DE102013215526B3 - Method and device for generating a slice image using a magnetic resonance tomography device - Google Patents

Abstract

The invention relates to a method for generating a sectional image (342) of an object to be recorded using a magnetic resonance tomography device. The method comprises a step of acquiring a series of projections (330, 332, 334, 336, 338) of the object to be recorded using a pulse sequence, in particular a balanced SSFP sequence, each with a phase cycle, a step of combining the A series of projections (330, 332, 334, 336, 338) in the k-space (340) and a step of applying (224) a reconstruction to the k-space (340) to produce a cross-sectional image (342) of the object to be captured as a function of the series of projections (330, 332, 334, 336, 338). The phase cycle (ΔΘ1, ΔΘ2, ΔΘ3, ΔΘ4, ΔΘn) is different for each of the pulse sequences, the series of projections (330, 332, 334, 336, 338) being superimposed in the center of a k-space (340);

Description

The present invention relates to a method for generating a slice image using a magnetic resonance tomography device, to a corresponding device for producing a slice image using a magnetic resonance tomography device, a corresponding magnetic resonance tomography device and to a corresponding computer program product.

Using magnetic resonance tomography, sectional images of objects can be made. One technique for acquiring tomograms using magnetic resonance tomography is the balanced SSFP sequence used in, for example, cardiac imaging, angiography and dynamic imaging. The term SSFP sequence stands for "steady state free precession" sequence. An intrinsic disadvantage of the balanced SSFP sequence is its strong susceptibility to off-resonance effects. Spins whose off-resonance angle falls within a signal minimum appear dark or black in the cross-section, which is referred to as a banding artifact.

A multiply phase-cycled steady state free precession sequence with a number of at least two subsequences with alternating high-frequency excitation pulses is known from US Pat DE 10 2007 045 996 A1 known.

The document J. L. Klaers et al. TR SSFP: ultra-high 0.29 mm isotropic resolution "in: Proceedings ISMRM, 18, 2010, p. 772 describes an ordinary phase-cycled measurement. That is, multiple images (in this case, two) each having a constant phase cycle are acquired.

The publication US 2006/0214660 A1 describes a method based on the acquisition of multiple images at different phase cycles.

The pamphlets US 2008/0084208 A1 . US Pat. No. 7,012,428 B1 and US 2005/0253579 A1 describe methods in which separate images are acquired and then combined, the phase cycle being constant during the acquisition of each of these images.

Against this background, the present invention provides a method for producing a slice image using a magnetic resonance tomography apparatus, a device for generating a slice image using a magnetic resonance tomography apparatus that uses this method, a corresponding computer program product and, finally, a corresponding magnetic resonance tomography apparatus presented according to the main claims.

Advantageous embodiments emerge from the respective subclaims and the following description.

By combining a, in particular radial, imaging by means of a balanced SSFP sequence with a dynamic phase cycle, a slice image can be generated which is free of banding artifacts. In successive recorded projections, the phase cycle is gradually increased. In the presented method, it is sufficient to detect a single k-space.

A method is presented for producing a sectional image of an object to be recorded using a magnetic resonance tomography apparatus, the method having the following steps:
Acquiring a series of projections of the object to be recorded using a respective one of a pulse sequence, in particular a balanced SSFP sequence, each having a phase cycle, the phase cycle being different for each of the pulse sequences, and wherein the series of projections is in the center of a k-space layered;
Combining the series of projections in the k-space; and
Apply a reconstruction to the k-space to obtain a cross-sectional image of the object to be photographed as a function of the series of projections.

Magnetic resonance imaging can be understood as magnetic resonance imaging or MRI for short. A pulse sequence can be understood to mean a balanced SSFP sequence or, in short, a sequence. The abbreviation SSFP stands for the English-language expression "steady state free precession". In this case, a sequence can be assigned a number of parameters. In this case, a projection can be assigned a sequence. Thus, the phase cycle of successively recorded projections can be gradually increased. A phase cycle may be understood to mean the increment by which the phase of the high-frequency excitation pulses is increased. In this case, a direction of incidence can be understood by phase. The use of a particular phase cycle shifts the frequency response function along the off-resonance angles (abscissa). Thus, a projection or a pulse sequence can be assigned a phase cycle. The projections acquired in the step of capturing can be combined in a star shape in the k-space. In the combining step, the projections can be weighted in the k-space combined. On the k-space thus detected, a reconstruction, in particular a radial reconstruction, to be applied. By means of a reconstruction, a sectional image of the object to be recorded can be generated from the k-space, or a signal representing a sectional image of the object to be recorded.

In one embodiment, the phase cycle is incrementally increased.

One aspect of the present invention is the combination of a balanced SSFP sequence, in particular a radial balanced SSFP sequence, with a dynamic phase cycle. While in a standard balanced SSFP sequence, a slice image is added to a fixed phase cycle, the phase cycle is dynamically increased here. That is, each projection can be recorded with a different phase cycle. The phase cycle increment between two successive projections can be adapted so that after filling the k-space a total of 360 °, that is, all possible phase cycle values, have been run through. The data thus acquired can be reconstructed in various ways, as described below. In contrast to Cartesian imaging, in radial imaging all recorded projections contribute to the k-space center and thus to the substantial contrast of the slice image. Thus, in the k-space center, there is a sum of the signals of the different projections at different phase cycles, that is, a sum of N phase cycles, where N denotes the number of acquired projections. If one carries out an ordinary reconstruction, in particular a radial reconstruction, of k-space, one thus obtains an image whose contrast is composed of the sum of the different projections and thus is free from banding artifacts.

Advantageously, the present invention provides robust removal of banding artifacts, even with large field inhomogeneities. In this case, a short measuring time can be maintained or achieved. Advantageously, it may be a single-shot measurement, that is, only a single k-space is acquired. A dynamic phase cycle can easily be inserted into a (radial) balanced SSFP sequence. The reconstruction of k-space is possible with a simple reconstruction method; Thus, a standard reconstruction, in particular a radial reconstruction, which is already implemented by default in a clinical scanner, for example, can be used.

