Magnetic resonance cine imaging

Abstract: In a method of magnetic resonance cine imaging a positive portion (52) of k-space and a negative portion (56) are both divided into n segments. In each cardiac cycle, a multiplicity of field echoes (106) are generated, which multiplicity of echoes are divided into groups of n contiguous echoes. Within each group, the echoes are all from either the positive portion of k-space or the negative portion of k-space. Preferably, every group of each cardiac cycle has the same views in the same order to generate like, time displaced frames of a cine image sequence. Within each group, the views from the central-most segment n are collected in the middle of the group, views from progressively less central, higher frequency segments are progressively less centrally located within each group with views of the most peripheral, highest frequency segments being collected at the ends of the group. The total number of segments is an integer multiple of four times the number of views per group to facilitate 0°-180° phase cycling. Quantitative flow images are generated by expanding each group to include corresponding views from the k-space for reference image data and a flow sensitized image data. The echo signals for the reference and flow sensitized images for each frame are separated and reconstructed into reference and flow sensitized images for each frame. The reference and flow sensitized images are subtracted to generate a quantitative flow image for each frame.

Description: This invention relates to magnetic resonance cine imaging. It finds particular application in conjunction with quantitative flow cine imaging and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with cardiac, angiography, circulatory, black blood, and other examinations in which flowing fluid is cine imaged. Cine images have commonly been acquired using field echoes. Field echoes permit a rapid repetition rate, e.g. 64 sequence repetitions in 600 msec. In cardiac imaging, this enables a view of data for each of 64-100 frames or images to be collected in a single cardiac cycle. Each frame is displaced by about 10 milliseconds. When displayed sequentially, the frame images illustrate the cardiac cycle with a resolution of about 10 msec. Generally, optimal cardiac gated cine images are achieved if all the views are collected within a single breath hold. Collecting all the views within a single breath hold eliminates the motion artifacts attributable to pulmonary motion. In the cardiac region, mis-representation of heart position between breath holds can be significant. A single breath hold is typically 16-20 heartbeats. Acquiring only one view per frame image in each cardiac cycle of a single breath hold would limit each fram image to 16-20 views. To increase the number of views per frame, data acquisition could be continued for several breath holds. However, the heart and surrounding tissue move with each pulmonary cycle and typically do not return accurately to the same position in subsequent breath holds. One technique described in "Cineangiography of the Heart in a Single Breath Hold with a Segmented TurboFLASH Sequence", Radiology, Vol. 178, pp. 357-360, Atkinson and Edelman (RSNA, 1991), 128 lines or views of k-space were grouped into 8 segments of 16 views each. The resultant magnetic resonance echoes of each cardiac cycle were grouped into 16 groups of 8 views each. The number of groups are dependent on the patient's heart rate. The 8 views within each group of 16-20 consecutive heart beats were differently phase encoded and processed as 128 views (16x8) of the same frame image. In this manner, multi-frame images of the cardiac cycle, each with 128 views per image, were generated. More specifically, in the first heart beat, views or lines 1, 17, 33, etc. were collected; in the second heart beat, lines 2, 18, 34, etc. were collected; and so forth. The raw data from these sets was combined or interleaved to form the 128 view data set for reconstruction into each of the multiple frames. One of the drawbacks of the Atkinson and Edelman segmentation of k-space was that the resultant frames suffered from blurring. The present inventor has recognized that this blurring is attributable to the acquisition of the central views, the views with the most signal power and image information, at different times in subsequent cardiac cycles. In the above-illustrated 8 segment, 16 heart beat sequence, the central views in each group alternated between two different time displaced points in the cardiac cycle. A common problem in magnetic resonance scanners is that eddy currents are generated in the cryostat by rapidly changing magnetic gradients, e.g. the phase encode gradients. These eddy currents cause a number of different image artifacts and distortions. Commonly, the