Passive field camera and method for operating a passive field camera
阅读说明：本技术 无源场照相机和运行无源场照相机的方法 (Passive field camera and method for operating a passive field camera ) 是由 S.坎宁吉瑟 R.雷纳 S.波派斯库 G.鲁伊特斯 M.维斯特 于 2020-03-11 设计创作，主要内容包括：本发明涉及一种无源场照相机和运行无源场照相机的方法。本发明涉及一种场照相机以及利用磁共振断层成像设备和场照相机测量磁场分布的方法。场照相机具有分布在待测定的空间体积上的数量M个样品和数量N个接收天线。在所述方法的一个步骤中,利用磁共振断层成像设备,针对每个接收天线处的每个样本,采集接收天线的灵敏度矩阵。在另一个步骤中,利用磁共振断层成像设备,借助N个接收天线,采集待测量的磁场中的M个样品的N个天线信号。最后,利用控制器M,依据灵敏度矩阵,根据N个天线信号,确定各个样品的M个磁共振信号。在另一个步骤中,可以根据磁共振信号,来确定样品的位置处的磁场强度。(The invention relates to a passive field camera and a method of operating a passive field camera. The present invention relates to a field camera and a method for measuring a magnetic field distribution using a magnetic resonance tomography apparatus and a field camera. The field camera has a number M of samples and a number N of receiving antennas distributed over the volume of space to be determined. In one step of the method, a sensitivity matrix of the receiving antennas is acquired for each sample at each receiving antenna with a magnetic resonance tomography apparatus. In a further step, N antenna signals of M samples in the magnetic field to be measured are acquired by means of N receiving antennas with a magnetic resonance tomography apparatus. Finally, the controller M is used to determine M magnetic resonance signals for each sample from the N antenna signals according to the sensitivity matrix. In a further step, the magnetic field strength at the location of the sample may be determined from the magnetic resonance signals.)
1. A field camera for recording a magnetic field distribution by means of magnetic resonance measurement, wherein the field camera (60) has:
a number M of samples (61) distributed over a volume of space (70) to be determined;
a number N of receiving antennas (62), wherein each receiving antenna (62) has a receiving volume (64) and is arranged relative to the spatial volume (70) to be determined such that at least two of the M samples (61) are arranged in one of the receiving volumes (64) and the receiving volumes (64) do not intersect at least in part, and at least one sample (61) is arranged in each receiving volume (64), wherein the number N of receiving antennas (62) is greater than or equal to the number M of samples (61).
2. The field camera as defined in claim 1, wherein the N receive antennas (62) at least partially circumferentially surround the volume of space (70) to be determined.
3. The field camera as claimed in one of the preceding claims, wherein at least one sample (61) has an inductively coupled first resonant circuit at the larmor frequency.
4. The field camera as defined in claim 3, wherein the sample (61) has a resonant circuit, wherein the first resonant circuit has a coil and a capacitance, wherein the capacitance is formed by twisted insulated conductor ends of the coil.
5. The field camera of claim 3 or 4, wherein the first resonant circuit has two different resonant frequencies.
6. The field camera of any of claims 3-5, wherein the field camera has a second resonant circuit, wherein the first resonant circuit is inductively coupled with the second resonant circuit, and the second resonant circuit has a larger inductive surface than the first resonant circuit.
7. A method of measuring a magnetic field distribution with a magnetic resonance tomography apparatus (1) and a field camera (60) according to claim 1 or 2, wherein the method has the steps of:
(S100) with the magnetic resonance tomography apparatus (1), a sensitivity matrix of the receiving antennas (62) is acquired with a sensitivity E for each sample (61) m at each receiving antenna (62) nmn；
(S200) using the magnetic resonance tomography device (1), N antenna signals A of M samples (61) in the magnetic field to be measured are acquired by means of N receiving antennas (62)n；
(S300) with the controller (23), depending on the sensitivity matrix EmnFrom N antenna signals AnDetermining M magnetic resonance signals S of each samplem。
8. The method according to claim 7, wherein the method further has the steps of:
(S310) determining a sensitivity matrix EmnInverse matrix of (I)nm；
(S320) by dividing the vector ANAnd the inverse matrix InmMultiplying to determine the magnetic resonance signal S of each samplem。
9. The method according to claim 7 or 8, wherein the magnetic resonance tomography apparatus (1) has a gradient system and the field camera (60) is arranged in the magnetic resonance tomography apparatus (1), wherein the step of acquiring sensitivity matrices (S100) further has the sub-steps of:
(S110) determining magnetic field gradients under which each sample (61) is subjected to a different magnetic field;
(S120) in the acquisition sensitivity matrix EmnDuring this, the determined magnetic field gradients are generated by means of the gradient system.
10. The method of claim 7, wherein the step of acquiring sensitivity matrices (S100) further has the sub-steps of:
(S130) weighting the antenna signals with a time dependent window function to sharpen the spectral distribution during the acquisition of the sensitivity matrix.
11. A computer program product directly loadable into a processor of a programmable controller (23), having program code means for performing all the steps of the method according to any of claims 7 to 10 when said program product is executed on said controller (23).
