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“Quality control (QC) consists of a series of standardized tests developed to detect changes in x-ray equipment function from its original level of performance. The objective of such tests, when carried out routinely, allows prompt corrective action to maintain x-ray image quality” (Periard and Chaloner, 1996). These tests can also help to avoid injury to the patient and radiologist or technician.
In a quality control program, there are 3 parts: environmental inspection, performance testing, and visual inspection. Each of these parts plays an important role in quality control of radiographic equipment. Environmental inspection “involves checking for mechanical and electrical safety.” (Papp, 2006) It can also be performed with visual inspection. Performance testing, in contrast, uses special testing instruments and gives an evaluation of the x-ray generator and x-ray tube. In this paper, the aspects of performance testing will be discussed.
PERFORMANCE TESTING: USING SPECIALIZED TEST INSTRUMENTS
Special test instruments can vary and range from phantoms to computerized systems. The computerized systems can make data collection time efficient, but are very costly. Many facilities choose to use multiple devices for their data collection purposes. Since most data obtained during performance testing involves radiation measurements, some form of a radiation detector is needed.
PERFORMANCE TESTING: MEASURING EXPOSURE
X-ray generators should always have the same amounts of radiation output (mR) for each unit of x-ray tube current and time (mAs). “In addition, similar types of x-ray generators and tubes within a department should emit the same values of mR per mAs so that technique charts can be valid in all rooms and the number of repeat examinations can be reduced. All beginning values are determined when the unit is installed, or when a quality control program starts. “The Joint Commission on the Accreditation of Healthcare Organizations (JAHO) requires the posting of this value to guarantee that the x-ray generator does not emit excessive amounts of radiation exposure for a given combination of kVp per mAs.
A. Filtration Checks
Filtration is needed to remove photons from x-ray beams. This test should be performed yearly, or any time servicing has been done to the x-ray tube or collimator.
“The best method to determine if adequate filtration exists is to measure the half-value layer (HVL) (the amount of filtration that reduces the exposure rate to one half of its initial value) because normally it is not possible to measure inherent filtration. This is due to filament evaporation that is continually taking place, which adds a layer of tungsten onto the inside of the x-ray tube window. The HVL should not vary from its original value (which is established at acceptance) or its value at the beginning of the quality control program. It is dependent on the kVp used, the total beam filtration, and the type of x-ray generator” (Papp, 2006)
The x-ray tube's voltage affects image contrast and density, as well as the patient's dose of radiation. The voltage level stated on the control panel should “produce an x-ray beam with a comparable and consistent amount of energy” (Papp, 2006). The accuracy can be measured by a specialized test cassette or a digital meter. This is an annual test, or it should be performed when the x-ray generator or tube is serviced.
A digital x-ray timer is the most effective method to test timer accuracy. These devices use a “solid-state detector that measures the total time of x-ray production and then displays the time by means of a digital readout” (Papp, 2006). There are older methods to test timer accuracy, which are more cost effective. The spinning top test is one of many of these methods. The spinning top includes a metal disk with an outside edge hole or slit.
D. Beam Restriction System
This system regulates the x-ray field size. It controls the amount of a patient's body is exposed to radiation. This system should be tested every six months, or when the system is serviced. “The factors to evaluate within the beam restriction system include light field-radiation field congruence, image receptor-radiation field alignment, accuracy of the X-Y scales, and illuminator bulb brightness” (Papp, 2006)
E. X-Ray Tube Heat Sensors
Although many x-ray generators have overload protection circuits that prevent excessive heat exposure to the tube, they do not protect against long-term heat exposure build up. “Radiographers can rely on anode cooling charts or housing cooling charts supplied by the tube manufacturer” (Papp, 2006) to prevent accumulated heat.
The purpose of quality control maintenance monitoring, specifically performance testing, is to “permit evaluation of the performance of the facility's x-ray system(s) in terms of the standards for image quality established by the facility and compliance with applicable Federal and State regulatory requirements” (Quality Assurance Programs for Diagnostic Radiological Facilities, 2006).
Papp, Jeffrey Quality Management in the Imaging Sciences (2006; pp.84-104) Mosby Publishing ISBN: 0323035671
Periard, M.A. and Chaloner, P. Diagnostic X-ray Imaging Quality Assurance: An Overview (Oct. 1996) accessed online at www.hc-sc.gc.ca/ewh-semt/pubs/radiation/qa-x_ray_image-aq/index_e.html
Quality Assurance Programs for Diagnostic Radiological Facilities accessed Jan. 2006
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