WO2001084172A1

WO2001084172A1 - Prospective multi-planar motion correction in mri

Info

Publication number: WO2001084172A1
Application number: PCT/US2001/011048
Authority: WO
Inventors: Clifford R. Jack; Heidi A. Ward; Christine Lee; Richard L. Ehman; Stephen J. Riederer
Original assignee: Mayo Foundation For Medical Education And Research
Priority date: 2000-05-04
Filing date: 2001-04-05
Publication date: 2001-11-08
Also published as: AU2001253160A1

Abstract

An MRI system performs a scan using an EPI pulse sequence to acquire a series of image frames used to produce functional MR images. Motion adjustment pulse sequences are interleaved with the image frame acquisition to produce navigator signals used to measure patient motion during the scan. MRI system scan parameters are prospectively adjusted during the scan to offset the effects of patient motion. The motion adjustment pulse sequence consist of a coarse and fine navigator echo sequence (314, 316) which are identical. In particular, two sets of three orthogonal orbital navigator echoes are generated prior to each image frame acquisition.

Description

PROSPECTIVE MULTI-PLANAR MOTION CORRECTION IN MRI

BACKGROUND OF THE INVENTION

The field of the invention is nuclear magnetic resonance imaging ("MRI") methods and systems. More particularly, the invention relates to reduction of artifacts caused by patient motion during the acquisition of time course MRI data in functional MRI and other clinical applications.

Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the gyromagnetic ratio γ of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".

When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field Brj), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M_z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B-] ) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M_z, may be rotated, or "tipped" into the x-y plane to produce a net transverse magnetic moment M , which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B-| is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance ("NMR") phenomena is exploited.

When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic field gradients (G_x, Gy, and G_z) which have the same direction as the polarizing field Q, but which vary linearly along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.

Functional MRI (fMRI) is a technique which permits the mapping of areas of specialized function in the brain noninvasively using standard clinical MRI equipment. As described in U.S. patent No. 5,603,322, during an fMRI scan the subject alternately performs specific mental tasks while a series of images of the brain are acquired over a period of time. The change in image signal intensity as a function of time in the resulting time course MRI data set is used to identify those areas of the brain which are active during functional tasks. Any source of MRI intensity signal fluctuation during the scan time which is not due to physiologic brain activation represents a source of artifact in the final fMRI map. The most significant source of this artifact is global motion which may occur between the acquisition of successive image frames.

The standard approach taken to correct for inter-image head motion during an fMRI scan has been retrospective realignment of individual images to a common baseline reference position. This is typically performed by searching for the alignment that best minimizes a measure of the difference between each acquired image frame and a baseline image.

U.S. patent No. 4,937,526 describes a different method for reducing motion artifacts in NMR images in which the NMR data set used to reconstruct the image is corrected after its acquisition using information from concurrently acquired NMR "navigator" signals. The navigator signals are produced by pulse sequences. The navigator signal is a projection along an axis defined by the readout gradient which is fixed in a direction throughout the scan, and encodes the current position of the patient. Correction for subject motion along this readout gradient axis can be made using the navigator echo signal. U.S. patent No. 5,539,312 describes an improvement to this approach in which the navigator echo signal is acquired while two, orthogonal readout gradients are applied. This enables correction for translational movements along the two orthogonal readout axes as well as rotational movement in the plane defined by the two orthogonal readout axes.

Such retrospective correction methods have not been entirely satisfactory. Any through-plane motion which occurs during the scan disrupts the magnetization history in the prescribed imaging slices. Retrospective correction cannot recapture information that was not acquired at the proper time during the scan because the spins were translated outside the excited slice by through-plane patient motion. In addition, rotation orthogonal to the plane of the navigator echoes disrupts the process of motion measurements within the plane of the navigator echoes, causing potentially inaccurate measurement.

SUMMARY OF THE INVENTION

The present invention is a method for acquiring MRI data, and more particularly, for reducing artifacts caused by inter-image subject motion. Navigator signals are acquired to establish a subject reference position and further navigator signals are acquired periodically and are compared to the reference position signals to measure current subject position. This information is used to prospectively adjust the MRI system gradient coordinate system or RF excitation frequencies to follow subject motion. The MRI system imaging coordinates are thus moved during the scan such that each image frame is acquired from the same location in the subject despite inter-image patient motion.

