9. Nuclear Physics Flashcards
band of stability
decay occurs to reach stablitybetween neutrons and protons (Z)
types of decay
beta minus (neutrons > protons)
beta plus/positron or electron capture (protons > neutrons)
alpha decay
beta minus decay
neutrons > protons; turn neutron into positron through beta particle emission
isobaric transition (mass does not change)
emission of beta particle changes charge of neutron from neutral to positive; and balance out energy loss
how to shield a beta emitter
plastic shielding
lead will generate bremmstahlung due to high Z
beta positive decay
protons > neutrons
positively charged proton converted into neutral neutron by addition of neutrino
511 keV positron emission
positron/electron collide and products with 511 keV photons are emitted 180 degrees apart from one another
electron capture
protons > neutrons
change proton into neutron by adding an electron
isobaric procedure; mass does not change
atomic number decreases since proton lost
often coupled to isomeric transition (emission of gamma photons)
isomeric transition
energy emission after isobaric transition
gamma emission usually
-multiple peaks for different tracers
gallium 4, indium 2
isobaric transition
beta emission, positiron emission, electron capture
how many peaks does gallium have
4
how many peaks does indium have
2
metastable
intermediate stae after transition from isobaric before isomeric transition
allows time to utilize nuclide before gamma emission for medical sue
alpha decay
heavy unstable atom with lots of tissue damage
positron emission
beta positive
production of tracers
cyclotron, nuclear reactor, radionuclide generator
cyclotron
produces elements via transmutation; carrier free (no parents to clean up/decay)
vacuum chamber in circular path accelerates particle to bombardment chamber to produce radioisotopes
nuclear reactor
spontaneous fissuino of uranium 235 into lighter fragments which will emit fission neutrons to produce unstable uranium 236
disadvantage: low yield and other undesired radioisotopes
cyclotron produced radioisotopes
molybdenum 99, fluorine 18, gallium 67m thallium 201
reactor produced radioisotopes
molybdenum 98 (can then go into cyclotron)
iodine 131
xenon 133
thallium 201
radionuclide generator
molybdenum is made in a Tc generator
molybendum vs tc halflife
Mo 67 hrs
Tc 6 hrs
Mo decays and is washed off with ____ to generate ___
saline to generate Tc stuck to Na
break through
Mo in a sample of Tc that washed off the generator
radionuclide purity test
evaluate for breakthrough Mo by evaluating photopeaks in sample
Mo assayed first: high energy Mo (740 keV) will NOT be attenuated by lead shield
NRC standar for radionuclide purity
NRC allows no more than 0.15 micro Ci of Mo per 1 milli Ci of Tc at the time of administration
if ratio <0.038 at time of elution, material will be suitable for injection/administration for at least 12 hrs
generator produced radionucldies
Tc99 from Moly 99
krypton 81m from Rubidium 81
chemical purity test
performed with pH paper
allowed amount is <10 microgram Al per 1 ml
generator column made of aluminum oxide which can wash off, clump with Tc and show up as liver activity or cause sulfur colloid
how does aluminum contamination manifest?
- Tc scan + LIVER activity
2. liver spleen scan with LUNG activity
radiochemical purity test
thin layer chromatography
after Tc comes out of generator as Na 99mtc)4, it must be reduced by adding SnCl2 (tin)
limits for free Tc
95% Na 99Tc O4
92% 99Tc sulfur collid (MAA)
91% all other Tc radiopharmaceuticals
is chemical purity testing NRC mandatory
not mandatory in NRC states
radionuclide purity ratio should be known at?
time of administration, not elution
which do you assay first for radionuclide purity?
Mo before Tc
free Tc cause
lack of stannous ions (reducing agent), air injection into vial which oxidizes
where does free Tc show up?
gastric, salivary, thyroid uptake
equilibria
parent and daughter isotopes are equal
transient equilibrium
half life of daughter shorter than parent; usually occurs after 4 half lives
secular equilibrium
halflife of daughter way shorter than parent
types of half life
physical, biologic, effective
physical half life
time reuired for radionuclide to decay to half of existence
biologic half life
time required for radionuclide to reach half level in body
effective half life
combination of biologic and physical
half life of Tc
6 hrs
I-131 biologic half life, effective half life
24 days; effective half life 6 days
equation for effective half life
1/effective = 1/physical + 1/biologic
how long to keep radioactive material?
