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Huda > Chapter 13 Nuclear Medicine I > Flashcards

Chapter 13 Nuclear Medicine I Flashcards

1
Q

what force holds protons and neutrons together?

A

strong force

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2
Q

what is mass number A

A

sum of protons and neutrons

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3
Q

isobar

A

nuclides having same mass number A

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4
Q

isotope

A

nuclei having same atomic number Z

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5
Q

stable nuclides have how many neutrons and protons?

A

if low A, equal number of neutrons and protons
if high A, more neutrons than protons

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6
Q

what is radioactive decay?

A

transformation of an unstable nuclide
-parent decays to daughters

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7
Q

what is transmutation

A

when decay involves change in number of protons, so element changes

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8
Q

most stable state

A

ground state

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9
Q

unstable states

A

higher energy states
excited states (isomeric states)

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10
Q

what is isomeric transition

A

excited state transforms to lower energy level, emits gamma rays
-no capture or emission of any particles

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11
Q

what are gamma rays

A

electromagnetic radiation originating in a nuclear process

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12
Q

99mTc gamma rays

A

140 kV
-ideal for nuc med because energy is high enough to escape from a patient but low enough to be easily detected

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13
Q

what is metastable?

A

isomeric state with very long lifetime
denoted by lower case m following mass number (99mTc)

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14
Q

beta minus decay

A

-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

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15
Q

beta plus decay

A

-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

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16
Q

what happens when positron loses all of its kinetic energy?

A

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

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17
Q

most popular positron emitter in Nuc Med

A

18F

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18
Q

popular beta emitted in Nuc Med

A

32 P

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19
Q

beta particle range in soft tissue

A

1 mm
-range increases with higher beta particle enrgy and in low density tissue like lung

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20
Q

describe electron capture

A

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

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21
Q

what 2 decay modes compete with each other?

A

electron capture and beta plus decay as both have too many protons

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22
Q

electron capture nuclides used in Nuc Med

A

67Ga, 111In, 123I

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23
Q

describe alpha decay

A

radionuclide emits alpha particle consisting of 2 neutrons and 2 protons

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24
Q

risk of alpha particles

A

-high risk if ingested, inhaled, or injected
-radioactive radon increases risk of lung cancer when deposited in respiratory tract

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25
Q

common alpha emitter in nature

A

226Ra- decays to 222Rn (another alpha emitter)

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26
Q

define activity

A

number of transformations per unit time

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27
Q

what is Bq and curie

A

1 Bq = 1 transformation/s
1 mCi = 37 MBq

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28
Q

what percent of activity remains after 10 half lives?

A

0.1%

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29
Q

relationship between decay constant and half life

A

T1/2 = ln2/lambda

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30
Q

what is effective half life

A

includes both radioactive decay and biologic clearance
-always shorter than physical and biological half lives

-1/Teff = 1/Tbio + 1/Tphys

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31
Q

describe neutron activation

A

-add neutrons to a stable nuclide to get radionuclide
-neutron activation products cannot be chemically separated and are therefore not carrier free

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32
Q

how do reactor-produced neutron activation radionuclides usually decay?

A

beta minus

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33
Q

describe cyclotron produced radionuclides

A

charged particles are added to stable nuclides
-include 123I, 18F, 67Ga, 111In

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34
Q

how do cyclotron produced radionuclides usually decay?

A

beta plus or electron capture

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35
Q

radionuclides as fission products

A

-heavy nuclides break up
-include 131I, 133Xe, 90Sr, 99Mo

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36
Q

how do fission product radionuclides usually decay?

A

beta minus

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37
Q

what radionuclide products are carrier free?

A

-cyclotron and fission products
-not neutron activation products

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38
Q

what happens in a generator?

A

useful radionuclide (daughter) is continuously produced by radioactive decay of a parent readionuclide

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39
Q

what is specific activity

A

activity per mass

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40
Q

longer half life is lower or higher specific activity?

A

lower

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41
Q

describe technetium generator

A

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

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42
Q

common isotopes from generators in nuc med

A

113mIn, 68Ga, 82Rb, 99mTc

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43
Q

how is 82Rb obtained from generator?

A

-obtained from 82Sr

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44
Q

where is 82Rb used?

A

-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

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45
Q

generator equilibrium

A

-as parent decays, daughter is produced
-at equilibrium, daughter and parent activities are the same

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46
Q

explain generator equilibrium for 99Mo-99mTc generator

A

-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

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47
Q

transient equlibrium

A

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).

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48
Q

secular equilibrium

A

parent is long lived (extremely long 1/2 life)
-parent and daguther activities are exactly equal

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49
Q

when do secular and transient equlibrium occur?

A

after 4 daughter half lives

50
Q

example of active transport

A

thyroid uptake scanning with iodine

51
Q

what is macro aggregated albumin labelled with 99mTc used to assess

A

lung perfusion (capillary blockade)

52
Q

where is 99mTc used for assessment?

A

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

53
Q

where is 18F used for assessment?

A

metabolism quantification
neuroreceptor imaging

54
Q

Mo breakthrough into saline elute

A

dose calibrators can determined the content of 99Mo each time the generator is eluted

55
Q

regulatory limit for 99Mo breakthrough

A

5.5 kBq of 99Mo for 37 MBq of 99mTc

56
Q

what can breakthrough into saline elute?

