Method for retrospectively gating NMR data

Abstract: NMR imaging data are acquired using a variable phase-­ encoding gradient. NMR non-imaging data are acquired using fixed phase-encoding, and motion data are derived from such NMR non-­ imaging data. Gating data are extracted from the motion data and used to select from the NMR imaging data to produce gated image data. The gated image data are used to produce a reconstructed image.

Description: BACKGROUND OF THE INVENTION The invention relates to NMR imaging, and more particularly relates to the production of NMR images which are gated in accordance with body motion of a living patient. In its most immediate sense, the invention relates to retrospective gating of NMR images without direct monitoring of a patient's body processes. Diagnosticians have long used gated images when diagnosing moving body parts. For example, when conducting a heart study, it is advantageous to isolate images which relate to one or more predetermined phases of the cardiac cycle. Thus, a physician may wish to view a series of images which all relate to the patient's heart at various points in his ECG. One way in which such gated images have been produced for cardiac imaging applications is to connect appropriate electrodes to the patient and to monitor the patient's ECG while the patient is undergoing an NMR study. This technique has proved unsatisfactory. This is because the intense magnetic fields and pulsed magnetic field gradients used in NMR often interfere with the patient's ECG signal. As a result, in an attempt to find electrode locations which produce satisfactory ECG signals, it is frequently necessary to repeatedly start an NMR study, stop the study when the ECG signal proves unsatisfactory, withdraw the patient from the NMR unit, and reposition the electrodes. All this is time-consuming, and therefore expensive (considering the high costs of operating an NMR unit). It is also known to gate NMR data using a priori assumptions about periodic motion which is used to drive the gating process. For example, European Patent Application No. 0 182 265 teaches the concept of empirically determining an average breathing rate using a constant phase-encoding gradient and then using that average rate to gate a subsequent imaging study. This is unsatisfactory because body processes are not exactly regular. It is additionally known to gate NMR data retrospectively. This is illustrated by U.S. Patent No. 4,710,717, which teaches the concept of varying the phase-encoding gradient in accordance with the cardiac cycle and interpolating the NMR data as necessary to produce appropriate images. This is not generally applicable because it is necessary to sense the ECG directly. U.S. Patent No. 4 761 613 teaches a method for the reduction of motion artifacts in MR imaging. A reference view having a first preselected phase encoding is derived before initiating a plurality of magnetic resonance excitation and data collection pulse sequences. At least some of the pulse sequences include collecting a monitor view with the reference phase encoding and collecting an image view with one of a plurality of image phase encodings. Each monitor view and the reference view is compared to assess validity of the corresponding image view. Motion data representing a body motion of interest is derived by comparing each monitor view and the reference view. Gating data, that assess the validity of the corresponding image view, is extracted by comparing the motion data to a predetermined threshold value. An NMR image is reconstructed using the valid image views. In none of these know techniques is it possible to simply and accurately gate NMR data retrospectively in accordance with any arbitrarily-selected body motion without undue consumption of expensive study time. SUMMARY OF THE INVENTION One object of the invention is to provide a method for retrospectively gating NMR data which can be used with any body motion, whether caused by voluntary muscular action or involuntary muscular action. Another object is to provide such a method which can be used to gate image data from one slice and possibly many slices through the body of a living patient using motion data derived from another slice through the patient. A further object is to provide such a method which can be used to gate image data from one echo using motion data derived from another echo. A still further object of the invention is to provide such a method which does not depend upon intrinsically inaccurate a priori assumptions about the regularity of body processes. Still another object is to provide such a method which can be carried out on existing NMR units and computer systems without requiring hardware modifications. Yet a further object is, in general, to improve on known methods of this general type. This object is solved as claimed in claim 1. In the preferred embodiment, NMR imaging data are calculated, as by interpolation, where existing data do not exactly correspond to the desired data. Advantageously, gating data are derived by software filtering and peak-detection of the motion data. The invention proceeds from a realization that when the same phase-encoding line is repetitively acquired, i.e. when NMR data (spin-echo data or gradient-echo data) of a particular slice of the patient's body is acquired using a net phase-encoding which does not change, changes in the data are reflective only of system noise and physical motion of the imaged slice. By reducing the noise, it is possible to accurately evaluate this physical motion, and to use it to gate conventional NMR imaging data (which are acquired using a varying phase-encoding gradient). Because the invention relies on simple physical motion of the living patient, it is generally useful and requires only routine adaptation for whatever application is desired. Thus, it is not only possible to conduct retrospectively gated cardiac studies, but it would also be possible to conduct a retrospectively gated study of the temporal mandibular joint by instructing the patient