Mammography - Why Do my Weekly Phantom Exposure Film Densities Differ with the Same Type of Film Over Time?"
Not only is it possible to see different measured densities on different emulsions of the same type of film from the same manufacturer; it's highly likely this will occur—for many reasons. The real question is HOW MUCH density variation in, for example, a Mid-Density step (i.e., "speed step") or ACR phantom density at a similar density level is considered reasonable or "within acceptable limits"?
A number of possible sources of measured density variability are discussed in this document. Reviewing this information will hopefully spare you possibly unnecessary or misdirected effort to address changes in film densities on ACR phantom images.
While a single factor can be the reason for density changes, there are often multiple variables involved, with each variable contributing some share of the total measured density difference. The following document offers the more frequently encountered system variables.
Some Possible Sources of Density variations on Processed FilmsPartial Checklist for x-ray exposures
- Different emulsion number or batch of film used/age
- Brand, type(s), mixing integrity, and age of chemicals
- "Seasoning" effects on chemicals from film processing
- Chemical replenishment rates
- Different x-ray equipment
- Calibration accuracy of x-ray exposure equipment/exposure repeatability over time
- X-ray exposure settings selected
- Selection of aec cell(s)/position of AEC cell(s)
- Positioning of phantom/body part over AEC cell
- Different collimation used
- Safelight integrity/condition
- Feeding mammography films into processor emulsion up & down rather than the same way consistently
- Feeding film into processor right side/left side/center rather than the same way consistently
- Different cassette used
- (Follows #14 above): different intensifying screen used
- Film not loaded/oriented correctly in cassette
- Different phantom used
- Film mix in non-dedicated processor(s)
- Variable time interval after exposure & before processing
- Film reciprocity characteristics
- DIFFERENT EMULSION NUMBER OR BATCH OF FILM USED/AGE
It is NOT atypical to have different emulsion batches of the same film from the same manufacturer show different sensitometric characteristics, such as average gradient (contrast), sensitivity (speed), gross fog, maximum density, etc. Manufacturing tolerances exist for all of these performance parameters, as a result of market-driven (or legislated) acceptance criteria and prudent manufacturing practices. Generally speaking, the higher the inherent contrast of the film (and all mammography films are "higher-contrast" films), the more pronounced can be the density changes as a result of small exposure variations, whether via a sensitometer or actual x-ray exposure. This is not new; it is how "high-contrast" films behave. Small exposure differences are "magnified" into larger density differences by a "high-contrast" film's steeper characteristic curve. The current trend toward even higher-contrast mammography images, whether arrived at through film design, increased developer immersion time, or "mammography" processing chemicals, has made this "density difference" issue occur with greater frequency.
In the 1995 ACR publication Recommended Specifications for New Mammography Equipment - Screen-Film Systems, Image Receptors, and Film Processors (order through the ACR at 1-800-227-7762) the authors suggest that up to a 0.30 optical density difference (at a density of about 1.25) between the same type and brand and approximate age films, when they are exposed and processed at the same time, reflects acceptable manufacturing variability. The suggested way for the user to adjust for this anticipated variability, according to this publication, is to change the AEC "density" control as necessary. A "split film" test for a density comparison is highly recommended by the authors of this ACR publication as well. Since there can be various discrepancies in using simulated screen light to predict a film's response vs. comparing film-screen response under actual x-ray exposure conditions, the authors caution against the use of commercial sensitometers to foretell film sensitivity or "speed" differences.
Those performing film-screen mammography should understand this possibility and be prepared to make compensating adjustments to automatic exposure control (AEC) devices on their dedicated mammography x-ray equipment. This amount of potential film emulsion variability also underscores the importance of doing a proper "crossover" calculation whenever a different emulsion batch of film for mammography Quality Control is introduced. If the crossover procedure is not done or done incorrectly, the ability to monitor processing conditions ( i.e., trust the QC charts) becomes immediately suspect.
It is also a fact that as most films age, their sensitometric response often changes. Typically "speed" and "contrast" measurements drop, and if recommended storage guidelines for temperature and relative humidity are not followed, sensitometric changes can occur more quickly. Such changes in "speed" and "contrast" can produce measurable density changes on sensitometer exposures, but these usually small and gradual changes over time may or may not always be noted when viewing clinical images.
Usually the film used for clinical imaging is turned over more quickly than the box or boxes of film set aside for Quality Control use, especially if only a single processor is being monitored. In such cases, film aging may need to be factored into an analysis of different processed film densities, and the film manufacturer should be able to provide emulsion age information upon request. If recommended temperature and relative humidity conditions cannot be met, it may be useful in some cases if the box of film used for Quality Control is changed more frequently.
- BRAND, TYPE(S), MIXING INTEGRITY, & AGE OF PROCESSING CHEMICALS
The condition of the processing chemicals is of vital importance to maintain film densities within acceptable limits. Different brands of chemicals have been shown to produce higher or lower "speed" and higher or lower "contrast" for the same film. Different types of chemistry within the same brand (e.g., specialized "mammography developer solutions") can have a similar and even more significant effect. Specific temperature recommendations for given types of processing solutions and film processors are most frequently obtained through the film and/or processor manufacturers or their authorized distributors.
