Abstract:
In a magnetic resonance imaging apparatus, a subject is disposed in an imaging region ( 10 ). A magnet assembly ( 16, 18 ) creates a main magnetic field (B o ) through the imaging region ( 10 ). A sequence of radio frequency pulses and gradient field manipulations excites and manipulates magnetic resonance within dipoles of the subject. The sequence comprises a 90° RF excitation pulse and a 180° refocusing pulse as is known in the art of standard spin echo imaging. The sequence includes gradient pulses that induce at least two gradient recalled RF echoes ( 98, 100 ) while suppressing spin echo signals. The sequence also includes diffusion sensitive gradients ( 76, 82 ) that sense the movement of water or other molecules during the imaging sequence. The gradient induced echoes ( 98, 100 ) are symmetrically disposed about a time (TE). Included in the apparatus is a reconstruction processor ( 54 ) that takes real and imaginary portions of the received magnetic resonance signals and converts them into magnitude data of the signals. The magnitude data is used in lieu of phase encoding in reconstruction.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the diagnostic imaging arts. It finds particular application in conjunction with open MRI systems operating diffusion detection sequences and will be described with particular reference thereto. It will be appreciated, however, that the present invention is useful in conjunction with other systems, such as higher field bore type systems, and is not limited to the aforementioned application. 
     Typically, in magnetic resonance imaging, a main magnetic field B o  is generated through an imaging region wherein is located a subject. Radio frequency (RF) coils transmit RF pulses into the imaging region exciting and manipulating dipoles within the subject. Gradient coils superimpose gradient magnetic fields on the main magnetic field in order to spatially and spectrally encode the excited dipoles. 
     Diffusion weighting sequences detect the movement of water in the subject on the cellular level. These sequences typically detect the movement of water on the order of a few microns, or about the distance it takes to cross a cell membrane. Echo planar imaging (EPI) sequences have been used to detect diffusion in a magnetic resonance scan. Although fast in data acquisition, EPI sequences tend to be very motion sensitive and have relatively low signal to noise ratios (SNR). 
     Rotating k-space diffusion sequences without phase encoding are also used. These sequences are typically more resilient to patient motion, and have higher SNR. However, they take much longer than an EPI sequence, typically on the order of three to four minutes. Another drawback of this type of sequence is that it tends to put heavy loads on the gradient equipment, and is not implementable in all present day MRI scanners. In addition, high, rapidly changing gradient fields can add to patient discomfort. 
     Fast spin echo (FSE) sequences have also been used to detect diffusion. Multi echo FSE sequences are faster than the rotating k-space sequences, but tend to have other limitations. Stimulated echoes and the primary echo of this type of sequence must have the same time and phase. In a normal FSE sequence, this is generally not a problem, but in a diffusion detection sequence, the phase becomes distorted unpredictably by patient motion and cannot be corrected. Thus, the primary echo and stimulated echoes cannot be summed properly. 
     Methods to restrict patient motion have been used, but are not desirable. Sedatives can be given to the patient to render the patient immobile, but there is always some degree of risk to the patient. Mechanical restraints are also used to restrict patient motion, but these are often imposing and uncomfortable to the patient. Such discomfort can often cause the patient to fidget and move more than if they were unrestrained. 
     The present invention provides a new and improved method a nd apparatus that overcomes the above referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a method of magnetic resonance is provided. Resonance is excited in selected dipoles of a subject in an imaging region. The resonance is refocused with an inversion pulse, and at least two gradient recalled echos are acquired. Data is collected until enough is present to be reconstructed into an image representation to the subject. 
     In accordance with another aspect of the present invention, a magnetic resonance apparatus is provided. A main magnetic assembly generates a main magnetic field through a subject in the imaging region. An RF coil assembly transmits RF pulses into the imaging region. A gradient coil assembly superimposes gradient fields on the main magnetic field inducing at least two gradient recalled echos. A receiver receives magnetic resonance signals from the imaging region, and a reconstruction processor reconstructs the received magnetic resonance into an image representation. 
     One advantage of the present invention resides in faster acquisition times. 
     Another advantage resides in higher signal to noise ratios in diffusion weighting imaging. 
     Another advantage resides in more robust data acquisition schemes. 
