METHOD FOR CALIBRATING A MEDICAL IMAGING DEVICE AND MEDICAL IMAGING DEVICE

Description

[0001] The invention relates to a method for calibrating a medical imaging device, in particular an endoscope system or an exoscope system, wherein the imaging device comprises an optical system with optics and an image sensor for capturing an image of a viewing area, and the image information is subject to spectral deviation due to an internal and / or external influence, in particular the optical system, illumination, and / or an environmental condition, in several steps. The invention further relates to a medical imaging device, in particular a medical endoscope or a medical exoscope.

[0002] Well-known medical imaging devices, such as endoscopes or exoscopes, are used both for acquiring images under confined geometric conditions and for analyzing specific parameters within a viewing area. For this purpose, so-called hyperspectral or multispectral imaging systems (HSI / MSI) for endoscopes or exoscopes are among the technologies used.

[0003] Furthermore, various endoscopes are known in which a corresponding optical element is rotatably designed at the tip of the respective endoscope. These can also be combined with HSI and / or MSI systems.

[0004] Furthermore, it is common practice to attach a camera to the eyepiece at the head of endoscopes or exoscopes with a simple eyepiece for viewing with the human eye, thus enabling external display of the image of a viewing area.

[0005] Common to known endoscopes or exoscopes is that when the distance to a viewing area or object, such as an organ, is changed, and / or when an optical system or lens is rotated, or when a camera is attached, the calibration necessary for a hyperspectral or multispectral imaging system exists only for a basic setting. Therefore, performing the described changes results in a change in image quality with respect to the obtained results, as the calibration is lost. Related prior art can also be found in publications JP2013090884 A, JP2009273676 A, US2019 / 129037 A1, JP2015134110 A, and US2018 / 299552 A1.

[0006] The purpose of the invention is to improve the state of the art.

[0007] The problem is solved by a method for calibrating a medical imaging device, in particular an endoscope system or an exoscope system, according to claim 1.

[0008] By determining the spectral distribution of the first image acquired in a real-world operating environment, the corresponding spectral deviation from a reference spectral distribution can be ascertained. This provides information for each initial image about the extent to which its spectral distribution deviates from the reference distribution. Based on this determined spectral deviation, the image information can then be corrected, resulting in a corrected image. The corrected image information thus compensates for the spectral deviation or is recalibrated for the specific operating situation with modified parameters, resulting in a reduced or eliminated spectral deviation.This allows for the precise determination of tissue parameters, for example using an MSI, in a given application environment, since the determination of these tissue parameters depends on a correspondingly accurate determination of the relevant spectral distributions.

[0009] The following terms should be explained here: In metrology or for a measurement process, "calibration" describes the determination and / or documentation of a deviation of the respective measuring instrument compared to another measuring instrument or a reference standard. The reference standard is also referred to as the "norm" in this case. Such calibration further includes a second step, namely taking the determined deviation into account when subsequently using the respective measuring instrument to correct the readings. A so-called calibrated measurement thus delivers more accurate or even exact results within a desired minimal deviation from reality compared to a non-calibrated measurement.

[0010] A "medical imaging device" is, for example, an endoscope, an endoscope system, or an exoscope or exoscope system. Furthermore, such a medical imaging device can be any instrument suitable for use with an imaging procedure in a medical setting, for example, to obtain a corresponding image or to determine parameters of a viewed area. An endoscope is a device with which, for example, an internal area of ​​a person, such as the abdomen, can be examined and parts within it manipulated. Such an endoscope includes, for example, both optical components and mechanical components for manipulating the viewed area or parts within it. The correct term for this is an endoscope system, where "endoscope" refers to the actual components used for image acquisition.However, these terms are often used synonymously. An exoscope, or exoscope system, is a comparable instrument for viewing and / or manipulating exposed areas of an organism, such as a human during open surgery. Tissue parameters or other parameters are determined, for example, by means of spectral analysis, often taking into account the targeted use of illumination devices for specific spectra. Thus, such a medical imaging device can be used, for example, for analysis via hyperspectral imaging (HSI) or multispectral imaging (MSI).

[0011] An "optical system" can be any component of a medical imaging device that is suitable or configured to receive, transmit, and / or output image information or other optical information. Such an optical system, for example, guides light from the viewing area to an image sensor or image capture chip.

