Patent Publication Number: US-7587231-B2

Title: Water fat separated magnetic resonance imaging method and system using steady-state free-precession

Description:
BACKGROUND OF THE INVENTION 
   Magnetic resonance imaging (MRI) with fully refocused steady state free-precession (SSFP) provides efficient signal-to-noise (SNR) performance and good image contrast. However, the application of SSFP has been hindered by the periodic spectral response of the SSFP sequence that results in band artifacts dependent on static magnetic field (B 0 ) inhomogeneities and time of repetition (TR). In addition, strong signals from fat limit the clinical usefulness of SSFP images in many applications. 
   There is a long felt need for robust methods for water and fat separation with SSFP imaging in the presence of B 0  static magnetic field inhomogeneities. Methods that require no additional scans are preferable as they minimize scan time and avoid problems caused by inter-scan variations. It is also desirable to have SSFP imaging methods that are applicable in low-magnetic field and mid-field MRI systems. 
   BRIEF DESCRIPTION OF THE INVENTION 
   A robust SSFP imaging technique has been developed to separate water and fat in the presence of B 0  field inhomogeneities. This technique combines with phase-cycled SSFP to achieve water-fat separation as well as to eliminate (or at least reduce) band artifacts. The technique is applicable to magnetic fields having low-, mid- and high field strengths. 
   The invention, in one embodiment, is a phase-cycled SSFP imaging method with a time of repetition (TR) and a time to echo (TE) selected such that water and fat signals, after echo isolation, are orthogonal in one data set and anti-parallel in another data set. A phase correction map is generated from the anti-parallel data set. The phase correction map is applied to correct for B 0  field inhomogeneities in the orthogonal data set to yield images having good water-fat separation. 
   In a second embodiment, the invention is a phase-cycled SSFP imaging method which uses the commonly applied TR=2TE selection such that water and fat signals are orthogonal in all data after echo isolation. A combinatory signal is constructed from two of the separated echo signals and used to generate a phase correction map. The phase correction map is applied to correct for B 0  field inhomogeneities to yield finally water-fat separation. 
   In a third embodiment, the invention is a phase-cycled SSFP imaging method having an “assist scan” used in conjunction with a master sequence SSFP scan. The assist scan is a preliminary scan set to low spatial resolution so as to reduce its scan time. The TR and TE of the assist scan are selected such that water and fat signals are anti-parallel in at least one of the data sets after echo isolation. The anti-parallel data is used to generate a phase correction map that is used to correct for B 0  inhomogeneities in the master SSFP scan to yield images with good water-fat separation. 
   In a fourth embodiment, the invention is a phase-cycled SSFP imaging method which uses the commonly applied TR=2TE selection such that water and fat signals are significantly orthogonal in all data after echo isolation. A field map already available from the scanner is used to correct for B 0  field inhomogeneities and to yield water-fat separation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an MRI system. 
       FIG. 2  is an exemplary SSFP sequence. 
       FIG. 3  is a flow chart of steps for acquiring SSFP data from which a phase correction map is generated and used to yield water-fat separated SSFP images. 
       FIGS. 4 and 5  are each a series of images acquired and processed using the techniques described herein. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Magnetic Resonance Imaging (MRI) is a widely accepted and commercially available non-invasive technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (NMR) also known as magnetic resonance (MR) phenomena. In MRI, imposing a strong main magnetic field (B 0 ) on the nuclei polarizes nuclei in the body of a patient to be imaged. 
   The nuclei are excited by a radio frequency (RF) signal at characteristic NMR (Larmor) frequencies. By spatially distributing localized magnetic fields surrounding the body and analyzing the resulting RF responses from the nuclei, a map or image of these nuclei responses as a function of their spatial location is generated and displayed. An image of the nuclei responses provides a non-invasive view of a patient&#39;s internal organs and of other tissues. 
