Abstract:
A system and method of MR imaging enables PROPELLER imaging to be feasibly carried out independently of slice orientation or anatomy of interest. The invention is directed to accelerated acquisition of blades of MR data that are rotated about a central region of k-space and reconstructing an image of arbitrary slice orientation from the blades of MR data that preserves contrast and reduces acceleration artifacts caused by signal amplitude variances.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to diagnostic imaging and, more particularly, to a system and method of filling partially filled blades of k-space acquired in parallel via a multi-coil array. The present invention is also directed to a process of reducing artifacts introduced in filling the partially filled blades. 
         [0002]    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, or “longitudinal magnetization”, M Z , may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M t . A signal is emitted by the excited spins after the excitation signal B 1  is terminated and this signal may be received and processed to form an image. 
         [0003]    When utilizing these signals to produce images, magnetic field gradients (G x , G y , and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. 
         [0004]    Fast Spin Echo (FSE) imaging is an MR imaging technique commonly used as an efficient method of collecting MRI data with minimal artifact. But even FSE images suffer from ghosting artifacts resulting from voluntary or involuntary patient motion as the image acquisition usually takes a few minutes. 
         [0005]    A number of imaging techniques have been developed to reduce motion artifacts of FSE images. One such FSE technique, which is referred to as Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) imaging, encodes an MR signal by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is acquired. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate acquisition of the necessary regions of k-space for a desired resolution image. 
         [0006]    Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction has been shown to be very effective in reducing artifacts associated with inadvertent subject translational and rotational movements in near axial head scans. PROPELLER is relatively insensitive to motion as the center of k-space is sampled multiple times during acquisition. In addition, explicit correction for rotation and shift are used to further reduce motion artifacts. Notwithstanding the advantages of PROPELLER imaging, its applicability has been limited to acquiring full blades of k-space data due to the complexity of PROPELLER data acquisition and reconstruction techniques. 
         [0007]    It would therefore be desirable to have a system and method of MR imaging implementing an accelerated PROPELLER imaging protocol. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    The present invention overcomes the aforementioned drawbacks with a system and method of reconstructing MR images from MR data acquired using a modified PROPELLER acquisition such that scan time and motion-related artifacts are reduced. 
         [0009]    Therefore, in accordance with one aspect of the invention, an MRI apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to, for each of a plurality of receiver coils, (A) acquire k-space data to partially fill a k-space blade of a plurality of k-space data lines extending through a central region of k-space, (B) apply a 1D Fourier Transform to the k-space data lines in a frequency encoding direction, and (C) fill a remainder of the partially filled and transformed blade with data determined from that sampled and transformed in each of the plurality of receiver coils to obtain a filled blade. 
         [0010]    In accordance with another aspect of the invention, a method of parallel imaging includes (A) acquiring a k-space data set representing a partially filled blade from each of a plurality of receiver coils, (B) acquiring a set of k-space calibration data lines from each of the plurality of receiver coils, and (C) transforming, in one dimension, the k-space data sets and the sets of k-space calibration data lines to hybrid space. The method also includes (D) determining reconstruction weights from the sets of hybrid space calibration data lines and (E) applying the reconstruction weights to the hybrid space data sets to synthesize a plurality of complete hybrid space blade data sets. A respective image from each of the plurality of complete hybrid space blade data sets is reconstructed. 
         [0011]    In accordance with yet another aspect of the invention, the invention is embodied in a computer program stored on a computer readable storage medium and representing a set of instructions that, when executed by a computer, causes the computer to acquire at least one k-space data set of a partially filled blade and fill a remainder of the partially filled blade with data determined from that acquired in the at least one k-space data set. The instructions also cause the computer to apply a T2 decay correction to data in the at least one k-space data set in a phase encoding direction on a per-blade basis such that k-space modulation artifacts are reduced. The instructions further cause the computer to reconstruct an image from the at least one k-space data set comprising T2 decay corrected data. 
         [0012]    Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
           [0014]    In the drawings: 
           [0015]      FIG. 1  is a schematic block diagram of an MR imaging system for use with the present invention. 
           [0016]      FIG. 2  is a schematic of a coil array usable with the MR imaging system shown in  FIG. 1 . 
