CA2023398A1 - Method for combining acquired nmr data to suppress motion artifacts - Google Patents

Abstract

Abstract of the Disclosure Displacement values which indicate respiration phase are acquired along with each view of an NMR scan. Two or more such NMR scans are conducted and the deviation of each displacement value from a smooth reference curve is employed to measure the integrity of its associated NMR image data.
The NMR image data from the separate scans are combined to reduce random noise and motion artifacts, and the combination is accomplished by weighting the data in accordance with its measured integrity.

Description

20,?~3~

15~M03463 A M~THOD FOR COMBINING ACQUIRED NMR
DA~A TO SUPPRESS ~CT~OU AR'IFACTS

Backaround o~ the Invention I This invention xelates to nuclear magne~ic resonance ¦ imaging methods. ~ore specifically, this invention relates I to a method for controlling image artifacts caused by -~
substantially periodic NMR signal variations due, for example, to subject motion in the course of an NMR scan.
NMR has been developed to obtain images of anatomical features of human patients. Such imageQ depict nuclear 3p n distribution (typically, protons assoclated with water and ;
!~ 15 tissue), spin-lattice relaxation time T1, and/or spin-spin relaxation time T2 and are of medical diagnostic value. ~MR
' data for constructlng images can be collected using one of ; many available techniques, such as multiple angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise a pulse sequence made up of a plurality of sequentially implemented views. Each view may include one or more NMR experiments, each of which comprises at least an -RF excitation pulse and a magnetic field gradient pulse to encode spatial information into the resulting NM~ signal. As . 25 is well-known, the NMR signal may be a free induction decay (FID~ or, preferably, a spin-echo cignal.
The preferred embodiments of the invention will be described in detail with reference to a variant of the well " ~

2 ~ 3 ~nown FT technique, which is frequently re~erred to as ~spin-warp~. It will be recognized, however, that the method of the invention is not limited to FT imaging methods, ~ut ~ay be advantageously practiced in conjunction wi~h other S techniques, such as multiple angle projection reconstruction -disclosed in U.S. Patent No. 4,471,306, and another variant of the FT tech~ique disclosed in U.S. eatent No. 4,070,611.
The spin-warp technique is discussed in an article entitled ; :
"Spin Warp NMR Imaging and Applications to Human Whole Body ~ -Imaging~ by W. A. Edelstein et al., ~ L~ Y:~ 91 Biolo~y, Vol. 25, pp. 751-756 (1980).
Briefly, the spin-warp technique employs a variable amplitude phase encoding magnetic fleld gradient pulse prior to the acquisition of NMR spin-echo sign~ls to phase encode lS spatial information in ~he direction of this gradient. In a two-dimensional implementation ~2DFT), for example, spatial ~
information is encoded in one direction by applying a phase~ ;
encoding gradient ~Gy) along that direction, and the observing a spin-echo signal in the presence of a second magnetlc field gradient ~Gx) in a direction orthogonal to the phase-encoding directlon. The Gx gradient present during the spin-echo encodes spatial information in the orthogonal dlrection. In a typical 2DFT pulse sequence, the magnitude of the phase-encoding gradient pulse Gy is incremented ~Gy) 25 monotonically in the sequence of views that are acquired to ~-produce a set of NMR data from which an entire image can be recon~tructed.
Ob~ect motion during the acquisition o NMR image data produces both blurring and ~'ghosts~' in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion, including cardlac and respiratory motion, each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition ,. . . , ~

