Ultrasonography of the Fetus and the Neonate

I. Transabdominal Fetal Ultrasound

1. Overview: 
   In many cases, the goal of transabdominal ultrasound in the pregnant mare is to document and monitor placental abnormalities. Mares with placentitis can be placed on appropriate antimicrobial therapy and ultrasound used to monitor treatment response. Although transrectal ultrasound is the technique of choice to diagnose ascending placentitis at the cervical star, infection can occur throughout the placenta and often requires a combination of transabdominal and transrectal ultrasound for accurate diagnosis. A normal transrectal ultrasound should not preclude a transabdominal ultrasound exam.
   
   Ultrasound is also useful to evaluate fetal viability, ranging from assessing well-being and growth on serial exams to confirmation of fetal demise. Ultrasonographic confirmation of fetal death can be surprisingly difficult, especially if a long bone obscures visualization of the fetal heart. In such cases, prolonged absence of fetal activity throughout the exam may be interpreted as fetal death. Repeat assessment should always be performed and results considered in conjunction with other parameters such as fetal ECG and rectal palpation.
   
   Transabdominal ultrasound can also be used to document twins in mid-gestation. If desired, ultrasound guided procedures can be performed to assist with fetal reduction.
   
2. Indications:

1. Vaginal discharge
2. Premature lactation/mammary development
3. Systemic disease
4. Recent colic surgery
5. Abdominal growth greater than stage of gestation, i.e., concern for twins or hydrops
6. Previous complicated pregnancy

3. Technique:
   Patient:

1. Preparation - Clipping often necessary, especially in late gestation. Alcohol saturation is less optimal and is difficult to apply to ventral surfaces. Pressurized garden sprayer is useful for ventral saturation.
2. Positioning - Examiner should sit on same side as fetus to prevent shoulder fatigue.
3. Sedation - Often not necessary. If necessary, small doses of detomidine HCl (0.005-0.01 mg/kg IV)) can be used.

4. Equipment:
   

1. Transducer (fetal exam): 2-5 MHz curvilinear required
2. Transducer (placenta/fetal fluids): 4-6 MHz curvilinear (microconvex) is ideal. 5-10 MHz rectal linear or tendon format transducer may be adequate in thin mares or during mid-gestation.
3. Program: Select "deep" program for fetal exam (equine or large animal abdominal programs). For placental imaging, "superficial" program may be used if mare is thin or in mid-gestation. In late term, fat mares or in mares with significant ventral edema, a "deep" program may be necessary to image placenta and fetal fluids.
4. Scanning depth - 15-30 cm for fetal examination, depending on stage of gestation. 5-15 cm for placental/fetal fluid examination.

5. Procedure:
   

1. Initial goal - Locate fetus.

1. Place transducer on ventral abdomen in longitudinal plane with indicator oriented towards mare’s head.
2. Look for fetal ribs and rib shadows. Once fetus located, sit on same side of mare as fetus. Adjust depth to maximize fetal visualization.

2. Locate fetal heart - Determine fetal positioning

1. Most foals in late gestation are in cranial presentation. Heart will be seen in fetal thorax as probe is slid caudally on mare. Thorax tapers towards heart in cranial thoracic region (Fig. 1).
2. If in caudal presentation, fetal thorax will taper towards heart as probe is slid cranially on mare (Fig. 2).
3. Twins - Visualization of two separate fetal hearts and rib cages. Assessment most easily performed in mid gestation when fetuses are small and fetal fluids constitute larger percentage of uterus. In late gestation, assessment is difficult due to large fetal size. A singleton’s heart can be seen over a large region of the mare’s abdomen, leading to misinterpretation as 2 fetal hearts. In contrast, a 2nd fetus may be located dorsal to a visible fetus, out of range of the ultrasound machine’s maximal depth.