           [0017]      FIG. 3  is a process map illustrating a preferred reconstruction flow in accordance with one aspect of the present invention. 
           [0018]      FIG. 4  is a process map illustrating a preferred reconstruction flow in accordance with another aspect of the present invention. 
           [0019]      FIG. 5  is an illustration showing a technique for fitting data for reconstruction weight calculation usable with an embodiment of the present invention. 
           [0020]      FIG. 6  is an illustration showing a technique for fitting data for reconstruction weight calculation incorporating the technique shown in  FIG. 5 . 
           [0021]      FIG. 7  is a graph showing modified view order signal amplitude correction according to one aspect of the present invention. 
           [0022]      FIG. 8  is an image showing a point spread function for PROPELLER data. 
           [0023]      FIG. 9  is an image showing a point spread function for PROPELLER data corrected for T2 decay. 
           [0024]      FIG. 10  is a schematic representation of a single blade of k-space that is filled using homodyne processing in accordance with another aspect of the present invention. 
           [0025]      FIG. 11  is an image illustrating a partially filled blade of k-space. 
           [0026]      FIG. 12  is an image illustrating a blade of k-space that was originally partially filled and subsequently filled using homodyne processing. 
           [0027]      FIG. 13  is a graph showing relative signal amplitude due to T2 decay after homodyne blade processing. 
           [0028]      FIG. 14  is an image of a DQA phantom reconstructed from blades of data acquired at different orientations using parallel imaging and a half NEX PROPELLER reconstruction technique. 
           [0029]      FIG. 15  is an image of a DQA phantom reconstructed from blades of data acquired at different orientations using parallel imaging with a modified half NEX PROPELLER reconstruction technique in accordance with one aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    The present invention is directed to a parallel imaging technique that is applicable to Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) imaging. K-space is well-known in the art of MR imaging as a matrix that functions as, or is equivalent to, a “frequency domain” repository for positive and negative spatial frequency values that are encoded as complex numbers, e.g., a +bi, i=sqrt(−1). That is, the k-space matrix is generally recognized as the repository for spatial frequency signals acquired during evolution and decay of an MR echo. The k-space matrix in PROPELLER imaging is typically filled with frequency encoded data collected during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate measurement of the necessary regions of k-space for a desired resolution. 
         [0031]    After the full k-space is obtained, the elements of the k-space matrix contain positionally-dependent phase change variations along the kx (frequency encode) and ky (phase encode) direction. A 2D inverse Fourier transform decodes the frequency domain information. The 2D Fourier transform is a two step process. First, a row-by-row 1D Fourier transform converts each row of k-space data. After the row-by-row Fourier transform, a column-by-column 1D Fourier transform is performed. Collectively, the pair of 1D Fourier transforms converts the k-space data from the frequency domain (k-space data) to the spatial domain (image space data). An image is then reconstructed from the image matrix illustrating spatial and contrast characteristics of the object imaged. 
         [0032]    “Hybrid space” refers to the intermediate matrix that results in the performance of one of the 1D Fourier transforms that comprise the 2D Fourier transform that converts k-space to image space. In other words, in 2D Fourier imaging, one Fourier transform is performed in the frequency encoding direction and another Fourier transform is performed in the phase encoding direction. The matrix after the first 1D Fourier transform is considered a “hybrid space”. That is, the data is no longer “untransformed” and therefore not considered k-space; however, the data, as a whole, is not yet in the spatial domain and, thus, not in “image space”. 
         [0033]    Referring to  FIG. 1 , the major components of a preferred magnetic resonance imaging (MRI) system  10  incorporating the present invention are shown. The operation of the system is controlled from an operator console  12  which includes a keyboard or other input device  13 , a control panel  14 , and a display screen  16 . The console  12  communicates through a link  18  with a separate computer system  20  that enables an operator to control the production and display of images on the display screen  16 . The computer system  20  includes a number of modules which communicate with each other through a backplane  20   a . These include an image processor module  22 , a CPU module  24  and a memory module  26 , known in the art as a frame buffer for storing image data arrays. The computer system  20  is linked to disk storage  28  and tape drive  30  for storage of image data and programs, and communicates with a separate system control  32  through a high speed serial link  34 . The input device  13  can include a mouse, joystick, keyboard, track ball, touch activated screen, light wand, voice control, or any similar or equivalent input device, and may be used for interactive geometry prescription. 
