Abstract:
A medical instrument for carrying out a percutaneous intervention in a patient is provided with a marker that is visible in an MR image. A real-time magnetic resonance image of the patient is created, so that the actual position of the marker can be identified in the real-time image. For assisting a person in the positioning of the medical instrument in an initial position suitable for the intervention, a desired position of the marker that correlates with the initial position is displayed in the real-time image. The positioning thus can be carried out relatively effortlessly and quickly.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention concerns a magnetic resonance tomography apparatus (MRT apparatus) for assisting a person when positioning a medical instrument for implementing a (MR-guided) percutaneous intervention in a patient. The invention also concerns a method for supporting such a person when positioning of such a medical instrument for implementing such a percutaneous intervention. 
         [0003]    2. Description of the Prior Art 
         [0004]    A percutaneous intervention is a medical intervention in which a medical instrument is introduced into the body of a patient so as to be as minimally invasive as possible. The aim of the intervention is usually to reach a lesion (abnormal tissue) inside the body with the medical instrument. The medical instrument is typically a needle or cannula or the like. Examples of such percutaneous interventions are biopsies, thermal ablations or local applications of drugs. 
         [0005]    To enable precise guidance of the instrument to the lesion in the body, imaging methods for supporting the person carrying out the intervention (hereinafter also called: “operator” for short) are conventionally used. A real-time image of the inside of the body or a body segment of the patient is conventionally displayed for the operator, so that the operator can follow the path of the medical instrument inside the body. Magnetic resonance tomography (MRT) is increasingly being used for real-time imaging, since lesions can be identified particularly well by the outstanding soft tissue contrast of MRT. Sometimes lesions can even be identified solely by means of MRT. 
         [0006]    MR imaging has the drawback, however, that the medical instrument used for the intervention cannot be seen outside of the body in the real-time image. The operator must therefore bring the medical instrument more or less “blind” into a position in which the instrument is suitably directed toward the lesion. 
         [0007]    Correct orientation of the medical instrument usually occurs therefore only after the entry thereof into the body and this can lead to unnecessary damage to body tissue. Furthermore, there are different tracking methods for the instrument, although these are associated with comparatively high outlay in teems of apparatus components. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the invention is to assist a person when positioning a medical instrument for implementing an MR-guided (i.e. guided by magnetic resonance tomography) percutaneous intervention, so as to indicate a pre-operative initial position suitable for the intervention. It should be possible to carry out the positioning relatively easily and quickly. 
         [0009]    A magnetic resonance tomography apparatus for assisting a person (an operator) when positioning a medical instrument for implementing a percutaneous intervention in a patient includes a marker that is visible in a magnetic resonance image and that is provided on the medical instrument. 
         [0010]    Furthermore, the MRT system has a magnetic resonance tomography scanner (MRT scanner). The MRT scanner is operated to produce a real-time image of the patient, so that—at least if the medical instrument is located in a region intended for the intervention—the marker can be seen in the real-time image. 
         [0011]    For assisting the operator in the pre-operative orientation of the medical instrument (generally not visible outside of the patient&#39;s body in the real-time image), the MRT system also has a processor configured to display in the real-time image a desired position of the marker that correlates with a predefined initial position of the medical instrument. 
         [0012]    For correct positioning of the instrument, the operator must then orient the medical instrument in space so that the actual position of the marker is matched to the desired position that is likewise displayed in the real-time image. 
         [0013]    An inventive method for assisting a person when positioning a medical instrument for implementing an MR-guided percutaneous intervention in a patient, included the steps of providing the medical instrument with a marker that is visible in an MR image. In principle this can be done in advance by an instrument manufacturer, but this step is preferably done by the operator during the course of preparation for the intervention. A real-time image of the patient is then generated by MRT, in which the actual position of the marker can be seen with at least approximately correct positioning of the medical instrument. The desired position of the marker that correlates with a predefined initial position of the medical instrument is also displayed in the real-time image, so that the operator can bring the medical instrument into the desired initial position by comparison of the actual position of the marker with the desired position of the marker. 
         [0014]    The method can advantageously be applied to any MR-compatible instrument. The method can be applied particularly advantageously to manually-guided instruments, since then the low additional outlay for implementing the method is shown to particular advantage. All that is necessary is for the instrument to be provided with the marker and a software program for displaying the desired position in the real-time image to be installed on a processor so as to be executable. 
