US 7408165 B2
The invention relates to a detector arrangement for the conversion of electromagnetic radiation into electrical signals. The detector arrangement includes sensitive areas (D1, D2, D3, D4), where each sensitive area corresponds to a respective electrical signal, and at least two of the sensitive areas mesh with one another in such a manner that non-overlapping envelopes (C1, C2, C3, C4) of the individual meshing sensitive areas also mesh with one another.
1. A detector arrangement for the conversion of electromagnetic radiation into electrical signals, which arrangement includes sensitive areas, each of which corresponds to a respective electrical signal, wherein at least two of the sensitive areas mesh with one another by positioning a portion of one of the sensitive areas in a fully enclosed free area of another one of the sensitive areas.
2. A detector arrangement as claimed in
3. A detector arrangement as claimed in
4. A detector arrangement as claimed in
5. A detector arrangement as claimed in
6. A detector arrangement as claimed in
7. An imaging X-ray system which includes a detector arrangement as claimed in
8. The detector arrangement of
9. The detector arrangement of
10. The detector arrangement of
11. A method for the conversion of electromagnetic radiation into electrical signals, which method comprises:
emission of electromagnetic radiation by a radiation source,
detection of the electromagnetic radiation by means of a detector arrangement which includes sensitive areas,
conversion of the electromagnetic radiation into electrical signals such that the sensitive areas of the detector arrangement correspond unambiguously to a respective electrical signal, wherein at least two of the sensitive areas mesh with one another so that a portion of one of the sensitive areas is positioned in a fully enclosed free area of another one of the sensitive areas, and
propagation of the electrical signals.
12. The detector arrangement of
13. The method of
14. The method of
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16. The method of
The invention relates to a detector arrangement which is intended to convert electromagnetic radiation into electrical signals and includes sensitive areas.
Detector arrangements which convert electromagnetic radiation into electrical signals are used in a variety of applications, for example, in CCD cameras or in X-ray detectors. Because detector arrangements of this kind are often manufactured in an integrated process, the sensitive areas (for example, photodiodes) have essentially a simple geometrical structure, for example, a rectangular shape, so that read-out leads and supply leads can be readily realized between the sensitive areas. A detector arrangement of this kind is usually intended to generate, by way of a source of electromagnetic radiation, a projection of an image (object image) which is converted into a measured image (data image) by the detector arrangement. The detector arrangement absorbs the electromagnetic radiation and converts it, directly or indirectly, into electrical signals (typically charge signals, current signals and/or voltage signals). As a result of the sensitive areas, localized scanning of the object image takes place. Ultimately, the data image (display image) is presented on a display medium, for example, a monitor, that is, possibly after post-processing of the data; the display image then consists of a discrete number of pixels, typically arranged in a matrix structure of n×m pixels, where n and m are integers. A pixel is usually associated with a sensitive area or with a group of sensitive areas, but the display image matrix may also be calculated by interpolation of the data image values. For example, the matrix of rectangularly arranged pixels can be calculated from a matrix of hexagonally arranged detector elements.
Detector arrangements of this kind are important in particular for medical imaging. When a detector arrangement of this kind consists of a scintillator layer and photodiodes which are situated therebelow, incident X-rays are converted into optical light quanta by the scintillator layer; the photodiodes, constituting the sensitive areas in this case, absorb the light quanta and charge carriers are generated. Because the process of converting X-rays into light quanta in a scintillator layer is a process in which the light quanta generated are isotropically emitted from the location in which they are generated, a photodiode cannot be unambiguously associated with a part of the surface of the scintillator layer. As a result, the object function of a sharp edge is not represented by a sharp “staircase structure” in the pixels of the display image, but by a “soft” edge structure; the “softness” of the edge, that is the width of the edge, is then dependent on the size of the sensitive areas and on the thickness and the type of scintillation material (for example, chemical composition, specific density, structuring) and further variables. Such softening (or low-pass filtering) of an object function by the detector is also described by the modulation transfer function (MTF) of the detector (electronic effects being ignored in this context). The “softening” leads to a reduction of high frequencies in the object function.
In the case of a spatially discrete scanning of the object function the modulation transfer function imposed by the finite dimensions of the sensitive areas of the photodiodes is strongly limited in the reproduction of high frequencies. When the object function of a sharp edge is scanned by such a photodiode arrangement, the high frequencies of the object function lead to disturbing aliasing effects. A scintillator layer significantly reduces aliasing effects due to the “softening” of the incident object function, that is, by suppressing the high frequencies. The low-pass filtering of the relevant object function, caused by the scintillator and taking place prior to the discrete scanning by the photodiodes of finite dimensions, is also referred to as pre-sampling filtering.
If the detector arrangement does not include a scintillator layer which suppresses the high frequencies of the object function already before the sensitive areas of the detector elements, disturbing aliasing effects occur. One example in this respect is an X-ray detector which consists of a directly converting material and comprises a non-structured top electrode at the X-ray side and single electrodes, constituting the sensitive areas in the present example, on the lower side of the directly converting material. Incident X-rays are absorbed by the directly converting material so as to be converted into charge carriers. The charge carriers are accelerated by means of a strong electrical field between the top electrode and the single electrodes so as to be converted into an electronic current signal. The strong electrical field between the top electrode and the single electrodes ensures that only little crosstalk occurs between the detector elements. Thus, the pre-sampling filtering which enhances the imaging properties is essentially absent in this case.
