PSMR2015, 17-20 May 2015, Isola d’Elba, Italy4th Conference on PET/MR and SPECT/MR

An innovative high rate preclinical PET/MR detector towards dynamic multimodal imaging.

Becker Robert, Cachemiche Jean-Pierre, Casella Chiara, Commichau Volker, Di Calafiori Diogo, Dissertori Günther, Fischer Jannis, Howard Alexander, Jeitler Astrik, Lustermann Werner, Morel Christian, Josep Oliver, Röser Ulf, Wang Qiulin, Weber Bruno

SAFIR - Small Animal Fast Insert for MRI

1. Introduction2. Construction and readout electronics3. NECR4. Spatial resolution5. High rate tests6. Summary / Outlook

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Objective: construction of a high rate PET insert for a Bruker BioSpin 70/30 USR MRI scanner (7T static field) with unprecedented temporal resolution (~5s), capable of truly simultaneous PET/MR acquisition.

Users:• Institute for Biomedical Engineering (IBE), ETH/University of Zurich, Prof. M. Rudin• Institute of Pharmacology and Toxicology (IPT), University of Zurich, Prof. B. Weber

Infrastructure:Bruker BioSpin 70/30 USR MRI scanner (7T static field) Installed at the Animal Imaging Center at Hoenggerberg(http://www.lifescience-zurichevents.ch/index.php?id=55&L=1)

Motivation:Quantitative dynamic PET imaging truly simultaneous with MRI using (among others) short lived isotopes 15O Study coronal blood flow and its underlying mechanism

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PET insert performance• High sensitivity ~5%• Good spatial resolution of <2mm FWHM• Excellent coincidence resolving time (CRT) ~300ps FWHM.

front-end electronics and DAQ• Operation at up to ~500 MBq source activity (10 times more than usual) data rates

~40kHz / ch• (almost) ZERO dead time, low pile-up• Large number of channels: ~16000

PET insert constraints• MR compatible: (i) no magnetic materials (ii) immune to high power RF switching fields

(300MHz) (iii) immune to static 7T field• Mechanical dimensions: Fitting inside the gradient coils (200 mm inner diameter) of the

MR system, leaving space for the receiving coils and the animal

image reconstruction• Quantitative, resolve temporal evolution of tracer concentration with precision of (5 – 10) s

> Such a PET detector is not existing > Several PET insert developments are ongoing, typically aiming at high spatial resolution, none at high temporal resolution

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Geometry:• Modules with 8x8 crystals:

one-to-one coupling of crystals to photo-sensors

8x8 crystal matrix:• 2.1 x 2.1 x 12mm3

• LYSO or LSO all sides polished

• 5 sides, 3M ESR foil + optical glue

8x8 SiPM matrix (TSV):• 2.0 x 2.0 mm2 sensors• 2.2 mm pitch• Hamamatsu

Kapton cable

FE card with readout ASCI

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Geometry:• Xtal dimensions: 2.1 x 2.1 mm2;

12 mm long• Modules with 8x8 crystals• Arranged into a ring of 24 modules• modules, ~130 mm inner diameter

Based on the reference design, a single ring system will be constructedStructure:• Mechanics will be planned for the

final system• Includes cooling interface to readout

electronics boards

Objective• Construction within ~1 year• Selection of components close

to completion

Design will be further optimized towards a full detector

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Geometry:• Build blocks of 5 modules• Integrate cooling for electronics

Kapton cable

FE cards with readout ASIC

Kapton cable

support

Five 8x8 crystal matrices

cooling/support

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Electronics compartment

Electronics compartmentSensor compartment

Dimensions:• Outer diameter ~200 mm, Inner diameter ~130 mm• Overall length ~1000 mm • Sensor compartment ~200 mm, electronic compartments ~400 mm

Modularity:• 10 rings with 24 modules of 8x8 crystals each 240 modules 240 SiPM arrays 15360 crystals 15360 SiPMs 15360 readout channels

• Total power consumption: (0.5-1.0) kW active liquid cooling• Operation temperature: ambient or lower (-20 degC)?

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Front-End ASIC 64 channel:• Pre-amplifier• Leading edge discriminator timing measurement with ~50 ps resolution• Time over threshold discriminator energy measurement with ~20 % FWHM• Two candidates TOFPET or STiC3 (both under investigation)• Power consumption (10 – 30) mW per channel• Data word (one hit): 48 bit• Digital data output: 1(2) LVDS links with 160 Mbit/sec 8/10 bit encoding

Data transmission: optical interfaces• 30 LVDS inputs 4.8 Gbit/sec• 1 optical output 5(10) Gbit/sec

DAQ• microTCA crate with optical receiver boards (AMC40)• Low jitter clock distribution (< 40 ps)• ASIC’s configuration

Auxiliary• Bias supply system and temperature monitoring including feedback• Power conversion system• Temperature and humidity monitoring

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NECR Noise Equivalent Count RateNECR = T2 / (T + f*R+ S): T true coincidences (trues)R random coincidences (randoms)S scatter coincidences (scatters)f=1, in the future singles based random estimation – almost error free