12. A computer-readable storage medium on which electronically readable control information is stored, which electronically readable control information is designed to carry out the method according to one of claims 7 to 10 when the storage medium is used in a controller (23) of a magnetic resonance tomography apparatus (1) with a field camera according to one of claims 1 to 6.
Technical Field
The invention relates to a field camera for acquiring a magnetic field distribution by means of magnetic resonance measurement using a plurality of samples distributed over a spatial volume, and to a method for performing field measurement using a field camera and a magnetic resonance tomography apparatus.
Background
A magnetic resonance tomography apparatus is an imaging device that aligns nuclear spins of an examination subject with an external strong magnetic field and excites the nuclear spins to precess around the alignment by an alternating magnetic field in order to image the examination subject. Precession or return of spins from this excited state to a state with less energy in response in turn produces an alternating magnetic field, which is received via an antenna.
These signals are position-encoded by means of gradient magnetic fields, which position-encoding subsequently enables the received signals to be correlated with the volume elements. The received signals are then analyzed and a three-dimensional imaging representation of the examination object is provided.
The quality of the generated image depends to a large extent on the homogeneity of the static magnetic field and the linearity of the gradient field used for spatial coding. These are also affected by dynamic effects, such as eddy currents, due to the rapidly alternating gradient fields. The device for spatially capturing the magnetic field is also referred to as a field camera.
For example, a method and a device are known from DE 102014213413, which use at least one field probe to measure a magnetic field in a magnetic resonance tomography apparatus, to which at least one erase signal can be applied in order to reduce the residual magnetization in the at least one field probe (FS).
For the analysis of the signals of the field probes, field cameras to date have individual antenna coils and individual receivers at the field probes, which means a considerable outlay for increasing the spatial resolution in the case of a plurality of field probes.
Disclosure of Invention
The object of the invention is to provide a magnetic field measurement in a magnetic resonance tomography apparatus in a simpler and less costly manner.
The above-described object is achieved by a field camera according to the invention and a method according to the invention for operating a magnetic resonance tomography apparatus having a field camera.
The field camera according to the invention has a number M of samples. The volumes in which the medium whose nuclear spins have magnetic resonance is present are referred to as samples. In the simplest case, this may be, for example, a bulb filled with water. Preferably the material of the bulb has no or only little diamagnetism or paramagnetism, such as polystyrene (Styropor), and is also formed as a spherical shell or ellipsoid to minimize field distortion of the magnetic field. Preferably the sample has a small volume relative to the volume of space to be determined, for example 10, 50, 100 or 1000 times smaller than the volume of space to be determined.
The sample is distributed over the volume of space to be determined. This is to be understood as meaning, for example, that the spatial volume is divided into M disjoint sub-volumes of equal size and in each of the sub-volumes only one sample is arranged in each case. It is also conceivable that the distance between two adjacent samples is not less than a predetermined minimum distance. In this case, the minimum distance is preferably more than 5, 10, 20 or 100 times greater than the maximum extent of the sample in one spatial direction. A "distribution" can also be regarded as only one sample being arranged in each case in a respective one of the receiving volumes described below. It is also conceivable to arrange the sample around the volume of space or on one or more concentric shells therein, for example spherical shells. The field equation based on the magnetic field makes it possible to deduce the magnetic field inside the housing from the measured values of the housing and the arrangement in the housing in an advantageous manner, so that the determination of the inverse matrix described below for the purpose of deducing the magnetic resonance signals of the individual samples from the signals of the receiving antennas is easier. In a preferred embodiment, there is at least one axis of projection for which, suitably in a plane perpendicular to the axis, only one sample is always arranged.
The field camera has a number N of receiving antennas for the magnetic resonance signals. The receiving antenna can be, for example, an antenna coil, such as is also used in local coils. The receiving antennas each have a receiving volume. The volume in which the magnetic resonance signals generated by the sample in the receiving antenna are not attenuated by more than 6dB, 12dB, 18dB, 40dB or 60dB relative to the maximum level that can be generated by the sample is considered here as the receiving volume. In the case of a circular antenna coil, the maximum magnetic resonance signal is achieved, for example, with the sample in close proximity to the coil conductor.
The receiving antennas are arranged relative to the spatial volume to be determined such that in at least one of the receiving volumes two of the M samples are arranged, wherein the receiving volumes do not intersect at least partially. In each of the receiving volumes, there is at least one sample. For example, the sample may be present in a cube or cylinder, arranged in a grid, with receiving antennas arranged in all three spatial directions on its outside. For example, it is conceivable that a prosthesis (Phantom) composed of structural elements in which a sample is distributed is arranged as a field camera in the head coil.
Here, the number N of receiving antennas is greater than or equal to the number M of samples.
In an advantageous manner, the field camera according to the invention makes it possible to reconstruct the magnetic resonance signals of the M samples from the N receive signals by means of N receive antennas arranged around the M samples, with the condition N > ═ M, thus making it possible to determine the magnetic field at M positions in the volume of space to be determined, as will be explained in more detail in the method below. The field camera according to the invention also enables the use of already existing local coils as part of the field camera, thus reducing the overhead.
