radionuclide imaging Flashcards
What are the main differences between Radionuclide imaging from radiography with x-rays?
It utilises gamma radiation from a radioactive substance
The radioactive substance (in the form of a radiopharmaceutical) is administered to the patient (i.e. it is introduced into the patient’s body) so that the patient becomes the source of radiation
Images are created using equipment known as a gamma camera
Image contrast is largely determined by organ uptake (which depends on physiological function) rather than physical factors such as density and average atomic number
The radiation dose to the patient is largely determined by the nature and quantity (activity) of the radiopharmaceutical
What does contrast in radionuclide imaging depend on?
Contrast in radionuclide images depends primarily on the difference in the amount of radioactivity (activity) that has localised in organs and tissues. Thus, it depends mainly on physiological function rather than physical factors, such as density and average atomic number, which determine contrast in x-radiography.
What is the most common interaction in soft tissue with gamma radiation?
In soft tissue, Compton scattering is by far the most likely interaction mechanism, although photoelectric absorption and coherent scattering are also possible.
How is HVL calculated?
ln2/LAC
How much energy dose a scattered photon of 99Tc have?
In the case of 99mTc, photons that have suffered only one Compton interaction have a range of energies from about 90 keV (corresponding to 180° or back-scattering) to nearly 140 keV (corresponding to low-angle forward scattering).
Photons that have been Compton scattered many times within the patient (multiply scattered) may have very low energies.
What are the main components of a gamma camera?
The main components of a gamma camera are:
A collimator and a large-area radiation detector (both contained within a shielded gamma camera head)
Electronics for radiation detection and signal processing
A computer (for control, further signal processing and image acquisition)
Facilities for image storage and display
What proportion of the radiation from a patient reches the detector?
typically 0.01-0.1%
What is the problem of scattered radiation in radionuclide imaging?
If scattered photons were allowed to contribute significantly to the radionuclide image, this would have an adverse effect on contrast and spatial resolution because the point at which a photon was last scattered is not the photon’s point of origin. The collimator cannot distinguish between scattered and unscattered photons. Fortunately, scattered photons have lower energy and thus generate smaller electrical signals from the radiation detector. These are electronically rejected by the gamma camera
How does noise and spatial resolution compare in radionuclide imaging and radiographic imaging?
Radionuclide images generally have more noise and poorer spatial resolution than radiographs. The high noise is due to the relatively small number of gamma photons that contribute to the image.
The poor spatial resolution is related to the way in which the points of origin of the photons are determined and the properties of the collimator.
How can organ position effect signal?
different depths in tissue, Thus the gamma radiation from more superficial structures is less attenuated and therefore gives a higher signal
What is SPECT?
By acquiring a series of projection images around the long axis of the patient, it is possible to reconstruct a set of transverse tomographic slices. This is most often done by rotating the gamma camera head around the patient. The technique is known as single-photon emission computed tomography (SPECT) or sometimes just single-photon emission tomography (SPET). Most modern gamma cameras have more than one head and this has the potential to reduce total image acquisition times in both planar and tomographic imaging
What is effective halflife?
the activity of a radiopharmaceutical in the body decreases at a rate described by the effective half-life Teff.
1/Teff = 1/physical half life + 1/biological halflife
This decrease is caused by two processes:
Radioactive decay - described by the physical half-life Tphys
Elimination from the body by physiological processes such as clearance by the kidneys - described by the biological half-life Tbiol
What is patient dose in radionuclide imaging dependent on?
It is determined by the nature of the radiopharmaceutical (including the properties of the radionuclide label), its activity, its route of administration and its effective half-life.
Energy is deposited in the body through ionisations caused by Compton recoil electrons and photoelectrons from gamma interactions with tissues.
There may also be contributions from beta particles, internal conversion electrons and Auger electrons.
With sufficient information, it is possible to estimate the absorbed and equivalent dose to organs and tissues and the effective dose to the whole body
A radiopharmaceutical:
A. Provides contrast in radionuclide images when it is uniformly distributed in body organs and tissues
B. May emit both beta and gamma radiation
C. Is usually administered to the patient by intravenous injection
D. May be described as a sealed source of radioactivity
E. Has a biological half-life which is always the same as the physical half-life of the radionuclide
A. False. Contrast arises from differences in the uptake of the radiopharmaceutical in different organs and tissues.
B. True. Beta and gamma radiation may be emitted by radiopharmaceuticals. The presence of a large amount of beta radiation is undesirable for in vivo studies because it is not sufficiently penetrating and it increases the radiation dose to the patient. However beta emitters may be used for some in vitro investigations.
