Normal Abdominal Ultrasonography of the Horse
Depth of View
In any ultrasound examination, it is important to be mindful of the depth of the field of view. The selection of the appropriate frequency for a transducer is the key to producing a high quality image that is most suitable for the depth of display. Recall that higher frequency probes provide sharp images; however, this clear resolution is compromised as the depth of the viewing field increases. If using a fixed single frequency probe, the 3.0 or 3.5 MHz curvilinear probe is the most suitable compromise to accommodate imaging of the majority of the abdominal viscera.4 You can simply orient yourself to the depth of view by looking at the scale of the displayed image. You can be easily tricked into believing that something is missing from the field of view, only to realize that the depth setting is too shallow to identify the structure of interest. It is also easy to fool yourself into thinking that a lesion is enormous, only to realize that the depth of view is only a few centimeters and therefore magnified. 5
How the Image is Generated
As ultrasound waves project through the body, they are reflected by tissue interfaces and sensed as ‘echoes’. If adjacent tissues have the same acoustic impedance, no sound is reflected and sound waves penetrate into the deeper tissues. While denser tissues have greater acoustic impedance, it is the interface between adjacent tissues, or between tissues within the same organ, that determines how much of the sound wave is reflected back to the transducer.1 The more sound reflected to the transducer, the "whiter" the image appears on the viewing screen. These tissue interfaces are called “echogenic” or “hyperechoic.” In contrast, less dense tissues reflect less sound and are called “anechoic” or “hypoechoic” and appear blacker. Knowing that tissue interface difference is what is primarily responsible for reflecting sound back to the transducer, more sound waves should echo back to the transducer if two adjacent interfaces have markedly different acoustic impedances. The interface between soft tissues and gas is an excellent example of this concept. The soft tissue of the gastrointestinal walls has an acoustic impedance that is several thousand fold greater than that of the free gas inside the adjacent lumen.1 Consequently, the image at this soft tissue to gas interface appears as a fuzzy hyperechoic border. Since most of the sound waves at this interface are reflected, and the free gas in the lumen has extremely low impedance, the rest of the lumen appears darker as sound is neither penetrating nor reflecting from the lumen. Remember, gas within a large viscus is actually one of the greatest limitations to gastrointestinal ultrasonography: the gas prevents visualization of deeper structures. While imaging a patient, keep in mind that fluid and heavier structures will fall to the dependent side and gas will float to the nondependent side and obstruct deeper views.
With a very high resolution linear array transducer, one may be able to see up to 5 layers to the gastrointestinal wall.4 Though most typically, when using the standard approach with a 3 to 3.5 MHz transducer, and depending on the surrounding tissue and the contents of the lumen, only three or less layers are normally visible (hyperechoic serosa, hypoechoic muscularis to mucosa, hyperechoic interface with lumen).
Integration of Knowledge of Normal Abdominal Anatomy
When scanning the equine abdomen it is helpful to use a systematic approach, scanning from top to bottom, rostral to caudal, left and right sides. Careful attention should be paid to the spatial relationship of the viscera, as this may be important in the distinction between normal and abnormal.5 The walls of some sections of the gastrointestinal tract appear strikingly similar and without knowledge of where the transducer is placed on the abdomen, they may not be distinguishable.6 Transabdominal ultrasonography not only provides structural information, but it can provide some idea of function i.e. motility. Recall that heavy sedation can cause transient ileus and mildly dilated small intestine.3