Normal Abdominal Ultrasonography of the Horse
The recent introduction of more affordable and portable ultrasound equipment makes it possible to perform transabdominal ultrasonography in the field situation on equine patients. The unique feature of this review does not lie in the information provided, but in the way that the information will be provided.* During the presentation, standard ultrasound images of the healthy equine abdomen will be accompanied by three-dimensional computer-generated animations that were designed to orient the viewer to normal anatomy.*
There are several different types of ultrasound transducers, though most imaging of the adult horse’s abdomen is done with a 2 to 5 MHz curvilinear array transducer.1 With the curvilinear transducer, ultrasound waves radiate from a curved footprint at the point of contact on the patient, and generate a curved “pie-shaped” image through the plane of projection into the horse. Ideally, prior to ultrasonography, the patient's hair should be clipped with a number 40 blade and the skin should be cleansed with isopropyl alcohol. Couplant gel should be liberally applied. If clipping the hair is not an option, soaking the hair with isopropyl alcohol will often suffice. Many horses will tolerate transabdominal ultrasonography without sedation. If however, sedation is needed, be mindful that alpha two agonists, such as xylazine and detomidine, will induce a transient state of ileus, and thus intestinal motility may be reduced and the luminal diameter of the small intestine may appear more dilated than in a patient that is not sedated.2,3
When performing ultrasonography, there are four key elements that will greatly facilitate the examination: being aware of the orientation of the probe with respect to the patient, using information on the depth of the viewing field, understanding how the image is generated by the ultrasound machine, and having knowledge of normal anatomy with respect to probe placement.
Orientation of the Probe
Ultrasound transducers have a physical mark on them that provides orientation of the transducer's placement on the patient relative to the projected ultrasound image on the viewing screen.1 Because much of abdominal ultrasonography is performed with the footprint of the transducer in an intercostal space, most imaging of the abdomen is performed with the transducer oriented in a slightly oblique transverse plane with the probe marker “up.” In other words, the transducer marker is at the 11 to 12 o’clock position or toward the “dorsal” aspect of the patient. When the image is displayed on the viewing screen, most ultrasound machines display the position marker on the screen. This screen position marker is intended to orient the ultrasonographer to the displayed image. For example, if the orientation marker on the transducer is 11 to 12 o’clock relative to the patient to obtain a transverse plane image (slicing across the long axis of the body), and if the ultrasound machine normally displays the probe orientation mark in the upper left hand corner of displayed image, then the ultrasonographer knows immediately that the portion of the image on the left hand side of the screen represents the dorsal aspect of the captured image. Knowing the orientation of the transducer marker relative to the patient and the way the ultrasound machine normally displays its images greatly facilitates orientation to the structures that appear in the image on the screen. Be mindful of the fact that you can manually "flip" the image display on most ultrasound machines by pressing a left/right or top/bottom inversion key.