What should the CT (Computed Tomography) head scan protocol be to rule out acute intracranial pathology?

CT Head Protocol for Ruling Out Acute Intracranial Pathology

A non-contrast CT head scan is the first-line imaging protocol for ruling out acute intracranial pathology, as it can be performed safely and rapidly in all patients with suspected acute intracranial conditions. 1

Primary Protocol Recommendation

Non-contrast CT Head

• Standard protocol without intravenous contrast administration
• Should include multiplanar reformatted images to increase diagnostic accuracy 2
• Advantages:
  • Rapid acquisition (crucial for unstable patients)
  • High sensitivity for acute hemorrhage
  • Ability to detect mass effect, hydrocephalus, and midline shift
  • Less susceptible to motion artifacts than MRI
  • Can be performed in patients with contraindications to MRI

Clinical Rationale

Non-contrast head CT is the imaging test of choice in this setting because:

• It rapidly detects life-threatening conditions requiring immediate intervention
• It effectively identifies hemorrhage, which appears hyperdense (50-100 Hounsfield units) 2
• It can detect mass effect, midline shift, and hydrocephalus
• The yield of acute contributory findings on CT ranges from 2% to 45% depending on clinical presentation 1

When to Consider Contrast Enhancement

Contrast-enhanced CT examinations should only be considered as a second step if:

• Initial non-contrast CT is negative but clinical suspicion remains high
• There is specific concern for intracranial infection, tumor, or inflammatory pathologies 1

However, contrast-enhanced head CTs as a first-line test in the acute setting may not add significant value over non-contrast head CT examinations 1, 3. A recent study found that most acute intracranial infections with enhancing CT findings also have correlative conspicuous non-contrast findings that would merit further evaluation 3.

Follow-up Imaging Considerations

If the initial non-contrast CT is negative but clinical suspicion for intracranial pathology remains high:

• MRI may be considered as a second-line test due to its higher sensitivity for:
  • Small ischemic infarcts
  • Encephalitis
  • Subtle cases of subarachnoid hemorrhage (SAH) 1
  • Detecting chronic hemorrhage 4

Common Pitfalls to Avoid

1. Delaying imaging: Prompt CT imaging is critical, as delays significantly increase morbidity and mortality risk 2
2. Over-reliance on CT for ischemic stroke: CT has limited sensitivity for early ischemic changes; MRI is more sensitive for acute infarcts 1
3. Missing subtle hemorrhage: Small subarachnoid hemorrhages can be difficult to detect on CT
4. Assuming all hemorrhages are traumatic: Consider other etiologies such as hypertension, cerebral amyloid angiopathy, or hemorrhagic transformation of ischemic stroke 5
5. Failing to consider background white matter changes: These may mask vasogenic edema surrounding intra-axial lesions on non-contrast CT 3

Special Considerations

• For patients with suspected stroke, focal neurologic deficit, seizure, or head trauma, specific ACR Appropriateness Criteria should be referenced 1
• In patients with altered mental status, a non-contrast head CT has a positive predictive value of 100% and negative predictive value of 97.7% for detecting markers associated with enhancing abnormalities 3
• Only approximately 3.6% of patients presenting with clinical stroke symptoms have intracranial hemorrhage on initial non-contrast CT scan 6

By following this protocol, clinicians can efficiently rule out acute intracranial pathology while minimizing unnecessary radiation exposure and contrast administration.