Abstract:
The various aspects relate to a method of determining a geometrical parameter of a side branch of a main branch of a blood vessel, comprising receiving a multitude of cross section data sets, the data sets representing geometries of cross sections of the main branch over a length of the main branch and receiving position data of a cut plane of the main branch, the cut plane intersecting the side branch and the cut plane being defined under an angle with the cross-sections. The method further comprises, based on data in at least a part of the cross section data sets, constructing image data of a constructed cross section of the main branch in the cut plane. The method comprises receiving contour data defining a contour in the constructed cross section, the contour being provided in the side branch and determining the parameter by determining a parameter of the contour.

Description:
TECHNICAL FIELD 
       [0001]    The various aspects relate to a method and device for determining at least one geometrical parameter of a side branch of a main branch of a blood vessel. 
       BACKGROUND 
       [0002]    Evaluation of a side branch ostium in coronary bifurcation lesions is challenging in clinical practice and science. Various methods of obtaining data by means of invasive methods are known, like Optical Coherence Tomography-OCT-and Intravascular Ultrasound-IVUS. Such methods require intervention in blood vessels of the human body. Although a skilled cardiologist is able to control such techniques without much risk, there still is a risk connected to such techniques, in particular a risk of damaging walls of blood vessels. 
         [0003]    Therefore, the least amount of invasive actions is preferred. This means it is advantageous to be able to obtain data on the ostium—or opening or cross section—of a specific blood vessel by means of data obtained from another blood vessel of which the specific blood vessel is a side branch. In the paper “First Presentation of 3-Dimensional Recontruction and Centerline-Guided Assessment of Coronary Bifurcation by Fusion of X-Ray Angiography and Optical Coherence Tomography” in the JACC Cardiovascular Interventions journal, a method is proposed by fusing data obtained by means of x-ray angiography—XA—and data obtained by means of OCT. With XA data, a centreline of a sidebranch of a coronary vessel is determined. Subsequently, using OCT data obtained in the main branch, the area of the ostium of the side branch is determined in a plane perpendicular to the centreline determined via the XA data. 
       SUMMARY 
       [0004]    It is preferred to determine the ostium of a blood vessel based on data obtained by means of one analysis technique. 
         [0005]    A first aspect provides method of determining at least one geometrical parameter of a side branch of a main branch of a blood vessel. The method comprises receiving a multitude of cross section data sets, the data sets representing geometries of cross sections of the main branch over a length of the main branch and receiving position data of a cut plane of the main branch, the cut plane at least partially intersecting the side branch and the cut plane being defined under an angle with the cross-sections. The method further comprises, based on data in at least a part of the multitude of cross section data sets, constructing image data of a constructed cross section of the main branch in the cut plane. The method also comprises receiving contour data defining a contour in the constructed cross section, the contour being provided in the side branch and determining the geometrical parameter by determining a geometrical parameter of the contour. 
         [0006]    This method may be carried out using OCT or IVUS data only, thus removing a need for XA data and the fusion procedure. XA data requires injection of a contrast fluid as well as x-ray photography or filming. This means the method as outlined above removes the need of another analysis method for which intervention of the patient is required. Furthermore, as only OCT or IVUS data is used, less processing power is required as no XA data is to be processed. 
         [0007]    Furthermore, as the resolution of OCT or IVUS data is higher than of XA data, this method is more accurate. 
         [0008]    In an embodiment of the first aspect, receiving the definition of the cut plane comprises: receiving a longitudinal angle of the cut plane, the longitudinal angle being defined between the cut plane and an axis of the main branch; and receiving a transversal angle of the cut plane, the transversal angle being defined between the cut plane and a direction perpendicular to the axis of the main branch. 
         [0009]    By using the main branch as a reference for the cut plane, rather than the side branch, using OCT or IVUS data obtained in the main branch suffices and no data on the side branch is required. 
         [0010]    Another embodiment of the first aspect comprises based on the multitude of cross section data sets, generating first image data for display of a first two-dimensional view of the main branch in a longitudinal direction. This embodiment further comprises receiving longitudinal location data of the cut plane indicating a location of the cut plane in the first two-dimensional view; based on the multitude of cross section data sets, generating second image data for display of a second two-dimensional view of the main branch in a transversal direction; and receiving transversal location data of the cut plane indicating a location of the cut plane in the second two-dimensional view. 
         [0011]    This embodiment allows accurate positioning of the cut plane, based on image data that can be derived from the OCT data without significant processing. For the longitudinal cross section, data from individual tomographic images are merged in one single image. And for the transversal cross section, only one tomographic image is required. 
         [0012]    A further embodiment of the first aspect comprises determining a discontinuity in a sequential series of the multitude of cross section data sets and defining the cut plane at the location of the discontinuity as a default location. 
