Abstract:
An energy selective radiation sensor has a photodetector and a transfer gate for controlling transfer of charge from the photodetector to a first sense node. A first readout circuit is provided for reading out charge from the first node. In use a first charge accumulated in the photodetector from a first predetermined portion of an energy spectrum of a radiation source is transferred to the first sense node. A second charge is accumulated in the photodetector from a second predetermined portion of the energy spectrum. The first charge is readout from the first sense node with the readout circuit, the second charge transferred to the first sense node and likewise readout. In embodiments of the invention, a second sense node and second transfer gate are provided and the first charge is read out through a first sense node and the second charge through the second sense node. In further embodiments a dump drain controlled by a dump gate is also provided to permit duty cycle control of sensitivity of the photodetector by selectively dumping and storing of charge accumulated in the photodetector.

Description:
CROSS-REFERENCE TO REALATED APPLICATIONS 
   This application claims priority from Application No. GB0517742.3filed in the United Kingdom on Aug. 31, 2005, the disclosure of which is incorporated herein by reference. 
   This invention relates to an energy selective radiation sensor and a method of operating a radiation sensor for energy selection. The invention has application to imaging using a radiation generator, such as an x-ray generator, which periodically generates at least two spectra of differing energies to generate at least two images in quick succession synchronized with the at least two different spectra. The two images may be processed and displayed either separately or combined, in order to produce an image with improved contrast or spectral information. 
   The invention has particular application to x-ray imaging and, in particular, to intra-oral dental x-ray imaging. 
   BACKGROUND OF THE INVENTION 
   Intra-oral x-ray imaging using a solid-state x-ray sensor is known. Bitewing x-ray images are used for crowns of teeth, that is a portion of the teeth above a gum, for both upper and lower teeth in a portion of a mouth. Bitewing x-rays are used to examine a thick, dense part of a tooth, e.g. enamel and dentine, and are used to determine thickness of enamel and the presence of decay (caries) between teeth. A best image contrast could in principle be obtained using a high-energy portion of a dental x-ray spectrum, e.g. between 35 keV and 70 keV, but typically a broad spectrum between 10 keV and 70 keV is used. 
   Periapical x-rays images show an entire tooth, including a root and surrounding bone. The root fits into fibrous bone below the gum, and is usually twice as long as the crown. Periapical x-rays examine a thin, less dense portion of the tooth and the low-density fibrous bone surrounding the root. Periapical x-rays are used to examine for root fracture, bone loss due to gum disease, and condition of root canal and periodontal ligament (which supports the tooth in its bony socket). Since bone loss can be caused by gum disease, detail of the gum where it meets the tooth is important. Periapical x-rays are also used to aid diagnosis following presentation of symptoms of pressure sensitivity, e.g. diagnosis of an abscess or cyst in the fibrous bone at the base of the root. In all cases, periapical x-rays are used to examine low density tooth and bone and image detail may be of soft tissue. Best image contrast could in principle be obtained by taking advantage of photo-electric x-ray attenuation in soft tissue using a low-energy portion of the dental x-ray spectrum, e.g. between 10 keV and 35 keV, but typically a broad spectrum between 10 keV and 70 keV is used. 
   Because different photon energies are required to obtain high resolution, high contrast images for different portions of a tooth and the surrounding tissue, it is not normally possible to obtain a single high resolution, high contrast image of a whole tooth and the surrounding tissue with a single exposure. 
   In other known systems:
         a second image is generated after a first image is read out, which for a large area sensor typical of intra-oral, mammo- or chest radiography, could take several seconds—registration of the two images will usually be adversely affected due to motion effects;   the two images may be generated from two different sensors and the spatial registration may be poor;   the two images may be generated via filtering in order to obtain high and low sensor channels, and such filtering can rarely achieve desirable characteristics such as sharp cut-off and discrimination of the pass-bands.       

   U.S. Pat. No. 6,381,301 describes a system which uses two complete mechanical scans of an object at two different x-ray energies. The two scan images are then combined. There is likely to be poor registration of the two images since each of the scans is likely to take at least 10seconds and the object may move between the scans. The registration also depends on generator and detector positioning accuracy. 
