Abstract:
The effects of complex motion of the heart and surrounding anatomy due to cardiac and respiratory motion is reduced in high resolution imaging of coronary arteries using a diminishing variance algorithm (DVA) using a navigator for tracking heart motion and iteratively reacquiring data frames where a data frame has a positional variance from a cumulative histogram of data. A target position is continually calculated from the cumulative histogram which is smoothed such as by low-pass filtering to continually provide a target position. An image histogram is developed based on a limited number of image frames which are iteratively replaced to attain desired image quality.

Description:
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0001] The U.S. government has rights in this invention pursuant to NIH Grant HL39297 and HL07846 with Stanford University, Assignee. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to Magnetic Resonance Imaging (MRI), and more particularly the invention relates to real time reduction of motion artifacts in MRI.  
           [0003]    One of the most challenging areas in Magnetic Resonance Imaging (MRI) research is high-resolution imaging of the coronary arteries. The complexity of the motion of the heart and its surrounding structures due to cardiac and respiratory motion makes obtaining useful coronary artery images very difficult. If this motion can be temporarily paused, better images can be obtained. In essence, this is achieved through cardiac-gated, breath-held scans. Cardiac gating allows the acquisition of data frames at the same point in the cardiac cycle, thus freezing the cardiac motion with respect to the other data frames, while breath holding is an easy way to remove respiratory motion artifacts. Fast scanning techniques can produce useful images in the span of one breath-hold. In three-dimensional (3D) imaging and submillimeter resolution imaging, multiple breath-holds are usually required. In reality, the practice will be to use breath-holds when possible because they are easy and effective, however, movement that can occur at the latter part of a breath-hold and, in a smaller percentage of patients, an inability to maintain a long breath-hold or reproduce duplicate breath-holds can cause problems and require other methods to acquire useful coronary artery images.  
           [0004]    Disclosed in U.S. Pat. No. 5,427,101 for “Diminishing Variance Process for Real Time Reduction of Motion Artifacts in MRI” of Sachs, Meyer, and Nishimura, is a method whereby motion can be detected in real time during the acquisition of MRI data. This enables the implementation of several algorithms to reduce or eliminate this motion from an image as it is being acquired. The method is an extension of the acceptance/rejection method algorithm called the diminishing variance algorithm (DVA). With this method, a complete set of preliminary data is acquired along with information about the relative motion position of each frame of data. After all the preliminary data is acquired, the position information is used to determine which lines are most corrupted by motion. Frames of data are then reacquired, starting with the most corrupted frame. The position information is continually updated in an iterative process, therefore each subsequent reacquisition is always done on the worst frame of data.  
           [0005]    [0005]FIG. 1 is a flow diagram of the diminishing variance process disclosed in Pat. No. 5,427,101. Initially, a complete set of data frames, as required to construct an image, is acquired with each frame having a mechanism for indicating a relative position of the frame. A histogram of positional shifts in the frames is then developed, and the positionally worst data frames due to motion are then reacquired. The histogram is updated, and the positionally worst data frames in the updated histogram are reacquired. This process is continued until an image which is motion free is realized. As used herein, a data frame is a portion of the data, typically a single line in k-space for 2DFT imaging or a simple spiral interleaf for interleaved spiral imagery.  
           [0006]    The present invention is an improvement to the diminishing variance process disclosed in U.S. Pat. No. 5,427,101.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    In accordance with the invention, a diminishing variance algorithm (DVA) is used to operate on MRI data frames. A cumulative histogram is obtained from all data frames, and an image histogram is obtained using a limited number (e.g. 36) frames with the outlier or worst frames in the image histogram being continually replaced.  
           [0008]    A target position is calculated for the imaged object based on the cumulative histogram. In accordance with a feature of the invention, in calculating the mode or target position the cumulative histogram is low-pass filtered to smooth the cumulative data. This can be done by averaging adjacent bin values. In accordance with another feature of the invention, the target position is recalculated based on the cumulative histogram throughout the scan, thereby allowing faster response to a patient&#39;s movement and faster convergence based on the new target position.  
           [0009]    The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a flow diagram illustrating a prior art application of DVA.  
         [0011]    [0011]FIGS. 2A and 2B are an interleaved spiral k-spaced trajectory with 36 data frames and a sample of corresponding heart positions given in number of resolution pixels from an initial position, respectively.  
