Acoustic radiation detector for high-energy radiation

Abstract

The invention relates to an acoustic radiation detector (4) for high-energy radiation, comprising a solid absorption layer (12) and a detector assembly (13) which is acoustically coupled to the absorption layer (12), wherein - the absorption layer (12) is designed, with respect to the material and thickness thereof, such that at least 50% of the particle radiation or electromagnetic radiation of the specified radiation energy is absorbed, and - the detector assembly (13) comprises a plurality of acoustic sensors (14) which are arranged in a matrix over a surface and are designed to measure sound waves emerging from the absorption layer (12). The invention also relates to an imaging medical system.

Description

[0001] Description

[0002] Acoustic radiation detector for high-energy radiation

[0003] The invention relates to an acoustic radiation detector for high-energy radiation and an imaging medical system.

[0004] Medical imaging frequently employs high-energy radiation. This radiation has the advantage of allowing the examination of a subject, such as an object or a patient, to be visualized, revealing internal structures. Often, electromagnetic radiation, such as X-rays, is used, but particle radiation can also be employed, as in PET imaging.

[0005] A significant disadvantage is that high-energy radiation can ionize molecules. This can have negative effects on a patient, but also on sensitive components such as electronic devices. The goal should be to capture the most accurate images possible with the lowest possible radiation dose.

[0006] For example, X-ray imaging is usually performed with image detectors that contain a scintillator, which converts the X-rays into (often visible) light, which is then converted into measurement signals by photodetectors. Only a small portion of the energy of the incoming photons is converted into light. This portion depends on the scintillator materials used and is typically only 15%. Furthermore, approximately 30% of the incoming photons pass through the scintillator and do not produce any light at all, and therefore no measurement signal. The majority of the losses (the remaining 55%) are due to thermal losses, mechanical losses (e.g., elastic collisions), and other interactions of the X-ray photons.

[0007] This means that with conventional image detectors, a large proportion of the photons striking the detector (85%) are not used for imaging. This poses a problem because a patient or object is thereby exposed to an unnecessarily high dose.

[0008] It is an object of the present invention to provide an acoustic radiation detector for high-energy radiation and a medical imaging system with which the disadvantages described above are avoided and, in particular, photons striking the detector are used more effectively for imaging. This object is achieved by an acoustic radiation detector according to claim 1 and a medical imaging system according to claim 12.

[0009] An acoustic radiation detector according to the invention for high-energy radiation comprises a solid absorption layer and a detector arrangement which is acoustically coupled to the absorption layer, wherein

[0010] - the absorption layer is designed with regard to its material and thickness in such a way that at least 50% of particle radiation or electromagnetic radiation of the specified radiation energy is absorbed,

[0011] - the detector arrangement comprises a multitude of acoustic sensors arranged in a matrix over an area, which are designed to measure sound waves emerging from the absorption layer.

[0012] The acoustic radiation detector according to the invention thus detects the incoming high-energy radiation (e.g., X-rays, electron beams, or proton beams) not via an optical detour, but by means of sound. This sound is generated by thermal pressure changes when high-energy radiation is absorbed by matter.

[0013] A special feature of the invention lies in the solid absorption layer. Instead of a liquid to conduct sound, a solid body, such as a lead plate, is used. When high-energy radiation strikes this absorption layer, vibrations are generated within it, which can then be detected by the detector arrangement.

[0014] Particularly preferably, air is located between the object to be recorded and the absorption layer. This allows the acoustic radiation detector according to the invention to be used in the same way as a conventional radiation detector.