10 half lives
Becquerel
1 disintegration/second
previously measured in Curie (Ci) = 3.7 x 10^10 disintegrations/second
specific activity
activity per unit mass of (Bq/g)
longer half life, lower specific activity
gamma camera
radionuclide > photon > light pulse > voltage > picture
collimator > crystal > photomultiplier > pulse height analyzer
collimator
reduces scatter and allows for correct localization of radionuclide events; discriminates direction
types of collimators
parallel, pinhole, converging hole/cone beam, diverging
parallel hole collimator
work horse
has low, medium, and high energy types based on plate thickness
parallel hole collimator: sensitivity/resolution relationship, distance, septal length, hole diameter
inverse relationship
high sensitivity collimators degrade resolution
distance affects resolution, no effect on sensitivity
short septa: low spatial resolution, better sensitivity
hole diameter: wide hole = low resolution, high sensitive
parallel hole collimator: low energy
1-200 keV, thinner plate
99Tc, 123I, 133 Xe, 201 Ti
parallel hole collimator: medium energy
200-400 keV
67Ga, 111 In
parallel hole collimator: high energy
> 400 keV, thicker plate
131 I (most energy peaks are medium)
septal length/holes and energy
high energy: long septa with wide holes
low energy: thin septa with narrow holes
pinhole collimator
magnifies/inverts image; cone shaped
used for thyroid and small parts
pinhole magnification ratio
pinhole to dector (F) to pinhole to patient (B)
F=B, no magnification
F>B, magnification
B>F, object minimized
effect of moving pinhole far from patient?
image smaller
sensitivity for pinhole camera
poor
converging hole collimator
cone beam
magnifies without inverting image
diverging collimator
opposite of converging
holes far apart on object side; close together on crystal side
objects minimized
scintillation crystal
sodium iodine doped with thallium; generates pulse of light when struck with photon
thick vs thin scintillation crystal
thick: better sensitivity, worse spatial resolution
thin: better spatial resolution, worse sensitivity
photomultipleir tube
detect light and convert to electric signal
records location and signal intensity
pulse height analyzer
discards background signal and an record multiple peaks
pulse height analyzer: radiotraers with multiple peaks
67 Ga, 111 In
downscatter
high energy photons can spill into window of low energy emitter
best to image lower photon energy tracers first
Xe then Tc in VQ scan
(Xe photopeak 81, Tc photopeak 140 keV)
downscatter: VQ scan
image Xe then Tc
downscatter: bone scan
Tc then Ga
gallium has 4 photo peaks with half life of 50 hrs
static vs dynamic gamma cameras
static
dynamic: rotates around patient; movement degrades imaging
gamma camera matrix size
128 x 128 has higher spatial resolution than 64 x 64
larger matrix means longer acquisition time and reduced density per pixel (impacts image contrast)
star artifact
septal penetration of hexagonal collimator holes
seen on thyroid bed after high therapeutic dose (medium energy collimator)
pattern of collimator holes with star artifact
hexagonal pattern
quality control for gamma camera parameters
field uniformity, window setting, image linearity, spatial resolution, center of rotation
field uniformity
subtle variations in the photomultiplier tube/crystal thickness
2-5% nonuniformity or 1% if SPECT is allowed
field uniformiy test
flood; evaluate if camera produces uniform imagie
1) extrinsinsically with collimator
2) intrinsically with Na99TcO4 or Co57 source
how often are extrinsic and intrinsic flood/field uniformity tests performed
extrinsic: daily; test collimator and crystal
intrinsic: no collimator; done weekly
bulls eye appearance of PMT
problem; defective crystals
quality control:energy window
performed daily
symmetric window centered at peak energy used in imaging test
source could be syringe, vial, or patient
Tc: 20% window centered at 140 keV
image linearity/spatial resolution
lead bar pphantoms with parallel lines are placed between Co sheet and collimator; tests image resolution and linearity
performed weekly
test for lines to be straight and resolution between bars
Center of rotation
gamma cameras used for SPECT have to be routinely monitored for allignment offset at the COR
5 small 99mTc point sources alongg axis of rotation; axis should be straight
performed monthly
why do NM techs not wear lead?
