SECURE SHARING OF MEDICAL IMAGES

Description

[0001] The present invention relates to a method for secure collaboration based on medical images, a system, a command unit and a pseudonymization unit as well as a computer program.

[0002] The invention lies in the fields of medical technology and security technology for cloud-based architectures.

[0003] In today's medical field, it's essential that multiple experts (for example, within an interdisciplinary council) can review medical images when addressing life-or-death issues. A cloud-based architecture makes this possible.

[0004] Various cloud-based image sharing services are known in the state of the art for this purpose, which enable images to be shared between hospital departments and between different clinics and research institutions that are located remotely from one another. This has the advantage that an image study can be sent to another workstation for review. In practice, however, it often proves necessary to review previous studies of the patient in order to make a final statement or a fundamental decision for further action. However, due to security requirements, the previous studies cannot be displayed on a remote system. This is primarily because the images also contain security-critical data from which conclusions could be drawn about the patient.

[0005] This state-of-the-art scenario is in Figure 1 A physician at a first hospital, A, wishes to share images stored in a PACS system with a person working at a second hospital, B, to display them at a workstation there. In known, state-of-the-art systems, the respective image data is anonymized or modified in such a way that no personally identifiable information is contained in the image. If previous studies are to be displayed at the workstation of Hospital B, this is not possible without violating security regulations.

[0006] DE 10 2014 003 889 A1 also concerns the anonymized provision of medical images. This involves making images available to a patient by sending the patient a link (URL) that points to an address on a web server accessible via the internet, where only the image data is stored without PHI information. The patient can then access their data via this link. Requesting additional images for images already displayed is not described here.

[0007] DE 10 2013 211 540 A1 concerns context-dependent pseudonymization of safety-critical medical data. This document aims to adapt the type of pseudonymization to the subsequent processing in order to optimally utilize the available network bandwidth. Requesting additional images for images already displayed is also not described here.

[0008] The present invention is based on the object of improving a system for sharing medical images such that previous studies can be requested for a shared image. Furthermore, security during collaboration in the medical field is to be improved. This object is achieved by a subject matter according to the independent, subordinate claims. Advantageous embodiments are the subject matter of the dependent claims, the description, and the drawings.

[0009] According to a first aspect, the object is achieved by a system for securely reloading previous studies to an image data study on a receiver node located remotely from an image transmitter node, comprising: A command unit and a signal sensor on a receiver node, wherein the signal sensor is displayed simultaneously with the currently displayed image data study and is intended to detect an image request signal for the currently displayed image data study, and wherein the command unit, after detecting the respective image request signal, is intended to generate a command and send it to a pseudonymization unit, wherein the command contains control instructions in order to load a predecessor study from the image sending node for the currently displayed image data study. The pseudonymization unit, which is arranged on the image sending node and comprises the following electronic or computer-based modules: An extractor, which is intended to extract an identification data set from the command upon receipt of the command. A loader, which is intended toaccess an image storage system to load the previous study based on the extracted identification data set. A pseudonymization processor designed to generate a pseudonymized image data set for the loaded previous study. A receiver node interface designed to send the pseudonymized image data set to the receiver node or to an alternative receiver node.

[0010] An image data study consists of at least one image or image data set acquired by a medical imaging modality (computer tomography, such as dual-source CT scanners or single-source CT scanners, magnetic resonance imaging, X-ray machines, etc.). It is preferably available in a standardized format (DICOM format). The image data study is assigned to a specific patient. Typically, several image studies are created for a patient within a certain period of time. Thus, an image study usually has predecessor studies (so-called prior studies).

[0011] The previous study is an image dataset from the same patient, preferably at a different examination time. However, it is also possible for the previous study to refer to the same or a different examination time as the image study, but to have been acquired using a different modality (e.g., MR instead of CT). In further developments of the invention, the previous study can also be a reference study with the same target volume represented in the image. It is also possible for an image study from a previous (not current) period to be shared and displayed on the receiver node, and for no previous study in the sense of the term to be requested, but rather a study from a later time, i.e., a follow-up study. This also lies within the scope of the invention.

[0012] The image sending node typically communicates directly with an image storage system, which serves as a central data archive for storing image data, via a secure network. The connection between the image storage system, which may be configured as a PACS system, and the image sending node with the pseudonymization unit is a secure local network (e.g., a local area network, LAN).

