Nucs Flashcards
What is the band of stability?
Happy place atoms go when they have a certain balance of protons and neutrons in the nucleus. Not exactly 1:1, but it’s close.
Depending on the imbalance of neutrons/protons different types of decays occur.
What kind of decay happens when you have lots of neutrons, not enough protons?
Beta minus decay
What is beta minus decay?
Have lots of neutrons, not enough protons
Take one of your neutrons and turn it into a proton - results in the emission of a “beta particle” from the nucleus - basically an electron emitted from the nucleus.
Need to change the charge of a neutron from neutral to positive (neutron is 1/2 negative and 1/2 positive - eject the negative, become all positive).
Need to balance out the energy loss - going from a neutron to proton - emit a massless “neutron” just to balance out the energy.
What is a “beta particle” and why is it ejected?
Ejected in beta minus decay when you have too many neutrons, not enough protons.
Basically an electron that comes from the nucleus.
- Ejected to change the charge of a neutron from neutral to positive (neutrons are basically 1/2 positive and 1/2 negative - eject the negative, then you become all positive).
- Balance out the energy loss - going from a neutron to proton, emit a massless particle called a “neutrino” just to balance out the energy.
What is emitted in beta minus decay?
Beta particle - negative particle
Neutrino - massless, to balance out the energy.
What kind of process is beta minus decay - chemistry term?
Isobaric transition - mass doesn’t change.
The atomic number changed (increased), but the mass of a neutron and proton are about the same - the mass number remained constant.
Is beta minus decay good for imaging?
No, but can cause some damage.
What kind of shielding do you want for a beta emitter?
Plastic b/c it has a low Z.
Lead’s high Z will result in Bremmstahlung x-rays.
What kind of decay happens when you have lots of protons and not enough neutrons?
Beta positive decay
Electron capture
What is Beta Positive Decay?
Too many protons and not enough neutrons.
The “rich guy” - has 1.02 MeV and not problem losing it.
A positively charged proton is converted into a neutral neutron by giving up a “positron” and a neutrino to keep the energy balanced.
Positron travels a very short distance and meets an electron - annihilate each other - results in two 511 keV photons emitted 180 degrees apart from each other.
511 keV is 1/2 of 1.02 MeV
What is Electron Capture?
Too many protons and not enough neutrons
The “poor guy” - don’t have the energy to kick out the positively charged proton to leave - adds a negative to a positive to make a neutral.
Pulls an electron into the neucleus from the K shell
What kind of process is Electron Capture - chemistry term?
Isobaric transition - just like beta minus and beta plus, mass does not change, but atomic number does - lose a proton.
Is Electron capture good for imaging?
Yes, it’s excellent b/c it’s often coupled to a process referred to as “isometric transition” which results in the emission of a characteristic gamma photons that can be imaged under a gamma camera.
After undergoing an isobaric transition (beta emission, positron emission, or electron capture), there is often left over energy - needs to be emitted to achieve final stability.
Isomeric transition - emitting energy and dropping to the ground state.
What is Gamma Emission from Isomeric Transition?
After undergoing isobaric transition (beta emission, positron emission, or electron capture), there is often left over energy - needs to be emitted prior to achieving final stability. Isomeric transition - emitting energy and dropping to the ground state.
Gamma Emimssion - the nucleus will emit its excess energy in the form of a gamma photon. The energy of these photons is variable and depends on the energy differences between the intermediate and final states of the nucleus undergoing isomeric transition. TOTAL energy emission for a given nuclei of a specific nuclide is the same - many times you have more than one intermediate state - with multiple energy gamma photons - why you see multiple peaks for different tracers.
What is Metastable?
In most cases the time spent in the intermediate state is very short. The transition from isobaric to isometric transitions is fast.
Situation where the intermediate state is prolonged - “metastable state”
The main value is the ability to separate out the electron radiation (bad) and the photon radiation (good). By using a nuclide that has already undergone an isobaric (electron emitting) transition - you can only expose the body to useful gamma emissions from the isometric transition.
Mo99 (isobaric transition via beta minus decay - emission of a beta particle (electron)) - Tc99m (hanging out in “metastable” limbo for a couple of hours) - Isometric transmission - emission of 140 keV photons - Tc99
What are the two types of isometric transition?
After undergoing isobaric transition (beta emission, positron emission, or electron capture), there is often left over energy - needs to be emitted prior to achieving final stability. Isomeric transition - emitting energy and dropping to the ground state.
