Screen-Film Mammography Technical Applications Unit 5. Bonnie A. Barnes, BA,R.T.(R)(M)(CT)(f) Xuan Ho, Ph.D., R.T. (R)

Transcription

1 Screen-Film Mammography Technical Applications Unit 5 Bonnie A. Barnes, BA,R.T.(R)(M)(CT)(f) Xuan Ho, Ph.D., R.T. (R)

2 Technical Applications Technical Variables Screen and Film Variables Processing

3 Technical Variables Density Contrast kvp mas Compression Automatic Exposure Control (AEC) Half-value layer (HVL) Reciprocity law failure Collimation

4 Technical Variables Density: It is very important to have the proper amount of density on a mammogram. Without it, the radiologist will have great difficulty determining whether the patient has a cancer. The proper amount of density used is determined by measuring the optical density on a phantom. The technologist then can determine if she should use +0, -1, or +1 density for the film-screen combination used in the facility. Image contrast will degrade with extreme variances of densities.

5 Technical Variables Patient with very dense breast tissue, needs higher kvp and density to penetrate the glandular areas of the breasts.

6 Technical Variables Contrast: is defined as the ability to visually compare adjacent anatomical densities. This is different from the term resolution in that resolution is the ability to visually separate adjacent anatomical details. It is of paramount importance in screen-film mammography to have good contrast. IT is difficult to maintain high contrast in images, because the breast itself has very little inherent subject contrast. To determine if there is good contrast on a mammogram, a phantom test should be performed, to see the fine detailed structures on the phantom. To achieve good contrast, beam quality, grids, film-screen system, processing, compression, and positioning all play key roles in the process.

7 Technical Variables kvp: The two methods by which X-rays are produced are the Compton effect and the photoelectric effect. Screen-film mammography employs the photoelectric effect to produce its very high-contrast X-ray spectrum. The range of kvp for mammography is between kvp. The ideal range is between kvp for grids. For non-grid studies, the optimum kvp range is kvp. The lower the kvp, the higher is the ability to achieve good contrast. The higher the kvp, the more grey the mammogram looks.

8 Technical Variables kvp (continued): With lower kvp, longer exposure time is used. With longer exposure time, there is not a linear increase in the density on the film. This is called Reciprocity Law Failure. If two identical films are exposed, one for a given length of time and the other for twice that long, the second would not be twice as dark in its density measurement as the first. Contrast would certainly diminish with longer exposure times, but density would not diminish as a direct effect.

9 Technical Variables mas: mas is the current applied to the filament of the cathode and determines the number or quantity of electrons available for target bombardment. mas is limited by focal spot size and is a function of tube capability. Time (seconds) is a function of the tube cooling system. mas is also limited by the use of repetitive exposures and kvp. ma may be fixed or variable. The time in seconds should be used in the shortest amount possible, given the ma and kvp selection. with Reciprocity law failure (RLF), density does not increase in a direct relationship with exposure time. It is important to keep exposure time to a minimum, 2 seconds or less. This minimizes the possibility of motion on the image.

10 Compression: Technical Variables MQSA states that compression shall be applied in a manner that minimizes the potential obscuring effect of overlying breast tissue and motion artifact. Compression is the single most important technique in producing high quality images. With more compression, less kvp and mas is used. The amount of radiation the patient receives is reduced. Contrast and detail of the mammogram are increased. Density is then lower, with lower mas.

11 Technical Variables Automatic exposure control (AEC): aka: phototiming AEC is the device which senses the radiation reaching the image receptor and automatically terminates the exposure at a preset value. With manual kvp selection, the AEC determines how long the exposure needs to be. With higher kvp, shorter exposure time is used. The purpose of the AEC is to produce consistent film density and image quality, which reduces the need for repeat films. AEC cells measure the required dose for the exposure and need to be placed over the dense-glandular portion of the breast. (retroareola)

12 Technical Variables Photocell placement: The photocell must be capable of assuming three positions. It must be fixed at midpoint lengthwise or laterally, the first position is at the chest wall. The second and third positions are at points further away.

13 Technical Variables Photocell placement (continued): It is very important that the photocell is completely covered by breast tissue. Adjustments in position may be necessary to accomplish this. The photocell is placed at either the thickest or the most dense part of the breast. The most dense part is difficult to estimate and varies with patient age, breast size and positioning. The glandular tissue in a large breast is usually further away from the chest.

14 Technical Variables Automatic exposure control (AEC) (continued): Reproducibility of the AEC must be established. Measure the typical entrance exposure for an average patient (approximately 4.2cm compressed breast thickness-50% adipose, 50% glandular composition). Calculate the associated average glandular dose, to assess short-term AEC reproducibility. This test is performed by a physicist.

