When is a computed tomography (CT) scan of the head indicated for a patient with a head injury that occurred 3 weeks ago?

CT Imaging for Head Injury 3 Weeks Post-Trauma

For a head injury that occurred 3 weeks ago, MRI is the preferred imaging modality rather than CT, as you are now in the subacute/chronic phase where the focus shifts from detecting acute hemorrhage to identifying subtle structural injuries that explain persistent symptoms. 1

Imaging Strategy Based on Clinical Presentation

When MRI is Indicated (Preferred at 3 Weeks)

• MRI is the most useful initial imaging for subacute or chronic head trauma (>7 days post-injury) when patients have unexplained cognitive or neurologic deficits, as it is more sensitive than CT for detecting subtle findings that explain persistent symptoms 1

• MRI detects focal encephalomalacia at the inferior frontal or anterior temporal lobes (chronic sequelae of previous contusions) that CT misses, particularly adjacent to the calvarium or skull base 1

• Small white matter lesions (microbleeds) from traumatic axonal injury or diffuse axonal injury are better visualized on MRI and may explain persistent cognitive or neurologic deficits 1

• MRI is more sensitive for posterior fossa lesions, small cortical contusions, and cerebellar injuries that CT may miss 2, 3

When CT Remains Appropriate at 3 Weeks

• CT is indicated if the patient presents with new or worsening symptoms suggesting delayed complications, such as gradual decline from a subacute or chronic subdural hematoma 1

• CT is appropriate when there is a specific clinical question that does not require high soft-tissue contrast resolution, such as possible shunt failure in patients with chronic severe TBI 1

• If the patient has new neurological deterioration at any point, immediate CT is strongly indicated regardless of time since injury 4, 2

Clinical Scenarios Requiring Imaging at 3 Weeks

Red Flags Warranting Immediate Imaging

• Any new or progressive neurological deficits (weakness, sensory changes, cranial nerve abnormalities) 1, 4

• Persistent or worsening headaches that are severe or different in character 1, 3

• New onset confusion, memory problems, or cognitive decline beyond initial injury 1

• Recurrent vomiting or new onset vomiting 1, 3

• Seizures occurring weeks after the initial injury 1, 3

• Progressive decline in level of consciousness or alertness 4, 2

Special Populations at Higher Risk

• Patients on anticoagulation (warfarin, apixaban, other DOACs) or antiplatelet therapy have higher risk of delayed hemorrhage and warrant lower threshold for imaging 4, 3

• Elderly patients (age >60-65 years) have increased risk of chronic subdural hematomas developing over weeks 1, 3

• Patients with history of alcohol use have higher risk of delayed complications 5, 6

When Imaging May Not Be Necessary

• If the patient is neurologically intact, at baseline mental status, and has no new or persistent symptoms 3 weeks post-injury, routine imaging is not indicated 4, 3

• Patients with mild TBI who had negative initial CT and have returned to baseline do not require routine follow-up imaging 4, 3

Common Pitfalls to Avoid

• Ordering CT when MRI is more appropriate for the subacute/chronic timeframe - CT misses subtle injuries that explain persistent symptoms at this stage 1, 2

• Failing to recognize that chronic subdural hematomas can present with gradual decline weeks after seemingly minor trauma, especially in elderly or anticoagulated patients 1, 4

• Dismissing persistent cognitive symptoms without imaging evaluation - these may represent diffuse axonal injury or other subtle structural damage visible only on MRI 1, 3

• Assuming a negative acute CT scan rules out all significant injury - many traumatic lesions become apparent only on delayed MRI 3, 7