[0012] An "optics" describes the entirety of optically conductive components. For example, such optics include lenses, light guides, or corresponding filters, but also optical components additionally attached to an endoscope system.

[0013] An "image sensor" is, for example, a device or assembly for electrically capturing light-based information. Such an image sensor is, for instance, a semiconductor-based component capable of detecting both visible and invisible light. Generally, such an image sensor can detect any type of electromagnetic radiation if it is configured for a corresponding wavelength range. Such an image sensor might be located, for example, at the tip, in the handle, or in the head of an endoscope or exoscope, or in an external camera attached to the endoscope or exoscope system.

[0014] An "image" refers to a representation or depiction of a corresponding viewing area, for example, as a color pixel or a group of color pixels, or an equivalent electronic function. Such an image need not be physical or visible to a human; it can also exist in electronic form during processing, storage, or preparation for display and encompass the information of a corresponding image of a viewing area.

[0015] A "viewing area" is, in particular, the area that is viewed with the medical imaging device; for example, such a viewing area is an abdominal cavity, an organ, or another component of an organism.

[0016] "Image information" refers to the informational content of the described image, for example, a corresponding number of pixels in a corresponding color, information about a corresponding spectral distribution of the respective image per pixel, or other information, such as meta-information, of the respective image.

[0017] An "internal and / or external influence" describes any influence on the image information that alters or degrades its quality or content, and which arises or is caused in particular by the optical system, illumination, and / or environmental conditions. Such an influence from the optical system could be, for example, an asymmetry of a lens, an uneven filtering effect across a surface, or, in the simplest case, contamination within the optical system. A corresponding influence from illumination arises, for example, from an uneven spectral distribution of the light source used for illumination.An environmental condition can be, for example, a suitable background around the viewing area, which distorts reflected or scattered light accordingly, for example by overlaying it with a different color spectrum, or by reflecting or absorbing it, thus changing the lighting intensity.

[0018] In this context, "illumination" refers specifically to targeted, multispectral, and / or with a defined light spectrum illumination of the viewing area, which is then used, for example, for MSI or HSI imaging. This illumination can also cause spectral deviations, as the light spectra emitted by the illumination can also deviate from the intended spectrum.

[0019] A "spectral deviation" can occur intentionally or unintentionally and can be, for example, the unintentional filtering of a particular light spectrum, the shadowing of certain spectral ranges, or an influence on the intensity of a particular spectral distribution in specific spectral ranges. An intentional spectral deviation can also be created, for instance, by using specific lighting to analyze tissue parameters based on the resulting light spectrum added to the image information. Such a spectral deviation can be inhomogeneously distributed across multiple pixels, across all pixels of the image, or even across the entire image.

[0020] "Capturing" image information describes, for example, the optical supply of the corresponding image information or a corresponding image towards the image sensor, as well as the conversion of the image information on the image sensor into, for example, an electronic signal.

[0021] A "recording" describes a concrete, identifiable and assignable image of the corresponding image area within the viewing area.

[0022] "Determining" a spectral distribution describes, for example, the generation of information in such a way that a corresponding intensity of the respective light present in the recording is assigned to certain spectra or to certain frequencies of the light, so that comprehensible information about a corresponding spectral distribution is available.

[0023] Determining a spectral deviation involves comparing the spectral distribution of a given recording with a reference spectral distribution, so that, for example, by calculating the difference to corresponding frequencies, information about a deviation in the intensity of the light arriving at that frequency is available.

[0024] A "reference spectral distribution" describes, for example, a spectral distribution determined during the manufacturing or quality assurance of a corresponding medical imaging device. This distribution is assigned to the respective medical imaging device and enables the most realistic possible representation of image information within the viewing area. Such a reference spectral distribution thus serves, for example, as a "normal" for the purpose of calibration. This reference spectral distribution is specifically inhomogeneous and / or definedly inhomogeneous across the image, allowing for calibration based on the resulting inhomogeneity. This enables pixel-precise calibration, unlike, for example, white balance.

[0025] Image correction describes the computational manipulation of image information by adjusting the spectral deviation so that the image information corresponds to the reference spectral distribution of the medical imaging device. One result of such correction is, for example, the "corrected image information."

[0026] In order to make the procedure advantageous in high-resolution imaging techniques and for common image information, the acquisition of image information, the determination of a spectral distribution, the determination of a spectral deviation and / or the correction of the image information is carried out for a pixel or for a respective pixel of the image and / or the image information.