The method according to the invention is used for measuring a magnetic field distribution with a magnetic resonance tomography apparatus and a field camera according to the invention. In one step of the method, the signal of the sample m received by each receiving antenna n is measured by measuring, for this antenna, the signal of this antennaSensitivity EmnTo determine the sensitivity matrix of the receiving antenna. This sensitivity can be given relative to the other antennas, or absolutely, for example, with a predetermined excitation of the sample m, the received magnetic resonance signal a of the receiving antenna nnThe microvolt (mikroolt) amplitude gives this sensitivity. The amplitude values are preferably measured with receivers of the magnetic resonance tomography apparatus, which are also used for image reconstruction. It is also conceivable to use a separate receiver for this purpose.
For this purpose, it is necessary to separate or differentiate the signals of the individual samples when the sensitivity matrix is acquired. For example, it is contemplated that upon receiving magnetic resonance signals, the frequencies of the individual samples are separated by gradients of the magnetic field, as will be described further below, whereby the signals can be distinguished. However, it is also conceivable that, if the individual samples are arranged in a manner adapted to the direction of the gradient, only the individual samples are selectively excited by means of a single gradient field when the nuclear spins are excited.
Finally, the selection of the individual samples can also be effected in a targeted manner by arranging suitable activation devices on the samples. For example, coils at the sample can be used for selection, with which additional local static magnetic fields are generated or coherent excitation is destroyed by an alternating magnetic field. It is also conceivable to selectively block the excitation or the radiation of the MRT signal by means of a switchable shielding at the respective sample.
It is also conceivable to determine the sensitivity of the individual receiving antennas by image acquisition of the samples with the receiving antennas and the body coil respectively, and to determine the relative sensitivity of the receiving antennas for the individual samples from the ratio of the intensity values for the individual samples.
Here, acceleration can be achieved if, instead of a complete 3-dimensional acquisition, only one or more projections from one or more different directions into two dimensions or even only one dimension are acquired and their intensity values are analyzed. The projection can be achieved by applying no magnetic field gradients, or only one magnetic field gradient (e.g., Gx or Gy) while the MR signals are acquired. That is, a complete scan of k-space is omitted. Thereby, phase encoding for scanning the entire space can be omitted.
If the magnetic resonance signals of the individual samples can be distinguished or only one sample is excited accordingly, the absolute or relative sensitivity of the N receiving antennas for the acquisition of the magnetic resonance signals of the sample m can be determined for each sample m. In this way, the sensitivity matrices E can be acquired in parallel or also sequentiallymnThe coefficient of (a).
In a further step of the method according to the invention, N antenna signals a of M samples in the magnetic field to be measured are acquired by means of N receiving antennasn. And for determining the sensitivity matrix EmnThe measurement signals are different, so that it is no longer necessary to acquire the magnetic resonance signals differently in frequency or sequentially. The acquisition of the antenna signals is in turn preferably carried out with a receiver of the magnetic resonance tomography apparatus.
In a further step of the method according to the invention, the sensitivity matrix E is relied uponmnFrom N antenna signals AnTo determine M magnetic resonance signals S of each samplem. It involves a magnetic resonance signal S from the samplemOpposite equation group An＝Emn×SmAnd (6) solving. Various advantageous methods for performing the solution are described in the following description and in the accompanying drawings.
The frequency of the separated magnetic resonance signals of the sample then provides a measure of the local static magnetic field at the location of the sample. The results may then be displayed or used to calibrate subsequent measurements.
In an advantageous manner, it is made possible to carry out the method according to the invention in parallel and thus quickly without additional equipment, with a relatively simple arrangement of the samples in the structure and in the local coil matrix, for example a head coil (header). Here, as the structural body, a material having no particularly unique electric or magnetic characteristics, that is, a relative dielectric constant and a magnetization constant is preferably used
Further advantageous embodiments are given below.
In a possible embodiment of the field camera according to the invention, the N receiving antennas surround the spatial volume to be determined at least partially on the outer circumference. The volume of space to be determined here has at least a volume surrounding the sample, in other words a continuous space in which the sample is arranged. In this case, in the sense of the present invention, the receiving antennas are arranged around the sample on at least three of the six cartesian spatial coordinates, considered as being at least partially enclosed on the outer circumference. For example, it is conceivable that the spatial volume describes a cuboid or a sphere which is arranged along the z-axis of the magnetic resonance tomography apparatus or in the direction of the B0 field and the receiving antennas are arranged on the outer walls in the +/-x direction and the +/-y direction. For example in a grid in which the sample is arranged or preferably on one or two concentric spherical shells. Two concentric spherical shells are also conceivable. A cylinder with a housing formed by a rolled antenna array is also possible. The receiving antennas can also be arranged, for example, on the surface of a surrounding prism or cube in all 6 directions of space.
By arranging the receiving antennas on the outer circumference, it is advantageously ensured that all samples are taken by the receiving antennas and that all samples are also weighted differently, so that the sensitivity matrix is not underdetermined (unsubscribed).