effects of eddy currents are minimized by using sequences that have gradients of alternating polarity such that the eddy currents tend to cancel. One problem with the Atkinson technique is that it acquired views from both positive and negative k-space in a single heartbeat or repetition. If there is a rewind pulse, this resulted in two phase encode gradients of the same polarity being applied back-to-back. The back-to-back applications of two gradients of the same polarity tended to cause the gradient currents to add, rather than cancel, increasing gradient current artifacts and distortions. In order to push DC artifacts and un-encoded signals from the center to the edge of an image, it is common to perform 0° - 180° phase cycling of the RF. For example the phase of the RF pulse is offset 180° in alternate views and cycles. With the Atkinson and Edelman technique, with some group and segment sizes, each group had both even and odd numbered views with even numbered views or odd numbered views back-to-back. This made 0° - 180° phase cycling impossible. According to a first aspect of the present invention there is provided a magnetic resonance cine imaging method in which the positive portion of k-space is segmented into n equally-sized segments, and the negative portion of k-space is segmented into corresponding n segments, the first segment of both positive and negative k-space containing views with the highest spatial frequency components, subsequent segments containing views with progressively lower spatial frequency components, and the n-th segment containing views with the central-most, lowest spatial frequency components, the method comprising: (a) monitoring cardiac cycles of a subject in a magnetic resonance imaging region; (b) in coordination with a cardiac cycle, generating a series of groups of consecutive echo sequences and receiving a corresponding plurality of echo signals, each echo signal corresponding to one view of k-space, each group of views corresponding to a distinct time interval in the subject's cardiac cycle, each group of views including a view from each of said segments of positive or negative k-space, and all of the views in each group being from one of the positive and negative k-space portions; (c) repeating step (b) with each of the groups at substantially the same time in each of a plurality of the monitored cardiac cycles, and with each group including a further view, different from the previously acquired views, from each of said segments of positive or negative k-space, views from the n-th segment of the positive or negative k-space portion being acquired at the same time within each group, whereby all the views from both the n-th segment of the positive and of the negative k-space portion are acquired at the same point in time in the subject's cardiac cycle; (d) repeating step (c) until all of the views in each segment have been acquired for both the positive and the negative portions of k-space; (e) sorting the echo signals from each of the plurality of monitored cycles by group; and (f) reconstructing a frame image representation from the echo signals of each group. According to a second aspect of the present invention there is provided a magnetic resonance cine imaging apparatus for carrying out the method according to the first aspect of the present invention in which the positive portion of k-space is segmented into n equally-sized segments, and the negative portion of k-space is segmented into corresponding n segments, the first segment of both positive and negative k-space containing views with the highest spatial frequency components, subsequent segments containing views with progressively lower spatial frequency components, and the n-th segment containing views with the central-most, lowest spatial frequency components, the apparatus comprising: means for (a) monitoring cardiac cycles of a subject in a magnetic resonance imaging region; means adapted for (b), in coordination with a cardiac cycle, generating a series of groups of consecutive echo sequences and receiving a corresponding plurality of echo signals, each echo signal corresponding to one view of k-space, each group of views corresponding to a distinct time interval in the subject's cardiac cycle, each group of views including a view from each of said segments of positive or negative k-space, and all of the views in each group being from one of the positive and negative k-space portions; means adapted for (c) repeating step (b) with each of the groups at substantially the same time in each of a plurality of the monitored cardiac cycles, and with each group including a further view, different from the previously acquired views, from each of said segments of positive or negative k-space, views from the n-th segment of the positive or negative k-space portion being