On a sagittal (side-view) MRI, Chiari malformation appears as the lower part of the cerebellum dropping below the base of the skull. The hallmark finding is the cerebellar tonsils, two rounded lobes at the bottom of the cerebellum, extending at least 5 millimeters past the foramen magnum, the opening where the skull meets the spinal canal. In a healthy brain, those tonsils sit above this opening. In Chiari, they’re pushed downward into space normally reserved for the spinal cord and spinal fluid.
The Key Measurement: Tonsillar Descent
The single most important feature radiologists look for is how far the cerebellar tonsils have dropped below the foramen magnum. This is measured on a mid-sagittal T1-weighted MRI, which provides a clear side-view slice right through the center of the brain. A descent of 5 millimeters or more is the established threshold for diagnosing Chiari Type I malformation. Some tonsils drop just past that line; others descend a centimeter or more. In surgical cases, herniation of 10 mm or greater is especially common, appearing in roughly 85% of patients who ultimately need an operation.
The tonsils themselves often change shape. Instead of their normal rounded appearance, they become elongated and pointed, sometimes described as “peg-like” in radiology reports. This happens because they’re being squeezed through an opening that’s too small for them, which reshapes the tissue over time.
A Crowded Posterior Fossa
The posterior fossa is the bowl-shaped compartment at the back and bottom of your skull that holds the cerebellum and brainstem. In Chiari Type I, this compartment is consistently smaller than normal. Several bones that form its walls, including the clivus (the slope of bone behind the nasal area) and the supraocciput (the bone forming the back wall), tend to be shorter in people with Chiari. The result is a space that’s simply too small for the brain tissue it contains.
This overcrowding shows up on MRI in a few ways. The cisterna magna, a pocket of spinal fluid that normally sits behind and below the cerebellum, becomes compressed or disappears entirely. Radiologists describe this as “effacement of the cisterna magna.” The fourth ventricle, a fluid-filled chamber nestled between the cerebellum and brainstem, also appears smaller than expected, even while the brain’s larger ventricles up top remain normal size. That mismatch is a visual clue that the posterior fossa is packed too tightly.
Blocked Spinal Fluid Flow
A standard MRI captures anatomy, but a specialized version called cine MRI captures motion. This technique uses the heartbeat to time a series of rapid images, creating a short loop that shows spinal fluid flowing around the base of the skull in real time. In a healthy scan, you’d see a brief upward pulse of fluid followed by a longer, steady downward flow, all moving freely around the brainstem and spinal cord.
In Chiari patients, this flow is visibly obstructed. The herniated tonsils act like a plug, slowing fluid velocity and shortening the period of normal downward flow. Instead of smooth, rhythmic movement, the fluid appears turbulent or stalled at the craniocervical junction. Some treatment centers use cine MRI before surgery to document exactly how much obstruction is present and afterward to confirm that flow has been restored.
Syringomyelia: The Spinal Cord Finding
About 43% of people with Chiari malformation also have a syrinx, a fluid-filled cavity inside the spinal cord itself. On a T2-weighted MRI (which makes fluid appear bright white), a syrinx shows up as a bright tube running through the center of the normally solid, gray spinal cord. It most commonly appears in the cervical and upper thoracic regions, the stretch of cord closest to the blockage at the skull base.
Syrinx cavities in Chiari tend to be centrally located within the cord and continuous rather than broken into segments. On an axial (cross-section) view, the syrinx appears as a bright white circle or oval in the middle of the cord, surrounded by a thin rim of normal spinal cord tissue. This finding matters because it indicates that blocked spinal fluid is being forced into the cord itself, which can cause progressive weakness, numbness, or pain if left untreated.
How Chiari Type II Looks Different
Chiari Type II is a more severe form almost always associated with spina bifida. Its MRI appearance is dramatically different from Type I. Instead of just the tonsils descending, the entire lower portion of the cerebellum (the vermis), the brainstem, and the fourth ventricle are all pulled downward into the spinal canal.
Several distinctive signs appear on imaging. The midbrain develops a pointed, triangular shape called “beaking of the tectum.” The cerebellum wraps around the brainstem in what radiologists call the “banana sign.” The frontal bones of the skull can appear indented, producing a “lemon sign” on cross-sectional images. Other common findings include an enlarged connection between the two halves of the thalamus, partial or complete absence of the wall separating the brain’s lateral ventricles, and hydrocephalus (excess fluid buildup in the brain’s chambers). The brainstem may appear kinked or buckled where it has been displaced downward.
What an Incidental Finding Means
Sometimes mild tonsillar descent shows up unexpectedly on an MRI done for an unrelated reason, like a headache workup or a concussion evaluation. A large study of brain MRIs in children found tonsillar ectopia, meaning tonsils sitting lower than usual, in about 0.4% of scans. Not all of these meet the 5 mm threshold for a formal Chiari diagnosis. Descent between 3 and 5 mm is sometimes called “tonsillar ectopia” rather than a true Chiari malformation, and many people with this finding have no symptoms at all.
If your radiology report mentions mild tonsillar descent without using the term “Chiari malformation,” it typically means the tonsils are low but haven’t crossed the diagnostic threshold. Even at or above 5 mm, some people remain completely asymptomatic. The MRI finding alone doesn’t determine whether treatment is needed. What matters is the combination of imaging features (degree of descent, presence of a syrinx, evidence of fluid flow obstruction) alongside the symptoms you’re actually experiencing.
Which MRI Sequences Are Used
A complete Chiari evaluation typically involves more than a single scan. The core study is a sagittal T1-weighted MRI of the brain, which provides the clearest view of the cerebellar tonsils relative to the foramen magnum. T2-weighted sequences add detail about fluid spaces and are essential for spotting a syrinx. Because a syrinx can extend well below the skull, imaging of the entire spine is often recommended alongside the brain MRI to check for fluid cavities or a tethered spinal cord.
When there’s a question about whether spinal fluid flow is blocked, cardiac-gated cine phase-contrast MRI is added. This specialized sequence times its images to the heartbeat and measures both the speed and direction of fluid movement at the base of the skull and upper cervical spine. It produces a dynamic loop rather than a still image, giving a functional picture of the obstruction rather than just an anatomical one.