MEDICAL PHYSICS
Compton scattering
(Arthur H. Compton, 18921962, American
physicist, Nobel laureate 1927). During Compton scattering, a photon impinges
on an electron in matter, and in this process transfers part of its energy to
it. The excited electron is termed a Compton electron and is ejected or moved
into an excited atomic state, while due to the law of conservation of energy
the photon energy is reduced.
Ionizing radiation
Radiation that is able to
produce ions via the photoelectric effect, Compton scattering or pair production
or the interaction of particles. As the atomic ionization energy is of the
order of a few electron volts (eV), rays having an energy of over a few tens of
electron volts are ionizing (ionization energy of hydrogen H: 13.6 eV), while
visible light has energies in the range of 1 eV and thus is not ionizing. In
imaging, the ionizing radiations are X-rays and gamma rays as well as particle
rays resulting from radioactive decay. These rays have energies ranging from 10
keV up to the high energy rays found in cosmic radiation which are in the 10
MeV (mega electron volt) range.
Annihilation reaction
Reaction in which a positron, generated by a beta
decay and slowed to "thermal speeds", reacts with an electron to
completely disappear (annihilate) and form two gamma rays (photons) that
radiate from the point of annihilation in opposite directions (conservation of
momentum). The energy of each gamma ray photon is 511 keV (the rest mass of an
electron and of a positron). This reaction is used in PET imaging.
Pair production
The process in which a high-energy photon is
completely transformed into an electron and a positron i.e. energy is
transformed into matter. It occurs only in the vicinity of atoms that act as a
sort of "catalyst". Since the rest masses of electron and
positron are 511 keV each, the minimum photon energy required for pair
production to occur is 1 022 MeV. The inverse reaction to pair production is
the annihilation reaction.
Intensity
Energy per unit area per unit
time.
Photoelectric effect
Effect in which a photon transfers its entire energy
to an electron in the material on which it impinges. The electron thereby
acquires enough energy either to free itself from the material to which it is
bound or to be elevated into the conduction band of a semiconductor or
insulator.
Absorption
Physical process by which the intensity of waves or the number of particles is diminished as they interact with matter. Waves lose energy through exciting molecular vibrations. High-energy photons interact with matter through the photoelectric effect, Compton scattering and pair production. Particles lose their energy by inelastic collisions with matter.
Attenuation
Process by which radiation loses power as it travels
through matter and interacts with it. Beam attenuation is the basis of the
contrast observed in all X-ray based imaging methods including computed tomography
CT. Attenuation degrades image quality in nuclear imaging and particularly
in the tomographic nuclear scanning methods SPECT imaging and PET imaging.
It also can lead to image artefacts in MR imaging, particularly in high-field
imaging, and it is at the basis of the varying penetration depth of sound
waves in ultrasonography.
It is very easy to detect the presence or absence of some radioactive materials even when they exist in very low concentrations. Radioisotopes can therefore be used to label molecules of biological samples in vitro (out of the body). Pathologists have devised hundreds of tests to determine the constituents of blood, serum, urine, hormones, antigens and many drugs by means of associated radioisotopes. These procedures are known as radioimmuno assays and, although the biochemistry is complex, kits manufactured for laboratory use are very easy to use and give accurate results.
DIAGNOSTIC RADIOPHARMACEUTICALS
Every organ in our bodies acts differently from a chemical point of view. Doctors and chemists have identified a number of chemicals which are absorbed by specific organs. The thyroid, for example, takes up iodine, the brain consumes quantities of glucose, and so on. With this knowledge, radiopharmacists are able to attach various radioisotopes to biologically active substances. Once a radioactive form of one of these substances enters the body, it is incorporated into the normal biological processes and excreted in the usual ways.
Diagnostic radiopharmaceuticals can be used to examine blood flow to the brain, functioning of the liver, lungs, heart or kidneys, to assess bone growth, and to confirm other diagnostic procedures. Another important use is to predict the effects of surgery and assess changes since treatment.
The amount of the radiopharmaceutical given to a patient is just sufficient to obtain the required information before its decay. The radiation dose received is medically insignificant. The patient experiences no discomfort during the test and after a short time there is no trace that the test was ever done. The non-invasive nature of this technology, together with the ability to observe an organ functioning from outside the body, makes this technique a powerful diagnostic tool.
A radioisotope used for diagnosis must emit gamma rays of sufficient energy to escape from the body and it must have a half-life short enough for it to decay away soon after imaging is completed.
The radioisotope most widely used in medicine is technetium-99m, employed in some 80% of all nuclear medicine procedures. It is an isotope of the artificially-produced element technetium and it has almost ideal characteristics for a nuclear medicine scan. These are:
* It has a half-life of six hours which is long enough to examine metabolic
processes yet short enough to minimise the radiation dose to the patient.