3. Fetal Heart Rates (HR):

1. Resting HR: Slide cursor into center of B-mode image of heart at any location and obtain M mode image. Image only needs to show 2-3 consecutive beats. Measure distance between heartbeats to calculate fetal HR with ultrasound machine software (Fig. 3).

i. Obtain 2-3 measurements

ii. Normal HR decreases with increasing fetal age:

iii. Month 6 of gestation: 113 ± 9.0 bpm [Bucca et al., 2005].

iv. Month 7-9 range: 112 ± 11.2 to 91 ± 6.6 bpm, respectively [Bucca et al., 2005].

v. Late gestation/month 12: 66.4 ± 6.5 bpm [Bucca et al., 2005]; 74.6 ± 7.4 bpm [Reef et al., 1995].

 vi. Persistently elevated or persistently depressed fetal heart rates have been associated with poor fetal outcome

vii. Hint: Utilizing ultrasound machine software to obtain HR values from M modes is more accurate than counting fetal heartbeats while scanning.

2. Activity HR - M mode acquisition during actual fetal movement is difficult. Heart rarely remains intersected by M mode cursor for sufficient length of time. Obtain immediate post-activity HR measurements instead (Fig. 4). Values should be elevated relative to resting HR. Hint: Obtain resting heart rates first, then continue with exam. Once fetal movement is detected, immediately return to heart to obtain "activity" HR.

4. Fetal Aorta:

1. Echogenic circle (Fig. 5) or 2 bright parallel lines (Fig. 2) located in center of heart.
2. Measure using "leading edge to leading edge" technique.
3. Aortic diameter increases with fetal growth and should be approximately 10% of thoracic diameter.
         i. Month 6-9 range: 10.28 ± 1.7 mm to 18.19 ± 1.6 mm [Bucca et al., 2005].
         ii. Late gestation: 22.8 ± 2.15 mm [Reef et al., 1995].
         iii. Measurement most closely associated with birth weight. Lower than expected aortic diameter has been correlated with low birth weight.

5. Thoracic diameter:

1. Maximal diameter obtained at fetal diaphragm
2. Diaphragm most visible in midgestation and becomes less apparent with fetal growth. Located at intersection of fetal lung and liver (Fig. 6). Fetal liver is less echogenic than fetal lung.
3. Measurements parallel diaphragm and extend beyond fetal ribs to include body wall. Measurements increase with gestational age.
4. Fig. 5: Circular appearance of aorta (arrowheads) located within center of fetal heart (arrows).

6. Allantoic and Amniotic Fluids - Evaluate vertical fluid depths to left and right of fetus. Locate and measure largest fluid pockets in cranial, mid and caudal quadrants on right and left sides, for a total of six quadrants.

1. Allantoic fluid:

i. Majority of visible fetal fluids. Located deep and adjacent to uteroplacental unit (Fig. 7).

ii. Measured fluid depths can vary substantially between quadrants. Seen in most quadrants, but may not be visible if fetus is laying directly on placenta (common in 1-2 quadrants).

iii. Maximal allantoic fluid depths: Late gestation: 13.4 ± 4.4 cm [Reef et al., 1995]

iv. Hippomane: Often visible within allantoic cavity. Is multilayered with a smooth elliptical shape (Fig. 8).

2. Amniotic fluid:

i. Differentiated from allantoic fluid by location deep to amniotic membrane (Fig. 9). Amniotic membrane has a thin smooth linear appearance and moves freely with fetal movement.

ii. Most easily seen in caudal quadrants. In other locations, amnion is often located adjacent to fetus, and amniotic fluid will not be visible.

iii. Maximal amniotic fluid depths:
         • Late gestation: 7.9 ± 3.5 cm [Reef et al., 1995].

3. Abnormalities/Variations:

i. Debris: Echogenic particulate material floating within amniotic and/or allantoic fluid is common in normal foals, especially during fetal movement.

ii. Herbie artifact (Fig. 9): Common artifact created by scanning through large quantities of fluid. Appearance differs between transducers and can be misinterpreted as debris.

iii. Hydrops amnion and hydrops allantois have been reported and are recognized by grossly increased fluid depths. With the exception of one case report, both conditions are associated with fetal or perinatal death and can result in significant physical/health risk to the mare

iv. Markedly decreased amniotic fluid depths have also been associated with fetal hypoxia and negative outcomes.

