Glossary of terms
The following are short definitions of many of the terms used in these pages.
For more detail, please read the relevant web page (via
hyperlinks in the text)
A
- Alpha particle A particle consisting of two protons plus
two neutrons. Emitted by a radionuclide. Alpha particles can be stopped by
a piece of paper and are only a concern when alpha-emitting isotopes are taken
into the body
B
- Background Radiation Radiation arising from natural sources.
These sources include: (1) terrestrial radiation from naturally occurring
radioactive isotopes in the soil; (2) cosmic radiation originating in outer
space; and (3) naturally occurring radioactive isotopes in the body.
All of
us were born with some naturally occurring radionuclides in our body which typically contains radioactive 14Carbon [~3.08kBq] and 40Pottasium [~4.28kBq]. This will vary of course with your local environment.
Background radiation averages 2.4mSv/year globally. Human produced sources
(other than for medical procedures) contribute about 0.01mSv/year in addition
to this.
- Becquerel Unit of radioactivity (Standard International
unit). One becquerel equals one radioactive disintegration per second.
- Beta particle An electron emitted by the nucleus of a radionuclide.
The electric charge may be positive, in which case the beta particle is called
a positron. Beta particles have a short range in air and even shorter range
in more dense material.
- Bone Densitometry
The assessment of bone strength to determine fracture risk and monitoring
of therapy.
Generally the lower spine and femur are measured, with typical imaging time
of 1-4 minutes per scan site (depending on the instrument)
- Bone Mineral Density
A measure of bone strength, express as g/cm2 (ie an area density).
This result is compared to a reference range to diagnose osteopaenia or osteoporosis.
C
- Computed tomography (CT,CAT) A diagnostic imaging procedure
that uses a combination of x-rays and computer technology to produce cross-sectional
images (often called slices), both horizontally and vertically, of the body.
A CT scan shows detailed images of any part of the body, including the bones,
muscles, fat, and organs. CT scans are more detailed than general x-rays
- Cyclotron A machine which accelerates charged atomic and sub-atomic particles in a vacuum using high frequency alternating voltage. A magnetic field is used to contain the particle beam in a spiral resulting in very high acceleration of the particles and an increase in the energy of the particle with each rotation, ultimately firing into a target which absorbs the particle. For PET imaging the particles are usually hydrogen or deuterium with electrons added (this increases the negative charge). The beam particle then passes through a carbon foil to strip the electrons and provide protons that strike the target to create a PET radioisotope.
The target material will be rich in one element which when bombarded with the protons produces a PET radioisotope such as 18F.
Further reading National Medical Cyclotron; HyperPhysics; Wikipaedia; Virtual Physics Laboratory demonstration of a cyclotron.
D
- DICOM Acronym for Digital Imaging and Communications in
Medicine. This is an extensive set of standards for handling, storing and
transmitting information in medical imaging. It includes a file format definition
and a network communications protocol (an application protocol which uses
TCP/IP to communicate between systems).
DICOM files can be exchanged between two entities that have the capability
to receive the information - image and patient data - in DICOM format (source
http://en.wikipedia.org/wiki/DICOM)
- Dual energy x-ray absorptiometry
(DXA, DEXA) A technique that uses two distinct x-ray energies
to determine the mass of bone at well described sites in the body, such as
the lower spine, femur or forearm. Unlike conventional x-rays, the radiation
exposure is very low.
E
F
- 18Fluorodeoxyglucose (FDG) The most commonly used radiopharmaceutical
in PET scanning. More information is available from our supplier Cyclotek.
- Fusion image combines two or more different images of the
related samples (such as mountain ranges) to form new images using mathematical
algorithms allowing new information to be extracted. In the case of a mountain
range, a combination of air infrared satellite images and topographic images
could be used to define temperature distribution with altitude.
In Medical Imaging, this is often the fusion of multimodal images such as
CT and SPECT or PET and CT or CT and MRI. These often allow very accurate
function maps of the organ imaged.
G
- Gamma camera
A camera that records the distribution of a radiopharmaceutical containing
a radionuclide that is attracted or taken up by a specific organ or tissue
of interest.
- Gamma ray A very high frequency form of electromagnetic
radiation that consists of photons emitted by radioactive elements. Gamma
rays can injure and destroy body cells and tissue, especially cell nuclei.