Gamma Emission - nucleus emits its excess energy in the form of a gamma photon.
Internal Conversion - The energy you would normally get rid of via gamma emission can be transferred to an electron w/in the atom. Vacancy can lead to a downward cascade from a higher level - and either the transmission of a characteristic x-ray or an Auger electron. Bad b/c you are emitting particles which cause harm, not imageable gamma photons which contribute to the study.
What is Internal Conversion?
Type of isometric transition- left over energy from an isobaric transition (beta emission, positron emission, or electron capture) needs to be emitted.
The energy instead of being emitted via gamma emission, is transferred to an electron w/in the atom. Vacancy can lead to a downward cascade from a higher level - and either the transmission of a characteristic x-ray or an Auger electron. Bad b/c you are emitting particles which cause harm, not imageable gamma photons which contribute to the study.
What is Alpha Decay?
Tends to occur in heavier unstable atoms.
Alpha particles are basically Helium nuclei (2 protons, 2 neutrons). Slow and fat, can’t penetrate a piece of paper, worthless for imaging, but bad in close proximity.
Used in treatment situations - bone cancer mets with Radium 223.
Two types of tracer production?
Bombardment (nuclear reactor or cyclotron) and Fission
What is Bombardment type of tracer production?
Striking target elements with either neutrons (in a nuclear reactor), or with charged particles (alpha particles, protons, or deuterons) in a cyclotron.
Cyclotron has the advantage of producing elements via transmutation, therefore you don’t have any parents to clean up “carrier free”.
Have to clean up the left over parent element.
Difference between bombardment production in nuclear reactor vs cyclotron.
Nuclear Reactor: bombard with neutrons
Cyclotron: bombard with charged particles - usually results in transmutation, so you don’t have to clean up the parent - CARRIER FREE
What is Fission type of tracer production?
Neutrons are fired into large atoms (like Uranium and Plutonium) and split them into pieces - a lot of random stuff being made - I-131, Xenon-133, Strontium-90m, Molybdenum-99, Cesium-137.
The desired isotope also has a bunch of fission products (contaminants) which have to be separated out - can be done with chemistry. Can demonstrate a bunch of different decay methods.
What is Neutron Activation?
Target atoms eat up neutrons to form a new isotope - don’t need to be accelerated.
Products are isotopes of the target atoms, they cannot easily be separated from each other - NOT carrier free.
Neutron rich products tend to decay to beta emission.
Difference between physical, biologic, and effective half life?
Physical - Time necessary for a radionuclide to be reduced to half its existing activity.
Biologic- how long it takes to shit or piss half the tracer out.
Effective- takes both of these into consideration.
How do you calculate effective half life?
1/Effective = 1/Physical + 1/Biologic
If you have a large mismatch with biological and physical half life, effective half life basically becomes the short one.
Xenon: if you breath it out in 15 seconds it doesn’t matter when the physical half life is.
How long do you have to keep radioactive material?
General rule is 10 half lives.
What is activity and what are the units?
Amount of disintegrations per second.
Curie (Ci)- 3.7x 10^10 disintegrations per second.
New SI unit is Becquerel (Bq) which is one disintegration per second.
What is Specific Activity?
Activity per unit of mass (Bq/g).
The longer the half life, the lower the specific activity.
How does half life affect specific activity?
Longer half life = lower specific activity.
Activity per unit of mass (Bq/g).
How does a gamma camera work basically?
Gamma camera takes photons emitted from the radionuclide, turns it into a light pulse, and then takes that light pulse and makes some voltage. The voltage makes a picture.
What is a collimator?
Reduce scatter and allow for correct localization of radionuclide events. Works by discriminating based on direction of travel - can NOT tell the difference in photon energy (that’s done by photon height analyzer).
What tracers is a low energy collimator used for?
1-200 keV
Tc99m, I-123, Xe-133, TI-201
What tracers is a medium energy collimator used for?
200-400 keV
Ga-67, In-111
What tracers is a high energy collimator used for?
> 400 keV
I-131
What is the relationship between sensitivity and resolution with collimators?
Have an inverse relationship.
High sensitivity collimator will allow twice as many counts to be imaged but will degrade the spatial resolution. High sensitivity collimators are important with dynamic imaging (like the flow phase of a bone scan).
Effect of distance on sensitivity and resolution
Distance has NO effect on sensitivity (increased distance reduces counts by inverse square, but the increased distance allows for a greater FOV = no change in counts).
