Method and apparatus for generating magnetic resonance images

Abstract

In a method, control computer and magnetic resonance (MR) apparatus for generating MR recordings of an examination object, first magnetic MR data are acquired in a first recording region inside a homogeneity volume of the scanner of the MR apparatus, and second MR raw data are acquired in a second recording region outside the homogeneity volume. First image data are reconstructed on the basis of the first MR raw data and second image data are reconstructed on the basis of the second MR raw data. The first image data and the second image data are combined to form combination image data, which cover a region that extends in the first recording region and in the second recording region.

Claims

1. A method for generating magnetic resonance (MR) recordings of an examination subject situated in an MR data acquisition scanner of an MR tomography apparatus, said method comprising: operating the MR data acquisition scanner to acquire first MR raw data from the subject, in a first recording region that is inside a homogeneity volume of the scanner, and to acquire second MR raw data from the subject, in a second recording region that is outside of said homogeneity volume; in a computer, reconstructing first image data based on said first MR raw data, and reconstructing second image data based on said second MR raw data; and in said computer, combining said first image data and said second image data so as to form combination image data, said combination image data covering a region that extends in said first recording region and in said second recording region.

2. A method as claimed in claim 1 comprising acquiring said first MR data in k-space, and reconstructing said first image data by executing a Fourier transformation of said first MR raw data in k-space, and reconstructing said second image data by executing a signal model-based image reconstruction method based on said second MR raw data.

3. A method as claimed in claim 2 comprising reconstructing said first image data so as to depict image points of said first image data on a first isotropic grid, and after reconstructing said second image data, regridding said second image data so as to depict image points of said second image data on an isotropic grid that is adjusted to the isotropic grid of said image points of said first image data.

4. A method as claimed in claim 1 wherein said homogeneity volume is produced by a basic magnetic field generated by a basic field magnet in said MR data acquisition scanner, and comprising acquiring said second MR data on at least one of iso-frequency contours of said basic magnetic field and iso-frequency surfaces of said basic magnetic field.

5. A method as claimed in claim 1 comprising acquiring at least one of said first MR raw data and second MR raw data as slice raw data.

6. A method as claimed in claim 1 comprising acquiring at least one of said first MR raw data and second MR raw data as volume raw data.

7. A method as claimed in claim 1 comprising acquiring at least one of said first MR raw data and said second MR raw data by executing an MR data acquisition sequence in said MR data acquisition scanner, selected from the group consisting of an MR data acquisition sequence that produces a gradient echo signal and an MR data acquisition sequence that produces a spin echo signal.

8. A method as claimed in claim 1 comprising acquiring said second MR raw data by operating said MR data acquisition scanner so as to radiate a slice-selective radio-frequency (RF) excitation pulse, and activating a bipolar gradient pulse coordinated with said slice-selective RF excitation pulse.

9. A method as claimed in claim 1 comprising acquiring said second MR raw data by operating said MR data acquisition scanner so as to radiate a non-slice-selective radio-frequency (RF) excitation pulse, and to activate a phase-encoding gradient pulse after radiating said non-slice-selective RF excitation pulse.

10. A method as claimed in claim 1 comprising producing said homogeneity volume in said scanner by generating a basic magnetic field in said scanner with a basic field magnet, and wherein acquiring said second MR raw data includes activating a gradient pulse that produces a non-linear gradient magnetic field, and wherein said method comprises, before acquiring said second MR raw data, determining, in said computer, a magnetic field distribution of at least one of said basic magnetic field and said non-linear gradient field, at least in said second recording region.

11. A control computer for controlling a magnetic resonance (MR) apparatus, having an MR data acquisition scanner, said control computer comprising: an acquisition processor configured to operate the MR data acquisition scanner to acquire first MR raw data from the subject, in a first recording region that is inside a homogeneity volume of the scanner, and to acquire second MR raw data from the subject, in a second recording region that is outside of said homogeneity volume; a reconstruction processor configured to reconstruct first image data based on said first MR raw data, and to reconstruct second image data based on said second MR raw data; and a combination processor configured to combine said first image data and said second image data so as to form combination image data, said combination image data covering a region that extends in said first recording region and in said second recording region.

