Ballistic Fundamentals
| Mechanism | Velocity | Injury Pattern |
|---|---|---|
| Low-velocity | <2,000 fps — handguns and most civilian firearms | Injury from permanent cavitation only (tissue crushed directly by projectile). Tissue destruction limited to projectile diameter + fragmentation. |
| High-velocity | >2,000 fps — rifles and military weapons | Permanent cavitation plus temporary cavitation: pressure wave expands a transient cavity far larger than the bullet diameter, causing indirect injury to structures not directly contacted. Organs tolerating stretch (lung, muscle) handle this better than those that do not (brain, liver, spleen, kidney, bowel). |
| Shotgun | Range-dependent | Close range (<3 m): high-energy injury comparable to rifle. Intermediate range: spreading pellet pattern. Distant range (>7 m): pellets in subcutaneous tissue only. |
Secondary projectiles: Bullet fragments and bone chips driven off by impact become high-velocity secondary missiles, extending the zone of injury well beyond the primary bullet path. When a bullet strikes cortical bone, resulting bone fragments can lacerate adjacent neurovascular structures, even if the primary projectile did not directly contact them. Always evaluate structures adjacent to any bone impact site.
CT Trajectory Assessment — Structures to Evaluate by Zone
| Zone | Critical Structures Along Path | Key CT Findings |
|---|---|---|
| Neck (Zones I–III) | Carotid arteries, vertebral arteries, jugular veins, trachea, esophagus, spinal cord, brachial plexus | Expanding hematoma; pseudoaneurysm; occlusion / dissection on CTA; tracheal air leak; esophageal perforation (periesophageal gas, fluid); epidural hematoma; cord signal change on MRI |
| Chest | Aorta / great vessels, heart / pericardium, trachea / bronchi, esophagus, lung parenchyma, diaphragm | Hemothorax; pneumothorax; pulmonary laceration; pericardial effusion / hemopericardium; mediastinal hematoma; diaphragmatic defect (herniated bowel / omentum); esophageal gas; aortic pseudoaneurysm on CTA |
| Abdomen / Pelvis | Liver, spleen, kidneys, bowel (small + large), mesenteric vessels, bladder, urethra, iliac vessels, lumbosacral plexus | Solid organ lacerations (AAST grade); free fluid (blood vs. bowel contents); pneumoperitoneum; mesenteric fat stranding / hematoma; active extravasation on CTA; pelvic fracture with vascular injury; perineal gas (rectal injury); extraperitoneal bladder rupture |
| Spine | Vertebral bodies, spinal cord / cauda equina, epidural space, paraspinal muscles | Vertebral body fracture / comminution; intracanalicular bullet or bone fragment; epidural hematoma; bullet trajectory crossing midline predicts cord injury; paraspinal hematoma |
| Extremities (MSK) | Long bones, major arteries / veins, peripheral nerves, joints, compartments | Comminuted fracture with bone fragment dispersal; periosteal stripping; intraarticular fragments; expanding soft tissue hematoma; absent distal flow on CTA; compartment syndrome (muscle edema, obliteration of fat planes) |
Intraarticular Bullet / Fragment
Report explicitly — clinical consequences are high:
| Consequence | Mechanism |
|---|---|
| Lead toxicity | Synovial fluid is mildly acidic and dissolves lead → systemic absorption → plumbism (anemia, encephalopathy, neuropathy). Risk is highest when the bullet is within a synovial joint or bursa; fragments in soft tissue or bone carry lower but real risk if near vascular structures. |
| Joint sepsis | Bullet carries skin flora along the tract; intraarticular entry creates a direct pathway for infection. |
| Articular cartilage injury | Metallic debris and bone chips cause mechanical damage and degenerative change. |
Surgical extraction is generally indicated for intraarticular projectiles.
Retained Projectile — MRI Considerations
Most civilian lead bullets and copper-jacketed rounds are non-ferromagnetic and are generally considered MRI-conditional or safe. Steel-jacketed military rounds (full metal jacket with steel core) may be ferromagnetic — obtain prior radiographs to assess composition before MRI. Metallic fragments in or near the orbit, spinal canal, or major vessels are high-risk regardless of composition. MRI artifact (susceptibility) can limit evaluation of adjacent structures — CT may be preferred for trajectory assessment near retained fragments.
Reporting Checklist — Gunshot Injuries
- Projectile accounting: number visible on scout / XR; correlate with entry / exit wounds to determine retained vs. passed through
- Trajectory: describe projected path (entry zone → exit zone or retained location); note crossing midline, traversing body cavities, proximity to named structures
- Bone injury: fracture pattern, degree of comminution; bone fragment dispersal (secondary missiles); periosteal stripping
- Intraarticular involvement: bullet or fragment within joint space — specify joint; note articular surface injury
- Vascular: expanding hematoma, pseudoaneurysm, occlusion, active extravasation (CTA if trajectory near major vessels)
- Hollow viscus: free air, mesenteric stranding / hematoma, bowel wall discontinuity (GI perforation → high infection risk from bacterial seeding along tract)
- Solid organ: laceration grade (AAST), subcapsular hematoma, devascularized segment, active extravasation
- Spine: vertebral fracture, intracanalicular fragment, epidural hematoma; trajectory crossing spinal canal
- Compartment syndrome: muscle edema, loss of fat plane definition, fascial integrity; flag if trajectory through a tight compartment (leg, forearm, thigh)
- Retained foreign material: location (intraarticular / soft tissue / bone / vascular / spinal canal); MRI safety note if relevant
- Recommended additional imaging: CTA for vascular injury, CT cystogram for pelvic trajectory near bladder, MRI spine for cord compression
References
Sodagari F et al. Gunshot Wounds: Mechanisms, Radiologic Manifestations, and Complications. RadioGraphics. 2020;40(6):1756–1788.