[0027] As a result, a calibrated spectral distribution can be generated for a specific pixel or, for example, for all pixels of a corresponding image or image information, so that even in high-resolution images with a large number of pixels, a very precise determination of, for example, a tissue property is possible using multispectral analysis.

[0028] A "pixel," also called a picture element, image cell, or image segment, refers to individual color or brightness values ​​in a digital raster graphic, as commonly used in electronic imaging processes. Such a pixel also refers to a corresponding area element, for example, of an image sensor or a screen, with a large number of pixels forming a complete image.

[0029] In one embodiment, image information is acquired, a spectral distribution is determined, a spectral deviation is identified and / or the image information is corrected for one or more spectral ranges.

[0030] This approach makes it possible to perform calibration as precisely as possible for the respective spectral range, so that, for example, calibration can be tailored to a multispectral analysis method for determining a tissue property.

[0031] A "spectral range" describes, for example, a section of the entire spectrum of a corresponding spectral distribution.

[0032] In one embodiment, the image information is then corrected based on the deviation from the reference spectral distribution for one or more spectral ranges, so that the image information is processed in such a way that the corrected image information is available with a reduced or eliminated spectral deviation for one or more respective spectral ranges.

[0033] According to the invention, a distance from the viewing area is determined and the reference spectral distribution is determined depending on this distance of the viewing area, wherein the correction of the image information is carried out based on the determined distance from the viewing area.

[0034] This distance allows for calibration with respect to distance-dependent spectral deviations. For example, a distance from the viewing area causes reflections from the background of the viewing area to lead to a spectral deviation.

[0035] To reliably determine this distance from the viewing area, the distance is determined using a laser, an ultrasound system and / or image analysis.

[0036] For example, a laser interferometer, a laser rangefinder, or a device for measuring the time-of-flight of a laser (also called "time-of-flight" analysis) can be used to reliably and precisely determine the distance. An ultrasound system, for instance, can determine the distance based on the echo travel time. Furthermore, it is possible to determine the distance using image analysis. This involves correlating or triangulating relevant image information, for example, in a stereo endoscope, so that the distance can be deduced from the results.

[0037] According to the invention, the correction of the image information is carried out based on a determined rotational position of the optical system and / or the optics.

[0038] Thus, for example, with an endoscope or exoscope featuring a rotating tip or a rotating camera mounted on an eyepiece, the image information can be corrected accordingly depending on the rotational position. Consequently, a corresponding calibration can be performed for each rotational position, which can be continuous and / or pixel-accurate. This involves correcting the image against a reference spectral distribution at each incremental rotational position.

[0039] A "rotational position" describes, for example, a corresponding angle relative to a zero angle or to a starting position of the optical system and / or the optics. The rotational position can, for instance, be specified as an angle in degrees. For each corresponding rotational position or for corresponding increments of rotational positions relative to a full circle, corresponding correction information in the form of a reference spectral distribution can be provided. In particular, this refers to the rotational position of the image sensor relative to the optical system and / or relative to the viewing area.

[0040] In order to reliably determine the rotational position without additional components, the rotational position is determined using an optical indicator for the rotational position assigned to the optical system and / or the optics.

[0041] For example, image analysis can be performed in such a way that an anomaly or geometric deviation of a corresponding image, which can be assigned to an angular position, is used to determine the rotational position.

[0042] An "optical indicator" can be, for example, a section of an otherwise circular image area, a marker, a point, or another geometric feature within the optical system and / or optics, which can be detected by the image sensor and / or connected components such as a display instrument with evaluation technology. Furthermore, a deviation or quality deviation generated during the manufacturing of a corresponding medical imaging device can also be used as an optical indicator. Alternatively or additionally, an optical indicator can also be generated by reference pixels from an image evaluation, provided these are present in the image and dependent on its rotation.

[0043] Alternatively, or to create redundancy, the rotational position is determined using a sensor.

[0044] A "sensor," also called a "measuring probe," is a technical component that can qualitatively and / or quantitatively detect physical or chemical properties and / or material properties or conditions in its respective environment. For example, it then generates a corresponding electronic signal that can be transmitted and / or processed.

[0045] The rotational position can be determined using a magnetic sensor, a Hall sensor, a laser sensor, a light sensor and / or an incremental encoder.

