Patent Description:
Radiation detectors may be used for many applications. One important application is imaging. Radiation imaging is a radiography technique and can be used to reveal the internal structure of a non-uniformly composed and opaque object such as the human body.

Early radiation detectors for imaging include photographic plates and photographic films. A photographic plate may be a glass plate with a coating of light-sensitive emulsion. Although photographic plates were replaced by photographic films, they may still be used in special situations due to the superior quality they offer and their extreme stability. A photographic film may be a plastic film (e.g., a strip or sheet) with a coating of light-sensitive emulsion.

In the <NUM>, photostimulable phosphor plates (PSP plates) became available. A PSP plate may contain a phosphor material with color centers in its lattice. When the PSP plate is exposed to radiation, electrons excited by radiation are trapped in the color centers until they are stimulated by a laser beam scanning over the plate surface. As the plate is scanned by laser, trapped excited electrons give off light, which is collected by a photomultiplier tube. The collected light is converted into a digital image. In contrast to photographic plates and photographic films, PSP plates can be reused.

Another kind of radiation detectors are radiation image intensifiers. Components of a radiation image intensifier are usually sealed in a vacuum. In contrast to photographic plates, photographic films, and PSP plates, Radiation image intensifiers may produce real-time images, i.e., do not require post-exposure processing to produce images. Radiation first hits an input phosphor (e.g., cesium iodide) and is converted to visible light. The visible light then hits a photocathode (e.g., a thin metal layer containing cesium and antimony compounds) and causes emission of electrons. The number of emitted electrons is proportional to the intensity of the incident Radiation. The emitted electrons are projected, through electron optics, onto an output phosphor and cause the output phosphor to produce a visible-light image.

Scintillators operate somewhat similarly to radiation image intensifiers in that scintillators (e.g., sodium iodide) absorb radiation and emit visible light, which can then be detected by a suitable image sensor for visible light. In scintillators, the visible light spreads and scatters in all directions and thus reduces spatial resolution. Reducing the scintillator thickness helps to improve the spatial resolution but also reduces absorption of radiation. A scintillator thus has to strike a compromise between absorption efficiency and resolution.

Semiconductor radiation detectors largely overcome this problem by direct conversion of radiation into electric signals. A semiconductor radiation detector may include a semiconductor layer that absorbs radiation in wavelengths of interest. When a radiation particle is absorbed in the semiconductor layer, multiple charge carriers (e.g., electrons and holes) are generated and swept under an electric field towards electric contacts on the semiconductor layer. Cumbersome heat management required in currently available semiconductor radiation detectors (e.g., Medipix) can make a detector with a large area and a large number of pixels difficult or impossible to produce. <CIT> discloses a radiographic image detector including an imaging plane formed by disposing pixel sections in a two-dimensional matrix. In use, the position of the radiographic image detector is changed by movement along an axis, and the inclination of the two-dimensional matrix relative to the movement axis is detected. The position is changed and the radiation image detector is irradiated twice so that common markers are imaged during each radiation irradiation. A reading operation is performed after each radiation irradiation to acquire image data, and the inclination is detected based on the positional relationship of marker images represented by the image data. <CIT> discloses an image sensor comprising: a plurality of X-ray detectors and an actuator configured to move the plurality of X-ray detectors to a plurality of positions. The image sensor captures images of portions of a scene at the positions and form an image of the scene by stitching the images of the portions.

Document <CIT> shows numerous degrees of freedom of linear as well as rotational motion of detector and source with collimator. The range of motion is for aligning the system for improved handling and work flow.

Document <CIT> discloses a method and image sensor, wherein a processor is configured for stitching portions of an image obtained at different locations of a subject to produce a radiation image. The source as well as the detector can be moved into a scanning location. The document provides for linear motion of the source in three perpendicular directions and linear motion of the detector in a vertical direction It furthermore shows rotation or tilting of the detector plane as well as rotation of the source system.

According to a first aspect of the present invention, there is provided a method comprising: aligning a collimator and a plurality of radiation detectors of an image sensor by: moving the radiation detectors along a first direction; moving the collimator along a second direction perpendicular to the first direction; rotating the collimator about an axis perpendicular to the first direction and the second direction; wherein the plurality of radiation detectors are configured to capture images of portions of a scene at different image capturing positions, respectively, and to form an image of the scene by stitching the images of the portions; and wherein the second direction is in a plane in which the radiation detectors are arranged.

