Artifact: a general radiology and nuclear medicine term which refers to any feature present within an image that is not actually present within the object being imaged. Examples include respiratory motion artifact, attenuation arti- fact, and various other technical artifacts.
Attenuation: the number of photons reaching a detector is reduced by a patient’s surrounding tissues. The photons emanating from an object of interest deep within a patient may not be sufficient to create an adequate image. Attenu- ation is a major source of image artifact; however, there are many compu- tational and imaging techniques which have been devised to help lessen the impact of attenuation. In PET and PET/CT, a transmission scan creates an attenuation map of the patient to help offset this problem.
11
C: carbon-11. This is a positron-emitting radionuclide which has a half-life of approximately 20 minutes.
11
C methionine: a PET radiopharmaceutical which can be used as a marker of protein synthesis. Carbon-11 (
11C) is one of the positron-emitting radionu- clides used in PET imaging.
Curie: a measure of radioactivity, abbreviated Ci. This term honors Marie Curie, a pioneer in the research of radioactivity and the discoverer of radium. 1 curie = 37 billion disintegrations per second. The typical FDG-PET dose for a whole-body scan is 10–15 mCi (140 µCi/kg).
Cyclotron: a type of particle accelerator. The cyclotron principle involves using an electric field to accelerate charged particles across a gap between two magnetic field regions. The magnetic field accelerates the particles in a semi- circle, during which time the electric field is reversed in polarity to acceler- ate the charged particle again as it moves across the gap in the opposite direction.
11C,
18F,
13N, and
15O are all produced in a cyclotron.
18
F: fluorine-18. This is the most widely used positron-emitting radionuclide in clinical medicine. It is most often in the form of the glucose analogue FDG.
18
F has a half-life of approximately 110 minutes.
FDG: fluorodeoxyglucose. The formal name is
18F-2-fluoro-2-deoxy-D-glucose.
This is an analogue of glucose and therefore a marker of metabolic activity.
FDG is the most frequently used radiopharmaceutical in PET and PET/CT imaging.
Gamma camera: a nuclear medicine scanner. The gamma camera is the funda- mental nuclear medicine imaging device. It is composed of a system of col- limators, crystals, photomultiplier tubes, and computers arranged to collect, analyze, and create images based upon gamma photon emissions from a patient who has been injected with a radiopharmaceutical.
Gamma decay: a type of radioactive decay in which a nucleus in a high energy state changes to a lower energy state by emitting electromagnetic radiation in the form of a photon, or gamma ray.
99mTechnetium is an example of a radionuclide that undergoes gamma decay also called isomeric transition.
Gamma ray: a photon. A photon is a discrete packet of energy emitted by many
substances as they undergo radioactive decay. This physical property is the
basis behind nuclear medicine imaging. Gamma photons are collected by a gamma camera and then processed to generate an image. See photon.
G-code: nomenclature used by the Centers for Medicare and Medicaid Services when first covering PET scans. In PET imaging, a G-code can be thought of as an indication for the study, for example initial staging of lung cancer.
Glycolysis: an energy-generating process occurring inside mitochondria in which glucose is converted to pyruvic acid. The metabolic breakdown of glucose produces energy in the form of adenosine triphosphate (ATP) which has high energy phosphate bonds. FDG behaves similarly to glucose and is therefore a marker for metabolic activity.
Half-life: a physical property that refers to the time necessary for one half of a radioactive substance to decay. This is denoted as t
1/2.
18F has a half-life of 110 minutes.
Image registration: a process of combining or fusing complementary but dif- ferent image data. PET/CT provides both metabolic and anatomic informa- tion in one fusion image.
13
N: nitrogen-13. This is a positron-emitting radionuclide which has a half-life of approximately 10 minutes.
13
N ammonia: a PET radiopharmaceutical which is used primarily in cardiac imaging as a myocardial perfusion agent. Nitrogen-13 (
13N) is one of the positron-emitting radionuclides used in PET imaging.
Negative predictive value: the percentage of patients who test negative and truly do not have the disease.
Nuclide: a type of atom characterized by the constitution of its nucleus; that is, the number of protons, number of neutrons, and its energy state.
15
O: oxygen-15. This is a positron-emitting radionuclide which has a half-life of approximately 2 minutes.
PET: positron emission tomography. This is a type of nuclear medicine scan which uses positron-emitting radionuclides to generate images of various physiological and pathophysiological processes within the body. The most common type of PET scan is an FDG-PET scan.
PET/CT: positron emission tomography/computed tomography. A PET/CT scanner has both PET imaging and CT imaging capabilities. The PET scanner and the CT scanner are built in-line as one unit so that the func- tional information obtained from the PET scan can be correlated with the highly accurate anatomic information from the CT scan during the same imaging session. Most of the new PET scanners sold today are actually PET/CT scanners.
Photon: a packet of energy. All nuclear medicine images are created by photons, called gamma photons (sometimes denoted γ). Photons are generated in a variety of ways. In conventional nuclear medicine, they are released by radioactive nuclei when those nuclei undergo decay. In the case of FDG- PET, the photons do not come from the
18F nucleus directly.
18F emits a positron, which then annihilates with an electron a short distance away from the nucleus. This annihilation reaction produces two high energy photons which travel in opposite directions. Photon energy is measured in electron volts (eV), and the photons created by positron and electron annihilation are 511 keV.
238 Glossary
Positive predictive value: the percentage of patients who test positive and truly have the disease.
Positron: a positively charged electron. Positrons are released from some radionuclides which are unstable due to an excess of protons within their nuclei. Positrons are often referred to as beta plus (or β
+) particles. See positron decay.
Positron decay: a type of radioactive decay in which a nucleus that has too many protons reaches a more stable state by emitting a positron. A positron is a positively charged electron. In positron decay, a proton is converted into a neutron, and a positron and a neutrino are emitted from the nucleus. A nucleus with too many protons undergoes positron decay when it does not have enough energy to emit an alpha particle.
Radionuclide: a nuclide that emits radioactivity; see nuclide.
Radiopharmaceutical: a combination of a radionuclide and a pharmaceutical agent; also referred to as radiotracer.
Radiotracer: another term for radiopharmaceutical. This term is sometimes used because the pharmaceutical portion of an administered radiopharma- ceutical in a typical nuclear medicine study is in trace amounts and has neg- ligible pharmacologic effect.
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