EP2748635A2

EP2748635A2 - Position-sensitive detector for detecting photon or particle distributions

Info

Publication number: EP2748635A2
Application number: EP12756098.5A
Authority: EP
Inventors: Peter Fischer
Original assignee: Universitaet Heidelberg
Current assignee: Universitaet Heidelberg
Priority date: 2011-08-25
Filing date: 2012-08-23
Publication date: 2014-07-02
Also published as: US20140203178A1; WO2013026577A3; WO2013026577A2; DE102011111432A1

Abstract

The invention relates to a position-sensitive detector for detecting photon or particle distributions, wherein the detector receiving surface (1) is formed by a plurality of detector cells (2) from individual detector elements that are connected to a plurality of readout channels (5). The assignment of the detector cells (2) to the readout channels (5) is chosen in such a way that a position of a focal point of a photon or particle distribution impinging on the detector receiving surface (1) can be determined locally from signals of the readout channels (5). The detector enables a high spatial resolution with a small number of readout channels.

Description

Location-sensitive detector for the detection of photons or particle distributions

Technical application

The present invention relates to a

A location sensitive detector for detecting photon or particle distributions, having a detector receiving surface formed by a plurality of detector cells of individual detector elements, and a number N of readout channels for the detector cells that is less than the number of detector cells, each detector cell being at least one of the readout channels assigned and connected to this.

Site-sensitive photodetectors are

used for example for the detection of gamma quanta in positron emission tomography (PET). The gamma quanta to be detected are in

Scintillation crystals that produce several thousand optical photons due to the interaction with the gamma quantum. This light must be detected with a spatial resolution of 0.5 to 3 mm. The scintillation crystals are usually subdivided into columns of 0.5 to 3 mm in width, owing to their high thickness of several cm required for the absorption of the gamma quanta, in order to obtain the required spatial resolution. The photons emitted by the individual columns must then be detected with a photodetector which also achieves this spatial resolution. An embodiment of the photodetector with a number of columns corresponding to the high number of channels However, it is expensive and expensive. A

The possibility of reducing the costs consists of distributing the light emerging from the scintillation crystals over several optical elements ("light spreaders") to several larger detector elements whose signals then interpolate the respective exit location of the light

Edge effects and greatly restricts the possible mechanical structure.

DE 102005055656 B3 describes a device for processing detector signals which has fewer read-out channels than detector elements. In this device, each detector element is with each

Readout channel connected. A position determination of incident photons is achieved by a suitable weighting of the detector signals with a binary code. US 2011/0001053 A1 discloses a detection device comprising a plurality of detector cells, in which individual channels are combined in signal processing units for the detector signals. The number of readout channels connecting the detector cells to the signal processing units thereby becomes

but not reduced.

The object of the present invention is to provide a location-sensitive detector for the detection of photon or particle distributions, which achieves a high spatial resolution with a small number of readout channels. The detector should be in an embodiment as a photodetector also suitable for use in a gamma detector.

Ways to carry out the invention

The task becomes with the location sensitive

Detector according to claim 1 solved. Advantageous embodiments of the detector are the subject of the dependent claims or can be the

refer to the following description and the exemplary embodiment.

The proposed detector has a detector receiving surface which is formed by a plurality of detector cells of individual detector elements. The

Detector receiving surface is thus in the individual

Segmented detector cells that can be read via readout channels of the detector. The detector has for this purpose a number N of readout channels for the

Detector cells that are much smaller than that

Number of detector cells is. The detector preferably has a number of N = 3 or N = 4 evaluation channels. The number of detector cells is preferably at least 30x30 detector cells. Each detector cell used for the detection is in this case assigned to at least one of the readout channels, preferably in each case exactly one readout channel, and connected thereto. In the proposed detector, the assignment of the readout channels to the detector cells is selected so that the signals of the readout channels

Position of the center of gravity of the incident on the detector receiving surface photon or particle distribution can be determined. Preferably, this assignment is chosen such that this Calculate position from the signals of the readout channels via a center of gravity. In a

preferred embodiment is the detector as

Photodetector formed with photodiodes as detector elements.