         [0034]    The system control  32  includes a set of modules connected together by a backplane  32   a . These include a CPU module  36  and a pulse generator module  38  which connects to the operator console  12  through a serial link  40 . It is through link  40  that the system control  32  receives commands from the operator to indicate the scan sequence that is to be performed. The pulse generator module  38  operates the system components to carry out the desired scan sequence and produces data which indicates the timing, strength and shape of the RF pulses produced, and the timing and length of the data acquisition window. The pulse generator module  38  connects to a set of gradient amplifiers  42 , to indicate the timing and shape of the gradient pulses that are produced during the scan. The pulse generator module  38  can also receive patient data from a physiological acquisition controller  44  that receives signals from a number of different sensors connected to the patient, such as ECG signals from electrodes attached to the patient. And finally, the pulse generator module  38  connects to a scan room interface circuit  46  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  46  that a patient positioning system  48  receives commands to move the patient to the desired position for the scan. 
         [0035]    The gradient waveforms produced by the pulse generator module  38  are applied to the gradient amplifier system  42  having Gx, Gy, and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in a gradient coil assembly generally designated  50  to produce the magnetic field gradients used for spatially encoding acquired signals. The gradient coil assembly  50  forms part of a magnet assembly  52  which includes a polarizing magnet  54  and a whole-body RF coil  56 . A transceiver module  58  in the system control  32  produces pulses which are amplified by an RF amplifier  60  and coupled to the RF coil  56  by a transmit/receive switch  62 . The resulting signals emitted by the excited nuclei in the patient may be sensed by the same RF coil  56  and coupled through the transmit/receive switch  62  to a preamplifier  64 . The amplified MR signals are demodulated, filtered, and digitized in the receiver section of the transceiver  58 . The transmit/receive switch  62  is controlled by a signal from the pulse generator module  38  to electrically connect the RF amplifier  60  to the coil  56  during the transmit mode and to connect the preamplifier  64  to the coil  56  during the receive mode. The transmit/receive switch  62  can also enable a separate RF coil (for example, a surface coil) to be used in either the transmit or receive mode. 
         [0036]    The MR signals picked up by the RF coil  56  are digitized by the transceiver module  58  and transferred to a memory module  66  in the system control  32 . A scan is complete when an array of raw k-space data has been acquired in the memory module  66 . This raw k-space data is rearranged into separate k-space data arrays for each image to be reconstructed, and each of these is input to an array processor  68  which operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link  34  to the computer system  20  where it is stored in memory, such as disk storage  28 . In response to commands received from the operator console  12 , this image data may be archived in long term storage, such as on the tape drive  30 , or it may be further processed by the image processor  22  and conveyed to the operator console  12  and presented on the display  16 . 
         [0037]    The MR system described in  FIG. 1  may also be equipped with a receive coil array that picks up the MR signals. Such coil arrays are well-known in the art and include whole body arrays as well as partial body arrays, such as head coil arrays, cardiac coil arrays, and spine coil arrays. As will be described, the invention includes a parallel imaging method wherein a region or volume of interest is sampled with an array of RF receive coils. In this regard, the invention is not limited to a particular coil array type or orientation. 
         [0038]    The present invention provides a system and method of MR imaging particularly applicable with protocols such as PROPELLER. In PROPELLER, data is collected during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate measurement of the necessary regions of k-space for a desired resolution. Multiple RF coils receive echoes that are used to create separate blades or strips in k-space. Preferably, each blade extends through the center of k-space. The echo train is sampled such that a blade does not have all lines filled with acquired data. A parallel imaging or partial Fourier technique is used to fill each blade partially acquired. The blades are incrementally rotated about the center of k-space with each echo train, are partially sampled, and are filled until a full set of blade data is acquired. 
         [0039]    Referring now to  FIG. 2 , a schematic representation of a conventional torso RF coil array  70  is illustrated. The torso RF coil array  70  is a surface coil used to acquire imaging data for a field-of-view (FOV) and includes eight separate coil elements  72 - 79 . Each coil element samples the FOV by detecting changes in excited nuclei in the FOV and transmits signals indicative of that which is detected to separate data acquisition channels  80 - 87  for each blade in PROPELLER imaging, respectively. According to one embodiment of the present invention, the data from each channel  80 - 87 , once all blades have been acquired, is then used to reconstruct a “coil” image  88 - 95  whereupon the respective coil images are combined into a composite image  96  using one of a number of known summation techniques, e.g., sum of squares. One skilled in the art will appreciate that the coil array illustrated in  FIG. 2  is exemplary and that the invention is not limited to parallel acquisition using such a coil array. 