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window. Blurring and ghosts, therefore, are due primarily to --the inconsistent appearance of the object from view to view, and in partic~lar, due to changes in the amplitude and/or phase of the NMR signal due to the motion.
Both blurring and ghosts can be reduced if the data acquisition is synchronized with the functional cycle of the -~
object. This method is known as gated NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the object "looks" the same in each view. The drawback of gating is that NMR data may be acquired only durins a small fraction of the object's functional cycle~ and even when the shortest acceptable pulse sequence is employed, the gating technique can significantly lengthen the data acquisition time.
One proposed method for eliminating ghost artifacts is disclosed in U.S. Patent No. 4,567,893, issued on February 4, l9a6, and is assigned to the same assignee as the present invention. In this prior applica~ion, it is recognized that the distance in the image between the ghosts and the object being imaged is maximized when the NMR pulse sequence repetition time is an odd multiple of one-fourth of the duration of the periodic signal variatlon (if two phase-alternated RF excitation pulses per view are used, as disclo~ed and claimed in commonly assigned U.S. Patent No.
4,443,760, issued April 17, 1984). It i~ recognized that this ratio can be used to alleviate ghosts due to respiratory motion. While thi~ method, indeed, improves image quality, it doe~ impose a constraint on the N~R pulse sequence repetition time and it often results in a longer total scan time. It also assumes that the motion is periodic. Its effectivene~s is diminished when the subject's breathing is irregular because the ghosts are blurred and can overlap t~.e image.

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Another method for reducing the undesirable effects due to periodic signal variations is disclosed in U.S. Patent ~o. ~-4,706,026 issued November 10, 1987 and entitled "A Method For Reducing Image Artifacts Due To Periodic Variations In NMR
S Imaging." In one embodiment of this method, an assumpt-o~ ~s made about the signal variation period (e.g. due, for example, to patient respiration~ and the view order is altered from the usual monotonically increasing phase- ~
encoding gradient to a preselected order. This involves -10 establishing the order in which either the gradient ;~
parameters, i.e. the amplitude of the phase-encoding gradien pulses (in the spin-warp method~ or the direction of the read-out gradient pulses (in the multiple angle projection reconstruction methodl are implemented. For a given signal variation period, a view order is chosen so as to make the NMR signal variation as a function of the phase-encoding amplitude (or gradient direction) be at a desired frequency.
In one embod~ment, the view order is selected such that the variation period appears to be equal to the total NMR scan ;
time (low frequency) so that the ghost artifacts are brough as close to the object being imaged as possible. In another embodiment (high frequency), the view order is chosen to make the variation period appear to be as short as possible so as to push the ghost artifacts as far from the object as possible.
Thi-~ prior method is effective in reducing artifacts, and is in some respects ideal if the variation is rather regular and at a known frequency. On the other hand, the method is not very robust if the assumption made about the motion temporal period does not hold (~.g., because the patient's breathing pattern changec or is irregular). If this occurs, the method loses some of its effectiveness because the focusing of the ghosts, either as close to the ~ ~-object or as far from the object as possible, becomes ~-s`:~

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' blurred. A solution to this problem is disclosed in U.S.
Patent No. 4,663,591 entitled "A Method For Reducing Image Art facts Due To Periodic Signal variations in NMR Imaging ~t In this method, the non-monotonic view order is determined as -~
the scan is executed and is responsive to changes in the period so as to produce a desired relationship ~low frequency or high frequency) between the signal variations and the gradient param2ter.
While the above described methods reduce motion -artifacts, they do rely on some regularity, or predictability, in the cyclic motion which is not always present. For example, irregularities may occur during a breathing cycle which introduces errors into the data acquired for one or more views of the scan. One method for reducing such spurlous errors is to perform two scans, or acquire twice the necessary data during a single scan and then average the information acquired for each view to produce an image of increased quality.
Summary of the Invention The present invention relates to an improved method and system for combining redundant NMR data for the purpose of reducing artifacts caused by patient motion. More specifically, the invention includes means for acquiring a plurality of sets of NMR image data, acquiring with each set of image data an associated set of motion data that indicates displacement of the subject during each view of acquired image data; means for generating a smooth reference curve for each set of acquir~d motion data; meanQ for determining the deviation of the acquired motion data from its reference curve; and combining the NMR image data for each view in the two sets of NMR image data a~ a function of the deviation of its associated motion data. Rather than merely averaging ~he redundant data for each view, the p~esent invention contemplates giving more weight to data which is more likely to provide reduced motion artifacts in the image, based o~
the deviation of its associated motion data from the reference curve.
A general ob~ect of the invention is to reduce motio~ -artifacts by intelligently combining redundant NMR image data. The associated motion data provides an indication of the integrity of the acquired NMR image data in each se~.
~ather than merely averaging the redundant data, therefore, more weight is given to the NMR data with higher integrity.
The result is a reduction in motion artifacts in the image which is reconstructed from the intelligently combined data.
Another object of the invention is to improve the quality of images which are reconstructed from NMR data acquired in a scan having a non-monotonic view order. The strategies which use a non-monotonic view order presume t~.er respiration ~ollows a smooth periodic cycle. To the extent that this is not true, the strategy does not work and motion artifacts are produced in the image. ~y acquiring redundant NMR data and combining it according to the teachings of the present invention, however, NMR data is produced which enhances the artifact suppressing mechanism of non-monotonic view ordered scans.
Tho foregoing and other objects and advantages of the invention will appea~ from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and ~n which there is shown by way of illustration a preferred embodiment of the invention.
Such embodiment doeq not neces~arily represent the full sco?e of the invention, however, and re erence i-~ made therefore ;o the claims herein for interpreting the scope of the invention.