   U.S. Pat. No. 6,285,740 describes a dual-energy x-ray system, in which the detector consists of a soft x-ray detector and a hard x-ray detector in tandem, with an inter-detector filter in between. Alternatively suitable high energy/low energy scintillators are arranged to produce light of different colours, which are then imaged by CCD detectors for the capture of separate images. The detector is likely to have poor selectivity of the two x-ray energies. The sensitivity for the higher x-ray energy is likely to be poor since the high energy photons have to pass through the low energy detector before being imaged. The registration of the two images is likely to be poor because two detectors are used, also the inter-detector filter may produce undesirable spatial offsets. 
   U.S. Pat. No. 5,841,833 describes a dual energy detector with high and low energy detecting elements each based on an x-ray sensitive scintillator and a photodiode, arranged alternately in at least two rows and with low and high energy detector elements also arranged in columns of alternating high and low energy detector elements perpendicular to the rows. There is interpolation of the signals from adjacent, e.g. high energy pixels, and the signals are combined to produce an interpolated value for a virtual detecting element of high energy. The system is based on a linear, not an area, array, and hence the object must be scanned. The arrangement of the detector pixels is described, but not how the adjacent low and high x-ray energy pixels are actually realised. There may, for instance, be poor discrimination of the two x-ray energies. 
   U.S. Pat. No. 6,683,934 describes first and second filters which are selectively switched so as to be disposed between a source and a space accommodating a body, independently and respectively, for first and second, different x-ray energy levels. The filters interposed can never be ideal and the system is likely to have poor discrimination of the two x-ray energies. 
   It is an object of the present invention at least to ameliorate the aforesaid shortcomings in the prior art. 
   BRIEF SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a method of selective energy imaging of X-radiation comprising the steps of: providing an imaging pixel structure having photodetector means for converting incident radiation to electrical charge; first charge storage means; first charge transfer control means for controlling transfer of charge from the photodetector to the first charge storage means; and first readout means for reading out charge from the first charge storage means; setting the charge transfer control means to prevent transfer of a first charge from the photodetector means to the first charge storage means; accumulating the first charge in the photodetector means from radiation incident on the photodetector means in a first portion of a time-varying energy spectrum emitted by a radiation source, corresponding to a first predetermined energy range selected from the energy spectrum; setting the first charge transfer control means to transfer the first charge from the photodetector means to the first charge storage means; and using the first readout means to read out the first charge from the first charge storage means. 
   Conveniently, the step of setting the first charge transfer control means to transfer the first charge includes the further steps thereafter of: setting the first charge transfer control means to prevent transfer of a second charge from the photodetector means to the first charge storage means; accumulating the second charge in the photodetector means from radiation incident on the photodetector in a second portion of the time-varying energy spectrum emitted by the radiation source corresponding to a second predetermined energy range selected from the energy spectrum; and the step of reading out the first charge includes the further steps thereafter of: setting the first charge transfer control means to transfer the second charge from the photodetector means to the first charge storage means; and reading out the second charge from the first charge storage means. 
   Advantageously, the step of providing an imaging pixel structure includes providing first reset means for eliminating charge from the first charge storage means and the step of setting the first charge transfer control means to prevent transfer of the first charge includes preceding steps of: setting the first charge transfer control means to transfer charge from the photodetector means to the first charge storage means; transferring dark current charge accumulated in the photodetector means to the first charge storage means; and using the first reset means to eliminate the dark current charge from the first charge storage means. 
   Conveniently, the step of providing photodetector means comprises providing a scintillator layer coupled to a pinned photodiode or a photo-gate. 
   Optionally, the step of providing photodetector means comprises providing a photoconductor layer coupled to a diode. 
   Optionally, the step of providing photodetector means comprises providing a pinned photodiode or a photo-gate which is intrinsically sensitive. 
   Advantageously, the step of providing an imaging pixel structure includes providing dump gate means arranged for controlling charge transfer from the photodetector means to dump drain means and the step of transferring charge to the charge storage means includes selectively transferring charge to the dump drain means to affect the effective sensitivity of the photodetector means. 
   Conveniently, the step of providing an imaging pixel structure includes providing scintillator means for converting radiation from the radiation source to output radiation suitable for detection by the photodetector means and the step of affecting the effective sensitivity of the photodetector means comprises matching the sensitivity of the photodetector means to the output of the scintillator means. 