Further, according to an embodiment, at least some of the plurality of radiation detectors are arranged in staggered rows.

According to a second aspect of the present invention, there is provided an image sensor comprising: a plurality of radiation detectors; a collimator; and an actuator configured to: move the radiation detectors along a first direction; move the collimator along a second direction perpendicular to the first direction; rotate the collimator about an axis perpendicular to the first direction and the second direction; wherein the plurality of radiation detectors are configured to capture images of portions of a scene at different image capturing positions, respectively, and to form an image of the scene by stitching the images of the portions; and wherein the second direction is in a plane in which the radiation detectors are arranged.

<FIG> schematically shows a cross-sectional view of a portion of an image sensor <NUM>, according to an embodiment. The image sensor <NUM> may have a plurality of radiation detectors <NUM> (e.g., a first radiation detector 100A, a second radiation detector 100B). The radiation detectors <NUM> may be spaced apart from one another in the image sensor <NUM>. The image sensor <NUM> may have a support <NUM>. The support <NUM> may have a curved surface <NUM>. The plurality of radiation detectors <NUM> may be arranged on the support <NUM>, for example, on the curved surface <NUM>, as shown in the example of <FIG>. The first radiation detector 100A may have a first planar surface 103A configured to receive radiation from a radiation source <NUM>. A second radiation detector 100B may have a second planar surface 103B configured to receive the radiation from the radiation source <NUM>. The first planar surface 103A and the second planar surface 103B may be not parallel. The radiation from the radiation source <NUM> may have passed through a scene <NUM> (e.g., a portion of a human body) before reaching the first radiation detector 100A or the second radiation detector 100B.

<FIG> schematically shows a cross-sectional view of one radiation detector <NUM> of the image sensor <NUM>, according to an embodiment. The radiation detector <NUM> may include a radiation absorption layer <NUM> and an electronics layer <NUM> (e.g., an ASIC) for processing or analyzing electrical signals incident radiation generates in the radiation absorption layer <NUM>. In an embodiment, the radiation detector <NUM> does not comprise a scintillator. The radiation absorption layer <NUM> may include a semiconductor material such as silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. The semiconductor may have a high mass attenuation coefficient for the radiation produced by the radiation source <NUM>.

As shown in a detailed cross-sectional view of the radiation detector <NUM> in <FIG>, according to an embodiment, the radiation absorption layer <NUM> may include one or more diodes (e.g., p-i-n or p-n) formed by a first doped region <NUM>, one or more discrete regions <NUM> of a second doped region <NUM>. The second doped region <NUM> may be separated from the first doped region <NUM> by an optional intrinsic region <NUM>. The discrete regions <NUM> are separated from one another by the first doped region <NUM> or the intrinsic region <NUM>. The first doped region <NUM> and the second doped region <NUM> have opposite types of doping (e.g., region <NUM> is p-type and region <NUM> is n-type, or region <NUM> is n-type and region <NUM> is p-type). In the example in <FIG>, each of the discrete regions <NUM> of the second doped region <NUM> forms a diode with the first doped region <NUM> and the optional intrinsic region <NUM>. Namely, in the example in <FIG>, the radiation absorption layer <NUM> has a plurality of diodes having the first doped region <NUM> as a shared electrode. The first doped region <NUM> may also have discrete portions.

When a particle of radiation hits the radiation absorption layer <NUM> including diodes, the particle of radiation may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of radiation may generate <NUM> to <NUM> charge carriers. The charge carriers may drift to the electrodes of one of the diodes under an electric field. The field may be an external electric field. The electrical contact 119B may include discrete portions each of which is in electrical contact with the discrete regions <NUM>. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of radiation are not substantially shared by two different discrete regions <NUM> ("not substantially shared" here means less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow to a different one of the discrete regions <NUM> than the rest of the charge carriers). Charge carriers generated by a particle of radiation incident around the footprint of one of these discrete regions <NUM> are not substantially shared with another of these discrete regions <NUM>. A pixel <NUM> associated with a discrete region <NUM> may be an area around the discrete region <NUM> in which substantially all (more than <NUM>%, more than <NUM>%, more than <NUM>%, or more than <NUM>% of) charge carriers generated by a particle of radiation incident therein flow to the discrete region <NUM>. Namely, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of the radiation detector <NUM> in <FIG>, according to an embodiment, the radiation absorption layer <NUM> may include a resistor of a semiconductor material such as, silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof, but does not include a diode. The semiconductor may have a high mass attenuation coefficient for the radiation produced by the radiation source <NUM>.

