Patent Publication Number: US-2010130869-A1

Title: Method and apparatus for displaying a field of a brain of a patient and navigation system for brain surgery

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. DE 10 2008 051 950.2, filed on Oct. 16, 2008. The disclosures of DE 10 2008 051 950.2 is hereby incorporated by reference in its entirety for all purposes. This application is also a continuation-in-part of U.S. patent application Ser. No. 12/104,919, filed on Apr. 17, 2008, entitled “Method And Apparatus For Displaying A Field Of A Brain Of A Patient And Navigation System For Brain Surgery,” the disclosure of which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method and an apparatus for displaying a field of a brain of a patient. In particular, this invention relates to a method for determining a continuous field in the brain which is activated or deactivated after a stimulation. Further, the invention relates to a navigation system for brain surgery which is suitable for displaying a field of the brain and determining a continuous field of the brain. 
     A variety of methods for examining the brain are known. Image-forming methods, that means methods which display extended areas of the brain are, for example, computer tomography (CT), magnetic resonance tomography (MRT) and ultrasonic methods. As methods for functionally examining the brain for example electro encephalography (EEG), magneto encephalography (MEG), positron emission tomography (PET) as well as functional magnetic resonance tomography (fMRT) are known. In recent years in particular the functional magnetic resonance tomography (fMRT) contributed to a large extend to the understanding of the construction of the brain. A neuronal activation of certain brain areas may cause an increase of the oxidized hemoglobin in the capillaries of the activated brain tissue. Using fMRT it is possible to locally detect the increase of the oxidized hemoglobin. A contrast due to a degree of oxygenation of the blood results from a changed susceptibility of the blood in dependence of the degree of oxygenation. This effect is referred to as BOLD (Blood Oxygenation Level Dependent)-contrast. With this technique, it is thus possible to measure brain activation of certain areas in the brain with a time resolution of a few seconds and thus to identify functional areas in the brain. A disadvantage of this technique is the relatively coarse time resolution and the high costs. Also, due to the relatively large required integration times of the measurement, it is mandatory that the patient does not move during this time span. Otherwise, due to movements of the patient, massive artifacts arise in the functional information which can later only hardly be corrected. For this, namely elaborate rotations and translations of the data sets relative to each other are necessary which may even comprise deformations of the data sets. In spite of these problems and of these disadvantages of fMRT this technique is to date the method of choice to identify functional areas in the brain. Thereby, typically fMRT-images before and after a stimulation are compared. From the differences between both images areas of the brain relating to the stimulation may be determined. In particular, these examinations revealed that an increased activity of the brain causes an increased blood flow and an increased oxygen concentration. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved method for identifying functional areas in the brain of a patient as well as to provide an apparatus for carrying out this method. A further object of the present invention is to provide an apparatus for brain surgeries. 
     According to a first aspect of the present invention a method of displaying a field of a brain of a patient is provided, wherein the method comprises: illuminating the field of the brain with measuring light and acquiring at least one first sequence of images of the illuminated field, then stimulating the patient, then illuminating the field of the brain with measuring light and acquiring at least one second sequence of images of the illuminated field, and evaluating the first and the second sequence of images by: (a) determining for at least one location a first analysis image value in dependence of at least one of amplitudes and frequencies of temporal changes of image values of those pixels in the first sequence of images to which the at least one location is imaged, (b) determining for the at least one location a second analysis image value in dependence of at least one of amplitudes and frequencies of temporal changes of image values of those pixels in the second sequence of images to which the at least one location is imaged, wherein the method further comprises: determining an output image from the first analysis image value and the second analysis image value; acquiring topography data representing a topography of the field of the brain; and displaying the output image in dependence of the topography data. 
     During performing this method typically the skullcap of the patient is opened. The first sequence of images acquires a reference state of the field of the brain. Then the patient is stimulated to provoke an activation of certain areas of the brain. The subsequently acquired second sequence of images may represent an activated state of the field of the brain. An image is given by image values in a plurality of pixels. The image values thereby correspond to detected intensities of measuring light which has interacted with the field of the brain and is returned from there. Image values of pixels are analyzed with respect to their temporal changes. In particular, at least one of amplitudes and frequencies of these temporal changes are used to determine for the first sequence of images a first analysis image value and to determine for the second sequence of images a second analysis image value. An output image is determined from first and second analysis image values and the output image is subsequently displayed. The displaying the output image is carried out in dependence of topography data representing a topography of the brain. The topography data represent a three-dimensional, i.e., defined by three coordinates independent from each other, surface structure of the brain. The topography data may for example comprise three-dimensional point cloud data, mesh data or the like. The topography data may also comprise tissue density values for locations of the brain, wherein the surface structure may be defined by locations at which the density values assume a particular value, lie below a particular value or exceed a particular value. Thus, a three-dimensional illustration of the brain is achieved which at the same time comprises the output image values, for example as grey scale values, as pseudo color values or the like. The displaying may be carried out in different manners, for example as (stereo) images or as a plurality of diagrams. Further, the displaying may be carried out in the form of a two dimensional image, a diagram, a plurality of diagrams and the like. The two dimensional image may be a grey scale image, a pseudo color image, or the like. Also three dimensional illustrations may be employed. A dependency of the first and second analysis image value on at least one of amplitudes and frequencies of temporal changes of image values is present if the first, respectively second, analysis image value changes due to temporal changes of the first, respectively second, image values. Thus, hereby, a constant dependency is not meant. The output image is thereby determined from first and second analyses image values, so that the output image changes, if either the first or the second analysis image values change. 
