Mastering X-ray Imaging: Essential Techniques for Crystal Clear Animal Radiographs
As veterinarians, we’re sure that you use X-rays commonly as a part of a diagnostic process. In order to achieve an accurate diagnosis, however, these radiographs must be correctly positioned and of high quality. If X-rays are taken poorly, lesions may be both under and over-diagnosed, due to problems with interpretation.
Having to repeat X-rays is far from ideal, both in terms of increased radiographic exposure for our patients, but also in terms of cost, time, and frustration for the veterinary professional.
We all want our X-rays to be the perfect first time – so how do we achieve that? Exposure problems are a common X-ray fault in animal hospitals, so in this blog, we will go through under-exposure, and how to avoid it. The opposite, over-exposure, is also a common issue – read more about that here.
What is exposure?
Exposure refers to the concentration of X-ray photons in the air at a specific point. If an X-ray is under-exposed, it means there was less penetration of the beam through the target tissue.
X-ray exposure is affected by four factors:
Kilovoltage (kV) – controls the energy of the X-rays, and therefore the penetrating power
Milliampere (mA) – the current applied to the tube to generate X-rays, and therefore the number of photons produced
Exposure time
Focus-film distance (FFD - the distance from the X-ray source to the patient) – as distance increases, the intensity of the beam decreases
It is a combination of these factors which determines the overall exposure of each X-ray.
Why does an under-exposed X-ray matter?
In conventional radiography, an under-exposed X-ray will look ‘lighter’ or ‘whiter’ than a properly exposed X-ray. This whitening effect can make it very difficult to spot subtle abnormalities and lesions. Under-exposure can also lead to increased mottling or cloudiness of an image, which can obscure parts of the radiograph, or lead to artifacts.
Most veterinary staff were taught the “Rule of Toast” to describe this – underdone toast is too white.
However, in a modern digital radiograph, the computer will automatically add additional filters. It is very, very rare for an underexposed digital radiograph to appear pale or white.
This can “trick” the inexperienced radiographer into thinking that the exposure is correct when, in reality, the computer is enhancing and filtering the image.
This enhancement adds an additional layer of complexity when diagnosing from the radiograph because there may be insufficient raw data to obtain a reliable image, despite excellent-appearing radiographs.
Mildly under-exposed images are unlikely to be diagnostically catastrophic, as the signal: noise ratio of a modern DDR plate is high enough that it is likely that sufficient data has been captured. However, the more underexposed the image is, the more serious the potential for errors.
In more severely under-exposed images, the most common sign is pixelation of the image. It may appear subtly grainy, or even blurred (like an image zoomed in too far). This can significantly obscure subtle or fine pathology, especially when viewing lung fields, or fine structures like small animal phalanges or dentitions.
Under-exposure is therefore a real problem when it comes to the interpretation of X-rays in an animal hospital. Poor X-ray quality can lead to both over and under-diagnosis of disease in our patients.
Why is my X-ray under-exposed?
Having problems with under-exposed X-rays in your veterinary clinic? Here’s where you might be going wrong!
A common error when it comes to exposure is changes in the distance between the X-ray machine and the patient. Even a relatively small increase in distance can have a large effect on exposure, and therefore X-ray quality, as exposure is controlled by the inverse square law of radiation dispersal.
A small increase in the FFD requires a corresponding significant increase in the mAs to avoid an underexposed X-ray. Common causes for a variable FFD include adjustable X-ray tables in small animal hospitals, or large animal veterinarians standing too far from the patient with a portable machine.
The kV and mA settings play a large role in the exposure. Low kV levels affect X-ray penetration and therefore result in lower exposure. If an image is under-exposed, this usually means that the kV or mAs need to be increased.
However, adjusting the kV will lead to contrast changes, so in most cases, unless the contrast is too high, the mAs is the adjustment of choice to correct exposure errors.
Exposure can also be low due to various common technical errors, such as using a grid unnecessarily or underestimating the weight or body size of a patient.
Tips for avoiding an under-exposed X-ray
Achieving the right exposure is vital for X-rays of good diagnostic quality. In basic terms, if radiographs are often being under-exposed at your veterinary hospital, then increases to the kV or mAs settings will be needed.
However, for an X-ray of true diagnostic quality, it should be remembered that an image needs to be of high quality, rather than just exposed correctly.
For example, different approaches may be used when imaging the thorax as compared to the abdomen. Thoracic radiographs require a high kV: there is high natural contrast between bone, soft tissue, and gas, and using a high kV value enhances detail in the soft tissue areas of the lung fields.
Conversely, the abdomen has low natural contrast due to the vast majority of content being soft tissue, and therefore abdominal X-rays require a lower kV to maximize contrast.
The mAs setting will therefore need to be adjusted appropriately to accommodate the change in kV whilst retaining good exposure.
Exposure charts can be a really useful way to give a good starting point for determining the best settings. By including appropriate settings for multiple species, breeds, and sizes of animals, as well as different areas of anatomy, quick reference charts can be invaluable for maximizing X-ray quality and exposure.
The other two exposure factors of distance and time should be standardized as much as possible. Using correct restraint of patients should minimize motion blur. It is also important to ensure your X-ray machine is regularly maintained and serviced, to minimize the likelihood of equipment error.
Most modern DR systems have embedded Exposure Indicators, and with “exposure creep” a significant issue in digital radiography, we strongly recommend you use this under-utilized tool to ensure that your exposures are optimal. https://newvetequipment.com/dr-digital
Summing up
Under-exposed X-rays lead to interpretation errors, especially as they can be difficult to detect on a digital radiograph. Under-exposure can be caused by changes to the exposure factors: kV, mAs, distance, and time.
Standardizing exposure factors, using correct calculations for kV and mAs, and using accurate Exposure Indicators will lead to correct exposure and X-rays of true diagnostic quality in your animal hospital.
Many instances of under or overexposure can be attributed to doctors failing to measure animals or consult the technique chart.
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