Since the presented invention can robustly remove banding artifacts even in the case of large field inhomogeneities, high-resolution measurements, high-field imaging, imaging of air-tissue interfaces and imaging of implants or metals are conceivable fields of application.

Since the signal minimums of the frequency response function (FRF) are indirectly proportional to the repetition time TR used, the repetition time TR is generally chosen to be very small in order to avoid banding artifacts in a standard balanced SSFP sequence. Since banding artifacts are completely removed by the present invention, therefore, the repetition time TR can be arbitrarily large. This allows the use of RF pulses of great duration, and thus SAR-reduced balanced SSFP recordings are possible. In this case, SAR (for "specific absorption rate") denotes the specific absorption rate. Furthermore, the use of arbitrarily low Slew-Rates very quiet balanced SSFP recordings and a reduction of eddy currents, which lead to artifacts. In this case, "slew rate" denotes the rate of increase of the gradient (in other words, the (slope) steepness or the voltage rise rate.

In the step of applying, a special k-space filter may be used for reconstruction, using only a small number of projections for the k-space center, gradually increasing the number of projections for outer areas of k-space. Alternatively, in the step of applying for reconstruction, a k-space weighted image contrast filter (KWIC) may be used to obtain the slice image. Alternatively or alternatively, a plurality of adapted sectional images can be obtained with a special k-space filter or a KWIC filter. From a k-space one can obtain a plurality of images at different phase cycles. The plurality of images can then be further processed.

In other words, instead of performing a normal, especially radial, reconstruction, it is possible to apply techniques such as a special k-space filter or a kwic filter to k-space. Hereby, when used several times from a single k-space, different images are obtained at different phase cycles, which can then be reused.

In the step of sensing, the series of projections may be radial projections. In the capture step, the series of projections may be a series of projections that intersect in the k-space center. Crossover of the projections may also be referred to as overlapping or cutting. In the step of detecting, the series of projections may also be a series of spirally arranged projections. If these are radial projections, simply create a two-dimensional slice in the step of applying. If it is a series of helically arranged projections, the projections in k-space can all intersect at a center. In the step of applying, a three-dimensional image can be generated.

Successive projections may be arranged in a linearly increasing angle and alternatively or additionally in a golden angle and alternatively or in addition to a pseudo-random and alternatively or additionally quasi-random angle in k-space. The respectively used phase cycle increment is incrementally increased in this case. A maximum phase cycle increment and thus a minimum number of used projections can be dependent on many factors, such as field strength, object size, object condition or object position as well as magnetic field inhomogeneities. For example, a phase cycle increment can be a maximum of 0.5 °. For example, a phase cycle increment may be a maximum of 0.72 °, which corresponds to a number of at least 500 projections. In this case, a better image quality can be achieved with a smaller phase cycle increment. If successive projections are arranged in a golden angle, the golden angle may be within a tolerance range of 111.2 °. In this case, adjacent phase cycle increments can be at most 0.5 ° or at most 0.72 ° apart.

The phase cycle increment can be varied during the measurement. For example, areas where the Frequency Response Function (FRF) is very steep may be sampled more accurately, that is, with a smaller phase cycle increment. For example, for areas where the frequency response function is flat, a larger phase cycle increment may be used.

In the step of detecting, as the series of projections, at least a predetermined number of projections can be detected. In particular, at least a number of 720 projections can be detected. In the step of applying good results can be achieved from 720 projections. Dividing the at least 720 projections into a 360 ° circle results in a phase cycle increment of less than or equal to 0.5 ° per projection. In particular, at least a number of 500 projections can be detected. In the step of applying good results from 500 projections can be achieved. Dividing the at least 500 projections into a 360 ° circle results in a phase cycle increment of less than or equal to 0.72 ° per projection.

The method may include a step of preparing the object to be photographed; this step can be done before the step of detecting. In the step of preparing, for example, a multiplicity of preparation pulses can be sent out without data acquisition. Due to the large number of preparation pulses, the excited spins can be put in a "steady state". The multiplicity of preparation pulses may be at least 200 or alternatively at least 500 or alternatively in particular at least 1000 or alternatively 1500 preparation pulses. In particular, it can be a sufficient number of preparation pulses. A sufficient number of preparation pulses can be understood to mean three to five times the ratio of the relaxation time T1 to the repetition time TR. Furthermore, other special sequences of preparation pulses (so-called preparation schemes) can also be used. A "steady state" is understood to mean a balance of magnetization or a stable state.

At the beginning of the acquisition of the first radial projection, the excited spins may already be in steady state. This can be ensured, for example, by means of a high number (typically more than 200 or alternatively more than 500 or alternatively more than 1000 or alternatively more than 1500) of preparation pulses without data acquisition. It is possible to record all these preparation pulses with a constant phase cycle. Furthermore, the phase cycle of the preparation pulses can already be increased stepwise.

After a first pass through the step of detecting, the step of combining and the step of applying, in a second pass in the step of detecting at least one further projection of the object to be recorded can be made using at least one further pulse sequence, that is to say another balanced SSFP sequence , are detected with at least one further phase cycle, and in the second pass, in the combining step, the at least one further projection can be combined with the series of projections in the k-space or, alternatively, in another k-space. In this case, at least one old projection of the series of projections can be rejected, this can be at least one chronologically oldest projection of the series of projections or a projection of the series of projections with one of the at least one further projection similar or the same assignable phase cycle. In the step of applying the reconstruction to the k-space or alternatively to the further k-space, a further sectional image can be obtained, wherein the further sectional image is a sectional image acquired temporally after the first sectional image.

