WO2009058092A1

WO2009058092A1 - X-ray detector

Info

Publication number: WO2009058092A1
Application number: PCT/SE2008/051256
Authority: WO
Inventors: Magnus Åslund
Original assignee: Sectra Mamea Ab
Priority date: 2007-11-01
Filing date: 2008-11-03
Publication date: 2009-05-07

Abstract

The present invention relates to an area detector (30) for photon counting image acquisition, comprising at least two edge-on line detectors for converting photons to electrical pulses. Each line detector is an array of sensors (31) and the line detectors are stacked closely next to each other.

Description

X-RAY DETECTOR

TECHNICAL FIELD

The invention relates to photon counting X-ray imaging detectors.

BACKGROUND

Photon counting X-ray imaging has a number of advantages such as complete rejection of all electronic noise and maximum use of the information in each X-ray. This was fully realized during the last decade and currently a number of the large companies currently dominating the global medical imaging market are officially pointing to photon counting as their future technology of choice for Computer Tomography (CT).

The clinical benefits of the photon counting technique in terms of reduced radiation dose and/or increased image quality are well documented. US patent No. 7,342,233, for example, teaches how photon counting enables measurement of photon energies and how to obtain a spectral view of the imaged object, and the clinical usefulness is under research.

The major strategy is to use exotic detector materials such as CdZnTe mainly motivated by the high atomic number (Z) and correspondingly high X-ray absorption per unit length. It is hard to estimate when we will see clinical non-research installations based on these technologies but an educated guess is that it will take several years.

On the contrary the photon counting technology is based on silicon sensors and high detection efficiency is enabled by edge-on (e.g. US patents No. 4,937,453, 7,212,605, and 5,434,417) detector geometries where the depth of material seen by the incident X- rays is made thick enough (~5mm) to absorb more than 90% of the X-rays for mammography imaging. For higher energies, the efficiency will drop due to increasing Compton scatter and reduced photo effect in the interaction of the X-rays with the silicon. For mammography energies the photo electric effect is dominating while at typical average X-ray energies for computed tomography (CT) at 57 keV the photo-electric effect is 50%. To detect the Compton scattered X-rays will be more challenging. In spite of a possible drop in efficiency we believe that this is well compensated by the advantages with a proven detector material such as silicon.

Typically, photon counting is implemented using a coupling material for converting a photon to an electrical pulse, a circuit for detecting the electrical pulse, and at least one or digital register (counter) that is changed when a photon is detected. In the simplest setups, the change is an increment of a single step. There are, however, more sophisticated photon counters, wherein the number of steps depends on pulse strength, which in turn depends on photon energy.

Bjorn Cederstrδm et al, "High-resolution X-ray imaging using the signal time-dependence on a double-sided silicon detector" (1998, Nuclear Instruments & Methods in Physics Research) teaches how a photon counting detector can be extended with a time- difference measurement, which provides a depth resolution in the silicon sensor.

Figs.1 and 2 show multi-slit X-ray scanner for acquisition of conventional 2-dimensional projection images for digital mammography. The patient is irradiated by a bundle of thin, X-ray beams, each of which is detected by a corresponding line detector. The narrow beams are created by letting the X-rays pass through a collimator 120, which is a metal plate with several narrow linear apertures, referred to as slits. For each slit, there is a corresponding line detector, which in turn is a silicon array of pixel detectors. The line detectors are arranged to scan virtually the same area of the patient, yielding redundant information and enabling noise reduction. In Fig.1 , the line detectors are mounted in a detector assembly 150. The multi-slit collimator is elevated away towards the X-ray source, hence the operator (nurse) can see and touch from a larger range of directions. Fig.4 illustrates a line detector 31 according to prior art. There is an array of electrodes 45, which defines the division into pixel sensors, also known as strips. The line detector is made from a coupling material, e.g. silicon, wherein photons are converted to electrical pulses. The pulses are fed through said electrodes 45 to electronic circuits for detecting the pulses. This type of line detector can be arranged to either receive photons edge-on (e.g. according to US 4,937,453), or slanted edge. In case of edge-on, photons enter through the edge surface 45. Unfortunately, due to manufacturing, the edge surface is covered by an inactive section, wherein many photons are absorbed without being converted to pulses. The inactive section is caused by cutting in manufacture, wherein the silicon within a distance from the cut is harmed with respect to its semiconductor properties.

