Dec 7th PET/SPECT
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Transcript of Dec 7th PET/SPECT
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Nuclear medicine
Pet/Spect
Chapters 18 to 22
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Activity
• Number of radioactive atoms undergoing nuclear transformation per unit time.
Change in radioactive atoms N in time dt
Number of radioactive atoms decreases with time (- minus sign)
€
A=−dNdt
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Activity
• Expressed in Curie – 3.7x1010 disintegrations per second dps
Becquerel discovers natural radioactive materials in 1896 the SI unit for radioactivity is the Becquerel. 1 becquerel = 1dps
€
A=−dNdt
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Nuclear medicine
• Therapeutic and diagnostic use of radioactive substances
• First artificial radioactive material produced by the Curies 1934 “Radioactivity,” “Radioactive
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Definitions: Nuclide
• Nuclide: Specie of atoms characterized by its number of neutron and protons
• Isotopes• Isotones• Isobars• (…)
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Definitions: Nuclide
• Isotopes are families of nucleide with same proton number but different neutron number.
• Nuclides of same atomic number Z but different A same element
• AZX
• A mass number, total # of protons and neutrons• Z atomic number (z# protons)
€
612C 6
13C
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Definitions: Nuclide
• Radionuclide: Nuclide with measurable decay rate
• A Radionuclide can be produced in a nuclear reactor by adding neutrons to nucleides 59Co + neurtron -> 60Co
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Radioactive Decay
• Disintegration of unstable atomic nucleus
• Number of atoms decaying per unit time is related to the number of unstable atoms N through the decay constant ()
€
−dNdt
=N
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Radioactive Decay
• Radioactive decay is a random process.
• When an atom undergoes radioactive decay -> radiation is emitted
• Fundamental decay equation (Number of radioactive atoms at time t -> Nt
€
Nt =N0e−t
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Radioactive Decay
• Father and daughter.
• Is Y is not stable will undergo more splitting (more daughters)
€
ZAX Z−2
A−3YFather Daughter
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Radioactive Decay Processes
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Radioactive Decay Processes
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Alpha decay
• Spontaneous nuclear emission of particles particles identical to helium nucleus -2 protons 2
neutrons
particles -> 4 times as heavy as proton carries twice the charge of proton
€
ZAX→ Z−2
A−4Y + 24He+2 +energy
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Alpha decay
• Occurs with heavy nuclides
• Followed by and characteristic X ray emission • Emitted with energies 2-10MeV
• NOT USED IN MEDICAL IMAGING
€
ZAX→ Z−2
A−4Y + 24He+2 +energy
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Positron emission +
• Decay caused by nuclear instability caused by too few neutrons
• Low N/Z ratio neutrons/protons
• A proton is converted into a neutron – with ejection of a positron and a neutrino€
ZA X→Z −1
AY + β + + ν + energy
positron neutrino
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Positron emission +
• Decrease of protons by 1 atom is transformed into a new element with atomic # Z-1
• The N/Z ratio is increased so “daughter” is more stable than parent€
ZA X→Z −1
AY + β + + ν + energy
positron neutrino
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Positron emission +
€
918F→ 8
18O+ + +ν +energy positron neutrinoFluorin oxygen
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Positron emission +
€
918F→ 8
18O+ + +ν +energy positron neutrinoFluorin oxygen
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Positron emission +
• Positron travels through materials loosing some kinetic energy
• When they come to rest react violently with their antiparticle -> Electron
• The entire rest mass of both is converted into energy and emitted in opposite direction
– Annihilation radiation used in PET
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Annihilation radiation
• Positron interacts with electron->annihilation• Entire mass of e and is converted into two 511keV photons
511keVenergy equivalent ofrest mass of electron
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- decay
• Happens to radionuclide that has excess number of neutron compared to proton
• A negatron is identical to an electron• Antineutrino neutral atomic subparticle
€
ZAX→ Z+1
AY + − +ν~
+energy negatron antineutrino
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Electron captive
• Alternative to positron decay for nuclide with few neutrons
• Nucleus capture an electron from an orbital (K or L)
€
ZAX +e−→ Z−1
AY + ν +energy neutrino
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Electron captive
• Nucleus capture an electron from an orbital (K or L)
• Converts protons into a neutron ->eject neutrino
• Atomic number is decreased by one –new element
€
ZAX +e−→ Z−1
AY + ν +energy neutrino
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Electron captive
• As the electron is captured a vacancy is formed
• Vacancy filled by higher level electron with Xray emission
• Used in studies of myocardial perfusion
€
81201Tl→ 89
201Hg+ ν +energy neutrino
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Isomeric transition
• During a radioactive decay a daughter is formed but she is unstable
• As the daughter rearrange herself to seek stability a ray is emitted
€
ZAmX→ Z
AX +energy ray
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Principle of radionuclide imaging
Principle of radionuclide imaging
Introduce radioactive substance into body
Allow for distribution and uptake/metabolism of compound Functional Imaging!
