Abstract:
In a method for determination of a diffusion-weighted image of an examination subject in a magnetic resonance system, a diffusion-weighted data set is acquired with magnetic diffusion gradients being activated; a diffusion-weighted image of the examination subject is calculated using this diffusion-weighted data set; dephasing or spoiler gradients are activated in order to reduce artifacts in the diffusion-weighted image due to additional signal echoes and the position and/or amplitude and/or polarity of the dephasing gradients is/are selected dependent on the diffusion gradients.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention concerns a method for determination of a diffusion-weighted image of an examination subject in a magnetic resonance system and a magnetic resonance system for this purpose. The invention in particular concerns a method and system of the type wherein the diffusion-weighted images are acquired using a double spin echo imaging sequence. 
   2. Description of the Prior Art 
   In magnetic resonance tomography imaging methods have been established with which the diffusion of the water molecules in the human body can be made visible. With the aid of diffusion imaging, magnetic resonance tomography in particular supplies valuable insights in the assessment of patho-physiological events and processes, for example in the human brain. It is in particular possible to localize stroke areas in the brain. 
   The difference arises due to the thermal translation movement of molecules. This is a random process that is also known as Brownian molecular motion. The distances covered by the diffused molecules on which the diffusion-weighted magnetic resonance tomography is based are very small. Strong magnetic field gradients (known as diffusion gradients) are used for diffusion-weighted imaging. These strong magnetic field gradients are used in addition to the gradient fields in the imaging sequence that are necessary for spatial coding. For diffusion imaging the physical effect is utilized that the magnetic resonance signal decreases given a difference movement of the molecules under the influence of magnetic field gradients. The phase of the magnetic resonance signal changes in proportion to the applied magnetic field gradients. If a bipolar diffusion magnetic field gradient is now activated, the phase development for resting (non-moving) spins now cancels out due to the bipolar gradient; for spins moving due to diffusion the moving spin experiences a different magnetic field. In the case of a bipolar diffusion gradient, for diffusing spins the signals at the end of both diffusion gradients are no longer phase coherent (as for resting spins) but rather are distributed. This entails a signal decrease. If a non-diffusion-weighted data set and a diffusion-weighted data set (given which additional diffusion gradients are activated) are now acquired, a conclusion about the diffusion can be reached from the difference of the two data sets. 
   The diffusion is typically a diffusion tensor that depends on the spatial direction. This diffusion tensor is a symmetrical tensor with six coefficients. To determine the diffusion tensor, non-diffusion-weighted data sets and at least six (for example 20-60) diffusion-weighted data sets are acquired in order to calculate the diffusion tensor. 
   Diffusion-weighted imaging techniques are very sensitive to gradient-induced eddy current effects. To avoid eddy current effects it was, for example, proposed in the conference volume of the Society of Magnetic Resonance in Medicine, page 799 in the year 2000, to use a double spin echo sequence with four diffusion gradients, i.e. two diffusion gradient pairs. 
   Furthermore, in such double spin echo sequences gradients known as spoilers or dephasing gradient fields are applied in order to avoid unwanted ancillary echoes. In spite of this, stripe artifacts in the magnetic resonance image have occurred under specific circumstances with such diffusion-weighted imaging sequences. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to generate diffusion-weighted magnetic resonance images with which such stripe artifacts can be reliably suppressed. 
   The above object is achieved in accordance with the invention by a method for determination of a diffusion-weighted image wherein: a diffusion-weighted data set is acquired, using diffusion gradients. Diffusion-weighted images of the examination subject can then be calculated on the basis of the diffusion-weighted data set. Dephasing or spoiler gradients are additionally activated in order to reduce artifacts in the diffusion-weighted image due to additional signal echoes. According to the present invention, the position and/or the amplitude and/or the polarity of the dephasing gradients is/are selected dependent on the diffusion gradients. In the prior art, constant and symmetrical dephasing gradients were used in the imaging sequence in order to reduce artifacts in the image. These constant, pre-programmed dephasing or spoiler gradients, however, could not reliably prevent the appearance of artifacts in the image. The invention is now based on the insight that the artifacts can be prevented in a reliable manner when the position and/or the amplitude and/or the polarity of the dephasing gradients is/are adapted to the current diffusion gradients. The diffusion-weighted image can also include the representation of the diffusion tensor. 