to chew gum during an NMR study. Furthermore, because the gating data need not come from the slice of the body which is being imaged, it would be possible to, e.g., retrospectively gate an NMR study of the liver or the cerebrospinal fluid using e.g. the cardiac cycle. BRIEF DESCRIPTION OF THE DRAWINGS Exemplary and non-limiting preferred embodiments of the invention are shown in the drawings, in which: Fig. 1 is a flowchart showing how a double gradient echo sequence, shown schematically, is used in an NMR imager to produce reconstructed NMR images; Fig. 2 is a flowchart showing how a preferred embodiment of the invention is used in an NMR imager to produce retrospectively gated NMR images; Fig. 3 shows, in schematic form, how gating data, filtered motion data and motion data relate to each other in accordance with the preferred embodiment of the invention; Fig. 4 shows how NMR data for a plurality of NMR images are acquired in a Siemens MAGNETOM and how such data are chronologically sorted in accordance with the preferred embodiment of the invention; Fig. 5 shows the low pass filter which is used in accordance with the preferred embodiment of the invention; and Fig. 6 shows the peak-detection algorithm which is used in accordance with the preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The following description has been written with specific reference to gated cardiac studies. This is only for purposes of illustration; the invention is not limited to cardiac studies. Furthermore, the following description has been written with specific reference to the Siemens MAGNETOM (not specifically shown). This is likewise for purposes of illustration and the invention can be used on any NMR imager/computer system with appropriate software modification. Additionally, the following description has been written on the assumption that a living patient (not shown), usually but not necessarily a human being, is subjected to an NMR study in the MAGNETOM and that at least one slice of the patient is imaged in the study. Fig. 1 will first be discussed to explain the environment in which the invention is used. As is shown there, a conventional NMR unit, such as a Siemens MAGNETOM, is used to produce NMR images of slices of a patient by using a double gradient-echo sequence. (This is not a part of the invention; a spin-echo sequence could be used as well.) First, the appropriate pulse sequence is selected and downloaded into the hardware. Then, data is acquired. Data acquisition proceeds, in each of the two echos, by stepping the gradient field between a minimum value and a maximum value in small steps. (In the Siemens unit, there are typically 256 such steps, but this is not part of the invention.) Thus, data are first acquired at the minimum (i.e. most negative) phase-encoding gradient (the so-called first phase-encoding line). Next, the phase-encoding gradient is increased, and data are acquired once more. This process is repeated until, at the center line of the gradient table (usually line 129), the phase-encoding gradient is zero. Next, the phase-encoding gradient is once again increased, to a positive value, data are acquired, and this process is repeated until the phase-encoding gradient is at its maximum (i.e. most positive) value, which typically occurs at line 256 of the gradient table. After all the data have been acquired in each of the two echos, the raw data is stored. The NMR images are then reconstructed. In accordance with the invention, the data acquisition process for the first echo is identical to that just described. However, as is schematically shown in Fig. 2, the data acquisition process for the second echo is different, in that such phase-encoding gradients are applied as will produce a net phase-encoding which is at a constant value, rather than being stepped through a gradient table, as in the case of the first echo. (In the preferred embodiment, this value is approximately 10% of the normal maximum phase-encoding. At this point it is only necessary to know that the phase-encoding is held constant and that the data acquisition process is repeated using the same gradient field.) Phenomena like cardiac activity, respiration, digestion, deliberate muscle movement etc. cause physical movement of the body. Where the second echo (the echo in which a constant net phase-encoding is repeatedly used) is derived from a slice of the patient's body which moves as a result of such phenomena, the variation in the NMR data will reflect such movement. Thus, changes in the NMR data acquired during the second echo represent motion; where the motion is periodic, as in the case of cardiac activity, respiration, periodic voluntary motion, etc., changes in the NMR data acquired represent not merely motion, but timing. The NMR data acquired while the phase-encoding amplitude is invariant are herein referred to as "NMR non-imaging data", and the changes in such data are herein referred to as "motion data". In accordance with the invention, the motion data are used as a source of gating data (e.g. ECG pulses). Then, NMR imaging data acquired during the first echo is selected, in accordance with the gating data, and only the selected data are used to form an image. Where the imaging data and the non-imaging data are acquired from the same slice and using a double echo technique, it is necessary to apply corrective phase-encoding gradients so that the net phase-encoding during the second echo is at the appropriate constant value. Where the imaging data and the non-imaging data are acquired from different slices or by using two single echos, no corrective phase-encoding gradients are needed because there is no residual phase-encoding remaining from a previous gradient pulse. The details of the preferred embodiment will be described further below, but the general results of the invention are illustrated in Fig. 3. Line A in Fig. 3 represents motion data which relates to a slice of a patient's body that includes the heart. (Because the motion of the heart is periodic, line A has a periodic