     According to an embodiment of the invention the acquiring the images of the first sequence and the second sequence comprises exposing detector elements of an image detector during an exposure time of less than 1 ms, in particular less than 0.1 ms, and further in particular less than 0.03 ms. Restricting the exposure time results in accumulating the light emanating from the illuminated field of the brain only over short times of less than 1 ms to detect image values. Thus, temporal fluctuations of high frequencies can be detected which is necessary for performing the method. 
     According to a further embodiment of the method the acquiring the images of the first and the second sequence comprises acquiring temporally subsequent images having a temporal interval of less than 1 ms, in particular less than 0.1 ms, and further in particular less than 0.05 ms. By choosing these temporal intervals between subsequent images it is possible, to detect frequencies of temporal changes of image values which comprise 0.5 kHz, 5 kHz and in particular 10 kHz. Thus, intensity fluctuations are detectable which result from Laser Doppler signals which are generated from moving particles having a velocity between 0.4 mm/s and 8 mm/s, when light of a wavelength of about 800 nm is used as illumination light. 
     According to a further embodiment of the method the acquiring the images comprises detecting measuring light, wherein intensities of the detected measuring light having wavelengths greater than 1300 nm and smaller than 600 nm, in particular greater than 820 nm and smaller than 780 nm, amount to less than 10% of a total intensity of the detected measuring light. Using light having wavelengths of a thus defined range enables a penetration depth into human tissue, in particular brain tissue, of about 1 mm to 2 mm. In particular, the wavelength range between 600 nm and 1300 nm is referred to as “diagnostic window”. Thus, Laser Doppler signals of particles which are present within this penetration depths and which move relative to each other can be detected. Thereby, the measuring light may exhibit a coherence length which substantially corresponds to twice the penetration depth of the measuring light into a tissue of the brain visible within the illuminated field. Using measuring light having this coherence length enables to detect an interference of light which was reflected by two particles which are a distance apart from each other corresponding to the penetration depth. Thus, it can be ensured that even the light penetrated in deepest locations of the field of the brain may interfere with light which was reflected at the surface of the field of the brain. Thus, a sensitivity may be increased. 
     According to an embodiment of the method the determining the output image from the first and the second analysis image values comprises computing at least one of a difference and a ratio of first and second terms, wherein the first terms depend on the first analysis image values and the second terms depend on the second analysis image values. The output image may thus for example be obtained by forming a difference of first and second analysis image values multiplied with different or same factors. Further, also a mathematical function may be applied to the first and second analysis image values, before forming a difference is carried out. These mathematical functions comprise for example forming a logarithm, forming a power, forming a product and the like. Upon similar modifications of the first and second analysis image values a ratio thereof may be formed to obtain the output image. A particular simple variant to determine a difference of the field of the brain before and after the stimulating is to determine the difference between the first and second analysis image. 
     According to an embodiment of the method a coordinate system is established relative to a head of the patient. Thus, a fixed relationship between locations in the field of the brain and pixels to which the light emanating from the field of the brain is imaged is given. The establishing a coordinate system may thereby comprise fixing a reference frame at the head of the patient. Thereby, the head of the patient may be held by at least three pins or pillows. The pins or pillows thereby contact the scull of the patient. Alternatively, the coordinate system may also be established in that predetermined reference locations at the head of the patient are detected. The detecting predetermined reference locations may thereby comprise optically detecting. Thereby, plural cameras may be provided to detect coordinates of at least three predetermined locations at the head of the patient. Further, the cameras are used, to determine coordinates of the apparatus for displaying a field of a brain. 
     According to this last mentioned embodiment, further, coordinates of locations of the field of the brain may be determined in the coordinate system, wherein the locations are imaged to selected pixels in the output image. 
     According to a preferred embodiment of the method the method further comprises determining at least one continuous region in the output image in which intensity values are greater than a threshold. It is thus possible, to mark regions in the output image which for example show large differences of analysis image values before and after the stimulation. These regions in the output image may correspond thereby to activated or deactivated areas in the field of the brain. This corresponds to identifying functional areas in the brain. 
     According to a preferred embodiment the at least one determined region may be displayed in superposition with the output image. Possible illustrations thereby comprise encircling the determined region using a line, a coloring the determined region, or the like. Again the illustration is carried out in dependence of the topography data so that the viewer is presented a three-dimensional illustration of the brain allowing an accurate localization of functional areas of the brain. 
     According to a preferred embodiment coordinates of locations which are imaged to the at least one determined region in the output image can be determined. Thus, locations of functional areas in the brain of the patient relative to a coordinate system of the head can be determined. 
