Mitglied der Helmholtz-Gemeinschaft A monolithic block detector design for a dedicated brain MR-PET scanner Karl Ziemons Forschungszentrum Juelich, Germany

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A monolithic block detector design for a dedicated brain MR-PET scanner

Karl Ziemons

Forschungszentrum Juelich, Germany

Member of the Crystal Clear Collaboration

09. Oct. 2009 | ICATPP, Como-Italy

09.10.2009, ICATPP A monolithic block detector concept –K.Ziemons Folie 2

Outline

1. ClearPET Project

2. From a pixelated to a monolithic tapered block concept

3. TOF – Time-of-Light

4. Hybrid imaging concept: MR-PET

5. Conclusion


Challenges in PET

fundamentals:

a) to obtain as many counts as possible

high sensitivity

b) to localize these counts as accurately as possible

high spatial resolution

high temporal resolution


ClearPET™ Project

R&D of 6 more or less identical small animal PET scanners

• Flandern (VUB a. Hosp. Gent)• Jülich (FZJ)• Lausanne (IHPE)• Lyon (CERMEP / UCBL a. CERN)• Raytest GmbH (Straubenhardt, Germany)• Samsung Medical Center (Seoul, Korea)

in collaboration with the Crystal Clear Network (CCC)

all scanners are called: ClearPET +extension


ClearPET™ Prototypes

ClearPET Petite (Samsung Med. C., Seoul)

ClearPET Rodent(VUB & Hosp. Gent)

ClearPET Prototype (Lausanne now Marseille)


ClearPET™ Neuro Scanner @ fz-juelich.de

@ Institute of Medicine, Forschungszentrum Jülich


ClearPET™: The Innovations

1. LSO & LuYAP crystals in a dual layer phoswich solution longer crystals improve the efficiency & estimate depth-of-Interaction for a better radial resolution

2. New concept implement into the frontend electronic:„Free Running Sampling“ mode allows digital pulse processing to identify the crystal layer & extrapolate timing and energy information

3. Modular design to realize different geometries

Crystal block: 8*8 LSO & LuYAP crystals, each 2*2*10mm3 coupled to multichannel PM


Depth Of Interaction in Principle:

parallax effect→ decreases the radial spatial resolution

information about the crystal layer→ improves the radial spatial resolution


ClearPET™ properties

1. Spatial resolution:

1. System peak sensitivity : 3.5%

2. Energy resolution ≈ 24%

3. Time resolution 5.5 ns FWHM

1.4mm

2.0mm

1.6mm

1.0mm

1.2m

m

1.8m

m

Derenzo phantom: Ø 40mm, filled with ~ 0.5mCi, 18F 6min scan time;

3D OSEM reconstruction

distance to centre 0 cm 2 cm 4 cm

radial resolution [mm] 1.32 1.96 2.59


Rat Scan:Comparison between HR+ & ClearPET


Beyond ClearPET™: Next Steps

1. from a pixelated to a monolithic block concept

• Increase sensitivity (no inter-crystal separations, reduced dead space)• 3D position information embedded in the light distribution• extract parallax-corrected incidence coordinates with good accuracy• continuous coordinates• easy to manufacture and to assemble



1. from a pixelated to a monolithic block concept

Geometry design of the ClearPET Neuro Geometry design of the BrainPET @CIEMAT

System peak sensitivity: > 15% (by the same geometry as the ClearPET Neuro)

Spatial resolution: < 1.3mm over the whole FOVEnergy resolution: ≈ 11%

Goal:


x

zLight distribution depends on the entry point on the front surface…and on the depth of interaction (DOI).

front

back

crystal

light sensor

Monolithic scintillator detectors


Technical Developments

1. APD detector arrays

Insensitive to magnetic field

Higher QE as PM but lower gain

compact

2. miniaturization of electronics

Multichannel preamplifier chips

Analog and digital preprocessing with FPGA’s (field programmable gate arrays) or ASIC’s (application specific integrated circuit)

S8550 Hamamatsu APD array


Block Configuration for Experimental Setup

readout with two S8550 APDs monolythic 20x20x10 mm3 LYSO:Ce

crystal Light Yield: 32000 photons per MeV of deposited

energy [*] optical photon spectrum: = 420 nm (maximum) attenuation lenght for 420 nm photons: 420 mm [*] refraction index: 1.81 [*]

crystal block wrapped by Teflon reflection coefficient: 0.95 [*]

optical coupling between LYSO:Ce and APD epoxy window

[*] data taken from product datasheet St GobainCoincidence Trigger

PMT

20x20x50 mmBaF2 crystal

Lead

22NaAPDmatrix

Rotationtable

LSO crystals

0o - 90o

High precisionX-Y stage

AND

32x cathode signal


Experimental resultscollimated beam scanned over X and Y

(x = +1.0) (x = +3.0) (x = +5.0) (x = +7.0) (x = +9.0)

(x = -9.0) (x = -7.0) (x = -5.0) (x = -3.0) (x = -1.0)

(y = -9.0)

(y = -7.0)

(y = -5.0)

(y = -3.0)

(y = -1.0)

(y = +3.0)

(y = +1.0)

(y = +5.0)


How to find photon position ?


