Abstract:
In a method and a magnetic resonance (MR) scanner for producing an MR image of a volume portion of a scanned object, MR data are acquired by repeatedly executing the following steps:
       radiating an RF excitation pulse,   applying magnetic field gradients for spatial encoding of a magnetization excited by the RF excitation pulse, with only two directions in space being encoded by the spatial encoding, and   acquiring essentially a single k-space point for each radiated RF excitation pulse.       
 
     An MR image is reconstructed from the MR data thusly acquired.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for producing magnetic resonance (MR) images wherein, for acquiring MR data, essentially only one point in k-space is sampled on the basis of an RF excitation pulse. The present invention also relates to a correspondingly designed magnetic resonance scanner. 
         [0003]    2. Description of the Prior Art 
         [0004]    For an MR scan, it is customary according to the prior art to fill so-called k-space, which corresponds to the Fourier transform of the signal measured in the image domain, with scan data. A memory organized as k-space is usually filled line-by-line by acquiring, on the basis of the same RF pulse, a series of k-space points along a line during readout. Apart from Cartesian line-by-line sampling, k-space can be filled along any trajectory, e.g. along radial or spiral trajectories. Always on the basis of the same RF pulse, a particular number of k-space points are acquired directly one after another on the respective trajectory during the same readout. 
         [0005]    More rarely according to the prior art, so-called single point imaging (SPI) is used. Here the MR data is acquired in the k-space such that, on the basis of the same RF pulse, generally only one measuring point or rather k-space point is captured, so that during a repetition only a single k-space point (or only a very small number of k-space points compared to the number of k-space points of a k-space line) is acquired. The advantage of single point imaging is that it is extremely robust, as the encoding time (i.e. the time between excitation and data acquisition) can be maintained constant for all the k-space points. 
         [0006]    However, the disadvantage of single point imaging is that it is very time-consuming. For example, to acquire the MR data at an isotropic resolution of 1 mm and a matrix size of 128*128*16, a scan time of 30 minutes may be required, which is much too long for everyday clinical practice. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to reduce the scan time for single point imaging compared to the prior art. 
         [0008]    According to the present invention, a method is provided for producing an MR image of a volume portion of an examination object by means of a magnetic resonance scanner. This method includes the following steps. 
         [0009]    MR data are acquired by repeatedly executing the following sub-steps. 
         [0010]    An RF excitation pulse is radiated into a subject. 
         [0011]    Apply magnetic field gradients for spatial encoding of the magnetization of nuclear spins in the subject excited by the RF excitation pulse, wherein only two directions in space (and not the third) are encoded by this spatial encoding. In other words, only two magnetic field gradients are used, so that the essentially single k-space point to be acquired in the next step is defined in only two directions in space (and not in the third) by the spatial encoding produced by the magnetic field gradients. 
         [0012]    Essentially a single k-space point is acquired. For every RF excitation pulse applied, only one or significantly fewer than the average number of k-space points of a k-space line are inventively acquired. If on average n k-space points are acquired for each k-space line, the essentially single k-space point can have less than n/10 k-space points. 
         [0013]    Following these sub-steps, the MR image is reconstructed according to the MR data acquired. 
         [0014]    In contrast to the prior art, only k-space points within a plane are acquired with the present invention. This advantageously enables the MR data acquisition time to be greatly reduced compared to the prior art where k-space points are acquired in three-dimensional space. 
         [0015]    Within the scope of the present invention, another method for producing an MR image of a volume portion of an examination object using a magnetic resonance scanner is provided. Similar to the first described method according to the invention, the other inventive method includes the steps of acquiring MR data, and reconstructing the MR image according to the MR data acquired. 
         [0016]    Here, the MR data acquisition includes a first acquisition of the MR data in a central region of k-space and a second acquisition of the MR data outside of this central region of k-space. The central region of k-space is a region that includes the center of k-space and in particular has a point-symmetric shape with respect to the k-space center. If k-space is, for example, only two-dimensional, i.e. slice-shaped, the central region can then be, for example, a circular surface whose central point corresponds to the center of k-space. 