One aspect of the invention is the method for measuring subject position and prospectively adjusting MRI system imaging coordinates prior to each image frame acquisition. A first set of coarse navigator echoes are acquired to measure subject position and make a coarse adjustment of the MRI system imaging coordinates. A second set of fine navigator echoes are then acquired to measure subject position and make a fine adjustment to the MRI system imaging coordinates. The image frame MR data is then acquired with consistent localization of image slices with respect to the subject's anatomy.

Another aspect of the invention is the method used to acquire navigator echo signals. A set of orbital navigator pulse sequences are used, and each orbital navigator pulse sequence includes a non-selective rf excitation pulse. This contrasts with prior navigator pulse sequences which excite only a portion of the imaged anatomy with selective rf excitation pulses and which selectively saturate corresponding spins. Image artifacts caused by this selective saturation are thus eliminated.

The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an MRI system which employs the present invention;

FIG. 2 is an electrical block diagram of the transceiver which forms part of the MRI system of FIG. 1 ;

FIG. 3 is a graphic representation of the EPI pulse sequence used to practice the preferred embodiment of the invention; FIG. 4A is a pictorial representation of the MRI data set acquired with the pulse sequence of FIG. 3 during a time course scan;

FIG. 4B is a graphic representation of a time domain voxel vector which forms part of the data set of FIG. 4A;

FIG. 5 is a graphic representation of the steps employed in the preferred time course scan according to the present invention; and

FIG. 6 is a graphic representation of a preferred navigator pulse sequence used in the time course scan of Fig. 5.

GENERAL DESCRIPTION OF THE INVENTION

The present invention is a method for acquiring a series of image frames in a time course MRI data acquisition scan and prospectively adjusting for patient motion prior to the acquisition of each image frame. Two sets of orbital navigator ("ONAV") echoes are acquired prior to each image frame acquisition to provide the necessary information needed to make the adjustment. Rotation and translation of an object within a given plane can be measured using the MR signal from an ONAV echo signal in which a circular k-space trajectory having a radius k_p is sampled. If S(k_p,θ) is the signal from the acquired ONAV echo, then the signal of an object rotated through the angle α and translated by an amount (Xo,y₀) can be described as

S'(k_p,θ) = S(k_p,θ -a)e ^kp(x^^{θ+ Smθ)} . (1 )

Here, θ is the azimuthal angle of the sample in k-space at the radius k_p of the ONAV echo and is proportional to the time t over which the echo is acquired. The ONAV echo signals from the first application of each of the orthogonal ONAVs at the beginning of the scan are retained as baseline reference signals. For subsequent ONAV sequences, the signal measured from each ONAV echo is compared with its respective baseline reference signal, and hence inter-image object motion can be tracked and corrected throughout the MRI scan.

From equation (1) it can be seen that object rotation is encoded only in the magnitude of the signal. Thus, measurement of any shift in the ONAV echo signal magnitude yields the detected rotation. Translation is encoded in the phase of the ONAV echo signal and can be extracted through analysis of the phase.

To correct for the detected rotation, the gradient rotation matrix, employed in the MRI system for oblique imaging, is rotated about the isocenter of the magnet. When the center of rotation of the object is not coincident with the isocenter, rotation of the gradient rotation matrix about isocenter results in an apparent translation of the object. Apparent translation is not due to a physical translation of the object with respect to the magnet bore, but rather to translation with respect to its new frame of reference. The sum of the actual and apparent translations is the amount of translation visible in the images that must be corrected. If the phase of the reference ONAV signal, ψ(θ), is shifted by the detected angular rotation, α, then the apparent translation is encoded in the difference of the phase of the reference ONAV signal and the current ONAV signal ψ'(θ), along with any actual translation.

Δψ(θ)=ψ'(θ)-ψ(θ-α)=k_p[Xo cos(θ-α)+y₀ sin(θ-α)]. (2)

Thus Xo and y₀ represent the sum of the actual and apparent translations in their respective directions and are used to correct the image.

After measurement of the rotations and translations, the values of the three measured rotations and the measured through-plane translations are applied to the MRI scanner. Correction of the rotations and through-plane translation are performed prospectively by appropriate adjustment of the MRI system gradient rotation matrix and RF excitation frequency prior to acquisition of the next image frame.