In one conceivable embodiment of the field camera according to the invention, the at least one sample has a first resonant circuit which is inductively coupled at the larmor frequency. The sample has a first resonance circuit it is to be understood that the first resonance circuit mainly interacts with one of the samples where the first resonance circuit is arranged. For example, the signal of the associated sample produces a resonance signal in the resonant circuit that exceeds the signal induced by the adjacent sample under the same excitation by more than 6dB, 12dB, or 18 dB. It is important here that the interaction of the resonant circuit with different samples is different in order to be able to distinguish it from the sensitivity matrix. This is achieved, for example, by the first resonant circuit having an inductance in the form of a coil which surrounds the sample at a distance which is smaller than the dimension of the sample in the plane of the coil. The larmor frequency is considered here to be the frequency at which the nuclear spins of the sample have magnetic resonance signals in the static B0 field of the magnetic resonance tomography apparatus to be measured, for example at 1.5T, 3T, 7T. To achieve resonance, the resonant circuit may have a capacitance in addition to the coil having its own capacitance.
In an advantageous manner, the first resonant circuit enhances the signal of the sample by means of a local increase in the field strength of the excitation pulse and a resonance enhancement of the magnetic resonance signal upon reception, so that magnetic field measurements can be carried out more quickly and more reliably.
In one conceivable embodiment, the field camera has a second resonant circuit having the same resonant frequency as the first resonant circuit. The first resonant circuit is inductively coupled to the second resonant circuit, and the second resonant circuit has a larger inductive surface than the first resonant circuit. In particular, inductive coupling is considered here if, in the case of a homogeneous passage of the alternating magnetic field through the two resonant circuits, the amplitude of the current in the first resonant circuit at the resonant frequency of the two resonant circuits exceeds the amplitude in the first resonant circuit without the second resonant circuit by 3dB, 6dB, 12dB or more. In this case, frequencies are considered to be identical resonant frequencies, which in the coupled system produce an amplitude in the respective resonant circuit that is attenuated by less than 12dB, 6dB or 3dB compared to the amplitude produced at the free natural resonant frequency of the resonant circuit with the same alternating field. In this case, it is also conceivable for the first resonant circuit to have a different spatial orientation relative to the second resonant circuit, for example for the angle between the normal vector of the antenna coil of the first resonant circuit and the normal vector of the antenna coil of the second resonant circuit to be greater than 10 degrees, 20 degrees or 30 degrees. It is also conceivable that a plurality of first resonant circuits inductively couple a plurality of samples with a common second resonant circuit.
Advantageously, the second resonant circuit, due to its larger area, can improve the coupling between the transmitting antenna for the excitation pulse and the sample and/or the coupling between the receiving antenna and the sample, so that the temporal resolution can be improved by shorter measurement times or the spatial resolution can be improved by smaller samples. Furthermore, in the case of different alignments, the coupling to the excitation pulses can be improved, which in the worst case may tend to zero if the field vector of the excitation field is parallel to the plane of the antenna coil of the first resonant circuit.
In one possible embodiment of the local coil according to the invention, the resonance circuit has a coil and a capacitance, wherein the capacitance is formed by twisted insulated conductor ends of the coil.
The lacquered stranded conductor ends form a high-quality, small capacitor for the resonant circuit by the opposing conductors insulated from one another in the capacitor region, which does not require additional mechanically weak solder joints and materials that could falsify the magnetic resonance measurement.
In this case, it is also conceivable, for example, for the resonant circuit to have two resonant circuits coupled to one another for different resonant frequencies, the resonant circuit having two resonant frequencies.
In an advantageous manner, the double-resonant circuit makes it possible to provide one field camera for two different B0 magnetic field strengths, for example 1.5T and 3T.
In one possible embodiment, the method according to the invention has the determination of the sensitivity matrix EmnInverse matrix of (I)nmThe step (2).
The term "Inverse matrix" is not limited to the more strict mathematical concepts for a square matrix with M ═ N, but includes, for example, a so-called pseudo-Inverse matrix with N > M, such as Moore-Penrose-Inverse (Moore-Penrose-Inverse).
For this purpose, a sensitivity matrix E is requiredmnThe system of equations of (a) is not underdetermined. In the case of a square matrix, this is given if the determinant is not equal to zero. This can be done by sample and interfaceA suitable distribution of the receiving antennas is achieved in such a way that in two different receiving antennas, different signals or signal combinations of the same sample are acquired. For this reason, the number N of receiving antennas needs to be at least as large as the number M of samples.
If the system of equations is overdetermined, i.e. the number of receiving antennas N is greater than the number of samples M, and the samples are arranged relative to the receiving antennas such that the same signal combination from the samples is acquired in different receiving antennas, the sensitivity matrix E can be determined from the singular value decomposition or boundary methodmnTo determine a pseudo-inverse matrix Inm. Here, a solution having a minimum distance and an improved signal-to-noise ratio may be determined using a least squares method.
The inversion of the matrix can be carried out, for example, in a control unit of the magnetic resonance tomography apparatus, but a separate calculation unit is also conceivable.
