Abstract:
In a SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions) or equivalent magnetic resonance imaging pulse sequence, the readout dephasing gradient is generated (activated) so as to occur immediately in front of the second refocusing pulse, thereby eliminating the long time duration that occurs in conventional SPACE or equivalent sequences between excitation and readout. This long time duration has been identified as a source for flow-related artifacts that occur in images reconstructed from data acquired according to conventional SPACE or equivalent sequences. By eliminating this long time duration, such flow-related artifacts are substantially reduced, if not eliminated.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention concerns a magnetic resonance system and an operating method therefor of the type wherein magnetic resonance data are acquired according to a SPACE (Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions) or equivalent pulse sequence, and in particular to such a system and method for flow artifact reduction in slab selective SPACE imaging. 
         [0003]    2. Description of the Prior Art 
         [0004]    Highly sophisticated spin-echo pulse sequences include a single-slab 3D turbo or fast spin-echo pulse sequence known, for example, as SPACE. Pulse sequences of this type allow an extremely large number of refocusing RF pulses (such as more than 300) by using a refocusing RF pulse train exhibiting pulses with respectively different flip angles throughout the duration of the echo train. The curve that represents the variation of the flip angles is designed to achieve desired signal strengths for different types of tissue (nuclear spins), and is referred to as the flip angle evolution. Such an evolution is usually designed for obtaining a specific contrast (such as in proton density-weighted images or T1-weighted images or T2-weighted images) between the tissues in the image. Such an imaging sequence can be used effectively in brain imaging, for example, wherein cerebral-spinal fluid (CSF), gray matter and white matter all exhibit markedly different signal intensities in T2-weighted images. Using the SPACE sequence, an optimal T2-weighted contrast among the various tissues can be obtained by setting the echo time around the middle portion of the echo train. 
         [0005]    A basic description of single slab SPACE imaging can found, for example, in U.S. Pat. No. 7,705,597 and in the article “Fat-Signal Suppression in Single-Slab 3D TSE (SPACE) Using Water-Selective Refocusing,” Mugler, III et al., Proc. Intl. Soc. Mag. Reson. Med., Vol. 19 (2011), page 2818. 
         [0006]    In slab-selective SPACE imaging, flow-related artifacts often occur in the readout direction, for example, in spine imaging due to the CSF flow. An example of such a flow-related artifact can be seen in  FIG. 1 , in the outlined region. In contrast to the phase-encoding direction, this problem primarily occurs in the readout direction, because this direction is more sensitive to flow. 
         [0007]    In an effort to address this problem, imaging sequence protocols are configured with the phase-encoding direction being aligned with the cranio-caudal or head-to-feet axis of the patient. This means, however, that a large number of phase-encoding steps are needed to cover the field of view (FOV) of interest, and a large number of phase-encoding steps are necessary for oversampling (normally about 50% to 80%), in order to avoid infolding artifacts. These factors result in a very long acquisition time when such imaging sequences are used. This situation is illustrated in  FIG. 2 , which shows a portion of the pulse-sequence elements for a conventional single slab SPACE imaging sequence in which the RF excitation pulse is shown at the top left, followed by the refocusing RF pulses of varying amplitude (labeled “RF signal data”). The sequence for the X gradient is shown below, with a relatively long time duration between the readout dephasing gradient (ROD) and the first application of a readout gradient, during which gradient data acquisition may occur. For completeness, the Z gradient is shown in  FIG. 2  as well, below the X gradient. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to provide a magnetic resonance imaging system and an operating method therefor wherein the aforementioned flow-related artifacts in SPACE or equivalent imaging are significantly reduced. A further object of the present invention is to provide a non-transitory, computer-readable data storage medium that, when loaded into a computerized processor that controls operation of magnetic resonance imaging system, causes such a method to be implemented in the operation of the magnetic resonance system. 