In case of slanted edge, the photons enter, in an oblique angle through the flat side below the inactive section layer. The slanted edge is not suitable for detectors.

SHORT DESCRIPTION

Thus, the present invention tries to solve one or several problems with occurring with the prior art.

The advantages of the invention include one or several of:

Improved Spatial Resolution - High spatial resolution is important in mammography for seeing micro-calcifications which can be an early sign of cancer. It is also important when diagnosing bone fractures and in other procedures. The current scanned-slit geometry for mammography has a very high resolution in the dimension along the detector (50 urn) but in the other dimension it is limited by the slit width. If the slit is narrowed, the resolution increases but the number of X-rays passing the slit will be reduced and this is not acceptable since the statistical noise in the image would increase. If a system could be developed that could measure the position of the X-ray interaction in the slit direction the slit could be made wider and the spatial resolution still significantly improved. If several thin detector modules could be stacked next to each other with much increased packing density the slit width could again be broadened without sacrificing spatial resolution. There is also a possibility to increase the spatial resolution in the detector dimension by measuring the individual charge collection to adjacent strips.

Improved contrast resolution: In principle X-rays have color just like visible light and with photon-counting the energy can be determined by measuring the induced charge for each photon. The high quality and purity of silicon and its relatively low energy to excite an electron-hole pair (3.6 keV) results in excellent inherent energy resolution. The X-ray image is normally a map of the combination of electron density and atomic number which together determines the X-ray attenuation. However, if the energy of each X-ray could be determined it would in principle be possible to single out the atomic number and this in turn would in many cases be of significant clinical value since the contrast of a particular tissue could be enhanced relative to others. To cope with the extra data volumes you likely need on-chip processing to preprocess and condense the information. Another feature that impacts the contrast resolution is simply the stopping ability of the sensor. Ideally approximately 100% of the X-rays incident on the detector should be detected. It is also important that the non-depleted "inactive section" where the X-rays enter is kept to a minimum since photons absorbed in this layer will not contribute to image contrast. Contrast resolution is often more of a limiting factor than spatial resolution.

Reduced radiation dose: There is always a tradeoff between image quality and radiation dose, the higher the dose the better the images is generally the rule. However, if the detection efficiency is improved and/or the information from each X-ray is used more efficiently there is a possibility to lower the radiation dose while simultaneously also increasing image quality. It is important to design the detector system for the right tradeoff between image quality and radiation dose.

Enabled imaging at higher energies (CT): For CT the count rates can be as high as several MHz per 50 μm pixel and the pulse counting circuit needs to be faster to cope with this and not run into dead-time problems which would increase the radiation dose and decrease the contrast. For higher energies the silicon detectors needs to have more stopping ability (-30 mm) and also they need to handle the higher fraction of Compton scattered X-rays. For CT it would be very valuable to be able to increase the packaging density between detector modules in order to enable imaging with one larger slot instead of a number of more narrow slots, this is much more practical for CT since there is no need for a pre-collimator close to the object which would interfere with positioning of the patient. In mammography this is less of a problem while in CT it is a severe constraint.

The present invention also makes it possible to build a narrower detector package. A narrow detector makes it possible to scan closer to the lateral sides of the patient support the sides of the patient support in the apparatus, e.g. according to Figs. 1 and 2. The scan time may also be shortened when increasing the total area of the slits, which reduces the risk for motion blur, due to patient motion. Possibility to have a one slit pre- collimator further away from the detector since the resolution is defined at the detector.

Reduced cost - There will be a heavy incitement to make healthcare more efficient in the future due to the aging population and lifecycle cost for any medical imaging modality will be very important and from a strategic point of view it is important to keep it low and this has to be kept in mind already in the early research phase. The present invention enables manufacture of detectors from inexpensive standard silicon wafers and/or increases the total irradiated area, and thus allows faster scan, less powerful X-ray source, and thus lower weight and consequently lower requirements of other mechanical parts and less powerful X-ray cooler.