When a particle of radiation hits the radiation absorption layer <NUM> including a resistor but not diodes, it may be absorbed and generate one or more charge carriers by a number of mechanisms. A particle of radiation may generate <NUM> to <NUM> charge carriers. The charge carriers may drift to the electrical contacts 119A and 119B under an electric field. The field may be an external electric field. The electrical contact 119B includes discrete portions. In an embodiment, the charge carriers may drift in directions such that the charge carriers generated by a single particle of radiation are not substantially shared by two different discrete portions of the electrical contact 119B ("not substantially shared" here means less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow to a different one of the discrete portions than the rest of the charge carriers). Charge carriers generated by a particle of radiation incident around the footprint of one of these discrete portions of the electrical contact 119B are not substantially shared with another of these discrete portions of the electrical contact 119B. A pixel <NUM> associated with a discrete portion of the electrical contact 119B may be an area around the discrete portion in which substantially all (more than <NUM>%, more than <NUM>%, more than <NUM>% or more than <NUM>% of) charge carriers generated by a particle of radiation incident therein flow to the discrete portion of the electrical contact 119B. Namely, less than <NUM>%, less than <NUM>%, less than <NUM>%, or less than <NUM>% of these charge carriers flow beyond the pixel associated with the one discrete portion of the electrical contact 119B.

The electronics layer <NUM> may include an electronic system <NUM> suitable for processing or interpreting signals generated by particles of radiation incident on the radiation absorption layer <NUM>. The electronic system <NUM> may include an analog circuitry such as a filter network, amplifiers, integrators, and comparators, or a digital circuit such as a microprocessor and a memory. The electronic system <NUM> may include components shared by the pixels or components dedicated to a single pixel. For example, the electronic system <NUM> may include an amplifier dedicated to each pixel and a microprocessor shared among all the pixels. The electronic system <NUM> may be electrically connected to the pixels by vias <NUM>. Space among the vias may be filled with a filler material <NUM>, which may increase the mechanical stability of the connection of the electronics layer <NUM> to the radiation absorption layer <NUM>. Other bonding techniques are possible to connect the electronic system <NUM> to the pixels without using vias.

<FIG> schematically shows that the radiation detector <NUM> may have an array of pixels <NUM>. The array may be a rectangular array, a honeycomb array, a hexagonal array or any other suitable array. Each pixel <NUM> may be configured to detect a particle of radiation incident thereon, measure the energy of the particle of radiation, or both. For example, each pixel <NUM> may be configured to count numbers of particles of radiation incident thereon whose energy falls in a plurality of bins, within a period of time. All the pixels <NUM> may be configured to count the numbers of particles of radiation incident thereon within a plurality of bins of energy within the same period of time. Each pixel <NUM> may have its own analog-to-digital converter (ADC) configured to digitize an analog signal representing the energy of an incident particle of radiation into a digital signal. The ADC may have a resolution of <NUM> bits or higher. Each pixel <NUM> may be configured to measure its dark current, such as before or concurrently with each particle of radiation incident thereon. Each pixel <NUM> may be configured to deduct the contribution of the dark current from the energy of the particle of radiation incident thereon. The pixels <NUM> may be configured to operate in parallel. For example, when one pixel <NUM> measures an incident particle of radiation, another pixel <NUM> may be waiting for a particle of radiation to arrive. The pixels <NUM> may be but do not have to be individually addressable.

<FIG> schematically shows that one or more of the radiation detectors <NUM> may be mounted on a printed circuit board (PCB) <NUM>. The term "PCB" as used herein is not limited to a particular material. For example, a PCB may include a semiconductor. The wiring between the radiation detectors <NUM> and the PCB <NUM> is not shown for the sake of clarity. The PCB <NUM> and the radiation detectors <NUM> mounted thereon may be called a package <NUM>. The PCB <NUM> may have an area not covered by the radiation detectors <NUM> (e.g., an area for accommodating bonding wires <NUM>). Each of the radiation detector <NUM> may have an active area <NUM>, which is where the pixels <NUM> are located. Each of the radiation detectors <NUM> may have a perimeter zone <NUM> near the edges. The perimeter zone <NUM> has no pixels and particles of radiation incident on the perimeter zone <NUM> are not detected.

