Demonstration of an Axial PET concept for Brain and Small Animal Imaging

C. Joram 4 Feb 2010 1

Demonstration of an Axial PET concept for Brain and Small Animal Imaging

• The AXIAL PET concept

• Design of the Demonstrator (2 modules)

• Characterization

• Simulation and Reconstruction

• Next steps / near future plans

https://cern.ch/twiki/bin/view/AXIALPET

Physics For Health in Europe

Christian Joram, CERN PH/DT

http://www.cern.ch/


The AX-PET collaboration

Istituto Nazionale di Fisica Nucleare Bari (INFN)

Ohio State University (OSU)

European Organization for Nuclear Research (CERN)

University of Michigan

University of Rome Sapienza (INFN)

Instituto de Fisica Corpuscular (IFIC)

Paul Scherrer Institute (PSI)

Eidgenössische Technische Hochschule (ETH)


P. Beltrame, A. Braem, V. Fanti, C. Joram, T. Schneider, J. Séguinot, C. Casella, G. Dissertori, L. Djambazov, W.

Lustermann, F. Nessi-Tedaldi, F. Pauss, D. Schinzel, P. Solevi , J. F. Oliver, M. Rafecas, R. De Leo, E. Nappi, E. Chesi, H. Kagan,

A. Rudge, P. Weilhammer, D. Renker, N. Clinthorne, E. Bolle, S. Stapnes , F. Meddi

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WHY AX-PET ?


C. Joram 4 Feb 2010 4Physics For Health in Europe

• Short, radially oriented crystals• readout in blocks by PMTs• Anger logic decoding• no depth of interaction (some exceptions)

sinLpNo DOI Parallax error

Find compromise between resolution and sensitivity

2

2 1

a

L

e Detection efficiency

Standard PET today L

adp


The AX-PET concept

From short radially oriented, block readout crystals …

… to long, axially oriented, individually readout crystals


C. Joram6

Our implementation of the AX-PET concept

x

yz

scintillation light

wavelength shifted light

• How to read crystals ?• How to measure axial coordinate ?

x

y

z


C. Joram 7

AX-PET components

The scintillator crystals are Ce doped LYSO (Lu1.8Y.2SiO5:Ce)


Dimensions: 3 x 3 x 100 mm3

The WLS strips are of type EJ-280-10x from Eljen Technologies


MPPC Kapton cable

The photodetectors are Geiger mode Avalanche Photodiodes (G-APD = SiPM) of type MPPC from Hamamatsu.

MPPC S10362-33-050C MPPC 3.22×1.19OCTAGON-SMD


AX-PET module geometry

• 48 crystals (6 layers x 8 crystals)• 156 WLS strips (6 layers x 26 strips)

7

3.5

Crystals are staggered by 2 mm. Crystals and WLS strips are read out on alternate sides to allow maximum packing density.

The layers are optically separated from each other.

2 modules= 408 ch.


C. Joram 9Physics For Health in Europe

Putting everything together….


Fully assembled module (48 crystals, 156 WLS stips)


… adding light protection cover and cables


AX-PET Frontend Electronics (1 channel, simplified)

• Analog readout of crystals and WLS strips• Sequential or sparse (only channels above threshold)• Fast energy sum of all crystals of 1 module• Trigger on 2 x 511 keV deposition in 2 modules

S&Hbias

trigger

VATAGP5 (128 ch.)

‘slow’

fastOPA486

SE (crystals only)

test

analog output:Light per crystal / WLS

Channels above threshold


MPPC


module

source

2-D moving station

PMT

tagger

single module characterization 2 modules in coincidence

• Module in coincidence with a tagging scintillator

• Use of different tagging crystals

module 2

module 1

2-D moving station

source

• Distance between modules = 15 cm

Test set-ups 22Na source (ø=250μm; A~900 kBq)


LYSO ENERGY CALIBRATION

• deviation from linearity due to MPPC saturation (3600 pixels), ~ 5% effect

• parameterization: logarithmic function.

Use intrinsic Lu radioactivity + Photopeak self-calibrating” device

[63 keV]

[202 keV]

[303 keV]

[511

keV]

Energy calibration[6

3 ke

V] [2

02 k

eV]

[303

keV

]

[511

keV

]


Energy resolution

Individual crystals:< R_FWHM > ~ 11.6% @511 keV (averaged on all crystals)

Sum signal (48 crystals)R_FWHM_Sum ~ 12.25% at 511 keV (on the summed distribution)


layer1 layer2

layer6

layer3

layer4 layer5

z coordinate = COG of hit WLS strips (typically 2-4)

s(d=0) ~ 0.76 mmFWHM ~ 1.8 mm(still includes size of source)

x

y

z

layer6

layer1

Axial (z) resolution

g

σi [i=1,6] include:• intrinsic spatial resolution • beam spot size on each layer


AXIAL PLANE TRANS-AXIAL PLANE

• Photoelectric events only (1 hit crystal per module)• Draw “LOR” (pure geometrical, no tomographic reconstruction)

First coincidence measurements


Estimate of Axial resolution

FWHM = 1.5 mm

intersection with plane (x=0)

Resolution still includes size of source. Finite positron range!(in water: <range> = 0.6 mm)

First tomographic reconstruction of point source data has been achieved last weekend. Will be discussed in AX-PET meeting tomorrow!


