Chapter 12

Malfunctions and Errors

 OBJECTIVES:

    * Radiographic equipment problems and trouble shooting.

    * Improper technique selection.

    * Common reasons why films have to be repeated.

 

By the end of this section, the student should be able to:

1. Determine the most likely causes of unacceptable radiographic images and how to correct each.

2. Explain how to perform a film/screen contact test and why it is important.

 

One of the most important skills learned by a radiologic technologist is the ability to recognize and correct errors that have made an image yield less than optimal diagnostic information.  Each film should be carefully evaluated. If there are problems with the image, the cause should be determined and appropriate action taken to remedy that problem. Some problems are quite easy to identify, while others are more difficult and may require a thorough investigation of all factors involved when the exposure was made.

 

EQUIPMENT PROBLEMS 157

No Exposure

Check the mAs and kVp settings to ensure that they are not set too high. Some exposures place an unacceptable heat load on the x-ray tube. Most machines have been programmed to recognize these extreme settings and shut the exposure off before the machine is damaged.  Usually a red light will appear on the console, indicating that there is a problem with the technique selected.

 

Modern x-ray rooms are equipped with an interlock switch attached to any doors leading out of the exam room. For safety, the switch is activated if the door is open when an exposure is being attempted. The door must be closed while any exposure is being made.

Check the settings on the console to ensure that the correct piece of x-ray equipment has been selected. If you are taking a chest x-ray on an upright chest bucky, but have selected the table bucky, the machine may recognize that there is no film in the bucky tray that you have selected. No exposure can be made until a film is placed into the tray, or the correct tray is selected.

 

Film, Bucky, and/or X-ray Tube Malalignment

It is critical that the x-ray central ray and center of the film are aligned. If they are not, part of the area of interest will be cut off from the image. This is one of the most common errors made by technologists. Often, the cause is because magnetic locks on the x-ray tube are not properly engaged. If the locks are not engaged, even though you have properly positioned the tube, film and patient, the tube slowly drifts away. Always re-check to verify that the x-ray tube is lined up with the bucky tray before making an exposure. When using a grid, make sure that it is perpendicular to the central ray.  It can not be tipped in any direction or grid cut-off will result.

 

Collimator Light Field Out of Adjustment

It is frustrating to position the central ray and film properly, but then end up with a film where part of the anatomy of interest has been cut off. The light field projected onto the film from the collimator must accurately represent where the central ray will actually be projected. Routine maintenance is performed to ensure that radiation is being delivered exactly where the collimator light field shows. If you think that the collimator light is not accurately positioned, report it immediately so that service can be performed.

 

Timer

The timer determines how long the exposure lasts, which controls mA. If the timer malfunctions, incorrect exposure results. Your films will come out too light or too dark, even though the technical factors you selected were correct.

 

mA Meter

This built-in meter gives a readout of the mA output produced during exposures. If one setting is selected, but the tube is not producing that amount of mA, the actual output may be more or less than what you expected. The film will be improperly exposed, resulting in repeat films and unnecessary patient dose.

 

Exposure Switch

This is the actual exposure switch built into the console. It starts and terminates the exposure.  If faulty, resultant exposures will be inaccurate. Films will be incorrectly exposed.

 

Automatic Exposure Control (AEC) adjusts the lead collimators to the size of film being used.  If it is out of adjustment, the area illuminated by the light field may not represent where radiation exposure will actually occur.

 

Collimator Light Timer

The collimator has a timer so that it shuts off when not in use, allowing it to cool.  If the collimator light stays on continuously, it will burn out prematurely.

 

SID Indicator

Correct subject to image (SID) distance from x-ray source (tube) to the film is critical in producing consistent films. Manufacturers of x-ray equipment install a tape measure in the collimator housing, easily accessible to the technologist. The tape is used to measure distance from tube to film.

 

Electrical Hazards

Within the x-ray tube housing there is an environment of very high voltage circuitry. High voltage cables leading into the tube are wrapped with a thick protective layer. Any crack or break in the covering should be reported. Any malfunction such as popping, a burning smell, sparking, or cables that are hot to the touch should be reported. The x-ray unit should be shut off if any safety hazard exists.

 

IMPROPER TECHNIQUE SELECTION

LOW DENSITY: When a film is far too light, it is likely that mAs was set too low. Although kVp contributes to density, mAs controls it. However, check the kVp setting to ensure that the kVp selected was appropriate for the thickness of the body part being examined. When mAs is adjusted, remember that it takes a 30% increase or decrease in mAs to make a noticeable difference in the image. When mAs is doubled, twice as many x-ray photons were produced, and the image will be twice as dark. Conversely, if the image is too dark, cutting the mAs in half will produce half the number of x-ray photons. The film will be half as dark.