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15N~103463 Br~f Des~ri~tion of ;he Dr~in~
FLg. ~ is an electrical block diagram of an NMR system which employs the present invention;
Fig. 2 is an exemplary imaging pulse sequence which s S executed by the system of Fig. 1;
Fig. 3 is a preferred em~odiment of the motion phase pulse sequence which is interleaved with the imaging pulse sequence o~ Fig. 2;
Fig. 4 is a pictorial representation of the data structures which are produced when practicing the preferred embodiment of the invention; and ~ -Fig. 5 is a flow chart of a program executed by the NMR
system of Fig. 1 to carry out the present invention.

Description of the Preferred Embodiment Fig. 1 is a simplified block diagram of an NMR imaging system which employs the preferred embodiment of the invention. The system includes a pulse control module 112 which provide3 properly timed pulse waveform signals, under the control of a host computer 114, to magnetic field gradient power 3upplies collectively deslgnated at 116.
These power supplieQ 116 energize gradient coils which form part of a gradient coil assembly generally indicated by block 118. The assembly contains coils which~produce the Gx, Gy and Gz magnetic field gradients directed in the x, y, and z direct~ons, respectively, of the Cartesian coordinate system.
The use of the Gx, Gy and Gz gradients in NM~ imaging applications will be described hereinafter with reference tO
Fig. 2.
Continuing with reference to Fig. 1, the pulse control module 112 provldes activating pulses to an RF synthesizer 120 which is part of an RE transceiver, portions of which are 2~,3~ ~

enclosed by dash-line block 122. The pulse control ~odule 112 also supplies signals to a modulator 124 which modulates the out?ut of the RF frequency synthesizer 120. The modulated RF signals are applied to an RF coil assembly 126 through an RF power amplifier 128 and a transmit/receive switch 130. The RF signals are used to excite nuclear splr.s in a sample object (not shown) which is to be imaged.
The NMR signals from the excited nuclear spins are sensed by the RF coil assembly 126 and applied through the transmit/receive switch 130 to an RF preamplifier 132. The amplified NMR signals are applied to a quadrature phase detector 134, and the detected signals are digitized by A/D
converter 136 and applied to computer 114 for storage and processing.
Reference is made to Fig. 2 which depicts two views of 2 conventional imaging pulse sequence of the type known as ~o-dimensional Fourier transforms (2DFT), and which is also referred to as two-dimensional "spin-warp". This pulse sequence is useful in obtaining, in a well known manner, imaging NMR data to reconstruct images of an object being investigated. The two views are indicated at ~A" and "B" and they are identical with the exception of the phase-encoding gradient field Gy~ ~ach view is a pulse sequence which utilizes phase-alternated RF excitation pulses which, as disclosed and claimed in the above-identified U.S. Patent No.
4,443,760, produce phase-alternated NMR signals Sl~t) and S1'(t) to cancel certain baseline error~ in the NMR system.
ReSerring now to View A in Fig. 2, there is shown in -;
interval 1 (indicated along the horizontal axis) a selective 90 RF excitation pulse applied in the presence of a positive Gz magnetic field gradient pulse. Pulse control module 112 (Fig. 1) provide3 the needed control signals to the frequenc-~synthesizer 120 and modulator 124 so that the resulting excitation pulse is of the correct phase and frequency to 2J ~ . d ~

