Radiation imaging device comprising array of Cd(Zn)Te detectors

BACKGROUND OF THE INVENTION

1. Field of the invention

0001The present invention relates to digital radiation imaging. In particular, the invention relates to the effective construction of optimized imaging areas of various size and shape by combining separate semiconductor pixel detectors side by side in a mosaic like manner so that the shape of the active area of the sensor is made to match as exactly as possible the X-ray beam shape (or shapes) of a specific application.

2. Description of related art

0002A major problem in constructing digital X-ray imaging devices is the manufacturing of large sensor areas. Commercial state of the art large area devices generally rely on flat panel technology. Sensors based on other technologies such as scintillator-CCD sensors or especially semiconductor-CMOS sensors produce higher image quality but suffer from a more limited active imaging area possible to manufacture.

0003The maximum continuous active area of single detector chip CCD and CMOS based sensors is typically a few square centimeters (cm2). These single detector chips may be combined by mounting them side by side in a mosaic like manner to form larger areas. Such mosaic constructions have been successfully applied in scanning systems in which the active imaging area has a slot shape to form a linear array of imaging elements. Rectangular shape imaging areas of a few tens of cm2 have also been successfully manufactured. Larger rectangular areas of CCD and CMOS based sensors are limited by the fact that one side of the single CCD and CMOS chips is always required for external electrical connections and prevents side by side mounting at that one side. Benefits of constructing imaging areas from separate small detector elements include flexibility to form areas of irregular shape and cost effective production. Prior art sensors, however, have not been able to address these needs.

0004Some applications may demand different sensor areas for different imaging modes. For example in modem digital dental extraoral X-ray imaging the same imaging system should be able to perform both fan beam panoramic scan acquisition and cone beam three dimensional (3D) imaging. In the scan mode a vertical relatively long slot like imaging area is preferred in order to match the sensor area with the X-ray fan beam shape and to optimize readout speed. In the 3D mode a vertically shorter but horizontally wider sensor area is optimal in providing appropriate cone beam coverage.

0005In the prior art, to match these partially contradicting sensor area requirements manufacturers of modem dental extraoral X-ray systems either have to use two separate available state of the art digital sensors or one large sensor (typically flat panel) with a sufficient area of rectangular shape to cover both the fan and the cone beam shapes. Both of these options introduce disadvantages in terms of cost, compactness and effective use. The present invention deals with this issue by introducing a novel digital X-ray sensor with a unique irregular shape of active area optimized for both of the imaging modes mentioned above. Dental extraoral X-ray imaging is here given as an example only. The invention can be used for benefit in any other X-ray-imaging application with similar requirements of imaging area. The invention can be realized especially well with semiconductor-CMOS detector technology but may also be realized with other technologies such as the scintillator-CCD technology.

0006Several ideas and methods of constructing larger mosaic type active imaging areas of single detector elements have been introduced and patented [<patcit id="pcit0001" dnum="US6207744B"><text>US Pat. 6,207,744</text></patcit>, <patcit id="pcit0002" dnum="US5812191A"><text>US Pat. 5,812,191</text></patcit>, <patcit id="pcit0003" dnum="EP0421869A"><text>EP0421869</text></patcit>, <patcit id="pcit0004" dnum="WO9708751A"><text>WO9708751</text></patcit>, <patcit id="pcit0005" dnum="EP0138647A"><text>EP0138647</text></patcit>]. The aim of such methods is generally to realize a large enough regular imaging area of either rectangular or slot shape. Most of the presented methods teach techniques to minimize the unavoidable dead space or blind region between the separate detector elements. The minimum gap between the active areas of adjacent detector elements is obviously achieved by mounting the elements in physical contact with each other. While eliminating or minimizing the inter-element dead space of multi element sensors is the ideal for acquiring uniform X-ray images it may not be feasible from the manufacturing point of view to assemble the separate detector elements physically touching each other. In addition to the optimal irregular shape of active area mentioned above the present invention introduces an effective manufacturing technique for multi element sensors. This technique is especially applicable to sensors based on semiconductor-CMOS technology and has specific relevance to CdTe-CMOS pixel detectors.

0007A device in accordance with the preamble of claim 1 has been presented at the 2007 convention of the German Society for Non-Destructive Testing (<nplcit id="ncit0001" npl-type="s"><text>P. Rost et al.: "Ein neues digitales Prüfsystem für die radiografische Prüfung von Rohr-Rohrboden-Verbindungen an Wärmetauschern mit computergestützter Auswertung", DGZfP-Jahrestagung, Fürth (DE), 14-16 May 2007</text></nplcit>).

SUMMARY OF THE INVENTION

0008The present invention provides a sensor structure leaving a finite, physical gap between adjacent detector elements. This structure is a departure from the prior art teachings of eliminating dead space between individual detector chips. This brings definite advantages in terms of production yield and long term endurance of the sensor. Since solid state semiconductors are generally fragile crystals, mounting them in physical contact increases greatly the risk of damaging the crystal edges with cracks or fractures during production. It also leaves the detector elements much more vulnerable to damage caused by thermal expansion or mechanical shock compared to the method of mounting the elements with an intermediate physical gap. Moreover, physical contact between the semiconductor detector crystals can lead to distortions in the signal collecting electric field applied to the crystals. The gap between the detector elements may be simply empty space or the gap may be created by placing some material such a mylar film in between the detector elements. The size of the gap is defined by the characterizing portion of claim 1.