Another application of the technique presented is the fat-water separation, based on the Dixon method. For this purpose, it is necessary to take several pictures (usually two to three) at different echo times (for example TE1, TE2, TE3). This can be realized on the basis of the presented technique as follows: The complete k-spaces are not picked up one after another, but nested: First, a projection is taken with TE1 (described in the case of the acquisition of three pictures with different echo times), then a projection with TE2, then a projection with TE3. Then another projection with TE1, one with TE2, one with TE3. The whole process is continued until sufficient projections are available for an image at echo time TE1, for an image at echo time TE2, and for an image at echo time TE3. The phase cycle increment of the various projections is thereby increased step by step. (For example, if 1000 projections per echo time are acquired, a total of 3000 projections are necessary, so to sample the 360 °, a dynamic phase cycle of 360 ° / 3000 = 0.12 ° is necessary).

The corresponding projections (all projections with TE1, all projections with TE2, all projections with TE3) are then combined and reconstructed to get the respective pictures. The images thus obtained are free from banding artifacts (due to the method underlying the invention). Furthermore, they are almost perfectly registered because they were nested. If they were taken one after the other, the images could, for example, reflect different breathing states of the patient and thus adversely affect the result of the fat-water separation. These images are then further processed with a suitable algorithm so as to obtain the desired separation images; in the case of fat-water separation: a picture showing only water and a picture showing only fat. However, the separation is also feasible with other substances (silicone, ...).

Since the acquisition of images at different echo times prolongs the necessary TR, corresponding images (when taken with a standard balanced SSFP) can also have more banding artifacts (context TR banding artifacts has already been described above).

The steps of detecting, combining, and applying may be performed repeatedly, creating a plurality of k-spaces and a plurality of slice images representing as a stack a three-dimensional image of the object to be photographed. Alternatively or additionally, in the step of capturing, the series of projections may describe any three-dimensional trajectory superimposed in a three-dimensional k-space in the center, wherein in the step of applying the reconstruction, reconstructing a three-dimensional slice image of the object to be photographed as a function of the series of projections becomes.

In one embodiment, only a single k-space, in particular a kartesian k-space, is recorded. Advantageously, by using the dynamic phase cycle, the waiting time and the preparation time between the individual images can be saved.

The present invention provides a device for producing a slice image of an object to be recorded for a magnetic resonance tomography device, the device being designed to perform the steps of a variant of a method for producing a slice image presented here using a magnetic resonance tomography device in corresponding devices or implement. The object underlying the invention can be solved quickly and efficiently by this embodiment in the form of a device.

A device can be understood as meaning an electrical device which processes signals, in particular sensor signals, and outputs control signals as a function of the signals and additionally or alternatively data signals. The device may comprise a hard- and / or software-trained interface. In the case of a hardware-based embodiment of the interface, this can be part of a so-called system ASIC, for example, which contains at least one function of the device. An interface may include proprietary integrated circuits or discrete components. In the case of a software-based design of the interface, these can be software modules that can be present, for example, on a controller in addition to other software modules or can be processed.

The present invention provides a magnetic resonance tomography apparatus having a device for producing a sectional image of an object to be recorded, the device being designed to perform the steps of a method for producing a sectional image of an object to be photographed in corresponding devices. A magnetic resonance tomography device can be understood to mean a nuclear spin tomography device.

It is a use of a variant of a method for generating a sectional image of a male object for a fat-water separation based on a Dixon method presented. Images can be captured at different echo times.

A computer program product with program code, which may be stored on a machine-readable medium and used for carrying out the method according to one of the embodiments described above, may be executed on a computer or on a device.

The invention will now be described by way of example with reference to the accompanying drawings. Show it:

1 a magnetic resonance imaging device;

2 a flowchart of a method according to an embodiment of the present invention;

3 a schematic flow diagram of a method according to an embodiment of the present invention;

4 an exemplary result of in vivo measurement according to an embodiment of the present invention;

5 a schematic flow diagram of a method with a special k-space filter or alternatively with a KWIC filter according to an embodiment of the present invention;

6a to 6d exemplary results of a method with a KWIC filter according to an embodiment of the present invention;

7 an exemplary sequence schematic of a balanced SSFP sequence;

8th a diagram of a frequency response function (FRF);

9 a detection scheme of images at different phase cycles with a balanced SSFP sequence;

10 a detection scheme of images of a frequency modulated SSFP sequence;

11 a detection scheme according to an embodiment of the present invention; and

12 a block diagram of a device according to an embodiment of the present invention.

In the following description of favorable embodiments of the present invention, the same or similar reference numerals are used for the elements shown in the various figures and similar acting, with a repeated description of these elements is omitted.

1 shows an overview of the structure of a magnetic resonance tomography device 100 according to an embodiment of the present invention; this corresponds to the structure of a conventional tomography device. A basic field magnet 102 generated for the polarization or alignment of the nuclear spins of the object to be picked up in the examination area 104 a temporally constant, strong magnetic field. In a spherical measuring volume, the required for a nuclear magnetic resonance measurement homogeneity of the basic magnetic field is defined. In the area of the basic field magnet 102 is a gradient coil system 106 arranged to generate a gradient field. Inside the gradient coil system 106 is a high frequency system 108 which generates an alternating magnetic field for exciting the nuclei and aligning the nuclear spin of the object to be examined. The high frequency system 108 is configured to convert an alternating field emanating from the magnetic resonance system, for example a nuclear spin echo signal caused by a pulse sequence, into a voltage. Furthermore, the high frequency system 108 designed to generate pulses for the excitation of nuclear magnetic resonance. The magnetic resonance imaging device 100 has a device 110 for producing a sectional image of an object to be recorded for a magnetic resonance tomography device 100 on. The device 110 is formed, facilities of the magnetic resonance tomography device 100 to drive to create a sectional image of an object to be recorded. Furthermore, the magnetic resonance tomography device 100 a user interface 112 on which one on the magnetic resonance imaging device 100 expired process for generating a sectional image of a male object parameterized and the sectional image can be displayed.

