CT Technique
Oral contrast is NOT required for CT evaluation of blunt bowel and mesenteric injury. Meta-analysis of 32 studies (Lee et al, 16,000+ patients) found no difference in accuracy with or without oral contrast. Sensitivity 95.0%, specificity 99.6% without oral contrast (Allen et al).
Protocol: portal venous phase (70-second delay) from lung bases through greater trochanters. Multiplanar reformats: axial, coronal, sagittal at ≤1.25 mm. Delayed phase (5 min) obtained when active hemorrhage is detected in real time — extravasation from mesenteric vessels can expand substantially on delayed images.
Arterial phase is NOT routinely performed for isolated blunt abdominal trauma; however, it may be added when high-energy mechanism (motor vehicle collision) triggers a chest CT that is extended inferiorly to include the splenic vasculature.
Mechanisms of Injury
Three mechanisms produce blunt bowel and mesenteric injury:
| Mechanism | Description | Anatomic Predilection |
|---|---|---|
| Shear | Rapid deceleration tears bowel at fixed points | Ligament of Treitz, ileocecal valve, sigmoid colon — where bowel is tethered to retroperitoneum |
| Crush | Bowel compressed between external force and osseous skeleton | Small bowel and mesentery; seatbelt-induced; incidence increased after introduction of seat belts |
| Burst | Intraluminal pressure rises to 120–140 mmHg → perforation | Single or multiple small perforations at antimesenteric border; predisposed by ileus, bowel obstruction, or Crohn disease; less associated force than shear/crush |
Direct CT Findings of Bowel Injury
| Finding | Specificity | Sensitivity | Notes |
|---|---|---|---|
| Discontinuous bowel wall | 100% | 5–10% | Most specific CT sign. Pathognomonic for bowel injury when present. Inspect all loops on coronal and sagittal reformats. |
| Free intraperitoneal air (pneumoperitoneum) | 95% | 30–60% | Fairly reliable but NOT pathognomonic. False positives: pneumothorax, chest tube, diaphragmatic injury, Foley catheter in bladder rupture. Free air + free fluid + seat-belt sign + focal bowel abnormality = highly predictive. |
| Pneumoretroperitoneum | High | Low | Gas in retroperitoneum suggests duodenal (2nd–4th portions), ascending colon, or descending colon injury. Periduodenal gas → duodenal perforation until proven otherwise. |
| Extraluminal oral contrast | Very high | Low | Only applicable if oral contrast was given (not routine in trauma). Highly specific — confirms perforation. |
| Bowel wall hypoenhancement / absent enhancement | High | 10–15% | Focal absent enhancement = devascularization or transmural infarction. Active arterial blush within bowel wall = active hemorrhage. |
Free air + free intraperitoneal fluid + seat-belt sign + focal bowel abnormality = highly predictive of bowel perforation requiring surgery. Do NOT dismiss free air as artifact when these are present together.
Indirect CT Findings of Bowel and Mesenteric Injury
| Finding | Specificity | Sensitivity | Notes |
|---|---|---|---|
| Focal bowel wall thickening (3–4 mm) | 90% | 55–75% | Reliable when focal. Diffuse small bowel thickening >10 mm = "shock bowel" (hypoperfusion complex) — NOT traumatic injury; resolves with resuscitation. |
| Abnormal bowel wall enhancement (Janus sign) | 90% | 10–15% | Adjacent enhancing and nonenhancing bowel loops. Janus sign = alternating hyperenhancing and hypoenhancing segments. Specific but insensitive. Focal mucosal hyperenhancement also seen in reperfusion after arterial injury. |
| Mesenteric infiltration / stranding | 40–90% | 70–77% | Hazy increased attenuation in mesenteric fat. Represents contusion or laceration. Stranding alone (without extravasation) may not require surgery. |
| Mesenteric contrast extravasation | Very high | Low | Active blush within mesentery = arterial injury. Much more specific than simple stranding. Generally requires surgical exploration. |
| Free fluid without solid organ injury | 15–25% | 90–100% | Most sensitive indirect sign. Hyperattenuating free fluid (>30 HU) raises suspicion for bowel injury. Up to 3% of male trauma patients have physiologic low-attenuation free pelvic fluid without injury. |
Approximately 3% of male trauma patients have a small amount of low-attenuation simple pelvic fluid without identifiable injury — this may be from aggressive IV hydration. However, hyperattenuating free fluid, free fluid + seat-belt sign, or free fluid + focal bowel abnormality is clinically significant and should prompt further evaluation.
