Radiography imaging system

Description of equivalent WO 2013167855 A2

Radiography Imaging System

Field, of the Invention

This invention relates to an apparatus and method for radiographic imaging.

Background of the Invention

Radiography is the process of creating images by means of using radiation other than visible light. Perhaps the most well-known example is that of X-rays whereby ionising radiation is passed over an object to produce an image on a radiosensitive surface which can be used e.g. in healthcare, industry and testing, airport security systems and so on. Figure 1 shows a conventional digital X-ray system 1. It comprises a source of X-ray photons 3, a scintillator 5, an amorphous silicon panel (ASP) 7, and read-out electronics 9. The scintillator 5 is formed of a material that emits visible light in response to X-ray absorption; it is usually in the form of a flat panel. The light is detected using the ASP 7 which is a relatively complex, expensive and bulky array of photodiodes or thin-film transistors (TFTs) that convert the light into electrical signals which are then interpreted and converted into data by the ^· read-out electronics 9 to represent a two-dimensional image form.

Such a system 1 has a high cost, which is dependent on size, not least because of the expense of the ASP 7 required. In addition, the useful life of such a system 1 is limited because the light sensing elements (photodiodes or TFTs) of the ASP 7 are in practise exposed to radiation which damages the semiconductor material. Such a system 1 is also susceptible to 'ghosting' which occurs when radiation causes electrical charge effectively to be trapped in the semiconductor material resulting in previous images (or parts thereof) to appear in new and/or subsequent images. The spatial resolution offered by such a system 1 is also limited by the pixel count, which results from the number of photodiodes and/or TFTS in the ASP 7 which cannot be varied once manufactured.

Additionally, such systems tend to have limited image readout speeds, meaning that the object or patient being imaged would have to be subjected to radiation for longer periods of time, increasing the dose received.

Summary of the Invention

A first aspect of the invention provides apparatus for radiography, comprising: a scintillator having a first surface for being exposed to radiation and a second surface for emitting visible light in response; and an imaging system comprising: an optical channel; a scanning mirror for scanning a two-dimensional region of the scintillator second surface along a predetermined scan path so as to direct light emitted from the scanned region to the optical channel; means for detecting the light received from the optical channel; and means for generating, using said received light and the corresponding scan path position, a signal or data for representing a two-dimensional image indicative of the light emitted from the two dimensional region of the scintillator.

Such an apparatus provides improvements over conventional digital radiography apparatus. By imaging a two-dimensional surface of the scintillator using a scanning mirror and effectively transmitting the resulting light in serial-fashion for the different scan path positions, the photodetection stage (s) can use much simpler and less expensive technology. Further, the overall apparatus can be made less bulky.

Further, the photodetection stage does not have to be adjacent nor in-line with the scintillator and can therefore be shielded from the radiation source to avoid adverse effects which may affect its operation.

The imaging system may further comprise an optical lens positioned between the scintillator and the scanning mirror for focussing light from the region of said second surface onto the scanning mirror.

The scanning mirror of the imaging system may be a micro electromechanical (MEM) mirror.

The scanning mirror of the imaging system may have an associated position control system configured to cause multi- axis movement of said mirror in accordance with the predetermined scan path.

The optical channel of the or each imaging system may be an optical fibre having an input end and an output end.

The apparatus may further comprise an optical lens positioned between the scanning mirror and the input end of the optical fibre for focussing light directed from the mirror into said input end.

The light detecting means may comprise a photodetector, e.g. a photodiode, for generating an electrical signal representing received light intensity. The light detecting means may be a single photodetector. The image generating means may comprise an analogue to digital converter (ADC) for converting the electrical signal generated by the photodetector into light intensity data and processing means for correlating the light intensity data to its corresponding scan path position.

The apparatus may further comprise a display for outputting the two-dimensional reconstructed image. The apparatus may further comprise a radiation source for directing radiation towards the scintillator first surface and wherein the light detecting means is shielded from the radiation source. In the preferred embodiment, a plurality of the above- described imaging systems are provided, each system being associated with a respective sub-region of the scintillator second surface. As a result of the parallel nature of the image capture, overall image capture speeds can be increased over conventional ASP's resulting in a reduction in dose to the patient or object being imaged. The apparatus may further comprise means for operating a subset of the imaging systems so as to scan its or their two- dimensional region at a greater resolution than those of the other imaging system(s). Said operating means may be configured to cause the scanning mirror of the or each subset to scan a smaller two-dimensional region than those of the other imaging system (s) . Said operating means may be configured to cause the image generating means of the or each subset to sample the electrical signal generated by the photodetector into light intensity data at a higher rate than those of the other imaging system (s) . The operating means may include a user interface to enable user selection of said one or more subsets to scan at a greater resolution.

