MRI

MRI

1. Uses differences in properties of different substances in magnetic fields to visualise brain

   • Water (CSF) behaves differently from fat

2. Requires use of 3 different magnetic fields to produce image of brain

   • Static field
     • Magnetic field of high field strength
       • Causes all atoms to orient in the same direction
       • Imagers are classified by the strength of this field (ie. 3 tesla)
   • Pulse sequence
     • Magnetic field that is quickly applied & then stopped in a pulsing manner
       • These pulses perturb static field, and can be tuned to energy of specific molecule under investigation
         • These cause the specific molecule to flip 180 degrees
   • Gradient field
     • Magnetic field that varies in intensity over the area being imaged & provides ability to localise points in space from which signals are emanating
   • Provide changes in magnetic field in particular regions of the brain
     • Changes are then detected using receiver coil that encircles head

3. When magnetised, molecules in the brain respond to exposure to radiowaves at particular frequency

   • Done by sending back a radiowave signal called "spin echo"
     • Pulse sequence magnetic field which produces the radiowaves is produced by radio antenna encircling the subject

4. Finally, gradient field is applied

   • This changes the signal subtly at different points in the brain so that the position of the active area can be identified

5. Software is used to image particular slices of the brain in more detail so that areas of damage or activity can be localised in detail

   • T1 weighted
     • Fat: bright
     • Water (CSF): dark
     • Image brain tissue
   • T2 weighted
     • Water (CSF): bright
     • Fat: dark
     • Image lesions
   • Diffusion Tensor Imaging (DTI)
     • Used to determine the direction in which fluids are best able to travel
     • Since axons are surrounded by myelin, fluid can travel along axon by not through myelin
     • Used to show axon fibres that connect one area to another

6. Fractional Anistropy (FA)

   • Degree to which water moves in one direction

7. Advantages

   • Precise anatomical image with high spatial resolution produced
   • Can be used to map different substances
   • Non-invasive

8. Disadvantages

   • Poor temporal resolution
   • Can't be used with metal or pacemaker
   • Head has to be still- no infants