         [0012]    [0012]FIGS. 3A and 3B are scatter plots and histograms of original sample data and converged data, respectively.  
         [0013]    FIGS.  4 A- 4 D are scatter plots and image histograms of position data from FIG. 2 as the DVA progresses from a target position of 3 after a) initial 36 acquisitions, b) 9 outlier replacements, c) 17 replacements, and d) 23 replacements.  
         [0014]    [0014]FIGS. 5A and 5B illustrate an original cumulative histogram and image histogram matched with 36 frames, and a smooth histogram with filtering.  
         [0015]    FIGS.  6 A- 6 D illustrate scatter plots and image histograms of position data as a DVA progresses with a target of −1 after a) initial 36 acquisitions, b) 9 outlier replacements, c) 14 replacements, and d) 19 replacements.  
         [0016]    [0016]FIG. 7 is a plot of a S/I direction mode during a DVA scan.  
         [0017]    [0017]FIG. 8 is a flow diagram of the DVA in accordance with the present invention.  
         [0018]    [0018]FIG. 9 is a plot of standard deviation of heart position data in an image histogram averaged from 13 scans of six volunteers.  
         [0019]    [0019]FIG. 10A- 10 C are cardiac gated interleaved spiral images of a) conventional 3D-DVA, b) 3D-DVA with histogram smoothing, and c) 3D-DVA with a continuously updated average. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    An embodiment of the invention will be described with respect to a spiral k-spaced trajectory as illustrated in FIG. 2A with 36 data frames or interleaves. A sample of corresponding heart positions in number of resolution pixels from an initial position is tabulated in FIG. 2B.  
         [0021]    Each spiral interleaf has a corresponding heart position. The DVA iteratively reacquires and replaces image histogram outliers with the goal of creating an image with data frames acquired with the heart in the same position. FIG. 3A shows the sample position data at the completion of the initial cycle, and FIG. 3B shows the goal position data for the DVA. Ideally, all of the position data will converge to a single bin in the histogram, as in FIG. 3B.  
         [0022]    In this example, if the position or mode is used to calculate the average, the target position is 3 and the first interleaves to be reacquired correspond to position −3 (interleaves 7 and 30). The overscan continues as described until complete convergence is made, or until the image is acceptable. A sample progression can be seen in FIGS.  4 A- 4 D which are scatter plots and image histograms of the position data from FIG. 2 as the DVA progresses with a target position of 3 after: a) initial 36 acquisitions, b) 9 outlier replacements, c) 17 replacements, d) 23 replacements. Since some reacquisitions can happen with the heart in a further position than it started in, the exact number of overscan iterations can be greater than the number of outlier replacements.  
         [0023]    The original DVA, as described up to this point, uses one navigator to trace the position of the heart (or diaphragm) in one direction. A navigator is described in U.S. Pat. No. 5,427,101 and in Ehman et al.,  Adaptive Technique for High - definition MR Imaging of Moving Structures, RADIOLOGY  173:255-263 (1989), and Korin et al.,  Compensation for Effects of Linear Motion in MR Imaging, MAGNETIC RESONANCE IN MEDICINE  12:99-113 (1989). A three-dimensional DVA (3D-DVA) uses three navigators to track the heart in the superior/inferior (S/I), anterior/posterior (A/P) and right/left (R/L) directions. These navigators are placed sequentially, immediately after the data acquisition. The 3D-DVA stores the position data for three directions and thus, maintains three image histograms and three cumulative histograms. Three distinct targets are calculated. A total distance from the target positions is determined for each data frame by adding the distance from the target in each direction. The data frame with the largest corresponding total distance is considered the ‘worst’ frames and is the next to be reacquired.  
         [0024]    Careful observation of the position data in FIG. 3A shows that the mode (position 3) does not reflect the most common general region for the heart. By applying a low-pass filter to the histogram, we smooth outliers and the new mode more accurately reflects the most common position of the heart. FIGS. 5A and 5B show a detailed example of the position data from FIG. 2 before and after a low-pass filter is applied. In this example, the filter has a width of three and basically replaces each bin with its original value added to its adjacent bins. While the original DVA would calculate a target of position 3, by filtering we avoid the outlier and the calculated target is position −1. The scatter plot in FIG.  3 A illustrates there are more data points around position −1, so it is a better choice for a target because it will allow for faster convergence and a better initial image.  