7. Uteroplacental Unit:

1. Located deep to body wall and retroperitoneal fat layer; adjacent and superficial to allantoic fluid (Fig. 10).
2. Evaluation of entire visible portion of uteroplacental unit should be performed and measurements obtained from representative sites in each of six quadrants, i.e., left cranial, mid and caudal and right cranial, mid and caudal.
3. Differentiation of layers of uteroplacental unit is not usually possible, and combined thickness measurements are used for assessment.
4. Normal appearance:

i. Combined thickness of uteroplacental unit (CTUP) can vary throughout gestation.

ii. Normal mid-gestation CTUP measurements range from 5-10 mm.

iii. In late gestation, CTUP tends to increase. Normal measurements range from 1.15 ± 0.24 cm [Reef et al., 1995].

iv. Mild undulation of placenta adjacent to allantoic space is within normal limits.

v. Non-fetal horn (Fig. 11): Has a thickened appearance with increased folding in normal mares. Often confused with placentitis.

8. Abnormal:

1. Placentitis: Increased uteroplacental thickness +/increased placental folding (Fig. 12). May appear edematous.
2. Placental separation: Creates linear anechoic space between uterus and placenta. Can be differentiated from placental blood vessels by rotating the transducer 90 degrees (Fig. 13). Small areas of separation have been seen in mares that delivered healthy foals. Large areas are considered significant.

Figure 1. Fetus in cranial presentation. Fetal thorax (arrows) tapers caudally within mare’s abdomen. H = heart, S = stomach, L = liver, Ab = fetal abdomen.

Figure 2. Fetus in caudal presentation. Fetal thorax (arrows) tapers cranially towards mare’s head. H = heart, Ao = aorta.

Figure 3. B mode image (top) showing cursor placement (arrowheads) within heart to obtain M mode image (bottom) in which 4 fetal heart beats can be seen (arrows). Heart rate calculated by ultrasound machine software after measuring distance between consecutive heart beats.

Figure 4. B/M mode image of fetal heart rate during activity. HR calculated from 2 consecutive heart beats (arrows) in M mode image. Heart not visible in B mode image due to fetal movement just before acquisition.

Figure 5. Circular appearance of aorta (arrowheads) located within center of fetal heart (arrows).

Figure 6. Fetal diaphragm (arrowheads) visualized between fetal liver (L) and lung. S = stomach, H = heart.

Figure 7. Allantoic fluid (Al) located deep and adjacent to uteroplacental unit (arrows). BW=body wall.

Figure 8. Hippomane (H) located within allantoic space (Al). Amniotic membrane (arrows) divides allantoic from amniotic (Amn) cavities. BW = body wall, CTUP = combined thickness of uteroplacental unit.  

Figure 9. Normal allantoic (Al) and amniotic (Amn) fluids separated by thin amniotic membrane (arrow). Note "Herbie" reverberation artifact (arrowheads) created by scanning through large fluid quantity.

Figure 10. Normal appearance of uteroplacental unit (arrows) located deep to body wall (BW) and retroperitoneal fat layer. Al = allantoic fluid.

Figure 11. Normal appearance of non-fetal horn (arrowheads) with its thickened appearance compared to the fetal horn (Fig. 7, Fig. 10).  

Figure 12. Abnormal appearance of uteroplacental unit showing increased thickness, placental edema and folding consistent with placentitis (arrows). BW = body wall, Al = allantoic fluid.

Figure 13. a) Longitudinal image of uteroplacental blood vessel (arrows). May be confused with placental separation. b) 90 degree rotation of transducer produces circular image, confirming blood vessel versus placental separation.

II. Umbilical Ultrasound

1. Overview:
   The value of umbilical remnant ultrasound is well established, as it can readily diagnose infection in foals without drainage from the umbilical stump or other palpable abnormalities. Additional sites of umbilical remnant infection can be detected in foals with visible stump abnormalities. Ultrasound can also identify the umbilicus as the source of infection in foals with sepsis, septic arthritis, pneumonia, etc.
   Ultrasonographic measurements of umbilical structures are helpful to identify abnormal structures; however, infection may be present despite measurements within reference ranges. Abnormal luminal contents, thickening of arterial, venous and urachal walls and peristructural thickening or inflammation are other important indicators of infection.
2. Anatomy:

1. Umbilical vein: Extends along ventral midline from umbilical stump to liver. Becomes falciform ligament.
2. Umbilical arteries: Extend caudally from umbilical stump to right and left of bladder in caudal inguinal region. Become round ligaments of the bladder.
3. Urachus: Extends from umbilical stump caudally to apex of bladder. Located between the right and left umbilical arteries.

