Abstract:
A medical apparatus, includes a medical instrument with an acquisition unit for a magnetic resonance signal, the acquisition unit exhibiting a non-constant sensitivity profile in the azimuthal direction relative to an axis of the medical instrument. An azimuthal orientation of the medical apparatus is determined by an evaluation unit from magnetic resonance signals received by the acquisition unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention concerns a method and apparatus for use with a magnetic resonance imaging apparatus, for determining the azimuthal orientation of a medical instrument from magnetic resonance (MR) signals.  
         [0003]     2. Description of the Prior Art  
         [0004]     For medical procedures involving in vivo insertion of a medical instrument, for example a catheter, monitoring of the position and alignment of the instrument is frequently necessary. To make placement of the catheter tip easier, the tip generally is formed with a bend. The catheter can thereby also be inserted into branched vessels. Whether the catheter reaches into a branch or passes by the branch depends on the rotation angle of the catheter around its own axis. This rotation angle in the following is designated as the azimuthal orientation. It is desirable to be able to determine the azimuthal orientation of the curved catheter tip during the procedure in order to make targeted placement in a vessel easier.  
         [0005]     It is generally not possible to discern the orientation of the catheter at the insertion point in the body from the orientation of the catheter tip, since the catheter is not resistant to buckling or bending over its entire length.  
         [0006]     One possibility for determining the azimuthal orientation of the catheter tip is to monitor using continuous fluoroscopic radiography. The azimuthal orientation of the catheter tip can be directly determined by this projection method. For this purpose, it is necessary to rotate the catheter, or to move the catheter forward and back, in order to detect the orientation. With radiographic observation, however, the vessel outlets into which the catheter should be inserted generally are not visible. For this reason, the catheter manipulation and probing place high demands on the experience of the treating physician and often require long radiography times, which is disadvantageous for the patient due to the x-ray dose associated therewith.  
         [0007]     The position and alignment of the catheter or a guide wire connected thereto can be shown in a magnetic resonance image by means of magnetic resonance measurements. For this purpose, for example, the catheter can be equipped with coils for acquisition of a magnetic resonance signal. The position of the tip can be determined from the acquired signal by comparison with signals of the other coils that are used, for example, for images. The catheter is not necessarily directly shown in the magnetic resonance image, but the position of the catheter tip is superimposed in a magnetic resonance image of the surrounding vessels. The position of the catheter so acquired in this manner can be used to track the position of the catheter tip in real time by ensuring that the measurement slice in which the tip is currently located is always currently displayed so the position of the type always remains visible in the magnetic resonance image. It has not been possible, however, to also assess or to determine the azimuthal orientation of the catheter tip in real time with this known method.  
         [0008]     The degree of the curvature of the tip depends on the size of the particular vessel in which the catheter is currently located. In narrow vessels, the tip is straightened by the pressure of the vessel walls, which reduces its degree of curvature in comparison with larger vessels in which the curvature is not altered. The significantly curved tip in large vessels can be shown in a magnetic resonance image. The actual curvature, and therewith the azimuthal orientation, however, can be detected only with difficulty even in high-resolution magnetic resonance images. In contrast to this, in narrow vessels the azimuthal orientation cannot be detected at all in the magnetic resonance image due to the straightened tip. Therefore it cannot to be seen in advance in which direction the catheter is bent immediately upon the vessel becoming wider again, or when the catheter arrives at a branch. A continuous representation of the azimuthal orientation of the tip during the entire procedure time thus has not been possible with magnetic resonance measurements.  
       SUMMARY OF THE INVENTION  
       [0009]     An object of the present invention is to provide a medical apparatus and an operating method therefor that allow the azimuthal orientation of a medical instrument to be determined by means of magnetic resonance measurements.  
