Device for calibrating a PET system

Abstract

Device (1) for calibrating a PET system (S), wherein the PET system (S) has a plurality of detectors, wherein the PET system (S) has an opening (O) into which an element to be measured can be inserted, the device (1) comprising • at least one radiation source (Q), • at least one first collimator (K1) so that radiation from the radiation source (Q) can exit through a provided first opening (A1) at the collimator (K1). • wherein the device (1) defines an axial direction (R) with which the device (1) can be inserted into the PET system (S) for measurement, wherein the device (1) furthermore has a radial extension about the axial direction (R) thus defined, • wherein the device (1) can be moved radially and / or axially in the opening (O) in a predetermined manner so that detectors of the PET system (S) can be calibrated, • characterized in that the first collimator (K1) has a second opening (A2) for the emission of radiation, wherein the second opening (A2) is designed in the same way as the first opening (A1), and wherein the second opening (A2) is arranged at the same axial position as the first opening (A1) and opposite it on the radial extent.

Description

[0001] The invention relates to a device for calibrating a PET system. background

[0002] Positron emission tomography (PET) is a well-known imaging technique in nuclear medicine and represents a variant of emission computed tomography. PET can generate cross-sectional images of living organisms by visualizing the distribution of a radioactive substance, known as a tracer. This allows for the imaging of biochemical and physiological functions.

[0003] PET is based on the simultaneous detection of two gamma radiation photons that are produced after the decay of a positron-emitting radionuclide (β+ decay). When a positron interacts with an electron (annihilation) in the body, two high-energy photons (e.g., several hundred keV, especially 511 keV) are emitted in opposite directions. This radiation is also known as annihilation radiation.

[0004] The PET scanner typically contains many photon detectors arranged in a ring around the patient. The principle of the PET scan is to record coincidences between any two opposing detectors. The temporal and spatial distribution of these recorded decay events is used to infer the spatial distribution of the radioactive substance within the body, and a series of cross-sectional images is generated.

[0005] Since the absorption of photons depends only on the thickness of the tissue through which the photons are irradiated, and not on the point of origin of the photons, this also allows for an accurate quantification of the distribution of the radioactive substance in the volume under investigation.

[0006] Most existing PET scanners work by stopping two high-energy photons (gamma photons) in crystals where a scintillation process generates optical photons. These crystals are therefore often called scintillation crystals. The photons are then detected by optical sensors and converted into electrical impulses.

[0007] Typically, these crystals and the photosensors are arranged in a ring-like structure, also known as a detector ring.

[0008] It is true that with increased spatial resolution, a better or more precise delineation of tissues becomes possible.

[0009] However, it turns out that parallax errors occur at measurement points outside the center, which are also referred to as radial astigmatism.

[0010] In order to better control parallax errors and improve spatial resolution, methods have been developed that determine the depth of interaction in the scintillation crystal (DOI).

[0011] Small detector rings are frequently used for certain examinations of body parts (e.g., organ-specific PET scanners for mammography and neuroimaging) as well as for examining smaller organisms. The aforementioned improvement is most pronounced with these small detector rings and with PET scanners that have a large axial field of view.

[0012] However, this requires finely structuring the scanners, and especially the scintillation crystals and associated photosensors, in the spatial directions so that gamma interaction with a resolution of 1-2 mm or less in the plane of the photosensors is enabled, as well as the detection of multiple DOI planes in the scintillation crystal.

[0013] To make this possible, scintillation crystals are segmented, for example. This segmentation is extremely expensive and, due to the interposed layers, also reduces the resolution sensitivity, as less material is now available for scintillation processes.

[0014] Therefore, systems with monolithic scintillation crystals have been developed in the past. Such systems have the capability to provide continuous DOI information.

[0015] Regardless of the type of scintillation crystals used, i.e., segmented or unsegmented, position calibration is a problem.