   Advantageously, charge accumulated from radiation incident on the photodetector means in a third portion of the time-varying spectrum emitted of the radiation source, corresponding to a third predetermined energy range of the energy spectrum, is dumped in the dump drain. 
   Conveniently, the step of providing an imaging pixel structure includes providing second charge storage means; and second charge transfer control means for controlling transfer of charge from the photodetector to the second charge storage means; and the step of reading out the first charge includes the further steps thereafter of: setting the second charge transfer control means to transfer the second charge from the photodetector means to the second charge storage means; and reading out the second charge from the second storage means. 
   Advantageously, the step of providing an imaging pixel structure includes providing a second readout means for reading out charge from the second charge storage means and the step of reading out the second charge from the second storage means comprises using the second readout means. 
   Alternatively, the step of reading out the second storage means comprises transferring the second charge sequentially to the photodetector and the first storage means and using the first readout means to read the second charge from the first storage means. 
   Conveniently, the radiation source is a DC x-ray source emitting x-ray pulses in the first predetermined energy range. 
   Conveniently, the radiation source is a DC x-ray source emitting x-ray pulses in the second predetermined energy range. 
   Alternatively, the radiation source is an AC x-ray source and the first predetermined energy range is a first continuum of energies emitted by the AC x-ray source using tube voltages within a first range. 
   Conveniently, the radiation source is an AC x-ray source and the second predetermined energy range is a second continuum of energies emitted by the AC x-ray source using tube voltages within a second range. 
   According to a second aspect of the invention, there is provided an imaging pixel structure including photodetector means for converting incident X-radiation to electrical charge; first charge storage means; first charge transfer control means for controlling transfer of charge from the photodetector means to the first charge storage means; and first readout means for reading out charge from the first charge storage means, characterised by: the pixel structure being arranged to accumulate a first charge in the photodetector means from radiation incident on the photodetector means in a first portion of a time-varying energy spectrum emitted by a radiation source, corresponding to a first predetermined energy range selected from the time-varying energy spectrum; to transfer the first charge from the photodetector means to the first charge storage means; and to use the first readout means to read out the first charge from the first charge storage means. 
   Advantageously, the imaging pixel structure is arranged to accumulate a second charge in the photodetector means from radiation incident on the photodetector means in a second portion of the time-varying energy spectrum corresponding to a second predetermined energy range selected from the energy spectrum; to transfer the second charge from the photodetector means to the first charge storage means; and to use the first readout means to read out the second charge from the first charge storage means. 
   Conveniently, the imaging pixel structure further comprises second charge storage means; second charge transfer control means for controlling transfer of charge from the photodetector to the second charge storage means; the pixel structure being arranged to accumulate a second charge in the photodetector means from radiation incident on the photodetector means in a second predetermined energy range selected from the energy spectrum of the radiation source; and to transfer the second charge from the photodetector means to the second charge storage means. 
   Conveniently, the imaging pixel structure further comprises second readout means arranged to read out charge from the second charge storage means. 
   Alternatively, the imaging pixel structure is arranged to transfer charge from the second charge storage means to the photodetector means and from the photodetector means to the first storage means to read out the second charge using the first readout means. 
   Conveniently, the imaging pixel structure further comprises dump charge transfer control means for transferring charge from the photodetector means to dump drain means arranged to vary an effective sensitivity of the photodetector means. 
   Advantageously, the imaging pixel structure further includes scintillator means for converting radiation from the radiation source to output radiation suitable for detection by the photodetector means, the pixel structure being arranged to vary the effective sensitivity of the photodetector means to match the output of the scintillator means. 
   Conveniently, the photodetector means comprises a scintillator layer coupled to a pinned photodiode or a photo-gate. 
   Optionally, the photodetector means comprises a photoconductor layer coupled to a diode. 
   Optionally, the photodetector means comprises a pinned photodiode or a photo-gate which is intrinsically sensitive. 
   Conveniently, the second charge storage means includes a storage gate. 
   Conveniently, at least one of the first and second charge storage means includes a sense node. 
   According to a third aspect of the invention, there is provided a sensor array comprising an imaging pixel structure as described above. 
   Advantageously, the sensor array is arranged for x-ray imaging. 
