The first step in this process is to establish what will be called our control film. To do this we will take a pan/ceph cassette load it with film and place it on the floor with the side that normally faces the pan or ceph head facing up in a room where we can aim an intra-oral head down at it. We will place our aluminum stepwedge on top of this cassette in one corner. Using something convenient like a yardstick, position the tube head 30 to 36 inches away from the cassette but directly over it facing down. The only reason for the distance is to make sure we have enough radiation spread to expose the whole cassette. Set the timer on the X-ray to a low setting somewhere around 8 pulses or .13 seconds @ 70kVp & 7mA. Make an exposure and process the film. When the film is out of the processor, there will be steps of increasing density on the processed film and one step will have a clear dot as a marker. What we want to see is that the bar with the dot on it is at about 1.0 Optical Density (O.D.) To determine the density we will use the medium of newspaper print. Taking the newspaper, lay it on top of a counter and place the processed film on top of the newspaper. One Optical Density is determined to be the density where you can just barely read the newspaper print through. If the bar with the marker is lighter than one O.D. then you will have to increase the X-ray timer and try this again. If the bar with the marker is darker than one O.D. then you will have to decrease the X-ray timer and try again. When you finally have a film that matches one O.D. at marked bar then you need to record all of the settings that it took to make that film; (Timer setting, which X-ray unit, which cassette, at what distance.) You will need to save this control film for daily comparison.
Now, daily you will take the same X-ray unit, the same cassette and make an exposure at the timer setting you used to get 1 O.D. at the bar with the marker on it. Process that film and lay it and the control side by side. They should be identical. If not, the test is telling you that you have a problem with your processing. It could be bad chemistry, over or under heated chemistry, bad process timing, light leakage, or replenishment problems. You should keep these films for at least three months. Now in addition to keeping the films you should set up a sheet for daily record keeping. I recommend a graph that has 1-31 on the X-axis representing the days of the month, and numbers from -5 lighter to 0 to +5 darker on the Y-axis. Each day when you take a stepwedge test film and compare it to the control. If they are equal you record a zero. If the test film is one step darker you would record a +1 and so on. Your safety zone is about -1 to +1. If you show further deviation than that you had better check your processor to ensure that it is working properly.
An Aluminum Stepwedge (Part# 608111) is available direct from Cone Instruments @ 800-321-6964 and costs under $60. (They will request a credit card payment.)
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