15N~103463 excite nuclear spins only in a predetermined region of the object being imaged. Typically, the excitation pulse can be ; amplitude modulated by a ~sin x)/x function. The frequency of the synthesizer 120 is dependent on the strength of t~.e S applied polarizing magnetic field and the particular NMR
species being imaged in accordance with the well known La~ or equation. The pulse control module 112 also applies activating signals to the gradient power supplies 116 tO
generate, in this case, the Gz gradient pulse.
; 10 Continuing with reference to Fig. 2, Gx, Gy and Gz gradient pulses are applied simultaneously in interval 2.
The Gz gradient in interval 2 is a rephasing pulse typically selected such that the time integral of the gradient wavefor~
over interval 2 is approximately equal to -'~ of the ti.~e i 15 integral of the Gz gradient waveform over interval 1. The function of the negative Gz pulse is to rephase the nuclear ; spins excited in interval 1. The Gy gradient pulse is a phase-encoding pulse selected to have a different amplitude in each of Views A, B, . . ., etc., to encode spatial information in the direction of the gradient. The number O r different Gy gradient amplitudeq is typlcally selected to equal at least the number of pixel resolution elements the reconstructed im2ge will have in the phase-encoding ~Y) direction. Typically, 128, 256, or 512 different gradient amplitudes Gy are selected and in the typical NMR system, ~..e Gy values are incremented a fixed amount from one view to the æ next until the NMR scan is complete.
The Gx gradient pulse in in~erval 2 is a dephasing pulse needed to dephase the excited nuclear spins by a 30 predetermined amoùnt to delay the time of occurrence of a spin-echo signal Sl(t) in interval 4. The spin-echo signal is produced typically ~y the application of a 180 RF pulse in interval 3. As is known, the 180 RF pulse is ~ pulse which reverses the direction of spin dephasing so as to ? æ

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produce the spin-echo signal. The spin-echo signal is sampled in interval 4 in the presence of a sradient pulse Gx to enc~de spatial infor~ation in the direction (x) of th s ~radient.
S As indicated above, baseline error components are eliminated by using an addltional NMR measurement in eac:~
view. This second measurement is substantially identical tO
the first with the exception that the RF excitation pulse i-.
interval S of View A is selected to be 180 out of phase (as suggested by the minus sign) relative to the excitation pulse in interval l of view A. As a result, the spin-echo signal Sl'(t) in interval 8 is 180 out of phase with the spin-echo signal Sl(t) in interval 4. If the signal Sl'(t) is subtracted from Sl(t), only those components of the signals with reversed sign in the signal S1'(t) are retained. The baseline error components thus cancel.
The process described above with reference to View A s repeated for View B and so on for all amplitudes of the phase-encoding Gy gradient. The NMR image data which is collected during this scan is stored in the host computer 114 ~ where it is processed to produce image data suitable for i controlling a CRT display.
As the above described conventional NMR scan i performed, NMR data is acquired from all physical locations ;~
25 within the plane, or slice, of the object being imaged. If ~;
an accurate image is to be reconstructed, then both the object and the measurement conditions must be stable, or fixed, during the time needed to complete the entire NMR
scan. The present invention deals with the very prac~ical situations in which this is noe the case, but instead, the measurement conditions change in some cyclic, or nearly cyclic, manner.
One such situation occur~ when an image is to be ~ -produced throu~h the abdomen of a human subject. In this .
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case, much of the material beLng imaged is in motion due to the s~bject's breathinq, and the time needed to acquire the NMR data for an entire image will often transcend many respiration cycles. If NMR data is acquired continuously S throughout the respiration cycles, the subject will be disposed differently from view to view and the reconstructed image will contain many motion artifacts.
Co-pending U.S. patent application serial no. 427,401 filed on October 10, 19a9 and entitled ~'A Method For Monitoring Respiration ~ith Acquired NMR Data~ describes a system in which the motion of the patient dl~e to respiration is monitored using a special NMR pulse se~uence which is interleaved with each view of the acquired image data. The disclosure of this co-pending application is expressly incorporated into this application. Referring again to Fig.
1, the motion NMR pulse sequence described in this co-pendir.g application is executed by the pulse control module 112 jus~
prior to the execution of each view in the image scan. The N~R signal which results is received by the computer 114 and is analyzed as described in the above cited co-pending U.S.
patent application to produce a displacement value ~Y which indicates the position of the patient's anterior abdominal wall with respect to a reference position. This displacement value ~Y is output by the computer 114 to a respiration proce~sor 188 which converts it in real time to a value wnich indicateq the current phase of the patient respiration cycle.
As descrlbed in the above cited U.S. Patent Nos. 4,663,591;
4,706,026 and 4,720,678, this phase value which is computed by processor 188 is applied to the pulse control module 112 to select the orde~ in which the amplitudes of the phase-encoding gradient pulses Gy are applied durlng the scan. In other word~, the phase values produced prior to each view a_e used by the pulse control module 112 to control the particular non-mono~onic view order which is being used to .~ ,, . .. . ~ . .

