Uses of ultrasound in equine internal medicine (Proceedings)

Ultrasound is an extremely useful tool to aid in diagnosis and treatment of a wide variety of diseases in equine medicine.  Trans-rectal ultrasound has long been used in equine reproductive medicine for assessment of uterine health, timing of ovulation and breeding, pregnancy diagnosis, and assessment of fetal well-being.  Ultrasound is also well known for its usefulness in diagnosis and prognosis in equine tendon and joint injuries.  However, the focus of this talk will be on the use of ultrasound in modern equine internal medicine.

Formation of an ultrasound wave

Sound is produced by rarefactions and compressions of molecules in air.  Ultrasound waves are simply very low frequency sound waves (well below the range of human hearing).  Ultrasound waves are produced by a "transducer", ie something that transduces electrical energy into mechanical energy.  Transducers are made of piezoelectric crystals which contain dipoles, particles that have a positive charge on one end and a negative charge on the other end.  In the natural state, these dipoles are dispersed randomly throughout the crystal, but if the crystal is heated slightly (which allows the dipoles to move), they can be aligned by passing an electric charge across them. 

If the crystal is cooled while the charge is maintained, the dipoles are trapped in alignment.  Thus, one side of the crystal is slightly positive, while the other side is slightly negative.  The crystal can be made to vibrate (and produce mechanical rarefactions and compressions in the surrounding air or tissue = sound waves) by passing an alternating electric current across it.  The frequency of the vibrations can be controlled by the rate at which the current alternates.

How does the ultrasound machine make a picture?

The different tissues and organs in the body have different densities and sound travels through them at slightly different rates. When the sound produced by the ultrasound transducer encounters a new tissue or organ, some of the sound wave continues onward, deeper into the body, but some of it is reflected back towards the transducer.  These reflected waves are detected by the transducer and are necessary to produce the image, sort of like sonar.  However, if the difference in speed of sound is very large, too much of the ultrasound wave will be reflected, and the rest of the wave will be unable to penetrate the tissue.

When this happens, the ultrasonographer is unable to visualize any tissue deep to the reflective barrier. In the body, this occurs whenever the sound wave encounters air (for example air in the lungs or gas in the bowel) or bone. As ultrasound waves pass deeper into tissue, they are attenuated by a combination of absorption, reflection, scatter and refraction.  The degree of attenuation is also influenced by tissue type and the frequency of the transducer. In general, lower frequency waves can penetrate deeper into tissue.

Resolution is the ability of ultrasound to differentiate between two closely spaced masses, rather than interpreting them as one, and includes both axial resolution and lateral resolution.  Axial resolution refers to differentiating between two points that are in line with each other longitudinally, whereas lateral resolution refers to differentiating between two points that are side by side with each other.

Axial resolution is affected by transducer frequency and number of wavelengths per pulse; lateral resolution is affected by beam width, frequency, and focusing.  In general, higher frequency transducers give better resolution. However, the trade-off is that higher frequency transducers do not penetrate as deeply into tissue as lower frequency transducers.  Thus, one usually picks the highest frequency available, provided that it still has enough power to image as deeply as needed to image the tissue of interest.

Tissues of interest in equine internal medicine diagnostic ultrasonography:

Lumps and Bumps

Ultrasound is useful to investigate lumps that appear on the body surface.  The echogenic appearance can help the clinician differentiate between an abscess, hematoma, edema, cellulitis, bone spur, neoplasia, etc.  Ultrasound is also helpful as a guide when aspirating a lump or swelling for cells or culture, or when performing a biopsy.

Abdominal ultrasound

Abdominal ultrasound allows visualization of organs that cannot be palpated rectally (eg right kidney, liver), as well as parts of the gastrointestinal tract, some of which can be palpated rectally, and some of which cannot. Ultrasound can be used to look for evidence of peritoneal effusion, abdominal tumors, sand in the GI tract, etc. It can also be used to guide abdominocentesis or biopsy.

Ultrasound of the kidneys may be useful in the diagnosis of congenital anomalies (renal cysts, renal ectopia, renal hypoplasia, renal agenesis), acute nephrosis, chronic renal disease, pyelonephritis, neoplasia (adenocarcinoma, lymphosarcoma), and renal calculi (stones). Ultrasound can detect perirenal edema or a distended renal pelvis or ureter.  The renal capsule can be imaged to determine whether it is smooth or irregular.

