How to use a veterinary exposure chart

We have all used exposure charts in veterinary practice and they should be the simplest thing to use, but like many simple things in life it is easy to get things slightly wrong!

So here are a few tips to help make your exposure chart work for you.

1. Measure

Not just the thickness of the part of the animal you are X-raying, but also if you can change the X-ray tube to plate/cassette distance check it is what you think it is.

It might sound daft, but if your generator has been off for a service, or you have a loan one, your Focal to Film Distance (FFD) may have changed, and as we all remember from the joys of the Inverse Square Law, a change in distance has a major effect on exposure settings. Just going from 80cm to 100cm will require you to double your mAs, or increase your kV by 10.

This is a major factor when doing equine radiography – remember, it’s the distance to the X-ray plate/cassette, not to the horse.

We often get calls about poor image quality on equine systems, and a frequent cause of this is ‘Distance Drift’, where accidently the operator has allowed the FFD to vary. On occasion you may need to alter the FFD, but just remember to alter the exposure too!

2. Alter – or write all over – your chart!

We can provide ‘suggested exposure’ charts, but we also leave gaps for you to add your own exposures; this is because (sadly) there is no one ‘magical chart’ that works for all X-ray generators and processing systems (including film, CR and DR). So what works for one machine may not be right for another. And the processing system makes as much of a difference as the generator.

Or if you change your grid you may need to alter your chart.

Plus different people want X-rays to look different, so you may need to alter your charts to personal preference.

3. Think about the animal

If an animal is very fit and muscular this can have a big effect on exposures – put simply, it’s easier to X-ray through fat than muscle so for a muscular dog you may need to increase your kV. Also, some medical conditions will alter the exposures that are needed. An animal with a thorax full of fluid will require a higher exposure to penetrate the chest than a healthy animal; the same applies with free fluid in the abdomen.

4. Consider what you are hoping to see

The exposure setting for a thorax should be different than for an abdomen. The reason for this is that the thorax is an area of naturally high contrast and you want to reduce this a bit so that you can see both lung detail and also rib and heart detail. By using a higher kV and lower mAs you will be able to see both. Conversely in the abdomen you want to increase contrast as the difference in soft tissue density between the various organs is poor, so a lower kV and higher mAs technique will improve contrast.

In addition, when looking at orthopaedic images you will get much better radiographs with a lower kV and higher mAs; the trabecular pattern in the bone is much easier to see, and you can also see soft tissue changes better too. If you have a higher kV you may well ‘burn out’ small bone chips. 

5. Question sudden changes

If for the last couple of years an exposure setting has worked and then suddenly it doesn’t, think about why. This may be an indication that your generator or processor has an issue. In hot weather some older wet processors can overheat, and this can lead to your X-rays looking too dark. In cold weather the opposite may happen. Also, in hot weather you may notice fogging on film or CR plates – this can make your image look grey and flat.

6. Ask for help if you need it!

One of the great things we like to do is offer help when people need it, and if you have an image that’s just not right from a technical point of view please ask for help!

From a simple bit of advice right the way through to help adapting exposure charts we are able to help, and if required we can arrange onsite training.

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