Module A - Anatomical and functional brain imaging Flashcards
What are the four main techniques of brain imaging?
- Radiography and X-ray angiography
- CT (computerised tomography)
- PET (positron emission tomography)
- MRI (magnetic resonance imaging)
Which imaging techniques can be combined to to show functional and anatomical information?
CT and PET
Which imaging techniques can be combined to reduce the radiation dose of functional and anatomical imaging?
PET and MRI
Describe the physical basis of radiography and X-ray angiography:
Absorption of X-rays by tissues (high energy visible light, short wavelength)
Describe the procedure of radiography and X-ray angiography:
Broad beam X-rays passes through the head onto X-ray film
Results in projection of 2D image of the 3D object
Intensity is in proportion to absorption of X-rays
For angiograms, contrast agent is injected intravascularly (contrast agent has high attenuation coefficient)
What is the attenuation coefficient?
A summation of all attenuations along the part the X-ray had to travel of tissue Thicker objects (higher density) have a high attenuation coefficient
Describe the uses of radiography and X-ray angiography:
Skull fractures, bone abnormalities (bone tumours) absorb X-rays as they are quite dense
Symmetry of structures can be observed (pineal gland, Ca2+ displaced to one side in abnormalities)
Describe the advantages of radiography and X-ray angiography:
Very good spatial resolution (0.05mm)
Good depication of the skull (fractures, bone tumours)
Suburb spatial resolution
Describe the disadvantages of radiography and X-ray angiography:
Projection images only (contrast not very good for soft tissue)
Poor intrinsic tissue contrast
Use of ionising radiation (interact with free-radicals to cause cancer)
Some risk associated with injection of contrast
Describe the physical basis of computerised tomography:
Absorption of X-rays by tissues (slices)
Describe the procedure of computerised tomography:
Very narrow X-ray beams are projected through the head onto detectors (array)The source is rotated about the head to acquire many views (projections)
2D image of the slice is reconstructed using filtered backprojection
An adjacent slice is imaged, building up a 3D image
Filtered back projection clears up the blurry image
There can be artifacts due to the limited number of projections
Describe the uses of computerised tomography:
Emergency situations (checking for internal damage/bleeding) Tumours, stroke, anatomical defects (haemorrhage, absess,, white/grey matter differences)
Describe the advantages of computerised tomography:
Tomographic
Can resolves grey and white matter, blood, CSF
High resolution (1mm)
Describe the disadvantages of computerised tomography:
Use of ionising radiation
Some risk associated with injection of contrast
Anatomical images only (not functional images)
Describe the physical basis of positron emission tomography:
Radioactive isotopes of certain elements (e.g. 11C, 13N, 15O, 18F) can be substituted/injected into biologically important compounds (glucose)
Decay of atoms emits positrons
Positrons collide with electrons, producing two gamma rays 180 degrees apart (matter meeting antimatter)
Detection of gamma rays reveals positron of labelled molecules
Like filtered back transmission
Describe the procedure of positron emission tomography:
Isotope labelled compounds are injected or inhaled
e.g. 18F labelled 2-deoxyglycose taken up by neurons, phosphorylated by hexokinase, cannot be further metabolised, builds up in cells
Gamma rays detected by array of crystal photomultipliers
Images of activity are reconstructed using tomographic techniques
Describe the uses of positron emission tomography:
Glucose imaging of metabolism
Blood flow imaging using labelled water (15O) (blood flow is a good indicator of local brain function through unknown mechanism)
Image subtraction can highlight area associated with specific tasks
Describe advantages of positron emission tomography:
Brain function can be imaged
Also distribution of receptors (e.g. dopamine)
Blood flow using labelled water
Describe the disadvantages of positron emission tomography:
Anatomical images not obtained (only functional) Poor resolution (2-8mm) (physical limitation of positron relocation until it annihilates)