Digital solid-state radiation detectors, that is to say digital X-ray detectors of different types are increasingly being used in the context of radiation image recording, be it in traditional radiography, fluoroscopy, angiography or cardioangiography. Such solid-state radiation detectors, also called flat detectors, are based on active pixel or read-out matrices which are composed e.g. of amorphous silicon. The incoming X-ray radiation is converted, in a scintillator layer functioning as an X-ray converter, into radiation which can be processed by the active pixel matrix and by which an electrical charge is generated in the photodiodes of the pixel matrix and is stored there.
The image quality of a solid-state radiation detector depends on a multiplicity of parameters. These include in particular the scintillator or converter material, cesium iodide (CsI) or gadolinium oxysulfide (GdO2S2) principally being used here, furthermore the design of the pixel matrix (size, filling factor, etc.) and also the read-out electronics etc. The image quality itself can be described by way of the modulation transfer functions MTF, the NPS value (NPS=noise power spectrum) and the effective quantum absorption DQE (DQE=detective quantum efficiency), the DQE being a derived variable. In solid-state detectors, the image quality is considerably reduced in particular by the so-called “low frequency drop” (LFD). The “low frequency drop” leads to a reduction of the MTF at low spatial frequencies, up to an order of magnitude of approximately 10%. This leads to significant losses in the DQE, which represents the actual variable which is relevant to image quality and which describes both the signal behavior and the noise behavior of the detector, to losses of up to approximately 20% since the MTF is incorporated quadratically in the calculation of the DQE.
In order consequently to improve the image quality of a solid-state radiation detector, it is therefore crucial to minimize the “low frequency drop”, which is one of the central causes of the reduction of the DQE.
At least one embodiment of the invention is based on the problem of specifying a solid-state radiation detector in which the “low frequency drop” caused by the occurrence of scattering effects of the converted radiation is to be reduced, and of improving the image quality.
In order to improve upon or even solve this problem, in the case of a solid-state radiation detector, at least one embodiment of the invention provides for at least one layer that at least partly absorbs the light that originates from the scintillator layer and has penetrated into the carrier to be provided at the carrier.
At least one embodiment of the invention is based on the insight that a non-negligible scattered light component is brought about by light or light quanta brought about through the transparent sections of the pixel matrix into the matrix carrier, which is transparent to the light originating from the scintillator layer. The light that has penetrated into the carrier is reflected therein; the carrier acts virtually like an optical waveguide.
After single or multiple reflection at a different transparent section of the pixel matrix, the reflected light again enters into the scintillator layer, where it is likewise reflected and impinges on a different pixel than the one assigned to the generation location. That is to say that light which enters into the matrix carrier in an undesirable manner is coupled into the pixel matrix possibly at a completely different location. This scattered light component, which is added to the possible scattered component within the scintillator layer itself, is not negligible.
In order to lessen or even avoid at least one of the resultant disadvantages, at least one embodiment of the invention provides for the provision of an absorption layer at the carrier, which absorption layer at least partly absorbs the scintillator light that has penetrated undesirably into the carrier. This layer, which is preferably provided at the carrier at the opposite side to the pixel matrix, diminishes or even prevents any reflection processes from actually occurring in the carrier material. The scattered component on the carrier side may thereby be reduced or even minimized through to completely reduced. This may be accompanied by a significant reduction of the “low frequency drop”, in association with a significant improvement of the MTF and the DQE.
The signal transfer behavior is consequently improved and the imaging properties are improved or even optimized. Comparable image qualities between detectors having DQE functions of varying quality can consequently be achieved with significantly lower X-ray doses, a lower DQE being tantamount to a higher dose requirement for obtaining a comparable image quality.
According to a first simple refinement of an embodiment of the invention, the absorption layer provided according to an embodiment of the invention may be a cured coating or a film. The coating may be for example an enamel coating or the like. The film may be a plastic film which is for example laminated onto the carrier or bonded to it in some other way.
With regard to the fact that the light emitted by the scintillator originates from a defined, known wavelength range, the cured coating may be a special color coating, the primary color of which is chosen such that precisely light having a wavelength corresponding to the scintillator light is absorbed. The film may correspondingly be a color film. Such a wavelength-specific absorption property is not mandatory, however; a black color coating or a black film that generally absorbs over the visible wavelength range may also be involved.
While the use of a simple coating or film is appropriate when the solid-state radiation detector does not have a reset light source serving for the defined resetting of the individual photodiodes of the pixel matrix, a solid-state radiation detector provided with a reset light source arranged adjacent to the carrier makes somewhat different requirements of the type or quality of the layer. In the case of such a solid-state radiation detector, according to an embodiment of the invention the layer is arranged between the carrier and the reset light source, which is preferably formed as a sheetlike reset light layer into which the reset light is coupled at a defined location.
At least when the reset light source is operated, that is to say therefore the resetting is effected, the layer is at least partly transparent to the light emitted by the reset light source. Here the layer has the task, on the one hand, of preventing part of the light converted at the scintillator from finding its way into the carrier, for example the glass substrate, and from there into the scintillator again and, consequently, impinging on a photodiode at a different location. On the other hand, it must be ensured that the reset light can pass via the carrier and the transparent regions in the active photodiode matrix into the scintillator and from there to the photodiodes. The absorption or transmission behavior of the layer must consequently be either adaptable or wavelength-selective.
In order to achieve this, the layer may preferably be variable or switchable in terms of its absorption behavior, preferably by way of an electrical control voltage that can be applied to the layer. This is possible, for example using an electrically drivable organic layer or an LCD layer. Both layers, which can be applied or produced in very thin fashion, make it possible, through application of a dedicated control or switching voltage, to switch or to vary the transmission or absorption behavior in specific regions. In both types of layer, an orientation of molecules integrated at the layer, for example of the liquid crystal molecules of an LCD or liquid crystal layer, is produced by means of the electric field generated upon application of the control voltage. This results in a change in the polarization properties of the layer, and consequently its transmission properties. The function of such organic or liquid crystal layers is generally known and does not require more detailed description.
If such a layer is used, it is thus possible, through application of a corresponding control voltage to the respective layer, for the absorption behavior either to be varied continuously variably between two limit values or to be switched between these two limit values.
As an alternative to the use of an electrically controllable layer, the solid-state radiation detector may, according to an embodiment of the invention, also be formed in such a way that the scintillator layer and the reset light source emit light from different wavelength ranges, the layer essentially only being absorbent for light from the wavelength range of the light emitted by the scintillator layer and essentially being transparent to the light emitted by the reset light source. This is based on the concept that the scintillator emits light from a relatively narrowly delimited wavelength range. Thus, CsI emits green light, for example.
If provision is then made of a layer which absorbs this light and is otherwise transparent to light outside this wavelength range, it is possible, with the use of a reset light source that emits light from such a different wavelength range, for this light readily to pass through the layer transparent to it into the carrier and from the latter to the scintillator or the pixel matrix. In this case, the reset light source emits in the red light range, for example. The layer used may in this case likewise be a cured coating, in particular a color coating, or a film, in particular a color film. Coatings or films which for example have special color centers or are naturally correspondingly colored are conceivable in this case.