Flashcards in Imaging in Inflammation and Infection Deck (35)
State the properties of the ideal isotope for labelling.
• Half-life similar to length of examination
• Gamma emitter, rather than alpha or beta
• Energy of gamma rays should be 50-300 keV
• Radionuclide should be readily available at hospital site
• Easily bound to pharmaceutical component
• Radiopharmaceutical should be simple to prepare
• Radiopharmaceutical should be eliminated in similar half-time to duration of examination
Define molecular imaging
• Molecular imaging is the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in humans and other living systems. Molecular imaging typically consists of 2- or 3-dimensional imaging as well as quantification over time.
• Molecular imaging originated from the field of radiopharmacology due to the need to better understand fundamental molecular pathways inside organisms in a noninvasive manner.
What are some examples of molecule imaging techniques?
Common techniques include:
o Radionuclide imaging
o Positron emission tomography
o Magnetic resonance imaging
o Optical imaging
Outline the scientific basis of radionuclide imaging
• Radionuclide scan is a nuclear medicine examination in which a minute amount of radioactive material is labelled to commonly found substances in the body such as iodine or phosphate. As those radioactive substances go to their normal positions in the body, images of bones, thyroid tissue, and other organs are created.
• A radiopharmaceutical agent is injected, which decays at a known rate, emitting gamma rays, that are detected by cameras to produce images
How are images produced in X-ray radiography?
• To create the image, a heterogeneous beam of X-rays is produced by an X-ray generator and is projected toward the object.
• A certain amount of X-ray is absorbed by the object, which is dependent on the particular density and composition of that object (attenuation).
• The X-rays that pass through the object are captured behind the object by a detector (either photographic film or a digital detector). The detector can then provide a superimposed 2D representation of all the object's internal structures.
Describe the characteristics of gamma radiation
•Gamma rays occur due to radioactive decay of unstable isotopes
• High energy, high frequency, very similar properties to x-rays
What are radiopharmaceuticals and what are they used for?
• Radiopharmaceuticals are used in the field of nuclear medicine as radioactive tracers in medical imaging and in therapy for many diseases (for example, brachytherapy).
What is a commonly used gamma emitting tracer nuclide used in radiopharmaceuticals?
Technetium-99m (Tc-99m) has many useful properties as a gamma-emitting tracer nuclide.
What should be the energy of gamma rays be?
• Energy of gamma rays should be 50-300 keV
What is hydroxy-diphosphonate used for?
In bone imaging for diagnosing bone cancer or injury
What is dimercapto-succinic acid used for?
In kidney imaging
What is hexamethyl-propine amine oxime (HMPAO used for?)
In brain imaging, useful in diagnosing stroke and dementia
What is macroaggregated albumin used in?
Outline the scientific basis of SPECT
• Single photon emission computed tomography (SPECT) is a nuclear medicine imaging technique using gamma rays.
• It may be used with any gamma-emitting isotope, including Tc-99m.
• In the use of technetium-99m, the radioisotope is administered to the patient and the escaping gamma rays are incident upon a moving gamma camera which computes and processes the image.
• To acquire SPECT images, the gamma camera is rotated around the patient.
• Projections are acquired at defined points during the rotation, typically every three to six degrees. In most cases, a full 360° rotation is used to obtain an optimal reconstruction.
• The time taken to obtain each projection is also variable, but 15–20 seconds are typical. This gives a total scan time of 15–20 minutes.
What can affect image quality in SPECT?
o Radiation dose – limits e.g. pregnancy
o Collimator (produces the parallel beams of radiation)
o Metal objects
o Proximity of area of interest to camera
What is hexamethyl-propine amine oxime (HMPAO used for?)
In brain imaging, useful in diagnosing stroke and dementia. Now in trials for the identification of ulcerative colitis and other inflammatory disorders
Outline the scientific basis of lung scintigraphy
Can be used as a ventilation/perfusion scan if the radionuclide is inhaled as an aerosol.
The perfusion phase of the test involves the intravenous injection of radioactive technetium macro aggregated albumin (Tc99m-MAA). A gamma camera acquires the images for both phases of the study.
How can lung scintigraphy be used to identify pulmonary embolism?
• Can establish where the clots are in the lungs by seeing where the aerosols can perfuse the lungs (white bits are blocked off by the embolism)
• CT pulmonary angiography can also be used to identify the location of pulmonary embolisms.
