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1.3.3 Ultrasonography of the eąuine fetus

Afier Day 60 of pregnancy, significance of transrec-tal ultrasonography for tnerely diagnosing pregnancies in the horse decreases. At this stage the emphasis of the value of the ultrasound examination shifts towards feta! diagnostics. Through sonographic observation of the fe-tus in utero, the depiction of its body parts and organs as well as taking feta! measurements - the so called feto-metry fetal deyelopment can be monitored and as-sessed (Kahn and Leidl 1987 a). Important conclu-sions for abnormal pregnancies can be madę in this manner. Sizes determined by fetometry can be used to assess gestational age in cases were unccrtainty exists about the exact service datę. Transcutaneous ultrasonography through the dam’s ventral abdominal wali may prove useful in some cases in order to visualize the fetus (Adams-Brendemuehl and Pipers 1987).

1.3.3.1    Imaging of fetal organs

The ultrasonic appearance of the various organs of equine fetuses generally resembles that of bovine fe-tuses (See Chapter 2.4.1). In the latter species the ultrasonography of the fetus has been well studied. Below foDows a description of particularly characteristic ultrasonic images of equine fetuses.

133.1.1    Head, neck and spinał column

Prominent structures on the head of the eąuine fetus are the eye and the cranial cavity (Fig. 1.84). The vit-reous body of the eye is anechoic and surrounded by the hyperechoic orbit. In the anterior portion of the eye bali Arch shaped structures become visible. The two convex lines that lie opposite one another represent the front and back wali of the lens.

The roof of the scull and the basal portion of the brain cavity form a hyperechoic, oval outline that is a

few millimctcrs thick (Fig. 1.84). The oval outline of the brain cavity can be dcpicted until about the 8th month of gestation. Afier that the ossification of the boncs of the cranium has progresscd so far that only the bony parts in prońmity of the transducer can be imagcd.

The most apparent structure in the neck is the cervi-cal spine (Fig. 1.85). The ossification centcrs of the ver-tebral bodies and arches produce disc like cchocs. Duc to the absorption of the sound waves, ultrasonic shad-ows are seen beyond the vertebrae.

When the probe is positioned favorably the spine can also be detected by ultrasonography in the thoracic, lumbar, sacral and even coccygeal regions. Usually the typical double row of hyperechoic discs is apparent. Only the taił vertebrae produce a single row of solid echoes.

1.33.1.2 Thorax and heart

The conically tapering images of the two halves of the chest cavity are obvious in the thoracic region (Fig. 1.86). Similar to the vertebrae, the cross sectional images of the ribs produce echoic circles. Ultrasonic shad-ows lie behind the ribs that produce them.

The heart will be evident in the apex of the thorax (Fig. 1.86).The echoic wali of the heart surrounds the hypoechoic lumina of the ventricles and atria which are separated fforn one another by bright valves and septa. The action of the heart can be demonstrated ąuite im-pressively if the probe is oriented properly.

The basal heart ratę of eąuine fetuses is 150 to 190 beats per minutę in the 3rd month of gestation and decreases slowly as pregnancy progresses (Fig. 1.87). In Month 7 the basal heart ratę will vaiy from 100 to 110, and near term 60 to 80 beats per minutę (Colles and Parkes 1978, Pipers and Adams-Brendemuehl 1984, Matsui et al. 1985, Adams-Brendemuehl and Pipers 1987). Generally, however, the heart ratę is rather var-iable and can rise well above the basal values during short periods of observation.