Thus, the invention relates to a two-dimensional area detector for photon counting image acquisition, comprising at least two edge-on line detectors for converting photons to electrical pulses. Each line detector is an array of sensors and the line detectors are stacked closely next to each other. Preferably, each of the line detectors is essentially a flat thin piece of a semiconductor material, and the line detectors are stacked facing each other's large flat surfaces, and furthermore, each of the line detector is arranged to receive photons through a narrow edge. The line detectors are electrically insulated from a neighboring line detector using an insulating space, comprising one of insulating material or gap. Preferably, the insulating material or gap comprises substantially thinner than the thickness of the line detectors. The line detectors are edge-on semi-conductor detectors. The line detectors may be grinded from standard wafer thickness to the thickness matching the required resolution of the imaging application. The line detectors have an entry surface located at one edge of the line detector for receiving photons, whereby the entry surface is obtained by one or several of removing an inactive section, using etching or laser cutting from the wafers. The detector may further comprise a set of circuits for detecting the pulses from each of the sensors. The line detectors are virtually obtained in a common piece of semiconductor, using time-difference measurement, the piece being thicker than the required resolution. The detector may comprise time- delayed integration used to simplify readout electronics. The line detectors comprise CdZnTe. The detector may comprise a gaseous detector. The detector may further comprise an insulating layer between the linear detectors, and a means for applying a bias-voltage across each of the sensors. The detector may further comprise a thin layer of high Z material between the line detectors to reduce scattered radiation transferring between the line detectors.

The invention also relates to a mammography scanner comprising an area detector as mentioned above.

The invention also relates to an image acquisition apparatus comprising an X-ray source and an area detector as mentioned above, and further comprising a means for moving the area detector during exposure from the X-ray source.

The invention also relates to an apparatus for Computed Tomography (CT) comprising an area detector according to above.

The invention also relates to an apparatus comprising the area detector according to above and a collimator comprising a plurality of slits or slots. The invention also relates to an apparatus for acquiring an image of an object, comprising of an X-ray source for emitting photons, and a detector for receiving photons, a scan means for moving the detector simultaneously as receiving photons, the detector comprising a stack of line detectors for conversion of the photons to electrical signals. Each line detector comprises an array of electrodes for receiving the electrical signals, and the line detectors are arranged to receive the photons through the edges of the line detectors, wherein the detector is characterized by the distance between the line detectors being smaller than the thickness of the active part of the line detectors. The X- ray apparatus may further comprise electrically insulating layers between the line detectors.

The invention also relates to a method for obtaining an image. The method comprising the steps of irradiating an object with X-ray photons, arranging a plurality of line detectors closely in a stack, such that the distance between adjacent line detectors is smaller than the thickness of the active section of the line detectors, and arranging the stack such that the line detectors receive the photons essentially through the narrow edge of the line detectors, arranging the line detectors to convert photons to electrical pulses, and detecting the electrical pulses. The method may further comprise the step of measuring the energy of the photons, based on the strength of the electrical pulses.

SHORT DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in a non-limiting way with reference to enclosed drawings, in which:

Figs.1 and 2 show a multi-slit X-ray mammography scanner, according to prior art, which can be upgraded to use the present invention,

Fig.3 shows a stack of line detectors, according to the present invention,

Fig.4 illustrates an exemplary single line detector, according to both prior art and the present invention, Fig.5 illustrates an exemplary stack of line detectors, according to the present invention, and Fig 6 shows a multi-slit X-ray mammography scanner, according to present invention.

DESCRIPTION OF THE EMBODIMENTS

Figs. 3 and 5 illustrate an area detector 30, according to the present invention, comprising a stack of line detectors 31 , according to the present invention. The present invention involves placing a plurality of line detectors 31 , corresponding to each slit of a collimator, on a substrate 35. Thus, the present invention acts like a narrow area detector, also known as a slot detector. A set of line detectors 31 are stacked closely next to each other. The primary intended usage is for scanning X-ray image capture, such as scanned mammography, tomosynthesis, CT or other devices with a moving detector.

