Imaging: Midterm to Final Flashcards
What are some common decay modes of radionuclides?
Common decay modes of radionuclides are:
- Beta minus decay
- Beta plus decay
- electron capture
- Internal conversion
- Alpha decay
What does the mass number A stand for?
A is the number of protons and neutrons.
What does the atomic number Z stand for?
Z is the number of protons.
Describe Isomeric transition.
An isomeric transition is a radioactive decay process that involves emission of a gamma ray from an atom where the nucleus is in an excited metastable state, referred to in its excited state, as a nuclear isomer.
The emission of a gamma ray from an excited nuclear state allows the nucleus to lose energy and reach a lower energy state, sometimes its ground state. In certain cases, the excited nuclear state following a nuclear reaction or other type of radioactive decay, has a half life that is more than 100 to 1000 times longer than the average 10−12 seconds, and this excited state is referred to as a metastable nuclear excited state. Some nuclei are able to stay in this metastable excited state for minutes, hours, days, or occasionally far longer, before undergoing gamma decay, in which they emit a gamma ray.
The process of isomeric transition (that is, the gamma decay of nuclear isomers), is therefore similar to any gamma emission from any excited nuclear state, but differs in that it involves excited metastable states of nuclei with longer half lives. These states are created, as in all nuclei that undergo gamma radioactive decay, following the emission of an alpha particle, beta particle, or occasionally other types of particles that leave the nucleus in an excited state.
The gamma ray may transfer its energy directly to one of the most tightly bound electrons causing that electron to be ejected from the atom, a process termed the photoelectric effect. This should not be confused with the internal conversion process, in which no gamma ray photon is produced as an intermediate particle.
What energy does 99mTc emit?
99mTc emits gamma rays of 140.5 keV.
What is the half life of 99mTc?
The half life of 99mTc is 6 hours.
What parent of 99mTc is used for its production for nuclear medicine?
99Mo is manufactured to, which decays to 99mTc inside the generator at the clinics.
After how many half-lives of 99mTc should one “milk the generator” for the best efficiency?
4 half-lives = 24 hrs for 99mTc.
What does SPECT stand for?
Single photon emission computed tomography.
What is used in conjuction with SPECT detectors to discriminate photons traveling in different directions?
Why is the emission energy of 99mTc a nice energy to work with?
140keV photons are nice because the photoelectric effect dominates at this energy, so the Compton continuum is not an issue. The energy is also nicely matched with the NaI(Ti) scintillator efficiency and spatial resolutions.
What is an acceptance window used for in nuclear imaging?
An acceptance window is the allowable energy range outside of the expected energy of the isomer that is accepted by the detector. It is used to discriminate signal from background.
How do you combine biological elimination and physical half-life to get an effective half life?
You add the activities:
lambda_e = lambda_b + lambda_p
where lambda_p=ln(2)/half-life
How is SPECT data reconstructed?
SPECT is reconstructed using filtered back projection or iterative reconstruction.
How big is the matrix for SPECT?
The matrix for SPECT is either 642 or 1282.
What type of filter is used for the FBP image reconstruction of SPECT?
What does the attenuation correction corecct for in SPECT? What does the image look like before correction?
The attenuation correction accounts for the fact that you will get more counts from near the surface of the object (that contains the isomer) than from the centre because the stuff in the centre had to travel more and got attenuated. Before the correction, the centre looks lighter.
How is spatial resolution measured for SPECT?
How many slices are typically used for PET?
PET uses ~109 slices.
What is FDG?
Fludeoxyglucose (18F) (INN), or fludeoxyglucose F 18 (USAN and USP), also commonly called fluorodeoxyglucose and abbreviated [18F]FDG, 18F-FDG or FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). 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.
What is the energy of photons from PET?
The photons originate from a positron anihilation, so there are two 511 keV photons.
How do the scintillators in PET help guide the photons in a straight path?
What is the formula for the random rate of PET detectors?
Rrandom = tau S1S2
Where tau=coincidence window
Si = count rate of detector i
What is the random/true ratio for PET?
Random/true ratio is the ratio of random events (incorrectly matched events) to properly matched events. It increases with activity and decreases with the time window.
What happens when you remove the septa in PET detectors?
The septa are the tungsten scatter grids. Removing them allow you detect in 3D instead of 2D, but it increases the random coincidence rate and the scatter fraction.
Which uses attenuation correction before reconstruction, PET or SPECT?
PET uses attenuation correction before reconstruction, SPECT uses attenuation correction during reconstruction. PET achieves this by using a CT scanner at the same time.
What radionuclide has been proposed to replace 99Tc?
82Rb has been proposed.
What is the half-life of FDG?
FDG, is a radiopharmaceutical used in the medical imaging modality positron emission tomography (PET). Its half-life is 110 min.
What is the principle of projection data collection for SPECT and PET?
SPECT uses collimation, PET uses annihilation coincidence detection.
How does attenuation affect SPECT and PET?
SPECT: Attenuation is less severe. Radioactive attenuation correction sources or x-ray CT scan can correct for attenuation.
PET: Attenuation more severe. Radioactive attenuation correction sources or x-ray CT can correct for attenuation.
What is meant by a Standardized Uptake Value (SUV)?
The SUV is the normalized uptake of FDG to:
- Administered activity
- radioactive decay from time of injection
- patient body mass
SUV = (activity concentration in a voxel or group of vodels)/(activity administered/body mass)
In other words, the SUV represents the ratio of (1) the image derived radioactivity concentration found in a selected part of the body at a certain time point, and (2) as reference the radioactivity concentration in the hypothetical case of an even distribution of the injected radioactivity across the whole body. Here, the two radioactivity measures need to be from the same time point, e.g. the time of injection or the time of the image (or “image frame”). For example in the latter case, the injected activity is to be corrected for the physical decay between time of injection (t=0) and the time of the image frame (t).