         [0040]    An embodiment of the present invention is directed to an autocalibrating parallel imaging technique to fill un-acquired blade data for accelerated PROPELLER imaging, such as generalized autocalibrating partially parallel acquisitions (GRAPPA), simultaneous acquisition of spatial harmonics (AutoSMASH), variable density AutoSMASH, autocalibrating reconstruction for Cartesian (ARC) sampling, and the like. While an exemplary embodiment of the present invention will be described below using a modified GRAPPA technique, one skilled in the art will readily recognize that the invention is not so limited and that other autocalibrating parallel imaging techniques such as those listed above are also applicable. 
         [0041]    In one embodiment of the invention, the autocalibrating parallel imaging technique is performed in hybrid space. The implementation is shown in  FIG. 3  for exemplary purposes as being for a coil array comprising two coils. In the illustrated implementation, two separate k-space data sets  97 ,  98  are acquired for one blade of a multi-blade scan. Those skilled in art will recognize that rotation of the k-space data sets  97 ,  98  will vary for each blade acquired. Each k-space data set comprises data sampled from a reduced FOV by a respective coil. Moreover, each k-space data set  97 ,  98  includes imaging data lines  99 ,  100  and calibration data lines  101 ,  102 , respectively. As shown, each k-space data set  97 ,  98  is undersampled in the phase encoding direction  105 . That is, a reduced number of phase encoding steps are undertaken so as to reduce scan time. As will be described, the missing phase encoding lines (views) will be synthesized mathematically from the acquired data. Those skilled in art will recognize that the calibration data can also be acquired separately rather than embedded within the scan, so that full acceleration can be accomplished for the parallel imaging scan. 
         [0042]    The k-space data sets  97 ,  98  are Fourier transformed in the one dimension to generate hybrid space data sets  103 ,  104 . Preferably, the 1D Fourier transformations are in the frequency encoding direction  107 . The reconstruction weights  106  are then estimated directly from hybrid space data preferably including a floating net-based fitting as described below in  FIG. 4 . 
         [0043]    Then, missing data in the hybrid spaces  103 ,  104 , i.e., the undersampled phase encoding locations, is synthesized efficiently from the acquired imaging and calibration data in hybrid space so as to yield complete blades  108 ,  110  in hybrid spaces  112 ,  114 . In other words, the phase encoding locations that were not sampled are filled. 
         [0044]    The complete blades  108 ,  110  are converted to k-space for further PROPELLER processing. In a preferred embodiment, a 1D Fourier Transform is applied to blades  108 ,  110  in the frequency encoding direction  107  to transform the complete blades  108 ,  110  to k-space data sets  116 ,  118 . In this manner, k-space data sets  116 ,  118  provide data to a standard multi-channel PROPELLER reconstruction pipeline  120 , the first step of which is applying a 2D Fourier Transform in the phase and frequency encoding directions  105 ,  107  to convert the k-space data sets  116 ,  118  to image space, applying phase correction to the images, summing the images into a combined blade image, transferring each blade to k-space for rotation/translation correction, gridding to a full k-space and converting to final images  122 . Alternatively, according to another embodiment of the present invention as shown in  FIG. 4 , the complete blades  108 ,  110  may be input into a modified multi-channel PROPELLER reconstruction pipeline  124 , which includes applying an inverse 1D Fourier Transform in the phase encoding direction  105  to convert the complete blades  108 ,  110  to image space, applying phase correction to the images, summing the images into a combined blade image, transferring each blade to k-space for rotation/translation correction, gridding to a full k-space and converting to final images  126 . 
         [0045]      FIG. 5  illustrates fitting data in hybrid space applicable to estimation of reconstruction weights  106  according to one embodiment of the present invention. A plurality of data lines  128  are shown including acquired lines  130 , autocalibrated signal (ACS)  132  lines, and lines  134  missing acquired data. In conventional fitting, a number of fitting blocks  136  are fixed resulting in a fixed number of training data sets. In floating net-based fitting, interpolation can be shifted to yield a number of fitting blocks  138  in addition to the conventional fitting blocks  136 . As shown in  FIG. 6 , a fitting block  136 ,  138  interpolates data in both the frequency encoding direction and the phase encoding direction  135 ,  137  for coils  1 -N. The fitting block  136 ,  138  of  FIG. 5  slides in the phase encoding direction according to the floating net-based fitting described above in  FIG. 5  as more training sets are determined. 
         [0046]    Table 1 illustrates view ordering of a 36-view blade including a full blade view order (VO) and accelerated blade view orders skipping every other view (phase encode location) of a blade in k-space. Views  18  and  20  are ACS lines, and the center of k-space is located in center views  17 ,  18 ,  19 , and  20 . 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Group 
                 Full Blade 
                 Accelerated 
                 Accelerated TE 
               