[0046] A "magnetic sensor" can be any sensor that uses magnetic properties to detect rotational position. For example, such a magnetic sensor comprises a magnetized ring which is scanned by a magnetic receiver to determine a rotational position.

[0047] Such a magnetic sensor can also be designed as a "Hall sensor," which can determine magnetic fields using the Hall effect. A corresponding magnetic component of the optical system and / or optics can be positioned relative to a Hall sensor, and a change in the rotational position of the optical system and / or optics will lead to a change in the signal at the Hall sensor.

[0048] A "laser sensor" can be any sensor that generates a measurement signal using a laser beam or by utilizing a laser beam. For example, such a laser sensor can scan a circumferential optical marker on the optical system and / or the optics, thus generating a signal regarding the rotational position.

[0049] Similarly, a rotary position can also be scanned using a light sensor, whereby, for example, an image of an increment ring or an increment encoder is read out and evaluated accordingly.

[0050] To reliably and precisely evaluate tissue properties using the medical imaging device, the corrected image information is analyzed for a high-sensitivity index (HSI) or a medium-sensitivity index (MSI). This is done in particular to determine properties of the viewed area, especially hemoglobin content, water content, and / or oxygen saturation, which are determined, for example, in the corresponding tissue.

[0051] In a further embodiment, the viewing area is illuminated with a light source, wherein the light source is particularly designed to illuminate the viewing area with a light spectrum corresponding to a spectral evaluation, in particular by means of an HSI or MSI.

[0052] In order to ensure reliable operation of the medical imaging device even in the absence of a corresponding reference spectral distribution for a specific rotational position and / or a specific distance, and / or to only have to determine an economical number of reference spectral distributions in the manufacturing process, the reference spectral distribution is determined by means of an interpolation from reference spectral distributions, in particular stored reference spectral distributions of a known distance or known distances and / or a known rotational position or known rotational positions.

[0053] For example, if a reference spectral distribution exists to the left of the current rotational position and another reference spectral distribution exists to the right of the current rotational position, a corresponding intermediate range can be covered by interpolation to cover the current rotational position, and calibration can be performed despite the lack of a reference spectral distribution for this current rotational position.

[0054] Interpolation describes a mathematical process in which a continuous function is found for given discrete data, such as measured values, that also represents intermediate values. The values ​​between the discrete data points are thus approximated and supplemented, so that unknown intermediate values ​​can be determined as precisely as possible.

[0055] In another aspect, the problem is solved by a medical imaging device, in particular a medical endoscope system or a medical exoscope system, which is set up to carry out a procedure according to one or more of the embodiments described above.

[0056] Such a medical imaging device can reliably and accurately determine images and / or image information of the viewing area, even if, for example, an optical system and / or optics of the medical imaging device is subject to rotation and / or the distance of the respective optical system and / or optics to the viewing area is changed.

[0057] For example, such an endoscope system is an optical endoscope with an eyepiece and a camera mounted on the eyepiece, with, for example, a rotation angle sensor to determine a rotational position of the camera in relation to the optical system of the optical endoscope.

[0058] The invention will now be explained in more detail using exemplary embodiments. These will show... Figure 1 shows an examination situation with an endoscope in a schematic side view; Figure 2 a) to c) shows a schematic representation of the rotation of an image with mathematical angle representation; Figure 3 shows a section of the view with a corresponding image of the endoscope. Figure 1 Figure 4 shows an enlargement of the tip of the endoscope. Figure 1 in a schematic side view, as well as Figure 5, a method for calibrating corresponding images.

[0059] An examination situation 101 shows an endoscope 103. The endoscope 103 has a handle 105 for grasping and operating the endoscope 103 by an operator, and an eyepiece 107 for viewing by the operator. A camera 108 is mounted on the eyepiece 107 of the endoscope 103, so that an image, which is actually processed and visible to the operator in the eyepiece 107, is captured by the camera 108. Furthermore, the endoscope 103 has a shaft 109 with a tip 111. Using the endoscope 103, an organ 123 within the abdominal wall 121 is examined, and an image of the organ 123 is generated and captured by the camera 108 through the eyepiece 107. The tip 111 of the endoscope 103 is positioned at a distance 131 from the organ 123. A rotation sensor 106, which is an optical incremental encoder, detects a rotation of the camera 108 relative to the eyepiece 107.

