iaea human health series publications - SEDIM

8.9. COLLIMATION SYSTEM8.9.1. Collimation assessment8.9.1.1. ScopeProper alignment of the edge of the compression paddle with the chest wall edge of the image receptor holderassembly is necessary for proper positioning and compression of the breast. If the edge of the compression paddleextends too far beyond the image receptor edge, the patient’s chest will be pushed away from the image receptorand some breast tissue will not be recorded in the image. If the edge of the compression paddle does not extend farenough, the breast tissue will not be properly pulled away from the chest wall and compressed for visualization inthe image, and a shadow of the vertical edge of the compression paddle will be visible in the image, possiblyobscuring clinical information.It is important to include as much of the breast tissue as possible in the mammogram to avoid missing thedetection of a cancer. Of necessity, there is some inactive region at the chest wall edge of the detector where breasttissue is excluded.— Objective: To determine the amount of breast tissue at the chest wall that is excluded from the image becauseof the imaging geometry or detector design; to ensure that the collimator allows for full coverage of the imagereceptor by the X ray field but does not allow radiation beyond the edges of the beam stop 27 except at the chestwall, and that the chest wall edge of the compression paddle aligns with the chest wall edge of the imagereceptor.— Minimum frequency: Annually and following service to, or replacement of, the X ray tube, collimator ordetector.8.9.1.2. Instrumentation(1) Two radiographic rulers (with 5 mm, or finer, markings and the ‘0’ point at the halfway point of the ruler), orone radiographic ruler and a coin.(2) Five phosphorescent screen pieces 28 , each approximately 20 mm × 50 mm.(3) Opaque material (e.g. metal foil) wide enough to cover the phosphorescent strips inside the acceptable zone.(4) Slabs of PMMA totalling 45 mm in thickness.8.9.1.3. MethodologyNote: If a CR digital mammography system is being tested, screen film (if available) or CR cassettes may beused for this test, and it is suggested that the method outlined in the report Quality AssuranceProgramme for Screen Film Mammography (IAEA Human Health Series No. 2 [34]) be observed.However, it should be noted that the objectives and tolerances defined in that report differ from thosethat apply for digital mammography. With that in mind, the methodology outlined here may bepreferred in all instances.(1) Create a ‘patient’ study with an appropriate name.(2) Tape four phosphorescent strips to the breast support, one on each edge, so that they overlap the edge of thebreast support. The long dimension should be perpendicular to the edge of the breast support (Fig. 34).(3) Tape a fifth phosphorescent strip in the middle of the X ray field on the breast support. This strip will providea check to ensure that the light generated by primary X ray interaction with the phosphor can be perceived.27‘Beam stop’ refers to the material below the image receptor that provides the effective barrier for the primary X ray beam.Normally, this is the breast support table.28The screens should be as ‘bright’ as possible and ideally should exhibit some persistence to facilitate observation.116