Veterinary Abdominal Ultrasound: Complete Clinical Guide

From machine setup and probe selection to organ-by-organ normal values and the AFAST protocol — a repeatable, systematic approach to veterinary abdominal ultrasound that covers artefacts, Doppler basics, measurement tables, and species-specific considerations.

1. Introduction: Why Ultrasound Belongs in Your Daily Practice

Point-of-care abdominal ultrasound has moved from the radiology suite to the consult room. It is now a core competency in small-animal practice, driven by portable machines that cost less than a dental X-ray unit and deliver diagnostic-quality images in seconds. This guide lays out a complete, repeatable system — from powering on the machine to interpreting an AFAST scan — so that any clinician can perform abdominal ultrasound with confidence and diagnostic accuracy.

We cover machine setup, probe selection, a standardised organ-by-organ scanning sequence, normal measurement values across dogs and cats, common artefacts, the basics of Doppler ultrasonography, the AFAST (Abdominal Focused Assessment with Sonography for Trauma) protocol, and species-specific anatomy that affects image acquisition. Every section includes clinical decision points and pitfalls to avoid.

2. Machine Setup: Getting the Basics Right

A poor image from a good machine is almost always a setup problem. Before you touch a patient, invest five minutes in optimising your settings. The most important controls are depth, gain, focus, and frequency — and they are interdependent.

2.1 Transducer Selection

2.2 Key Controls

• Depth: Set so the deepest structure of interest sits in the lower third of the screen. A depth that is too shallow clips the far field; too deep wastes pixels and reduces frame rate.
• Gain: Adjust until the liver parenchyma is a uniform medium grey. Over-gain fills cysts and vessels with noise; under-gain hides subtle hypoechoic lesions.
• Time-gain compensation (TGC): Compensates for attenuation with depth. Set TGC sliders to produce a uniform grey scale from near-to-far field in normal liver.
• Focus: Place the focal zone(s) at or just below the depth of the structure you are examining. Multiple foci reduce temporal resolution.
• Frequency: Higher frequency = better resolution but shallower penetration. Start at the highest frequency that penetrates to your depth of interest, then drop if penetration is inadequate.

2.3 Patient Preparation

A full abdomen ultrasound examination requires a patient that has been fasted for 8–12 hours. Food in the stomach and gas in the intestines are the two biggest enemies of image quality. The animal should have the opportunity to urinate and defecate before scanning. Clip the entire ventral abdomen from xiphoid to pubis, and laterally to the margins of the paralumbar fossae. Apply coupling gel liberally — or, in a pinch, alcohol (isopropyl 70%) works well but evaporates quickly. Position the animal in dorsal recumbency in a padded V-trough; lateral recumbency is acceptable for focused examinations or dyspnoeic patients.

3. The Standardised Scanning Sequence

A systematic, organ-by-organ approach prevents the most common ultrasound error — stopping at the first abnormality and missing a second, unrelated lesion. Follow this sequence every time, regardless of the clinical question. It takes approximately 15–25 minutes for a complete exam in an experienced clinician's hands.

1. Liver and gall bladder: From the xiphoid, angle the transducer cranially under the rib cage. Fan through the entire liver in sagittal and transverse planes. Evaluate echogenicity, size, margins, and intrahepatic vasculature. Measure gall bladder wall thickness.
2. Spleen: Sweep the left cranial quadrant. The spleen is superficial, homogeneous, and highly vascular. Follow its entire length — it can extend from the left cranial abdomen to the pelvic inlet. Note: splenic size is highly variable in sedated dogs.
3. Left kidney: Located retroperitoneally, just caudal to the last rib and ventral to the epaxial muscles on the left. Measure length, width, and cortical thickness in sagittal plane.
4. Left adrenal gland: Medial to the left kidney, between the aorta and the left renal artery. A small hypoechoic structure with a hyperechoic centre in transverse view.
5. Stomach: The gastric fundus sits to the left of midline, the pylorus to the right. Assess wall thickness (< 5 mm in dogs, < 3.5 mm in cats), peristalsis, and contents.
6. Duodenum and pancreas: Follow the descending duodenum along the right body wall. The right pancreatic lobe is adjacent to the duodenum; the left lobe follows the splenic vein. Normal pancreas is isoechoic to fat — you are looking for enlargement or altered echogenicity.
7. Right kidney: More cranial than the left, tucked under the last rib. The right kidney contacts the caudate lobe of the liver cranially — the "renal–hepatic" interface.
8. Small intestine: Scan from duodenum to ileocolic junction. Measure wall thickness in transverse plane, assess layering (five layers should be visible), peristalsis, and luminal content.
9. Ileocolic junction and colon: The ileocolic junction is a fixed landmark in the right mid-abdomen. The colon is gas-filled and often obscures deeper structures — apply gentle transducer pressure to displace.
10. Right adrenal gland: Just medial to the cranial pole of the right kidney, adjacent to the caudal vena cava. Best imaged in a transverse or dorsal plane.
11. Urinary bladder: Evaluate wall thickness, mucosal surface, and contents. Measure wall thickness at the apex (should be < 2 mm when distended). Assess for calculi, masses, or sediment.
12. Uterus/prostate: In intact females, scan the uterine body and horns (only visible if pathological or during oestrus). In males, evaluate prostate size, echogenicity, and margins.
13. Mesenteric and sublumbar lymph nodes: Assess mesenteric (jejunal) lymph nodes for size and echotexture. The medial iliac lymph nodes are visible lateral to the caudal aorta.

