Transformation determination device and method for determining a transformation for image registration

A method is for determining a transformation for image registration of a first image relative to a second image. The method includes ascertaining a test series of test elements including a test transformation and a test value, the ascertaining including ascertaining the test transformation based on a sequence of test transformations and/or based on previously ascertained test elements, transforming the first image via the ascertained test transformation, ascertaining a difference image, and ascertaining the test value of the test element based on the difference image such that the test value is a measure for an extension of a frequency distribution of values of pixels of the difference image in a direction of pixel value increase. It further includes determining a minimum test value based on test values encompassed by the test elements and determining the transformation which is the test transformation of a test element including the minimum test value.

PRIORITY STATEMENT

The present application hereby claims priority under 35 U.S.C. § 119 to German patent application number DE 102015217317.8 filed Sep. 10, 2015, the entire contents of which are hereby incorporated herein by reference.

FIELD

At least one embodiment of the invention generally relates to a method for determining a transformation for image registration of a first image relative to a second image. At least one embodiment of the invention further generally relates to a method for determining a subtraction angiography image, to a transformation determination device, to an image determination device, to an imaging apparatus, to a computer program product, and/or to a computer-readable medium.

BACKGROUND

In digital image processing, an image registration serves in particular to establish a correlation between a plurality of images of a region of interest that is to be imaged in such a way that when the plurality of images are combined pixel by pixel, those pixels are combined with one another which relate to the same part of the region to be imaged. During the image registration of a first image relative to a second image, for example, the first image can be transformed in such a way that those pixels which relate to the same part of the region to be imaged are located at the same position in the first image and in the second image.

In digital subtraction angiography (DSA), an original image pair set containing one or more original image pairs is typically acquired of the same region of a patient that is to be imaged, wherein each original image pair of the original image pair set has a contrast agent image and a mask image. The contrast agent image is typically acquired while a contrast agent is present in the region to be imaged. The mask image is typically acquired while no or an extremely small amount of contrast agent is present in the region to be imaged. Via a pixel-by-pixel subtraction of the mask image from the contrast agent image it is possible for a structure of the region to be imaged in which contrast agent is present to be visualized more clearly with respect to an environment of the structure that is free of contrast agent.

In three-dimensional digital subtraction angiography (3D-DSA), a 3D subtraction angiography image is determined based on an original image pair set containing a plurality of original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image. In particular, the 3D subtraction angiography image can be determined via a technique known as dual volume reconstruction. Using said technique, a 3D mask image dataset can be determined based on the mask images and/or transformed mask images and a 3D contrast agent image dataset can be determined based on the contrast agent images. The 3D subtraction angiography image can be determined through combination of the 3D mask image dataset with the 3D contrast agent image dataset.

The mask image and the contrast agent image are acquired at different points in time. In the time interval lying therebetween, the position of the region to be imaged can change. Possible reasons for this are e.g. a movement of the patient, mechanical vibrations of the raw data acquisition apparatus and/or of the C-arm, fluctuations in an X-ray beam focus or pulsating blood vessels. This can lead to a misregistration of the mask image relative to the contrast agent image such that in a pixel-by-pixel combination, in particular a subtraction, of the mask image and the contrast agent image, those pixels which relate to the same part of the region to be imaged are not combined with one another. The misregistration can result in the quality of a subtraction angiography image which is determined based on the mask image and the contrast agent image being degraded due to artifacts, and consequently can lead to misdiagnoses. Typically, the degradation in image quality is particularly severe when metal, e.g. a coil inserted into an aneurysm, is present in the region to be imaged. In 3D-DSA, a type of artifact known as an eggshell artifact can be produced in this way. In particular in the case of eggshell artifacts it is often difficult to decide to what extent a circulation of blood in the aneurysm is suppressed due to the coil.

SUMMARY

The inventors recognize that the image registration of the mask image relative to the contrast agent image takes on huge importance. An example of a known image registration method is the flexible pixel shift algorithm, which is implemented e.g. in the 3D reconstruction software of the syngo X workplace product.

In at least one embodiment of the invention, an image registration of a first image relative to a second image is improved.

At least one embodiment is directed to a method; at least one embodiment is directed to a transformation determination device; at least one embodiment is directed to an image determination device; at least one embodiment is directed to an imaging apparatus; at least one embodiment is directed to a computer program product; and at least one embodiment is directed to a computer-readable medium.

In the method according to at least one embodiment of the invention for determining a transformation for image registration of a first image relative to a second image, a test series of test elements is ascertained, wherein each test element comprises a test transformation and a test value, wherein each test element of the test series is ascertained by at least the following:

ascertaining the test transformation of the test element based on a predetermined sequence of test transformations and/or based on one or more previously ascertained test elements,

transforming the first image by way of the ascertained test transformation,

ascertaining a difference image based on the transformed first image and the second image,

ascertaining the test value of the test element based on the difference image in such a way that the test value is a measure for an extension of a frequency distribution of values of pixels of the difference image in the direction in which the values of the pixels increase.

