Lecture 11
Production of Positron
Emitters, Continued
The Positron Tomograph
Radionuclide Production
Methods by which radionuclides are
produced. Radionuclides can be produced
in a nuclear reactor, in a cyclotron or in a
radionuclide generator.
Cyclotron
A cyclotron consists of a pair of hollow, semicircular metal electrodes (called "dees" because of
their shape), positioned between the poles of a large electromagnet (not shown). The dees are
separated from one another by a narrow gap. Near the center of the dees is an ion source (typically
an electrical arc device in a gas) that is used to generate charged particles.
Cyclotron
Principle of Operation
Period, T, of revolution is constant
Velocity increases with each revolution.
Radius of revolution increases with each
new period.
This enables the application of alternating
electric fields, thus accelerating the
particles.
Cyclotron Produced Radionuclides
• Cyclotron produced radionuclides include
all PET nuclides in common use such as
fluorine F -18, oxygen O -15, nitrogen N -13
and carbon C -11, which are activated by
proton irradiation.
Reaction Equations
• X (n, p) Y
• Means neutron, proton reaction
• X, the parent nucleus is bombarded with a
neutron.
• The parent nucleus absorbs the particle and
exists in an excited state.
• The parent nucleus promptly decays to the
product nucleus (which is the positron
emitter)
Examples of Reaction
Equations
20Ne
(d, a) 18F
This means: A neon target is
bombarded by deuterons (one
proton, one neutron). The mass
number increases by 2, the atomic
number increases by 1. This is
however, instantaneous.
Examples of Reaction
Equations, Continued
The new species, decays with the
emission of an alpha particle.The
mass number therefore decreases by
four, and the atomic number
decreases by two, to Fluorine-18.
Examples of Reaction
Equations, Continued
The nucleus of Fluorine-18 remains in an
excited state. It decays by positron
emission to Oxygen 18, with a 120 minute
half life.
However, not all cyclotron
produced radionuclides are
positron emitters.
For example:
201Hg (d, 2n) 201Tl
Specific Activity and
Radioconcentration
• Specific Activity: decay rate of atoms / total
# of same atoms in sample.
• Low specific activity can influence uptake,
and physiological characteristics.
• Low specific activity occurs more in an
radionuclide where the atomic number is
not changed due to the difficulty of
separation.
Radioconcentration
• Decay rate (amount of dps) per total volume
of sample.
PET Tomograph
Detector
PET Data Acquisition
• If 2 photons are simultaneously detected by 2 small
detectors, we can infer that the annihilation has
occurred along the line connecting the 2 detectors.
This line is referred to as the "line of response," or
"LOR." To increase the sensitivity of the scanner, the
object is surrounded by a "ring" of small detectors
rather than only 2. Such a ring is shown in Figure 1A.
To image multiple planes simultaneously, several
such rings are placed back-to-back.
PET Data Acquisition
• It is therefore convenient to organize the
detectors into 2D arrays, called detector
"blocks," where the detectors along the x-axis
go around the ring and those in the y-direction
go axially into the ring. An array of
photomultiplier tubes are placed behind the
detector block to collect the scintillation light
and determine within which detector the event
occurred
Lines of Response
Parallel LORs
Photomultiplier tubes
• Photomultiplier tubes (or PMTs) are photodetectors which provide extremely high
sensitivity and ultra-fast response. A typical
PMT consists of a photoemissive cathode
(photocathode) followed by focusing
electrodes, an electron multiplier and an
electron collector (anode) in a vacuum tube.
Diagram of PMT
How the PMT works
• When light enters the tube and strikes the photocathode, the
photocathode emits photoelectrons into the vacuum. These
photoelectrons are then directed by the focusing electrode
voltages towards the electron multiplier where electrons are
multiplied by the process of secondary emission. The
multiplied electrons are collected by the anode as an output
signal. Because of secondary-emission multiplication,
photomultiplier tubes provide extremely high sensitivity and
exceptionally low noise among the photosensitive devices
currently used to detect radiant energy in the ultraviolet,
visible, and near infrared regions. A photon striking the
photocathode would usually yield the emission of a single
electron but the multiplier can create a final output of one
million electrons for each electron emitted. This is the gain of
the PMT and it is enormous!
PMT Continued,
• The electron multiplier section consists of nine (or
more, some PMTs use up to 19) electrodes called
dynodes. Each dynode is charged with about 100
volts more positive charge than the previous dynode
in the chain. As electrons are emitted from a previous
dynode they are focussed to the next dynode by
means of this increasing positive voltage. The
electrons strike that dynode and are multiplied and
the cascade of emitted electrons continues to grow at
each dynode. Finally the stream of electrons, which
began perhaps as a single electron, is collected by an
anode where it appears as an electrical current.
PMT Continued,
• The entire PMT is powered by a source of about 1000
Volts. The photocathode being the most negative
electrode, each dynode is succession is more positive
than the last. The potential difference required is
easily derived using a resistive voltage divider
consisting of a chain of one Megaohm resistors in
series. The final dynode has a potential of 1000 volts
positive relative to the photocathode. Finally the
electron stream is collected by the anode.
Detector Materials
Detector Blocks
2D vs 3D Configurations
Range in mm before annihalation
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What is the average distance traveled before annihilation of the following radionuclides
Rb-82
_______________2.6mm_________________
Ge-68
________________________________
N-13
_______________1.4 mm________________
O-15
_______________1.5mm_________________
C-11
_______________0.3mm_________________
F-18
_______________0.2mm_________________