2 is a flowchart of a method 200 according to an embodiment of the present invention. The procedure 200 For example, on an embodiment of the in 1 described magnetic resonance tomography device 100 be executed. The procedure 200 for generating a sectional image of an object to be photographed using a magnetic resonance tomography apparatus comprises a step 220 acquiring a series of projections of the object to be recorded using one pulse sequence (balanced SSFP sequence) each a phase cycle, a step 222 combining the series of projections in the k-space and a step 224 applying a reconstruction to the k-space to obtain a sectional image of the object to be photographed as a function of the series of projections. In this case, the phase cycle is different for each of the pulse sequences, with the series of projections superimposed in the center of k-space. The procedure of the procedure 200 also shows in the embodiment of the following figure in 3 ,

In one embodiment of the method 200 is in the step 224 Applying for reconstruction, a special k-space filter or k-space weighted image contrast filter (KWIC) filter is used to obtain the slice image and simultaneously or alternatively obtain a plurality of matched slice images. In one embodiment, the step 224 , ie the special k-space filter or alternatively the KWIC filter, applied several times. Per application (ie per reconstruction) you get a picture. In 5 and 6 this is explained clearly.

In one embodiment, it is in the step 220 of capturing the series of projections around a series of projections that cross in the k-space center, here radial projections. Alternatively, it is in one embodiment in the step 220 capturing a series of spirally arranged projections.

In one embodiment, projections are arranged with a linearly increasing angle, under a golden angle, with a pseudo-random or a quasi-random angle in k-space.

In step 220 In one exemplary embodiment of the acquisition, at least a predetermined number of projections are detected as a series of projections, in particular at least a number of 500 projections are recorded, in particular at least a number of 720 projections are detected.

In order to put the excited spins in a "steady state", can in an optional step 226 the preparation of the object to be recorded, for example, a plurality of preparation pulses are emitted. The optional step 226 of dissection can be before the step 220 of detecting. Thus, in the step of preparing, for example, at least 50, in particular at least 100, in particular at least 200, in particular at least 500, in particular at least 1000, in particular at least 1500 preparation pulses can be emitted with a constant or dynamically increasing phase cycle.

The steps of comprehending 220 , of combining 222 and applying 224 can be repeatedly executed in an embodiment, wherein a plurality of k-spaces and a plurality of sectional images are created, representing as a stack, a three-dimensional image of the object to be recorded; simultaneously or alternatively, in the embodiment in step 220 of capturing the series of projections describe any three-dimensional trajectory that overlap in a three-dimensional k-space in the center, being in step 224 of the reconstruction, a three-dimensional sectional image of the object to be photographed is reconstructed as a function of the series of projections.

3 shows a schematic flow diagram of a method according to an embodiment of the present invention. The method may be an embodiment of the in 2 described method 200 for generating a sectional image of a male object using a magnetic resonance tomography device. projections 330 . 332 . 334 . 336 . 338 are in a k-space 340 ; In this case, the embodiment shown is a radially accommodated k-space 340 , If all projections 330 . 332 . 334 . 336 . 338 captured and combined in k-space, so in the subsequent step 224 applying a banding-free image 342 generated, which can be provided at an interface to the display. In the embodiment, the first projection 330 a first phase cycle ΔΘ ₁ assigned. The second projection 332 a second phase cycle ΔΘ _{2 is} assigned. The third projection 334 a third phase cycle ΔΘ _{3 is} assigned. The fourth projection 336 a fourth phase cycle ΔΘ _{4 is} assigned. Substituting these series continued, so this results in that the n-th projection is assigned to an n-th cycle phase ΔΘ _n.

So shows 3 a method for rapidly removing banding artefacts, alternatively or alternatively, for acquiring images with different phase cycles based on a radial balanced SSFP sequence in combination with a dynamic phase cycle. An alternative embodiment or further development is in 5 as a method of acquiring images having different phase cycles based on radial balanced SSFP in combination with a dynamic phase cycle.

One aspect of the present invention is the combination of radial imaging using balanced SSFP and the use of a dynamic phase cycle. That is, the phase cycle of successive radial projections is gradually increased. On the sun Afterwards, an ordinary reconstruction for radial imaging can be applied to the acquired k-space. You get an image that is free from banding artifacts. Advantageously, it is sufficient here to acquire a single k-space.

4 shows an exemplary result of in vivo measurement according to an embodiment of the present invention. The example result may be a banding-free slice image 342 as it is in 3 is described, act. The sectional view in 4 shows the result of an in vivo measurement, usually reconstructed. When the cross-sectional image may be after the in 2 produced section described method 342 act. In the embodiment shown, the following parameters were used: 1000 projections with a phase cycle increment of 0.36 ° per projection, a repetition time TR = 8 ms, a flip angle of 40 ° and a resolution of 1.37 × 1.37 mm ² .

5 shows a schematic flow diagram of a method 200 with a KWIC filter according to an embodiment of the present invention. In the process 200 it can be in the 2 or 4 shown method 200 for generating a sectional image of a male object using a magnetic resonance tomography device. A radially absorbed k-space 340 has a predetermined number of projections 330 . 332 . 334 . 336 . 338 which overlap in the center of k-space. In step 224 When applied, a so-called k-space weighted image contrast filter or KWIC filter is applied to the recorded k-space. Each application of the KWIC filter results in a picture. Thus one obtains from a k-space a multitude of pictures of different phase cycles. As a rule, a KWIC filter for the k-space center uses not just a single projection but several projections. The resulting contrast is thus difficult to assign to a very particular phase cycle (such as ΔΘ ₁ ), but rather to a range of adjacent phase cycles. A picture 554 can be assigned to a phase cycle. A picture 556 can be assigned to a further phase cycle etc.