0009The invention applies to X-ray imaging sensors made of CdTe-CMOS pixel detectors

0010The invention is to be used in particular in dental extraoral X-ray imaging but is beneficial in other application as well.

BRIEF DESCRIPTION OF DRAWINGS

0011<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> illustrates various options of irregular shape of the active area of the invented X-ray sensor.</li><li><figref idref="f0001">Figure 2</figref> illustrates a preferred shape of the active area of the invented X-ray sensor designed for dental extraoral X-ray imaging.</li><li><figref idref="f0002">Figure 3</figref> illustrates preferred shapes of the active area of the invented X-ray sensor designed for dental extraoral X-ray imaging.</li><li><figref idref="f0002">Figure 4</figref> illustrates a preferred shape of the active area of the invented X-ray sensor designed for dental extraoral X-ray imaging</li><li><figref idref="f0003">Figure 5</figref> illustrates plural views of one detector element.</li><li><figref idref="f0003">Figure 6</figref> illustrates a slot sensor with three separate detector elements, which is an embodiment of the present invention</li><li><figref idref="f0004">Figure 7</figref> illustrates two identical sensor substrates populated with detector elements to form different active sensor areas.</li><li><figref idref="f0005">Figure 8</figref> illustrates a sensor constructed of two separate sensor substrates.</li><li><figref idref="f0006">Figure 9</figref> is a schematic of a dental extra oral X-ray imaging system.</li></ul>

DESCRIPTION OF PREFERRED EMBODIMENTS

0012A radiation imaging device includes plural individual detectors defining an irregular rectangular active area responsive to x-rays and with different widths along a length of the active area. The individual detectors may be of different rectangular shapes and mounted on a motherboard. The motherboard may be formed of a first module mounting a first of two individual detectors and a second module detachable connected to the first module and mounting a second of two individual detectors.

0013A preferred shape 5 of the active area of the X-ray sensor is shown in <figref idref="f0001">Figure 2</figref>.

0014The shape 5 is in this example constructed from nine individual detector elements 8 and is designed for the needs of modem dental extraoral X-ray imaging in which a fan beam 3 is, at the same time, used for panoramic scan imaging and a rectangular cone beam 4 is used for 3D tomographic imaging. Typical dimensions of the preferred active area are given in <figref idref="f0001">Figure 2</figref>. As can be observed from <figref idref="f0001">Figure 2</figref>, the beam coverage of the preferred active sensor area 5 of irregular shape is much better than that of a large conventional rectangular area 6. The rectangular shape 6 leaves much more useless sensor area 7 both in the panoramic mode (fan beam) and in the 3D mode (cone beam).

0015As illustrated by <figref idref="f0001">Figures 1-2</figref>, in each case the radiation imaging device includes an active area responsive to x-rays. The active area has an irregular rectangular shape with an overall length y (150 mm in <figref idref="f0001">Figure 2</figref>) and an overall width x (50 mm in <figref idref="f0001">figure 2</figref>) . Advantageously, each imaging device has different local widths (50 mm, 6 mm in <figref idref="f0001">figure 2</figref>) for corresponding different ranges along the length (respectively the lower and upper halves of the <figref idref="f0001">Figure 2</figref> device).

0016<figref idref="f0002">Figure 3</figref> shows three other preferred shapes 19, 20 and 21 of active sensor area in the application of dental extraoral X-ray imaging. Many other similar shapes can also be used in dental extraoral imaging. The choice of shape depends on the X-ray beam shapes of the application. The shape 19 provides coverage for a larger cone beam which may be desired to acquire image data from a larger area for more comprehensive 3D or transverse slicing (tomographic) imaging. The shapes 20 and 21 provide less cone beam coverage resulting in a more economic sensor solution. In shape 21 the cone beam area 22 is lifted higher up than in shape 20. Shape 21 is a desirable sensor shape if the X-ray cone beam is centered higher up above the chin to cover more efficiently the teeth region.

0017<figref idref="f0002">Figure 4</figref> shows another preferred shape 9 of the active area optimized not only for dental panoramic and tomographic imaging but also for cephalometric imaging. In cephalometric imaging an image of the complete human skull is acquired and, therefore, the vertical dimension of the slot part of the active sensor area has to be longer.

0018<figref idref="f0003">Figure 5</figref> shows a drawing of one detector element 10 used to construct larger imaging areas in a radiation imaging device according to the present invention. Top and side views 23 and 24, respectively, of the element 10 are shown. The structure of the element 10 reflects the structure of a CdTe-CMOS detector which comprises a CdTe crystal 16 connected to a CMOS readout circuit 17. The element 10 has electrical connections 11 (typically ultrasonic wire bonds) at one side preventing side by side mounting of elements at this side. Other elements can be mounted very close or in physical contact to this one element 10 at all other sides 12.