         [0015]    At least one section of the medical instrument can already be seen outside of the body in the MR image due to the marker. The operator can consequently advantageously already precisely position the instrument before it enters the body. The orientation of the instrument can be carried out easily and intuitively hereby. 
         [0016]    Within the context of the invention “marker” generally designates an object which is made at least partly from a material whose nuclear spins can be excited by the MRT device, and which can therefore be seen in an MR image. In a preferred embodiment the instrument is a thin, elongated instrument that is preferably (but not exclusively) suitable for carrying out a biopsy, for carrying out a thermal ablation or for carrying out a local drug application. In particular the instrument can be a needle, electrode or cannula appropriate to the respective application. 
         [0017]    An image or MR image designates a depiction which is produced from measurement data acquired by the MRT device. The MR image (the depiction) is expediently displayed on a display unit that is situated in the vicinity of the MRT device. 
         [0018]    An MR image is characterized as a “real-time” image when the image is produced at a scan rate that is sufficiently high for online tracking of the actual position of the marker. The image produced is updated at a scan rate of, for example, two images or more per second. 
         [0019]    Within the context of the invention the term “initial position” designates a position of the medical instrument in space that is an insertion position or entry position for the medical instrument according to an intervention plan produced before the procedure. This means the initial position describes the position of the instrument immediately before the start of the intervention. The initial position is chosen such that a longitudinal extension of the instrument aligns with a planned intervention path. The initial position is preferably fixed by specifying two points of the instrument. The first point can be the instrument tip, which in the initial position is placed at a predefined entry point in the body. To fix the second point the marker is provided on the instrument at a defined spacing from the instrument tip. The marker is small compared with the longitudinal extent of the instrument. It is also within the context of the invention, however, for multiple markers to be provided on the instrument, so then for orientation of the instrument all of the markers have to be brought into an appropriate desired position displayed in the real-time image. 
         [0020]    Within the context of the invention, the desired position to be schematically can be shown in the real-time image solely by a marking point. In a preferred embodiment, however, an external contour of the marker is displayed at the desired position by the processor, so that the overlaying of the actual position of the marker with its desired position may advantageously be carried out particularly precisely. 
         [0021]    For additional assistance, the processor can be configured to incorporate the geometry of the instrument provided with the marker, to determine the location of the desired position from a predefined entry point of the instrument in the body of the patient and a predefined target point inside the body of the patient. Entry point and target point are fixed by the operator during the course of intervention planning using a previously acquired image of the patient. For example, the operator can “click” the entry point and the target point in a planning image, from which the processor first determines the respective positions in space and from this determines the position of the marker in space by incorporating the geometry of the instrument provided with the marker. The initial position is specified such that a longitudinal orientation of the instrument—if it is located with its tip at the entry point—aligns precisely with an intervention path that leads from the entry point to the target point. 
         [0022]    For particularly straightforward display and overlaying (matching), the marker is rotationally symmetrical in design, with its axis of symmetry preferably oriented coaxially to a longitudinal extent of the instrument. The marker is preferably approximately spherical in design. The marker is preferably small in dimension compared to the longitudinal extent of the medical instrument. 
         [0023]    The marker preferably is a hollow body that contains a medium that can be depicted by the MRT device, in particular water to which gadolinium has been added, or Vitamin E. 
         [0024]    For provision of the marker on the medical instrument, the marker preferably has a passage, such as a central passage, with which it can be placed on the medical instrument. The passage is dimensioned such that the marker can be held in place on a port of a cannula or directly on a cannula of the medical instrument. 
         [0025]    The marker is preferably in the form of a component part that is separate from the instrument, with the marker only being provided on the instrument only by the operator. Commercially available medical instruments can advantageously be used hereby for the MRT system described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  schematically illustrates an MRT apparatus for assisting a person when positioning a medical instrument for a percutaneous intervention in a patient. 
           [0027]      FIG. 2  schematically illustrates the medical instrument provided with a marker that can be depicted in an MR image. 
           [0028]      FIG. 3  also shows the marker according to  FIG. 2 . 
           [0029]      FIG. 4  shows a real-time image produced with the MRT apparatus of  FIG. 1  according to the invention. 
           [0030]      FIG. 5  is a flowchart of a method for assisting a person when positioning a medical instrument for a percutaneous intervention in a patient according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Corresponding parts are provided with the same reference characters in all figures. 
