Download Gamma-camera SPECT PET Gamma radiation

Gamma-camera
SPECT
PET
17.04.2012.
Gamma radiation
• Electromagnetic radiation (f>1019Hz, E>100keV (1.6*10-14J),
λ<10-12m)
• gamma-decay (the atomic nucleus can get into a more stable
state by photon emission) - α- and β-decay frequently
accompanied by gamma radiation
• Ionizing radiation (the energy is used to remove (detach) the
electrons from the atoms)
• Discovered by Paul Ulrich Villard in 1900, french chemistphysicis
• 1903 - Ernest Rutherford used its name at first
• radiation emitted by the atomic nucleus ↔ x-ray: radiation
emitted by the atom due to electron transitions
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γ-ray and matter interaction
• photoelectric effect
– photon (E<50keV) + electron → ejected electron
• Compton effect (scattering)
– photon (E1:100 keV-10 MeV) + electron → photon
(E2<E1, altered direction) + ejected electron
• Pair production
– photon (E>1.02MeV=2*0.51MeV) + the electric field of
the atomic nucleus → electron-positron pair (higher than
1.02MeV → Ekinetic)
How to produce the γ-ray
• together with α- and β-decay
• relaxation of nuclear isomer (a metastable
state of an atomic nucleus caused by the
excitation of one or more of its protons or
neutrons)
– Co → Ni* → Ni
– 99mTc → 99Tc
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Isotopes
• Isotope = same place in the periodic table
• atoms having the same atomic number
(p+) but different mass numbers (p+ + n0)
• Similar chemical and biological properties
• Different physical properties (radioactive
isotopes)
•
pl.
Protium: 1H (H) - 99,985 %
2H
Deuterium:
(D)
Tritium: 3H (T)
(1p + Øn + 1e)
(1p + 1n + 1e)
(1p + 2n + 1e)
George Charles de Hevesy
(1885–1966)
• Hungarian radiochemist
• Discovered that the radioactive isotopes can be traced
(1913)
• Nobel prize in chemistry (1943) for his work with radioactive
tracers
Radioactive tracing technique
– Investigating metabolic processes with radioactive
isotopes by replacing part of stable isotopes with small
quantities of the radioactive isotopes (same biology
different physics).
– radiopharmacon: radioactive isotope + biologically
relevant (active) molecule
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Gamma-camera
Gamma photon detecting device to image gamma radiation emitting
radioisotopes (2D image).
Scintigraphy (radioactive tracer + scintillation counter)
Detecting gamma-ray emitting isotopes.
• 99mTc (metastable Technetium isotope)
only emmits gamma ray
half-life ~ 6 hours
PMT
Scintillation crystal (NaI; CsF; BaF2; Bi4Ge3O12)
scintillation = a spark, a flash
Scintillation crystal
Collimator (lateral resolution!)
can turn radiation into parallel (non-divergent)
beams
Gamma-camera
• disadvantages:
– Low sensitivity (high loss on the collimator).
– Bad spatial resolution (1.8 cm / 5 cm
(distance from the detector)).
• advantage: relatively cheap
Spatial resolution: the minimum distance between distinguishable
objects in an image
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SPECT – Single Photon Emisson
Computed Tomography
Imaging technique that works with gamma camera which is capable to record 2D
images from different angles (360°). A computer will reconstruct a 3D image.
Result: 3D image.
Based on the usage of gamma decaying isotopes (99mTc; 123I; 131I; 133Xe).
Acqusition time ~ 15-20 minutes (can be better with a multidetector system)
disadvantage: bad spatial resolution (~1cm)
Advantages:
• relatively cheap
• functional information can be gained
Usage: - monitoring the the myocardial function (CO; MI)
- monitoring cerebral functions
Weak points of SPECT
• Low sensitivity (high loss on the
collimator).
• Bad spatial resolution (~1cm)
• Long acquisition time (positioning of the
detector).