Associated Findings
Chance Fracture — Hyperflexion injury of the lumbar spine (posterior element distraction + vertebral body compression). Chance-type fractures have a 40% rate of associated intra-abdominal injury. Bowel injury is particularly likely when the fracture has a burst component. Always evaluate the entire bowel when a Chance fracture is identified.
Multiple Solid Organ Injuries — The incidence of bowel and mesenteric injury rises substantially with increasing number of solid organ injuries:
- 1 solid organ injured → 7.3% bowel injury rate
- 2 solid organs injured → 15.4%
- 3 solid organs injured → 34.4%
Pancreatic Injuries — Associated with duodenal injury in ~20% of cases. When deep pancreatic laceration involves >50% of gland thickness, main pancreatic duct injury should be presumed and MRCP obtained.
Segment-Specific Pearls
| Segment | Key CT Findings | Pitfalls |
|---|---|---|
| Duodenum | Pneumoretroperitoneum adjacent to D2–D4; peripancreatic hematoma; duodenal wall thickening or discontinuity; retroperitoneal fluid tracking; duodenal hematoma = intramural filling defect on contrast CT | Retroperitoneal location makes free air hard to detect; delayed phase may show retroperitoneal contrast leak; MRCP for pancreatic duct co-injury |
| Small bowel (jejunum/ileum) | Most commonly injured hollow viscus in blunt trauma; look for focal wall thickening, free air, adjacent mesenteric hematoma; Chance fracture association; lap belt bruising (seat-belt sign) | Free air may be tiny — search perihepatic and subdiaphragmatic spaces on soft tissue and lung windows |
| Colon | Ascending and sigmoid most vulnerable; blunt colonic injury less common than small bowel; look for pericolic fat stranding, wall thickening, focal hypoenhancement, free air tracking retroperitoneally | Colonic injury often requires laparotomy; right colon injury associated with shear at ileocecal junction; sigmoid fixed at peritoneal reflection |
Management Decision Points
| Scenario | Action |
|---|---|
| Peritonitis, uncontrolled hemorrhage, hemodynamic instability | Emergent laparotomy — do not delay for CT |
| Direct CT signs of perforation (discontinuous wall, free air + bowel signs) | Surgical exploration; consult trauma surgery immediately |
| Indirect signs only (stranding, free fluid, wall thickening) — clinically stable | Admit for observation; repeat CT abdomen/pelvis at 12–24 h with oral contrast if clinical concern persists |
| Questionable findings at initial CT — high-risk mechanism, seat-belt sign, Chance fracture | Repeat CT at 12–24 h (oral contrast recommended); bowel injury prediction score (McNutt BIPS) may assist triage |
| Isolated low-attenuation pelvic free fluid in male patient — no bowel signs | Observe; serial abdominal exams; low threshold for repeat CT |
Reporting Checklist — Blunt Bowel and Mesenteric Injury
- Pneumoperitoneum: present / absent — location (subdiaphragmatic, perihepatic, periduodenal, retroperitoneal)
- Pneumoretroperitoneum: present / absent — suggests D2–D4, ascending, or descending colon injury
- Discontinuous bowel wall: present / absent — segment, location
- Focal bowel wall thickening: present / absent — segment; focally >3 mm (traumatic) vs diffusely >10 mm (shock bowel — do not confuse)
- Bowel wall enhancement: normal / focally absent (ischemia) / active extravasation (blush)
- Mesenteric findings: none / stranding / hematoma / active extravasation — location (root, right, left, sigmoid)
- Free intraperitoneal fluid: absent / low-attenuation / hyperattenuating (>30 HU); volume; distribution
- Seat-belt sign: subcutaneous fat stranding of anterior abdominal wall
- Chance fracture: present / absent — level
- Solid organ injuries: number and organs involved (correlates with hollow viscus risk)
- Recommendation: if indirect signs only in stable patient → recommend repeat CT in 12–24 h with oral contrast
Reference
Bates DDB, Wasserman M, Malek A, et al. Multidetector CT of Surgically Proven Blunt Bowel and Mesenteric Injury. RadioGraphics. 2017;37(2):613–625. doi:10.1148/rg.2017160092