WO2009141760A2 - Détecteur de photons à bloc convertisseur - Google Patents

Détecteur de photons à bloc convertisseur Download PDF

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Publication number
WO2009141760A2
WO2009141760A2 PCT/IB2009/051954 IB2009051954W WO2009141760A2 WO 2009141760 A2 WO2009141760 A2 WO 2009141760A2 IB 2009051954 W IB2009051954 W IB 2009051954W WO 2009141760 A2 WO2009141760 A2 WO 2009141760A2
Authority
WO
WIPO (PCT)
Prior art keywords
magnetic field
converter unit
photon detector
charges
photons
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/IB2009/051954
Other languages
English (en)
Other versions
WO2009141760A3 (fr
Inventor
Ewald R Roessl
Roland Proksa
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Philips Intellectual Property and Standards GmbH
Koninklijke Philips NV
Original Assignee
Philips Intellectual Property and Standards GmbH
Koninklijke Philips Electronics NV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Philips Intellectual Property and Standards GmbH, Koninklijke Philips Electronics NV filed Critical Philips Intellectual Property and Standards GmbH
Publication of WO2009141760A2 publication Critical patent/WO2009141760A2/fr
Publication of WO2009141760A3 publication Critical patent/WO2009141760A3/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/16—Measuring radiation intensity
    • G01T1/24—Measuring radiation intensity with semiconductor detectors

