Usual Sources of Minus Density Artifacts
This document discusses the usual sources of minus density artifacts. Minus density artifacts cover a broad range of "defects" seen on films; the common denominator of these types of artifacts is that they are lighter in optical density than the area surrounding them.
Although not limited to appearing on single-emulsion films, minus density film artifacts are often detected more frequently on such films. Single-emulsion films include those for mammography, video recording, laser printers, etc. Because there is not a second emulsion on the opposite side of the film support or base to "hide" shadow images, processor scratches, etc., the visibility of minus density artifacts on single-emulsion films is inherently greater than with double-emulsion films. For best results, it is important to remember that many different variables must be controlled: darkroom construction materials, processor maintenance, air ventilation systems, housekeeping/cleaning procedures and frequency, film box storage, intensifying screen cleanliness, and more. This document will briefly discuss the above as they relate to minus density artifacts on films.
Discussion in this document regarding the use of Screen Cleaner and Antistatic Solution pertains only to its use with MIN-R Cassettes and screens, etc.
Mishandling of most types of films (with some exceptions, such as T-Mat Films for medical radiography) prior to exposure can result in unwanted minus density artifacts. The fix for such artifacts is usually straightforward and centers upon the film handler(s)--radiologic technologist, darkroom personnel, stockroom personnel, loading dock and delivery personnel. In some cases film-handling equipment (i.e., chest changers, angiographic rapid serial film changers, etc.) can cause minus density artifacts if they are not properly adjusted. Obviously, processor rollers and other film processor components can cause emulsion scratches and other emulsion "digs" and these components need to be routinely cleaned and examined in order to eliminate these sources of minus density artifacts. It is also possible for minus density artifacts to be traced to certain film manufacturing and/or packaging stages.
In the majority of cases the single biggest contributor to minus density artifacts is airborne dust contaminants, lint, and other debris which find their way into cassettes and onto screens and film. In some cases, debris which has adhered to an intensifying screen will produce a minus density artifact with the same shape, size, and location on the film. If cassettes and screens are individually numbered for Quality Control test purposes, isolation of the affected cassettes should not take long.
Proper cleaning of the cassette(s) and screen(s) usually brings about a cure. However, if the air filtration is not adequate, or if darkroom "housekeeping" is not stringent, simple cassette and screen cleaning will offer only a very temporary respite.
Often overlooked is the air filtration/circulation system in the darkroom. Positive air pressure in the darkroom can help restrict the influx of airborne particles as people enter and exit this room. The commonly used "bag filters" at the heating/AC system level may not provide fine enough filtration for darkrooms where single emulsion films (e.g., mammography films) and cassettes are handled. Surgical suites and critical care areas within a hospital probably have higher air filtration levels, but a particular film darkroom may not. The efficiency of bag filters can be greatly reduced or even negated by dirty post-filtration ductwork, poor or deteriorated gasketing which allows particle "bypass," and other variables.
The use of pre-filters or "pleated" filters ahead of bag filters is sometimes seen as a measure to prolong bag filter life. Like bag filters, pre-filters can be had with different filtration efficiency percentage numbers and varying micron size filtration specifications. Such filters, depending upon their specifications, condition, and the condition of ductwork and gaskets may not provide the level of particle filtration deemed adequate by the film interpreter.
If even "cleaner" air is desired, other types of air filtration methods can be employed. Popular approaches include electrostatic air cleaners and high efficiency particulate air ("HEPA") filters. Electrostatic filters are good at capturing or "scrubbing" the comparatively larger "smoke size" particles, but this may not be sufficient for some medical films and exams. Regular cleaning of such devices is recommended and this sometimes is neglected, causing the collection plates to become loaded. In such circumstances, the filtration efficiency may suffer.
Very high levels of air filtration can be achieved through the use of a terminal-ducted, HEPA filter. These filters usually post specifications of 99.9% efficiency in removing particles down to around 0.3 microns (the diameter of a single human hair is about 75-100 microns). A step below this level in both efficiency and particulate size filtration would be a minimum rated HEPA filter without a fan. Filtration efficiency is often quoted in the mid-90% range and these filters are advertised to be near 100% effective in removing particles above one micron in size. This may be an effective measure to employ, as it offers both lower cost and less pressure drop vs. a HEPA filter + fan, so that other rooms, which may feed off the same ductwork, are not "starved" for ventilation.
A professional, not-for-profit organization, The Institute of Environmental Sciences (Illinois; 847-255-1561; FAX # 847-255-1699), is a good source for "recommended practices and standards" regarding air filtration methods and testing. This organization also offers various reports and videotapes on a variety of subjects for a fee. Subjects include cleanroom design and housekeeping, clothing recommendations, movement and behavior recommendations in cleanroom environments, laminar flow clean air devices, microorganism control, and related issues. Although a number of the above documents involve "cleanroom" environments which exceed normal photographic and/or radiographic darkroom environments, there is good information to be found for air filtration information.
As for darkroom construction, it is recommended that the darkroom walls and ceiling have solid, smooth surfaces if possible. If painted, a semi-gloss finish is preferred over a flat finish. This allows for easier cleaning via wiping and/or vacuuming. If a suspended ceiling is in place, there are a number of drop-in panels available which do not "shed" fibers or drop debris. Improper selection of ceiling tile material and/or improper installation (for darkroom conditions) can be a significant source of airborne debris continually introduced into the darkroom, which can reduce the effectiveness or even defeat other housekeeping efforts such as air filtration, screen cleaning, etc.
