Spatial

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

2
The 3D PET cameras
Standard radial concept
axis
axis
Many rings of crystalphotodetector blocks
radially displaced Lc 1.5-3cm
Concept made possible by CERN development of
rectangular segmented 5 HPDs with integrated
self-triggering electronics
3
A HPD-PET CAMERA MODULE

• Array of 208 scintill.s (LSO, LYSO, YAP, LaBr3,
  n1.8-1.9)
• 16x13 crystals (3.2Rxx3.2Ryx150Lc mm3)
• tx51mm, ty42mm ? tX,Ygt3la (LSO) ? e2g(?)
  90
• with spacing 4x4 mm2 Rx 64 mm, Ry 52
  mm

Rx
Gain HPD 3.103 (Uop 12 kV ) or 5.103 (Uop
20 kV)
Ry
Lc

• "Proximity focused HPDs
  
• sapphire window (n1.8)
• (better light transmission)
• (crystal-window refractive indices
  matching)
• optical transport image 1 1
• segmented (4x4 mm2) silicon detector pads
• (each crystal readout by its pad, no
  cross-talk)
• each pad with integrated lecture electronics (2
  VATA-Gp5)

4
Reference Radial PET The High Resolution
Research Tomograph (HRRT) (CTI, MPI,
Karolinska )
K. Wienhard et al., IEEE Trans. Nucl. Sci. 49
(2002) 10410

• ANGER LOGIC a block seen by 4 PMTs
• CM of signals in 4 PMTs ?
• ? interac. point (x,y) of g
• DOI (z) from PHOSWICH tecnique
• Pulse Shape Discrimin. (PSD)

Lc
g
z
g
x
y

• 8 panels with 9 ? 13 blocks
• 2 ? 64 crystals per block
• 120000 crystals 2.1?2.1x7.5mm3
• 936 (20x20 mm2) PMTs

5
Inorganic Scintillation crystals

• Criteria to be taken into account light yield,
  absorption length, photofraction, self
  absorption, decay time, availability,
  machinability, price.

BGO
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)
6
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
• s(x,y) 3.2 mm/v12 0.92 mm
• Dx,Dy (FWHM) 2.2 mm
• z (DOI) from the ratio of the photoelectrons
  
• detected at the two crystal ends
• s(z) linked to the scint. choice
• Reduced of photodet., scint., electr.
• (12 module PET only 24 HPDs)
• No limit to module radial (x,y) dimension
• higher efficiency
• Double scatt. events in one module
• (Compton-photoel.) reconstruction
• higher efficiency

z
y
g
g
(Ry)
x
(Rx)
HPD2
208 4 x 4 mm2 Si pads centred on crystal
matrix
7
2) ADVANTAGES OF THE AXIAL HPD-PET CONCEPT
possibility to reconstruct the int. point of
part of gs that suffers a double (Compton
photoelectric) event in the same module
COMPTON PHOTOELECTRIC events-
25 Compton events (50 keV energy cut lt E lt
170 keV) followed by a photoelectric one in
the same module can unambiguously be
reconstructed
detection efficiency increases but spatial (DOI)
resolution worsens
8
Lc, leff, No KEY PARAMETERS OF THE HPD-PET
CONCEPT

• Lc crystal length
• leff attenuation length of scint. photons
• 1/leff 1/(lbulk cos q)
  c/(cabs)
• No light yield p.e.s (511keV g) in a Lc0
  crystal
• (nph/keV, sci.ph.transport, q.e.
  wind. of photodet.)

HPD1
Z
sz, sE/E, st (only statistical)
g
cabs
g
q
a) crystal axial length (Lc) worsens all
resolutions
HPD2
limit of Lc 10 15 cm
b) light yield (No) improves all resolutions
c) contrasting effects of leff on sz sE/E,
st

• optimize leff value by wrapping or coating
  the crystal lateral surface

9
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 (F8mm, nw1.47,bialkali)
• B8850Quantacon(F5cm,nw1.47,bialkali)
• ?linear translator M-511(Phys.Instrum.)