     According to a preferred embodiment of the invention at least one of the first analysis image value and the second analysis image value comprises a value representing one of a perfusion, a concentration, an average velocity, a parameter of a velocity distribution, in particular a standard deviation of the velocity distribution of particles moving relative to each other, in particular blood cells, and a combination thereof. Thus, it is possible, to detect and to quantify a blood motion using the claimed method. The thus specified analysis image value may thereby be obtained using a frequency spectrum of the temporal changes of image values. In particular, the concentration of particles moving relative to each other may be obtained as a 0th moment of the frequency distribution. The perfusion may be obtained as a 1st moment of the frequency distribution. An average velocity or a parameter of the velocity distribution may be obtained as a ratio between the perfusion and the concentration of particles moving relative to each other. 
     According to an embodiment of the inventive method the method further comprises repeating the acquiring the at least one of the first and the second sequence of images. Further, the repeatedly acquired sequences of images are evaluated as described above. Thus, temporal sequences of first and second analysis image values are obtained. This may represent a movie of first and second analysis image values. Using such a movie a temporal course of an activation or a deactivation of brain areas after a simulation may be inferred. 
     According to an embodiment the temporal sequences of the first, respectively the second, analysis image values may be at least one of displayed and stored. Displaying the temporal sequences of the first, respectively second, analysis image values may thereby correspond to playing a movie of the first, respectively second, analysis image values. A storing the sequences of the first, respectively second, analysis image values may thereby also comprise a storing the coordinates of the field of the brain in the coordinate system of the head of the patient, as well as the storing a representation of the stimulation carried out between the first and second sequences. 
     According to a preferred embodiment of the method the stimulating comprises requesting the patient, to move a particular body part. This body part may for example be an extremity, such as a finger. 
     According to a preferred embodiment of the present invention the stimulating comprises applying an agent to the patient. Thereby, the agent may be incorporated or may be externally applied. The applying the agent may also comprise irradiating the patient, such as with electro magnetic waves. 
     According to a preferred embodiment of the invention the stimulating comprises applying of at least one of a visual stimulus, an acoustic stimulus, a gestation stimulus, an olfaction stimulus, and a tactition stimulus to the patient. Thereby, images, such as photographs, paintings, text, may for example be presented to the patient or sounds, such as music or speech, may be played. The patient may also be supplied with food and/or beverages. The stimulus may also comprise a request to carry out a conscious action. This action may for example comprise moving one or more fingers. 
     Also different kinds of food or beverages may be tasted by the patient, by applying the food or the beverages to the tongue of the patient, in particular by an application device. Physical properties of these applied nourishments, such as their temperature, may be controlled. Further the patient may be allowed to smell a substance by bringing the substance in proximity to its nose or introducing the substance into the patient&#39;s nose, in particular by a technical appliance. Thereby olfactory receptor neurons may be activated or deactivated. 
     Further, the patient may be allowed to experience touch or a somatic sensation. For this, objects having different roughness of their surfaces, different surface shapes, or different temperatures may contact a portion of the skin of the patient or may be slid over portions of the skin of the patient applying a varying amount of pressure. In particular the effect of anesthetics may be probed by applying this kind of stimulus. 
     Also the stimulus may comprise a request to carry out a conscious action. This action may for example comprise moving a finger. 
     According to a preferred embodiment of the inventive method the method further comprises removing a portion of the brain in dependence of at least one of the output image, the determined coordinates, and the at least one continuous region in the output image. The portion of the brain may comprise several regions. Thereby, a predetermined continuous region in the field of the brain may for example be circumvented to gain access to regions of the brain which are to removed. The regions to be removed may thereby be situated in layers of the brain which are located in a depth which is deeper than the penetration depth of the measuring light. 
     According to a preferred embodiment of the inventive method the method further comprises acquiring an image of the field of the brain in a visible wavelength range. Further, the image in a visible wavelength region may be displayed in superposition with at least one of the first analysis image values, the second analysis image values, and the output image. Here, in particular blood vessels situated at the surface of the brain may be imaged and concurrently the determined blood flow characteristics may be displayed. 
     According to an embodiment of the present invention the topography data are established by applying at least one of optical coherence tomography, stripe projection, a runtime method, photogrammetry, triangulation and stereo Laser Doppler imaging. 
     Optical coherence tomography (OCT) is an optical interferometric method in which measuring light reflected at a particular layer of the object to be examined is superimposed with reference light having a defined phase relation to the measuring light and the superimposed light is detected. Structural information (such as three-dimensional density values of a reflectivity) of the examined object may be obtained from which data representing a topography of the object may be obtained. For applying the method of stripe projection the object is illuminated with at least one pattern, while a camera acquires an image of the illuminated object from a known viewing angle. From a plurality of such camera images a topography of the object may be computed. 
     A runtime measuring method comprises an optical distance measurement by measuring the runtime of measuring light (in particular infrared light) from a light source to a point at the object. Thereby the measuring light may in particular be modulated in its amplitude. 