Definitions

  • a photon detector with converter unit A photon detector with converter unit
  • the invention relates to a photon detector comprising a converter unit for converting incident photons into electrical conduction charges. Moreover, it relates to a method for the detection of photons with such a converter unit and an imaging system comprising such a photon detector.
  • Photon detectors are for example required in X-ray devices like a CT (Computed Tomography) scanner for the detection of X-rays after their transmission through an object of interest.
  • CT Computer Tomography
  • solid-state detectors like the one that is known from the
  • a converter unit consisting of a semiconductor material is used for converting incident photons into a cloud of electrical conduction charges, i.e. electrons and holes that are (temporarily and with certain restrictions) free to move in the semiconductor material.
  • the charges can be detected by electrodes which generate an electrical field in the converter unit. Problems arise when the associated detection electronics has to deal with high photon fluxes as they are typical in for example CT applications. In this case it can be tried to make the pixel volumes associated to each electrode small in order to limit the counting rates said electrode has to deal with.
  • the charges will however move in a more or less extended cloud through the converter unit an therefore give rise to signals in several neighboring small electrodes that may be misinterpreted as being generated by more than one photon.
  • the photon detector according to the present invention serves for the detection of electromagnetic radiation, particularly of X-ray photons or ⁇ photons. It comprises the following components: a) A converter unit for converting incident photons into electrical conduction charges. A photon that interacts with the material of the converter unit will typically generate, depending on its energy, not just one but a more or less extended cloud of electrical charges, i.e.
  • An electrode arrangement for generating an electrical field in the converter unit when a voltage is applied to it. When for example a high potential is applied to a first electrode ("cathode”) and a lower potential to a second electrode (“anode”) of the arrangement, an electrical field will develop between the electrodes along which conduction electrons drift to the anode.
  • a magnetic field generator for generating a magnetic field inside the converter unit that can affect the movement of the conduction charges in said converter unit. Typically, said magnetic field will be static with respect to the timescale on which a movement of electrical conduction charges generated by incident photons to an associated electrode takes place.
  • the described photon detector allows to take influence on the movement of electrical conduction charges inside the converter unit by the magnetic field that is generated with the magnetic field generator. Thus it is possible to control and limit or even eliminate an undesired spread of said charges, which would normally impair the spatial resolution of the device and/or which might lead to a misinterpretation of charges arriving at the "wrong" electrodes.
  • the possibility to confine electrical charges to smaller movement corridors also allows the design of smaller pixels that can cope with higher photons fluxes.
  • the magnetic field generator can be built in many different ways, particularly as a permanent magnet and/or as an electromagnet.
  • the use of an electromagnet has the advantage that the magnetic field can be switched on and off and/or be adjusted to a desired magnitude if necessary.
  • the magnetic field generator is designed and disposed such that the magnetic field that is generated by it is directed along a desired movement path of the conduction charges in the converter unit. Due to the effect of the Lorentz force, moving charges will rotate around the field lines of the magnetic field in a distance that depends on their charge, mass, and velocity as well as on the field magnitude. The movement of the particles will therefore be restricted to a cylinder around the magnetic field lines.
  • the picture In a semi-conductor the picture is more complex as the charges are not free.
  • the band structure determines the allowed energy states and electron orbits (for conduction band electrons) in the presence of the magnetic field. Of importance is also the level of purity since the magnetic electron orbits will be followed only in between scattering events.
  • the magnetic field generator may further be designed and disposed such that the magnetic field that is generated by it inside the converter unit is substantially parallel to the electrical field that is generated by the electrode arrangement.
  • This design is akin to the aforementioned one, as the direction of the electrical field will usually correspond to the desired movement paths of the conduction charges. It should be noted that the orientations of the parallel magnetic and the electrical fields may be identical or opposite, i.e. the associated field vectors may point into the same or into opposite directions.
  • the magnetic field generator shall be designed such that it can generate a magnetic field which is strong enough to affect the movement of conduction charges in the converter unit.
  • the magnitude of the magnetic field will be larger than about 0.1 T (Tesla), preferably larger than about 1 T, depending on band-structure, impurity concentration, and electric field strength.
  • the movement of conduction charges can be restricted to reasonably small volumes.
  • the electrode arrangement comprises a plurality of single electrodes, particularly a plurality of anodes. Each of these single electrodes is then associated to a corresponding pixel- volume of the converter unit which is defined by the condition that it comprises all electrical field lines that end at the considered single (pixel-)electrode.
  • the photon detector comprises a detection device to which the electrodes of the electrode arrangement are connected, said detection device being adapted to detect free charges that have reached the electrodes. In this way it is possible to detect the conversion of a photon in the converter unit and to assign it to a particular (pixel-)electrode, i.e. to a position in the converter unit.
  • the aforementioned detection device is preferably further adapted to count and/or classify (with respect to their shape or magnitude) electrical pulses generated by electrical conduction charges that originate from the conversion of a photon.
  • Suitable materials for the converter unit are any materials that provide the desired conversion of incident photons into electrical conduction charges. They comprise for example Si, Ge, GaAs, HgI, CZT and/or CdTe.
  • the invention further relates to an X-ray detector comprising a photon detector of the kind described above that is sensitive to X-rays. Moreover, it relates to an imaging system comprising a photon detector of the kind described above, wherein said imaging device may particularly be an X-ray, CT (Computed Tomography), PET (Positron Emission Tomography), SPECT (Single Photon Emission Computed Tomography) or nuclear imaging device.
  • CT Computer Tomography
  • PET Positron Emission Tomography
  • SPECT Single Photon Emission Computed Tomography
  • nuclear imaging device may particularly be an X-ray, CT (Computed Tomography), PET (Positron Emission Tomography), SPECT (Single Photon Emission Computed Tomography) or nuclear imaging device.
  • the invention also relates to a method for the detection of photons, particularly of X-ray photons or ⁇ photons, comprising the following steps: a) Converting incident photons into electrical conduction charges in a converter unit. b) Generating an electrical field in said converter unit. c) Generating a magnetic field in said converter unit that affects the movement of the conduction charges.
  • the method comprises in general form the steps that can be executed with a photon detector of the kind described above. Therefore, reference is made to the preceding description for more information on the details, advantages and improvements of that method.
  • Fig. 1 illustrates an examination apparatus comprising a photon detector according to the present invention
  • Fig. 2 schematically shows a section through a photon detector according to the present invention
  • Fig. 3 illustrates the movement of a conduction electron in the converter unit of a photon detector according to the invention.
  • Fig. 1 shows schematically a side view of a Computed Tomography (CT) system 1 as one example of a device that requires the detection of photons.
  • the CT system 1 comprises an X-ray source 2 and a corresponding photon detector 100 that are arranged opposite to each other on a rotatable ring. Between the X-ray source 2 and the detector 100, a patient 3 can be located on a table such that the detector 100 will sense the flux of X-ray photons X transmitted through the patient.
  • projection images of the patient 3 can be generated from different directions, from which an associated control and evaluation unit 4 (e.g. a workstation) can reconstruct three-dimensional slice images.
  • an associated control and evaluation unit 4 e.g. a workstation
  • Fig. 2 schematically sketches a section through the photon detector 100 or, more precisely, through one super-pixel of the detector as the detector will usually comprise a two-dimensional array of several thousands of such super-pixels. For simplicity the following discussion will however identify the photon detector and the (single) super-pixel.
  • the detector 100 comprises as a central component a converter unit 10 consisting of a material like CZT which can convert incident X-ray photons X (impinging from the space above the drawing plane, i.e. along the z-axis) into a cloud 11 of electrical conduction charges, i.e. electrons and holes.
  • a converter unit 10 consisting of a material like CZT which can convert incident X-ray photons X (impinging from the space above the drawing plane, i.e. along the z-axis) into a cloud 11 of electrical conduction charges, i.e. electrons and holes.
  • electrode arrangements 21 and 22 are disposed, wherein each single electrode of these arrangements is individually connected to a detection device 40.
  • the first electrode arrangement 21 is constituted by a large, single electrode that is operated as a cathode
  • the second electrode arrangement 22 is constituted by a plurality of single, isolated electrodes 22a that are operated as anodes.
  • an electrical field E is created inside the converter unit 10 which drives the conduction charges in x-direction towards the electrodes, for example the electrons to the single electrodes 22a that are operated as anodes.
  • the area of the converter unit 10 is thus effectively divided into stripes or "pixel-areas" defined by the sub-volumes from which electrons are (ideally) collected at an associated single "pixel-electrode" 22a.
  • One possible way to reduce charge sharing without simultaneously reducing the DQE of the detection system is to apply the electrical bias field in a direction perpendicular to the incoming photons (in Fig. 2 realized by an E-field in x-direction with the photons impinging from the z-direction).
  • the drift length i.e. the distance to the next electrode
  • the spread of the charge cloud during its collection at the electrodes.
  • a magnetic field in the direction of the charge carrier motion i.e. usually the direction of the electric field.
  • a magnetic field will force the electrons to move on spirals in addition to their movement towards the anodes.
  • any transverse motion can be suppressed at high enough field strengths and with it the transverse spread of the charge cloud upon arrival at the electrode.
  • the aforementioned magnetic field B is generated inside the converter unit 10 by a permanent magnet 30 the north pole of which is disposed next to the cathode 21 while its south pole is located next to the anode(s) 22a.
  • the magnetic field B (parallel to the electric field) forces the charges to spiral around the direction of the fields on the surface of a cylinder (for homogenous fields).
  • the relation between the Larmor radius r of the transverse motion is related to the mass m of the electron, its charge e, its velocity perpendicular to the B-field v, and the magnetic field B via the formula:
  • the above picture is inadequate due to collisions and due to different effective masses of the electrons in a solid.
  • the charge collection time tc is not too large compared to the mean time interval between two collisions of the electrons, the effect of drift suppression is however noticeable.
  • the impurity scattering will be strongly reduced and the effect of the magnetic field on the particle orbits will be important.
  • Fig. 3 shows a possible electron orbit inside the active volume of a semiconductor crystal (converter unit).
  • the invention relates to a detector 100, particularly for X-ray photons, that comprises a converter unit 10 in which incident photons are converted into electrical conduction charges and electrodes on opposite sides of the converter unit that generate an electrical field and at which said charges are collected. Furthermore, it comprises a magnetic field generator for generating a magnetic field inside the converter unit that affects the movement of the electrical conduction charges, particularly by restricting possible drifts of the charges in directions perpendicular to the electrical field.
  • the invention can be used in any direct conversion X-ray detector system in which signals are generated through the collection of charges produced by the incoming X-rays. The only prerequisite is that providing the magnetic field is compatible with the performance of the detector from both a geometrical and operational point of view.

