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Counting and integrating microelectronics development for direct conversion x-ray imaging [Elektronische Ressource] / von Edgar Kraft. Universität Bonn, Physikalisches Institut

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149 pages
..UNIVERSITAT BONNPhysikalisches InstitutCounting and Integrating Microelectronics Development forDirect Conversion X-ray ImagingvonEdgar KraftA novel signal processing concept for X-ray imaging with directly con-verting pixelated semiconductor sensors is presented. The novelty of thisapproach compared to existing concepts is the combination of chargeintegration and photon counting in every single pixel. Simultaneous ope-ration of both signal processing chains extends the dynamic range beyondthe limits of the individual schemes and allows determination of the meanphoton energy. Medical applications such as X-ray computed tomographycan benefit from this additional spectral information through improvedcontrast and the ability to determine the hardening of the tube spec-trum due to attenuation by the scanned object. A prototype chip in0.35-micrometer technology has been successfully tested. The pixel elec-tronics are designed using a low-swing differential current mode logic.Key element is a configurable feedback circuit for the charge sensitiveamplifier which provides continuous reset, leakage current compensationand replicates the input signal for the integrator. The electronic characte-rization of a second generation prototype chip is described and a detaileddiscussion of the circuit design is given.
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UNIVERSITAT BONN
Physikalisches Institut
Counting and Integrating Microelectronics Development for
Direct Conversion X-ray Imaging
von
Edgar Kraft
A novel signal processing concept for X-ray imaging with directly con-
verting pixelated semiconductor sensors is presented. The novelty of this
approach compared to existing concepts is the combination of charge
integration and photon counting in every single pixel. Simultaneous ope-
ration of both signal processing chains extends the dynamic range beyond
the limits of the individual schemes and allows determination of the mean
photon energy. Medical applications such as X-ray computed tomography
can benefit from this additional spectral information through improved
contrast and the ability to determine the hardening of the tube spec-
trum due to attenuation by the scanned object. A prototype chip in
0.35-micrometer technology has been successfully tested. The pixel elec-
tronics are designed using a low-swing differential current mode logic.
Key element is a configurable feedback circuit for the charge sensitive
amplifier which provides continuous reset, leakage current compensation
and replicates the input signal for the integrator. The electronic characte-
rization of a second generation prototype chip is described and a detailed
discussion of the circuit design is given.
Post address: BONN-IR-2008-01
Nussallee 12 Bonn University
53115 Bonn February 2008
Germany ISSN-0172-8741Counting and Integrating
Microelectronics Development for
Direct Conversion X-ray Imaging
Dissertation
zur
Erlangung des Doktorgrades (Dr. rer. nat.)
der
Mathematisch-Naturwissenschaftlichen Fakult at
der
Rheinischen Friedrich-Wilhelms-Universit at Bonn
vorgelegt von
Edgar Kraft
aus
Koblenz
Bonn 2007Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen
Fakult at der Rheinischen Friedrich-Wilhelms-Universit at Bonn
Referent: Prof. Dr. N. Wermes
Korreferent: Prof. Dr. I. Brock
Tag der Promotion: 18.10.2007
Erscheinungsjahr: 2008
Diese Dissertation ist auf dem Hochschulschriftenserver der ULB Bonn
http://hss.ulb.uni-bonn.de/diss online elektronisch publiziert.Abstract
A novel signal processing concept for X-ray imaging with directly converting
pixelated semiconductor sensors is presented. The novelty of this approach
compared to existing concepts is the combination of charge integration
and photon counting in every single pixel. Simultaneous operation of both
signal processing chains extends the dynamic range beyond the limits of the
individual schemes and allows determination of the mean photon energy.
