage npp e = V max -V; V min X 100 [5-6]maxA comparison of the voltage ripple for various x-ray generators is shown in Fig.5-33. In theory, a single-phase generator, whether one-pulse or two-pulse output,