The right kidney can be found high in the right paralumbar fossa or just under the last ribs, and is typically 15-18 cm wide by 13-15 cm long. The left kidney is found in the left paralumbar fossa and lies deep to the spleen. It is typically 15-18 cm long by 11-15 cm wide. Increased size may be indicative of renal swelling in acute disease, whereas decreased size may support a diagnosis of chronic renal disease. The normal renal cortex is slightly more echogenic than the medulla and should measure approximately 1-2 cm thick.

Poor ability to differentiate between the cortex and medulla may indicate loss of renal architecture due to chronic renal disease, or may occur in large or fat horses if the transducer is not powerful enough to give a good image. The renal pelvis is usually lined with a bright echogenic line. This is due to the presence of fat, and also may be due to calcium carbonate crystals in the urine. Renal calculi are hyperechoic and cast a shadow. They are often accompanied by distention of the renal pelvis. The bladder can be imaged both trans-abdominally and rectally.  Abnormalities of the bladder that can be visualized with ultrasound include a thickened bladder wall, bladder stones or crystals, or ruptured bladder. 

Ultrasound of the liver provides information on the size of the liver, fibrosis, abscessation, cholelithiasis, congestion, fat accumulation, and neoplasia.  It also helps determine the best place to perform a percutaneous biopsy.  While it is not really possible to measure the actual size of the liver because much of it is hidden behind the lungs, one can still get an idea of whether the liver is atrophied or swollen, based on how much liver is visible at each rib space, just ventral to the edge of the lung field. The liver should appear as homogeneous tissue with hepatic veins and portal vessels running through it. Portal vessels can be differentiated from hepatic veins because they have thicker, more echogenic walls.

The spleen is a very homogeneous, relatively echogenic organ found on the left side of the abdomen. Abnormalities of the spleen that can be diagnosed by ultrasonography include abscessation and neoplasia. The position and size of the spleen also provide useful information, particularly when examining horses with colic. The spleen is displaced medially and the left kidney cannot be imaged deep to the spleen in a horse with a left dorsal displacement of the colon (nephrosplenic ligament entrapment). The spleen is often displaced ventrally and to the right in horses with epiploic foreamen entrapment.

Ultrasound of normal gastrointestinal tract is often unrewarding due to gas in the lumen of the bowel.  However, trans-abdominal ultrasound is useful to determine whether different parts of the bowel are where they are supposed to be in the abdomen.  For example, ultrasonography of the left nephrosplenic ligament can be helpful in the diagnosis of nephrosplenic ligament entrapment in horses, especially in cases in which the entrapped piece of bowel is not distended, making diagnosis by rectal palpation difficult.

Ultrasonography of the gut also reveals information regarding wall thickness, lumen size, and motility.  As such, abdominal ultrasonography can be very helpful in examination of horses with colic, especially when making decisions about whether or not a horse has a problem that requires surgery for correction.  Gastrointestinal lesions that can be diagnosed by ultrasound include colitis, enteritis, intussusception, ileus, strangulated bowel, and diaphragmatic hernia.

The stomach can be found cranially on the left side of the abdomen. The wall of the stomach has a very characteristic and somewhat unique appearance compared to other parts of the gastrointestinal tract, and can measure up to 7.5 mm thick.

Normal small intestine may be difficult to find; the best chance of finding some is usually in the left inguinal area. However, the duodenum is routinely found on the right side of the abdomen, between the liver and right dorsal colon. Distended small intestine can be appreciated much more easily than normal small intestine. Ultrasound can be used to gauge motility, degree of distention, and wall thickness.

Normal small intestinal wall thickness should not exceed 3-4 mm, although the ileum may be slightly thicker. Degree of bowel distention should also be considered when deciding whether the wall thickness is normal or not – the more distended the bowel, the thinner the wall thickness should be.  Simple obstruction of the small bowel is characterized by multiple loops of distended small intestine with thin walls. With stasis, one can also often see sedimentation of particulate matter within the bowel lumen. The bowel wall becomes thickened with strangulating obstructions or enteritis.