The energy of a gamma ray is equivalent to the difference in energy of the
initial and final atomic state minus the binding energy of the electron.
- Gray One gray is the absorption of one
joule of radiation energy by one kilogram of matter, and is a physical measure
of radiation energy absorbed only. Biological measures of the effect are expressed
in Sieverts.
H
- Half life (T½) - the time it takes for
the amount of a material to be reduced by half.
In the case of radioactive materials, the physical half life is the time for
the isotope to decay to half its activity.
eg 18F (the PET isotope) has a half life of 110 minutes. 100 MBq
of activity will be reduced to 50MBq after 110 minutes.
There is also a biological half life for many of the pharmaceuticals used
in nuclear medicine, and is the time taken for half the material to be excreted
by the body by natural processes.
- Helicobacter pylori
Helicobacter pylori is a gram negative, microaerophilic, curved bacillus.
It is motile, has flagellae and has a special affinity for human gastric mucosa.
Since its initial discovery it has been implicated in the pathogenesis of
a number of gastroduodenal disorders including acute and chronic gastritis,
gastric and duodenal ulceration, gastric cancer and gastric MALT lymphoma.
I
- Iodine The radioisotope 131I is often used in
nuclear medicine for both imaging
and treatment
- Intrinsic factor A material produced by the stomachs parietal cells which is necessary for the absorption of Vitamin B12 in the small intestine.
- Isotope atoms with the same number of protons but different
numbers of neutrons. Atoms with an unstable number of neutrons that disintegrate,
releasing rays of subatomic particles, are called radioisotopes.
K
L
M
- Mass spectrometer A device for determining the chemical composition of a sample by ionising (splitting the compound into ions of each atom present) and sorting the ions by mass with an electromagnetic field. The result gives the proportion of each ion in the sample. As isotopes masses also vary, a mass spectrometer also allows the relative abundance of an isotope such as 13C to be determined compared to 14C. See Wikipaedia for additional information
N
O
- Osteopenia
Low bone density, but not osteoporotic. T scores between less than -1 and
greater than -2.5 are considered osteopaenic
- Osteoporosis
Very low bone density (as defined by the World Health Organisation). T scores
lower than -2.5 are considered osteoporotic. Bone becomes less resistant to
fracture due to a loss of bone mass and changes in its structure
- Osteopetrosis
Literally stony bones. There are many causes of high bone density which may
need further investigation, some of which we list in the hyperlink.
P
- PET/CT Combines PET and CT images using image fusion to better localise lesions. PET/CT significantly improves sensitivity and specificity of lesion identification over either modality. Most new PET scanners now have an integrated CT
- Pernicious anaemia The inability of the stomach parietal cells to produce intrinsic factor and hence the malabsorption of Vitamin B12. This vitamin is essential for the production of haemoglobin.
- Photomultiplier tubes are used extensively in nuclear medicine to detect gamma rays (although other devices may also be used). These vacuum tube devices will amplify an incoming photon by first converting it to an electron by the photoelectric effect with a photocathode. The electron is then drawn into a dynode array (an array of electrodes which increase the number of electrons on each as each electron strikes it - each dynode has an increasingly positive charge) and finally to an anode at the end of the tube, which produces an electrical signal. A good illustration of this process may be found at Florida State University's Molecular Expressions (requires Java)
- Precision is a measure of the ability to reproduce results.
It is often calculated as the % coefficient of variation (%CV) - the variability
of the measurements around the mean of a series of repeated measurements.
The lower the precision, the more confident one can be of change in serial
examinations.
- Predictive value
- Positive predictive value (PV+): If the test is positive what is the probability of being sick
- Negative predictive value (PV-): If the test is negative what is the probability of being healthy
- Positron Emission Tomography
(PET or a PET scan) The most advanced branch of nuclear medicine
based on the use of short-duration radioisotopes integrated into molecules
present in the human body and emitting positrons allowing evaluation of the
metabolic activity of cells. Because it delivers high-quality functional images,
PET is particularly valuable for detecting and locating cancerous tumors and
unhealthy cells; it has equally valuable applications in cardiology and neurology.
Q
R
- Radiation The process of emitting energy as waves or particles.
The energy thus radiated. Frequently used for ionising radiation except when
it is necessary to avoid confusion with non-ionising radiation. Sunlight is
a form of radiation. Without that radiation, we would not exist. Thus radiation
is necessary for life on our planet, as we know it.