In an alternative embodiment, in step 224 of applying to the acquired k-space, a special k-space filter that uses only a low number of projections for the k-space center, the projection number for outer areas of the k-space is gradually increased.

6a to 6d show exemplary results of a method 200 with a KWIC filter according to an embodiment of the present invention. In the sectional images as a result of a process 200 it can be a result of in 2 or in 5 described method 200 for generating a sectional image of a male object using a magnetic resonance tomography device. The four figures are the same record as in 4 based. The four different mappings are reconstructed to different phase cycles with a KWIC filter. So will in 6a a first sectional picture 554 that is, an image of phase cycle ΔΘ ₁ and a range of adjacent phase cycles, 6b shows a second sectional image 556 that is, an image to a range of phase cycle ΔΘ _{2 of} adjacent phase cycles. 6c shows a third sectional view 658 that is, an image to a range of phase cycle ΔΘ _{3 of} adjacent phase cycles and 6d shows a fourth sectional image 659 that is, an image to a range of phase cycle ΔΘ _{4 of} adjacent phase cycles.

7 shows an exemplary sequence schematic of a balanced SSFP sequence according to an embodiment of the present invention. The representation can be understood as an RF pulse and a readout gradient or readout gradient. The waveform of the RF pulse has two peaks that are spaced apart by a repetition time TR. The gradient pulse is arranged symmetrically at a time TR / 2 in the middle between the two peaks of the RF pulse. Relative to the Cartesian coordinate system, time is plotted on the abscissa and amplitude on the ordinate.

In an alternative embodiment, the gradient pulse is arranged symmetrically about another point (TE ≠ TR / 2). The gradient must be completely balanced after a repetition time TR, ie the sum over the gradient moment must be zero. Thus, there is no dephasing of the magnetization due to gradients.

The balanced SSFP sequence, also referred to as bSSFP, TrueFISP, FIESTA, balanced FFE, is a fast gradient echo sequence (repetition time TR 3-15 ms). There is no gradient spoiling or radio frequency (RF) spillover. As a result, no magnetization is destroyed, resulting in a very high signal-to-noise ratio (SNR). Under certain conditions (constant flip angle, constant repetition time TR, constant phase cycle) after a certain number N of RF pulses (N ~ 3 * T1 / TR to N ~ 5 * T1 / TR, depending on the source), an equilibrium of the magnetization is formed , also called "Steady State", out. The contrast depends on the spin density as well as the tissue-specific relaxation times T1 and T2 and is typically weighted T2 / T1, but may be due to the use of Magnetisierungspäparationen be influenced.

8th shows a diagram of a frequency response function (FRF) 870 according to an embodiment of the present invention. In a Cartesian coordinate system is called frequency response function 870 the signal strength plotted against the off-resonance angle. Thus, on the abscissa an off-resonance angle Θ in degrees [°] and on the ordinate a signal strength S _stst indicated. The scale of the ordinate begins with zero at the origin and extends up to the value 0.4. The scale of the abscissa ranges from -500 ° over 0 ° to + 500 °. The frequency response function 870 has two banding artifacts at approximately -180 ° and + 180 °. on; otherwise it will range between 0.3 and 0.4 on the ordinate.

An intrinsic disadvantage of the balanced SSFP sequence (bSSFP sequence) is the strong susceptibility to off-resonance effects. Local B ₀ field inhomogeneities ΔB ₀ impress the spins on a phase angle θ = γ · ΔB ₀ · TR (γ is the gyromagnetic ratio in this case). In contrast to spotted sequences (FLASH etc.), the signal strength of the bSSFP sequence varies widely at different off-resonances, shown in 8th , This curve is also called frequency response function (FRF). Spins whose off-resonance angle falls within a signal minimum appear dark / black in the picture, which is referred to as a so-called banding artifact. These banding artifacts can significantly reduce or even eliminate the diagnostic utility of an image.

B ₀ field inhomogeneities, and thus also banding artefacts, occur more frequently when using higher field strengths as well as at transitions with large susceptibility jumps, such as air-tissue interfaces or implants. With long repetition times TR, the spins are imprinted with a larger phase angle, which leads to more banding artifacts occurring. Length repetition times TR can be necessary, for example, if a high resolution is desired.

The phase angle imposed on the spins is proportional to the repetition time TR. Consequently, the signal minimums of a frequency response function, or "Frequency Response Function", and thus also the occurrence of banding artifacts, are indirectly proportional to the repetition time TR. Thus, by using short repetition times TR, the plateau of the frequency response function FRF can be stretched to provide equal / similar signal strength for a wide range of off-resonances. Due to hardware limitations and to improve patient comfort and maintain patient safety due to peripheral nerve stimulation, the repetition time TR can only be reduced to a certain lower limit (~ 3 ms). Thus, while banding artifacts caused by low field inhomogeneities can be effectively prevented, banding artifacts still occur with larger field inhomogeneities. Furthermore, when using small repetition times TR as well as large flip angles (> 40 °) and / or high or ultra-high field strengths (3 T, 7 T, 9.4 T, ...), the specific absorption rate (SAR) limit very quickly reached or exceeded. The Specific Absorption Rate (SAR) describes the RF energy deposited in a patient and is subject to limits.

9 Figure 4 shows a detection scheme of images at different phase cycles with a balanced SSFP sequence. In a Cartesian coordinate system, the abscissa shows the time and the ordinate of the RF phase cycle. With a phase cycle of 90 °, 180 °, 270 °, 360 °, an auxiliary line is drawn into the diagram. Time intervals are marked on the time axis. On a preparation phase 980 follows a detection of a first image 982 followed by a wait 984 , After the wait 984 may be a new cycle with a preparation phase 980 start around in the following phase 986 capture a second image, followed by a wait 984 , The first preparation phase 980 as well as the capture 982 of the first image take place at a phase cycle of 0 °, the second preparation phase 980 as well as the capture 986 of the second image take place at a phase cycle of 90 °. The cycle described above can be continued continuously.

