MAGNETIC RESONANCE TOMOGRAPHY UNIT FOR LOCALIZING METALLIC OBJECTS AND OPERATING METHOD

Abstract

A magnetic resonance tomography unit for localizing metallic objects and an operating method are provided. In one act of the method, an excitation pulse is used to excite nuclear spins in a region surrounding a compact metallic object. Magnetic resonance data is acquired with samplings along a plurality of trajectories, where the samplings take place using a bSSFP sequence, and the nuclear spins are dephased by a gradient. A position of a geometric focal point of the compact metallic object is ascertained based on a position of a visual focal point of acquired artifacts.

Claims

1. A method for localizing a metallic object that is compact in three dimensions or a linear metallic object that is compact in two dimensions using a magnetic resonance tomography unit, the method comprising: exciting nuclear spins in a region surrounding the compact linear metallic object using an excitation pulse; and acquiring magnetic resonance data with samplings along a plurality of trajectories, wherein the nuclear spins are dephased by a gradient, wherein the samplings take place using a bSSFP sequence, wherein a position of a geometric focal point of the compact linear metallic object is ascertained based on a position of a visual focal point of acquired artifacts that are generated by the compact linear metallic object in a mapping reconstructed from the magnetic resonance data.

2. The method of claim 1, wherein the compact linear metallic object is spherically symmetrical, and the sampling is a three-dimensional (3D) sampling of a volume with the compact linear metallic object.

3. The method of claim 1, wherein the compact linear metallic object is a metallic object that is compact in two dimensions, wherein at least two two-dimensional (2D) samplings take place in two planes spaced apart along an axis of a longitudinal extension of the compact linear metallic object, and wherein the two planes intersect the compact linear metallic object.

4. The method of claim 1, wherein the magnetic resonance data is acquired for the plurality of samplings along radial trajectories, and wherein the plurality of trajectories are evenly distributed in an angle over a full circle in a plane through a center of the k-space or over a sphere about the center of the k-space.

5. The method of claim 1, wherein the plurality of trajectories are sampled in both directions.

6. A magnetic resonance tomography unit for localizing a metallic object that is compact in three dimensions or a linear metallic object that is compact in two dimensions, wherein the magnetic resonance tomography unit is configured to: excite nuclear spins in a region surrounding the compact linear metallic object using an excitation pulse; acquire magnetic resonance data with samplings along a plurality of trajectories; dephase the nuclear spins using a gradient; carry out the samplings using a bSSFP sequence; ascertain a position of a geometric focal point of the compact linear metallic object based on a position of a visual focal point of an acquired artifact that is generated by the compact linear metallic object in a mapping reconstructed from the magnetic resonance data.

7. The magnetic resonance tomography unit of claim 6, wherein the sampling is a three-dimensional (3D) sampling of a volume with the compact linear metallic object.

8. The magnetic resonance tomography unit of claim 6, wherein the magnetic resonance tomography unit is further configured to carry out at least two two-dimensional (2D) samplings in two planes spaced apart along an axis of a longitudinal extension of the compact linear metallic object, and wherein the planes intersect the compact linear metallic object.

9. The magnetic resonance tomography unit of claim 6, wherein the magnetic resonance tomography unit is configured to acquire the magnetic resonance data for a plurality of samplings along radial trajectories, and wherein the plurality of trajectories are evenly distributed in an angle over a full circle in a plane through a center of the k-space or over a sphere about the center of the k-space.

10. The magnetic resonance tomography unit of claim 6, wherein the magnetic resonance tomography unit is further configured to sample the plurality of trajectories in both directions.

11. In a non-transitory computer-readable storage medium that stores instructions executable by a magnetic resonance tomography unit for localizing a metallic object that is compact in three dimensions or a linear metallic object that is compact in two dimensions, the instructions comprising: exciting nuclear spins in a region surrounding the compact linear metallic object using an excitation pulse; and acquiring magnetic resonance data with samplings along a plurality of trajectories, wherein the nuclear spins are dephased by a gradient, wherein the samplings take place using a bSSFP sequence, wherein a position of a geometric focal point of the compact linear metallic object is ascertained based on a position of a visual focal point of acquired artifacts that are generated by the compact linear metallic object in a mapping reconstructed from the magnetic resonance data.

12. The non-transitory computer-readable storage medium of claim 11, wherein the compact linear metallic object is spherically symmetrical, and the sampling is a three-dimensional (3D) sampling of a volume with the compact linear metallic object.

13. The non-transitory computer-readable storage medium of claim 11, wherein the compact linear metallic object is a metallic object that is compact in two dimensions, wherein at least two two-dimensional (2D) samplings take place in two planes spaced apart along an axis of a longitudinal extension of the compact linear metallic object, and wherein the two planes intersect the compact linear metallic object.