     When applying the method of photogrammetry at least two images of the object are acquired from different viewing directions. From the image coordinates of object points in the at least two images the three-dimensional position of the object points may be determined. 
     When applying the method of triangulation an object point is aimed from two different positions having a known relative distance to measure angle positions. From the angle positions and the known distance the three-dimensional location of the object point may be determined. 
     Stereo Laser Doppler imaging comprises recording sequences of images of the object to be examined from two different directions. The sequences of images are evaluated according to the above described method for determining data representing a motion of particles, such as perfusion data. The perfusion data may be displayed, for example stereoscopically. 
     According to another aspect of the present invention a navigation system for brain surgery is provided which comprises a position acquisition apparatus to acquire coordinates of locations of a brain of a patient, an apparatus for acquiring topography data representing a topography of the brain, and a measuring apparatus for acquiring images which is configured to illuminate a field of the brain with measuring light, to acquire at least one sequence of images of the illuminated field, and to process the sequence of images by determining for at least one location a first analysis image value in dependence of at least one of amplitudes and frequencies of temporal changes of image values of those pixels in the first sequence of images to which the at least one location is imaged; and a display system which is adapted to display first analysis image values in dependence of the topography data. 
     On one hand with this system Laser Doppler images of a field of a brain can be acquired. On the other hand the provided position acquisition apparatus enables to associate individual points in these images to locations of the brain. Further, a topography, in particular a surface structure, of the brain may be determined. Thus, locations in the brain may be identified whose Laser Doppler signals change after a simulation. 
     The position acquisition apparatus may thereby be designed in different ways. One possibility is the provision of a head coupling system which is adapted to hold the head of the patient by at least three pins or pillows in a fixed position relative to an object plane of the measuring apparatus for acquiring images. The position acquisition apparatus may also be an optical position acquisition apparatus, wherein the coordinates of predetermined locations of the head of the patient are determined. Concurrently thereto the coordinates of predetermined locations of the measuring apparatus for acquiring images are determined. Thus, a relationship between a coordinate system of the measuring apparatus for acquiring images and a coordinate system of the brain of the patient can be determined. Thus, a mapping between locations of images acquired by the measuring apparatus and locations of the brain of the patient is given. The measuring apparatus for acquiring images is adapted to temporally subsequently acquire a sequence of images of the field of the brain. This acquired sequence of images is then evaluated by analyzing intensity values of the light emanating from the field of the brain with respect to its temporal changes, wherein the intensity values are obtained in every pixel. An analysis image value obtained from this analysis of the temporal changes of image values may thereby obtained using a frequency spectrum which can be determined from the temporal changes of the image values. In particular, moments of the frequency spectrum of the temporal changes may thereby be comprised. 
     The display system displays the first analysis image values in dependence of the topography data. Thereby, the surface structure of the brain may be illustrated superimposed with the associated first analysis image values, such as color coded. 
     According to an embodiment of the present invention the measuring apparatus is further adapted to acquire a second sequence of images of the illuminated field; to process the second sequence of images by determining for the at least one location a second analysis image value in dependence of at least one of amplitudes and frequencies of temporal changes of image values of those pixels in the second sequence of images to which the at least one location is imaged; and to determine an output image based on the first analysis image values and the second analysis image values, wherein the display system is adapted to display the output image in dependence of the topography data. 
     The output image may for example represent a change of a perfusion of blood in the field of the brain due to a stimulation of the patient. 
     According to an embodiment of this aspect of the present invention the measuring apparatus for acquiring images comprises a camera having an exposure integration time of less than 1 ms, in particular less than 0.1 ms, and further in particular less than 0.03 ms. The exposure integration time may be adjustable. Thus, a detectable frequency of a change of the temporal changes of the image values is greater than 1 kHz, in particular greater than 10 kHz, and further in particular greater than 30 kHz. According to a preferred embodiment of the present invention the measuring apparatus for acquiring images comprises a camera having a frame rate greater than 1 kHz, in particular greater than 10 kHz, and further in particular greater than 20 kHz. A frame-rate of the camera is a frequency indicating a number of images acquired by the camera in a certain time span. If the frame-rate is for example 10 kHz the camera is enabled to acquire 10,000 images in one second. Thus it is possible, to sample the temporal changes of the image values with a frequency which is higher than 1 kHz, in particular higher as 10 kHz, and further in particular higher than 20 kHz. In particular, detectors of the camera comprise CMOS-sensors. A frequency analysis of such an acquired signal of temporal changes allows determining frequency components contributing to the signal of the temporal changes. According to the Nyquist-Theorems thus frequency components may be determined whose frequencies are greater than 0.5 kHz, in particular greater than 5 kHz, and further in particular greater than 10 kHz. An analysis of the temporal changes of the image values with respect to the determining a frequency spectrum comprises thereby the determining the amplitudes and frequencies of the frequency components which, as a sum, result to the acquired temporal changes. 