         [0010]     This object is achieved by a medical apparatus with a medical instrument having an acquisition unit for acquiring a magnetic resonance signal that is firmly connected to the medical instrument and exhibits a non-constant (non-uniform) reception sensitivity profile in the azimuthal direction relative to an axis of the medical instrument. Furthermore, the medical apparatus has an evaluation unit that is connected with the acquisition unit and that determines the azimuthal orientation of the medical instrument from the magnetic resonance signal acquired by the acquisition unit. The azimuthal orientation of the instrument can always be determined from the measured magnetic resonance signals due to the non-constant sensitivity profile of the acquisition unit in the azimuthal direction and the fixed connection with the medical instrument. In the ideal case, the sensitivity profile is asymmetric, such that the azimuthal orientation of the tip can be unambiguously determined. Sensitivity profiles in which an ambiguity occurs, for example modulo 180°, can also be used. Even with this limitation, an efficient manipulation of the medical apparatus is possible since, in comparison to previous techniques, only two possibilities are available for selection and the correct orientation can quickly be found by testing. The information thus acquired about the azimuthal orientation of the medical instrument can be superimposed into an overview image of the surrounding anatomy of the patient in the planning (image preparation) for magnetic resonance measurements. Thus, for example, the vessels can be shown in the magnetic resonance image and, as is known, the position of the inserted medical instrument can be displayed. The azimuthal orientation of the medical apparatus can additionally be superimposed, so manipulation of the instrument is made easier for the physician. The use of magnetic resonance measurements for determination of the azimuthal orientation moreover has the advantage that the patient is exposed to no radiation load as occurs, for example, in radiography.  
         [0011]     In a preferred embodiment, the acquisition unit has a half-open coaxial cable for acquisition of the magnetic resonance signal. The sensitivity profile is asymmetrical, such that magnetic resonance signals can be acquired only from a limited angle range in the azimuthal direction.  
         [0012]     In a preferred embodiment, the medical instrument has a reference unit for acquisition of the magnetic resonance signal, the reference unit being connected with the evaluation unit. The sensitivity profile of the reference unit differs from the sensitivity profile of the acquisition unit in the azimuthal direction, at least in a limited range. An evaluation of the magnetic resonance measurement implemented with the acquisition unit is made easier by the reference measurement.  
         [0013]     In a method according to the invention for determination of the azimuthal orientation, the magnetic resonance signal measured with the acquisition unit to the evaluation unit, which determines therefrom the azimuthal orientation dependent on the non-constant sensitivity profile.  
         [0014]     In an embodiment the method, the magnetic resonance signal is acquired with the reference unit and is likewise supplied to the evaluation unit, which determines from this a non-azimuthal orientation of the axis of the medical instrument. For example, the orientation and position of the medical instrument can be determined, which is of great importance for accurate placement. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic of a portion of a medical instrument in the embodiment of a catheter with an acquisition unit and a reference unit according to the invention.  
         [0016]      FIG. 2  schematically illustrates the signals of the acquisition unit and the reference unit in the plane of a magnetic resonance image.  
         [0017]      FIG. 3  schematically illustrates the sensitivity profile of the half-open coaxial cable used as the acquisition unit in the embodiment.  
         [0018]      FIG. 4  schematically illustrates a further embodiment of the acquisition unit, as a tilted dipole coil.  
         [0019]      FIG. 5  schematically illustrates another embodiment of the acquisition unit, as a transversal dipole coil.  
         [0020]      FIG. 6  is a flowchart of the inventive method for determination of the orientation of the medical instrument.  
         [0021]      FIG. 7  schematically illustrates an MR apparatus embodying the inventive apparatus, showing a magnetic resonance image for explaining the invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]      FIG. 1  shows a section of a catheter  2  with a curved tip  4 . The azimuthal orientation of a tip  4  is designated by the angle φ, which is defined relative to the catheter axis  6 . The catheter  2  has an acquisition unit, which in this embodiment is a half-open coaxial cable  8 , and a reference unit formed by two coils  10  and  12 . The axes of symmetry of the coils  10  and  12  and of the coaxial cable  8  coincide with the catheter axis  6 . The catheter  2  is resistant to buckling over its entire length, i.e. the coaxial cable  8 , both coils  10  and  12  and the tip  4  are in a fixed alignment relative to one another in the azimuthal direction. The coaxial cable  8  is connected in series with the coils  10  and  12 , with the coaxial cable  8  between the coils  10  and  12 . The distance of the coaxial cable  8  from the coil  10  is thereby larger than the distance to the coil  12 . An asymmetry is thereby created in the arrangement from which the orientation of the catheter axis  6  can be determined, as explained below using  FIG. 2 . The coaxial cable  8  has an inner conductor  14  that runs along the catheter axis  6  and an outer conductor  16  partially shielding the inner conductor  14 . The outer conductor  16  has the shape of a cylinder cut in half parallel to its axis of symmetry, the cylinder likewise being aligned parallel to the catheter axis  6 . The coils  10  and  12  are formed by windings of two cables  18  that are connected with the outer conductor  16  of the coaxial cable  8  at two contact points  20 . The inner conductor  14  of the coaxial cable  8  is continued beyond the outer conductor  16  and crosses both coils  10  and  12  along their axes of symmetry  22 . Furthermore, the inner conductor is connected with the cable  18  at a contact point  24  near the tip  4  of the catheter  2 , such that a circuit is created.  
