Patent Publication Number: US-6335621-B1

Title: Method of compensating for phase error of phase encoding gradient pulse in fast spin echo imaging method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the magnetic resonance art and, in particular, to a method for compensating for a phase error of a phase encoding gradient pulse in a fast spin echo (FSE) imaging technique. 
     2. Description of the Related Art 
     According to an FSE imaging method, different phase encoding gradients are applied to acquire the position information from an echo signal generated from a plurality of radio frequency (RF) pulses so as to obtain data corresponding to each line on a k-space and thereby construct an image. The FSE imaging method has an advantage of decreasing the imaging time to 1/4, 1/8 or shorter as compared to a conventional spin echo (SE) imaging method without image quality degradation. Accordingly, clinical attention has been focused on the FSE imaging technique. 
     However, in the FSE imaging technique of constructing an image from data obtained by applying different phase encoding gradients at different times, which corresponds to each line on a k-space, a constructed image may have a ringing artifact or a blurring due to a phase error when a phase encoding gradient is not linearly increased. Accordingly, a signal-to-noise ratio (SNR) or a degree of contrast may be decreased, which in turns degrades the quality of the image. 
     FIG. 1A illustrates pulse sequences for a conventional FSE imaging method. Corresponding to a phase encoding gradient pulse sequence shown in FIG. 1A, a k-space is filled with data obtained using phase encoding gradients, as shown in FIG.  1 B. In FIG. 1B, the Kx-axis indicates a frequency encoding direction, and the Ky-axis indicates a phase encoding direction. In FIG. 1, reference numerals  1  through  8  denote gradient pulses used for application of phases. The eight gradient pulses have different amplitude. To recover the phases subjected to the gradient pulses  1  through  8  to original states, gradient pulses  11  through  18  are applied. Each gradient pulse  11  through  18  has the same amplitude as a corresponding gradient pulse  1  through  8 , respectively, but with a different polarity. Therefore, the phases at positions  21  through  28  are the same. 
     However, it frequently happens that actually, the phases at positions  21  through  28  are not the same due to a variety of causes. In this case, a phase error occurs in the k-space. In a magnetic resonance imaging (MRI) apparatus, phase errors due to phase encoding gradients occur depending on a gradient system. Gradient offset, non-linearity of a gradient and eddy current are related to occurrence of a phase error. In particular, eddy current is fatal to the FSE imaging technique. Accordingly, an active shielded gradient is used to fundamentally prevent a magnetic field from changing due to eddy current. However, when this technique is used, the price of a gradient system becomes significantly increased, while the space for a patient in an MRI apparatus is decreased. 
     Meanwhile, in low-field MRI, a method of pre-emphasized gradient waveform using an eddy current compensation circuit is employed. However, since this method does not fundamentally remove eddy current, an operation of adjusting the amplitude of a gradient pulse is required. Moreover, this method cannot remove a phase error due to an external factor. Consequently, a compensation operation with respect to a phase encoding gradient pulse sequence is required to remove a phase error. 
     SUMMARY OF THE INVENTION 
     In an attempt to address the above problems, it is a feature of the present invention to provide a method of compensating for a phase error of a phase encoding gradient pulse in a fast spin echo (FSE) imaging method to improve the quality of an image. 
     Accordingly, in a preferred embodiment of the present invention, there is provided a method of compensating for a phase error of a phase pulse in fast spin echo imaging. The method includes the steps of (a) generating phase encoding gradient pulses for tuning encoding gradient between a 90° RF signal and a first 180° RF signal, the phase encoding gradient pulses for tuning having opposite polarities to each other and the same amplitudes as those of respectively phase encoding gradient pulses corresponding to the first 180° RF signal, (b) adjusting the amplitude of either of the phase encoding gradient pulses for tuning such that a phase before the phase encoding gradient pulses for tuning is the same as a phase behind the phase encoding gradient pulses for tuning and obtaining the adjusted amplitude, and (c) compensating for the phase errors of the phase encoding gradient pulses corresponding to the first 180° RF signal using the adjusted amplitude. 
     The method preferably further includes the step of (d) sequentially changing the amplitudes of the phase encoding gradient pulses for tuning to the amplitudes of phase encoding gradient pulses corresponding to second through n-th (n is an integer greater than 1) 180° RF signals and repeating the steps (b) and (c) at each change to compensate for the phase errors of the phase encoding gradient pulses corresponding to the second through n-th 180° RF signals, after the step (c). 
