Should I obtain a computed tomography (CT) scan or magnetic resonance imaging (MRI) for a patient with new onset left-sided weakness 4 weeks after a concussion?

MRI is Indicated for New-Onset Left-Sided Weakness 4 Weeks After Concussion

For a patient with new-onset left-sided weakness 4 weeks after a concussion, MRI is strongly recommended over CT as it provides superior detection of subacute brain injuries that may explain the neurological deficit. 1

Rationale for MRI Selection

Clinical Presentation Analysis

• New-onset left-sided weakness appearing 4 weeks after initial injury represents a persistent neurologic deficit
• This timing (4 weeks post-injury) places the patient in the subacute phase of traumatic brain injury
• The development of focal neurological deficits that were not present initially requires thorough investigation

Evidence-Based Imaging Selection

1. MRI Superiority for Subacute Evaluation:

   • MRI is significantly more sensitive than CT for detecting:
     • Small cortical contusions
     • Subdural hematomas (especially near the skull base)
     • Diffuse axonal injury (DAI)
     • Non-hemorrhagic axonal injury
     • Ischemic changes 1

2. CT Limitations:

   • CT has limited sensitivity for non-hemorrhagic axonal injury
   • Only about 10% of diffuse axonal injuries are detectable on CT (>80% of DAI lesions are non-hemorrhagic) 1
   • CT may miss subtle findings that could explain new neurological deficits 1

3. Specific Indications for MRI:

   • The ACR Appropriateness Criteria explicitly states: "MRI may be indicated in acute TBI when results on NCCT are normal and there are persistent unexplained neurologic findings, such as new-onset, progressive, or worsening symptoms" (Class I recommendation) 1
   • Brain MRI is recommended as the most useful initial imaging for evaluation of subacute or chronic head trauma 1

Recommended MRI Protocol

• T1-weighted imaging
• T2-weighted imaging
• T2*-weighted/gradient echo sequences (for detecting microhemorrhages)
• Diffusion-weighted imaging (to detect ischemic changes)
• FLAIR sequences (fluid-attenuated inversion recovery) 1

Clinical Considerations

Potential Pathologies to Evaluate

• Subacute subdural hematoma
• Small cortical contusions that evolved over time
• Diffuse axonal injury with delayed manifestation
• Ischemic changes secondary to vascular compromise
• Delayed neurological complications of TBI 1, 2

Important Caveats

• While CT is the first-line imaging in acute trauma (0-7 days), MRI becomes more valuable in the subacute phase (>7 days) 1
• The Transforming Research and Clinical Knowledge in TBI study found that 27% of patients with mild TBI and normal CT had abnormalities on MRI 1
• Left-sided weakness suggests a right-sided brain lesion that may have been too subtle to detect on initial evaluation or may have evolved over time

Algorithm for Decision-Making in Post-Concussion Neurological Changes

1. For new focal neurological deficits (like left-sided weakness):

   • MRI is first-line imaging 1

2. For worsening symptoms without focal deficits:

   • Consider MRI if symptoms are progressive or persistent 1

3. For stable symptoms without new deficits:

   • Clinical follow-up may be sufficient without additional imaging 1

MRI provides the most comprehensive evaluation for a patient with new neurological deficits in the subacute phase after concussion, offering the best opportunity to identify the underlying cause and guide appropriate management.