Answer to Question #10461 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Equipment

The following question was answered by an expert in the appropriate field:

Q
Can you suggest a method for testing 0.5-mm lead aprons using a radiation detection kit (I have the Swedish-made Unfors Xi Prestige Platinum kit) which also includes a survey detector? I have read the mentioned techniques of testing using the methods of fluoroscopy and CT but would like to know about any technique from your end using specifically the radiography mode and the measuring kit. If it can be done, then please mention the procedure and also the method of doing the calculations. Please note that no phantom is available. Can we simulate the function of the phantom using any other readily available alternate item?
A
Unfortunately, while the Unfors Xi Prestige Platinum is a fine piece of equipment for the measurement of a wide variety of parameters on diagnostic x-ray equipment, it's just not the right device for testing lead aprons. Fortunately, you probably have the right equipment "on hand" already.

The best way to routinely check lead aprons is to first lay them out on a flat surface and visually check all of the seams and the outer and inner covers for any visible damage. Also, check the belts and fastening devices to confirm they are in good shape. Then using your hands, feel the surface of the apron for any lumps, cracks, or evidence of separation from the seams or sagging. If it passes this initial test, it is probably in pretty good shape and can be returned to service.

If the apron is found to be suspect, only then should it be inspected radiographically. If a fluoroscope is available, you should set it to manual mode with the highest kVp available and a low mA setting to protect the x-ray tube and lower the dose rate to the person performing the inspection. Also, you should only fluoroscopically inspect the suspect area(s). The typical maximum fluoroscopic field size is 40 cm in diameter. The typical wraparound, full-length, lead apron is about 100–150 cm wide and about 100–120 cm long. Consequently, to cover the entire apron, the inspector has to move the apron around a lot to inspect it completely and receives unnecessary radiation exposure. Most fluoroscopes are normally used in automatic brightness mode which will ramp up both kVp and mA to high levels in an attempt to penetrate the lead apron. Consequently, untrained operators risk burning out the fluoroscopic x-ray tube (which typically would cost >$10,000 plus labor) and exposing themselves to much higher than normal x-ray exposures.

If you only have radiographic equipment, then place the image receptor under the suspect area and use a high kVp technique to get a radiographic image of the suspect area. For criteria on accepting or rejecting aprons with defects, I would recommend a paper by Kent Lambert and Tara McKeon published in Operational Radiation Safety.

If the underlying lead shielding is in good shape but the outer covering or belts and fasteners are worn out, you can typically send them to be refurbished for about half the price of a new apron. If you decide to retire the apron, make sure it gets disposed of properly. Lead aprons should not be disposed as normal trash. Most manufacturers will recycle your old apron if you buy a new one from them. Also, to preserve the apron and prevent cracks in the first place, it is important that the aprons are properly stored on a heavy-duty hanger when not in use. Folding aprons or dropping them in a heap on the floor will reduce their working life span.

If you are just interested in checking that a new lead apron is 0.5-mm lead equivalent, then using your Unfors Xi you would make a radiographic exposure at a fixed distance in air and then place the apron between the x-ray source and the detector and repeat the exposure with the same technical factors. The percent of transmission will vary with the kVp setting of the x-ray source, but for a 0.5-mm lead apron it would be about 0.02 percent at 50kVp, 0.82 percent at 70 kVp, 3.9 percent at 100 kVp, 5.2 percent at 120 kVp and 6.6 percent at140 kVp (transmission data derived from CALKUX © 1990 Douglas J. Simpkin).  

Mike Bohan
Radiation Safety Officer
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