• Exposure!
– Gamma ray - burnes, tumor, mutations
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PET
Positron Emission Tomography –
computertomography
History
• 1973 – St. Louis (Missouri, USA) Washington
University
Edward J. Hoffman & Michael Phelps
The first PET scanner
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Definition
• 3D imaging technique that can monitor
functional changes within the living
systems by using positron emitting
isotopes.
• Qualitative analysis of the metabolic
processes!
positron
• Elementary particle: not to be made up of smaller particles. Subatomic particle.
• Antimatter of electron (same weight (9.1 x 10-31kg), same energy (0.51MeV =
8.2 x 10-14 J), same charge with different (opposite) polarity (+1.6 x 10-19C).
• The first proven antimatter.
• Chung-Yao Chao (student at California Institute of Technology – 1930): the first
scientist who captured the positively charged electron (positron)
• The egsistence of positrons was suggested at first by Paul Dirac in 1928.
• Carl D. Anderson – 1932: the first who called it as positron.
• production:
- pair production
- positron emission (+ β decay)
• Application: PET.
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Origin of positrons
• Pair production (↔annihilation)
• β-decay
Generating positrons I.
Pair production (↔annihilation)
e-
Atomic nucleus
photon
e+
• Ephoton > 1.022 MeV
Annihilation: opposite of pair production - 1e- + 1e+ = 2 gamma photon (180°!)
Latin nihil = nothing
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Generating positrons II.
β+ radiaton (nuclear decay)
1
0
n →11p + + 00e − + ν ( antineutrino )
1
1
p + →01n + 00e + + ν ( neutrino )
137
55
22
11
−
Cs→137
56 Ba + e + ν
22
Na →10
Ne + e + + ν
β+ radiation emitting atoms
11
6
13
7
15
8
18
9
C
N
O
F
124
53
82
37
I
isotopes
Half life (t1/2)
carrier
What can be
followed
usage
C-11
20 min.
Amino acids
Amino acid
metabolism
Metabolic
changes, protein
synthesis, tumors
N-13
2 min.
ammonia
Blood perfusion
Viability of the
myocardium
O-15
10 min.
O-15 labeled
gases
Oxygen transport
Oxygen
metabolism
F-18
110 min.
Deoxiglucose
(FDG)
Glucose
metabolism
Metabolic
changes, tumors
I-124
4.15 day
Tumor detection
Testing the
Thyroid
Rb-82
25 day
Perfusion of the
myocardium
Coronary vessel
diseases
Rb
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Conditions
• PET machine (~3 million Euros)
• Cyclotron (particle accelerator) to produce
the isotopes (~3 million Euros)
• Onkology center (85-90% oncological
patient)?!?!?
Basic units of the PET laboratory
PET camera
cyclotron
isotopes
synthesis of the
radiopharmacon
injection
Data acquisition
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Basics of PET
Detector ring
t<20ns
photomultiplier
Scintillation crystal
CsFBaF2Bi4Ge3O12-
180o
0.511MeV
annihilation
Basics of PET
Detector ring
t<20ns
180o
0.511MeV
annihilation
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What is it good for?
• diagnosis
• Checking the efficiency of the therapy
Indications
• Oncology
– Lung cancer, breast cancer, ….
• cardiology
– Myocardial viability, myocardial perfusion
• Neuropsychiatry
– epilepsy, dementia, Parkinson-disease,
Huntington-disease
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Advantages and disadvantages of PET
•
•
•
•
sensitive, specific
selective (radiopharmacon)
tomography technique
information about functional changes within the living
systems (metabolic pathways)
• early diagnosis (before the structural changes)
• good spatial resolution (4-6mm) (better than SPECT)
• no structural information → fusion techniques (PET-CT)
• high cost (expensive to establish it (3+3 M Euros) and
expensive to operate (800-1200€/investigation)
• low accessibility
• time consuming (preparation+measurement)
The end!
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