     In another preferred embodiment of the present invention, there is provided a method of compensating for a phase error of a phase encoding gradient pulse in fast spin echo imaging. The method includes the steps of (a) embedding a 180° RF signal between a 90° RF signal and a first 180° RF signal and generating a selection gradient pulse and a frequency encoding gradient pulse, which correspond to the embedded 180° RF signal, (b) generating phase encoding gradient pulses for tuning between the embedded 180° RF signal and the first 180° RF signal, each of the phase encoding gradient pulses for tuning having pulse elements which have polarities that are opposite to each other and the same amplitudes as those of respective phase encoding gradient pulses corresponding to the first 180° RF signal, (c) adjusting the amplitude of either of the phase encoding gradient pulses for tuning such that a phase before the phase encoding gradient pulses for tuning is the same as a phase behind the phase encoding gradient pulses for tuning and obtaining the adjusted amplitude, and (d) compensating for the phase error of the phase encoding gradient pulse corresponding to the first 180° RF signal using the adjusted amplitude. 
     The method preferably further includes the step of (e) sequentially changing the amplitudes of the phase encoding gradient pulses for tuning to the amplitudes of phase encoding gradient pulses corresponding to second through n-th (n is an integer greater than 1) 180° RF signals and repeating the steps (c) and (d) at each change to compensate for the phase errors of the phase encoding gradient pulses corresponding to the second through n-th 180° RF signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1A illustrates a conventional fast spin echo (FSE) pulse sequence; 
     FIG. 1B illustrates a k-space scan method corresponding to FIG. 1A; 
     FIG. 2 is a flowchart illustrating an embodiment of a method of compensating a phase error of a phase encoding gradient pulse according to an embodiment of the present invention; 
     FIG. 3 illustrates a pulse sequence for the method of FIG. 2; 
     FIG. 4 is a flowchart illustrating a method of compensating a phase error of a phase encoding gradient pulse according to another embodiment of the present invention; and 
     FIG. 5 illustrates a pulse sequence for the method of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     The present invention provides a method of obtaining a compensation value by applying an additional gradient pulse for compensating for a phase error of a phase encoding gradient pulse in a fast spin echo (FSE) imaging method. In other words, the present invention relates to a method for making the phases to be the same at the positions  21  through  28  of FIG. 1A showing a phase encoding gradient pulse sequence for a usual FSE imaging method. 
     The present invention will be described on the basis of a FSE imaging method using a pulse sequence generating 8 echo signals as shown in FIG.  1 A. In this FSE imaging method, a k-space is largely divided into 8 regions, and phase encoding gradients in the 8 regions have different initial amplitudes and are increased according to a predetermined step. 
     Referring to FIGS. 2 and 3, in an embodiment of the present invention, tuning pulses  31  and  32  having opposite polarities relative to each other and the same amplitude as phase encoding gradient pulses  1  and  11  corresponding to a first 180° radio frequency (RF) pulse are generated between a 90° RF signal and the first 180° RF signal, in step  210 . 
     Here, the tuning pulse  31  and the tuning pulse  32  have the same magnitude and different signs, so a phase at a position  21  is ideally the same as a phase at a position  20 . However, even if the tuning pulses  31  and  32  have the same magnitude, the phases at final positions  20  and  21  may actually be different. In this case, the amplitude of the tuning pulse  31  or  32  can be fine tuned by increasing or decreasing the magnitude  41  or  42  so that the phases at positions  20  and  21  can be the same, in steps  220  and  230 . Since an MR signal or a spectrum varies with a change in a phase, the magnitude  41  or  42  can be adjusted while checking the MR signal or the spectrum. 
     In step  240 , the adjusted magnitude  41  or  42  is applied to the phase encoding gradient pulses  1  and  11  corresponding to the first 180° RF signal. Here, the magnitude  41  or  42  indicates the difference between the magnitude of an initially generated phase encoding gradient pulse for tuning and the magnitude of the adjusted phase encoding gradient pulse for tuning, and is used as a tuning value or a compensation value. 
     Thereafter, while the amplitudes of the tuning pulses  31  and  32  are sequentially changed into the amplitudes of pairs of phase encoding gradient pulses  2  through  8  and  12  through  18  corresponding to second through eighth 180° RF signals, respectively, the steps  210  through  240  are repeated, thereby compensating phase errors of the phase encoding gradient pulses corresponding to the respective second through eighth 180° RF signals. Steps  200 ,  250  and  260  in FIG. 2 are required to repeat the steps  210  through  240  n times in the case of an n-echo train FSE imaging method. 
