Evaluation of the minimum iodine concentration for contrast-enhanced subtraction mammography

Early manifestation of breast cancer is often very subtle and is displayed in a complex and variable pattern of normal anatomy that may obscure the disease. The use of dual-energy techniques, that can remove the structural noise, and contrast media, that enhance the region surrounding the tumour, could help us to improve the detectability of the lesions. The aim of this work is to investigate the use of an iodine-based contrast medium in mammography with two different double exposure techniques: K-edge subtraction mammography and temporal subtraction mammography. Both techniques have been investigated by using an ideal source, like monochromatic beams produced at a synchrotron radiation facility and a clinical digital mammography system. A dedicated three-component phantom containing cavities filled with different iodine concentrations has been developed and used for measurements. For each technique, information about the minimum iodine concentration, which provides a significant enhancement of the detectability of the pathology by minimizing the risk due to high dose and high concentration of contrast medium, has been obtained. In particular, for cavities of 5 and 8 mm in diameter filled with iodine solutions, the minimum concentration needed to obtain a contrast-to-noise ratio of 5 with a mean glandular dose of 2 mGy has been calculated. The minimum concentrations estimated with monochromatic beams and K-edge subtraction mammography are 0.9 mg ml-1 and 1.34 mg ml-1 for the biggest and smallest details, respectively, while for temporal subtraction mammography they are 0.84 mg ml-1 and 1.31 mg ml-1. With the conventional clinical system the minimum concentrations for the K-edge subtraction mammography are 4.13 mg ml-1 (8 mm diameter) and 5.75 mg ml-1 (5 mm diameter), while for the temporal subtraction mammography they are 1.01 mg ml-1 (8 mm diameter) and 1.57 mg ml-1 (5 mm diameter).