   According to a preferred embodiment of the invention, in the workflow of the imaging sequence, the amplitude and/or position and/or polarity of the dephasing gradients for each imaging sequence is/are selected dependent on the current imaging sequence workflow and/or the direction of the applied diffusion gradients. The dephasing gradients were previously programmed into the sequence workflow in a fixed manner. According to the invention, the dephasing gradients are calculated dependent on the direction of the applied diffusion gradients and are inserted into the sequence workflow in the planning of the sequence workflow, when this is set, or even at run time. 
   Furthermore, a diffusion-weighted data set and a non-diffusion-weighted data set can be acquired or data sets with different diffusion weightings can be acquired, and diffusion-weighted images can be generated on the basis of the different diffusion weightings of the acquired data sets. 
   For example, an imaging sequence with doubled spin echo can be used for acquisition of the diffusion-weighted data set and the non-diffusion-weighted data set. Such an imaging sequence has a 90° radio-frequency pulse and two 180° radio-frequency pulses. These imaging sequences (known as double spin echo sequences) can be used for diffusion weighting. With these double spin echo sequences with two 180° radio-frequency pulses, according to the invention one dephasing gradient can be activated just before each 180° pulse and one dephasing gradient can be activated just after each 180° pulse. The amplitude and the polarity of these dephasing gradients depend on the selection and the parameters of the diffusion gradients. 
   In a preferred embodiment of the invention, the amplitude and/or polarity and/or duration and/or position of the diffusion gradients is/are determined. The amplitude and the polarity of the dephasing gradients can then be calculated from the specific values. As noted above, the diffusion constant is a tensor with components in the various spatial directions. For calculation of the tensor different diffusion-weighted data sets are acquired, with diffusion gradients with different intensities in the different three spatial directions being used. The position and the presence of the diffusion gradients are different for the various imaging sequences. 
   The gradient moments of the diffusion gradients, meaning the product of the gradient amplitude and the activation duration (more precisely the surface integral under the activated gradient on the time axis), are preferably determined. 
   In a preferred embodiment, the gradient moment of the dephasing gradients that are activated before and after the first 180° pulse differs in magnitude from the gradient moment of the dephasing gradients that are switched before and after the second 180° pulse. This has the following cause. 
   Four diffusion gradients with alternating polarity, i.e. two bipolar diffusion gradient pairs for one spatial direction, are advantageously used in the diffusion-weighted double spin echo sequence. A first diffusion gradient pair encloses the second diffusion gradient pair. This means that the first diffusion gradient pair is the first gradient and the fourth gradient that enclose the second diffusion gradient pair (the second and the third diffusion gradients) in the temporal sequence. The total gradient moment of the first diffusion gradient pair is now different. This means that the area under the first diffusion gradient differs from the area under the fourth diffusion gradient. This difference in the gradient moments corresponds to a net gradient moment that acts like an additional dephasing or spoiler gradient. The dephasing gradients placed between the two 180° pulses must now be selected such that this net gradient moment is amplified and is not canceled. 
   The difference of the gradient moments of the first diffusion gradient pair determines the polarity of the dephasing gradients around the 180° pulses. 
   According to a further aspect of the invention, the dephasing gradients are activated with the larger gradient moment around the 180° pulse that lies closer to the diffusion gradient of the diffusion gradient pair that has a larger gradient moment than the other diffusion gradient of the first diffusion gradient pair. It is thus ensured that the aforementioned net moment of the diffusion gradients is amplified and not canceled. This amplification is necessary so that the artifacts occurring in the image can be avoided. As is explained below in the exemplary embodiments, the formation of additional unwanted echoes that would lead to unwanted artifacts in the images thus can be effectively prevented. 
   Furthermore, the gradient moment of the readout gradient is determined. Given the selection of the dephasing gradients that are activated around the 180° pulses, according to a preferred embodiment of the invention the gradient moment of these dephasing gradients is at least as large as the gradient moment of the signal readout gradients. The amplitudes of the dephasing gradients can be selected dependent on the gradient moment of the signal readout gradient. The gradient moment of the dephasing gradient is preferably between 1.3-1.7 times as large as the gradient moment of the readout gradient. Furthermore, the gradient moment of the dephasing gradients is preferably 1.5 times as large as the gradient moment of the signal readout gradients. 
   For example, for acquisition of the diffusion-weighted data sets and non-diffusion-weighted data sets, the echoplanar technique can be used in order to reduce the acquisition time of the diffusion-weighted images. Naturally other suitable imaging sequence can also be used. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an example of a pulse sequence of a diffusion-weighted double spin echo sequence according to the prior art. 