In an embodiment of the inventive method for determining a subtraction angiography image based on an original image pair set containing one or more original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image, an original image pair set is provided. A registered image pair set containing one or more registered image pairs is ascertained by performing at least the following for each original image pair of the original image pair set:

determining a transformation for image registration by way of an embodiment of the inventive method for determining a transformation for image registration of a first image relative to a second image, wherein the first image is the mask image of the original image pair and the second image is the contrast agent image of the original image pair, and

transforming the mask image by way of the determined transformation, wherein the transformed mask image and the contrast agent image form a registered image pair of the registered image pair set.

An embodiment of the inventive transformation determination device is embodied for determining a transformation for image registration of a first image relative to a second image and has a test series ascertainment module, a minimum test value determination module and a transformation determination module. The test series ascertainment module is embodied for ascertaining a test series of test elements, wherein each test element comprises a test transformation and a test value. The test series ascertainment module is embodied for performing at least the following for each test element of the test series:

ascertaining the test transformation of the test element based on a predetermined sequence of test transformations and/or based on one or more previously ascertained test elements,

transforming the first image via the ascertained test transformation,

ascertaining a difference image based on the transformed first image and the second image, and

ascertaining the test value of the test element based on the difference image in such a way that the test value is a measure for an extension of a frequency distribution of values of pixels of the difference image in the direction in which the values of the pixels increase.

According to an aspect of an embodiment of the invention, the inventive transformation determination device is embodied for performing an embodiment of the inventive method for determining a transformation for image registration. In particular, the ascertainment of the test series of test elements via the test series ascertainment module, the determining of the minimum test value via the minimum test value determination module and the determining of the transformation which is the test transformation of a test element comprising the minimum test value can be performed via the transformation determination module.

The image determination device according to an embodiment of the invention is embodied for determining a subtraction angiography image based on an original image pair set containing one or more original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image. An embodiment of the inventive image determination device has an original image pair set provider module, a registered image pair ascertainment module and an image determination module.

The original image pair set provider module is embodied for providing the original image pair set. The registered image pair ascertainment module has an embodiment of the inventive transformation determination device and a transformation module. The registered image pair ascertainment module is embodied for ascertaining a registered image pair set containing one or more registered image pairs by performing at least the following for each original image pair of the original image pair set:

determining a transformation via the inventive transformation determination device, wherein the first image is the mask image of the original image pair and the second image is the contrast agent image of the original image pair, and

transforming the mask image by way of the determined transformation via the transformation module, wherein the transformed mask image and the contrast agent image form a registered image pair of the registered image pair set.

An embodiment of an inventive determination device is to be understood as a device that is chosen from the group including an embodiment of an inventive transformation determination device and an embodiment of an inventive image determination device. An embodiment of the inventive imaging apparatus includes an embodiment of the inventive determination device.

An embodiment variant of the invention provides that the imaging apparatus is a medical imaging apparatus and/or that the imaging apparatus has a raw data acquisition apparatus and/or a patient support apparatus. The raw data acquisition apparatus is embodied for acquiring a raw data set. In particular, the raw data acquisition apparatus can comprise a radiation source and a detector. In a C-arm X-ray machine, the radiation source can be an X-ray source and the detector an X-ray detector. In a magnetic resonance tomography system, the radiation source can be a radiofrequency antenna and the detector the same radiofrequency antenna or a further radiofrequency antenna. The patient support apparatus is embodied for supporting and positioning a patient.

An embodiment variant of the invention provides that an embodiment of the inventive determination device and/or one or more components of the inventive determination device are realized at least in part in the form of software on a processor system. In particular, the test series ascertainment module, the minimum test value determination module, the transformation determination module, the original image pair set provider module, the registered image pair ascertainment module, the transformation module and the image determination module in each case form a component of an embodiment of the inventive determination device. An embodiment variant of the invention provides that an embodiment of the inventive determination device and/or one or more components of an embodiment of the inventive determination device are realized at least in part in the form of software-assisted hardware, for example FPGAs or the like.

An embodiment variant of the invention provides that the original image pair set is provided with the aid of a data transfer from the original image pair set provider module to the registered image pair ascertainment module. An embodiment variant of the invention provides that the original image pair set provider module has at least one access module for accessing a memory area, e.g. of a computer and/or control device of an imaging apparatus, in which the original image pair set is stored. The data transfer can be accomplished e.g. by way of a suitable interface. An embodiment variant of the invention provides that interfaces for data transfer to and/or from components of the inventive determination device are realized at least in part in the form of software. In particular, the interfaces can have at least one access module for accessing suitable memory areas in which data can be suitably buffered, retrieved and updated. The interfaces can also be embodied as hardware-based interfaces which are controlled via suitable software.

A largely software-based implementation has the advantage that existing control devices already used in the prior art can also be easily upgraded via a software update in order to operate in the inventive manner. In this respect an embodiment relates to a corresponding computer program product having a computer program which can be loaded into a memory device of a computer, wherein an embodiment of an inventive method is performed by way of the computer program when the computer program is executed in the control device. As well as the computer program, such a computer program product can comprise additional software components, e.g. documentation, and/or hardware components, e.g. a hardware key (dongle, etc.) to enable use of the software.

In an embodiment, in order to transport the computer program to the control device and/or to store the computer program on or in the control device, a computer-readable medium can be used, for example a memory stick, a hard disk or some other transportable or permanently installed data medium on which a computer program which can be loaded into a memory device of a computer is stored, wherein an embodiment of an inventive method is performed by way of the computer program when the computer program is executed on the computer. An embodiment variant of the invention provides that the control device has a processor system. The processor system can be formed e.g. by one or more cooperating microprocessors.

