What is the best initial imaging modality, CT (Computed Tomography) or US (Ultrasound), for evaluating the head?

CT is Superior to Ultrasound for Initial Head Imaging

CT scan is the most appropriate initial imaging modality for evaluating the head, not ultrasound. 1 CT is the imaging modality of choice for acute head evaluation, particularly in trauma settings, due to its high sensitivity for detecting acute hemorrhage, fractures, and other neurosurgically significant lesions.

Advantages of CT for Head Imaging

• Speed and Accessibility: CT scans can be performed rapidly, which is critical in emergency situations 1
• Superior Detection: CT has near 100% sensitivity for detecting acute hemorrhage, mass effect, and other neurosurgical lesions 1
• Bone Visualization: CT provides excellent visualization of skull fractures and bony abnormalities 2
• Multiplanar Capabilities: Reformatted images increase diagnostic accuracy and should be included in head CT protocols 1

Limited Role of Ultrasound in Head Imaging

Ultrasound has extremely limited applications for head imaging in most clinical scenarios:

• No mention of ultrasound as an appropriate imaging modality for head trauma in the ACR Appropriateness Criteria 1
• Ultrasound is not listed as an appropriate imaging modality for headache evaluation in children 1
• Ultrasound has significantly poorer interobserver agreement compared to CT for detecting intracranial hemorrhage and ischemia 3

Clinical Decision Algorithm for Head Imaging

1. Acute trauma setting:

   • CT without contrast is first-line imaging 1
   • Multiplanar reformatted images should be included 1
   • Follow with MRI only if neurologic deficits remain unexplained after CT 1

2. Suspected vascular abnormalities:

   • Initial non-contrast CT to detect hemorrhage 1
   • If hemorrhage is detected, proceed to CTA, MRA, or conventional angiography 1

3. Subacute or chronic evaluation:

   • MRI is preferred over CT for evaluating subtle findings and white matter lesions 1
   • MRI is more sensitive than CT for detecting small cortical contusions and subdural hematomas 1

Important Considerations and Pitfalls

• Radiation Exposure: CT scanning involves radiation exposure, with predictions that 2% of future cancers may be attributable to CT radiation 4. This is particularly important in pediatric patients.

• Follow-up Imaging: Serial brain imaging may not be required for all children with intracranial injury 5. Unnecessary repeat scans increase radiation exposure without clinical benefit.

• Special Populations: In neonates, while ultrasound has traditionally been used due to open fontanelles, both CT and MRI demonstrate better sensitivity and interobserver agreement for detecting intracranial pathology 3.

• Contrast Use: Non-contrast CT is preferred in trauma settings as contrast may obscure small hemorrhages 1.

In conclusion, CT remains the gold standard for initial head imaging in most clinical scenarios, particularly in acute settings, while MRI offers advantages in subacute and chronic evaluation. Ultrasound has very limited applications for head imaging outside of specific neonatal contexts.