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 displaced Lc =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

A HPD-PET CAMERA MODULE Array of 208 scintill.s (LSO, LYSO, YAP, LaBr 3, n= ) 16x13 crystals (3.2[Rx]x3.2[Ry]x150[Lc] mm 3 ) t x =51mm, t y =42mm  t X,Y >3· a (LSO)   2   ⊥ )~ 90% with spacing: 4x4 mm 2 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 mm 2 ) silicon detector pads (each crystal readout by its pad, no cross-talk) each pad with integrated lecture electronics (2 VATA-Gp5) Gain HPD : ~ (U op = 12 kV ) or (U op = 20 kV) RxRx RyLc

“Reference Radial PET”: The High Resolution Research Tomograph (HRRT) (CTI, MPI, Karolinska …) AFOV 25 cm 31 cm 8 panels with 9  13 blocks 2  64 crystals per block crystals 2.1  2.1x7.5mm (20x20 mm 2 ) PMTs PMT2PMT1 LSO  = 7 ns 8  8 matrix (2x2 cm 2 )  4PMTs  z (FWHM) ~ 5 mm  V= (FWHM) ~ 20 mm 3 Lc =15mm ~ 1 a   2  ( ⊥ )~ 53%  E/E (511 keV) ~ 17 % Heavy electronics (PSD) 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:CeLSO:CeLuAP:CeLaBr 3 :Ce Density ρ (g/cm 3 ) Effective atomic charge Z Scintillation light output (photons / MeV) ~10000~61000 wavelength max of max. emission (nm) Refractive index n at max ~1.88 Bulk light abs. length bulk (cm) at max ~20~40 Principal decay time (ns) ±5 Mean γ atten. length a at 511keV (mm) ~20 Photo fraction at 511 keV (%)4, Energy resolution (FWHM) at 663 keV Inorganic Scintillation crystals 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)

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 x 4 mm 2 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 σ z, σ E /E, σ t : (only statistical) Lc, eff, No: KEY PARAMETERS OF THE HPD-PET CONCEPT Z Lc: crystal length eff : attenuation length of scint. photons 1/ eff = 1/( bulk * cos  ) + c’/(c abs ) No: light yield ≡ p.e.’s (511keV  ) in a Lc ~ 0 crystal ( n ph /keV, sci.ph.transport, q.e. & wind. of photodet. ) c abs  a) crystal axial length (Lc) worsens all resolutions limit of Lc: 10 ~ 15 cm c) contrasting effects of eff on σ z & σ E /E, σ t  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  BaF 2 (used with a 22 Na source)  Pb collimator Pb + source  YAP (Preciosa Co) LYSO (Photonic Materials) (3.2 x 3.2 x mm 3 ) PMT H (  =8mm, n w = 1.47,bialkali ) B8850 Quantacon(  =5cm,n w = 1.47,bialkali )  linear translator M-511( Phys.Instrum. )

polished 3x3x100 mm 3 YAP-LYSO comparison z=1 cm z = 5 cm z =9 cm YAP+H Q L +Q R (5cm)= 1692 ch LYSO+H Q L +Q R (5cm)= 2295 ch 22 Na 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(n 2 =1) 3x3x100 mm 3 YAP-LYSO LYSO = 42.6±0.9 cm YAP = 20.8±0.4 cm QLQL LYSO more transparent (higher eff than YAP) too high -eff values (poor  z ) both for LYSO and YAP exp(-z/ eff ), ≈ n 1 /n 2, n 1 /n W 1/z 2 ≈ n 1 /n W No/2

Crystal wrappings or metal-coatings change light attenuation length of a YAP (3.2 x 3.2 x 100 mm 3) best solution polished No/2 at z=0: N 0 (teflon) > N 0 (polished) (diffusing wrapping) eff (polished) / eff (teflon) = 1.9 possibility to tune eff value with metal coatings metal coating (n 2 ) reduces No QLQL

N O = 510±18 pe N O = 753±34 pe  E /E,  z,  tdc (z=5cm) in coated 10cm YAP & LYSO (511 keV) vs eff N O = 724±34 pe YAP LYSO a) statistical b) phenomen.

very l ow 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 reflection T transmission D Lambertian diffusion S R diffuse reflect. (smear) S T diffuse transmiss. long and thin cylindrical crystal (n 1, N ph /keV) lat.surface: polished, smeared (  ), wrapped ( n wrap ), coated (n wrap,ik,t) polished bases coupled to PMTs (n win, t,q.e.) Polar diagram of reflected-refracted scintillation photons

n wrap n win n wrap does not change eff decreases No worsens  E /E,  z,  t (n win =1.47) (n wrap =1.0) n win decreases eff increases No improves  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 (N 1 no more exp.)

Geant4 reproduction of exp. results YAP + H PMTs

Geant4 predictions for engraved crystals mechanical or laser ablation engravings engravings (effects similar to absorp.) decrease eff improve spatial resol. worsen energy-time resol. & high reproducibility to the N 0 and eff values of the many HPD-PET crystals

Full ring scanner A possible final configuration for a HPD-PET R = 170 mm 12 modules  =  34 cm Lc = 15 cm 2496 crystals 3. 2x3.2x150mm ” rect. HPDs det.Vol. 3834cm 3 det.depth 41mm  ~  TOT (LSO ) (%) ~ 8.5 (ph) (Co-rec)  TOT (LaBr 3 )(%) ~ 1.9 (ph) (Co-rec) xy z HRRT vs. HPD-PET 8 panels 9x13 blocks  =  31 cm AFOV = 25 cm crystals 2.1x2.1x7.5mm PMTs 2x2cm 2 det.Vol. 3962cm 3 det.depth 15mm  ~  TOT (LSO)(%) (exp) ~ 6.9 AFOV 25 cm 31 cm

HPD-PET (Lc=10cm) vs HRRT crystalnwnw No (pe) eff (cm)  z FWHM (mm)  E /E FWHM (%) HPD-PETHPD-PET exPexP YAP polish YAP teflon LYSO polish LYSO teflon MCMC YAP polish YAP teflon HRRTHRRT exPexP LSO 5 17