A

99Mo
alumina (test with color indicator paper)

57
Q

radionuclide purity

A

identified by photopeak energies using gamma ray spectroscopy

58
Q

radiochemical purity

A

chemical purity of isotope, checked with thin later chromatography

59
Q

what is chemical purity

A

amount of unwanted chemical contaminants in the agent

60
Q

frequency of severe reactions from radiopharmaceuticals

A

2/100,000

61
Q

what testing is done before agent is administered to patient

A

sterility
pyrogen testing

62
Q

describe how scintillator works

A

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)

63
Q

photopeak for 123I

A

160 keV

64
Q

photopeak for 99mTc

A

140 keV

65
Q

detection efficiency

A

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)

66
Q

full mean half value

A

width of photopeak
statistical fluctuations

67
Q

detection efficiency for 99mTc

A

90%

68
Q

energy resolution

A

photopeak broadening, as % of photopeak energy
-10% is common
-10% energy resolution for 99mTc is FWHM width of 14 keV

69
Q

pulsed height analyzer

A

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

70
Q

what has signal lower than Slow?

A

compton scatter in a patient

71
Q

what has signal higher than Shigh?

A

coincidence events where 2 gamma rays interact at same time

72
Q

how big is pulse height analyzer signal width (Shigh-Slow)

A

twice the energy resolution

73
Q

describe well counter

A

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

74
Q

describe uptake probe

A

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

75
Q

describe dose calibrator

A

-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

76
Q

how is dose calibrator response checked?

A

daily
137Cs (T1/2 = 30 years)
-day to day measurements should vary by less than 5 %

77
Q

how are dose calibrators QAd?

A

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

78
Q

what does gamma camera show

A

projection images of distribution of radioactivity in patients, built up one gamma ray at a time

79
Q

components of gamma camera and how they work together

A

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.

80
Q

how thick is the NaI crystal?

A

10 mm

81
Q

what does PMT do to light signal?

A

converts to electrical signal that is used to estimate total energy of each event

82
Q

what is done for radionuclides with multiple gamma energies?

A

accept multiple photopeak events

83
Q

how is location of gamma ray interaction determined?

A

pulse arithmatic circuit based on relative strength of signals from each PMT

84
Q

describe gamma camera collimators

A

-lead with holes
-lead strips between holes are septa

85
Q

parrallel hole collimators

A

project same object size onto camera
FOV doesn’t change with distance

86
Q

converging collimator

A

produce magnified image

87
Q

diverging collimator

A

project image size that is smaller than object size

88
Q

pinhole collimator

A

cone shaped with single hole at apex
images are magnified and inverted

89
Q

low energy collimators

A

used with 99mTc and 123I
-have thin seta

90
Q

medium energy collimators

A

used with 67Ga and 111In
-have thick septa

91
Q

what is collimator sensitivity

A

count rate per MBq being imaged
-low and independent of distance from collimator
-0.01% of incident photons

92
Q

what happens to parrallel hole collimator resolution with distance

A

it falls off

93
Q

what is system resolution dominated by?

A

collimator resolution
-blur introduced by scintillator is of littler importance

94
Q

high sensitivity collimators

A

large holes
thinner
transmit more photons

95
Q

high resolution collimators

A

small holes
thicker
better localize activity

96
Q

how many counts are typically acquired for a scintillation camera image?

A

500,000

97
Q

nuc med matrix size

A

64^2 or 128^
lower than CT (512^2)

98
Q

what corrections are done to the gamma camera image?

A

-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

99
Q

contrast in gamma camera image

A

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)

100
Q

what degrades contrast

A

septal penetration
scatter

101
Q

counts/area for lung images

A

10 photons/mm^2

chest radiographs have 100,000 x-ray photons/mm^2

-nuc med images are mottled due to low counts

102
Q

how to increase counts

A

increase administered activity
increase imaging time
use more sensitive collimator

103
Q

gamma camera intrinsic resolution (without collimator) vs with collimator

A

3 mm
FWHM 8 mm with collimator
-this is 0.06 lp/mm (10x worse than CT and 100 X worse than mammo)

104
Q

gamma camera artifacts

A

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

105
Q

describe edge packing

A

-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

106
Q

describe septal penetration

A
  • 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
107
Q

describe artifact from off-peak images

A

-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

108
Q

describe metal object artifact

A

metal objects produce photopenic areas that can mimic pathological cold lesions

109
Q

describe extravasation artifact

A

injecting the radiopharmaceutical at a spot gives hot spot at injection site and excessive scatter from adjacent tissues

110
Q

what is typical administered activity to a patient

A

700 MBq or 20 mCi

111
Q

how often is FWHM measured?

A

annually

112
Q

when is geometric efficiency checked?

A

at installation

113
Q

typical counts per pixel for gamma camera

A

100 counts/pixel

114
Q

time integrated activity coefficient

A

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

115
Q

what is plotted in gamma ray spectrum from a scintillator?

A

of counts vs detected light

116
Q

number of 140 keV photons totally absorbed by NaI scintillator?

A

2/3

117
Q

how often is gamma camera resolution checked?

A

annually

118
Q

how often is constancy checked?

A

daily

119
Q

how often is accuracy checked?

A

annually

120
Q

how often is geometric efficiency checked?

A

at installation

121
Q

number of counts in typical NM image

A

500000
for 64^2 pixels, this is about 100 counts per pixel

122
Q

activity administered to patient should be within what % of prescribed dose?

A

10-20%

Huda (20 decks)

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