12. A magnetic resonance (MR) apparatus comprising: an MR data acquisition scanner; a computer configured to operate the MR data acquisition scanner to acquire first MR raw data from the subject, in a first recording region that is inside a homogeneity volume of the scanner, and to acquire second MR raw data from the subject, in a second recording region that is outside of said homogeneity volume; said computer being configured to reconstruct first image data based on said first MR raw data, and reconstructing second image data based on said second MR raw data; and said computer being configured to combine said first image data and said second image data so as to form combination image data, said combination image data covering a region that extends in said first recording region and in said second recording region.

13. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a computer of a magnetic resonance (MR) apparatus comprising an MR data acquisition scanner, said programming instructions causing said computer to: operate the MR data acquisition scanner to acquire first MR raw data from the subject, in a first recording region that is inside a homogeneity volume of the scanner, and to acquire second MR raw data from the subject, in a second recording region that is outside of said homogeneity volume; reconstruct first image data based on said first MR raw data, and reconstructing second image data based on said second MR raw data; and combine said first image data and said second image data so as to form combination image data, said combination image data covering a region that extends in said first recording region and in said second recording region.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates a magnetic resonance tomography system according to one exemplary embodiment of the invention.

(2) FIG. 2 is a schematic diagram of the spherical image volume SIV of the static B.sub.0 magnetic field (B.sub.0 homogeneity volume SHV) and a B.sub.0 inhomogeneity volume SIHV outside the spherical image volume SIV of a magnetic resonance tomography apparatus.

(3) FIG. 3 is a section through the B.sub.0 homogeneity volume SHV and the B.sub.0 inhomogeneity volume SIHV of FIG. 2 in a transverse slice plane.

(4) FIG. 4 is a section through the B.sub.0 homogeneity volume SHV and the B.sub.0 inhomogeneity volume SIHV of FIG. 2 in a sagittal slice plane.

(5) FIG. 5 is a section through the B.sub.0 homogeneity volume SHV and the B.sub.0 inhomogeneity volume SIHV of FIG. 2 in a coronal slice plane.

(6) FIG. 6 is a block diagram for an embodiment of a sequence of the inventive method.

(7) FIG. 7 is an exemplary pulse sequence that can be used for carrying out the inventive method.

(8) FIG. 8 is a further exemplary pulse sequence that can be used for carrying out the inventive method.

(9) FIG. 9 is a schematic diagram of a slice that can be acquired with the inventive method for generating slice image data, the slice extending continuously inside a homogeneity volume and outside a homogeneity volume of the B.sub.0 magnetic field.

(10) FIG. 10 is a schematic diagram of a spatial volume region that can be acquired by means of the inventive method for generating volume image data, which region extends continuously inside a homogeneity volume and outside a homogeneity volume of the B.sub.0 magnetic field.

(11) FIG. 11 is a schematic diagram of an isotropic grid.

(12) FIG. 12 is a schematic diagram of an anisotropic grid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) FIG. 1 schematically illustrates a magnetic resonance tomography apparatus 1. It includes the actual magnetic resonance tomography scanner 2 having an examination space 3 or patient tunnel. The whole body of a patient or test subject constitutes the actual examination object O, or a particular body region constitutes the examination object O. The patient or test subject is moved through the examination space 3 on a bed 8.

(14) The magnetic resonance scanner 2 is conventionally fitted with a basic field magnet 4, a gradient system 6, and an RF transmit antenna system 5, and an RF receive antenna system 7. In the illustrated exemplary embodiment, the RF transmit antenna system 5 is a body coil permanently installed in the magnetic resonance scanner 2. The RF receive antenna system 7 includes local coils for arrangement on the patient or test subject (symbolized only by a single local coil in FIG. 5). Basically, however, the body coil can also be used as an RF receive antenna system and the local coils as an RF transmit antenna system provided these coils can each be switched over into different operating modes. The gradient system 6 conventionally has individually controllable gradient coils in order to be able to switch independently of each other in the x, y or z direction. In addition, the magnetic resonance scanner 2 contains shim coils (not shown) that are conventionally designed.

