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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

Nuclear medicinePet/Spect

Chapters 18 to 22

activity
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)

activity3
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

nuclear medicine
Nuclear medicine
  • Therapeutic and diagnostic use of radioactive substances
  • First artificial radioactive material produced by the Curies 1934  “Radioactivity,” “Radioactive
definitions nuclide
Definitions: Nuclide
  • Nuclide: Specie of atoms characterized by its number of neutron and protons
  • Isotopes
  • Isotones
  • Isobars
  • (…)
definitions nuclide6
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)
definitions nuclide7
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
radioactive decay
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)
radioactive decay9
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
radioactive decay10
Radioactive Decay
  • Father and daughter.
  • Is Y is not stable will undergo more splitting (more daughters)

Daughter

Father

alpha decay
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
alpha decay14
Alpha decay
  • Occurs with heavy nuclides
  • Followed by g and characteristic X ray emission
  • Emitted with energies 2-10MeV
  • NOT USED IN MEDICAL IMAGING
positron emission b
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
positron emission b16
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
positron emission b17
Positron emission b+

Fluorin oxygen

positron emission b18
Positron emission b+

Fluorin oxygen

positron emission b19
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
annihilation radiation
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

b decay
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
electron captive e
Electron captive e
  • Alternative to positron decay for nuclide with few neutrons
  • Nucleus capture an electron from an orbital (K or L)
electron captive e23
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
electron captive e24
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
isomeric transition
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
slide26

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)

radioactive nuclide
Radioactive nuclide
  • Produced into a cyclotron
  • Tagged to a neutral body (glucose/water/ammonia)
  • Administered through injection
  • Scan time 30-40 min
positron emission b29
Positron emission b+

Fluorin oxygen

slide30

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

principles
Principles
  • Uses annihilation coincidence detection (ACD)
  • Simultaneous acquisition of 45 slices over a 16 cm distance
  • Based on Fluorine 18 fluorodexyglucose (FDG)
slide32
PET
  • Ring of detectors surrounds the patient
  • Obtains two projection at opposite directions
  • Patient is injected with a 18 fluorine fluorodeoxyglucose (FDG)
pet principle
Pet principle
  • Ring of detectors
annihilation radiation34
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

true coincidence
True coincidence

Detector 1

Detector 2

random coincidence
Random coincidence
  • Emission from different nuclear transformation interact with same detector

Detector 1

Detector 2

scatter coincidence
Scatter coincidence
  • One or both photons are scattered and don’t have a simple line trajectory

Detector 1

False

coincidence

Detector 2

total signal is the sum of the coincidences
Total signal is the sum of the coincidences

Ctotal = Ctrue+Cscattered+Crandom

slide39

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):
pet detectors
Pet detectors

NAI (TI) Sodium iodide doped with thallium

BGO bismuth germanate

LSO lutetium oxyorthosilicate

slide41

PET

MRI

PET resolution

  • Modern PET ~ 2-3 mm resolution (1.3 mm)
spect
SPECT
  • Single photon emission computed tomography
  •  rays and x-ray emitting nuclides in patient
spect cnt
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
spect cnt45
SPECT cnt
  • 60-130 projections
  • Technetium is the isothope
  • Decays with  ray emission
  • Filtered back projection to reconstruct an image of a solid
typical studies
Typical studies
  • Bone scan
  • Myocardial perfusion
  • Brain
  • Tumor
slide47

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
slide48

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
slide49

d

L

Collimators

  • Purpose: Image formation (acts as “optic”)
  • Parallel collimatorSimplest, most common 1:1 magnification
  • Resolution
  • Geometric efficiency
  • Tradeoff: Resolution  Efficiency

Aopen

Aunit

slide50

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

slide52

SPECT applications

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