Since application of orthogonal rotation matrices is non-commutative, and the angles of rotation are measured about the axes of the original gradient rotation matrix, the standard rotation matrices used to describe rotation of an object do not apply. Instead, the three orthogonal angles of rotation can be considered components of a single rotation of magnitude φ about the unit vector h , where

φ a² + β² +y²

and (3)

„ a . β „ γ n = —e_l + — e₂ H — e₃

Φ Φ Φ

α is the measured rotation about the vector e, (sagittal rotation), β is the measured rotation about the unit vector e₂ (coronal rotation), γ is the measured rotation about the unit vector e₃ (axial rotation), and ( e, , e,, e₃ ) are the perpendicular unit vectors describing the original frame of reference, or in other words, the gradient rotation matrix. If 9*(φ, « ) is the rotation through angle φ about unit vector h , the new frame of reference ( , , ,, ₃ ) can mathematically be described as

I = W(φ,ή)e, = e_l cosφ + (n χ e_l

cosφ) (i=1 ,2,3) (4)

Note that this equation reduces to the standard rotation matrices given only one non-zero component of , and that for a rotation of zero, is undefined. Thus, the gradient rotation matrix is updated to correct for rotation according to Eq. 4 only when rotation is detected.

Correction for in-plane translation can be performed retrospectively with no loss of information by applying a phase modulation to the complex data set after row-flipping the even echoes relative to the odd echoes of the EPI data set and before EPI image reconstruction. Alternatively, correction for in-plane translation can be performed prospectively by adjusting the carrier frequency of the receiver according to the polarity of the acquired echo to correct for translation in the readout direction, and adjusting the phase of each echo to correct for translation in the phase-encode direction.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1 , there is shown the major components of a preferred NMR system which incorporates the present invention and which is sold by the General Electric Company under the trademark "SIGNA". The operation of the system is controlled from an operator console 100 which includes a console processor 101 that scans a keyboard 102 and receives inputs from a human operator through a control panel 103 and a plasma display/touch screen 104. The console processor 101 communicates through a communications link 116 with an applications interface module 117 in a separate computer system 107. Through the keyboard 102 and controls 103, an operator controls the production and display of images by an image processor 106 in the computer system 107, which connects directly to a video display 118' on the console 100 through a video cable 105.

The computer system 107 is formed about a backplane bus which conforms with the VME standards, and it includes a number of modules which communicate with each other through this backplane. In addition to the application interface 117 and the image processor 106, these include a CPU module 108 that controls the VME backplane, and an SCSI interface module 109 that connects the computer system 107 through a bus 110 to a set of peripheral devices, including disk storage 111 and tape drive 112. The computer system 107 also includes a memory module 113, known in the art as a frame buffer for storing image data arrays, and a serial interface module 114 that links the computer system 107 through a high speed serial link 115 to a system interface module 120 located in a separate system control cabinet 122.

The system control 122 includes a series of modules which are connected together by a common backplane 118. The backplane 118 is comprised of a number of bus structures, including a bus structure which is controlled by a CPU module 119. The serial interface module 120 connects this backplane 118 to the high speed serial link 115, and pulse generator module 121 connects the backplane 118 to the operator console 100 through a serial link 125. It is through this link 125 that the system control 122 receives commands from the operator which indicate the scan sequence that is to be performed. The pulse generator module 121 operates the system components to carry out the desired scan sequence. It produces data which indicates the timing, strength and shape of the RF pulses which are to be produced, and the timing of and length of the data acquisition window. The pulse generator module 121 also connects through serial link 126 to a set of gradient amplifiers 127, and it conveys data thereto which indicates the timing and shape of the gradient pulses that are to be produced during the scan. The pulse generator module 121 also receives patient data through a serial link 128 from a physiological acquisition controller 129. The physiological acquisition control 129 can receive a signal from a number of different sensors connected to the patient. For example, it may receive ECG signals from electrodes or respiratory signals from a bellows and produce pulses for the pulse generator module 121 that synchronizes the scan with the patient's cardiac cycle or respiratory cycle. And finally, the pulse generator module 121 connects through a serial link 132 to scan room interface circuit 133 which receives signals at inputs 135 from various sensors associated with the position and condition of the patient and the magnet system. It is also through the scan room interface circuit 133 that a patient positioning system 134 receives commands which move the patient cradle and transport the patient to the desired position for the scan.