15 Technical Variables Automatic exposure control (AEC) (continued): MQSA requires that the AEC shall be capable of maintaining an optical density within +/ of the mean optical density when thickness of a homogeneous material is varied over a range of 2 to 6cm, and the kvp is varied appropriately for such thicknesses over the kvp range used clinically in the facility. If this requirement cannot be met, a technique chart shall be developed showing appropriate techniques for different breast thicknesses and compositions that must be used so that optical densities within +/-0.30 of the average under phototimed conditions can be produced.

16 Technical Variables Half-value Layer (HVL): Affects radiographic contrast and dose. Should be as close to 0.30mm equivalence of aluminum at 30kVp as possible. (MQSA standards) Dose decreases as the HVL increases. As the kvp increases, so does the HVL. Image quality deteriorates, due to decreased contrast with increasing filtration.

17 Technical Variables Reciprocity law failure (RLF): Film used in conjunction with an intensifying screen states that as the length of time of exposure increases, there is not a linear increase in the density on the film. Since grids are more commonly used to clean up scatter radiation, an increase in either kvp, the ma, time or some combination of these factors to accommodate the use of a grid. Since most mammography units have a fixed ma station, the technologist is limited to changing either the kvp or time.

18 Reciprocity: The ability of the film to respond in a constant manner to a constant exposure (intensity x time). Reciprocity failure occurs during very long or very short exposures, requiring exposure increase.

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20 low radiation levels over a longer period have less effect than high radiation levels over a shorter period, even though the sum of light intensity and exposure time is the same.

21 Technical Variables Reciprocity law failure (RLF) (continued): Time determines density and kvp determines contrast. In order to use the optimal kv settings that will produce high contrast, as a compromise the length of the exposure should be increased up to the point at which RLF has a major effect. Exposures that continue beyond this point have adverse effects such as: motion, scattered radiation on the image, and increased dose to the patient.

22 Collimation: Technical Variables Used to limit field size and reduce secondary radiation found in the primary beam. Three designs are currently used: fixed apertures, a set of interchangeable cones or various sizes, and an internal set of collimating blades.

23 It is important to have collimation that fully covers the image receptor by the X-ray field, but does not allow significant radiation beyond its edges, and that the chest-wall edge of the compression paddle aligns with the chest-wall edge of the film. MQSA regulations state that the X-ray field shall not extend beyond any edge of the image receptor by more that 2% of the SID.

24 Screen and Film Variables Screens Image receptors (film systems)

25 Screen and Film Variables Screens: Intensifying differences Single screens Double screens Cassettes Care and maintenance of screens MQSA requirements and tests Artifacts

26 Screen and Film Variables Intensifying differences: The state-of-the-art recording system today consists of a single-emulsion orthochromatic X- ray film that provides high contrast and good resolution. This film is encased in a specially designed mammographic cassette that has a rare-earth intensifying screen. This screen is located on one-half on the cassette, so that it will not act as an attenuator of the X-ray beam.

27 Screen and Film Variables

28 Screen and Film Variables Single and double screens: Using a single screen with a single emulsion film will improve image quality. Resolution is increased because receptor blur is reduced. Single emulsion film contains higher amounts of silver halide and gelatin than does double emulsion film. Double emulsion/double-screen systems yield lower contrast and have faster screens, thus radiation to the patient is reduced.

29 Screen and Film Variables Cassettes: Mammography cassettes are specialized cassettes used only in mammography. The standard sizes are 18x24 and 24x30 cm. It is a rigid device made of low absorption or carbon fiber which provides a light-tight holder for the film and screen. It is also designed to provide great screen-film contact.

30 Cassette It is to be identified with numbers, on the outside of the cassette, as well as on the screen inside of the cassette. The number must be placed in an area of the screen to not obscure breast tissue, but so that it will be documented permanently on the film used. The purpose of this is so that any artifacts showing up on the film can quickly be identified, so that the cassette can be cleaned.

31 Screen and Film Variables This is a dedicated Q.C. cassette used only for testing purposes. It is also numbered and labeled.

32 Screen and Film Variables Care and maintenance of screens: Mammography screens are very sensitive and can be damaged if handled inappropriately. It is critical to clean screens at least once a week. A specialized cleaning solution is used with a lint-free cloth to clean these screens, along with a soft-bristle dust brush, and canned air under pressure. According to MQSA standards, screens must be kept clean. This means they must be cleaned and documented at a minimum of once per week. If any artifacts are to show up on film, the screen showing the artifact must be cleaned right away.