thin lead will not stop gamma rays
high energy rays will collide with lead and turn into penetrating bremmsstrahlung xrays
where should film badge and ring badge be worn?
film badge: collar at chest/neck level
ring badge: dominant hand, index finger; label in towards saurce, under glove to avoid contamination
gamma instruments
sodium iodine well counter, thyroid probe, geiger muller counter, ion chamber
sodium iodine well counter
small gamma counter with single PMT
may under report if sample exceeds 5000 counts/second (in vitro blood/urine samples)
good for wipe in test
thryoid probe
modified well counter to calculate thyroid uptake values; positioning guide to keep distance constant
device has shielding; dose compared to calibrated capsule of same radionuclide
geiger muller counter
detects small amounts of radioactive contamination
gas-filled chamber which becomes ionized when in contact with radiation and creates voltage
detects radiation amount, not type
dead time for geiger
overloaded by large dose of radiation
maximum dose: 100 mR/h
device will click and stop
ion chamber
used when high doses are expected; no problems with dead time
detect exposures from 0.1 to 100 R/hr (higher than geiger)
intraoperative probe
used for lymphoscintigraphy
Q/A on dose calibrator parameters/ionizing chamber
consistency, linearity, accuracy, geometry
ionization chamber QA: consistency
daily; should be within 5% of computed activity
ionization chamber QA: linearity
quarterly; readout for range of activities typically used with sheets of varied thickness of lead which simulates decay over time
ionization chamber QA: accuracy
performed at device installation and annually
standard measurements of radiotracers measured and compared to activity
ionization chamber QA: geometry
installation and anytime device moved
correct for positioning and size of different volumes of liquids
minor vs major spills, who cleans
major: call radiation safety
types of personal dosimeters
pocket ionization detector, solid state dosimeter, film badge, otically stimulated dosimeter, thermo-luminescent dosimeter
solid state vs optically stimulated dosimeter
solid state dosimeter: accumulated dose with LCD display
optically stimulated dosimeter: replaced film badge; chips/strips placed under filter
problems with film badge
damaged by temperature and humidity
degree of darkening corresponds to dose
pocket ionization detector
miniature ionization chamber for real time estimated dose, but must be charged and zero-d prior to use
no longer used
CFR part 19, 20, 35
code of federal regulations
19: inspections, notices, reports
20: radiation protection
35: human use of radioisotopes
NRC
governing body in charge of enforcing directives
agreement states
individual states reach agreement with federal government on guidelines; can be more strict but not more lenient than national agency
major spil qualifications
> 100 Ci Tc 99m, TI 201
> 10 mCi In-111, 1-123, Ga 67
> 1 mCi I 131
what to do if there is a major spill
clean area, cover spill with absorbent paper, indicate boundaries of spill, shield source, notify radiation safety office, decontaminate people
what to do if minor spills
protect patient then spill
confine spill/secure area, clean spill using PPE, survey cleanup items and decay for 10 half lives, survey cleanup people by RSO
what to do if contamination on clothes vs skin
clothes: ungarb and give to RSO for decay
skin: wash, but don’t break skin
what to do if there is a xenon leak?
