Magnetic resonance tomography apparatus and method for generating a magnetic resonance image of a subject with different pulse sequences in K-space

[0002] 1. Field of the Invention

[0003] The present invention relates to a magnetic resonance tomography apparatus and to a method for generating a magnetic resonance image of a subject, of the type wherein k-space is scanned by different pulse sequences.

[0004] 2. Description of the Prior Art

[0005] In a magnetic resonance tomography apparatus for generating a magnetic resonance image of a subject, such as a slice of the human body, successive pulse sequences of one or more high-frequency pulses and one or more gradient pulses are emitted into the area of the subject to be scanned. The resulting echo signals are scanned and are used for generating a magnetic resonance image of the area. The local space, in which the subject is situated, or in which the part of the subject to be scanned is situated, is scanned in Cartesian coordinates (x, y or x, y, z) in k-space, which is a Fourier space that is inverse to the local space. The parameter k is also referred to as “normalized time” or “spatial frequency” and results from the product of the gyromagnetic relationship with the gradient field (-portion) and the time. When k-space is scanned by successive pulse sequences, the magnetization dependent on the spatial frequency k results as a measurement signal. On the basis of a fast Fourier transformation (FFT), the desired spatial distribution of the magnetization in the region to be scanned results. K-space can be two-dimensionally scanned or three-dimensionally scanned. Given a two-dimensional scanning of k-space in Cartesian coordinates, kx normally indicates the scanning direction, whereas k_yrefers to the phase coding direction. The basic magnetic field extends in the z-direction. Therefore, k-space is scanned by rows in the k_xdirection by means of a readout gradient G_x. Modulation of the phase coding gradient G_yenables a step-by-step scanning of k-space in a row by row fashion. The sequence of the scanning of k_xrows can vary dependent on the application. The middle region of k-space, i.e., the region having smaller k-values, contains the bits of information for the low spatial frequencies and therefore is controlling for the contrast of the received image. The outer regions of k-space, i.e., the larger k-values, contain the bits of information for the higher spatial frequencies and therefore determine the resolution of the received image.

[0006] In order to optimize the two opposing goals of image quality and short measuring time, different proposals have been made. One of the proposals is to utilize different pulse sequences during the scanning of k-pace. This utilization of sequences referred to as hybrid sequences includes the utilization of different pulse sequences, for example, such as FLASH pulses in the central area of k-space and EPI pulses in the outer areas of k-space, as proposed by P. M. Jacob et al. in “MR-CAT SCAN: Cardiac Imaging with a new Hybrid Approach”, ISMRM 1999, No. 1306. It has also been proposed to use different pulse sequences of one single pulse sequence type, however, with different parameters, see Hillenbrand et al. in “MR CAT Scan: A Modular Approach for Hybrid Imaging”, ESMRMB 1999, No. 261.

[0007] A problem during the use of different pulse sequences or heterogenous pulse sequences during the scanning of k-space is that strip artifacts caused by phase jumps and amplitude jumps occur in the reconstructed image. A cause of these artifacts can be discontinuities between scanning steps that do not immediately follow one another and between different segments in the scanning of k-space. FIGS. 3A, 3B and 3C herein show an example. FIG. 3A shows a scan in two-dimensional k-space, whereby the readout direction corresponds to the k_xdirection and the phase direction to the k_ydirection, for example. K-space is scanned by two sequences (sequence 1 and sequence 2), with sequence 1 used in the middle region and sequence 2 in the outside regions. The sectional view of FIG. 3B shows the amplitude jumps in the measuring signal given the transition between sequence 1 and sequence 2. Subsequent to the transformation of the received measuring values by means of a fast Fourier transformation in the spatial domain, these jumps lead to strip artifacts in the reconstructed image, as shown in FIG. 3C. The prior art approaches have tried to minimize the jumps occurring as a result of the use of different pulse sequences when k-space is scanned by raw data filters, such as Hanning filters. FIGS. 4A, 4B and 4C show this course of action. FIG. 4A schematically shows the application of a raw data filter to the scanning shown in the FIGS. 3A and 3B. On the basis of the applied raw data filter, the measuring values received during the scan are smoothed with respect to a continuous curve (see FIG. 4B). The Fourier transformation of the smoothed raw data results in the subject profile as shown in FIG. 4C. The disadvantages of the application of raw data filters, such as a considerable loss in image sharpness, are obvious when the FIGS. 4C and 3C are compared.