   Advantageously, the sensor array is arranged for intra-oral dental x-ray imaging. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEW OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic diagram of a known first pixel structure of an x-ray solid-state sensor, suitable for use with the invention, having a pinned photodiode with a transfer gate; 
       FIG. 2  is a schematic diagram of a second pixel structure of an x-ray solid-state sensor, suitable for use with the invention, having a pinned photodiode with a transfer gate and a dump gate and dump drain; 
       FIG. 3  is a schematic diagram of a known third pixel structure of an x-ray solid-state sensor, suitable for use with the invention, having a photo-gate with a transfer gate; 
       FIG. 4  is a schematic diagram of a fourth pixel structure of an x-ray solid-state sensor, suitable for use with the invention, with a photo-gate, a transfer gate and a dump gate and dump drain; 
       FIG. 5  is a schematic diagram of a fifth pixel structure of an x-ray solid-state sensor suitable for use with the invention, having a pinned photodiode with two transfer gates and separate readout circuits, one associated with each transfer gate respectively; 
       FIG. 6  is a schematic diagram of a sixth pixel structure of an x-ray solid-state sensor according to the invention, having a pinned photodiode, with two transfer gates, a dump gate and dump drain and separate readout circuits, one associated with each transfer gate respectively; 
       FIG. 7  is a schematic diagram of a seventh pixel structure of an x-ray solid-state sensor suitable for use with the invention, having a photo-gate with fist and second transfer gates and a storage gate; 
       FIG. 8  is a schematic diagram of an eighth pixel structure of an x-ray solid-state sensor according to the invention, having a photo-gate with first and second transfer gates, a storage gate, a sensor node and a dump gate and dump drain; 
       FIG. 9  is a schematic diagram of output of a DC x-ray generator operating at two different energies, suitable for use with the invention; and 
       FIG. 10  is a schematic diagram of output of an AC x-ray generator, suitable for use with the invention. 
   

   In the Figures like reference numerals denote like parts. 
   DETAILED DESCRIPTION OF THE INVENTION 
   It is an object of the invention to provide an x-ray sensor, which includes a facility to synchronize its signal charge collection with either a pulsed DC, or an AC, broad-spectrum x-ray source, in such a way as preferentially to select low-energy x-ray photons, or high energy x-ray photons, in a particular collection/readout frame. For a pulsed DC x-ray source, x-ray exposure may consist of a series of short pulses with different x-ray tube voltages. For an AC x-ray source, x-ray output has a cyclical variation of energy at the line frequency. 
   Currently available DC x-ray generators often have a selectable tube voltage, eg 60 kV (giving a broad spectrum from 10 keV to 60 keV) or 70 kV (giving a broad spectrum from 10 keV to 70 keV). It is preferable to change the two voltages to 35 kV and 70 kV to obtain the periapical and bitewing spectra for use in the method described below. Currently available AC x-ray generators already have a known periodic variation of x-ray energy over the required range and should be able to be used with the method described below without modification. 
   An objective is to enhance contrast of a particular image frame for either the low, or the high density, parts of an object under x-ray examination, whilst still using a same, broad spectrum, x-ray source. Alternatively, two frames may be taken in quick succession, a first optimised in contrast for low density, and a second for high density, portions of an object. The two frames are then combined with good registration to show detail in both the low and high density portions of the object—even when the object is moving or changing rapidly. This provides an advantage over two examinations carried out one after the other, which may not capture an object in a same part of a frame. 
   A particular application of the invention is in intra-oral dental x-ray imaging. Contrast of a particular image frame can be optimized for low, or high, density portions of teeth under examination. For low-density bone and soft tissue (periapical x-rays), photo-electric attenuation is accentuated by use of a low-energy portion of a dental x-ray spectrum. To provide good contrast for the dense crown of the tooth (bitewing x-rays), a high-energy portion of the dental x-ray spectrum is used. Alternatively two frames may be taken in quick succession, the first optimised in contrast for the low density, and the second for the high density, portions of the teeth. The two frames are then combined with good registration to show detail in both the low and high density portions. The capture of these two images in a single exposure ensures accurate registration between the two to facilitate image processing and may also minimise a dose delivered to a patient. 
   Energy selection, as described herein, can also be combined with selective sensitivity of an x-ray sensor, for example for scintillator matching, as disclosed in the Applicant&#39;s co-pending application entitled “Image sensor” under the Applicant&#39;s reference number P110450GB, in which charge a generated in a sensor is selectively stored and dumped, for example by storing charge in an on-off duty cycle. 