(15) The basic field magnet 4 includes multiple magnetic coils and is conventionally designed so as to generate a basic magnetic field B.sub.0 in the longitudinal direction of the patient, in other words along the longitudinal axis of the magnetic resonance scanner 2 running in the z direction. A homogeneity region of the basic magnetic field B.sub.0 in which the basic magnetic field B.sub.0 is homogeneous is present tin the scanner 2. The homogeneity region corresponds here to a spherical homogeneity volume SHV, which is shown in dashed lines and is located inside the patient tunnel 3. The iso-center of the magnetic resonance scanner 2 is located in the homogeneity volume SHV.

(16) The magnetic resonance tomography apparatus 1 also has a central control computer 13 that controls the entire magnetic resonance tomography apparatus 1. The central control computer 13 has an acquisition controller or image recording processor 14 for pulse sequence control. The sequences of RF pulses and gradient pulses are controlled in this processor as a function of a selected imaging sequence. For emitting the individual RF pulses, the central control computer 13 has an RF controller 15 and for controlling the gradient pulses, a gradient controller 16, which communicate accordingly with the image recording processor 14 in order to emit the pulse sequences. The RF controller 15 includes not only a transmit unit, in order to emit the RF pulse sequences, but also a reception unit in order to acquire magnetic resonance raw data in a coordinated manner.

(17) A reconstruction processor 20 accepts the acquired raw data and reconstructs the image data therefrom. The image data can then be stored, for example, in a memory 17.

(18) The acquisition of raw data radiation of RF pulses and the generation of gradient fields, and the reconstruction of MR images therefrom are basically known to those skilled in the art, and thus need not be explained in more detail herein.

(19) The reconstructed image data can then be combinedas will be explained belowin a combination processor 18, so as to form combination image data BDK to obtain a final homogeneous MR image.

(20) The central control computer 13 can be controlled via a terminal interface 21 by the use of an input unit 24 and a display unit 23, with which the entire magnetic resonance tomography apparatus 1 can be operated by an operator. MR images can be displayed on the display unit 23, and measurements can be planned and started by using the input unit 24, optionally in combination with the display unit 23.

(21) FIG. 2 shows an example of a magnetic field distribution with the homogeneity volume SHV from FIG. 1. In this homogeneity volume SHV a variation in the prevailing basic magnetic field B.sub.0 is less than 20 ppm peak-to-peak or less than 3 ppm RMS. Previously, the acquisition of magnetic resonance raw data RD1, RD2 has been limited to this measurement volume, the spherical image volume SIV with a diameter DSV, which contains the above-mentioned tolerance indication. This means that a field of view matches the spherical image volume SIV.

(22) Outside of the homogeneity volume SHV there is an inhomogeneity volume SIHV. The surface or the iso-frequency surface of the magnetic field deviation is very pronounced here and undulates or meanders in a complex manner, with the homogeneity of the static magnetic field B.sub.0 having a greater deviation than, for example, 20 ppm peak-to-peak or greater than 3 ppm RMS.

(23) FIGS. 3 to 5 show different sections through the magnetic field distribution in FIG. 1. FIG. 3 shows the magnetic field deviation in a transverse slice, in other words in the x-y plane perpendicular to the z-axis (see FIG. 1), FIG. 4 shows the magnetic field deviation in a sagittal slice, in other words in the y-z plane perpendicular to the x-axis, and FIG. 5 shows a magnetic field deviation in a coronal slice, in other words in the x-z plane perpendicular to the y-axis.

(24) The transverse section through the basic magnetic field B.sub.0 shown in FIG. 3 shows three lines IR which represent the three Iso-frequency contours IR on which the magnetic field deviation run, constantly in each case. FIG. 3 shows the iso-frequency contours IR for a deviation around 5 ppm, around 10 ppm and around 50 ppm. As can be seen in FIG. 3, the deviation of the magnetic field B.sub.0 increases significantly as the distance from the iso-center IZ increases, in other words the closer it extends to an edge region of the ring tunnel.

