Radiology Continuing Education Series
Course 4 of 6
Poor Quality Films-Causes and Corrections
Course 1 – Physics of Radiology
Course 2 – Choosing the Appropriate Exposure Factors
Course 3 – Recording the Image
Course 4 – Poor Quality Films-Causes and Corrections
Course 5 – Radiation Safety-Importance and Procedures
Course 6 – Pros and Cons of Digital Radiography-CR vs. DR
Poor Quality Films – Causes and Corrections:
Due to the multiple steps in the production of the x-ray image there are also multiple places for things to go wrong and cause films of poor quality. There are 3 Major Categories of poor image quality.
1. Inappropriate film blackness
2. Poor image detail
3. Poor image contrast
1. Inappropriate film blackness:
This can be expressed as a film that is overall too black. One that is overall not black enough (too white); one with focal or smaller regions of abnormal blackness; or one with focal or smaller regions of abnormal whiteness. In the films where an overall effect is noted check the film appearance in the area around the patient. An appropriately exposed and processed film should have this area be of blackness such as you cannot see you fingers between the film and the light from the view box.
Overall Too Black – Causes & Corrections:
Overexposure:
Selection of too high mAs such that too many x-rays reach the film.
Check the line on your technique chart to be sure that you read the correct mA an exposure times.
Check the control panel to be sure that you selected the correct mA and exposure times.
If this is a persistent problem and you are reading the chart correctly and setting the appropriate numbers on the control panel this may be a problem with the incoming electrical supply to the machine or a malfunction to the circuitry of the machine. These latter two require assessment by a x-ray machine service person.
Inadvertent exposure of the film to light prior to and during development. The film must be kept in the dark until the development stage is completed and the film has been in the fixer solution for at least 1 Minute. Failure to do so will result in unwanted blackening.
Check that stored film is not exposed to stray light.
Check darkroom safe lighting – cracks in the safelight filter, light bulb of way too high wattage, safelight too close to the working surface; light leaks under or around door frames. You can check the duration of safe time that you have to work with the film without it being adversely exposed. Begin in the totally darkened darkroom. Place a new film on your darkroom counter. Cover the film with a light proof cardboard or black plastic sheet with the exception of 1 inch. Turn on the safelight. After 1 minute uncover an additional inch of film leaving the 2 inches exposed for another minute. Continue this incremental exposure until all the film has been uncovered except for the final inch. Then turn off the lights and process the film in total darkness. Ideally the entire film will be clear- not exposed and thus not blackened in the developer solution. If however there is a light leak one or several segments of the test film will be black or a dark shade of gray. If a leak is discovered it is best to fix it. If that is not possible then you at least know how long you can work in the light-leaking area without adding substantial non-diagnostic blackness to the image.
Exposure to stray x-radiation. It is best not to store the unexposed film in the x-ray room. It is possible that the constant scatter radiation may expose the film.
Exposure to excess heat or humidity. Both are capable of stimulating the silver halide emulsion to the point of allowing development.
Over Development:
Remember that this is a chemical reaction governed by time, temperature, and chemical concentration.
Mix the developer chemical according to manufacturer’s direction. An abnormally high concentration results in excess blackening.
Carefully check the temperature of the developer. An abnormally high temperature results in increased blackening.
Abnormally long time in the developer. This will cause excessive blackening. If using automatic processing the roller transport may be too slow.
Focal areas of Increased Blackness – Causes & Corrections:
Light leakage reaching only a portion of the film.
Lid off the film box during storage. The light may reach only the top portion of the film creating a black band on the exposed films. Always be sure the lid is tightly on the box of the films. Lightproof storage bins are useful but care must also be taken that they are not inadvertently opened when the white light are turned on.
Light leakage into the corner of the cassette. This may appear in the form of a streak or a broader triangular shape. This happens most when the cassette backing is incompletely closed.
Static Electricity discharges. Two patterns are typically observed – tree branching type and smudged spots. This is most likely to occur when there is low humidity. The friction that causes it can be reduced to some degree if the film is not rapidly slid over the counter surface of pulled from the storage box. Cleaning the screens and the film tray with a commercial anti-static product may be useful.
Crescent shaped pressure marks. This is due to rough handling of the film. This will occur if the film is pinched between fingernails and when larger films bend when flipped.
Overall Too White – Causes & Corrections:
Underexposure:
Selection Of too low mAs such that too few x-rays reached the film.
Check the line on your technique chart to be sure that you read the correct mA an exposure times.
Check the control panel to be sure that you selected the correct mA and exposure times.
If this is a persistent problem and you are reading the chart correctly and setting the appropriate numbers on the control panel this may be a problem with the incoming electrical supply to the machine or a malfunction to the circuitry of the machine. These latter two require assessment by a x-ray machine service person.
Under Development:
Remember that this is a chemical reaction governed by time, temperature, and chemical concentration.
Mix the developer chemical according to manufacturer’s direction. An abnormally low concentration results in insufficient blackening.
Carefully check the temperature of the developer. An abnormally low temperature results in decreased blackening.