     Meanwhile, when the data of image is actually acquired, the tuning pulses  31  and  32  may be removed from the phase encoding gradient pulse sequence, in step  270 . Thereafter, the constructed image is verified after data acquisition, as shown in FIG.  1 B. Here, the data for constructing the image is obtained from an echo train E 1  through E 8 . 
     In the embodiment shown in FIGS. 2 and 3, phase encoding gradient pulses for tuning  31  and  32  do not need to be generated at the same time when all phase encoding gradient pulses are generated. In addition, the phase encoding gradient pulses for tuning  31  and  32  are independent from the effect of a selection gradient pulse and a frequency encoding gradient pulse. 
     Referring to FIGS. 4 and 5, in another embodiment of the present invention, a 180° RF signal  71  is embedded between a 90° RF signal and a first 180° RF signal, and a selection gradient pulse  72  and a frequency encoding gradient pulse  73  which correspond to the embedded RF signal  71  are generated, in step  400 . Next, phase encoding gradient pulses for tuning  51  and  52  each having pulse elements, which have different polarities to each other and the same amplitude as those of phase encoding gradient pulses  1  and  11  corresponding to the first 180° RF signal, are generated between the embedded 180° RF signal  71  and the first 180° RF signal, in step  410 . 
     Here, each of the phase encoding gradient pulses for tuning  51  and  52  has two pulse elements having the same magnitude and opposite polarities, so a phase at a position  21  is ideally the same as a phase at a position  20 . However, the phases at positions  20  and  21  may actually be different due to a variety of causes. In this case, the amplitude of the tuning phase encoding gradient pulse  51  or  52  can be fine tuned or adjusted by increasing or decreasing a magnitude  61  or  62  (when an image is acquired using positive phase encoding gradient pulses) or a magnitude  63  or  64  (when an image is acquired using positive phase encoding gradient pulses) so that the phases at positions  20  and  21  can be the same, in steps  420  and  430 . Since an MR signal or a spectrum varies with a change in a phase, the magnitude  61 ,  62 ,  63  or  64  can be adjusted while checking the MR signal or the spectrum. 
     In step  440 , the adjusted magnitude  61 ,  62 ,  63  or  64  is applied to the phase encoding gradient pulses  1  and  11  corresponding to the first 180° RF signal. Thereafter, while the amplitudes of the phase encoding gradient pulses for tuning  51  and  52  are sequentially changed into the amplitudes of pairs of phase encoding gradient pulses  2  through  8  and  12  through  18 , corresponding to second through eighth 180° RF signals, respectively, the steps  410  through  440  are repeated, thereby compensating phase errors of the phase encoding gradient pulses corresponding to the respective second through eighth 180° RF signals. A step  400  of initializing n to  1  and steps  450  and  460  in FIG. 4 are required to repeat the steps  410  through  440  n times in the case of an n-echo train FSE imaging method. 
     Meanwhile, when the data of image is actually acquired, the 180° RF signal and the selection gradient pulse and the frequency encoding gradient pulse corresponding to the 180° RF signal, which are embedded in step  400 , and the phase encoding gradient pulses for tuning  51  and  52  generated in step  410  may be removed in step  470 . Thereafter, the constructed image is verified after data acquisition as shown in FIG.  1 B. Here, the data for constructing the image is obtained from an echo train E 1  through E 8 . 
     The embodiment of FIGS. 2 and 3 is different from the embodiment of FIGS. 4 and 5 in the following term. In FIG. 3, a tuning gradient pulse for compensating a phase encoding gradient pulse is positioned between the 90° RF signal and the first 180° RF signal. In FIG. 5, an additional RF signal is embedded between the 90° RF signal and the first 180° RF signal, and a tuning gradient pulse for compensating a phase encoding gradient pulse is positioned between the embedded RF signal and the first 180° RF signal. 
     In these embodiments of the present invention, an effective echo time (TE) may be changed, and accordingly, the arrangement of phase encoding gradient pulses for an echo train may also be changed. 
     According to the present invention, the ringing artifact or blurring of an image can be decreased in a FSE imaging method, and the contrast of an image can be improved. In addition, time for the development of a pulse sequence and the maintenance of a system can be saved, and information for grasping the problems of a gradient system can be obtained.