PHYSICS - Nucs Flashcards
Radiopharmaceutical definition
radio-isotope + pharmaceutical
Radionuclides definition
unstable radio-isotopes; decay to more stable form resulting in release of radiation
Mass number
A (for mAss); protons + neutrons
Atomic number
Z; protons
Isotopes
same number of protons
Isotones
same number of neutrons
Isomers
same number of protons and neutrons, different energy
Isobars
same mass number (A)
Gamma rays originate from…
nucleus (as opposed to x-rays which originate from orbital electron interactions)
1 mCi is how many MBq?
37 MBq; 1 Bq = 1 decay/second
SI units
Bq, Gy, and Sv (NOT Ci, rad, or rem)
Activity definition
decays per unit time
Specific activity definition
activity per unit mass; measured in Ci/g or Bq/g
Relationship between decay constant and time constant
inversely related; _ = 1 / _
Effect of a larger decay constant on half-life
shorter half life
Effective half-life should be…
longer than the examination time
Effective half-life relative to biological and physical half-life
effective half-life is always shorter than either the biological and physical half-life
Alpha decay
release of 2 protons + 2 neutrons (alpha particle)
What is a beta particle?
essentially an electron released from the nucleus; in beta-negative decay
Beta-minus decay
release of beta particle + anti-neutrino; N => P; new element formed
Example of beta-minus decay
Mo-99 => Tc-99m
Electron capture
orbital electron captured by nucleus; P => N; new element formed; always accompanied by characteristic x-ray release
Beta-positive decay
release of positron + neutrino; P => N; new element formed
Decay type(s) with neutron excess
beta-minus decay
Decay type(s) with proton excess
beta-positive decay, electron capture
Preferred decay type with proton excess and sufficient energy
beta-positive decay (positron emission)
Preferred decay type with proton excess and insufficient energy
electron capture
Excess energy after radioactive decay is released via…
internal conversion or isomeric transition
Decay type for Tc-99m => Tc-99
isomeric transition (remember the definition of isomers)
Isomeric transition
excess energy released as gamma photons
Internal conversion
excess energy transmitted to orbital electron which is ejected => orbital hole filling results in production of characteristic x-ray or Auger electron
Decay type(s) resulting in characteristic x-ray production
internal conversion, electron capture
Nuclear reactor production
production via bombardment with neutrons
Cyclotron production
production via bombardment with charged particles
Radionuclide production - products have neutron excess
neutron bombardment, nuclear fission; decay by beta-negative decay
Radionuclide production - products are neutron deficient
cyclotron; decay by electron capture or beta-positive decay
Radionuclide production - products are carrier-free
cyclotron, nuclear fission
Carrier-free definition
none of stable element accompanying products
Parent half-life is much greater than daughter half-life
secular equilibrium
Parent half-life is slightly greater than daughter half-life
transient equilibrium
How many daughter half-lives for daughter activity to equal parent activity in secular and transient equilibrium?
4 daughter half-lives for both (per Huda)
Equilibrium type for Mo-99 => Tc-99m
transient equilibrium
Ideal time to “milk” Mo-Tc generator
every 4 half-lives (24 hours or 1 day)
How many days until Molybdenum is used up in Mo-Tc generator?
5 days (or 1 week)
Isotopes produced from a generator
Tc-99m, rubidium; generators are used to produce short-lived isotopes
SPECT (acronym)
Single Photon Emission Computed Tomography
Cardiac SPECT
L-mode, 180 degree arc (-45 RAO to +135 LPO), images sorted by phase of cardiac cycle using ECG to create cines, 64 x 64 matrix
Causes of image degradation (PET)
positron travel, angle of photon emission, scatter and absorption, simultaneous separate events
2-D PET
less sensitive, less scatter => reduced noise; fewer true events detected => longer study
3-D PET
more sensitive; increased scatter and random events detected
Time-of-flight systems (3-D PET) most improve image quality in…
obese patients
Alternate names for gamma camera
scintillation camera, Anger camera
Spatial information in planar/SPECT vs. PET
collimators in planar/SPECT, coincidence detection in PET
Low-energy collimator radionuclides
Tc-99m, I-123, Xe-133, Tl-201
Medium-energy collimator radionuclides
Ga-67, In-111
High-energy collimator radionuclides
I-131
Pinhole collimator
magnified and inverted image; FOV increases with distance between patient and collimator; magnification decreases with distance between the patient and collimator, and increases with distance between the collimator and detector
Parallel hole collimator
no magnification; FOV does not change with distance
Converging collimator