[0008] An object of the present invention therefore is to provide a magnetic resonance tomography apparatus for generating a magnetic resonance image of a subject and to provide a method for generating a magnetic resonance image of a subject, wherein an optimally efficient image quality is achieved despite the utilization of different pulse sequences when k-space is scanned.

[0009] This object is achieved in a magnetic resonance tomography apparatus having a gradient field system for generating gradient pulses, a high-frequency system for generating high-frequency pulses and for scanning echo signals, and a control apparatus for driving the gradient field system and the high-frequency system such that k-space corresponding to a region of the subject to be scanned is scanned by successive pulse sequences composed of one or more high-frequency pulses and one or more gradient pulses with at least one first pulse sequence and a second pulse sequence that is different from the first one being used, and wherein a transition area, within which the first pulse sequence is converted step-by-step into the second pulse sequence, is defined between the first pulse sequence and the second pulse sequence.

[0010] It has shown that the strip artifacts and jumps in the reconstructed image, which are caused by the utilization of different pulse sequences, can be significantly reduced or even completely prevented by such a step-by-step transition of the one pulse sequence into the other pulse sequence. It is thus possible to optimally adjust the measuring parameters desired for the generation of a magnetic resonance image, such as image quality and measuring time, in a simple way.

[0011] The first pulse sequence and the second pulse sequence preferably belong to the same pulse sequence type and in the transition area, the sequence parameters of the first pulse sequence are adapted step-by-step to the sequence parameters of the second pulse sequence. This enables an almost-continuous transition of the first pulse sequence into the second pulse sequence. The sequence parameters, which are adapted step-by-step, are one or more parameters from the parameter group of echo time, repetition time, readout bandwidth, excitation angle etc. For example, the first pulse sequence and the second pulse sequence can be spin echo sequences, with the readout bandwidth of the echo being increased and the echo time and the repetition time being reduced in the transition area from the first pulse sequence to the second pulse sequence. The excitation angle can be simultaneously reduced in the transition area.

[0012] Alternatively, the first pulse sequence and the second pulse sequence can belong to different pulse sequence types and in the transition area, the pulse sequence type of the first pulse sequence is adapted step-by-step to the pulse sequence type of the second pulse sequence. An advantage is to calculate the step-by-step transition of the fist pulse sequence into the second pulse sequence on the basis of phase values and amplitude values of the two pulse sequences.

[0013] FIG. 1 is a schematic block diagram showing basic components of a magnetic resonance tomography apparatus according to the present invention.

[0014] FIG. 2A shows an example of an inventive scanning of k-space.

[0015] FIG. 2B shows a sectional view of the scan shown in FIG. 2A.

[0016] FIG. 2C shows a representation of a subject profile of the scan shown in the FIGS. 2A and 2B.

[0017] FIG. 3A, as noted above, shows a known scan of k-space.

[0018] FIG. 3B, as noted above, shows a sectional view of the known scan shown in FIG. 3A.

[0019] FIG. 3C, as noted above, shows a representation of a subject profile of the known scan shown in the FIGS. 3A and 3B.

[0020] FIG. 4A, as noted above, shows the application of a raw data filter to the scan shown in FIG. 3A.

[0021] FIG. 4B, as noted above, shows a sectional view of the representation of FIG. 4A.

[0022] FIG. 4C, as noted above, shows a representation of a subject profile of the filtered scan shown in the FIGS. 4A and 4B.

[0023] FIG. 1 is a schematic representation of a magnetic resonance tomography apparatus for generating a magnetic resonance image of a subject according to the present invention. The structure of the magnetic resonance tomography apparatus corresponds to the structure of a conventional tomography apparatus. A basic field magnet 1 generates a chronologically constant strong magnetic field for the polarization or orientation of the nuclear spins in the examination area of a subject, such as a part of a human body to be examined. The high homogeneity of the basic magnetic field, which is necessary for obtaining magnetic resonance measurements, is defined in a spherical measuring volume M, into which a part of the human body to be examined is introduced. For supporting the homogeneity requirements and particularly for eliminating chronologically invariable influences, shim elements composed of ferromagnetic material are provided at suitable locations. Chronologically variable influences are eliminated by shim coils 2, which are driven by a shim current supply 15.