15N~103463 suppress artifacts caused by patient respiratiOn. Therefore, to t~e extent that a phase value deviates from the ~rue, or expeceed, respiration cycle of the patient, it can be anticipated that the integrity of the image data which s acquired during the subsequent view may suffer. More particularly, the motion artifact suppression strategy may ~e less effective and the data acquired durinq the subsequent view may produce more artifacts in the reconstructed image.
By using the present invention, motion artifacts can be further suppressed. As explained above, during a complete scan at least one set of image data from a digitized NMR
signal is obtained at each phase encoding value, ox ~view number". Associated with each such set of image data is a displacement value ~Y which indicates the measured respiration phase when the NMR image data set was acquired.
In addition, it is also possible to obtain more than one set of NMR image data and an associated displacement value ~Y for ;
each view number. For example, the pulse sequence of Fig. 2 can be executed twice for each phase encoding value during . ..
the scan, or a second scan can be executed. In either case, redundant NMR image data is acquired and may be combined to reduce noise and motion artifacts in accordance with the present invention.
. . - .
It is, of course, common practice to combine redundant NMR image data to reduce random noise. Such a com~ination is accompllshed by taking the average value of the corresponding data element~ in each array o~ acquired image data. For example, if two values for each element have been acquired, their values are added and the result i~ divided by two.
Such averaging provides a ~ reduction in random noise. This well known method weights each value equally in arriving at the average.
The present invention combines redundant NMR data to improve image quality, but it weights the acquired values ~ . : , . . .

being combined as a function of their measured integrity. .~s will now be explained in detail, the integrity of the NMR
data s measured by uslng the associated displacement values ~Y.
S Referring particularly to Fig. 3, the displacement values ~Y for each view in two successive scans are plotted.
A "low frequency" sort as described in the above cited Paten.
Nos. 4,663,591; 4,706,026 and 4,720,678 was used to acauire the associated image data, and as a result, the measured displacement values should follow a slowly changing smooth cur~e as indicated at 200 and 201. It is readily apparent that the displacement values deviate from these smooth curves, and it is a teaching of the present invention that the extent of this deviation is an indication of the propensity for motion artifact generation when the associated image data is used to produce an image. Such deviations, therefore, provide a quantitative measure of the integrity o the image data. It should be noted that these deviations are not necessarily measurement errors, but may instead be caused by spurious variations in the patient's breathing pattern which has disrupted the motion artifact suppression strategy.
Accordingly, the present invention combines the redundant NMR
image data to reduce both noise and motion artifacts by -weighting the data as a function of the deviations in their 2S corresponding displacement values.
The invention is carried out by thç computer 114 under ~-the direction of a program which is executed after the data sets have been acquired. The operatlon of this program will now be described with reference to the flow chart in Fig. 5.
As indicated in Fig. 4, the acquired data includes a two-dimensional array of image data 205 and an associated one-dimensional array 206 of displacement data such as that illustrated in Fig. 3. Similar arrays 207 and 208 are stored for the second scan, or second set of acquired NMR data. The ;~

