Abstract:
MRI acquisitions are performed to acquire data from which separate water and fat images may be reconstructed using a Dixon technique. Fat signal components intermixed with the water image are suppressed by shifting the pixels in the fat image and subtracting a fraction of this pixel shifted fat image from the water image.

Description:
BACKGROUND OF THE INVENTION 
     The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the separation of fat and water signals in MR images produced using the Dixon method. 
     When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . An NMR signal is emitted by the excited spins, and after the excitation signal B 1  is terminated, this signal may be received and processed to form an image. 
     Materials other than water, principally fat, are found in biological tissue and have different gyromagnetic ratios. The Larmor frequency of protons in fat is shifted approximately 225 Hz from those of protons in water in a 1.5 Tesla polarizing magnetic field B 0 . The difference between the Larmor frequencies of such different species of the same nucleus, viz., protons, is termed chemical shift, reflecting the differing chemical environments of the two species. 
     Often it is desired to “decompose” the NMR image into its several chemical shift components. In the exemplary case of protons, which will be used hereafter for illustration, it may be desired to portray as separate images the water and fat components of the subject. One method of accomplishing this is to acquire two images S 0  and S −1  with the fat and water components of the images in phase, and out of phase by π radians, respectively (the “Dixon” technique). Adding and subtracting these images provides separate fat and water images. The phase shift between the fat and water components of the images may be controlled by timing the RF pulses of the NMR sequence so that the signal from the fat image evolves in phase with respect to the water by the proper angle of exactly π, before the NMR signal is acquired. 
     In the ideal case above, the frequency of the RF transmitter is adjusted to match the Larmor frequency of the water. If the polarizing magnetic field B 0  is uniform, this resonance condition is achieved through out the entire subject. Similarly, the out-of-phase condition (π radians) for the fat component is achieved for all locations in the subject under homogeneous field conditions. In this case, the decomposition into the separate images is ideal in that fat is completely suppressed in the water image, and vice versa. 
     When the polarizing field is inhomogeneous, however, there are locations in the subject for which the water is not on resonance. In this case, the accuracy of the decomposition breaks down and the water and fat images contain admixtures of the two species. Field inhomogeneities may result from improper adjustment or shimming of the polarizing magnetic field B 0 , but are more typically the result of “demagnetization” effects caused by the variations in magnetic susceptibility of the imaged tissue, which locally distort the polarizing magnetic field B 0 . These demagnetization effects may be of short spatial extent but of conventional linear or higher order shimming techniques. 
     The influence of demagnetization may be accommodated, however, by a three-point Dixon imaging technique that uses three acquired images S 0 , S 1  and S −1 , with the phase evolution times adjusted so that the fat and water components of the images are in phase, out of phase by π, and out of phase by −π respectively. The complex pixels in each of the three images after conventional reconstruction may be processed as described, for example, in U.S. Pat. No. 5,144,235 to produce a separate water and a separate fat image. 
     An important assumption in Dixon imaging is that the spectral composition of living tissues is made of two distinct δ-peaks, one corresponding to the water proton resonance and the other corresponding to a loosely termed “fat” resonance peak. The latter is approximately 3.35 ppm, or 225 Hz at 1.5 Tesla field strength, apart from the water resonance frequency. In reality, the “fat” is composed of multiple spectral components. Table 1, lists the major spectral components of corn oil that was measured at 1.5 Tesla. 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Amplitude, T1 and T2 of the corn oil sample at 1.5 Tesla 
               
               
                 (assuming water chemical shift is 4.7 ppm.) 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Chemical 
                 Freq Shift 
               
               
                   
                   
                 Amplitude 
                 T1(n) 
                 T2(n) 
                 Shift 
                 From Water 
               
               
                 No. 
                 Component 
                 A(n) 
                 (ms) 
                 (ms) 
                 (ppm) 
                 Signal (Hz) 
               
               
                   
               
             
          
           
               
                 1 
                 CH 2   
                 0.26 
                 577 
                 227 
                 0.8 
                 −250 
               
               
                 2 
                 (CH 2 ) n   
                 1.00 
                 223 
                 107 
                 1.2 
                 −220 
               
               
                 3 
                 O═C—CH 2 CH 2   
                 0.10 
                 185 
                 43 
                 1.5 
                 −200 
               
               
                 4 
                 C═C—CH 2   
                 0.21 
                 209 
                 67 
                 1.9 
                 −180 
               
               
                 5 
                 O═C—CH 2   
                 0.11 
                 210 
                 71 
                 2.15 
                 −160 
               
               
                 6 
                 C—CH 2 —C═ 
                 0.05 
                 245 
                 183 
                 2.6 
                 −135 
               
               
                 7 
                 CH 2 O(right) 
                 0.04 
                 237 
                 36 
                 3.95 
                 −48 
               
               
                 8 
                 CH 2 O(left) 
                 0.04 
                 242 
                 38 
                 4.15 
                 −35 
               
               
                 9 
                 CH═CH AND CHO 
                 0.15 
                 204 
                 137 
                 5.2 
                 30 
               
               
                   
               
             
          
         
       
     