- Radioisotope/Radionuclide In a group of atoms with the
same number of protons but different numbers of neutrons, the atoms with an
unstable number of neutrons that disintegrate, releasing rays of subatomic
particles, are called radioisotopes.
- Radioactivity The spontaneous emission of radiation from
unstable atoms. Radionuclides lose particles (e.g., alpha or beta) and energy
through radioactive decay.
- Radiopharmaceutical
Radioactive pharmaceuticals, often used in nuclear medicine studies, to examine
the function of specific organs. The principal underlying this is that a pharmaceutical
may be targeted to specific organs (eg kidneys, lungs, liver, brain) which
may then be imaged with a gamma camera to determine distribution and rate
of uptake or clearance of the radiopharmaceutical.
- RAID Redundant Array of Independent Disks. This computer
storage system uses multiple hard disk drives to share data, so that if one
disk drive should fail, the data may be recovered from the other disks. There
are many RAID architectures. For more information, see http://en.wikipedia.org/wiki/RAID
S
- Sensitivity Statistical term specifying how many of the diseased will be detected by the test - usually expressed as a percentage. The higher the better
- Single Photon Emission
Computed Tomography (SPECT or SPET) A nuclear medicine imaging
technique which produces a 3 dimensional reconstruction of radio-pharmaceutical
distribution in an organ. The image may be viewed either as slices through
the organ or as a pseudo 3 dimensional image.
- Sievert (Sv) is the unit of measure
for the radiation dose equivalent for biological tissues, and allows for the
the biological effect on different tissues - as sensitivity will
vary with the type of particle and the tissue. The physical measure of absorbed
dose is the Gray.
For most Nuclear Medicine procedures, equivalent doses are in the microSievert
(µSv) to milliSievert (mSv) range.
- Specific activity A radioisotopes radioactivity per unit mass or volume - (the number of decays per second per unit amount)
- Specificity Statistical term specifying how many of the non-diseased will be classified as normal - usually expressed as a percentage. The higher the better
- SPECT/CT A nuclear medicine imaging device with an integral CT scanner. This type of image fuses
Nuclear Medicine images with X-ray CT images improving disease and lesion localisation. See PET/CT
T
- Technetium A chemical element with atomic number 43. Technetium
was discovered in 1937 at the University of California at Berkeley by Enrico
Fermi and Carlo Perrier. Its chemistry allows easy labelling of many pharmaceuticals.
The short half life radioactive form (99mTc with a half life of
6.02 hours) is used for nuclear medicine imaging procedures.
The longest lived isotope of technetium is 98Tc which has a half
life of 1.5 billion years.
The bulk of the department's Technetium is supplied by ANSTO through their ARI division
- Tomography - from the Greek words "to cut or section"
(tomos) and "to write" (graphein). In Nuclear Medicine, it is a
method of separating interference from the area of interest by imaging a cut
section of the object.
- T score A DXA
derived measurement (in standard deviations) of how far from the average young
adults value (sex matched) an individuals bone mineral density is. 66% of
individuals in the population of young normals will be ± 1 standard
deviation (T score unit) from the average.
For every T score unit below 0, fracture risk approximately doubles.
Therefore a T score of -1 has a fracture risk approximately twice the normal
for a young adult, whilst a T score of -2 is approximately four times that
of a young adult. A high risk does not indicate a fracture
will occur.
U
- Urea breath test
Test for the presence of Helicbacter pylori. The patient swallows
carbon- labelled urea which is metabolised by H. pylori produced
urease to produce labelled carbon dioxide. This is absorbed into the blood
stream and then exhaled in the breath of infected individuals. Measuring abnormal
amounts of carbon dioxide indicates an infection.
V
W
X
- X-ray Photons or electromagnetic radiation produced by
the de-excitation of bound atomic electrons. The energy of an x-ray is equivalent
to the difference in energy of the initial and final atomic state minus the
binding energy of the electron.
Y
- Yttrium A rare earth metal used for procedures
such as synovectomy as the silicone salt of radioisotope 90Y.
Z
- Z score A DXA
derived measurement (in standard deviations) of how far from the average age,
sex (and sometimes weight) matched value an individuals bone mineral density
is. How a person compares to someone their own age.