As in 9 One method of suppressing banding artifacts is to capture multiple images at different phase cycles. Here, the phase of the RF pulse is increased by various increments (180 ° phase cycle: α _x, α _-x, α _x, α _-x, α _x, α _-x, α _x, α _-x, ... 0 ° phase cycle: α _x , α _x , α _x , α _x , α _x , α _x , α _x , α _x , ..., 90 ° phase cycle: α _x , α _-y , α _-x , α _y , α _x, α _y, α _-x, α _y, ...; the index respectively designated the axis, ie the direction in which the RF pulse is irradiated α). The acquisition of the next image is started when the spins are again in "steady state", possibly after a waiting time and after loading of preparation pulses without data acquisition, flip angle preparation or the like. The resulting images can be subsequently combined in a variety of different ways, so as to obtain an (almost) artifact-free image. Usually, at least two, usually four images are acquired at different phase cycles. While with this kind of methods in the acquisition of a correspondingly large number of pictures at different phase cycles Banding artifacts are effectively suppressed even with larger field inhomogeneities, the limiting factor here is the measurement time. In order to obtain a banding-free balanced SSFP image, multiple images must be acquired. Together with the waiting time required between the images and the time to reach the "steady state", the clinical operational capability is thus limited. In regions with very large field inhomogeneities (eg implants, metal) signal variations caused by off-resonance are often visible.

As stated below by means of 10 1, a method to avoid the need to wait for the spins to relax and the steady state to be achieved between images at different phase cycles and thus shorten the measurement time is based on using a dynamic phase cycle. In this case, the phase of the RF pulses is not linearly increased, but quadratic. The phase cycle used therefore increases by a constant value for each RF excitation. If this value is small enough (<~ 4 °), the "steady state" remains. After reaching the "steady state" N images are taken during a continuous shift of the phase cycle by a total of 360 °. Corresponding k-space lines thus have a difference of their underlying phase cycle by 360 ° / N. These lines are then combined and the (banding-free) image is reconstructed from the resulting k-space. Since no waiting time between the acquisition of the different images is necessary with this method, the measuring time is compared to the 9 shortened significantly. Nevertheless, here too only a very limited number of images are recorded at different phase cycles, so that banding artifacts can remain with strong field inhomogeneities.

10 Figure 4 shows a detection scheme of images of a frequency modulated SSFP sequence. In a Cartesian coordinate system, the time on the abscissa and a phase cycle in force on the ordinate are shown. On a preparation phase 980 follows the capture 982 a first picture 554 to then successively take a second picture 556 capture 986 as well as the capture 1090 a third picture 1094 and finally the recording 1092 a fourth picture 1096 , In every phase 982 . 986 . 1090 . 1092 of capturing an image 554 . 556 . 1094 . 1096 a complete k-space is detected. In the preparation phase 980 the phase cycle is 0 °. A first picture 554 associated phase cycle is 45 °, a phase cycle associated with the second image is 135 °, a phase cycle associated with the third image is 225 °, and a phase cycle associated with the fourth image is 315 °. An arrow marks a fully captured k-space, with a phase cycle of 45 °. The arrow marks the k-space during the first phase 982 capturing the first slice image 554 is detected.

In one embodiment, the phase cycles of the first image differ 554 and simultaneously or alternatively the second image 556 from the in 5 described phase cycles.

11 shows a detection scheme according to an embodiment of the present invention. The detection scheme may be another representation of the in 2 described method 200 for generating a sectional image of an object to be recorded for a magnetic resonance tomography device. In a Cartesian coordinate system, the time is plotted on the abscissa and the ordinate of the phase cycle is plotted on the abscissa. A preparation phase 980 is from a capture 1198 followed by k-space. During acquisition, the phase cycle is adjusted for each projection. In the illustrated embodiment, the angle of the projection is selected on the principle of the golden angle. At the end of the collection period 1198 was once the entire phase cycle through 360 °. Depending on the embodiment, adjacent phase cycles have a constant or alternatively a variable distance.

In terms of in 2 The method described can be the preparation phase 980 the optional step 226 be assigned to the dissection. The step 220 of comprehension may be in the temporal history in 11 the phase 1198 to be associated with the detection of k-space. So can the phase 1198 of grasping the step 222 of combining and the step 224 of applying. The step 222 Combining may be parallel to the step in one embodiment 220 of detecting. The step of detecting may be performed once a k-space has been completely detected.

In one embodiment, after a first pass through the step 220 of grasping, the step 222 of combining and step 224 applying in a second pass in the step of detecting 220 at least one further projection of the object to be recorded is detected using at least one further pulse sequence (balanced SSFP sequence) with at least one further phase cycle and in the second pass in the step 222 combining combines the at least one further projection with the series of projections in another k-space. The reference numerals correspond to those in FIG 2 described method 200 and relate to this. In this case, at least one old projection of the series of projections is discarded, wherein the at least one old projection at least one temporally the oldest projection of the series of projections or a projection of the series of projections with one of the at least one further projection similar or the same assignable phase cycle represents. In step 224 applying the reconstruction to the further k-space, a further sectional image is obtained, wherein the further sectional image is a sectional image recorded temporally after the first sectional image. Relative to the illustration in 11 In other words, this means that in one embodiment, the first detected, in particular radial, projection is rejected and replaced by a current projection produced in particular with the same parameters. Thus, a current cross-sectional image can always be reconstructed after capturing a single projection.

An exemplary embodiment of a corresponding application is the already described fat-water separation.