[0013] The term "reload" refers to the primary embodiment of the invention, in which the previous study is related to or associated with an image data study currently being displayed on the receiver node. The image data study is thus loaded on the receiver node and displayed there, and the previous study is to be reloaded for this displayed study.

[0014] Basically, the system comprises two different computer-based nodes or computer systems (networks with processor kernels in real or virtual form): A receiver node on which the image data study is to be displayed (shared for collaboration) and an image sending node that is in data exchange with an image storage system and that acts as a sending instance and from which the image data studies are to be shared.

[0015] Both nodes interact via a network, in particular via a public, unsecured network connection (e.g., the Internet). For the sake of simplicity, the invention is described below in the embodiment in which an image data set is to be sent from the image sending node to only one receiver node for collaboration. However, it is also within the scope of the invention for one or more image studies to be sent in parallel to several receiver nodes, in particular according to a broadcasting principle. A receiver node can also receive image data sets from different image sending nodes. It is also possible for the image request signal to comprise several parameters in which, for example, it can be specified that a previous study to be requested should not be loaded on the current, currently determined receiver node, but on another node. In this case, a further input field can be provided in which the input of an address (e.g.,an IP address or a name) can be entered.

[0016] For secure data transmission between the image sending and receiving nodes, security measures must be implemented to ensure that, in the event of an interception of the network connection, the patient's identity cannot be deduced from the transmitted data. It must be taken into account that medical image datasets typically contain security-critical content that must be processed and handled with access protection. It must be ensured that patient-identifying information (e.g., name, age, gender, place of residence, etc.) does not, in combination with image data or image data excerpts, escape beyond the boundaries of the respective access-protected environment. This access-protected content is referred to as "protected health information (PHI)." The pseudonymization unit is then used to remove the PHI components from the respective image dataset.Preferably, a mapping between the original image dataset (with PHI data) and the pseudonymized image dataset (without PHI data) is stored. Preferably, the following mapping is stored: an identifier for the original image data study (hereinafter also referred to as "study"), an identifier for the pseudonymized study, an identifier for the respective patient's original PHI data, and the identification data for the PHI data after pseudonymization.

[0017] The pseudonymized image dataset can be generated using different methods depending on the embodiment of the invention. Pseudonymized image data is characterized by the fact that all sections or portions that identify the patient have been manipulated or removed from the dataset. The image data can be de-identified using various measures so that it no longer contains any identifying information (for the patient). For this purpose, predeterminable parameters can be specifically and explicitly eliminated, generalized, replaced, or otherwise manipulated from the dataset, such as names, geographical data, contact numbers or addresses, including email addresses, security numbers, other ID numbers (e.g., from health cards or bank cards, etc.), biometric data, photos, etc.An alternative measure involves the application of statistical methods based on testing the statistical probability of whether the original data can be derived from the de-identified dataset. Alternatively, anonymization of the PHI data can be used, which generally does not maintain a central authority to correlate the original PHI data with the pseudonym. The anonymized or pseudonymized image data is generally visible and readable by anyone (as plain text)—i.e., unencrypted.

[0018] The image prompt signal is preferably a signal that can be entered by operating an input field on a (preferably graphical) user interface. Alternatively, however, the image prompt signal can also be another optical or acoustic signal.

[0019] The image request signal received or captured by the command unit is used to generate a command. The command contains control instructions to load a previous study from the image sending node for the displayed image study. The command contains an identification data record that identifies the displayed study and / or the desired previous study. Furthermore, the command can include additional criteria, e.g., which of the available previous studies should be loaded. This can be done by specifying the time period, the modality, the location of the respective anatomy, or other medical parameters.

[0020] A key advantage of the invention is that the respective pseudonymized datasets are generated only when needed and upon request, i.e., when a study is to be shared or when the recipient of the shared study requests previous studies. This significantly reduces the resource requirements for storing and calculating pseudonymized datasets.

[0021] According to an advantageous embodiment of the invention, the pseudonymization unit comprises a pseudonymization frontend and a pseudonymization backend. The pseudonymization frontend is located on the image sending node, and the pseudonymization backend is located remotely. The backend can, in particular, be installed on a network node.

[0022] According to a further advantageous embodiment of the invention, the command unit comprises a command frontend and a command backend, wherein the command frontend is arranged on the receiver node and the command backend is arranged on a remote instance and in particular on a network node.