[0016] A movable calibration collimator is known from the prior art according to US patent US 5,021,667 A. Likewise, a scanner system is known from the prior art according to US patent application US 2006 / 0180,766 A1. However, these systems are complex in design and not suitable for rapid measurements.

[0017] Against this background, the object of the invention is to provide a means of quick and easy position calibration for PET scanners.

[0018] The problem is solved by a device according to claim 1. Further advantageous embodiments are in particular the subject of the dependent claims, the description and the figures.

[0019] The invention is explained in more detail below with reference to the figures. These show: Fig. 1 a schematic perspective representation of elements of a PET system with a device according to the invention, Fig. 2 a view into the opening of the PET system with a device according to the invention, Fig. 3 a view of one side of a PET system with a device according to the invention, Fig. 4 a schematic top view of elements of a device according to the invention in embodiments of the invention, Fig. 5 a schematic top view of elements of a device according to the invention in embodiments of the invention, Fig. 6 a schematic top view of elements of a device according to the invention in embodiments of the invention, Fig. 7 a schematic top view of elements of a device according to the invention in embodiments of the invention, Fig. 8 a schematic top view of elements of a device according to the invention in embodiments of the invention. Fig. 9 a schematic top view of elements of a device according to the invention in embodiments of the invention, and Fig. 10 a schematic top view of elements of a device according to the invention in embodiments of the invention

[0020] The invention will now be described in more detail with reference to the figures. It should be noted that different aspects are described, each of which can be used individually or in combination. That is, each aspect can be used with different embodiments of the invention, unless explicitly presented as a pure alternative.

[0021] Furthermore, for the sake of simplicity, reference will generally be made to only one entity at a time. Unless explicitly stated otherwise, the invention may also include several of the entities concerned. Therefore, the use of the words "a," "an," and "one" should only be understood as an indication that at least one entity is used in a simple embodiment.

[0022] Unless explicitly stated otherwise, the individual steps of a procedure described below can be arranged and / or combined in any order. Furthermore, the procedures can be combined with each other unless expressly indicated otherwise.

[0023] Information with numerical values ​​should generally not be understood as exact values, but also include a tolerance of + / - 1% to + / - 10%.

[0024] In embodiments of the invention, which are shown in the figures, a device 1 is provided for calibrating a PET system S.

[0025] As previously described, a PET system comprises a plurality of detectors D. These detectors are arranged in pairs opposite a measuring center in the opening O of the PET system. The exact number and shape of the detectors D, whether monolithic or segmented, are not relevant to the invention.

[0026] The PET system S has an opening Ö. An object to be examined / measured can be inserted into this opening Ö. Subsequently, the device 1 according to the invention is to be inserted for the purpose of calibration.

[0027] The device 1 has at least one radiation source Q. The radiation source Q can be of different types. For example, the radiation source Q can be a gamma emitter or a beta emitter, in particular a beta+ emitter, such as e.g. 22 That may be. However, for the purposes of understanding this, it is irrelevant, and in principle any suitable radiation source that emits radiation measurable by the detectors D can be used. For example, radiation with an energy of approximately 511 keV can be used.

[0028] The device 1 has at least one first collimator K1, so that radiation from the radiation source Q can exit the collimator K1 through at least one opening A1 provided there.

[0029] Device 1 defines - as from Fig. As can be seen in Figures 1-3, there is an axial direction R. This axial direction coincides with the axial direction of the PET system S. The device 1 has a radial extension around this axial direction R.

[0030] It should be noted that the terminology "axial" and "radial" does not require that cylindrical devices 1 necessarily be provided. Rather, segment-like arrangements can also be used, as will be discussed later in relation to the Fig. 9 and Fig. Section 10 will be explained. In this respect, axial and radial essentially define a relationship to the opening of the PET system S.

[0031] Once the device 1 is inserted into the opening O, it can be moved there in a controlled manner and in a predetermined way. Suitable drives can be provided for this purpose, which move the device in a coordinated manner within the PET system S.

