Written by Kat Evans, Applications Specialist
Why does field of view matter on CT?
When preparing for a CT scan, it is important to consider the size of your field of view (FOV). This can have a massive impact on image quality. The size of the chosen FOV will actually have a much greater impact on image quality than the resolution of your detector.
Historically CT and MRI scanners were configured to export images on a 512x512 matrix, (the number of pixels on the digital image) which was set to tie in to the resolution of the monitors used for viewing images, and the capabilities of the medical laser printers used when printing images. This matrix has since been adopted as an industry standard within human medical imaging; 1024x1024 is the only other option if a higher resolution image was required. Remember, though, that increasing your matrix from 512x512 up to 1024x1024 actually quadruples the size of the file, and when you may have 300 slices within a CT scan you can very quickly have a massive file to handle, which will quickly fill space on your image storage system.
You should always set the field of view to be as small as possible, whilst still covering all your required anatomy. When you later view your scan, the anatomy should fill the scan area- and thus the screen. If when you view an image you are always having to magnify it, you will have used valuable storage space to save images of ‘fresh air’, and will have reduced your image quality
If you look at these basic examples you can see this (I have used a very small matrix).
On the first image the scan is only covering 6x6 pixels, so when this is scaled up to 512x512 you will get a very ‘pixelated’ image.
On the second example the scan is covering 15x15 pixels, so when this is scaled up to 512x512 this will give you a much better image quality.
Keeping the field of view as small as possible will ensure you are getting the best image quality.
There is no change in radiation dose to the patient by having a larger or smaller field of view.
Very often you can do retrospective reconstructions to help improve your image; this can often rescue you if you have accidentally clipped anatomy. Retrospective recontruction can also enable you to indivually reconstruct structures that have been scanned at the same time- for example stifles.
But it shouldn’t be used to try to disguise that extra body parts have been scanned, for example if the dogs feet had been left near its head during a brain scan.
So remember, try to keep your field of view as small as possible , which will give you nice large images, and will help optimise your CT scan image quality.