         [0017]    For the first acquisition of the MR data, the following steps are repeatedly implemented: an RF excitation pulse is radiated, magnetic field gradients are applied for spatial encoding of the magnetization of nuclear spins excited by the RF excitation pulse, wherein only two directions in space are encoded by this spatial encoding, and essentially a single k-space point is acquired for the RF excitation pulse that was radiated. 
         [0018]    In other words the first MR data acquisition corresponds to the MR data acquisition step from the first described method according to the invention. 
         [0019]    For the second MR data acquisition, the following steps are repeatedly carried out in the following sequence: another RF excitation pulse is radiated and k-space points lying on a radial spoke which in turn corresponds to a section of a straight line passing through the k-space center. This straight line section or spoke begins at the outer edge of the central k-space region and ends at an outer boundary of k-space. 
         [0020]    For the second MR data acquisition, MR data thus are not acquired using single point imaging, but rather line-by-line, i.e. all the k-space points of a spoke are acquired on the basis of an RF excitation pulse. 
         [0021]    In other words, the other method according to the invention combines the advantages of the inventive single point imaging, which is carried out according to the first described method according to the invention, with the advantages of line-by-line acquisition of the MR data. Advantageously, the important k-space points lying in the central region of k-space are acquired using the inventive single point imaging, whereas the k-space points lying outside the central region are effectively acquired using a known and quick method. 
         [0022]    As described for the two methods according to the invention, spatial encoding takes place in two spatial directions only, so that effectively a position within a plane is encoded. For the third spatial direction, which is perpendicular to the plane, the two following variants are used in accordance with the invention. 
         [0023]    In a first variant, the RF excitation pulse is selectively radiated, by a slice selection gradient being simultaneously applied during the selective RF excitation pulse. This slice selection gradient is in a direction perpendicular to the aforementioned in which the spatial encoding occurs. In other words, a slice is excited by the RF excitation pulse, so that spatial encoding in two directions is sufficient to acquire a k-space point. 
         [0024]    In the second variant, during the acquisition of the MR data, no other magnetic field gradient (either for slice selection or spatial encoding) is applied in the third direction perpendicular to the spatial encoding plane. Therefore, for the acquisition of a k-space point, effectively all the information along the third direction is acquired. The reconstructed MR image corresponds to a so-called projection scan. The RF excitation pulse is not selective in this variant. 
         [0025]    In addition to the advantage that only two-dimensional k-space has to be sampled for acquiring the MR data, the advantage of the two variants described above is that no fold-overs can occur in the third spatial direction which is often a problem with conventional single point imaging, for example, even if this were only to acquire k-space points within one plane. 
         [0026]    Advantageously, the two magnetic field gradients in the last described variant (projection scan) can also be applied during injection of the non-selective RF excitation pulse. 
         [0027]    The advantage of applying the magnetic field gradients during injection of the RF excitation pulse is that the magnetic field gradients can effectively produce their effect immediately after the RF excitation pulse, so that the encoding time (i.e. the time between RF excitation and the instant of acquisition of the k-space point or points), and therefore the time taken to acquire all the MR data is in total somewhat shorter than if the magnetic field gradients do not have to be, or must not be, ramped up until after the end of the RF excitation pulse. In addition, a scan during which the magnetic field gradients must be switched on and off for each scan frequency is very noisy, which can be a problem in everyday clinical practice and which is avoided with a projection scan (i.e. the second variant). 
         [0028]    According to a preferred embodiment of the invention, the essentially one k-space point corresponds to just a single k-space point. In this embodiment, a single measuring point in k-space is sampled after or in respect of each RF excitation by in particular acquiring the free induction signal. Because of the short echo time and the direct or pure phase encoding for spatial encoding, the single point imaging is virtually unaffected by B0 field inhomogeneities, by magnetic susceptibility variations, and by chemical shift artifacts. 