[0015] The detector arrangement is acoustically coupled to the absorption layer, meaning that the detector arrangement is positioned on the absorption layer in such a way that it can measure, and in particular directly measure, the structure-borne sound of the absorption layer. This means that sound waves from the absorption layer can penetrate into the detector arrangement without having to travel through air. Preferably, the acoustic sensors of the detector arrangement are connected to the absorption layer with their sensitive side directly or via another solid, e.g., pressed or glued to it. However, the acoustic sensors can also simply touch the absorption layer or a layer connected to it.The absorption layer is designed with respect to its material and thickness such that particle radiation or electromagnetic radiation of a predetermined energy is absorbed at least 50%, preferably at least 60%, particularly at least 70%, and most preferably at least 80% or even more than 90%. The absorption properties of materials for X-rays of a predetermined energy are known, and the required thickness can therefore be calculated. For example, the thickness a material must have to absorb 50% of a known radiation is called the "half-value thickness." This predetermined radiation energy should be the maximum energy of the beam with which investigations are to be carried out with the radiation detector as intended.

[0016] In X-ray examinations, radiation energies typically range between 20 keV and 120 keV. A preferred predefined radiation energy is therefore 120 keV, or 160 keV when used in a system with slightly higher radiation energies, or 80 keV when used in a system with slightly lower radiation energies.

[0017] At such beam energies, a layer of a material typically used as an absorber (especially with an average atomic mass >160 u) should preferably be at least 0.1 mm thick, preferably at least 0.5 mm, and particularly at least 1 mm thick. However, the thickness should not exceed 1 cm and, in particular, should be a maximum of 5 mm. Considering an average atomic mass of A (units are considered dimensionless), the material should preferably have a thickness of at least 100 / A mm, more preferably at least 150 / A mm, and in particular at least 180 / A mm. To avoid using too much material, the thickness should preferably be a maximum of 400 / A mm, more preferably a maximum of 300 / A mm, and in particular at least 250 / A mm. The unit unit is neglected in this calculation, and the atomic mass in units is considered dimensionless. For example, for a given beam energy of 120 keV for lead (atomic mass A approx. 208 u), a thickness of 1 mm could be considered.1 mm can be used for an absorption layer made of lead.

[0018] The detector arrangement comprises a plurality of acoustic sensors arranged in a matrix over a surface. Suitable acoustic sensors are known in the prior art. These should be matched to the expected sound waves in the absorption layer, which are typically in the ultrasonic range, particularly above 20 kHz. Therefore, the acoustic sensors should be ultrasonic sensors. Preferred acoustic sensors are piezoelectric sensors or capacitive sensors, e.g., capacitive micromachined ultrasound transducers (cMUTs). Optical sensors, such as interferometric sensors or silicon photonic sensors, e.g., fiber Bragg grating sensors, are also preferred. The arrangement of the acoustic sensors should correspond to that of a known image detector.

[0019] Its pixels are usually arranged in even rows and columns.

[0020] An imaging medical system according to the invention is designed to acquire images by detecting electromagnetic radiation or particle radiation using a detector. This is the typical task of imaging systems. The special feature of this system is that it includes an acoustic radiation detector according to the invention. It may also include other image detectors, e.g., a conventional optical image detector.

[0021] The invention thus utilizes the photoacoustic effect to achieve imaging with lower conversion losses than conventional image detectors. The photoacoustic effect describes the conversion of radiant energy into mechanical energy in the form of acoustic waves or shock waves in a solid (the absorption layer). Detection of a radiation quantum occurs such that it is first absorbed in the absorption layer, thereby causing a local thermoelastic expansion due to the absorption of the electromagnetic energy in this layer. This generates an acoustic (shock) wave, which can be measured by the acoustic sensors.

[0022] The beam should advantageously be pulsed to achieve good temporal resolution of the measured values.

[0023] Further, particularly advantageous embodiments and developments of the invention result from the dependent claims and the following description, wherein the claims of one claim category may also be further developed analogously to the claims and description parts of another claim category and, in particular, individual features of different embodiments or variants may be combined to form new embodiments or variants.

[0024] A preferred radiation detector is characterized by the fact that adjacent acoustic sensors of the detector arrangement are acoustically decoupled from each other with respect to structure-borne sound. This means, in particular, that adjacent acoustic sensors do not directly touch each other. Preferably, there is a space (vacuum or air) between the acoustic sensors, but a sound-absorbing layer, e.g., made of foam, can also be arranged between adjacent acoustic sensors.