         [0023]     When the catheter  2  is located in a patient in an MR scanner  63  (See  FIG. 7 ), magnetic resonance signals can be acquired with the coaxial cable  8  and the coils  10  and  12  and supplied via a cable  65  (connected at the left end of the catheter  4  shown in  FIG. 1 ) to an evaluation unit  64 . A spatial variation of the magnetic field strength, and with it a variation of the Larmor frequency, is set by the magnetic field gradients used in magnetic resonance examinations. Signals at respectively different frequencies are therefore detected (received) by the coaxial cable  8  and the coils  10  and  12 , dependent on the respective reception sensitivity profile. The evaluation unit  64  can resolve these signal components at different frequencies by spectral analysis, for example. The orientation of the catheter axis  6  and the position of the tip  4  can be determined from those signals and shown in a magnetic resonance image acquired in a known manner with a local coil located outside of the patient.  
         [0024]      FIG. 2  shows the orientation of the coaxial cable  8  and of both coils  10  and  12  from  FIG. 1  using the positions  26  of the respective magnetic resonance signals acquired with them. Since the spacings between the coaxial cable  8  and both coils  10  and  12  are of different sizes, the orientation of the catheter axis  6  and of the tip  4  can be determined.  
         [0025]     In a schematic representation,  FIG. 3  shows the reception sensitivity profile  28  of the coaxial cable  8  and the reception sensitivity profile  30  of both coils  10  and  12  from  FIG. 1  in a plane perpendicular to the catheter axis  6 . While the sensitivity profiles  30  of the coils  10  and  12  are symmetrical around the catheter axis  6 , due to the half-open shielding the sensitivity profile  28  of the coaxial cable  8  exhibits an asymmetry so that it is only sensitive to magnetic resonance signals from the direction of the open side. An unambiguous indication of the azimuthal orientation of the tip for guidance of the catheter  4  is established by the fixed azimuthal position of the coaxial cable  8  relative to the tip.  
         [0026]     In a second exemplary embodiment, a tilted dipole coil is fixed to the catheter  4  as an acquisition unit instead of the half-open coaxial cable  8  of  FIG. 1 . Since the remaining design is identical to that shown in  FIG. 1 ,  FIG. 4  shows only the tilted dipole coil and the catheter axis  6 . The titled dipole coil is composed of two coil parts  32  with windings  34  wound at an angle, the windings  34  of both coil parts  32  being tilted opposite to one another at an angle  36 . An asymmetry of the sensitivity profile of the dipole coil results from this arrangement, which is represented by the sensitivity profile amplitude line  38  being off-center relative to the axis  6 . The azimuthal orientation of the tip of the catheter can be determined by means of the dipole coil in a manner analogous to the exemplary embodiment described in  FIG. 1 .  
         [0027]     Both of the aforementioned exemplary embodiments detect the intensity of the magnetic resonance signal, it is also possible to use the phase of the magnetic resonance signal to determine the azimuthal orientation. It is thereby necessary to eliminate possible phase shifts due to signal delays in the cables as well as frequency- and position-dependent phases, by calibration. As in the exemplary embodiments explained above, it is necessary to determine the azimuthal orientation of the catheter axis and its position.  
