This is really a common complaint and a huge misconception in the field. Collimators are NOT the primary source of scatter radiation. Collimators made in the US are designed under Federal BRH guidelines, and are generally “certified” unless designed specifically for veterinary use. As such they contain lead and steel to guide the x-ray beam and keep it from “leaking” out of the collimator except at the exit port. While there is an apparent component of scatter called “off focus radiation” and another component called “leakage radiation” they are really not scatter radiation but are a component of “non image forming radiation” of which scatter may be included. Leakage radiation is generally “soft” and is primarily controlled through design of the tube housing and proper collimator filtering with a filter inserted between the tube and collimator.
The (typically) aluminum filter inserted between the collimator and the x-ray tube port blocks the harmful “soft” rays so that they are no longer part of the useful x-ray beam. Scatter radiation is produced by x-rays being deflected by patient anatomy; particularly bone. The amount and proportion of “non image forming” radiation (bad) versus “image forming” radiation (good) is a function primarily of the volume of anatomy being radiographed. Thus any time you are radiographing any patient you can expect scatter radiation to occur, and for such procedures as chest films you can expect as much scatter radiation as primary radiation. The upshot of this effect is a lowering of subject contrast. That is why a grid (10:1 preferred; 8:1 acceptable) is mandatory to “clean up” scatter radiation. Since such areas as extremities do not scatter much radiation, a grid does not do much good and a table top technique is advised.
Total radiation can be much less for a table top technique as the extra radiation required to get through the top of the table and the grid is usually about double the MAS or you must add 10 to 15 KVP over and above the table top technique. Total scatter is a function of anatomical volume, KVP and MAS and it is always present. It does not matter whose name is on the equipment as it is a physics phenomenon and not due to the manufacturer. THEREFORE….particularly with veterinarians and vet techs….WEAR PROTECTIVE APPAREL AT ALL TIMES WHEN WORKING AT THE X-RAY TABLE, INCLUDING GLOVES, APRONS, GLASSES AND THYROID COLLARS. Be sure to collimate your beam to the size of anatomical interest (patient parts not x-rayed cannot contribute to scatter) and if you are dividing a film use lead rubber dividers, not just the collimator.
Remember, the scatter is there whether you believe it or not and you cannot see it. Wear your dosimetry badge and do not store them next to your x-ray machine. The inverse square law applies to scatter radiation so the more distance between you and the patient, the better. The dosage drops with the square of the distance. At 4′ distance you are receiving 1/4 of the scatter radiation as at 2′. When you see the effects of scatter on your film it serves as a reminder to protect yourself and staff. Some screen film combinations are more susceptible to showing scatter (they are more sensitive to radiation) than others but the scatter is present (whether you can see it or not on your film).
Anyone who truly believes they have a dangerous situation due to leakage or equipment design should contact their respective State Radiation authorities.
The following comments are from Mr. Richard Warner, RT & Health Physicist for the State of Washington. These comments were generated in a conversation with Roy Duncan regarding scatter radiation and are particularly interesting with respect to image field size.
“Dear Roy; Thanks so much for the good info on scatter and off-focus radiation. I especially was impressed on your compiled optical densities in and outside the collimated field. Yes, KVP and the opacity of the subject (and their bulk size), all have an influence to the degree of secondary generated, but the area of the collimated beam has the most dramatic.
I like to set up a situation to simulate a lateral-stress view of an adult knee being done in a sports medicine clinic. I set the FFD @ 40″ use 60KVP, and place the meter at arms length from a phantom, and I will measure around 2.0 millroentgens per hour through the standard lead apron. I will expect to measure around 200 Mr/hr above the apron, to let the staff appreciate the 100:1 ratio of protective attenuation. Then without changing any other parameters, I then open the collimation from a 8 x 10 field to a 10 x 12. Now logic dictates that a beam field size going from eighty square inches to one hundred and twenty, would result in a fifty percent increase level of secondary exposure. No so! I find that the rate of secondary will dramatically jump from two hundred to more than four hundred or five hundred millroentgens per hour!!! I love to show this to veterinarians when I preach good practices according to ALARA. All to often the vet will remark that they don’t really care how much the animal gets. My reply is, “I don’t either, but look what is coming back in your face!”/ So to me, the shutters in the collimator housing are just as important as the lead in their aprons and gloves.”
Comments from Summit Industries, the maker of the Innovet series of radiographic equipment. In reviewing my comments with Don Matson and Greg Krause (both engineers for Summit with extensive experience in the radiographic field) the following information was provided and is paraphrased and reproduced with their permission.
Why is there an image on this film outside of the collimated field? Assuming you have read the above comments we will only add that a grid will cleanup most of this undesirable “non image forming” radiation and that the most effective way to reduce it is to use the grid cabinet and cassette tray. This will dramatically reduce the film density outside of the collimated area. The sources of scatter and off focus radiation cannot be eliminated. One can only take action to reduce their effect on the film.
How much off focus radiation is typical? Because off focus radiation comes from the anode, it is stronger in the “cross” direction than in the “long” direction, aligning itself with the direction of the face of the anode. In samples taken, the off focus radiation is about 10% of the radiation within the collimated field. However some articles state that it can be as much as 25% or higher.
So why does the film darken outside of the collimated area? There are many factors involved. One is the amount of the off focus radiation; another is the HD curve of the film (which is the amount of film darkening that occurs for a given amount of radiation), and the two areas of non linear response to radiation on the HD curve. If the collimated field has an optical density of less than 2.0, the small area of off focus radiation will be virtually invisible. If the collimated field has a density of more than 2.5, the off focus radiation will be sufficient to darken the film. As the density within the collimated field approaches saturation, the film responds less if at all to the collimated field due to the non linear response of the film to the abnormally high dosage, and only the off focus radiation area will darken. (This would only occur with a high overdose of radiation). Reducing the MAS by 25 to 50% will often almost eliminate the off focus and scatter viewed outside the collimated area while improving image quality.
A client recently sent me a 14 x 17 film with a cross table collimation (not longitudinal) and was very concerned about the expected non image forming radiation out side the collimated area. After reading the above , they were still skeptical, but less so, until I assured them that they could go to any x-ray machine and duplicate their concern. Put a 14 x 17 in the cassette tray, collimate so you have some unexposed borders, dial up 75KV and 15 or 20 MAS and make an exposure. You will see some darkening outside the collimated image. Reduce the MAS to around 1.5 or 2 and you will not see much of the non image forming radiation. Increase it to 60 and you will see a lot!