[0032] This can – as in Fig. 1 and Fig. As shown in Figure 2, the device 1 can be rotated about its axis R by defined angles. Alternatively or additionally, the device 1 can also be rotated – as shown in Figure 2. Fig. 1 and Fig. 3 is shown - to be moved along the axis R.

[0033] The displacement paths and rotation angles result from the detector arrangement to be measured.

[0034] By moving and / or rotating the device 1 in a predetermined manner, the detectors can now be calibrated based on the known position of the radiation exits from the opening(s).

[0035] That is, the collimator K1 limits the radiation from a radiation source Q to a specific area. This area can emerge from an opening A1 of the collimator as a single (one-dimensional) beam or as a wider, fanned-out curtain.

[0036] Is a single hole opening A1 - as in Fig. As shown in Figure 4, using this method results in a relatively long measurement time, meaning that calibration often cannot be performed within a reasonable timeframe. Such a calibration would require moving the hole opening both radially and axially to measure (preferably each) detector D.

[0037] If, however, a fan-shaped beam is used from a strip-shaped aperture, it becomes necessary to measure across the width of the fan in at least two different orientations relative to the detectors. That is, if one imagines a flat detector surface in the xy direction, it would be advantageous to move the fan once in the x-direction across the detectors and once in the y-direction. Thus, each individual detector would be addressed twice. This approach can significantly reduce the time required for calibration.

[0038] Calibration with respect to position can be performed by a suitable (self-learning) algorithm.

[0039] Typically, calibration is performed during factory assembly.

[0040] However, if it becomes necessary to replace detectors in an existing PET system S, for example, because a detector D is defective or because more powerful detectors are to be integrated into an existing system, such recalibration is not possible with current methods. In particular, monolithic detectors have previously required calibration for position determination before actual use. This means that, according to the state of the art, disassembly of the system would be necessary for any required recalibration.

[0041] However, the invention now makes this possible, as the calibration of the detectors can be carried out completely and cost-effectively at any time without major effort.

[0042] The approach of the invention is not to calibrate a single detector D outside the PET system S, but rather to calibrate detectors within the PET system S.

[0043] For this purpose, the device 1 is moved in the PET system by suitable drives, whereby certain locations are irradiated.

[0044] As previously indicated, the opening A1 on the first collimator K1 can have different shapes.

[0045] For example, the A1 opening can be used - as in the Fig. Figures 4-6 and 9 show a hole opening. However, it is also possible that opening A1 has a strip / slot opening – as shown in the Fig. 7, Fig. 8 and Fig. 10 shown. Fig. Figure 7 shows, for example, a collimator K1 in which a slot-like opening A1 is shown in axial extension, while in collimator K2 a radial slot-like opening A3 is shown.

[0046] Without limiting the generality, multiple collimators K1 and K2 can also be provided. As shown in the figures, essentially identical openings, i.e., hole-like or strip / slit-like openings, can be provided, or (not shown) the opening(s) in a first collimator K1 can be designed differently from the opening(s) in a second collimator K2.

[0047] To enable faster calibration, multiple openings can be provided on a collimator instead of a single opening.

[0048] For example, in Fig. 5 and Fig. Figure 6 shows a first collimator K1, which has a first opening A1 and a second opening A2 for the emission of radiation. The second opening A2 is designed identically to the first opening A1. The second opening A2 is located at the same axial position as the first opening A1, which is offset by an angle α along its radial extent. In a particularly simple embodiment, the angle is 180°, meaning the second opening is positioned opposite the first.

[0049] For example, in Fig. Figure 8 shows a first collimator K1, which has a first strip-shaped / slit-like opening A1 and a second strip-shaped / slit-like opening A2 for the emission of radiation. That is, the second opening A2 is designed identically to the first opening A1. The second opening A2 is located at the same axial position as the first opening A1, which is offset by an angle α along its radial extent. In a particularly simple embodiment, the angle is 180°, meaning the second opening is arranged opposite the first opening.