Medical applications such as X-ray computed tomography can bene t from
this additional spectral information through improved contrast and the ability
to determine the hardening of the tube spectrum due to attenuation by the
scanned object. A prototype chip in 0.35-micrometer technology has been
successfully tested. The pixel electronics are designed using a low-swing
di erential current mode logic. Key element is a con gurable feedback circuit
for the charge sensitive ampli er which provides continuous reset, leakage
current compensation and replicates the input signal for the integrator. The
thesis focusses on the electronic characterization of a second generation
prototype chip and gives a detailed discussion of the circuit design.Contents
1. Introduction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 1
2. Fundamentals : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 5
2.1 Photo E ect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Compton E ect . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Absorption Coe cient . . . . . . . . . . . . . . . . . . . . . . 7
3. X-ray Imaging : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 9
3.1 Photographic Film . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 Photostimulable Phosphor Plate . . . . . . . . . . . . . . . . 10
3.3 Image Intensi cation . . . . . . . . . . . . . . . . . . . . . . . 12
3.4 Flat Panel Detectors . . . . . . . . . . . . . . . . . . . . . . . 14
3.5 Direct Conversion Hybrid Pixel Detectors . . . . . . . . . . . 16
3.5.1 Integrating Pixel Detectors . . . . . . . . . . . . . . . 17
3.5.2 Photon Counting Pixel Detectors . . . . . . . . . . . . 18
3.6 Computed Tomography . . . . . . . . . . . . . . . . . . . . . 20
4. Counting and Integrating Readout Concept : : : : : : : : : : : : 23
4.1 Prototype ASICs . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2 Photon Counter . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.3 Integrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3.1 Integrator Charge Pumps . . . . . . . . . . . . . . . . 35
4.3.2 In Logic . . . . . . . . . . . . . . . . . . . . . 37
4.4 Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.4.1 Feedback and Signal Duplication . . . . . . . . . . . . 38
4.4.2 Static Leakage Current Compensation . . . . . . . . . 39
4.4.3 Sampling . . . . . . . . . . . . . . . . . . . . . . . . . 40
4.4.4 Continuous Leakage Current Compensation . . . . . . 41
4.4.5 Controlled Redirection . . . . . . . . . . . . . . . . . . 43
4.4.6 Comparison of the Feedback Modes . . . . . . . . . . 46
4.4.7 Integrator O set Correction . . . . . . . . . . . . . . . 47
4.5 Charge Injection and Signal Generation . . . . . . . . . . . . 48
4.6 Di erential Current Steering Logic . . . . . . . . . . . . . . . 50
4.7 Digital Readout Scheme . . . . . . . . . . . . . . . . . . . . . 53
4.8 Data Acquisition and Analysis . . . . . . . . . . . . . . . . . 550
5. Experimental Results : : : : : : : : : : : : : : : : : : : : : : : : : 59
5.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Photon Counter . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2.1 Threshold Dispersion and Tuning . . . . . . . . . . . . 64
5.2.2 Dynamic Range . . . . . . . . . . . . . . . . . . . . . . 68
5.2.3 Electronic noise . . . . . . . . . . . . . . . . . . . . . . 70
5.2.4 Noise Count Rate . . . . . . . . . . . . . . . . . . . . 70
5.2.5 Charge Injection . . . . . . . . . . . . . . . . . . . . . 74
5.2.6 Ballistic De cit . . . . . . . . . . . . . . . . . . . . . . 75
5.2.7 Double Pulse Resolution . . . . . . . . . . . . . . . . . 76
5.2.8 Poisson-distributed Pulse Spacings . . . . . . . . . . . 77
5.2.9 Measurements with Poisson-distributed Pulses . . . . 79
5.2.10 Photon Counter Breakdown Behavior . . . . . . . . . 81
5.3 Integrator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.3.1 Dynamic Range . . . . . . . . . . . . . . . . . . . . . . 83
5.3.2 Noise Performance . . . . . . . . . . . . . . . . . . . . 84
5.4 Feedback Circuit . . . . . . . . . . . . . . . . . . . . . . . . . 90
5.4.1 Signal Reproduction . . . . . . . . . . . . . . . . . . . 90
5.4.2 Feedback Noise Performance (Continuous Currents) . 94
5.4.3 Leakage Compensation . . . . . . . . . . . . . . . . . . 96
5.5 Simultaneous Photon Counting and Integration . . . . . . . . 101
5.5.1 Observability of Fluctuations in the Photon Flux . . . 101
5.5.2 Dynamic Range and Energy Resolution . . . . . . . . 102
5.5.3 Spectral Hardening . . . . . . . . . . . . . . . . . . . . 107
5.6 Digital Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . 111
5.6.1 Power Consumption . . . . . . . . . . . . . . . . . . . 111
5.6.2 Propagation Delay and Power-Delay-Product . . . . . 111
5.6.3 Power Optimization . . . . . . . . . . . . . . . . . . . 113
5.6.4 Dead-time Free Readout . . . . . . . . . . . . . . . . . 117
5.7 Summary of the Experimental Results . . . . . . . . . . . . . 119
6. Conclusions and Outlook : : : : : : : : : : : : : : : : : : : : : : : 123
Bibliography : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 129
Appendix 131
A. Detailed Implementation Descriptions : : : : : : : : : : : : : : : 133
A.1 Three-Transistor Charge Pump Type . . . . . . . . . . . . . . 133
A.2 Implementation of the Readout Scheme . . . . . . . . . . . . 136
A.2.1 Bus Receiver . . . . . . . . . . . . . . . . . . . . . . . 138
A.2.2 Address Sequencer . . . . . . . . . . . . . . . . . . . . 139
A.3 Counter Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . 1401. Introduction
This thesis discusses the implementation and characterization of a new signal
processing concept for semiconductor X-ray sensors for the use in computed
tomography (CT) and medical imaging in general. The research was con-
ducted as part of an activity which is pursued jointly by the Universities of
Bonn and Mannheim and the Philips Research Laboratories Aachen.
