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. MRI
signal can be "weighted" to accentuate some properties and not
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.
(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; gray 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.
This was, and may yet still be the "gold-Standard" for very
difficult spinal diagnostic problems.. It involves a lumbar
puncture using a 22 gauge spinal needle (smaller needle than
when your blood is drawn!), and then a "radiopaque "dye" is
injected. The Neurosurgeon, or neuroradiologist can then move
the patient about, watching the dye flow over the nerve structures.
It is most helpful when there has been prior surgery, particularly
when metal instrumentation is present. Even still, the MRI has
nearly made this test obsolete...but not yet!
SPECT/PET (single photon/positron emission computed tomography)
When radio-labeled 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.
A. Johnson, M.D., Harvard Medical School
series of cases by Tumor Type