What factors affect SUV?
Factors affecting Standardized Uptake Value:
- Accuracy of administered activity (material left in syringe)
- Leakage of activity during administration (??)
- PET/CT Calibration and attenuation correction
- Elapsed time before imaging (accuracy)
- Patient Physiological state (fasting, insulin etc)
- Body composition
- Size of tumor (partial volume effects)
- Motion (respiratory)
- Region of Interest Selection
What is the half life of FLT?
The half life of FLT is 109 minutes and emits positrons with Emax = 1.65 MeV. It accululates in cells that are undergoing cell division (tumors divite a lot). This way you can look directly for dividing cells instead of metabolic uptake.
What particle interaction dominates for PET?
Compton interactions dominate for PET, with 511 keV photons.
What type of particle interactions dominate for CT scans?
At 70 keV, photoelectric effect dominates for CT scans.
Explain how a 99Mo/99mTc generator works.
A technetium-99m generator, or colloquially a technetium cow or moly cow, is a device used to extract the metastable isotope 99mTc of technetium from a source of decaying molybdenum-99. 99Mo has a half-life of 66 hours and can be easily transported over long distances to hospitals where its decay product technetium-99m (with a half-life of only 6 hours) is extracted.
The column, usually glass, containing a bed of aluminium oxide (alumina) as a support for the parent radionuclide. 99Mo (molybdate) will bind strongly to this support media and is not washed off during the subsequent elution of the daughter radionuclide 99mTc (pertechnetate).
Pouring normal saline solution through the column of immobilized 99Mo elutes the soluble 99mTc, resulting in a saline solution containing the 99mTc as the pertechnetate, with sodium as the counterbalancing cation.
When the generator is left unused, 99Mo decays to 99mTc, which in turn decays to 99Tc. The half-life of 99Tc is far longer than its metastable isomer, so the ratio of 99Tc to 99mTc increases over time. Both isomers are carried out by the elution process and react equally well with the ligand, but the 99Tc is an impurity useless to imaging.
How often should you milk the 99mTc generator for the best efficiency?
You should milk it every 4 half-lives, or 24 hrs for the best efficiency.
What does a scintillator do and how does it work?
A scintillator converts high energy EM radiation into low energy visible light.Photons release electrons through ionizations and these energetic free electrons prodce electron-hole pairs.
Inorganic ones, such as NaI, BGO:
The scintillation process in inorganic materials is due to the electronic band structure found in crystals.
- An incoming particle excites an electron from the valence band to the conduction band.
- This leaves a hole behind in the valence band
- Electrons from the conduction band can fall back into the hole, emitting a photon in the visible light range.
- Impurities are added so that there are more acvitation sites closer to the valence band
- The photons emitted by the transitions of electrons from upper to lower states will be lower in energy than in the pure crystal: The emission spectrum is shifted to longer wavelengths and will
not be influenced by the optical absorption band of the bulk crystal. - The photons are emitted in the visible range.
Where does the visible light emission spectrum peak for NaI?
The visible light emission spectrum peaks for NaI at 415nm.
What are the typical gains for a PMT?
Typical gains for a PMT are on the order of 107 to 1010.
What is SPECT used for?
A SPECT scan is primarily used to view how blood flows through arteries and veins in the brain. Tests have shown that it might be more sensitive to brain injury than either MRI or CT scanning because it can detect reduced blood flow to injured sites.
The main advantages of SPECT/CT are represented by better attenuation correction, increased specificity, and accurate depiction of the localization of disease and of possible involvement of adjacent tissues. Endocrine and neuroendocrine tumours are accurately localized and characterized by SPECT/CT, as also are solitary pulmonary nodules and lung cancers, brain tumours, lymphoma, prostate cancer, malignant and benign bone lesions, and infection. Furthermore, hybrid SPECT/CT imaging is especially suited to support the increasing applications of minimally invasive surgery, as well as to precisely define the diagnostic and prognostic profile of cardiovascular patients. Finally, the applications of SPECT/CT to other clinical disorders or malignant tumours is currently under extensive investigation, with encouraging results in terms of diagnostic accuracy.
What types of imaging modalities are used to verify patient positioning before and during treatment?
kV CBCT, MV CBCT, Ultrasound, 2D kV, MRI, PET, and MV portal imaging are all used to verify patient positioning. In almost all cases, these images are compared to the kV fan-bam CT that is used to create a patient’s treatment plan.
What is the mean dose given to a patient with an on-board cone-beam CT imaging system?
3-35 mGy as of 2009.
Who performs contouring?
The radiation oncologist performs contouring, with simple structures occasionally contoured by the radiation therapist.
What needs to be taken into account in planning the margins of contouring?
The full range of the tumour motion (due to breathing, etc) needs to be captured in the contour.
What is the MIP?
The Maximum Intensity Projection: when comparing a set of voxels with the same planar coordinates, the intensity of the voxel with the maximum value is projected into a single image. This is mostly used when images are taken over time to show movement. For some region, the maximum value during movement is assigned to the voxel. The tumor is assumed to be more dense than the surrounding region. This is often used for lungs, where this is definitely true.
What is a digitally reconstructed radiograph?
A digitally reconstructed radiograph is an image obtained from a CT that has been reconstructed to look like an x-ray film. The radiological thickness along each ray line are summed together to achieve this.