       FIG. 2  shows an exemplary sequence of the radio-frequency pulses and gradients for explanation of the physical background of the invention. 
       FIG. 3  shows the sequence of  FIG. 2  with additional dephasing gradients according to the prior art. 
       FIG. 4  shows the sequence of  FIG. 2  with the inventive insertion of dephasing gradients. 
       FIG. 5  shows a sequence with activation of the dephasing gradients according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An example of a pulse sequence of a double spin echoplanar sequence without switching of diffusion gradients according to the prior art is shown in  FIG. 1 . In this double spin echo imaging sequence a 90° radio-frequency pulse  11  is radiated into the examination subject (not shown), followed by two 180° pulses  12  and  13 . A magnetic field gradient GS for slice selection is switched (gradient  14 ) at the same time as the 90° pulse. The switching of the addition diffusion gradients  15 - 18  ensues in the slice direction after the slice-selection gradient in the slice direction. 
   Furthermore, a phase coding gradient G p  is activated in the phase coding direction. This gradient  19  corresponds to the phase coding gradient of an echoplanar imaging sequence. A readout gradient G A  is likewise activated in the signal readout direction (gradients  20  and  21 ), whereby the signal readout ensues during the switching of the gradient  21 . A central spin echo  22  (that is read out during the signal readout upon switching of the gradient  21 ) ensues via the switching of the 90° pulse and both 180° pulses  12  and  13 . In the sequence order shown in  FIG. 1 , no additional spoiler or dephasing gradients ensue in order to avoid unwanted echoes. 
   In connection with  FIG. 2  it is explained in detail how these unwanted echoes arise in order to understand the basis for avoiding unwanted echoes. In principle, three different types of echoes are to be differentiated, namely the spin echo that occurs given the activation from a 90° pulse and 180° pulse that was discovered by Hahn, as well as stimulated and anti-stimulated echoes that additionally occur. The anti-stimulated echoes have been described by Ordige in the conference volume of the Society of Magnetic Resonance in Medicine, 1995, page 670. The 180° pulses  12  and  13  from  FIG. 1  and  FIG. 2  are considered as 90° pulses  12 ′ and  13 ′ for understanding of the stimulated and anti-stimulated echoes. This is based on the fact that no radio-frequency pulse is so ideal that 90° signal portions do not also occur given a 180° pulse. Furthermore, the first diffusion pulse  15  and the fourth diffusion pulse  18  of the diffusion-weighted gradient switching are shown. After the first 90° pulse  11  the spins are tilted in the transversal plane and there experience the magnetic field gradient  15 . They are tilted in the vertical plane by the second 90° pulse  12 ′, such that here no transversal component exists. For this reason both gradient switchings  16  and  17  can be disregarded, or do not have to be taken into account. After the third 90° pulse  13 ′, the spins are tilted again in the transversal plane where they are subjected to the diffusion gradient  18 . The diffusion gradients  15  and  18  form the first bipolar diffusion gradient pair that surrounds the second bipolar diffusion gradient pair  16  and  17  from  FIG. 1 . 
   As can be seen in  FIG. 2 , the gradient moment (i.e. the area under the switched gradient) is larger for the gradient  15  than for the gradient  18 . This net gradient moment is provided in order to avoid unwanted echoes and in order to prevent distortions due to eddy currents. The gradient moments of the gradients  15  and  18 , however, do not necessarily differ strongly, such that an anti-stimulated echo is not reliably avoided. When both moments are identical, meaning that their sums cancel out, it leads to an anti-stimulated echo in the k-space center. When these two moments are not identical or do not strongly differ, this echo of the signal acquisition in k-space or Fourier space is displaced from the center, which can then lead to the unwanted stripe artifacts in the image. 
   In  FIG. 3  it is shown how these echoes were previously avoided in the prior art. In addition to the diffusion gradients  15  and  16 , two dephasing or spoiler gradients  23  and  14  activated around the pulses  12 ′ and  13 ′. The gradient pair is switched around the pulse  12 ′, the gradient pair  24  around the radio-frequency pulse  13 ′. As can be seen in  FIG. 3 , according to the prior art symmetrical gradients with the same amplitude were used. These symmetrical gradients, however, could not reliably prevent the occurrence of the anti-stimulated echoes and therewith the stripe artifacts in the image. The spoiler gradients  23  and  24  must be activated around the 180° pulses in order to obtain the spin echo. Furthermore, these dephasing gradients must be activated as briefly as possible in order to be able to minimize the echo time. 