In this case, the assignment of the detector cells to the readout channels is preferably locally approximated to a distribution function which, in an ideal case of a reception area which is not discrete by detector cells of size, would enable a clear determination of the position of a single impinging photon or particle at each location. Each read channel is doing a position around the

Associated with the detector receiving surface or at the detector receiving surface, said N positions span an area in which the detector receiving surface is located. The assignment of the detector cells to the readout channels is then locally approximated to the chosen distribution function. The

Distribution function allocates signal components of the detector cells to each readout channel preferably as a linear or nonlinear function of the relative position of the respective detector cell to the position associated with the respective readout channel. The approach is accomplished by viewing areas comprising multiple detector cells. In these areas, the assignment of the individual detector cells to the

Selection channels are selected so that over the respective area considered approximately a division of the signal components results on the readout channels, as they are by the distribution function for a focus the area arranged detector cell is obtained.

In an embodiment of the detector in which the individual detector cells are approximately rectangular

Have receiving surfaces and a rectangular

Arrangement with vertical rows and columns, are preferably a total of N = 4 readout channels

used with the corners of the rectangular

Arrangement correspond. Although it is equally possible to use only N = 3 read-out channels in the case of a rectangular arrangement, use of four read-out channels results in a lower noise component. However, use of a total of N = 3 readout channels is advantageous, for example, for a triangular arrangement of detector cells.

The individual detector cells may be avalanche photodiodes in the proposed photodetector. Preferably, the entire photodetector is as

Silicon photomultiplier formed, which has a high

Has amplification for the incident photon distributions. For example, in the case of a detector for the detection of particle distributions, the detector cells may be MAPS (monolithic active pixel sensors).

In a further advantageous embodiment, a scintillator of a plurality of scintillation crystals is arranged for the detection of X-ray or gamma quanta over the detector-receiving surface, which converts the incident X-ray or gamma quantum into optical photons with photodiodes as

Detector elements are detectable. The scintillator can in this case, for example, in individual columns

be divided as is known to achieve a high spatial resolution of the prior art.

The proposed detector can be found in the

Use design as a photodetector, for example, in a gamma detector in conjunction with scintillation crystals. Application examples for this are the already mentioned PET as well as applications in the material sciences. Also in the field of research, such a photodetector can be used for applications in which the high spatial resolution is required with as few electronic readout channels as possible. Brief description of the drawings

The proposed detector will be briefly explained again with reference to an embodiment in conjunction with the drawings. Hereby show:

Fig. 1 shows two examples of an assignment of

individual detector cells of the proposed detector to total

Readout channels;

Fig. 2 in four subpictures another

Example of an assignment of the individual detector cells of the proposed

Detector to the four readout channels;

3 shows an example of a simulation ("Flood

Map ") with oblique arrangement of a

Scintillators in conjunction with the proposed detector; and Fig. 4 is a highly schematic representation of a plan view of a possible

Design of the proposed

Detector in the cutout; and

Fig. 5 is a schematic representation of a

Arrangement of several adjacent

Detectors.

In the example below, the detector is

Silicon Photomultiplier (SiPM) is formed, in which the detector receiving surface of many single cells, referred to in the present patent application as detector cells composed. The detector cells are in turn formed in a known manner by avalanche photodiodes with series resistor. In which

proposed photodetector, the detector cells are not all connected to an electrode or a readout or output channel, but to several

Distribution channels distributed. In the present example of a rectangular detector receiving surface, N = 4

Readout channels used. The assignment of the detector cells to the read-out channels is selected such that the position of the center of gravity of a photon distribution impinging on the detector surface can be determined from the signals of the readout channels via a center of gravity formation. When a region of detector cells is hit by photons, signals are produced at the N outputs which correspond in each case to the number of cells of the region hit by the photons, which are connected to the respective readout channel. From these signals can then due to the Cell allocation by focusing on the

Position of the area to be recalculated.

The assignment of the cells to the read-out or output channels takes place in such a way that this is achieved locally as well as possible-within the discretization accuracy by the finite size of the individual cells.

Focusing is just a preferred example based on a special distribution function. For the formation of the centroid, the selected positions or coordinates {x _Ka nai, i, y ^" Kanai, i to which the read-out channels i (i = l..N) have been assigned are weighted with the signals (Signali) measured there is normalized to the total signal:

Sum [{x _Ka nai, i, Y _K anai. i} * Signali] / Sum [Signali] where {xrek Yrek} corresponds to the coordinate of the position to be determined.

