Abstract:
In a method for image generation by magnetic resonance, a first MR raw data set is acquired with a first resolution in k-space, at least one further raw data set is acquired with a resolution in k-space that is reduced relative to that of the first raw data set, the acquired raw data sets are transferred into three-dimensional space for generation of image data the various image data are averaged for generation of an averaged image data set and the averaged image data set is displayed as an MR image.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention concerns in general magnetic resonance systems (or magnetic resonance tomography system) as used in medicine for examination of patients.  
         [0003]     2. Description of the Prior Art  
         [0004]     Magnetic resonance tomography is based on the physical phenomenon of nuclear magnetic resonance and is successfully used as an imaging method in medicine and biophysics. The water protons present in the body, or their nuclear spins, align in a strong, constant magnetic field. These nuclear spins can be brought out of the equilibrium state by the radiation of a radio-frequency pulse, and they relax again into this equilibrium state with specific relaxation times. These relaxation phenomena can be detected with suitable acquisition coils. The spatial resolution ensues by switching of additional currents in gradient coils for generation of magnetic field gradients. The precession frequency of the spins changes dependent on the location.  
         [0005]     The acquisition of the data in magnetic resonance tomography ensues in k-space, or Fourier space. The magnetic resonance image (MR image) considered by the physician is obtained by a Fourier transformation of the MR raw data. Generally, gradients are activated in all three spatial directions for spatial coding of an examined subject. Differentiation is made between the slice-selection gradients, the phase coding gradients and the gradients during which the signal readout ensues.  
         [0006]     In order to improve the signal-to-noise ratio, it is known to identically repeat a specific measurement sequence (i.e. a specific sequence of gradient switchings) and ultimately to average the signals of both measurements in order to present an averaged MR image that has a better signal-to-noise ratio. However, this leads to significantly longer acquisition times.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to provide a method for image generation by means of magnetic resonance with which the signal-to-noise ratio is improved without having to increase the time for the image acquisition to the same degree.  
         [0008]     The above object is achieved in accordance with the invention by a method for image generation by magnetic resonance that includes the following steps. First, a first MR raw data set is acquired with a first resolution in k-space. A further raw data set is acquired in k-space that has a resolution that is reduced relative to the first raw data set. The acquired raw data sets are transformed in three-dimensional space or (image space), thereby generating image data. The image data are then averaged to generate an averaged image data set and this averaged image data set is presented as an MR image. By the averaging of an MR raw data set with a first resolution and an MR raw data set with a second resolution, either the acquisition time is reduced relative to averagings of identical MR raw data sets or (if the same acquisition time as for typical averagings is used) the signal-to-noise ratio can be improved.  
         [0009]     According to a preferred embodiment of the invention, the resolution of the raw data set is reduced in one spatial direction that is perpendicular to the direction of the signal readout. By the reduction of the resolution in one k-space direction in the further acquired raw data set, and the subsequent averaging of the Fourier-transformed image data, an MR image is obtained having spatial resolution in the one direction that is just as good as in the case of an averaging of two identical raw data sets. It is accepted in exchange that the spatial resolution in the other spatial direction is poorer.  
         [0010]     Multiple MR raw data sets are advantageously acquired, with the resolution of one raw data set relative to the resolution of the previously-acquired raw data set being reduced. The resolution of a raw data set is advantageously essentially halved relative to the next raw data set. This means that the number of the k-space lines that are read out during a signal readout is essentially halved given each further acquired MR raw data set.  
         [0011]     The portions of the raw data that are not acquired in a data set with reduced resolution are filled with zeros until the same k-space volume has been acquired as in the first MR raw data set. The resolution in the phase coding direction is advantageously reduced. For example, for an MR raw data set with reduced resolution, the k-space line that is not acquired can then, for example, be filled with zeros.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a flow chart showing the basic inventive steps for acquisition of an MR image with improved signal-to-noise ratio.  
         [0013]      FIG. 2  shows an MR image with conventional averaging and an MR image with averaging according to the invention.  
         [0014]      FIG. 3  shows an MR image of an edge with a conventional averaging and an averaging according to the invention.  
         [0015]      FIG. 4  shows the noise power spectrum given a conventional averaging and an averaging according to the invention.  