Data from GEANT4 simulation with additional:- Gaussian energy smearing: 20% FWHM- Gaussian time smearing (according to CRT)- Energy cut: (350-650) keV- Multiple window coincidence sorting

02468

101214

0 500 1000 1500 2000 2500

NEC

R in

Mcp

s

CTW in ps

NECR maximum for given CTW:- Depends on CRT- Slightly depends on activity use NECR optimum for 500 MBq Example: CRT 600 ps FWHM

NECR for different CRT’s, optimum CTW

CRT in ps 300 400 500 (3400)

CTW in ps 284 344 404 3400

0

5

10

15

20

25

0 200 400 600 800 1000

NEC

R in

Mcp

s

activity in MBq

300 ps

400 ps

500 ps

3.4 ns

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Crystal CRT FWHM

SIPAT LYSO:Ce 446 ps

SIPAT LSO:Ce:Ca(0.5) 327 ps

Crystals 1.5x1.5x12 mm3

Setup 22Na + 2 LYSO crystals in coincidence

Sensors Digital SiPM (PDPC, Philips)

Interface Air coupling (no grease)

Wrapping None

Digital SIPM (Philips) LSO:Ce:CaLYSO:Ce

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ToFPET: 411 ps FWHM

Coincidence setup, LYSO:Ce crystals, ‘air-coupled’ to Hamamatsu MPPCs22Na source ~2.5 MBq activity Improved results expected using optical glue and crystal wrapping

STiC: 388 ps FWHM

Work to achieve similar results using LYSO matrices instead of single crystals is ongoing

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Spatial resolution- Following NEMA- STIR: FORE, 2DFBP- Crystals 1.5x1.5x12 mm3

> Slightly smaller crystals than in the reference design

Radial: ~1.5 mm FWHMTang.: ~1.5 mm FWHMAxial: ~(1-3) mm FWHM depending on ring difference

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30

reso

lutio

n FW

HM

in m

m

radial distance from center in mm

radial tangential axial ring diff 5 axial ring diff 9

NEMA National Electrical Manufacturers AssociationSTIR Software for Tomographic Image ReconstructionFORE Fourier Rebinning 2D FBP two-dimensional filtered back-projection

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1.0 mm1.2 mm

4.0

mm

3.2 mm2.4 mm1.

6 m

m

Line profiles

Derenzo phantom simulation: GEANT4Simulation 1s, 500 MBq in the spheres

Background Rat phantom, ZERO activityCrystals 2.0x2.0x12 mm3, pitch 2.2 mm (uniform)

Geometry 91 rings (uniform), 180 xtals / ring

Reconstruction OSMAPOSL (STIR)

1.6 mm

2.4 mm

4.0 mm3.2 mm

Voxel size: 0.55x0.55x1.1 mm3

Spheres- Concentration:1.42 MBq / µl

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1.0 mm1.2 mm

4.0

mm

3.2 mm2.4 mm1.

6 m

m

Line profiles

Derenzo phantom simulation: GEANT4Simulation 1s, 500 MBq in the spheres

Background Rat phantom, 1.74 kBq / µl Crystals 2.0x2.0x12 mm3, pitch 2.2 mm (uniform)

Geometry 91 rings (uniform), 180 xtals / ring

Reconstruction OSMAPOSL (STIR)

1.6 mm

2.4 mm

4.0 mm3.2 mm

Spheres- Concentration:100 kBq / µl

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2) CRT setup: • LYSO:Ce crystals matrices with ESR foil

(Hilger), crystal size: 3.1x3.1x12 mm3

0.0

200.0

400.0

600.0

800.0

0.00 20.00 40.00 60.00 80.00

CRT

FWH

M in

ps

rate per channel in kHz

18F UniZ Hospital 22Na lab

0.00

20.00

40.00

60.00

80.00

0 100 200 300 400

rate

per

cha

nnel

in k

Hz

activity in MBq

TOFPET: System tests at Univ. of Zurich Hospital with FDG1) Setup: • Detector heads: MPPC array plus LYSO matrices• Hamamatsu MPPC array 4x4, TSV, pixel size

50x50 µm2, sensor size 3x3 mm2, pitch 3.2 mm• 1 head on one side on one ASIC, 1 head opposite• TOFPET single channel rate was limited to 90 kHz

in our DAQ configuration

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Converging on a single ring configuration:• LSO/LYSO crystals: 2.1x2.1x12mm3

• Hamamatsu 8x8 MPPC array, 2x2 mm2 sensors, 50x50 µm2 pixel, TSV• 24 detector heads• Readout ASIC – ToFPET or STiC

First ring system available mid of 2016:• Tests in the Bruker 70/30 MR system• Data acquisition and processing software• Reconstruction software including first 4D algorithm

In parallel to first ring system• Continue detector head performance optimization

• Geometry: spatial resolution• Sensor and crystal: timing resolution

• Optimize readout electronics system• Optimize reconstruction software

Full system 2017++• Test of the complete ring system and analysis of results

This work was supported by the ETH Zurich Foundation through ETH Research Grant ETH-30 14-2, and by an SSSTC Exchange Grant EG 02-03201.