Chapter 13 Nuclear Medicine I Flashcards
what force holds protons and neutrons together?
strong force
what is mass number A
sum of protons and neutrons
isobar
nuclides having same mass number A
isotope
nuclei having same atomic number Z
stable nuclides have how many neutrons and protons?
if low A, equal number of neutrons and protons
if high A, more neutrons than protons
what is radioactive decay?
transformation of an unstable nuclide
-parent decays to daughters
what is transmutation
when decay involves change in number of protons, so element changes
most stable state
ground state
unstable states
higher energy states
excited states (isomeric states)
what is isomeric transition
excited state transforms to lower energy level, emits gamma rays
-no capture or emission of any particles
what are gamma rays
electromagnetic radiation originating in a nuclear process
99mTc gamma rays
140 kV
-ideal for nuc med because energy is high enough to escape from a patient but low enough to be easily detected
what is metastable?
isomeric state with very long lifetime
denoted by lower case m following mass number (99mTc)
beta minus decay
-neutron inside nucleus is converted to a proton
-occurs when there is excess of neutrons
-daughter product has mass number A, atomic number Z + 1, neutron number N-1
beta plus decay
-proton inside nucleus is converted into neutron with the emission of a positron
-occurs in nuclei with too many protons
-daughter has mass number A, atomic number, Z-1, neutron number N+1
what happens when positron loses all of its kinetic energy?
annihilates with an electron
-mass of positron and electron (511 keV each) is converted into 2 511 keV photons that are emmitted 180 degrees apart
most popular positron emitter in Nuc Med
18F
popular beta emitted in Nuc Med
32 P
beta particle range in soft tissue
1 mm
-range increases with higher beta particle enrgy and in low density tissue like lung
describe electron capture
proton is converted into a neutron by capturing an atomic electron, usually in the K-shell
-occurs in nuclei with too many protons
-when the electron is captured fromthe K-shell, the vacancy is filled by outer shell electrons, emitting characteristic x-rays
what 2 decay modes compete with each other?
electron capture and beta plus decay as both have too many protons
electron capture nuclides used in Nuc Med
67Ga, 111In, 123I
describe alpha decay
radionuclide emits alpha particle consisting of 2 neutrons and 2 protons
risk of alpha particles
-high risk if ingested, inhaled, or injected
-radioactive radon increases risk of lung cancer when deposited in respiratory tract
common alpha emitter in nature
226Ra- decays to 222Rn (another alpha emitter)
define activity
number of transformations per unit time
what is Bq and curie
1 Bq = 1 transformation/s
1 mCi = 37 MBq
what percent of activity remains after 10 half lives?
0.1%
relationship between decay constant and half life
T1/2 = ln2/lambda
what is effective half life
includes both radioactive decay and biologic clearance
-always shorter than physical and biological half lives
-1/Teff = 1/Tbio + 1/Tphys
describe neutron activation
-add neutrons to a stable nuclide to get radionuclide
-neutron activation products cannot be chemically separated and are therefore not carrier free
how do reactor-produced neutron activation radionuclides usually decay?
beta minus
describe cyclotron produced radionuclides
charged particles are added to stable nuclides
-include 123I, 18F, 67Ga, 111In
how do cyclotron produced radionuclides usually decay?
beta plus or electron capture
radionuclides as fission products
-heavy nuclides break up
-include 131I, 133Xe, 90Sr, 99Mo
how do fission product radionuclides usually decay?
beta minus
what radionuclide products are carrier free?
-cyclotron and fission products
-not neutron activation products
what happens in a generator?
useful radionuclide (daughter) is continuously produced by radioactive decay of a parent readionuclide
what is specific activity
activity per mass
longer half life is lower or higher specific activity?
lower
describe technetium generator
alumina column is coated with 99Mo
99Mo decays to 99mTc
-saline is passed through the column to elute (wash off) 99mTc as sodium pertechnetate
-Mo is not soluble and stays in column
common isotopes from generators in nuc med
113mIn, 68Ga, 82Rb, 99mTc
how is 82Rb obtained from generator?
-obtained from 82Sr
where is 82Rb used?
