Developing a systematic approach to evaluation of the abdomen with ultrasound is crucial for accurate and efficient ultrasound examinations.1 Similar to doing a physical examination, it does not necessarily matter the order of organ/system evaluation as long as long as everything that needs to be looked at for a complete exam is evaluated. Additionally, if done in the same order each time, you can be confident that the exam is thorough with consistent evaluation of all structures.
Different approaches to systematic evaluation of the abdomen with ultrasound include organ-by-organ approach and quadrant approach. Obtaining orthogonal views (scanning in both long and short axis) of organs is recommended and allows the sonographer to mentally reconstruct the 2-D images into a 3-D representation of the organ. Knowledge of normal anatomy is extremely important for the ultrasonographer. Taking every opportunity to scan healthy animals will give the ultrasonographer a good understanding of normal ultrasonographic anatomy and the normal internal architecture of different organs so that recognition of abnormalities is heightened and false positive findings are decreased. Knowledge of species differences will also increase accuracy of the ultrasound examination.
When first learning ultrasound, often the emphasis is on organ identification; however, the diagnostic potential of an ultrasound examination is dependent on thorough organ evaluation. Each organ/structure should be evaluated for number (How many? Is this normal?), location (Is this organ in a normal location? Is it displaced?), size (be familiar with normal reference ranges), shape, margination, echogenicity (anechoic, hypoechoic, hyperechoic, isoechoic, mixed/heterogenous) and echotexture (coarse vs. fine, is echotexture appropriate for organ?).
Structures that should be evaluated on an abdominal ultrasound examination include the following:
- Liver (including the gall bladder)
- Adrenal glands
- Urinary bladder
- Gastrointestinal tract (stomach, small intestine, colon, gastroduodenal junction/pylorus, ileocecocolic junction)
- Lymph nodes (Typically medial iliac and jejunal lymph nodes are visible in all dogs; colonic lymph nodes are visible in cats. Other lymph nodes may be visible when abnormal.)
- Greater vessels (caudal vena cava, aorta, portal vein)
- Peritoneal and retroperitoneal spaces (evaluate for the fusion and abnormal mesentery/fat)
Systematic evaluation should include labeling of ultrasound images. This is helpful when looking at the images after the examination, comparing ultrasound findings to a previous ultrasound examination, and when sending images to a radiologist for interpretation or as part of the medical record for patients referred for specialist care. Knowledge of ultrasound artifacts (in particular, distal acoustic enhancement, distal acoustic shadowing and reverberation) will be helpful for accurate interpretation of ultrasound images.2
Distal acoustic enhancement: This artifact occurs deep to fluid-filled structures. Tissues distal to a fluid-filled structure will be brighter than the adjacent tissues at the same depth. This is helpful for accurate identification of fluid-filled structures such as cysts.
Distal acoustic shadowing: This artifact occurs when the ultrasound beam encounters structures that completely absorb and/or reflect soundwaves. Deep to these structures will be a hypo- to anechoic region (shadow). In some respects, distal acoustic shadowing is a nuisance as it is the reason intrapelvic structures cannot be visualized, but it also helps with recognition of abnormalities such as abnormal areas of mineralization (e.g., urinary stones, prostatic mineralization) and gastrointestinal foreign material.
Reverberation: This artifact occurs when the ultrasound beam encounters gas and results in the visualization of successive hyperechoic parallel lines deep to the gas. This is often referred to as a “dirty shadow.” In general, gas is not our friend in ultrasound as it inhibits visualization of structures deep to it (such as the lung parenchyma and far wall of gas-filled bowel); however, if reverberation artifact is detected within a mass lesion it can indicate necrosis/abscessation. Also ultrasound is very sensitive for small volumes of free peritoneal gas due to identification of reverberation artifact.
1. Mattoon JS, Berry CR, Nyland TG. Abdominal ultrasound scanning technique. In: Mattoon JS, Nyland TG, eds. Small Animal Diagnostic Ultrasound. 3rd ed. Saunders; 2015.
2. Mattoon JS, Nyland TG. Fundamentals of diagnostic ultrasound. In: Mattoon JS, Nyland TG, eds. Small Animal Diagnostic Ultrasound. 3rd ed. Saunders; 2015.