Abstract:
In a detector arrangement for a medical diagnosis apparatus as well as a medical imaging diagnostic method, radiation quanta striking a detector are detected and evaluated with spatial resolution. The use of collimators is thereby omitted, such that radiation quanta that strike at a non-perpendicular angle to the detector are also evaluated. For spatial resolution of the origin of the radiation quantum, the spatial distribution of sensor elements of the detector that emit a signal upon incidence of a radiation quantum is evaluated. A higher efficiency of the radiation evaluation is enabled and, particularly in nuclear medical diagnostic methods, the radiation exposure of the patient can be distinctly reduced.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention concerns a radiation detector arrangement for a medical diagnostic apparatus, a medical diagnostic system with such a detector arrangement, as well as a medical imaging diagnostic method.  
         [0003]     2. Description of the Prior Art  
         [0004]     Various imaging diagnostic methods are known in the field of medicine, such as, for example, computed tomography (CT), in which the patient to be examined is irradiated with x-rays from a number of different directions and the x-rays are detected by a detector as projection datasets, which are combined for generation of an image.  
         [0005]     In addition to this method based on x-ray radiation, what are known as nuclear medicine methods also are known such as, for example, PET diagnostics (positron emission tomography) and SPECT diagnostics (single photon emission computed tomography). In each of these modalities, the patient is injected with a radioactive substance and the radiation emanating from this substance is detected with the detector and used for image processing.  
         [0006]     For precise spatial resolution of the point of origin of the emitted or transmitted radiation, collimators are disposed upstream of the detectors, the collimators being formed by a number of individual collimator plates (known as septa (singular: septum) that are oriented so that essentially only radiation portions incoming perpendicularly on the detector surface are detected. Therefore only a part of the total radiation is used for the image generation. In order to have sufficient information for the image generation, the patient therefore must be exposed with a relatively high radiation dose.  
       SUMMARY OF THE INVENTION  
       [0007]     The invention is based on the object to enable an improved efficiency in the detection of radiation in medical imaging methods.  
         [0008]     This object is achieved according to the invention by a detector arrangement for a medical diagnostic apparatus having a detector with a radiation-sensitive detector element and a sensor field (associated with the detector element) with a number of sensor elements arranged like a matrix. An evaluation unit for spatially-resolved evaluation of electrical signals from the sensor elements is connected with the sensor field. This evaluation unit is fashioned so that, upon a radiation quantum striking the detector element, the sensor element affected by this event is identified and the radiation direction of the detected radiation quantum is derived from its spatial distribution.  
         [0009]     In comparison a conventional detector arrangement with the collimator placed in front of the detectors, the inventive detector arrangement in that it also allows lateral radiation incidence and radiation incidence oriented at non-perpendicular angle onto the detector. In order to be able to localize the origin of this radiation, the radiation direction (thus the direction of incidence of the respective radiation quantum) is derived from the detected measurement values. The detector arrangement therefore also enables the spatial resolution required for the medical imaging without the use of collimators. The decisive advantage is thereby achieved of no collimators being necessary and a significantly higher fraction of the available radiation is evaluated and used for the imaging. In particular in the nuclear imaging modalities (SPECT, PET), in which the radiation is distributed over the entire examination volume and is not specific due to the nuclear decay, the invention achieves a significant increase in efficiency, i.e. the available radiation is used more efficiently for image generation. In the reverse, this means that the dose to be administered to the patient can be smaller. The exposure of the patient is thereby reduced.  
         [0010]     Additionally, a significant technical simplification in the technical complexity of the apparatus and therewith a significant cost savings are achieved via the omission of the collimators. The collimators are typically extremely delicate, position-sensitive and therewith very elaborate to manufacture.  
