Pearls and Pitfalls in Emergency Radiology: Variants and Other Difficult Diagnoses

Martin L. Gunn

University of Washington School of Medicine

Subdural hematoma is the most common extra-axial collection and is present in up to 5% of trauma patients. Subdural hemorrhages are located between the arachnoid and inner layer of the dura and typically appear as a crescent-shaped fluid collection. As a rule, subdural hemorrhages can cross sutures, but cannot breach the dural attachments. Subdural hemorrhage will displace the cortical vascular structures medially and compress and mildly displace the underlying brain. The typical imaging appearance of subdural hemorrhage can vary depending on acuity. Hemorrhage can be classified as acute (6 hours to 3 days), subacute (3 days to 3 weeks), and chronic (> 3 weeks). The CT density decreases by approximately 1.5 Hounsfield Units (HU) per day as the hemorrhage resolves.

Subdural hemorrhage that is isodense to brain parenchyma is typically subacute. Rarely, isodense subdural hemorrhage can be acute in patients who are anemic (serum hemoglobin < 8–10 g/dL) (Figure 1.1A). Either case can easily be missed because small collections may be difficult to see when they are the same density as the underlying cortex [1].

Contrast-enhanced CT can detect isodense subdural hemorrhage. Enhancement of the dura, displacement of the cortical veins away from the skull, and cortical enhancement all help make the hemorrhage more conspicuous (Figure 1.1B) [2].

Based on the paramagnetic properties of blood at various stages of breakdown, MRI has become an excellent tool for the evaluation of isodense subdural hemorrhage (Table 1.1) [3]. Subacute blood products are composed of methemoglobin and are hyperintense on T1- and T2-weighted MR (Figure 1.2).