         [0016]      FIG. 5  is an estimation of the relative modulation transfer function for a method according to the invention in comparison to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The basic steps for acquisition of an MR image according to the invention are shown in  FIG. 1 . A first MR raw data set with a first resolution is acquired in a first step  11 . The Fourier transformation of this data set in three-dimensional or, respectively, image space ensues in a step  12 . A further raw data set is acquired in a further step  13 , whereby this raw data set acquired during the step  13  has a reduced resolution relative to the first raw data set. The resolution is advantageously essentially halved relative to the first raw data set, This means that only approximately half of the k-space volume is acquired in step  13  in comparison to the acquisition of k-space in step  11 , the acquisition time for this second data set also essentially being halved. The second raw data set is Fourier-transformed in image space in step  14 . The Fourier-transformed image data sets are then added to obtain an averaged image data set in step  15 . The averaged image data set is then displayed in step  16 . Further MR raw data sets naturally can be acquired before step  15 , with the resolution of the raw data set respectively essentially being halved from one raw data set to the next. The portion of the k-space volume that was not acquired in comparison with the first MR raw data set is filled with zeros until the size of k-space is identical to the first raw data. Phantom images are exemplarily shown in  FIG. 2 . The MR image  21  is an MR image that was acquired with a conventional averaging methodology. This means that an MR image with a predetermined k-space resolution is acquired twice and is subsequently averaged. The inventive averaging method with identical measurement time was applied in the MR image  22 . For example, gradient echo exposures of a phantom were acquired with the conventional averaging method according to image  21  with a repetition time of 100 ms and a k-space size of 1024×1024. The acquisition time of a single image was 1:44 min for the example from  FIG. 2 . In the shown image  21  a technique known as an “averaging equals 2” was used, meaning that the same image was measured again with identical resolution and added, which leads to a total measurement time of 3:28 min.  
         [0018]     In the method used in image  22  a first k-space volume was likewise acquired with 1024×1024 points in k-space and a repetition time of 100 ms, which likewise leads to a measurement time of 1:44 min. A second MR raw data set with a resolution of 512×1024 k-space points was acquired in a further step, which leads to a measurement time of 53 s. A k-space raw data set with 256×1024 points was subsequently acquired with a measurement time of 27 s. An MR acquisition with an MR raw data set of 133×1024 points subsequently ensued with a measurement time of 15 s, and finally an acquisition of an MR raw data set with 64×1024 k-space points with a measurement time of 8 s. As can be seen from the example above, the resolution in the phase coding direction was respectively halved. If the measurement times for the acquisition of the raw data from 1024×1024 to 64×1024 are now added, a total measurement time of 3:27 min for the MR image shown in  FIG. 2  is obtained. This is essentially identical with the acquisition time of image  21 . As is to be recognized from the two images, the signal-to-noise ratio in image  22  according to the present invention is distinctly better relative to a method with conventional averaging,  
         [0019]     The relative modulation transfer function for both methods is shown in  FIG. 5 . The modulation transfer function is a measure for the spatial resolution, i.e. up to which pitch two lines can be shown separate from one another. In image  5  the relative modulation transfer function is plotted over the spatial frequency from zero up to the Nyquist frequency. The horizontal relative modulation transfer functions of both methods approximately correspond. However, as expected the relative vertical modulation transfer function clearly decreases perpendicular to this (in the phase coding direction) up to the image edge of k-space. As can be seen from  FIGS. 2 and 5  in synopsis, a significant improvement of the signal-to-noise ratio is obtained with a factor of approximately 2.5. This improved signal-to-noise ratio is bought via a reduced spatial resolution in one spatial direction. However. overall the image does not appear to be correspondingly less sharp to the observer because the spatial direction in the other spatial direction is the same as in the comparison method.  
         [0020]     Sections from edge images of phantom measurements that confirm the reduced resolution in the phase coding direction are shown in  FIG. 3 . Image  31  shows the edge of a phantom with a conventional averaging of two identical raw data; MR image  32  shows the same MR image that was acquired with approximately the same acquisition time with a method according to the invention. The signal-to-noise ratio or the contrast for the observer is essentially improved in image  32 , however the edge in the vertical direction is rendered more softly than in the comparison image  31 .  
         [0021]     The noise power spectrum (NPS) is shown in  FIG. 4  for a conventional averaging and an averaging according to the invention. The noise power spectrum in a conventional averaging shows an orientation-independent distribution of the noise while, contrarily, an orientation-dependent distribution of the noise appears in the noise power spectrum according to the present invention. A smaller noise in the vertical direction partially compensates for the lower vertical resolution.  
         [0022]     In summary, the multi-scalar approach with MR raw data of different resolutions that are averaged into an MR image enables an increase of the signal-to-noise ratio given equivalent measurement time in comparison to conventional averaging methods. The improved signal-to-noise ratio is “achieved” with a loss of the spatial resolution in one spatial direction but this does not significantly matter to the observer since the resolution in the other direction is not impaired.  
         [0023]     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.