<FIG> schematically shows that the image sensor <NUM> may have a system PCB <NUM> with multiple packages <NUM> mounted on it. The image sensor <NUM> may include one or more such system PCBs <NUM>. The electrical connection between the PCBs <NUM> in the packages <NUM> and the system PCB <NUM> may be made by bonding wires <NUM>. In order to accommodate the bonding wires <NUM> on the PCB <NUM>, the PCB <NUM> has an area <NUM> not covered by the radiation detectors <NUM>. In order to accommodate the bonding wires <NUM> on the system PCB <NUM>, the packages <NUM> have gaps in between. The active areas <NUM> of the radiation detectors <NUM> in the image sensor <NUM> are collectively called the active area of the image sensor <NUM>. The other areas of the image sensor <NUM>, radiation incident on which cannot be detected by the image sensor <NUM>, such as the perimeter zones <NUM>, the area <NUM> or the gaps between the packages <NUM>, are collectively called the dead zone of the image sensor <NUM>. The radiation detectors 100A and 100B shown in <FIG> may be embodiments of the radiation detector <NUM> and mounted on respective PCB <NUM> as shown in <FIG> in a similar manner, and the support <NUM> may be part of the PCB <NUM> in one embodiment or part of the system PCB <NUM> in another embodiment.

<FIG> schematically shows movements of the radiation detectors <NUM> (e.g., 100A and 100B) relative to the radiation source <NUM>, according to an embodiment. In the examples of <FIG>, only a portion of the image sensor <NUM> with the first radiation detector 100A and the second radiation detector 100B is shown. The first radiation detector 100A and the second radiation detector 100B may be arranged on the support <NUM>. A relative position of the first radiation detector 100A with respect to the second radiation detector 100B remains the same at the multiple positions. The relative position of the first radiation detector 100A with respect to the second radiation detector 100B may but does not necessarily remain the same while they move from one of the multiple positions to another.

As shown in the example of <FIG>, the first radiation detector 100A and the second radiation detector 100B translate along a first direction <NUM> from position 506A to position 506B, relative to the radiation source <NUM>. The first radiation detector 100A and the second radiation detector 100B may translate along a second direction <NUM>. The second direction <NUM> is different from the first direction <NUM>. For example, the second direction <NUM> may be perpendicular to the first direction <NUM>. As shown in the example of <FIG>, the first radiation detector 100A and the second radiation detector 100B can translate from position 506A to position 506C, along the second direction <NUM>. The first direction <NUM> or the second direction <NUM> may be parallel to both, either or neither of the first planar surface 103A and the second planar surface 103B. For example, the first direction <NUM> may be parallel to the first planar surface 103A, but not parallel to the second planar surface 103B.

<FIG> schematically shows that the image sensor <NUM> comprises a collimator <NUM>. The collimator <NUM> comprises a plurality of radiation transmitting zones <NUM> and a radiation blocking zone <NUM>, according to an embodiment. The radiation blocking zone <NUM> substantially blocks radiation that would otherwise incident on the dead zone <NUM> of the image sensor <NUM>, and the radiation transmitting zones <NUM> allow at least a portion of radiation that would incident on the active areas <NUM> of the image sensor <NUM> to pass. The radiation transmitting zones <NUM> may be holes through the collimator <NUM> and the rest of the collimator <NUM> may function as the radiation blocking zone <NUM>. The collimator <NUM> may be disposed close to the radiation detectors <NUM> or away from the radiation detectors <NUM>. For example, the scene <NUM> may be between the collimator <NUM> and the radiation detectors <NUM>. The radiation transmitting zones <NUM> may have different sizes or positions from those of the active areas <NUM>.

The relative position of the collimator <NUM> and the radiation detectors <NUM> may not be fixed. For example, if the radiation from the radiation source <NUM> is not parallel rays, different relative positions of the collimator <NUM> and the radiation detectors <NUM> may be needed to keep the radiation transmitting zones <NUM> aligned with the active areas <NUM> when the image sensor <NUM> are at different positions relative to the radiation source <NUM>.

