WO2011135484A1

WO2011135484A1 - Device and method for providing subtraction images

Info

Publication number: WO2011135484A1
Application number: PCT/IB2011/051671
Authority: WO
Inventors: Jens Von Berg; Ulrich Neitzel; Dirk Manke
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2010-04-26
Filing date: 2011-04-18
Publication date: 2011-11-03

Abstract

A device (2) for providing subtraction images comprises a suppression unit (4) for suppressing bony structures, a registration unit (6) for registering the first image and the second image and a subtraction unit (8) for subtracting the registered second image from the registered first image. Thereby the registration especially of thorax X-Ray images does not depend on bony structures and subtraction images comprise a clearly 10 improved accuracy.

Description

DEVICE AND METHOD FOR PROVIDING SUBTRACTION IMAGES

FIELD OF THE INVENTION

The invention relates to imaging systems which inter alia can be used for obtaining image data of tumourous tissues. In particular the invention relates to a device and a method for providing subtraction images and to an imaging system capable of conducting such a method.

BACKGROUND OF THE INVENTION

The detection, localization and interpretation of objects in medical images, such as X-ray images, is often complicated by noise and partial obscuration of objects of interest. For interpretation of X-ray images of the chest or thorax it is a common practice to compare recently obtained X-ray images with older X-ray images, which method is accomplished by subtraction images. These are a good means to visualize interval changes between follow-up images. This technique has been mainly applied to thorax X-ray images.

Typically, a non-rigid 2D image registration method is used prior to subtraction of the current image from the prior image in order to align corresponding anatomical parts. Ideally, the subtraction image should become medium grey except for the places where interval changes occurred. There, a tumour growth may appear bright at the subtraction image (or dark, respectively).

The quality of temporal chest subtraction images is often impaired by different positions of the patient in the two acquisitions which may be delay ed/separated by a time interval of weeks or even months. Especially anterior-posterior inclination is problematic with thorax images, because it causes a shift of posterior ribs quite different to those of other important structures, for example lung vessels. Image registration of follow-up images typically leads to the alignment of the most contrasted structure in the lung fields which are typically the posterior ribs. For this reason there exist methods to generate soft tissue images made with dual energy techniques instead of standard chest radiographs. Lacking bones in these images the image registration of soft tissue follow-up images typically aligns the most contrasted remaining structure in these images, which are typically the lung vessels. Dual energy, however requires special equipment that is not widely available. It may also mean a higher dose of radiation to the patient.

US 6,067,373 shows a method for iterative image warping prior to temporal subtraction of chest radiographs in the detection of interval changes. SUMMARY OF THE INVENTION

The main inherent drawback of methods known in the prior art for producing subtraction images is that the scanning geometry might be different in the two subsequent scans for obtaining the prior image and the current image. This usually does not allow for a compensation with image registration.

It would therefore be advantageous to provide a device, a method or an imaging system to provide a reliable and precise subtraction image for detecting, localizing and interpreting objects of interest in a set of timely delayed images.

To better address one or more of these concerns, in a first aspect of the invention there is provided a device for generating subtraction images.

The device comprises a suppression unit adapted for bony structure reduction processing in a first image and in a second image, a registration unit adapted for registering the first image and the second image processed by the suppression unit and a subtraction unit adapted for subtracting the registered second image from the registered first image. The device according to the invention is adapted for conducting the suppression of a bony structure prior to transferring the modified images with suppressed bony structure to the registration unit. Also, the first and second image are transferred to the subtraction unit after the registration has been conducted.

The suppression unit is useful for virtually remove or at least suppress bony structures and especially ribs or clavicles in chest radiographs. There are methods readily available in the prior art to deal with this issue. For example, the suppression may be conducted based on an independent component analysis which performs a blind source separation based on the probability distribution of the individual components. Also, the suppression may be conducted by regression, which is a supervised technique from pattern recognition theory in which a mapping from a number of input variables

(features) to a continuous output variable is learned from a set of examples from which both input and output are known. The regression may be applied on a pixel level. The suppression may also be conducted by a multi-resolution massive training artificial neural network that is a highly non-linear filter that can be trained by use of input chest radiographs and the corresponding teaching images. Finally, suppression may also be conducted by detecting the contours of bones and then remove the shadows of the bones. A detailed algorithm and the realization of such a suppression unit is not elementary to this invention. It merely is important to provide such a suppression unit in order to have a suitable means for suppressing bone information or bony structures in X-ray images or the such.

