System and Method for Low X-Ray Dose Breast Density Evaluation

Abstract

A system and method for measuring volumetric breast density using a low dose of radiation are provided. This low-dose image can be added to a standard mammographic screening protocol with less than a two percent increase in radiation dose imparted to the subject. This low-dose image can also be used as a single standalone test to determine breast density for younger women for the purposes of risk determination or screening regimen planning. The breast density measurement is more accurate than measurements that can be obtained with existing systems and methods by making use of a compression assembly that maintains a parallel alignment between the compression paddle and breast support table. Additionally, the compression assembly maintains a uniform known thickness of the compressed breast. The system and method have the added benefit that they can be readily implemented on a conventional digital x-ray unit with low cost.

1 . A compression assembly for compressing a breast in use with a mammography system, comprising:
 a rigid plate;   at least one spacer coupled to the rigid plate and sized such that the spacer maintains the rigid plate in a substantially parallel alignment at a fixed distance from a breast support plate of an x-ray mammography system, thereby providing a region for receiving and compressing a breast at a substantially uniform thickness.   
     
     
         2 . The compression assembly as recited in  claim 1  in which the at least one spacer comprises at least two spacers and each of the at least two spacers is composed of a different material having x-ray attenuation characteristics equivalent to a known composition of breast tissue. 
     
     
         3 . The compression assembly as recited in  claim 1  in which a thickness and a material of the rigid plate are chosen such that x-rays transmitted through the rigid plate are hardened sufficient to decrease a dose delivered to the compressed breast. 
     
     
         4 . The compression assembly as recited in  claim 3  in which the rigid plate is at least about one centimeter thick. 
     
     
         5 . The compression assembly as recited in  claim 1  in which the at least one spacer has a cross section that is polygonal. 
     
     
         6 . The compression assembly as recited in  claim 5  in which the cross section is at least one of square, rectangular, and circular. 
     
     
         7 . The compression assembly as recited in  claim 1  in which the at least one spacer is composed of a material having a known relationship to x-ray attenuation characteristics of breast tissue of a known composition. 
     
     
         8 . The compression assembly as recited in  claim 7  in which the at least one spacer is composed of a material having a known x-ray attenuation. 
     
     
         9 . The compression assembly as recited in  claim 8  in which the known attenuation is selected as equivalent to at least one of fat tissue, fibroglandular tissue, and combinations thereof. 
     
     
         10 . The compression assembly as recited in  claim 1  in which the at least one spacer is composed of a plurality of different materials, each of the plurality of different materials having a known relationship to x-ray attenuation characteristics of breast tissue of a known composition. 
     
     
         11 . The compression assembly as recited in  claim 10  in which the at least one spacer is composed of a plurality of materials, each of the plurality of materials having a known x-ray attenuation. 
     
     
         12 . The compression assembly as recited in  claim 1  in which the rigid plate has a curved surface and the at least one spacer has a contact face that is curved so as to uniformly contact the curved surface of the rigid plate, thereby maintaining the rigid plate in a substantially parallel alignment at a fixed distance from a breast support plate of an x-ray mammography system. 
     
     
         13 . A method for measuring a density of a subject's breast with a mammography system, the steps of the method comprising:
 a) compressing the subject's breast to a defined uniform thickness using a rigid compression paddle that is parallel to a breast support plate and spaced apart from the breast support plate at a fixed distance by at least one spacer;   b) acquiring data with the mammography system by exposing the compressed breast to x-rays that traverse the rigid compression paddle before traversing the compressed breast and impinging upon an image plane of an x-ray detector;   c) determining attenuation values from the acquired data at all points in the image plane;   d) converting the determined attenuation values to mammographic density values;   e) estimating volumetric density content by:
 i) calculating a total volume of dense tissue by summing the mammographic density values across a total volume of the breast; and 
 ii) dividing the total volume of dense tissue by the total volume of the breast. 
   
     
     
         14 . The method as recited in  claim 13  in which step a) includes compressing the subject's breast using a lower compression force than conventional mammography. 
     
     
         15 . The method as recited in  claim 13  in which step b) includes operating the mammography system using a kV in a range of 25-50 kV so as to reduce dose imparted to the subject's breast. 
     
     
         16 . The method as recited in  claim 13  in which step e) includes estimating the total breast volume by determining a projected area of the breast and a contact area of the breast with the rigid compression paddle and the breast support plate. 
     
     
         17 . The method as recited in  claim 16  in which step e) includes using an assumed shape of a profile of the breast outside the contact area to estimate path length. 
     
     
         18 . The method as recited in  claim 13  in which step d) includes, for each point in the image plane, calculating a ratio of signal values at that point to an average signal value calculated from pixels in a region not associated with the breast. 
     
     
         19 . A compression assembly for compressing a breast in use with a mammography system, comprising:
 a rigid compression paddle;   a support plate;   a parallel linkage device coupled to the rigid compression paddle and configured to maintain the rigid compression paddle and the support plate in a parallel alignment; and   wherein the parallel linkage is configured to maintain the rigid compression paddle and the support plate at a substantially uniform separation distance when in a configuration for compressing a subject's breast.   
     
     
         20 . The compression assembly as recited in  claim 19  in which the parallel linkage is a straight line linkage.