PET & Nuclear Medicine Flashcards
What is PET?
PET stands for Positron Emission Tomography.
” It is a nuclear medicine functional imaging technique that is used to observe metabolic processes in the body as an aid to the diagnosis of disease.” - WikiPedia
“Positron emission tomography (PET) is a nuclear medical imaging technique that produces a three-dimensional image of functional processes in the body. The system detects pairs of gamma rays emitted during annihilation of positron produced by a positron-emitting radionuclide.”
- non-invasive
- measures metabolic activity in the body
- measures functionality of the body.
- 3D functional image
When was PET developed?
PET was developed in the mid 70s.
“It was the first scanning method giving functional information about the brain.”
True or False: PET systems rely on the detection of positrons to reconstruct an image.
False.
Positrons are the anti-particles of electrons. The range of positrons are very short. This is because when a positron interacts with an electron, they annihilate each other, releasing energy in the form of two
gamma photons. Since electrons are very abundant in nature, positron-electron annihilation is certain and happens shortly after positron emission.
PET systems thus rely on the detection of pairs of gamma photons of the same energy travelling in perfectly opposite directions for imaging.
What are the main radioactive isotopes used in PET?
Carbon-11
Nitrogen-13
Oxygen-15
Fluorine-18
What is Nuclear Medicine?
Nuclear medicine is the study and utilization of radioactive compounds in medicine to image and treat human disease. It relies on the ‘tracer principle’ first
espoused by Georg Karl von Hevesy in the early 1920s. The tracer principle is the study of the fate of compounds in vivo using minute amounts of radioactive
tracers which do not elicit any pharmacological response by the body to the tracer. Today, the same principle is used to study many aspects of physiology, such as cellular metabolism, DNA (deoxyribonucleic acid) proliferation, blood flow in organs, organ function, receptor expression and abnormal physiology, externally using sensitive imaging devices. Larger amounts of radionuclides are also applied to treat patients with radionuclide therapy, especially in disseminated diseases such as advanced metastatic cancer, as this form of therapy has the ability to target abnormal cells to treat the disease anywhere in the body.
Nuclear medicine relies on function. For this reason, it is referred to as ‘functional imaging’. Rather than just imaging a portion of the body believed to have some abnormality, as is done with X ray imaging in radiology, nuclear medicine scans often depict the whole body distribution of the radioactive compound often acquired as a sequence of images over time showing the temporal course of the radiotracer in the body.
What is the most common type of PET scan?
Cancer metastasis scan using fluorodeoxyglucose (FDG) , analogous of glucose. FDG is said to be a biologically active molecule. Higher concentrations of the tracer would indicate higher tissue metabolism (which is indicative of cancer).
What is the most common radiopharmaceutical for PET?
Fluorodeoxyglucose (FDG) is used to indicate tissues with higher metabolisms/ cellular activity.
Higher than normal cellular activity is indicative of cancerous cells.
Give the main chemical reaction for the production of fluorodeoxyglucose (FDG).
proton + oxygen-18 –> fluorine-18 + neutron
Name some examples of diagnostic procedures in Nuclear Medicine.
(1) Whole-body bone scans for various bone-related pathology
(2) Thyroid scan for thyroid disease
(3) Myocardial perfusion scan for coronary artery disease
List the steps in positron-emission & PET.
(1) Positron Decay
(2) Positron-Electron Annihilation
(3) Detection by Detector system
(4) Coincidence Processing Unit
(5) Sinogram
(6) Image Reconstruction
What is positron decay?
Positron (beta +) decay falls under beta decay.
It is the process by which a proton in the nucleus is converted to a neutron, positron and electron neutrino.
proton –> neutron + beta+ + electron neutrino .
In the process, the element is changed since the proton number is one less, but the mass number stays the same.
(i) List the structural components of a gamma camera
(ii) State their functions
(iii) Explain how these components may be adjusted to improve image quality.
Collimator: used to separate gamma rays
and keep scattered rays from entering the
scintillation crystal
–Resolution and sensitivity are terms used to
describe the physical characteristics of
collimators
–Made of material with a high atomic number,
lead, to absorb scattered gamma rays
Crystals
• Scintillation crystals commonly used are sodium
iodide with trace amounts of thallium to increase light
production
– Thicker crystals are better for imaging
radiopharmaceuticals with higher energies,
but have decreased resolution
– Thinner crystals provide improved resolution
but not efficient with higher Kv
PMTs
• Photomultiplier tubes are attached to the back of
the crystals
• PMTs are used to detect and convert light photons
emitted from the crystal into and electronic signal
that amplifies the original photon signal
• The electronic signal is displayed on a cathode ray
tube for filming or reading
What is a gamma camera?
A gamma camera (γ-camera), also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes.
How does the gamma camera function?
“A gamma camera consists of one or more flat crystal planes (or detectors) optically coupled to an array of photomultiplier tubes in an assembly known as a “head”, mounted on a gantry. The gantry is connected to a computer system that both controls the operation of the camera and acquires and stores images.[2]:82 The construction of a gamma camera is sometimes known as a compartmental radiation construction.
The system accumulates events, or counts, of gamma photons that are absorbed by the crystal in the camera. Usually a large flat crystal of sodium iodide with thallium doping in a light-sealed housing is used.
The crystal scintillates in response to incident gamma radiation. When a gamma photon leaves the patient (who has been injected with a radioactive pharmaceutical), it knocks an electron loose from an iodine atom in the crystal, and a faint flash of light is produced when the dislocated electron again finds a minimal energy state. The initial phenomenon of the excited electron is similar to the photoelectric effect and (particularly with gamma rays) the Compton effect. After the flash of light is produced, it is detected. Photomultiplier tubes (PMTs) behind the crystal detect the fluorescent flashes (events) and a computer sums the counts. The computer reconstructs and displays a two dimensional image of the relative spatial count density on a monitor. This reconstructed image reflects the distribution and relative concentration of radioactive tracer elements present in the organs and tissues imaged.” - WikiPedia
What are three gamma camera systems?
Standard camera: Single detector that is moved in
various positions around the PT
• Dual-head camera: Allows simultaneous anterior and
posterior imaging and may be used for whole-body
bone or tumor imaging.
• Triple-head systems may be used for brain and heart
studies