The "logical" gradient waveforms produced by the pulse generator module 121 are rotated by a rotation matrix (not shown) to form physical gradient waveforms which are applied to a gradient amplifier system 127 comprised of G_x, Gy and G_z amplifiers 136, 137 and 138, respectively. Each amplifier 136, 137 and 138 is utilized to excite a corresponding gradient coil in an assembly generally designated 139. The rotation of the gradient waveforms is described for example in U.S. Pat. No. 4,743,851 which is incorporated herein by reference. Adjustments to the rotation can be made during the scan to adjust the orientation of the region from which image data is acquired.

The gradient coil assembly 139 forms part of a magnet assembly 155 which includes a polarizing magnet 140 that produces a 1.5 Tesla polarizing field that extends horizontally through a bore. The gradient coils 139 encircle the bore, and when energized, they generate magnetic fields In the same direction as the main polarizing magnetic field, but with gradients G_x, G_y and

G_z directed in the orthogonal x-, y- and z-axis directions of a Cartesian coordinate system. That is, if the magnetic field generated by the main magnet 140 is directed in the z direction and is termed Bo, and the total magnetic field in the z direction is referred to as B_z, then G Q B_z/5 x,

Gy=5 B_z/δ y and G_z=d B_z/δ z, and the magnetic field at any point (x, y, z) in the bore of the magnet assembly 141 is given by B(x, y, Z)=BQ+ G_XX+

G_yyG_zz. The gradient magnetic fields are utilized to encode spatial information into the NMR signals emanating from the patient being scanned. Located within the bore 142 is a circular cylindrical whole-body RF coil

152. This coil 152 produces a circularly polarized RF field in response to RF pulses provided by a transceiver module 150 in the system control cabinet 122. These pulses are amplified by an RF amplifier 151 and coupled to the RF coil 152 by a transmit/receive switch 154 which forms an integral part of the RF coil assembly. Waveforms and control signals are provided by the pulse generator module 121 and utilized by the transceiver module 150 for RF carrier modulation and mode control. The resulting NMR signals radiated by the excited nuclei in the patient may be sensed by the same RF coil 152 and coupled through the transmit/receive switch 154 to a preamplifier 153. The amplified NMR signals are demodulated, filtered, and digitized in the receiver section of the transceiver 150.

The transmit/receive switch 154 is controlled by a signal from the pulse generator module 121 to electrically connect the RF amplifier 151 to the coil 152 during the transmit mode and to connect the preamplifier 153 during the receive mode. The transmit/receive switch 154 also enables a separate local RF head coil to be used in the receive mode to improve the signal-to-noise ratio of the received NMR signals. With currently available NMR systems such a local RF coil is necessary in order to detect the small variations in NMR signal produced by brain functions.

In addition to supporting the polarizing magnet 140 and the gradient coils 139 and RF coil 152, the main magnet assembly 141 also supports a set of shim coils 156 associated with the main magnet 140 and used to correct inhomogeneities in the polarizing magnet field. The main power supply 157 is utilized to bring the polarizing field produced by the superconductive main magnet 140 to the proper operating strength and is then removed. The NMR signals picked up by the RF coil are digitized by the transceiver module 150 and transferred to a memory module 160 which is also part of the system control 122. When the scan is completed and an entire array of data has been acquired in the memory modules 160, an array processor 161 operates to Fourier transform the data into an array of image data. This image data is conveyed through the serial link 115 to the computer system 107 where it is stored in the disk memory 111. In response to commands received from the operator console 100, this image data may be archived on the tape drive 112, or it may be further processed by the image processor 106 and conveyed to the operator console 100 and presented on the video display 118' as will be described in more detail hereinafter. Referring particularly to FIGS. 1 and 2, the transceiver 150 includes components which produce the RF excitation field B-| through power amplifier