The apparatus may further comprise means for receiving each signal or data set representing the reconstructed two- dimensional image for a sub-region and generating a joined two-dimensional image indicative of the light emitted from the overall second surface.

A second aspect of the invention provides apparatus for use in radiography, comprising: a scintillator having a first surface for being exposed to radiation and a second surface for emitting visible light in response; and an imaging system comprising: a plurality of scanning mirrors, each associated with a respective sub-region of the scintillator second surface, each scanning mirror being mounted and controlled so as to re-direct light from along a predetermined scan path within the respective sub-region towards a respective optical channel; photodetection means associated with each scanning mirror and optical channel for receiving the re-directing light and generating an electrical signal representing light intensity; and processing means for receiving the electrical signal from each photodetection means and the corresponding position of each mirror to generate therefrom a reconstructed two-dimensional image indicative of the light emitted from each scintillator sub-region. A third aspect of the invention provides a method for performing radiography, comprising: operating a scanning mirror to scan a two-dimensional region of a scintillator light-emitting surface along a predetermined scan path so as to direct light emitted from the scanned region to a light- carrying optical channel; detecting the light received from the scanning mirror by means of the optical channel; and generating, using said received light and the corresponding scan path position from which the light was received, a signal or data for representing a two-dimensional image indicative of the light emitted from the two dimensional region of the scintillator .

A fourth aspect of the invention provides a computer program comprising instructions that when executed by a computer apparatus control it to perform the method defined in the third aspect.

A fifth aspect of the invention provides a non-transitory computer-readable storage medium having stored thereon computer-readable code, which, when executed by computing apparatus, causes the computing apparatus to perform a method comprising: operating a scanning mirror to scan a two- dimensional region of a scintillator light-emitting surface along a predetermined scan path so as to direct light emitted from the scanned region to an optical channel/ detecting the light received from the scanning mirror; and generating, using said received light and the corresponding scan path position, a signal or data for representing a two-dimensional image indicative of the light emitted from the two dimensional region of the scintillator.

Brief Description of the Drawings

Embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a conventional digital X- ray system; Figure 2 is a schematic diagram of a digital X-ray system according to the invention;

Figure 3 is a plan view of a printed circuit board (PCB) on which is mounted micro electro-mechanical (MEM) mirrors used in the system of Figure 2;

Figure 4 is a plan view of the PCB of Figure 3 overlaid with a surface of a scintillator used in the system of Figure 2;

Figure 5 is a schematic diagram of one part of the digital X- ray system of Figure 2 which is useful for understanding its operation;

Figure 6 is a schematic diagram of a control module of the Figure 2 system; and

Figure 7 is a flow diagram indicative of processing steps performed by the control module of the Figure 2 system.

Detailed Description of a Preferred Embodiment

Referring to Figure 2, a digital X-ray system 11 in accordance with a first embodiment comprises a radiation source 13, a flat-panel scintillator 15, a Micro-Electromechanical (MEM) mirror module 17, an optical channel module 19, a photo detection module 21, and a control and processing module (hereafter "control module") 23. The radiation source 13 is a conventional source of ionizing radiation for use in medical, industrial or security applications .

The scintillator 15 is also conventional in terms of it having the same properties and form of commercially available flat- panel scintillators configured for medical, industrial or security applications. The scintillator 15 in this case has a first, upper side which is exposed to radiation and an opposed, lower side which emits visible light in response to absorption of the radiation. Further details of scintillator technology can be found at http://www. flatpaneldr. com/?p=443 to give one example. It is assumed in this case that the scintillator upper and lower sides have a substantially rectangular, planar form.

In use, a subject such as person or article is positioned between the source 13 and the scintillator 15; the radiation absorbed by the scintillator after it has passed around/through the subject is interpreted by the subsequent stages in order to generate the X-ray image.

The MEM mirror module 17 comprises an array of multiple (n) MEM mirrors. As will be known, a MEM mirror is a silicon (or integrated circuit (IC) ) device having on one surface a small mirror connected typically to small flexures allowing it to move about multiple axes in response to control signals which cause the flexures to move. MEM mirrors currently find useful application in scanning systems, for example in image capture and image display technologies whereby a two-dimensional image can be reproduced or displayed pixel-by-pixel by means of the mirror scanning along a predetermined scan path.

