iaea human health series publications - SEDIM

5.2.2. Electronic versus geometric zoomElectronic zoom has replaced the magnifying lens. The number of pixels that can be displayed on a monitor isusually less than the number contained in a digital mammogram (and certainly less than that number when multipleimages are displayed on the same monitor). Therefore, zooming is often necessary to see the image at full resolutionwith 1:1 pixel display and is always required for full resolution in systems with a pixel size of less than 100 μm.When an image is displayed at other than 1:1 (or 2:1) resolution, the pixels are ‘interpolated’ into the display imagematrix. This means that information may be smoothed across a number of pixels or may be dropped out completely.Often there is aliasing (see Section 2.3.3.5), but unless tested with an appropriate test pattern, this cannot bediscerned. While the display zoom may eliminate the need for some spot magnification techniques, it does notreplace the need for geometric magnification, which provides more detail and information with a higher signal tonoise ratio (SNR) in most instances. Display zoom makes the appearance of the object larger (making it easier forthe radiologist to see structures if they are resolved), but it does not improve the spatial resolution of the image.Using these and other tools such as coning and annotating increases the time required to review and report a digitalmammogram.5.3. IMAGE INTERPRETATION5.3.1. Soft copy readingA functional radiologist workstation requires three monitors: two 5 MP image monitors and one lowerresolution monitor for RIS display. Workstation ergonomics (height of table, angle of monitors, etc.) is moreimportant for digital mammography than for screen film mammography, and it is difficult to find generalizedconditions to suit all radiologists. Room lighting is critical, and care must be taken to eliminate glare from othermonitors and viewboxes, for example.With soft copy reading, hanging protocols should be customized for different radiologists, allowing display ofprotocols for a full range of frequently used mammography options (e.g. routine 4 views, views plus 1, 2 or 3, etc.,extra views, prosthesis views, large breast format, mastectomy format) so as to avoid the need to frequently ‘dragand drop’ images onto the monitor. Different clinical scenarios should be incorporated, including screening,diagnosis, recalls and short term follow-up. Automatic fetching and display of prior images would be needed forbreast screening applications. It is preferable for all acquired images to be displayed to the reader automatically.This does not always happen with PACS workstations, so a special effort must be made to ensure that no images aremissed. There should also be features to indicate when the displayed image is not completely demonstrated or is notbeing displayed at full resolution. Image annotation (patient name, etc.) must be displayed such that it does notobscure the breast image. Moreover, it should be noted that soft copy display protocols will not hang properly andwill not be synchronized for zoom, etc., unless care is taken with image labelling/tagging.Soft copy review of digital mammograms (i.e. on the monitor) has the advantage of window and leveladjustment. This reduces the problems of over- and underexposed images seen with screen film mammography, andtherefore fewer repeats are required for poorly exposed images. This can be particularly useful with the augmentedbreast. There are multiple tools to aid in the review and comparison of images, such as magnification, quadrantzoom, windowing, panning and marking areas of interest on the image. These tools should be set up on a dedicatedradiologist keypad for easy use.5.3.2. Reporting speed issuesAs working with soft copy requires time to display a case and to move through a display protocol, it takeslonger to report digital mammograms, regardless of the radiologist’s familiarity with the system or the speed of thenetwork. There is also the additional opportunity for the radiologist to adjust the image display, such as windowwidth and window level, and to zoom in on and roam across the image. All these operations necessitate additionalreporting time.26