appearance. Also, because the motion of the heart is not completely uniform, line A is not precisely periodic.) Line B in Fig. 3 represents the results of filtering the line A data through a low-pass software filter, and line C in Fig. 3 represents the results of subjecting the line B data to a software peak-detection algorithm. Line C is therefore an artificially-generated ECG signal, which can be used to gate the NMR imaging data. While the low-pass filtering and the peak-detection are carried out by software in the preferred embodiment, it would alternatively be possible to realize these functions in hardware. The motion data are derived from successive changes in the NMR non-image data. Such changes are only meaningful if the NMR non-image data are ordered chronologically. Therefore, if the NMR non-image data are not, after acquisition, stored in chronological order, they must be sorted into chronological order before being used to derive motion data. This is what was done in the preferred embodiment of the invention, as is described below. The preferred embodiment of the invention was realized on a Siemens MAGNETOM, with appropriately modified software. The nature of the modifications and reasons for them will now be explained with reference to Fig. 4 and later Figures. The Siemens MAGNETOM, like all other conventional NMR imagers, is set up to acquire a plurality of images. To do this, the gradient field is set initially at the minimum phase-encoding gradient. Then, the data for the first image are acquired, the data for the second image are acquired, the data for the third image are acquired and so on until data for all the desired images have been acquired. Only after all this has taken place is the gradient field stepped to the next (here, next larger) phase-encoding gradient. Then, the data for the desired images are acquired at this next line (i.e. line 2). After data for all the desired images have been acquired, the phase-encoding gradient is incremented once again, and the image data for line 3 are then acquired. This process is then repeated until all image data for all the images desired have been acquired for all (normally, 256) lines. In implementing the invention, the software used together with the Siemens MAGNETOM was modified. The modification was that the NMR non-image data were acquired with a constant net gradient amplitude, rather than with gradient amplitudes which were stepped through a table. Fig. 4 illustrates that the NMR non-image data therefore needed to be sorted chronologically, so that changes in the data would be correlated with the physical motion in the slice of interest. Therefore, in the preferred embodiment, and with specific reference to Fig. 2 once again, the NMR non-image data is sorted chronologically and a particular column of NMR non-image data is chosen as the preferred source for motion data. Advantageously, the column is chosen first and the sorting is carried out afterward; this reduces the volume of data which must be sorted and therefore reduces the time required for processing the data. The chosen and sorted data are then used to derive motion data. The motion data are then filtered using a software low-pass filter which is shown in Fig. 5 and subjected to a software peak-detection algorithm which is shown in Fig. 6. This produces gating data. Then, the gating data which is so derived, and which in this example is an artificially-generated ECG signal, is used to select NMR imaging data of interest from all of the NMR imaging data. Advantageously, where such NMR imaging data cannot be selected because they do not exist, the data are computed (as by some interpolation scheme) between known data points. Finally, the selected or computed data are used to reconstruct the final NMR image. In practice, the motion data contains noise and such noise must be reduced to provide usable gating data. In the preferred embodiment, the first step in such noise reduction is low-pass filtration. The filter used is recursive (i.e. the filter uses feedback) and has the characteristics shown in Fig. 5. Experiments suggest that an appropriate optimization of the low-pass filter occurs when the variable A is set at 0.85, but this is not part of the invention because there are many low-pass filter algorithms and the algorithm chosen will in part depend on the type of motion which is being used as a source of gating data. The results of the preferred low-pass filter are illustrated at line B, Fig. 3. As is evident therefrom, there is still a substantial variation in amplitude and it would be advantageous, if not absolutely necessary, to detect the peaks in line B. This is carried out by the algorithm illustrated in Fig. 6. The algorithm operates as follows: The filtered motion data is scanned and all local maxima and minima are noted. Then, for each local maximum, the amplitude differences between that maximum and the two closest local minima are computed. Whichever difference is greater is the difference which is used. If the difference exceeds a predetermined threshold, the local maximum is taken to be a genuine peak and is used as gating data. If, on the other hand, the difference does not exceed this threshold, the local maximum is taken to be a noise spike and is rejected as gating data. In this example, the noise threshold is established at 1.5 units, but this is not part of the invention and will vary in accordance with the motion which is being monitored. Generally, the peak-detection algorithm identifies local maxima which are so far separated in amplitude from adjacent local minima that they can be said to represent physical motion and not merely noise spikes. Thus, in the example illustrated, maxima occur at times A, B, C and D. The differences between the two local minima adjacent time A are three units and two units respectively, and the difference which is used for purposes of threshold comparison is the larger of these, namely three. Since this exceeds the threshold of 1.5 units, time A is taken as a genuine unit of gating data. For the maximum at time