What are the indications for a computed tomography (CT) scan in an emergency setting, particularly in patients with severe head trauma, suspected stroke, acute abdominal pain, or severe chest pain, and considering factors such as history of kidney disease, allergy to contrast agents, or pregnancy?

CT Indications in Emergency Settings

CT scanning is the first-line imaging modality in emergency settings for trauma patients, suspected stroke, acute abdominal pain, and other life-threatening conditions, with specific protocols tailored to the clinical presentation and hemodynamic stability.

Head Trauma

Non-Contrast Head CT Indications

• Perform non-contrast head CT immediately for patients with head trauma who have abnormal neurologic examination, altered mental status, or focal neurologic deficits 1, 2, 3
• CT is superior for detecting acute hemorrhage, skull fractures, mass effect, and herniation—all requiring urgent neurosurgical evaluation 1, 3
• Do NOT routinely obtain CT scans for mild head injuries in pediatric patients who have returned to baseline mental status with normal neurologic examination 1
• CT performed within 6 hours of symptom onset has 98.7% sensitivity for subarachnoid hemorrhage 2

When to Add Vascular Imaging

• Add CTA of head and neck when vertebral artery dissection is suspected following trauma with ataxia or posterior circulation symptoms 1
• CTA is indicated for suspected blunt mediastinal vascular injury or wide mediastinum on chest radiograph 1

Common Pitfall

Do not delay CT imaging if MRI is unavailable when acute hemorrhage or stroke is suspected—CT's rapid acquisition time is critical in emergencies 2, 3

Suspected Stroke

Immediate Non-Contrast Head CT

• Non-contrast head CT is mandatory for all patients with suspected acute stroke to exclude hemorrhage before thrombolytic therapy 2, 3
• CT must be performed emergently to determine eligibility for time-sensitive interventions 2
• If CT is negative but clinical suspicion for subarachnoid hemorrhage remains high (especially >6 hours from onset), perform lumbar puncture for xanthochromia 2

Red Flag Features Requiring Urgent CT

• Thunderclap headache (sudden, severe onset) 2
• New-onset headache in patients ≥50 years old 2, 4
• Progressively worsening headache over days to weeks 2, 4
• Headache awakening patient from sleep 2
• Headache worsened by Valsalva maneuver 2

Abdominal Trauma

Hemodynamically Stable Patients

• Perform contrast-enhanced CT of chest, abdomen, and pelvis for all hemodynamically stable patients with suspected abdominal trauma 1
• CT has sensitivity of 98% and specificity of 98% for solid organ injuries 1
• Contrast-enhanced CT reduces mortality in polytrauma patients and identifies all bleeding sources 1
• Whole-body CT scanning time can be reduced to less than 30 seconds with modern multi-slice CT 1

Hemodynamically Unstable Patients

• Begin with bedside ultrasonography (FAST exam) to detect free intraperitoneal fluid 1
• If FAST is positive with systolic blood pressure <90 mmHg, proceed directly to urgent surgical intervention without CT 1
• Do not transport unstable patients to CT scanner—this delays life-saving intervention 1

Performance Characteristics

• CT sensitivity for hollow viscus injuries is lower (85%) compared to solid organ injuries (98%) 1
• Pneumoperitoneum and mesenteric infiltration have poor sensitivity (9%) for hollow organ injury—do not rely exclusively on CT 1
• For penetrating abdominal trauma, CT has 81% sensitivity and 85% specificity 1

Contrast Enhancement Protocol

• Always use IV contrast for abdominal trauma CT unless contraindicated 1
• Contrast pooling in peritoneal cavity indicates active massive bleeding requiring emergent surgery 1
• Intra-parenchymal contrast pooling with intact capsule suggests self-limited hemorrhage amenable to non-operative management 1

Acute Non-Localized Abdominal Pain (Non-Traumatic)

CT Abdomen and Pelvis with IV Contrast

• CT with IV contrast is the primary imaging modality for acute non-localized abdominal pain in adults 1
• CT changed management in 42% of patients with suspected mesenteric ischemia 1
• Reduced segmental bowel-wall enhancement on contrast CT is 100% specific for bowel infarction 1

When to Consider Alternative Imaging First

• In pediatric patients with suspected appendicitis, begin with ultrasonography 1
• Use Pediatric Appendicitis Score or Alvarado score for risk stratification 1
• If ultrasound is equivocal (non-visualized appendix), perform follow-up ultrasound after 6-12 hours of observation 1
• MRI without contrast is an alternative to CT in pediatric patients when ultrasound is non-diagnostic 1

Pregnancy Considerations

• In pregnant patients with acute abdominal pain, begin with ultrasonography 1
• If ultrasound is non-diagnostic, proceed to MRI without contrast 1
• Reserve CT for emergent situations when MRI is unavailable and ultrasound is inadequate 1

Diagnostic Yield Considerations

• Repeat CT after initially negative CT has low diagnostic yield (5.9% on fourth CT or greater) 1
• CT in patients with abdominal pain plus diarrhea changed management in only 11% versus 53% for abdominal pain alone 1
• Negative predictive value of CT for upper abdominal pain is relatively low (64%), commonly missing pancreaticobiliary inflammation and gastritis 1

Chest Trauma

CT Chest Indications

• CT chest with IV contrast is indicated for suspected blunt mediastinal vascular injury 1
• Perform CT chest for wide mediastinum on chest radiograph 1
• CT chest is indicated for penetrating thoracic trauma 1
• Do NOT perform routine whole-body CT in pediatric trauma patients—use selective imaging based on clinical findings 1

Diaphragmatic Hernia

Diagnostic Approach

• Begin with chest radiograph (anteroposterior and lateral) for patients without trauma history presenting with respiratory symptoms 1
• Chest radiograph has 2-60% sensitivity for left-sided hernia and 17-33% for right-sided hernia 1
• For stable trauma patients with suspected diaphragmatic hernia, perform contrast-enhanced CT of chest and abdomen 1
• CT is the gold standard with 14-82% sensitivity and 87% specificity 1

Special Populations

• In pregnant patients with suspected non-traumatic diaphragmatic hernia, begin with ultrasonography 1
• If ultrasound is non-diagnostic in pregnant patients, proceed to MRI 1

Nephrolithiasis

• Perform low-dose non-contrast CT (stone protocol) for suspected kidney stones 1
• Ultrasonography is an alternative first-line option in pediatric patients 1

Key Contraindications and Precautions

Contrast Allergy

• Use non-contrast protocols when contrast allergy is documented 1
• For abdominal trauma, contrast is essential—consider premedication protocols if history of mild reaction 1

Renal Dysfunction

• Assess renal function before contrast administration 1
• In trauma settings with life-threatening hemorrhage, contrast CT benefits outweigh nephrotoxicity risks 1

Radiation Exposure

• Between 322-1250 whole-body CTs are required to cause one lethal cancer 1
• With trauma mortality of 17% and odds ratio of 0.7 for CT benefit, only 20-40 patients need scanning to save one life—benefit outweighs radiation risk 1
• Avoid routine CT in pediatric patients with simple febrile seizures, uncomplicated headaches meeting migraine criteria, or mild head injuries with normal examination 1