Spectral x-ray photon counting

ASIC Development for Energy Resolved
Spectral Counting and Imaging of X-rays
S. Azman, W. Dang, C. Gheorghe, M. Gjetanger, T.M. Johansen,
D. Meier, S. Mikkelsen, G. Mæhlum, J. Talebi, T. Ødegaard

Gamma Medica – Ideas (Norway) AS

CERN, 4.-6. April 2011

dirk.meier@gm-ideas.com 2011-April, CERN, SpecX-Ray Workshop 1

ASIC Designs Overview


Radiation Detector Design and Development for Science

Science: e.g. space, physics
Requirements,
Preliminary
JAXA, BepiColombo Specifications ASIC wafer

Design
Simulation

Implementation,
Rule checks, simulation
ATHENA Manufacture, Foundry
Radiation sensor
Assembly

Integration

Tests,
Verification and
Validation


Abstract – ASIC for Spectral X-ray Photon Counting

We describe an integrated circuit (IC) for spectral counting and imaging of
polychromatic x-rays.

The circuit has been designed for the readout of cadmium telluride (CdTe) radiation
sensors with up to 64 pixel electrodes. Each readout channel contains a charge
sensitive amplifier, up to 6 discriminators and 6 counters which increment when the
amplitude exceeds a given threshold. The energy dynamic range is about 120 keV.
Subsequently we designed and assembled camera prototypes from 1D-linear arrays of
CdTe read out by 32 ASICs. We have tested the cameras with imaging phantoms and
polychromatic x-rays in a micro-computed tomography (microCT) setup. We studied
the use of the cameras for material separation, especially with gadolinium and iodine
based contrast agents, for medical imaging. The presentation will focus on the ASIC
design specifications which were originally set for industrial scanning, but also discuss
the requirements for x-ray microCT and medical imaging.


Spectral X-ray Photon Counting - Principle


ASIC Channel Architecture – 1 out of 32/64

Mikkelsen et al. (2008), Proc. IEEE
NSS


ASIC Specifications


ASIC Floorplan, Padframe, Block Diagram


ASIC Block Diagram


Spectral X-ray Photon Counting – Applications

Security
e.g. explosives
Food safety
e.g. foreign objects
Separtion & sorting
e.g. metals, waste....
NDT
Medical
e.g. contrast agents
Physics research


Spectral X-ray Counting Detector

X-ray Line Camera

Modules: CdTe + ASICs

Wang et al. (2009), Proc. IEEE NSS


Radiation Sensor - CdTe Crystal with Electrodes

-500V
3 mm

0.4 mm Anode Pitch


Spectral X-ray Counting Detector - Specifications


Photographs of X-ray Line Detector


X-Ray Line Camera - Hardware Details

 External COUNT signal
synchronized with conveyor belt
or CT system
 1500 frames/seconds maximum
(0.67 ms frames)
 Short dead time between frames
(40us)
 Interface to PC through National
Instruments I/O card


Example Material Identification – Luggage Phantom

Grey scale image based on Color coded based on the ratio of
attenuation at one energy attenuation at two energies, [80,100]
keV and [35,50] keV


Food Safety - Pieces of Glass in Raisins

Grey scale image based on Dual-energy
attenuation at one energy information, enhanced to ’see’
only glass and eliminate raisins


Material Separation – Hard Disk Drive

Grey scale image based on Regions color coded based on
attenuation at one energy attenuation ratio between high
and low energy


Count Rate and Spectra

Mono-energetic x-rays

Customer data


Spectral Distortion in Photon Counting Detector

Comparison between
• GM-I Spectral Photon
Counting Detector
• AMTEK CdTe
Spectrometer

Effects:
• Energy resolution
• Pulse pileup
• Characteristic x-ray
• Charge sharing


Charge Sharing

Customer data
80 keV
Summed Count rate Summed energy


Spectra with Gd Contrast Agent

2.8 Water
PMMA
POM
2.4 PTFE
Cortical bone
Iodine solution
2 Gd solution

1.6

1.2

0.8

0.4 Wang, 2011
30 40 50 60 70 80 90 100
Energy (keV)

Energy-dependent x-ray attenuation


CT Phantom Images and Energy Thresholds Trimming


Summary

We have built a X-ray Line Camera capable of Spectral X-ray Photon
Counting. The design is based on CdTe custom made ASICs.

The prototype cameras allow one to study the potentials
of spectral x-ray photon counting.

It is important to understand and improve:

• Spectral response vs. count rate
– Charge sharing in CdTe
– Rate capability of CdTe pixels and ASIC, especially for CT
• Threshold uniformity and trimming


References

[1] S. Mikkelsen et al. (2008), ”An ASIC for Multi-Energy X-ray Counting”, Proc. IEEE NSS 2008.

[2] X. Wang et al. (2009), ”A Digital Line-Camera for Energy Resolved X-ray Photon Counting”,
Proc. IEEE MIC 2009.

[3] X. Wang et al. (2011), “Material Separation in X-ray CT with Energy Resolved Photon
Counting Detectors", Medical Physics (0094-2405), 3/1/2011. Vol.38,Iss.3;p.1534-1546.

[4] X.Wang et al. (2011), “MicroCT with energy-resolved photon-counting detectors2011“, Phys.
Med. Biol. 56 2791–2816.


Acknowledgments

We like to thank our customers and collaborators.

and

X. Wang, E. Frey, K. Taguchi, B.M.W. Tsui at Johns Hopkins
University

D. Wagenaar at Gamma Medica, Northridge

B. Sundal at Corentium, Norway

Thank you


Spectral x-ray photon counting

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