14. The non-transitory computer-readable storage medium of claim 11, wherein the magnetic resonance data is acquired for the plurality of samplings along radial trajectories, and wherein the plurality of trajectories are evenly distributed in an angle over a full circle in a plane through a center of the k-space or over a sphere about the center of the k-space.

15. The non-transitory computer-readable storage medium of claim 11, wherein the plurality of trajectories are sampled in both directions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows a schematic representation of an embodiment of a magnetic resonance tomography unit for carrying out a method;

[0037] FIG. 2 shows a schematic flow diagram of an embodiment of a method;

[0038] FIG. 3 shows a schematic example of a three-dimensional (3D) sampling scheme according to an embodiment in the k-space;

[0039] FIG. 4 shows a schematic flow diagram of an embodiment of the method; and

[0040] FIG. 5 shows a schematic example of a two-dimensional (2D) sampling scheme.

DETAILED DESCRIPTION

[0041] FIG. 1 shows a schematic representation of an embodiment of a magnetic resonance tomography unit 1 for carrying out a method according to the present embodiments.

[0042] A magnet unit 10 has a field magnet 11 that produces a static magnetic field B0 for aligning nuclear spins of samples or of a patient 100 in an acquisition region. The acquisition region is characterized by an extremely homogeneous static magnetic field B0. The homogeneity relates, for example, to the magnetic field strength or magnitude. The acquisition region is approximately spherical and located in a patient tunnel 16 that extends through the magnet unit 10 in a longitudinal direction 2.

[0043] A patient couch 30 may be moved inside the patient tunnel 16 by the travel unit 36.

[0044] Typically, the field magnet 11 is a superconducting magnet that may provide magnetic fields having a magnetic flux density of up to 3 T or even more in more recent devices. For lower field strengths, it is, however, possible to also use permanent magnets or electromagnets having normally conductive coils.

[0045] The magnet unit 10 also has gradient coils 12 that are configured to superimpose variable magnetic fields in three spatial directions on the magnetic field B0 for the purpose of spatial discrimination of the acquired mapping regions in the examination volume. The gradient coils 12 may be coils made of normally conductive wires that may generate mutually orthogonal fields in the examination volume.

[0046] The magnet unit 10 also has a body coil 14 that is configured to radiate a radiofrequency signal supplied via a signal line into the examination volume, to receive resonance signals emitted by the patient 100, and to output the resonance signals via a signal line. The term transmit antenna denotes below an antenna via which the radiofrequency signal is emitted for exciting the nuclear spins. This may be the body coil 14, but may also be a local coil 50 having a transmit function.

[0047] A control unit 20 supplies the magnet unit 10 with the various signals for the gradient coils 12 and the body coil 14, and analyzes the received signals.

[0048] Thus, the control unit 20 has a gradient controller 21 that is configured to supply the gradient coils 12 via supply lines with variable currents that provide, coordinated in time, the desired gradient fields in the examination volume.

[0049] In addition, the control unit 20 has a radiofrequency unit 22 that is configured to produce a radiofrequency pulse having a predefined variation over time, amplitude, and spectral power distribution for the purpose of exciting magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range may be achieved in this case. The excitation signals may be radiated via the body coil 14 or via a local transmit antenna into the patient 100.

[0050] A controller 23 communicates via a signal bus 25 with the gradient controller 21 and the radiofrequency unit 22.

[0051] A local coil 50 is arranged on the patient 100 and is connected to the radiofrequency unit 22 and its receiver via a connecting lead 33. In one embodiment, however, the body coil 14 may be a receiving antenna.

[0052] A compact metallic object 70 (e.g., a biopsy needle) may be inserted into the patient 100. There may, however, also be objects 70 in the body of the patient 100 that are already present there and the location of which is to be acquired. One example of such objects are seeds (e.g., capsules that contain a radiation source and are used for radiotherapy). Seeds may be reference markers for an X-ray examination or an irradiation controlled by X-ray acquisition. The precise geometric location relative to a tumor may be decisive. The object 70 is metallic and generates artifacts during magnetic resonance imaging, as alternating electromagnetic fields are attenuated. The metal also causes a change in the static or semistatic magnetic fields (e.g., the magnetic field B0 and the gradient fields) on account of its different susceptibility in comparison with the surrounding region. As a result, same magnetic field magnitudes occur at a number of (e.g., several) different geometric positions in the region surrounding the object and are mapped onto a common point via position encoding. In conventional image acquisition, the region surrounding the object is distorted by the variation in the magnetic field and transposed to a changed location relative to the surrounding organs.

[0053] The magnetic resonance tomography unit 1 may also be integrated via a signal connection to an external signal processing resource such as a cloud 80 as part of the system in which parts of the method according to the present embodiments are carried out.

