306
8 Ionizing Radiation and Life
Electron microscopes can resolve objects as small as one ångströms (0.0001 μ),
while a (UV) light microscope is limited to about 0.2 μ. As an example, to get a
resolution of 10 Å, one would need electrons with an energy of the order of 2 keV.
If the specimen is cut very thin, an electron wave image might be generated
by having the electron beam pass through the object. This technique is used in
a ‘transmission electron microscope’. Alternatively, the initial electron beam can
scatter back from the object. The beam is often highly focused and then scanned
across the specimen. This forms what is called a ‘scanning electron microscope’.
In an electron microscope, the electrons are generated either by thermionic
emission (boiling electrons off a hot metal, such as LaB 6 -coated tungsten), or
by field emission from the sharp tip of a cold cathode. However, field emission
requires the chamber to have an ultra-high vacuum (less than 10 −8 torr). Images
can be created by having the electrons from the specimen hit a fluorescent screen,
or through the analysis of signals from one or more solid-state detectors, such as a
CCD, placed under a scintillator which converts the relatively high electron energies
into light. (The electrons in the beam have energies in the keV range, which is too
large to impinge directly on a silicon device, which only requires a few eV to release
an electron, and would be damaged by keV electrons.)
8.9 PET Scans and Gamma Emission Tomography
When information is needed about body functions, such as specific metabolic
processes, a ‘Positron Emission Tomography’ technique is one option when simpler
options are ruled out. The idea is that positrons (anti-electrons) can be generated
by the nuclear decay of specific isotopes. Almost immediately, those positrons are
annihilated by local electrons, producing two gamma rays with energy of about
0.511 MeV sent in directions opposite to each other. Such gamma rays easily pass
through tissue, and are detected external to the body by a set of detectors on a
surrounding ring. Tracing the line of emission of one of these gamma rays (in
coincidence with the second) will define a thin cylinder in the body where the
emitting tissue must lie. Analysis over angles and along the tissue produces an image
of the positions of the emitters.
The source of the positron in clinical applications is often 18 F within fluorodeoxyglycose, in effective doses of about 14 mSv. This compound accumulates
in cells with a high glucose uptake, such as the brain, the liver, and most cancer
cells.
Brain neuroimaging is effected by using 15 O, but with a half-life of only 2 min,
the scan must be done adjacent to a cyclotron production facility.
Isotopes which are positron emitters and that are used in medicine are shown
in Table 8.3. Carbon-11, Nitrogen-13, and Oxygen-15 are used in studying brain
physiology and pathology, in particular for localizing epileptic focus, and in
dementia, psychiatry and neuropharmacology studies. They also have a significant
role in cardiology. Note that the very short half-life of 15 O means that a medical
8 Ionizing Radiation and Life
Electron microscopes can resolve objects as small as one ångströms (0.0001 μ),
while a (UV) light microscope is limited to about 0.2 μ. As an example, to get a
resolution of 10 Å, one would need electrons with an energy of the order of 2 keV.
If the specimen is cut very thin, an electron wave image might be generated
by having the electron beam pass through the object. This technique is used in
a ‘transmission electron microscope’. Alternatively, the initial electron beam can
scatter back from the object. The beam is often highly focused and then scanned
across the specimen. This forms what is called a ‘scanning electron microscope’.
In an electron microscope, the electrons are generated either by thermionic
emission (boiling electrons off a hot metal, such as LaB 6 -coated tungsten), or
by field emission from the sharp tip of a cold cathode. However, field emission
requires the chamber to have an ultra-high vacuum (less than 10 −8 torr). Images
can be created by having the electrons from the specimen hit a fluorescent screen,
or through the analysis of signals from one or more solid-state detectors, such as a
CCD, placed under a scintillator which converts the relatively high electron energies
into light. (The electrons in the beam have energies in the keV range, which is too
large to impinge directly on a silicon device, which only requires a few eV to release
an electron, and would be damaged by keV electrons.)
8.9 PET Scans and Gamma Emission Tomography
When information is needed about body functions, such as specific metabolic
processes, a ‘Positron Emission Tomography’ technique is one option when simpler
options are ruled out. The idea is that positrons (anti-electrons) can be generated
by the nuclear decay of specific isotopes. Almost immediately, those positrons are
annihilated by local electrons, producing two gamma rays with energy of about
0.511 MeV sent in directions opposite to each other. Such gamma rays easily pass
through tissue, and are detected external to the body by a set of detectors on a
surrounding ring. Tracing the line of emission of one of these gamma rays (in
coincidence with the second) will define a thin cylinder in the body where the
emitting tissue must lie. Analysis over angles and along the tissue produces an image
of the positions of the emitters.
The source of the positron in clinical applications is often 18 F within fluorodeoxyglycose, in effective doses of about 14 mSv. This compound accumulates
in cells with a high glucose uptake, such as the brain, the liver, and most cancer
cells.
Brain neuroimaging is effected by using 15 O, but with a half-life of only 2 min,
the scan must be done adjacent to a cyclotron production facility.
Isotopes which are positron emitters and that are used in medicine are shown
in Table 8.3. Carbon-11, Nitrogen-13, and Oxygen-15 are used in studying brain
physiology and pathology, in particular for localizing epileptic focus, and in
dementia, psychiatry and neuropharmacology studies. They also have a significant
role in cardiology. Note that the very short half-life of 15 O means that a medical