Distance DOES affect resolution (septa are no longer able to eliminate photons from oblique angles as distance increases).
Effect of septal length on sensitivity and resolution?
Short septa give a crappy resolution, but better sensitivity
Long septa give excellent spatial resolution, but crappy sensitivity (noisier image).
Effect of hole diameter on sensitivity and resolution?
Wide holes = highly sensitive, low resolution.
Narrow holes = low sensitivity, high resolution.
General trend for high energy and low energy septa?
High energy - want to use long thick septa + wide holes
Lower energy - Short thin septa + narrow holes
What is a converging hole collimator?
Cone Beam - holes are close together on the object side and far apart on the crystal/camera side - magnifies WITHOUT inverting the image (as opposed to a pinhole).
What is a diverging hold collimator?
Holes are far apart on the object side, and close together on crystal/camera side.
Takes a large object and minimizes it - can mage a large part of the body on a small crystal.
Increased area, decreased sensitivity and resolution.
What do photomultiplier tubes do?
Detect the light produced by a photon hitting the scintillation crystal and convert it into an electric signal or measurable magnitude.
More PMTs = more light you can pick up and greater resolution.
PMTs can record two things:
- Location- on X and Y axis
- Signal Intensity - which goes into a pulse height analyzer.
What is the function of a pulse height analyzer?
Discard background information and only look at photons from the tracer you are looking for.
What is the difference between background from a point source vs background from a person?
Compton scatter from a person is a lot closer to the energy you want to image, but really degrades the images.
Set the window that will exclude higher or lower to the desired peak.
What is Downscatter?
High energy photons can spill into the window of a low energy emitter, mainly resulting from Compton scatter effects.
LOWER PHOTON ENERGY TRACERS MUST BE USED FIRST.
How does matrix size affect acquisition time?
Larger matrix has superior resolution, but means longer acquisition time.
What is Star Artifact?
Focal intense energy - septal penetration of the hexagonal collimator holes.
Thyroid bed after a high therapeutic dose (using a medium energy collimator, instead of high).
What are the four parameters measured for gamma camera QC?
Field Uniformity - daily (extrinsic) and weekly (intrinsic) Window Setting - Daily Image Linearity - Weekly Spatial Resolution - Weekly Center of Rotation - Weekly
What is Field Uniformity?
The PMTs can have subtle variability in what voltage they assign to a given photon of light and the crystal isn’t totally uniform, and has subtle variations in thickness.
Want to try and keep these two things as uniform as possible.
A 2-5% non-uniformity is allowed (1% if SPECT).
Tested with a “flood”
What is a “Flood” test?
Test field uniformity- used to see if the camera can produce a uniform image along the entire crystal surface.
Extrinsically - with a collimator
Intrinsically - NO collimator
Use either Na99mTcO4 or Co57 source
Recommended counts for both extrinsic and intrinsic is between 5-10 million.
Recommended counts for extrinsic and intrinsic flood test?
5-10 million counts
How often should extrinsic flood test be done?
WITH collimator - done daily.
Tests the collimator and crystals
How often should intrinsic flood test be done?
WITHOUT collimator - done weekly.
What is the Energy Window test?
Correct window needs to be used prior to each study, so should be done DAILY.
Use a symmetric window centered at the peak energy used in the imaging test.
Source can be a syringe, a vial, or if absolutely necessary the patient.
For Tc you would use a 20% window centered at 140keV.
What is image linearity and spatial resolution test?
Lead bar phantoms with parallel lines placed between collimator and a Co57 sheet.
Done weekly
Resolution is ability to differentiate between two distinct points (can you tell the bars are separate).
Linearity is tested by looking to see if all bars are straight (some distortion at the edges is ok). Bars should NOT look wavy.
What is center of rotation test?
Gamma cameras that are used for SPECT have to be routinely monitored for alignment at the COR (center of rotation).
Done with 5 small Tc99m point sources along the axis of rotation. The axis should be straight with minimal deviation.
WEEKLY.
Why aren’t lead aprons used in NM?
- Thin lead doesn’t stop gamma rays - would have to be a big apron.
- When high energy gamma rays collide with a dense material that is not thick enough to stop them (lead apron) they rapidly slow down and lose energy which turns them into deeply penetrating Bremsstrahlung x-rays which actually makes the dose worse.
How should a ring badge be worn?