Abnormally short time in the developer. This will cause insufficient blackening. If using automatic processing the roller transport may be too fast.
Focal areas That are too White – Causes & Corrections:
Debris within the film cassette – any material that is in the cassette between the intensifying screen and the film will block the light exposure of the film. This includes hair, straw, stains on the screen surface, Etc. The shape and pattern of the material will show as a sharp white image on all studies made with that cassette. The screens should be routinely checked for debris and cleaned. If you have a number of cassettes it may be helpful to mark the screens in one corner with small press on letters. Place the same letters on the outside of the cassette. These letters show on the processed image and allow you to quickly find the dirty cassette creating the artifact.
Film contact during development-if films are in contact during development the chemical cannot get to those areas to effect development. When passed through the fixer the undeveloped region will be removed leaving a focal less blackened area. The size and shape of the area depends of the surface area of contact. Generally the edges are smooth but may be undulant. This type of artifact is most likely to occur with the manual processing ad is one of the main reasons that the time should be agitated periodically while in the developer. It also can occur in the automatic processor if a film is fed into the system before the previous film has sufficiently advanced.
Band of white at one end of the film:
Insufficient volume of developer to cover the film but appropriate depth of fixer to cover the film. The undeveloped emulsion is removed by the fixer.
Improper alignment of the x-ray tube to the film:
This causes an unexposed region to appear at one end or one side of the image.
2. Poor Image Detail:
This is seen as lack of crispness to the image shadows. Many components of the x-ray machine system, patient, and film recording system can cause poor image detail.
Focal spot size – this is the area on the target of the x-ray tube from which the x-rays are produced. The smaller this spot the smaller the “beam” of x-rays and the better the detail in the image. Its effect can be thought of the same as creating shadows on the wall with flashlights. The smaller the light beam, the crisper the edges of the shadow. There is a practical limit to the focal spot size that is pretty well correlated with the mA capabilities of the x-ray tube. The conversion of the electrons kinetic energy to x-rays is very inefficient. The majority of the conversion is to heat (99%), so the focal spot is also subjected to a lot of heat. The larger the focal spot, the more heat can be distributed and the longer the usefulness of the target. Those machines such as the small portable machines have stationary targets with relatively large focal spot size. The fixed x-ray machines usually have rotating targets that provide a greater surface area for heat dissipation and thus the focal spot size can be made smaller.
X-Ray tube to film distance- The farther the tube is from the patient the sharper the image edges. There is again a practical limit to this distance after which the exposure intensity drops off and higher Exposure Factors must be used. The typical distances used are 40 inches for small animal imaging and 27-36 inches for large animal extremity imaging.
Patient to film Distance – The greater this distance the blurrier the edges of the shadows will be. This is due to magnification. Any degree of magnification will blur the edges. To some degree this is tolerable and does not result in poor quality film. However the greater the magnification the poorer the detail.
Exposure Time – The longer the exposure time the greater the chance for blurring of the image. Usually the respiratory motion of the patient causes blurring. The effect is worse in the panting patient; however this can also be a problem when portable x-ray machines are hand held during the exposure. If the holder is shaking the motion artifact is due to the movement of the tube. Similarly if the stand or person holding the x-ray film cassette is shaking motion artifact will be created. Unless your patient is sedated or anesthetized, always try to use the shortest exposure time possible that will produce the needed degree Of Film blackening. This is where the x-ray machines with high mA capabilities are advantageous.
Film-Screen Speed – The faster the speed of these combinations the greater the loss of image detail. This is the offset that is accepted when the choice to use the increases speed is made. The loss of detail due to diffusion of light and to some degree fewer “beams” of light used to expose the film.
3. Poor Image Contrast:
Contrast refers to the visual difference between regions in the image. If there is no visual difference between areas there is no contrast. Poor image contrast may be due to pathological changes in the patient. It can also however be artificially created. Improper exposure setting, excessive scatter radiation reaching the film, fogged film, and poor processing can all result in poor contrast.
Exposure Setting – Both over and under exposure produce less than optimal image contrast.
Scatter Radiation
Production – Most scatter radiation is produced by the patient. The larger the surface area of the patient exposed to the x-ray beam, the larger the amount of scatter radiation produced. The best way to reduce the production of scatter radiation is to limit the surface area irradiated. Collimation of the beam to only the part of interest makes a significant improvement in the image contrast. Whole body images should be avoided
Scatter Removal – Some of the scatter produced by the patient can be removed from the x-ray beam before it reaches the film. The grid does this (refer back to the previous section on Grids).
Fogged Film – fogged film is film that has been exposed to undesired excess radiation or light, excesses of temperature, humidity, or pressure that results in an overall increase in the blackening of the film. Typically the film would have been exposed to this prior to the intended x-ray exposure. The effect on contrast is that the added blackness degrades the visual differences between regions on the film. To prevent film fogging proper storage and handling of the film in the darkroom is important.
Film Processing – Over or under development will not depict the true patient contrast.
A Quality Film is one with as true a depiction of the patients anatomy as possible. It is a balance of film blackness, Image Detail, And Image Contrast