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Digital Mammography Systems

2 November, 2010 | Mammography Unit

Mammography X-ray systems are specially designed to produce radiographic images of the breast. The main clinical applications of these systems are in breast cancer screening, staging and grading, and differential diagnoses in symptomatic patients. Most of the mammographs allow magnification views and spot images. In general, mammography systems have different ergonomics requirements compared to those of general radiographic systems. A special stereotactic attachment enables the performance of stereotactic biopsy procedures.

Digital mammography images can be obtained either by a full field (large sized) digital detector, or by using CR cassettes and a CR reader. Also, a small-sized digital detector can be incorporated into an analogue mammography for image spotting and as a way of guiding stereotactic biopsies.

The major components of a mammographic system are:

  • The pedestal support for the tube, the breast platform and the cassette holder or detector
  • The X-ray tube assembly, including the collimator and the filters to reduce low energy radiation
  • The breast-holding platform and compression paddle
  • The detector or cassette holder

In general, the digital mammographic system should provide a high degree of spatial resolution, wide dynamic range and as low as possible exposure of the patient to X-ray radiation.

There are several benefits of using digital radiography:

  • More efficient storage of and access to images
  • Fewer retakes
  • Better visualization of dense breasts
  • Availability of image post-processing and image manipulatio

There are several types of full field detectors. The major technologies in use are:

  • Indirect X-ray conversion detectors – the X-ray is converted to light, and the light is converted to digital electric signals.
  • Direct X-ray conversion detectors – the X-ray produces a charge, and the charge is read out to produce a digital matrix of the image.
  • Slot scanning systems, which scan the breast by a slot of detectors.

The best spatial resolution in digital radiography is performed by the CR systems, however the X-ray dose to the patient is higher. Clinical studies have been reported and generally suggest that digital mammography provides either equal or better imaging performance than film or screen imaging. Digital mammography systems usually have a deeper dynamic range. It is important to note that pixel size is not a good indicator of spatial resolution, as the noise and blurring effects in the detector system can have a significant effect on resolution. In addition, different types of detector technologies have different noise and blurring characteristic.

Small field digital mammography units are typically available as add-on components to conventional mammography units and are used for spot imaging and stereotactic biopsy procedures, in conjunction with a stereotactic attachment.    It is important to note the performance can vary significantly between different manufacturers.  Most full field digital systems will use the full field detector for spot imaging and stereotactic procedures.

Digital mammography images must be reported using a suitable reporting workstation. Currently there are no universally accepted formats in terms of the number of mammograms that need to be displayed simultaneously. In general, two high resolution monitors will be required for a reporting workstation, as usually two images need to be compared at a given time. A third, lower resolution monitor may be required to display patient information, work lists, etc.

The user interface should enable full visualization of image data. Standard imaging processing typically includes:

  • Magnification, zoom and roam functionalities
  • Window and leveling (contrast and brightness)
  • Image flip and rotation
  • Edge enhancement and noise reduction
  • Black/white inversion

Digital imaging can be processed using sophisticated pattern recognition software and “suspicious” features are highlighted on the image and brought to the attention of the reader. It is necessary to understand that the existing workflow will probably not work at maximum efficiency once the department has moved to digital imaging. To achieve an efficient workflow, links to a PACS (Picture Archiving and Communications System) must be considered.

Reducing patient dosage to levels which are as low as reasonably practicable (ALARP) is both an ethical and legal requirement, falling under the Ionising Radiation (Medical Exposure) regulations. There is a responsibility to reduce the long- term risks of radiation exposure (such as cancer reduction) to levels where the potential benefit of the examination outweighs the risk of any adverse effect. This is of particular importance in mammography, as the breast is a radiosensitive organ and mammography is currently the technique of choice for breast cancer screening, which results in a large number of disease-free women being imaged.