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Welcome to a VIA Online Learning Tutorial: This site has been developed to provide learning materials to enable you to create X-ray exposure charts.
Select Chapter:
1. INTRODUCTION [print this page]
2. STARTING
3. CREATING
4. OPTIMISED
Basics
A radiograph exposure chart can be constructed to suit any X-ray machine. When correctly formulated it identifies reliable exposure factors (mA, kVp and exposure time) for known tissue thicknesses.
Exposure charts may be kVp-variable or mAs-variable. A kVp-variable chart uses constant settings for mA and exposure time (seconds) and indicates the appropriate kVp for specific tissue thicknesses and types. The alternative, a mAs-variable chart, recommends various mAs settings for specific tissue thicknesses and types, allowing the operator to keep the kVp constant for all exposures.
Which type of exposure chart is most suited to your practice depends on your X-ray machine’s range of available settings for kVp, mA and seconds, and whether the kVp and mA settings are linked.
In a perfect world, all veterinary radiographers would have machines suited to kVp-variable radiography, which allows the operator to vary the penetration of the X-ray beam in proportion to changes in the thickness of the patient.
However mAs-variable charts are more appropriate for machines with a limited number of exposure settings because they enable changes in photon output to compensate for the restricted selection of beam energies.
If you are using a digital system, this construction guide will not help you to establish a chart of exposures. The latitude inherent in digital radiography produces similar appearing radiographs at different exposure settings. However, image resolution with digital systems is directly related to exposure which is influenced by the signal to noise ratio. Resolution is lower at lower exposures and greater at higher exposures. Our recommendation is to modify any existing successful technique chart to one that would work successfully for 200 speed receptors, and then apply the chart to your digital system. For example, if you are using an exposure chart with 400 speed receptors, doubling the mAs will convert the chart to a 200 speed chart.
Aims
Regardless of which system best suits your machine, you will need two basic charts -- one to list exposures to use in conjunction with a grid, the other to indicate the settings for gridless radiography. A grid is recommended for radiographing thoraxes deeper than 14cm and any other body part thicker than 10cm.
You may also decide to create specialised charts for determining exposures for extremities, small birds, exotic species and cats. If much of your time is spent on one type of radiography such as hip dysplasia, you may wish to construct a chart specific to that body region. The aim of the following sections is to help you produce an initial chart. Once you have accomplished this, other charts can be made to complement your requirements.
Assumed knowledge
You are expected to be familiar with your X-ray machine and ancillary equipment and you must also be able to process a radiograph in an appropriate manner. It is assumed your X-ray machine is in good working order and the settings for kVp, mA and seconds are properly calibrated. If you are unsure whether your equipment is reliably calibrated, arrange a visit from a licensed service technician before proceeding further.
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Standardise
Standardise wherever possible to minimise the number of variables in your method. For example, always measure at the same place (I recommend the 12th rib for thoracic and abdominal radiography) and avoid using a mixture of intensifying screens unless they are spectrally matched and the same speed.
The anode-to-film distance should be fixed (usually 100cm for high-output machines, 75cm for portable or low-output units). Finally, ensure your darkroom is free from extraneous light, and that the safelight is, in fact, safe.
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