3. Indications:

1. Palpable enlargement and/or purulent drainage from umbilical stump
2. Patent urachus
3. ADR, FUO (fever of unknown origin), hyperfibrinogenemia
4. Septic arthritis/tenosynovitis
5. Pneumonia

4. Technique
5. Patient:

1. Preparation: Clip 3-4” strip of hair from sternum to umbilical stump along ventral midline. Extend clipped area into right and left inguinal regions.
2. Positioning: Standing or lateral recumbency – both require adequate restraint
3. Sedation: Important for examiner safety.
   Butorphanol (DOSE mg/kg) added to Xylazine or Valium may help to decrease kicking.

6. Equipment:

1. Ideal transducer: 7-14 MHz linear ("tendon" probe)
2. Adequate transducer: Rectal linear
3. Useful transducer: 4-8 MHz curvilinear (microconvex). Small size of probe allows easier placement into inguinal region for evaluation of umbilical arteries.
4. Program: Select superficial program (i.e., tendon, musculoskeletal, vascular or small parts program name of program will vary by machine)
5. Scanning depth: 3-5 cm
6. Orientation: Transverse views of all structures are most useful and easiest to obtain compared to longitudinal views.

7. Umbilical Vein
   Procedure: Begin immediately cranial to umbilical stump, locate vein deep to linea alba (between rectus abdominus muscle bellies), follow vein cranially to liver along ventral midline. Measure the diameter cranial to umbilical stump, in midvein region and at liver.
   Normal appearance: (Fig. 14)

1. Diameter: cranial to stump = 0.61 ± 0.20 cm, midvein = 0.52 ± 0.19 cm, liver = 0.60 ± 0.19 cm (Reef & Collatos, 1988)
2. Thin walls with no to minimal anechoic luminal contents
3. Variation: Small amount of homogeneously hypoechoic contents can be present to 7-10 days of age (Fig. 15). Typically represents blood clot formation but can also be seen with early infection. Recheck in 3-7 days if remain suspicious for infection

8. Abnormal appearance: Omphalophlebitis

1. Increased diameter measurements
2. Thickened walls (Fig. 16)
3. Hyperechoic luminal contents (Fig. 17)
4. Increased accumulation of echogenic, especially heterogeneous, material within lumen.

9. Umbilical Stump - "ET View"
   Procedure: Place transducer immediately caudal to umbilical stump. Visualize urachus between right and left umbilical arteries (Fig. 18). Follow urachus to bladder apex.
   Normal appearance:

1. Combined diameters: 1.75 ± 0.37 cm (Reef & Collatos, 1988)
2. Thin walls of all structures. Minimal luminal contents.
3. Variation: Hypoechoic clot material within arteries is common to 7-10 days of age. Recheck if remain suspicious for infection.

10. Abnormal appearance:

1. Enlargement of entire structure or of individual structures
2. Thickened arterial and/or urachal walls. Periurachal or periarterial thickening (Fig. 19).
3. Hyperechoic luminal contents (Fig. 19).
4. Urachal abscess: Large accumulation of echogenic material within urachus (arrows, Fig. 20).
5. Patent urachus: Anechoic space within urachal lumen (Fig. 18). Patency may not be visible sonographically along length of urachus due to pressure from transducer

11. Umbilical Arteries Procedure: From "ET View", follow each umbilical artery individually to right and left of bladder as far caudally as possible. Adequate restraint/control of hind limbs is most important for this portion of exam.
    Normal appearance:

1. Diameter: 0.85 ± 0.21 cm (Reef & Collatos, 1988)
2. Thin walls with minimal to no luminal contents (Fig. 21)
3. Variation: Hypoechoic clot material may be present to 710 days of age (Fig. 22). Recheck in 3-7 days, if suspicious for infection.

12. Abnormal appearance: Omphaloarteritis

1. Increased diameter measurements
2. Hyperechoic luminal contents (Fig. 23)
3. Thickened arterial walls (Fig. 24)
4. Large accumulation of echogenic material within lumen is consistent with abscessation

Figure 14. Normal umbilical vein (arrows) located deep and adjacent to rectus abdominus (RA) muscle bellies near linea alba (LA).