10
polished 3x3x100 mm3 YAP-LYSO comparison
z1 cm z 5 cm z 9 cm
22Na source
YAPH3164-10 QLQR (5cm) 1692 ch
photoelectric peak (511 keV)
Compton
10
?E/E(FWHM)
14
LYSOH3164-10 QLQR (5cm) 2295 ch

• LYSO produces more light (pes) than YAP
• LYSO has a higher photofraction, lower energy
  resolution than YAP

11
?eff in polished(n21) 3x3x100 mm3 YAP-LYSO
No/2
?LYSO 42.60.9 cm
QL
exp(-z/leff), n1/n2, n1/nW
?YAP 20.80.4 cm
1/z2 n1/nW

• LYSO more transparent (higher leff than YAP)
• too high l-eff values (poor sz) both for LYSO
  and YAP

12
Crystal wrappings or metal-coatings change light
attenuation length of a YAP (3.2 x 3.2 x 100 mm3)
No/2
polished
QL
best solution

• at z0 N0(teflon) gt N0(polished) (diffusing
  wrapping)
• leff (polished) / leff(teflon) 1.9
• possibility to tune leff value with metal
  coatings
• metal coating (n2) reduces No

13
?E/E, ?z, ?tdc (z5cm) in coated 10cm YAP LYSO
(511 keV) vs leff
YAP
LYSO
a) statistical b) phenomen.
NO 51018 pe
NO 72434 pe
NO 75334 pe
14
very low leff in a Lc5 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 sz but z-dependent, bad sE/E

15
?z , ?E/E in coated-smeared YAP LYSO vs z,
leff, Lc, Eg

• crystal length worsens sz, does not influence
  much sE/E
• worse sz and sE/E values at lower Eg
• very low leff (raw lat.surf) values ? Lc
  limited, z-dep. of sz

16
Geant4 simulations for YAP long crystals PMTs
long and thin cylindrical crystal (n1,
Nph/keV) lat.surface polished, smeared (s),
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 reflection T
transmission D Lambertian diffusion SR
diffuse reflect. (smear) ST diffuse transmiss.
17
nwrap nwin

(nwin1.47)
(nwrap1.0)
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
18
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.)

19
Geant4 reproduction of exp. results

YAP H3164-10 PMTs
20
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

to be exp. proven
21
HRRT vs. HPD-PET
Full ring scanner A possible final configuration
for a HPD-PET
R 170 mm

• 12 modules
• F 34 cm
• Lc 15 cm
• 2496 crystals 3.2x3.2x150mm3
• 24 5 rect. HPDs
• det.Vol. 3834cm3
• det.depth 41mm
• DW/4p 0.165
• eTOT(LSO) () 8.5 (ph) 7.5 (Co-rec)
• eTOT(LaBr3)() 1.9 (ph) 4.9 (Co-rec)

• 8 panels 9x13 blocks
• F 31 cm
• AFOV 25 cm
• 120000 crystals 2.1x2.1x7.5mm3
• 936 PMTs 2x2cm2
• det.Vol. 3962cm3
• det.depth 15mm
• DW/4p 0.344
• eTOT (LSO)() (exp) 6.9

z
x
y
22
HPD-PET(Lc10cm) vs HRRT
crystal nw No (pe) leff (cm) Dz FWHM (mm) DE/E FWHM ()
H P D - P E T e x P YAP polish 1.47 950 21 19.3 10.8
H P D - P E T e x P YAP teflon 1.47 1120 10.5 12.7 11.2
H P D - P E T e x P LYSO polish 1.47 1200 42 34.5 14.6
H P D - P E T e x P LYSO teflon 1.47 750 20 22.1 16.4
H P D - P E T MC YAP polish 1.8 1500 16 8.9 8.9
H P D - P E T MC YAP teflon 1.8 1640 9 7.7 9.4
H RRT e x P LSO 5 17