In one embodiment, the k-space is acquired by means of a radial balanced SSFP sequence. The phase cycle increment is chosen so that a total of 360 ° has been passed after the acquisition of all projections. Alternatively, multiples of 360 ° can be traversed. The k-space can be filled in different ways with radial projections. In practice, an arrangement has been found in which each successive radial projections according to the golden angle (111.2 °) are arranged. Other arrangement schemes are possible as described below, for example three paragraphs below.

Although theoretically any number of radial projections can be measured, it has been shown in practice that the best results from about 500, in particular 720, projections, that is, with a phase cycle increment of at most 360 ° / 500 projections = 0, 72 ° / projection or alternatively of at most 360 ° / 720 projections = 0.5 ° / projection can be achieved. A further increase in the number of projections (= reduction of the phase cycle increment) gives better results, a reduction has a negative effect on the image quality. It should be noted at this point that this value is independent of the readout resolution used, the phase cycle increment is the limiting factor. Consequently, it is recommended to use high-resolution (eg. 384 or 512 or even beyond), because of the phase cycle increment already many projections are required and thus the Nyquist criterion is already met for higher resolutions. For example, a 2D measurement with a layer thickness of 5 mm can be performed. 3D measurements are also possible.

At the beginning of the acquisition or acquisition of the first, in particular radial, projection, the excited spins should already be in "steady state". This can be ensured, for example, by means of a sufficient number (typically greater than 100, alternatively greater than 200, alternatively greater than 500, alternatively greater than 1000, alternatively greater than 1500) of preparation pulses without data acquisition. It is possible here to record all these preparation pulses with a constant phase cycle. Furthermore, the phase cycle of the preparation pulses can already be incrementally increased, which achieved better results in practice. In both cases, the first projection is recorded with the phase cycle following the last preparation pulse, and the phase cycle of the subsequent projection is then increased accordingly. Image acquisition is also possible if spins are not yet in "steady state", but this leads to a lowering of image quality.

In addition to an arrangement of the radial projections according to the golden angle (~ 111.2 °), other schemes are conceivable: So have been in an arrangement according to pseudo or quasi random numbers also shown good results. Furthermore, a linear arrangement is conceivable.

The presented 2D sequence can be extended to 3D. Here, a "stack of stars" coverage is possible in which the third dimension is scanned incrementally analogous to a Cartesian 3D imaging sequence. Here, the phase cycle increment in each partition must go through 360 °, that is, in each partition, the number of projections described above must be recorded.

Furthermore, a 3D radial measurement can be performed, with each projection passing through the k-space center. Since in this case a very high number of projections is necessary to fulfill the Nyquist criterion, the number of projections to be acquired can thereby be determined. Consequently, no additional projections need to be acquired to ensure a smaller phase cycle increment.

Instead of recording a fixed number of radial projections, projections can be taken continuously. From this images can be continuously reconstructed, always using the last recorded projections. The number of projections used depends on the phase cycle increment used, such that 360 ° are passed in each picture. Possible field of application for this alternative embodiment is dynamic imaging, such as surgical navigation or image guided surgery, also referred to as "image guided surgery".

Instead of the preparation pulses previously used for the preparation without data acquisition, a faster preparation scheme can be used. The prerequisite is that this reliably puts all spins (ie on- and off-resonant) in the "steady state".

If a radial trajectory is referred to in the description, then the statements also apply analogously to other trajectories, for example several spirals. The only requirement is that the recorded lines or projections overlap in the k-space center.

12 shows a block diagram of a device 110 according to an embodiment of the present invention. The device 110 has a facility 1202 to capture, a device 1204 to combine as well as a device 1206 to apply to. In the in 12 embodiment shown, the device 110 for generating a sectional image of an object to be photographed for a magnetic resonance tomography device, an optional device 1208 to prepare for.

The device 1202 for detecting is adapted to record a series of projections of the object to be recorded using a pulse sequence each having a phase cycle, wherein the phase cycle for each of the pulse sequences is different. The series of projections is superimposed in the center of a k-space. The device 1204 For combining, it is designed to combine the detected series of projections in the k-space so that the projections are superimposed in the center of the k-space. The device 1206 for applying, it is arranged to apply a reconstruction to k-space to obtain a sectional image of the object to be photographed as a function of the series of projections.

The device 110 For generating a sectional image of an object to be recorded, for example, in an in 1 described magnetic resonance imaging apparatus can be used.

The embodiments described and shown in the figures are chosen only by way of example. Different embodiments may be combined together or in relation to individual features. Also, an embodiment can be supplemented by features of another embodiment.

Furthermore, method steps according to the invention can be repeated as well as carried out in a sequence other than that described.

If an exemplary embodiment comprises a "and / or" link between a first feature and a second feature, then this is to be read so that the embodiment according to one embodiment, both the first feature and the second feature and according to another embodiment either only first feature or only the second feature.

LIST OF REFERENCE NUMBERS

100

Magnetic resonance imaging device

102

Basic field magnet

104

Object to be recorded

106

gradient coil

108

RF System

110

contraption

112

User interface

200

method

220

Step of grasping

222

Step of combining

224

Step of applying

226

(optional) step of dissection

330

first projection

332

second projection

334

third projection

336

fourth projection

338

nth projection

340

k-space

ΔΘ ₁

first phase cycle

ΔΘ ₂

second phase cycle

ΔΘ ₃

third phase cycle

ΔΘ ₄

fourth phase cycle

ΔΘ _n

nth phase cycle

342

Banding-free cut

552

special k-space filter / KWIC filter

554

Picture to phase cycle ΔΘ ₁

556

Picture to phase cycle ΔΘ ₂

558

Picture to phase cycle ΔΘ _n

658

Image for phase cycle ΔΘ ₃

659

Picture to phase cycle ΔΘ ₄

760

RF pulses

762

gradient

870

Frequency response function

980

preparation phase

982

Capture first picture

984

waiting period

986

Capture second image

1090

Capture third picture

1092

Capture fourth image

1094

Picture to phase cycle 225 °

1096

Picture to phase cycle 315 °

1198

Phase of the detection of k-space

1202

Device for detecting

1204

Device for combining

1206

Means for applying

1208

Device for preparing

Claims (12)