[0060] The endoscope 103 also has devices for illuminating the organ 123, so that the organ is visible and, as a side effect, reflections 141 also occur within the abdominal wall 121. These reflections 141 distort the image recorded by the endoscope 103 and presented to the viewer.

[0061] Furthermore, the endoscope 103 has an evaluation unit 161, on which both the image of the organ 123 captured by the camera 108 and additional information are displayed. Thus, in addition to the image of the organ 123, the evaluation unit 161 can display a superimposed image from a multispectral imaging (MSI) scan, and blood flow or oxygen saturation values ​​of the organ 123 can be shown.

[0062] Image 201 depicts a corresponding image of organ 123. Image 201 has a circular border 203 and an indicator 205 associated with a zero radius 251. Image 201 also has a rotation axis 211. Image areas 207 and 209, which exhibit spectral distortions due to reflections 141, are shown here as examples.

[0063] If image 201 is rotated from its orientation to the zero radius 251 to the rotation radius 252 by a rotation angle 253, the image surfaces 207 and 209 shown as examples also rotate with image 201. Furthermore, the axis of rotation 211 in image 201 can shift due to precession. This can be caused, for example, by the off-center positioning of the corresponding axis of rotation of the camera 108 relative to the eyepiece 107.

[0064] If image 201 is rotated from its orientation to the zero radius 251 to an orientation to the rotation radius 252 by the rotation angle 253, then the corresponding reference points 254 shown in example in image 201 also move along with it (compare Figure 2c )). Based on these reference points, which for example depict significant color differences or contrast differences, the rotation angle 253 is determined by means of image analysis.

[0065] Alternatively, the rotation angle 253 can be determined using an image analysis of indicator 205. This can also be done redundantly by determining the rotation angle 253 using reference points 254. In another alternative, which can also be used redundantly with the image analysis and / or the image analysis of indicator 205, the rotation angle 253 is determined using the rotation sensor 106.

[0066] A view 301 with a background 303 shows a possible display on the evaluation unit 161. An example image 304 is shown here, which contains both an optical image and superimposed color information relating to a multispectral imaging. A corner 305 of the image 304 serves as an indicator for a rotational position, thus allowing a rotation angle of the image 304 to be determined analogously to the rotation angle 253.

[0067] A magnified image 401 of the endoscope 103 shows, in addition to part of the shaft 109 and the tip 111, an image guide 403. The image guide 403 transmits the image of the organ 123 and converts it into a visible image within the endoscope 103. This visible image can then be displayed on the evaluation unit 161 using the eyepiece 107 and the camera 108. Furthermore, an illumination device 405 is arranged at an end region 404 of the tip 111. The illumination device 405 is a multispectral illumination device, allowing individual spectral ranges to be selectively generated to illuminate the organ 123. This makes it possible to determine tissue properties by means of spectral analysis.

[0068] Furthermore, the tip 111 has a laser sensor 407 for determining the distance 131.

[0069] Alternatively, the tip of an endoscope 103 or exoscope can also be rotatable, for example, if an integrated image sensor is used in the endoscope instead of a camera 108. A rotation sensor 409 scans an increment ring (not shown) on the shaft 109 of the endoscope 103. The rotation angle 253 can be determined using the rotation sensor 409, either as an alternative or redundant method to the image evaluation described above.

[0070] Consequently, in the examination situation 101, both the distance 131 to the organ 123 and the rotation angle 253, depending on the camera 108 or alternatively or additionally on the tip 111, can be reliably and accurately determined on the endoscope 103.

[0071] A corresponding procedure 501 for calibrating the endoscope 103 is described in detail as follows:

[0072] An image of organ 123 is acquired at a distance of 131 with a rotation angle of 253. This results in a distorted image of organ 123. The spectral distribution of this acquired image information is then determined, so that the spectral distribution is available for further evaluation. This spectral distribution is used to determine a spectral deviation. This determination uses a reference spectral distribution, and a deviation from this reference spectral distribution is calculated by taking a difference. The deviation is the difference between the determined spectral distribution and the reference spectral distribution.