Figure 15. Blood clot formation within umbilical vein (arrows) in a 3-day-old foal. Thin walls support diagnosis of hematoma versus infection.

Figure 16. Abnormal umbilical vein (arrows) with increased wall thickness and abnormal luminal contents in a foal with omphalophlebitis.

Figure 17. Abnormal umbilical vein (arrows) showing hyperechoic luminal contents in a foal with omphalophlebitis.

Figure 18. "ET" view showing left umbilical artery (LUA), right umbilical artery (RUA) and urachus (Ur) in a 4-day-old foal. Anechoic fluid within urachus is consistent with patency.

Figure 19. Marked periurachal/urachal inflammation with increased luminal contents of left and right umbilical arteries.

Figure 20. Urachal abscess (arrows) with left and right omphaloarteritis (hyperechoic luminal contents).

Figure 21. Normal umbilical artery (arrows) adjacent to bladder.

Figure 22. Hypoechoic luminal contents within left umbilical artery, consistent with blood clot formation.

Figure 23. Hyperechoic luminal contents within right umbilical artery, consistent with infection/omphaloarteritis. (Image courtesy of Dr. Olga Seco).

Figure 24. Hyperechoic luminal contents and increased wall thickness in a 19-day-old foal with omphaloarteritis of left umbilical artery.

III. Thoracic Ultrasound

1. Overview:
   Thoracic ultrasound is the imaging modality of choice for the ambulatory practitioner. A thorough exam can be easily performed in the field with a standard ultrasound machine and a rectal transducer. Interpretation is relatively straightforward compared to other regions and often helps to clarify radiographic findings. While aerated lung may obscure pathology deep within the lung, most abnormalities are visible on ultrasound.
2. Anatomy:
   The equine lung field generally extends from the 3rd to 15th ICS, from the level of the tuber coxae dorsally to the point of the elbow ventrally.
3. Indications:

1. ADR
2. Fever of unknown origin
3. Hyperfibrinogenemia
4. Cough
5. Nasal discharge
6. Abnormal thoracic radiographic findings

4. Technique
   Patient:

1. Preparation: Alcohol saturation. Clipping not necessary in most foals.
2. Position: Standing preferred
3. Sedation: Light, if necessary

5. Equipment:

1. Ideal transducer: 7-14 MHz linear (tendon), 5-10 MHz rectal linear
2. Useful transducer: 4-8 MHz curvilinear (microconvex) - Useful when increased penetration is necessary in foals with large areas of consolidation.
3. Program selection: Majority of lung field is located within 1-5 cm of skin surface, and therefore any "superficial" program can be used in foals. If severe pathology requires an increased depth of penetration, use "abdomen" type program (small animal abdomen, equine abdomen) to enhance penetration and visualization.
4. Scanning depth: 4-8 cm if no consolidation is present; 10-15 cm if have large areas of consolidation or severe effusion.

6. Procedure:
   Orient transducer parallel to ribs in last (17th) ICS, slide transducer from dorsal to ventral to view entire pleural surface, making sure to image ventral lung margin. Repeat technique in each rib space.
   Normal appearance (Fig. 25)

1. Pleural surface (hyperechoic line deep to intercostal muscles) should be smooth and glide freely with inspiration and expiration. Mild undulation as lung glides against ribs and intercostal muscles is normal.
2. Portion of image located deep to normal pleural surface is artifactual due to sound wave reflection by normally aerated lung.
3. Variation - Very small comet tails are normal, especially cranioventrally (Fig. 26)

7. Abnormal appearance:

1. Comet tails: Diffuse small comet tails are suspicious for infection - Recheck if clinical signs persist. Large comet tails are significant for infection and are often due to small areas of consolidation on pleural surface (Fig. 27).
2. Consolidation (Fig. 28, arrows): Cellular and/or fluid infiltrate allows penetration of ultrasound waves into pulmonary tissue. Consolidated lung is diffusely hypoechoic and may appear similar to liver ("hepatized lung"). Most commonly found in cranioventral lung field.
3. Pleural effusion: Relatively uncommon in foals. If present, will be found cranioventrally.
4. Abscessation: Most common in older foals (3-6 months) due to Rhodococcus equi. Uncommon in younger foals. Abscesses can appear similar to consolidation but are usually well demarcated (Fig. 29). Older abscesses may show encapsulation.
5. Any combination of these findings may be present in foals with severe pneumonia or pleuropneumonia.