Each of the line detectors 31 is essentially a flat thin piece of a semiconductor material. The line detectors 31 are stacked facing each other's large flat surfaces, and furthermore, each line detector is arranged to receive photons through a narrow edge. The line detectors 31 may be grinded from standard wafer thickness to the thickness matching the required resolution of the imaging application. Each line detector may have an entry surface located at one edge of the line detector for receiving photons The entry surface can be obtained by one or several of removing an inactive section, using etching or laser cutting from the wafers.

In one embodiment of the present invention, the line detectors thickness is in the range 50-1000 μm, and that thickness limits the image resolution along that direction. For example, if the thickness is 300 μm thick and 20 of the detectors are stacked without spacing in between, which gives a 6 mm thick detector. In this case, this detector should be matched by pre-collimator slit of width less than 6 mm. The resolution is then defined in the detector and the pre-collimator 120 (as shown in Fig. 6) only serves to collimate the beam. Between the line detectors 31 insulating layers or gaps 32 are arranged, which can be 0.5 to 3 μm preferably 1.5 μm on either side of the sensor and still obtaining a fill-factor above 99%. In another embodiment, the line detectors are substantially thicker than the required resolution, preferably in the range 400 μm to 1500 μm, preferably 700 μm. A finer resolution is obtained using a circuit with the timing technology for measuring position from time difference, and obtaining a resolution that is not limited by the sensor thickness, preferably 50 μm for two-dimensional mammography. The linear one- dimensional detectors are edge-on double sided Si-detectors, several times thicker than a pixel. The two strips of each one-dimensional pixel are connected to a photon counting circuit that measures the position of a photon interaction within the pixel and thereby improves the resolution to that required by the application.

A set of circuits for detecting the pulses from each of the sensors may be arranged as external or embedded electronics.

In yet another embodiment, a time-delayed integration (WO/2001/026382) may be used to simplify readout electronics.

In other embodiments, the line detectors may comprise of CdZnTe or a gas.

The one-dimensional detector may have a bias-voltage across the sensor and an insulating layer is used between the linear detectors. It may also have a bias-voltage across the sensor and one side of the sensor has an insulating layer.

In one embodiment, a thin layer of high Z material can be placed between the sensors to reduce scattered radiation transferring between the sensors.

In one embodiment, the detector package may be used in a mammography scanner. In another embodiment, it may be used in a multi-slice CT. Both embodiments comprise a scanning means either a multi-slot or multi-slit collimator. The X-ray apparatus 100 according to one embodiment of the invention illustrated in Fig 6, is intended for acquiring an image of an object, and comprises an X-ray source 110 for emitting photons, and a detector 150 for receiving photons, a scan arrangement for moving the detector simultaneously as receiving photons. The detector 30 comprises a stack of line detectors for conversion of the photons to electrical signals, as described earlier. The line detectors are configured to receive the photons having a first edge from which the photons enter. The distance between line detectors is smaller than their thicknesses. Insulating layers are arranged between the line detectors for avoiding direct contact to adjacent line detectors.

An X-ray slot sensor according to invention for imaging comprises a silicon sensor, and a time difference detecting circuit for measuring the position of the point where the photon is converted to charge, thus obtaining a virtual stack of line detectors.

Thus, the invention relates to a two dimensional area detector 30 for counting photons, comprising a set of linear one-dimensional detectors 31 stacked closely next to each other. Preferably, the distance between adjacent line detectors is smaller than a pixel. The line detectors are electrically insulated from neighboring detectors using an insulating material or gap 32, which is thin enough to allow line detectors to be stacked at a distance, which is smaller than the thickness of the line detectors.

The linear one-dimensional detectors are edge-on semi-conductor detectors. The edge- on detectors are silicon (Si) strip detectors can be grinded from standard wafer thickness to the thickness matching the required resolution of the imaging application.

Preferably, the Si-strip detectors have minimized thickness of the inactive section of the entry surface (45), i.e. the edge where the photons enter. Preferably, such silicon line detectors can be manufactured using etching or laser cutting from the wafers as opposed to standard blade-diced sensors. The allowable thickness of the inactive section surface depends on the application, in particular the spectrum of the X-rays, i.e. the energies of the photons to detect. In typical embodiments, such requirements are much harder for mammography than for high-energy CT.

Each line detector constitutes an array of pixels arranged at a pitch length of a pixel and is connected to a circuit for counting electrical pulses from the photons.