If a conventional door is fitted to the darkroom, the vibrations and possibly air pressure changes produced by opening and closing the door many times a day can result in unnoticed movement of the ceiling tiles. Such movement can cause ceiling tile particles to dislodge and fall. To help relieve changes in air pressure caused by opening and closing the door, installation of a lightproof vent in the door or in a wall of the light lock may be considered. Revolving darkroom doors or "barrel doors" alleviate the air pressure transfer somewhat, but these can transmit vibrations into the walls and ceiling structure too. If special ceiling tiles cannot be used, some effort to seal the tile edges and surfaces (via painting with a semi-gloss paint, polyurethane, or other means) should be considered. The tiles should also be checked for movement within the suspended grid and this movement should be minimized as much as possible, taking into consideration ceiling access needs and especially local building and fire codes, any of which may prohibit certain measures to "lock" or otherwise adhere ceiling panels into place.
Continuing with darkroom design considerations, the floor beneath benches, sinks, and equipment should be accessible for cleaning as much as possible. Storage facilities below the countertops should be closed cabinets. Shelving should not be open. Dust-holding ledges and overhead surfaces that are too high for easy cleaning should be minimized. Any storage above the countertops should be closed cabinets that preferably reach all the way to the ceiling. Non-essential items such as newspapers, magazines, facial tissues, notebooks, writing pads, etc., should not be kept in the darkroom loading area where they could attract or create dust and lint.
Darkroom countertops and the processor feed tray should be wiped with a damp, lint-free damp sponge or cloth a number of times daily to remove settled dust and lint. A chamois cloth, available at most auto parts stores and discount stores, often works well.
Be sure to use an appropriate solution that will not leave a flaky residue after drying to moisten cleaning sponges, mops, or cloths. Obviously, such solutions should not cause any adverse effect to films, cassettes, countertops, feed tray, walls, etc. Floors and open shelves should be damp wiped or mopped at least daily. Often it is advisable to damp mop at the end of the day so that any dust settles overnight, before cassettes are reloaded the next morning. This timing can sometimes have a positive effect on the incidence of dust/lint artifacts.
The corrugated shipping boxes with boxes of film inside should not be opened in the darkroom--this can introduce cardboard fibers into the darkroom. The stiffener boards commonly found in film boxes should be removed from the boxes and from the darkroom. The same should be done with the tops of the film boxes. Do not remove the film from the envelope or store the film loose in the film bin. Film(s) kept in a film bin should be sufficiently immobilized to prevent shifting back and forth as the bin drawer is opened and closed. This can help minimize or eliminate "chucking" abrasions on films, which are multidirectional scratches, usually grouped together in approximately 1-2 mm diameter clusters. Cases and boxes of film should be stored vertically to help prevent (with most films) minus density marks due to pressure.
The use of a darkroom patient I.D. stamper or printer requires the handling of many paper I.D. cards in the darkroom, which can create another source of fibers and debris in very close proximity usually to films and open cassettes. A daylight-type (i.e., non-darkroom) I.D. camera is preferred for not only this reason, but such a device can reduce the possibility of mis-labeled films, be faster in operation, and produce a more clear I.D. imprint in most cases.
Selection of garments worn by those working in the darkroom is important too. To the best extent possible, lint-free clothing is desirable. If such clothing is not provided or worn, uniform smocks and other lint-free gear worn over clothing are usually very helpful. Wool clothing should be avoided, and articles of clothing should not be hung in the darkroom.
An ultraviolet radiation or "light" device (commonly sold to check intensifying screens), used with the proper precautions, can be an excellent aid to help illuminate dust and lint on counter surfaces, feed trays, walls, etc. Not all dust particles fluoresce, however. If an ultraviolet light is utilized, it is advisable to check all the areas of the darkroom before cleaning and again after cleaning to see the effect of the cleaning efforts. ALWAYS follow the recommendations of the manufacturer for eye protection and handling when using such UV devices.
Relative humidity and temperature play a part in the generation of film artifacts too. A range of 30-50% relative humidity is prescribed for opened boxes of most types of Carstream medical x-ray films. Increased humidity reduces the potential for static discharge and electrostatic attraction of dust and lint to film and screens. However at higher humidity drying becomes less effective and films may be tacky on exit from the processor or exhibit more pronounced dryer pattern.
Very low humidity can cause respiratory discomfort in people, static buildup on film and equipment, and curl and brittleness in films. Static charges on film and cassettes attract dust; they can also cause static discharge marks (artifacts) on processed images. Longer-term storage in very low humidity conditions can cause another type of unwanted film artifact: emulsion cracking. Temperature recommendations for opened packages of film are 50-75 degrees F.
Cleaning of cassettes and screens can be accomplished using both wet and dry means.
The use of intensifying screen cleaner and antistatic solution as recommended by the manufacturer of the screens is highly recommended. Depending upon individual circumstances, the frequency of cleaning can vary widely--from weekly to daily to more than once each day, as necessary. Excessive rubbing of plastic or polycarbonate cassettes (seen commonly in mammography cassettes) when cleaning should be avoided as this can possibly increase the static attraction of dirt, dust, lint, etc.
In addition to the use of screen cleaner, a solution of 70% isopropyl may be used occasionally when thorough cleaning of MIN-R Intensifying Screens is necessary. If this practice is employed, it is highly recommended that the user follow the alcohol cleaning with an application of Screen Cleaner.
Such antistatic brushes can be used to clean the internal surfaces and channels of the cassette, and it may be additionally effective (or recommended by the manufacturer) to hold the opened cassette either vertically or upside down while using the antistatic brush to allow gravity to assist in debris removal. Be sure to follow the brush manufacturer.s directions for handling, use, and replacement (if applicable) of brush components. Other products such as canned air and a rubberized roller device with a special adhesive, which have been used in the graphic industry and in the photographic trade, have been marketed for screen cleaning too.