67607 P1130021

1J Uterine structures in the marę 1.3.1 Non-pregnant uterus

A sagittal cross section of a physiologic, nonpregnant uterine hom is sonographically seen as round to oval image (Fig. 1.47 to 1.49). The uterine wali appears as a gray structure of modę ratę echogenicity. The peritoneal border of the uterus can be seen as a thin, hypoechoic linę. Concentrically positioned to this linę, another linę can occasionally be identified in the middle of the uterine wali (KAhn and Leidl 1985). This hypoechoic zonę represents the transition from endometrium to myo-metrium (Fig. 1.47 and 1.48). In the center of the uterus a smali area of high echogenicity can sometimes be found. It is caused by reflections from the echoic surface of opposing layers of the endometrium which lie in close apposition in the empty uterus and project as an echoic spot when seen in cross section or as anechoic linę when riewed longitudmally (Fig 1.6).

The structure of the uterine wali is homogenous dur-ing anestrus and diestrus (Fig. 1.47). A distinct layering cannot be recognized. Around the time of estrus a elear

distinction between areas of differing echogenicity can be observed in the transverse view of the uterus of many mares (Fig. 1.48 to 1.50).The ring like structure in the area between endo- and myometrium which lies con-centrically oriented to the outer, peritoncal wali of the uterus is now very prominent. From this zonę hypoechoic stripes run towards the center of the lumen where they meet to form a star (spoke-wheel) patiem. Hyperechoic segments lie between these areas of Iow echogenicity. This pattem is caused by the endometrial folds which buige towards the center of the uterine lumen and consist of an echoic base, the hypoechoic, strongly edematous adluminal part and the hyperechoic luminal epithelium. This wagon wheel pattem of the uterus is typical of estrus and can cleariy be seen in about 50% of all estrous mares (Ginther and Pierson 1984 c). It is associated with the estrogenisation during estrus and becomes apparent 6 to 10 days before ovula-tion (Hayes et al. 1985). The pattem is best developed about 1 to 4 days before ovulation and then gradually and progressively weakens until it disappears 2 to 6 days after ovulation.