Normal pressure hydrocephalus (NPH) is a condition where cerebrospinal fluid (CSF) accumulates in the brain’s ventricles, causing them to enlarge without a corresponding increase in pressure. Magnetic Resonance Imaging (MRI) is the most effective tool for detecting NPH because it provides detailed images of brain structures and CSF flow, allowing doctors to identify characteristic changes associated with the condition.
MRI detects NPH primarily by visualizing **ventriculomegaly**, which means the lateral and third ventricles of the brain are abnormally enlarged. This enlargement is disproportionate to the size of the surrounding brain tissue and cortical sulci (the grooves on the brain surface), which helps differentiate NPH from brain atrophy where ventricles enlarge due to tissue loss. One key measurement used is the **Evans index**, which is the ratio of the width of the frontal horns of the lateral ventricles to the widest internal diameter of the skull; an Evans index greater than 0.3 suggests ventriculomegaly typical of NPH.
Other morphological features seen on MRI include:
– **Widening of the temporal horns** of the lateral ventricles beyond 6 mm, which is not explained by hippocampal atrophy.
– A **narrow callosal angle** (less than 90 degrees) seen on coronal images, indicating the shape of the ventricles is altered.
– **Upward bowing of the corpus callosum**, the thick band of nerve fibers connecting the brain’s two hemispheres, due to ventricular enlargement.
– **Disproportionate enlargement of subarachnoid spaces**, especially dilation of the Sylvian fissures and insular cisterns, while the sulci at the vertex (top of the brain) are compressed or effaced. This pattern is sometimes called **disproportionately enlarged subarachnoid space hydrocephalus (DESH)**.
– The **cingulate sulcus sign**, where the posterior half of the cingulate sulcus is narrower than the anterior half, reflecting crowding of brain tissue near the ventricles.
MRI also allows for advanced techniques beyond just structural imaging. **CSF flow studies** using phase-contrast MRI can measure the movement of cerebrospinal fluid through the aqueduct of Sylvius (a narrow channel connecting the third and fourth ventricles). In NPH, abnormal CSF flow dynamics may be detected, such as increased stroke volume or altered flow patterns, which support the diagnosis.
Additionally, **diffusion tensor imaging (DTI)**, a specialized MRI sequence, can assess the integrity of white matter tracts. In NPH, prolonged ventricular enlargement stretches and damages these tracts, which DTI can detect by showing changes in water diffusion patterns within the brain tissue.
MRI is superior to CT scans for NPH diagnosis because it provides better soft tissue contrast, allowing clearer visualization of subtle changes in brain structures and CSF spaces. It can also identify other causes of hydrocephalus, such as tumors or cysts, that might block CSF flow.
In summary, MRI detects normal pressure hydrocephalus by revealing a combination of enlarged ventricles, specific changes in brain shape and sulcal patterns, and abnormal CSF flow, all of which help distinguish NPH from other neurological conditions. This detailed imaging is crucial for accurate diagnosis and guiding treatment decisions.
