Point-of-care ultrasound (POCUS) can detect abdominal free fluid with a sensitivity of 96% in less than 3 minutes using the AFAST protocol. This bedside tool has transformed emergency triage in veterinary medicine. Use the X-Ray Analyzer AI to correlate radiographic findings and the Radiology Specialist for interpretation guidance.
The two primary transducers for abdominal ultrasound in veterinary medicine are:
Curvilinear (convex) probe (3-8 MHz): The workhorse for abdominal imaging. Lower frequency provides deeper penetration for large-breed dogs. The wider footprint gives a broader field of view, ideal for liver, spleen, and kidneys.
Microconvex probe (5-8 MHz): Smaller footprint fits between ribs and into tight intercostal spaces. Excellent for cats, small dogs, and focused organ assessment. Higher frequency provides better resolution for superficial structures.
For general practice POCUS, a 5-8 MHz microconvex probe is the most versatile single-probe choice. Coupling gel should be applied liberally. Air between the probe and skin creates artifact that degrades the image.
Fasting: 12-hour fast is ideal to reduce gastric gas artifact and distend the gallbladder. For emergency AFAST, scanning can be performed without preparation.
Clipping: Clip the ventral abdomen from xiphoid to pubis and laterally to the flank folds. In emergency settings or for client-sensitive situations, applying alcohol liberally and parting the fur can substitute but image quality is reduced.
Positioning: Right lateral recumbency is standard for systematic abdominal scanning. Dorsal recumbency is acceptable and preferred by some operators, particularly for AFAST and bladder assessment. Left lateral recumbency optimizes gallbladder visualization (gallbladder falls toward the dependent side).
The Abdominal Focused Assessment with Sonography for Trauma (AFAST) is a rapid, standardized protocol to detect free abdominal fluid. Developed by Dr. Gregory Lisciandro, it uses four acoustic windows:
1. Diaphragmatico-hepatic (DH): Subxiphoid, angled cranially. Views the interface between diaphragm and liver. Most sensitive site for free fluid in dogs.
2. Spleno-renal (SR): Left flank, caudal to the last rib. Views left kidney and spleen. Fluid accumulates in the dependent left gutter.
3. Cysto-colic (CC): Ventral midline, just cranial to the bladder. Views the space between bladder and colon. Fluid pools in the pelvic inlet.
4. Hepato-renal (HR): Right flank, caudal to the last rib. Views right kidney and liver. The hepato-renal recess (Morison's pouch) is a key fluid collection site.
AFAST is scored 0-4 based on the number of positive sites. An AFAST score of 3-4 in a trauma patient correlates with significant hemorrhage requiring intervention. Serial AFAST monitoring tracks fluid accumulation or resolution over time.
| Organ | Normal Appearance | Normal Size (Dog) |
|---|---|---|
| Liver | Homogeneous, medium echogenicity; hepatic veins visible | Does not extend past costal arch |
| Gallbladder | Anechoic (black) fluid, thin wall (<2-3 mm) | Variable; pear-shaped |
| Spleen | Homogeneous, hyperechoic relative to liver and left kidney cortex | Head and body visible; smooth capsule |
| Left kidney | Cortex hypoechoic to spleen; distinct corticomedullary junction | 5.5-9.0 cm (breed-dependent) |
| Right kidney | Same as left; may be slightly smaller | 5.5-9.0 cm (breed-dependent) |
| Bladder | Anechoic urine; smooth, thin wall (<2 mm when distended) | Variable with filling |
| Adrenals | Hypoechoic, bilobed (peanut-shaped); left easier to find | <7.4 mm width (dogs) |
| GI tract | 5-layer wall pattern visible; wall thickness <5 mm (small intestine) | Peristalsis present |
Free fluid (effusion): Anechoic triangular pockets between organs. Characterize by cellularity, echogenicity, and clinical context (transudate, exudate, hemorrhage).
Masses: Splenic masses are common in older dogs (hemangiosarcoma vs hematoma vs nodular hyperplasia). Hepatic masses, renal masses, and adrenal masses each have differential lists.
Uroliths: Hyperechoic structures with distal acoustic shadowing in the bladder, ureters, or kidneys.
Pyometra: Distended, fluid-filled uterine horns with variable echogenicity. Measure uterine diameter; >1 cm is abnormal in non-pregnant dogs.
GI obstruction: Dilated fluid-filled small intestinal loops proximal to obstruction; plication pattern with linear foreign bodies.
Warning: A splenic mass identified on ultrasound should be treated as hemangiosarcoma until proven otherwise. Two-thirds of splenic masses in dogs are malignant. Aspirating splenic masses carries a risk of hemorrhage and is generally not recommended; surgical excision with histopathology is the definitive diagnostic approach.
General practice POCUS is excellent for triage and focused questions (free fluid? bladder stones? pyometra?). Refer for a complete diagnostic ultrasound performed by a radiologist or ultrasonographer when: a mass is identified and staging is needed, hepatic or pancreatic disease is suspected, adrenal assessment is complex, Doppler evaluation is required, or fine-needle aspirate guidance under ultrasound is needed for deep structures.
- AFAST is a rapid, 4-view protocol to detect free abdominal fluid with 96% sensitivity.
- A 5-8 MHz microconvex probe is the most versatile choice for general practice POCUS.
- Patient preparation (fasting, clipping, positioning) significantly impacts image quality.
- Learn normal sonographic appearance of abdominal organs before attempting to identify pathology.
- Splenic masses should be treated as potentially malignant; refer for surgical excision and histopathology.
- Refer for full diagnostic ultrasound when staging, Doppler, or guided aspiration is needed.