In a further possible step of the method according to the invention, the vector A of the N antenna signals is usedNAnd inverse matrix InmMultiplying to recover the magnetic resonance signals S of the respective samples acquiredm. This calculation can also be performed by a control unit or an image reconstruction unit of the magnetic resonance tomography apparatus.
It is particularly advantageous here if the relative arrangement of the receiving antennas with respect to the sample is not changed, so that the inverse matrix can be applied repeatedly to subsequent measurements of the antenna signals. In this way, magnetic field measurements and analysis can be performed at a high repetition rate.
In principle, however, it is also conceivable to solve the system of equations again separately. For overdetermined systems of equations with such deficiencies in coefficients, there are a number of mathematical solutions. It is thus conceivable that, especially in the case of overdetermination of the system of equations, a solution can be found more quickly and/or more accurately in the specific case of a special input vector or a sensitivity matrix that is only partially occupied.
In a conceivable embodiment of the method according to the invention, the magnetic resonance tomography apparatus has a gradient system and the field camera is arranged in the magnetic resonance tomography apparatus. The step of acquiring the sensitivity matrix here also has the step of determining the field gradient with which each sample is subjected to a different magnetic field. This condition is met if more than one sample is not placed on any plane perpendicular to the gradient vector of the field gradient. This determination is already possible, for example, when the field camera is constructed by appropriately arranging the sample in space with respect to the predetermined gradient vectors. It is also conceivable, however, to determine the gradient vector for a given distribution of the sample. In this case, it is also conceivable to determine different gradient vectors.
In a further step of the method according to the invention, a sensitivity matrix E is acquired by means of a gradient systemmnDuring which, in particular during the nuclear spins, the determined field gradients are output.
However, rather than selectively exciting a single sample, it is also conceivable to obtain the sensitivity matrix via a combination of multiple projections of the sample along different axes onto one or two dimensions. This can be achieved by scanning k-space along only one axis, or by acquiring only the amplitude values.
In a possible embodiment of the method according to the invention, the step of acquiring the sensitivity matrix further has the sub-steps of: the antenna signals are weighted in the time domain with a time-dependent window function in order to sharpen the spectral distribution during the acquisition of the sensitivity matrix. For example, a decreasing exponential function or Hann-function (Hann-fusion) is conceivable.
By means of the field gradient the magnetic resonance signals of the individual samples are shifted to different larmor frequencies so that the signals become distinguishable. However, due to the limited length of the scanning window (Abtastfenster) and the duration of the magnetic resonance signals that can be acquired, the resolution in the Fourier domain may still be insufficient for separation. In an advantageous manner, a better resolution is achieved in the frequency domain by suitable weighting with a window function which also takes into account the exponentially decreasing signal strengths.
Drawings
The above described features, characteristics and advantages of the present invention and its implementation will become clearer and more easily understood in conjunction with the following description of the embodiments, which are described in detail in conjunction with the accompanying drawings.
Fig. 1 shows a schematic overview of a magnetic resonance tomography apparatus with a field camera according to the invention;
FIG. 2 shows a schematic diagram of an arrangement of samples of a field camera according to the invention;
FIG. 3 shows a schematic diagram of a receiving antenna of a field camera according to the present invention;
fig. 4 shows a schematic flow chart of a method according to the invention.
Detailed Description
Fig. 1 shows a schematic illustration of an embodiment of a magnetic resonance tomography apparatus 1 with a field camera 60 according to the invention.
The magnet unit 10 has a field magnet 11, and the field magnet 11 generates a static magnetic field B0 in the recording region for aligning the nuclear spins of the sample or patient. The recording region is characterized by an extremely homogeneous static magnetic field B0, wherein the homogeneity relates in particular to the magnetic field strength or magnitude. The recording area is almost spherical and is arranged in a patient tunnel 16, which patient tunnel 16 extends through the magnet unit 10 in the longitudinal direction 2. The moving unit 36 may move the patient bed 30 in the patient tunnel 16. The field magnet 11 is typically a superconducting magnet that can provide a magnetic field with a flux density of up to 3T, even higher with the latest devices. However, for smaller field strengths, permanent magnets or electromagnets with normally conducting coils can also be used.
Furthermore, the magnet unit 10 has a gradient coil 12, the gradient coil 12 being designed for superimposing the magnetic field B0 with the variable magnetic field in three spatial directions for spatial discrimination of an imaging region in the acquired examination volume. The gradient coils 12 are typically coils of normally conductive wire, which can generate mutually orthogonal fields in the examination volume.
The magnet unit 10 also has a body coil 14, the body coil 14 being designed for radiating high-frequency signals fed via signal conductors into the examination volume, receiving resonance signals emitted by the patient 100, and outputting via the signal conductors.
The control unit 20 supplies the magnet unit 10 with different signals for the gradient coil 12 and the body coil 14 and analyzes the received signals.
The control unit 20 therefore has a gradient controller 21, the gradient controller 21 being designed for supplying variable currents to the gradient coils 12 via the feed lines, the variable currents providing the desired gradient fields in the examination volume in a time-coordinated manner.