         [0025]    FIGS.  6 A- 6 D show how the DVA will progress more quickly using this new target. These figures show scatter plots and image histograms of the position data as the DVA progresses with a target of −1, instead of −3 after: a) initial 36 acquisitions, b) 9 outlier replacements, c) 14 replacements, d) 19 replacements. Here, the image histogram mode changes from 3 to −3 to −1. The DVA reacquires and replaces positions furthest from the target. Since the heart position during the new acquisitions is unknown, the mode of the image histogram may change while the DVA is attempting to converge on a target.  
         [0026]    A number of factors can cause longer scan times in the original DVA.  
         [0027]    One of the most common problems is a change in the patient&#39;s movement. A shift on the scanner bed or respiratory drift can cause the heart to be in a different set of positions than it was in during the initial DVA cycle. In terms of the algorithm, this means that the current heart position could be far away from the target position that was calculated at the end of the initial cycle. This results in the same data frame being reacquired many times without getting closer to the target, thus wasting many iterations scanning the same data frame without improving the image. FIG. 7 shows an example of how a mode or position can change during the scan. Here, the mode is calculated from one of 512 positions. Position 256 corresponds to the initial.  
         [0028]    In another modification, instead of calculating the target heart position only once (immediately after the initial cycle is completed), we continuously update the mode during the overscan based on the cumulative histogram of all heart positions collected. The new 3D-DVA has three continuously updating modes. If a target changes (one or more of the three directions), we update to a full data set with corresponding heart positions closest to the new targets and continue with the overscan based on the new target positions, thus minimizing the time to convergence and yielding the best possible image as early as possible.  
         [0029]    [0029]FIG. 8 is a flowchart of the DVA as modified in accordance with the invention. Initially, data frames for an image are acquired at  80 , and heart positions for each frame are stored using a navigator for each frame. Cumulative and image histograms are created using the acquired data. The mode or position of the heart is calculated from the cumulative histogram at  82 , and then the frame with heart position farthest from the mode is identified at  84 . This frame is reacquired at  86 , and the image histogram is updated at  88 . The cumulative histogram is smoothed at  90 , by way of low pass filtering for example, and the image is reconstructed with the replacement frame at  92 . If the image quality is not satisfactory, then the sequence is repeated beginning with mode calculation at  82 . Otherwise, the algorithm execution is finished.  
         [0030]    Navigator echoes use a 2 cm diameter cylindrical excitation with a 1D readout along the excitation axis. During each cardiac cycle, a data frame is acquired. For 3D-DVA, three navigators are acquired after the data frame to avoid saturation. Navigators yield projections in the S/I, A/P and R/L directions that are cross-correlated with reference projections. A number of methods can be used to determine the relative position including: (1) the maximum value of the cross-correlation, (2) edge detection (more applicable, for navigators that are pointed at the diaphragm), and (3) object detection and translation determination. The maximum value of the cross-correlation can be used with cardiac gating since there is time to perform the computations.  
         [0031]    The low-pass filters can have any width. Applying the low-pass filter, in the easiest case, just adds or averages adjacent bins together. More advanced filtering methods can be applied to include more bins or give more weight to the central bin(s). In all filtering cases, the goal is to modify the cumulative histogram so the calculated target position more accurately represents the most common position of the heart. A filter width of three can give the best improvement on the calculation of the target position.  
         [0032]    The DVA can use any trajectory. Each has the goal of compiling a complete set of data frames acquired with the heart in the same relative position. A spiral k-space trajectory provides nice flow properties, but any trajectory can be used.  
         [0033]    To implement the DVA, communication is required between a real-time display application and the scanner. The real-time application controls the navigator&#39;s position and size, displays the images and calculates the next frame to request from the scanner. The requested frame is sent to the scanner. After the scanner completes the next scan interval, the real-time application retrieves the new data frame and the new navigator information so the position data can be calculated.  
         [0034]    This invention has been implemented on a GE Signa 1.5 T CV/i scanner (General Electric Inc., Milwaukee, Wis.). The pulse sequences were designed for gradients capable of 40 mT/m magnitude and 150 mT/m/msec slew rate, with a receiver capable of 4 μsec sampling (±125 kHz). A body coil was used for RF transmission and a 5-inch surface coil was used for signal reception. An external workstation was used to provide interactive control over the navigators.  
         [0035]    Scans were performed on volunteers and the total distance was presumed from the targets (for each direction) for each of the 36 interleaves used in the