Transcript Imaging_Techniques

Imaging Techniques
CAT,PET,MRI
Different uses
 Brain Imaging
 􀂄 Structural Imaging
 􀂄 CAT
 􀂄 MRI
 􀂄 Functional Imaging
 􀂄 PET
CAT
 Computerized Axial Tomography (CAT
or Computed Tomography, CT)
 􀂄 Introduced commercially in 1983
 Involves a procedure using x-rays
CAT Procedure
􀂄 CAT machine involves x-ray source and
detectors
􀂄 These are fixed at opposite sides of the
scanner
􀂄 They rotate, so a radiologist can shoot xrays in all possible directions
CAT Image Reconstruction
 􀂄 X-rays
are generally projected from multiple
angles
 􀂄 Using computer algorithms, a three
dimensional density image is obtained
 􀂄 White indicates greatest density, black
indicates lowest density
CAT Procedure Example
CAT Measure
 􀂄 X-rays are absorbed by the brain tissue
 􀂄 The more dense the tissue, the more xrays are absorbed
 􀂄 CAT is a measure of brain density, bone
is more dense, than brain tissue, which is
more dense than blood
CAT Image Reconstruction
 􀂄 X-rays are generally projected from
multiple angles
 􀂄 Using computer algorithms, a three
dimensional density image is obtained
 􀂄 White indicates greatest density, black
indicates lowest density
CAT Example
MRI
MRI Magnets
 􀂄 MRI requires a magnet to work
 􀂄 Resistive Magnet – coils of wire around
cylinder, huge electricity demands
 􀂄 Permanent Magnet – very heavy
 􀂄 Superconducting Magnet – need to be cooled
Magnets are Dangerous
Components of MRI
 􀂄 The main magnet creates a uniform,
stable magnetic field
 􀂄 Gradient magnets – three of them, much
lower strength, variable field
 􀂄 RF coil – coil that generates a radio
frequency pulse
How MRI Works
 􀂄 Hydrogen atoms in water create a tiny
magnetic field
 􀂄 The main magnetic field causes these
atoms to align with it
 􀂄 Only about one in a million of the
hydrogen atoms aligns (isn’t cancelled out)
How MRI Works
 􀂄 RF pulses cause the hydrogen atoms to
become unaligned with the magnet
 􀂄 The hydrogen atoms eventually return to
their aligning with the magnetic field
 􀂄 They release energy (gained from the
RF pulse) as they do so
Hydrogen and MRI
How MRI Works
 􀂄 The energy is detected by the RF coil

􀂄 The three gradient magnets
determine which part of the brain is
imaged
 􀂄 When turned on and, they vary the local
magnetic field in a way that is
necessary to get the picture
How MRI Works
 􀂄 Different tissues respond differently to
changes in the local magnetic field
 􀂄 The tissue changes the energy signal
 􀂄 This allows the contrast seen on images
 􀂄 Contrast agents can also be used
MRI Detail
MRI vs. CAT
 􀂄 MRI provides greater resolution
 􀂄 CAT is cheaper
 􀂄 MRI is not good for claustrophobic or people
with metal in their body
 􀂄 CAT is thought to increase chance of cancer
(no ref available)
PET
Radioactive Oxygen
 􀂄 Water with radioactive oxygen is
injected into the body (half-life, 123
seconds)
 􀂄 Positrons break off from unstable
oxygen, collide with electrons, release
photons
 􀂄 The photons are picked up by detectors
Imaging and Abnormality
 􀂄 Brain imaging will not detect many
abnormal disorders reliably
 􀂄 Brain imaging is a very expensive
assessment tool
 􀂄 Maybe a better research tool than an
everyday assessment tool