C. True. Intravenous injection is the most common route of administration, although other routes may be used for particular investigations (e.g. inhalation for lung ventilation studies).
D. False. Radiopharmaceuticals are unsealed sources of radioactivity.
E. False. The biological half life refers to the rate of clearance of a radiopharmaceutical from the body. The physical half life refers to the rate of radioactive decay. In general, the two values will be different.
Regarding a gamma camera collimator:
A. It is usually made of tungsten
B. It must have holes of circular cross-section
C. It attenuates gamma radiation mainly by photoelectric interactions
D. It acts in the same way as a lens
E. It has no direct effect on the radiation dose to the patient
A. Incorrect. Gamma camera collimators are made of lead.
B. Incorrect. The holes in a collimator may have any one of a variety of cross-sections (e.g. round, square, hexagonal).
C. Correct. In lead, most photons interact by photoelectric absorption.
D. Incorrect. There is no lens for gamma radiation. A collimator acts by selecting gamma photons according to their direction of travel
E. Correct. The radiation dose to the patient is determined by the nature of the radiopharmaceutical, its activity, its route of administration and the rate at which it is eliminated from the body. The collimator has no direct effect on the dose.
What are three methods of radionuclide production?
Cyclotron - produces radionuclides by bombarding stable nuclei with high-energy charged particles.
Nuclear reactor - nuclear fission or neutron activation of a stable target material.
Radionuclide generator - Radionuclides with short half-lives can be produced from a radionuclide generator, by decay of the parent radionuclide.
The generator allows the parent and daughter product to be easily separated.
How does a cyclotron work?
ions travel in a vacuum inside two D-shaped electrodes. A magnetic field causes the ions to travel in circular paths.
Application of an alternating voltage of the correct frequency between the Ds causes ions to be accelerated across the gap, thus gaining velocity and kinetic energy. As ion velocity increases, so does the radius of the circular path.
Ions with the greatest velocity and kinetic energy travel near the outside of the Ds, and are deflected onto a suitable target.
Nuclear reactions occur in the target to produce the required radionuclide
How are radionuclides produced in a nuclear reactor?
A naturally occurring uranium-235 (235U) nucleus absorbs a neutron which causes fission (breaking apart of the nucleus) with the release of more neutrons.
The fission is controlled by control rods, which can be inserted or withdrawn from the reactor core. These are made of a material which absorbs neutrons without producing fission.
Suitable target materials can be lowered into holes (ports) in the reactor so that they are irradiated by the neutrons. Neutron-capture reactions create radioisotopes of the target element; an example is the creation of radioactive molybdenum-99 (99Mo) from stable molybdenum-98 (98Mo).
The fission of 235U nuclei creates a mixture of radionuclides of lower mass number and lower atomic number. Some of these products of fission, such as iodine-131 (131I), are useful in nuclear medicine. Molybdenum-99 is also a fission product, and so there are two ways in which this radionuclide may be obtained from a nuclear reactor.
How is 99mTc produced?
beta-minus (β-) decay of parent radionuclide nuclide 99Mo (half-life 67 hours) produces a daughter radionuclide 99mTc (half-life 6 hours). Technetium-99m then decays by isomeric transition (IT) to 99Tc with the emission of 140 kiloelectron volt (keV) gamma rays that are suitable for medical imaging
What is the half life of 99mMo?
2.8 days (β- decay).
how is 99mTc extracted from the generator?
Technetium-99m is extracted from the generator by a process called elution. Immediately after each elution, the activity of 99mTc is reduced to zero.
How is a 99mTc generator made?
Fission produced 99Mo is adsorbed on alumina in a sterilised glass column surrounded by a lead or depleted uranium shield.
Technetium-99m is eluted from the alumina column, by an ion exchange mechanism when sterile saline is passed through it.
The resulting eluate is sodium pertechnetate (NaTcO4).
What is the half life of 99m Tc?
6-hour half-life
Long enough to be useful for imaging, but not so long as to result in an unacceptable radiation dose to the patient.
How does 99mTc decay?
Decay mode-isomeric transition
Only gamma rays are emitted, thus limiting radiation dose, since alpha and beta particles give a high localised radiation dose.