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E2102. A Pictorial Review: Normal Pressure Hydrocephalus Structural MRI and Scintigraphic Imaging
Authors
  1. Kunal Patel; Mount Sinai Medical Center
  2. Jyostna Kochiyil; Mount Sinai Medical Center
  3. Vinay Bhatia; Mount Sinai Medical Center
  4. Chetan Rajadhyaksha; Mount Sinai Medical Center
Background
Idiopathic normal pressure hydrocephalus (NPH) was initially described as a treatable syndrome in the 1960s, and was established upon pneumoencephalography. Given the degree of difficulty to differentiate from cerebral atrophy, multimodality imaging has played a role in the diagnostic imaging workup. NPH is often defined by clinical symptoms of urinary incontinence, gait abnormalities, and cognitive derangement. Ventriculomegaly is an imaging hallmark despite normal cerebrospinal fluid (CSF) pressures. Imaging such as structural MRI and cisternograms are useful tools to supplement a patient’s diagnostic evaluation, especially in equivocal cases. Prior to the development of CSF flow studies, structural MRI and cisternograms were the most suitable exams to aid in CSF flow dynamics and diagnosis. Correlation between structural MRI and cisternograms help understand CSF flow dynamics and improve future study evaluations when findings are corroborated.

Educational Goals / Teaching Points
Discuss pertinent anatomy, clinical presentation, and proposed pathophysiology of normal pressure hydrocephalus. Review salient imaging findings of normal pressure hydrocephalus on structural MRI and nuclear medicine cisternograms. Reveal pearls and pitfalls of structural MRI and nuclear medicine cisternograms when imaging normal pressure hydrocephalus patients.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Three vital factors in structural MRI help determine ventriculomegaly: Evan’s Index (EI), callosal angle, and ventriculosulcal disproportion. These defined assessments are helpful in diagnosing NPH from a purely imaging point of view. Pitfalls of structural MRI range from technique to availability in addition to contraindications. Technique of a standard MRI brain can affect measurements of callosal angle when T2 weighted coronal images are not acquired perpendicular to the anterior-posterior commissure plane, as the angle is measured at the level of posterior commissure. In contrast, T2 space images allow for volumetric acquisition, allowing retrospective reconstruction for appropriate measurements. Structural analysis is important in patients with suspected NPH or ventriculomegaly, although CSF flow dynamics will ultimately support neurosurgical evaluation. CSF flow dynamics are dependent on the cardiac cycle with craniocaudal net flux via the cerebral aqueduct. Scintigraphic evaluation can be performed to evaluate NPH with Indium 111-DTPA via an intrathecal injection. Radiotracer within the CSF helps delineate qualitative reflux into the ventricles with persistence, suggesting hyperdynamic bi-directional flow through the cerebral aqueduct and a caudocranial net flux through the cerebral aqueduct. Alternatively, initial reflux of radiotracer into the lateral ventricles without persistence may indicate an alternate etiology, such as cerebral atrophy.

Conclusion
A review of case examples helps demonstrate typical imaging findings of NPH through structural MRI and cisternograms. Understanding the appropriate techniques, pearls, and pitfalls of each help delineate what a radiologist should know to help direct clinical decision-making.