In an embodiment, the radiation detectors <NUM> of the image sensor <NUM> can move to multiple positions ("image capturing positions"), relative to the radiation source <NUM>. The image sensor <NUM> may use the radiation detectors <NUM> and with the radiation from the radiation source <NUM> to capture images of multiple portions of the scene <NUM> respectively at the multiple positions. The image sensor <NUM> can stitch these images to form an image of the entire scene <NUM>. As shown in <FIG>, according to an embodiment, the image sensor <NUM> may include an actuator <NUM> configured to move the radiation detectors <NUM> to the multiple positions. The actuator <NUM> may include a controller <NUM>. The actuator <NUM> may move the radiation detectors <NUM> to change their position relative to the collimator <NUM> and move the collimator <NUM> to change its position and orientation relative to the radiation detectors <NUM>. The positions and orientations may be determined by the controller <NUM>. After the radiation detectors <NUM> are moved to one of the image capturing positions, the collimator <NUM> and the radiation detectors <NUM> may be aligned. As shown in <FIG>, the collimator <NUM> and the radiation detectors <NUM> are aligned by moving the radiation detectors <NUM> along a first direction 799X, moving the collimator <NUM> in a second direction 799Y perpendicular to the first direction 799X, and rotating the collimator <NUM> about an axis 799Z perpendicular to the first direction 799X and the second direction 799Y. The image capturing positions may be displaced from one another along the first direction 799X. In one embodiment, during operation to capture an image of a scene, the positions may be selected such that the active areas of the image sensor <NUM> collectively tessellate the entire scene at the multiple positions.

<FIG> schematically shows that the image sensor <NUM> can capture images of portions of the scene <NUM>. In the example shown in <FIG>, the radiation detectors <NUM> move to three positions A, B and C, for example, by using the actuator <NUM>. Respectively at the positions A, B and C, the image sensor <NUM> captures images 51A, 51B and 51C of portions of the scene <NUM>. The image sensor <NUM> can stitch the images 51A, 51B and 51C of the portions to form an image of the scene <NUM>. The images 51A, 51B and 51C of the portions may have overlap among one another to facilitate stitching. Every portion of the scene <NUM> may be in at least one of the images captured when the detectors are at the multiple positions. Namely, the images of the portions when stitched together may cover the entire scene <NUM>.

The radiation detectors <NUM> may be arranged in a variety of ways in the image sensor <NUM>. <FIG> schematically shows one arrangement, according to an embodiment, where the radiation detectors <NUM> are arranged in staggered rows. For example, radiation detectors 100J and <NUM> are in the same row, aligned in the Y direction, and uniform in size; radiation detectors 100C and 100D are in the same row, aligned in the Y direction, and uniform in size. Radiation detectors 100J and <NUM> are staggered in the X direction with respect to radiation detectors 100C and 100D. According to an embodiment, a distance X2 between two neighboring radiation detectors 100J and <NUM> in the same row is greater than a width X1 (i.e., dimension in the X direction, which is the extending direction of the row) of one detector in the same row and is less than twice the width X1. Radiation detectors 100J and 100E are in a same column, aligned in the X direction, and uniform in size; a distance Y2 between two neighboring radiation detectors 100J and 100E in the same column is less than a width Y1 (i.e., dimension in the Y direction) of one detector in the same column.

<FIG> schematically shows another arrangement, according to an embodiment, where the radiation detectors <NUM> are arranged in a rectangular grid. For example, the radiation detectors <NUM> may include radiation detectors 100J, <NUM>, 100E and 100F as arranged exactly in <FIG>, without radiation detectors 100C, 100D, <NUM>, or <NUM> in <FIG>.

Other arrangements may also be possible. For example, in <FIG>, the radiation detectors <NUM> may span the whole width of the image sensor <NUM> in the X-direction, with a distance Y2 between two neighboring radiation detectors <NUM> being less than a width of one detector Y1. Assuming the width of the detectors in the X direction is greater than the width of the scene in the X direction, the image of the scene may be stitched from two images of portions of the scene captured at two positions spaced apart in the Y direction.

According to an embodiment, the radiation source <NUM> and the image sensor <NUM> may collectively rotate around the object about multiple axes.

The radiation detectors <NUM> in the image sensor <NUM> have any suitable sizes and shapes. According to an embodiment (e.g., in <FIG>), at least some of the radiation detectors <NUM> are rectangular in shape. According to an embodiment, as shown in <FIG>, at least some of the radiation detectors are hexagonal in shape. In such radiation detectors, the radiation detectors and the corresponding collimators that are aligned may have the same shape.

