When looking to buy a grid there are many different options out there and I want to try to help you understand them better, as there is a wide range of prices and types out there!
The reason for using a grid is to reduce internal scatter by preventing it from hitting your receptor, whether this is film or a CR plate, and you should use a grid for all radiographs where the patient is over 10cm thick, especially if they are very obese. This should include cats, as many of these are now over 10cm thick, especially Maine Coons and similar.
The two main types of grid are Focused and Parallel.
Parallel grids are composed of parallel lines of lead interspaced with aluminium, and when radiation that is not heading straight towards the cassette hits the grid it is absorbed, and prevented from reaching the plate.
Parallel grids can be used at any distance, and can normally be used either way up.
Focused grids are made with the grid lines focusing towards a point, thus they actually allow more ‘good radiation through’ and are less likely to appear as grid lines on an image, but they must be used at the correct distance, and you must be correctly centred over the grid when taking the image, as well as having the grid the right way up. They are still lines of lead interspaced with aluminium.
This diagram shows you how the different types look inside – obviously the diagram is simplified!
When using a grid it is vital that the primary beam is perpendicular to the cassette and grid – so at right angles; if it is not you will get really bad cut off (i.e. parts of your image will be missing).
How many lines?
When choosing a grid the more lines per cm (or inch) the better the image quality; historically grids have tended to be around 40 lines per cm, but now they are often 70 line or higher. The great advantage to this is that you will see less of the grid lines on the image – resulting in better image quality, but the higher number of lines does normally require a greater increase in exposure settings.
How much to increase exposure
Normally if you either increase your kV by 10 or double the mAs from an ungridded exposure that will work, but you may have to increase more for some grids.
Many grids come with a clip-on cover to make it easy to attach them straight to the cassette and this can be very useful, but you can also buy grids separately, and these work very well if you are using a Bucky tray under a table, as often the grid lids can be too chunky to fit into the Bucky tray. One real benefit of a grid lid is if the grid gets dropped you can damage them, and the lid will help to protect it.
Film Grids v CR/DR grids
You will see grids being described as for film or CR/DR; the exact differences can vary from manufactures, but normally a film grid is a bit less refined, with an aluminium interspacing, and cover.
Whereas the CR/DR grids have finer lines, which tend to be very precise, and often coated in carbon fibre – but this does mean they can cost up to 5 times what a film grid does!
CR/DR system will often show the grid lines more than you are used to noticing them on a film image, so the software should include grid suppression software to help with this.
In the past some people have said if you have a CR system you don’t need a grid, but this is not true.
DR systems do cope better without grids, but you should still ideally consider a grid, if the DR software has grid options. For a DR system you really do need the highest number of lines per cm- ideally 70 or more
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