acquired at the same time within each group, whereby all the views from both the n-th segment of the positive and of the negative k-space portion are acquired at the same point in time in the subject's cardiac cycle; means adapted for (d) repeating step (c) until all of the views in each segment have been acquired for both the positive and the negative portions of k-space; means for (e) sorting the echo signals from each of the plurality of monitored cycles by group; and means for (f) reconstructing a frame image representation from the echo signals of each group. Preferably, in each group, echo signals corresponding to views from the nth segment of k-space are generated most central within each group and echo signals corresponding to views from the first and second segmentations of k-space are at the ends of each group, whereby the central views are collected in the middle of each distinct time interval of the subject's cardiac cycle. One advantage of the present invention is that it can be implemented on a standard MR scanner with no special hardware. Another advantage of the present invention is that the resulting images have higher spatial resolution and higher signal to noise ratio than echo planar images. Another advantage of the present invention is that all central views are acquired at the same point in each group. hence the same point in each cardiac cycle, which minimizes blurring. Another advantage of the present invention is that it minimizes eddy current effects by treating positive k-space and negative k-space separately. Yet another advantage of the present invention is that it enables the generation of quantitative flow cine images. One magnetic resonance cine imaging apparatus and methods of use of the apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic illustration of the apparatus; Figure 2 illustrates a preferred segmentation of k-space used in the methods; Figure 3 illustrates exemplary motion sensitized and reference magnetic resonance sequences used in the methods; Figure 4 depicts a heartbeat of a cardiac gated cine acquisition in relative time sequence with grouped views in accordance with a preferred method of the invention; Figure 5 illustrates a 5-way symmetric, centrally-ordered segmentation of k-space with a 40 view imaging sequence; Figure 6 illustrates the views collected in each of several cardiac cycles; and, Figure 7 illustrates an interleaved grouping of motion sensitized and reference views within each group. Referring to Figure 1, the apparatus comprises a main magnetic field control means 10 which controls superconducting or resistive magnets 12 such that a substantially uniform main magnetic field is created longitudinally along a z-axis through an examination region 14. A magnetic resonance echo generating means applies sequences of RF and magnetic field pulses to cause magnetic resonance echoes, preferably field or gradient echoes, to be generated. More specifically, gradient pulse amplifiers 20 apply current pulses to gradient coils 22 to create gradient magnetic fields along orthgonal x, y and z-axes of the examination region. A radio frequency transmitter 24 transmits RF pulses to an RF coil 26 to transmit RF pulses into the examination region 14 to excite magnetic resonance and manipulate excited magnetic resonance. A radio frequency receiver 28 receives magnetic resonance signals emanating from the examination region that are picked up by the RF coil 26 or by surface coils (not shown). A sequence control means 30 controls the gradient pulse amplifiers 20 and the transmitter 24 to generate a series of field or gradient echo imaging sequences. More specifically to the preferred embodiment, a master sequence control 32 is gated by a cardiac monitor 34 to start a series of field echo sequences. The master sequence control means 32 controls a motion sensitized sequence control 36 and a reference or motion insensitive sequence control 38 . The motion sensitive and reference sequence controls implement analogous field echo imaging sequences such as sequences in which the gradients along one of the axes are time or amplitude shifted relative to the other, such as shown for example, in U.S. Patent No. 4,689,560 of Nayler and Pattany. A polarity alternating means 40 causes 0°-180° cycling of the RF. In the preferred embodiment, the phase alternating means includes a means 40 which causes the phase of the radio frequency excitation pulse and the receiver 28 , preferably a digital receiver, to be reversed in alternate views. With reference to FIGURE 1, each magnetic resonance echo signal received by the receiver 28 corresponds to one view or line of data for a corresponding one of the frames of the cine image sequence. Each view includes a plurality, e.g. 256 frequency encoded digital values. Each view is phase encoded with