FAST scanning: principles, technique and CT correlation in abdominal trauma

Overview

Focused Assessment with Sonography for Trauma (FAST) is a rapid, bedside, point-of-care ultrasound examination designed to detect free intraperitoneal fluid (haemoperitoneum), pericardial effusion, and pleural fluid in the acutely injured patient. It forms a core component of the primary imaging survey in trauma resuscitation, performed alongside the portable anteroposterior chest radiograph and pelvic radiograph. The Extended FAST (eFAST) incorporates bilateral anterior chest wall assessment for pneumothorax, broadening diagnostic utility in thoracoabdominal trauma.

FAST is a triage tool, not a diagnostic examination. Its primary role is to determine whether haemoperitoneum or haemopericardium is present, directing the haemodynamically unstable patient toward immediate surgery or interventional radiology, or guiding the stable patient toward definitive CT assessment.

FAST does not characterise solid organ injury, assess the retroperitoneum reliably, or replace CT for management planning. MDCT of the abdomen and pelvis with IV contrast remains the gold standard for comprehensive trauma evaluation in haemodynamically stable patients.

Principles of FAST Scanning

Physical Basis

Free intraperitoneal fluid, predominantly blood in trauma, collects in dependent peritoneal recesses under gravity and appears as anechoic (black) fluid on ultrasound. Fresh liquid blood is anechoic; clotted haematoma appears echogenic or heterogeneous. The hepatorenal recess (Morison's pouch) and the pelvic cul-de-sac are the two most dependent recesses in the supine patient, connected via the paracolic gutters.

Standard FAST Windows

eFAST Additional Views

Bilateral anterior chest wall scanning for pneumothorax:

• Loss of lung sliding (pleural line movement with respiration)
• Loss of B-lines (comet-tail artefacts from normal subpleural lung)
• Lung point sign (transition zone between sliding and non-sliding pleura): pathognomonic for pneumothorax
• M-mode: normal lung → "seashore sign"; pneumothorax → "barcode/stratosphere sign"

Normal Sonographic Appearances

Peritoneal Spaces

• No visible fluid in Morison's pouch, splenorenal recess, or pelvis
• Small physiological fluid may be present in the cul-de-sac of premenopausal women (not pathological in isolation)
• Hepatic and splenic parenchyma homogeneously echogenic without contour disruption

Pericardium

• Thin echogenic pericardial line surrounding the heart
• No anechoic stripe between visceral and parietal pericardium (a trivial posterior stripe ≤2 mm may be normal)

Pleural Spaces

• Echogenic, moving lung-pleural interface with visible lung sliding
• No anechoic fluid above the diaphragm

Pathological FAST Findings

Haemoperitoneum

Free fluid appears as an anechoic or hypoechoic stripe in dependent recesses:

• Morison's pouch: Fluid between the inferior right hepatic lobe and right kidney, the most sensitive and earliest-detected location in the supine patient
• Splenorenal / left subphrenic: Fluid lateral to the spleen or between spleen and left hemidiaphragm; slightly less sensitive due to overlying stomach gas and rib shadowing
• Pelvis: Fluid in the rectovesical pouch or pouch of Douglas; may be the only site in pelvic solid organ, bowel, or bladder injuries

Haemopericardium

• Anechoic stripe between echogenic pericardium and myocardium
• Posterior collections detected first (gravity-dependent)
• Tamponade features: large effusion, right ventricular diastolic collapse, hyperdynamic left ventricle, IVC plethora

Haemothorax / Pleural Effusion

• Anechoic fluid above the diaphragm at RUQ or LUQ windows
• Diaphragm is the key landmark: fluid above = pleural; fluid below = peritoneal
• Mirror artefact of the liver is lost when pleural fluid is present

Limitations of FAST

A negative FAST does not exclude significant abdominal injury, particularly bowel perforation, mesenteric injury, and retroperitoneal haemorrhage. In any patient with a high-mechanism injury or ongoing clinical suspicion, CT should follow regardless of FAST result.

CT in Trauma: Protocol and Correlation with FAST

Indications for CT After FAST

CT Protocol

• MDCT with IV contrast is standard; oral contrast is not routinely administered in acute blunt trauma (delays scanning, limited additional benefit)
• Dual-phase (arterial + portal venous): preferred for solid organ injury, detects active extravasation and characterises parenchymal lacerations; arterial phase is particularly important for splenic injury assessment
• Split-bolus single-pass technique combines phases, reducing scan number and radiation dose, widely used in contemporary trauma protocols
• Delayed phase (~5-10 min, urographic): added when ureteric or bladder injury is suspected
• Multiplanar reformats (coronal and sagittal) are essential for complete interpretation
• Whole-body CT is increasingly employed in polytrauma; newer lower-dose protocols have lowered the threshold for CT in the severely injured patient

CT Signs of Intraperitoneal Injury

Haemoperitoneum

The sentinel clot sign: a focal hyperdense clot adjacent to the injured organ, surrounded by lower-attenuation liquid blood. Its location guides the radiologist to the injury source even when the organ laceration is subtle.