33 Screen and Film Variables Artifacts: Some artifacts known to show up on mammograms are lint, dirt, nail polish, hair, pick-off, patient s chin, glasses, earrings, necklaces, the patient s other breast, gown, fingers, and many more. To prevent most of these artifacts from occurring, the technologist must be aware of positioning artifacts before making an exposure. Lint, dirt, pick-off and nail polish can all be prevented by cleaning the cassettes regularly. Nail polish should not be worn the day of cassette cleaning.

34 Processing Darkroom Dedicated and non-dedicated processing Standard and extended processing Artifacts MQSA requirements

35 Processing Darkroom: The darkroom used for mammography must have a safelight standard for its use. This means that the safelight can not be too intense, so as to fog mammography film. A safelight has to be at a minimum of 4 feet above the work area. The proper wattage light bulb is used to prevent fogging. (no more than 15-W in an overhead ceiling fixture, and no more than 7.5-W in closer fixtures.

36 Processing Airflow (ventilation): The temperature of the darkroom should be kept at approximately 70 degrees F. When temperature rises, emulsion on film becomes softer and more susceptible to scratching. A cooler temperature can cause cracking and peeling. There should be a positive airflow to the processor. Humidity also plays a key role in that static may result as an artifact on film, if the air is too dry. Humidity should be maintained at 50-60%. Design: the design of the darkroom should be so that it is light-leak tight, and so that lint artifacts are minimized. Any type of light leak can fog mammography films. Shelves are not to be placed above the work area.

37 Darkroom (continued): Processing MQSA requires that there should be no eating, smoking, or drinking in the darkroom. There should be any object on the countertop that is used for loading and unloading cassettes. Items on the countertop make cleaning more difficult and provide a convenient place for dust and dirt to accumulate. The ceiling of the darkroom should be constructed of a solid material such as drywall. The vent for heating and air conditioning should not enter the room over the counter used for handling cassettes.

38 Processing Dedicated and non-dedicated processing: Many facilities have dedicated mammography processors. In this situation, a daily quality control chemistry strip is processed to be sure of stability of the chemistry. In facilities where a processor is shared with Radiodiagnostic, extended processing time may need to be manually selected by the mammographer. In this situation, the mammographer will need to process a q.c. strip and a phantom, before developing mammography film through it. The phantom is processed to be sure the detail of mammography film has not been compromised.

39 Processing

40 Processing Standard and extended processing: Standard processing A 90-second processing cycle. Film is immersed in the developer for a minimal time to achieve average contrast and speed. This type of processing can accommodate single and double emulsion films.

41 Processing Extended processing Is developed to achieve the best possible image quality on a mammographic film by enhancing its speed and contrast. Sometimes, a higher developer temperature is established. Film is immersed in developer solution for a longer period of time. Double emulsion film has not been found to have better speed and contrast with extended processing. With extended processing, there is improved quality, increased contrast, reduction in radiation dose, increased tube life and reduced reciprocity law failure. Units must be cleaned and maintained on a regular basis.

42 Processing Standard and extended processing (continued): The chemistry of a mammography processor is recommended by the manufacturer of the film-screen system being used. Fresh chemistry must be maintained in a mammography processor, to give the best image quality possible. The temperature of a mammography processor must be maintained at 95 degrees. If the temperature varies by 1 degree, image quality will decrease and dose to the patient will increase.

43 Under replenishment of fixer results in poor fixation, insufficient hardening, inadequate washing and drying, and possible failure to transport in the fixer rack or at any point beyond. Replenishment rates: Processing Accurate replenishment of developer and fixer solutions is essential to proper processing of the film and to the long life of the processing solutions. If the solutions are not properly replenished the films may not be dry or be transported correctly. Over replenishment of developer will result in lower contrast and lower maximum density. Over replenishment of fixer does not affect the transport of the film or its quality, but is wasteful.

44 Processing Artifacts: An artifact is a detail or pattern caused by materials introduced into the imaging and processing systems. The types of artifacts are unlimited. Artifacts associated with processing can be guide shoe marks, roller marks, pickoff, or under replenishment of fixer.

45 Processing MQSA requirements: Cleaning: processors must be kept clean on a routine basis. For processing mammography films, the facility shall use chemical solutions that are capable of developing the films used by the facility in a manner equivalent to the minimum requirements specified by the film manufacturer. To consistently obtain quality radiographs, a quality assurance program should be implemented. Crossover racks should be cleaned at least once a day.

46 Processing MQSA requirements (continued): Chemical replenishment should be monitored, and chemical tanks should be checked periodically. Transport clean-up film should be processed every morning and during the day as needed. Chemicals should be changed and tanks should be cleaned routinely, according to the type of processing being used. Maintenance of processor quality control logs is required on a daily basis.