Detect regional variations of radioactivity as indication of presence or absence of specific physiologic function
Detection by “gamma camera” or detector array
(Image reconstruction)
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Radioactive nuclide
• Produced into a cyclotron
• Tagged to a neutral body (glucose/water/ammonia)
• Administered through injection• Scan time 30-40 min
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Positron Emission Tomography
Tomography?
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Positron emission +
€
918F→ 8
18O+ + +ν +energy positron neutrinoFluorin oxygen
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• Cancer detection
• Examine changes due to cancer therapy– Biochemical changes
• Heart scarring & heart muscle malfunction
• Brain scan for memory loss– Brain tumors, seizures Lymphoma
melanoma
PET Positron emission tomography
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Principles
• Uses annihilation coincidence detection (ACD)
• Simultaneous acquisition of 45 slices over a 16 cm distance
• Based on Fluorine 18 fluorodexyglucose (FDG)
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PET
• Ring of detectors surrounds the patient
• Obtains two projection at opposite directions
• Patient is injected with a 18 fluorine fluorodeoxyglucose (FDG)
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Pet principle
• Ring of detectors
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Annihilation radiation
• Positron travel short distances in solids and liquids before annihilation
• Annihilation COINCIDENCE -> photons reach detectors, we collect the photons that happen almost at the same time – coincidence? I don’t think so!
Detector 1
Detector 2
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True coincidence
Detector 1
Detector 2
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Random coincidence
• Emission from different nuclear transformation interact with same detector
Detector 1
Detector 2
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Scatter coincidence
• One or both photons are scattered and don’t have a simple line trajectory
Detector 1
Detector 2
Falsecoincidence
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Total signal is the sum of the coincidences
Ctotal = Ctrue+Cscattered+Crandom
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PET noise sourcesPET noise sources
O T S Aij ij ij ijC C C C= + +
• Noise sources:– Accidental (random)
coincidences– Scattered coincidences
• Signal-to-noise ratio given by ratio of true coincidences to noise events
• Overall count rate for detector pair (i,j):
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Pet detectors
NAI (TI) Sodium iodide doped with thallium
BGO bismuth germanate
LSO lutetium oxyorthosilicate
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PET resolutionPET resolution
• Modern PET ~ 2-3 mm resolution (1.3 mm)
MRI
PET
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PET evolutionPET evolution
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SPECT
• Single photon emission computed tomography
rays and x-ray emitting nuclides in patient
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SPECT cnt
• One or more camera heads rotating about the patient
• In cardiac -180o rotations• In brain - 360o rotations• It is cheaper than MRI and PET
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SPECT cnt
• 60-130 projections
• Technetium is the isothope
• Decays with ray emission
• Filtered back projection to reconstruct an image of a solid
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Typical studies
• Bone scan
• Myocardial perfusion
• Brain
• Tumor
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Scintillation (Anger) cameraScintillation (Anger) camera
1. Enclosure
2. Shielding
3. Collimator
4. NI(Tl) Crystal
5. PMT
• Imaging of radionuclide distribution in 2D• Replaced “Rectilinear Scanner”, faster, increased efficiency,
dynamic imaging (uptake/washout)• Application in SPECT and PET• One large crystal (38-50 cm-dia.) coupled to array of PMT
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Anger logicAnger logic
• Position encoding example: PMTs 6,11,12 each register 1/3 of total Photocurrent, i.e.:
I6 = I11 = I12 = 1/3 Ip
• Total induced photo current (Ip) is obtained through summing all current outputs
• Intrinsic resolution ~ 4 mm
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Ld
CollimatorsCollimators
• Purpose: Image formation (acts as “optic”)
• Parallel collimatorSimplest, most common 1:1 magnification
• Resolution
• Geometric efficiency
• Tradeoff: Resolution Efficiency
( )2a L z bR
L
+ +=
24open open
unit
A AG
d Aπ=
Aopen
Aunit
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Collimator typesCollimator types
Tradeoff between resolution and field-of view (FOV) for different types:
Converging: resolution, FOV
Diverging: resolution, FOV
Pinhole (~ mm):High resolution of small organs at close
distances
Diverging
L
d
d
Converging
L
d
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SPECT applicationsSPECT applications
• Brain: – Perfusion (stroke, epilepsy,
schizophrenia, dementia [Alzheimer])
– Tumors• Heart:
– Coronary artery disease– Myocardial infarcts
• Respiratory• Liver• Kidney