The cerebellum, often called the “little brain,” plays a primary role in controlling movement and balance. It coordinates functions throughout the body, accounting for about 10% of the brain’s total weight. Magnetic Resonance Imaging (MRI) offers detailed views of this structure, providing insights into its health and potential abnormalities.
The Cerebellum’s Function and Location
The cerebellum is positioned at the back of the head, nestled beneath the larger cerebrum and behind the brainstem, which connects the brain to the spinal cord. This region forms a half-circle shape around the brainstem and features horizontal grooves or folds on its surface.
The cerebellum’s primary functions involve muscle control, coordination, balance, and movement. It contains specialized sensors that detect shifts in balance and movement, sending signals to the body to adjust smoothly. The cerebellum also helps time muscle actions, allowing for fluid body movements, and coordinates eye movements. Beyond motor control, it contributes to motor learning, the process of acquiring movements that require practice and fine-tuning. Emerging research also suggests its involvement in cognitive functions like language processing, attention, memory, and emotional regulation.
How MRI Visualizes the Cerebellum
MRI is a preferred imaging method for examining the cerebellum due to its ability to generate detailed images of soft tissues within the brain. Unlike X-rays or CT scans, MRI does not use ionizing radiation, relying instead on strong magnets and radio waves to produce images. The MRI scanner creates a temporary magnetic field that aligns hydrogen atoms in the body’s water molecules.
Radiofrequency waves are pulsed, temporarily disrupting these aligned atoms. As the atoms realign with the magnetic field, they emit radio signals, which the MRI machine detects. Different tissues, such as the gray matter and white matter of the cerebellum, realign at different rates. A computer translates these varying signals into a grayscale image, enabling radiologists to differentiate between brain structures and identify subtle changes.
Recognizing a Healthy Cerebellum on MRI
A healthy cerebellum on an MRI scan exhibits a characteristic appearance. It typically presents as two large lateral structures, the cerebellar hemispheres, separated by a central, midline bulge called the vermis. The surface is covered with parallel grooves and folds, known as folia, giving it a convoluted or “cauliflower-like” texture.
Internally, the cerebellum consists of an outer layer of gray matter and an inner core of white matter. The gray matter, or cerebellar cortex, appears as a darker signal on T1-weighted MRI scans. The white matter, known as the arbor vitae, appears brighter. Radiologists assess the overall size and symmetry of both hemispheres and the vermis, ensuring no unexpected areas of abnormal signal intensity, which could indicate swelling, fluid, or lesions. The deep cerebellar nuclei, embedded within the white matter, are also evaluated.
Common Cerebellar Conditions Seen on MRI
MRI is instrumental in identifying various conditions affecting the cerebellum.
Strokes
Strokes, which can be ischemic infarcts (due to blocked blood flow) or hemorrhages (due to bleeding), appear differently depending on their age. Acute cerebellar infarcts typically show as bright areas on diffusion-weighted imaging (DWI) with low signal on apparent diffusion coefficient (ADC) maps. Over time, these infarcts become hyperintense on FLAIR and T2-weighted images, and later may shrink.
Tumors
Cerebellar tumors also have varied appearances on MRI. Medulloblastomas, common in children but also seen in adults, can originate in the vermis or cerebellar hemispheres. Their MRI appearance is variable, sometimes showing low-density areas consistent with cystic or necrotic degeneration. Hemangioblastomas often appear as well-defined cystic lesions with a vividly enhancing mural nodule, which is typically hypointense on T1-weighted and hyperintense on T2-weighted sequences. Metastatic tumors, originating from cancers elsewhere in the body, can present with diverse signal intensities on MRI, frequently showing low signal on T1-weighted and high signal on T2-weighted images. High-grade astrocytomas exhibit non-specific MRI findings and may present as multiple small lesions.
Atrophy
Atrophy, or shrinkage of the cerebellum, can be observed on MRI as a reduction in cerebellar volume, often involving both the vermis and hemispheres. This can be a sign of various neurodegenerative disorders, such as ataxia-telangiectasia or Friedreich ataxia, or can result from toxic causes like alcoholic cerebellar degeneration or certain medications.
Inflammatory Conditions
Inflammatory conditions, such as multiple sclerosis (MS), can manifest as lesions in the cerebellum’s white and gray matter. These lesions, often seen as bright spots on T2-weighted and FLAIR sequences, can disrupt nerve signals and contribute to symptoms like coordination difficulties.
Developmental Malformations
Developmental malformations, such as Chiari malformation and Dandy-Walker syndrome, are also well-visualized with MRI. Chiari malformation type I involves the downward displacement of the cerebellar tonsils through the foramen magnum into the spinal canal. On MRI, these tonsils may appear pointed or peg-like, and there can be crowding of the foramen magnum and associated fluid collections in the spinal cord (syringomyelia). Dandy-Walker syndrome is characterized by hypoplasia or absence of the cerebellar vermis, cystic dilation of the fourth ventricle, and an enlarged posterior fossa. MRI findings typically include an abnormally high position of the tentorium and torcular herophili, and the cerebellar hemispheres may appear displaced anterolaterally.
Interpreting these findings requires the expertise of a medical professional.