4. Organ-by-Organ Normal Values and Measurements

Normal values are population references, not absolute cut-offs. Always interpret measurements alongside clinical signs, signalment, and body condition. Below are the most commonly referenced canine and feline norms, drawn from published studies.

4.1 Renal Resistive Index (RI)

The renal resistive index (RI) is calculated from spectral Doppler waveforms obtained at the interlobar or arcuate arteries:

RI = (Peak Systolic Velocity – End Diastolic Velocity) / Peak Systolic Velocity

Normal canine renal RI: < 0.70. An RI > 0.70 suggests increased intrarenal vascular resistance, seen in acute tubular necrosis, renal vein thrombosis, ureteral obstruction, or chronic renal disease. An RI < 0.50 is rare but can occur with renal artery stenosis proximal to the sampling site.

5. Ultrasound Artefacts: Recognising What Is Not There

Artefacts are not imaging failures — they are physical consequences of how ultrasound interacts with tissue. Recognising them prevents misdiagnosis and occasionally provides diagnostic information in its own right.

6. Doppler Basics for the Abdominal Clinician

Doppler ultrasonography detects movement — specifically the frequency shift of ultrasound reflected from moving red blood cells. The two modes you will use are colour-flow Doppler (CFD) and spectral (pulsed-wave) Doppler.

6.1 Colour-Flow Doppler

CFD maps the mean velocity and direction of blood flow onto a greyscale B-mode image. By convention, flow towards the transducer is red, flow away is blue. Turbulent or high-velocity flow appears as a mosaic of colours (aliasing). Clinical applications include:

• Confirming vessel patency — e.g. portal vein thrombosis in a dog with ascites and hypoalbuminaemia.
• Differentiating avascular (cyst, abscess, haematoma) from vascular (neoplastic) structures.
• Identifying the "wall thump" sign — flow signal visible within the wall of a structure confirms it is vascular.
• Assessing renal perfusion in acute kidney injury.

6.2 Spectral Doppler

Pulsed-wave Doppler produces a velocity–time waveform from a sample volume placed within a vessel. The key parameters are:

• Peak systolic velocity (PSV): Maximum velocity during systole.
• End-diastolic velocity (EDV): Minimum velocity at end diastole.
• Resistive index (RI): (PSV − EDV) / PSV. Reflects downstream vascular resistance.

The renal RI is the most clinically useful abdominal Doppler measurement. Obtain it at the interlobar or arcuate arteries using a sample volume of 1–2 mm and an angle of insonation < 60°. Average measurements from three cardiac cycles at three different locations within each kidney.

6.3 Portal Vein Flow

The portal vein normally demonstrates a monophasic, slightly undulating waveform with flow towards the liver (hepatopetal). A portal vein velocity > 20 cm/s or < 10 cm/s is abnormal. Reversed (hepatofugal) flow indicates portal hypertension and is a grave prognostic sign.

7. The AFAST Protocol: Abdominal Fluid Scoring

AFAST (Abdominal Focused Assessment with Sonography for Trauma, Triage, and Tracking) is a rapid, standardised ultrasound protocol used to detect free abdominal fluid. It was developed from the human FAST exam and validated in dogs and cats. It takes 3–7 minutes and should be performed as part of the initial assessment of any traumatised or collapsed patient.

