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.