[0050] Without limiting the generality, multiple collimators can of course be used. For example, in Fig. Sections 5-8 show the use of multiple collimators, whereby reference will subsequently be made only to collimators K1 and K2, without excluding other collimators.

[0051] For example, in Fig. 5 and Fig. Six identical collimators K1 and K2 are shown. These have perforated openings. The perforations can be – as in Fig. 5 shown - arranged on a parallel to the axial direction R, or - as shown in Fig. 6 shown - arranged offset from each other. That is, in Fig. 6. Opening A3 is offset by an angle relative to both opening A1 and opening A2.

[0052] By an order according to Fig. 6. With a suitable number of offset collimators and thus offset openings, a PET system S or the detectors D in it can be calibrated with fewer axial rotations, down to a single axial pass.

[0053] If this is the goal, the number of collimators is also related to the number of openings in a single collimator. As the number of openings per collimator increases, the number of collimators decreases.

[0054] Similarly, a device 1 can also be designed with strip-like openings. The arrangement can be as follows: Fig. 8 will be modified accordingly.

[0055] In general, the device can be used - as in Fig. 7 and Fig. 8 shown - have at least a first opening A1 and a second opening A3, wherein the first opening A1 allows the irradiation of a first solid angle and the second opening A3 allows the irradiation of a second solid angle, wherein the first solid angle and the second solid angle are of different sizes.

[0056] For example, the strip opening A3 allows radial emission of radiation.

[0057] Without limiting the generality, the material of a collimator may be a material with an atomic number of 70 or more, but in particular tungsten (atomic number 74) or lead (atomic number 82), e.g. in pure form or as an alloy.

[0058] Without limiting the generality, the radiation that occurs can be gamma radiation or beta radiation and can originate from a suitable source Q.

[0059] The invention proposes, for example, an arrangement to create a hole-like opening A1 according to Fig. 4 - 6 to perform a calibration.

[0060] In the case of the Fig. 4. The hole-like opening would have to address each individual detector D. Different schemes can be used for this; for example, measurements could first be taken (successively) at a single axial position at different solid angles before the device 1 is moved axially and measurements are taken again (successively) at different solid angles. Alternatively, measurements could first be taken (successively) at different axial positions for a specific solid angle before the device 1 is rotated by a specific solid angle to take further measurements (successively) at different axial positions.

[0061] Preferably, a measurement is carried out at opposite positions, i.e. the collimator K1 is then designed as a so-called coincidence setup.

[0062] The design can of course be as in Fig. Figure 5 shows how this can be extended. Identical collimators can be arranged one behind the other at a predetermined distance. The minimum distance is determined by the distinguishability of gamma interactions without prior calibration. Preferably, the predetermined distance is chosen to match the scintillation crystals, since each scintillation crystal element can be identified even without calibration. In most detector designs, the center of the radiation distribution can be strongly correlated with the radiation position (of the gamma particle), so that the distance between the collimators with hole-shaped openings can be smaller than that between a single scintillation crystal element.

[0063] The invention also proposes, for example, an arrangement to create a hole-like opening A1 according to Fig. 7-8 to perform a calibration.

[0064] In Fig. Figure 7 shows, for example, a collimator with an opening A1 for a fan-shaped radiation emission in the axial direction and a collimator with an opening A3 for a fan-shaped radiation emission in the radial direction. Again, similar to what is shown in Fig. 5 and Fig. 6 - by using multiple arrangements, the speed can be increased or the need for rotation eliminated.

[0065] However, especially for PET systems S with a large opening Ö, e.g. whole body scanners (opening approximately 70 cm), it may be advantageous to use a lighter design instead of a large-volume and therefore heavy and expensive collimator that almost fills the space of the opening.

[0066] It would be possible, as in the Fig. As indicated in Figures 1-3, a non-space-consuming device 1 should also be provided in relation to the opening O. However, this results in high demands on the openings and / or on the precise guidance through the opening in the axial direction. This is because the beam from the opening(s) widens with increasing distance from the opening towards a detector.

