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

Standard radial concept

Many rings of crystal–photodetector blocks

radially displacedLc =1.5-3cm

New 3D axial HPD-PET concept

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

p

PMTs

axis

axis

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·a (LSO) 2⊥)~ 90%

• with spacing: 4x4 mm2 Rx = 64 mm, Ry = 52 mm

"Proximity focused“ HPDs

• sapphire window (n~1.8):

(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)

Gain HPD : ~ 3.103 (Uop= 12 kV ) or 5.103 (Uop = 20 kV)

Rx

Ry Lc

LYSO crystals

sapphire window

ring electrodes

silicon sensor

readout chips

Bi-alkali photocathode

“Reference Radial PET”: The High Resolution Research Tomograph (HRRT) (CTI, MPI, Karolinska …)

AFOV25 cm

31 cm

• 8 panels with 9 13 blocks

• 2 64 crystals per block

• 120000 crystals 2.12.1x7.5mm3

• 936 (20x20 mm2) PMTs

PM

T2

PM

T1

LSO

LSO = 7 ns

88 matrix (2x2 cm2) 4PMTs

z (FWHM) ~ 5 mm V= (FWHM) ~ 20 mm3

Lc =15mm ~ 1 a 2( )~ 53% ⊥

E/E (511 keV) ~ 17%Heavy electronics (PSD)

7.5

7.5

K. Wienhard et al., IEEE Trans. Nucl. Sci. 49 (2002) 104–10

Lc

•ANGER LOGIC: a block seen by 4 PMTs CM of signals in 4 PMTs interac. point (x,y) of

•DOI (z) from PHOSWICH tecnique Pulse Shape Discrimin. (PSD)

x

y

z

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

Density ρ (g/cm3) 5.55 7.4 8.34 5.3

Effective atomic charge Z 32 66 65 46.9

Scintillation light output (photons / MeV) 18000 23000 ~10000 ~61000

wavelength max of max. emission (nm) 370 420 370 356

Refractive index n at max 1.94 1.82 1.95 ~1.88

Bulk light abs. length bulk (cm) at max

~20 ~40    

Principal decay time (ns) 27 40 18 30±5

Mean γ atten. length a at 511keV (mm) 22.4 11.5 10.5 ~20

Photo fraction at 511 keV (%) 4,5 32.5 30.5 15

Energy resolution (FWHM) at 663 keV 4.5 8   2.9

Inorganic Scintillation crystals

BGO

7.13

75

~9000

480

~2.15

 

300

~11.6

41.5

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)

• x,y from fired scintillator σ(x,y) = 3.2 mm/√12= 0.92 mm x,y (FWHM) = 2.2 mm

• z (DOI) from the ratio of the photoelectrons detected at the two crystal ends σ(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

HPD2

HPD1

x

y

z

208 4 x 4 mm2 Si ‘pads’ centred on crystal matrix

1) ADVANTAGES OF THE AXIAL HPD-PET CONCEPT

High Granularity exact

reconstruction of the interaction point (no parallax error)

(Rx)

(Ry)

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

possibility to reconstruct the int. point of part of ’s that suffers a double (Compton + photoelectric) event in the same module

2) ADVANTAGES OF THE AXIAL HPD-PET CONCEPT

detection efficiency increases but spatial (DOI) resolution worsens

HPD2

HPD1

σz, σE/E, σt : (only statistical)

Lc, eff, No: KEY PARAMETERS OF THE HPD-PET CONCEPT

Z

, exp exp2

, ln

2

12/1

2

1

eff

C

effo

eff

zCeff

zLz

N

ENFL

N

Nz

, )(

exp2

, exp2

02

01

eff

C

eff

zLNN

zNN

.21 NNzN pe

, ,Rint pe

Tpe

E

N

c

N

ENF

E

• Lc: crystal length

• eff: attenuation length of scint. photons

1/eff = 1/(bulk* cos) + c’/(cabs)

• No: light yield ≡ p.e.’s (511keV ) in a Lc~0 crystal

(nph/keV, sci.ph.transport, q.e. & wind. of photodet.)