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I5N~103463 lmage data and the associa~ed displacement data have been sor~ed into sequential v~ew number order, with each row of image data array 205 and 207 being associated with a corresponding element in its respective displacement ar ay 206 and 208.
As indicated by process block S0 in Fig. 5, the fi-st step of the processing is to fit a quadratic equation to ~he values in the displacement arrays 206 and 208 using a well known least-squares technique. This effectively establishes the smooth curves 200 and 201 (Fig. 3) which serve as the I reference from which deviation values can be computed. These j are identified as reference functions rl(ky) and r2t~y). As -~
t indicated by process block 51, the scan with the smallest I variance in displacement values from their reference curve is .
~ 15 then computed as follows~
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N
!' variance - (l/N) ~ (m(ky) - r(ky))2 (l) ~ky ~

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where: N - the total number of views in the scan, - ~-m(ky) - values in the displacement array, and r(ky) - values of the reference function.
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Absolute devia~ions between ~he displacement values m(ky) and their reference values r(ky) are then calculated subject to a lower threshold deviation e ~ . 003 in the preferred embodiment)~ as indicated by process block S2.
That is, one-dimensional deviation arrays 209 and 210 are 25 produced by finding the absolute value of the difference .
m(ky) - r(ky). If the difference does not exceed the threshold value ( e ), then the corresponding image data elements are weighted equally (i.e., .5).

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15N~S03463 Using the deviation values D1(ky) and D2(ky) in the respective arrays 209 and 210, the image data in the arrays 205 and 207 is now combined. More specifically, each row of data S1(ky) in the first image data array 204 is combined with its corresponding row of data S2~ky) in the second image data array 207 to form a row of data S(ky) in a combined image data array 211 as indicated by process blocks 53 and 54.

S(~y) - Sl(ky) (W~(ky)) + S2~ky) (W2(ky)) (2) where: W1(ky) = D2(ky)/(Dl(ky) + D2(ky)) (3) W2(ky) - Dl~ky)/(Dl~ky) + D2(ky)) As indicated above, the weighting factors Wl and W2 are set equal to .S in cases where both deviationq are less than the threshold amount (e).
The resulting image data array 211 is then processed as indicated at block 55 using the conventional reconstruction technique (2DFT in the preferred embodiment) to produce a two-dimensional display data array 212 which indicates the intensity of each display pixel.
It should be apparent to those skilled in the art that while two sets of data are acquired in the preferred embodiment of the invention, the invention is also applicable when three or more sets of data are acquired. Regardless of the number of set~ acquired, the present invention can be employed to select the best data or to weight the averaging of the data to minimize motion artifacts.

Claims (6)

1. An NMR system for producing an image of a subject undergoing motion, the combination comprising:
means for acquiring a plurality of sets of NMR image data;
means for acquiring with each set of image data an associated set of motion data that indicates motion of the subject during each view of acquired image data;
means for generating a smooth reference curve for each set of acquired motion data;
means for determining the deviation of the acquired motion data from a reference curve; and means for combining each view in the sets of NMR image data by weighting the NMR image data as a function of the deviation of its associated motion data.

2. The NMR system as recited in claim 1 in which the means for acquiring the sets of motion data includes means for producing a motion NMR pulse sequence and the means for acquiring the sets of NMR image data includes means for producing an image NMR pulse sequence; and each of said data sets is acquired in a scan comprised of a plurality of image NMR pulse sequences interleaved with a plurality of the motion NMR pulse sequences.

3. The NMR system as recited in claim 2 in which the motion data in each set indicates the displacement of a feature in the subject from a reference position, and the smooth reference curve is generated by fitting a curve to the displacement values in the motion data set.

4. The NMR system as recited in claim 3 in which the means for combining each view weights the NMR image data in each set inversely proportional to the magnitude of the deviation of its associated motion data from the reference curve.

5. The system as recited in claim 1 in which two sets of NMR image data S1(ky) and S2(ky) are acquired along with two sets of associated motion NMR data, the means for determining deviation produces two sets of deviation data D1(ky) and D2(ky), and the means for combining each view in the sets of NMR image data performs the following calculations:
S(ky) - S1(ky) (W1(ky)) + S2(ky) (W2(ky));
where: W1(ky) = D2(ky)/(D1(ky) + D2(ky)); and W2(ky) = D1(ky)/(D1(ky) + D2(ky)).

6. The invention as defined in any of the preceding claims including any further features of novelty disclosed.