Hyperattenuating peritoneal fluid in the absence of an identifiable solid organ injury should raise suspicion for bowel or mesenteric haemorrhage. Exception: small volumes of low-attenuation free fluid in the pelvis of women of reproductive age may be physiological.

Pneumoperitoneum and Bowel Perforation

Free intraperitoneal gas is the hallmark of hollow viscus perforation. Lung windows are mandatory ($W \approx 1500,\ L \approx -600$); small bubbles of free gas are invisible on soft-tissue windows.

CT signs of pneumoperitoneum:

• Gas anterior to the liver (CT equivalent of falciform ligament / football sign)
• Gas outlining both sides of the bowel wall (Rigler sign equivalent on supine CT)
• Small bubbles of gas in the mesentery, retroperitoneum, or porta hepatis
• Portal venous gas or mesenteric venous gas: peripheral branching gas within the liver or mesenteric vessels, implies severe bowel ischaemia or infarction (cf. benign pneumatosis coli)

Most common cause of pneumoperitoneum: duodenal or gastric ulcer perforation. In the trauma context, bowel perforation, recent surgery/laparoscopy, or infection with gas-producing organisms must also be considered.

CT Signs of Bowel and Mesenteric Injury

Shock bowel: diffuse small bowel wall thickening with marked mucosal hyperenhancement and fluid-filled loops in the setting of haemodynamic compromise. This is a systemic response to hypoperfusion and must not be misinterpreted as bowel perforation requiring surgery.

Systematic CT Search Pattern in Trauma

1. Lung bases and pleural spaces: pneumothorax, haemothorax, pulmonary contusion
2. Pericardium and heart: pericardial effusion, cardiac contusion
3. Solid organs (liver, spleen, kidneys, pancreas, adrenals): laceration depth, subcapsular haematoma, active extravasation; apply AAST organ injury grading
4. Bowel and mesentery: free gas on lung windows (mandatory), bowel wall integrity, mesenteric haematoma or stranding
5. Vascular structures: aorta, IVC, mesenteric vessels, extravasation, pseudoaneurysm, dissection
6. Retroperitoneum: haematoma, renal injury (poor FAST sensitivity), ureteric injury (delayed phase)
7. Bladder and pelvis: intra- vs extraperitoneal bladder rupture; pelvic fractures and associated haematoma
8. Bones: vertebral fractures, pelvic ring disruption

Differential Diagnosis: Free Fluid on FAST / CT

Key Pitfalls

FAST Pitfalls

• False-negative FAST for retroperitoneal haemorrhage (renal, aortic): no intraperitoneal free fluid despite major haemorrhage; FAST has poor sensitivity for renal trauma specifically
• False-negative FAST for early bowel perforation: insufficient fluid accumulation immediately post-injury
• Subcapsular haematoma: contained injury produces no free peritoneal fluid
• Perinephric fat: echogenic fat may mimic early haemoperitoneum; optimise gain settings
• Bladder artefact: posterior acoustic enhancement from a full bladder may mimic pelvic free fluid; assess true cul-de-sac posterior and superior to bladder
• Paediatric population: NPV ~50% for abdominal injury in stable children; CT should not be withheld based on negative FAST alone in a child with significant mechanism

CT Pitfalls

• Missing free gas: always review lung windows; small bubbles invisible on soft-tissue windows
• Shock bowel misinterpreted as perforation: bowel wall thickening with hyperenhancement in hypotension reflects hypoperfusion, not necessarily rupture; do not operate on this sign alone
• Delayed haemorrhage: initial CT may show minimal findings; active extravasation may only appear at delayed scanning or on repeat imaging
• Pseudo-enhancement artefact: may mimic active extravasation; compare arterial and portal venous phases
• Missed urinary extravasation: omitting delayed phase when renal pelvic or ureteric injury is suspected
• Subtle mesenteric injury: fat stranding and a small haematoma without free gas may be the only signs of a significant mesenteric tear at risk of subsequent bowel ischaemia; do not dismiss

Summary: FAST vs CT

Clinical Integration

FAST functions as a binary triage tool:

• Positive FAST + haemodynamic instability → immediate operative management; CT bypassed
• Positive FAST + haemodynamic stability → CT for injury characterisation and management planning
• Negative FAST + high-mechanism injury or clinical suspicion → CT mandatory; negative FAST does not exclude significant injury

MDCT with IV contrast, interpreted with meticulous attention to lung windows for free gas, dual-phase assessment for active haemorrhage, and systematic organ-by-organ review, remains the definitive imaging investigation in the haemodynamically stable injured patient and is the cornerstone of evidence-based trauma radiology practice.

Sources

• ACR Appropriateness Criteria (American College of Radiology)
• RANZCR position papers & standards
• Radiopaedia (open educational resource)