Transcript Slide 1

Spatial & Energy resolutions (exp.& MC)
for the Axial HPD-PET concept
with YAP and LYSO crystals
from the thesis works of
Ignazio Vilardi
Anna Palasciano
Francesca Ciocia
The 3D PET cameras
New 3D axial HPD-PET concept
Standard radial concept
PMTs
p
axis
axis
Many rings of
crystal–photodetector blocks
radially displaced
Lc =1.5-3cm
Arrays of long (Lc~10-15 cm) crystal bars
read out at both sides by segmented HPDs
Concept made possible by CERN development of rectangular segmented 5‘’ HPDs
with integrated self-triggering electronics
A HPD-PET CAMERA MODULE
• Array of 208 scintill.s (LSO, LYSO, YAP, LaBr3, n=1.8-1.9)
• 16x13 crystals (3.2[Rx]x3.2[Ry]x150[Lc] mm3)
tx=51mm, ty=42mm  tX,Y>3·la (LSO)  e2g(⊥)~ 90%
• with spacing: 4x4 mm2
Rx
"Proximity focused“ HPDs
•
Ry
Rx = 64 mm, Ry = 52 mm
Gain HPD : ~ 3.103 (Uop= 12 kV )
or 5.103 (Uop = 20 kV)
Lc
sapphire window (n~1.8):
(better light transmission)
LYSO crystals
(crystal-window refractive indices matching)
•
optical transport image 1 : 1
•
segmented (4x4 mm2) silicon detector pads
sapphire window
Bi-alkali photocathode
ring electrodes
silicon sensor
(each crystal readout by its pad, no cross-talk)
•
each pad with integrated lecture electronics (2 VATA-Gp5)
readout chips
“Reference Radial PET”: The High Resolution Research Tomograph (HRRT)
(CTI, MPI, Karolinska …)
K. Wienhard et al., IEEE Trans. Nucl. Sci. 49 (2002) 104–10
•ANGER LOGIC: a block seen by 4 PMTs
CM of signals in 4 PMTs 
 interac. point (x,y) of g
PMT1
PMT2
AFOV
25 cm
•DOI (z) from PHOSWICH tecnique
Pulse Shape Discrimin. (PSD)
Lc
LSO
7.5
31 cm
LSO
g
7.5
z
g
x
• 8 panels with 9  13 blocks
• 2  64 crystals per block
• 120000 crystals 2.12.1x7.5mm3
• 936 (20x20 mm2) PMTs
Dt = 7 ns
88 matrix (2x2 cm2)  4PMTs
y
Dz (FWHM) ~ 5 mm
DV= (FWHM) ~ 20 mm3
Lc =15mm ~ 1 la  e2g(⊥)~ 53%
DE/E (511 keV) ~ 17%
Heavy electronics (PSD)
Inorganic Scintillation crystals
Criteria to be taken into account: light yield, absorption length, photofraction, self
absorption, decay time, availability, machinability, price.
YAP:Ce
LSO:Ce
LuAP:Ce
LaBr3:Ce
5.55
7.4
8.34
5.3
7.13
32
66
65
46.9
75
Scintillation light output (photons / MeV)
18000
23000
~10000
~61000
~9000
wavelength lmax of max. emission (nm)
370
420
370
356
480
Refractive index n at lmax
1.94
1.82
1.95
~1.88
~2.15
Bulk light abs. length lbulk (cm) at lmax
~20
~40
27
40
18
30±5
300
22.4
11.5
10.5
~20
~11.6
Photo fraction at 511 keV (%)
4,5
32.5
30.5
15
41.5
Energy resolution (FWHM) at 663 keV
4.5
8
Density ρ (g/cm3)
Effective atomic charge Z
Principal decay time (ns)
Mean γ atten. length la at 511keV (mm)
2.9
LSO (LYSO) is the most interesting crystal scintillator :
fast (40 ns), short att. length (~12mm) at 511keV, high photofraction (32%),
not hygroscopic, but high intrinsic energy resolution (~ 5% FWHM)
BGO
1) ADVANTAGES OF THE AXIAL HPD-PET CONCEPT
High Granularity  exact reconstruction of
the
•
•
g
interaction point (no parallax error)
HPD1
x,y from fired scintillator
σ(x,y) = 3.2 mm/√12= 0.92 mm
Dx,Dy (FWHM) = 2.2 mm
z (DOI) from the ratio of the photoelectrons
detected at the two crystal ends
z
σ(z) linked to the scint. choice
Reduced # of photodet., scint., electr.
(12 module PET: only 24 HPDs)
y
(Ry)
x
(Rx)
g
No limit to module radial (x,y) dimension

