CHAPTER 7 Quality Control of X-Ray Generators and Ancillary Radiographic Equipment KEYTERMS actual focal spot automatic exposure control comparator coulomb/kilogram detector effective focal spot focal spot blooming grid latitude grid uniformity half-value layer high-frequency homogenous phantom ion chamber kilowatt rating Law of Reciprocity linear tomography line focus principle linearity mobile x-ray generator objective plane photodetector pluridirectional tomography portable x-ray generator reciprocity reproducibility roentgen sensor single-phase solid state detector three-phase voltage ripple OBJECTIVES 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 exposure control (AEC) system • Perform quality control testing of various AEC parameters • Describe the quality control parameters for conventional tomographic systems • Discuss the importance of grid uniformity and alignment on image quality • Explain the quality control tests performed on mobile equipment OUTLINE X-Ray Generators 86 Single-Phase Generator 86 Half-Wave Rectified 86 Full-Wave Rectified 87 Three-Phase Generator 87 Three-Phase, Six-Pulse 87 Three-Phase, 12-Pulse 87 High-Frequency Generator 88 Voltage Ripple 88 Power Ratings 88 Control or Operating Console 88 High-Voltage Generator 89 X-Ray Tube, Tube Accessories, and X-Ray Table 89 Quality Control Program for Radiographic Units 89 Visual Inspection 89 Control Panel 89 Overhead Tube Crane 90 Radiographic Table 90 Protective Lead Apparel 90 85
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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