-cardiac imaging
-myocardium rapidly takes up 82Rb and is used to asses cardiac perfusion
-half life of 82Rb is 1.25 minutes, which permits sequwntial studies every 5 minutes
generator equilibrium
-as parent decays, daughter is produced
-at equilibrium, daughter and parent activities are the same
explain generator equilibrium for 99Mo-99mTc generator
-equlibrium is reached after 4 daughter half-lives
-1/2 life of 99mTc is 6 h, so this is 24 h
-because half-life of 99Mo is 66 hours, generators stay useful for 5 working days (2 parent half lives)
-after five days, yield of 99mTC is < 25 % of that on day 1 of generator
transient equlibrium
parent radionuclide is short lived (but still longer half life than daughter)
-parent and daughter activities are only approximately equal and directly proportional to each other
Ad = Ap Tp/(Tp - Td).
secular equilibrium
parent is long lived (extremely long 1/2 life)
-parent and daguther activities are exactly equal
when do secular and transient equlibrium occur?
after 4 daughter half lives
example of active transport
thyroid uptake scanning with iodine
what is macro aggregated albumin labelled with 99mTc used to assess
lung perfusion (capillary blockade)
where is 99mTc used for assessment?
lung perfusion (capillary blockage)
compartmental localization (blood pool scanning)
compartmental leakage (GI bleeding)
kidney filtration (diffusion)
liver and spleen function (phagocytosis)
bone scans (physiochemical adsorption)
spleen scanning (cell sequestration)
antibody-antigen reactions
where is 18F used for assessment?
metabolism quantification
neuroreceptor imaging
Mo breakthrough into saline elute
dose calibrators can determined the content of 99Mo each time the generator is eluted
regulatory limit for 99Mo breakthrough
5.5 kBq of 99Mo for 37 MBq of 99mTc
what can breakthrough into saline elute?
99Mo
alumina (test with color indicator paper)
radionuclide purity
identified by photopeak energies using gamma ray spectroscopy
radiochemical purity
chemical purity of isotope, checked with thin later chromatography
what is chemical purity
amount of unwanted chemical contaminants in the agent
frequency of severe reactions from radiopharmaceuticals
2/100,000
what testing is done before agent is administered to patient
sterility
pyrogen testing
describe how scintillator works
gamma rays are absorbed in scintillator
absorbed energy produces light photons
light produced is detected by photomultiplier tube
scintillators display number of counts at a given energy vs given energy
gamma rays may undergo compton- only part of photon energy is deposited in scintillator (scatter produces less light than the PE effect)
photopeak for 123I
160 keV
photopeak for 99mTc
140 keV
detection efficiency
percentage of incident gamma rays totally absorbed in scintillator by PE effect
-increasing photon energy decreases efficiency (PE effect is proportional to 1/E^3)
full mean half value
width of photopeak
statistical fluctuations
detection efficiency for 99mTc
90%
energy resolution
photopeak broadening, as % of photopeak energy
-10% is common
-10% energy resolution for 99mTc is FWHM width of 14 keV
pulsed height analyzer
used to identify which of the detected gamma ray interactions will be used to create the nuc med image
-sets Slow and Shigh- signals outside these boundaries are excluded
what has signal lower than Slow?
compton scatter in a patient
what has signal higher than Shigh?
coincidence events where 2 gamma rays interact at same time
how big is pulse height analyzer signal width (Shigh-Slow)
twice the energy resolution
describe well counter
samples inserted into a well within the crystal, which maximizes sensitivity by detecting most of the emitted gamma rays
-identifiees radionuclides based on photopeak energy
-can quantify amount of activity by using a calibration factor
describe uptake probe
quantifies uptake of radioiodine in patients, by comparing patient activity with a known amount in a neck phantom
-usually done 24 h after iodine is given
-uptake is measured at standard distance from a scintillator
-neck counts are corrected for background
-also used to monitor iodine in workers who handle the chemical
describe dose calibrator
-ion chamber used to measure activity of radioisotope dose
-activity is measured in a syringe but prior to injection into a patient
-NRC requires administered dose is within 20% of prescribed dose
how is dose calibrator response checked?
daily
137Cs (T1/2 = 30 years)
-day to day measurements should vary by less than 5 %
how are dose calibrators QAd?
response is checked daily: 137Cs (T1/2 = 30 years)
-day to day measurements should vary by less than 5 %
-accuracy is checked at installation and annualyl using calibrated sources
-linearity is checked quaterly by measuring decay of 99mTc over 72 hours
-can also check linearity by using a source placed into cylinders of lead which attenuate the source by known amounts
what does gamma camera show
projection images of distribution of radioactivity in patients, built up one gamma ray at a time
components of gamma camera and how they work together
collimator
NaI scintillator
PMT arrays
processing of PMT signals
gamma rays that are travelling vertically from patient pass through the collimator. A fraction of the absorbed energy in the NaI scintillator is converted to light. Light is detected by 2D array of PMT tools. Pulsed height analyzer identifies photopeak events and pattern of light gives positional info. Corrections are applied for non-random distortions. Image is displayed as number of detected photons in each pixel.
how thick is the NaI crystal?