[0023] The command unit and the pseudonymization unit interact via a digital network, in particular a public, insecure network. Therefore, according to the invention, no image data containing PHI information is transmitted, but only pseudonymized image data sets without PHI data.

[0024] The backend of the command unit and / or the backend of the pseudonymization unit can be implemented as web services and hosted on a server accessible via the Internet. In one embodiment of the invention, the backend of the command unit and the backend of the pseudonymization unit are hosted in different cloud systems.

[0025] According to one aspect, the invention relates to a command unit for operation in a system described above, and in particular for operation in a receiver node. The command unit comprises a signal sensor that is displayed simultaneously with the image data study currently being displayed and is intended to detect an image request signal for the image data study currently being displayed. After detecting an image request signal, the command unit is intended to generate a command and send it via a pseudonymization unit interface. The signal sensor can be designed to detect input signals from a user and, in particular, as an input field on a graphical user interface. If the user then places a check mark or cross in this input field, this is considered a signal that a previous study is being requested from the remote image storage system.In a preferred development of the invention, further parameters can be detected in addition to the image request signal, e.g. to specify a special previous study from a set of available previous studies or to specify the time of loading on the receiver node or to determine one or more further receiver nodes on which the previous study should also be displayed.

[0026] According to a further aspect, the invention relates to a pseudonymization unit for operation in a system described above. The pseudonymization unit exchanges data with an image storage system, in particular via an internal secure network (e.g., LAN). The pseudonymization unit comprises the following electronic or computer-based modules: An extractor, which, upon receipt of a command, is designed to extract an identification data set from the command. With regard to the command and the identification data set, the statements below regarding the description of the method apply accordingly. A loader, which is designed to access the image storage system based on the extracted identification data set to load one or more previous studies. A pseudonymization processor, which is designed to generate a pseudonymized image data set or several corresponding data sets for the loaded previous study. A receiver node interface, which is designed to send the pseudonymized image data set to the receiver node or to an alternative receiver node.

[0027] According to a further aspect, the invention relates to a method for securely reloading previous studies for an image data study on a receiver node located remotely from an image transmitter node, the method comprising the following method steps: Displaying a signal sensor simultaneously with the image data study currently being displayed. Detecting an image request signal on the signal sensor of the receiving node. Generating a command, wherein the command contains control instructions to load a predecessor study from the image sending node for the image data study currently being displayed, and sending the command to a pseudonymization unit, wherein the command preferably comprises an identification data record. Alternatively, the identification data record can also be read in and provided via an additional communication channel. The identification data record uniquely identifies the patient and / or the patient's image study. Receiving the command on the pseudonymization unit. Extracting an identification data record from the received command. Loading at least one predecessor study from the local image storage system based on the extracted identification data record.Generate at least one pseudonymized image dataset for the previous study(s). Send the generated pseudonymized image dataset over the network to the recipient node and / or to other specified recipient nodes.

[0028] The method comprises several method steps that are executed on different entities, namely the image sending node and the receiving node. Preferably, the first two method steps are executed on the receiving node, and the remaining ones on the image sending node. However, it will be apparent to those skilled in the art that additional nodes can also be included and interposed.

[0029] In a preferred embodiment of the invention, an association between the (original) image data set, identification data set, and / or pseudonymized image data set is stored. This association is preferably stored locally on the pseudonymization unit, i.e., on the system on which the image storage is located and / or on which a pseudonymized data set is created.

[0030] In a preferred embodiment of the invention, it is provided that the image request signal can only be entered for a currently displayed image data set in order to automatically generate an assignment to the image study. This is achieved by providing an input field on a screen interface of the receiver node only while an image data set is being displayed. If an image request signal is then detected on the signal sensor, this simultaneously defines the image data set for which a previous study is being requested. Otherwise, the image request signal can also be entered independently of the display of a split image study. In this case, definition means are provided with which the image study can be identified, e.g., via an additional input field.

[0031] The pseudonymization unit is preferably operated in passive mode. This means that it only becomes active upon receiving a command from the command unit of the receiving node. The pseudonymization unit is only activated upon a request command for further previous studies and creates a pseudonymized image dataset for the previously shared image data. This has the advantage that the method can be operated very efficiently in terms of processor usage, as unnecessary pseudonymizations do not have to be calculated, stored, and / or sent. Alternative embodiments of the invention provide an active mode for operating the pseudonymization unit, in which, based on events, a pseudonymized image dataset is always simultaneously generated and stored when a new study or a new image dataset is stored in memory. This pseudonymized image dataset can then be immediately reloaded for subsequent requests.