   As shown in  FIG. 1 , an MRI system typically includes a large magnet  10  to impose the static magnetic field (B 0 ), gradient coils  14  for imposing spatially distributed gradient magnetic fields (G x , G y , and G z ) along three orthogonal coordinates, and RF coils  15  and  16  to transmit and receive RF signals to and from the selected nuclei of the body being imaged. The patient  13  lies on a movable patient table  12  such that a portion of the patient to be imaged is moved, in three-dimensions, into an “imaging volume”  11  between the magnet and coils, which defines a field of view (FOV) of the MRI system. 
     FIG. 2  schematically shows a three-dimensional (3D) magnetic resonance imaging sequence with fully refocused steady-state free precession (SSFP). A radio frequency (RF) excitation pulse  30  is applied to the body being scanned. The RF pulse  30  is periodically applied at a certain time of repetition (TR). An RF echo signal (S), is received during a data acquisition (ADC) period  32 . The received echo signal occurs after a time to echo (TE) period that begins with the center of the RF pulse  30 . Gradient magnetic fields are applied in the x, y and z directions to select and encode a set of slices (G slice ) of the body for imaging and to spatially phase encode (G phase ) the excited nuclei of the selected body slices. A readout magnetic field gradient (G read ) is applied during the ADC period to frequency-encode the received echo signal. 
   To acquire MRI data, the MRI system generates magnetic gradient and RF nutation pulses via MRI pulse sequence controllers  17  and  18  that operate under the control of a programmable processor  19 , e.g., a workstation computer. In addition, the processor  19  controls gradient pulse amplifier  20 , and RF source and amplifier circuits  21  and  22 . The MR signal circuits (RF detector)  25  are suitably interfaced with MR signal RF coils  15 ,  16  located within the shielded MRI system gantry. The received MR RF echo signal responses are digitized by a digitizer  23  and passed to the processor  19 , which may include an array processors or the like for image processing and suitable computer program storage media (not shown) wherein programs are stored and selectively utilized so as to control the acquisition and processing of MR signal data and to produce image displays on a CRT of control terminal  24 . The MRI system control terminal  24  may include suitable keyboard switches and the like for exerting operator control. Images may also be recorded directly on film or on other suitable media by printing device. 
   Steady-state free precession (SSFP) is a conventional technique used to generate MRI signals from precessing hydrogen nuclei that do not completely return to their thermal equilibrium state. The SSFP sequence uses a series of RF excitation pulses and magnetic gradient pulses that are applied at repetition times (TR) significantly shorter than the spin-lattice (T 1 ) and the spin-spin (T 2 ) relaxation times of hydrogen nuclei within the subject being imaged. The magnetic gradient pulses are applied to reverse the magnetic field gradients in a predetermined manner to enhance the echo signal. In each TR period, the magnetic gradient pulses are fully balanced, i.e., the total areas of all gradient pulses are zero for each TR period. 
   MR signals acquired in a fully-refocused SSFP sequence, such as that shown in  FIG. 2 , comprise multiple echo sub-signals with different echo-formation pathways. The acquired MR signal, S, can be collectively represented as: 
                 S   =       ∑     n   =     -   ∞         +   ∞       ⁢     S   n               (   1   )               
where S n  denotes n th  echo sub-signal. S 0 , S −1  and S 1  are the three most significant echo components. The echo sub-signals can be separated by RF phase cycling or frequency shifting. For a two component system of water and fat, the dominant echo sub-signals, S 0 , S −1  and S 1 , can be described by
   S   0 =( W   0   +F   0   e   iψ     0   ) e   iφ     0     (2)   S   −1 =( W   −1   +F   −1   e   iψ     −1   ) e   iφ     −1     (3)   S   1 =( W   1   +F   1   e   iψ     1   ) e   iφ     1     (4) 
where
 ψ 0 =2πΔfTE  (5) ψ −1 =−2πΔ f ( TR−TE )  (6) ψ 1 =2πΔ f ( TR+TE )  (7) φ 0   =γΔB   0 TE  (8) φ −1   =−γΔB   0 ( TR−TE )  (9) φ 1   =γΔB   0 ( TR+TE )  (10) 
wherein W 0  and F 0  are amplitudes of water and fat signals, respectively, in S 0 ; W −1  and F −1  are amplitudes of water and fat signals, respectively, in S −1 ; W 1  and F 1  are amplitudes of water and fat signals, respectively, in S 1 ; ΔB 0  represents B 0  inhomogeneities; TR and TE are repetition and echo times, respectively, of the SSFP sequence, such as shown in  FIG. 2 ; and Δf is the difference in resonance frequency between the water and fat signals.
 