Furthermore, the control unit 20 has a radio-frequency unit 22, the radio-frequency unit 22 being designed for generating radio-frequency pulses having a predetermined temporal profile, amplitude and spectral power distribution for exciting magnetic resonances of the nuclear spins in the patient 100. In this case, pulse powers in the kilowatt range can be achieved. The excitation pulses may be radiated into the patient 100 via the body coil 14 or also via a local transmitting antenna.
The controller 23 communicates with the gradient controller 21 and the high-frequency unit 22 via a signal bus 25.
Instead of the patient, a field camera 60 according to the invention is arranged on the patient bed 30 to measure the magnetic field in the patient tunnel 16. As shown in fig. 2 and 3, the field camera 60 has a sample 61 and a receiving antenna 62, wherein the receiving antenna 62 is connected to the receiver signal of the high-frequency unit 22 via a connecting line 33. The signal connection may also be made wirelessly.
In fig. 2 a sample 61 of a field camera 60 according to the invention is schematically shown.
The field camera 60 has a number M of samples 61, which samples 61 are preferably distributed over the surface of the volume of space 70 to be determined. Sample 61 has an active material relative to nuclear magnetic resonance. Here, a hydrogen-containing sample 61, for example water or a hydrocarbon, is possible. The liquid sample 61 can be contained in a bulb (Kuvetten) or a vial (Phiolen). The sample 61 may be embedded in a matrix or structure that does not itself have nuclear magnetic resonance or that is active at other frequencies. Here, the size of the sample 61 is a compromise between spatial resolution and sensitivity. The larger the sample 61, the lower the spatial resolution.
The spatial volume 70 is shown here as a cube, so that the orientation of the spatial axes can be shown more easily. However, the volume of space 70 may also take any other shape, for example spheres, ellipsoids, cylinders, prisms or similar arrangements at least partially filling the recording area are also conceivable. One or more concentric spherical shells, in which the sample 61 is arranged, are also advantageous, because the internal field can be determined from the field on the surface of a passive or zero divergence (quelenfreie) volume, based on the magnetic field law.
In this case, it is advantageous for the method according to the invention to arrange the samples 61 such that always only one sample lies on a plane perpendicular to the gradient vector. This enables, in the method explained in relation to fig. 4, the sample 61 to be distinguished according to the larmor frequency under the influence of the magnetic gradient.
This can be achieved by arranging the samples 61 in a regular grid, the symmetry axes of the grid being suitably inclined with respect to the basic axes x, y and z of the gradient coil 12. However, a suitable random or regular distribution is also conceivable. Finally, it is also conceivable that, although the axis of the arrangement of the sample 61 is parallel to the axis of the gradient coil 12, the magnetic field gradients are generated by appropriately superimposing the magnetic fields of the gradient coil 12, in particular in such a way that conditions are met.
In order to speed up the measurement, it is also conceivable to arrange interference coils 63 around the respective sample 61, to which interference coils 63 direct currents or high-frequency currents can be applied in order to attenuate the still present excitation more quickly, so that the next measurement can be carried out more quickly.
Finally, as is also explained with regard to the method, the relative sensitivity of the respective receiving antenna 62 to the respective sample 61 can also be determined by a plurality of magnetic resonance imaging of the sample 61 with a projection onto a two-dimensional plane or a one-dimensional line.
For the sake of clarity, the receiving antenna 62 of the field camera 61 is shown by way of example in fig. 3 alone. The receiving antennas 62 each have a receiving volume 64, and the receiving antennas 62 are arranged relative to the spatial volume to be determined such that two of the M samples are arranged in at least one of the receiving volumes 64 and the receiving volumes 64 do not intersect at least in part, and at least one sample is arranged in each receiving volume. Here, the number N of receiving antennas is greater than or equal to the number M of samples.
One possible configuration of the receiving antenna 62 of the field camera 60 according to the invention is shown in fig. 3. Here, the receiving antenna 62 is exemplarily shown as an antenna coil. The spatial volume 70 is at least partially surrounded on the outer circumference by the receiving antenna 62. For the sake of clarity, the sample 61 inside the spatial volume 70 is not shown in fig. 3, but as shown in fig. 2, the sample 61 inside the spatial volume 70 is arranged inside the spatial volume 70.
In fig. 3, the receiving antenna 61 is arranged on the outer circumference of the hollow body. As shown, it may also be a head coil, for example, which receives magnetic resonance signals from the inner space by arranging the sample 61 to a volume of space 70, as in the known model, with 64 separate receiving coils as receiving antennas 62. Thus, by using a cost-effective passive (passiv) matrix with the sample 61 in the presence of the head coil, a field camera can be provided with little overhead.
Here, the idea of the present invention is that if the signals of the M samples 61 and the N antenna reception signals form a solvable system of linear equations, the signals of the respective samples 61 can be recovered from the reception signals of the reception antennas 62.
