Grid Cut-Off in the Digital World

Grid Cut-Off in the Digital World

After years of wondering about it, sometimes noticing it and finally researching it, I’ve come to the conclusion that grid cut-off isn’t mentioned anymore because no one even knows it’s happening.  With film it was totally obvious because grid lines covered 50-100% of the image.  That is not the case at all with digital images.  This is because the post processing has grid suppression software built in that does just that – it suppresses the grid lines so they don’t appear in the image.

This is a little good and a lot bad.  Good because the image doesn’t have annoying grid lines in it which could superimpose anatomy or pathology.  Bad because the radiographer does not know that there is grid cut-off occurring, with all the ill effects that grid cut-off creates.  These include:  decreased contrast, increased brightness, increased Exposure Index (EI) number, more technique (mAs) needed and possible decreased sharpness of detail.

Whenever you are using a grid and the image has any problem with contrast, brightness, detail or EI number, it is almost a given that you have grid cut-off.  It is happening all the time and almost no one knows it.  In fact, most of the grids that come with both CR and DR equipment are incorrect for use with chest x-rays.

There are many different styles of grids, but the 2 most common are with the grid lines running parallel with the floor (called a horizontal grid) and 90 degrees to the floor (called a “decubitus” or “short axis” grid).  When using the grid for any AP chest work where the central ray (CR) is being angled caudally into the patient and grid, the grid needs to be short axis so that the grid lines are still parallel with the CR.

I did some experiments recently with the grids that came with both our GE and Siemens DR equipment and both of them were horizontal grids.  This isn’t written anywhere on them, but I proved it by taking chest phantom shots with a 10 degree caudal tube angle and the grid both lengthwise and crosswise.  With the GE grid it needed to be put in behind the patient with the handle to the side which is very difficult and the Siemens grid needed to be put in portrait and not landscape.  With our CR grids we had to purchase separate grids that were short axis so that we could continue to put the grid behind the patient crosswise.

When you get grid cut-off, it means you have used more mAs than needed to get a proper EI number.  From my research I’ve concluded it’s around 30%.

I was thinking I could have you go to the Presentation Download on my home page and look at the slides that discuss grid cut-off, but I do not have time to cover it in my 2 hour lecture.  It is only in my 6 and 8 hour presentations.  Sorry about that.  Hopefully one day I will have it on my site in the Current Research section.

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