Film Processors

The days of fumbling around in the dark looking for a handle on an X-Ray film storage cabinet is becoming a thing of the past. Film processors are moving more towards digital processing every day. There have been remarkable strides in the development of image quality in machines like CR’s and DR’s. There are still a few smelly wet processors around but not many and they are usually used for special applications. First we need a little lesson in radiography in order to understand what happens in the processor.

Radiographic images are produced by exposing a light sensitive piece of film or image screen to an X-Ray beam. Different parts of the body have different densities. Bone is denser than surrounding muscle and tissue so X-Ray’s do not penetrate the bone as well. X-Rays are normally a reverse or negative image of the body part in question. Because bone is denser fewer X-Rays can get through to the film and causes it to appear white or unexposed. Muscle tissue varies in density and gray scale images of it can be seen around the bone because it absorbs some of the X-Rays but passes the rest. To learn more about X-Ray technology click here to read my other blog about general X-Ray equipment.

Film that is used in wet processors has a coating of a light sensitive emulsion affixed to one or sometimes both sides of the film. When the exposed film is run through the wet processor it first enters a tank full of developer chemicals. This causes the emulsion on the section of film that was exposed to radiation to adhere more to the film and other sections that got less radiation lets loose and get recycled in the silver mercury recovery unit underneath the processor. Next the film travels through a series of rollers and gets dumped into a fixer chemical tank. This tank of chemicals fixes the image permanently on the film. Next the film is rolled through to a water bath that rinses all chemicals and residual emulsion away. Next the film is run through a dryer that removes all the water from the film and finally it falls into a catch tray at the opposite end of the processor.

The most important thing to remember about processors is that you must NEVER allow any fixer not even a single drop to get into the developer tank. Extreme caution must be used when removing the racks that the rollers are attached to. The reason is that the processor gets a Quality Control film run on it daily for mammography and weekly for general X-Ray. The density of the QC film is then read with a machine called a sensitometer. The numbers generated by the sensitometer are the readings for varying densities placed on the film in the dark room by a densitometer.

If the numbers are out of range the films run after that will be of questionable diagnostic value. Areas of the film may not be readable enough to spot things like cancers or malignant areas of tissue. Mixing the chemicals even minutely like one drop can throw the numbers way out of proportion. Getting it back into range requires dumping the chemical in the infected tank and refilling it. Starter solution is normally added and this is where it gets hairy. The starter solution must be added proportionally to the degree that the numbers were out of range. In the few times I have seen this chemical contamination happen it required a phone call to the third party vendor we use for service on the machine to come in and get it back in range.

Temperature also plays a major factor in film quality of wet processors. The temperature must be kept constant in the developer tank with a variance of usually less than one degree. The temperature in the fixer and water tank is unimportant. The processor must constantly dump a small amount of chemicals while it is turned on and fresh chemicals are running through it to replenish the supply at a replenish rate set in the processor. If a large number of films are being run through and the replenish rate is too low, after several films are run it can be noticed that they are getting lighter, which means the quality is getting worse. A new QC film will usually verify that the numbers have drifted out of acceptable range and the chemicals must be replenished faster. Fortunately there is a button on most processors that you hold down for a minute or 2 and the replenishing motor will run constantly pumping in a few ounces of fresh chemicals. This normally fixes the problem.

CR’s are a digital form of processor that uses an intensifying screen inside a film cassette that the exposure is made on. The term CR means Computed Radiography and is just that. A cassette is tagged at the work station with the patient demographic data then inserted into the X-Ray machine. The X-Ray is taken and the cassette is inserted into the CR processor. The processor reads the tag and knows which X-Ray machine was used and attaches the patient information to the digital image it will produce. The cassette is opened up inside the machine and the intensifying screen is removed. The image is scanned with a laser and the digital image is produced as a jpeg image on the CR monitor for the X-Ray technicians and doctors to read.

After the image is uploaded to the monitoring computer the intensifying screen is exposed to light erasing the current image. The screen is then reinserted into the cassette and the cassette is ejected back to the technicians to use again. CR cassettes are expensive but will pay for themselves very quickly because they are reusable. Each cassette is capable of several thousand exposures before becoming useless. Each CR manufacturer uses basically the same technology to read and erase the intensifying screens. The major difference is how the cassette is physically handled by the machine. AGFA machines open the cassette, develop the image and then spin a large assembly inside that holds the cassette. The screen is then reinserted and ejected out to the user. Kodak and Fuji CR’s open the cassette, read It, and reinsert it without moving the cassette around. Each have their pros and cons.

DR’s or Digital Radiography units use the same image production process but with one thing removed. The cassette is taken out of the equation. The image is transferred digitally from the X-Ray machine right to the image processing software of the DR. The advantage is no moving parts, no machinery to take up space like the CR, and images can be seen almost instantly. The disadvantage is that only newer X-Ray machines may be capable of making full use of the technology. Older X-Ray machines may not be upgradeable to digital imaging. Some X-Ray machines also have software compatibility issues with the newer digital software. As an example most X-Ray machines use MS DOS as the primary operating system for communication. I’m talking about pre-Windows 95 operating systems.

The advantage of using a CR or DR is in image quality control. In wet processors the chemicals control the quality of the image. With CR’s and DR’s the doctor controls the image. He can use the software to increase density, contrast, sharpness, or several other parameters to get the best quality image he can. CR’s and DR’s are much better for radiographic diagnostics because of that feature. The Doctor can enhance any area of the image to reveal things that would otherwise have been lost on wet processed film.