         [0032]      FIG. 1  shows schematically (and not-to-scale) illustrates a magnetic resonance tomography apparatus (MRT apparatus  1  for short), having a magnetic resonance tomography scanner  2  (MRT scanner  2  for short). A patient bed  3  for supporting a person to be examined or treated (hereinafter “patient  5 ”) is associated with the MRT scanner  2 . The MRT system  1  also has a processor  6 , which is used for operating the MRT scanner  2  and for causing a magnetic resonance image (MR image  7 ), reconstructed in the processor  6 , to be shown on a display unit  8 . 
         [0033]    The MRT scanner  2  is constructed in a conventional manner. It has a basic field main magnet for generating a basic magnetic field, radio-frequency coils for resonant excitation of nuclear spins of certain body tissue of the patient  5 , and a gradient coil system for spatial resolution of the magnetic resonance signal (MR signal MR) resulting from the resonant excitation. A coordinate system  10  of the scanner  2  is defined by the gradient coil system. Three coordinates of the coordinate system  10  are clearly associated with each volume element of the acquired MR data. 
         [0034]    For image generation, the processor  6  derives an image data record B from the MR signal. An image point (voxel) of the image data record B is associated with each volume element considered (defined by its 3D coordinates). From the MR signal MR the processor  6  determines for each image point a gray scale value that represents the tissue properties of the associated volume element. 
         [0035]    The processor  6  produces one or more two-dimensional MR image(s)  7  (e.g. in the form of sectional views or rendered scenes) from the three-dimensional image data record B and emits electronic signals represented by each MR image  7  to the display unit  8  of the MRT systems  1  as the MR image  7 . Different views in particular  7 ′ can be produced from a single image data record B.  FIG. 4  shows a sectional view of this as an example. 
         [0036]    In the present case the MRT system  1  is used for supporting a percutaneous intervention in the patient  5 . The example of a biopsy as the intervention is used below. A tissue sample in the region of a lesion inside the body of the patient  5  is to be extracted using a medical instrument  20 . The person carrying out the biopsy will be called the “operator” below. 
         [0037]    The medical instrument  20  is shown in  FIG. 2  in a side view. The instrument is a commercially available, MR-compatible (biopsy) needle. The instrument  20  is formed by a cannula  21  with a connecting element  22  made from plastic. The connecting element  22  is used to conventionally connect the cannula  21  to a vacuum device for generating suction for removal of tissue. A tip  23  is formed on the cannula  21  at the longitudinal end that faces the connecting element  22 . Connecting to the vacuum device is optional, however. The instrument  20  can alternatively also be designed as a biopsy needle, which cuts or punches out the tissue sample to be removed without the application of a vacuum. 
         [0038]    The instrument  20  itself cannot be depicted by MRT and is therefore not visible in the MR image  7  outside of the body. The instrument  20  can only be seen inside the body for the MRT as a consequence of susceptibility artifacts produced thereby. A marker  30  that can be depicted by MRT is nevertheless provided on the connecting element  22 . 
         [0039]    The marker  30  (shown in a perspective view in  FIG. 3 ) has an approximately spherical hollow body  31  having a defined spherical radius R of, for example, approximately 0.5 cm. The hollow body  31  is filled with a medium that can be depicted by MR, in this case with vitamin E. The wall of the hollow body  31  is made from a rubbery material. The marker  30  is placed as intended on the connecting element  22  of the instrument  20  (see  FIG. 2 ) with a continuous central passage  35 . The diameter of the passage  35  is dimensioned such that the marker  30  is held by friction on the connecting element  22 . The center of the marker is then at a defined spacing A from the tip  23 . In an alternative embodiment the diameter is dimensioned such that the marker  30  can be placed on the cannula  31 . 
         [0040]      FIG. 4  shows the image data record B in one of the views  7 ′ according to  FIG. 1 , with a section through the body  40  of the patient  5  being shown here. 
         [0041]    A (suspected) lesion  41  can be seen inside the body  40 . Shown in the region of the lesion  41  is a target point  42  at which the tissue sample is to be removed. Fixed on the surface of the body is an entry point  43  at which the instrument  20  should be introduced into the body  40 . The depiction of entry point  43 , target point  42  and intervention path  44  in the MR image  7  is optional. 
         [0042]    In any case, a contour  51  corresponding to the marker dimensioning is overlaid on the MR image  7  at a desired position  50 . The desired position  50  for supporting the operator in the positioning of the medical instrument  20  represents the position that the marker  30  adopts if the instrument  20  is located in an initial position  55  (likewise optionally depicted in the MR image  7 ) with its tip  23  at the entry point  43  and oriented in the direction of the intervention path  44 . 