It is an object of the present invention to improve a detector arrangement for the conversion of electromagnetic radiation into electrical signals in such a manner that aliasing effects are reduced.
This object is achieved by means of a detector arrangement for the conversion of electromagnetic radiation into electrical signals which includes sensitive areas, each of which corresponds to a respective electrical signal, it being arranged that at least two of the sensitive areas mesh with one another.
This is particularly advantageous for detector arrangements without pre-sampling filtering in which, therefore, the sensitive areas define essentially the sampling of the object function. However, when the sensitive areas mesh with one another, there will be parts of a sensitive area which not only adjoin parts of another sensitive area, but partly or completely enclose such parts or are enclosed thereby, or there will be parts of sensitive areas which are enclosed completely by parts of another sensitive area. Such meshing leads to less sharp confinement of the sampling and hence to a reduction of the aliasing effects.
Claim 2 discloses a particularly advantageous embodiment of the invention in which the meshing of the sensitive areas with one another is realized by means of dentation (coherent meshing) or by interleaving (incoherent meshing).
In respect of their sampling properties for the object function the sensitive areas are defined in conformity with claim 3, that is, by way of respective associated sensitive surfaces which mesh with one another.
Sensitive areas which are sensitive either to electromagnetic radiation or to the conversion products of a conversion layer which forms part of the detector arrangement are advantageously realized way of photodiodes or electrodes as described in claim 4. In these cases the surfaces of the sensitive areas define the sampling properties of the sensitive areas.
Claim 5 discloses an advantageous embodiment of the invention which comprises sensitive areas of equal size. This results in homogenization of the detector properties (for example, signal level, dark current etc.). It is also advantageous when the shape of the sensitive areas varies, so that moiré like effects can be avoided (claim 6).
The invention also relates to an X-ray apparatus, notably an imaging X-ray apparatus, in which the detector arrangement in accordance with the invention is employed.
The invention also relates to a method of detecting electromagnetic radiation.
Several embodiments of the invention will be described in detail hereinafter with reference to the Figures. Therein:
With each photodiode in the present embodiment of a flat X-ray detector there is associated a switching transistor which propagates the charge stored in the capacitance of the photodiode during the read-out operation. This switching transistor is realized each time in the free (not-sensitive) area S1, S2 associated with the photodiode. In addition there are provided (not shown) read-out leads and bias voltage leads which extend between the photodiodes. Because of the leads extending between the photodiodes, it is difficult to indicate spatially separate areas which can be unambiguously associated with a detector element, since, for example, the line driver leads are associated with many detector elements in common. Therefore, the subdivision into rectangular areas C1, C2 as denoted by heavy lines need not necessarily be understood to be a physical subdivision into detector elements. The subdivision, however, shows that the detector surface can be subdivided into convex (in this case rectangular) elements in such a manner that each convex area C1, C2 encloses the entire sensitive surface of a detector element. In this context the term “detector element” is to be understood as the combination consisting of the sensitive area and the associated electronic circuitry. A convex subdivision into its elements can also be found for detectors whose detector rows or columns are offset relative to one another or for a hexagonal detector element arrangement. It is to be noted that the sensitive areas shown in the Figures may also be situated on a detector surface which is curved in space, for example, as is the case for detector arrangements for computer tomography apparatus which are arranged around the focus of the radiation source as the center on a segment of the surface of a cylinder. Curvature in both dimensions (for example, like a segment of a spherical surface) is also permissible.
When such a detector is read out, an electrical signal is read out from each detector element (for example, from a sample-and-hold stage). Each of the electrical signals corresponds to a respective sensitive surface. This results in a two-dimensional data image which corresponds to the object image MTF filtered by the detector.
In a detector which is based on optically encapsulated scintillator crystals (for example, scintillator crystals separated from one another by means of reflectors), the scintillator crystals do not serve for the pre-sampling filtering as is the case in a non-structured scintillator. In this case pre-sampling filtering can also be realized by the meshing of the scintillator crystals which are optically encapsulated from one another. The sensitive areas are then formed by the scintillator crystals and the meshing can be configured so as to be three-dimensional.
As opposed to, for example, a capacitive coupling between neighboring sensitive surfaces, the meshing ensures that the sharing of signal contributions occurs only in the dentated areas, whereas a capacitive coupling mixes the signals from the entire sensitive surface of neighboring detector elements. The latter corresponds to a filtering after the sampling by means of the sampling function of the object image which is given by the sensitive surfaces, so that aliasing effects cannot be reduced.
As for the dentations, it is to be understood again that the interleaving can be carried out by way of free areas of different geometrical shape and sensitive surfaces of neighboring detector elements which are realized therein, and also the type of interleaving may vary etc. Furthermore, dentation and interleaving may also be combined.