According to an aspect of the invention, the imaging apparatus is chosen from the group including a C-arm X-ray machine, a computed tomography device, a single photon emission computed tomography device (SPECT device), a positron emission tomography device (PET device), a magnetic resonance tomography device, and combinations thereof. In particular, the imaging apparatus can comprise an X-ray machine, an ultrasound device and similar. The imaging apparatus can furthermore be a combination of a plurality of imaging and/or irradiation modalities. In this case an irradiation modality can comprise for example an irradiation device for radiation therapy.

DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS

In the method according to at least one embodiment of the invention for determining a transformation for image registration of a first image relative to a second image, a test series of test elements is ascertained, wherein each test element comprises a test transformation and a test value, wherein each test element of the test series is ascertained by at least the following:ascertaining the test transformation of the test element based on a predetermined sequence of test transformations and/or based on one or more previously ascertained test elements,transforming the first image via the ascertained test transformation,ascertaining a difference image based on the transformed first image and the second image,ascertaining the test value of the test element based on the difference image in such a way that the test value is a measure for an extension of a frequency distribution of values of pixels of the difference image in the direction in which the values of the pixels increase.

A minimum test value is determined based on the test values encompassed by the test elements of the test series. The transformation which is the test transformation of a test element comprising the minimum test value is determined.

The test value is a measure for the extension of a frequency distribution of values of pixels in the direction in which the values of the pixels increase in particular when the test value is dependent on the values of the pixels in such a way that a change in the values of the pixels that corresponds to a shifting and/or stretching of the frequency distribution in the direction in which the values of the pixels increase leads to an increase in the test value.

The inventors recognize that an optimal transformation for image registration can be determined by testing different test transformations in relation to their suitability for image registration and rating them with a test value. The inventors propose that the test value be ascertained in such a way that a test transformation which results in the difference image having one or more pixels with comparatively larger values is penalized more heavily, in particular is rated as less suitable for the image registration. The test value can be ascertained in particular based on the values of the pixels of the difference image. In this case the one pixel or the plurality of pixels having the greatest, in other words maximum, values can be weighted more heavily compared to pixels having smaller values.

An embodiment variant of the invention provides that test transformations which are less suitable for image registration are assigned larger test values. The optimal transformation for image registration can therefore be determined by identifying, among the test transformations of the test series, the test transformation having the minimum test value. The method according to the invention can in this regard be described as a “Minimum of Maximum” (MoM) method.

An embodiment variant of the invention provides that the first image and the second image are acquired with the aid of a radiation dose penetrating the region to be imaged and that the values of the pixels of the first image and the values of the pixels of the second image are dependent on an absorption of the radiation in such a way that a stronger absorption of the radiation in a part of the region to be imaged to which a given pixel relates results in a smaller value of the given pixel. The values of the pixels can be in particular grayscale values and/or brightness values. The values of the pixels can be in particular absorption values and/or intensity values.

An embodiment variant of the invention provides that the difference image is ascertained by subtracting the transformed first image from the second image one pixel at a time. Typically, the difference image has a pixel with a comparatively large value when a small value of a pixel of the transformed first image is subtracted from a large value of a pixel of the second image. This case occurs in particular when, on account of the misregistration, a pixel relating to a part of the region to be imaged exhibiting stronger absorption is subtracted from a pixel relating to a part of the region to be imaged exhibiting weaker absorption. A part of the region to be imaged exhibiting stronger absorption can comprise e.g. a part of a blood vessel and/or of an aneurysm and/or of a metal object, in particular a coil inserted into an aneurysm. A part of the region to be imaged exhibiting weaker absorption can comprise e.g. a part of a tissue.

By way of the method according to at least one embodiment of the invention, it is possible in particular to determine a transformation for image registration in such a way that, during the ascertainment of the difference image, pixels of the second image having in each case a large value or exhibiting strong absorption have subtracted from them pixels of the first image, likewise having in each case a large value or exhibiting weak absorption. This results in a reduction of the extension of the frequency distribution of values of pixels of the difference image in the direction in which the values of the pixels increase. The test value associated with said transformation is therefore comparatively small, with the result that the transformation can be determined from the set of test transformations of the test series via a minimum search relating to the test value.

According to an aspect of at least one embodiment of the invention, the test value is ascertained based on a pixel group of the difference image, wherein a pixel position condition relating to a position of the pixel and/or a pixel value condition relating to a value of the pixel is met for each pixel of the pixel group.

An embodiment variant of the invention provides that the pixel group comprises the pixels of the difference image for which a position condition relating to a position of the respective pixel and/or a pixel value condition relating to a value of the respective pixel is met.

The frequency distribution can relate to the entire difference image and/or to a pixel group of the difference image, wherein a pixel position condition relating to a position of the pixel and/or a pixel value condition relating to a value of the pixel is met for each pixel of the pixel group. An embodiment variant of the invention provides that a pixel group of pixels of the difference image is ascertained, wherein a pixel position condition relating to a position of the pixel and/or a pixel value condition relating to a value of the pixel is met for each pixel of the pixel group. An embodiment variant of the invention provides that the test value of the test element is ascertained and/or calculated based on the values of the pixels of the pixel group and/or that the test value is a measure for the values of the pixels of the pixel group. Ascertaining the test value in such a way that a plurality of values of pixels contribute in each case to the test value enables the error susceptibility of the method in relation to outliers to be reduced.