10 mm
what does PMT do to light signal?
converts to electrical signal that is used to estimate total energy of each event
what is done for radionuclides with multiple gamma energies?
accept multiple photopeak events
how is location of gamma ray interaction determined?
pulse arithmatic circuit based on relative strength of signals from each PMT
describe gamma camera collimators
-lead with holes
-lead strips between holes are septa
parrallel hole collimators
project same object size onto camera
FOV doesn’t change with distance
converging collimator
produce magnified image
diverging collimator
project image size that is smaller than object size
pinhole collimator
cone shaped with single hole at apex
images are magnified and inverted
low energy collimators
used with 99mTc and 123I
-have thin seta
medium energy collimators
used with 67Ga and 111In
-have thick septa
what is collimator sensitivity
count rate per MBq being imaged
-low and independent of distance from collimator
-0.01% of incident photons
what happens to parrallel hole collimator resolution with distance
it falls off
what is system resolution dominated by?
collimator resolution
-blur introduced by scintillator is of littler importance
high sensitivity collimators
large holes
thinner
transmit more photons
high resolution collimators
small holes
thicker
better localize activity
how many counts are typically acquired for a scintillation camera image?
500,000
nuc med matrix size
64^2 or 128^
lower than CT (512^2)
what corrections are done to the gamma camera image?
-correct for system spatial non-linearity
-corrections for non-uniformities in detector response (these are computed from flood images obtained with a higher number of counts to minimize random noise)
-corrections to account for differences in amount of light generated by varying locations
contrast in gamma camera image
difference in counts in any abnormality compared to counts in normal anatomy (background)
occurs when radiopharmaceuticals localize in uptake (hot spot) or don’t uptake (cold spot)
what degrades contrast
septal penetration
scatter
counts/area for lung images
10 photons/mm^2
chest radiographs have 100,000 x-ray photons/mm^2
-nuc med images are mottled due to low counts
how to increase counts
increase administered activity
increase imaging time
use more sensitive collimator
gamma camera intrinsic resolution (without collimator) vs with collimator
3 mm
FWHM 8 mm with collimator
-this is 0.06 lp/mm (10x worse than CT and 100 X worse than mammo)
gamma camera artifacts
patient motion
damaged collimators
septal penetration
cracked crystals- defects in image reflect cracks
PMT failure - get cold defect
edge packing
off-peak images
metal object artifact
extravasation
describe edge packing
-increased brightness at edge of crystal due to internal reflection of light and absence of PMTs
-crystals are made larger than image FOV to minimize edge packing
describe septal penetration
- when a low energy collimator is used to image higher energy photons and the photons can go through the septa- get hexagonal pattern around hot spots
describe artifact from off-peak images
-on “low” side of photopeak contain excessive compton scatter
-when PHA window is not centered on photopeak, the resultant image will show location of individual PMTs
describe metal object artifact
metal objects produce photopenic areas that can mimic pathological cold lesions
describe extravasation artifact
injecting the radiopharmaceutical at a spot gives hot spot at injection site and excessive scatter from adjacent tissues
what is typical administered activity to a patient
700 MBq or 20 mCi
how often is FWHM measured?
annually
when is geometric efficiency checked?
at installation
typical counts per pixel for gamma camera
100 counts/pixel
time integrated activity coefficient
time integrated activity/injected activity
Eighty-five percent of an injection of 99mTc sulfur colloid is cleared from the blood by the liver. What is the time-integrated activity coefficient in the liver? (Assume a biological half-life of 3 hours.)
Lamda_eff = 0.693/3 h + 0.693/6 h = 0.3465/h (2 h) 0.85*2 h/0.693=2.5 h
what is plotted in gamma ray spectrum from a scintillator?
of counts vs detected light
number of 140 keV photons totally absorbed by NaI scintillator?
2/3
how often is gamma camera resolution checked?
annually
how often is constancy checked?
daily
how often is accuracy checked?
annually
how often is geometric efficiency checked?
at installation
number of counts in typical NM image
500000
for 64^2 pixels, this is about 100 counts per pixel
activity administered to patient should be within what % of prescribed dose?
10-20%