cabs

a) crystal axial length (Lc) worsens all resolutions limit of Lc: 10 ~ 15 cm

c) contrasting effects of eff on σz & σE/E, σt

optimize eff value by wrapping or coating the crystal lateral surface

b) light yield (No) improves all resolutions

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 (=8mm, nw=1.47,bialkali)• B8850Quantacon(=5cm,nw=1.47,bialkali)

linear translator M-511(Phys.Instrum.)

polished 3x3x100 mm3 YAP-LYSO comparison

z=1 cm z = 5 cm z =9 cm

YAP+H3164-10QL+QR (5cm)= 1692 ch

LYSO+H3164-10QL+QR (5cm)= 2295 ch

22Na source

photoelectric peak (511 keV) Compton

ΔE/E(FWHM) ~ 10%

LYSO produces more light (pe’s) than YAP LYSO has a higher photofraction, lower energy resolution than YAP

~ 14%

eff in polished(n2=1) 3x3x100 mm3 YAP-LYSO

LYSO= 42.6±0.9 cm

YAP= 20.8±0.4 cm

QL

LYSO more transparent (higher eff than YAP)

too high -eff values (poor z) both for LYSO and YAP

exp(-z/eff), ≈ n1/n2, n1/nW

1/z2 ≈ n1/nW

No/2

Crystal wrappings or metal-coatings change light attenuation length of a YAP (3.2 x 3.2 x 100 mm3)

best solution

polished

No/2

at z=0: N0(teflon) > N0(polished) (diffusing wrapping)

eff (polished) / eff(teflon) = 1.9

possibility to tune eff value with metal coatings

metal coating (n2) reduces No

QL

NO = 510±18 pe

NO = 753±34 pe

E/E, z, tdc (z=5cm) in coated 10cm YAP & LYSO (511 keV) vs

eff

NO = 724±34 pe

/

0

22

zz e

N

ENF

int/

0

ReN

ENF

E zStat

E

/

''

mzte

ca

YAP LYSOa) statisticalb) phenomen.

very low eff 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 eff , but dependent with z

good z but z-dependent, bad E/E

crystal length worsens z, does not influence much E/E

worse z and E/E values at lower E

very low eff (raw lat.surf) values Lc limited, z-dep. of z

z , E/E in coated-smeared YAP & LYSO vs z, eff, Lc, E

Geant4 simulations for YAP long crystals + PMTs

I incident unpol. opt. photons

A absorption

R reflectionT transmission

D Lambertian diffusion

SR diffuse reflect. (smear)ST diffuse transmiss.

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

nwrap nwin

nwrap

does not change eff

decreases Noworsens E/E, z, t

(nwin=1.47) (nwrap=1.0)

nwin decreases eff

increases Noimproves E/E, z, t

*refr.ind.match

*

absorption diffusion smearing

absorp. & diffus. •similar effects•decrease eff

(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 mechanical or laser ablation engravingsengravings (effects similar to absorp.)•decrease eff

•improve spatial resol.•worsen energy-time resol. &• high reproducibility to the N0 and eff values of the many HPD-PET crystals

Full ring scannerA possible final configuration for a HPD-PET

R = 170 mm12 modules

=34 cm

Lc = 15 cm

2496 crystals 3.2x3.2x150mm3

24 5” rect. HPDs

det.Vol. 3834cm3

det.depth 41mm

~ 0.165

TOT(LSO) (%) ~ 8.5 (ph) + 7.5 (Co-rec)

TOT(LaBr3)(%) ~ 1.9 (ph) + 4.9 (Co-rec)

xy

z

HRRT vs. HPD-PET

8 panels 9x13 blocks

=31 cm

AFOV = 25 cm

120000 crystals 2.1x2.1x7.5mm3

936 PMTs 2x2cm2

det.Vol. 3962cm3

det.depth 15mm

~ 0.344

TOT (LSO)(%) (exp) ~ 6.9

AFOV25 cm

31 cm

HPD-PET(Lc=10cm) vs HRRT

crystal nw No(pe)

eff

(cm)z FWHM

(mm)

E/E FWHM

(%)

HPD-PET

exP

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

MC

YAP polish 1.8 1500 16 8.9 8.9

YAP teflon 1.8 1640 9 7.7 9.4

HRRT

exP

LSO 5 17