According to an aspect of an embodiment of the invention, the test value is a parameter of the pixel group and/or of the difference image and/or the test value is ascertained based on a parameter of the pixel group and/or of the difference image, wherein the parameter is chosen from the group which includes a maximum of the values of the pixels, a sum of the values of the pixels and a location parameter of a frequency distribution of the values of the pixels. An embodiment variant of the invention provides that the test value is ascertained based on a plurality of parameters of the pixel group and/or of the difference image, wherein each of the parameters of the plurality of parameters is chosen from the group which includes a maximum of the values of the pixels, a sum of the values of the pixels and a location parameter of a frequency distribution of the values of the pixels. An embodiment variant of the invention provides that the test value is ascertained based on the parameter and/or based on the plurality of parameters, wherein the test value is dependent in a monotonically increasing manner on the parameter or on each of the parameters of the plurality of parameters. An embodiment variant of the invention provides that the test value is the maximum of the values of the pixels of the pixel group and/or of the difference image. An embodiment variant of the invention provides that the test value is the sum of the values of the pixels of the pixel group. According to an aspect of the invention, the location parameter is chosen from the group which includes a mean value, a mode and a quantile.

According to an aspect of an embodiment of the invention, the pixel position condition for a given pixel of the difference image is met when the position of the given pixel is located in a region of interest (RoI). With the aid of the pixel position condition it is possible to restrict the pixel group in relation to the positions of the pixels of the pixel group. Preferably, the region of interest is determined in such a way that the part of the region to be imaged affected by the pixels of the pixel group is particularly sensitive in relation to a misregistration. This is typically the case when a section having large pixel values is contiguous with a section having small pixel values in the region of interest. In such a case even a slight misregistration corresponding e.g. to a translation by one pixel results in large pixel values in the difference image and consequently a severe degradation in quality, e.g. of the DSA image, due to artifacts.

An embodiment variant of the invention provides that a subregion is ascertained in the first image and/or in the second image, wherein the pixels of the subregion have smaller values relative to an environment of the subregion and/or wherein the subregion comprises both pixels having large values and pixels having small values and/or wherein the values of the pixels of the subregion form a gradient whose gradient magnitude exceeds a gradient magnitude threshold value. An embodiment variant of the invention provides that the region of interest comprises the subregion and/or the environment of the subregion.

According to an aspect of an embodiment of the invention, the pixel value condition for a given pixel of the difference image is met when the value of the given pixel exceeds a pixel threshold value. With the aid of the pixel position condition and/or of the pixel value condition, pixels can be selected for the pixel group from regions that are particularly relevant for the image registration and/or pixels can be excluded from the pixel group that are less relevant for the image registration. In this way it is possible to achieve a reduction in the computational overhead and in the error susceptibility of the method.

According to an aspect of an embodiment of the invention, the test transformation comprises a translation and/or a rotation. An embodiment variant of the invention provides that the test transformation alternatively or additionally comprises a further image operation, e.g. a local or a global image operation. An embodiment variant of the invention provides that the test transformation is a test transformation in the plane of the first image and/or that the translation is a translation in the plane of the first image and/or that the rotation is a rotation in the plane of the first image.

An embodiment variant of the invention provides that a sequence of test transformations is predetermined. The predetermined sequence of test transformations can include one test transformation or a plurality of test transformations. Starting with a first test transformation of the predetermined sequence of test transformations, a first test value is ascertained. The first test transformation of the predetermined sequence and the first test value form the first test element of the test series. Next, a further test value can be ascertained via a further test transformation of the predetermined sequence of test transformations. The further test transformation of the predetermined sequence and the further test value form a further test element of the test series. This step can be repeated e.g. for one or more or all test transformations of the predetermined sequence of test transformations.

The test transformation of the test element can be ascertained in particular based on a predetermined sequence of test transformations. For example, with the exception of the first test transformation of the predetermined sequence, each test transformation of the predetermined sequence of test transformations can differ compared to the respective preceding test transformation by a translation by one pixel in the same or in a different direction. For example, the predetermined sequence of test transformations can comprise all translations up to a given number of pixels, e.g. up to five pixels.

According to an aspect of an embodiment of the invention, the test transformation is ascertained via a search algorithm. An embodiment variant of the invention provides that the test transformation of the test element is ascertained via a search algorithm based on one or more previously ascertained test elements. The search algorithm can be a minimum search algorithm and/or e.g. a downhill simplex algorithm.

According to an aspect of an embodiment of the invention, the test value is compared with a test threshold value, wherein the ascertainment of the test series is not continued further if the test value falls below the test threshold value. An embodiment variant of the invention provides that the test value is compared with a test threshold value, wherein the ascertainment of the test series is terminated if the test value falls below the test threshold value.

