Neuroimaging Primer
Keith A. Johnson, M.D., Harvard Medical School
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Tome is Greek for slice. The standard slice orientation in most brain imaging is transaxial or "axial". Left is shown at right. Note that, like the "lower organs", we look up to the brain. Other standard planes of view arecoronal and sagittal. Non-tomographic images represent "projections" from a single point of view and include bolus contrast x-ray angiograms and MR angiograms.
Tomographic images are made up of little squares called "pixels" (picture elements), each of which takes a grey-scale value from 1 (black) to 256 (white). Each pixel represents brain tissue which is about 1 mm. on each of two sides. The thickness of the slice is often 3 or 5 mm, thus creating a three-dimensional volume element, or "voxel", which is shaped like a shoe box. Pixel intensity represents an average from tissue within the voxel.
Image types
- CT (roentgen-ray computed tomography) A beam of x-rays is shot straight through the brain. As it comes out the other side, the beam is blunted slightly because it has hit dense living tissues on the way through. Blunting or "attenuation" of the x-ray comes from the density of the tissue encountered along the way. Very dense tissue like bone blocks lots of x-rays; grey matter blocks some and fluid even less. X-ray detectors positioned around the circumference of the scanner collect attenuation readings from multiple angles. A computerized algorithm reconstructs an image of each slice. (example)
- MRI (magnetic resonance imaging) When protons (here brain protons) are placed in a magnetic field, they become capable of receiving and then transmitting electromagnetic energy. The strength of the transmitted energy is proportional to the number of protons in the tissue. Signal strength is modified by properties of each proton's microenvironment, such as its mobility and the local homogeneity of the magnetic field. MR signal can be "weighted" to accentuate some properties and not others.When an additional magnetic field is superimposed, one which is carefully varied in strength at different points in space, each point in space has a unique radio frequency at which the signal is received and transmitted. This makes constructing an image possible. It represents the spatial encoding of frequency, just like a piano. (example). More details of MR here.
- SPECT/PET (single photon/positron emission computed tomography) When radiolabeled compounds are injected in tracer amounts, their photon emissions can be detected much like x-rays in CT. The images made represent the accumulation of the labeled compound. The compound may reflect, for example, blood flow, oxygen or glucose metabolism, or dopamine transporter concentration. Often these images are shown with a color scale. (example)