[0054] FIG. 2 shows a schematic flow diagram of an embodiment of the method.

[0055] In act S10, the magnetic resonance tomography unit uses an excitation pulse to excite nuclear spins at least in a region surrounding the compact metallic object. The excitation takes place according to the present embodiments in accordance with the bSSFP sequence used.

[0056] The bSSFP sequence is characterized in that the zeroth gradient moment on all axes is reversed again (e.g., by all gradients that are played out after the excitation pulse being played out with opposite polarity before the next excitation).

[0057] In a further act S20, the magnetic resonance tomography unit 1 acquires magnetic resonance data by sampling along a radial trajectory 90.

[0058] The nuclear spins are dephased in act S21 by a gradient in order to hide the background of the body. Only the region immediately surrounding the object is not affected by the magnetic field changes produced by the susceptibility difference, as the duration and/or strength of the gradient is selected precisely so that the duration and/or strength of the gradient dephase the magnetic resonance signals in the unaffected, more remote tissue.

[0059] In act S22, the magnetic resonance tomography unit acquires MR signals with a sampling scheme along a radial trajectory 90 in the k-space.

[0060] Further, in act S23, the magnetic resonance tomography unit 1 acquires MR signals with a sampling scheme along the radial trajectory 90 in the opposite direction in the k-space.

[0061] FIG. 3 schematically shows radial trajectories 90 in the k-space. The trajectories 90 extend through the center or the coordinate point of origin of the k-space. FIG. 3 also shows a three-dimensional (3D) sampling. A sampling takes place in all three spatial coordinates of the k-space and not just in one plane. Trajectories 90 along the coordinate axes are specified here by way of example. The sampling may, however, take place with any radial trajectories 90 that span a three-dimensional space for the 3D sampling or a two-dimensional (2D) space for the 2D sampling in the k-space. The number of the trajectories 90 may be greater (e.g., three times, ten times, or more), and the trajectories are distributed equally over all spatial directions or directions in the plane.

[0062] In one embodiment, as already described, radial trajectories 90 that are distributed evenly over a circle or a full sphere result in symmetrical delocalized artifacts in an image reconstructed therefrom, which cancel each other out in the image during focusing. This effect may, however, also be achieved for non-radial (e.g., Cartesian) trajectories if these are sampled in both directions in each case along the trajectory 90 during acquisition of the magnetic resonance signals.

[0063] An even distribution of the directions of the trajectory over a circle or a sphere enables a time saving with higher spatial resolution compared to sampling in opposite directions through acquisition of a number of (e.g., several) different k-space rows.

[0064] The acts S22 and S23 are repeated for each trajectory 90.

[0065] Then, in act S30, image reconstruction takes place with the acquired MR signals in the magnetic resonance tomography unit 1 or on an external device in the cloud 80. The generated images show the metallic object essentially without background through the surrounding tissue on account of the white marker gradient or the dephasing of the object 70, albeit with the already mentioned artifacts such as distortion and dislocation.

[0066] Finally, in act S40, a focal point of the mapping of the object 70 is determined. A weighting factor may be applied to the position vectors, corresponding, for example, to a brightness value of the corresponding pixel or voxel. A threshold value function that considers only image points with an amplitude greater than a predetermined threshold value may also be provided.

[0067] Radial trajectories 90 that are distributed evenly over a circle or sphere in the k-space also result in a distortion or dislocation in the image space that is distributed evenly over the directions, so that during the subsequent averaging, when the focal point is determined, these cancel each other out and are eliminated without quantitative knowledge of the distortion and dislocation when the focal point is determined.

[0068] A comparable effect may also be achieved through sampling of the trajectory 90 in both directions. In the bSSFP sequence used, this may result in a symmetrically mirrored distortion or dislocation in the image space, so that during the subsequent averaging, when the focal point is determined, these cancel each other out and are eliminated without quantitative knowledge of the distortion and dislocation when the focal point is determined.

[0069] The object 70 may then be output to a user by the magnetic resonance tomography unit with the actual position ascertained (e.g., in a mapping), or also as a navigation aid in optical, acoustic, or tactile form.

[0070] The sequence shown in FIG. 2 corresponds essentially to the method according to the present embodiments for a compact object 70 having essentially the same dimensions and, for example, also symmetries in all spatial directions (e.g., a sphere, a polyhedron, or also irregularly shaped compact bodies).

[0071] FIG. 4 shows a flow diagram of an embodiment of the method for a metallic object 70, as shown in FIG. 5. The object 70 in FIG. 5 extends along an axis 71. The extension along the axis is significantly greater than the dimensions of the object 70 at right angles to the axis 71. In the context of the present embodiments, an object 70 of this kind is referred to as linear and compact in two dimensions. This is apparent, for example, from the method sequence in FIG. 4, which corresponds to a large extent to that for the compact object 70 in three dimensions.