<FIG> schematically shows a flowchart for a method of aligning the collimator <NUM> and the radiation detectors <NUM>, according to an embodiment. In procedure <NUM>, the radiation detectors <NUM> are moved along a first direction (e.g., the direction 799X in <FIG>). The first direction may be in a plane in which the radiation detectors <NUM> are arranged. In procedure <NUM>, the collimator <NUM> are moved along a second direction (e.g., the direction 799Y in <FIG>) perpendicular to the first direction. The second direction is in a plane in which the radiation detectors <NUM> are arranged. In procedure <NUM>, the collimator <NUM> is rotated about an axis (e.g., axis 799Z in <FIG>) perpendicular to the first direction and the second direction. In optional procedure <NUM>, the radiation detectors <NUM> are moved relative to the source <NUM>, for example, to one of the image capturing positions.

<FIG> and <FIG> each show a component diagram of the electronic system <NUM>, according to an embodiment. The electronic system <NUM> may include a first voltage comparator <NUM>, a second voltage comparator <NUM>, a counter <NUM>, a switch <NUM>, a voltmeter <NUM> and a controller <NUM>.

The first voltage comparator <NUM> is configured to compare the voltage of an electrode of a diode <NUM> to a first threshold. The diode may be a diode formed by the first doped region <NUM>, one of the discrete regions <NUM> of the second doped region <NUM>, and the optional intrinsic region <NUM>. Alternatively, the first voltage comparator <NUM> is configured to compare the voltage of an electrical contact (e.g., a discrete portion of electrical contact 119B) to a first threshold. The first voltage comparator <NUM> may be configured to monitor the voltage directly or calculate the voltage by integrating an electric current flowing through the diode or electrical contact over a period of time. The first voltage comparator <NUM> may be controllably activated or deactivated by the controller <NUM>. The first voltage comparator <NUM> may be a continuous comparator. Namely, the first voltage comparator <NUM> may be configured to be activated continuously and monitor the voltage continuously. The first voltage comparator <NUM> configured as a continuous comparator reduces the chance that the system <NUM> misses signals generated by an incident particle of radiation. The first voltage comparator <NUM> configured as a continuous comparator is especially suitable when the incident radiation intensity is relatively high. The first voltage comparator <NUM> may be a clocked comparator, which has the benefit of lower power consumption. The first voltage comparator <NUM> configured as a clocked comparator may cause the system <NUM> to miss signals generated by some incident particles of radiation. When the incident radiation intensity is low, the chance of missing an incident particle of radiation is low because the time interval between two successive particles is relatively long. Therefore, the first voltage comparator <NUM> configured as a clocked comparator is especially suitable when the incident radiation intensity is relatively low. The first threshold may be <NUM>-<NUM>%, <NUM>%-<NUM>%, <NUM>-<NUM>%, <NUM>-<NUM>% or <NUM>-<NUM>% of the maximum voltage one incident particle of radiation may generate in the diode or the resistor. The maximum voltage may depend on the energy of the incident particle of radiation (i.e., the wavelength of the incident radiation), the material of the radiation absorption layer <NUM>, and other factors. For example, the first threshold may be <NUM> mV, <NUM> mV, <NUM> mV, or <NUM> mV.

The second voltage comparator <NUM> is configured to compare the voltage to a second threshold. The second voltage comparator <NUM> may be configured to monitor the voltage directly or calculate the voltage by integrating an electric current flowing through the diode or the electrical contact over a period of time. The second voltage comparator <NUM> may be a continuous comparator. The second voltage comparator <NUM> may be controllably activated or deactivated by the controller <NUM>. When the second voltage comparator <NUM> is deactivated, the power consumption of the second voltage comparator <NUM> may be less than <NUM>%, less than <NUM>%, less than <NUM>% or less than <NUM>% of the power consumption when the second voltage comparator <NUM> is activated. The absolute value of the second threshold is greater than the absolute value of the first threshold. As used herein, the term "absolute value" or "modulus" |x| of a real number x is the non-negative value of x without regard to its sign. Namely, |x| = <MAT>. The second threshold may be <NUM>%-<NUM>% of the first threshold. The second threshold may be at least <NUM>% of the maximum voltage one incident particle of radiation may generate in the diode or resistor. For example, the second threshold may be <NUM> mV, <NUM> mV, <NUM> mV, <NUM> mV or <NUM> mV. The second voltage comparator <NUM> and the first voltage comparator <NUM> may be the same component. Namely, the system <NUM> may have one voltage comparator that can compare a voltage with two different thresholds at different times.