The principal idea of the new signal processing concept is to include single
photon counting and charge integrating signal processing channels into every
picture element (pixel) of the detector system. If the circuit is designed in
such a way that both channels can operate simultaneously on the same input
signal, the dynamic range of the system can be extended beyond the limits
of the individual channels. Furthermore, additional spectral information is
obtained in the region where the dynamic ranges of both channels overlap.
This essentially adds a new dimension to the acquired data by not only
measuring the signal intensity but also the average energy of the photons
constituting the signal. The additional information is obtained without the
need to increase the radiation dose.
These properties of the new signal processing concept should allow to build
1X-ray detector systems which deliver ‘colored’ X-ray images with a high
contrast, high dynamic range and an electronic noise performance which is
better than the expected quantum noise due to photon number uctuations
in the X-ray signal. The intended application in a CT system imposes two
additional requirements: a high image acquisition rate of several kHz and
the possibility of a dead-time free operation.
The signal processing chip itself does not actually detect X-ray photons,
it analyzes the signal created in a pixelated sensor whose electrodes are
connected to the inputs of the picture elements on the chip. This is the
reason why the chip is often referred to as a readout chip. In the course of this
thesis, two generations of prototype chips were developed and characterized.
The main focus of this thesis is the discussion of the results obtained with
the second prototype chip. Even though the prototype chip features input
pads for the connection to a sensor, the measurements discussed here will
only deal with the electronic characterization of the chip with test signals
produced by circuits on the chip. The evaluation of the imaging performance
with actual X-ray signals and di erent sensor materials lies beyond the scope
1
The term ‘colored’ refers to the obtained spectral information.2 1. Introduction
2of this work and will be covered elsewhere . The extensive set of test circuits
on the prototype chip, however, allows an exhaustive investigation of all
aspects of the prototype and the signal processing concept itself.
The following discussion is structured as follows:
2. Fundamentals This chapter brie y discusses the basic physical processes
underlying medical X-ray imaging.
3. X-ray Imaging reviews the technologies underlying the detector systems
commonly used in medical imaging, starting with conventional lm-
based systems and covering computed radiography, digital radiography
and uoroscopy systems up to the direct-converting hybrid pixel systems
which are currently under development.
4. Counting and Integrating Readout Concept contains the detailed ex-
planation of the concepts and circuits involved in the implementation
of the new signal processing scheme. It starts with a brief explanation
of the motivation for the new concept and gives an introduction to the
prototype chips involved. The subsequent sections explain the photon
counter and the integrator, followed by a detailed discussion of the
feedback circuit, which is the central (and arguably most complicated)
element of the signal processing concept. Sections reviewing the charge
injection circuits, the digital logic and the data acquisition system
conclude the chapter. The research of the underlying transistor-level
concepts necessary for the implementation of the signal processing
scheme was conducted as part of the doctoral thesis of I. Peric and is
documented in [1].
5. Experimental Results This chapter provides experimental evidence for
the claims made in this introduction. It contains measurements inves-
tigating the performance of photon counter and integrator separately,
followed by a characterization of the feedback circuit and the behavior
in simultaneous counting and integration mode. After a discussion
of the power consumption and a demonstration of the dead-time free
image acquisition mode, the section nishes with a summary of the
obtained results.
6. Conclusions and Outlook summarizes the ndings of this thesis and
gives an overview over potential improvements to the prototype.
Appendix The appendix contains descriptions covering some subcircuits
in more technical detail. They are provided rather for matters of
completeness and are not fundamentally relevant for the understanding
of the signal processing concept.
2
At the time of writing, measurements on such an X-ray imaging system are being
carried out by Johannes Fink, a colleague of the author, as part of his doctoral thesis.