[0032] According to another aspect, the invention is solved by an image sending node with a pseudonymization unit as described above.

[0033] According to another aspect, the invention is solved by a receiver node with a command unit as described above.

[0034] In the above description of the invention with its various aspects and embodiments, mentioned features, advantages, and / or alternative embodiments can also be transferred to the other claimed subject matter, and vice versa. In other words, the subject claims (which are directed, for example, to a system or a computer program product) can also be developed with the features described or claimed in connection with the method. The corresponding functional features of the method are implemented by corresponding subject modules, in particular by hardware modules or microprocessor modules, of the system or product, and vice versa.

[0035] A further solution to the problem consists in a computer program product that is loaded or loadable into a memory of a computer with computer program code for carrying out the method described in more detail above when the computer program product is executed on the computer.

[0036] Another solution to the problem involves a computer program with computer program code for carrying out all steps of the method described in more detail above when the computer program is executed on a computer. It is also possible for the computer program to be stored on a computer-readable medium.

[0037] The following detailed description of the figures discusses non-limiting embodiments, their features, and other advantages based on the drawings. These show: Fig. 1 is a schematic overview of a system for sharing image data according to the prior art; Fig. 2 is a block diagram of a system according to a preferred embodiment of the invention; Fig. 3 is a block diagram of a system according to a preferred embodiment of the invention in an overview; and Fig. 4 is a flowchart for method steps of a method according to a preferred embodiment of the invention that are executed on distributed units of the system. Detailed description of the characters

[0038] In the following, the invention is described in connection with Figure 2 described in more detail.

[0039] In a first clinic A, a physician wishes to share a specific image data study (BDS) stored locally in a PACS (picture archiving system) with a colleague working in a distant second clinic B in order to initiate further medical measures for a patient. However, the two units, or clinics A and B, can only exchange data via an unsecured network (NW), e.g., the Internet. To meet security requirements, it is therefore required that image data studies (BDS) are generally only transmitted over the unsecured network (NW) in a pseudonymized or anonymized manner, i.e., without any PHI data (PHI: Protected Health Information) that identifies the patient.

[0040] For this purpose, the image data studies BDS to be shared are stored on a producer system, the image sending node 10 (Clinic A in Figure 1 ), pseudonymized and sent as a pseudonymized image data set BDS` to a receiver node 20 (Clinic B in Figure 1) and displayed there on a monitor M. The colleague can now also view the pseudonymized image data set BDS` and discuss the further steps with his colleague in Clinic A. If it turns out that the colleague at the receiver node 20 still needs previous studies in order to be able to make a statement, it was previously not possible in the state of the art to trigger the reloading of previous studies for the already shared and displayed study on the receiver node 20.

[0041] The invention only proposes providing a command unit 200 on the receiver node 20 next to the monitor M for displaying the shared pseudonymized image data set BDS`. The command unit 200 communicates with a signal sensor 202, which serves to detect an image request signal S1. The image request signal S1 can be detected via a user action (such as pressing a button on the user interface and / or via another input device) and forwarded to the command unit 200. The command unit then calculates a command Be from this. The command Be comprises an identification data set ID, which identifies the study to be reloaded and / or the currently displayed study BDS` to which a previous study is to be loaded. In a preferred embodiment of the invention, this can be achieved by displaying the signal sensor 202 together (simultaneously) with the shared image data study BDS` on the monitor M.An assignment of the image request signal S1 detected via the signal sensor 202 and the respectively displayed image data study BDS' can then be automatically recorded. Otherwise, corresponding additional input fields can be provided, via which the user can specify both an image data study BDS' and a previous study BDS-V.

[0042] For data protection reasons, according to the invention, only pseudonymized or anonymized image data, which thus do not contain any personal data sets that identify the patient, are displayed, loaded, and / or processed on the (external) receiving node 20. The original image data sets containing the PHI data are only available locally on the image sending node or on the local system of the image sending node 10.