               
                 No. 
                 VO 
                 VO 
                 Preserving VO 
               
               
                   
               
             
             
               
                 1 
                 01, 02, 03, 04, 
                 01, 03, 05, 07, 
                 01, 35, 03, 33, 
               
               
                 2 
                 05, 06, 07, 08, 
                 09, 11, 13, 15, 
                 05, 31, 07, 29, 
               
               
                 3 
                 09, 10, 11, 12, 
                 17, 18, 19, 20, 
                 09, 27, 11, 25, 
               
               
                 4 
                 13, 14, 15, 16, 
                 21, 23, 25, 27, 
                 13, 23, 15, 21, 
               
               
                 5 
                 17, 18, 19, 20, 
                 29, 31, 33, 35 
                 17, 18, 19, 20 
               
               
                 6 
                 21, 22, 23, 24, 
               
               
                 7 
                 25, 26, 27, 28, 
               
               
                 8 
                 29, 30, 31, 32, 
               
               
                 9 
                 33, 34, 35, 36 
               
               
                 column 1 
                 column 2 
                 column 3 
                 column 4 
               
               
                   
               
             
          
         
       
     
         [0047]    The full blade view order, shown in column 2 of Table 1, shows an arrangement of the views according to a chronological order beginning with view  01  and ending with view  36 . The center views of the full blade view order occur in group number  5  beginning at view number  17 . T2 decay causes the signal amplitude in each succeeding view to be less than the signal amplitude of previous views. 
         [0048]    An accelerated view order, shown in column 3 of Table 1, shows an arrangement of the accelerated views maintaining a chronological order beginning with view  01  and ending with view  35 . The center views of the accelerated view order occur in group number  3 , beginning eight views prior to the center views  17 - 20  of the full blade view order. In this manner, the signal amplitudes of the center views of the accelerated view order will be higher than the signal amplitudes of the center views in the full blade view order. Also, views  03 - 15  and views  21 - 35  will also have signal amplitudes higher than the signal amplitudes of their counterpart views in the full blade view order. Only view  01  will have a signal amplitude substantially equal to the signal amplitude of view  01  the full blade view order. 
         [0049]    Acquiring signal amplitudes using the accelerated view order that have higher amplitudes than counterpart views in the full blade view order results in change of contrast in the final image. To preserve contrast, the accelerated TE preserving view order shown in column 4 of Table 1 rearranges the order of the views such that odd-numbered views  01 - 15  of the accelerated view order and that the center views have signal amplitudes substantially equal to the signal amplitudes of their counterparts in the full blade view order. The rest of the views will have signal amplitudes greater than their counterparts in the full blade view order. 
         [0050]      FIG. 7  shows correction of signal amplitudes due to T2 decay according to an embodiment of the present invention. A curve  140  shows signal amplitudes for views  01 - 36  for a full blade view order as described above. As illustrated, the signal amplitudes of echoes acquired in each view become smaller with each succeeding view. Accelerating blade acquisition by acquiring every other view according to the accelerated TE preserving view order of Table 1 causes views  21 - 35  to be acquired at higher signal amplitudes than in a full blade acquisition. To preserve contrast of the views acquired at signal amplitudes higher than in a full blade acquisition, for example, views  21 - 35 , curve  140  of a single un-accelerated reference blade acquired using zero phase encoding gradients is used to correct the higher signal amplitudes. 