     According to an embodiment of this aspect of the invention a band-pass filter is arranged in a beam path upstream of the camera, wherein the transmission characteristics of the band-pass filter is adapted such that a transmission of light having wavelengths greater than 1300 nm and smaller than 600 nm, in particular greater than 820 nm and smaller than 780 nm, amounts to less than 10% of a total transmission of the band-pass filter. This enables that only desired light emanating from the field of the brain is detected by the camera. Hereby it is for example possible to use as measuring light a small spectral range of visible light and to concurrently illuminate the field of the brain for visual inspection with white light (light comprising wavelength in the visible range) avoiding disturbing the measurement by this white light reflected at the field of the brain. 
     According to an embodiment of the present invention the measuring apparatus comprises a light source having an emission spectrum formed such that an emitted intensity of light having wavelengths greater than 1300 nm and smaller than 600 nm, in particular greater than 820 nm and smaller than 780 nm, amounts to less than 10% of a total intensity. When in this embodiment no further perturbation light is incident onto the field of the brain it is thus ensured that the camera detects only light within a particular wavelength range. 
     According to an embodiment of this aspect of the present invention the navigation system further comprises an apparatus for acquiring images of the brain of the patient in a visible wavelength range. With this provision it is possible to acquire beside Laser Doppler images also images in the visible wavelength range. This may be advantageous to associate details of the field of the brain recognizable in the visible wavelength range with details in the Laser Doppler images. For example, a blood flow in blood vessels at the surface of the brain may thus be examined. In particular, thereby also a blood flow may be examined in areas not pervaded by blood vessels recognizable in the visible wavelength range. 
     According to an embodiment of this aspect of the present invention the navigation system further comprises a display apparatus for displaying an image in the visible wavelength range, in particular in superposition with analysis image values. Thus, it is further enabled to superimpose details recognizable in the visible wavelength range with details recognizable in the Laser Doppler images. This facilitates an interpretation of the Laser Doppler images by an observer. 
     According to an embodiment of the present invention the apparatus for acquiring topography data comprises at least one of an optical coherence tomography system, a stripe projection system, a runtime measuring system, a photogrammetry system, a triangulation system, and a stereo Laser Doppler imaging system. 
     According to an embodiment of this aspect of the invention the navigation system is adapted to perform methods of the first aspect of the present invention. Thus, with the navigation system, Laser Doppler images may be acquired before and after a stimulation and may be displayed and their changes may be determined. Thus, this apparatus is suited to examine a change of a blood flow characteristics in the brain triggered by a stimulation. These areas may thereby be interpreted as activated or deactivated areas. 
     The invention will now be described with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a navigation system for brain surgeries according to an embodiment of the present invention; 
         FIG. 1B  shows an apparatus for acquiring topography data representing a topography of the brain which may be employed in the navigation system illustrated in  FIG. 1A ; 
         FIG. 2  shows an evaluation method according to an embodiment of the present invention; 
         FIG. 3  shows a method for displaying a field of a brain of a patient according to an embodiment of the present invention; 
         FIG. 4  shows principles of a measuring process according to an embodiment of the present invention; 
         FIG. 5  shows a field of a brain of a patient, wherein  FIG. 5A  shows a white light image and  FIG. 5B  shows a Laser Doppler image; 
         FIG. 6  shows a diagram according to an analysis method according to an embodiment; and 
         FIG. 7A  and  FIG. 7B  show a superposition of a white light image and an output image gained according to an embodiment of a method of the present invention, wherein the output image is obtained from Laser Doppler images acquired before and after a stimulation. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
       FIG. 1A  shows an embodiment of a navigation system for brain surgeries according to an embodiment of the present invention. The navigation system  1  comprises an imaging system  2 , a measuring light camera  3 , a control and evaluation unit  4 , a white light camera  6 , a stimulation apparatus  5 , a position acquisition apparatus  7  and an apparatus  100  for acquiring topography data. 
     In an object plane  9  of the imaging system  2  a field  11  of a brain  13  of a head  15  is arranged. The head  15  of the patient is connected to a base  19  using a head coupling apparatus  17  via an arrangement of levers  21 . Thereby, the head of the patient is held using pins  17   a ,  17   b  and a not illustrated pin  17   c  which are connected to the head coupling apparatus. A coordinate system  23  is connected to the base  19  and thus also to the head coupling apparatus  17 . This coordinate system  23  is defined by coordinate axis x, y and z. The imaging system  2  is also connected via the arrangement of levers  21  to the base  19 . 
     The imaging system  2  comprises a light source  25  emitting light  27 . In the embodiment illustrated here the light source is a laser emitting light having a wavelength of 808 nm. The coherence length of the light amounts to some mm. The light  27  emanating from the light source  25  is shaped by a beam shaping optics  29  to form measuring light  31  substantially comprising plane wave fronts. Measuring light  31  traverses a semi-transparent mirror  33  and a semi-transparent mirror  35 . Subsequently the measuring light is incident onto the field  11  of the brain  13 . Measuring light having interacted at a location x, y in the field  11  of the brain  13  emanates from this location x, y, traverses the semi-transparent mirror  35  and is reflected at the semi-transparent mirror  33  to form light  37 . Light  37  enters the measuring light camera  3 . Thereby the light  37  traverses a band-pass filter  39 . The band-pass filter allows light within a wavelength range of 780 nm to 820 nm to pass. Subsequently the filtered light  37  traverses a camera objective  41  to be detected at a pixel (i,j) of a detector  43 . The detector  43  comprises CMOS-sensors and is connected to the control and evaluation unit  4 . 
     The control and evaluation unit comprises a processing unit  45 , a display unit  47 , an input unit  49  and a storage unit  51 . The processing unit  45  is adapted to evaluate a sequence of images each of which is given by image values I(i,j). The detailed evaluation method is described further below. 