Referring to Figure 3, the MEM mirror module 17 comprises an array of nine MEM mirror ICs (MEMs) 30 provided on a printed circuit board (PCB) 32, the MEMs being arranged in a 3 x 3 configuration as shown. Instead of a PCB, a mechanical plate can be used which tends to be less expensive. The PCB 32 has approximately the same rectangular shape as the scintillator 15, although this is not essential, and is arranged below its lower surface in a vertical stack-like fashion. The mirror of each MEM 30 is configured to move about a vertical and horizontal axis in accordance with control signals received from the control module 23 to scan a predefined two- dimensional planar portion of the scintillator. The control signals are transmitted to each MEM 30 through respective wires 33 of an MEM bus, indicated by the dotted lines in Figure 3. The mirror of each MEM 30 is arranged at an obtuse angle to the scintillator surface so as to direct light generally sideways from the scan path towards the capturing part of the system to be described below. In the example shown in Figure 2, the angle of redirection is substantially normal to the 'stacking axis' X-X. This helps reduce the dimensions of the system 11 because the image capturing parts of the system, particularly the photodetection and control modules 21, 23, are not stacked in alignment with the stacking axis X-X as is the case in conventional systems, for which see Figure 1.

The scan path can follow any two-dimensional path and in this case follows a raster scan path. As shown in Figure 4, which shows schematically the PCB 32 overlaid on the scintillator 15 lower surface (with one MEM 30 shown for ease of explanation) each MEM is arranged so as to scan a respective one of nine two-dimensional portions or ^footprints' 36 of the scintillator's light-emitting surface. The raster scan path is indicated by dotted line 34. It will be appreciated that scan paths can overlap on the periphery of each footprint 36 with image post-processing being used to handle recognition and removal of the overlapping content.

Referring back to Figure 2, the optical channel 19 is arranged to receive the light reflected by each MEM 30 and carry said light to the capturing part of the system 11, specifically to the photodetection module 21. The optical channel 19 here comprises n(=9) optical fibres, so that each MEM 30 is paired with a corresponding optical fibre. A receiving end of each optical fibre is positioned so as to receive the light from the MEM with which it is paired and through total internal reflection carries it to an emitting end of said fibre at which point it is passed to the photodetection module 21. The above-described radiation source 13, scintillator 15, and MEM mirror array 17 are arranged within a shielded casing 25 or similar container which shields, absorbs and/or limits the amount of ionising radiation leaving the casing or containment which may otherwise cause the abovementioned adverse effects outlined in the background. The optical channel 19 can be within the casing 25, completely or partially, and carries light to the external location where the photodetection and control and processing modules 21, 23 are located. By using flexible optical fibres, the light signals can be carried in any direction relative to the other parts of the system 11 which is convenient in terms of size.

The light carried by each optical fibre represents the light emitted from part of the scan path of a particular region. As the MEM 30 scans along the scan path, the light intensity will vary and hence the optical fibre carries a serial representation of what is a two-dimensional image at a given point in time. This means that fewer photodetectors are required to convert the light signal to an electrical signal representing the two-dimensional image.

The photodetection module 21 comprises n photodiodes, one photodiode being associated with each optical fibre of the optical channel 19. Each photodiode is configured to generate an electrical signal representing the light received via the MEM and optical fibre from a respective region of the scintillator 15. Each photodiode is connected to an analogue- to-digital converter (ADC) for generating data indicative of the light intensity for the region. It will be appreciated that a small number of relatively inexpensive photodiodes is therefore utilised instead of the bulky and expensive ASP module referred to in the background.

The control module 23 is configured to perform the following functions :

(1) to generate control signals for each of the nine MEMs 30 to control their vertical and horizontal position to move in accordance with the scan path. The control module 23 includes a memory storing data representing said scan path. The control module 23 therefore knows the scan path position at a given instant in a scan cycle for each MEM 30;

(2) to construct a two-dimensional image for each of the n scintillator regions using the received light intensity data and corresponding position data; and

(3) to construct a two-dimensional image representing the joining of the images constructed in (2) above, to represent the overall scintillator output.

The above functions can be performed in real time in the case of a moving subject, e.g. an item of luggage passing through a security scanner.

Preferably, an optical lens is provided between each scintillator region and its corresponding MEM 30 in order to focus emitted light onto the MEM mirror.

A further optical lens can also be provided between the MEM 30 and the input end of its corresponding optical fibre, again for focussing light reflected from the MEM mirror into said input end. Referring now to Figure 5, for completeness, an explanation of the system 11 will now described in terms of its operation with respect to just one scintillator region, namely Region 1 shown in Figure 4.

The scintillator is again indicated by reference numeral 43. A lens 45 is disposed underneath Region 1 of the scintillator 43, between it and its MEM device 49 having an MEM mirror 47 axially aligned beneath the lens 45. The lens 45 focuses the light received from Region 1 onto the MEM mirror 47 which scans over the focussed image and directs it sideways, generally normal to the scan axis, towards a corresponding optical fibre 53. A further lens 51 is disposed between the MEM mirror 47 and the input end of the optical fibre 53.