In an embodiment of the inventive method for determining a subtraction angiography image based on an original image pair set containing one or more original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image, an original image pair set is provided. A registered image pair set containing one or more registered image pairs is ascertained by performing at least the following for each original image pair of the original image pair set:

determining a transformation for image registration by way of an embodiment of the inventive method for determining a transformation for image registration of a first image relative to a second image, wherein the first image is the mask image of the original image pair and the second image is the contrast agent image of the original image pair, and

transforming the mask image via the determined transformation, wherein the transformed mask image and the contrast agent image form a registered image pair of the registered image pair set.

In an embodiment, the subtraction angiography image is determined based on the registered image pair set.

An embodiment variant of the invention provides that the first image is two-dimensional (2D) and/or that the second image is two-dimensional (2D) and/or that the image registration is a 2D-2D image registration. An embodiment variant of the invention provides that the subtraction angiography image is a 2D subtraction angiography image or that the subtraction angiography image is a 3D subtraction angiography image. The method according to an embodiment of the invention can furthermore be advantageously employed also in the case of a method for determining so-called 2D roadmap images and/or fluoroscopic images.

By a strong and/or a stronger absorption may be understood in particular an absorption by a blood vessel filled with contrast agent and/or an absorption by a metal object. By a weak and/or a weaker absorption may be understood in particular an absorption by a tissue in which one or more blood vessels are embedded. By a small and/or a smaller value of a pixel may be understood in particular a value from the value range, which value range is formed by the values of the pixels of the second image and/or of the contrast agent image which relate to one or more blood vessels filled with contrast agent and/or to a metal object. By a large and/or a larger value of a pixel may be understood in particular a value from the value range, which value range is formed by the values of the pixels of the first image and/or of the mask image which relate to a tissue in which one or more blood vessels are embedded.

An embodiment of the invention enables an improved image registration of the first image relative to the second image. In particular, an embodiment of the invention enables eggshell artifacts to be effectively suppressed and/or avoided.

An embodiment of the inventive transformation determination device is embodied for determining a transformation for image registration of a first image relative to a second image and has a test series ascertainment module, a minimum test value determination module and a transformation determination module. The test series ascertainment module is embodied for ascertaining a test series of test elements, wherein each test element comprises a test transformation and a test value. The test series ascertainment module is embodied for performing at least the following for each test element of the test series:

ascertaining the test transformation of the test element based on a predetermined sequence of test transformations and/or based on one or more previously ascertained test elements,

transforming the first image via the ascertained test transformation,

ascertaining a difference image based on the transformed first image and the second image, and

ascertaining the test value of the test element based on the difference image in such a way that the test value is a measure for an extension of a frequency distribution of values of pixels of the difference image in the direction in which the values of the pixels increase.

The minimum test value determination module is embodied for determining a minimum test value based on the test values encompassed by the test elements of the test series. The transformation determination module is embodied for determining the transformation which is the test transformation of a test element comprising the minimum test value.

According to an aspect of an embodiment of the invention, the inventive transformation determination device is embodied for performing an embodiment of the inventive method for determining a transformation for image registration. In particular, the ascertainment of the test series of test elements via the test series ascertainment module, the determining of the minimum test value via the minimum test value determination module and the determining of the transformation which is the test transformation of a test element comprising the minimum test value can be performed via the transformation determination module.

The image determination device according to an embodiment of the invention is embodied for determining a subtraction angiography image based on an original image pair set containing one or more original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image. An embodiment of the inventive image determination device has an original image pair set provider module, a registered image pair ascertainment module and an image determination module.

The original image pair set provider module is embodied for providing the original image pair set. The registered image pair ascertainment module has an embodiment of the inventive transformation determination device and a transformation module. The registered image pair ascertainment module is embodied for ascertaining a registered image pair set containing one or more registered image pairs by performing at least the following for each original image pair of the original image pair set:

determining a transformation by way of an embodiment of the inventive transformation determination device, wherein the first image is the mask image of the original image pair and the second image is the contrast agent image of the original image pair, and

transforming the mask image by way of the determined transformation via the transformation module, wherein the transformed mask image and the contrast agent image form a registered image pair of the registered image pair set.

The image determination module is embodied for determining the subtraction angiography image based on the registered image pair set.

According to an aspect of an embodiment of the invention, an embodiment of the inventive image determination device is embodied for performing an embodiment of the inventive method for determining a subtraction angiography image. In particular, the providing of the original image pair set via the original image pair set provider module, the ascertaining of a registered image pair set via the registered image pair ascertainment module, the determining of a transformation for image registration via the transformation determination device, the transforming of the mask image via the transformation module, and the determining of the subtraction angiography image based on the registered image pair set can be performed via the image determination module.

An embodiment of an inventive determination device is to be understood as a device that is chosen from the group including an embodiment of an inventive transformation determination device and an embodiment of an inventive image determination device. An embodiment of the inventive imaging apparatus includes an embodiment of the inventive determination device.

An embodiment variant of the invention provides that the imaging apparatus is a medical imaging apparatus and/or that the imaging apparatus has a raw data acquisition apparatus and/or a patient support apparatus. The raw data acquisition apparatus is embodied for acquiring a raw data set. In particular, the raw data acquisition apparatus can comprise a radiation source and a detector. In a C-arm X-ray machine, the radiation source can be an X-ray source and the detector an X-ray detector. In a magnetic resonance tomography system, the radiation source can be a radiofrequency antenna and the detector the same radiofrequency antenna or a further radiofrequency antenna. The patient support apparatus is embodied for supporting and positioning a patient.

