Gayle D. Hallowell, MA, VetMB, CertVA, DACVIM, MRCVS
Improved, more affordable, mobile ultrasound equipment has never made it easier to use ultrasonography as a diagnostic tool in food animal patients. In addition, although full abdominal examinations are not possible with 10 MHz linear transrectal probes, information on specific abnormal organs can be gained, particularly regarding ruminal and reticular wall thickness and abscessation and for identification of appropriate areas for organ biopsy. When examining gastro-intestinal viscera, motility (patterns) and wall thickness can be assessed. In equine abdominal ultrasound there has been interest in both of these areas, whereas until now the focus in food animals has been on gastro-intestinal motility patterns. These are obviously extremely important and provide valuable information, but are not always as easy to perform in the field and can be very time-consuming. Changes in gastro-intestinal wall thickness occur frequently with many common ruminant diseases and can help with diagnosis and formation of appropriate diagnostic plans as well as potentially monitoring response to therapy.
Ultrasound is a valuable tool in animals that present with pyrexia, 'not doing right' or with unexplained weight loss or colic. It is also an adjunctive tool for organ biopsies and for sampling masses. It should however, be used in conjunction with history, physical examination, rectal examination, haematology and biochemistry, and other appropriate diagnostic tests.
What Abdominal Ultrasonography is Not
Transabdominal ultrasonography should not be used as a 'fishing device' or screening test that replaces other diagnostic modalities. In some ways it is very sensitive in diagnosing abnormalities within the abdomen; however, interpretation of these findings is difficult without other information. On the other hand, abnormalities that do not lie peripherally can easily be missed, particularly in adult cattle or if the abnormality lies within or beneath a gas-filled viscous.
1. Develop a system for ultrasonographic examination of the abdomen or of an individual organ and stick to it so that you don't miss things. A logical examination order includes starting the examination caudally and moving cranially on the left, then examining the ventral abdomen and then repeating the same procedure on the right. Examination per rectum and transrectal ultrasound are both integral parts of this investigation.
2. Choose the most appropriate probe and frequency for what you wish to examine. In order to maximise visualisation of organs beneath the ribs, a convex, microconvex or phased-array probe is a logical choice. Linear or microconvex probes are most useful for ultrasonographic examination per rectum. For examination of the rumen, reticulum and caecum, frequencies of 10-7.5 MHz are ideal whereas examination of the spleen, liver, small intestine and when examining deeper structures 5-2.5 MHz are required. The lower frequencies (2.5MHz) are more likely to be required in large cattle where the musculature of the body wall is significant, whereas 5MHz is often ideal for examination of most structures in sheep and goats. Never underestimate, however, what you can see with 10MHz linear probe if that is all you have available to you.
3. Prepare the area of skin overlying the area of interest appropriately. In an ideal world all hair or wool would be removed, the skin would be cleaned and coupling gel would be applied. Done properly this can take as long or longer than the ultrasonographic examination itself. An alternative, which may not give such superior images, but is more practical in field situations includes soaking the skin with water or alcohol and using a coupling gel on the probe or parting the wool/ fibre and using alcohol or gel directly on the skin
4. Start the examination dorsally in the paralumbar fossa and move ventrally angling the probe cranially and caudally in order to maximise the area of visualisation. Don't forget you will constantly need to alter machine settings--gain and depth particularly in order to maximise the information that can be obtained from the ultrasound scan.
In order to fully appreciate and interpret ultrasonographic images well, an understanding of the abdominal anatomy is required. Some of the published anatomical images of the ruminant abdomen are inaccurate when compared to ultrasonographic findings--this may be due to organ displacement post-mortem or changing structure location when animals are lying in dorsal or lateral recumbency compared to the living standing animal. In addition, air within viscera will obscure views of organs even if they are not that far from the ultrasound probe and this should be taken into account. If normal ultrasonographic anatomy cannot be appreciated, it will not be possible to interpret and thus diagnose abnormalities.
As would be expected, the rumen and reticulum make up the majority of the ultrasonographic anatomy of the left side of the abdomen, with the spleen also visible overlying the rumen in the mid abdominal area. Although the rumen is split physiologically into layers with gas, fluid and more solid ingesta, ultrasonographically there is enough gas throughout its structure not to be able to view anything but the wall in the near field. The same is true of the normal appearance of the reticulum. The rumen lies from the paralumbar fossa to the 8-9th rib space and the reticulum in the 5-7th rib space with the ventral blind sac of the rumen between these two structures (7th to 8th). The rumenal groove lies approximately equidistant between the dorsal and ventral body walls. The spleen lies between the body wall and the spleen between the 11th and 9th rib spaces in the dorsal half of the abdomen--thus being larger than is usually described anatomically or seen at post-mortem. As you move further forward, more and more of the abdominal organs are obscured by the lung surface, another interference to abdominal organ visualization.