     As illustrated in Table 1, the loosely-termed “fat” peak is actually composed of a series of peaks (Peaks 1-6) dominated by Peak #2 that corresponds to the methylene protons. In addition, there is actually another group of peaks (Peaks 7-9) whose frequencies fall more closely to the water resonance frequency. The signals from these protons that generate these latter peaks intermix with the “water” signals and are not separated properly by the Dixon method. 
     SUMMARY OF THE INVENTION 
     The present invention is an improved method for producing separate water and fat images. More specifically, the invention includes acquiring MRI data with an MRI system using a pulse sequence in which separate water and fat images may be reconstructed, producing a pixel shifted fat image from a reconstructed fat image which indicates fat signal components intermixed with a reconstructed water image, multiplying the pixel shifted fat image by a factor a, and subtracting the result from the water image. Fat signal components that are not separated from the water signal are emulated by the pixel shifted fat image and subtracted from the water image to remove them therefrom. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an MRI system which employs the present invention; 
     FIG. 2 is a graphic illustration of a pulse sequence employed by the MRI system of FIG. 1 to acquire data; and 
     FIG. 3 is a flow chart of the process used to reconstruct an image according to the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is shown the major components of a preferred MRI system which incorporates the present invention. The operation of the system is controlled from an operator console  100  which includes a keyboard and control panel  102  and a display  104 . The console  100  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on the screen  104 . The computer system  107  includes a number of modules which communicate with each other through a backplane. These include an image processor module  106 , a CPU module  108  and a memory module  113 , known in the art as a frame buffer for storing image data arrays. The computer system  107  is linked to a disk storage  111  and a tape drive  112  for storage of image data and programs, and it communicates with a separate system control  122  through a high speed serial link  115 . 
     The system control  122  includes a set of modules connected together by a backplane. These include a CPU module  119  and a pulse generator module  121  which connects to the operator console  100  through a serial link  125 . It is through this link  125  that the system control  122  receives commands from the operator which indicate the scan sequence that is to be performed. The pulse generator module  121  operates the system components to carry out the desired scan sequence. It produces data which indicates the timing, strength and shape of the RF pulses which are to be produced, and the timing of and length of the data acquisition window. The pulse generator module  121  connects to a set of gradient amplifiers  127 , to indicate the timing and shape of the gradient pulses to be produced during the scan. The pulse generator module  121  also receives patient data from a physiological acquisition controller  129  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes or respiratory signals from a bellows. And finally, the pulse generator module  121  connects to a scan room interface circuit  133  which receives signals from various sensors associated with the condition of the patient and the magnet system. It is also through the scan room interface circuit  133  that a patient positioning system  134  receives commands to move the patient to the desired position for the scan. 
     The gradient waveforms produced by the pulse generator module  121  are applied to a gradient amplifier system  127  comprised of G x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding gradient coil in an assembly generally designated  139  to produce the magnetic field gradients used for position encoding acquired signals. The gradient coil assembly  139  forms part of a magnet assembly  141  which includes a polarizing magnet  140  and a whole-body RF coil  152 . A transceiver module  150  in the system control  122  produces pulses which are amplified by an RF amplifier  151  and coupled to the RF coil  152  by a transmit/receive switch  154 . The resulting signals radiated by the excited nuclei in the patient may be sensed by the same RF coil  152  and coupled through the transmit/receive switch  154  to a preamplifier  153 . The amplified NMR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  150 . The transmit/receive switch  154  is controlled by a signal from the pulse generator module  121  to electrically connect the RF amplifier  151  to the coil  152  during the transmit mode and to connect the preamplifier  153  during the receive mode. The transmit/receive switch  154  also enables a separate RF coil (for example, a head coil or surface coil) to be used in either the transmit or receive mode. 
     The NMR signals picked up by the RF coil  152  are digitized by the transceiver module  150  and transferred to a memory module  160  in the system control  122 . When the scan is completed and an entire array of data has been acquired in the memory module  160 , an array processor  161  operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  115  to the computer system  107  where it is stored in the disk memory  111 . In response to commands received from the operator console  100 , this image data may be archived on the tape drive  112 , or it may be further processed by the image processor  106  and conveyed to the operator console  100  and presented on the display  104 . For a more detailed description of the transceiver  150 , reference is made to U.S. Pat. Nos. 4,952,877 and 4,992,736 which are incorporated herein by reference. 
     The following discussion considers a spin echo pulse sequence employed by the above described apparatus and suitable for use with the present invention. It should be understood, however, that the invention may be used with other pulse sequences as will be apparent to one skilled in the art. 
     Referring to FIG. 4, a spin echo pulse sequence begins with the transmission of a narrow bandwidth radio frequency (F) pulse  50 . The energy and the phase of this initial RF pulse  50  may be controlled such that at its termination, the magnetic moments of the individual nuclei are precessing around the z axis within the x-y plane. A pulse of such energy and duration is termed a 90° RF pulse. 
     The result of a combination of RF pulse  50  and a z axis gradient pulse G z  (not shown) is that the nuclear spins of a narrow slice in the imaged object along an x-y plane are excited into resonance. Only those spins with a Larmor frequency, under the combined field G z  and B 0 , equal to the frequencies of the RF pulse  50  will be excited. Hence the position of the slice may be controlled by the gradient G z  offset or the RF frequency. 
     After the 90° RF pulse  50  the precessing spins begin to dephase according to their chemical shifts which cause the spins of certain chemical species to precess faster than others and the action of the applied gradients which cause spatially dependent off resonance conditions. At time TE/2 after the application of 90° RF pulse  50 , a 180° RF pulse  54  may be applied which as the effect of rephasing the spins to produce a spin echo  56  at time TE after the 90° RF pulse  50 . This spin echo signal  56  is acquired during a readout gradient  53 . As is understood in the art, a dephaser pulse  52  is applied after the 90° RF pulse but before the readout gradient to center the spin echo within the read out gradient. The spin echo signal  56  is sampled at a rate which determines the resolution of the acquired image along the readout gradient axis. For example, 256 samples are typically acquired. 
     With the 180° RF pulse  54  centered at time TE/2, any constant off resonance-induced phase shifts, including the chemical shift of fat relative to water, will be completely rephased at the time of the spin echo  56 . This timing produces an S 0  signal which, consequently, will have no off resonance phase encoding. The time of the 180° pulse  54 , however, may be shifted forward to back by time τ from the time TE/2. In this case, the fat and water proton spins will not be in phase but will be shifted with respect to each other by 2τω cs , where ω cs  is the difference in Larmor frequencies between water and fat. The value of the phase shift between the fat and water images caused by their chemical shift will be designated θ. At the same time, any constant resonance offset ω 0  will be phase encoded by an amount φ=2ω 0 τ. This sequence is repeated with different G y  gradient pulses  57 , as is understood in the art, to acquire a k-space NMR data set from which an image of the imaged object may be reconstructed according to conventional reconstruction techniques using the Fourier transform. The number of separate phase encodings employed in the scan determines image resolution along the phase encoding gradient axis. For example, 128 phase encodings are typically acquired. 
     In the three point Dixon method, three images S i  are acquired using this pulse sequence. Each image S i  has a different phase shift value θ between the water and fat components, as follows: S 0  where θ=0, S 1  where θ=π, and S 2 , where θ=−π. 
     Referring particularly to FIG. 3, the MRI system is operated with the pulse sequence of FIG. 2 to acquire three images as indicated at process block  200 . The images are acquired with θ=0, π and −π such that a water image (w) and a fat image (F) may be reconstructed using the 3-point Dixon method as indicated at process blocks  202  and  204 . This assumes that the tissue spectra are composed of two δ-peaks, one at 3.35 ppm and the other at the water resonance frequency. In the preferred embodiment the 3-point Dixon method is employed, but it can be appreciated that other water/fat separation methods such as the 2-point and 1-point Dixon methods may also be employed. 
     The water image is refined according to the present invention using information in the fat image. More specifically, the fat image is shifted along the frequency encoding axis by an amount D which corresponds to the 3.35 ppm separation between the predominant fat peak and the water peak. The amount D is determined at process block  206 , and it is calculated as a function of the readout bandwidth in Hertz (BW), the image matrix size (N x ) along the readout axis, and the water/fat separation (3.35 ppm). At 1.5 Tesla field strength, the shift amount D is given as: 
     