     FIG. 4  shows how the inventive embodiment of the dephasing gradients must ensue in order to reliably avoid the occurrence of stripe artifacts in the image. As is explained in connection with  FIG. 2 , the gradient moment of the diffusion gradients  15  and  18  are different. In the shown example the gradient moment of the gradient  15  is greater than that of the gradient  18 . Furthermore, the spoiler gradients  25  and  26  are shown in  FIG. 4 . These spoiler gradients  25 ,  26  inventively have a different gradient moment, which was not the case given the gradient moment of the gradients  23  and  24 . Since the switching time for the spoiler gradients  25 ,  26  should be as short as possible and is therewith equal, the amplitudes of the spoiler gradients  25  and  26  are different. The amplitude of the spoiler gradients  25  and  26  is selected such that the difference of the gradient moments from the gradient moments  25  and  26  is at least as large as the gradient moment of the readout gradient. The difference of the gradient moments of the gradient  25  and  26  is advantageously 1.5 times as large as the gradient moment of the readout gradient. 
   In order to amplify the net gradient moment of both diffusion gradients  15  and  18 , the position of the two spoiler gradients  25  with the larger amplitude is selected such that they lie closer to the diffusion gradient that has a larger gradient moment. The net difference can thereby be amplified in the gradient moment between the diffusion gradients  15  and  18 . 
   In the prior art the spoiler gradients  23  and  24  from  FIG. 3  were kept constant. However, the diffusion gradients differ in terms of their amplitude and polarity given generation of the various diffusion-weighted data sets in order to be able to calculate the diffusion tensor overall. For this reason the switching shown in  FIG. 3  is not able to effectively avoid stimulated or, respectively, anti-stimulated echoes. The polarity and the amplitude of the spoiler gradients now inventively depend on the diffusion gradients. Since the diffusion gradients are not the same for all diffusion-weighted data sets, the polarity must be calculated given each acquisition of a data set in the progression of the imaging sequence. During progression of the imaging sequence, the net gradient moment must be calculated from the first diffusion gradient  15  and the fourth diffusion gradient  18 . The polarity of the spoiler gradients  25  and  26  must be selected dependent on which moment is greater. Among other things, the amplitude of the spoiler gradients  25  and  26  depends on the gradient moment of the readout gradient. 
   An inventive imaging sequence for generation of diffusion-weighted images is shown in  FIG. 5 . In the shown embodiment diffusion gradients are activated in the phase coding direction and in the slice-selection direction. As in the exemplary embodiment from  FIG. 1 , this is a double spin echo sequence with a 90° pulse  11  and two 180° pulses  12  and  13 . A first diffusion gradient pair  31  and  32  that temporally surrounds the second diffusion gradient pair with the gradients  33  and  34  is switched in the phase coding direction. Dephasing gradients  35  are additionally activated around the first 180° pulse  12  and dephasing gradients  36  are activated around the second 180° pulse  13 . As can be seen in  FIG. 5 , the amplitudes of these two dephasing gradients  35  and  36  differ. This likewise applies for the two dephasing gradients  37  and  38  that are activated in the readout direction. During the 90° pulse  11  the gradient  14  is activated in the slice-selection direction before the first gradient  39  of the diffusion gradient pair (namely the gradients  39  and  40 ) is activated. Two further diffusion gradients  41  and  42  are likewise switched in the slice-selection direction between the two 180° pulses  12  and  13 . The dephasing gradients  43  and  44  differ in turn in terms of their amplitude in the slice-selection direction. 
   As is to be learned in  FIG. 5 , the gradient moment of the gradient  32  was selected larger than the gradient moment of the gradient  31 , such that the gradient moment of the dephasing gradients  36  was selected larger than the gradient moment of the dephasing gradients  35 . This likewise applies for the slice-selection gradient, given which the gradient moment of the gradient  40  is greater than the gradient moment of the diffusion gradient  39 . For amplification of the net moment the amplitude of the dephasing gradients  43  and  44  was correspondingly selected in order to amplify the net moment. 
   In summary, the adaptation of the amplitude and the polarity of the dephasing gradients to the current imaging sequence enable the avoidance of unwanted artifacts in a diffusion-weighted image. 
   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.