phys586-lec02b
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Transcript phys586-lec02b
Common Lab Sources
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Radioactive Sources
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Radionuclides in the
AZ Particle Lab
Gamma
Co @ 1uC
241Am, 133Ba, 137Cs,
57Co @ 10 uC
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60Co, 88Y, 22Na, 64Mg, 203Hg,
X-ray
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Fe
5.90 keV (24.4%) and 6.49 keV (2.86%)
Beta
90Sr/90Y
@ 50 mCi, 5 mCi, 2mCi, 0.5mCi
Alpha
241
Am @ 5 mCi
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Radionuclides in Medicine
Nuclear medicine
Diagnostic
Permits functional imaging (biochemistry and metabolism
versus anatomical structure)
>80% of all procedures use 99mTc
Radiotherapy
Therapeutic
Primarily for cancer treatment
External beam – teletherapy using 60Co units
Internal – brachytherapy using small, encapsulated sources
Notes
90% of all radionuclide use in medicine is diagnostic
Use of term “radioisotope” is common
Will there be a shortage of radionuclides in the future?
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Radionuclides in Medicine
George de Hevesy
Nobel in 1943 for use of isotopes as tracers
for chemical processes
A failed experiment to separate Radium-D
(210-lead) from lead (206-lead)
The landlady’s leftovers
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Radionuclides for Diagnosis
What are the characteristics of an ideal
radionuclide for diagnosis?
Half-life?
Effective half-life 1/teff = 1/tradioactivity +
1/tbiological
Type and energy of radiation?
Production and expense?
Purity?
Target area to non-target ratio?
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Radionuclides for Diagnosis
The ideal gamma energy (for gamma
camera use) is between 100 and 250
keV
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Nuclear Medicine
99mTc is used in ~ 80% of diagnostic
procedures
Tc pertechnetate (TcO4-) is mixed with an
appropriate pharmaceutical (biological construct)
for use for
99m
Cardiac imaging and function
Skeletal and bone marrow imaging
Pulmonary perfusion
Liver and spleen function
Cerebral perfusion
Mammography
Venous thrombosis
Tumor location
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Technetium – 99m
Half-life t1/2=6.02 hrs
Decay scheme
Which is (are) the medically useful
gamma(s)?
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Technetium – 99m
A closer look
There is no g1
emission, it IC’s
IC competes
with g2
IC competes
with g3
X-ray and Auger
electron
emission can
also occur
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Radionuclides for Therapy
Brachytherapy
Brachys = short
Brachytherapy uses encapsulated radioactive
sources to deliver a high dose to tissues near the
source
Provides localized delivery of dose
But the tumor must be well localized and small
Proposed by Pierre Curie and, independently,
Alexander Graham Bell shortly after the discovery
of radioactivity
Inverse square law determines most of the
dosimetric effect
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Brachytherapy
Used to treat a variety of cancers
Prostate
Gynecological
Eye
Skin
Only ~10% of radiotherapy patients are
treated via brachytherapy
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Brachytherapy
Sources
Most of the sources used emit gammas
Lower gamma energies are preferred for
radioprotection
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Brachytherapy
Sources
But a few emit betas
90Sr/90Y for eye lesions
90Sr/90Y ,
90Y, 32P
angioplasty
for preventing restenosis after
In general, alphas and betas are absorbed
by encapsulation to avoid tissue necrosis
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around the source
Nanotargeted Radionuclides
Use monoclonal antibodies to carry a
radionuclide payload
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Brachytherapy
Sources
226Ra
->
222Rn
+ a -> … ->
206Pb
Although rarely used now, it’s a good reaction
to know given its historical significance
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Brachytherapy
Sources
226Ra
->
222Rn
+ a -> … ->
206Pb
Which equilibrium is achieved (t1/2(226Ra) =
1600 years)?