The large colon normally rests along the ventral body wall. The ventral colon can be imaged through the ventral body wall, but usually all one can appreciate is the wall thickness, which should be less than 3-4 mm. Gas trapped in the mucosa or present in normal ingesta usually stops further penetration of the ultrasound wave.  A very bright echogenic reflection may suggest the presence of sand. Edema in the colon wall or colitis results in increased wall thickness. Edema can occur with hypoalbumenemia or colon torsion.

Severity of disease and/or progress in recovery can be monitored by sequential measurements of colon wall thickness.  In cases of colitis, the colon contents can often be seen as hypoechoic swirling liquid containing hyperechoic gas bubbles.  The right dorsal colon can be imaged on the right side of the abdomen just deep to the edge of the liver. Right dorsal colitis can be diagnosed by measuring increased wall thickness. Often the increase in thickness is irregular as well. Observation of distended colon vessels along the right side of the abdomen is suggestive of a colon displacement, usually right dorsal displacement.

Cardiac ultrasound (echocardiography)

One of the most common reasons to have a cardiac ultrasound performed is discovery of a cardiac murmur.  M-mode (motion mode), 2-D (or real time), doppler, and color flow ultrasonography are useful in diagnosing and predicting prognosis for congenital and acquired cardiac diseases.  The most commonly diagnosed congenital abnormality is ventricular septal defect (a hole in one of  the heart walls).  Examples of acquired diseases in which ultrasound is useful for diagnosis and prognosis include valvular insufficiencies or stenoses, vegetative lesions (bacterial endocarditis), pericarditis, decreased contractility due to myositis or cardiomyopathy, and ruptured chordae tendonae.  Cardiac ultrasonography will be discussed in more detail in the cardiac lectures.

Lungs / pleura

Normal lungs cannot be imaged by ultrasound because of the air in them.  The sound waves are all reflected at the lung surface and none of the deeper tissues can be visualized.  However, ultrasonography is extremely useful in diagnosis of pleuritis, pleuropneumonia, lung surface abscesses, diaphragmatic hernia, and other thoracic conditions, providing information that cannot be obtained radiographically. Ultrasound can be used to locate the optimum place to drain fluid from a chest or to aspirate an abscess.  

Ultrasound also can be used to diagnose tumors in the chest. The lungs should be scanned on each side of the chest in a systematic manner, moving the transducer sequentially down each rib space until the ventral border of the lung is reached. The normal pleural surface is imaged as a bright, smooth hyperechoic line that moves back and forth (gliding sign) during inspiration and expiration.  Pneumothorax can be differentiated from the normal pleural surface by virtue of the fact that there is no back and forth (gliding) movement with respiration.

Inflammation of the pleural surface results in irregularities of the pleural surface, imaged as “comet tails”, or rays of echogenicity that travel from the pleural surface, appearing to go deeper into the tissue. Pleural effusion is readily diagnosed by ultrasound. The character and amount of fluid present contribute to decisions regarding when and where to remove the fluid. Abnormalities that occur or extend to the surface of the lung can be imaged ultrasonographically.  Examples include lung atelectasis, consolidation, abscessation, neoplasia, etc.

Foals

Ultrasound is very useful pre-natally to diagnose twins and to monitor fetal health and well-being.  Ultrasound is used in late-term pregnant mares that are sick to determine the health of the placenta.  Transrectal ultrasonography is more useful in early gestation, whereas both transrectal and transabdominal ultrasound are useful in mid to late gestation.  In late gestation, transrectal ultrasound allows the operator to evaluate placental integrity in the cervical area, the place where ascending infections are most likely to begin.  Transabdominal ultrasound allows for a more complete evaluation of the fetus and the uteroplacental unit, especially where the horns join the body, an area where nocardial infection is most likely to occur. 

Ultrasound examination reveals placental integrity, fetal vitality, quality/quantity of the allantoic and amniotic fluid, and can also be used to assess fetal heart rate, look for twins, and to confirm fetal position.  Fetal size (and to some degree gestational age/maturity) can be estimated from aortic diameter measurement (2.2-2.5 cm in normal term horse fetus) and from measurement of the size of the fetal orbit.  However, the fetal head can be difficult to locate in late pregnancy.