         [0009]    The magnetic resonance system and operating method in accordance with the present invention are based on the insight that the aforementioned flow related artifacts in single slab SPACE imaging arise due to the aforementioned long time duration between ROD and the first application of a readout gradient, that occurs directly after the excitation RF pulse. Due to the long duration of the excitation RF pulse compared to the subsequent RF pulses, the first echo spacing (ESP) is very long, resulting in the aforementioned long time duration between ROD and the first application of a readout gradient, thereby making the sequence overly sensitive to flow in the readout direction. In other words, due to said long time duration, the net effect of the ROD and first application of a readout gradient on the excited (transverse) magnetization of moving nuclear spins may be, depending on the degree of motion, substantially different than that corresponding to stationary nuclear spins, which leads to a motion-induced phase difference in the detected magnetic resonance signal from moving nuclear spins. This is the source of the signal voids that can be seen in  FIG. 1 , resulting from the conventional single slab SPACE sequence shown in  FIG. 2 . In the method according to the present invention, therefore, magnetic resonance imaging data are acquired by operating a magnetic resonance imaging system with a single slab SPACE or equivalent imaging sequence wherein the ROD is shifted from its conventional position after the excitation RF pulse in order to instead be activated immediately in front of the second refocusing RF pulse, replacing the application of the readout gradient just before the second refocusing RF pulse in the conventional pulse sequence. Thus data is not measured just before the second refocusing RF pulse. 
         [0010]    In a further embodiment of the inventive method, in order to reduce sensitivity to inhomogeneities in the basic magnetic field (B 0  inhomogeneities), spoiler gradients are activated around the first refocusing pulse on the Z-axis. In a further embodiment, such spoiler gradients can also be activated along the Y-axis and the X-axis. This latter embodiment is particularly useful in other SPACE variants, such as inner volume SPACE, or SPACE with a water-selective first refocusing pulse. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1 , as noted above, is an image obtained with a conventional single slab selective SPACE imaging sequence, embodying flow-related artifacts. 
           [0012]      FIG. 2  schematically illustrates a portion of the pulse-sequence elements for a conventional single slab selective SPACE imaging sequence. 
           [0013]      FIG. 3  schematically illustrates analogous pulse sequence elements for a single slab SPACE imaging sequence according to the present invention. 
           [0014]      FIG. 4  schematically illustrates a further embodiment of the single slab SPACE imaging sequence according to the present invention. 
           [0015]      FIG. 5  shows two images for comparison between conventional single slab selective SPACE imaging and single slab selective SPACE imaging according to the present invention. 
           [0016]      FIG. 6  shows two images for comparison between single slab selective SPACE imaging according to the present invention, and single slab selective SPACE imaging with swapped phase encoding. 
           [0017]      FIG. 7  shows further images for comparison between single slab selective SPACE imaging according to the invention, and conventional single slab SPACE imaging. 
           [0018]      FIG. 8  schematically illustrates a magnetic resonance imaging system constructed and operating according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Turning first to  FIG. 8 ,  FIG. 8  is a schematic representation of a magnetic resonance system  5  and a central control unit  10  as can be used both for nuclear magnetic resonance imaging and for magnetic resonance spectroscopy.  FIG. 8  is meant only to represent an exemplary magnetic resonance imaging/spectroscopy system, whereas the present invention applies to any of the many permutations of such a system for magnetic resonance imaging/spectroscopy that are used in the field. 
         [0020]    A basic field magnet field  1  generates a temporally constant strong magnetic field B 0  for polarization or for alignment of the nuclear spins in a region under investigation of an object O, such as, for example, a part of a human body to be examined, the part being shifted on a table  23  lying in the magnetic resonance system  5 . The high homogeneity of the fundamental magnetic field required for nuclear spin resonance tomography is defined in a measurement volume M. For support of the homogeneity requirements and in particular for the elimination of time invariable influences, shims made of ferromagnetic material are mounted at a suitable location. Time variable influences are eliminated by shim coils  2  according to signals from shim coils amplifier  23 . 
         [0021]    A cylindrical gradient coil system  3  is inserted into the fundamental field magnet  1 , composed of three windings. The windings are provided with power by respective amplifiers for the generation of linear (also time modifiable) gradient field in the respective directions of the Cartesian coordinate system. The first winding of the gradient field system  3  generates a gradient Gx in the x-direction, the second winding generates a gradient Gy in the y-direction and the third winding generates a gradient Gz in the z-direction. Each amplifier has a digital-to-analog converter that is controlled by a sequence controller  18  for the generation of gradient pulses at the correct time. 