         [0051]    As illustrated in  FIG. 7 , view  35 , for example, will have a signal amplitude similar to a signal amplitude of view  2  since view  35  is acquired instead of view  2  in the accelerated sequence. However, the signal amplitude of view  35  in curve  140  is less than the signal amplitude of view  2 . The acquired view amplitude  142  is compared to the amplitude  144  of the view in curve  140 , and the acquired view amplitude  142  is corrected so as to be substantially similar to amplitude  144 . In this manner, differences in contrast between an image reconstructed from accelerated data and an image reconstructed from un-accelerated data are reduced. 
         [0052]    For acceleration factors greater than two, the blade width is increased to maintain TE equal to the TE of a fully acquired blade of the same blade width in addition to modifying the view order to preserve TE as described above. With an increased blade width, fewer blades are needed to sample k-space, thus scan time is further reduced. 
         [0053]    The T2 decay correction described above may additionally include weighing each echo by its inverse exponential decay factor to achieve substantially uniform signal amplitude across all views within a blade assuming no phase encoding. In this manner, a point spread function for PROPELLER reconstruction may yield improved results.  FIG. 8  shows a point spread function for combined propeller blades with T2 decay.  FIG. 9  shows a point spread function for combined propeller blades with T2 uniform signal amplitude compensation. 
         [0054]    According to another embodiment, the present invention includes homodyne processing of a half NEX blade of k-space to accelerate processing using a single coil or a multi-coil array. This is illustrated in  FIG. 10 . For purposes of illustration, a single blade of k-space will be described, but one skilled in the art will appreciate that the homodyne processing technique described herein is applicable to each acquired blade of k-space. 
         [0055]    K-space blade  146  extends through a central region  148  or origin of k-space such during subsequent rotation of the blade  146 , the central region  148  will be over-sampled. K-space blade  146  is preferably defined by a number of k-space data lines  150 , wherein each line  150  is characterized as either as sampled (solid line) or unsampled (dashed line). Further illustrated in  FIGS. 5 and 6  is that not all phase encode locations (views) of the k-space blade  146  is filled with sampled MR data. Since less than all phase encode locations have been sampled, the number of phase encoding steps can be reduced. Alternatively, the number of phase encoding locations (and thus phase encoding steps) can be increased such that after partial filling of each k-space data line, the total number of phase encoding steps equals that of an acquisition not carrying out homodyne processing. As will be described, the un-sampled phase encode locations will be filled with data based on sampled phase encode locations. 
         [0056]    While a number of partial Fourier reconstruction techniques may be used to determine data for un-sampled phase encode locations, one preferred technique is homodyne processing. Homodyne processing uses a pair of filters for Hermitian conjugate symmetry to determine data for un-sampled phase encode locations based on the data of sampled phase encode locations. In addition to providing data for un-sampled phase encode locations; homodyne processing also corrects phase errors or variations in the sampled data often attributable to variations in resonance frequency, flow, and motion. With homodyne processing or other partial Fourier reconstruction technique, un-sampled phase encoded locations  152  may be “effectively” sampled without requiring the phase encoding steps that would otherwise be required. Additionally, the time needed to sample an echo (TE) is effectively reduced, which also reduces scan time and increases subject throughput. 
         [0057]    It is preferred for the sampled phase encode locations to include slightly more than one-half of a column of k-space. In this regard, spatial-frequency data is acquired for phase encode locations about the center of the k-space data line. Thus, it is contemplated that the number of phase encoding steps can be reduced by slightly less than one-half. On the other hand, the number of phase encoding gradients applied are not reduced, but redefined such that almost all sampled phase encode locations are positioned in slightly more than one-half of the k-space data line. As a result, scan time is not reduced, but spatial resolution is improved for each blade. In effect, the number of phase encode locations can be doubled without requiring a doubling in the number of phase encoding steps. 
