Mammography (radiography of the breast)
Mammography technique and dose
Mammography can be used for diagnosis or for screening asymptomatic patients.
Mammography is a highly effective imaging method for detecting, diagnosing, and managing a variety of breast diseases, especially cancer. It is an application where an emphasis on patient dose management and risk reduction is required. This is because of a combination of two factors. First, breast tissue has a relatively high sensitivity to some adverse effects of radiation, and second, mammography requires a higher exposure than other radiographic procedures to produce the required image quality. The higher exposure, compared to other radiographic procedures, is because the breast is composed of soft tissue (no bones or air) and has very low contrast. Therefore, more radiation is required to produce visible images of both normal breast anatomy and signs of disease.
In mammography, the objective is to produce images that provide maximum visualization of breast anatomy and the signs of disease without subjecting the patient to unnecessary radiation.
Technique
- Why is mammography performed on a dedicated breast imaging equipment?
- Are specialized film-screen combination required for mammography?
- Can mammography films be processed in an ordinary film processor?
- Why is a higher X ray exposure required to produce a mammogram than other radiographic images, such as that of the chest?
- Can digital mammography be used, and are there advantages?
Radiation dose
- Should we be adjusting our mammographic techniques to produce images with the lowest possible radiation dose to the patient?
- What determines the radiation dose to the breast of my patients in mammography?
- Which quantity is used to express the exposure to radiation of a mammography patient?
- Is it possible to measure the mean glandular dose to the breast of my patient in mammography?
- What is the best kV value to use in mammography to reduce the absorbed dose to the breast?
- Should I tell my patients the radiation exposure for their mammographic procedures?
- Is compression necessary, even if the patient has sensitive breasts?
1. Why is mammography performed on dedicated breast imaging equipment?
Early breast cancer is difficult to detect because it appears similar to normal breast tissue. The difference between cancer and normal tissue needs to be amplified, and for this the use of lower energy X rays is required. This can be achieved only with specially designed radiographic equipment dedicated to mammography
2. Are specialized film-screen combinations required for mammography?
Yes. The intensifying screens used for mammography are designed to produce very low blurring and good visibility of small structures and objects, especially microcalcifications. In order to detect small changes (in attenuation and in size) in breast tissue, it is necessary to have a film-screen combination which can detect very small differences at a low dose.
3. Can mammography films be processed in an ordinary film processor?
Ideally the best results for standard non-digital mammography are obtained in a well adjusted film processor dedicated for mammography film. Ordinary processors can be used, but the quality of the mammograms produced might not be as good. Irrespective of which type of processor is used, it is vital that it is subject to rigorous quality control to ensure that optimum processing occurs all the time
4. Why is a higher X ray exposure required to produce a mammogram than other radiographic images, such as that of the chest?
The short answer is image information required. Now some explanation. In all X ray imaging methods there is always a balance between image information (visibility of signs of disease) and the absorbed dose to the patient. In mammography, especially for detection of cancer at the early stage when it can be treated most effectively, images with high contrast sensitivity, high detail, and low visual noise are required. Each of these characteristics requires a relatively high X ray exposure. Because the normal composition of the breast and the usual signs of cancer are in soft tissue with very little difference or physical contrast, X ray beams with low penetrating characteristics must be used to produce visible images. This results in higher exposure. Another valuable sign of early cancer are small (micro-) calcifications. The imaging receptors necessary for their visibility require a higher exposure than receptors for other radiographic procedures.
5. Can digital mammography be used and are there advantages?
Yes. Digital imaging is replacing conventional imaging in general and also in mammography. Information is quickly available on the screen for making a diagnosis, it can be transmitted electronically to others and images can be printed on films as necessary. Image storage is easier and less bulky. Digital imaging also has the potential to reduce patient doses because there may be fewer repeat images. Staff operating the equipment find operation easier owing to improved equipment ergonomics. However, it may happen that images of quality higher than necessary are taken, as is observed for many other digital imaging techniques, and this can increase patient dose without adding diagnostic information. Chemical processing is not required to produce the images and this reduces the environmental impact as there are no waste chemicals. For further details »
6. Should we be adjusting our mammographic techniques to produce images with the lowest possible radiation dose to the patient?