[0072] A linear object 70 of this kind may be determined precisely in its orientation if the axis 71, which is a straight line, is determined precisely by two points. This may be achieved in that intersection points of the linear object 70 with two planes, which are spaced apart from one another, are ascertained.

[0073] The magnetic resonance tomography unit 1 is to ascertain these two planes in act S05. The magnetic resonance tomography unit 1 may, for example, have stored data from a treatment plan or from a user input. The stored data specifies a rough, error-prone location and position of the object 70. In one embodiment, the magnetic resonance tomography unit determines the location roughly with an error-prone conventional sequence of the magnetic resonance tomography unit 1. Based on this location, the magnetic resonance tomography unit 1 may use linear algebra means to ascertain a first plane 72 and a second plane 73 that, with certainty, have in each case an intersection point with the object 70 (e.g., by taking into consideration safety distances of the planes to the extremal coordinates of the object 70). The planes may be parallel to one another and at right angles to the axis 71, but other arrangements may also be provided as long as the planes do not coincide and are not aligned parallel to the axis 71.

[0074] In act S10, the magnetic resonance tomography unit uses an excitation pulse to excite nuclear spins in a region surrounding the compact metallic object. The excitation takes place according to the present embodiments in accordance with the bSSFP sequence used. In contrast to the method from FIG. 2, only 2D samplings of the planes take place in this embodiment of the method. The excitation pulse for the planes 72, 73 may be a slice excitation pulse under corresponding gradient fields for selection of a slice in the respective plane.

[0075] In a further act S50, the magnetic resonance tomography unit 1 acquires magnetic resonance data by sampling along a trajectory 90 in the first plane 72, as shown in FIG. 5. The trajectories 90 in the first plane 72 and the second plane 73 are accordingly likewise arranged in a plane in the k-space.

[0076] As already explained with reference to FIG. 2, the trajectories 90 may be radial trajectories 90 and may be distributed evenly, for example, over a full circle about the zero point in the plane 72, 73 in order to achieve the advantageous effect of symmetry of the artifacts in the mapping. Alternatively, this advantageous effect may also be achieved in a plane 72, 73 in that the trajectory 90 is sampled in each case in both directions, even if it is not a radial trajectory 90 but instead runs for example parallel to the Cartesian coordinates.

[0077] The nuclear spins are, for example, dephased in act S51 using a gradient in the first plane 72 in order to hide the background of the body. Only the region immediately surrounding the object is not affected by the magnetic field changes produced by the susceptibility difference, as the duration and/or strength of the gradient is selected precisely so that the duration and/or strength of the gradient dephase the magnetic resonance signals in the unaffected, more remote tissue.

[0078] In act S52, the magnetic resonance tomography unit acquires MR signals with a sampling scheme in the first plane 72 along a radial trajectory 90 in the k-space.

[0079] Further, in act S53, the magnetic resonance tomography unit acquires MR signals with a sampling scheme along the radial trajectory 90 in the opposite direction in the k-space.

[0080] The acts S52 and S53 are repeated for each trajectory 90 in the first plane 72.

[0081] Then, in act S60, image reconstruction takes place with the ascertained MR signals in the magnetic resonance tomography unit 1 or on an external device in the cloud 80 (e.g., a 2D image reconstruction for the first plane 72). The generated images show the metallic object essentially without background through the surrounding tissue on account of the white marker gradient or the dephasing of the object 70, albeit with the already mentioned artifacts such as distortion and dislocation.

[0082] In act S70, a focal point of the mapping of the object 70 in the first plane 72 is determined. A weighting factor may be applied to the position vectors, corresponding, for example, to a brightness value of the corresponding pixel. A threshold value function that considers only image points with an amplitude greater than a predetermined threshold value may also be provided.

[0083] In the bSSFP sequence used, the even distribution of the radial trajectories 90 or the acquisition of the trajectories 90 in both directions may result in a symmetrically mirrored distortion or dislocation in the image space, so that during the subsequent averaging, when the focal point is determined, these cancel each other out and are eliminated without quantitative knowledge of the distortion and dislocation when the focal point is determined.

[0084] Acts S50 to S70 are repeated as acts S80, S81, S82, S83, S90, and S100 in the same manner for the second plane 73. In one embodiment, the acts S50 to S70 and S80 to S100 may be carried out in parallel in a multislice acquisition for the first plane 72 and the second plane 73.

[0085] Two positions of the linear object 70 are determined with the focal points in both planes. An alignment may thus be determined as a straight line between the points and output to the user (e.g., in a representation of the patient or via an interface as an optical, acoustic, or tactile navigation aid).

[0086] The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.

[0087] While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.