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Raymond F. Muzic, Jr., Ph.D.
Raymond Muzic Protons
Neu
tron
s
0 40 80
80
40
0
120
Stable 12C Unstable 11C
Unstable 14C
3
Reactor- produced
Cyclotron- produced
Raymond Muzic
Insufficient number of n/too many p+
Competing decay processes Electron Capture: p+ + e– n + �̅�+ E Positron Decay: p+ n + β+ + 𝜈 + E
β+ decay Predominant over EC at lower Z 1.02 MeV threshold
Raymond Muzic
82Rb 76 sec 15O 122 sec 13N 10 min 11C 20 min 68Ga 68 min 18F 110 min 64Cu 12.7 hr 124I 5.2 d
Raymond Muzic
18O (p,n) 18F 14N (p, α) 11C 10B (d,n) 11C 16O (p,α) 13N 12C (d,n) 13N 14N (d,n) 15O 15N (p,n) 15O
Raymond Muzic
Raymond Muzic
H gas + e – in magn field plasma: p +,H –, H H – pulled from ion source chamber 2 or 4 dees are electrodes, radiofrequency Dees In high vacuum, 10 – 6 mTorr (10 – 9 atm) or better High magnetic field, 1 T or greater
Ions accelerated across gaps in dees, coast through dees on curved path
Saha Basics of PET Imaging, Chap 7
Raymond Muzic
Accelerate H+ vs H– or 2H+ vs 2H– H+ cyclotron housing becomes activated H –
beam can be split dual targets
Raymond Muzic
q charge M magnetic field, Gauss r radius of cyclotron m mass of particle
𝐾𝐾 = 𝑞𝑞𝑞 2
2𝑚
10 to 20 MeV typical for medical cyclotrons
Raymond Muzic
Raymond Muzic
Raymond Muzic
Power ~150 A at 208 V, 3-phase
Cooling ~5x104 Btu/h
Air exhaust > 500 cfm
Shielding Cyclotron ~25 cm Pb, ~75 cm concrete both with B Hot cell ~75 mm Pb
Floor loading Cyclotron ~4x104 kg; Hot cell 104 kg
Raymond Muzic
Shielding: HVLs, narrow-beam, ignore build-up Lead 4.1 mm Concrete 3.4 cm
Distance Can be more economical than shielding
Time Inject patient then stay away
Raymond Muzic
Raymond Muzic
Raymond Muzic
Nucleus ejects positron Positron slows, meets with
electron: annihilates Two photons rays created 180o (±0.25o FWHM) 0.511 MeV simultaneous β−
0.511 MeV
0.511 MeV
β+
~.25-3mm
Radionuclide
Raymond Muzic
Photons are emitted from radionuclide inside subject
0.511 MeV
0.511 MeV
Raymond Muzic
Photons detected in temporal coincidence same nucleus
Recorded coincidence data reflect sum of activity along line
Data are tomographically reconstructed to make image
Raymond Muzic
d1=35 cm
d2=45 cm
t1 = T+d1/c = T+1.2 ns
t2 = T+d2/c = T+1.5 ns
t2 - t1 = 0.3 ns
Confidence-weight backprojection Current scanners have timing resolution of 500 to 600 ps FWHM resolution 7.5 to 9 cm
∆𝑥 =𝑐 ∆𝑡
2
Raymond Muzic
Crystal materials for PET Old: NaI, CsF, BaF2, GSO, BGO Modern: LSO, LYSO, (LaBr3?)
Raymond Muzic
Decode analogous to Anger camera n PMTs read light from m crystals n < m
Raymond Muzic
Singles Trues Randoms Scatter
Raymond Muzic
The majority of photons that the scanner detects are not in coincidence with another photon
Raymond Muzic
The location of the annihilation is on the line between the detectors that sensed the photons in temporal coincidence
Raymond Muzic
By random chance two annihilations occur nearly simultaneously
Scanner detects one photon from each mispositioned coincidence
Keep activity “low” to minimize probability of random coincidence
Estimate and statistically correct using delayed coincidence window
Raymond Muzic
Photon undergoing Compton interaction is deflected or scattered
Scanner mispositions event
Energy discrimination can reduce
Software corrections for scatter
Raymond Muzic
Ahmad Hatami Zhenghong Lee Valdir Colussi
Raymond Muzic