one of 2n of phase encodings. That is, in each repetition of the field echo sequence, such as illustrated in FIGURE 3, a phase encode gradient 42 is applied with one of n positive (or zero) or one of n negative steps. A sorting means 44 (FIGURE 1) sorts the echo signals corresponding to the same frame of the cine image among a plurality of frame image memories 46 . As shown in FIGURE 2, the data in each of memories 46 is stored as a matrix of data which is phase encoded in one direction and frequency coded in another, commonly known as k-space. A central phase encoded view 50 has zero phase encoding. Each view on a positive side 52 of k-space has progressively an additional step of phase encoding up to a top view 54 which has the most or highest frequency phase encoding. Analogously, the views phase encoded with an opposite polarity are in a negative portion 56 of k-space. Each view in the negative portion of k-space has progressively one step greater phase encoding of an opposite polarity up to a bottom view 58 which has a maximum or highest frequency negative polarity phase encoding. The views closest to the central or zero phase encoding view 50 carry the most image information and the best signal to noise ratio. With reference to FIGURE 4 and continuing reference to FIGURE 2, an earlier cardiac cycle 60 causes the cardiac monitor 34 to trigger a series of field echo sequences, e.g. 64 or more sequences with about a 10 msec. repeat time. The 64 repetitions of the gradient echo sequence occur in the interval before a subsequent R-wave 62 , i.e. spanning a single cardiac cycle. The k-space is then divided into 2n segments with segments n and -n being most central and segments 1 and -1 being most peripheral, i.e. having the highest frequency components. Analogously, the repetitions of the field echo sequence within each heart beat are divided into groups of n views. Each segment of k-space has y/2 views, where y is the number of imaged cardiac cycles within a single breath hold. The generated magnetic resonance echoes are analogously divided into groups of n contiguous echoes or views. Views from the nth or central-most k-space segment occur at the same, central location within each group. In the example illustrated FIGURE 4 in which n=5, the central-most echo of each group comes from the fifth segment. The two views most closely adjoining the central-most view come from the next two lowest segments, i.e. the third and fourth segments of k-space. The next most central views are arranged next, and so on, until the views from the least central segments which are located at the ends of each group. Table 1 illustrates the order of the segments from which the views come within each group for group sizes between 3 and n. Group Size Segment Order Number 3 1, 3, 2 4 1, 3, 4, 2 5 1, 3, 5, 4, 2 6 1, 3, 5, 6, 4, 2 7 1, 3, 5, 7, 6, 4, 2 8 1, 3, 5, 7, 8, 6, 4, 2 9 1, 3, 5, 7, 9, 8, 6, 4, 2 n (odd) 1, 3, 5, ..., (n-2), n, (n-1), ..., 6, 4, 2 n (even) 1, 3, 5, ..., (n-1), n, (n-2), ..., 6, 4, 2 After collecting the positive k-space data, the series of sequences is repeated for the views of the negative half of k-space. The views from the corresponding negative segments occur at the same relative location within each group, i.e. at the same time in the cardiac cycle. When the total number of views collected is an integer multiple of 2 (but not an integer multiple of 4), then, even and odd views can occur back-to-back, interfering with 0°-180° phase cycling. In order to provide for 0°-180° phase cycling, the number of views is preferably a multiple of 4 times the group size. FIGURES 5 and 6 give an example in which the number of segments is 5 and the total number of views is 40. As illustrated in FIGURE 5, the central-most view 50 is denoted as the 0th view. The nth or 5th segment on the positive side of k-space includes views 0 through 3, the fourth segment includes views 4 through 7, ..., and the first segment includes views 16 through 19. Analogously, in the negative half of k-space, the nth or 5th segment includes views -1 through -4, the fourth segment includes views -5 through -8, ..., and the first segment includes views -17 through -20. In this manner, k-space includes 40 views which are divided into 5 segments which enables the entirety of k-space to be sampled in eight cardiac cycles. Only even numbered views in the positive half of k-space are sampled in a given cardiac cycle, only odd numbered views in the positive half of k-space are sampled in a given cardiac cycle, only odd views in negative k-space are sampled in a given cardiac cycle, and only even views in the negative half of k-space are sampled in a given cycle. As illustrated in FIGURE 6, in a first cardiac cycle, the views of k-space are sampled in the order 19, 11, 3, 7, 15. In the second cardiac cycle, the