         [0028]      FIG. 5  shows a section through a transversal dipole coil that is formed by three conductor loops  40 . The conductor loops  40  are perpendicular to the drawing plane, and exit the drawing plane at points  24  and re-enter into the drawings plane at crosses  44 . The catheter axis  6  likewise is perpendicular to the drawing plane and thus is parallel to the conductor loops  40  of the transversal dipole coil. In a manner analogous to the exemplary embodiment in  FIG. 1 , the dipole coil is fixed at the catheter  4  so that the relative orientation of the dipole coil and the tip of the catheter is also fixed. The azimuthal orientation is again represented by the angle φ. The phase of the magnetic resonance signal in the environment of the dipole coil is represented by arrows  46 , as an example. Given a half-rotation around the catheter axis  6 , the phase completely rotates once, i.e. the phase is identical at two opposite sides of the dipole coil. The phase changes linearly with the azimuthal orientation φ. By analysis of the magnetic resonance signal acquired with the transversal dipole coil, the orientation of the dipole coil can be determined from the linear relation of the phase to the azimuthal angle, from which in turn the azimuthal orientation of the tip of the catheter can be determined. The determination of the azimuthal orientation is in this case established as modulo 180°. The ambiguity can be resolved by testing of the two different orientations. A different possibility to prevent this ambiguity is to use an asymmetrically-wound transversal dipole coil in which the conductor loops  40  are closer together on one side than on the other side. The phase then no longer changes linearly with the azimuthal angle and is thus unambiguously established.  
         [0029]     In the flowchart shown in  FIG. 6  of an embodiment of the inventive method for determination of the azimuthal orientation, magnetic resonance is excited in a step S 2 . In a second step S 4 , a spatial coding is effected by means of gradient fields. In a subsequently-implemented step S 6 , the magnetic resonance signals are acquired with the acquisition unit and the reference unit. In a step S 8 , the corresponding measurement values are transmitted to the evaluation unit  64 . In a step S 10 , the position of the catheter tip and the orientation of the catheter axis are initially determined from these measurement values. In a step S 12 , the azimuthal orientation of the catheter tip is determined. The determined azimuthal orientation and the position of the catheter tip, as well as the orientation of the catheter axis, are subsequently shown in a magnetic resonance image in a step S 14 .  
         [0030]      FIG. 7  schematically shows a magnetic resonance image of a branched vessel  48 , obtained in a known manner, in which information is obtained in accordance with the invention also is displayed. The magnetic resonance image shown in  FIG. 7  (except for the instrument orientation information obtained in accordance with the invention) is acquired in a known manner, using the magnetic resonance scanner  63  to acquire magnetic resonance imaging data, that are supplied to a magnetic resonance image computer  66 . The signals acquired from the catheter  4 , in the manner described above, are supplied via a cable  65  to the evaluation unit  64 , which supplies the orientation information to the magnetic resonance image computer  66  for inclusion in the magnetic resonance image that is presented at a display  67 .  
         [0031]     A point  50  that shows the position of the catheter tip is superimposed in the magnetic resonance image. A line  52  indicating the orientation of the catheter axis  6  is shown originating from the point  50 . An arrow  54  that indicates the azimuthal orientation of the curved tip of the catheter  4  likewise originates from the point  50 . The azimuthal orientation cannot be unambiguously indicated with the arrow  54 . The arrow  54  can only indicate a projection of the tip of the catheter  4  on the image plane. If the tip does not lie in the image plane, it is necessary for the physician to rotate the catheter  4  so that the tip lies in the image plane and can be inserted into the branching vessel. A cross  56  that unambiguously indicates the azimuthal orientation of the tip is superimposed into the image as an orientation aid for the physician. A line  58  indicates the position of the image plane and an arrow  60  indicates the orientation of the tip. The catheter axis  6  is perpendicular to the display plane, which is illustrated by the point  62 . The representation of the arrow  60  is selected as it would appear looking along the catheter axis  6  toward the tip. The physician thus knows at any point in time in which direction he or she must rotate the catheter  4  so that the tip arrives in the image plane. As an additional or alternative orientation aid, the arrow  54  can assume various colors, which cannot be shown in  FIG. 7 . A green arrow  54  can indicate that the tip lies in the image plane. If the tip has not yet reached the image plane, the arrow  54  can be red; if the tip is beyond the image plane, the arrow can be yellow.  
         [0032]     Upon movement of the catheter  4 , the slice used for acquisition of the magnetic resonance image is automatically guided along using the coils fastened to the catheter  4  so that an efficient and easy placement of the tip of the catheter  4  is possible.  
         [0033]     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.