If the image is properly exposed on part of the film, but too light on the rest of the image, it is possible that grid cut-off has occurred. Grid cut-off is usually asymmetric. If the central ray is not perpendicular to the film or if the grid is tipped, grid cut-off occurs. If a grid is used upside down, the center of the image will be exposed normally, but the edges all the way around will be too light.

 

EXCESSIVE DENSITY

When a film has excessive density (too dark), mAs must be reduced. Although kVp contributes to density, mAs is the controlling factor (unless kVp selected is unreasonably high).

 

LOW CONTRAST

kVp controls contrast.  When a film has a "flat" appearance with similar shades of gray but few blacks or whites, kVp was set too high. As kVp is reduced, image contrast will increase.  Such an image may also occur if the body part being examined was thick enough (11 cm. or more) to merit the use of a grid, but no grid was used. In such an image, an unacceptable amount of scatter was produced, degrading the image by destroying image contrast.

Base fog also decreases image contrast. Essentially, fog is exposure to the film from heat, excess moisture, light, radiation, storage, aging, or rough handling. A clean, unexposed sheet of film can be taken out of the film bin and sent through the processor. Most film inherently has a blue tinted base. It the film appears gray instead of clear or very light blue, it has been fogged. The amount of fog can be measured using a sensitometer.

 

HIGH CONTRAST

kVp controls contrast. As kVp goes up, the amount of contrast seen on the image goes down.  Film/screen combinations have some effect on contrast because certain films are manufactured to have high or low contrast latitude. If the film has blacks and whites, but few grays in between, kVp was likely set too low. There were not enough strong x-ray photons to penetrate through the body tissue being examined, resulting in a high amount of absorption. Higher absorption means that the patient received a higher radiation dose.

 

LACK OF DETAIL (FUZZINESS)

Motion is the single most common reason for images that lack fine detail, especially of edges and lines. Motion can result from the patient or the equipment. Using the fastest exposure time possible reduces motion significantly. Immobilization devices can be helpful in many circumstances. Patient comfort is important. If a patient is in pain, the likelihood of motion is increased.  Good communication also reduces motion because the patient understands what is expected and what they need to do to make the exam successful.

 

QUANTUM MOTTLE:

Quantum mottle is seen on some images where fast film/screen combinations are used.  Because the fast systems are so efficient at producing light that exposes the film, very little mAs is required. However, if so few photons are produced that numerous crystals of phosphor in the screen or silver crystals in the film undergo no interaction with an incoming x-ray photon, the image will appear grainy. There are two possible solutions to reducing mottle. First, a slower speed film/screen system can be used because slow speed systems do not produce as much mottle effect. Second, kVp can be decreased 15% and mAs can be doubled. This will maintain the same radiographic density, but will produce twice as many x-ray photons.

 

FOCAL SPOT SIZE:

Focal spot selection has a large bearing on radiographic detail. When a large focal spot is used, x-rays fan out in a wider pattern as they travel towards the patient, decreasing detail. The x-ray beam produced by small focal spots do not fan out as far, yielding higher detail. Large focal spots must be used when high techniques settings are used, such as hip, lumbar spine, abdomen and chest.

 

DARKROOM AND FILM PROBLEMS

FOG

Most x-ray film has a bluish tint to it if the film has not been exposed to light. Fog decreases image quality, radiographic detail, and contrast of an image. There are many factors that will cause a film to become partially exposed.

    * heat    * chemical fumes    * moisture    * improper handling    * improper storage- boxes of film should be kept upright and not tightly packed against one another.    * exposure to light   
* exposure to radiation    * defective safe (red) lights in the dark room    * prolonged exposure to safe lights (such as when the film bin is left open with the safe light shining down on the film).

CASSETTE IS DEFECTIVE OR WAS OPENED OUTSIDE OF THE DARKROOM

It is not uncommon for the latches of a film cassette to either not lock tightly or to accidentally be opened and exposed to light. When loading sheets of film into a cassette, make sure that the latches lock into place. When handling cassettes outside of the darkroom, be careful not to grab the latches or hook the latch on tabletops, processor, pass box, bucky tray or other objects that might force the latch open. When film is exposed in an open cassette, the film will be black on the side that was open. If the cassette only opened a very little distance, only the edge of the film will be blackened. Depending on how wide or how long the cassette opened will determine how much of the film inside is ruined.