views are sampled in the order 18, 10, 2, 6, 14. In the third and fourth cardiac cycles, the rest of positive k-space is sampled. In the fifth cardiac cycle, the views are sampled -17, -9, -1, -5, -13. In a sixth cardiac cycle, the views are sampled -18, -10, -2, -6, -14. The rest of negative k-space is sampled in the seventh and eighth cardiac cycles. The complete collection of all 40 views in this example is summarized in Table 2 below. Cardiac Cycle Views in Each Group 1 19, 11, 3, 7, 15 2 18, 10, 2, 6, 14 3 17, 9, 1, 5, 13 4 16, 8, 0, 4, 12 5 -17, -9, -1, -5, -13 6 -18, -10, -2, -6, -14 7 -19, -11, -3, -7, -15 8 -20, -12, -4, -8, -16 Returning again to FIGURE 1, the sorting means 44 sorts the views corresponding to each frame into a corresponding one of the frame memories 46 . The views are sorted to create a k-space matrix essentially as shown in FIGURE 5, for the example of five views per group. A two-dimensional inverse Fourier transform or other image reconstruction means 70 reconstructs each matrix into a corresponding image representation which is stored in a corresponding frame image memory 72 . An image selecting means 74 is controlled by an operator control panel 76 or the like to select one or more of the frame images for display. A video formatting means 78 adjusts the format of each selected image into an appropriate format for display on a video monitor 80 . Preferably, the operator can control the image selecting means to select each frame image in order at a selectable rate such that the display on the video monitor illustrates the movement of the heart during the cardiac cycle. This same technique can be utilized to generate quantitative flow images. Within a single breath hold, and preferably within a single cardiac cycle, the gradient echo sequences are applied to generate field echoes corresponding to views of a motion sensitive image and a reference image. The views of a reference image for each frame are sorted by the sorting means and stored in a frame reference memory for each imaged frame. Analogously, the views of a motion sensitized frame are sorted by frame into corresponding ones of frame memories 46 . The reference and motion sensitized views are reconstructed frame by frame to construct reference and motion sensitized images for each of the frames by the reconstruction means 70 . A subtracting means 82 subtracts the reference and motion sensitized images corresponding to each frame to create a difference or quantized motion image which is stored in corresponding quantized motion image memories 84 . The image selecting means 74 is connected with the quantized motion image memory 84 such that a cine sequence of quantized motion images is displayable on the video monitor 80 . With reference to FIGURE 3, both the reference and motion sensitive sequences are initiated by the simultaneous application of an RF pulse 90 and a slice select gradient 92 of a first polarity. A complementary slice select pulse 94 of the opposite polarity and a third slice select pulse 96 of the first polarity follow. For simplicity of illustration, the three slice select pulses are of identical amplitudes with durations such that the pulse 96 is half the duration of pulse 94 and approximately matches the section of pulse 92 which occurs after the center of the RF excitation pulse 90 . The magnitudes and duration of the pulses in the slice select direction are selected such that the magnetization of material moving perpendicular to the slice plane is rephased. In an orthogonal direction, a read gradient pulse 100 of one polarity is followed by a second read gradient pulse 102 of an opposite polarity, followed by a read gradient 104 which frequency encodes a resultant field or gradient echo 106 . For simplicity of illustration, the read gradient pulses 100 , 102 , and 104 are illustrated as being of identical amplitude with the pulse 100 being about half the duration of pulse 102 and approximately matching the duration between the initiation of gradient pulse 104 and the center of the resultant echo of 106 . The magnitudes and duration of the slice selection pulses are selected such that the magnetization of material moving parallel to the read axis is rephased. The phase encode gradient 42 is applied before the gradient echo 106 with one of the phase encode gradient steps or views of k-space as described, for example in FIGURE 5. Following the echo, an unwind phase gradient 108 is applied with the opposite polarity to the phase encode gradient 42 to remove the phase encoding from the system. This prevents the phase encoding from being carried forward into the next repetition of the sequence. The gradient pulse in the direction of the flow component which is to be phase mapped is altered to encode flow into the gradient echo. In the illustrated embodiment, in