The first voltage comparator <NUM> or the second voltage comparator <NUM> may include one or more op-amps or any other suitable circuitry. The first voltage comparator <NUM> or the second voltage comparator <NUM> may have a high speed to allow the system <NUM> to operate under a high flux of incident radiation. However, having a high speed is often at the cost of power consumption.

The counter <NUM> is configured to register a number of particles of radiation reaching the diode or resistor. The counter <NUM> may be a software component (e.g., a number stored in a computer memory) or a hardware component (e.g., a <NUM> IC and a <NUM> IC).

The controller <NUM> may be a hardware component such as a microcontroller and a microprocessor. The controller <NUM> is configured to start a time delay from a time at which the first voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold (e.g., the absolute value of the voltage increases from below the absolute value of the first threshold to a value equal to or above the absolute value of the first threshold). The absolute value is used here because the voltage may be negative or positive, depending on whether the voltage of the cathode or the anode of the diode or which electrical contact is used. The controller <NUM> may be configured to keep deactivated the second voltage comparator <NUM>, the counter <NUM> and any other circuits the operation of the first voltage comparator <NUM> does not require, before the time at which the first voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold. The time delay may expire before or after the voltage becomes stable, i.e., the rate of change of the voltage is substantially zero. The phase "the rate of change of the voltage is substantially zero" means that temporal change of the voltage is less than <NUM>%/ns. The phase "the rate of change of the voltage is substantially non-zero" means that temporal change of the voltage is at least <NUM>%/ns.

The controller <NUM> may be configured to activate the second voltage comparator during (including the beginning and the expiration) the time delay. In an embodiment, the controller <NUM> is configured to activate the second voltage comparator at the beginning of the time delay. The term "activate" means causing the component to enter an operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by providing power, etc.). The term "deactivate" means causing the component to enter a non-operational state (e.g., by sending a signal such as a voltage pulse or a logic level, by cutting off power, etc.). The operational state may have higher power consumption (e.g., <NUM> times higher, <NUM> times higher, <NUM> times higher) than the non-operational state. The controller <NUM> itself may be deactivated until the output of the first voltage comparator <NUM> activates the controller <NUM> when the absolute value of the voltage equals or exceeds the absolute value of the first threshold.

The controller <NUM> may be configured to cause the number registered by the counter <NUM> to increase by one, if, during the time delay, the second voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold.

The controller <NUM> may be configured to cause the voltmeter <NUM> to measure the voltage upon expiration of the time delay. The controller <NUM> may be configured to connect the electrode to an electrical ground, so as to reset the voltage and discharge any charge carriers accumulated on the electrode. In an embodiment, the electrode is connected to an electrical ground after the expiration of the time delay. In an embodiment, the electrode is connected to an electrical ground for a finite reset time period. The controller <NUM> may connect the electrode to the electrical ground by controlling the switch <NUM>. The switch may be a transistor such as a field-effect transistor (FET).

In an embodiment, the system <NUM> has no analog filter network (e.g., a RC network). In an embodiment, the system <NUM> has no analog circuitry.

The voltmeter <NUM> may feed the voltage it measures to the controller <NUM> as an analog or digital signal.

The system <NUM> may include an integrator <NUM> electrically connected to the electrode of the diode <NUM> or which electrical contact, wherein the integrator is configured to collect charge carriers from the electrode. The integrator can include a capacitor in the feedback path of an amplifier. The amplifier configured as such is called a capacitive transimpedance amplifier (CTIA). CTIA has high dynamic range by keeping the amplifier from saturating and improves the signal-to-noise ratio by limiting the bandwidth in the signal path. Charge carriers from the electrode accumulate on the capacitor over a period of time ("integration period") (e.g., as shown in <FIG>, between t<NUM> to t<NUM>, or t<NUM>-t<NUM>). After the integration period has expired, the capacitor voltage is sampled and then reset by a reset switch. The integrator can include a capacitor directly connected to the electrode.