151 at a coil 152A and components which receive the resulting NMR signal induced in a coil 152B. As indicated above, the coils 152A and B may be a single whole-body coil, but the best results are achieved with a single local RF coil specially designed for the head. The base, or carrier, frequency of the RF excitation field is produced under control of a frequency synthesizer 200 which receives a set of digital signals (CF) through the backplane 118 from the CPU module 119 and pulse generator module 121. These digital signals indicate the frequency and phase of the RF carrier signal which is produced at an output 201. As indicated above and described in more detail below, the frequency of this carrier signal may be adjusted during a scan to adjust the location from which image data is acquired. The commanded RF carrier is applied to a modulator and up converter 202 where its amplitude is modulated in response to a signal R(t) also received through the backplane 1 18 from the pulse generator module 121. The signal R(t) defines the envelope, and therefore the bandwidth, of the RF excitation pulse to be produced. It is produced in the module 121 by sequentially reading out a series of stored digital values that represent the desired envelope. These stored digital values may, in turn, be changed from the operator console 100 to enable any desired RF pulse envelope to be produced. The modulator and up converter 202 produces an RF pulse at the desired Larmor frequency at an output 205.

The magnitude of the RF excitation pulse output through line 205 is attenuated by an exciter attenuator circuit 206 which receives a digital command, TA, from the backplane 118. The attenuated RF excitation pulses are applied to the power amplifier 151 that drives the RF coil 152A. For a more detailed description of this portion of the transceiver 122, reference is made to U.S. Pat. No. 4,952,877 which is incorporated herein by reference. Referring still to FIG. 1 and 2 the NMR signal produced by the subject is picked up by the receiver coil 152B and applied through the preamplifier 153 to the input of a receiver attenuator 207. The receiver attenuator 207 further amplifies the NMR signal and this is attenuated by an amount determined by a digital attenuation signal (RA) received from the backplane 118. The receive attenuator 207 is also turned on and off by a signal from the pulse generator module 121 such that it is not overloaded during RF excitation.

The received NMR signal is at or around the Larmor frequency, which in the preferred embodiment is around 63.86 MHz for 1.5 Tesla. This high frequency signal is down converted in a two step process by a down converter 208 which first mixes the NMR signal with the carrier signal on line 201 and then mixes the resulting difference signal with the 2.5 MHz reference signal on line 204. The resulting down converted NMR signal on line 212 has a maximum bandwidth of 125 kHz and it is centered at a frequency of 187.5 kHz. The down converted NMR signal is applied to the input of an analog-to- digital (A/D) converter 209 which samples and digitizes the analog signal at a rate of 250 kHz. The output of the A/D converter 209 is applied to a digital detector and signal processor 210 which produce 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received digital signal. The resulting stream of digitized I and Q values of the received NMR signal is output through backplane 118 to the memory module 160 where they are employed to reconstruct an image.

To preserve the phase information contained in the received NMR signal, both the modulator and up converter 202 in the exciter section and the down converter 208 in the receiver section are operated with common signals. More particularly, the carrier signal at the output 201 of the frequency synthesizer 200 and the 2.5 MHz reference signal at the output 204 of the reference frequency generator 203 are employed in both frequency conversion processes. Phase consistency is thus maintained and phase changes in the detected NMR signal accurately indicate phase changes produced by the excited spins. The 2.5 MHz reference signal as well as 5, 10 and 60 MHz reference signals are produced by the reference frequency generator 203 from a common 20 MHz master clock signal. The latter three reference signals are employed by the frequency synthesizer 200 to produce the carrier signal on output 201. For a more detailed description of the receiver, reference is made to U.S. Pat. No. 4,992,736 which is incorporated herein by reference.

The EPI pulse sequence employed in the preferred embodiment of the invention is illustrated in FIG. 3. A spatial-spectral RF excitation pulse 250 is applied in the presence of a G_z slice select gradient pulse 251 to produce transverse magnetization in a slice through the brain ranging from 3.1 to 25 mm thick. The excited spins are rephased by a negative lobe 252 on the slice select gradient G_z and then a time interval elapses before the readout sequence begins. Separate NMR echo signals, indicated generally at 253, are acquired during the EPI pulse sequence. Each NMR echo signal 253 is a different view which is separately phase encoded to scan ky-space in monotonic order. The readout sequence is positioned such that the view acquired at ky=0 occurs at the desired echo time (TE). Echo times typically range from TE=20 to 120 ms, in brain studies.