         [0058]    Referring now to  FIGS. 11 and 12 , two blade images illustrate the blade homodyne processing steps described above with respect to  FIG. 10 .  FIG. 11  is a ten view blade image showing eight views in one-half of k-space and two views in the other half of k-space. After homodyne processing, the ten views are converted to a sixteen view blade showing eight views in one-half of k-space and eight views in the other half of k-space as illustrated in  FIG. 12 . As a result of the blade homodyne processing, six additional views of data are used during image reconstruction which improves spatial resolution without six additional and time consuming phase encoding steps during data acquisition. 
         [0059]    The additional views of data obtained via the blade homodyne process described above results in the signal amplitudes of the additional views after homodyne processing being greater than the signal amplitudes of the corresponding views in a full blade acquisition.  FIG. 13  shows a plurality of views  154  (views  15 - 36 ) acquired as described above with regard to  FIG. 10 . Following homodyne processing, the signal amplitudes of views  156  (views  1 - 14 ) are substantially similar to the signal amplitudes of views  36 - 22 , respectively. Applying a T2 decay correction as described above with respect to  FIG. 7  in a phase encoding direction on a per-blade basis reduces differences in high frequency content between a homodyne-processed image reconstructed from accelerated data and a non-homodyne-processed image reconstructed from un-accelerated data. 
         [0060]      FIGS. 14 and 15  further illustrate the advantages achieved with the present invention. Specifically, as shown in  FIG. 14 , conventional PROPELLER reconstruction of all half NEX blades acquired and homodyne processed from an object results in artifacts introduced into the image. The introduced artifacts are the result, at least in part, of higher signal amplitudes in the additional views obtained through homodyne processing. On the other hand,  FIG. 15 , which is an image of the same object imaged in  FIG. 14 , but with the T2 decay correction described herein, shows reduced image artifacts. The image of  FIG. 15  is considerably more uniform than the image of  FIG. 14 , and thus, of greater diagnostic value. 
         [0061]    Therefore, in accordance with one embodiment of the invention, an MRI apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. The MRI apparatus also includes a computer programmed to, for each of a plurality of receiver coils, (A) acquire k-space data to partially fill a k-space blade of a plurality of k-space data lines extending through a central region of k-space, (B) apply a 1D Fourier Transform to the k-space data lines in a frequency encoding direction, and (C) fill a remainder of the partially filled and transformed blade with data determined from that sampled and transformed in each of the plurality of receiver coils to obtain a filled blade. 
         [0062]    In accordance with another embodiment of the invention, a method of parallel imaging includes (A) acquiring a k-space data set representing a partially filled blade from each of a plurality of receiver coils, (B) acquiring a set of k-space calibration data lines from each of the plurality of receiver coils, and (C) transforming, in one dimension, the k-space data sets and the sets of k-space calibration data lines to hybrid space. The method also includes (D) determining reconstruction weights from the sets of hybrid space calibration data lines and (E) applying the reconstruction weights to the hybrid space data sets to synthesize a plurality of complete hybrid space blade data sets. A respective image from each of the plurality of complete hybrid space blade data sets is reconstructed. 
         [0063]    In accordance with yet another embodiment of the invention, the invention is embodied in a computer program stored on a computer readable storage medium and representing a set of instructions that, when executed by a computer, causes the computer to acquire at least one k-space data set of a partially filled blade and fill a remainder of the partially filled blade with data determined from that acquired in the at least one k-space data set. The instructions also cause the computer to apply a T2 decay correction to data in the at least one k-space data set in a phase encoding direction on a per-blade basis such that k-space modulation artifacts are reduced. The instructions further cause the computer to reconstruct an image from the at least one k-space data set comprising T2 decay corrected data. 
         [0064]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.