* Technetium-99m decays by a process called "isomeric"; which emits
gamma rays and low energy electrons. Since there is no high energy beta emission
the radiation dose to the patient is low.
* The low energy gamma rays it emits easily escape the human body and are
accurately detected by a gamma camera. Once again the radiation dose to the
patient is minimised.
* The chemistry of technetium is so versatile it can form tracers by being
incorporated into a range of biologically-active substances to ensure that
it concentrates in the tissue or organ of interest.
Its logistics also favour its use. Technetium generators, a lead pot enclosing a glass tube containing the radioisotope, are supplied to hospitals from the nuclear reactor where the isotopes are made. They contain molybdenum-99, with a half-life of 66 hours, which progressively decays to technetium-99. The Tc-99 is washed out of the lead pot by saline solution when it is required. After two weeks or less the generator is returned for recharging.
A similar generator system is used to produce rubidium-82 for PET imaging from strontium-82 - which has a half-life of 25 days.
Myocardial Perfusion Imaging (MPI) uses thallium-201 chloride or technetium-99m and is important for detection and prognosis of coronary artery disease.
For PET imaging, the main radiopharmaceutical is Fluoro-deoxy glucose (FDG) incorporating F-18 - with a half-life of just under two hours, as a tracer. The FDG is readily incorporated into the cell without being broken down, and is a good indicator of cell metabolism.
In diagnostic medicine, there is a strong trend to using more cyclotron-produced isotopes such as F-18 as PET and CT/PET become more widely available. However, the procedure needs to be undertaken within two hours of a cyclotron.
THERAPEUTIC RADIOPHARMACEUTICALS
For some medical conditions, it is useful to destroy or weaken malfunctioning cells using radiation. The radioisotope that generates the radiation can be localised in the required organ in the same way it is used for diagnosis - through a radioactive element following its usual biological path, or through the element being attached to a suitable biological compound. In most cases, it is beta radiation which causes the destruction of the damaged cells. This is radiotherapy. Short-range radiotherapy is known as brachytherapy.
Although radiotherapy is less common than diagnostic use of radioactive material in medicine, it is nevertheless widespread, important and growing. An ideal therapeutic radioisotope is a strong beta emitter with just enough gamma to enable imaging, eg lutetium-177. This is prepared from ytterbium-176 which is irradiated to become Yb-177 which decays rapidly to Lu-177.
Iodine-131 and phosphorus-32 are examples of two radioisotopes used for therapy. Iodine-131 is used to treat the thyroid for cancers and other abnormal conditions such as hyperthyroidism (over-active thyroid). In a disease called Polycythemia vera, an excess of red blood cells is produced in the bone marrow. Phosphorus-32 is used to control this excess.
A new and still experimental procedure uses boron-10 which concentrates in the tumor. The patient is then irradiated with neutrons which are strongly absorbed by the boron, to produce high-energy alpha particles which kill the cancer.
For targeted alpha therapy (TAT), actinium-225 is readily available now, from which the daughter Bi-213 can be obtained (via 3 alpha decays) to label targeting molecules.
Considerable medical research is being conducted worldwide into the use of radionuclides attached to highly specific biological chemicals such as immunoglobulin molecules (monoclonal antibodies). The eventual tagging of these cells with a therapeutic dose of radiation may lead to the regression - or even cure - of some diseases.
What are radioisotopes?
Many of the chemical elements have a number of isotopes. The isotopes of an element have the same number of protons in their atoms (atomic number) but different masses due to different numbers of neutrons. In an atom in the neutral state, the number of external electrons also equals the atomic number. These electrons determine the chemistry of the atom. The atomic mass is the sum of the protons and neutrons. There are 82 stable elements and about 275 stable isotopes of these elements.
When a combination of neutrons and protons, which does not already exist in nature, is produced artificially, the atom will be unstable and is called a radioactive isotope or radioisotope. There are also a number of unstable natural isotopes arising from the decay of primordial uranium and thorium. Overall there are some 1800 radioisotopes.
At present there are up to 200 radioisotopes used on a regular basis, and most must be produced artificially.
Radioisotopes can be manufactured in several ways. The most common is by neutron activation in a nuclear reactor. This involves the capture of a neutron by the nucleus of an atom resulting in an excess of neutrons (neutron rich). Some radioisotopes are manufactured in a cyclotron in which protons are introduced to the nucleus resulting in a deficiency of neutrons (proton rich).
The nucleus of a radioisotope usually becomes stable by emitting an alpha and/or beta particle (or positron). These particles may be accompanied by the emission of energy in the form of electromagnetic radiation known as gamma rays. This process is known as radioactive decay.
Radioactive products which are used in medicine are referred to as radiopharmaceuticals.