No. While it is possible to select mammography imaging components (e.g. film, intensifying screens) and technique factors that will produce images with very low X ray exposure, the images would have very low quality and not provide the necessary diagnostic results. The correct action is to optimize each imaging procedure or technique so that the mammography equipment and imaging factors are selected and adjusted to provide the necessary image information and visualization with the lowest radiation exposure.
7. What determines the radiation dose to the breast of my patients in mammography?
The dose to the breast of an individual patient is determined by a combination of three factors: (i) the characteristics of the equipment being used, (ii) the technique factors selected for the examination, and (iii) the size and density of the patient’s breasts. The radiation beam of each mammography machine is usually calibrated so that it delivers a specific and known absorbed dose to a reference (average size) breast when imaged with a specific set of technique factors. The major factors determining this dose are the sensitivity of the receptor (film- screen combination, characteristics of digital receptors), and the setting of the automatic exposure control (AEC) level to produce a specific film density. The dose generally increases with increased breast size and density, for a given optical density. With regard to the technique, the selected kV (typical range of 24kV to 32kV) and anode/filter combination are the major factors that determine the dose. Lower kV values are used to enhance contrast but do not provide sufficient penetration through thick or dense breast tissue where the higher kV values are required. When the kV is reduced in a specific examination, the mAs (in the units of mAs) must be increased to produce the necessary receptor exposure. The combination of reduced kV and increased mAs results in a higher dose. Dose generally increases with increased breast size and density because more radiation is required to penetrate the breast and expose the receptor.
8. Which quantity is used to express the exposure to radiation of a mammography patient?
The quantity that is generally used is the mean glandular dose (MGD). This is based on the assumption that it is the glandular tissue in the breast, and not the adipose tissue or fat that is most sensitive to radiation effects. The MGD is defined as the mean (average) dose to the glandular tissue. It is considered to be a reasonable quantity for comparing relative risk from different mammography procedures.
9. Is it possible to measure the mean glandular dose to the breast of my patient in mammography?
The MGD cannot be measured directly because it occurs within the breast. Estimates of the MGD for a specific patient can be calculated (usually by a physicist) from the tube output, which is directly measurable, and the exposure parameters to obtain the image. However, the exact composition of the breast is not known and it also influences the MGD.Some digital mammography systems automatically calculate and display the MGD for each patient.
10. What is the best kV value to use in mammography to reduce the absorbed dose to the breast?
That is not really the appropriate question! The better question is ‘what is the best kV value to provide the necessary image contrast and visibility with the lowest possible radiation absorbed dose to the breast?’ The consideration for image information must come first. The kV value that provides the optimum balance between image information (visibility of the signs of cancer) and absorbed dose depends on the compressed size and density of the breast and will generally be in the range of 24 to 32. If the kV is set too low for a specific breast size or density the breast will not be properly penetrated and the dose will be higher than necessary. If the kV is too high then the contrast and, therefore the visibility, will be reduced.
To ensure the necessary image quality with the lowest possible dose, mammography should be performed by highly qualified radiographers with the ability to evaluate each patient’s breast and then select the optimum kV and other exposure parameters.
11. Should I tell my patients the radiation exposure for their mammographic procedures?
There are several factors to consider. First, you will not know the dose for each patient unless there has been a special effort to calculate it from the exposure parameters and the equipment calibration data (beam output).
What should be known in each mammography facility is the absorbed dose for a reference average breast size and for the range of breast sizes encountered. These data are determined by a physicist. The output is directly measurable and MGD is estimated from output and the exposure parameters.
What should be avoided is any encouragement for patients to go 'dose shopping' by searching the facility that does mammography with the lowest dose.
The appropriate approach is to provide patients with professionally prepared information covering the benefits and risk of mammography.
12. Is compression necessary, even if the patient has sensitive breasts?
Yes. Each breast is gently but firmly compressed using a clear plastic device just for a few seconds. It is important that the breast is compressed to obtain the clearest possible picture and the lowest radiation dose. Compression stops the breast moving during the exposure. Some women find mammography uncomfortable, similar to have the blood pressure taken.
- Information for
- Health Professionals
- Member States
- Patients