Nuclear medicine Flashcards
Learn the main concepts and improve PET and SPECT
What is nuclear medicine?
medical specialty that uses radioactive tracers (radiopharmaceuticals) to assess body functionality and diagnose and treat malignant diseases.
What we image is the distribution of the radionuclide into the patient.
What is a radiopharmaceutical?
It is obtained by the combination of a radionuclide, undergoing a nuclear decay and releasing the photons that are imaged, and a pharmaceutical which only acts as carrier of the radionuclide and delivers it to the target organ.
What makes a good radionuclide?
1) Should only emit gamma rays.
2) The emitted photons should have energies between 50 and 300 keV.
3) The half-life should be comparable with the duration of the study.
4) Should be readily available at the hospital site.
5) Should combine good with the radiopharmaceutical.
What makes a good radiopharmaceutical?
1) Should not be simple to prepare and not be toxic
2) Should preferably localize in the area of interest
3) Should be eliminated from the body with a HL comparable to the duration of the study.
Describe the working principle of a gamma camera
The gamma camera is the device we use to collect the signal of the radionuclide and form an image.
1) The emitted gamma rays pass through a collimator (why do we need it?)
2) Gamma rays reach the detector.
- Usually made out of a crystal which is sensitive to gamma radiation and emits in turn light pulses when stimulated (photoelectric effect).
3) The pulse is amplified through photomultiplier tubes
4) The amplified signal is transmitted via optical fiber into a CCD chip which creates the image.
What is another problem we can encounter in the imaging process, which is related to the energy of the incident photon energy? What is a solution to this problem? What is a limit?
A) DETECTED FALSE EVENTS: The problem is related to photons that get possibly scattered in the patient’s body (Compton scattering events) because for them the spatial correlation is not guaranteed.
B) A solution to this problem would be to apply an energy cut-off in the detection process that ignores all signals lower than the energy threshold we expect the emitted photons to have (typically 140 keV)
C) A limit to this method is that by cutting off the low signals we increase the noise. By leaving a tolerance window of 20%, the 30% of the detected photons will be scattered and the image quality will be impacted.
Collimator resolution: how is it related to the image quality? What quantities does in depend on? What is a limit to enhancing this quantity above a certain threshold?
A) The collimator resolution is related to the image quality through a relation involving the different error sources making up the total PSF (3)
B) It depends on d and b directly, those being the spatial separation of the collimator and b the distance from the source to the patient.
C) The limit to the excessive collimator resolution is the collimator geometric efficiency, proportional to d^4.
Explain the concept of tomography
Tomographic imaging is a technique that consists in acquiring multiple rotational projection images of the body at different angles, superpose and filter them in order to produce a volumetric dose distribution that can be visualized.
SPECT: how does it work? Why is the filtering so important?
A) Single Photon Emission Computed Tomography
B) Detects photons emitted from the gamma decay of 99Tc with a pair of gamma cameras that rotate around the patient.
C) The filtering cut-off determines the quality and the spatial resolution of the final image.
PET: how does it work? what are possible artifacts?
A) Positron Emission Tomography
B) The radionuclide used is 18F, which emits positrons. They annihilate with neighboring electrons and produce a couple of 180° divergent photons with energy E = 511 keV
C) The photons are detected in coincidence and the source is assumed to be somewhere along the line connecting those two points.
D) Problems in the detection may come from multiple coincidences detected at the same time, Compton scattering events in the patient that alter the 180° angular deviation and accidental coincidences.
What are the advantages and disadvantages of nuclear medicine imaging?
(General with respect to radiography, PET, SPECT/SCINTIGRAPHY)
GENERAL ADV: we get functional information and the displayed dose distribution is in 3D
GENERAL DIS: It can take some time to image
PET ADV: the detection is simple, the images have high spatial resolution, we image some biologically interesting radionuclides which are also present in the body
PET DIS: we need a cyclotron to produce the radionuclides
SCINTI/SPECT ADV: we don’t necessarily need a cyclotron to produce the radionuclides
SCINTI/SPECT DIS: the spatial resolution is low.
What is a PET/SPECT-CT used for? What is the difference in the images produced by nuclear medicine and those from CT scans?
PET and SPECT scans image the activity of the radionuclide into the body and provide functional information, whereas CT scans are used to obtain an anatomy picture of the body. The combination of the two images gives the functional information on top of the anatomy.