Figure 25. Normal pleural surface (arrows). Fine comet tails as shown in this image are within normal limits. Significance should be considered in conjunction with other clinical and ultrasonographic findings.

Figure 26. Occasional small comet tails (arrows) are a common finding in normal foals.

Figure 27. Multiple large comet tails with small areas of consolidation (arrows) are consistent with pneumonia.

Figure 28. Cranioventral consolidation (arrows) in a foal with pneumonia. Note visible bronchial tree branching within the consolidated wedge of lung.

Figure 29. Well demarcated and encapsulated pulmonary abscess (arrows) in a 5-month-old foal with pneumonia due to Rhodococcus equi.

IV. Abdominal Ultrasound

1. Overview:
   The use of abdominal ultrasound continues to increase in foals and adult horses alike, as more practitioners recognize its value beyond the acute abdomen. This portion of the chapter will focus on the most common ultrasonographic abnormalities to affect foals, including gastric distention, duodenitis, enteritis, intussusception, ileus, ruptured bladder and abscessation. Potential abnormalities are numerous, and a complete examination should be performed whenever possible.
2. Indications:

1. ADR
2. Fever of unknown origin
3. Hyperfibrinogenemia
4. Colic
5. Bruxism
6. Abdominal distention

3. Technique:
   Patient:

1. Preparation: Alcohol saturation provides adequate visualization in most foals. Clipping the hair will improve image quality, especially in heavily coated foals.
2. Positioning: Exams should be performed in the standing position whenever possible. Small intestine abnormalities are common and are usually found on the ventrum due to their weight and dependent nature. Such abnormalities may be missed if foal is scanned in right, left or ventral recumbency.
3. Sedation: Light, if necessary

4. Equipment:

1. Ideal transducer: 4-8 MHz curvilinear (microconvex)
2. Adequate transducer: 2-5 MHz curvilinear (low frequency results in poorer resolution in neonates)
3. Adequate for some (superficial) abnormalities: 5-10 MHz rectal linear
4. Program selection: If using microconvex transducer, choose either small animal or equine abdominal program, depending on foal size.
5. Scanning depth: 5-15 cm, depending on structure being evaluated
6. Technique: Individual structures will be addressed below. For complete exam, evaluate entire paralumbar fossa/flank region, each ICS from ventral lung margin to costochondral junctions and entire ventrum from sternal to inguinal regions. Repeat for other side of abdomen.

5. Common Ultrasonographic Findings:
   Duodenitis: Descending duodenum is seen in its fixed location ventral to the right kidney in the right 15-17th ICS (Fig. 30) and deep to the right liver lobe in the right 11-15th ICS (Fig. 31). Affected foals show an increased wall thickness (>3 mm) and often present with clinical signs consistent with gastric ulceration. Distention and/or hypomotility may also be present. Gastroscopy can be negative in affected foals.
   Enteritis: (Fig. 32) Affected SI loops show an increased wall thickness (>3 mm) and will be found in the dependent portion of the abdomen due to their increased weight (ventral if standing). Hypomotility with some degree of distention is usually present.
   Intussusception: Nearly always found in dependent portion of abdomen (ventral if standing). Characteristic "target sign" formed by three layers of intussusception (Fig. 33). Distention and hypomotility may be seen proximal to intussusception (Fig. 34).
   Gastric distention: Stomach is left sided structure seen in mid-thoracic intercostal spaces. A severely distended stomach has a large radius of curvature and is often visible to the last (17th) ICS (Fig. 35). Fluid contents often visible ventrally.
   Abscessation: Generally seen in older foals (3-6 months). Abscesses may be filled with hypoechoic fluid contents and have a well-defined capsule (Fig. 36) or may be heterogeneous with poorly marginated borders (Fig. 37). Rhodococcus equi abscesses can be quite large.
   Ruptured Bladder: Most prominent feature is severe anechoic peritoneal effusion. A small spherical bladder may be seen due to residual urine contents. Bladder wall defect often not visible.

