C HA P T E R

7Quality Control of X-Ray

Generators and Ancillary

Radiographic Equipment

K E Y T E R M S

actual focal spotautomatic exposure controlcomparatorcoulomb/kilogramdetectoreffective focal spotfocal spot bloominggrid latitudegrid uniformityhalf-value layerhigh-frequency

homogenous phantomion chamberkilowatt ratingLaw of Reciprocitylinear tomographyline focus principlelinearitymobile x-ray generatorobjective planephotodetectorpluridirectional tomography

portable x-ray generatorreciprocityreproducibilityroentgensensorsingle-phasesolid state detectorthree-phasevoltage ripple

O B J E C T I V E S

At the completion of this chapter the reader should be able to do the following:• Explain the difference between single-phase, three-

phase, and high-frequency x-ray generators• Recognize the voltage waveform characteristics of the

three types of x-ray generators• List the voltage ripple values for the three types of x-ray

generators• Calculate the power output rating for the three types of

x-ray generators• List the three main parts of a quality control program

for radiographic equipment• List and describe the performance tests for radiographic

equipment

• List the main components of an automatic exposurecontrol (AEC) system

• Perform quality control testing of various AECparameters

• Describe the quality control parameters for conventionaltomographic systems

• Discuss the importance of grid uniformity and alignmenton image quality

• Explain the quality control tests performed on mobileequipment

O U T L I N E

X-Ray Generators 86Single-Phase Generator 86

Half-Wave Rectified 86Full-Wave Rectified 87

Three-Phase Generator 87Three-Phase, Six-Pulse 87Three-Phase, 12-Pulse 87

High-Frequency Generator 88Voltage Ripple 88Power Ratings 88

Control or Operating Console 88High-Voltage Generator 89X-Ray Tube, Tube Accessories,and X-Ray Table 89

Quality Control Program forRadiographic Units 89Visual Inspection 89

Control Panel 89Overhead Tube Crane 90Radiographic Table 90Protective Lead Apparel 90

85

Miscellaneous Equipment 90Environmental Inspection 90Performance Testing 91

Radiation Measurement 91Reproducibility ofExposure 92

Radiation Output 92Filtration Check 93Kilovolt (Peak) Accuracy 93Timer Accuracy 95Voltage Waveform 97Milliampere and Exposure TimeLinearity and Reciprocity 97

Focal Spot Size 98Beam Restriction System 100Beam Alignment 102Source-to-Image Distance andTube AngulationIndicators 102

Overload Protection 103X-Ray Tube HeatSensors 103

Ancillary Equipment 104Automatic Exposure ControlSystems 104Detectors 104

Photodetectors 105Ion Chambers 105Solid-State Detectors 105

Comparator 105Automatic Exposure ControlTesting 105Backup, or MaximumExposure, Time 106

Minimum Exposure Time 106Quality Control for AutomaticExposure Control 106Consistency of Exposure withVarying Milliampere 106

Consistency of Exposure withVarying Kilovolt (Peak) 106

Consistency of Exposure withVarying Part Thickness 106

Consistency of Exposure withVarying Field Sizes 107

Consistency of AutomaticExposure ControlDetectors 107

Reproducibility 107Density Control Function 107

Reciprocity Law Failure 107

Conventional TomographicSystems 108Quality Control of TomographicSystems 108

Section Level 109Section Thickness 109Level Incrementation 110Exposure Angle 110Spatial Resolution 110Section Uniformity and BeamPath 110

Patient Exposure 111Grids 111Grid Uniformity 111Grid Alignment 111

Portable and Mobile X-RayGenerators 112Portable X-Ray Generator 112Mobile X-Ray Generator 112

Direct-Power Units 112Capacitor Discharge Units 113Cordless, or Battery-Powered,Mobile Units 113

High-Frequency MobileUnits 113

Summary 113

Several of the previous chapters have dealt with film/screen image receptors (digital image receptors are dis-cussed in a later chapter) and the importance of qualitycontrol testing to avoid poor quality images. However,many other components of diagnostic imaging depart-ments are subject to variability and must have separatequality control protocols established to ensure safeoperation and function. One such component is theequipment used as the x-ray source in conventional radi-ography. This includes the x-ray generator, control oroperating console, x-ray tube, and accessory devicessuch as the x-ray table and support mechanism.

X-RAY GENERATORS

The x-ray generator is the largest component of the radio-graphic unit. It contains the high-voltage transformers,rectifiers, timing circuitry, and milliampere (mA) andkilovolt (peak) (kVp) selectors. Single-phase, three-phase,and high-frequency x-ray generators are available.

Single-Phase Generator

A single source of alternating current is used to power thegenerator in a single-phase unit. A graphic representationof single-phase alternating current is shown in Figure 7-1.