         [0011]     The basic underlying concept of this collimator-free detector arrangement is that a number of photons, which are subsequently detected by the sensor elements, are generated along the path (signal track) of the radiation quantum in the scintillator upon impact of an x-ray or gamma quantum on the detector element (for example a scintillator). The size of the light spot imaged (mapped) upon impact of the radiation quantum on the sensor field depends on the radiation direction of the radiation quantum with regard to the surface normal of the detector element. If the radiation quantum strikes nearly parallel to the surface normal, photons are generated only in the immediate impact area. By contrast, if the radiation strikes at a comparably large angle relative to the surface normal, photons are generated in a larger-area region. The sensor elements are therefore more or less affected by the impact event dependent on the incoming radiation direction. The spatial distribution of the sensor elements of the sensor field thus emit a signal that varies dependent on the direction of incoming radiation.  
         [0012]     In an embodiment, the evaluation electronic is designed so that the signal intensity and/or the signal energy of the individual signals transmitted by the sensor elements are additionally detected. Using this additional information, not only is information obtained about in the spatial region within which a signal has been generated, but also information about the intensity and/or energy, which can be evaluated in order to more precisely determine the its angle of incidence from the track path of the radiation quantum.  
         [0013]     The evaluation electronics furthermore can be designed so that the signal track generated by the radiation quantum is evaluated with regard to its length, its start location, its end location as well as with regard to its intensity/energy. This embodiment is based on the recognition that the three-dimensional orientation of the radiation direction can be determined from the two-dimensional position and orientation of the signal track on the sensor matrix field, by inclusion of the intensity/energy measured for the signal track. In fact, if a radiation quantum strikes at only a small angle with regard to the surface normal of the detector, the signal track is shorter in comparison with a radiation quantum striking the detector at a flatter angle.  
         [0014]     In order to achieve a sufficiently high spatial resolution, the average separation between two adjacent sensor elements is &lt;1 mm according to an embodiment. The size (aperture) of each sensor element is therefore &lt;1 mm. The average separation preferably is in the range between 10 μm and 500 μm.  
         [0015]     In an embodiment the detector arrangement is designed for an angular resolution of approximately 0.01° to 0.50°, in particular for an angular resolution of at least 0.15°. This means that rays that deviate from one another at these specified angles are evaluated as originating from different locations. Given a typical separation of 40 cm from the patient, a spatial resolution of 1 mm can be achieved with the angular resolution of at least 0.15°; this means that examination volumes that are separated from one another b 1 mm are differentiable.  
         [0016]     In an embodiment of the detector, the detector element is a scintillator and the sensor elements are photosensor elements.  
         [0017]     In a further embodiment, the detector element is what is known as a direct transducer element, at one side of which a first planar electrode is arranged. The sensor elements are counter-electrodes arranged like a matrix on the side opposite the first electrode. The direct transducer element is, for example, made from a system composed of CdTe, CdZnTe or Hgl.  
         [0018]     Such a detector arrangement is preferably used in a medical diagnostic system for a nuclear medical diagnostic modality such as PET or SPECT. As an alternative to use in a nuclear medical diagnostic modality, the detector arrangement can be used in an x-ray medical diagnostic method, in particular in a computed tomography system. The use of the detector arrangement leads to a higher efficiency of the radiation evaluation, as well as to an apparatus simplification due to the omission of a collimator, and additionally to a lower exposure of the patient, in particular in the nuclear medical diagnostic modality (which is of decisive importance in the medical field).  
         [0019]     The above object is furthermore achieved according to the invention by a collimator-free medical imaging diagnostic method wherein the incoming directions of respective radiation quanta are detected and evaluated. The advantages and preferred embodiments mentioned with regard to the detector arrangement are applicable to the method.  
         [0020]     For determination of the radiation direction of the incident radiation quantum, in an embodiment of the method the respective detected measurement values are compared with reference values stored in a memory of the evaluation unit and the radiation direction is determined from this comparison. The reference values are determined, for example, by simulation or by reference measurements with defined radiation intensity and radiation direction.  