[0025] It is preferred that the absorption layer is also divided into acoustically separated areas, with these areas being acoustically coupled to one or more acoustic sensors. No acoustic sensor should be acoustically coupled to two of these areas, as this would result in the acoustic coupling of the two areas via the acoustic sensor. The acoustic separation can be achieved by means of acoustic decoupling of the acoustic sensors, such as gaps or a sound-absorbing layer between adjacent areas. Alternatively, each acoustic sensor could have its own absorption layer, with adjacent acoustic sensors then preferably arranged in a matrix with acoustic decoupling.

[0026] It should be noted that acoustic decoupling is not strictly necessary. In particular, the absorption layer can be a continuous layer over the (optionally acoustically decoupled) detector array, and an event in the absorption layer can be registered by multiple acoustic sensors. Based on temporal information and information about measured amplitudes, conclusions can be drawn about the point of impact of an X-ray photon. However, sound reflections should be suppressed for this purpose.

[0027] A preferred radiation detector is characterized by a detector arrangement comprising ultrasonic sensors, preferably piezoelectric sensors or capacitive sensors, e.g., capacitive micromachined ultrasound transducers (cMUTs), and / or silicon photonic sensors, e.g., fiber Bragg grating sensors. Such sensors are advantageous for imaging because they can be manufactured very small and have good sensitivity to the sound waves occurring in the absorption layer. Preferably, optical sensors are spectrally decoupled from both the high-energy radiation and the detector frequency (electrical signals), which enables interference-free detection.

[0028] A preferred radiation detector is characterized in that the absorption layer is a solid layer and comprises at least 20% by weight, preferably more than 40% by weight, elements with an atomic number greater than 40, preferably greater than 70, particularly preferably tungsten, bismuth, and / or lead. Such materials are good absorbers for the radiation used in medical imaging.

[0029] A preferred radiation detector is characterized by an absorption layer that is a metallic layer made of a metal or metal alloy. Such layers possess both good absorption properties and good acoustic properties.

[0030] A preferred radiation detector comprises an impedance matching layer between the detector assembly and the absorption layer, which matches the acoustic coupling of the detector assembly with the absorption layer. Matching the acoustic coupling reduces reflections at the layer boundaries and thus improves sound measurement. Preferably, the acoustic impedance Zm in the matching layer lies between the acoustic impedance Zd of the detector assembly and the acoustic impedance Za of the absorption layer. Particularly preferably, Zm is the square root of the product of Za and Zd.

[0031] Preferably, the matching layer is formed from several layers with different acoustic impedances. The layers are preferably arranged such that the acoustic impedance decreases continuously towards the detector array. As in the calculation above, it is preferred that the acoustic impedance of a matching layer is the square root of the product of the impedances of the two adjacent layers.

[0032] A preferred radiation detector is characterized by an acoustic impedance of less than 10 MRayl, preferably less than 3 MRayl. For example, a silicon-based ultrasonic sensor has an acoustic impedance of 1.42 MRayl.

[0033] Preferably, the acoustic impedance of the absorption layer is greater than 50 MRayl, preferably greater than 80 MRayl. For example, a tungsten layer has an acoustic impedance of approximately 98 MRayl.

[0034] Preferably, the matching layer is formed from a number of layers from the group consisting of magnesium layer (acoustic impedance approx. 10 MRayl), aluminum layer (acoustic impedance approx. 17 MRayl), steel layer (acoustic impedance approx. 45 MRayl), gold layer (acoustic impedance approx. 63 MRayl) or a glass or ceramic layer (e.g. made of aluminum oxide with an acoustic impedance of approx. 10 MRayl or zirconium with an acoustic impedance of approx. 30 MRayl).