The graph in Figure 7-1 plots the voltage on they-axis versus time on the x-axis. The peaks in the graphrepresent the flow of electricity changing directionthroughout the circuit. Voltage values range from zerovolts to a peak value (hence the term kilovolts [peak])and back to zero volts. The two types of single-phase

generators used in diagnostic radiography are half-waverectified and full-wave rectified.Half-Wave Rectified. In a half-wave rectified genera-tor, one half of the normal alternating current wave isused to power the x-ray tube, and the other half is shutoff by the addition of one or two rectifiers. This causesthe normal single-phase alternating current waveformgraph to appear as shown in Figure 7-2.

Because the standard frequency of alternating currentin the United States is 60 Hz, or 60 cycles per second (c/s)(a cycle represents the current flowing in each directionone time), only 60 pulses of electricity per second canbe used to create radiographs. This means that the radio-graphs are emitted in pulses, or spurts, and therefore alonger amount of time is required to obtain a specificquantity of radiographs. For this reason, half-wave recti-fied units are generally used in dental x-ray units andsome small portable x-ray units.

Time1/60 sec

FIGURE 7-1 Voltage waveform graph of single-phase alternatingcurrent.

86 CHAPTER 7 Quality Control of X-Ray Generators and Ancillary Radiographic Equipment

Full-Wave Rectified. Full-wave rectified generators usea combination of four rectifiers to channel all of thepulses through the x-ray tube during x-ray production.The resultant waveform graph for this type of unitappears in Figure 7-3.

Because 120 pulses of electricity per second can beused to create x-rays, twice as many x-rays can be createdin a given period as compared with the half-wave unit.This allows full-wave rectified units to be used for manyconventional radiographic procedures. However, the x-rays are still emitted in pulses (as demonstrated by thenumber of times the pulses reach zero on the waveformgraph) and therefore still require some time to achieve aspecific quantity of x-rays. The shortest exposure timeavailable for single-phase x-ray generators is 1/120 second.For this reason, full-wave rectified units are seldom foundin larger hospitals but are frequently found in doctors’offices and small clinics because of their relatively lowpurchase price and installation cost (in comparison to 3-phase generators discussed below).

Three-Phase Generator

Three-phase x-ray generators are powered by three sepa-rate sources of alternating current that have been staggeredso that they are “out of phase” with each other by 120degrees or one third of a cycle. The voltagewaveformgraphfor three-phase alternating current appears in Figure 7-4.

By the time one pulse of current begins to drop towardzero voltage, another pulse is heading back up to the maxi-mumvalue, so the voltage never reaches zero and x-rays arecontinuously produced (eliminating the pulsed effect of sin-gle-phase units). This allows exposure time values as low as1/1000 second (1 ms). The x-rays created with three-phaseunits also have a higher average energy than those of sin-gle-phase units because the voltage is near the peak valuefor a higher percentage of the time during x-ray production(which can lower patient dose compared with single-phaseunits). The main disadvantages of three-phase equipmentare higher capital cost (at least twice as expensive as sin-gle-phase) and the size of the unit (because of the additionalelectronic components required). The advantages have gen-erally outweighed the disadvantages because the three-phase x-ray generator has been the most common type ofunit in major hospitals and medical centers sincethe 1970s. The two types of three-phase generators are 6-pulse and 12-pulse generators.Three-Phase, Six-Pulse. The six-pulse type of three-phase unit uses six rectifiers and one half of the three-phase alternating current pulses. The resulting voltagewaveform appears in Figure 7-5.

As mentioned previously, one cycle of single-phasealternating current referred to one pulse of electricitytraveling each direction one time so that two pulses arefound in one cycle. Because 60 cycles occur each second,one cycle requires a time of 1/60 second. In a three-phase,six-pulse x-ray generator, six pulses of electricity existduring the same cycle or 1/60-second time interval(instead of two pulses per 1/60 second in single-phase),hence the name three-phase, six-pulse. This means that360 voltage pulses are now available per second.Three-Phase, 12-Pulse. The 3-phase, 12-pulse type ofx-ray generator uses 12 rectifiers (four rectifiers perphase) that direct all of the three-phase alternating cur-rent pulses through the x-ray tube during x-ray produc-tion. This yields 12 pulses of electricity per one-cycle

Time

Voltage

FIGURE 7-4 Voltage waveform graph of 3-phase alternatingcurrent.

Time1/60 sec

1/120sec

FIGURE 7-2 Voltage waveform graph of half-wave rectified,single-phase current.

Time1/60 sec

FIGURE 7-3 Voltage waveform graph of full-wave rectified,single-phase current.

Voltage

�V max

Time

1/60 sec

FIGURE 7-5 Voltage waveform graph of 3-phase, 6-pulse current.

87CHAPTER 7 Quality Control of X-Ray Generators and Ancillary Radiographic Equipment