Image Processing Reference
Figure4.26 Schematic diagram of a PET scan.
This characteristic is used to discriminate between positron-induced gamma-
ray signals and unwanted “background” signals produced by naturally occurring
radiation. The computer only counts pairs of signals that come very close in time
to one another from opposing detectors in the detector ring. A large number of
gamma-ray detections are sufficient to create a picture of the concentration of the
tracer as a function of position within the tissue, just as thin x-ray beams can be
used to recreate a 3D map of density within a person (or mummy).
Figure 4.27 shows two different PET scans of a human heart in a person with
a history of heart attacks. The left image shows blood flow in muscle using a
radioactive ammonia tracer. Arrows point to tissue that has been rendered inactive
by a loss of blood flow. The question is whether a heart bypass could restore heart
function in the damaged area. This question can be answered with a PET scan
using a different tracer to indicate a different biochemical process, in this case
metabolic activity, using radioactive glucose. The right image shows the presence
of metabolic activity in the areas that appear dark (inactive) in the left image. This
indicates that the heart muscle in the area is in “metabolic hibernation” and could
possibly be restored to nearly normal function with a bypass. The color bar between
the two images indicates the relationship between the pseudocolor in the image and
the relative intensity of gamma-ray signals.