Ultrasound imaging of mammary glands in dairy heifers at different stages of growth.

To obtain B-mode ultrasound images of mammary glands in dairy heifers at different stages of growth, 25 clinically normal Holstein heifers were used. The heifers were divided into 5 groups (n=5/group) by stage of their growth: 2-month-old (group 1), 5-month-old (group 2), postpuberty (group 3), mid (group 4), and late (group 5) pregnancy. Furthermore, the sections of mammary glands were observed grossly at postmortem examination in one heifer in each group. Ultrasound images varied with the development of mammary glands. In group 1, the mammary glands had distinctive ultrasonographic findings: an oval to fusiform homogeneous hypoechoic structure. In all groups except group 1, mammary tissue consists of two major areas: a homogeneous, medium echogenic area and a poorly-defined, heterogeneous, hypoechoic area mostly in the superficial part. The superficial hypoechoic area spread more extensively and more irregularly with the development of mammary glands. Most pregnant heifers had irregular and extremely hypoechoic or anechoic areas like lactiferous sinus in the glands. The gross findings of mammary glands suggested that the hypoechoic areas of various shapes represented the lactiferous sinus and ducts. Thus, these results indicate that B-mode ultrasound imaging can visualize the internal structures of udders and could be a useful tool for evaluation of mammary glands in heifers.