Figure 30. Location of descending duodenum (arrows) deep to right kidney in right caudal intercostal spaces. Wall thickness is increased (6-7 mm, arrowhead) in this foal with colic due to duodenitis.

Figure 31. Location of descending duodenum (arrows) deep to right liver lobe in right mid intercostal spaces. Note the markedly increased wall thickness (arrowhead, 8-9 mm) in this foal with chronic duodenitis.

Figure 32. Multiple thickened small intestine loops (arrowheads) found in the ventral abdomen in a foal with diarrhea and chronic enteritis.

Figure 33. Small intestinal intussusception located in the ventral abdomen. The inner intussusceptum (arrowheads) is surrounded by the outer intussuscipiens (arrows). (Image provided courtesy of Dr. Olga Seco, University of Pennsylvania).

Figure 34. Severe small intestinal distention and ileus in a 2-weekold foal secondary to distal jejunal obstruction.  

Figure 35. Gastric distention with increased ventral fluid accumulation. Note the large radius of curvature of the gastric wall (arrowheads) adjacent to the spleen.  

Figure 36. Large intra-abdominal Rhodococcus equi abscess located deep to the spleen. Note the homogeneous fluid contents and clear encapsulation compared to that in the similarly affected foal shown in Fig 37.  

Figure 37. Large intra-abdominal Rhodococcus equi abscess (arrows) visualized from the ventral abdomen. Note heterogeneous appearance of abscess contents and absence of visible encapsulation, compared to Fig. 36.

V. Musculoskeletal Imaging

1. Overview:
   Evaluation for septic arthritis/tenosynovitis and/or septic physitis is the most common indication for musculoskeletal imaging in foals. Findings are often complementary to radiographs and may result in an earlier diagnosis of infection. Ultrasound has also been shown to be useful and superior to radiographs to diagnose rib fractures in foals.
2. Technique:
   Clipping produces superior images. A high frequency linear transducer designed for musculoskeletal use is preferred. Depth settings: 3-8 cm, depending on structure being evaluated. Program selection: Superficial (musculoskeletal, tendon, small parts)
3. Ultrasonographic Findings:
   Normal fetal cartilage/physes: Depending on joint/region being evaluated, foals have prominent cartilage layers due to normal juvenile incomplete ossification. Cartilage has hypoechoic to anechoic appearance and multiple pinpoint echoes that can easily be mistaken for cellular synovial fluid (Fig. 38). In addition, the underlying subchondral bone is often irregular (Fig. 39) and can be misinterpreted as osteomyelitis. Normal physes show a" V" shape in the bony surface deep to the cartilage layer.
   Osteomyelitis: Affected bone may show a "fluffy" or thickened cortical surface with an overlying hypoechoic to anechoic layer (Fig. 40). Chronic infection shows obvious bony destruction (Fig. 41).
   Septic synovial structures: While severe effusion can cause concern for sepsis (Fig. 42), ultrasound more commonly reveals markedly thickened synovium with a lacey, edematous appearance (Fig. 43). Ultrasound can aid with synoviocentesis in such foals to confirm infection.
   Rib Fractures: Rib fractures occur with regularity in foals during parturition. While affected foals may appear clinically normal, fractures can penetrate vital structures, such as the lung and heart. Similar to other fractures, ultrasound will reveal step defects in the normally smooth bony surface (Fig. 44). Local hemorrhage and/or muscle tearing can be present.

Figure 38. Normal appearance of lateral stifle in a 1-day-old foal. Note the thick cartilage layer (arrows) with its speckled appearance and the "V" shaped appearance of the proximal tibial physis. LM=lateral meniscus.

Figure 39. Normal appearance of medial and lateral lobes (ML,LL) of the biceps tendon and proximal humeral tubercles in a 4 month-old foal. Note the thick cartilage layer of the intermediate tubercle (IT) and the normal irregular contour of the subchondral bone (arrows).

Figure 40. Osteomyelitis of the proximal scapula (supraspinous fossa) in a 6-week-old foal showing a thickened and fluffy periosteal/cortical surface (large arrows) with an overlying hypoechoic layer (arrowheads). Normal cortical surface is seen to the left (small arrows).

