Comparative sensitivity of computed tomography vs. magnetic resonance imaging for detecting acute posterior fossa infarct

Abstract

Background: Posterior fossa strokes, particularly those related to basilar occlusion, pose a high risk for progression and poor neurological outcomes. The clinical history and examination are often not adequately sensitive or specific for detection.

Study objectives: Because this population stands to benefit from acute interventions such as intravenous and intra-arterial tissue plasminogen activator, mechanical thrombectomy, and intensive monitoring for neurologic deterioration, this study examined the sensitivity of non-contrast head computed tomography (NCCT) for diagnosing posterior fossa strokes in the emergency department.

Methods: This study analyzed a prospectively collected database of acute ischemic stroke patients who underwent head NCCT within 30 h of symptom onset and who were subsequently found to have a posterior fossa infarct on brain magnetic resonance imaging (MRI) performed within 6 h of the NCCT.

Results: There were 67 patients identified who had restricted diffusion on MRI in the posterior fossa. The National Institutes of Health Stroke Scale (NIHSS) scores ranged from 0 to 36, median 3. Only 28 patients had evidence of infarction on the initial NCCT scan. The timing of NCCT scans ranged from 1.2 to 28.9 h after symptom onset. The sensitivity of NCCT was 41.8% (95% confidence interval 30.1-54.4). The longest period of time between symptom onset and a negative NCCT with a subsequent positive diffusion-weighted imaging MRI was 26.7 h.

Conclusions: Head NCCT imaging is frequently insensitive for detecting posterior fossa infarction. Temporal evolution of strokes in this distribution, coupled with beam-hardening artifact, may contribute to this limitation. When a posterior fossa stroke is suspected and the NCCT is non-diagnostic, MRI is the preferred imaging modality to exclude posterior fossa infarction.

Figure 1

Additional strokes detected by DWI MRI for a patient whose head NCCT was interpreted as positive for posterior fossa infarct. The top and bottom left panels show two slices from the head NCCT scan of the patient, who presented initially with symptoms of dizziness and confusion. The NCCT scan was interpreted by an attending neuroradiologist as having a possible infarct in the right hemipons. The top and bottom right panels show two corresponding sections from the DWI sequence of the same patient’s brain MRI. There is clear involvement of both sides of the pons, as well as infarction in the cerebellum. This particular patient also had extensive involvement of territories supplied by both the right and left posterior cerebral arteries. The patient eventually became comatose with extensor posturing in the ED.