Ventrally the rumen lies to the left of the midline, with a small amount of abomasum to the left of the udder. Abomasum and small intestine lie ventrally to the right of the midline, with some large intestine visible caudally.
On the right, large intestine is visible in the paralumbar fossa, with small intestine in the most ventral segment. This is the most reliable place to find small intestine, although it is sometimes obscured by gas-filled large intestine. Kidney is visible just caudal to or beneath the liver between the 12th-10th rib spaces. Liver is visible in the 12th-7th rib spaces with the gallbladder most commonly seen in the 11th and 10th rib spaces. Omasum lies beneath the liver from the 11th-7th rib spaces and small intestine can be visualised ventrally from the paralumbar fossa to the 9th rib space.
Sheep and Goats
As in cattle, the rumen and reticulum make up the majority of the ultrasonographic anatomy of the left side of the abdomen, with the spleen also visible. The sheep and goat have similar, although not identical ultrasonographic appearance of organs within the abdomen. The rumen lies from the paralumbar fossa to the 8th rib space and the reticulum in the 5-6th rib space with the ventral blind sac of the rumen between these two structures (6th to 8th). The rumenal groove lies in a similar anatomical position to that reported in cattle, but is less large and obvious. The spleen lies between the body wall and the spleen from the paralumbar fossa to the 10th rib spaces in the dorsal half of the abdomen; again this is more extensive than reported in anatomy texts.
Ventrally the rumen lies to the left of the midline. The omasum lies in the cranial right ventral abdomen. Small and some large intestine lie in the right caudolateral ventral abdomen and the abomasum lies in the right caudomedial ventral abdomen.
On the right side of the abdomen, the left kidney lies in the dorsal region of the paralumbar fossa and the right kidney in the dorsal region beneath the 11th and 12th rib spaces. Large intestine is visible in the paralumbar fossa to the 8th rib space, with small intestine in the ventral sections of these rib spaces. Small intestine is much easier to find in small ruminants than in cattle and will be found on any ultrasonographic examination of the abdomen. Liver is visible in the 12th-6th rib spaces with the gallbladder most commonly seen in the 11th rib space. Omasum lies beneath the liver from the 11th-7th rib spaces.
Individual Abdominal Structures
There have been several studies investigating rumenal motility in cattle using ultrasound. However, there have been no ultrasonographic studies looking at normal wall thickness and how it changes with disease. This is surprising as it is a commonly used technique to diagnose intestinal disease in small animals and horses. In addition, it is known that common gastro-intestinal diseases in ruminants lead to pathological thickening of the affected area. To the author's knowledge, no work has been published investigating the ultrasonographic appearance of the gastro-intestinal system in sheep and goats.
Studies (Potter and Hallowell--unpublished observations) have shown that the rumen of adult dairy cattle is approximately 0.5-0.6 cm thick except at the area of the rumenal groove where it may be up to 1-1.2cm in thickness. Pathological diseases such as rumenal acidosis lead to increased wall thickening. Unpublished data in sheep and goats (Hallowell and Potter) has shown that rumen wall thickness is similar between the two species and is approximately 0.2-0.35 cm and the rumenal groove between 0.5-0.75 cm in diameter.
Braun and colleagues have performed various studies1-4 investigating normal motility of the reticulum in dairy cattle and evaluated changes in these parameters in cattle with traumatic reticuloperitonitis. Motility studies are time-consuming and require patience. It has been shown that the reticulum has three biphasic contractions of which the first is incomplete. These occur over a three-minute period. Velocities and length of contraction are available in the original papers but are beyond the scope of these notes. In traumatic reticuloperitonitis there is a reduction or abolishment of reticular contractions, with a reduction in amplitude and increased duration of the length of contraction. A change in reticular contour is reported by this group in traumatic reticulitis cases, but actual wall dimensions were not reported. Normal reticular wall thickness is approximately 0.4-0.55 cm in diameter in adult cattle and 0.2-0.35 cm in adult sheep and goats (Potter and Hallowell and Hallowell and Potter, unpublished observations).
This is easily visualised in ruminants beneath the liver. It has a very smooth edge with a thick, hyperechoic edge associated with the gas cap. Omasal wall thickness measured in cattle is approximately 0.3-0.45 cm in adult dairy cattle and 0.25-0.35 cm in sheep and goats (Potter and Hallowell and Hallowell and Potter, unpublished observations).