higher efficiency
Double scatt. events in one module
(Compton-photoel.) reconstruction

higher efficiency
HPD2
208 4 x 4 mm2 Si ‘pads’
centred on crystal matrix
g
2) ADVANTAGES OF THE AXIAL HPD-PET CONCEPT
possibility to reconstruct the int. point of part of g’s that suffers a double
(Compton + photoelectric) event in the same module
COMPTON + PHOTOELECTRIC events-
~ 25% Compton events (50 keV [energy cut] < E < 170 keV) followed by
a photoelectric one in the same module can unambiguously be reconstructed
detection efficiency increases but spatial (DOI) resolution worsens
Lc, leff, No: KEY PARAMETERS OF THE HPD-PET CONCEPT
• Lc:
crystal length
leff: attenuation length of scint.
•
photons
HPD1
1/leff = 1/(lbulk* cos q) + c’/(cabs)
• No:
light yield ≡ p.e.’s (511keV g) in a Lc~0 crystal
(nph/keV, sci.ph.transport, q.e. & wind. of photodet.)
σz, σE/E, σt :
N1 
z 
N0
,
exp 
l 
2
 eff 

1
N
z   leff ln 1  LC  ,
2
N2

E
E

Z
(only statistical)
N2 
  ( LC  z ) 
N0
,
exp 
 l

2
eff


T 
cabs
1/ 2
ENF leff 

 exp z  exp LC  z  ,
z 
leff
leff 
2 N o 
ENF
 Rint ,
N pe
g
N pe ( z   N1  N 2 .
g
q
c
,
N pe
a) crystal axial length (Lc) worsens all resolutions
limit of Lc: 10 ~ 15 cm
HPD2
b) light yield (No) improves all resolutions
c) contrasting effects of leff on σz & σE/E, σt
 optimize leff value by wrapping or coating the crystal lateral surface
PROOF of the HPD-PET CONCEPT with YAP and LYSO crystals and PMTs
 BaF2 (used with a 22Na source)
 Pb collimator Pb + source
 YAP (Preciosa Co)
LYSO (Photonic Materials)
(3.2 x 3.2 x 50-150 mm3)
• PMT H3164-10 (F=8mm, nw=1.47,bialkali)
• B8850Quantacon(F=5cm,nw=1.47,bialkali)
linear translator M-511(Phys.Instrum.)
polished 3x3x100 mm3 YAP-LYSO comparison
z=1 cm
photoelectric peak
(511 keV)
z = 5 cm
z =9 cm
Compton
22Na
source
YAP+H3164-10
QL+QR (5cm)= 1692 ch
~ 10%
ΔE/E(FWHM)
~ 14%
LYSO+H3164-10
QL+QR (5cm)= 2295 ch
LYSO produces more light (pe’s) than YAP
LYSO has a higher photofraction, lower energy resolution than YAP
leff in polished(n2=1) 3x3x100 mm3 YAP-LYSO
No/2
lLYSO= 42.6±0.9 cm
QL
exp(-z/leff), ≈ n1/n2, n1/nW
1/z2 ≈ n1/nW
lYAP= 20.8±0.4 cm
LYSO more transparent (higher leff than YAP)
too high l-eff values (poor z) both for LYSO and YAP
No/2
Crystal wrappings or metal-coatings
change light attenuation length of a YAP (3.2 x 3.2 x 100 mm3)
polished
QL
best solution
at z=0: N0(teflon) > N0(polished) (diffusing wrapping)
leff (polished) / leff(teflon) = 1.9
possibility to tune leff value with metal coatings
metal coating (n2) reduces No
E/E, z, tdc (z=5cm) in coated 10cm YAP & LYSO (511 keV)
YAP
a) statistical
b) phenomen.
NO = 510±18 pe
NO = 753±34 pe
E 