 

SCREENS NEED CLEANING

Although screens are durable, they are fragile and must be handled with care. A fingernail scraping along the screen while you are trying to remove or replace a film can cut into the screen. Laying one cassette on top of another while the cassette is open may scratch or cut into the screen. Artifacts may accumulate on a screen, such as dust and other debris. These should be carefully removed using the appropriate screen cleaning solution provided by the vendor. Not only does screen cleaner clean the screen, but it contains an anti-static solution to help reduce the buildup of static electrical charges. The exterior of the cassette should also be routinely checked for cracks, warping, deteriorating foam sponges that form the cassette seal, or loose latches. The exterior should also be cleaned periodically. Any barium, contrast or other material on the outside of the cassette will interfere with incoming x-ray photons, leaving an undesirable artifact on the film.

 

FILM/SCREEN CONTACT

Sometimes air gets between the sheet of film and the screen if the cassette is slammed shut when loading a new sheet of film. When this occurs, an area of increased density will be present on the film because x-rays were able to penetrate through the air pocket easier than other areas. The area may also appear distorted, unsharp and lack detail because the film is lifted above the screen surface similar to a bubble.

A mesh-screen test can be performed using a special wire mesh that is placed over the screen after which an exposure is made using low kVp. If the screen is warped or defective so that a sheet of film can not rest firmly against it, the screen test will show an area of distortion and unsharpness as shown in the images at right. Damaged cassettes should be replaced.

   * Top: Dark area represents poor film/screen contact.

    * Bottom: Normal film/screen contact

 

FILM ARTIFACTS

STAINS ON THE FILM

Small rounded spots on the film are caused by water drops that were not removed before the film started drying, similar to a car not being dried after a wash. Yellow stains on the film indicates that the developer was oxidized, the wash cycle was inadequate, or the fixer chemicals are exhausted.

 

MARKS ON THE FILM

Crinkle marks are curved lines that are shaped similar to a crescent moon. These are due to bending of the film while it is being handled. White spots or irregular lines on a film are usually due to foreign bodies such as dust, hair or other debris collecting on the screens, cassette, or film. Crinkle marks are the result of bending a film, usually when grasping it out of the film bin, or when placing it into our removing it from a cassette. A black crescent usually results from bending the film after the film has been exposed. A white crescent means that the film was bent before it was exposed.

 

SMUDGES

Most smudges are caused by handling the film with lotion, perspiration or wet hands.  Sometimes an actual fingerprint will be visible on the film. Gripping the film too tightly may also cause smudges. Sliding the film against the cassette, counter top or film processor tray can also result in smudges.

 

SLOW DRYING

If the film processor has not been turned long enough for the heater to bring the developing solutions up to normal operating temperature, the films may not develop properly, including the important step that dries the film. Exhausted or contaminated fixer lacks the ability to harden film after development. A film passing through inadequate fixer will have a hard time drying.  Another possible cause of films that will not dry is the heater element within the film processor. The heat exchanger runs continually as long as the processor is turned on. Eventually it wears out and must be replaced.

 

BRITTLE FILMS

Improper fixer. Either the fixer chemicals were exhausted or contaminated, or the film was not processed for the correct amount of time. This was a common problem back in the days of hand processing before automatic machines were widely used. Another common reason that films are brittle is that too high of temperature was used when the film was being dried.

STREAKS ON FILMS

Streaks are generally caused by inadequate agitation of the film while it is in the developer.  It can also be caused by improper wash or uneven drying. Maintenance should be performed on the film processor if streaks are present on numerous films.

 

STATIC

Static can appear in several forms, ranging from black dots that are all in a line, up to a lightning bolt or tree-like artifact. Some static marks are very small, but others may significantly interfere with image visibility. Static occurs when humidity is too low in the room where film is stored. Low humidity in the darkroom is a common problem. Dry, cold weather may also lead to static build-up. Since film contains metallic silver, is can collect static electrical charges if it is handled roughly, if films are slid against one another, or the film is slid across the screen rapidly. Static is frequently seen when film is being taken out of the storage box and placed into the film bin. Static can also occur from rubbing sheets of film against one another or against counter tops or film processor trays.

SCRATCHES

Defects from rough handling, fingernails, crimping or folding the film may all cause scratches. If the scratches go all the way across the film, are evenly spaced, and parallel to one another, one of the processing racks is out of adjustment. The films are being scratched as the travel through the rollers and rub against an uneven surface of the maladjusted roller.

 

Key Points Remember from this Chapter

- Crinkle marks are curved lines that are shaped similar to a crescent moon. These are due to bending of the film while it is being handled. A black crescent usually results from bending the film after the film has been exposed. A white crescent means that the film has been bent before it was exposed.

- Most smudges are caused by handling the film with lotion, perspiration or wet hands.

- Static occurs when humidity is too low in the room where film is stored. Low humidity in the darkroom is a common problem. Dry, cold weather may also lead to static build-up.

 

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