Due to the frequency of abomasal disease, more ultrasonographic studies investigating size, position and contents have been performed.4-6 It is described in these papers as having a thin wall. The abomasum lies to the right of the midline and caudal to the reticulum.6 In cattle with left displaced abomasum, the abomasum was visible between the body wall and the rumen. In this abnormal position, a hyperechoic line beneath the wall can be seen indicative of gas or fluid highlighting the folds may be visible. It can be distinguished from rumen by its much thinner wall.5 Abomasal motility is not possible to measure. In adult dairy cattle it has been shown that the abomasal wall thickness was approximately 0.2-0.25 cm (Potter and Hallowell, unpublished observations) and in sheep and goats approximately 0.15-0.25 cm (Hallowell and Potter, unpublished observations).
Diseases of the small intestine are common in ruminants. Normal small intestinal wall thickness in cattle was found to be approximately 0.25-0.35 cm in cattle and 0.2-0.3 cm in sheep and goats (Potter and Hallowell and Hallowell and Potter, unpublished observations). Normal diameter of small intestine varies between 1 and 5 cm in adult dairy cattle7 and 1 to 2.5cm in sheep and goats (Hallowell and Potter, unpublished observations). Subjectively, more collapsed loops of small intestine were seen in cattle than in sheep and goats. Thickening of the small intestinal wall is seen with inflammation and neoplastic infiltrates such as intestinal lymphoma.
Ultrasonographically it is not possible to categorically distinguish between different areas of the large intestine. The caecum subjectively has more haustrations than other areas of the large intestine, which can be appreciated ultrasonographically, particularly in small ruminants. The large intestinal wall thickness of adult dairy cattle was found to be approximately 0.3-0.5 cm and in sheep and goats 0.15-0.25 cm (Potter and Hallowell and Hallowell and Potter, unpublished observations). With caecal dilation and torsion, it has been shown that it often contains fluid rather than gas and thus diameter can be assessed.8
The liver is less homogenous and more hypoechoic than the spleen. Liver disease is of great importance in ruminant medicine. Metabolic disturbances can cause fatty degeneration, diffuse changes in liver texture, organ enlargement and rounding of the organ margins. Abnormalities seen include liver abscessation often leading to hypo- or hyperechoic variable-shaped masses and biliary obstruction. Biliary dilation is a qualitative assessment as it depends where visualised and also varies between animals and on time of day. It has been suggested that gall bladder enlargement should be suspected if its circumference exceeds 30cm in cattle and 9cm in sheep.
The spleen is a homogeneous organ that contains small regular anechoic blood vessels. It is approximately 2-3cm in depth. Splenic torsion causes the viscus to be enlarged and more heterogeneous due to distended vessels. Spleens with splenic abscessation often secondary to wires also appear heterogeneous with an increased number of an- or hypoechoic pockets.
In cattle the hypoechoic renal parenchyma is slightly less echogenic than liver parenchyma and can be easily differentiated from the hyperechogenic renal sinus. The renal pyramids are seen as hypoechoic areas in the renal parenchyma. The vertical diameter of the kidney has been reported as around 5cm whilst the horizontal diameter is in the region of 10cm.9 In sheep the surface of the kidney appears smooth, with the capsule being seen as a white hyperechogenic band. The hyperechogenic renal sinus can be easily differentiated from the surrounding hypoechogenic cortex and medulla.
In conclusion, abdominal ultrasound is an easily performed, non-invasive, practical technique as a diagnostic aid for a variety of conditions in ruminants. The key to interpretation is an appreciation of what is normal in terms of normal appearance, wall thickness and vessel size, motility (GI) and location.
1. Braun U, Gotz M, Marmier O. Vet Rec 1993;133:416-422.
2. Braun U, Gotz M. Am J Vet Res 1994;55:325-332.
3. Braun U, Iselin U, Lischer C, et al. Vet Rec 1998;142:184-189.
4. Braun U. Vet J 2003;166:112-124.
5. Braun U, Pusterla N, Schonmann M. Vet Rec 1997;141:331-335.
6. Braun U, Wild K, Guscetti F. Vet Rec 1997;140:93-98.
7. Braun U, Marmier O. Vet Rec 1995;136:239-244.
8. Braun U, Amrein E, Koller U, et al. Vet Rec 2002;150:75-79.
9. Braun U. Am J Vet Res 1991;52:1933-1939.