         [0043]    As can be seen from  FIG. 4 , a depiction of the marker  30  can be seen in the MR image  7  moreover, and, more precisely, at its actual position  56  at which it is currently located in the position of the instrument  20  shown according to  FIG. 1 . 
         [0044]    With knowledge of the desired position  50 , the operator is able to orient the instrument  20  in the desired initial position  55  by moving the instrument  20  in space, with simultaneous MR imaging, until the current depiction of the marker  30  covers the contour  51  (the “virtual image” of the marker  30 ) at its desired position  50 . 
         [0045]    A method for assisting a person in the positioning of the medical instrument  20  is explained using the flowchart in  FIG. 5 . 
         [0046]    In a first step  60  the operator carries out an intervention plan in preparation for the biopsy. Using either the MRT scanner  2  or another modality, an image of the patient  5  is recorded in advance, in which the lesion  41  to be treated can be seen. The operator fixes target point  42  ( FIG. 4 ) and entry point  43  ( FIG. 4 ) by “clicking” or some other form of marking in this image. Using the marked image points the processor  6  determines the 3D coordinates of target point  42  and entry point  43  within the coordinate system  10 . 
         [0047]    In a second step  61  the operator chooses a suitable medical instrument  20  for carrying out the intervention (by way of example the needle according to  FIG. 2 ) and provides this with the suitable marker  30 . The spacing A and the spherical radius R are fed to the processor  6  as geometric data of the medical instrument  20  provided with the marker  30 . The operator inputs the data manually by way of example, or he has the option of choosing the instrument  20  and the marker  30  from a list, with the associated geometric data A, R being retrievably stored for the processor  6 . 
         [0048]    In a further step  62 , the processor  6  determines from the 3D coordinates of target point  42  and entry point  43 , as well as the spacing A firstly the 3D coordinates of the desired position  50  at which the center of the sphere of the marker  30  must be located if the instrument  20  is oriented in the initial position  55 . Furthermore, the processor  6  determines the position of the contour  51  with spacing R from the desired position  50 . 
         [0049]    During the course of the actual intervention the operator is then firstly assisted in step  63 , as is known, for example, from US 2013/0218003 A1, in finding the entry point  43  on the body of the patient  5 . Alternatively the operator finds the entry point  43  in an image-assisted manner by placing a finger (which can be depicted by MR) or by MR-visible marking points which are provided on the skin of the patient  5  in the region of the anticipated entry point  43 . Once the entry point  43  has been found and prepared for the intervention the operator places the instrument  20  with its tip  23  at the entry point  43  (according to the diagram in  FIG. 1 ). 
         [0050]    In step  64 , the processor  6  activates the MRT device  2  to start a data acquisition protocol that is capable of producing the MR image  7  in real-time. A data acquisition protocol of this kind is, for example, a balanced SSFP sequence (“Steady State Free Precession”). 
         [0051]    Finally, in step  65  the processor  6  combines the MR signal MR of the MRT device  2  with the determined desired position  50 . The processor  6  generates an image data record B in which the contour  51  with spacing R from the desired position  50  is provided. In other words, the processor  6  synthetizes a virtual image of the marker  30 , specifically of its external contour, and overlays this virtual image on the MR image at the calculated desired position  50 . 
         [0052]    In step  66 , the MR image  7  modified in this way is shown to the operator on the display unit  8 , so that the operator can then position the instrument  20  in the desired initial position  55  by optical feedback. 
         [0053]    The operator first moves the instrument  20  until the depiction of the marker  30  appears in the MR image  7 . The operator then performs the orientation within the cutting plane. Typically the operator has even more views  7 ′ of the image data record B available, however, wherein the actual position  56  of the marker  30  must then be aligned in all views  7 ′ with the desired position  50 . The sectional images are ideally chosen such that the intervention path  44  is located in the image plane (analogously to  FIG. 4 ). 
         [0054]    In an alternative embodiment, the MR image  7  can be formed by two projections that are perpendicular to each other. As a further alternative, the MR image  7  can be a volume depiction. 
         [0055]    Once the instrument  20  has been positioned, the tip  23  is finally guided—again with real-time imaging—along the intervention path  44  to the target point  42  and the tissue sample removed. 
         [0056]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.