target="_blank" rel="nofollow" href="#ulink_a2e3eb72-474f-5ac7-a2c7-b287fa60a780">Figure 8.15. Biliary tract distension. (A) Cat with bile duct tortuosity which can be a normal variant and common finding. This variant can be easily mistaken for a variety of pathology (nodules, cysts, masses) by the novice or hasty sonographer. Careful correlation with other clinical findings is imperative for an accurate assessment (CBD, common bile duct; GB, gallbladder). (B) A cat with suppurative cholangitis with concurrent pathological tortuosity and dilation of the bile duct. The bile duct is seen ventral to the portal vein and can be distinguished from vessels with color flow Doppler evaluation. In cats, a bile duct greater than 4 mm is considered to be consistent with extrahepatic biliary obstruction. When the biliary duct wall is irregular or unevenly thickened, neoplasia should be a differential. (C) Cholelithiasis can be seen anywhere along the biliary tract and presents challenges for the novice sonographer. This image demonstrates a hyperechoic cholelith at the level of the duodenal papilla in a dog. Note the strong distal acoustic shadow (clean shadow) and lack of normal architecture of the duodenal papilla (duod, duodenum). This example emphasizes the need for a complete detailed abdominal ultrasound evaluation by a veterinary radiologist or specialist with advanced ultrasound training.
Routine Add‐on of AFAST and Abdominal Fluid Scoring System (or Global FAST)
In the authors’ experience, it is extremely valuable to perform an AFAST examination with assignment of an abdominal fluid score in either lateral recumbency for the best approach. The positioning of small animals in dorsal recumbency for POCUS abdominal organ exams can obscure small‐volume peritoneal effusion and underestimate the volume of effusion present because peritoneal effusion will flow into the less sonographically accessible diaphragmatic recesses. AFAST improves the diagnostic potential of the ultrasound exam by detecting peritoneal effusion. The early detection of abdominal effusion is clinically important and helps direct additional diagnostic recommendations while avoiding morbidity, complications, and patient mortality in the event the effusion is missed.
The Global FAST approach is used as a screening test to rapidly discriminate between localized and disseminated disease, comorbidities, and complications. Global FAST offers a rapid assessment to survey for pleural and pericardial effusion and cardiac and pulmonary abnormalities, including lung nodules, with minimal restraint upon initial patient triage.
Pearls and Pitfalls, The Final Say
Normal echogenicity rule of thumb. The spleen (S) is more echogenic (hyperechoic, brighter) than the liver (Li), which is the same or slightly more echogenic (hyperechic, brighter) than the cortex of the kidney (CK). Remember this as S > Li ≥ CK or the mnemonic “SLiCK” for dogs. For the feline, hepatic echogenicity is often compared to the adjacent falciform fat and the feline liver should be isoechoic to slightly hypoechoic to the falciform fat.
Benign conditions may appear ultrasonographically “malignant.” Hepatic echogenicity can be difficult to interpret and significant liver disease may still be present even with an unremarkable exam. Ultimately, tissue sampling is necessary for definitive diagnosis of hepatic parenchymal diseases.
Be familiar with the possible artifacts related to the liver and gallbladder. The fluid‐filled gallbladder is subject to several artifacts including side‐lobe, edge shadowing, and far‐field acoustic enhancement. A mirror image artifact of the gallbladder and liver may appear on the thoracic side of the diaphragm due to the air interface between the diaphragm and lung, and can be mistaken for diaphragmatic hernia, pleural and pericardial effusion.
The origin of a large midabdominal mass can sometimes be difficult to determine when the mass contacts multiple origins. In this case, try repositioning the patient from dorsal to lateral recumbency. This will often separate the liver and spleen from one another and help determine the origin of the mass.
Always look into the thorax through the diaphragmatic hepatic interface when evaluating liver to investigate the pleural and pericardial spaces, and the lung surface along the diaphragm.
An AFAST and assignment of an abdominal fluid score should be considered as routine for POCUS liver and gallbladder examinations and the Global FAST approach should be performed so as not to miss comorbidities including intraabdominal bleeding or peritonitis as well as intrathoracic conditions. The AFAST and its fluid scoring system should be used on initial patient evaluation and also to monitor for complications such as hemorrhage or peritonitis after interventional procedures including liver and gallbladder surgeries, percutaneous aspirates and biopsies, and laparoscopic procedures. The Global FAST approach is used for postinterventional intrathoracic complications including aspiration pneumonia (see Chapters 23 and 36).
Quick Reference of Normals and Rules of Thumb
Liver assessment is subjective. Edges of the liver lobes should be sharply pointed. Rounded margins of liver lobes and hepatic extension (especially the left lobes) beyond the costal arch suggest enlargement. Abdominal radiography is more reliable for the determination of hepatomegaly.
Target lesions. Single target lesions have a high predictive value for malignancy. When multiple target lesions are seen, the positive predictive value of malignancy increases from 74% to 81% (Cuccovillo and Lamb 2002), and Global FAST is a rapid and quick assessment test for localized versus disseminated disease by surveying the remainder of the abdomen (AFAST), the thorax (TFAST) including heart (TFAST), and lung (Vet BLUE).
Normal echogenicity rule of thumb. The spleen (S) is more echogenic (hyperechoic) than the liver (Li) which is the same or slightly more echogenic (brighter) than the cortex of the kidney (CK), remembered as S > Li ≥ CK or by the mnemonic “SLiCK.” For the feline, hepatic echogenicity is often compared to the adjacent falciform fat and the feline liver should be isoechoic to slightly hypoechoic to the falciform fat.
Gallbladder wall thickness is normally <1 mm in cats and <3 mm in dogs (Hittmair et al. 2001; Spaulding 1993; Quantz et al. 2009).
Gallbladder luminal content is generally homogeneous anechoic bile although degrees of gallbladder sludge may be considered as clinically irrelevant or may indicate cholestasis based on the patient’s clinical profile (Tsukagoshi et al. 2012).
Being fluid filled, the gallbladder is subject to several artifacts including side‐lobe, slice thickness, edge shadowing, and acoustic enhancement in the far‐field; and the liver and gallbladder, being adjacent to the strong soft tissue–air interface between the diaphragm and the aerated lung, are subject to mirror image artifact.
In cats, tortuosity of the bile duct can be a normal variation; however, a bile duct greater than 4 mm (≥5 mm) is considered to be consistent with extrahepatic biliary obstruction.
When you are unable to effectively visualize the gallbladder using ultrasound (likely due to mineralized material or air), a good rule of thumb is to add radiography.
References
1 Choi J, Kim A, Keh S, et al. 2014. Comparison between ultrasonographic and clinical findings in 43 dogs with gallbladder mucoceles. Vet Radiol Ultrasound 55:202–207.
2 Crowe DT, Lorenz MD, Hardie EM, et al. 1984. Chronic peritoneal effusion due to partial obstruction caudal vena caval obstruction following blunt abdominal trauma: diagnosis and successful surgical management. J Am Anim Hosp Assoc 20:231–238.
3 Cuccovillo