By suppressing bony structures in the first image and the second image it is possible to register the first image and the second image based on characteristic features in the soft tissue, such as lung vessels. The effect of strong anterior-posterior inclination between the first image acquisition and the second image acquisition may thereby be eliminated as lung vessels or other features make interval change detection much easier.

As the registration is much more precise and accurate since disturbing bony structures are eliminated or highly suppressed the subtraction image formed by the subtraction unit is also much more precise, leading to a better ability for detection, localization and interpreting interval changes in soft tissue images and especially to an improvement in the ability to detect tumourous tissues or other tissues of interest.

The gist of the invention lies in acquiring a first image and a second image during longer time intervals, which first and second image are processed by means of a suppression unit, then registered so that soft structures are used for aligning the first and the second image in order to improve the accuracy and finally subtracting the images to achieve a highly precise subtraction image where objects of interest are having a highly improved visibility.

According to yet another aspect of the invention a method for providing subtraction images is provided. The method according to the invention basically comprises the steps of obtaining a first image of a region of interest, obtaining a second image of the region of interest, processing the first and the second image by a suppression method, eliminating or reducing the visibility of bony structures, registering the processed first image and the processed second image and finally subtracting the second image from the first image in order to provide a subtraction image.

In another exemplary embodiment of the present invention a computer program or a computer program element is provided, which computer program element is a part of a computer program adapted for controlling a device, e.g. an imaging apparatus, according to one of the above-described aspects, which, when being executed by a processing unit, is adapted to perform corresponding method steps according to the invention. The computer program element may therefore be stored on a computing unit, a calculating device or an electronic device, which may also be part of an

embodiment of the present invention. The computing unit may be adapted to perform or initiate a performing of method steps associated with the above-described device.

Moreover, it may be adapted to operate the components of the above-described device. A computer program element may be loaded into a working memory of a data processor, which data processor may thus be equipped to carry out the method according to the invention.

This exemplary embodiment of the invention covers both, a computer program element is adapted for using the invention right from the start and a computer program element that is adapted for using the invention through being integrated by means of an update to turn an existing program into a program that uses the invention.

Further, the computer program element may also be able to provide all necessary steps to fulfill the steps of an exemplary embodiment of the method according to the invention as described above.

According to a further exemplary embodiment of the present invention, a computer readable medium, such as a CD-ROM or the like, is presented wherein the computer readable medium has a computer program element stored on it, which computer program element is described by the preceding section.

However, the computer program element may also be presented over a network like the World Wide Web and may be downloadable into the working memory of a data processor from such a network.

According to a further exemplary embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is adapted to perform a method according to one of the previously described embodiments of the invention.

Additional embodiments deriving from additional suppression, registering or subtracting method steps are conceivable and are understood to be included in the invention described in this disclosure.

The described functionality may be applied either by replacing or supplementing known temporal subtraction techniques. It may also possible to selectively replace only a subset of bony structures. The method according to the invention may be realized as a plug-in for a Picture Archiving and Communication System (also known as

"PACS") in a hospital, as well as a dedicated viewing station with access to such a PACS. It has to be noted that exemplary embodiments of the invention are described with reference to different subject matters. In particular, some exemplary embodiments are described with reference to method type claims whereas other embodiments are described with reference to the apparatus type claims.

It has to be noted that exemplary embodiments of the invention are described with reference to different subject matters. In particular, some exemplary embodiments are described with reference to apparatus type claims whereas other exemplary embodiments are described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims, is considered to be disclosed with this application. However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 shows an exemplary embodiment of the device according to the invention.

Fig. 2 shows an exemplary embodiment of the method according to the invention in a schematic drawing.