[0024] A cylindrical gradient coil system 3, which is composed of three sub-windings, is arranged in the basic magnet 1. Each sub-winding is provided with current by an amplifier 14 for generating a linear gradient field in one direction of a Cartesian coordinate system. The first sub-winding of the gradient field system 3 thereby generates a gradient G_xin the x-direction, the second sub-winding generates a gradient G_yin the y-direction and the third sub-winding generates a gradient G_zin the z-direction. Each amplifier 14 has a digital-analog converter, which is driven by a sequence control 18 for the chronologically correct generation of gradient pulses.

[0025] A high-frequency antenna, which converts the high-frequency pulses emitted by a high-frequency power amplifier 30 into a magnetic alternating field for exciting the nuclei and orienting the nuclear spins of the subject to be examined, or the area of the subject to be examined, is situated within the grading field system 3. The alternating field produced by the precessing nuclei, i.e., normally the magnetic resonance echo signals caused by a pulse sequence composed of one or more high-frequency pulses and one or more gradient pulses, is also translated into a voltage by the high-frequency antenna 4. This voltage is supplied via an amplifier 7 to a high-frequency reception channel 8 of a high-frequency system 22. The high-frequency system 22 also has a transmission channel 9, in which the high-frequency pulses are generated for exciting the nuclear spins. The respective high-frequency pulses are digitally represented as a sequence of complex numbers in the sequence control 18 dependent on a pulse sequence prescribed by a system computer 20. This number sequence, as a real part and as an imaginary part, is supplied via an input 12 to a digital-analog converter in the high-frequency system 22 and from there is supplied to a transmission channel 9. In the transmission channel 9, the pulse sequences are modulated onto a high-frequency carrier signal, whose base frequency corresponds to the resonant frequency of the nuclear spins in the measuring volume.

[0026] The changeover from the transmitting operation to the receiving operation ensues via a transmission-reception switch 6. The high-frequency antenna 4 radiates the high-frequency pulses for exciting the magnetic resonances into the measuring volume M and receives resulting echo signals. The correspondingly acquired magnetic resonance signals are phase-dependently demodulated in the reception channel 8 of the high-frequency system 22 and are converted via a respective analog-digital converter into a real part and an imaginary part of the measuring signal. In the image computer 17, an image is reconstructed from the thus-acquired measuring data. The measuring data, image data and control programs are administered by the system computer 20. By means of control programs, the sequence control 18 controls the generation of the respectively desired pulse sequences and the corresponding scanning of k-space. The sequence control 18 particularly controls the chronologically correct switching of the gradients, the transmission of the high-frequency pulses with a defined phase and amplitude, and the reception of the magnetic resonance signals. A synthesizer 19 provides the time basis for the high-frequency system 22 and the sequence control 18. The selection of corresponding control programs for generating a magnetic resonance image and the representation of the generated magnetic resonance image ensues via a user interface terminal 21 having a keyboard and one or more screens.

[0027] The scanner components are contained in a shielded room or chamber indicated by dot-dash lines, with all connections proceeding through a feed-through panel 10.

[0028] In accordance with the invention, k-space corresponding to the area of a subject to be represented is scanned (read out) by successive pulse sequences composed of one or more gradient pulses and one or more high-frequency pulses, using at least one first pulse sequence and a second pulse sequence that is different from the first, and with a transition area, within which the first pulse sequence is converted step-by-step into the second pulse sequence, being defined in k-space between the first pulse sequence and the second pulse sequence. FIG. 2A shows an example in the scanning of two-dimensional k-space in Cartesian coordinates. The readout direction corresponds to the k_x-direction, for example, whereas the phase direction or phase encoding direction corresponds to the k_y-direction. The invention also is employable for scanning three-dimensional k-space. In the example shown in FIG. 2A, the middle area of k-space, i.e., the area of small k_y-values, is scanned by a first pulse sequence (sequence 1), whereas the outer areas, i.e., the areas having larger k_y-values, are scanned by a second pulse sequence (sequence 2). For the transition from the first pulse sequence to the second pulse sequence, the transition area (sequence 1/2) is respectively defined, in which the first pulse sequence is converted step-by-step into the second pulse sequence.