   A first basic three transistor pixel  10 , as shown in  FIG. 1 , suitable for use with the invention, has a pinned photodiode  11  and includes a transfer gate  12  controlling charge transfer from the pinned diode  11  to a sense node  13 . The pixel includes a reset transistor  14  having an output connected to the sense node  13 , a source follower, buffer transistor  15  with its gate connected to the sense node  13  and a pixel switch transistor  16  which connects an output of the source follower  15  to a video out line  161 . 
   A second pixel structure, shown in  FIG. 2 , also suitable for use with the invention, is similar to the first pixel structure of  FIG. 1 , but has in addition a dump gate  18  controlling charge transfer from the pinned photodiode  11  to a dump drain  17 . 
   A third pixel structure, shown in  FIG. 3 , is also suitable for use with the invention, and is identical to the first structure of  FIG. 1 , but with the pinned photodiode  11  of the first pixel structure replaced by a photo-gate  31 . 
   Similarly, a fourth pixel structure, shown in  FIG. 4 , is also suitable for use with the invention, and is identical to the second structure of  FIG. 2 , but with the pinned photodiode  11  of the second pixel structure replaced by a photo-gate  31 . 
   Referring to  FIGS. 5 to 8 , fifth, sixth, seventh and eighth pixel structures  50 ,  60 ,  70 ,  80  comprise a pinned photodiode  51 ,  61  or photo-gate  71 ,  81  with two independent transfer gates  521 ,  522 ;  621 ,  622 ;  721 ,  722 ;  821 ,  822 , which allow charge transfer to two separate storage locations/sense nodes  531 ,  532 ;  631 ,  632 ;  731 ,  74 ;  831 ,  84  respectively. 
   Referring to  FIGS. 5 and 6 , the fifth and sixth pixel structures  50 ,  60  each include two readout circuits  541 ,  542  each comprising a sense node  531 ,  532 ;  631 ,  632 , a reset switch  5411 ,  5421  having an output connected to the sense node, a source follower  5412 ,  5422  having a gate connected to the sense node and a select switch  5413 ,  5423  connected to an output of the source follower and having a video output  5414 ,  5424  with first and second transfer gates  521 ,  522 ;  621 ,  622  connecting a common photodiode  51 ,  61  to the sense nodes  531 ,  532 ;  631 ,  632 . Although the connections to the two readout circuits are shown in  FIGS. 5 and 6  as separate, at least some of the power supply, reset gate  5411 ,  5421 , row select  5413 ,  5423  and video out  5414 ,  5424  connections could alternatively be common to the two readout circuits of the fifth and sixth pixel structures  50 ,  60 . If the row select  5413 ,  5423  line is shared, separate video out  5414 ,  5424  connections are required and the two signals are output simultaneously. If the video out  5414 ,  5424  connection is shared, separate row select  5413 ,  5423  connections are required and the two signals are output sequentially. 
   Referring, to  FIG. 6 , the sixth pixel structure  60  is the same as the fifth pixel structure  50  apart from an additional dump gate  62  controlling charge transfer from the pinned photodiode  61  to a dump drain  63 . 
   Referring to  FIG. 7 , a seventh pixel structure  70  uses a sense node  731  as a first storage location and an additional storage gate  74  as a second storage location. Therefore, only a single readout circuit  542 , connected to the sense node  731  is provided. A means to transfer charge from the second storage location  74  to the sense node  731  is therefore required, and is described below. This is in effect a mini CCD in each pixel. 
   Referring to  FIG. 8 , an eighth pixel structure  80  is similar to the seventh pixel structure, but has a dump gate  82  and drain structure  83  for dumping charge from the photo-gate  81 . 
   Referring to  FIG. 9 , a DC x-ray source for use with the invention is arranged to output a first portion  91  of a programmed exposure  90  at a first energy and a second portion  92  at a second energy, higher than the first energy, with a short synchronizing pause  93  in a middle of the exposure between the first portion and the second portion. For dental x-ray imaging, a suitable duration of the total pulse  90  is 80 ms, and a suitable short synchronising pause is 2 ms. 