An embodiment variant of the invention provides that the imaging apparatus has a control device and an image reconstruction device. The control device can be a computer, for example. In particular, the inventive determination device can be embodied as part of a control device of an imaging apparatus. The image reconstruction device is embodied for ascertaining a first image and a second image, in particular a mask image and a contrast agent image, of a region to be imaged of an object examined via the imaging apparatus based on a raw data set, e.g. using a reconstruction method.

An embodiment variant of the invention provides that an embodiment of the inventive determination device and/or one or more components of the inventive determination device are realized at least in part in the form of software on a processor system. In particular, the test series ascertainment module, the minimum test value determination module, the transformation determination module, the original image pair set provider module, the registered image pair ascertainment module, the transformation module and the image determination module in each case form a component of an embodiment of the inventive determination device. An embodiment variant of the invention provides that an embodiment of the inventive determination device and/or one or more components of an embodiment of the inventive determination device are realized at least in part in the form of software-assisted hardware, for example FPGAs or the like.

An embodiment variant of the invention provides that the original image pair set is provided with the aid of a data transfer from the original image pair set provider module to the registered image pair ascertainment module. An embodiment variant of the invention provides that the original image pair set provider module has at least one access module for accessing a memory area, e.g. of a computer and/or control device of an imaging apparatus, in which the original image pair set is stored. The data transfer can be accomplished e.g. by way of a suitable interface. An embodiment variant of the invention provides that interfaces for data transfer to and/or from components of the inventive determination device are realized at least in part in the form of software. In particular, the interfaces can have at least one access module for accessing suitable memory areas in which data can be suitably buffered, retrieved and updated. The interfaces can also be embodied as hardware-based interfaces which are controlled via suitable software.

A largely software-based implementation has the advantage that existing control devices already used in the prior art can also be easily upgraded via a software update in order to operate in the inventive manner. In this respect an embodiment relates to a corresponding computer program product having a computer program which can be loaded into a memory device of a computer, wherein an embodiment of an inventive method is performed by way of the computer program when the computer program is executed in the control device. As well as the computer program, such a computer program product can comprise additional software components, e.g. documentation, and/or hardware components, e.g. a hardware key (dongle, etc.) to enable use of the software.

In an embodiment, in order to transport the computer program to the control device and/or to store the computer program on or in the control device, a computer-readable medium can be used, for example a memory stick, a hard disk or some other transportable or permanently installed data medium on which a computer program which can be loaded into a memory device of a computer is stored, wherein an embodiment of an inventive method is performed by way of the computer program when the computer program is executed on the computer. An embodiment variant of the invention provides that the control device has a processor system. The processor system can be formed e.g. by one or more cooperating microprocessors.

According to an aspect of an embodiment of the invention, the imaging apparatus is chosen from the group including a C-arm X-ray machine, a computed tomography device, a single photon emission computed tomography device (SPECT device), a positron emission tomography device (PET device), a magnetic resonance tomography device, and combinations thereof. In particular, the imaging apparatus can comprise an X-ray machine, an ultrasound device and similar. The imaging apparatus can furthermore be a combination of a plurality of imaging and/or irradiation modalities. In this case an irradiation modality can comprise for example an irradiation device for radiation therapy.

Within the scope of the invention, features which are described in relation to different embodiment variants and/or different claims categories (method, device, etc.) can be combined to form further embodiment variants. In particular, the features, advantages and embodiment variants described in relation to the inventive method can also be applied to the inventive determination device, the inventive imaging apparatus, the inventive computer program product, and the inventive computer-readable medium, and vice versa. In other words, the object-related claims can also be developed on the basis of the features which are described or claimed in connection with a method. Functional features of an inventive method can in this case be implemented by correspondingly embodied components or modules of the inventive transformation determination device and/or of the inventive image determination device. The use of the indefinite articles “a” or “an” does not preclude the possibility that a plurality of the features in question may also be present.

The described method, the described transformation determination device, the described image determination device and the described imaging apparatus are simply embodiment variants of the invention. The invention can be varied by the person skilled in the art without leaving the scope of the invention insofar as this is defined by the claims.

FIG. 1shows a flowchart of a method for determining a transformation for image registration of a first image relative to a second image according to a first embodiment variant of the invention. At step DTR, a test series of test elements is ascertained, wherein each test element comprises a test transformation and a test value. Each test element of the test series is ascertained by performing steps TT, T1, DI and TV in each case. At step TT, the test transformation of the test element is ascertained based on a predetermined sequence of test transformations and/or based on one or more previously ascertained test elements. At step T1, the first image I1is transformed via the ascertained test transformation. At step DI, a difference image is ascertained based on the transformed first image TI1and the second image I2. At step TV, the test value of the test element is ascertained based on the difference image in such a way that the test value is a measure for an extension of a frequency distribution of values of pixels of the difference image in the direction in which the values of the pixels increase. At step MIN, a minimum test value is determined based on the test values encompassed by the test elements of the test series. At step DT1, the transformation which is the test transformation of a test element comprising the minimum test value is determined.