[0043] Preferably, an association between the original image dataset BDS and the pseudonymized image dataset BDS' associated with it is stored locally in the image storage system PACS or on the image sending node 10 or an instance associated with it, so that it is also possible to apply the method iteratively. This means that in a first step, it is possible to share an original image dataset BDS (for display on the monitor M of the receiving node 20). Then, in a second step, a first predecessor study BDS-V1 is requested for the displayed image dataset BDS. This first predecessor study BDS-V1 is determined on the image sending node system and pseudonymized again and sent to the receiving node 20 in the form of a pseudonymized image dataset BDS-V1'. If a second predecessor study BDS-V2 is to be requested in a third step, this is also possible.To this end, in a further step, the second previous study BDS-V2 is identified on the image sending node system, pseudonymized again, and sent to the receiving node 20 in the form of a pseudonymized image dataset BDS-V2'. This procedure can be repeated as often as required, as the association between the original study and its pseudonymized form is saved each time. Alternatively or cumulatively, an identification dataset can also be saved for the respective study.

[0044] The generated command Be can include, in addition to the identification data set ID, further additional information Z, which is intended, for example, for further specification of the predecessor study BDS-V (e.g., regarding the modality, including time period information, etc.). The command Be is then forwarded to a receiving unit 101 on the image sending node 10. An extractor 102 extracts the identification data set ID from the received command Be and forwards it to a loader 104, which is intended to use it to access the image storage system PACS in order to load the predecessor study BDS-V into the image sending node 10. A pseudonym processor 106 is used to generate a pseudonymized predecessor study BDS-V` for the loaded predecessor study BDS-V and to send this via a receiver account interface 110 to the receiver node 20 for display on the monitor M.Since the previous study is also pseudonymized, it can be transmitted over an insecure network (NW) without violating security regulations.

[0045] In a preferred embodiment of the invention, a memory MEM can be provided on the image transmission node 10, on which an association between the original image data set BDS, its pseudonymized form BDS`, the PHI data of the BDS study and / or the predecessor study BDS-V, and / or the identification data set ID is stored. The following association is preferably stored: A Study Instance UID of the original image data study BDS An identifier for the Study Instance UID after pseudonymization Patient identifier in the original, i.e. in particular the patient's PHI data An identifier for the patient ID after pseudonymization.

[0046] Figure 3shows that the pseudonymization unit 100 is installed on the image sending node 10 and the command unit 200 is installed on the receiving node 20. Preferably, the two units 100, 200 each comprise a frontend and a backend. The respective frontend can be loaded, provided, and / or operated as a browser-based application in a window of the respective node. A pseudonymization frontend 100-F is installed on the image sending node 10, and a pseudonymization backend 100-B can be installed remotely, for example, on a remote network node 30. A command unit frontend 200-F is installed on the receiving node 20, while a command unit backend 200-B can be installed remotely, for example, also on the same or a different network node 30.

[0047] Figure 4explains which method steps are carried out on which computer-based units. In step a, an image request signal S1 is detected. This preferably takes place on the signal sensor 202. In step b, a command Be is generated based on the signal S1 and sent to the pseudonymization unit 100. In step c, the command Be is received, and in step d, the identification data set ID is extracted from this command. In step e, the PACS can then be accessed based on the extracted identification data set ID in order to download the respective predecessor study to the already shared study. In step f, the pseudonymized predecessor study data set BDS-V` is generated, which is then sent to the recipient account 20 in step g. In step h, the predecessor study BDS-V` can be displayed in pseudonymized form on the monitor M.Steps a, b and h are executed on the receiver node 20 and steps c to g are executed on the image transmitting node 10.

[0048] Finally, it should be noted that the description of the invention and the exemplary embodiments are fundamentally not to be understood as limiting any particular physical implementation of the invention. All features explained and illustrated in connection with individual embodiments of the invention can be provided in various combinations in the subject matter of the invention in order to simultaneously realize their advantageous effects. It is particularly obvious to a person skilled in the art that the invention can be applied not only to image studies and previous studies, but also to other medical data sets that were acquired at different time phases and may only be transmitted in a protected form over a public network. Furthermore, the invention can be operated with other methods for securing PHI data in addition to pseudonymization.The components of the system can also be distributed across several physical products.

[0049] The scope of the present invention is defined by the following claims and is not limited by the features explained in the description or shown in the figures.

