Computerised Tomography (CT scan) - Movable X-Ray tube is rotated around head of patient. Opposite the tube are X-ray detectors much more sensitive than film. Exposure times are shorter; intensity information from many positions around the head is combined to generate a 3-d image which can be virtually "sliced" along various planes.
Now superceded by Magnetic Resonance Imaging (MRI) which uses NMR. Hydrogen nuclei in the body are momentarily aligned by a magnetic field and then ``tapped'' with a brief RF pulse. The emitted radio frequencies are detected and used to reconstruct extraordinarily detailed images of the brain. The angle of the sections is not limited as it is in a CT scan.
Normally field strengths of 1.5 Tesla are used which provide mm resolution. For fMRI 3-4 Tesla.
functional MRI (fMRI) is still better as it depends on detection of signals intrinsic to the brain. Oxyhemoglobin and deoxyhemoglobin differ in their MRI signals. Brain areas activated by a specific task (eg. occipital cortex during visual behaviour) utilise more oxygen, thus initially decreasing levels of oxhb and increasing deoxhb. Within seconds however the flow of oxygen-rich blood to this area increases leading to an increase in the oxhb:deoxhb ratio. Gives good spatial (mms) and temporal (secs) resolution.
Positron Emission Tomography (PET) and Single-photon Emission Computerised Tomography (SPECT) also depend on detection of increased blood flow to active areas of the brain. In PET unstable positron emitting isotopes are synthesised in a cyclotron (^15O- 2 mins., ^18F- 110 mins. and ^11C- 20 mins. half-life) and incorporated into reagents (like water, precursor molecules of specific neurotransmitters, or glucose) and injected into the bloodstream. Oxygen and glucose accumulate in metabolically active areas, the transmitter probes in their appropriate regions. Extra proton in the atom decays into a neutron and a positron; the positron in turn within a few mms annihilates with an electron producing two oppositely directed gamma rays which are detected by 180 deg placed detectors.
SPECT similarly uses radio-labelled compounds which bind to red blood cells. SPECT probes give lower resolution (8 mm.) but are more easily available unlike the PET probes which need an on-site cyclotron.