[0035] A preferred radiation detector includes a termination layer behind the acoustic sensors. This termination layer is designed and arranged to prevent reflections from a rear wall of the acoustic sensors and / or the detector assembly. The termination layer serves to suppress reflections of sound from the rear of the acoustic sensors. Such reflections can distort a measurement. The termination layer should have a high acoustic attenuation coefficient to effectively prevent reflections. Preferred materials for the termination layer are tungsten (preferably in powder form), silver, epoxy resin, or a mixture of these materials. For example, a mixture of powdered tungsten in an epoxy resin matrix can be used as the termination layer because this material has a high acoustic attenuation coefficient.A preferred radiation detector is characterized in that the thickness of a layer, preferably of all layers, of the matching layer and / or the capping layer corresponds to one-quarter of the mean wavelength of the sound waves in the respective material of the layer. This is based on the mean wavelength of the sound waves after generation by radiation of a predetermined energy in the absorption layer and the transmission of the sound waves into the respective layer of the matching layer. It should be noted that the spectrum of the propagating acoustic wave is normally Gaussian distributed in the frequency domain and depends on the pulse duration of the absorbed X-rays. The sound frequency is inversely proportional to the pulse duration of the excitation. The maximum of this Gaussian curve then indicates the mean wavelength, with frequency and wavelength being coupled via the speed of sound in the medium.With an exemplary pulse duration in the range of 100 ns - 1000 ns, sound waves with a mean frequency of several MHz can be generated.

[0036] A preferred radiation detector further comprises an optical image detector, in particular with a scintillation layer and photosensors. It is particularly preferred that the absorption layer and the acoustic sensors are arranged behind the photosensors, or that the scintillation layer is designed as an absorption layer and the acoustic sensors are arranged between or behind the photosensors. This allows the advantages of optical and acoustic measurements to be combined. As indicated above, a portion of the radiation passes through the image detector and can be measured with the acoustic detector.

[0037] A preferred radiation detector is characterized by the fact that the largest diameter of an acoustic sensor in a plane parallel to the absorption layer is less than 0.05 mm, preferably less than 0.03 mm. This allows for good spatial resolution of the radiation detector. With the aforementioned matrix-shaped arrangement of the acoustic sensors, resolutions greater than 300 dpi can be achieved with such dimensions.

[0038] A preferred medical imaging system is based on particle radiation or electromagnetic radiation. Such a system is preferably a radiography system, a CT system, a PET system, a tomosynthesis system, an angiography system, or a mammography system.

[0039] A preferred medical imaging system comprises a measuring device that receives signals from acoustic sensors and is designed to measure signal intensity and / or (absolute or relative) reception time of signals from the acoustic sensors. A preferred medical imaging system comprises a number, preferably a plurality, of acoustic radiation detectors according to the invention. At least some of these are particularly preferably arranged for measuring scattered radiation from an object. A conventional image detector, an acoustic radiation detector according to the invention, or a combination of both detectors can be arranged directly in the beam direction, and a plurality of further acoustic radiation detectors can be arranged around the imaging area of ​​the medical imaging system for measuring scattered radiation.

[0040] The advantages of the invention are a higher X-ray detection efficiency for clinical imaging through the absorption layer, the possibility of measuring new image content or image information from the acoustic readouts, a cost-efficient design through inexpensive acoustic sensors, no additional X-ray dose due to the effective absorption and use of radiation, and a possible design for the cost-efficient detection of scattered X-rays.

[0041] The invention is explained in more detail below with reference to the accompanying figures and exemplary embodiments. The same components are designated with identical reference numerals in the various figures. The figures are generally not to scale. They show:

[0042] Figure 1 shows a rough schematic representation of a CT system according to the state of the art,

[0043] Figure 2 is a sketch of an imaging system according to the state of the art,

[0044] Figure 3 shows a sketch of an imaging system with an acoustic radiation detector according to the invention,

[0045] Figure 4 shows a special embodiment of a sound-decoupled radiation detector,

[0046] Figure 5 shows another special embodiment of the sound-decoupled radiation detector,

[0047] Figure 6 shows a special embodiment of the radiation detector with an matching layer,

[0048] Figure 7 shows a particular embodiment of the radiation detector with a multilayer matching layer; Figure 8 shows a particular embodiment of the radiation detector in combination with an optical detector.