The NMR echo signals 253 are gradient recalled echoes produced by the application of an oscillating G_x readout gradient field 255. The readout sequence is started with a negative readout gradient lobe 256 and the echo signals 253 are produced as the readout gradient oscillates between positive and negative values. Samples are taken of each NMR echo signal 253 during each readout gradient pulse 255. The successive NMR echo signals 253 are separately phase encoded by a series of Gy phase encoding gradient pulses 258. The first pulse is a negative lobe 259 that occurs before the echo signals are acquired to encode the first view. Subsequent phase encoding pulses 258 occur as the readout gradient pulses 255 switch polarity, and they step the phase encoding monotonically upward through ky space.

At the completion of the EPI pulse sequence, therefore, separate frequency encoded samples of separately phase encoded NMR echo signals 253 have been acquired. This element array of complex numbers is Fourier transformed along both of its dimensions (k_y and k_x) to produce an array of image data that indicates the NMR signal magnitude along each of its two dimensions (y and x). For functional MRI, the EPI pulse sequence is repeated n times to acquire time course NMR data for n image frames of each prescribed slice. In a typical fMRI scan 20 to 25 slices are prescribed and it requires a number of minutes to acquire n image frames for all the slices. As a result, a typical time course acquisition spans a 2 to 6 minute time period. During that time period the subject is asked to perform a specific function in a predetermined pattern, or a stimulus is applied to the subject in a predetermined pattern. For example, the subject may be instructed to touch each finger to his thumb in a sequential, self-paced, and repetitive manner, or the subject may be subjected to a sensory stimulus such as a smell or visual pattern in a periodic manner.

The acquired NMR data is processed in the conventional manner to produce an NMR image data set for n image frames. As explained above, a two dimensional Fourier transformation is performed by the array processor 161 (FIG. 1 ) and the resulting NMR image data set for each slice is stored in the disk 111 for further processing by the image processor 106.

Referring to FIG. 4A, this NMR image data set is organized as a set of element 2D arrays 300 in which each element therein stores the magnitude of the NMR signal from one voxel in the scanned slice. Each array 300 can be used to directly produce an anatomical image of the brain slice for output to the video display 118. While each array 300 is a "snap shot" of the brain slice at a particular time during the time course study, the NMR image data set may also be viewed as a single 3D array 301 in which the third dimension is time. The time course NMR image data for one voxel in the array 301 is referred to herein as a time course voxel vector. One such voxel vector is illustrated in FIG. 4A by the dashed line 302. Each time course voxel vector 302 indicates the magnitude of the NMR signal at a voxel in the image slice over the time course study. It may be used to produce a graphic display as shown in FIG. 4B. The resulting time domain voxel graph 303 reveals very clearly variations in the activity of the brain in the region of the voxel. Regions which are responsive to a sensory stimulus, for example, can be located by identifying time domain voxel graphs which vary at the same repetition rate as the applied stimulus.

It should be apparent that the above-described time course acquisition of fMRI data must accurately acquire the data from exactly the same locations in the subject's brain throughout the scan. If the subject moves during the scan, each voxel vector will contain signals from different locations in the brain, and the time domain voxel graphs will not accurately represent the brain activity. Physical restraints may be used to stabilize the subject's head and prevent head motion, but these are only partially successful. In addition, with most subjects, highly rigid head fixation (e.g. a dental molded bit bar) is not possible. Referring particularly to Fig. 5, the fMRI scan according to the preferred embodiment of the invention is comprised of a series of image frame acquisitions 310 as described above, which are interleaved with motion adjustment sequences indicated generally at 312. As will be described in more detail below, each motion adjustment sequence is comprised of a coarse navigator echo sequence 314 followed by a fine navigator echo sequence 316. During the initial motion adjustment sequence the acquired navigator echo signals are digitized and stored as reference signals. These reference signals indicate the position of the patient's head at the beginning of the scan, and then serve as a reference against which subsequently acquired navigator signals can be compared.

The coarse and fine navigator echo sequences 314 and 316 are identical and are shown in more detail in Fig. 6. Each sequence is comprised of three non-selective RF excitation pulses 320, 321 , and 322 which produce transverse magnetization that is read out as three corresponding navigator signals 324, 325 and 326. The transmit repeat time (TR) between RF pulses is 11 milliseconds and the flip angle of each RF pulse 320, 321 and 322 is 10°.

