In breast tomosynthesis as well as in conventional mammography and in fusion systems that can selectively carry out either or both types of imaging, a source support arm carries an x-ray source and rotates or otherwise moves from one imaging position to another. In tomosynthesis, the patient's breast remains immobilized while the x-ray source moves to allow taking a number of projection x-ray images at respective angles of the x-ray beam to the breast. The x-ray imaging receptor may or may not move in this process, depending on the design of the system. The projection x-ray images are computer-processed to reconstruct values for voxels (volume elements) that are in a three-dimensional arrangement in the breast. The set of voxel values is further computer-processed to derive tomosynthesis slice images, each representing a respective slice of the breast that has a selected orientation and a selected thickness that typically is made up of several voxels. In conventional mammography, typically only one x-ray image is taken while the patient's breast remains immobilized but another image may be taken after releasing the breast, rotating the x-ray source and the imaging x-ray receptor to a new angle relative to the breast and again immobilizing the breast. For example, in screening mammography in this country, typically two images are taken of each breast, a CC image and an MLO image. In addition, a scout shot may be taken before the imaging shots, to help select appropriate AEC (Automatic Exposure Control) settings. In a fusion system, a conventional mammogram may be taken in addition to projection tomosynthesis x-ray images while the breast remains immobilized. Alternatively, a fusion system may be used to take only one or more conventional mammography images of a breast or to take only tomosynthesis x-ray projection images (each of which requires a much lower x-ray dose than a conventional mammography x-ray image).
A number of conflicting requirements and challenges need to be addressed in the design and operation of breast x-ray image system. One is to promote image quality and patient flow. Another is to ensure patient comfort with moving system components, which means not only preventing collisions with moving components but also providing the patient with psychological comfort against concern with components that move close to the patient's body and particularly the patient's head. Yet another is to guard against the possibility that the patient's hands, arms or head may enter the x-ray field at inappropriate times. Still another is to provide the x-ray technologist with convenient and effective access to the patient's breast and chest wall tissue before and while the breast is being compressed between a breast platform and a compression paddle. The technologist's work in this respect is vital, both to the reduce patient's discomfort and to ensure image quality, because typically it is necessary to pull patient tissue away from the chest wall and into the x-ray field and also to make sure that the compressed breast is as uniform in thickness as practical, for reasons such as reducing overlap of tissue of interest along the direction of the x-ray beam, reducing scatter of x-ray during passage through the breast, and making the overall density of the image more uniform.
Patient shields have been used in an effort to meet some of these and other concerns in breast imaging. For example, commonly assigned U.S. Pat. No. 7,245,694 describes a fused mammography/tomosynthesis system that takes tomosynthesis projection x-ray images while the x-ray source moves through a trajectory that is generally to one side of the patient's head rather than symmetrically to both sides. A patient shield separates the patient's head from the x-ray source trajectory and moves to one position when the trajectory is to the right of the patient's head and to another position when the source trajectory is to the left of the patient's head. This improves over earlier proposals for shields that do not move relative to the x-ray source. U.S. Pat. No. 7,315,607 proposes a face shield that retracts up into the x-ray emitter head or extends down from the head for imaging. This is said to improve over prior art shields that were manually slipped over the x-ray emitter to prevent the patient's head from entering the x-ray beam path, and to have allowed the use of different shields for different examinations. However, it is not practical for tomosynthesis, as the shield that is attached to the x-ray emitter head would have to move as the x-ray source moves to different positions relative to the patient during tomosynthesis imaging. U.S. Pat. No. 6,999,554 proposes a shield behind which the x-ray source would rotate. Published patent application US2005/0078797 A1 proposes yet another shield. The patents and other publications identified above and below in this patent specification are hereby incorporated by reference herein.