[0049] Figure 9 shows another special embodiment of the radiation detector in combination with an optical detector,

[0050] Figure 10 shows a sketch of an imaging system with several acoustic radiation detectors according to the invention for measuring scattered radiation.

[0051] Figure 1 shows a computed tomography (CT) system 1 with a radiation detector 4 and a radiation source 5. The radiation source 5 is configured to expose the radiation detector 4 with radiation. The CT system 1 shown comprises a gantry 2 with a rotor 3. The rotor 3 includes an X-ray source 5 as the radiation source 5 and the radiation detector 4, which is configured to detect X-rays.

[0052] The rotor 3 is rotatable about the axis of rotation 8. The patient 6 is positioned on the patient table 7 and can be moved along the axis of rotation 8 by the gantry 2. The processing unit 9 is provided for controlling the CT system 1 and / or for generating an image data set based on signals detected by the radiation detector 4.

[0053] Typically, a (raw) X-ray image dataset of the patient 6 is acquired from a variety of angular directions using the radiation detector 4. Subsequently, a (final) image dataset can be reconstructed based on the (raw) X-ray image dataset using a mathematical procedure, for example, including a filtered backprojection or an iterative reconstruction method.

[0054] The processing unit 9 serves here as a control unit 9 for controlling the CT system 1. An input device 10 and an output device 11 are connected to this processing unit 9. The input device 10 and the output device 11 can, for example, enable interaction by a user or the display of a generated image data set.

[0055] It is noted that the invention can be used for any X-ray imaging application. Its use in a CT scanner is just one of many examples, and the invention is also advantageous for many other X-ray systems and systems that operate with high-energy particle radiation and electromagnetic radiation, such as radiography systems, PET systems, angiography systems, or mammography systems. Figure 2 shows a sketch of an imaging system according to the prior art. An object 6 is illuminated by a beam (dashed lines) emitted by a radiation source 5. This beam can be a particle beam or an electromagnetic beam; for example, an X-ray source 5 can emit an X-ray beam onto the object 6. This beam is converted into light by a scintillator S and measured by a photodetector P of a conventional image detector.

[0056] Figure 3 shows a sketch of an imaging system with a radiation detector 4 according to the invention. This is very similar to Figure 2, with the significant difference that the radiation is measured here with an acoustic radiation detector 4, which in this example is formed from a solid absorption layer 12 and a detector arrangement 13, which is acoustically coupled to the absorption layer 12. The absorption layer 12 is designed with respect to its material and thickness such that the radiation is absorbed by at least 50%. The detector arrangement 13 comprises a plurality of acoustic sensors 14 arranged in a matrix over a surface, which are configured to measure sound waves emanating from the absorption layer 12.

[0057] Figures 4 to 9 show different embodiments of the acoustic radiation detector 4. It can be assumed that the detector arrangement 13 is composed of ultrasonic sensors, e.g., piezoelectric sensors, capacitive sensors, or silicon photonic sensors. The absorption layer 12 can be, for example, a layer of tungsten, bismuth, or lead.

[0058] Figure 4 shows a particular embodiment of a sound-decoupled radiation detector 4. In this detector, adjacent acoustic sensors 14 of the detector arrangement 13 are acoustically decoupled from each other with respect to structure-borne sound. This is achieved here by a gap Z between the acoustic sensors 14. The gaps Z contain air.

[0059] Figure 5 shows another special embodiment of the sound-decoupled radiation detector, which is a modification of Figure 4. Here, too, the absorption layer 12 is divided into acoustically separated areas, each of which is acoustically coupled to an acoustic sensor 14. The spaces Z in the absorption layer 12 are also filled with air.

[0060] Figure 6 shows a particular embodiment of the radiation detector 4 with a

[0061] Matching layer 15 between detector array 13 and absorption layer 12. This

[0062] The matching layer 15 serves to match the acoustic coupling of the detector arrangement 13 with the absorption layer 12. For example, the acoustic impedance of the detector arrangement 13 might be one MRayl and the acoustic impedance of the absorption layer 12 one hundred MRayl. In this case, it is preferred that the acoustic impedance of the matching layer 15 be in the range of ten (the square root of the product of 1 and 100). This could be achieved by a magnesium layer.