[0029] FIG. 2B shows a section along the line IIA—IIA of FIG. 2A. The measuring signals, which are received by the first sequence (sequence 1) and which have smaller k_y-values, are represented in the middle region, whereas the outer regions show the measurement values received as a result of the scan with the second pulse sequence (sequence 2). As a result of the step-by-step transition from the first pulse sequence into the second pulse sequence, a soft and relatively smooth transition is obtained in the transition region. This results, after the subsequently carried out fast Fourier transformation, in a subject profile as shown in FIG. 2C, for example. It can be seen from FIG. 2C that—despite the difference between the pulse sequences—a sharp image that is high in contrast can be produced by the step-by-step transition between the first pulse sequence and the second pulse sequence. The transition pulse sequences used in the transition region between the first pulse sequence and the second pulse sequence are calculated in the system computer 20, which correspondingly drives the sequence control 18 of the magnetic resonance tomography apparatus of FIG. 1.

[0030] Dependent on the form of the first pulse sequence and the second pulse sequence, the pulse sequences used in the transition area are set in different ways. If the first pulse sequence and the second pulse sequence belong to the same pulse sequence type, the sequence parameters of the first pulse sequence type are adapted step-by-step to the sequence parameters of the second pulse sequence. The modified parameters that are adapted to one another can be the echo time, the repetition time, the readout bandwidth and/or the excitation angle etc., for example. The system computer 20 thereby performs a type of dissolving calculation determining the degree and the speed for the transition between the first pulse sequence and the second pulse sequence. For example, if both the first pulse sequence and also the second pulse sequence are a spin echo sequence, the readout bandwidth can be increased in the transition region from the first pulse sequence to the second pulse sequence and the echo time and repetition time can be simultaneously reduced. Furthermore, the excitation angle can also be reduced in the transition area. A complete scanning of k-space thus is achieved given a simultaneous reduction of the measuring time and maintenance of a sufficiently high image quality. In order to increase the signal-to-noise ratio, it can be started in the center of k-space (k_y=0) with a lower readout bandwidth than usual.

[0031] In a more general case, the first pulse sequence and the second pulse sequence belong to different pulse sequence types, and the pulse sequence type of the first pulse sequence is adapted step-by-step in the transition area. The system computer 20, in turn, carries out a type of dissolving calculation, which determines the degree and the speed of this transition. Hybrid pulse sequences simultaneously having features of the first pulse sequence type and of the second pulse sequence type can thereby arise in the transition area. Regardless of whether the first pulse sequence and the second pulse sequence belongs to the same or different pulse sequence types, the phase values and the amplitude values of the respective high-frequency pulses and gradient pulses are used as a basis for calculating the transition sequences. For example, in the center of k-space (small k_y-values), it can be started with a SE sequence, which accelerates in a multishot-SE-EPI pulse sequence in the outer k-space areas with an increasing EPI factor, given the utilization of different pulse sequence types. In another version, it could be started with a gradient echo sequence, which converts into a FID-EPI sequence. A reduction of the measuring time and a simultaneous maintenance of the image quality, particularly of the image sharpness, is obtained by both possibilities. Furthermore, it can be started with a TSE sequence in the center of k-space, the TSE converting into a TGSE sequence by additional gradient echoes. It can also be started with a gradient echo sequence, which successively converts into a turbo GSE sequence having an increasing echo length. A reduction in measuring time and a simultaneous maintenance of the image quality is also obtained by these two possibilities.

[0032] It should be noted that the present invention is not limited to the utilization of a first pulse sequence and a second pulse sequence with a transition region situated therebetween, but an arbitrary number of different pulse sequences can be used with transition areas situated therebetween.

[0033] Although modifications and changes may be suggested by those skilled in the art, it is in the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.