   The DC x-ray source may be used with any of the pixel structures described herein in a basic energy selection mode but for simplicity operation is initially described with the first pixel structure  10  shown in  FIG. 1 . 
   In standby mode, dark current charge is continuously, or periodically, transferred from the pinned photodiode  11  to the sense node  13  by holding, or pulsing, respectively, the transfer gate  12  high, and the sense node  13  is reset by holding, or pulsing, a reset pulse high at the reset gate  14 . 
   At a beginning of the first, low energy, x-ray period  91 , the reset gate  14  is turned off so that all photo-generated charge is collected in the pinned photodiode  11  and then transferred to the sense node  13  by keeping, or pulsing, the transfer gate  12  high. At an end of the first period  91  all generated charge is therefore on the sense node  13 . 
   The transfer gate  12  is then taken low and held low. Charge generated from the higher energy second x-ray period  92 , is collected in the pinned photodiode  11 . 
   After an end of the second x-ray period  92 , signals collected are read out in a three-step process. Signal from first period  91  is read out from the sense node, the sense node  13  is reset and signal from second period  92  is transferred from the pinned photodiode to the sense node  13  and read out 
   Some variations of this procedure are possible in common with normal CMOS readout strategies—for example, readout of the reset level between the two signal levels for fixed pattern correction. 
   Readout of the two sets of data from an array of pixels, corresponding to the first period  91  and the second period  92  could be either as two separate frames with global reset and transfer gate pulses, or on a line interleaved basis with rolling reset and transfer gate pulses. The latter may allow CDS to reduce kTC noise on the second sample. 
   The energy selection feature can be combined with the sensitivity matching technique described in the Applicant&#39;s co-pending application entitled “Image sensor” under reference number P110450GB. For example, energy selection as described may be combined with scintillator matching using the DC x-ray source. That is, the DC x-ray source may be used with the second or sixth pixel structures  20  and  60 , shown in  FIGS. 2 and 6 , in a mode which combines energy selection &amp; scintillator matching. 
   Charge collection may be performed in any desired part of the first (e.g. low energy) period  91  by appropriate pulsing of the transfer gates  12 ;  621 ,  622  and dump gates  18 ;  62 . In this case, a duty cycle can be chosen to match sensitivity of the second sensor pixel structure  20  or the sixth sensor pixel structure  60  to the scintillator output. In the second, e.g. high energy, period  92 , however, only charge from a beginning of the period  92  can be dumped for sensitivity control, as pulsing the gate will remove all signal from the photodiode  11 ;  61 . 
   To select only parts of the signal from the second period  92  for sensitivity control requires the sixth pixel structure  60  or eighth pixel structure  80  illustrated in  FIGS. 6 and 8  respectively. The energy selection feature can be combined with sensitivity control, for example, for scintillator matching, in both the low and high energy periods. 
   Referring to  FIG. 6 , in the sixth pixel structure  60  charge collection may be performed in any desired part of the first (e.g. low energy) period  91  by appropriate pulsing of the transfer gates  621  and dump gate  62  for the first storage location  631 . The duty cycle can be chosen in order to match the sensitivity of the sensor pixel  60  to the scintillator output In the second (e.g. high energy) period  92 , pulsing of the dump gate  62  and transfer gate  622  for the second storage location  632  takes place. The duty cycle can be chosen in order to match sensitivity of the sensor pixel  60  to the scintillator output. 
   Referring to  FIG. 8 , in the eighth pixel structure  80 , charge collection may be performed in any desired part of the first (e.g. low energy) period  91  by appropriate pulsing of the transfer gates  821  and dump gate  82  for the first storage location  831 . The duty cycle can be chosen, for example, in order to match the sensitivity of the sensor pixel  80  to the scintillator output. In the second (e.g. high energy) period  92 , pulsing of the dump gate  82  and transfer gate  822  takes place for the second storage location  84 . The duty cycle can be chosen, for example, in order to match sensitivity of the sensor pixel  80  to the scintillator output. 
   A standard AC x-ray source may also be used with the invention. X-ray energy  100  emitted by the source varies cyclically in a similar way to a fill wave rectified waveform, as illustrated in  FIG. 10 . Of the described pixel structures, only the sixth pixel structure  60  illustrated in  FIG. 6 , or the eighth pixel structure  80 , illustrated in  FIG. 8  can be used with the AC x-ray source for energy selection. 