For this purpose, the number N of receiving antennas 62 needs to be greater than or at least equal to the number M of samples. Furthermore, the signals from all samples 61 must be received by at least one receiving antenna 62 accordingly. The receiving volume 64 of the receiving antenna 62 is exemplarily shown in fig. 3. The contour in this case gives the surface on which the signal from the sample 61 is attenuated by a predetermined value, for example by 6dB, 12dB, 18dB or 40dB, relative to the sample in the middle of the antenna coil. The receiving volume 64 is maximally limited by the distance the received signal falls below the noise level. In fig. 3, the receiving volume 64 of the receiving antenna 62 is a rod-like structure extending radially inwards. The radially outward rods (Keule) are not shown for clarity.
In order for the system of equations to be solvable at N ═ M, it is necessary that the receiving volumes 64 of the two receiving antennas 62 differ in the sense that, for the same sample 61, the two receiving antennas 62 provide the same or scaled signal levels by the same factor. But it is not necessary that the receiving antenna 62 and the sample 61 are spaced apart by 1: 1, some of the receiving antennas 62 may also cover a large receiving volume 64, while other receiving antennas 62 only detect a single sample 61.
In order to determine the system of linear equations between the signals of the samples 61 and the receiving antennas 62, one approach is to determine the received signals of all N receiving antennas 62 for all M samples 61, respectively for a single sample 61 in turn. This is possible, for example, if only one sample 61 is excited, or the signals are distinguishable.
This can be achieved, for example, by applying a magnetic field gradient in which each sample 61 is subjected to a further static magnetic field consisting of the magnetic field B0 and a gradient field. Only a single sample 61 can then be excited with a narrow band excitation pulse and the single signal of this sample 61 is analyzed by the respective receiving antenna 62.
Or in the case of broadband excitation, the signals of all samples 61 may be received with all receiving antennas 62 and the signals of all samples 61 may be separated according to different frequencies in the frequency domain. In the following, measures for performing an improved frequency separation are described with respect to the method according to the invention.
However, it is also conceivable, in the embodiment with interference coils 63 around the respective sample, to selectively excite the sample via these interference coils 63 with an excitation pulse accordingly.
That is, an exemplary head coil having a number of 64 receiving antennas 62 may incorporate up to 64 samples 61 to measure the magnetic field inside the head coil. For this purpose, as already described, only a matrix or a shaped body is required
It is also conceivable to improve the coupling of the sample 61 to the excitation pulse and/or the receiving antenna 62 by means of a resonance coil at the sample as first resonance circuit, thus improving the signal strength and the SNR. In this case, for example, by coupling two resonant circuits, the first resonant circuit can also have two different resonant frequencies, so that signal amplification can also take place in different static B0 fields.
Coupling can also be improved further by a two-stage design with a first resonant circuit directly at the sample and a second resonant circuit with a greater distance, but for this purpose also with a larger inductive surface of the antenna coil.
A schematic flow chart of a method according to the invention for measuring a magnetic field distribution with a magnetic resonance tomography apparatus 1 and a field camera 60 according to the invention is given in fig. 4.
In step S100, a sensitivity matrix of receiving antennas is acquired, the sensitivity matrix having a sensitivity E for each sample m at each receiving antenna nmn. For this purpose, at least M, preferably N (where N > M), signal responses of the receiving antennas 62 need to be acquired for a predetermined excitation of each sample M. For magnetic resonance signals S with M samples 61mAnd a received signal a having N receiving antennas 62nThe following system of equations is obtained:
An＝Emn×Sm
one possible embodiment of the matrix elements for determining the sensitivity matrix is to excite each of the M samples 61 individually in turn. This can be achieved, for example, by generating a magnetic field gradient in step S120, at which each of the M samples 61 is subjected to a magnetic field differing in magnitude. As already described, this can be achieved by arranging the samples with respect to the magnetic field gradient such that in a plane perpendicular to the gradient vector, only one sample 61 is arranged accordingly. In this case, it is sufficient to align the sample 61 in the X, Y or Z direction in accordance with the gradient vector of each gradient coil 12, so that a current is applied to the respective gradient coil 12. Alternatively, in step S110, a suitable gradient vector is determined and in step S120, the gradient vector is generated in a suitable direction with respect to the sample 61 by superimposing the fields of the three gradient coils 12.
If each of the M samples is subjected to a different magnetic field combined by the gradient magnetic field and the static magnetic field B0, respectively, the M samples also have different larmor frequencies. Then, the sensitivity matrix E can be acquired by correspondingly individually exciting the single sample by means of narrow-band excitation pulses having the corresponding larmor frequency, and subsequently receiving the magnetic resonance signals of the one sample 61 by all N receiving antennas 62mnCorresponding matrix element of (a).
It is also conceivable that, under the influence of the same magnetic field gradient, a broadband excitation pulse is emitted which, under the influence of the magnetic field gradient and of the static magnetic field B0, comprises a signal component with the larmor frequency of all samples. Then, all M samples were excited simultaneously. Subsequently, if magnetic resonance signals are recorded with the receiving antenna 62 with the magnetic field unchanged, these magnetic resonance signals are a superposition of a plurality of magnetic resonance signals of different samples 61. However, since the larmor frequencies are different, it is possible to determine the sensitivity matrix E by performing separation in the frequency domain after performing fourier transform, for examplemnEach element of (1).