(25) As can be seen from the iso-frequency contours IR, likewise shown in FIGS. 4 and 5, for the deviations around 5 ppm, around 10 ppm and around 50 ppm, the magnetic field strength of the magnetic Felds B.sub.0 periodically spatially increases and decreases at the edge of the field of view of the magnetic resonance scanner 2 in the other planes or sections. The reason for this is the conventional arrangement of the magnetic field coils for the basic magnetic field B.sub.0 in the magnetic resonance scanner 2. In the case shown in FIGS. 4 and 5 five magnetic field coils lead to corresponding magnetic field increases (in the coronal representation on the left/right side respectively) close to these coils, and to corresponding magnetic field reductions between the magnetic field coils. This results in the illustrated periodic magnetic field behavior.

(26) FIG. 6 is a flowchart of an embodiment of the method according to the invention.

(27) In a first method step Ia first magnetic resonance raw data RD1 is acquired in a first recording region B1.sub.2D, B1.sub.3D inside a homogeneity volume SHV.

(28) At the same time or at different times (before or after) to the acquisition of the first magnetic resonance raw data RD1, second magnetic resonance raw data RD2 can be acquired in a method step Ib in a second recording region B2.sub.2D, B2.sub.3D outside the homogeneity volume SHV.

(29) For the acquisition of the magnetic resonance raw data in a second recording region B2.sub.2D FIG. 7 shows a pulse graph, in other words the course over time of a pulse sequence, for a 2D-MR sequence according to an exemplary embodiment of the invention. A first RF excitation pulse HF1 and a second RF excitation pulse HF2 are shown in the first row RF. The second row G.sub.S shows a bipolar slice gradient pulse G.sub.S1, in other words a slice-selective gradient pulse. The frequency offsets of the RF excitation pulse HF1, together with the amplitude of the bipolar slice gradient pulse G.sub.S1, determine the position and or the width of the excited slices. The frequency offsets determine an excited iso-frequency contour IR. The slice gradient pulse G.sub.S1 determines the excited slices along this iso-frequency contour IR, or iso contour IR. Due to the bipolarity of the slice gradient pulse G.sub.S1 there is a refocusing of the applied spins. In the fourth row G.sub.F a frequency-encoding gradient G.sub.F1 is activated after the bipolar slice gradient pulse G.sub.S1. With the likewise bipolar frequency-encoding gradient G.sub.F1, the momentum is equalized at instant TE or set to 0. The fifth row R.sub.O shows a readout window R.sub.O1. The readout or receipt of the magnetic resonance signals (the actual acquisition of the raw data) occurs temporally symmetrically around this 0-line of the momentum since then all spins are encompassed. After the repetition time TR an RF excitation pulse HF2 can then be radiated again in order to repeat the sequence iteratively. A phase-encoding gradient is not necessary here since a number of parallel receive coils are used.

(30) With the aid of the frequency-encoding gradient G.sub.F1 k-space is scanned for the slices selected with the bipolar slice gradient pulse G.sub.S1. The separation of the acquired raw data or reconstructed image data of the different slices is then carried out in an evaluation step on the basis of the spatial sensibility profiles of the receive coils, for example with the known Grappa method or the SENSE method.

(31) The first magnetic resonance raw data RD1 and the second magnetic resonance raw data RD2 can be acquired by a 3D-MR sequence as well, however.

(32) FIG. 8 shows in this regard, analogously to FIG. 7, a pulse graph for a 3D-MR sequence in a second recording region B2.sub.3D. The first row RF shows an RF excitation pulse HF1 which is played out during an excitation phase. The RF excitation pulse HF1 also has a limited bandwidth here. The RF excitation pulse or frequency offsets therefore determine the iso-frequency surfaces and, by way of the limited bandwidth, also the thickness of the excited iso-frequency surface IO. A slice gradient pulse is not necessary here. The third row G.sub.P shows a phase-encoding gradient G.sub.P1. This can be the Gx, Gy or Gz gradient or a combination of these gradients. The fourth row G.sub.F shows a frequency-encoding pulse G.sub.F1 which is used to generate an MR gradient echo signal at time TE. The frequency-encoding gradient G.sub.F1 can be the Gx, Gy, Gz gradient or a combination of these gradients. Here too a readout window R.sub.01, shown in the fifth row R.sub.0, again around the 0-line of the momentum since then, as already mentioned, all spins are in phase.

(33) In further method steps IIa, IIb (see FIG. 6 again) the image data is then reconstructed on the basis of the acquired magnetic resonance raw data.

