From the Guidelines
In the emergency room setting, when ordering a non-contrast head CT, the most sensitive and specific findings are acute intracranial hemorrhages, particularly subarachnoid hemorrhage and intraparenchymal hemorrhage. Fresh blood appears hyperdense (bright white) compared to brain tissue, making these hemorrhages readily identifiable within minutes of onset. Non-contrast head CT can detect as little as 2-3 ml of acute blood with nearly 100% sensitivity for acute hemorrhage if performed within 6 hours of symptom onset, as supported by the American Stroke Association guidelines 1. This high sensitivity for acute bleeding is why non-contrast head CT remains the first-line imaging study for patients presenting with acute neurological symptoms, suspected stroke, or head trauma. The density of blood changes over time, decreasing from hyperdense to isodense and eventually hypodense, which affects detection sensitivity.
Other findings visible on non-contrast head CT include:
- Midline shift
- Mass effect
- Hydrocephalus
- Skull fractures
- Large vessel ischemic strokes (after several hours) Though these generally have lower sensitivity and specificity compared to acute hemorrhage detection. According to the most recent study from 2024, CT head without IV contrast is usually performed first in the emergency setting for workup of acute stroke, and hemorrhage can be excluded rapidly to select patients for thrombolysis 1.
It's worth noting that while non-contrast head CT is highly sensitive for acute hemorrhage, it may not be as sensitive for other conditions such as posterior fossa ischemic strokes or subtle findings adjacent to the calvarium or skull base, for which MRI may be more sensitive 1. However, in the emergency room setting, non-contrast head CT remains the most appropriate initial imaging study due to its rapidity and high sensitivity for acute hemorrhage. The use of non-contrast head CT as the first-line imaging study is supported by the most recent and highest quality evidence, and it is essential for guiding treatment decisions in patients with suspected stroke or head trauma.
From the Research
Sensitive and Specific Findings on Noncontrast Head CT
The most sensitive and specific findings that can be seen on a head CT without contrast in the emergency room setting include:
- Acute hemorrhage
- Hydrocephalus
- Herniation
- Interval worsening of a prior finding 2, 3 These findings are considered positive if they necessitate a change in acute management.
Vascular Abnormalities
Noncontrast head CT can also detect vascular abnormalities, such as:
- Increased density and/or size of the vessels
- Filling defects
- Parenchymal and bony changes 4 However, dedicated vascular imaging such as CT/MR angiogram, venogram, or catheter angiogram may be necessary for confirmation and better characterization.
Predictive Value of Noncontrast Head CT
The predictive value of noncontrast head CT is influenced by various factors, including:
- Patient age
- Time of exam (daytime vs. nighttime)
- Clinical indication (e.g., malignancy, focal neurologic deficit, trauma, vertigo/dizziness) 3 These factors can affect the likelihood of positive findings on noncontrast head CT.
Emergency Computed Tomography
Emergency computed tomography, including noncontrast head CT, plays a crucial role in the assessment of trauma and nontrauma emergencies 5. While noncontrast head CT is a valuable diagnostic tool, it is essential to consider the clinical context and potential limitations of the examination.