What is 99mTc HDP bone scintigraphy used for?
o Cancer of the bone or cancers that have spread (metastasized) to the bone (darker areas of image)
o Locating some sources of bone inflammation (e.g. bone pain such as lower back pain due to a fracture)
o The diagnosis of fractures that may not be visible in traditional X-ray images
o The detection of damage to bones due to certain infections and other problems
What is scintigraphy?
Scintigraphy is a diagnostic test in nuclear medicine, where radioisotopes attached to drugs that travel to a specific organ or tissue (radiopharmaceuticals) are taken internally and the emitted radiation is captured by external detectors (gamma cameras) to form 2D images in a similar process to the capture of x-ray images.
When might 99mTc HDP bone scintigraphy not be effective and what can be used instead?
Two-dimensional projections of scintigraphy may be enough, but in order to view small lesions (less than 1 cm) especially in the spine, single photon emission computed tomography (SPECT) imaging technique might be required.
What factors affect the reliability of bone scintigraphy images?
Size of the tumour
Need for 3D images
Growth plates and areas of bone with high turnover usually have a high uptake so can be misleading
What is the use of 99mTc MAG 3 radionuclide renograms?
Medical imaging of kidneys
After injection into the venous system, along with the administration of a diuretic (furesomide) the compound is excreted by the kidneys and its progress through the renal system can be tracked with a gamma camera, and is useful in evaluating the functioning of the kidneys.
If the kidney is not getting blood for example, it will not be viewed at all, even if it looks structurally normal in medical ultrasonography or magnetic resonance imaging. If the kidney is getting blood, but there is an obstruction inferior to the kidney in the bladder or ureters, the radioisotope will not pass beyond the level of the obstruction, whereas if there is a partial obstruction then there is a delayed transit time for the MAG3 to pass.
What is 99mTc DMSA renography used for?
Used to identify any cortical scarring or damage
Describe the scientific basis of SPECT
Single-photon emission computed tomography (SPECT) is a nuclear medicine tomographic imaging technique using gamma rays which requires delivery of a gamma-emitting radioisotope (a radionuclide) into the patient, normally through injection into the bloodstream. A SPECT scan monitors level of biological activity at each place in the 3-D region analyzed. Emissions from the radionuclide indicate amounts of blood flow in the capillaries of the imaged regions. Can be used to analyse blood flow to the brain and heart e.g. dopamine receptor SPECT to identify Parkinson's.
What is posterior cortical atrophy?
Varient of Alzheimer's where patients cannot recognised faces and facial expressions e.g. Terry Pratchett
Give an example of a radioisotope used in SPECT
Describe the scientific basis of positron emission tomography (PET)
• Molecular imaging used to image biologically interesting processes by using radionuclides that decay by positron emission: proton → neutron + positron
• To conduct the scan, a short-lived radioactive tracer isotope incorporated into a biologically active molecules is injected into the living subject. During the scan, a record of tissue concentration is made as the tracer decays.
• As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, an antiparticle of the electron with opposite charge. The emitted positron travels in tissue for a short distance, during which time it loses kinetic energy, until it decelerates to a point where it can interact with an electron. The encounter annihilates both electron and positron, producing a pair of annihilation (gamma) photons moving in approximately opposite directions. These are detected when they reach a scintillator on either side in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes (Si APD).
What is the half life of PET radionuclide 18F?
What is the half life of PET radionuclide 11C?
What is the half life of PET radionuclide 13N?
What is the half life of PET radionuclide 15O?
What is a cyclotron and what are they used for?
• A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1934 in which charged particles accelerate outwards from the centre along a spiral path. The particles are held to a spiral trajectory by a static magnetic field and accelerated by a rapidly varying (radio frequency) electric field.
• Cyclotron beams can be used to bombard other atoms to produce short-lived positron-emitting isotopes suitable for PET imaging.
What is fludeoxyglucose (18F) used for?
• Fludeoxyglucose (18F), often abbreviated to [18F]FDG, 18F-FDG or FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). Chemically, it is 2-deoxy-2-(18F)fluoro-D-glucose, a glucose analog, with the positron-emitting radionuclide fluorine-18 substituted for the normal hydroxyl group at the C-2 position in the glucose molecule.
• The uptake of 18F-FDG by tissues is a marker for the tissue uptake of glucose, which in turn is closely correlated with certain types of tissue metabolism. After 18F-FDG is injected into a patient, a PET scanner can form two-dimensional or three-dimensional images of the distribution of 18F-FDG within the body.