Ultrasound can detect nonpalpable lesions in the mammary gland of transgenic mice. (A) Fourth mammary gland ultrasound image from a control wild-type mouse illustrating the homogeneous texture of the mammary gland tissue that surrounds the centrally located lymph node (*) with lines indicating the border of the #4 mammary gland. (B) Ultrasound image of the left #4 mammary gland of a tTA/TAg/ERα mouse with nonpalpable mammary lesions (thick arrows). The dense (less echogenic) tissue represents an area of increased cell number in the tTA/TAg/ERα mammary gland and correlates with the presence of hyperplastic alveolar nodules (HANs) identified after carmine-alum whole mount staining of the same mammary gland at necropsy (C). Image: 0.5×, insert: 4×. Thick arrows: HANs, white (ultrasound) or black (whole mount), thin white arrows: skin surface, *: lymph node. Scale for all ultrasound images: in mm. (D) Correlation between 2-D ultrasound and 3-D in situ measurements of mammary preneoplasias (n = 7 mice); R = 0.96, p < 0.001.