Low attenuation of acute traumatic hemoperitoneum on CT scans

This study was undertaken to determine the incidence of low attenuation values in intraperitoneal hemorrhage, which could be confused with ascites.

We retrospectively analyzed the CT scans of 42 consecutive patients with hepatic and splenic lacerations and intraperitoneal fluid after blunt abdominal trauma. Patients were excluded if they had prior peritoneal lavage, bladder or bowel injury, or low hematocrit values. Intraperitoneal fluid was categorized by the site of accumulation (perihepatic, perisplenic, Morison's pouch, paracolic gutters, or pelvis). The amount of fluid in each intraperitoneal location was categorized as small, moderate, or large. Attenuation values were obtained from each intraperitoneal site, and overall mean attenuation values were determined for each patient. We correlated the size of each fluid collection with the attenuation value. We also compared attenuation values at locations adjacent to the site of each injury with those at other intraperitoneal sites. We then evaluated technical factors that could have lowered attenuation values, including CT miscalibration, volume averaging, and beam-hardening artifacts.

For the 42 patients, we measured 131 separate attenuation values. Attenuation values ranged from 0 to 80 H, with attenuation of 24% of sites (32/131) measuring less than 20 H. Only 16% of sites (21/131) had attenuation values greater than 45 H. Attenuation at the remaining 78 sites (60%) measured from 20 to 45 H. All intraperitoneal locations except the pelvis had mean attenuation values significantly lower then 40 H. Mean attenuation values (determined by averaging measurements from different intraperitoneal sites) were also calculated for each patient. Only 6 (14%) of 42 patients had mean attenuation values greater than 40 H, whereas 4 (10%) of 42 patients had mean attenuation values less than 20 H. The remaining 32 patients (76%) had mean attenuation values between 21 and 40 H. Patients with hepatic lacerations showed no significant difference (p = .3509) in attenuation between perihepatic fluid and the remainder of intraperitoneal fluid. However, in patients with splenic lacerations, perisplenic fluid had a significantly higher (p = .0013) attenuation value (43 H) than did fluid at other intraperitoneal locations.

Low attenuation measurements for acute hemoperitoneum represented a common finding that was not attributable to technical factors or underlying anemia. Fluid with attenuation values less than 20 H in acute trauma should not be dismissed as ascitic fluid.