text: Breast cancer and other breast lesions continue to be a significant threat to women's health. X-ray mammography currently is the most widely used tool for early detection and diagnosis, and is the modality approved by the U.S. Food and Drug Administration to screen for breast cancer in women who do not show symptoms of breast disease. Breast tomosynthesis is a more recently developed modality and is expected to become more widely used, for diagnosis and possibly as a screening tool. An even more recent development is multi-modality breast imaging systems that have both mammography and tomosynthesis capabilities and can provide either or both of mammograms and tomosynthesis images of a patient's breast, in the same or different immobilizations of the breast.
  A typical x-ray mammography system immobilizes a patient's breast on a breast platform that is between an x-ray source and an x-ray imaging receptor, and takes a projection x-ray image (called here a conventional mammogram or simply mammogram) using a collimated cone or pyramid or fan beam of x-rays at appropriate factors such as mA (current), kVp (voltage) or keV (energy), and msec (exposure time). In the United States, typically two views are taken of each breast, one from above (cranial-caudal, or CC, with the image plane generally at a 0° angle to the horizontal) and one from the side (mediolateral-oblique, or MLO, with the image plane at an angle of typically around 45° to the horizontal). Different typical views may be taken for other purposes or in other countries. The x-ray source typically is an x-ray tube operating at or in the neighborhood of 25-30 kVp, using a molybdenum, rhodium, or tungsten rotating anode with a focal spot of about 0.3 to 0.4 mm and, in some cases, 0.1 mm or less. An anti-scatter grid between the breast and the imager can be used to reduce the effects of x-ray scatter. The breast is compressed to reduce patient motion and also for reasons such as reducing scatter, separating overlapping structures in the breast, reducing the x-ray thickness of the imaged breast and making it more uniform, and providing more uniform x-ray exposure. Traditionally, the imager has been a film/screen unit in which the x-rays impinging on the screen generate light that exposes the film. In the last several years, mammography systems using electronic digital flat panel x-ray receptors have made significant inroads. A Selenia™ digital mammography system with such a digital flat panel x-ray receptor or imager is offered by Lorad, a division of the assignee hereof, Hologic, Inc. of Bedford, Mass. See brochure “Lorad Selenia™” Document B-BI-SEO US/Intl (5/06) copyright Hologic 2006. Digital mammography has significant advantages and in time may fully supplant film/screen systems.
  Digital tomosynthesis also has made advances and the assignee hereof has exhibited breast tomosynthesis systems at trade shows and has carried out clinical testing. It is a three-dimensional process in which several two-dimensional projection views are acquired at respective different angles but at lower x-ray dose each compared to a conventional mammogram, and are reconstructed into tomosynthesis slice views that can be along any desired plane in the breast and can represent any thickness of breast tissue. For tomosynthesis, the breast is still immobilized, by compression to the same or lesser extent than in conventional mammography. See, e.g., International Application WO 2006/058160 A2 published under the Patent Cooperation Treaty on Jun. 1, 2006 and Patent Application Publication No. 2001/0038681 A1 PCT application International Publication No. WO 03/020114 A2 published Mar. 13, 2003, U.S. Pat. Nos. 7,142,633, 6,885,724, 6,647,092, 6,289,235, 5,051,904, 5,359,637, and 4,496,557, and published patent applications US 2004/0109529 A1, US 2004/0066884 A1, US 2005/0105679 A1, US 20050129172A1, and Digital Clinical Reports, Tomosynthesis, GE Brochure 98-5493, 11/98. A tomosynthesis system specifically for imaging patients' breast is disclosed in commonly owned U.S. Pat. Nos. 7,123,684 and 7,245,694. The publications identified in this patent specification are hereby incorporated by reference herein.
  Further, the assignee hereof has developed multi-mode systems in which the same x-ray data acquisition equipment can be used for either or both of mammography and tomosynthesis imaging. A mammogram and tomosynthesis images can be acquired while the patient's breast remains immobilized, or they can be acquired at different times or patient's visits. One such system is known as Selenia Dimensions™ and another is known as Gemini™. See Smith, A., Fundamentals of Breast Tomosynthesis, White Paper, Hologic Inc., WP-00007, June 2008. Additional information regarding digital mammography, tomosynthesis and multi-mode systems offered by the common assignee can be found at <www.hologic.com>.
  When digital flat panel x-ray imaging receptors are used, one of the practical requirements is to provide gain calibration. The imaging receptor may comprise a two-dimensional array of millions of imaging pixels, and there may be inherent differences in the response of different imaging pixels to impinging x-rays. When all imaging pixels receive the same x-ray exposure, ideally each should provide the same electrical output signal (pixel value). However, in practice this may not be the case and typically there are differences between the pixel values that different imaging pixels provide when exposed to the same x-ray input. In addition, incident x-ray intensity across the detector surface usually is non-uniform; for example, due to the “heel effect” the x-ray intensity drops along the direction from the chest wall to the nipple. To correct for differences in pixel values in response to uniform x-ray exposure, and to correct for the non-uniform x-ray intensity distribution across the x-ray imaging detector surface area, various gain calibration and image correction techniques are employed. Typically, in conventional x-ray mammography the flat panel imager is exposed to an x-ray field through a “flat-field” phantom that simulates a patient's breast but has a uniform thickness and is made of a uniform material, the differences between pixel values are recorded, and a gain correction map is generated that accounts for such differences. This can be done periodically during the service life of the flat panel x-ray receptor. The gain map is stored in the imaging system and, when x-ray images of a patient's breast are taken, software in the system corrects the acquired pixel values according to the gain map to bring them closer to the pixel values that would have been produced if all the imaging pixels had the same response to uniform exposure to x-ray energy.
  For conventional mammography, usually one gain map is acquired for each viewing mode or x-ray filter mode. For use in this country, this may translate to one gain map for each of the CC and MLO views, for each of the filter modes, with possible consideration for the presence or the absence of an anti-scatter grid and for possible use of magnification. Gain calibration thus can be used to compensate for sensitivity differences between detector pixels and non-uniform x-ray field intensity given a particular physical relationship between the x-ray source and imaging detector. However, tomosynthesis imaging is characterized by a much greater number of changes in x-ray source projection angle during x-ray exposure, much lower x-ray exposure of the breast at any one of the projection angles, and other significant differences from conventional mammography imaging. As a result, gain maps typical for conventional mammography cannot be expected to work well in tomosynthesis image acquisition and image correction, particularly if the tomosynthesis projection angles may change depending on imaging protocol or decisions or preferences of the health professional in charge.