AX-PET simulation

• Geant4 (multi-purpose Monte Carlo tool, optical transport, dedicated geometry)

• GATE (PET dedicated MC, including time dependent phenomena, scanner rotation, source/phantoms...)

Energy - ELOW=40 keV, ESUM=[400,600] keV

excellent agreement data / MC


A dedicated reconstruction code, based on MLEM (Maximum Likelihood Expectation Maximization) has been developed.The geometrical component of the system matrix has been computed using Siddon's ray-tracing technique. Takes also account for crystal attenuation and penetration effects.

Monte-Carlo data for a cylindrical source: D = 10 mm, h = 10 mm

• FOV: 25x25x25 mm3

• Voxel: 25x25x25 vox3

• #steps = 6• Distance = 10 cm

AX-PET reconstruction

10 mm

10

Projections (x, y, z) 3D image

Reconstruction of real data very soon!


AX-PET main features

• parallax-free 3D localization of photons.

• Spatial resolution (crystal and WLS strip dimensions) and sensitivity (additional layers) can be optimized independently. Physical limits in reach.

• The 3D capability should allow to identify a significant fraction of Compton interactions (Inter Crystal Scatter). ICS events can either be discarded (resolution fully maintained) or reconstructed (increased sensitivity).

• AX-PET concept can be scaled in size and number of layers to match specific needs. small animal PET brain PET full body PET PEM (mammography)

• Concept and components are in principle MRI compatible and TOF extendable.



What’s next ?

• Mount set-up on a horizontal gantry (rotating source + 1 module rotation +/- 60°).

At ETH Zurich, in cooperation with Center for radio-pharmaceutical Science (Prof. A. Schubiger), as of ~April 2010:

• Tomographic reconstruction of small animal phantoms (FDG)

• Optimization of M.C. and reconstruction code

Time scale 1 year

• Performance extrapolation (M.C.) to full scanner and specific geometries.

• … towards commercialization (contacts with Finnish PET teams and companies)



BACK-UP



AX-PET components

Density [g/cm3] 7.1Attenuation length for 511 keV [cm] 1.2Wavelength of maximum emission [nm] 420Refractive index at W.L. of max. emission 1.81Light yield [photons/keV] 32Average temperature coefficient [%/K] -0.28Decay time [ns] 41Intrinsic energy resolution [%, FWHM] ~8Natural radioactivity [Bq/cm3] ~300Effective optical absorption length [mm] ~ 420

The scintillator crystals are Ce doped LYSO (Lu1.8Y.2SiO5:Ce) single crystals, fabricated by Saint Gobain and commercialized under the trade name PreLude 420.

The main characteristics are:


One end is read out, the other end is mirror-coated (evaporated Al-film).



300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

LY

SO

, w

avel

ength

of

max

. em

issi

on

WLS sheet ELJEN EJ-280-10x

0.7 mm 1.1 mm 1.5 mm

tran

smis

sion T

wavelength (nm)

The WLS strips are of type EJ-280-10x from Eljen Technologies

• Shift light from blue to green• Density: 1.023 g/cm3• Absorption length for blue light: 0.4mm (10 x

standard concentration)• Index of reflection: 1.58• Decay time: 8.5ns• Size: 0.9×3×40mm3

One end is read out, the other end is mirror-coated (evaporated Al-film).



AX-PET Mechanics

- holds crystals, WLS strips, and MPPCs in place.

- carries Kapton cables and mini patch panels- shields against ambient light- Made from relatively light materials (Al)- relatively complex because it’s designed for

full de-mountability



AX-PET Frontend / Readout / Bias (for 1 module)

based on NIM modules

G-APDBias

Supply(256 ch.)

VATAGP5 ASIC


TIMING PERFORMANCE

First estimate of the time resolution : - measure delay of coincidence wrt Module2 - measurement from the scope [Lecroy Waverunner LT584 L 1GHz]

LYSO SUM - MODULE 1

LYSO SUM - MODULE2

Trigger

time resolution : σ ~ 800 ps


Documents

Demonstration of an Axial PET concept for Brain and Small Animal Imaging