[0067] Therefore, it is also possible to provide only one detector with a precise beam, while the opposite detector (in the detector pair) is irradiated without a collimator. That is, the opposite detector is used solely for coincidence measurements.

[0068] For example, as in Fig. Figure 9 shows that only one segment can be arranged as a collimator in a device 1. It is understood that several sources (corresponding to the openings) can be arranged in such a segment. For example, a 30° segment may be sufficient for a whole-body scanner. For example, as shown in Fig. 10 shown - also a semicircular segment with a single source Q is provided.

[0069] Without limiting the generality of the invention, the device according to the invention can also be used to determine the precise position of the scintillation crystal elements. The precise determination of the position of the scintillation crystal elements allows for improved reconstruction of PET images.

[0070] In contrast to the prior art, the invention allows the measurement of an entire PET system S, in particular a PET system S which uses monolithic scintillation crystals in the detectors D.

Claims

[1] Device (1) for calibrating a PET system (S), wherein the PET system (S) has a plurality of detectors, wherein the PET system (S) has an opening (O) into which an element to be measured can be inserted, the device (1) comprising • at least one radiation source (Q), • at least one first collimator (K1) so that radiation from the radiation source (Q) can exit through a provided first opening (A1) at the collimator (K1). • wherein the device (1) defines an axial direction (R) with which the device (1) can be inserted into the PET system (S) for measurement, wherein the device (1) furthermore has a radial extension about the axial direction (R) thus defined, • wherein the device (1) can be moved radially and / or axially in the opening (O) in a predetermined manner so that detectors of the PET system (S) can be calibrated, • characterized by, that the first collimator (K1) has a second opening (A2) for the emission of radiation, wherein the second opening (A2) is designed in the same way as the first opening (A1), and wherein the second opening (A2) is arranged at the same axial position as the first opening (A1) and opposite it on the radial extent. [2] Device according to claim 1, characterized by that the device has a second collimator (K2), wherein the second collimator (K2) is arranged axially offset to the first collimator (K1). [3] Device according to claim 3, characterized by , that the second collimator (K2) has a first hole opening (A3) as an opening for the exit of radiation. [4] Device according to claim 3, characterized by , that the second collimator (K2) has a first strip aperture (A3) as an opening for the emission of radiation. [5] Device according to one of the preceding claims 3 or 4, characterized by , that the second collimator (K2) has a second opening (A4) for the emission of radiation, wherein the second opening (A4) of the second collimator (K2) is designed in the same way as the first opening (A3) of the second collimator (K2), and wherein the second opening (A4) is arranged at the same axial location but opposite it on the radial extent. [6] Device according to claim 5, characterized by , that the first opening (A3) of the second collimator (K2) is offset from the second opening (A4) of the second collimator (K2) by an angle (α), where the angle is greater than 0° and less than 180°. [7] Device according to any one of the preceding claims 2 to 6, characterized by, that the openings (A1, A2, A3, A4) in the first collimator (K1) and in the second collimator (K2) are designed in the same way, wherein the opening(s) (A1, A2) of the first collimator (K1) is arranged offset by an angle to an opening (A3, A4) of the second collimator (K2) on the axial extent, wherein the angle is greater than 0° and less than 180°. [8] Device according to any one of the preceding claims, characterized by , that the first collimator (K1) comprises a material with an atomic number of 70 or more, preferably 74 or 82. [9] Device according to any one of the preceding claims, characterized by that the radiation emitted is gamma radiation or beta radiation. [10] Device according to any one of the preceding claims, characterized by , that the first collimator (K1) has a first hole opening as an opening (A1) for the emission of radiation. [11] Device according to any one of the preceding claims, characterized by, that the first collimator (K1) has a first strip aperture (A1) as an opening for the emission of radiation. [12] Device according to claim 11, characterized by , that the strip opening (A1) allows radial emission of radiation.

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