         [0013]    A discontinuity in the vessel wall, shown by OCT or IVUS data, may indicate presence of a side branch of the main branch. This information may be used to position the cut plane. The position of the cut plane may be adjusted to obtain more accurate information subsequently. 
         [0014]    Yet another embodiment of the first aspect comprises determining increase of a cross section geometry parameter being above a pre-determined threshold; determining a slope of either side of the main branch relative to a centre line of the main branch; determining a first side of the main branch having the largest slope; and defining the cut plane to be positioned perpendicular to the first side. 
         [0015]    This embodiment allows for further automation of determining an ostium of the side branch. 
         [0016]    A second aspect provides a computer programme product comprising computer executable code that, when loaded in a processing module of a computer, cause the computer to execute the method according to the first aspect. 
         [0017]    A third aspect provides a device for determining at least one geometrical parameter of a side branch of a main branch of a blood vessel. The device comprises a data input module arranged to receive a multitude of cross section data sets, the data sets representing geometries of cross sections of the main branch over a length of the main branch. The device further comprises a processing unit arranged to receive position data of a cut plane of the main branch, the cut plane at least partially intersecting the side branch and the cut plane being defined under an angle with the cross-sections; based on data in at least a part of the multitude of cross section data sets, construct image data of a constructed cross section of the main branch in the cut plane; receive contour data defining a contour in the constructed cross section, the contour being provided in the side branch; and determine the geometrical parameter by determining a geometrical parameter of the contour. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The various aspects and embodiments thereof will now be discussed in conjunction with Figures. In the Figures, 
           [0019]      FIG. 1 : shows a data analysis device 
           [0020]      FIG. 2 : shows a main branch and a side branch of a coronary artery with an OCT probe; 
           [0021]      FIG. 3 : shows a flowchart depicting a method as embodiment of the first aspect; 
           [0022]      FIG. 4 : shows the main branch and the side branch with several transversal cross sections; 
           [0023]      FIG. 5 : shows the main branch and the side branch with a cut plane; 
           [0024]      FIG. 6 : shows a transversal cross section of the main branch and the side branch; 
           [0025]      FIG. 7  A: shows cross section of the main branch and the side branch in the cut plane; and 
           [0026]      FIG. 7  B: shows a detail of the cross section of  FIG. 7  A with a contour provided in it. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  shows a data analysis device  110  for processing data obtained by means of Optical Coherence Tomography-OCT. OCT is a method for obtaining image data from within a blood vessel. With OCT, a probe is provided in the vessel and slowly pulled back while collecting data. The data can be obtained from the direct inner surface of the vessel wall, as well as of material deeper in the vessel wall. Output data is obtained in data sets representing 2D images, each image providing a transversal cross section image of the blood vessel. The data analysis device  110  comprises a microprocessor  112  as a processing module, a memory module  114 , a measurement processing module  116 , a user input module  118  and an image rendering module  120 . The microprocessor  112  is arranged for controlling the data analysis device  110  and various components thereof. The memory module is arranged for storing measurement data, processed data and computer executable code enabling the microprocessor  112  to execute methods as discussed below. 
         [0028]    The measurement processing module  116  is arranged for receiving raw data from an OCT probe  160  and for providing light—infrared light in particular—to the probe  160  for collecting data. The user input module  118  is arranged for receiving user input data from a keyboard  152  and a mouse  154 . The image rendering module  120  is arranged for rendering image data in any format for providing the rendered image in ready to display data to a screen  140 . The screen  140  may be provided as a touch screen, enabling a user of the data analysis device  110  to control the data analysis device  110 . 
         [0029]      FIG. 2  shows a first blood vessel  210  as a main branch and a second blood vessel  220  as a side branch. The first blood vessel  210  has a first vessel wall  212  and the second blood vessel  220  has a second vessel wall  222 . In the first blood vessel  210 , the OCT probe  160  is introduced. To the OCT probe  160 , a feed wire  254  is connected for moving the OCT probe  160 . The OCT probe  160  is connected to a guide wire  252  for guidance of the probe  160  and for inserting the probe. During acquisition of data, the probe  160  is pulled back through the first blood vessel  210  at a constant velocity. The pulling back is preferably done in a motorised way, using an electromotor controlled by the data analysis device  110 . 
         [0030]    Using OCT, cross section data of the first blood vessel  210  is obtained. From the cross section data, a cross section dimension of the first blood vessel  210  may be obtained in a relatively accurate way. For accurately determining a cross section dimension of the second blood vessel, ideally the OCT probe  160  is inserted in the second blood vessel  220  as well. A disadvantage of this method, however, is that this may increase a risk of puncture of the first vessel wall  212  of the first blood vessel  210  or the second vessel wall  222  of the second blood vessel  220 . Furthermore, this method also increases the workload of a cardiologist. 