The optical fibre 53 carries the varying light signal to a position external to the shielded environment 25. At this external position, a single photodiode 55 receives the light and converts it into an electrical signal representing the varying light intensity along the scan path of Region 1. The electrical signal is converted to data by an ADC 57. The data for Region 1 is then outputted to the control module 59. The control module 59 is responsible for generating control signals for input to the MEM device 49 which moves the mirror 47 by means of its flexures. Therefore, the control module 59 is also configured to identify from which part of the scan path (and therefore the two dimensional part of Region 1) the received signal corresponds. Data corresponding to the light intensity at each of a plurality of scan positions is therefore generated and stored as pixel data on memory of the control module 59 for processing and display. The control module 59 therefore generates in real-time (^"or near real-time) an image representing Region 1 of the scintillator 43. The same process is performed in parallel for each of Regions 2 - 9 shown in Figure 4 and the control module 59 generates a joined image representing all regions in the 3 x 3 configuration shown in Figure 4.

The control module 59 may comprise a dedicated processing module, e.g. a microcontroller or FPGA configured to operate under the control of a program. Figure 6 shows functional components, including a controller 61, RAM 63, a memory 65 an input bus for the light data, and output buses for the positional data and image data. The controller 61 is connected to each of the other devices to control operation thereof. RAM 63 is used by the controller 61 for the temporary storage of data. The memory 65 stores one or more programs having code which, when executed by the controller 61 in conjunction with RAM 63, controls the overall system 11. The memory also stores data 69 representing pixels of the two dimensional scan images for output to a display system, such as a computer monitor.

Referring to Figure 7, the main steps performed by the program 67 will now be described in relation to one of the scan regions, e.g. Region 1. Those steps shown in dotted line are not part of the program itself but are useful for understanding the process.

The process starts at step 7.1 which is at the start of a scan cycle. In step 7.2, the MEM position data is generated for each step of a scan cycle. In step 7.3, the position data for the current step is received by the MEM and the mirror scans to the appropriate position. In step 7.4, light is received from the scanning mirror and is carried to the photodiode. In step 7.5, the signal from the photodiode is analogue-to- digital converted. In step 7.6, a correlation is made between the received light intensity data and the positional data for the current step of the scan cycle so that each pixel of the 2D Region is generated and stored. In step 7.7 the resulting image is joined with images generated in the above manner for the other Regions.

Spatial resolution for a given scan region is governed by its 2D scan area and/or the sampling speed of the ADC 57. By reducing the scan area, for example, the spatial resolution is improved allowing smaller features to be imaged. This is in effect an optical zoom feature. The scan area may also be offset to allow the zooming to be targeted to within a particular area within the MEM's field of view. This gives the option of generating a composite image where, say, one mirror is operated to scan at a higher resolution over a relatively small area with the other mirrors scanning at, relatively speaking, a lower resolution with a wider field of view to provide 100% coverage. The overall image produced can therefore show an area-of-interest in higher detail than the rest of the image, enabling for example an area of the body to be imaged in greater detail than the rest. A user interface associated with the control module 59 may be configured to allow selection of a subset (one or more) of the regions for higher resolution scanning by causing the mirror and/or ADC of said region (s) to operate accordingly. The extent to which the scan area of the selected region (s) is or are reduced can be user defined through the interface. The remaining region(s)' wider field-of-view may be determined automatically to obtain full coverage of the overall imaging area.

In the above embodiment, the use of electromagnetically actuated MEM mirrors with associated lens and optical fibre parts means the active, contained, area 2B of the overall system 11 is effectively radiation hardened, making it useful for high energy radiation imaging such as that used on radiotherapy equipment where it is desirable to use treatment radiation for portal imaging. Also, because the photodetector module 21 is outside the radiation field, such a system should not suffer from the ghosting found in conventional imaging devices which will improve image quality. As a result of the parallel nature of the image capture, overall image capture speeds can be increased over conventional ASP' s resulting in a reduction in dose to the patient or object being imaged. It will be appreciated that the above described embodiments are purely illustrative and are not limiting on the scope of the invention. Other variations and modifications will be apparent to persons skilled in the art upon reading the present application.

Moreover, the disclosure of the present application should be understood to include any novel features or any novel combination of features either explicitly or implicitly disclosed herein or any generalization thereof and during the prosecution of the present application or of any application derived therefrom, new claims may be formulated to cover any such features and/or combination of such features.