Chemical mixing methods can vary with both human and automated chemical mixers, and it is the responsibility of those who mix the chemistry to ensure that the mixed solutions fall within the manufacturer's specifications. Commonly performed tests for chemical integrity such as pH and specific gravity do NOT give "the whole picture"; a correct pH measurement by itself does not guarantee that a solution was properly mixed. A specific gravity measurement by itself is not specific enough to determine whether critical components, such as developer anti-foggants, are missing or removed from the solution. In addition, test equipment calibration and the temperatures at which measurements are made are very important in order to yield accurate data.
Oxidation of chemicals and/or age of the mixed chemicals are additional variables which can affect processed film densities. Following the manufacturer's recommendations for chemical turnover, the use of floating lids in replenisher tanks, proper processor venting, etc., will reduce variability due to age and reduce chemical oxidation to a minimum.
- FILM PROCESSING CHEMICALS "SEASONING" EFFECTS
Chemical changes occur in fresh processing solutions as a result of films being processed. These changes are collectively called "seasoning" and seasoning has definite effects upon processed film densities. This is a complex issue, as seasoning is affected by the number and types of films fed into the processor, which means that film densities can change over time, as the degree of seasoning changes.
The addition of developer "starter" solution is designed to bring freshly mixed chemistry to a seasoned state very quickly, so that processed film densities experience less variation. The by-products of film development, however, add additional seasoning effects so in most cases the addition of developer starter solution does not completely eliminate some density changes over time due to this factor.
All developer starter solutions are not formulated the same, so this can be a possible source of density variability too. Certain brands of developer starter solution(s) may not provide the sensitometric performance equivalent to that specified by the film manufacturer. For these reasons, it is often a good idea to use developer starter solution made by the same manufacturer whose developer is in use, as these solutions were designed to work together.
- CHEMICAL REPLENISHMENT RATES FOR FILM PROCESSING
The establishment of proper chemical replenishment rates is critical for maintaining the stability of processed film densities within acceptable limits. Consult the film and/or chemical manufacturer for proper replenishment rate suggestions. Replenishment rates will differ depending upon many factors: the numbers and types of film(s) processed, film density, the film orientation as sheets are fed into the processor, whether the films are fed individually or fed two at a time, whether the processor is "dedicated" to processing one type of film or multiple types/brands of films, and so on. In some cases modifications to the film processor's replenishment calculation software may be recommended. Changes in chemical formulations over time and/or film emulsion modifications may prompt revised replenishment recommendations as well. Recommended replenishment rate data is usually obtainable through the manufacturer directly or through chemical solution/service distributors.
Service Bulletin #30 deals with recommended replenishment rates for Carestream films.
This document is also available via the FAXBACK system 1-800-336-4722 (document # 11).
- DIFFERENT X-RAY EQUIPMENT
Different x-ray generating equipment can introduce many variables which will affect processed film densities. Even two similar models of equipment from the same manufacturer can vary due to unit-to-unit variability, possibly different amounts of beam filtration, and so forth. It is always advisable to use the same piece of x-ray equipment when producing films which are being checked and monitored for specific density measurements. Of course, even the same piece of x-ray exposure equipment can produce different exposure levels over time, even when the same parameters are set. Consult a medical physicist on equipment issues.
- X-RAY EXPOSURE EQUIPMENT CALIBRATION ACCURACY OVER TIME
Periodic tests should be made to ensure the exposure accuracy and repeatability of x-ray equipment. This will reduce or eliminate this potential source of processed film density variability.
- X-RAY EXPOSURE SETTINGS SELECTED
Are the same exposure parameters being set? Even when the same exposure settings are set, remember the emulsion-to-emulsion variability over time explained in #1, and all the other possible sources of variability outlined in this document.
- SELECTION OF AEC CELL(S)/POSITION OF AEC CELL(S)
By selecting the same AEC cell(s) and making sure their position has not changed, the potential for exposure variation and thus film density variation is reduced.
- POSITIONING OF PHANTOM/BODY PART OVER AEC CELL(S)
Paying special attention to patient and/or phantom positioning with respect to AEC cell position will reduce or eliminate this variable.
- DIFFERENT COLLIMATION USED
Changing collimation usually impacts the production of scatter radiation, which affects overall exposure, image contrast, and patient dose. If collimation is changed, this increases the likelihood of altered processed film densities.
- SAFELIGHT INTEGRITY/CONDITION
Improper safelight conditions can cause increased fog levels on films, increased "speed" or Mid-Density measurements, and produce lower image contrast and Density Difference measurements. Periodic tests should be made of all darkroom safelights to ensure appropriate safe handling times for the film(s) used in that darkroom. Carestream offers a pamphlet with information on safelights and safelight testing (Catalog # 848 0733; $1.00 each). It can be ordered by calling 1-800-677-9933 or downloaded from the Internet.