[0063] In this example, the radiation detector 4 is equipped with a termination layer 16 behind the acoustic sensors 14, which is designed and arranged to prevent reflections on a rear wall of the acoustic sensors 14.

[0064] Figure 7 shows a particular embodiment of the radiation detector 4 with an impedance matching layer 15, which is formed from several layers with different acoustic impedances. The layers are arranged such that the acoustic impedance decreases continuously towards the detector assembly 13. This allows the acoustic impedance between the absorption layer 12 and the detector assembly 13 to be matched in a smooth, non-linear transition.

[0065] Figure 8 shows a particular embodiment of the radiation detector 4 as a combination of photodetectors P and acoustic sensors 14. In this example, the scintillator S acts as an absorption layer 13, which can be achieved, for example, using a lead crystal. This is optically and acoustically coupled to an alternating arrangement of photodetectors P and acoustic sensors 14. Since both photodetectors P and acoustic sensors 14 can be made relatively small, such a detector can be implemented without significant limitations for subsequent image analysis, as the resolution is within an acceptable range. Furthermore, the resolution of the photodetectors P and acoustic sensors 14 can be mutually optimized using computational methods.

[0066] Figure 9 shows another particular embodiment of the radiation detector 4 in combination with an optical detector. In this example, a radiation detector 4 according to the invention, as shown, for example, in Figure 3, is arranged behind a conventional image detector, as shown, for example, in Figure 2. Radiation that penetrates the image detector (which can easily exceed 50% of the incident radiation) is absorbed by the absorption layer 13, which is then detected by the acoustic sensors 14. Compared to Figure 8, the highest possible resolution can be achieved here.

[0067] Figure 10 shows a sketch of an imaging system with several acoustic radiation detectors 4 according to the invention for measuring scattered radiation. A conventional image detector is arranged in the beam direction for image acquisition (although this could certainly be replaced by a combination according to Figures 8 or 9). Acoustic radiation detectors 4 according to the invention are positioned around the object 6 under investigation. These detectors detect scattered radiation from the object under investigation. Due to the effective quantum efficiency of the acoustic radiation detectors 4 according to the invention, usable measurement results can be obtained despite the low overall intensity of the scattered radiation. Furthermore, spatial reconstruction can even be achieved through structural measures (e.g., upstream apertures) or computational measures (e.g., based on the temporal propagation of a spherical switching wave in the absorption layer 12).

[0068] Finally, it should be noted once again that the invention described in detail above merely represents exemplary embodiments, which can be modified in various ways by a person skilled in the art without departing from the scope of the invention. Furthermore, the use of the indefinite articles "a" or "an" does not preclude the possibility that the features in question may be present multiple times. Likewise, terms such as "unit" do not preclude the possibility that the components in question consist of several interacting sub-components, which may also be spatially distributed. The term "a number" should be read as "at least one." Regardless of the grammatical gender of a particular term, persons of male, female, or other gender identities are included.

Claims

Patent claims 1. Acoustic radiation detector (4) for high-energy radiation, comprising a solid absorption layer (12) and a detector arrangement (13) which is acoustically coupled to the absorption layer (12), wherein - the absorption layer (12) is designed with respect to its material and thickness such that at least 50% of particle radiation or electromagnetic radiation of the specified radiation energy is absorbed, - the detector arrangement (13) comprises a plurality of acoustic sensors (14) arranged in a matrix over a surface, which are designed to measure sound waves emerging from the absorption layer (12).

2. Radiation detector (4) according to claim 1, wherein adjacent acoustic sensors (14) of the detector arrangement (13) are acoustically decoupled from each other with respect to structure-borne sound, preferably wherein an intermediate space is provided between the acoustic sensors (14), preferably wherein the absorption layer (12) is also divided into acoustically separated areas, wherein these areas are acoustically coupled to one or more acoustic sensors (14).