<FIG> schematically shows a temporal change of the electric current flowing through the electrode (upper curve) caused by charge carriers generated by a particle of radiation incident on the diode or the resistor, and a corresponding temporal change of the voltage of the electrode (lower curve). The voltage may be an integral of the electric current with respect to time. At time t<NUM>, the particle of radiation hits the diode or the resistor, charge carriers start being generated in the diode or the resistor, electric current starts to flow through the electrode of the diode or the resistor, and the absolute value of the voltage of the electrode or electrical contact starts to increase. At time t<NUM>, the first voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the first threshold V1, and the controller <NUM> starts the time delay TD1 and the controller <NUM> may deactivate the first voltage comparator <NUM> at the beginning of TD1. If the controller <NUM> is deactivated before t<NUM>, the controller <NUM> is activated at t<NUM>. During TD1, the controller <NUM> activates the second voltage comparator <NUM>. The term "during" a time delay as used here means the beginning and the expiration (i.e., the end) and any time in between. For example, the controller <NUM> may activate the second voltage comparator <NUM> at the expiration of TD1. If during TD1, the second voltage comparator <NUM> determines that the absolute value of the voltage equals or exceeds the absolute value of the second threshold V2 at time t<NUM>, the controller <NUM> causes the number registered by the counter <NUM> to increase by one. At time te, all charge carriers generated by the particle of radiation drift out of the radiation absorption layer <NUM>. At time ts, the time delay TD1 expires. In the example of <FIG>, time ts is after time te; namely TD1 expires after all charge carriers generated by the particle of radiation drift out of the radiation absorption layer <NUM>. The rate of change of the voltage is thus substantially zero at ts. The controller <NUM> may be configured to deactivate the second voltage comparator <NUM> at expiration of TD1 or at t<NUM>, or any time in between.

The controller <NUM> may be configured to cause the voltmeter <NUM> to measure the voltage upon expiration of the time delay TD1. In an embodiment, the controller <NUM> causes the voltmeter <NUM> to measure the voltage after the rate of change of the voltage becomes substantially zero after the expiration of the time delay TD1. The voltage at this moment is proportional to the amount of charge carriers generated by a particle of radiation, which relates to the energy of the particle of radiation. The controller <NUM> may be configured to determine the energy of the particle of radiation based on voltage the voltmeter <NUM> measures. One way to determine the energy is by binning the voltage. The counter <NUM> may have a sub-counter for each bin. When the controller <NUM> determines that the energy of the particle of radiation falls in a bin, the controller <NUM> may cause the number registered in the sub-counter for that bin to increase by one. Therefore, the system <NUM> may be able to detect a radiation image and may be able to resolve particle of radiation energies of each particle of radiation.

After TD1 expires, the controller <NUM> connects the electrode to an electric ground for a reset period RST to allow charge carriers accumulated on the electrode to flow to the ground and reset the voltage. After RST, the system <NUM> is ready to detect another incident particle of radiation. Implicitly, the rate of incident particles of radiation the system <NUM> can handle in the example of <FIG> is limited by <NUM>/(TD1+RST). If the first voltage comparator <NUM> has been deactivated, the controller <NUM> can activate it at any time before RST expires. If the controller <NUM> has been deactivated, it may be activated before RST expires.

The image sensor <NUM> described above may be used in various detection systems, such as but not limited to, medical imaging such as dental Radiation radiography; a cargo scanning or non-intrusive inspection (NII) system for inspecting and identifying goods in transportation systems such as shipping containers, vehicles, ships, luggage, etc.; a full-body scanner; a radiation computed tomography (Radiation CT) system; an electron microscope; a system for performing energy-dispersive radiation spectroscopy (EDS).

The image sensor <NUM> may also have other applications such as in a radiation telescope, radiation mammography, industrial radiation defect detection, radiation microscopy or microradiography, radiation casting inspection, radiation non-destructive testing, radiation weld inspection, radiation digital subtraction angiography, etc. It may also be suitable to use the image sensor <NUM> in place of a photographic plate, a photographic film, a PSP plate, an radiation image intensifier, a scintillator, or another semiconductor radiation detector.

Claim 1:
A method comprising:
aligning a collimator (<NUM>) and a plurality of radiation detectors (<NUM>) of an image sensor (<NUM>) by:
moving (<NUM>) the radiation detectors along a first direction (799X);
moving (<NUM>) the collimator along a second direction (799Y) perpendicular to the first direction;
rotating (<NUM>) the collimator about an axis (799Z) perpendicular to the first direction and the second direction;
wherein the plurality of radiation detectors are configured to capture images of portions of a scene (<NUM>) at different image capturing positions, respectively, and to form an image of the scene by stitching the images of the portions; and
characterized in that: the second direction is in a plane in which the radiation detectors are arranged.