FIG. 2shows an illustration of a transformation determination device35according to a second embodiment variant of the invention. The transformation determination device35is embodied for determining a transformation for image registration of a first image I1relative to a second image I2and has a test series ascertainment module51, a minimum test value determination module52and a transformation determination module53.

The transformation determination device35is embodied in particular for performing the method according to the first embodiment variant of the invention. In particular, the ascertaining DTR of the test series of test elements via the test series ascertainment module51, the determining MIN of the minimum test value via the minimum test value determination module52and the determining DT1of the transformation which is the test transformation of a test element comprising the minimum test value can be performed via the transformation determination module53.

In the embodiment variants shown below, invention-developing features in particular are described with reference to the respective hereintofore-explained embodiment variants. Features, in particular steps and components, which remain substantially the same are labeled with the same reference signs.

FIG. 3shows a flowchart of a method for determining a subtraction angiography image based on an original image pair set containing one or more original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image, according to a third embodiment variant of the invention.

At step PI, the original image pair set is provided. At step RI, a registered image pair set containing one or more registered image pairs is ascertained by performing steps DT3and T3for each original image pair of the original image pair set. At step DT3, a transformation for image registration is determined by way of the method according to the first embodiment variant of the invention, wherein the first image is the mask image of the original image pair and the second image is the contrast agent image of the original image pair. At step T3, the mask image is transformed via the determined transformation, wherein the transformed mask image and the contrast agent image form a registered image pair of the registered image pair set. At step DS, the subtraction angiography image is determined based on the registered image pair set.

FIG. 4shows an illustration of an image determination device60according to a fourth embodiment variant of the invention. The image determination device60is embodied for determining a subtraction angiography image based on an original image pair set containing one or more original image pairs, wherein each original image pair of the original image pair set has a mask image and a contrast agent image. The image determination device60has an original image pair set provider module61, a registered image pair ascertainment module62and an image determination module64. The registered image pair ascertainment module62has a transformation determination device35according to the second embodiment variant of the invention and a transformation module63.

The image determination device60is embodied in particular for performing the method according to the third embodiment variant of the invention. In particular, the providing PI of the original image pair set via the original image pair set provider module61, the ascertaining RI of a registered image pair set via the registered image pair ascertainment module62, the determining DT3of a transformation via the transformation determination device35, the transforming T3of the mask image via the transformation module63, and the determining DS of the subtraction angiography image based on the registered image pair set can be performed via the image determination module64.

FIG. 5shows an imaging apparatus1according to a fifth embodiment variant of the invention. Without limitation of the general inventive concept, a C-arm X-ray machine1is shown by way of example for the imaging apparatus1. The fifth embodiment variant of the invention provides that the raw data acquisition apparatus26,28is a projection data acquisition apparatus26,28, that the raw data set is a projection dataset, that the radiation source26is an X-ray source26, and that the detector28is an X-ray detector28, in particular a flat-panel image detector.

The imaging apparatus1has a C-arm apparatus20, a patient support apparatus10and a projection data acquisition apparatus26,28. The patient support apparatus10has a support table11and a support tabletop12for supporting and positioning the patient13. The C-arm apparatus20has a stationary supporting frame21and a C-arm24. The C-arm24is movable, in particular able to pivot, relative to the patient support apparatus10. This enables different arrangements of the radiation source26and the detector28to be set relative to the region to be imaged of the patient13.

The projection data acquisition apparatus26,28is embodied for acquiring a projection dataset. The projection data acquisition apparatus26,28has a radiation source26which is embodied for emitting radiation quanta27and a detector28which is embodied for detecting the radiation quanta27. A region to be imaged of an object, in particular of the patient13, can be arranged between the radiation source26and the detector28. The radiation quanta27are able to travel from the radiation source26to the region to be imaged of the patient13and impinge on the detector28following an interaction with the region to be imaged. In this way a projection profile of the region to be imaged can be acquired. The projection profile can be in particular two-dimensional. Projection data acquired by the projection data acquisition apparatus26,28is forwarded to the control device30. The control device30is embodied in particular for acquiring, storing, retrieving, post-processing and providing the projection data in the form of a projection profile and/or in the form of a projection dataset. A projection dataset can comprise one or more projection profiles.

By moving the projection data acquisition apparatus26,28relative to the patient support apparatus it is possible to acquire at least one projection profile in each case for different arrangements of the radiation source26and the detector28relative to the region to be imaged.

The first image and/or the mask image can be in particular a first projection profile and/or be ascertained based on a first projection dataset. In this case the first projection profile and/or the first projection dataset can relate to a mask examination status of the region to be imaged in which the region to be imaged has no contrast agent or contrast agent in a concentration that is not relevant for the contrast agent imaging.

The second image and/or the contrast agent image can be in particular a second projection profile and/or be ascertained based on a second projection dataset. In this case the second projection profile and/or the second projection dataset can relate to a contrast agent examination status of the region to be imaged in which the region to be imaged has a contrast agent, in particular in a concentration that is suitable for contrast agent imaging.

Optionally, the imaging apparatus1can have a contrast agent injection apparatus for injecting a contrast agent into the patient13. A contrast agent injection can be e.g. at least partially automated with the aid of a contrast agent injection module of the control device30and/or be performed manually by an operator.