[0073] The deviation from the reference spectral distribution is then superimposed on the originally recorded image information to generate a corrected image information 513. This corrected image information 513 contains the recorded image information, with the spectral distribution adjusted to match the actual spectral distribution of the image information of organ 123. Thus, this calibrated measurement with the corrected image information allows for precise evaluation and display of the image information on the evaluation unit 161. Reference symbol list

[0074] 101 Examination situation 103 Endoscope 105 Handle 107 Eyepiece 108 Camera 109 Shaft 111 Tip 121 Abdominal wall 123 Organ 131 Distance 141 Reflection 161 Evaluation unit 201 Image 203 Edge 205 Indicator 207 Image area 209 Image area 211 Axis of rotation 251 Zero radius 252 Radius of rotation 253 Angle of rotation 254 Reference points 301 Viewing area 303 Background 304 Image 305 Corner 401 Magnification 403 Image guide 404 End area 405 Illumination 407 Laser sensor 409 Rotation sensor 501 Calibration method 503 Acquisition 505 Determination 507 Determination 509 Reference spectral distribution 511 Correct 513 Corrected image information

Claims

1. Method for calibrating a medical imaging device (103, 108, 161), in particular an endoscope system or an exoscope system, wherein the medical imaging device (103) has an optical system (111) which has an optical unit (403) and an image sensor (108) for capturing an image (301) of a viewing region (123), and image information regarding the image is subject to a spectral deviation due to an internal and / or external influence, in particular due to the optical system (111, 108), due to an illumination (405) and / or due to an environmental condition (131), the method comprising the following steps: - recording (503) image information regarding an image region so that a first recording is obtained, - identifying (505) a spectral distribution of the first recording so that a spectral distribution of the first recording is obtained, - determining (507) a spectral deviation based on the spectral distribution of the first recording and a reference spectral distribution (509) so that a deviation from the reference spectral distribution is obtained, - correcting (511) the image information on the basis of the deviation from the reference spectral distribution (509) so that corrected image information is obtained, so that the image information is processed such that the corrected image information is obtained with a reduced or eliminated spectral deviation, wherein the correction of the image information is carried out based on a determined rotational position of the optical system and / or the optical unit, and / or wherein a distance (131) from the viewing region is determined and the reference spectral distribution (509) is identified depending on this distance (131) from the viewing region, wherein the correction of the image information is carried out based on the determined distance (131) from the viewing region.

2. Method according to claim 1, characterized in that the recording of image information, the identification of a spectral distribution, the determination of a spectral deviation and / or the correction of the image information is carried out for a pixel or for a relevant pixel of the image and / or of the image information.

3. Method according to claim 1 or 2, characterized in that the recording of image information, the identification of a spectral distribution, the determination of a spectral deviation and / or the correction of the image information is carried out for one spectral region or a plurality of spectral regions.

4. Method according to claim 3, characterized in that a correction of the image information is carried out on the basis of the deviation from the reference spectral distribution (509) for one spectral region or a plurality of spectral regions so that the image information is processed such that the corrected image information is obtained with a reduced or eliminated spectral deviation for one relevant spectral region or a plurality of respective spectral regions.

5. Method according to any of the preceding claims, characterized in that the determined distance is identified by means of a laser (407), an ultrasound system and / or by means of image analysis.

6. Method according to any of the preceding claims, characterized in that the rotational position is determined using an optical indicator (205, 305) for the rotational position, which is assigned to the optical system and / or the optical unit.

7. Method according to claim 6, characterized in that the rotational position is determined by means of image evaluation to detect the optical indicator (205, 305) for the rotational position.

8. Method according to any of the preceding claims, characterized in that the rotational position is determined using a sensor (106, 409).

9. Method according to claim 8, characterized in that the rotational position is determined using a magnetic sensor, a Hall sensor, a laser sensor, a light sensor and / or an incremental encoder.

10. Method according to any of the preceding claims, characterized in that the corrected image information is evaluated for an HSI or an MSI, in particular for identifying properties of the viewing region, in particular a hemoglobin content, a water content and / or an oxygen saturation.

11. Method according to any of the preceding claims, characterized in that the viewing region (123) is illuminated using a light source (405), the light source (405) being configured in particular to illuminate the viewing region (123) using a light spectrum corresponding to a spectral evaluation, in particular by means of an HSI or an MSI.

12. Method according to any of the preceding claims, characterized in that the reference spectral distribution (509) is determined by means of an interpolation from reference spectral distributions, in particular stored reference spectral distributions of a known distance or known distances and / or a known rotational position or known rotational positions.

13. Medical imaging device, in particular a medical endoscope system (103, 108, 161) or medical exoscope system, which is configured to carry out a method according to any of claims 1 to 12.