       
           D =225 *N   x   /BW.   
       
     
     At a readout bandwidth BW of +/−16 kHz and with N x =256, for example, the shift amount D is approximately 2 pixels. The fat image is shifted by the amount D as indicated at process block  208  and a loop is entered at  210  in which the water image is corrected using this shifted fat image (shift (F)). When the shift factor D is not an integral number, it is also possible to shift the fat image and interpolate between its pixels to produce a corrective image having the desired non-integral shift factor D. 
     As indicated at process block  212 , the corrected water image (W′) is produced by subtracting a small fraction (α) of the shift fat image (shift (F)) from the acquired water image (W): 
     
       
           W′=W −α*shift ( F ). 
       
     
     The factor α is determined by the relative amounts of the different fat components, the relaxation time constants of these components and the timing parameters (e.g. TR, TE, echo train length) in the pulse sequence used to acquire the MRI data. The value of α can be determined empirically or modeled using empirical data. For example, in a T1-weighted spin echo acquisition, α is usually in the range of 0.05 to 0.10. 
     This initial value of α is entered and a corrected water image is produced at  212  and displayed to the operator. If the image is optimal, the image is stored as indicated at process block  214  and the process ends. Otherwise, the process branches at decision block  216  and the factor α is adjusted at process block  218  before producing another corrected water image at process block  212 . Adjustments are made until the fat signal is optimally removed from the water image. 
     The optimal value of the scaling factor α is application specific. For example, for a quantitative water/fat image, the value of α may be chosen for best fat suppression. On the other hand, such an image may not be optimal for visualization from a clinical diagnostic standpoint. For example, a certain amount of fat signal may be maintained in the image so as not to create dark voids that may be distractive. In such cases the water image is “fat-managed” by choosing a value of α that results in the best visualization of the anatomy.