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Nuclear medicine Pet/Spect. Chapters 18 to 22. 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). Activity. Expressed in Curie

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Nuclear medicine pet spect l.jpg

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 -> 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 -> radiation is emitted

  • Fundamental decay equation (Number of radioactive atoms at time t -> 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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Alpha decay

  • Spontaneous nuclear emission of a particles

  • a particles identical to helium nucleus -2 protons 2 neutrons

  • a particles -> 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 -> 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

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 ->eject 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 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 -> 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

False

coincidence

Detector 2


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Total signal is the sum of the coincidences

Ctotal = Ctrue+Cscattered+Crandom


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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

MRI

PET resolution

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



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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

  • Enclosure

  • Shielding

  • Collimator

  • NI(Tl) Crystal

  • 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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d

L

Collimators

  • Purpose: Image formation (acts as “optic”)

  • Parallel collimatorSimplest, most common 1:1 magnification

  • Resolution

  • Geometric efficiency

  • Tradeoff: Resolution  Efficiency

Aopen

Aunit


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Converging

d

L

L

d

d

Diverging

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


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