   As shown in equations (2-10), S 0 , S −1  and S 1  are modulated differently by the chemical shift and B 0  field inhomogeneities depending on the values of TR and TE. This modulation difference can be harvested to assist with the separation of water and fat data. 
   A phase-cycled or frequency-shifted SSFP sequence is used to acquire signals that can be isolated into sub-signals (S 0 , S −1 , S 1 , etc.) to yield artifact-free images. Phase-cycled or frequency-shifted SSFP, and isolation of the signals are conventional and well-known in the prior art. The isolated sub-signals are applied to achieve water-fat separation that are free of banding artifacts and provide good water-fat separation in the presence of B 0  field inhomogeneities. 
     FIG. 3  is a flow chart of process steps to be performed in order to separate water and fat images. SSFP signals can be acquired for purposes of water-fat separation in at least four manners, briefly: 
   First—Single SSFP Scan: Select TR and TE of the SSFP scan sequence so that water and fat signals are orthogonal in S 0  and anti-parallel in S −1 , or vice versa, in step  100 . An SSFP scan is performed,  102 , with the selected TR and TE values to obtain echo signals from a body. The echo signals are separated into orthogonal S 0  and anti-parallel S −1  data sets, in step  104 . A magnetic field map is generated from the anti-parallel data in step  106 . The field map is applied as a phase correction factor to the orthogonal data set(s), in step  108 . Water and fat images can be constructed from the orthogonal data after applying the phase correction to compensate for B 0  field inhomogeneities, in step  110 . 
   Second—Single SSFP Scan: Select TR and TE of the SSFP scan sequence with TR equal to 2TE and such that water and fat signals are orthogonal in all separated signals,  100 . Apply the imaging sequence and acquire MR data,  102 . Isolate the acquired SSFP MR data to have S −1 , S 0 , and S 1 ,  104 . Generate a field map from a combinatory signal of S 0 S 1 ,  106 . Generate water-fat images constructed from S 0  and S −1 ,  110 , after applying the field map for phase correction,  108 . 
   Third—Assist and Master SSFP Scans: Select TR and TE for a master SSFP scan sequence, TR m  and TE m , such that water and fat signals are significantly orthogonal in all separated signals,  100 . Select TR and TE for a low spatial resolution “assist scan” sequence, TR a  and TE a , such that water and fat signals are anti-parallel in at least one of the separated signals,  100 . Apply both the “master” scan and the “assist” scan in a single session,  102 . Generate a field map from the anti-parallel data of the “assist” scan,  106 . The water-fat images are constructed from the master scan data,  110 , to which the field map from the “assist” scan is applied for phase correction,  108 . 
   Fourth—Single SSFP Scan with Known Field Map: Select TR and TE for an SSFP sequence such that water and fat signals are significantly orthogonal in all separated signals,  100 . Apply the imaging sequence to acquire MR data,  102 . Isolate the acquired SSFP MR data to have S 0  and S −1 ,  104 . Obtain a magnetic field map available from the scanner,  106 . Apply the field map to phase-correct S 0  and S − ,  108 , and generate images with water-fat separation,  110 . 
   In a first embodiment, TR and TE are selected so that water and fat signals are orthogonal in S 0  and anti-parallel in S −1 . This can be accomplished, for example, by selecting TR and TE according to:
 
 TE =(2 j+ 1)/(4 Δf ) and  TR =(6 j+ 3)/(4 Δf ), j=0, 1, 2,  (11)
 
At 0.35 Telsa, one such selection corresponds to TR=15 milliseconds (ms) and TE=5 ms (j=0). The acquired SSFP signals after echo isolation can be described as:
 
 S   0 =( W   0   +iF   0 ) e   iφ   (12)
 
 S   −1 =( W   −1   −F   −1 ) e   −i2φ   (13)
 
A field correction map can be estimated from S −1  according to the following relationship:
 
−4φ=unwrap{arg( S   −1   2 )}  (14)
 
where unwrap{ } stands for the mathematical process for phase-unwrapping such as is described in U.S. Pat. No. 5,909,119, (which is incorporated by reference) and arg( ) returns the principle phase value of its complex input.
 