(34) The reconstruction method differs here, however, for image data that are to be reconstructed on the basis of magnetic resonance raw data that were acquired inside a homogeneity volume SHV, and for image data that are to be reconstructed on the basis of magnetic resonance raw data that were acquired inside an inhomogeneity volume SIHV.

(35) If the first magnetic resonance raw data RD1 were acquired in the homogeneity volume SHV, thenas customarythe data are acquired in (entered into) k-space. Therefore, the first image data BD1 can be reconstructed on the basis of the first magnetic resonance raw data RD1, here in the method step IIa, by execution of a Fast Fourier Transform FFT.

(36) The second magnetic resonance raw data RD2, which were acquired in the inhomogeneity volume SIHV, are not stored in (entered into) k-space and thus are not acquired in k-space. In a method step IIb, the second image data BD2 are therefore reconstructed by execution of a signal model-based image reconstruction method SMR that proceeds on the basis of the second magnetic resonance raw data RD2. In this case a signal model is generated which returns the expected amplitudes and phases of measured echo signals as a function of the encoding scheme.

(37) FIG. 9 shows an example in this regard of image points that have been reconstructed as slice image data. A first recording region B1.sub.2D, in which image data reconstructed inside the homogeneity volume SHV are located, and a second recording region B2.sub.2D, in which image data reconstructed outside the homogeneity volume SHV are located, are shown. If the reconstructed image data are located inside the homogeneity volume SHV, they are isotropic pixels Pi. The reconstructed image data in the second recording region B2.sub.2D outside the homogeneity volume SHV are anisotropic pixels Pa, however. This is because the magnetic field B.sub.0 in the inhomogeneous region or inhomogeneity region is distorted. The magnetic field B.sub.0 has different values in different directions (x, y and z directions). In the second recording region B2.sub.2D, outside the homogeneity volume SHV, the anisotropic pixels Pa therefore cannot, as already explained, be stored in k-space and be reconstructed with a Fast Fourier Transform FFT.

(38) As shown in FIG. 10 in comparison with FIG. 9, the behavior is similar if the image points have been reconstructed as volume image data. The image points, which have been reconstructed inside the first recording region B1.sub.3D in the homogeneity volume SHV, are isotropic voxels Vi. Due to the magnetic field B.sub.0 with different extents, the image point shown by way of example here in the second recording region B2.sub.3D outside the homogeneity volume SHV is an anisotropic voxel Va. A Fast Fourier Transform is not possible in the second recording region B2.sub.3D here either.

(39) The method steps IIa, IIb shown in FIG. 6 for the reconstruction of the image data BD1, BD2 can take place independently of each other time-wise, in other words one after the other as desired or simultaneously or also slightly staggered.

(40) In a following method step III for generating a final homogeneous magnetic resonance scan, the first image data BD1 reconstructed in method step IIa and the second image data BD2 reconstructed in method step IIb are joined together to form combination image data BDK.

(41) FIG. 11 shows in this regard an isotropic grid 25 on which the isotropic pixels Pi, or voxels Vi from FIG. 9 or FIG. 10 can be displayed. In contrast thereto, FIG. 12 shows an example of an anisotropic grid 26. In order obtain a homogeneous image overall, the first recording region B1.sub.2D, B1.sub.3D and the second recording region B2.sub.2D, B2.sub.3D are merged with the aid of what is known as a regridding method on an isotropic grid. Specifically, the image data of the second recording region B2.sub.2D, B2.sub.3D can be adapted here with the aid of the regridding method to the image data of the first recording region B1.sub.2D, B1.sub.3D, in other words the isotropic grid that already exists for this. This is achieved, for example, by execution of a correction of the magnetic field non-linearity and an adjustment of the pixel or voxel size and the spatial encoding.

(42) In conclusion, it is noted once again that the methods described in detail above and the illustrated magnetic resonance tomography system are only exemplary embodiments, which can be modified by those skilled in the art in a wide variety of ways without departing from the scope of the invention. Furthermore, use of the indefinite article a or an does not preclude the relevant features from also being present several times. Similarly, the terms unit and module do not preclude the relevant components from comprising a plurality of cooperating sub-components which, optionally, can be spatially distributed as well.