the motion sensitized imaging sequence, the slice select gradient component 96' is shifted an amount Δ relative to gradient component 96 in the reference sequence. Analogously, in the motion sensitized sequence read gradient component 100' may be shifted relative to reference sequence gradient component 100 to encode the flow component in the read direction. To read the flow component oblique to both the slice select and read directions, slice select and read gradient components are both shifted. Alternately, the gradient pulses may be amplitude shifted or otherwise altered as explained in greater detail in U.S. Patent No. 4,689,560. To encode velocity the gradient pulses are amplitude and time scaled such that the 0th moment between the center of the excitation pulse 90 and the field echo 106 is zeroed, but such that the first moment changes between the reference motion sensitive sequences. Preferably, second and higher order moments are also zeroed in both. In this manner, the difference represents the first moment or velocity. Analogously, by zeroing the first and third moments but not the second, acceleration can be imaged. Analogously, by creating a non-zero 0th moment, the static material is imaged. In some instances, it may be advantageous to display an image of the static material at half intensity or in black and white concurrently with a superimposed color display of flow velocity to improve the viewers spatial reference. First, the motion encoded and reference images can be collected in two separate scans in two breath holds. This maximizes the number of frame images available, but creates the possibility of mis-registration between the flow encoded and reference images. Second, the reference and motion sensitized images may be acquired in alternate cardiac cycles of the same breath hold. This is advantageous because the reference and motion sensitized scans occur at the same point in the cardiac cycle, but this is difficult to implement on many commercially available scanners. Third, the reference and motion sensitized groups of views may be alternated or interleaved within a single cardiac cycle and the full set of data collected in a single breath hold. However, this results in a time mis-registration between the corresponding reference and motion sensitized views. This mis-registration is about a 13 msec. times the number of views per group, i.e. about 65 msec. for groups of five. Fourth and preferred, the corresponding views of the reference and motion sensitized images are interleaved within each group. With reference to FIGURE 7, in which the five view grouping example is again continued, each group now has five reference views from segments 1-5 (not underlined) and the corresponding five motion sensitized views from segments 1-5 (underlined), expanding each group to ten views. This, of course, reduces the number of frames by half. The entire sequence is illustrated in Table 3 below where underlined views are motion sensitized and non-underlined views are reference views. Cardiac Cycle Views in Each Group 1 19, 19 , 11, 11 , 3, 3 , 7, 7 , 15, 15 2 18, 18 , 10, 10 , 2, 2 , 6, 6 , 14, 14 3 17, 17 , 9, 9 , 1, 1 , 5, 5 , 13, 13 4 16, 16 , 8, 8 , 0, 0 , 4, 4 , 12, 12 5 -17, -17 , -9, -9 , -1, -1 , -5, -5 , -13, -13 6 -18, -18 , -10, -10 , -2, -2 , -6, -6 , -14, -14 7 -19, -19 , -11, -11 , -3, -3 , -7, -7 , -15, -15 8 -20, -20 , 12, 12 , 4, -4 , -8, -8 , -16, -16 In this manner, corresponding reference and motion sensitized views are time offset by only a single gradient echo sequence repetition time, about 13 msec. in the preferred embodiment. Although illustrated in conjunction with the central weighting of the low frequency phase encode data, it is to be appreciated that this quantitative motion imaging technique is also applicable to techniques that are not centrally ordered, such as in the 40 view k-space example illustrated in Table 4 below. Cardiac Cycle Views in Each Group 1 0, 0 , 4, 4 , 8, 8 , 12, 12 , 16, 16 2 -1, -1 , -5, -5 , -9 -9 , -13, -13 , -17, -17 3 1, 1 , 5, 5 , 9, 9 , 13, 13 , 17, 17 4 -2, -2 , -6, -6 , -10, -10 , -14, -14 , -18, -18 5 2, 2 , 6, 6 , 10, 10 , 14, 14 , 18, 18 6 -3, -3 , -7, -7 , -11, -11 , -15, -15 , -19, -19 7 3, 3 , 7, 7 , 11, 11 , 15, 15 , 19, 19 8 -4, -4 , -8, -8 , -12, -12 , -16, -16 , -20, -20 Other echo sequences for flow and static imaging are also contemplated such as the sequences of prior U.S. Patents 4,689,560 and 4,683,431, and prior art articles "Blood Flow: Magnetic Resonance Imaging" Bradley, et al. (RSNA 1985); "A Flow Velocity Zeugmatographic Interlace for NMR Imaging in Humans", Moran, Magnetic Resonance Imaging, Vol. 1, pp. 197-203 (1982); and "Measurement of Flow with NMR Imaging Using a Gradient Pulse and Phase Difference Technique", Bryant, et al., J. Comp. Asst. Tomography 8(4):588-593 (1984).

