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 constantVelocity 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 photo-detectors 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• 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_________________ •  •  •