27th September 2013
Inconsistency in X-ray image exposure through incorrect application of the X-ray trigger.
The X-ray trigger should be depressed half-way to prime the X-ray generator. This is known as prepping the generator. For a rotating anode generator, this allows the anode to come up to speed prior to the exposure.
You then depress the trigger the rest of the way to fire the generator. This full depression should be maintained until the X-ray generator stops beeping or there is a visual indication of a red light to stop.
Being able to prep and then fire the generator separately is really useful, as this enables you to know exactly when the generator will fire. This is especially useful on chest X-rays as it allows you to catch the thorax fully inflated.
Prepping the generator will help prevent under exposure of the X-ray. In the case of DR X-ray systems, this is particularly prevalent as there are more communication requirements between the X-ray plate and the PC, which occurs while the generator is prepping.
If you take your finger off the trigger too soon, this will cause the output of the generator to be cut short. If you don't give the generator enough time to prep, you can also get a misfire.
Inconsistent image quality through varying focal distances.
The X-ray exposure will vary with different focal distances in relation to the X-ray plate i.e a shorter focal distance at a given kV and mAs will produce an over-exposed image as opposed to a given kV at a greater focal distance.
This is due to the Inverse Square law. Put simply, when you increase the distance from the generator to the receiver/plate you need to increase the exposure. For an increase from 60 to 80 cm this relates to roughly doubling the mAs or increasing the kV by 10.
How to fix this:
Ensure consistency on exposures for different anatomy. 9 times out of 10, small animal X-rays will be taken at a fixed distance. However, for equine applications, this is certainly not the case. For equine, it is a question of judgement. It is important to ensure that the same focal distances are applied for each anatomical structure once a good image is achieved for that area.
The correct procedure for an equine vet would be to have a built-in tape measure or have the laser pointers in the generator focused at a particular distance.
A poor quality exposure chart will lead to varying X-ray image quality. Also, the RCVS requires an up-to-date exposure chart to be on display/available in the vet practice.
When an exposure chart hasn’t been kept up to date or is not accurate enough, it can lead to poor quality images. Ideally, an exposure chart should be looking at the thickness of the region you are X-raying, not the weight of the animal. A missing or old exposure chart can lead to time being wasted through poor X-ray quality with re-takes of the X-ray often being required.
Always try to keep your exposure charts up to date and clearly visible in the area the X-rays are being taken. Ideally, your chart should be adapted to your X-ray generator and processing system. Film, CR and DR all need different exposures, and sadly there is no such thing as one totally universal exposure chart!
IF you do not currently have a working X-ray exposure chart, it is possible to download an example one from the BCF website, and there are gaps left on here for you so that you can adapt these charts to your own equipment.
We are all human and we can all make mistakes, but it is vital from both a patient care and legal point of view to ensure that all X-ray images are correctly labelled. This is especially the case for horses being vetted, or dogs having Hips or Elbow radiographed for the BVA schemes.
Whether you are using film, or a digital system you should still include:
This means that a later date you know exactly whose images you are looking at.
If the patient is having a lot of images taken it can also be really helpful to include the time of the X-ray too- and most digital systems will time stamp the images automatically. Another thing to be aware of is when using a digital system ensure you have ended your patient before you start a new one- this is also important when ultrasounding too!
Also, it’s important to make sure that left and right markers should be within the primary X-ray beam.
This is where the user is not erasing the X-ray cassette, thus not removing scatter/background radiation from the cassette. This can result in you having excess noise on your images and thus reduces image quality.
All you need to do is simply erase the CR cassettes if they haven’t been used for a few days. This is especially important if they have been left close to the X-ray generator or in an excessively warm location.
This cassette had been left by a radiator, with some cables in front of it.
The same things that can fog a film cassette will fog a CR cassette, but can have a greater impact on image quality. If you are not sure if you are suffering from fogging you can simply process a blank cassette and see if you have an image on it. Or if you think a radiograph has fogging on it, have a look under things like markers as you may be able to see the fogging there.
This image shows fogging that was on the cassette before the image was taken.