0019<figref idref="f0003">Figure 6</figref> illustrates the invented radiation imaging device with detector elements 13 side by side with a finite physical gap 14 between the elements 13. The gap 14 between the outside-most edges of the detector elements can be empty or it can be filled by placing an electrically isolating material 15 such as a mylar film between the elements 13. The width of the gap 14 is in relation to the size of the detector pixel dimension and it can be smaller than the pixel size as long as electrical isolation is maintained and physical contact is avoided. This is important since the edges may be "rough" or diced with some acceptable degree of "chipping" and therefore the distance between the edges may be more appropriately be referred to as average distance. A typical pixel size may be 0.1 mm. However, the invention also applies to pixels sized from 0.05 mm to 0.1 mm. For the 0.1 mm pixel, the gap is within the range of 0.005 mm to 0. 10 mm, since it offers adequate spacing but also a small enough gap compared to the pixel size.

0020In one embodiment, the inventive radiation imaging device is made of individual detectors Cd(Zn)Te detectors juxtaposed next to each other with an average physical gap of 0.005 mm - 0.10 mm between the edges of the Cd(Zn)Te detectors This gap can be provided by a film, e.g., by a mylar thick 0.005 mm - 0.10 mm thick or alternatively by accurately placing the Cd(Zn)Te detectors using a microscope having an average gap in the above range.

0021<figref idref="f0004">Figure 7</figref> shows how two different shapes 32 and 33 of active sensor area can be constructed on identical sensor substrates 34 (also referred to as detector module (s)). In this illustration the substrate/detector module 34 is a printed circuit board (PCB). The same substrate can accommodate a varying number of detector elements of different or identical shape and size to form a desired active sensor area. The benefit of this is cost and time effective production.

0022<figref idref="f0004">Figure 7</figref> shows, for each of the identical sensor substrates/detector modules 34, a plurality of individual detectors defining an active area responsive to x-rays. The active areas each have a rectangular shape with an overall length y and an overall width x. In each case the substrate/ detector module 34 serves as a common motherboard with the active area comprises individual detectors of different rectangular shape commonly mounted on the mother board. As shown, a first of the detectors has a first active length and a first active width x1, and a second of the two detectors has a second active length y2 and a second active width x2. As the two types of individual detectors have different rectangular shapes, at least one the first active length y1 and the second active length y2 are different or the first active width x1 and the second active width x2 are different. This allows the construction of the active area to be such that the active width x varies along the active length y (for example see active width x and x' in <figref idref="f0004">figure 7</figref>).

0023<figref idref="f0005">Figure 8</figref> shows how a desired sensor area can be constructed by combining two different sensor substrates 35 and 36 side by side in a detachable manner. In this example substrate 35 accommodates a slot like linear array sensor and substrate 36 accommodates a square shape sensor. The benefit of this is as above effective production and product development. Using plural substrates allows the separate substrates to be used independently and flexibly in applications where an irregular shape of the sensor area is not required or in combination to form a shape of active area according to the present invention.

0024<figref idref="f0005">Figure 8</figref> shows a radiation imaging device comprised a first module 35 (i.e., first detector substrate) mounting a first individual detector 37 and a second module 36 (ie second detector substrate) detachable connected to the first module 35 and mounting a second individual detector 38. Together the first and second detectors define the active area responsive to x-rays. As shown, the first and second detectors are of different rectangular shape with at least one of their lengths and widths being different.

0025<figref idref="f0006">Figure 9</figref> illustrates the application to dental extra oral X-ray imaging. A patient 25 is placed between an X-ray source 26 and an X-ray imaging sensor 27. The image acquisition is performed as a rotational scan around the head of the patient. The X-ray beam shape 28 is in this illustration optimized for simultaneous acquisition of a panoramic image and a tomographic image. The tomographic image data is collected by the lower and wider part 29 of the sensor area while the narrower slot like full length 30 of the active area is used to collect the panoramic image data.

0026The data is acquired at a predefined rate as image frames each frame corresponding to a certain position of the X-ray source and sensor along the rotational path of the scan. The data is sent to a computer 31 for image reconstruction and display. A full panoramic layer or a local part of a panoramic layer as well as a transverse slice or a 3D image corresponding to a local part of a panoramic layer can be reconstructed from the data frames.

0027The system of <figref idref="f0006">Figure 9</figref> thus provides an extra-oral dental x-ray imaging system. The x-ray source 26 generates x-rays for exposure of such x-rays to the patient to be imaged. The x-ray imaging devices 28, as disclosed above, are used for producing multiple frames during at least part of the exposure. At least one of the x-ray source and imaging device rotate around at least one rotational axis 37 defined by a spline 38, the axis being located between the x-ray source focal point and the x-ray imaging device and changing position along directions 39 and 40 during the scan.

0028The computer 31, a processor, processes the frames of a single exposure to compose selectively at least two of a group of elements, the elements comprising (a) a predetermined dental panoramic layer image, (b) a local part of a non-predetermined dental panoramic layer image, (c) a transverse slice to a local part of a dental panoramic layer image; and (d) 3-D reconstruction of a volume corresponding to some local part of a dental panoramic layer.