The first navigator signal 324 is acquired in the presence of sinusoidal readout gradients 328 and 330 directed along the x-axis and z-axis respectively. As described in more detail in the above-cited U.S. Patent No. 5,539,312 which is incorporated herein by reference, the sinusoidal readout gradients 328 and 330 sample a circular trajectory in the k_x-k_z plane (i.e. coronal). Because the RF excitation is non-selective, the first navigator signal 324 is a projection along the k_y-axis.

The second navigator signal 325 is acquired in the presence of sinusoidal readout gradients 332 and 334 directed along the k_y-axis and k_z- axis respectively. These readout gradients produce a circular sampling trajectory in the k_y-k_z plane (i.e. sagittal) and the second navigator signal 325 is a projection along the k_x-axis.

The third navigator signal 326 is acquired in the presence of sinusoidal readout gradients 336 and 338 directed along the k_x-axis and k_y-axis respectively. These readout gradients produce a circular sampling trajectory in the k_x-k_y plane (i.e. axial) and the third navigator signal 326 is a projection along the k_z-axis.

The navigator signals 324, 325 and 326 are digitized as described above and processed to calculate the translational movement of the patient along the respective x, y and z axes and the rotational movement in each of the coronal, sagittal and axial planes. The movements are calculated by comparing the acquired navigator signals with the reference signals acquired at the beginning of the scan. The rotational movement is calculated by determining the shift α that minimizes the error function ε_rot(α), which is the sum of the squared difference between the reference navigator magnitude and the current navigator magnitude:

After the rotation angle is calculated, the in plane translational motion is calculated by comparing the phase of the current navigator signal with the phase of the reference navigator signal. From equation (2), we see that the displacements, X_Q and y₀, could be determined by regressing the waveform against cosinusoidal and sinusoidal waveforms. However, magnetic inhomogeneities and non-ideal gradients may lead to imper ect k-space trajectories, causing linear drift in the phase signal. To account for these imperfections, linear terms for slope (m) and intercept (b) were added to the regression. In addition, regions of the waveform having low signal magnitude have poor detection of phase signal. Therefore, a weighting function, w(θ)=S'(k_p, θ)S(k_p, θ-α), is used such that areas of high signal magnitude and thus high confidence have a greater contribution to the linear regression. With these two modifications, the error function, ε_tra^Xo.yo.m.b), minimized by linear regression is given by:

£trans (^X0 <yo > ^m<b) =

∑{ω(θ)[AΨ(θ)- k_p{x₀ cos(θ -α) + y₀ sm(θ - α) + m(θ -α) + b}² (6)

Within each set of ONAVs, the translation in each of the three orthogonal directions is measured by two of the three orthogonal ONAVs. Thus, for translation in any particular direction, the two measured values may be averaged.

The above calculations are carried out on the first navigator signal 324 in the array processor 161 as the second navigator signal 325 is being acquired and digitized. Similarly, the calculations are performed on the second navigator signal 325 as the third navigator signal 326 is being acquired and digitized. And finally, the three rotational corrections and three translational corrections are used to prospectively adjust the MRI system operating parameters prior to the acquisition of the next image frame. The total time required to acquire the three navigator signals 324, 325 and 326 and prospectively adjust the MRI system parameters to correct for patient motion is currently from 100 to 160 milliseconds. This adds about 15 seconds to a 2 to 6 minute scan when both coarse and fine navigator echo sequences are employed.