Another aspect of the present invention is to manufacture the geometrical dimensions for the readout electronics. In one embodiment, a thick wafer is grinded to a thinner wafer used with the technology.

The present invention is based on edge-on geometry. With the edge-on geometry, as illustrated in Figs. 3 and 5, the line detectors 31 can be placed without spacing in between as the silicon dioxide forms an insulating layer. In Fig. 3, 30 denotes the detector package and 33 a collimated beam.

The above mentioned and described embodiments are only given as examples and should not be limiting to the present invention. Other solutions, uses, objectives, and functions within the scope of the invention as claimed in the below described patent claims should be apparent for the person skilled in the art.

Claims

1. A two-dimensional area detector (30) for photon counting image acquisition, comprising at least two edge-on line detectors for converting photons to electrical pulses, characterized in that each line detector is an array of sensors (31 ) and that said line detectors are stacked closely next to each other.

2. The detector of claim 1 , wherein each of said line detectors is essentially a flat thin piece of a semiconductor material, and said line detectors are stacked facing each other's large flat surfaces, and furthermore, each of said line detector is arranged to receive photons through a narrow edge.

3. The detector of claim 1 , wherein said line detectors are electrically insulated from a neighboring line detector using an insulating space, comprising one of insulating material or gap

4. The detector of claim 3, wherein said insulating material or gap comprises substantially thinner than the thickness of said line detectors.

5. The detector of claim 1 , wherein said line detectors are edge-on semi-conductor detectors.

6. The detector of claim 5, wherein said line detectors are grinded from standard wafer thickness to the thickness matching the required resolution of the imaging application.

7. The detector of claim 1 , wherein said line detectors have an entry surface located at one edge of said line detector for receiving photons, whereby said entry surface is obtained by one or several of removing an inactive section, using etching or laser cutting from the wafers.

8. The detector of claim 1 , further comprising a set of circuits for detecting said pulses from each of said sensors.

9. The detector of claim 1 , wherein said line detectors are virtually obtained in a common piece of semiconductor, using time-difference measurement, said piece being thicker than the required resolution.

10. The detector of claim 1 , comprising time-delayed integration used to simplify readout electronics.

11. The detector of claim 1 , wherein said line detectors comprise CdZnTe.

12.The detector of claim 1 , comprising a gaseous detector.

13. The detector of claim 1 , further comprising an insulating layer between the linear detectors, and a means for applying a bias-voltage across each of said sensors.

14. The detector of claim 1 , further comprising a thin layer of high Z material between said line detectors to reduce scattered radiation transferring between said line detectors.

15. A mammography scanner comprising an area detector according to any of claims claim 1 to 14.

16. An image acquisition apparatus (100) comprising an X-ray source (110) and an area detector according to any one of claims 1 to 14, and further comprising a means for moving said area detector during exposure from said X-ray source.

17. An apparatus for Computed Tomography (CT) comprising an area detector according to any of claims 1 to 14.

18. An apparatus comprising the area detector according to any of claims 1 to 14 and a collimator comprising a plurality of slits or slots. \

19. An apparatus for acquiring an image of an object, the apparatus comprising of an X- ray source for emitting photons, and a detector for receiving photons, a scan means for moving said detector simultaneously as receiving photons, said detector comprising a stack of line detectors for conversion of said photons to electrical signals, characterized in that each line detector comprises an array of electrodes for receiving said electrical signals, and said line detectors are arranged to receive said photons through the edges of said line detectors, wherein said detector is characterized by the distance between said line detectors being smaller than the thickness of the active part of said line detectors..

20. The X-ray apparatus according to claim 19, further comprising electrically insulating layers between said line detectors.

21. A method for obtaining an image, the method comprising the steps of irradiating an object with X-ray photons, arranging a plurality of line detectors closely in a stack, such that the distance between adjacent line detectors is smaller than the thickness of the active section of said line detectors, and arranging said stack such that said line detectors receive said photons essentially through the narrow edge of said line detectors, arranging said line detectors to convert photons to electrical pulses, and detecting said electrical pulses.

22. The method in claim 20, further comprising the step of measuring the energy of said photons, based on the strength of said electrical pulses.