         [0022]    Located within the gradient field system  3  is one (or more) radio-frequency (RF) antenna  4 , which convert the high frequency pulses emitted by a radio frequency power amplifier  24  to a magnetic AC field for excitation of the nuclear spins and alignment of the nuclear spins of the object O to be examined or of the region to be examined of the object O. Each high frequency antenna  4  has one or more RF transmission coils and one or more RF reception coils in the shape of an annular, preferably linear or matrix-shaped arrangement of component coils. From the RF reception coils of the respective high frequency antenna  4  the AC field proceeding from the precessing nuclear spin, usually the nuclear spin echo signals generated from a pulse sequence of one or more high frequency pulses and one or more gradient pulses, is also converted into a voltage (measurement signal) that is fed via an amplifier  7  to a high frequency receive channel  8  of a high frequency system  22 . The high frequency system  22  additionally has a transmission channel  9  in which the high frequency pulses are generated for the excitation of the nuclear magnetic resonance. In the process the respective high frequency pulses are digitally represented as a sequence of complex numbers on the basis of a pulse sequence in the sequence controller  18  predefined by the system computer  20 . This number sequence is fed as a real part and as an imaginary part via inputs  12  to a digital analog converter in the high frequency system  22  and from the system  22  to a transmission channel  9 . In the transmission channel  9  the pulse sequences are modulated to a high frequency carrier signal whose basic frequency corresponds to the resonance frequency of the nuclear spin in the measurement volume. 
         [0023]    The switchover between send-receive modes occurs via a diplexer  6 . The RF transmission coils of the high frequency antenna(e)  4  irradiate the high frequency pulses for excitation of the nuclear spin to the measurement volume M and resulting echo signals are scanned via the RF reception coil(s). The obtained nuclear resonance signals are phase-sensitively demodulated in the receive channel  8 ′ (first demodulator) of the high frequency system  22  to an intermediate frequency and are digitized in the analog-digital converter (ADC). This signal is also demodulated to the basic frequency. The demodulation to the basic frequency and the separation into real and imaginary parts at outputs  11  takes place, after the digitization, in the digital domain in a second demodulator  8 . An image processor  17  reconstructs an MR image from the measurement data obtained in such a way. The administration of the measurement data, the image data and of the control programs occurs via a system computer  20 . By means of a specification with control programs the sequence controller  18  controls the generation of the respective desired pulse sequences and the corresponding scanning of the k-space. 
         [0024]    The sequence controller  18  controls the switching of the gradients at the correct time, the transmission of the high frequency pulses with defined phase amplitude as well as the reception of the nuclear resonance signals. The time base for the high frequency system  22  and the sequence controller  18  are made available by a synthesizer  19 . The selection of corresponding control programs for the generation of an MR image, said control programs being e.g. stored on a DVD  21 , as well as the representation of the generated MR image occurs via a terminal  13  comprising a keyboard  15 , a mouse  16  and a monitor  14 . 
         [0025]      FIG. 3  schematically illustrates a single slab selective SPACE imaging sequence according to the present invention wherein the ROD has been shifted from its conventional position following the excitation RF pulse to a position that immediately precedes the second refocusing RF pulse. The long time duration between the ROD and readout thus no longer occurs, and the flow-related artifacts that result from that long time duration no longer occur, or are significantly reduced, in magnetic resonance images that are reconstructed from the data acquired according to the imaging sequence shown in  FIG. 3 . 
         [0026]      FIG. 3  also shows a further embodiment of the invention wherein spoiler gradients are added around the first refocusing pulse. Spoilers can be added around the refocusing RE pulses on more than one axis, as shown in  FIG. 4 . 
         [0027]      FIGS. 5 ,  6  and  7  show, for comparison purposes, magnetic resonance images reconstructed according to data acquired with a conventional single slab selective SPACE imaging sequence, and data acquired with the single slab selective SPACE imaging sequence according to the invention. The outlined portion of each image shows the region of interest in which, in the conventionally-generated images, flow-related artifacts are visible. In the outlined regions of the images obtained according to the invention, such artifacts are virtually non-existent. 
         [0028]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.