   Correcting the S 0  image by the phase correction map yields:
 
 S   0   corr   =S   0   e   −iφ   =W   0   +iF   0   (15)
 
and water-fat images can finally be constructed directly from S 0   corr  according to:
 
 I   W =|real{ S   0   corr }|  (16)
 
 I   F =|imag{ S   0   corr }|  (17)
 
where real{ } and imag{ } stand for mathematical routines for returning the real and imaginary part, respectively, of the complex input.
 
   In a second embodiment, TR and TE are selected according to a commonly used TR=2TE condition such that water and fat signals are orthogonal in all isolated echo signals. This can be accomplished by selecting TR=2TE=(2j+1)/(2Δf) with j=0, 1, 2, . . . At 0.35 T, one such selection corresponds to TR=10 ms, TE=5 ms. After isolation, the echo signals, can be described as:
 
 S   0 =( W   0   +iF   0 ) e   iφ   (18)
 
 S   −1 =( W   −1   −iF   −1 ) e   −iφ   (19)
 
 S   1 =( W   1   −iF   1 ) e   i3φ   (20)
 
A combinatory signal is constructed using the following relationship:
 
 S   0   S   1 =[( W   0   W   1   +F   0   F   1 )+ i ( F   0   W   1   −W   0   F   1 )] e   i4Φ   (21)
 
Since it is mostly true that |W 0 W 1 +F 0 F 1 |&gt;&gt;|F 0 W 1 −W 0 F 1 |, S 0 S 1  can be approximated by
 
 S   0   S   1 ≅( W   0   W   1   +F   0   F   1 ) e   i4φ ,  (22)
 
meaning that the phase of the combinatory signal is dominated by the inhomogeneity angle (φ). A field correction map can be generated according to:
 
4φ=unwrap{arg(S 0 S 1 )}  (23)
 
The S 0  and S −1  images are phase-corrected using the field correction map (4φ) and used to finally yield water and fat images (I w  and I f  respectively) described as follows:
 
 S   0   corr   =S   0   e   −iφ   =W   0   +iF   0   (24)
 
 S   −1   corr   =S   −1   e   iφ   =W   −1   −iF   −1   (25)
 
 I   W =|real( S   0   corr )|  (26)
 
 I   F =|imag( S   0   corr )|  (27)
 
or
 
 I   W =|real( S   −1   corr )|  (28)
 
 I   F =|imag( S   −1   corr )|  (29)
 
or
 
   
     
       
         
           
             
               
                 
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   In a third embodiment, a phase-cycled SSFP master scan is accompanied with a low spatial resolution “assist” scan. For the assist scan, the TR and TE values, TR a  and TE a , are selected such that water and fat signals are orthogonal in S 0  and anti-parallel in S −1 . The TR and TE values, TR m  and TE m , are selected for the master scan such that water and fat signals are significantly orthogonal in both S 0  and S −1 . We therefore have:
 
 S   0   a =( W   0   a   +iF   0   a ) e   iφ     0       a     (32)
 
 S   −1   a =( W   −1   a   −F   −1   a ) e   −iφ     −1       a     (33)
 
 S   0   m =( W   0   m   +F   0   m   e   iψ     0       m   ) e   iφ     0       m     (34)
 
 S   −1   m =( W   −1   m   +F   −1   m   e   −iψ     −1       m   ) e   −iφ     −1       m     (35)
 
where
 
φ 0   a =γΔB 0 TE a   (36)
 
φ −1   a   =γΔB   0 ( TR   a   −TE   a )  (37)
 
φ 0   m =γΔB 0 TE m   (38)
 
φ −1   m   =γΔB   0 ( TR   m   −TE   m )  (39)
 
ψ 0   m =2πΔfTE m   (40)
 
ψ −1   m =2 πΔf ( TR   m   −TE   m )  (41)
 
A low spatial resolution field correction map is generated from S −   a  according to:
 
−2φ −1   a =unwrap{arg( S   −1   a   ·S   −1   a )}  (42)
 
The correction map is applied to correct the isolated echo images from the master scan to yield:
 