     The white light camera  6  comprises a camera objective  42  and a detector  44  for acquiring light in the visible wavelength range. A white light source not illustrated in  FIG. 1  illuminates the field  11  of the brain. White light reflected at a location (x, y) of the field of the brain is reflected at the semi-transparent mirror  35 , traverses a camera objective  42  and is detected at the pixel (i,j) of the detector  44 . The white light image detected by the detector is displayed at the display unit  47 . 
     Concurrently the field  11  of the brain  13  having a surface  13 ′ is examined by the apparatus  100  for acquiring topography data. The apparatus  100  for acquiring topography data allows to acquire topography data representing a topography of the field  11  of the brain  13  by optical means. In particular, the topography data may represent a geometry of the surface  13 ′ of the field  11  of the brain  13 . Thus, the surface structure of the field  11  of the brain  13  may be determined. In different embodiments the apparatus  100  for acquiring topography data is adapted in different manners, such as an optical coherence tomography system, as a stripe projection system, as a runtime measuring system, as a photogrammetry system, as a triangulation system and/or as a stereo Laser Doppler imaging system or the like. One embodiment is described below with reference to  FIG. 1B . 
     The topography data may describe a surface geometry or surface structure of the field  11  of the brain  13 . The topography data are supplied to the control and evaluation unit  4  via a data line  103 . The perfusion data of the field  11  obtained as explained below are displayed in dependence of the topography data at the display unit  47  to enable a user to perceive a three-dimensional impression of the surface structure of the field  11  of the brain  13  with superimposed perfusion data. 
     The navigation system for brain surgeries further comprises a stimulation apparatus  5  for stimulating a patient. The simulation apparatus  5  comprises a stimulation controller  53  as well as a variety of apparatuses for stimulating the patient. These apparatuses comprise in this embodiment a display unit  55 , a speaker  57  and an injection robot  59 . The display unit  55  is arranged in a field of view  56  of the patient. Also, the speaker  59  is within earshot of an ear  58  of the patient. At one arm  60  of the patient an infusion valve  61  is connected to a blood vessel of the patient. A supply tube  63  is inserted into the infusion valve. Via the supply tube  63  the injection robot is enabled to apply a variety of agents into the vascular system of the patient. 
       FIG. 1B  shows an apparatus  100  for acquiring topography data to acquire topography data representing a topography of the brain  13  having a surface  13 ′. The apparatus  100  for acquiring topography data may be employed in the navigation system illustrated in  FIG. 1A . The apparatus  100  for acquiring topography data operates according to a photogrammetry method, for which it comprises two cameras  105  and  107  which are arranged with a distance from each other so that each of the cameras  105 ,  107  may acquire an image of the surface  13 ′ of the field  11  of the brain  13  of the patient. Because of the distance of the two cameras  105 ,  107  from each other the field  11  is imaged from two different viewing angles. Because of these different viewing angles the recordings acquired by the two cameras  105 ,  107  differ, as is apparent from exemplary surface points  109 ,  110  indicated in  FIG. 1B . Upon suitable illumination light  111  emanates from the points  109 ,  110 . Thus, the points  109 ,  110  are imaged onto an image plane of the camera  105  in images  109 ′,  110 ′, respectively which are spaced apart from each other by a distance x 1 . This distance x 1  is greater than a distance x 2  being the distance of image points  109 ″ and  110 ″ from each other in an image plane of the camera  107  which is spaced apart from camera  105 . After displacing the apparatus  100  for acquiring topography data additional images may be acquired by the cameras to obtain an extensive measuring data set of distances of the points measured from different viewing angels. 
     By suitable evaluation of the images supplied from the cameras  105  and  107  the control and evaluation unit  4  calculates topography data of the field  11  of the brain  13  of the patient. These topography data may represent a three-dimensional data model of the surface  13 ′ of the field  11 . 
     Additional examples for photogrammetry methods and systems are for example described in U.S. Pat. No. 6,165,181 which disclosure is incorporated into this application by reference in its entirety. The references cited in this document disclose further examples for photogrammetry methods. 
     With reference to  FIG. 2  an evaluation method according to an embodiment of the present invention is illustrated. In this method a sequence  65  of images  66  is acquired by the detector  43  of  FIG. 1A . In the illustrated embodiment every image  66  comprises a quadratic field of pixels (i,j). The number of pixels in this embodiment is 512×512 pixels. Each pixel (i,j) of each image  66  of the sequence of images  65  is associated by the acquisition to an image value I (i,j). Thus, for each pixel (i,j) a temporal course of image values I (i,j,t) is established. The here illustrated method comprises an analysis of the temporal changes of these image values. In the diagram shown here (upper right in  FIG. 2 ) the image values are composed of a constant part not changing over time and a fluctuation part representing a change of the image values over time. Then, in the method, a frequency spectrum of the temporal course of the image values is computed. The frequency spectrum may for example be obtained by a Fourier transformation. In particular, in the present embodiment, the FFT (fast Fourier transform) is employed. In the lower left part of  FIG. 2  the thus obtained frequency spectrum S(ν) of the temporal changes of the image values is depicted and indicated by reference number  68 . On the X-axis the frequency ν is depicted and on the Y-axis the amplitude A of the respective frequency component is depicted. The frequency spectrum S(ν) is commonly also referred to as power spectrum. It represents the amplitudes and frequencies of the temporally changing image values I (i,j,t). Then a first moment of the power spectrum is computed to compute for every pixel of the sequence  65  of images an analysis image value. This first moment is also referred to as a perfusion. Alternatively to the perfusion also a concentration C, or a velocity V of particles moving relative to each other may be computed as analysis image value according to the following formulas: 
     
