Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

TABLE 5-4. ROOT-MEAN-SQUAREVOLTAGE (Vrms) AS A FRACTION OFPEAK VOLTAGE {V peak)V,msas a fractionVpeakofSingle-phaseThree-phase six-pulseThree-phase 12-pulseHigh-frequencyConstant-potential0.710.950.990.95-0.991.00generator waveform and cable capacitance properties, is used in the equation todetermine the power delivered over the lOO-msec period. For all but constantpotentialgenerators, the average kV is less than the kVp (due to voltage ripple), andit is substantially less for a single-phase generator. Exact determination of the powerrating requires multiplication by the "root-mean-square" voltage (V,ms), as listed inTable 5-4 for various x-ray generator voltage waveforms. (V,msis the constant voltagethat would deliver the same power as the varying voltage waveform). This correctionis usually ignored for all but the single-phase generator.These two examples illustrate the power delivery capability of the generator andpower deposition tolerance of the focal spot. Table 5-5 classifies the x-ray generator interms of power output, and Table 5-6 lists typical x-ray tube focal spot power ratings.X-ray generators have circuits to prohibit combinations of kVp, mA, and exposuretime that will exceed the single-exposure power deposition tolerance of the focal spot.Typical PowerPower Level RequirementsGenerator Type Usage Applications (kW)Single-pulse, one-phase Very low Dental and handheld :Qfluoroscopy unitsTwo-pulse, one-phase Low to medium General fluoroscopy and ~10 and ~50radiographySix-pulse, three-phase Medium to high Light special procedures ~30 and ~100Twelve-pulse, three-phase High Interventional and cardiac ~50 and ~150angiographyConsta nt-potentia I High Interventional and cardiac ~80 and ~200angiographyHigh-frequency All ranges All radiographic procedures ~2 and ~150