Claims: A magnetic resonance cine imaging method in which the positive portion of k-space is segmented into n equally-sized segments, and the negative portion of k-space is segmented into corresponding n segments, the first segment of both positive and negative k-space containing views with the highest spatial frequency components, subsequent segments containing views with progressively lower spatial frequency components, and the n-th segment containing views with the central-most, lowest spatial frequency components, the method comprising: (a) monitoring cardiac cycles of a subject in a magnetic resonance imaging region; (b) in coordination with a cardiac cycle, generating a series of groups of consecutive echo sequences and receiving a corresponding plurality of echo signals, each echo signal corresponding to one view of k-space, each group of views corresponding to a distinct time interval in the subject's cardiac cycle, each group of views including a view from each of said segments of positive or negative k-space, and all of the views in each group being from one of the positive and negative k-space portions; (c) repeating step (b) with each of the groups at substantially the same time in each of a plurality of the monitored cardiac cycles, and with each group including a further view, different from the previously acquired views, from each of said segments of positive or negative k-space, views from the n-th segment of the positive or negative k-space portion being acquired at the same time within each group, whereby all the views from both the n-th segment of the positive and of the negative k-space portion are acquired at the same point in time in the subject's cardiac cycle; (d) repeating step (c) until all of the views in each segment have been acquired for both the positive and the negative portions of k-space; (e) sorting the echo signals from each of the plurality of monitored cycles by group; and (f) reconstructing a frame image representation from the echo signals of each group. A method according to Claim 1, wherein in each group, echo signals corresponding to views from the nth segment of k-space are generated most central within each group and echo signals corresponding to views from the first and second segmentations of k-space are at the ends of each group, whereby the central views are collected in the middle of each distinct time interval of the subject's cardiac cycle. A method according to claim 1 or 2, wherein the views in k-space are numbered sequentially and wherein in each group, all of the views are one of even and odd numbered views. A method according to any one of the preceding claims, wherein the echo sequence generating step includes generating a motion sensitized echo sequence and a reference echo sequence and receiving a motion sensitized echo signal and a reference echo signal corresponding to each view of k-space; in the sorting step, the reference and motion sensitized echo signals corresponding to each frame are separated; in the reconstructing step, the motion sensitized and reference echo signals of each group are reconstructed into separate reference and motion sensitized frame images; and further including subtracting the reference and motion sensitized frame images corresponding to the same group to create a flow image, whereby a plurality of flow images each corresponding to one of the distinct time intervals of the subject's cardiac cycle are generated. A method according to Claim 4, wherein in each group, corresponding reference and motion sensitive views are collected back-to-back in the same group. A magnetic resonance cine imaging apparatus for carrying out the method of claim 1 in which the positive portion of k-space is segmented into n equally-sized segments, and the negative portion of k-space is segmented into corresponding n segments, the first segment of both positive and negative k-space containing views with the highest spatial frequency components, subsequent segments containing views with progressively lower spatial frequency components, and the n-th segment containing views with the central-most, lowest spatial frequency components, the apparatus comprising: means (34) for (a) monitoring cardiac cycles of a subject in a magnetic resonance imaging region (14); means (12,20,22,24,26,28,30,40) adapted for (b), in coordination with a cardiac cycle, generating a series of groups of consecutive echo sequences and receiving a corresponding plurality of echo signals, each echo signal corresponding to one view of k-space, each group of views corresponding to a distinct time interval in the subject's cardiac cycle, each group of views including a view from each of said segments of positive or negative k-space, and all of the views in each group being from one of the positive and negative k-space portions; means (12,20,22,24,26,28,30,40) adapted for (c) repeating step (b) with each of the groups at substantially the same time in each of a plurality of the monitored cardiac cyclcs, and with each group including a further view, different from the previously acquired views, from each of said segments of positive or negative k-space, views from the n-th segment of the positive or negative k-space portion being acquired at the same time within each group, whereby all the views from both the n-th segment of the positive and of the negative k-space portion are acquired at the same point in time in the subject's cardiac cycle; means (10,12,20,22,24,26,28,30,40) adapted for (d) repeating step (c) until all of the views in each segment have been acquired for both the positive and the negative portions of k-space; means (44) for (e) sorting the echo signals from each of the plurality of monitored cycles by group; and means (70) for (f) reconstructing a frame image representation from the echo signals of each group.

Inventors: NESSAIVER MORIEL S, NESSAIVER, MORIEL S.
Applicants: PICKER INT INC, PICKER INTERNATIONAL, INC.
Application year: 1993
Main classification: G01R33
IPC codes: G01R33, A61B5