magnified image, FOV decreases with distance (CDD)
Diverging collimator
minified image, FOV increases with distance (DID)
Pinhole collimator - sensitivity and spatial resolution
low sensitivity, high spatial resolution; varies with pinhole size
Parallel hole collimator - sensitivity and spatial resolution (distance)
spatial resolution decreases with increased distance; distance has no effect on sensitivity
Parallel hole collimator - higher sensitivity (septa)
thinner septa, shorter septa, larger inter-spaces
Parallel hole collimator - higher spatial resolution (septa)
longer septa, smaller inter-spaces, decreased distance
Parallel hole collimator - decreased penetration (septa)
thicker septa, longer septa
LEHR vs. LEAP collimators
LEHR has longer septa with smaller inter-spaces => decreased sensitivity, increased spatial resolution
Effect of switching from a low energy to a high energy collimator
decreased penetration, decreased sensitivity, decreased resolution (all because of thicker septa)
Effect of increased scintillator crystal thickness
increases sensitivity, decreased spatial resolution
Light to electrical signal conversion component (gamma camera)
photomultiplier tubes (PMTs)
Artifact: focal photopenic defect
single bad PMT
Pile-up (gamma camera)
summation of two events occurring before integration time
Pile-up increases with…
increased integration time, higher radionuclide activity level
Normal SPECT matrix size (non-cardiac)
128 x 128; cardiac is 64 x 64
Elliptical orbit of gamma camera (SPECT) improves…
spatial resolution
SPECT improves ______ relative to planar imaging
contrast (by removing overlapping structures)
How to: fix inhomogeneities (gamma camera)
pre-scan flood field used to adjust for inhomogeneities
Downscatter
Compton scatter from higher energy radionuclide is picked up in photopeak window for subsequently administered lower energy radionuclide
Effect of downscatter
decreased CNR
How to: avoid downscatter
use lower energy radionuclides first
Standard Uptake Value (SUV)
mean ROI activity / (administered activity / lean body weight)
Diffusely decreased SUV values (PET)
high blood glucose
Contrast is negatively affected by…
scatter and septal penetration
Intrinsic resolution
performance of gamma camera without collimator
Energy resolution
ability of an imaging system to distinguish between photon energies (to remove scatter)
Spatial resolution - planar vs. SPECT
planar has better spatial resolution (always)
Survey meters
gas-based detectors; in order of highest-to-lowest sensitivity: GM > proportional counter > ionization chamber
Pancake probe
a.k.a. Geiger-Mueller counter; higher voltage applied across ionization chamber
Cutie pie
a.k.a. ionization chamber; lower voltage applied across ionization chamber
Dose calibrator
essentially an ionization chamber; quantifies radiation in clinical doses; must select type of radioisotope prior to measuring
Well counter
NaI-lined lead tube; for detecting activity <1 uCi; saturates quickly; used for wipe test
Geometric efficiency of well counter
very high (all gamma rays interact with NaI crystal)
Thyroid probe
for calculating %RAIU; single NaI crystal, PMT, and a single hole collimator; no image created
Attenuation correction is performed for which of planar, SPECT, and PET?
SPECT and PET (not planar)
Attenuation of gamma rays in patient is dependent upon…
originating location in patient, size of patient, energy of gamma rays (lower energy => more attenuated)
Scanning line source attenuation correction
attenuation map has low SNR; uses isotope with a long half-life (gadolinium)
Defect on extrinsic uniformity QC, but not intrinsic uniformity QC
issue with collimator
Order of “peaks” from a point source (left-to-right)
Compton scatter, Compton edge, backscatter, iodine escape peak
Artifact: star artifact (SPECT)
septal penetration
Artifact: diffusely decreased counts (SPECT)
incorrect energy window (for type of isotope and study)
Artifact: 2 lines in one direction and 1 line in the other direction
tuning fork artifact; center of rotation error; occurs in cardiac SPECT
Causes of attenuation correction artifact
implanted hardware, IV contrast, oral barium, external jewelry, dense calcification
Examples of scintillators (in nuclear medicine)
gamma camera, well counter, thyroid probe
Decay type(s) with neutrino release
beta-minus, beta-positive decay
Amount of energy required for beta-positive decay
1.022 MeV
Mo-99 is produced by…
fission in a nuclear reactor
FWHM of a photopeak is a measurement of…
system energy resolution
Rubidium vs. F-18 positrons
rubidium positrons are higher energy => travel further prior to annihilation (gamma rays are still 511 keV)
Linear energy transfer (LET)
amount of energy deposited in mass per unit length by the incident radiation; high LET particles deposit a large amount of radiation in a short distance (e.g. alpha particles)