leave room, close door
annual allowable dose to public
100 mrem annually, no greater than 2mrem/hr in an unrestricted area
allowable dose in a restricted area
> 2 mrem/h
signs necessary for radiation safety areas
radiation area (0.0005 rem/hr at 30 cm)
high radiation area (0.1 rem/hr at 30 cm)
very high radiation area: > 500 rads/5 gray in 1 hr at 1 m
NRC exposure limits
- total body dose/yr
- dose to ocular lens
- total equivalent organ dose
- total equivalent extremity dose
- total dose to embryo over 9 mo
- total body dose/yr: 5 rem/50 mSv
- dose to ocular lens: 15/150 mSv
- total equivalent organ dose: 50 rem/500 mSv
- total equivalent extremity dose: 50 rem/500 mSv
- total dose to embryo over 9 mo: 0.5 rem/5 mSv
unit conversion between rad and rem, mSv
1 rad= 1 rem = 10 mSv
1 mSv = 100 mRem / 0.1 rem
reportable medical event
wrong drug, route, patient
wrong dose (> 20% of dose or 10% in agreement states)
dose to body site other than treatment site that >50% than expected
dose > 5 rem to body or single organ >50 rem
what to do if there is a medical event
call ordering doc (24 hrs), patient, NRC/state (15 days)
how long to keep record of recordable events
5 years
receiving radioactive material protocol
within 3 hours survey package with GM counter test at surface and 1 meter from package; wipe surfaces; keep in hot lab
allowable limit: <6600 dpm/300 cm2
contact shipper/NRC if beyond allowable limit
packaging labels:
white
yellow2
yellow3
white: no special handling, surface dose rate <0.5 mRem/hr, 0 mRem at 1 m
yellow 1: special handling, surface dose rate <50, 1 m < 1 mRem/hr
yellow 2: special handling, surface dose rate <200 mRem/hr, 1 meter <10 mRem
transport index :
white
yellow2
yellow3
max dose at 1 m
white: none
yellow2: < 1 mrem/hr
yellow3: > 1 mrem/hr
common carriers
truck that carries regular packages and radiactive material
TI should not exceed 10 mrem/hr; surface rate < 200 mrem
multiple packages
shipped together; sum should not exceed 50 mrem
critical vs target organ
critical: organ limiting radiopharmaceutical dose due to increased risk for cancer; usually where tracer spends most time in
target organ: organ of interest
critical organs:
- liver
- spleen
- stomach
- gallbladder wall
- renal cortex
- proximal colon
- distal colon
- bladder
- liver: In prostascint, I 131 MIBG, sulfur colloid (IV)
- spleen: octreotide, damaged RBC, In-WBC
- stomach: pertechnetate
- gallbladder wall: HIDA
- renal cortex: thallium, DMSA
- proximal colon: sulfur colloid (oral), sestamibi
- distal colon: gallium
- bladder: MAG 3, I 123 MIBG, MDP
SPECT
single photon emission CT
rotation of gamma camera around patient for 3D image ; uses single photon not like PET which uses positron annihilation
speed, sensitivity, matrix size of SPECT
slow, each proection takes 30 sec, can take up to 20 min
matrix size: 128 x 128 and uses iterative reconstruction for picture
sensitivity depth dependent with different tissue originas attenuating differently
collimators with longer holes may be used to improve photon collection
fan beam collimation
brain SPECT
types of radiopharmaceuticals that can be used for SPECT
must not redistribute or decay within the 20-40 min need to complete scan; must not have short half life
SPECT vs PET
emission, spatial resolution, sensistivity
emission: SPECT is 1 photon (gamma), PET is 2 photons (positron)
spatial resolution: SEPCT 10 mm, PET 5 mm
sensitivity: SPECT is depth dependent, not PET
strategies to minimize spect artifact
decrease pt motion, injection site outside field of view, image chest/abd withhands above head
cardiac spect positioning
heart is located off center with camers in L mode and images obtained 180 degrees (RAO to LPO)
use of cardiac gating to deal with wall motion
8 framers are aquired per cardiac cycle and processed into 64 x 64 matrix
tuning fork artifact
occurs when center of 180 degree cardiac spect rotation is not a point source (misregistration error)
will appear as a tuning fork rather than a single point
PET annihilation event energy
annihilation splits the 1.02 MeV
two 511 keV photons are emitted 180 degrees from each other
SPECT vs PET
- cameras
- energy level
- counts
cameras: PET has a complete set of detectors
energy level: SPECT medium, PET high; thicker more robust crystals used in PET
counts: PET > SPECT
non collinearity
degree in which photons in SPECt do not travel in a completely 180 from each other
SPECT: photon evaluation
evaluated for spatial information, energy, arrival time
determining if photons in SPECT are a pair
- within energy window (full width half maximum) centered at 511 photopeak
- stay within timing window (5-10 nanoseconds)