B, the relevant differences are 3 and 1.5 units respectively, and time B is taken as a genuine unit of gating data because the larger difference (3) exceeds the threshold (1.5). The maximum at time C likewise indicates that time C is a genuine unit of gating data, but the maximum at time D is not. This is because the relevant differences are 0.5 and 0.25 units, respectively, and the larger of these (0.5) is smaller than the threshold (1.5). There is thus produced a set of gating data which can be used to select from the available set of NMR imaging data. In practice, a diagnostician identifies times at which he wishes to have NMR images, and these times are then related to the gating data which have actually been acquired. Then, selections are made from the set of NMR imaging data in accordance with the times so identified; if appropriate NMR imaging data does not exist for a particular time, the data are produced by calculation between times for which the data is available. The phase-encoding gradient selected in the preferred embodiment is that which corresponds to line 143 in the gradient table. While line 143 does not have the same signal-to-noise ratio as does line 129 (the signal-to-noise ratio is maximum at line 129 and decreases steadily towards lines 1 and 256), the contrast-to-noise ratio of this line is adequate and it is more sensitive to motion of the patient; this sensitivity is more important than signal-to-noise ratio for purposes of this invention. The NMR imaging data may be, but need not be, derived from the same slice as the NMR non-imaging data. In for example the case of a cardiac study, the slice will likely be the same in both cases, but this need not be so. However, if the diagnostician wishes to gate a liver study as a function of cardiac activity, the slices will differ. Furthermore, the motion involved need not be of the involuntary type (e.g. pulse, respiration, etc.) Where a diagnostician wishes to conduct a study of the temporal mandibular joint, the patient may be instructed to chew gum while undergoing an NMR study. In a preferred embodiment a plurality of points in the cardiac cycle of the patient is identified and a plurality of NMR images is reconstructed. Those skilled in the art will understand that changes can be made in the preferred embodiments here described, and that theses embodiments can be used for other purposes. Such changes and uses are within the scope of the invention, which is limited only by the claims which follow.

Claims: A method for gating NMR data in accordance with body motion of a living patient, comprising the following steps: acquiring a set of NMR imaging data using a variable phase-encoding gradient; a set of NMR non-imaging data which represent the body motion using fixed phase-encoding, each of said NMR non-imaging data acquired in temporal correspondence with the NMR imaging data ; storing said imaging data set and said non-imaging data set; selecting at least one point in time related to a body motion for which NMR images are to be acquired ; deriving motion data representing a body motion of interest exclusively from said NMR non-imaging data placed in chronological order ; extracting gating data from said motion data after said steps of acquiring NMR imaging and non-imaging data have been completed, said gating data representing said at least one point in time related to said body motion ; selecting, from said stored imaging data and in accordance with said gating data, gated image data which relates to at least one specific point in time of the body motion of interest; and using said gated image data to produce a reconstructed NMR image. The method of claim 1, wherein a slice of the patient's body is established, which experiences the body motion of interest The method of claim 1 or 2, wherein the body motion is periodic and wherein said step of deriving motion data comprises the steps of: deriving timing data from said NMR non-imaging data; extracting an artificially-generated timing signal from said timing data; and wherein the gating data of said selecting-step is said artificially-generated timing signal. The method of claim 3, wherein the periodic body motion is a cardiac cycle, wherein said non-imaging data represents the cardiac cycle of the patient's heart; wherein said timing data is a cardiac timing data; and wherein said artificially-generated timing signal is an ECG signal The method of one of claims 1 to 4, wherein said step of selecting comprises the step of generating said image data by computation. The method of one of claims 1 to 5, wherein said extracting step comprises the steps of filtering said motion data through a low-pass filter and subjecting the resulting filtered motion data to a peak-detection algorithm. The method of one of claims 1 to 6, wherein said imaging data acquisition step is carried out using one echo and said non-imaging data acquisition step is carried out using another echo. The method of claim 7, further comprising the step of chronologically sorting the NMR data which are acquired during said another echo, and wherein said extracting step comprises the step of extracting, from said chronologically sorted NMR data, a column of data. The method of one of claims 1 to 8, wherein said NMR imaging data is acquired from one slice through the body of the living patient and said non-imaging data is acquired from another slice through said body and said one slice is different from said another slice. The method of one of claims 1 to 8, wherein said acquisition steps are carried out with respect to a single slice through the body of the living patient. The method of one of claims 1 to 10, wherein said selecting step comprises the step of identifying a plurality of points of a periodic motion and said reconstructing step comprises the step of reconstructing a plurality of NMR images.

Inventors: OWENS STEVEN F, SPRAGGINS THOMAS A, OWENS, STEVEN F., SPRAGGINS, THOMAS A.
Applicants: SIEMENS AG, SIEMENS AKTIENGESELLSCHAFT
Application year: 1989
Main classification: G01R33
IPC codes: G01R33