On dominant hand, index finger, label in towards source (usually means towards the palm), under a glove (to avoid contamination).
What is a well counter?
Basically a small gamma camera with PMT. HOle in the block of NaI crystal into which the sample is placed (so it’s surrounding the sample)
Great efficiency, but can be overwhelmed. If sample exceeds 5000 counts per second (a lot less than a micro curie), it will be under reported.
Limitation of a well counter?
Great efficiency, but can be overwhelmed. If sample exceeds 5000 counts per second (a lot less than a micro curie), it will be under reported.
What is “dead time” in a Geiger-Muller Counter?
GM counters are very sensitive but also vulnerable to being overloaded by a large dose of radiation. Then you have “dead time” before it can respond again.
Max dose it can handle is about 100 mR/h
Can NOT provide info on radiation type.
What is the maximum dose a Geiger-Muller counter can handle?
100 mR/h.
What is an ionization chamber?
Same concept as a Geiger-Muller Counter but when higher doses are expected. Don’t have dead time problem.
Can detect exposure rates from 0.1 to 100 R/h (GM counter is 100mR/h) - unit change.
Dose calibrator is ionization chamber.
Exposure rates detected with ionization chamber?
0.1 to 100 R/h
GM is 100 mR/h - unit change.
Differences between a Geiger-Muller Counter and Ionizing Chamber?
GM Counter - Very sensitive, great for low-level radioactive survey, terrible for very high radiation fields (“dead time”)
Ionizing Chamber - lower sensitivity, stable across a wide voltage range - excellent for accurate estimates (or exposure).
What are the types of personal dosimeters?
Pocket ionization detector
Solid state dosimeter
Film badge
Optically stimulated dosimeter
Thermo-luminescent dosimeter
What should the constancy measurement be for a dose calibrator and when should it be checked?
Should be within 5% of computed activity. Checked with reference sources - checked DAILY.
What is linearity for a dose calibrator and when should it be checked?
Accurate readout for activities over the whole range of potentially encountered activities - checked with a large activity of Tc (around 200 mCi) and decaying it down to less than the smallest activity you would measure for use.
Or use a Calicheck or LIneator kit (contains sheets of varied thickness of lead, simulating decay over time).
QUARTERLY.
What is Accuracy for a dose calibrator and when should it be checked?
Standard measurements of radiotracers measured and compared to what the activity should be
At installation of the device and annually.
What is Geometry for a dose calibrator and when should it be checked?
Correction for different positioning and size (different volumes of liquids) of the sample (performed at installation and any time you move the device).
Where are the rules and regulations for NM found?
Code of Federal Regulations (CFR)
Part 19- Inspections
Part 20- Radiation protection
Part 35- Human use of radioisotopes
The NRC is the governing body that has been charged with the task of enforcing all the directives.
Activity greater than what for Tc99m, TI-201, In-111, Ga-67, and I-131 is considered a major spill?
Greater than 100 mCi of Tc-99m is considered
Greater than 100 mCi of TI-201 is considered
Greater than 10 mCi of In-111 is considered to represent
Greater than 10 mCi of Ga-67 is considered to represent
Greater than 1 mCi of I-131 is considered to constitute
What do you do for a major spill vs a minor spill?
Minor spill = You clean it up
Major Spill = Don’t clean it up, call the radiation safety officer.
Activity of Tc-99m to be considered a major spill?
100 mCi
Activity of Tl-201 to be considered a major spill?
100 mCi
Activity of In-111 to be considered to represent a major spill?
10 mCi
Activity of Ga-67 to be considered to represent a major spill?
10 mCi
Activity of I-131 to be considered a major spill?
I-131
What do you do during a minor spill?
- Protect the patient- address the patient first if in distress, then address the spill.
- Confine the spill/limit the spread - don’t let people come in and track it all over the place.
- Clean up the spill - use gloves and wear shoes
- Survey cleanup items - anything used in the clean up needs to be surveyed or presumed contaminated (held until they decay to safe levels - rule is 10 half lives).
- Survey cleanup people - surveyed by the radiation safety officer.
What do you do for a major spill?
- Clear area
- Cover spill with absorbent paper. Do NOT clean it up.
- Clearly indicate boundaries of spill area. Limit movement of contaminated persons.
- Shield source if possible
- Notify the Radiation Safety Officer immediately
- Decontaminate persons
What do you do if a spill gets on your clothes?
Take them off - will be held by the RSO until decayed to safe levels.