3. Radiation detector (4) according to one of the preceding claims, wherein the detector arrangement (13) comprises ultrasonic sensors, preferably piezoelectric sensors or capacitive sensors and / or silicon photonic sensors.

4. Radiation detector (4) according to one of the preceding claims, wherein the absorption layer (12) is a solid layer and comprises at least 20% by weight, preferably more than 40% by weight, elements with an atomic number greater than 40, preferably greater than 70, particularly preferably tungsten, bismuth and / or lead.

5. Radiation detector (4) according to any of the preceding claims, wherein the absorption layer (12) is a metallic layer made of a metal or a metal alloy.

6. Radiation detector (4) according to one of the preceding claims, comprising an matching layer (15) between detector arrangement (13) and absorption layer (12), which matches the acoustic coupling of the detector arrangement (13) with the absorption layer (12), preferably wherein the acoustic impedance Zm in the matching layer (15) lies between the acoustic impedance Zd of the detector arrangement (13) and the acoustic impedance Za of the absorption layer (12), wherein Zm is preferably the square root of the product of Za and Zd, preferably wherein the matching layer (15) is formed from several layers with different acoustic impedances and the layers are arranged such that the acoustic impedance decreases continuously towards the detector arrangement (13), preferably wherein the acoustic impedance of a matching layer (15) is the square root of the product of the impedances of the two adjacent layers.

7. Radiation detector (4) according to one of the preceding claims, wherein the acoustic impedance of the detector arrangement (13) is less than 10 MRayl, preferably less than 3 MRayl and / or the acoustic impedance of the absorption layer (12) is greater than 50 MRayl, preferably greater than 80 MRayl, and / or wherein the matching layer (15) according to claim 6 is formed from a number of layers from the group consisting of magnesium layer, gold layer, aluminum layer, steel layer, glass layer, ceramic layer.

8. Radiation detector (4) according to one of the preceding claims, comprising a termination layer (16) behind the acoustic sensors (14) which is designed and arranged to prevent reflections at a rear wall of the acoustic sensors (14) and / or the detector arrangement (13), wherein this termination layer (16) comprises tungsten, in particular powdered tungsten, silver and / or epoxy resin.

9. Radiation detector (4) according to one of the preceding claims, wherein the thickness of a layer, preferably of all layers, of the matching layer (15) corresponds to one quarter of the mean wavelength of the sound waves in the respective material of the layer, based on the mean wavelength of the sound waves after generation of sound waves by radiation of a predetermined radiation energy in the absorption layer (12) and transition of the sound waves into the respective layer of the matching layer (15).

10. Radiation detector (4) according to one of the preceding claims, further comprising an optical image detector, in particular with a scintillation layer and photosensors, preferably wherein the absorption layer (12) and the acoustic sensors (14) are arranged behind the photosensors or the scintillation layer is designed as an absorption layer (12) and the acoustic sensors (14) are arranged between or behind the photosensors.

11. Radiation detector (4) according to one of the preceding claims, wherein a largest diameter of an acoustic sensor (14) in a surface parallel to the absorption layer (12) is less than 0.05 mm, preferably less than 0.03 mm.

12. Medical imaging system designed to take images by detecting electromagnetic radiation or particle radiation using a detector, comprising an acoustic radiation detector (4) according to any of the preceding claims.

13. Medical imaging system according to claim 12, wherein the medical imaging system is an imaging system based on particle radiation or electromagnetic radiation, preferably a radiography system, a CT system (1), a PET system, a tomosynthesis system, an angiography system or a mammography system.

14. Medical imaging system according to claim 12 or 13, wherein the medical imaging system comprises a measuring device which receives signals from the acoustic sensors (14) and is designed to measure a signal intensity and / or a reception time of signals from the acoustic sensors (14).

15. Medical imaging system according to one of claims 12 to 14, comprising a number of acoustic radiation detectors (4) according to one of claims 1 to 11, arranged for measuring scattered radiation from an object.

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