Procedure ( 200 ) for generating a sectional image ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) one object to be recorded ( 104 ) using a magnetic resonance tomography device ( 100 ), the process ( 200 ) has the following steps: capture ( 220 ) of a series of projections ( 330 . 332 . 334 . 336 . 338 ) of the object to be recorded ( 104 ) using a pulse sequence, in particular a balanced SSFP sequence, each having a dynamically increased phase cycle per projection, the phase cycle (ΔΘ ₁ , ΔΘ ₂ , ΔΘ ₃ , ΔΘ ₄ , ΔΘ _n ) being different for each of the projections, and where the projections ( 330 . 332 . 334 . 336 . 338 ) of the series of projections ( 330 . 332 . 334 . 336 . 338 ) in the center of a k-space ( 340 superimpose); Combine ( 222 ) of the projections ( 330 . 332 . 334 . 336 . 338 ) of the series of projections ( 330 . 332 . 334 . 336 . 338 ) in the k-space ( 340 ); and apply ( 224 ) of a reconstruction on k-space ( 340 ), to a sectional view ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) of the object to be recorded ( 104 ) as a function of the series of projections ( 330 . 332 . 334 . 336 . 338 ) to obtain. Procedure ( 200 ) according to claim 1, wherein in step ( 224 ) of applying for reconstruction, a special k-space filter is used, the special k-space filter for a k-space center using only a low number of projections, the number of projections for outer areas of k-space is gradually increased and / or in step ( 224 ) of applying for reconstruction, a k-space weighted image contrast filter (KWIC) ( 552 ) is applied to the sectional image ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) to obtain. Procedure ( 200 ) according to one of the preceding claims, wherein in step ( 220 ) of detecting it in the series of projections ( 330 . 332 . 334 . 336 . 338 ) are a series of projections that cross in the k-space center and / or are radial projections and / or are a series of spirally-arranged projections. Procedure ( 200 ) according to one of the preceding claims, wherein the successive projections are arranged in a linearly increasing angle and / or in a golden angle and / or with a pseudorandom and / or quasi-random angle in k-space. Procedure ( 200 ) according to one of the preceding claims, in which in step ( 220 ) of capturing as the series of projections ( 330 . 332 . 334 . 336 . 338 ) at least a predetermined number of projections ( 330 . 332 . 334 . 336 . 338 ), in particular at least a number of 500 projections, in particular at least a number of 720 projections is detected. Procedure ( 200 ) according to one of the preceding claims, with a step ( 226 ) of preparing the object to be photographed ( 104 ), wherein a plurality of preparation pulses is emitted. Procedure ( 200 ) according to one of the preceding claims, wherein after a first pass through the step of detecting ( 220 ), the step of combining ( 222 ) and the step of applying ( 224 ) in a second pass in the step of detecting ( 220 ) at least one further projection ( 330 . 332 . 334 . 336 . 338 ) of the object to be recorded using at least one further pulse sequence, in particular a balanced SSFP sequence, with at least one further phase cycle (ΔΘ ₁ , ΔΘ ₂ , ΔΘ ₃ , ΔΘ ₄ , ΔΘ _n ) is detected and in the second pass in the step of Combining ( 222 ) the at least one further projection ( 338 ) with the series of projections ( 330 . 332 . 334 . 336 . 338 ) is combined in a further k-space (340), wherein at least one old projection ( 330 . 332 . 334 . 336 ) of the series of projections ( 330 . 332 . 334 . 336 . 338 ), the at least one old projection ( 330 . 332 . 334 . 336 ) at least one temporally oldest projection of the series of projections ( 330 . 332 . 334 . 336 . 338 ) or a projection of the series of projections ( 330 . 332 . 334 . 336 . 338 ) with one of the at least one further projection ( 338 ) is a similar or identifiable phase cycle, and wherein in the step of applying ( 224 ) of the reconstruction on the further k-space ( 340 ), a further sectional image is obtained, wherein the further sectional image is a time after the sectional image ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) captured sectional image represents. Procedure ( 200 ) according to one of the preceding claims, in which the steps of detecting ( 220 ), Combining ( 222 ) and applying ( 224 ), wherein a plurality of k-spaces ( 340 ) and a plurality of sectional images ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) which, as a stack, forms a three-dimensional image of the object to be photographed ( 104 ) and / or in step ( 220 ) of capturing the series of projections ( 330 . 332 . 334 . 336 . 338 ) describe a three-dimensional radial trajectory superimposed in a three-dimensional k-space in the center, wherein in step ( 224 ) of applying the reconstruction, a three-dimensional sectional image of the object to be photographed ( 104 ) is reconstructed as a function of the series of projections. Contraption ( 110 ) for generating a sectional image ( 342 ; 554 . 556 . 558 ; 20 1094 . 1096 ) of an object to be recorded ( 104 ) for a magnetic resonance tomography device ( 100 ), which is adapted to the steps of a method ( 200 ) according to one of Claims 1 to 8 in corresponding facilities ( 1202 . 1204 . 1206 . 1208 ). Magnetic Resonance Imaging Device ( 100 ) with a device ( 110 ) for generating a sectional image ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) according to claim 9. Using a procedure ( 200 ) for generating a sectional image ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) of an object to be recorded ( 104 ) according to any one of claims 1 to 8 for a fat-water separation based on a Dixon method. Computer program product with program code for carrying out the method ( 200 ) according to one of claims 1 to 8, when the program product is stored on a device ( 110 ) for generating a sectional image ( 342 ; 554 . 556 . 558 ; 1094 . 1096 ) of an object to be recorded ( 104 ) for a magnetic resonance tomography device ( 100 ) is performed.

Images