Transabdominal ultrasonography should be performed frequently in late gestation in high risk mares to detect early signs of fetal distress.  It has been recommended that ultrasound examination be performed weekly until or unless an abnormality is noted, after which daily monitoring is recommended.  Normal uteroplacental thickness should be 0.7-1.5 cm with minimal to no evidence of separation. 

The placenta will have a more corrugated appearance in the non-pregnant horn, and this should not be confused with placental separation.  Increased uteroplacental thickness suggests placental edema or inflammation.  Amniotic and allantoic fluid should be mainly hypoechoic with a minimal amount of small echogenic particles floating in them.  The echogenic particles increase as parturition approaches. 

Amniotic fluid depth ranges from 0-8 cm, and allantoic fluid depth is up to 13 cm.  Abnormally turbid fluid may indicate meconium passage, hemorrhage (eg placental separation) or placental infection.  Collection of allantoic or amniotic fluid has been described, but is not practiced routinely.  Perhaps this is because the procedure is not without risk and the information provided has not been considered to be crucial.  Fluid obtained by centesis can be used for cytologic analysis, culture and sensitivity testing, analysis of fetal karyotype, and analysis of lung maturation.

Fetal vitality is estimated by fetal heart rate, movement, tone and breathing.  In an unsedated mare, bradycardia indicates fetal stress.  Severe tachycardia often occurs just prior to fetal death.  Poor oxygen delivery to the fetus causes loss of fetal heart rate reactivity (ie the heart does not accelerate with fetal movement), followed by decreased fetal breathing, decreased fetal movement, and decreased fetal tone.  Decreased amount of amniotic fluid may indicate chronic intrauterine stress and hypoxia. 

Chronic placental insufficiency results in redistribution of blood flow to the fetal brain and heart, resulting in intrauterine growth retardation and disproportionate body parts.  The head is unusually large for the body.  Decreased renal perfusion leads to decreased urine production and decreased amniotic fluid.  This in turn can compound an intrauterine hypoxia problem by compressing the umbilical cord and decreasing blood supply further.

In the neonatal period, ultrasound examination of the umbilical structures will reveal subclinical problems.  Starting at the umbilical stump, the umbilical vein is identified and followed cranially to the liver, looking for evidence of enlargement or infection.  The vein should measure < 1 cm. Posterior to the stump, the umbilical arteries and urachus are followed to the bladder.  Both external and internal abscessation of the urachus can be diagnosed in this manner.  The umbilical arteries should measure < 1.2 cm, from external wall to external wall. 

Measurement of both umbilical arteries and the urachal stump just at the apex of the bladder should be < 2.5 cm. Presence of anechoic urine in the bladder should be confirmed. An intact bladder should be round or oval, with varying degrees of fullness.  The walls of a ruptured bladder often do not appear rounded, and may even appear folded. Urine or free fluid can readily be imaged in the peritoneal space. However, in some cases with only a small hole in the bladder or the urachus, the bladder wall can appear to be intact, despite the presence of uroabdomen. 

Ultrasound of the neonatal gut can be helpful to diagnose meconium impaction, enteritis, intussusception, vovulus or strangulation, and increased or decreased motility. This information is useful during evaluation of colicky foals. It also helps to determine whether a sick foal can tolerate oral feeding.

Ultrasound of the neonatal thorax is similar to that described for adults. In neonates, ultrasound can also be useful to detect rib fractures. Cardiac ultrasonography is useful to detect congenital abnormalities, and also can give information on blood volume (poor cardiac filling) or sepsis/shock (poor contractility).

Ultrasound of neonatal cuboidal bones will reveal if they are incompletely ossified.  The information obtained may not be as quantitative as that supplied by radiographs, but the technique is simple and quick to perform, and can be used to determine whether or not radiographs are necessary. Ultrasound of joints will show evidence of joint effusion/infection before it is possible to palpate problems; ie these problems can be detected earlier than by other means.  Ultrasound can also be used to find infected physes.

Summary

In summary, ultrasound is a very useful tool for the equine internist, allowing him or her to “see” tissues that cannot be imaged by other means.  It is a safe, non-invasive procedure.  Equipment capable of transabdominal or cardiac ultrasonography can be expensive, so is not available in every private practice. However, ultrasound exams are a common reason for referral to a secondary or tertiary care facility.