Understanding MRI's
Magnetic Resonance Images (MRI) can be hard to read when you are new. This will give you a quick overview so you can know some basic terminology and basic anatomy.
MRI's are a type of scan that uses strong magnets and radiofrequency pulses to generate signals from the body, which are then detected by a radio antenna. These signals are then processed by a computer to create the images we use to see what is going on in our body. There is no radiation used. MRI's magnets come in different strengths represented as a number followed by a T (i.e 3T) which stands for Tesla. The higher the number the stronger the magnet and better quality of the image. Currently, a 3T MRI is the strongest commercially available in Australia (and most of the world outside of research facilities). Researchers are working on MRI's that are upwards of 10.5T. As MRI's use magnets their use is contraindicated in certain people who have internal metal devices such as pacemakers, cochlear implants, some shunts etc.
Some cases will need a contrast agent such as Gadolinium Contrast. This is injected into the body to improve the clarity of the scans. People who are pregnant, have had allergic reactions or kidney issues are usually advised against its use. In most cases related to Chiari and/or Syringomyelia contrast is not something that is regularly needed for MRI scans. If you are concerned speak to the doctor or radiologist recommending it.
Views
MR Images are two dimensional and we humans are three-dimensional. To ensure all parts of the body are seen the MRI takes images of multiple views and creates "slices" (like slicing a loaf of bread). . These different views are Sagittal, Axial and Coronal. Each of these views are shown below.
MRI studies
A sagittal MRI looks at the body from side on. This creates slices from ear to ear.
An axial brain MRI looks at the body from the top down. Head to toe.
Looks at the body from the back of the head to the front of the head.
MRI Imaging Sequences
MRI's also have several sequences they use to best visualise and contrast the structures in our body. The most common MRI sequences used are T1 and T2 weighted images (not to be confused with the Tesla strength). Tissue can be characterized by two different relaxation times – T1 and T2. T1 (longitudinal relaxation time) READ THE PHYSICS HERE
T1- and T2-weighted images can be easily differentiated by looking at the CSF which is dark on T1-weighted imaging and bright on T2-weighted imaging.
The third commonly used sequence is the Fluid Attenuated Inversion Recovery (Flair). The Flair sequence is similar to a T2-weighted but due to changes in the timing of the sequence abnormalities remain bright but normal CSF fluid is made dark. This sequence is very sensitive to pathology and makes the differentiation between CSF and an abnormality much easier.
CSF: Dark
Spinal Cord: Gray
Fat: Bright
Muscle: Gray
Air: Very Dark
CSF: Bright
Spinal Cord: Light Gray
Fat: Light
Muscle: Dark Gray
Air: Very Dark
CSF: Dark
White Matter: Dark Gray
Fat: Light
CINE MRI
A CINE MRI is a specialised study used to observe the CSF flow. One of the ways CSF is moved throughout the body is with each heartbeat. The CSF goes through the brain's ventricles into the cisterna magna and finally to the spinal canal. This then works in reverse when the heart relaxes after it beats. To visualise the CSF movements through the CINE MRI and oximeter device is attached to your finger to measure the heart-rate. A program is used to take images in conjunction with the heart-rate to create a movie-like MRI. This can help determine is fluid is blocked and by how much. While this is a commonly used scan for Chiari it is not always essential if the static images show full blockage by the cerebellar tonsils.