         [0031]      FIG. 3  shows a flowchart  300  depicting a method for more accurately determining a cross section geometry of the second blood vessel  220  from OCT measurements of the first blood vessel  210  only. 
         [0032]    The process starts in a terminator  302  and continues to step  304  in which data sets are being received. Data may be buffered by the measurement processing module  116  before storing it in the memory module  114 . Alternatively, data received by the measurement processing module  116  is stored directly in the memory module  114 . In step  306 , a longitudinal cross section image of the main vessel  210  is created.  FIG. 4  shows a longitudinal image of the first blood vessel  210  and the second blood vessel. 
         [0033]    As data is obtained from a position in the first blood vessel  210 , a part of the second vessel wall  222  of the second blood is occluded from the probe  160 . This part of the second vessel wall  222  is provided in dotted lines in  FIG. 4 . The occlusion results in a discontinuity of obtained data at an armpit section  480  between the first blood vessel  210  and the second blood vessel  220 . The longitudinal cross section as shown by  FIG. 4  is obtained by combining data from multiple sequential transversal cross section data sets in step  306 . 
         [0034]    The angle under which the longitudinal cross section as shown by  FIG. 4  is shown, may be determined by an operator of the data analysis device  110 . Alternatively, the angle may be obtained by detecting discontinuities or occlusions in cross section data sets or in the reconstructed 2D image as shown by  FIG. 4 . Subsequently, the image data is processed such that a discontinuity is shown at the top or bottom of an image. Alternatively, the cut plane  500  may be placed at a random position or a pre-determined default position. 
         [0035]      FIG. 4  shows five cross section images of the first blood vessel  210  and the second blood vessel  220 . A first cross section image  410  is obtained in a cross section between A and A′. At this location, a substantially circular cross section is found. A second cross section image  420  obtained between B and B′ shows a small bulge  422  where the second blood vessel  220  stems from the first blood vessel  210 . In a third cross section  420  between C and C′, the cross section of the second blood vessel  420  is already larger. A location between D and D′ is a last location, from left to right, where data on a part of the second blood vessel  220  may be obtained. Beyond that point, the second blood vessel  220  is occluded by the first vessel wall  212  of the first blood vessel  210 . 
         [0036]    And already at the armpit section  480 , part of the second vessel wall  222  of the second blood vessel  220  is already occluded. This is indicated by a dotted line in the fourth cross section image  440 . At the dotted locations, data cannot be obtained by an OCT probe  160  in the first blood vessel  210  as these locations are occluded to the OCT probe  160  by the first vessel wall  212  and the second vessel wall  222 . Data of the fifth cross section image  450  obtained between E and E′ shows a substantially circular cross section again. Although a cross section of a blood vessel may be approximated by a circle, in practice the cross section will seldom be a perfect circle. 
         [0037]    After the longitudinal cross section has been obtained, data on a position of a cut plane  500  as shown by  FIG. 5  is obtained in step  308 . The data may be obtained by means of the keyboard  152 , the mouse  154 , the touch screen  140 , other, or a combination thereof, thus by receiving user input indicating a position of the cut plane  500 . Alternatively or additionally, the cut plane provided at a default location in an image. In yet another alternative, the cut plane  500  is provided at or near the armpit section  480 . The armpit section  480  may be determined by determining a discontinuity in data obtained on the walls of the vessels on which data is obtained. 
         [0038]    The cut plane  500  is preferably defined by five parameters: an x-coordinate relative to a reference point in the obtained data, a y-coordinate, a z-coordinate, a first angle of the cut plane relative to a longitudinal axis  224  of the first blood vessel  222  and a second angle of the cut plane  500  relative to a transversal axis  602  shown by  FIG. 6 . The Cartesian coordinates are preferably coordinates of a hinge point  510  over which the cut plane  500  may be tilted in an image provided by  FIG. 5  by means of keyboard  152 , the mouse  154 , the touch screen  140 , other, or a combination thereof. 
         [0039]    And the Cartesian coordinates preferably also indicate the hinge point  510  over which the cut plane may be tilted in an image shown by  FIG. 6 , by means of the keyboard  152 , the mouse  154 , the touch screen  140 , other, or a combination thereof. Also the Cartesian coordinates of the cut plane  500  may be modified using the input means. In this way, an operator of the data analysis device  110  is able to move the cut plane  500  to a location where the operator deems a cross section or other geometry of the second blood vessel  200  may be determined best. 
         [0040]    First, a location of the cut plane  500  in the longitudinal cross-section is received by the data analysis device  110 . After a preferred position of the cut plane  500  is determined in the longitudinal cross-section, a transversal cross-section of the first blood vessel  210  is displayed, the displayed transversal cross-section being at a location where the hinge point  510  is determined in the longitudinal cross-section. 