- FEEDING FILM EMULSION-UP VERSUS EMULSION-DOWN
Depending upon the film, the brand and model of the processor, and the recirculation characteristics within a given processor, different film densities can occur when single-emulsion films are fed into the processor emulsion-up versus emulsion-down. This possibility, plus other considerations such as the reduction of certain types of processing artifacts, usually promote film and/or processor manufacturers to specify which way to orient a particular single-emulsion film when processing. However, it's a good idea to evaluate each processor individually and determine which film-feeding orientation provides the best overall processing results. Once this decision is made, everyone should be trained to feed film(s) the same way to ensure consistency and minimize possible density variations on processed films.
- FEEDING FILMS ON RIGHT/LEFT/CENTER OF FEED TRAY
Due to dynamic recirculation patterns within a given film processor's internal tanks and rack components, and the condition of a given processor's recirculation system, local development activity can sometimes differ slightly from one side of the tank across to the other side. This possible slight development difference should not affect normal clinical image viewing. However, density measurements of simple phantom objects and/or sensitometric strip areas could detect these density differences if and when they might occur.
- DIFFERENT CASSETTE USED
Different cassettes of the same type and brand can have slightly different x-ray attenuation/transmission characteristics, which can translate into different densities on processed films. Such density variations from this source should not normally affect clinical image viewing, but when density measurements are being made and compared on films exposed in cassettes, it is recommended that the same cassette be used to eliminate this possible variable.
- DIFFERENT INTENSIFYING SCREEN(S) USED
Intensifying screen "speed" can sometimes differ with the same screen type and brand, due to phosphor coating variability, screen construction revisions over time, the type of screen cleaner used over time, and other factors. For these reasons, the use of the same screen(s) and cassette are recommended when making phantom exposures or any radiographic images where the processed film densities in selected areas are measured and/or compared. The ACR manual recommends an upper limit of 0.30 on measured optical density differences among all mammography cassettes/screens in a department. Testing for this variability should be done following the guidelines published in the American College of Radiology Quality Control Manual.
- FILM EMULSION NOT ORIENTED PROPERLY IN CASSETTE
Mis-orientation of film emulsion-to-screen when loading single-emulsion film(s) will produce marked system speed differences which should be immediately apparent as significant density differences on processed films.
- DIFFERENT PHANTOM/TEST OBJECT USED
Slight construction and/or x-ray attenuation differences may exist in phantoms and test objects which could result in different densities on processed films. Because of this, it is wise to use the same phantom or test object when making exposures to produce and measure certain densities on films over time.
- FILM MIX IN NON-DEDICATED PROCESSOR(S)
Tests have shown that in "non-dedicated" processors where mammography films are processed along with other types/brands of film(s), certain mixes and volumes of different films can affect the "seasoning" of the chemicals which in turn could affect processed film densities. In such non-dedicated processing circumstances, it may be prudent to first consult with the film and/or chemicals manufacturer to see if there is any additional information or caveats involving a particular mix of film types /brands.
- VARIABLE TIME INTERVAL BEFORE PROCESSING FILMS
Density changes can occur when the same films are processed after waiting different lengths of time after exposure. This variable, known as "latent image keeping" (LIK) may need to be considered if films are not always processed within the same time interval after x-ray or sensitometric exposure. Generally speaking, LIK characteristics tend to affect film "speed" more than film average gradient on a percentage loss basis, when both measurements are taken at the same time intervals (i.e., zero to 48 hours post-exposure). In mobile van situations where immediate post-exposure processing may not occur, this could become an important variable when specific density measurements are made and compared.
Not all films exhibit the same latent image keeping characteristics; the film manufacturer should be able to provide this data upon request. The ACR recommends a limit of not more than 0.15 optical density loss due to LIK at a density of approximately 1.25 for mammography films. If LIK characteristics are an issue in some processing circumstances, the ACR recommends using the same delay time before processing for consistency and compensating adjustment of AEC circuitry as necessary.
- FILM RECIPROCITY CHARACTERISTICS
"Reciprocity failure" is a term used to describe the non-linear response or sensitivity of a given film emulsion, usually at certain extremes of either very short exposure times, very long exposure times, or both. Thus, a processed film's densities may not be reproducible under certain exposure conditions,depending upon the exposure factors selected manually or by automatic exposure control (AEC circuitry).
The majority of mammographic exposures on clinical patients and phantoms today are performed using automatic exposure control. It could occur as a result of the system speed of the film-screen combination in use, the generation and condition of the exposure equipment and AEC, the patient/phantom, and other variables, that some exposures fall into the range where film reciprocity characteristics must be considered. The preferred length of exposure at present is approximately 1.0 to 1.5 seconds. Exposure times longer than 2 seconds may invite either patient motion (causing image blur) or reciprocity failure, or both. Depending upon the x-ray exposure equipment, the film-screen combination, etc., it may be necessary to raise the kVp to keep exposures below 2 seconds. A medical physicist should be consulted for this determination.
Most newer dedicated x-ray mammography units offer some type of automatic compensation or safeguards to minimize this reciprocity variable