222Rn is a radioactive gas
About 50 gamma energies are possible ranging
from 0.184 to 2.45 MeV, though on average
there are 2.2 gammas emitted for each decay
The average energy (filtered by 0.5 mm of Pt)
is 0.83 MeV
The exposure rate constant (assuming 0.5 mm
of Pt) is G = 8.25 R-cm2/hr-mCi
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Brachytherapy
Sources
More modern replacements for
137Cs
226Ra
are
Familiar gamma ray spectrum with E=0.662
MeV
t1/2=30 yrs and G=3.26 R-cm2/hr-mCi
and
192Ir
More complicated gamma ray spectrum with
<E> = 0.38 MeV
t1/2=73.8 days and G=4.69 R-cm2/hr-mCi
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Brachytherapy
Methods of delivery
LDR (0.4-2 Gy/hr) versus HDR (> 12 Gy/hr)
Temporary versus permanent
Intracavity versus interstitial
Also surface, intraluminal, intravascular,
intraoperative
Seeds, needles, tubes, pellets, wire
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Brachytherapy
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Radionuclide Production
How are radionuclides made?
Primary sources
Nuclear reactors
235U
fission produced
Neutron activated
Both produce neutron rich radionuclides
Cyclotrons
Uses charged particle beams (p, d, t, a)
Produces proton rich radionuclides
Secondary source
Radionuclide generators
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Nuclear Fission
Fission of 236U* yields two fission nuclei
plus several fast neutrons
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Nuclear Reactors
Nuclear reactor schematic
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Fission Production
Nuclei such as 99Mo, 131I, and 133 Xe are
produced in the fission products using an
enriched 235U target (HEU – 90%)
Complex chemical processing (digestion or
dissolution) and purification separates the
99Mo from chemically similar elements and
radiocontaminents
The result is a high specific activity (Bq/kg),
carrier free nuclide
This means there is no stable isotope of the element of
interest
Some negatives are the potential proliferation of HEU
targets and radioactive waste
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Neutron Activation
An alternative use of reactors is to produce
radionuclides via neutron activation
A
X
98
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X n, g
A1
X
X
Mon, g Mo, Pn, g P
124
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99
42
31
15
Xen, g Xe I
125
54
32
15
125
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Two drawbacks of this method are
Small activation fraction
Chemically similar carrier that cannot be separated
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Cyclotrons
We will cover accelerator physics later
in the course
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Cyclotron Production
Cyclotron energies can be a
few MeV to a few GeV
Laboratory/university or hospital
based
Beam currents of 40-60 uA
Produces Ci-level radioisotopes
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7
N ( p, a ) C
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8
O( p, a )137N
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7
N ( p, n)158O
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Siemens Eclipse
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6
18
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O ( p, n) F
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Cyclotron Production
The reactions shown on the previous page
Are proton rich -> decay by e+ emission or EC
18F is the most common radionuclide in PET
oncology
Are important elements of all biological
processes hence make excellent tracers
18F is used to label FDG (18F-fluorodeoxyglucose)
Useful because malignant tumors show a high
uptake of FDG because of their high glucose
consumption compared with normal cells
Have short lifetimes (O(minutes))
Except t1/2 for
18F
= 110 minutes
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Cyclotron Production
18F in PET/CT
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Cyclotron Production
Alzheimer’s diagnosis
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Radionuclide Generators
Generates a radionuclide by exploiting
transient equilibrium
Most important application are moly generators
99Mo
(67 hours) decaying to
99mTc
(6 hours)
Sodium pertechnetate (NaTcO4) results which can
then be combined with an appropriate
pharmaceutical
Developed at BNL, a particle and nuclear physics
lab
Other generators also exist (69Ge to 68Ga, 82Sr to
82Rb, …)
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Radionuclide Generators
Procedure
A glass column is filled with
aluminum oxide that serves as an
adsorbent
Ammonia molybdenate attaches to
the surface of the resin
A sterile saline (the eluant) solution
is drawn through the column
The chloride ions exchange with the
TcO4- but not the MoO4The elute is thus Na+TcO4- (sodium
pertechnetate)
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Radionuclide Generators
Technetium cow
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Radionuclide Generators
Generator schematic
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Radionuclide Generators
Generally shipped weekly and milked
daily
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Gamma Camera
These images are made using gamma
cameras
We will cover the details of these (and
similar detectors) in upcoming lectures
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Gamma Camera
A schematic of a standard gamma
camera
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