Point-of-Care Ultrasound Techniques for the Small Animal Practitioner. Группа авторов
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total bilirubin, and gamma‐glutamyl transferase (GGT).
Cranial abdominal organomegaly discovered on physical examination/palpation and abdominal radiography (or both).
Abnormal serum bile acid levels or resting ammonia levels.
Decreased albumin, cholesterol, blood urea nitrogen (BUN).
Persistent hypoglycemia.
Prolonged anorexia (especially cats).
Vomiting.
Objectives
Recognize liver masses (single, multifocal).
Recognize regional changes in liver echogenicity.
Recognize venous congestion.
Recognize gallbladder wall abnormalities.
Recognize gallbladder luminal conditions such as mucocele, choleliths, biliary sludge, and gallbladder mass lesions.
Recognize signs supportive of biliary obstruction, abnormal biliary contents (choleliths and inspissated bile), which when found should prompt referral for a complete detailed abdominal ultrasound study by a veterinary radiologist or internist with advanced training.
Ultrasound Settings and Positioning
When imaging the liver, a 5–10 MHz curvilinear probe may be used depending on the size of the animal. Generally, higher frequency probes (7.5–10 MHz) are used for smaller dogs and cats and lower frequency probes (5 MHz) are used for large dogs to allow for deeper penetration. The depth of field should be adjusted initially (increased) to visualize the interface of the diaphragm and liver in the far‐field. Gain should also be adjusted to maximize image quality. Adjustments in depth, gain, and focus position will be made repeatedly during the exam to optimize penetration and image quality.
The patient is most commonly imaged in dorsal recumbency with the transducer placed on the ventral abdomen just caudal to the xiphoid process, and the liver scanned in transverse and sagittal planes. This position generally allows for adequate visualization of the liver in smaller dogs and cats. In deep‐chested dogs or larger animals, additional approaches through the right and left intercostal spaces may also be needed. The presence of an overlying gas‐distended stomach or colon may also interfere with adequate visualization of the liver in dorsal recumbency. In these instances, positioning the patient in right and/or left lateral recumbency will cause the gas to rise to the upside (least gravity‐dependent) of the patient, and scanning planes can be obtained from below the gas‐filled structure.
Pearl: Best practice is to perform a Global FAST prior to moving and restraining your patient in dorsal recumbency to make sure the patient is stable (ruling out pleural and pericardial effusion, obvious cardiac and pulmonary conditions, poor volume status via caudal vena cava charcterization) and thus not hemodynamically fragile in which dorsal recumbency risks patient decompensation.
How to Perform the POCUS Liver and Gallbladder Examination
The patient is positioned in dorsal recumbency and the hair is clipped from the ventral abdomen and as needed over intercostal spaces. Acoustic gel is applied to the abdomen, often preceded by 70% isopropyl alcohol that helps strip lipid and remove air from the patient's hair follicles. The transducer is placed caudal to the xiphoid process with the probe marker facing toward the head of the patient using a subcostal approach. Generally, this provides a sagittal view through the midline of the liver. The pleural margin of the diaphragm should be evident as a hyperechoic (bright white line) in the far‐field (Figures 8.1A and 8.2B–E). If this is not initially visualized, then depth of field should be adjusted (increased) to bring it into view. Falciform fat may be seen along the ventral margin of the liver in the near‐field, especially in overweight animals and should not be mistaken for hepatic parenchyma (see Figures 8.2A–D and 39.2).
The probe should then be fanned to the right side of the patient where the gallbladder should come into view, and can then be rotated to obtain a long‐axis image of the gallbladder. The probe is then fanned to the patient’s left so that the entirety of the sonographically accessible liver can be evaluated. In larger animals, it may be necessary to move the probe to the right and left along the costal arch to visualize the entire liver. As the probe is fanned to the left, the stomach will come into view. After thoroughly scanning in the sagittal view, the probe is then rotated with the marker facing to the right side (“turn left” or counterclockwise) of the patient and the liver is scanned from right to left again in the transverse view. The hepatic and portal veins are imaged within (or immediately adjacent to) the hepatic parenchyma. Portal veins are distinguished by their hyperechoic (brighter) walls.
Figure 8.1. Comparative echogenicity of the liver to the cortex of the right kidney and spleen. (A) The liver (R LIVER) is normally slightly more echogenic (brighter) or the same echogenicity (isoechoic) as the cortex of the kidney (RK). In (B) another example with the right kidney (RK) compared to the echogenicity of the liver because the RK is located within the renal fossa of the liver's caudate lobe. (C) Normally, the spleen is more echogenic than the liver (unlabeled). (D) Same image as (C) with the liver and spleen labeled. When it is difficult to image the organs being compared in the same image frame, use of the split‐screen function of your ultrasound machine facilitates comparative echogenicities of the liver to the spleen and kidney.
Large volumes of gas or ingesta can prevent complete visualization of the left side of the liver. A large amount of gas or stool in the descending colon may also interfere with imaging of the left side of the liver. If the stomach or colon interferes with imaging, the patient should be repositioned in right lateral recumbency and a left intercostal approach used. Alternatively, the patient can be reimaged following fasting and/or colonic evacuation if necessary and if the patient is stable. However, in the authors’ opinion, some food within the stomach and a full colon can provide natural contrast for evaluation of the gastric and colon walls, and evacuation of the colon is rarely needed.
In deep‐chested dogs, the liver may be difficult to image completely using a subcostal approach since the entire liver may lie under the ribcage. In these instances, right and left intercostal approaches are often needed to evaluate portions of the liver. From intercostal approaches, evaluation may further be hindered by gas shadowing from the caudal lung. The sonographer should be aware that complete ultrasonographic interrogation of the liver may not be possible especially in large‐breed dogs, for which computed tomography (CT) may be elected for a more comprehensive evaluation.
Figure 8.2. Liver margination and comparative echogenicity to the falciform fat. (A) Image of the falciform fat. Note the linear subcutaneous facial planes and falciform fat just distal to the point of probe contact. Liver echogenicity can be similar to the falciform fat. This especially depends on sonographic gain settings and hence comparative echogenicity can vary. Notice delineation between the falciform fat and normal liver by the thin echogenic line (arrow) representing the liver capsule. (B) Another example of differences in echogenicity between the liver and falciform fat. Note the sharp, normal liver margin. Compare to (A) and (B) and note that the liver capsule can be seen surrounding the hepatic parenchymal and at its contact with the diaphragm