fig1 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 fig2 . 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 . when the catheter 2 is located in a patient in an mr scanner 63 ( see fig7 ), 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 fig1 ) 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 . fig2 shows the orientation of the coaxial cable 8 and of both coils 10 and 12 from fig1 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 . in a schematic representation , fig3 shows the reception sensitivity profile 28 of the coaxial cable 8 and the reception sensitivity profile 30 of both coils 10 and 12 from fig1 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 . 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 fig1 . since the remaining design is identical to that shown in fig1 , fig4 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 fig1 . 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 . fig5 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 fig1 , 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 . in the flowchart shown in fig6 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 . fig7 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 fig7 ( 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 . 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 fig7 . 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 . 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 . 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 .
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 .