In a patient with a small right frontal subarachnoid hemorrhage and bilateral basal ganglia hypodensity consistent with hypoxic‑ischemic injury on CT, what is the recommended next imaging study and which MRI sequences should be obtained to assess the injury and detect any residual hemorrhage?

MRI Brain is the Recommended Next Imaging Study

In a patient with small right frontal subarachnoid hemorrhage and bilateral basal ganglia hypodensity consistent with hypoxic-ischemic injury, MRI brain should be obtained as the next imaging study, with specific sequences including diffusion-weighted imaging (DWI), T2-weighted imaging, T2-weighted/gradient recalled echo (GRE) or susceptibility-weighted imaging (SWI), and FLAIR sequences.* 1

Rationale for MRI Over Repeat CT

• MRI is superior to CT for detecting the full extent of hypoxic-ischemic injury, particularly in identifying subtle cortical contusions, white matter lesions, and diffuse axonal injury that may explain persistent neurologic deficits after the initial CT findings 1

• The ACR Appropriateness Criteria explicitly recommend MRI for follow-up when persistent neurologic deficits remain unexplained after head CT, as MRI is more sensitive for subtle findings adjacent to the calvarium or skull base and for small white matter lesions 1

• In the context of post-cardiac arrest hypoxic-ischemic injury, MRI provides superior accuracy compared to CT in assessing regional injury severity, with DWI sequences particularly valuable for detecting cytotoxic edema and predicting neurological outcomes 1

Essential MRI Sequences and Their Specific Purposes

For Hypoxic-Ischemic Injury Assessment:

• Diffusion-weighted imaging (DWI) is critical for identifying areas of cytotoxic edema and acute ischemic injury, with lesions in the basal ganglia and cerebral cortex being particularly prognostic 1

• T2-weighted imaging helps characterize the extent of vasogenic edema and cortical laminar necrosis that develops in hypoxic-ischemic injury 1

• FLAIR sequences are useful for detecting subtle cortical injury and differentiating acute from chronic changes 1

For Hemorrhage Detection:

• T2-weighted gradient recalled echo (GRE) or susceptibility-weighted imaging (SWI)* are essential for detecting residual hemorrhage, as these sequences are highly sensitive to blood products and can identify microbleeds not visible on CT 1, 2, 3

• MRI can detect acute intracerebral hemorrhage with accuracy equivalent to CT (96% concordance), and is actually more sensitive for detecting hemorrhagic transformation and chronic hemorrhage 3

• Hyperacute hemorrhage (within 2 hours) shows a distinctive three-zone pattern on MRI: a center that is isointense to hyperintense on T2*/T2-weighted imaging, a hypointense periphery from susceptibility effects, and a rim of vasogenic edema 2

Timing Considerations

• MRI should ideally be performed within 72 hours post-injury for optimal prognostic value in patients with moderate to severe brain injury, as this timing captures critical pathophysiological changes 4

• In the context of post-cardiac arrest care, MRI performed within the first 2 weeks after injury (particularly within the first week) provides the most valuable prognostic information regarding basal ganglia and cortical lesions 1

• The repeat CT already performed shows evolution of injury with cortical laminar necrosis, making this an appropriate time for MRI to fully characterize the extent of injury 1

Clinical Pitfalls to Avoid

• Do not assume CT is sufficient for complete evaluation of hypoxic-ischemic injury—CT has limited sensitivity for subtle cortical contusions, diffuse axonal injury, and early ischemic changes that MRI readily detects 1

• Standard MRI sequences have low sensitivity for skull fractures, so if fracture assessment is needed, CT remains superior 1

• Avoid using MRI findings from the acute phase alone to make definitive prognostic decisions, as some patients with regional lesions on MRI can still have favorable outcomes; MRI should be integrated with clinical examination and other prognostic markers 1

• In the setting of traumatic SAH (as opposed to aneurysmal SAH), the risk of delayed ischemic injury and vasospasm is significantly lower, so the primary focus should be on characterizing the hypoxic-ischemic injury rather than vascular complications 5

Practical Implementation

• The MRI protocol should include at minimum: DWI, T2-weighted imaging, T2*/GRE or SWI, FLAIR, and T1-weighted imaging 1

• Multiplanar and 3D-reconstructed images increase sensitivity for small hemorrhages and should be performed when possible 1

• MRI is more sensitive than CT for detecting small subdural hematomas, particularly in the posterior fossa, which is relevant given the SAH in this patient 1

• The presence of bilateral basal ganglia hypodensity on CT strongly suggests hypoxic-ischemic injury, and MRI with DWI will definitively characterize the extent and distribution of this injury, which has significant prognostic implications 1