16. January 2007Status Report On Compton Imaging Projects 1 Status Of Compton Imaging Projects Carried Out In The CIMA Collaboration HPD Brain PET Meeting

16. January 2007 Status Report On Compton Imaging Projects

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Status Of Compton Imaging Projects Carried Out In The CIMA Collaboration

HPD Brain PET Meeting Bari

16. 01. 2007

P. Weilhammer

INFN Perugia/CERN


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1. Photon Detection with Silicon Detectors (A few trivial notions)

2. Two Projects Involving the Compton Camera Concept Using Silicon Radiation Detectors

OUTLINE of Presentation


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Detection of Photons and Energetic Electrons in Semiconductor Detectors

Medical imaging requires good ability of detection of photons, in reality detection of energetic electrons created inside the material ( an advantage!), over a wide range of energies.

Energy Ranges:

•Single Photon Emission Tomography (SPECT): detect -rays for a big variety of isotopes used in different tracer molecules, e.g.

•99mTc 140 keV

•111In 185 and 245 keV

•31I 360 keV

Positron Emission Tomography (PET): 511 keV or ~50 keV to 350 keV Compton recoil electrons


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Photon Interactions in Silicon

Only two out of all photon interactions are important for medical imaging:

In traditional imaging applications the “wanted” one: Photoelectric Absorption (total absorption of or X-ray)

= 4√2 4 5Th

with the Thomson cross-section Th.= 8/3 r20 = 6.652 bars per electron.


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The “unwanted” one: Compton scattering

)cos1(12

cm

E

EE

e

)cos1(1

11

2

cm

EEE

e

e

The recoil electron ( from K-shell or L-shell or valence band) creates (eh) pairs in the semiconductor bulk through ionization

Kinetic energy of recoil electron


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Attenuation of incoming photons in material

In 1mm thick silicon for 20 keV photons

Photoelectric interaction: ~ 97%

Compton interactions: ~3%

Interactions/m for Si versus photon energy

Interesting region for medical imaging


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Range of Electrons in Materials

The range of electrons in materials expressed as range * density is very similar for many different materials

Typical Range of Compton recoils:

50 keV electron in silicon: ~20 m

200 keV : ~200 m

500 keV :~ 600 m

For Compton interaction the medical imaging “point-like” domain is between 10 keV and 250 to 300 keV!

10-2

Range*density [g/cm2]

100 keV

Si

NaI


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Some inherent physical limitations in different imaging modalities with scintillators are:

Spatial extension of the photon interaction in the detector material due to the nature of photon interactions (in most materials interaction cascades are frequent before final absorption). The typical extension of a photon interaction in many detector materials ( at 500 keV) can be considered to be confined in a sphere of ~1 cm in diameter.

Depth of Interaction Parallax error

Finite path length of positrons and recoil electrons

Compton scattering in tissue.

In PET: Finite momentum of e+e- compound at the moment of decay Acolinearity

Accidental coincidences.

………

~1cm through multiple

interactions in scintillator

Incoming


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An Observation:

What about Silicon Radiation Sensors which are NOW part of Scanners or Cameras used in Hospitals

Photo-diode arrays in present day X-Ray CT (mostly Hamamatsu )

Low Dose digital Mammography Scanner from SECTRA (single sided strip detectors and VLSI Front-end) See www.sectra.com

Autoradiography Camera from BIOMOLEX (double sided strips and VLSI Front-end) See www.biomolex.no

And some more……… but it is a niche market except fror photo-diodes

http://www.biomolex.no/


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R&D Projects using Silicon Detectors in Medical Imaging within the CIMA

Collaboration

•Novel axial brain PET Scanner

•Compton Camera and Probes

•High resolution small animal PET scanner based on Compton interactions


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Compton Imaging


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The main features of Compton Imaging are:The Mechanical collimator in the Anger Camera is replaced by “Electronic

Collimation”. This removes the coupling between sensitivity and spatial resolution.

This is achieved by having two detectors in coincidence:In the first detector the gamma rays are scattered by Compton Scattering

on electrons in the detector materialIn the second detector the scattered gamma ray is absorbed


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The measured quantities in Compton imaging are:x, y, z-co-ordinates in the first detector

x, y, z-co-ordinates in the second detectorEnergy of recoil electron in first detector

Energy of scattered photon in second detector

Not measurable with Compton Cameras for medical applications: Direction of recoil electron, which leads to the conical ambiguity. This

leads to more complicated image reconstruction algorithms.