       
         
           
             
               Perfusion 
               = 
               
                 
                   M 
                   1 
                 
                 = 
                 
                   
                     ∫ 
                     0 
                     ∞ 
                   
                    
                   
                     
                       vS 
                        
                       
                         ( 
                         v 
                         ) 
                       
                     
                      
                     
                         
                     
                      
                     
                        
                       v 
                     
                   
                 
               
             
             , 
             
               
 
             
              
             
               C 
               = 
               
                 
                   M 
                   0 
                 
                 = 
                 
                   
                     ∫ 
                     0 
                     ∞ 
                   
                    
                   
                     
                       vS 
                        
                       
                         ( 
                         v 
                         ) 
                       
                     
                      
                     
                         
                     
                      
                     
                        
                       v 
                     
                   
                 
               
             
             , 
             
               
 
             
              
             
               V 
               = 
               
                 
                   M 
                   1 
                 
                 
                   M 
                   0 
                 
               
             
             , 
             
               
 
             
              
             
               
                 S 
                  
                 
                   ( 
                   v 
                   ) 
                 
               
               = 
               
                 
                   
                      
                     
                       
                         ∫ 
                         0 
                         ∞ 
                       
                        
                       
                         
                           I 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                          
                         
                           exp 
                            
                           
                             ( 
                             
                               
                                 - 
                                  
                               
                                
                               
                                   
                               
                                
                               2 
                                
                               π 
                                
                               
                                   
                               
                                
                               vt 
                             
                             ) 
                           
                         
                          
                         
                             
                         
                          
                         
                            
                           t 
                         
                       
                     
                      
                   
                   2 
                 
                 . 
               
             
           
         
       