     An other advantage resides in reduced RF and gradient loads. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus in accordance with the present invention; 
     FIG. 2 is a pulse sequence diagram illustrating a diffusion weighting pulse sequence in accordance with the present invention; 
     FIG. 3 is a preferred trajectory through k-space, generated by multiple iterations of the pulse sequence of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a subject is disposed in an imaging region  10  of an magnetic resonance apparatus. Preferably, an open magnetic resonance apparatus is utilized, however, bore type machines have also been contemplated. The subject is disposed in the apparatus between an upper pole assembly  12  and a lower pole assembly  14 . Annular magnets  16 ,  18  preferably resistive magnets, generate a static, main magnetic field B o  through the imaging region  10 , between upper and lower pole pieces  20 ,  22 . It is to be understood that superconducting magnets are also contemplated. 
     For spatially encoding the B o  main field, gradient coils  24 ,  26  transmit gradient pulses into the imaging region. The pulses are controlled by a gradient field controller  28  and amplified by gradient amplifiers  30  then transmitted to the gradient coils  24 , 26 . In the preferred embodiment, the gradient field controller  28  includes three specific gradient synthesizers which are utilized in constructing the gradient sequence. A diffusion gradient synthesizer  32  synthesizes gradients that sensitize the magnetic resonance apparatus to the diffusion of water or other molecules in the subject. A frequency encoding synthesizer  34  synthesizes gradient pulses that frequency encode resonance in the subject as well as induce gradient recalled echoes in the subject. The third gradient synthesizer is a slice select gradient synthesizer  36 . The slice select synthesizer creates gradient pulses that will limit a region of interest within the subject to a slab or a slice. 
     Upper and lower RF coils  40 ,  42  transmit RF pulses into the imaging region  10 . The pulses are transmitted to the coils  40 ,  42  by at least one RF transmitter  44 , preferably digital. The RF pulses are generated by an RF pulse controller  46 . The gradient field controller  28  and the RF pulse controller  46  are both controlled by a sequence controller  48  which retrieves desired sequences from a sequence memory  50 . 
     When the selected pulse sequence induces the desired magnetic resonance manipulations in the subject, the RF coils  40 ,  42  or other local receive coils detect the resonance signals. The resonance signals are received by at least one receiver  52  and processed by a reconstruction processor  54  which applies a one dimensional Fourier transform or other appropriate reconstruction algorithm to the detected resonance signals. The processed signals are stored in a magnitude memory  56 . A backprojection processor  58  backprojects the image magnitudes producing an image representation that is stored in a volumetric image memory  60 . An image processor  62  extracts portions of the image representation from the image memory  60  and formats them for display on a human readable display  62  such as a video monitor, LCD display, active matrix monitor, or the like. 
     With reference to FIG. 2, and with continuing reference to FIG. 1, a preferred pulse sequence is illustrated. Generally, the preferred pulse sequence generates multiple gradient recalled echoes. Magnetic resonance data is read from these echoes in the presence of a plurality of different gradients. Slice select gradients are applied to reduce the imaging volume to a selected slice or slab within the subject. Diffusion gradients and frequency encoding gradients are applied preferably such that their sensitivities are orthogonal to one another. In the preferred embodiment, data is read in the absence of phase encoding gradients. 
     After the subject has been positioned properly within the imaging region  10 , the sequence is initiated. A 90° RF excitation pulse  70  is applied in the presence of a slab select gradient  72 . The excitation pulse  70  preferably tips magnetic dipoles selected by the slab select gradient  72  into the transverse plane, that is, approximately 90°. 90° of tip yields a relatively strong signal, in comparison to lesser degrees of initial tip. It is to be understood that other tip angles are possible, with 90° being preferred as it yields a higher SNR with a possibility for the most gradient recalled echoes. 
     Immediately following the excitation pulse  70  a dephase gradient  74  is applied to the imaging region. The dephase gradient is composed of a y-component  74   a  and an x-component  74   b.  The x and y directions refer to the orthogonal directions in the excited slab, and they also define the directions k x  and k y  in k-space. The purpose of the dephase gradient is to form an echo in the first read gradient event. Alternatively, it may be applied after the refocusing pulse with opposite sign. The two portions of the dephase gradient  74  combine to define a directional vector of sensitivity in k-space. 
     Following the dephase gradient  74 , a first portion of a diffusion sensitivity gradient  76  is applied. Similar to the dephase gradient  74 , the first portion of the diffusion gradient is divided into a y-component  76   a  and an x-component  76   b.  Subsequent to the first portion of the diffusion gradient  76  is a 180° refocusing RF pulse  78 . Concurrent with the refocusing pulse  78 , a second slice select gradient  80  is applied. The first and second slice select gradients  72 ,  80  select the same region within the subject such that the excitation pulse  70  and the refocusing pulse  78  affect the same region. Following the second slice select gradient is a second portion of the diffusion sensitivity gradient  82 . Similar to the first portion  76  the second portion  82  is divided into a y-component  82   a  and an x-component  82   b.  Collectively, the diffusion sensitivity gradient  76 ,  82  detects the diffusion of selected molecules, preferably water, during times when it is active. The first part  76  and the second part  82  of the diffusion sensitivity gradient are preferably disposed symmetrically in time about the 180° refocusing RF pulse  78 . Preferably, the gradient lobes  76  and  82  should have the same area. 
