What is an over-exposed X-ray and how can I avoid that in my animal clinic's X-ray room?
X-rays are a vital and commonly used tool in every animal hospital. However, they are only of use if the X-ray image is of good diagnostic quality. If radiographs are of poor quality, for example through inadequate positioning or incorrect exposure, this can lead to errors in interpretation.
If X-rays taken in the animal clinic are over-exposed, this can be very frustrating to veterinary staff. The radiographs may need to be repeated, leading to increased exposure to X-ray beams for patients, and higher time and cost penalties.
X-rays being over or under-exposed is a common problem in veterinary clinics. In this blog, we’ll go through over-exposure, why it happens, and how to help. To learn more about the opposite problem, under-exposure, check out our blog here.
What is exposure?
Exposure is the term used to describe the number of X-ray photons present at a certain point. Over-exposure to animal X-rays happens when the concentration of these photons is too high, leading to excessive darkening of the film.
Four radiographic factors affect exposure:
Kilovoltage (kV) – the voltage applied across the X-ray generator, affecting the energy of the X-ray, and therefore the penetrating power of the beam
Milliampere (mA) – the current applied to the cathode to generate X-rays, affecting the number of electrons and thus of X-ray photons
Exposure time
The distance from the X-ray source to the patient (FFD – focus-film distance) – as distance decreases, the intensity of the beam increases.
The exposure of the X-ray is determined by changes to any of these four factors.
Why does an over-exposed X-ray matter?
A radiograph should be properly exposed so that all structures in the targeted anatomical region can be visualized.
In a film-based radiograph, over-exposure makes an X-ray very dark, making it hard to interpret but easy to detect. Using an over-exposed X-ray as a diagnostic tool may lead to subtle lesions being missed, or to artifacts being seen.
However, in a DR system, there are very few signs of over-exposure, as the computer will automatically filter the image and return an “optimal” radiograph.
If the exposure is massively excessive, however, there may be other artifacts generated, in particular, blocky or geometric shapes superimposed over the image.
This is more apparent in some systems than others but seems to be due to the local vs regional adjustment patterns generated by the filtering software.
So, an over-exposed DR radiograph rarely leads to a non-diagnostic image. However, over-exposure is also a safety concern, with animals and potentially staff being exposed to unnecessary levels of X-rays.
Correctly exposed X-rays are important for accurate diagnosis but above all for safety in the animal hospital.
"Exposure Creep” is a common problem with digital radiography, and with our increasing knowledge of the health concerns associated with cumulative X-ray exposure, something that all clinicians need to work to minimize – even in jurisdictions with relatively relaxed radiation safety limits, such as the USA.
Why is my X-ray over-exposed?
It can be frustrating to have an over-exposed X-ray, and difficult to determine the underlying problem. Here are some common reasons for over-exposure in animal radiography.
A common issue when struggling with exposure is non-deliberate changes in the distance between the film and the X-ray generator.
A small change in distance can have a huge effect on exposure, as the relationship between FFD and exposure is an exponential function.
If your X-ray is overexposed, the FFD may be too small, and require adjustment – or a corresponding change to the mAs.
In animal hospitals, moveable and adjustable X-ray tables can make changes to the FFD a common problem.
Technical errors in the choice of kV and mA levels are also common. If the kV setting is too high, the X-rays will have more power and penetrate straight through the patient, leaving a film that is overexposed and too dark to interpret.
Over-exposed X-rays require a decrease in the kV level and mAs. The omission of a grid when one is needed – or accounted for in the exposure chart - can also affect exposure.
Tips for avoiding an over-exposed X-ray
Interpreting X-rays requires films of high quality, excellent positioning, and good exposure.
A simple response to avoid over-exposed radiographs in the animal clinic is to ensure the kV and mA settings are correct. Over-exposure implies the settings are too high.
The use of an exposure chart can be invaluable to ensure accurate levels. A comprehensive chart, with suggested settings for all different species and sizes of animals, as well as differing anatomical locations, can help avoid mistakes when calculating appropriate settings.
It should be remembered that X-rays need to be of good quality and exposed correctly for the anatomical area. Different bodily areas have varying needs for good interpretation.
For example, the thorax has both soft tissue and bone which all need to be detailed whereas the abdomen contains high volumes of soft tissue structures, requiring excellent contrast.
Understanding this principle may lead to small adjustments to kV and mAs to maximize the quality of the X-ray.
Most DR systems are now equipped with Exposure Indicators, and these are invaluable for detecting higher-than-optimal exposures.
Ensure that you are familiar with how this works on your system and that you know how to interpret the numbers generated. LINK?
Remember to keep X-ray machines well-maintained and regularly serviced, for optimal performance.
Summing up
One of the reasons for observing overexposed X-rays is the failure to make necessary adjustments to the imaging technique when transitioning from film to CR to DR, or between different DR panels.
X-rays are a regularly used tool in animal clinics and have great diagnostic value. However, accurate interpretation relies upon good-quality X-rays.
Over-exposure rarely leads to a non-diagnostic radiograph but does lead to excessive radiation exposure to the patient and, potentially, staff.
Over-exposure can be caused by changes to the exposure factors: kV, mA, time, and distance.
Using accurate settings for the size, species, and anatomical location of the desired image, and knowing how to interpret the Exposure Indicator, are essential for optimal exposure and good quality X-ray.
Many instances of under or overexposure can be attributed to doctors failing to measure animals or consult the technique chart.
The DynaVue Duo x-ray machine for veterinarians automatically adjusts the exposure based on animal size, optimizing imaging and reducing radiation exposure.
This feature saves time, minimizes errors, and enhances diagnostic quality, improving veterinary care.
References
Mattoon, J. (2006) ‘Digital Radiography’ Vet Comp Orthop Traumatol 19(03) pp.123-132
Kirberger, R. (2005) ‘Radiograph quality evaluation for exposure variables – a review’ Veterinary Radiology and Ultrasound 40(3) pp.220-226