   Referring to  FIG. 6 , in standby mode, dark current charge is continuously, or periodically, transferred from the photodiode  61  to the dump drain  63  or a sense node  631 , by holding, or pulsing, respectively, the dump gate  62  and the appropriate transfer gate  621  high, and resetting the sense node  631  by holding, or pulsing, the reset gate  5411  high. 
   During low energy x-ray periods  101 , the reset gates  5411 ,  5421  are turned off so all photo-generated charge is collected in the photodiode  61  and then transferred to the sense node  631  by keeping, or pulsing, the appropriate transfer gate  621  high. 
   During high energy x-ray periods  102 , the reset gates  5411 ,  5421  are turned off so all photo-generated charge is collected in the photodiode  61  and then transferred to the second sense node  632  by keeping, or pulsing, the second transfer gate  622  high. 
   Referring to  FIG. 8 , in standby mode, dark current charge is continuously, or periodically, transferred from the photo-gate  81  to the dump drain  83  or the sense node  831 , by holding, or pulsing, respectively, the dump gate  82  and the appropriate transfer gate  821  high, and resetting the sense node  831  by holding, or pulsing, the reset gate high. 
   During low energy x-ray periods  101 , the reset gate is turned off so all photo-generated charge is collected in the photodiode  81  and then transferred to the sense node  831  by keeping, or pulsing, the appropriate transfer gate  821  high. 
   During high energy x-ray periods  102 , the reset gate is turned off so all photo-generated charge is collected in the photodiode  81  and then transferred to the storage gate  84  by keeping, or pulsing, the second transfer gate  822  high. 
   With the sixth pixel structure  60 , or the eighth pixel structure  80 , a third band of x-ray energies (e.g. corresponding to very low energies of minimal diagnostic value) can be defined for which the signal is always dumped directly to the dump drain  63 ,  83 . 
   After an end of the x-ray exposure  100 , the signals collected are read out as follows. For the fifth and sixth pixel structures  50  and  60  the two sense nodes  531 ,  532 ;  631 , 632  are read out by addressing them sequentially or simultaneously depending on a manner in which connections are commoned, see above. 
   For the seventh pixel structure  70  and the eighth pixel structure  80 , a first signal from the sense node  731 ,  831  is read out. The sense node  731 ,  831  is reset. A second signal from the storage gate  74 ,  84  is transferred to sense node  731 ,  831  by pulsing the storage gate transfer gate  722 ,  822  and photo-gate  71 ,  81  in a typical CCD charge transfer sequence. The second signal from the sense node  731 ,  831  is read out and the sense node  731 ,  831  is reset. 
   Energy selection may be combined with sensitivity matching using the AC x-ray source using the sixth pixel structure  60 , or the eighth pixel structure  80 . Charge collection may be performed in any desired part of the low energy periods  101  by appropriate pulsing of the first transfer gate  621 ,  821  and dump gate  62 ,  82  for the first storage location  631 ,  831 . The duty cycle can be chosen in order to match the sensor pixel sensitivity to output from the scintillator. In the high energy periods  102 , pulsing of the second transfer gate  622 ,  822  and the dump gate  62 ,  82  for the second storage location  632 ,  84  takes place. The duty cycle can be chosen in order to match sensitivity of the sensor pixel  60 ,  80  to scintillator output. 
   Advantages provided by the invention include:
         two images are generated from a same sensor with same optics and separated by only 1 or 2 ms in time;   registration of the two images is excellent, since the same photo-sites are used and the radiation generator is fixed in position;   motion artefacts are minimized, since the two images are taken in quick succession; and   there is excellent separation of high and low input spectra (via time division).       

   Although the invention has been described in relation to image sensors having photodiodes and photo-gates, it will be understood that the invention is equally applicable to image sensors with other types of photodetector, such as, for example, photoconductor-based image sensors and intrinsically sensitive image sensors. In a photoconductor-based imager an imager is coated with, or attached to, a photoconductor layer. This layer has a bias field V bias  applied, for example by means of opposed metal contacts. When charge is generated in the photoconductor, the bias field sweeps the generated charge into a pixel structure. In an intrinsically sensitive imager signal is directly generated in the imager pixel.