     Following the second part of the diffusion gradient  82  a first frequency encode gradient  84 , commonly known as a read gradient, is applied. Similar to the previous gradients, the first read gradient  84  is composed of a y-component  84   a  and an x-component  84   b.  Preferably, the two components define a directional vector that is approximately perpendicular to the directional sensitivity of the diffusion gradient  76 ,  82  and has the same direction as the dephasing gradient  74 . In the preferred embodiment, the area underneath lobes of the first read gradient  84  is approximately twice the size of the area underneath the dephase gradient  74 . 
     In order to induce gradient recalled echoes, a reversal gradient  86  is applied to the imaging region. The reversal gradient, similar to the other gradients has a y-component  86   a  and an x-component  86   b.  The reversal gradient effectively resets the data readout in k-space, and defines where one data line stops and the next one begins. Subsequently, another read gradient  88  is applied to form a second data line in k-space. The second read gradient comprises a y-component  88   a  and an x-component  88   b.  As illustrated, the second read gradient  88  is slightly different than the first read gradient  86 . Preferably, the read direction of the second read gradient  88  is shifted less than one degree from the read direction of the first read gradient  84 . 
     In the preferred embodiment, and as illustrated in FIG. 3, data lines are stepped in a rotational fashion about a center of k-space  100 . The first read gradient  84  generates a data line  102  in k-space. The second read gradient  88  generates a second data line  103  with a slightly different read direction than the first data line  102 . The preferred sequence makes  256  steps around 180° making the angle  106  between each data line approximately 0.70°. Of course, more or fewer steps can be taken, it is preferred, however, that the angle between data lines be no greater than 10° as the sequence will become sensitive to motion of the subject, as discussed in the background. Additional data lines are generated with additional cycles of the described two echo embodiment. 
     Alternately, additional data lines can be read by utilizing additional reversal gradients. In a three echo embodiment, the second reversal gradient is approximately the same amplitude of the reversal gradient  86 , with a small change in the direction of the third read gradient, producing data line  104 . As a practical limit, about four gradient recalled echoes may be induced from one RF excitation pulse. After four echoes, the phase errors accumulated in the gradient recalled echoes may become too large. 
     In the illustrated two echo embodiment, RF echoes  90  and  94  occur in time windows  92 , and  96  respectively and are disposed symmetrically about a time TE. In additional echo embodiments, the group of echoes is centered on the time TE, for example, in a three echo embodiment, the second echo is centered on the time TE. The 180° refocusing pulse  78  occurs at a time TE/2, as implied, the time TE/2 occurs at half the value of time TE. 
     As was stated earlier, the RF echoes are received in the absence of phase encoding. Projection reconstruction methods are used to form the image. Because the gradient echo signal typically contains phase errors, reconstruction methods using magnitude data are preferred. In the preferred embodiment, the receiver  52  receives a real part and an imaginary part of the magnetic resonance signals. The reconstruction processor  54 , as part of the reconstruction, adds the squares of the real and imaginary parts and takes the square root. Simplified, if the real part is x and the imaginary part is y the reconstruction processor performs the operation {square root over (x 2 +L +y 2 +L )} to obtain the magnitude data of the received resonance signals. 
     In an alternate embodiment, no reversal gradients are used. Instead, subsequent gradients are substantially opposite each other. That is, if the first gradient lobe is positive, the second is negative, and so forth. In this embodiment, the frequency encoding gradients run subsequently with no interruption, or possibly with a small gradient pulse in-between to adjust the starting point of the data line. In this embodiment, the data lines are read in opposite directions, and the arrowheads in FIG. 3 would likewise alternate. 
     In an alternate embodiment, the diffusion gradient pulses  76 ,  82  are both applied before the refocusing pulse  80 . In this embodiment, the pulses have opposite signs. 
     In an alternate embodiment, the diffusion gradient pulses  76 ,  82  contain a component in the direction of the slice select gradient. Despite this modification, the requirement that the diffusion gradient be orthogonal to the read gradient can still be fulfilled. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.