Biomedical Engineering Reference
In-Depth Information
odontoidectomy. Figure 3.15a is an axial MRI section (T2-weighted)
at the level of the C2 vertebra which shows a multi-lobulated tumor
extending into the prevertebral soft tissues bilaterally (labeled
L and R) and appearing to extend posteriorly and to be in contact
with the spinal canal (labeled
*
). Figure 3.15b is a double-contrast
CT myelogram which shows the central portion of the MRI-
*
enhanced mass ( ) to be a pseudomeningiocele which is sand-
wiched between the low attenuation left (L) and right (R) lateral
masses. The CT finding dramatically changed the target volume
and, consequently, the plan of treatment.
P
OSITRON
-E
MISSION
T
OMOGRAPHY
(PET)
Positron emission tomography (PET) measures the three-dimensional
distribution of a positron-emitting radioactive isotope within the
body. Positrons are the positively charged antiparticles of electrons.
They have the special property that, when they encounter an electron
the two particles annihilate one another and emit a pair of gamma rays
of equal energy (0.511 MeV each) and moving in opposite directions,
back-to-back. Thus, a positron emitted by a radioactive isotope
annihilates with an electron in close proximity to its point of origin
and, if the locations of both of the emitted gamma rays are detected in
an array of photosensitive detectors surrounding the patient, one
can deduce that the original positron-emitting nucleus must have lain
closely on the line between the two detected locations. By detecting a
large number of such lines, the spatial distribution of the radioactive
isotope can be deduced using a mathematical algorithm which is
conceptually the same as that used in CT reconstruction.
6
The main isotopes used in PET are:
11
C,
15
O, and
18
F, and there are
several others. They are all relatively short lived, which poses a
technical but not a fundamental problem. The unique and valuable
aspect of PET is that these isotopes can be incorporated into
molecules with specific biological properties which can be injected to
concentrate in various body compartments. Typically, one can design
tracers to identify concentrations of cells: with a high rate of
metabolism; which are rapidly proliferating; or which are hypoxic.
6
The reconstruction is complicated by the fact that some of the emitted
gamma rays are lost by attenuation, so corrections have to be made to
account for this.