         [0021]     For an optimally efficient evaluation, all radiation quanta that occur at an angle of incidence of 0° to approximately 70° (relative to the surface normals of the detector) are detected and evaluated.  
         [0022]     Given use for computed tomography, radiation portions that (relative to the surface normals) strike at an angle on the detector are preferably identified as scatter radiation and are not further used for imaging. Interfering scatter radiation portions therefore can be eliminated in this manner without the use of a collimator. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  schematically illustrates a detector arrangement according to a first embodiment of the invention.  
         [0024]      FIG. 2  is an intensity curve detected by the inventive detector.  
         [0025]      FIG. 3  schematically illustrates a second embodiment of a detector arrangement according to the invention.  
         [0026]      FIG. 4  is a simplified representation of an inventive medical imaging system in the embodiment of a PET diagnostic system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     As shown in  FIGS. 1 and 3 , a detector arrangement HAS a detector  2 , an evaluation unit  4  as well as a display element  6  connected with the evaluation unit  4 . The detector  2  in  FIG. 1  has a detector element  8 A as well as a sensor field, arranged on the underside of the detector element  8 A, with a number of sensor elements  10 A arranged in a matrix. The detector  2  in  FIG. 3  has a detector element  8 B and sensor elements  10 B. In each embodiment, the average separation a of two adjacent sensor elements  10 A,  10 B (thus the average separation a between their center points) is in the range between 10 μm-500 μm. Each of the sensor elements  10 A,  10 B in the matrix is connected with the evaluation unit  4 . For better clarity, in the  FIGS. 1 and 3  only some of the sensor elements  10 A,  10 B are shown connected with the evaluation unit  4 .  
         [0028]     In the exemplary embodiment of  FIG. 1 , the detector element  8 A is a scintillator and the sensor elements  10 A are photosensor elements. The sensor field is formed as a matrix field comprised of individual semiconductor sensors.  
         [0029]     If an x-ray quantum or a gamma quantum (designated as a radiation quantum  12  in the following) strikes the scintillator  8 A, the radiation quantum  12  leaves a “scintillation track” in the scintillator  10 A and multiple photons (secondary radiation) are generated along the path of the radiation quantum  12 . In the exemplary embodiment, the radiation quantum  12  is incident at an angle to the surface normal  14  of the scintillator  8 A. Since photons are generated over the entire path, a number of the individual photosensor elements  10 A react to this incident radiation quantum  12  (impact event) and each emits an electrical signal to the evaluation unit  4 . The affected sensor elements  10 A are characterized in  FIG. 1  by a hatching.  
         [0030]     The signal emitted to the evaluation unit  4  is in particular a current signal, the intensity of which depends on the number of incident photons per readout cycle on the respective photosensor element  10 A. “Readout cycle,” means the time period between two readout events for a respective photosensor element  10 A. The incident photons within this time span are “added”; the photosensor element  10 A therefore forms an integral over the incident photons.  
         [0031]     Subsequently, the affected sensor elements  10 A,  10 B are identified for evaluation and determination of the radiation direction, meaning that the evaluation unit  4  detects which sensor elements  10 A,  10 B are affected by the incidence of the radiation quantum  12  using the signals transmitted from the affected sensor elements  10 A,  10 B. The evaluation electronics and evaluation logic of the evaluation unit  4  are designed so that, given the expected number of incident radiation quanta  12  per time unit, each individual event is detected as such and evaluated. In a PET examination, this number is approximately 1000 events per second and per mm 2  of detector surface.  
         [0032]     A determined spatial distribution that corresponds to the sensor area irradiated by the photons results via the identification of the sensor elements  10 A affected by the respective event. The signal intensities (the current peak of the signals emitted by the individual sensor elements  10 A) are also detected and evaluated by the evaluation unity. A three-dimensional intensity curve (of which a two-dimensional version is shown in  FIG. 2 ) can be determined using these measurement values. In this representation, the intensity J is plotted against the position of the respective sensor element  10 A in the X-direction of the sensor field. The intensity distribution in the Y-direction is also simultaneously detected (not shown here).  