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  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • High Energy & Nuclear Physics (AREA)
  • Molecular Biology (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Measurement Of Radiation (AREA)
  • Light Receiving Elements (AREA)

Abstract

L'invention porte sur un détecteur (100), en particulier pour des photons de rayons X, qui comprend un bloc convertisseur (10) dans lequel des photons incidents (X) sont convertis en charges de conduction électrique (11) et des électrodes (21, 22) qui génèrent un champ électrique et au niveau desquelles lesdites charges (11) sont collectées. En outre, le détecteur comprend un générateur de champ magnétique (30), pour générer un champ magnétique (B) à l'intérieur du bloc convertisseur (10), qui modifie le mouvement des charges de conduction électrique (11), en particulier par restriction de dérives possibles des charges dans des directions perpendiculaires au champ électrique (E).
PCT/IB2009/051954 2008-05-19 2009-05-12 Détecteur de photons à bloc convertisseur Ceased WO2009141760A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP08156428 2008-05-19
EP08156428.8 2008-05-19

Publications (2)

Publication Number Publication Date
WO2009141760A2 true WO2009141760A2 (fr) 2009-11-26
WO2009141760A3 WO2009141760A3 (fr) 2010-07-29

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PCT/IB2009/051954 Ceased WO2009141760A2 (fr) 2008-05-19 2009-05-12 Détecteur de photons à bloc convertisseur

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WO (1) WO2009141760A2 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9301378B2 (en) 2011-10-19 2016-03-29 Koninklijke Philips N.V. Photon counting detector

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH0483380A (ja) * 1990-07-26 1992-03-17 Jeol Ltd 半導体ｘ線検出器
WO1994017428A1 (fr) * 1993-01-26 1994-08-04 Zlatimir Dimcovski Procede et dispositif d'imagerie de tissus biologiques utilisant des detecteurs a l'etat solide avec microelectrodes
US6201257B1 (en) * 1996-10-10 2001-03-13 Advanced Scientific Concepts, Inc. Semiconductor X-ray photocathodes devices

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9301378B2 (en) 2011-10-19 2016-03-29 Koninklijke Philips N.V. Photon counting detector

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WO2009141760A3 (fr) 2010-07-29

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