Figure 41. Chronic osteomyelitis of the proximal scapula (infraspinous fossa) in a 3-month-old foal due to Rhodococcus equi showing a large area of bony destruction (arrows) and a hypoechoic layer (arrowheads) overlying the abnormal bone.

Figure 42. Nonseptic navicular bursitis (NB) with anechoic effusion and minimal synovial thickening. Image obtained proximal to navicular bone with transducer placed transversely between heelbulbs and directed towards toe. (DDFT = deep digital flexor tendon, P2 = second phalanx).

Figure 43. Septic navicular bursitis (NB) in a 2-month-old foal showing severe synovial thickening and minimal fluid pocketing. Image obtained as in Fig. 42.  

Figure 44. Rib fracture with a large step defect in the normally smooth bony surface (arrows). (Image provided courtesy of Dr. Johanna Reimer, Lexington, KY).

VI. Echocardiography (Practical Approach)

See Also Echocardiography

1. Overview:
   This portion of the chapter will focus on congenital abnormalities most likely to be encountered in practice.
2. Indications:

1. Murmurs other than those consistent with patent ductus arteriosus
2. Foals with clinical signs of heart failure

3. Technique: Equipment:

1. Ideal transducer: 2-5 MHz phased array
2. Adequate transducer: 4-6 MHz curvilinear microconvex transducer in young foals. 3-5 MHz mechanical sector transducer can also be used, but detail resolution is typically suboptimal.
3. Program: Equine cardiac program. If not available, use small animal cardiac or abdominal program.
4. Depth: 10-20 cm

4. Procedure:

1. Right parasternal views obtained from right 4th ICS. Three long axis views (right outflow, left outflow, four chambers) and three transverse views (aortic valve, mitral valve, left ventricle) should be obtained.
2. Left parasternal views are especially useful if mitral abnormalities are suspected.
3. While the structural defects discussed below can be seen with B mode imaging, color Doppler is required to determine the size and extent of shunts or regurgitant jets.

5. Ventricular septal defects (VSD)

1. Most common congenital abnormality
2. Auscultation - Bilateral systolic murmur, usually loudest on right.

1. Right sided murmur created by left to right shunt through VSD
2. Left sided murmur loudest over pulmonic valve region (murmur of relative pulmonic stenosis).
3. Accurate assessment of left sided point of maximal intensity is important to differentiate VSD from mitral insufficiency, in which case murmur is loudest in mitral to aortic valve region.

3. Subaortic defects most common. Best seen on left outflow tract view (long axis) as defect in membranous portion of septum just below aortic valve (Fig. 45). On transverse views, best seen on aortic view as defect in aortic wall (Fig. 46).
4. Affected foals should have normal life expectancy if:

1. VSD measures less than 2.5 cm in its widest dimension
2. Peak shunt velocity is greater than 4 m/sec
3. No aortic insufficiency or additional congenital abnormalities.

5. Tetralogy of Fallot: Murmur will sound similar to VSD murmur, but left murmur is louder than right murmur. In addition to VSD, echocardiography will reveal pulmonic stenosis, overriding aorta and right ventricular hypertrophy.

6. Atrial septal defects:

1. Less commonly identified.
2. Best visualized on four chamber long axis view as defect in septum between left and right atria (Fig. 47).
3. Should be differentiated from patent foramen ovale. PFO is common in normal foals and is best seen on four chamber view. Two membranous layers of PFO will be seen to open and close within interatrial septum (Fig. 48).

Figure 45. Left outflow tract view obtained from right parasternal window showing most common location of subaortic ventricular septal defect (arrow). RV = right ventricle, RA = right atrium, LV = left ventricle, Ao = aorta.

Figure 46. Transverse view of aortic valve obtained from right parasternal window showing subaortic VSD (arrows) seen in Fig. 45.

Figure 47. Four chamber view obtained from left parasternal window showing atrial septal defect (arrow). LV = left ventricle, LA = left atrium, RV = right ventricle, RA = right atrium.

Figure 48. Four chamber view obtained from right parasternal window showing patent foramen ovale. Note the two visible layers of the foramen ovale (arrows).