7.1 The Five AFAST Views

1. Diaphragmatico-hepatic (DH) view: Transducer caudal to the xiphoid, angled cranially. Evaluate the interface between the liver and diaphragm for anechoic fluid. Also assess the gall bladder for the "halo sign" (fluid surrounding the gall bladder wall indicating free abdominal fluid adjacent to the gall bladder).
2. Spleno-renal (SR) view: Left flank, caudal to the last rib. The spleen and left kidney are imaged together. Fluid collects between the spleen and the left body wall.
3. Cysto-colic (CC) view: Caudal ventral abdomen, over the bladder. Fluid pools in the rectogenital pouch (males) or vesicogenital pouch (females) — the most dependent point of the abdomen.
4. Hepato-renal (HR) view: Right flank, caudal to the last rib. The right kidney and caudate liver lobe are imaged. Fluid collects in the hepatorenal recess.
5. Umbilical (UMB) view: A scan of the ventral midline at the umbilicus, surveying for fluid in the dependent loops of intestine.

7.2 Abdominal Fluid Score (AFS)

The AFS quantifies the amount of free fluid detected on AFAST:

7.3 Abdominocentesis Technique Under Ultrasound Guidance

Use a 22 G 1.5-inch needle or over-the-needle catheter attached to a 3–5 mL syringe. Identify the largest pocket of fluid. Insert the needle with the bevel facing the transducer so you can track its tip. Aspirate gently. If no fluid is obtained, perform a four-quadrant tap (right cranial, left cranial, right caudal, left caudal). A sample PCV that matches peripheral PCV with fluid that does not clot = haemorrhage. A PCV significantly lower than peripheral with a nucleated cell count > 10,000/μL suggests septic peritonitis.

8. Species-Specific Considerations

8.1 Canine

• The canine spleen is extremely mobile and can be found anywhere from the left cranial abdomen to the pelvic inlet. Always follow it in its entirety — a splenic mass at the tail can be missed if you only scan the head.
• Gastric wall thickness is best assessed with the stomach moderately distended with water (via a gastric tube if necessary).
• Canine adrenal glands enlarge symmetrically with pituitary-dependent hyperadrenocorticism. The caudal pole thickness is the most reliable single measurement.

8.2 Feline

• The feline mesentery is fatty and echogenic, which makes the normal pancreas nearly indistinguishable from surrounding fat. Look for hypoechoic enlargement.
• Feline kidneys are more mobile than canine kidneys and can be imaged via a lateral approach in some cats.
• The feline ileocolic junction is a prominent landmark — the ileum intussuscepting into the colon is the most common feline intussusception.
• Feline intestinal wall layering can be more difficult to resolve than canine; use a high-frequency linear probe (≥ 10 MHz) for best results.

8.3 Exotic and Large Animal Notes

• Ferrets: The spleen enlarges dramatically with isoflurane anaesthesia — do not mistake this for splenomegaly. Adrenal glands are a common site of neoplasia in middle-aged ferrets.
• Rabbits: The stomach is never empty in a healthy rabbit. The caecum occupies a large proportion of the right abdomen and contains gas and ingesta — it should not be mistaken for pathology.
• Equine: Transabdominal ultrasound in adult horses is limited to the ventral abdomen (foals allow deeper imaging). The left kidney is accessible per rectum in most horses. Use a 2–5 MHz curvilinear probe for the adult equine abdomen.

9. Putting It All Together: Clinical Decision Algorithm

Use the following decision tree after completing your abdominal ultrasound:

1. Free fluid present? → AFAST score ≥ 1. Sample fluid: compare PCV to peripheral. If haemorrhage → surgical exploration. If septic → laparotomy after stabilisation. If transudate → investigate heart, liver, protein-losing disease.
2. Wall thickening of intestine? → Measure all segments. If jejunal wall > 4 mm (dog) or > 3 mm (cat) → assess layering. If layering preserved → IBD vs dietary sensitivity. If layering effaced → biopsy (lymphoma vs fungal vs severe IBD).
3. Renal asymmetry? → If one kidney is enlarged and the contralateral kidney is small → the small kidney is the chronically diseased one; the large kidney is compensating. If both enlarged → acute nephritis, lymphoma, or ethylene glycol toxicity.
4. Hepatomegaly with hypoechoic parenchyma? → consider passive congestion (right-sided CHF), acute hepatitis, or diffuse neoplasia (lymphoma, mast cell). Assess hepatic veins and caudal vena cava diameter for evidence of congestion.
5. Adrenal mass? → If unilateral and > 2 cm → adrenalectomy candidate. If bilateral enlargement with preserved shape → likely PDH. Assess for vascular invasion of the caudal vena cava.

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