Also other about the receiving surface non-linear

Distribution functions can be chosen if, for example, a higher spatial resolution in the center of the receiving surface than at the edges is desired. The assignment is dependent on the selected

Distribution function performed, these

Distribution function is then approximated by the local map as well as possible. When choosing a

Distribution function, which has the shape of a sinh (hyperbolic sine) in the directions parallel to the edges of the detector receiving surface, will be advantageous over the entire receiving surface of the same spatial errors and compared to a distribution function for center of gravity higher average spatial resolution achieved (given noise).

Fig. 1 shows two examples of this

Assignment of the detector cells 2 of a detector receiving surface 1 to the four readout channels for two different discretizations. For illustrative purposes, in the upper part of the figure, the receiving surface consists only of 16 × 16 detector cells 2, in the lower part of the figure 32 × 32 detector cells 2. In the case of photodetectors used in practice, the number of cells can be even higher, for example between 40 × 40 and 160 x 160 cells or above. The different assignment of the individual detector cells 2 to the four readout channels is indicated by the different representation of the cells. With such an assignment of the detector cells 2 to the four readout channels, a distribution function is approximated, with which the position of the center of gravity of the incident photon distribution over a center of gravity of the signals of the four readout channels

can be determined. This leads to a nearly identical spatial resolution over the entire receiving area, whereby the determination error for each area of the detector receiving area 1 is approximately the same.

2 shows in the four partial images a to d the respective assignment of the detector cells 2 to one of the readout channels for a size of the detector receiving surface 1 of 80 × 80 cells. The dots in the respective partial illustrations mark the cells that - lo

the respective channel are assigned. Each cell is assigned exactly one channel, so that one

Overlaying the four-part image would result in a completely black area.

The following is an example of a procedure for the assignment of the detector cells to the

Readout channels explained with the assignments of Figures 1 and 2 were generated. The following steps are carried out:

1) For each detector cell (pixel), in a first step, the ideal percentage distribution Ii of a (unit) signal on the N readout channels

calculated (i = 1 ... N). This is the chosen

Distribution function across the surface of the cell

integrated. In this example, the sum of the N components results in the value 1 due to the unit signal. 2) The individual detector cells are not initially assigned to any channel.

3) As the initial block size M x M, a size of 2 x 2 detector cells is set.

4) Start with a cluster in a corner of the detector receiving surface.

5) The sum of the x M division shares Ii is calculated for this cluster. Ii is usually not integer. Already assigning pixels in the cluster to a readout channel is added up in Fi. Fi is an integer for all i = 1 .... 6) As long as any Ii is greater than the associated Fi by more than 1, another pixel must be assigned to channel i in the cluster:

an unassigned pixel in the cluster is randomly selected and assigned to channel i;

Fi is increased by 1. This step is repeated until all

Differences Ii - Fi are less than 1.

7) The cluster is shifted N to the right / left or up / down and from step 5) the

Repeat the process until the entire detector reception area has been processed. Of course, this can in principle also elsewhere

Receiving area to be started or the processing of the total area according to another scheme.

8) In the next step, the cluster size is increased, preferably doubled, the maximum size being limited by the size of the detector receiving area, and started again at step 4). This takes place until all detector cells or pixels are assigned to a readout channel.

The approach to the desired distribution function is performed the better, the more individual detector cells are available on the detector receiving surface. This also applies to the later

Error in positioning, which also increases with the size of each illuminated area decreases, because then the statistics will be better. With a detector of 8mm edge length and cells of 50μπ \ x 50μπι (square) can be about 10 blocks of 0.8mm

Differ edge length.

A particular advantage of the proposed

Photodetector is that the spatial resolution and determination accuracy is independent of the position of a scintillator on the receiving surface. Fig. 3 shows an example of this over the

Edges of the detector-receiving surface 1 twisted

Scintillator 3 with 7 x 7 individual scintillator crystals. The ones emitted by the crystals

Photons were simulated with a finite number of photons and the assignment of the

Detector cells adopted to the readout channels.

Here, a detector receiving surface was simulated with 100 x 100 cells. From the locations 4 of this so-called flood map calculated by the assignment, it can be seen that, despite the rotation, the positions of the individual scintillator crystals can be well resolved. The photodetector is thus very tolerant of a misalignment of any scintillator used in conjunction with the detector.

