Optimal equipment to scan the umbilical remnant in the foal is a high-frequency, 10–12 MHz linear probe, although a cursory examination can be performed with a transrectal, 5–7.5 MHz linear probe. Ideally, clip the hair with a #40 surgical clipper blade from the xiphoid caudal to the inguinal region and apply coupling gel. However, wetting the area with alcohol is often sufficient. The probe should be covered with an examination glove or similar to prevent damage to the probe from alcohol (Figure 27.1).

Although most foals tolerate this procedure without sedation, it may be required in some foals to minimize stress of the procedure. In the author’s experience, the best way to perform this examination is to stand beside the foal, with an assistant restraining the foal at the shoulder, next to the mare. One hand holds the probe while the other stretches the skin. The umbilical remnant can also be scanned with the foal in lateral recumbency. The ultrasonographer is behind the foal’s back with head and legs restrained by an assistant.

Orientation of the probe must be kept constant over time to obtain comparable images. In the longitudinal axis view, orientate the probe so the caudal abdomen is on the left side of the screen, and the proximal abdomen is on the right. In the transverse (short) axis view, orientate the probe so the right abdomen is on the left side of the screen and the left abdomen is on the right (Figure 27.2).

Structures imaged when scanning the umbilical area include the external umbilical remnant, umbilical arteries, urachus, and umbilical hernia if present. The umbilical remnant is located along the ventral midline of the abdomen (Figure 27.3), a few centimeters from the surface. To locate and recognize the different structures, the easiest way is to start with the cross-sectional view of the external umbilical remnant and move cranially to the xiphoid cartilage. Then, start at the external umbilical remnant again and move caudally to the pelvic rim. In the male, the presence of the penis makes scanning the most caudal part difficult. If an abnormal structure is suspected, a longitudinal scan should be performed. This scan plane is more difficult to visualize due to the tubular structure of the umbilical structures.

External Umbilical Remnant

The external umbilical remnant contains the umbilical vein, the two umbilical arteries, and the urachus (Figure 27.4). These structures normally atrophy within a few days after birth and detach from the skin.

Infection is the main pathology of the external umbilical remnant. Clinically, this pathology is characterized by a local swelling and a draining tract (Figure 27.5A). Sonographically, it is characterized by an enlarged external umbilical remnant filled with hypoechoic to echoic fluid (Figures 27.5B and 27.5C).

Umbilical Vein

The umbilical vein runs cranially from the umbilicus to the xiphoid cartilage, along the midline, very close to the skin surface (Figure 27.6A). When it reaches the xiphoid cartilage, it courses deep to reach the liver (Figure 27.6B). In the fetus, the umbilical vein transports blood from the placenta to the fetal liver. The umbilical vein is connected to the abdominal wall by a reflection of the peritoneum (Figure 27.7). It atrophies after birth and becomes the round ligament of the liver, part of the falciform ligament. The umbilical vein is an oval or elliptical structure with a thin echogenic wall and a hypoechoic center. Its diameter is 10 mm (or less) near the umbilicus (Figure 27.8A), but diameter decreases moving cranially (Figure 27.8B). It is located directly over the linea alba and this is used as the landmark to identify the vein. The vein can move laterally only a few centimeters from this landmark due to the peritoneal reflection. However, this slight lateral movement may occur when the scan is performed in lateral recumbency.

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