         [0029]    However, the methods according to the invention are also designed to include so-called enhanced single point imaging. With said enhanced single point imaging, the free induction signal is repeatedly sampled at successive (e.g. identical) time intervals, wherein a plurality of measurement points (e.g. ten) in k-space are acquired without, for example, the RF excitation pulse being re-generated each time to acquire each of said k-space points, as is the case with the previously described ‘pure’ single point imaging. 
         [0030]    In addition, it is possible for k-space to be only incompletely sampled for acquisition of the MR data. For this purpose, for example, random k-space points, which are acquired by means of single point imaging according to the invention, are determined using an algorithm as a function of a predefined acceleration factor. This algorithm is designed such that the resulting undersampling is less, the shorter the distance of the k-space point from the k-space center. In other words, the probability of a k-space point being acquired according to the invention is higher, the shorter the distance of the k-space point from the k-space center. The information concerning k-space points not acquired is ascertained by reconstructing the MR image using compressed sampling (“compressed sensing”). The information missing due to the k-space points not acquired can be replaced by iterative reconstruction of the MR image and a priori knowledge about the MR image to be reconstructed. 
         [0031]    Using compressed sampling enables acceleration by up to a factor of 16 to be achieved. With the usual line-by-line sampling of k-space according to the prior art, only complete lines can be omitted for undersampling. On the other hand, single point imaging according to the invention allows k-space to be randomly sampled as required, resulting in higher acceleration factors. 
         [0032]    It is inventively possible, after acquisition of the essentially single k-space point and prior to injection of the next RF excitation pulse, to apply, instead of the two magnetic field gradients, a so-called spoiler gradient with which the magnetization is dephased. 
         [0033]    The advantage of using such spoiler gradients is that the effect of the latest scan on the immediately succeeding scan is virtually non-existent, as the respective spoiler gradient effectively destroys the magnetization produced by the magnetic field previously applied for spatial encoding. 
         [0034]    Instead of the two magnetic field gradients, it is also inventively possible, following acquisition of the essentially single k-space point and prior to injection of the next RF excitation pulse, to apply two other magnetic field gradients for refocussing. 
         [0035]    The refocussing causes the magnetization due to the two magnetic field gradients for spatial encoding to be effectively undone again, so that also in this variant the effect of the last scan on the immediately succeeding scan is virtually non-existent. 
         [0036]    The methods according to the invention can be used in particular for the imaging of metal parts, e.g. metal implants. 
         [0037]    As described above, with single point imaging according to the invention, the encoding time can be kept constant for all the k-space points, so that the methods according to the invention are much less prone to abrupt changes in susceptibility occurring in the vicinity of metal implants than is the case with prior art methods. 
         [0038]    Also provided, as part of the present invention, is a magnetic resonance scanner for producing an MR image of a volume portion within an examination object. The magnetic resonance scanner has a basic field magnet, a gradient field system, one or more RF antennas and a control device for controlling the gradient field system and the RF antenna(s), for receiving the scan signals picked up by the RF antenna(s), and for evaluating the scan signals, as well as for producing the MR data. The magnetic resonance scanner is designed to radiate an RF excitation pulse using the RF antenna(s), to apply, by operation of the gradient field system, magnetic field gradients for spatial encoding of a magnetization excited by the RF excitation pulse, and to acquire essentially a single k-space point using the RF antenna(s), wherein only two directions in space are encoded by the spatial encoding. The magnetic resonance scanner reconstructs the MR image from the MR data acquired. 