Fig. 4 shows still a highly schematic

Example of a construction of the photodetector in

Top view, with only a section with 4 x 3 detector cells can be seen. The receiving surfaces of the individual detector cells 2 are rectangular and arranged in mutually perpendicular rows and columns. Run between the individual lines each two readout channels 5, so that each

Detector cell 2 between four readout channels 5 is located. The readout channels themselves run at the edge of the detector receiving surface in the column direction. The connection of the individual detector cells 2 with the respective

Readout channels 5 is schematic in the figure

indicated. This connection can also be formed below the detector cells. FIG. 5 shows, by way of example, an arrangement of several adjacent detectors with a triangular detector receiving surface 1. Each of these detectors has three readout channels which correspond to the corners of the detector

Detector receiving surfaces 1 are assigned. In an advantageous embodiment, in each case one or more readout channels of respectively adjacent detectors are connected to one another, so they are used together. Thus, in the example of Figure 5 each of the

Corner point 6 associated readout channel shared by all six adjacent detectors. The same applies to the other corner points. This can additionally reduce the number of readout channels in such an arrangement.

LIST OF REFERENCE NUMBERS

Detector receiving surface

Detector cells or pixels

scintillator

Simulated places of incidence

readout channels

vertex

Claims

claims

Location-sensitive detector for Detektxon of

Photon or particle distributions, with

- A detector-receiving surface (1) by a plurality of detector cells (2) of individual

Detector elements is formed, and

a number N of read-out channels (5) for the detector cells (2), which is smaller than the number of detector cells (2),

- each used for detection

Detector cell (2) at least one of

Readout channels (5) associated with and connected to, and

- The assignment of the detector cells (2) to the readout channels (5) is selected such

that from signals of the readout channels (5) the

Position of a focus on the

Detector receiving surface (1) incident

Photon or particle distribution can be determined.

Detector according to claim 1,

characterized,

that the assignment of the detector cells (2) to the readout channels (5) each locally

In an ideal case of a receiving surface which is not discretized by detector cells of finite size, an unambiguous determination of the

Position of a single incident photon or particle would allow.

Detector according to claim 1 or 2,

characterized,

that each readout channel (5) is assigned a position around the detector receiving surface (1) or on the detector receiving surface (1), the positions spanning an area in which the detector receiving surface (1) lies, and

Approximation function is approximated that assigns each readout channel (5) signal components of the detector cells (2) as a linear or non-linear function of a relative position of the respective detector cell (2) to the position associated with the respective readout channel (5).

Detector according to claim 3,

characterized,

in that the positions assigned to the readout channels (5) are corner points of the detector receiving surface (1).

Detector according to one of Claims 1 to 4, characterized

that the assignment of the detector cells (2) to the

Readout channels (5) is selected such

that from signals of the readout channels (5) via a

Focusing the position of the center of gravity of the detector receiving surface (1)

incident photon or particle distribution can be calculated.

Detector according to one of Claims 1 to 5, characterized

in that each detector cell (2) used for detection is assigned to and connected to only one of the readout channels (5).

Detector according to one of Claims 1 to 6, characterized

in that the detector cells (2) have rectangular receiving surfaces and form a rectangular arrangement with mutually orthogonal rows and columns, wherein the detector has a total of four

Has readout channels (5).

Detector according to claim 7,

characterized,

Distribution function approximated in

Directions parallel to edges of the detector receiving surface (1) the shape of a sine

Hyperbolic has.

Detector according to one of Claims 1 to 6, characterized

the detector cells (2) are triangular

Form arrangement, wherein the detector has a total of three readout channels (5).

10. A detector according to any one of claims 1 to 9,

characterized, the detector cells (5) are formed by avalanche photodiodes.

Detector according to one of Claims 1 to 9, characterized

that the detector is designed as a silicon photomultiplier.

Detector according to one of Claims 1 to 11, characterized

in that one or more scintillation crystals (3) are arranged above the detector receiving surface (1) and convert incident X-ray or gamma quanta into optical photons, for which the

Detector elements are sensitive.

Arrangement of a plurality of adjacently arranged detectors according to one or more of

preceding claims, wherein one or more readout channels (5) of adjacent detectors are connected to each other.

Use of the detector or arrangement according to one or more of the preceding

Claims as a photodetector in one

Detector for positron emission tomography.