 This is a head CT on a four year old boy with a headache head CT at first glance looks fairly normal. We're looking through it. We're not seeing any signs of hydrocephalus any mass or anything like that. But as we go down lower, we can see at the level of the frame Magnum there's fullness there's soft tissue like material posterior to the brain step. With modern CT scans we have the ability to have multi planar reformats, and this sagittal reformat shows us cerebellar tonsils protrude codle to the level of the frame and Magnum. This can be approximated on CT, but the best characterization for this is on MR. MR, if we zoom in, allows us to see several landmarks. There is the Bayesian, which is the inferior aspect of the clivis, in particular the Bayesian ox put portion of the clivis. And then we have the epistheon, the Bayesian and epistheon represent the anterior and posterior margins of the frame and Magnum, we can use that to draw a line. We can bring the Bayesian and the epistheon to approximate the plane of the frame and Magnum. And then we can make a measurement perpendicular to that line to see how far coddle to that plane, the cerebellar tonsils extend. And here they extend approximately nine and a half millimeters below the plane of the frame and Magnum. And that is typically considered to be a carry type one malformation. How do we determine how significant this is? Well, the extent of tonsillar descent is one mechanism. But another tool we have is CSF flow studies. This is a dynamically acquired, created phase contrast image. This is acquired sagittally at the level of the frame and Magnum. We can see the basic anatomic details that we see in the structural image. And as we scroll through the city, we can see different phases of CSF pulsations. And we can see here, the brainstem, the upper cervical cord. And so this is the cervical medullary junction. And we can see that ventral to the brainstem and cervical medullary junction, we have patent bidirectional flow of cerebral spinal fluid. We can see it going from dark to bright. So that is showing two different directions of flow. We're also seeing flow posterior to the brainstem at the level of the frame and Magnum. This is a sign that at the time the study is performed, there's not a significant alteration in CSF flow dynamics at the level of the frame and Magnum. Now, one of the other things that's a part of a carry type one malformation imaging workup is imaging of the cervical spine. And we can see here the cerebellar tonsillar ectopia extending below the plane of the frame and Magnum. And what we see of the cervical cord looks normal. We do not see any signs of a fluid collection within either hydromyelia or syringo hydromyelia. So this case is an example of a mild carry type one malformation without a perishable alteration in CSF flow dynamics and without evidence of syringo hydromyelia.