     
     If this method is carried out for all pixels in the sequence  65  of images, an analysis image  70  is obtained showing perfusion related data of the field  11  of the brain  13 . 
       FIG. 3  shows an embodiment of a method according to the invention. Thereby, a field  11  of a brain  13  of a patient is arranged in the object plane  9  of the imaging system  2  of the navigation system  1  for brain surgeries. The scull of the patient is opened to allow measuring light  31  to be incident onto a field  11  of the brain of the patient. In  FIG. 3A  first a first sequence  65   a  of images  66   a  is acquired. Thereby the images  66   a  are acquired having a temporal interval of less than 0.1 ms. The acquired first sequence  65   a  of images  66   a  is then evaluated according to the evaluation method described with reference to  FIG. 2 . Thus, a first analysis image  70   a  is obtained representing information about a blood flow within the field  11  of the brain  13 . The analysis image  70   a  may also be displayed at the display unit  47  in dependence of the topography data, for example as three-dimensional surface illustration, wherein the surface may be coloured according to the perfusion values or may be represented by corresponding grey values. 
     In  FIG. 3B  the patient is then stimulated. In the illustrated embodiment the stimulating comprises a request to lift a finger  64 . On the right hand side in  FIG. 3B  it is shown that the patient follows the request and lifts the finger  64 . 
     In  FIG. 3C  then a second sequence  65   b  of images  66   b  of the field of the brain of the patient is acquired. Again, a temporal interval between acquisitions of subsequent images  66   b  is smaller than 0.1 ms. As in the case of the first sequence of images the second sequence of images  66   b  is evaluated to obtain a second analysis image  70   b . Then an output image  72  is computed from a computation involving the first and the second analysis image, as illustrated in  FIG. 3D . In the example illustrated here the computing comprises forming a difference between the first and the second analysis image. The analysis image  72  is illustrated using contour lines in  FIG. 3D . It is apparent that in a region around the pixel (i,j) the output image  72  exhibits enhanced values. These enhanced values around the pixel (i,j) may indicate that areas around the corresponding location (x,y) of the field of the brain of the patient have been activated or deactivated by the stimulating illustrated in  FIG. 3B . 
       FIG. 4  illustrates principles of the measurement according to an embodiment of the present invention. Light  31  is incident onto a field  11  of a brain of a patient. In this embodiment the light has a wavelength of 808 nm and is generated by a laser. A penetration depth of light of this wavelength into brain tissue amounts to about 1 mm to 2 mm. The spectral width of the light  31  having wavelength 880 nm generated by the laser is chosen such that a coherence length of the light results which lies in the range of the penetration depth of the light. Depending on the tissue structures present in the examined field  11  of the brain the penetration depth may amount to more or less than 1 mm to 2 mm. In the field  11  of the brain the incident light  31  is incident onto blood cells  73  and blood cells  75 . Blood cells  73  (illustrated as circles having a white filling) are substantially at rest with respect to the examined field  11  of the brain. In contrast, blood cells  75  (illustrated as circles having a striped filling) move relative to the examined field  11  of the brain according to a certain velocity whose magnitude and direction is indicated by arrows originating at the respective blood cell in  FIG. 4 . The light  31  is reflected at the blood cells  73  and  75  and is scattered. While blood cells  73  being at rest relative to the field  11  reflect light  31  as light having the same wavelength λ 0  as the one of the incident light  31 , blood cells  75  moving relative to the field  11  reflect the incident light  31  with wavelengths λ 0 +Δλ different from the wavelengths of the incident light. If blood cells  75  move in a direction of the incident light  31 , they reflect light with a wavelength which is greater than the wavelength of the incident light  31 . If blood cells  75  move in a direction opposite to the incoming light  31 , they reflect the light with wavelengths which are smaller than the wavelengths of the incident light. A plurality of blood cells  73  and  75  are hit by the incident light  31 . Thus, the light reflected from this plurality of blood cells comprises, dependent on the velocity of the plurality of blood cells, wavelengths which are greater and smaller than the wavelength of the incident light. This velocity dependent shift Δλ of the wavelengths of the light is commonly referred to as Doppler-Effect. Reflected light having different wavelengths and different relative phases superimposes, to be detected at a pixel of the detector  43 . A superposition of light having slightly different wavelengths results in a temporally fluctuating signal which may also be denoted as beat signal. From the beat signal the different wavelengths generating this beat signals can be inferred. 
     From the different wavelengths then the different velocities of the blood cells may be inferred which have generated the detected light by reflection. Using this method thereby information about a blood motion can be obtained. In particular, this method is very simple and cost effective. A further great advantage of this method is a high time resolution which lies in the range of below one second. Thus, in particular changes of a blood motion due to external influences or stimulations may be examined. Thus it is possible, to identify functional areas in the brain of a human. Identified areas in the brain may also be accounted for during brain surgeries. In particular functional areas may be circumvented to gain access to a region of the brain to be removed which region lies in deeper layers of the brain. The area to be removed may in particular be a tumour in the brain of the patient. 
       FIG. 5A  shows a white light image  74  of a field of a brain of a patient, wherein the patient&#39;s skullcap is opened. On the surface of the brain of the patient several blood vessels  90 ,  91 ,  92 ,  93  are visible. These blood vessels comprise arteries and veins. The blood vessels branch out more and more to continue into micro capillaries. Between the blood vessels white brain tissue is visible. 
       FIG. 5B  shows a Laser Doppler image acquired from the same field of the brain of the patient. Higher values of a blood flow, respectively perfusion, are depicted by brighter grey values. One recognizes a correspondence of structures in the white light image illustrated in  FIG. 5A  and the Laser Doppler image illustrated in  FIG. 5B . In particular, it is apparent that areas of the field of the brain which show blood vessels in the white light image indicate in the Laser Doppler image a rather decreased blood motion. A possible explanation may be that a blood motion in the blood vessels apparent in the white light image is too fast that it may not anymore be acquired using the embodiment of the measuring apparatus illustrated here. Indeed, blood flow velocities in aortas amount to up to some meters per second. Blood flow velocities decrease in middle sized and smaller capillaries to some centimeters per second. Only in very small micro capillaries blood flow velocities of some millimeters per second are found which may be detected by the measuring apparatus of the present embodiment. In the embodiment the white light image of  FIG. 5A  and the Laser-Doppler image of  FIG. 5B  are obtained by simultaneous illumination with laser light and white light and simultaneous detection. 
     The Laser Doppler image shown in  FIG. 5B  may be overlaid or adapted onto a three-dimensional surface illustration of the field of the brain which was obtained by the acquired topography data to obtain a spatial perception of the distribution of perfusion values in the field of the brain. 
       FIG. 6  shows a diagram depicting an average of analysis image values of all pixels of a Laser Doppler image of  FIG. 5B  in dependence of time t in units of seconds s. Thus, the diagram in  FIG. 6  represents an average blood motion in a field of a brain of a human over time. In this example the patient was requested at the time points  82  to count loudly. At the time points denoted by reference number  83  the patient was requested to stop counting. This sequence of requests was repeatedly performed, wherein the requests for counting and for stopping the counting were performed for different time spans. One recognizes a dependency of the intensity signal depicted on the y-axis from the time points  82  of the request to count and the time points  83  of the requests to stop counting. Thus, it is shown that it is possible using the inventive method to detect changes in a blood flow in the brain due to stimulations. 
       FIGS. 7A and 7B  show output images according to the method for displaying a field of a brain superimposed with white light images of the same field of the brain. Alternatively, the output images may be displayed in dependence of the topography data for a three-dimensional illustration. 
     The numbers along the image edges indicate pixel indices. The white light image thereby is illustrated as grey scale image in  FIGS. 7A and 7B , respectively. The output image is illustrated as contour line image. The output image was obtained in the following way: First and second sequences of images were acquired, wherein the patient moved a finger during the acquisition of the second sequence of images. Thereby, eight sequences of images were acquired, where upon the patient moved the finger and alternating thereto eight sequences of images were acquired, where upon the patient kept the finger calmly. Thereby, the durations of the sequences were not uniformly, but amounted sometimes to 20 s, sometimes to 10 s, and sometimes to 30 s. The output images depicted in  FIGS. 7A and 7B  were obtained by comparison of the average value of the images of the first sequences with the average value of the images of the second sequences. While contour lines in the image of  FIG. 7A  indicate an increase of a perfusion or a blood motion in the field of the brain, contour lines in the image of  FIG. 7B  indicate a decrease of the perfusion or the blood motion of the field of the brain due to the moving a finger. It is apparent that in a central lower area of the acquired field of the brain a pronounced decrease of the blood motion was detected during the moving the finger. In other areas of the acquired field of the brain such detected decreases or increases of a blood motion continuous over a certain area do not occur. A possible interpretation of this finding is that the continuous area of the field of the brain identified by a significant decrease is activated or deactivated during moving a finger. Conversely also a finger movement may occur due to an activation or deactivation of this area. Interestingly, exactly this area was identified from independent electro stimulation experiments as an area which is related to a finger movement. Other areas shown in  FIGS. 7A and 7B  were not detected as areas which are related to a finger movement. Thus, the inventive method proves to be a non-invasive method to identify functional areas in the brain of a human.