                   S   0     m   ,   corr       =         S   0   m     ⁢     ⅇ     -       ⅈϕ     -   1     a     ⁡     (       TE   m         TR   a     -     TE   a         )             =       W   0   m     +       F   0   m     ⁢     ⅇ     ⅈψ   0   m                     (   43   )                 S     -   1       m   ,   corr       =         S     -   1     m     ⁢     ⅇ     -       ⅈϕ     -   1     a     ⁡     (         TR   m     -     TE   m           TR   a     -     TE   a         )             =       W     -   1     m     +       F     -   1     m     ⁢     ⅇ     -     ⅈψ     -   1     m                       (   44   )               
Water and fat images can be constructed according to:
   I   W =|Real( S   0   m,corr )−Imag( S   0   m,corr )·cot(ψ 0   m )|  (45) 
                   I   F     =            Imag   ⁡     (     S   0     m   ,   corr       )         sin   ⁡     (     ψ   0   m     )                      (   46   )               
or
   I   W =|Real( S   −1   m,corr )−Imag( S   −1   m,corr )·cot(ψ −1   m )|  (47) 
   
     
       
         
           
             
               
                 
                   
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   In a fourth embodiment, when a field map of the scanner is available for use, TR and TE are selected for a phase-cycled SSFP imaging sequence such that water and fat signals are significantly orthogonal in all sub-signals after echo isolation. The isolated echo signals are phase-corrected by the field map (ΔB 0 ), that is available either from a prior session of field shimming or from another procedure that measures the magnetic field distribution. Water and fat images are constructed from the phase-corrected sub-signals according to:
 
 S   0   corr   =S   0   e   −iγΔB     0     TE =( W   0   +F   0   e   i2πΔfTE )  (51)
 
 S   −1   corr   =S   −1   e   iγΔB     0     (TR−TE) =( W   −1   +F   −1   e   −i2πΔf(TR−TE) )  (52)
 
 I   W =|Real( S   0   corr )−Imag( S   0   corr )·cot(2 πΔfTE )|  (53)
 
                   I   F     =            Imag   ⁡     (     S   0   corr     )         sin   ⁡     (     2   ⁢   πΔ   ⁢           ⁢   fTE     )                      (   54   )               
or
   I   W =|Real( S   −1   corr )−Imag( S   −1   corr )·cot(2 πΔf ( TR−TE ))|  (55) 
   
     
       
         
           
             
               
                 
                   
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                 ( 
                 57 
                 ) 
               
             
           
           
             
               
                 
                   I 
                   F 
                 
                 - 
                 
                   
                     1 
                     2 
                   
                   ⁢ 
                   
                      
                     
                       
                         Imag 
                         ⁡ 
                         
                           ( 
                           
                             S 
                             0 
                             corr 
                           
                           ) 
                         
                       
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             2 
                             ⁢ 
                             πΔ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             fTE 
                           
                           ) 
                         
                       
                     
                      
                   
                 
                 + 
                 
                   
                     1 
                     2 
                   
                   ⁢ 
                   
                      
                     
                       
                         Imag 
                         ⁡ 
                         
                           ( 
                           
                             S 
                             
                               - 
                               1 
                             
                             corr 
                           
                           ) 
                         
                       
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             2 
                             ⁢ 
                             πΔ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               f 
                               ⁡ 
                               
                                 ( 
                                 
                                   TR 
                                   - 
                                   TE 
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                     
                      
                   
                 
               
             
             
               
                 ( 
                 58 
                 ) 
               
             
           
         
       
     
   
     FIG. 4  are computer-generated images showing water and fat separated SSFP images generated according to the first embodiment of the invention. The phase-cycled SSFP imaging sequence had TE of 5 ms and TR of 15 ms. The phase of the RF pulse was incremented by 0, 60, 120, 180, 240, and 300 degrees, respectively. The left image  40  is the isolated SSFP image of S 0 . The middle image  42  shows the separated water data. The right image  44  shows the separated fat data. These images show an absence of banding and a high quality of water-fat separation achieved. 
     FIG. 5  shows the combined image of S 0  and S −1    50 , separated water  52  and fat  54  SSFP images generated according to the second embodiment. The phase-cycled SSFP imaging sequence had a TE of 5 ms and TR of 10 ms. The phase of the RF pulse was incremented by 0, 60, 120, 180, 240, and 300 degrees, respectively. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.