Radiographic Equipment Testing (Part 1)
Many people confuse the terms, “Quality Control” and “Quality Assurance”.
Quality Assurance is a managerial philosophy that encompasses all aspects of patient care,
image production and image interpretation.
This can include a radiation protection program for patients and personnel, the provision
of technique charts, patient and physician satisfaction surveys, and also elements of
Quality Control.
Quality control in a radiology department specifically refers to the calibration and
monitoring of equipment, along with the discussion of continuing the value of repeat analysis.
The technologist used to be responsible for many quality control tests for radiographic
equipment, but is now mostly performed by a physicist.
Still, it is useful to be aware of these types of tests in order to document problems with
equipment, and potentially troubleshoot and isolate the problem that may be occurring.
The next two videos will cover some basic quality control tests for radiographic equipment,
and we will later discuss some common QC testing procedures for fluoroscopic equipment.
Exposure Timer Accuracy can be checked with an ion chamber or photodiode device.
If the exposure timer set at the control panel is not accurately producing an x-ray beam
at the set time, 100% of the time, then actual exposure values may vary from what the technologist
sets, leading to potentially incorrect exposure values for each exam, causing unnecessary
repeat exposures and increased dose to the patient.
There is a plus or minus 5% variance allowable, and this should be checked annually.
mA Linearity is defined as the alignment of a particular mA station relative to other
stations in its output of radiation.
This test assures that the mA station set at the console matches the radiation output
of the x-ray tube.
To test for mA linearity, a series of exposures can be taken at various mA stations on the
console, and the dose received by the ion chamber should be recorded.
Theoretically, a doubling in mA should double the radiation dose at the ion chamber.
If a variance of plus or minus 10% is noticed, the equipment should be serviced.
This test is also performed manually.
Exposure Reproducibility is defined as the ability to repeat the same overall technique
settings and receive the same result in exposure.
Again, you can use the ion chamber to measure the dose put out by the x-ray tube.
Your textbook recommends 10 exposures at the same settings, and recording the ion chamber
reading.
A variation of plus or minus 5% is acceptable and this should be performed annually.
Half-Value Layer, or HVL, is defined as the amount of absorbing material required to reduce
the x-ray beam into half its original value.
We did an experiment in your exposure principles course to determine the half value layer of
the beam if you recall using different Aluminum Filters of varying thickness.
As a QC test, you can expose the ion chamber with a set mA, exposure time, and kVp combination
that will produce a substantial reading with no absorber over the chamber.
A combination of these aluminum filters should be used as absorbers with the same technical
factors to reduce the beam to 50% of its original value at the ion chamber.
Once that is accomplished, the thickness of the absorber should be added up to determine
the actual HVL.
There are minimum required aluminum-equivalent standards at different kVp ranges and with
different generators depicted here.
To pass the test, the HVL must be greater than the minimum listed, and this should be
tested annually.
HVL results are valuable in the process of Kilovoltage Peak Calibration, and there are
different methods of testing for kVp consistency.
One of the simplest ways is with a digital kVp meter where a series of test exposures
are taken with the beam centered to the meter, and a digital readout of kVp produced by the
beam is displayed.
There’s a +/- 5% range in the State of California, but Federally, the acceptance range is +/-
10%, and this should also be tested annually.