The imaging apparatus1has a control device30, an input device38and an output device39. The control device30is a computer, in particular a digital computer, and is embodied for controlling the imaging apparatus1. The input device38is embodied for inputting control information, e.g. image reconstruction parameters and/or examination parameters. The output device39is embodied for outputting control information and/or images, in particular for outputting the subtraction angiography image. The control device30has a memory device31. The memory device31is embodied for loading a computer program, in which case the steps of an inventive method are performed via the computer program when the computer program is executed on the control device30.

The imaging apparatus1has an image reconstruction device34, the transformation determination device35according to the second embodiment variant of the invention and the image determination device60according to the fourth embodiment variant of the invention. Both the image reconstruction device34and the image determination device60together with transformation determination device35are implemented in the form of software on a processor system of the control device30.

Via the image reconstruction device34, an image dataset can be ascertained based on a projection dataset. The image dataset can comprise e.g. the first image and/or the second image and/or one or more mask images and/or one or more contrast agent images and/or one or more original image pairs and/or an original image pair set. The image dataset can be stored in particular by the image reconstruction device34and/or by the control device30in a memory area of the control device30.

FIG. 6shows, in a greatly simplified illustration, a first image I1, a second image I2and a transformed first image TI1according to a sixth embodiment variant of the invention. The sixth embodiment variant of the invention provides that the values of the pixels of the first image I1and the values of the pixels of the second image I2are grayscale values. InFIG. 6, a brighter gray shade of a pixel corresponds to a larger value, in particular grayscale value, of the pixel. The first image I1is a mask image. The second image I2is a contrast agent image. On account of the contrast agent, the value of the pixels relating to the blood vessel BV is significantly smaller in the second image I2than in the first image. The blood vessel BV is embedded in a tissue ST having a large grayscale value. The value of the pixel relating to a metal coil CA inserted into an aneurysm is particularly small. A misregistration of the first image I1relative to the second image I2is present. The consequence of the misregistration is in particular that in a pixel-by-pixel combination of the first image I1and the second image I2, the pixel of the first image I1relating to the metal coil CA inserted into an aneurysm is combined with a pixel of the second image I2relating to the tissue ST. In particular, a difference image which is ascertained by pixel-by-pixel subtraction of the first image I1from the second image I2has a large value at the position at which the pixel relating to a metal coil CA inserted into an aneurysm is located in the first image I1. This value is approximately equal to the value of the pixels relating to the tissue ST.

The transformed first image TI1has been produced from the first image via a transformation for image registration that was determined with the aid of a method according to the first embodiment variant of the invention. In a pixel-by-pixel combination of the transformed first image TI1and the second image I2, the pixels of the second image I2which relate to the tissue ST are combined with the pixels of the transformed first image TI1which relate to the tissue ST. In particular, a difference image which is ascertained via a pixel-by-pixel subtraction of the transformed first image TI1from the second image I2does not have a large value at any position.

The fact that the contrast agent image comprises pixels which relate to a blood vessel BV filled with contrast agent and which therefore have small values does not have a disrupting effect on the method for determining the transformation for image registration according to the first embodiment variant of the invention, because the transformation is determined in such a way that large values of pixels in the difference image are avoided. In conventional methods for determining a transformation for 2D-2D image registration it can happen that pixels of the second image which relate to a blood vessel filled with contrast agent and pixels of the first image which relate to a metal object are registered onto one another and/or are combined with one another during the pixel-by-pixel combining of the transformed first image and the second image.

Advantages of embodiments of the invention taking clinical data as an example are explained with reference toFIGS. 7 to 10.

FIG. 7shows a difference image which has been ascertained by pixel-by-pixel subtraction of a mask image from a contrast agent image, wherein the mask image is misregistered relative to the contrast agent image. Contours of a metal coil which has been inserted into an aneurysm are clearly recognizable and are marked by an arrow.

FIG. 8shows a difference image which has been ascertained by pixel-by-pixel subtraction of a transformed mask image from a contrast agent image. In this case the mask image and the contrast agent image are the same as those on which the difference image shown inFIG. 7is also based. The mask image has been transformed via a transformation that was determined with the aid of an inventive method for determining a transformation for image registration. The misregistration as well as the contours of the coil are avoided as a result.

FIG. 9shows a 3D subtraction angiography image (3D-DSA image) which has been determined based on an original image pair set. In this case the original image pair set has an original image pair comprising the mask image and the contrast agent image on which the difference image shown inFIG. 7is based. On account of the misregistration of the mask image relative to the contrast agent image shown inFIG. 7, the 3D-DSA image has an artifact, in particular of a type known as an eggshell artifact. The arrow marks the aneurysm into which the coil has been inserted. On the basis of the 3D-DSA image shown inFIG. 9the impression could be created that a blood flow is taking place into the aneurysm into which the coil has been inserted, which could result in a misdiagnosis.

FIG. 10shows a 3D subtraction angiography image (3D-DSA image) which has been determined by way of an inventive method for determining a subtraction angiography image. The 3D-DSA image shown inFIG. 9and the 3D-DSA image shown inFIG. 10are based on the same original image pair set. In the determining of the 3D-DSA image shown inFIG. 10, the mask image for each original image pair of the original image pair set has been transformed by way of a transformation that was determined with the aid of an inventive method for determining a transformation for image registration. The eggshell artifact is avoided as a result.