Fig. 3 shows a block diagram of an imaging system according to the present invention.

Fig. 4a, 4b, 4c, 4d, 4e and 4f show three pairs of subtraction images of three patients, wherein one image of each patient is generated by a method according to the prior art and the other one is generated by a method according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

A first exemplary embodiment will be described in the following. Fig. 1 schematically shows a device 2 for providing subtraction images according to the first embodiment of the invention. The device 2 comprises a suppression unit 4 which is provided with image data ID. This image data comprises a first image obtained at a first point of time and a second image obtained at a second point of time. Usually these two points of time may be distanced, for example, by days, weeks or months. Therefore, the patient needs to be placed on a table or the such of an imaging system at each one of the two points of time, which makes aligning or placing the patient in exactly the same manner rather cumbersome.

The first image and the second image may contain bony structures, for example in thorax X-ray images, wherein the bony structures may not be aligned exactly the same relative to the soft tissue when comparing the first image and the second image. The suppression unit is adapted for processing the first image and the second image independently in that bony structures are reduced or completely eliminated. There are a number of known methods and algorithms available in prior art to conduct such an image processing so that it is not explained in detail hereinafter. The suppression unit 4 is further adapted to provide the independently processed first and second image for further processing.

The device 2 further comprises a registration unit 6 which is provided with the processed image data PID. Since the bony structures are reduced or eliminated in the processed image data the registration unit 6 registers the processed first image and the processed second image in order to compensate for a patient/organ motion, perspective changes, or any other difference occurring between the two points of time in which the first image and the second image are obtained. The registering process itself may be rigid or non-rigid, thus deformable. The registering unit 6 provides a set of registered images represented by registered image data RID.

The device 2 according to the invention further comprises a subtraction unit 8 that receives the registered image data RID and is adapted for subtracting the registered second image from the registered first image. Ideally, the subtraction is conducted in such a manner that spots of the tissue of interest which remain unchanged between the first and second image acquisition lead to a medium grey appearance in the resulting subtraction image, wherein spots of the tissue of interest which change during the interval between the first and second image acquisition are emphazised within the subtraction image. For example, a tumourous tissue has a stronger opacity in the second registered image than in the first registered image and leads to generating a bright spot in the resulting subtraction image, wherein a tissue fraction with a decreasing opacity in time may lead to a darker spot in the subtraction image or vice versa.

The device 2 according to the invention has the advantage over the prior art that bony structures that are movable relative to soft tissues are not used for registering images of soft tissue, therefore leading to a clearly higher precision in generating subtraction images with respect to soft tissue contrast and therefore leading to a clearly higher ability to localize, measure and interpret changes in the appearance of soft tissues.

The device 2 according to the invention provides a subtraction image, for example at an output of the subtraction unit 8, delivering a subtraction image data SID.

In Fig. 2 an exemplary embodiment of the method according to the invention is shown in a schematic drawing. The method may begin with obtaining 10 a first image of the patient or the object of interest. Secondly, a second image may be obtained 12, wherein the acquisition time delay between the first image and the second image may be a couple of days, weeks, months, or years. Therefore, the method according to the invention may also comprise the step of accessing 14 a storage unit in order to store or load at least one first image.

After obtaining the second image 12 the first image and the second image are processed 16 in order to reduce or eliminate bony structures. This may be conducted by a suppression algorithm that may be selected out of a number of already known suppression algorithms readily available in the prior art.

Furthermore, the method according to the invention may comprise the step of registration 18 of the first processed image and of the second processed image so that these images are aligned and scaled for enabling the subtraction process. The registration may also be conducted by a registration algorithm or method already known in the prior art.

Further, the second registered image is subtracted 20 from the first registered image in order to achieve a subtraction image that enables the visibility of interval changes of the first and second image. Since the registration is not based on bony structures the precision of the subtraction images generated by the method according to the invention is clearly improved. Finally, the subtracted image is output 22.

Fig. 3 shows a block diagram of an imaging system 24 according to the present invention. The imaging system 24 comprises a radiation source 26, a radiation detection module 28, a data processing unit 30 with a central processing unit, a memory and a device 2 according to the first embodiment of the invention, described in regard to Fig. 1.