Expected improvements over Anger Camera:

•Factor ~5 in spatial resolution for probes

•Factor 5 to 50 improvement in sensitivity


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Results from a Demonstrator Test in 2005


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10 cm

All possible solutions need to be cheap and standard technology readily available in Industry. Modifications to a technology need to be available in the standard industrial processes

The Silicon Pad Sensors

Schematic cross section of double metal pad sensor

A processed wafer 1mm thick

Details of routing technology on pads via double metal vias

Routing lines end at external bond pad rows for connection to readout chip


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Silicon detector and stack of 5 detectors


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A Demonstrator set-up with stack of 5 Silicon pad sensor and 3 camera heads


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Main Results

Spatial resolution was measured for 4 energies; 57Co (122 keV) and

133Ba (272,302 and 356 keV). For the highest energy with a source-first detector distance of 11.3 cm: 5mm FWHM

With a source Si distance of 3 cm this gives (simulation) 2 -3 mm FWHM


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Status:

Spatial resolution in Silicon Demonstrated

Next Demonstrator test foreseen before end of 2006 with much improved camera head and improved silicon ( lower thresholds possible)


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A High Resolution Small Animal PET Scanner based on Compton Scatter Events in Silicon Pad

Detectors

Harris Kagan Imaging 2006, June 26-30, Stockholm

Resolution Limitations for Conventional PET

Scintillator

~ 1 cm

Inter-Crystal Scattering Depth of Interaction Uncertainty

• Multiple Interactions• Energy deposited over a volume• ~ 1 cm mean path

• Penetration into crystals widens LOR

Best Resolution ~ 1.5-2 mm


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BGOdetector

Sidetector

Si-Si

BGO-BGO

Si-BGO

Si-Si : Very High Resolution

Si-BGO : High Resolution

BGO-BGO : Conventional PET Resolution

Three Major Coincidence Events

A Very High Resolution PET Scanner for small animals based on Compton Scattering

events is proposed:

The Concept


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Detection Efficiency (%)

Radial Posn. (mm)

Single – Single Single – BGO BGO - BGO

0 1.05 8.83 20.84

6 0.96 8.96 20.69

12 1.04 8.94 19.70

18 1.19 9.06 18.17

Calculated for point source in center plane. Only single scattering or absorption interactions in the silicon detector are included. Back scattered photons from BGO and events without full energy deposition are excluded.

Simulation results with this configuration

Efficiency for different event classesBGO ring


Compton PET Test Bench

Silicon detector BGO detector

4.5 cm 2.2 cm and 1 mm thick 3216 (512) pads, 1.4 mm 1.4 mm pixel sizeEnergy Resolution 1.39 keV FWHM for Tc 99m

5.3 cm 5 cm and 3 cm thick 84 array, 12.5 mm 5.25 mm crystal sizeEnergy Resolution 22% FWHM for Na-22

HAMAMATSU PMT R2497

VATAGP3


Prototype PET Instrument

Single-slice instrument using silicon and BGO

Disassembled Assembled

Silicon detector Silicon detector


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Resolution Uniformity

0 1 2 3 4 5 cm

5

4

3

2

1

0

Source pairs at 5, 10, 15, & 20mm off-axis

Sinogram

The sources in each pair are clearly separated at appropriate sinogram angles


Compton PET: Intrinsic Resolution

Needle 25G (ID = 0.254 mm, OD = 0.5mm, SS_steel wall = 0.127 mm)

5

4

3

2

1

0

0 1 2 3 4 5 cm

0.254 mm

0.127 mm

Image Resolution= 700 m FWHM

SS_steel wall

F-18

5

4

3

2

1

00 1 2 3 4 5 cm


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Run PET in strong magnetic field (Raylman,Hammer,…)

•Positrons spiral transverse to magnetic B-Field vector

• Potentially useful for emitters with higher e+ energy than 128F like 124I, 94mTc,..

Measurement was done recently in a 9 Tesla magnet at OSU with one of the modules. The performance of the detector did not change.

BUT: the bond wires need to be fixed with epoxy!


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This work goes on

For the PET project we got NIH funding and will go for FP7 European funduing

For the ”PET in magnetic field” study a funding proposal has been submitted to NIH.

For Compton PET project a funding request is prepared to develop a VATA”Compton” chip together with GM-I

Documents

16. January 2007Status Report On Compton Imaging Projects 1 Status Of Compton Imaging Projects Carried Out In The CIMA Collaboration HPD Brain PET Meeting