         [0039]    Also provided as part of the present invention is another magnetic resonance scanner for producing an MR image of a volume portion within an examination object. This magnetic resonance scanner has a basic field magnet, a gradient field system, one or more RF antennas and a control device for controlling the gradient field system and the RF antenna(s), receiving the scan signals picked up by the RF antenna(s), and evaluating the scan signals, as well as for producing the MR data. The magnetic resonance scanner is designed to acquire MR data and reconstruct the MR image according to the MR data acquired. The acquisition of the MR data includes a first acquisition of the MR data in a central region of k-space and a second acquisition of the MR data outside the central region. The magnetic resonance scanner is designed to radiate an RF excitation pulse during the first acquisition of the MR data using the RF antenna(s), to introduce, by operation of the gradient field system, magnetic field gradients for spatial encoding of the magnetization of nuclear spins excited by the RF excitation pulse, and to acquire essentially a single k-space point using the RF antenna(s), wherein only two directions in space are encoded by means of spatial encoding. The magnetic resonance scanner is also designed to inject another RF excitation pulse during the second acquisition of the MR data using the RF antenna(s) and to acquire k-space points lying on a radial spoke which is on a straight line passing through the k-space center and begins at an edge of the central k-space region and ends at a k-space boundary. 
         [0040]    The advantages of the magnetic resonance scanners according to the invention are essentially the same as the advantages of the methods according to the invention that have been described in detail above. 
         [0041]    The present invention also encompasses a non-transitory, computer-readable data storage medium that can be loaded into a memory of a programmable control device or of a processing unit of a magnetic resonance scanner. The storage medium is encoded with programming instructions that cause the computer to perform all or various of the above described embodiments of the inventive methods. The storage medium may require program means, e.g. libraries and auxiliary functions, in order to implement the corresponding embodiments of the methods. 
         [0042]    The electronically readable data carrier may be a DVD, a magnetic tape or a USB stick, on which the electronically readable control information, is stored. When this control information is read from the data carrier and stored in a control device or processing unit of a magnetic resonance scanner, all the inventive embodiments of the above described methods can be executed. 
         [0043]    The present invention is particularly suitable for acquiring MR data within a volume portion in which, or in the vicinity of which a metal implant is located. Self-evidently, the present invention is not restricted to this preferred field of application, as the present invention can be used wherever there are magnetic field inhomogeneities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  shows a magnetic resonance scanner according to the invention. 
           [0045]      FIG. 2  shows k-space undersampled according to the invention. 
           [0046]      FIG. 3  shows instructions according to the invention for sampling k-space. 
           [0047]      FIG. 4  shows a sequence according to the invention for a projection scan. 
           [0048]      FIG. 5  shows a sequence according to the invention for slice-selective acquisition of k-space. 
           [0049]      FIG. 6  is a flowchart of a method according to the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0050]      FIG. 1  schematically illustrates a magnetic resonance scanner  5  (a magnetic resonance imaging or tomography machine). A basic field magnet  1  generates a strong, time-invariant magnetic field for polarization or rather orientation of the nuclear spin in a volume portion of an object O, such as e.g. a part under examination of a human body lying on a table  23  in the magnetic resonance scanner  5 . The high homogeneity of the main magnetic field required for MR scanning is defined in a typically spherical measurement volume M in which the human body parts to be examined are disposed, wherein this homogeneity may be disturbed by metal objects. To support the homogeneity requirements and in particular eliminate time-invariant effects, shim pieces of ferromagnetic material are mounted at suitable points. Time-variable effects are eliminated by shim coils  2 . 
         [0051]    A cylindrical gradient field system  3  composed of three sub-windings is used in the basic field magnet  1 . Each sub-winding is supplied with power by an amplifier to produce a linear (also time-variable) gradient field in the respective direction of the Cartesian coordinate system. The first sub-winding of the gradient field system  3  produces a gradient G x  in the x-direction, the second sub-winding a gradient G y  in the y-direction, and the third part-winding a gradient G z  in the z-direction. Each amplifier has a digital/analog converter which is controlled by a sequence controller  18  for properly-timed generation of the gradient pulses. 