         [0041]    Alternatively, the location of the cut plane  500  may be determined fully automatically by the microprocessor  112 , in which case the microprocessor  112  receives the position data of the cut plane  500  from the microprocessor  112  itself. First, a discontinuity is detected in the vessel wall  212 . Such discontinuity may be an abrupt ending in data collected on a vessel wall in a longitudinal cross section or a transversal cross-section. Subsequently, a location of the second blood vessel  220 , the side branch, is determined in the vicinity of the discontinuity. Having determined the side branch, a first slope of the second blood vessel  220  is determined using the available OCT probe data. The first slope of the second blood vessel  220  is a basic orientation of the second vessel wall  222  in the view as depicted by  FIG. 5 , relative to the longitudinal axis  224  of the first blood vessel. Subsequently, the cut plane  500  is determined to be provided perpendicular to the determined slope in the view as depicted by  FIG. 5 . This determines a longitudinal angle of the cut plane  500  with the longitudinal axis of the first blood vessel  210 . 
         [0042]    Alternatively, increase of vessel width is determined over a pre-determined trajectory in the first blood vessel  210 . If the width of the first blood vessel  210  increases with and/or above a pre-determined width, it is determined that at this location or over this trajectory, the second blood vessel is present as a side branch of the first blood vessel  210 . In order to determine at which side of the first blood vessel  210  the second blood vessel  220  starts, increase of distance from a centre line of the first blood vessel  210  and the first vessel wall  212  and/or the second vessel wall  222  is determined. To this end, the radius of the first blood vessel  210  is determined at one and preferably more angles transversal to the axis of the first blood vessel  210 . The location of the second blood vessel  220  relative to the first blood vessel  210  is determined as a location where the radius increases most. Alternatively, a slope of a vessel wall is determined as a location and/or angle where a slope increases most. 
         [0043]    Having determined the longitudinal angle, a transversal angle may be determined, being the angle of the cut plane with the transversal axis  602  shown by  FIG. 6 . This may be done by aligning the cut plane  500  with a general orientation of the second vessel wall perpendicular to the view of  FIG. 5  and the view of  FIG. 6 . Subsequently, Cartesian coordinates of a central point or a hinge point of the cut plane  500 . are determined such that the cut plane  500  touches the armpit section  480 . In this way, the cut plane  500  is positioned perpendicular to the length of the second blood vessel  220 . 
         [0044]    Having received position data of the position of the cut plane  500  in the data set comprising imaging data of the first blood vessel  210  and the second blood vessel  220 , cross section image data is determined in step  310 . The cross section image data represents a cross section of the first blood vessel  210  and the second blood vessel  220  in the cut plane  500 .  FIG. 7  A shows a cut plane image  700  showing reconstructed image data in the cut plane  500 , showing the cross section of the second blood vessel  210  in a substantially circular way. As indicated before, this is an ideal situation not very likely to be found in nature. It is an approximation of a usual situation, though. 
         [0045]      FIG. 7  B shows the cross section of  FIG. 7  A in further detail. In the cut plane image  700 , a contour  710  is provided in dotted lines, approximating an inner perimeter of the second blood vessel  220 . The contour  710  may be generated automatically by determining differences in contrast in data obtained by means of the probe  160 . Alternatively, the contour  710  may be drawn by an operator of the data analysis device  110  by means of the keyboard  152 , the mouse  154 , the touch screen  140 , other, or a combination thereof. In another embodiment, the contour  710  is generated automatically and may be adjusted manually. In this way, contour data is obtained in step  312 . 
         [0046]    Subsequently, using the contour following or at least approximating the inner side of the second vessel wall  222 , geometrical parameters of the second blood vessel  220  may be determined.  FIG. 7  B shows a dashed line  720  through the middle of the contour  710  and crossed by a first marker  722  and a second marker  724 . The cross section may be determined as the distance between the first marker  722  and the second marker  724 . Also the total area of the lumen of the second blood vessel  220  may be determined using data of the contour  710 . This may be done by using data on a determined cross-section and a simple formula. Alternatively, this may be done using numerical calculations. In yet another alternative, two cross-section parameters are determined in directions perpendicular to one another and an area is determined using an elliptical approximation of the shape of the lumen of the second blood vessel. In another embodiment, the curve is a parameterised curve. In this embodiment, parameters determining the curve may be used for determining any geometrical parameter desired to be obtained. 
         [0047]    Thus far, various embodiments have been described using data obtained by means of optical coherence tomography. However, the various embodiment may also be used with other techniques suitable for obtaining cross section data sets representing image data of the inside of a blood vessel. This means that the techniques described above may also be used on data obtained by means of intravascular ultrasound or other techniques for obtaining tomographic data within a blood vessel.