Claims

1. System for secure reloading of previous studies (BDS-V) relating to an image data study (BDS) on a receiver node (20) from a remote image sending node (10), comprising: - the receiver node (20) for representing image data studies, which comprises a command unit (200) with a signal sensor (202), wherein the signal sensor (202) is indicated at the same time as the image data study (BDS') currently shown and is designed to capture an image request signal (S1) relating to the image data study (BDS') currently shown and wherein the command unit (200) is designed, after capturing the respective image request signal (S1), to generate a command (Be) and to send the same to a pseudonymisation unit (100), wherein the command (Be) contains control instructions, in order to load a previous study (BDS-V) from the image sending node (10) relating to the image data study (BDS') currently shown, - the pseudonymisation unit (100) on the image sending node (10), comprising: - an extractor (102), which is designed to extract an identification data set (ID) from the command (Be) upon receipt of the command (Be), - a loader (104), which is designed to access the image storage system (PACS) for loading the previous study (BDS-V) on the basis of the extracted identification data set (ID), wherein the previous study (BDS-V) comprises an image data set, - a pseudonymisation processor (106), which is designed to generate a pseudonymised image data set (BDS-V') based on the image data set of the loaded previous study (BDS-V) relating to the loaded previous study (BDS-V), - a receiver node interface (110), which is designed to send the pseudonymised image data set (BDS-V') to the receiver node (20).

2. System according to the preceding claim 1, in which the pseudonymisation unit (100) comprises a pseudonymisation frontend (100-F) and a pseudonymisation backend (100-B), wherein the pseudonymisation frontend (100-F) is arranged on the image sending node (10) and the pseudonymisation backend (100-B) is arranged remote from the image sending node (10).

3. System according to one of the preceding system claims, in which the command unit (200) comprises a command frontend (200-F) and a command backend (200-B), wherein the command frontend (200-F) and the command backend (200-B) are arranged on the receiver node (20) and remote from the receiver node (20) in each instance.

4. System according to one of the preceding system claims, in which the command unit (200) and the pseudonymisation unit (100) exchange data with one another by way of a network (NW), in particular by way of a public network.

5. Method for secure reloading of previous studies (BDS-V) relating to an image data study (BDS) on a receiver node (20), which is arranged remote from an image sending node (10), wherein the method comprises the following method steps: - indicating a signal sensor (202) on the receiver node (20) at the same time as the image data study (BDS') currently shown on the receiver node (10), - capturing an image request signal (S1) on the signal sensor (202) of the receiver node (20), - after capturing the image request signal (S1), generating a command (Be) by means of a command unit (200) of the receiver node (20), wherein the command (Be) contains control instructions in order to load a previous study (BDS-V) from the image sending node (10) relating to the image data study (BDS') currently shown, - and sending the command (Be) to a pseudonymisation unit (100) of the image sending node (10) by means of the receiver node (20), - receiving the command (Be) of the command unit (200) on the pseudonymisation unit (100) - extracting an identification dataset (ID) from the command (Be) by means of the pseudonymisation unit (100) - loading the previous study (BDS-V) from an image storage system (PACS) on the basis of the extracted identification data set (ID) by means of the pseudonymisation unit (100), wherein the previous study (BDS-V) comprises an image data set, - generating a pseudonymised image data set relating to the previous study (BDS-V') on the basis of the image data set of the loaded previous study (BDS-V) by means of the pseudonymisation unit (100), - sending the generated pseudonymised image data set relating to the previous study (BDS-V') to the receiver node (20) by means of the pseudonymisation unit (100).

6. Method according to the preceding method claim, in which an assignment between image data set (BDS), identification data set (ID) and / or pseudonymised image data set is stored relating to the previous study (BDS-V').

7. Method according to the preceding method claim, in which the assignment is stored on the pseudonymisation unit (100).

8. Method according to one of the preceding method claims, in which the command (Be) comprises an identification data set (ID) .

9. Method according to the preceding method claim, in which the identification data set (ID) identifies the patient and / or the image study (BDS) one-to-one.

10. Method according to one of the preceding method claims, in which the image request signal (S1) can only be input relating to an indicated image data set (BDS') by an input field only being provided during the display of the image data set on an image screen of the receiver node (20).

11. Computer program product which can be loaded into an internal memory of a digital computer and comprises software routines, with which the steps of the method according to the preceding method claims are executed, when the software routines are executed on the respective digital computer.