         [0033]     Intensity curves for an angled incidence (solid curve) and a substantially perpendicular incidence (dashed curve) of the radiation quantum  12  are shown in  FIG. 2 . Given a large angle of incidence α relative to the surface normal  14  of the detector element  8 A, the sensor field is irradiated by photons over a large area so that a relatively broad spatial distribution is formed (solid line). In contrast to this, if the radiation quantum  12  strikes essentially perpendicularly, the angle of incidence α is very small or zero, and photons are only generated in a very narrowly limited area and only a few sensor elements  10 A are irradiated. Only a narrowly limited local distribution of the affected sensor elements  10 A and a rather tight intensity distribution therefore result.  
         [0034]     For the determination of the angle of incidence α, it is sufficient in a first approximation to identify the affected sensor elements  10 A and to determine the irradiated surface area of the sensor field. A specific angle of incidence of the invention α is statistically associated with each area size.  
         [0035]     The signal intensity and/or the signal energy preferably is/are additionally used for determination of the angle of incidence α. A more precise determination can be effected using the intensity distribution. Iterative conclusions about the radiation curve can be derived given an asymmetrical intensity distribution. Overall an unambiguous association with the angle of incidence α is enabled using the signal width. The angle of incidence of the radiation quantum  12  relative to the X-Y plane directly results from the position and orientation of the appertaining sensor elements  10 A of the sensor field.  
         [0036]     The radiation direction is respectively calculated dynamically (online) in the evaluation unit  4  from the individual measurement data. As an alternative, a storage element (not shown) can be provided in which reference values (in particular reference aspects or reference distributions) are stored. A specific radiation direction is thereby associated with a respective curve, such that the radiation direction is determined by a comparison with these reference curves.  
         [0037]     The evaluation with the detector arrangement of  FIG. 3  ensues according to the same principle. In contrast to the exemplary embodiment according to  FIG. 1 , the detector element  8 B is a direct transducer element. A first electrode  16  is arranged on its upper side and a number of counter-electrodes arranged like a matrix, and forming the individual sensor elements  10 B, are arranged on the opposite side. The direct transducer element  8 B is based on the principle that, upon impact of a radiation quantum  12 , electrons are released in the direct transducer element and these released electrons are attracted toward the counter-electrodes  8 B by a voltage applied across the electrodes  16 ,  8 B. As in the exemplary embodiment according to  FIG. 1 , a spatially resolved detection and a spatially resolved intensity distribution are thus likewise enabled.  
         [0038]     The use of the detector arrangement in a PET system  17  is illustrated in  FIG. 4 . Such a diagnostic system typically has an examination tunnel  18  within which the patient  22  to be examined is located on a patient bed  20 . Radiopharmaceuticals are injected into the patient  22  prior to the examination. In a PET examination these are positron radiators. The injected radiopharmaceuticals particularly concentrate in regions with high metabolism. Upon decay, positrons are created that immediately combine with electrons. Two gamma quanta are thereby created in opposite directions. The evaluation in a PET system is based on a coincidence measurement. Two opposite detectors  22  are therefore provided in a PET system. Both are connected with a common evaluation unit  4  that is in turn connected to a display element  6 . Furthermore, the system  17  has operating, input and output devices (not shown in detail here). The detectors  2  can be moved around the patient  22 . As an alternative, a number of circular detectors  2  arranged around the patient  22  can also be provided.  
         [0039]     Due to the use of the detector arrangement  2 , such a medical diagnostic system  17  is characterized by collimator-free detection of the radiation over a large angular range. Due to the omission of collimators, not just radiation quanta striking parallel to the surface normal  14  are detected. Radiation quanta also are detected that strike the detector  2  through a very large range of the inclination angle α of nearly 0 to 70°.  
         [0040]     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.