 
 E  Stat
z 
ENF
 R int
N 0e  z / l
l ENF
2N0
 t  a' 
2  ez /l
c'
e  mz / l
vs leff
LYSO
NO = 724±34 pe
very low leff in a Lc=5 cm YAP with
raw (smeared) lateral surfaces
no exp behaviour of Q1 (fermi function)
no coincident Q1-Q2 signals in a Lc = 10 cm YAP
very low leff , but dependent with z 
good z but z-dependent, bad E/E
z , E/E
in coated-smeared YAP & LYSO vs z, leff, Lc, Eg
crystal length worsens z, does not influence much E/E
worse z and E/E values at lower Eg
very low leff (raw lat.surf) values  Lc limited, z-dep. of z
Geant4 simulations for YAP long crystals + PMTs
long and thin cylindrical crystal (n1, Nph/keV)
lat.surface: polished, smeared (), wrapped (nwrap), coated (nwrap,ik,t)
polished bases coupled to PMTs (nwin, t,q.e.)
Polar diagram of reflected-refracted scintillation photons
I
incident unpol. opt. photons
A
absorption
R
T
reflection
transmission
D
Lambertian diffusion
SR diffuse reflect. (smear)
ST diffuse transmiss.
nwrap
(nwin=1.47)
nwin
(nwrap=1.0)
*refr.ind.match
*
nwrap
does not change leff
decreases No
worsens E/E, z, t
nwin
decreases leff
increases No
improves E/E, z, t
absorption
diffusion
smearing
absorp. & diffus.
•similar effects
•decrease leff
(diff. increases No)
•improve z
•worsen E/E, t
smearing
•to be avoided
(N1 no more exp.)
Geant4 reproduction of exp. results
YAP + H3164-10 PMTs
Geant4 predictions for engraved crystals
mechanical or laser ablation
engravings
(effects similar to absorp.)
•decrease leff
•improve spatial resol.
•worsen energy-time resol.
&
• high reproducibility
to the N0 and leff
values of the many
HPD-PET crystals
HRRT
vs.
HPD-PET
AFOV
25 cm
Full ring scanner
A possible final configuration for a HPD-PET
31 cm
8 panels 9x13 blocks
12 modules
F = 31 cm
F = 34 cm
AFOV = 25 cm
Lc = 15 cm
120000 crystals 2.1x2.1x7.5mm3
2496 crystals 3.2x3.2x150mm3
936 PMTs 2x2cm2
24 5” rect. HPDs
det.Vol. 3962cm3
det.Vol. 3834cm3
det.depth 15mm
det.depth 41mm
DW/4p ~ 0.344
DW/4p ~ 0.165
eTOT (LSO)(%) (exp) ~ 6.9
eTOT(LSO) (%) ~ 8.5 (ph) + 7.5 (Co-rec)
eTOT(LaBr3)(%) ~ 1.9 (ph) + 4.9 (Co-rec)
z
y
x
HPD-PET(Lc=10cm) vs HRRT
crystal
H
P
D
P
E
T
H
R
R
T
e
x
P
M
C
e
x
P
nw
No
(pe)
leff
(cm)
FWHM
(mm)
(%)
Dz
DE/E
FWHM
YAP polish
1.47
950
21
19.3
10.8
YAP teflon
1.47
1120
10.5
12.7
11.2
LYSO polish
1.47
1200
42
34.5
14.6
LYSO teflon
1.47
750
20
22.1
16.4
YAP polish
1.8
1500
16
8.9
8.9
YAP teflon
1.8
1640
9
7.7
9.4
5
17
LSO