Nuclear medicine Pet/Spect

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Nuclear medicinePet/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)

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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

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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)

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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 -gt 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 (l)

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Radioactive Decay

• Radioactive decay is a random process.
• When an atom undergoes radioactive decay -gt
  radiation is emitted
• Fundamental decay equation (Number of radioactive
  atoms at time t -gt Nt

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Radioactive Decay

• Father and daughter.
• Is Y is not stable will undergo more splitting
  (more daughters)

Daughter
Father
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Radioactive Decay Processes
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Radioactive Decay Processes
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Alpha decay

• Spontaneous nuclear emission of a particles
• a particles identical to helium nucleus -2
  protons 2 neutrons
• a particles -gt 4 times as heavy as proton
  carries twice the charge of proton

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Alpha decay

• Occurs with heavy nuclides
• Followed by g and characteristic X ray emission
• Emitted with energies 2-10MeV
• NOT USED IN MEDICAL IMAGING

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Positron emission b

• 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

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Positron emission b

• 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

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Positron emission b
Fluorin oxygen
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Positron emission b
Fluorin oxygen
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Positron emission b

• Positron travels through materials loosing some
  kinetic energy
• When they come to rest react violently with their
  antiparticle -gt 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-gtannihilation
• Entire mass of e and ?? is converted into
• two 511keV photons

511keV energy equivalent of rest mass of electron
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b- decay

• Happens to radionuclide that has excess number of
  neutron compared to proton
• A negatron is identical to an electron
• Antineutrino neutral atomic subparticle

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Electron captive e

• Alternative to positron decay for nuclide with
  few neutrons
• Nucleus capture an electron from an orbital (K or
  L)

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Electron captive e

• Nucleus capture an electron from an orbital (K or
  L)
• Converts protons into a neutron -gteject neutrino
• Atomic number is decreased by one new element

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Electron captive e

• As the electron is captured a vacancy is formed
• Vacancy filled by higher level electron with Xray
  emission
• Used in studies of myocardial perfusion

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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 g ray is emitted

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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
b
Tomography?
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Positron emission b
Fluorin oxygen
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PET Positron emission tomography

• 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
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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 -gt photons reach
  detectors, we collect the photons that happen
  almost at the same time
• coincidence? I dont 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 dont have
  a simple line trajectory

Detector 1
False coincidence
Detector 2
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Total signal is the sum of the coincidences

• Ctotal CtrueCscatteredCrando
  m

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PET noise sources

• 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 resolution

• Modern PET 2-3 mm resolution (1.3 mm)

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PET 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) camera

• 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

1. Enclosure
2. Shielding
3. Collimator
4. NI(Tl) Crystal
5. PMT

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Anger 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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Collimators

• Purpose Image formation (acts as optic)
• Parallel collimatorSimplest, most common 11
  magnification
• Resolution
• Geometric efficiency
• Tradeoff Resolution ? Efficiency

Aopen
Aunit
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Collimator 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
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SPECT applications

• Brain
• Perfusion (stroke, epilepsy, schizophrenia,
  dementia Alzheimer)
• Tumors
• Heart
• Coronary artery disease
• Myocardial infarcts
• Respiratory
• Liver
• Kidney