photons are then registered as a pair/event will be calculated along the line of response (LOR)
rejection rate of photons in SPECT
> 90% of generated photons
true coincidence
photons from same annihilation reaction detected in same coincident window
scatter coincidence
one photon has compton interaction and is deflected, but still within window (wrong location)
compton will also lower energy; reduce this error by narrowing photo peak
random coincidence
two photons from different annihlation reactions land within the same coincidence window; false calculation
increased number of random events occur with higher doses
decrease with lower dose/counts and narrower window setting
noise equivalent counts
NEC; signal to noise ratio
ratio of true coincidences/total coincedences
higher NEC will be one that can achieve superior contrast to noise and SNR
interative reconstruction
ordered subset expectation maximization (OSEM); conversion of LOR photons into sinogram and reformatted
crystals: SPECT vs PEt
SPECT: NaI activated with thallium scintillator crystlas; great with low medium energy photons
PET: crystals handl 511 photons; thick, high density, high atomic numbers
BGO, GSO, LSO
common pet scintillators
bismuth germanate, gadolinium oxyorthosilicae, lutetium oxyorthosiciliate
most commonly used PET scintillator
lutetium oxyorthosilicate
limitations to PET
crystal thickness, positron range, angulation, scatter
crystal thickness in PET
thick crystals needed to evaluate high energy 511 keV photons but it decreases spatial resolution
positron range in PET
detectors track position of annihilation, not location of emission
maximum range of 1 mm for FDG PET
Rb 82 can travel a few mm
positron range => “ultimate limit of spatial resolution”
angulation/non-collinearity in PET
small deviation from 180 degrees during collision
septa in 2D or 3D
2D systems use lead or tungsten to block scatter radiation
3D systems DO not use septa
-typically used in CNS and peds imaging due to small object size
disadvantages of 3D PET
dead time if the detectors have high count rates, increased amount of scatter and more random events
ways to decrease PET dead time
crystals with faster scintillation times (such as LSO) or add more photomultiplier tubes
time of flight
estimates point of annihilation; used on large objects with low contrast
improves spatial resolution and image contrast
PET attenuation correction
relies on data from transmission scan using positron emitter compared to blank scan QA
removal of compton and photoelectric interactions
depth independent; sum of distance required for photons to move in 180 directions is constant
however tissue dependent
attenuation correction with PET CT
xrays help attenuate correct for PET
advantage of PET over SPECT
corrected vs uncorrected PET
skin is hot on uncorrected as is lungs
how do metal objects affect PET
generated overcorrected false increased activity at site
metal is outside convential range that is processed
happens with calcified lymph nodes and IV contrast bolus injection sites
SUV calculation
(tissue radioactivity x patient weight) / injected dose activity
SUV values with patient body habitus
SUV in large patients are overestimated; can be corrected with lean body mass calculations
factors affecting SUV calculations
body habitus, timing, glucose levels, size of object, dose extravasations, reconstruction, attenuation correction
timing of study on SUV values
lag time to allow for FDG uptake will increase SUV values
how do glucose levels affect SUV
high glucose => lower SUV
size and SUV values
size threshold for PET is usually 1 cm
dose extravasation and SUV values
lower FDG given IV results in less SUV in the soft tissues, less SUV
reconstruction type and SUV
iterations increase SUV
attenuation correction and SUV
makes comparing SUVs difficult
truncation artifact
lesion appears hot on margin due to soft tissue outside the field of view
large body habitus => abnormal SUV on peripheral lesions
FDG PET preparations
fasting at least 4 hours prior, minimizing cardiac activity (for thoracic cancers)
- adequate hydration
- decrease exercise (muscle uptake)
- brown fat (keep room warm, valium/propranolol)
how often to blank scan PET CT
daily
-positron source (Ge 68 or Cs 137) in scanner. No phantom
- uniform cylinder with 511 keV positron emiting 68Ge/Ga placed at center of FOV
why is blank scan performed
uniform data to help zero for attenuation correction calculation
analog for daily flodo scan for planar gamma camera to evaluate detector response
why is Ge68 used over F18 as a cylinder for blank scan?
longer half life; 270 days