Preferably the imaging system 24 further comprises a display unit 32, which display unit 32 is connected with the data processing unit 30. The device 2 receives image data ID from the radiation detection module 28.

It is not substantial for the present invention how the radiation source 26 and the radiation detection module 28 are realized, as long as the imaging data ID is being supplied to the device 2 according to the first embodiment of the present invention.

Preferably, the data processing unit 30 is adapted for storing a first image obtained at an earlier time than the second image so that the device 2 according to the first embodiment of the present invention is capable of conducting the method steps according to the invention at a later time. Furthermore, the data processing unit 30 may also be adapted for sending image data ID to an external storage unit 34 such as a server, an external mass storage unit or the such. Thereby, the imaging system 24 has a higher flexibility and is not limited to conduct the above-described method steps to image data ID that has been obtained by the imaging system 24 according to the invention itself.

Fig. 4a and 4b both show subtraction images from a pair of follow-up thorax radiographs. While Fig. 4a shows the classical subtraction image, Fig. 4b shows the one after bone suppression in both radiographs. Figs. 4c and 4d show the same for a different patient, as well as Figs. 4e and 4f.

It is clearly visible, that the subtraction images based on the classical method in Figs. 4a, 4c and 4e are rather disturbed since the subsequent images comprise an angular shift that renders a rib-based registration imprecise regarding the soft tissue. Fig. 4d demonstrates a clearly more precise alignment of a soft lung tissue. Especially in Fig. 4f a a tumor growth 36 has a much better visibility compared to the classical method in Fig. 4e.

Two positive effects of bone suppression are visible. First, rib artefacts are reduced that previously masked soft tissue visibility. Second, the registration no longer aligns the ribs with each other, because they are no longer the most contrasted structures in the image. Instead, lung tissue gets aligned, making interval change detection of soft tissue structures easier.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered to be disclosed with this application. However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SIGNS:

2 device for providing subtraction images

4 suppression unit

6 registration unit

8 subtraction unit

10 obtaining a first image

12 obtaining a second image

14 accessing (storing and loading)

16 processing

18 registration

20 subtraction

22 outputting

24 imaging system

26 radiation source

28 radiation detection module

30 data processing unit

32 display unit

34 storage unit

36 tumor growth

Claims

CLAIMS:

1. A device (2) for providing subtraction images, comprising:

a suppression unit (4) adapted for receiving a first image and a second image and adapted for bony structure reduction processing of the first image and in the second image;

a registration unit (6) adapted for registering the first image and the second image processed by the suppression unit; and

a subtraction unit (8) adapted for subtracting the registered second image from the registered first image.

2. A device (2) according to claim 1, further comprising:

a storage unit (34) for storing a first image and for providing the first image to the suppression unit (4).

3. A device according to claim 1 or 2,

wherein the suppression unit (4) is adapted for selectively replacing only a subset of bony structures of the first image and the second image.

4. An imaging system (24), comprising:

a radiation source (26);

- a radiation detection module (28);

a data processing unit (30) comprising a central processing unit, a memory, and a device (2) according to one of the claims 1 to 3; and

a display unit (32) connected with the data processing unit;

wherein the device (2) receives image data from the radiation detection module (28).

5. A method for providing subtraction images, comprising the steps:

obtaining (10) a first image;

obtaining (12) a second image; bony structure suppression processing (16) of the first image and of the second image;

registering (18) the first processed image and of the second processed image;

- subtracting (20) the second registered image from the first registered image.

6. The method according to claim 5,

wherein the bony suppression processing of the first image and of the second image comprises selectively replacing bony structures.

7. The method according to claim 6, further comprising

accessing (14) a storage unit (34) in order to store the first image after obtaining (10) it or to load the first image after obtaining (12) the second image.

8. A computer program element for controlling a device (2) according to one of the claims 1 to 3, which, when being executed by a processing unit (30), is adapted to perform the method steps of one of the claims 5 to 7.

A computer readable medium having stored the program element of claim 8.