If the frequency distance is not sufficient for complete separation, it is also conceivable to increase the separation accuracy by weighting the received signals of the receiving antennas 62 with a window function such as a hann function or a gaussian function
It is also conceivable to improve the separation accuracy by combining selective excitation with analysis in the frequency domain. The individual samples may be excited selectively or may also be excited in a plurality of layers spaced apart from one another.
Finally, it is also conceivable, in the embodiment with interference coils 63 around the individual samples 61, to excite the interference coils 63 by means of excitation pulses applied to the respective interference coils 63 in order to selectively locally excite the individual samples 61.
In step S310, a sensitivity matrix E is determinedmnInverse matrix of (I)nm. For this reason, the number N of receiving antennas needs to be at least as large as the number M of samples. Here, the sensitivity matrix EmnGiving a system of equations that are reversible under certain conditions so that the received signal or amplitude a from the receive antenna 62 can be givennThe magnetic resonance signal of each sample 61 is deduced.
Computationally, this utilizes EmnInverse matrix of (I)nmTo proceed with. Here, if the determinant is not equal to 0, the square matrix EnmIs reversible. Then, according to Sm＝Inm×AnReception of signal A by a reception antenna 62nAnd obtaining a vector of the magnetic resonance signals.
If the number N of receiving antennas 62 is greater than the number M of samples 61, the concept "Inverse matrix" is not limited to the more strict mathematical concept of a square matrix with M ═ N, but here also includes so-called pseudo-Inverse matrices with N > M, such as Moore-Penrose-Inverse (Moore-Penrose-Inverse).
For this purpose, a sensitivity matrix E is requiredmnThe system of equations of (a) is not underdetermined. If the system of equations is overdetermined, i.e. the number N of receiving antennas 62 is greater than the number M of samples 61, and the samples 61 are arranged relative to the receiving antennas 62 such that, in different receiving antennas 62, unequal linear combinations of the signals from the samples 61 are acquired, singular value decomposition (selbstwertzerlenging) or the boundary method can be usedDetermination of a pseudo-inverse matrix I from a sensitivity matrixnm. Thus, the least square method can be used to confirmDetermining the magnetic resonance signal S of the sample 61mWith a minimum distance and an improved signal-to-noise ratio.
The inversion of the matrix can be carried out, for example, in the control unit 20 of the magnetic resonance tomography apparatus 1, but a separate computing unit or a computing unit for image reconstruction is also conceivable.
In a further step S200, N antenna signals a of M samples 61 in the magnetic field to be measured are acquired by means of N receiving antennas 62n. For this purpose, the sample 61 first needs to be excited by an excitation pulse having a larmor frequency in the desired magnetic field B0. Due to the subsequent use of the sensitivity matrix EmnOr the inverse I thereofnmThe magnetic resonance signals of the individual samples 61 are separated, so that it is not necessary to subject the individual samples 61 to different magnetic fields by means of gradients. Thus, excitation can be performed with a common narrow band excitation pulse having a frequency that is the larmor frequency of the sample 61 in the expected static magnetic field B0. The bandwidth of the excitation pulse only needs to be large enough to compensate for the inhomogeneity of the static magnetic field B0. Subsequently, the antenna signals a of the M samples 61 are acquired by the receiving antenna 62nN magnitudes or vectors.
Subsequently, in step S320, by applying vector ANAnd inverse matrix InmAre multiplied to determine the magnetic resonance signal S for each sample 62m. For N>Instead of the inverse square matrix, an overdetermined matrix of M is used, for example, a system of equations that can be solved by the least squares method or a pseudo-inverse matrix.
In an advantageous manner, the inverse matrix is kept unchanged here as long as the sensitivity matrix does not change, for example, due to spatial variations. Thus, the inverse matrix can be used to rapidly calculate the magnetic resonance signals to rapidly take multiple measurements in succession.
However, it is also conceivable to use the underlying system of equations a in a special configuration of the receiving antenna 62 and/or the sample 61n＝Emn×SmOther solution methods of (3) provide more accurate or faster results. It is therefore likewise conceivable for the parties to be carried out separately after each measurement, depending on the respective antenna signalSolving the equation method and matching the solution of the equation method with the corresponding antenna signal.
The individual magnetic resonance signals S of the individual samples 61 thus obtained are then frequency-wise, for example by fourier transformationmThe analysis is performed again to obtain a value directly proportional to the magnetic field at the location of the sample 61. The analysis is preferably performed on the control unit 20 of the magnetic resonance tomography apparatus 1.
The magnetic field values so determined may be output to a user via a display to assess the uniformity of magnetic field B0. However, it is also conceivable for the control unit 20 to determine the settings of the shim currents by the shim coils of the magnetic resonance tomography apparatus 1 from the B0 value and to output them in order to improve the homogeneity of the static magnetic field B0.
While the invention has been shown and described in further detail with reference to preferred embodiments thereof, the invention is not limited to the examples disclosed, and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.
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