         [0052]    Inside the gradient field system  3  are one or more antennas  4 , which convert the RF pulses emitted by an RF power amplifier into an alternating magnetic field to excite the nuclei and orientation of the nuclear spin of the object O to be scanned or more specifically of the region of the object O to be scanned. Each RF antenna  4  consists of one or more RF transmitter coils and one or more RF receiver coils in the form of an annular, preferably linear or matrix-shaped arrangement of component coils. The RF receiver coils of the respective radio-frequency antenna  4  also convert the alternating field arising from the processing nuclear spins, i.e. in general the spin echo signals produced by a pulse sequence of one or more radio-frequency pulses and one or more gradient pulses, into a voltage (measurement signal) which is fed via an amplifier  7  to an RF receive channel  8  of a radio-frequency system  22 . The radio-frequency system  22  which is part of a control device  10  of the magnetic resonance scanner  5  additionally comprises a transmit channel  9  in which the radio-frequency pulses exciting nuclear magnetic resonance are produced. On the basis of a pulse sequence predefined by the system computer  20 , the respective radio-frequency pulses are represented digitally in the sequence controller  18  as a sequence of complex numbers. This sequence of numbers is fed as a real part and imaginary part via respective inputs  12  to a digital/analog converter in the radio-frequency system  22  and from there to a transmit channel  9 . In the transmit channel  9 , the pulse sequences are modulated onto a radio-frequency carrier signal whose base frequency corresponds to the center frequency. 
         [0053]    The switchover from transmitting to receiving mode takes place via a diplexer  6 . The RF transmitter coils of the radio-frequency antenna(s)  4  inject the radio-frequency pulses for exciting the nuclear spin into the measurement volume M and resulting echo signals are sampled via the RF receiver coil(s). The correspondingly obtained nuclear resonance signals are demodulated in a phase-sensitive manner to an intermediate frequency in the receive channel  8 ′ (first demodulator) of the radio-frequency system  22 , digitized in the analog/digital converter (ADC), and fed out via the output  11 . This signal is again demodulated to the frequency 0. Demodulation to the frequency 0 and separation into a real component and an imaginary component takes place after digitization in the digital domain in a second demodulator  8 . An MR image or more precisely a three-dimensional image dataset is reconstructed from the thus obtained scan data by an image processor  17 . The scan data, the image data and the control programs are administered via the system computer  20 . On the basis of predefined control programs, the sequence controller  18  controls the generation of the required pulse sequences and the corresponding sampling of k-space. In particular, the sequence controller  18  controls the properly-timed application of the gradients, the emission of the radio-frequency pulses with defined phase amplitude, and reception of the nuclear resonance signals. The time base for the radio-frequency system  22  and the sequence controller  18  is provided by a synthesizer  19 . Corresponding control programs for generating an MR image, which are stored e.g. on a DVD  21 , are selected and the MR image is displayed via a terminal  13  comprising a keyboard  15 , a mouse  16  and a screen  14 . 
         [0054]      FIG. 2  shows an example the k-space points  31  which are acquired in the case of randomly sampled, two-dimensional k-space  30  extending in the spatial directions x and y. Whereas in the case of linear acquisition of the k-space points a certain statistical dependence exists between the k-space points acquired, the advantage of the single point imaging according to the invention is that any sampling scheme can be implemented, as no dependence of any kind exists between the k-space points to be acquired.  FIG. 3  shows another inventive sampling scheme for sampling two-dimensional k-space  30 . Within a central region  33  of k-space  30 , k-space points are acquired using single point imaging according to the invention. On the other hand, the k-space points outside this central region  33  are acquired on the basis of radial spokes  32  extending in a straight line from the edge  36  of the circular central region  33  to the edge  34  or  35  of k-space  30 . The reference character  34  applies to circular k-space  30  and the reference character  35  to square k-space  30 . The k-space points lying on a spoke  32  are acquired using a single readout on the basis of a single RF excitation pulse. 