Parallax correction during training

Train neural network to reproduce the DOI independent incidence position ATrain neural network to reproduce the DOI independent incidence position A

Photo detectors


Performance of APD-based systems

20x10x10 mm LSO block read out by S8550 Hamamatsu APD array 1.5 mm FWHM average resolution 0.4 mm FWHM degradation at 30o incidence 11.5 % energy resolution ~ 2 ns FWHM time resolution


rotating source holder

detector blocks

two detector blocks aligned face to face 400 mm apartrotating precision source holder for tomographic imaging

BrainPET laboratory prototype


BrainPET prototype performance

single 1.0 MBq 0.25 mm diameter Na-22 sourceimages acquired for a single source at two positions 6.0 mm apart along radial directionindividual images superposed to calculate profiles and spatial resolutions

Image size: 32 mm 32 mm (64 64 pixels), 0.5 mm slice thicknessReconstruction: 3D SSRB + 2D FBP with ramp filter at half the Nyquist frequencyVisualization: 3D rendering with AMIDE viewer


SNR limitation of APD based detector


SiPM or GeigerMode-APD: a new photodetector

Courtesy by C.Jackson, SensL


The output signal is proportional to the number of fired cells

as long as

NPh < Ncells

B.Dolgoshein, NIM 2003

SiPM: Saturation Effect


SiPM Key Parameter


Linearity Measurement

Courtesy by C.Jackson, SensL


SiPM Array

SensL SPMArray 3035G16:Pixel Chip Area 3.16 x 3.16 mm2 Pixel Active Area 2.85 x 2.85 mm2 Operating Voltage (typical) 29.5 V +2V above Vbr, λ = 520nm Array Details 4 x 4 PixelMicrocell Gain >1x106 - Total Pixel Effective Area 13.4 x 13.4 mm2 Number of Microcells 3640 Per pixelPhoton Detection Efficiency 10-20 %

1V to 4V above Vbr Dark Rate 8 MHz Per pixel


Influence of pixel size

Simulation study of a 20x20x10 mm LSO block read out by a pixelated photo detector.

Perfect detection of scintillation photons


Expected performance of SiPM array design

∆E = 8.7 % FWM

Array Size : 8 x 8 Pixel size : 3 mm x 3 mm Pixel Pitch : 3.4 mm Detection eff. : 30 % Cell size : 100 µm x 100 µm Crosstalk : < 20 %



2. TOF – Time of Flight PET

• Can localize source along line of flight

• Time of flight information reduces noise in images

• Variance reduction given by 2D/ct.

• 500ps timing resolution 5x reduction in variance


Conventional:• Detected event projeted to all

voxels between detector pairs• Lots of coupling between voxels Many iterations to converge

Adding Time-of-Flight to Reconstruction Faster Convergence

Time-of-Flight:• Detected event projeted only to

voxels consistent w measured time• Little coupling between voxels Few iterations to converge

Data courtesy by W.Moses


Clear improvement of contrast enhancement visually!

Whole Body – Time of Flight Simulation

Data courtesy by Mike Casey, CPS Innovation


22Na pulse height spectrumCoincidence timing spectrum

(two LaBr3:Ce3+/SiPM detectors)

3 x 3 x 5 mm3 LaBr3:Ce3+ on 3 x 3 mm2 Hamamatsu S10362-33-025C SiPM

LaBr3:Ce3+ with SiPMs: First results(data from D.Schaart et al., IEEE Oct.2008)



3. Hybrid imaging concept: MR-PET

PET & MRI are medical imaging techniques

that in widespread use both for patient diagnosis and management, and in clinical research

playing a key role in a wide range of fields from mapping of the human brain

to the development of new treatments for cancer


Complementary Nature of MRI & PET

Parameter MRI PET

Anatomical Detail Excellent Poor

Spatial Resolution Excellent Compromised

Clinical Penetration Excellent Limited

Sensitivity Poor Excellent

Molecular imaging Limited Excellent

Hence: The Sum of PET and MRI should be excellent and even better MRI + PET << MRI-PET


Why MRI-PET Hybrid Imaging?

MRI

• Want true simultaneous data acquisition in a single device

• Want combined functional and morphological data acquisition at the same time

• Want multi modal functional acquisitions at the same time (fMRI / MRS - PET)

• Want to cross-validate activations measured with PET and fMRI under the same conditions, at the same time, in the same status

but still want quantitative PET image


Installed in Jülich in autumn 2008:MAGNETOM Trio with a BrainPET


Our first MR-FDG-PET images

20-50 min p.i. 18FDG-PET

AW-OSEM3Dfiltered with 2.5 mm Gaussian

SimultaneousT1 MPRAGE

Fusion


Beyond ClearPET™: BrainPET Project

SiPM PET insert based on monolithic block design:

Aim: Development and implementation of a hybrid 9.4Tesla MR-PET animal / human scanner

Design of SiPM PET insert Hand made housing of the PET insert including the gradient coil


Summary

BrainPET insert is being developed using monolithic LSO blocks and S8550 Hamamatsu APD arrays

Using an array of 3x3 mm SiPM pixels to read out monolithic LSO blocks has

similar spatial resolution than APDs (~ 1.3 mm FWHM) slightly better energy resolution (8.7 % versus 11 %) better timing resolution (sub-nanosecond versus 2 ns with APDs

Detection efficiency is the most important SiPM parameter to optimize Compared to APDs

signals are easier to handle we can add more rows or columns because amplifier noise is negligible

Future : Experimental studies using SiPMs and monolithic scintillator in preparation of a fully MR compatible PET insert with high resolution and high sensitivity performance.


Conclusion

PET/CT was a medical revolution and a technical evolution

MR/PET seems to be a technical revolution and a medical evolution


Acknowledgments

Thank‘s to P.Bruyndonckx and S.Tavernier@VUB, Belgium

J.Perez and P.Rato Mendes@CIEMAT, Spain

H.Larue, C.Parl, M.Streun (ZEL)N.J.Shah, H.Herzog and U.Pietrzyk (INM) @Forschungszentrum Jülich

and

all the CCC members

Documents

Mitglied der Helmholtz-Gemeinschaft A monolithic block detector design for a dedicated brain MR-PET scanner Karl Ziemons Forschungszentrum Juelich, Germany