         [0055]      FIG. 4  shows a sequence diagram of single point imaging according to the invention for a projection scan. For each k-space point to be acquired, a non-selective RF excitation pulse  41  is applied at a first point in time  44 . Using magnetic field gradients  62 ,  63 , wherein one magnetic field gradient  62  is oriented along the x-direction and the other magnetic field gradient  63  along the y-direction, spatial encoding takes place. After an echo time TE or encoding time, the MR data of the k-space point is acquired at a second point in time  45 . This sequence is then repeated, resulting in the specified repetition time TR. 
         [0056]    As no selective RF excitation pulses  41  are used for the projection scan shown in  FIG. 4 , the magnetic field gradients  62 ,  63  can remain applied even during injection of the RF excitation pulses  41 . 
         [0057]      FIG. 5  shows another sequence diagram of single point imaging according to the invention. Here, simultaneously with the selective RF excitation pulse  43 , a slice selection gradient  64  is applied at the first point in time  44  in order to selectively excite the magnetization within a predetermined slice. Because of the selective excitation, the two magnetic field gradients  62 ,  63  for spatial encoding in the x-direction and the y-direction are not ramped up until after the RF excitation pulse  43 . The echo time TE is therefore somewhat longer than in the sequence shown in  FIG. 4 . The slice selection gradient  64  is oriented along the z-direction and is therefore perpendicular to the directions x, y of the magnetic field gradients  62 ,  63 . 
         [0058]    At the second point in time  45  the single k-space point is acquired, which is specified by a time interval  42  at which analog/digital conversion (ADC) of the measured signal takes place. After this acquisition of the k-space point, instead of the magnetic field gradients  62 ,  63  a spoiler gradient  65  is applied in each case in order to dephase the magnetization. As a result of these spoiler gradients  65 , at the third time instant  46 , i.e. at the end of the repetition time interval TR or at the start of the next repetition time interval TR, virtually the same conditions in respect of the magnetization are present as at the start of the first repetition time interval TR shown in  FIG. 5 . 
         [0059]    Instead of the two spoiler gradients  65 , refocussing gradients can also be applied. Similarly to the spoiler gradients  65 , these refocussing gradients are applied after acquisition of the k-space point and are matched in amplitude and duration to the previously applied magnetic field gradients  62 ,  63  such that they again effectively undo the effect of the latter, i.e. phase shift. In other words, the moments of the respective refocussing gradient  65  and corresponding spatial encoding gradient  62 ,  63  add up to zero. The amplitude of one of the refocussing gradients  65  therefore has an opposite sign to the amplitude of the corresponding magnetic field gradient  62 ,  63 . Similarly to the spoiler gradients, the refocussing gradients  65  ensure that at the start of the next repetition time interval TR virtually the same conditions in respect of the magnetization are present as at the start of the previous repetition time interval TR. 
         [0060]      FIG. 6  shows a flow chart of an inventive method for producing an MR image. 
         [0061]    In step S 1 , a slice selection gradient is applied, wherein a selective RF excitation pulse is simultaneously injected in step S 2 , causing the magnetization to be excited within a slice. For spatial encoding in two directions, each perpendicular to the direction of the slice selection gradient, magnetic field gradients are applied in step S 3 . In step S 4 , a k-space point is acquired at the echo time. 
         [0062]    Steps S 1  to S 4  are repeated until a central region of two-dimensional k-space has been sampled. 
         [0063]    In the second part of the flow chart shown in  FIG. 6 , a slice selection gradient is again applied (step S 5 ), while in step S 6  a selective RF excitation pulse is injected. The k-space points lying on a radial spoke are then acquired, for which purpose at least one magnetic field gradient for frequency encoding is generally applied. 
         [0064]    Steps S 5  to S 7  are repeated until the region of two-dimensional k-space outside the central region has been sampled. 
         [0065]    From the thus acquired MR data, an MR image is reconstructed in step S 8 . 
         [0066]    It should be noted that steps S 1  to S 4  can also be carried out after steps S 5  to S 7  or that steps S 1  to S 4  can also be carried out interleaved with steps S 5  to S 7 . 
         [0067]    Although modifications and changes may be suggested by those skilled in the art, it is 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.