Acquiring two sets of three orthogonal ONAVs improves the precision with which motion can be measured. A fundamental limitation to using a single set of three orthogonal ONAVs to detect global head motion, is that after rotation, the ONAVs sample a slightly different projection of the head when compared to the baseline reference set of ONAVs. This limits the precision with which rotational motion can be measured. However, in the preferred embodiment of the invention the rotations with respect to the reference position are measured with the first set of ONAVs and are corrected prior to playing out the second set of ONAVs. Thus, the second set of ONAVs sample roughly the same projection of the subject as did the baseline reference set. This step is necessary to achieve one millimeter and one degree accuracy which is necessary for fMRI. Although in this preferred embodiment we describe two sets of ONAVs, in theory it is possible to extend this to three or four, etc. sets. The limiting factor is the tradeoff between the desired precision of inter-image correction and the time expanded for acquiring the ONAV data. The use of multiple navigator echo sequences during the scan presents another problem which is addressed by the present invention. Known navigator implementations employ RF excitation pulses which are selective to a subdivision of the body part being sampled. For example, for the head, a single slice is employed, or for tracking diaphragmatic motion, a "pencil beam" excitation profile is usually employed. A disadvantage of exciting only a portion of the body part whose motion is being tracked is that the RF excitation pulses saturate the longitudinal magnetization of a part of the anatomy being imaged. This produces undesirable artifacts in the images. In the preferred embodiment of the invention we employ an ONAV excitation of low flip angle which encompasses the entire head, essentially acting as a nonspatially selective navigator pulse. This provides accurate tracking of motion, while eliminating the undesirable saturation artifacts caused by prior methods. While the preferred embodiment of the invention is employed to acquire a series of image frames for fMRI analysis, the present invention has other MR applications where global subject motion is a problem. For example in cardiac imaging the successive images are aligned such that the heart remains substantially fixed in location despite patient respiration. The present invention is also applicable to non-human studies as well. Also, images acquired at different angles or using different imaging pulse sequences can be aligned using the present invention. For example, in a typical head MRI examination the first imaging pulse sequence may acquire sagittal spin echo data, the second pulse sequence acquire axial spin echo data, and the third pulse sequence acquire coronal gradient echo data.

The present invention may also be employed in combination with known presaturation methods. Spatial presaturation as described in U.S. Pat. No. 4,715,383 may be interleaved during the scan to suppress signals from anatomy of no interest or anatomy which does not move the same as that used to make motion adjustments. Similarly, spectrally selective presaturation pulses may be interleaved to suppress signal from fat.

Claims

CLAIMS We claim:

1. A method for acquiring image data from a patient with a magnetic resonance imaging (MRI) system, the steps comprising: a) performing a reference motion adjustment pulse sequence with the MRI system to acquire navigator echo signals for establishing a patient reference position; b) performing an imaging pulse sequence with the MRI system to acquire image data; c) performing a motion adjustment pulse sequence with the MRI system to acquire navigator echo signals for measuring the current position of the patient, the motion adjustment pulse sequence including the steps of: i) performing a coarse navigator echo sequence with the MRI system to measure current patient position; ii) adjusting an MRI system scan parameter using the measured current patient position; and iii) performing a fine navigator echo sequence with the MRI system to produce the navigator echo signals; d) adjusting an MRI system scan parameter using information derived from the navigator echo signals acquired in steps a) and c); and e) repeating steps b), c) and d) to acquire image data.

2. The method as recited in claim 1 in which the fine navigator echo sequence includes:

producing two, orthogonal readout magnetic field gradients during the readout of the navigator echo signal such that the navigator signal samples a substantially circular path in k-space.

3. The method as recited in claim 2 in which the fine navigator echo sequence produces three navigator echo signals which sample k-space in three orthogonal planes.

4. The method as recited in claim 1 in which the motion adjustment pulse sequence includes producing rf excitation pulses which produce transverse magnetization, and the rf excitation pulses are non-selective such that transverse magnetization is produced substantially uniformly throughout a field of view of the acquired image data.

5. The method as recited in claim 1 in which the imaging pulse sequence is an echo-planar imaging pulse sequence.

6. The method as recited in claim 1 in which the image data is acquired from the head of the patient and the image data is analyzed to provide an image of the patient's brain functions.

7. The method as recited in claim 1 in which the scan parameter which is adjusted is the frequency of an rf signal employed during the performance of the imaging pulse sequence.

8. The method as recited in claim 1 in which the scan parameter is the orientation of magnetic field gradients employed during the performance of the imaging pulse sequence.

9. The method as recited in claim 1 in which a series of image frames are reconstructed from the acquired image data.

10. The method as recited in claim 1 in which the navigator echo sequences employ an rf excitation pulse and said rf excitation pulse is substantially non-selective.

11. A method for performing a functional magnetic resonance imaging scan of a patient's brain with a magnetic resonance imaging (MRI) system, the steps comprising: a) acquiring a series of image frames over a period of time by performing a series of imaging pulse sequences with the MRI system to acquire image data from the patient's brain; b) interleaving motion adjustment pulse sequences with the series of imaging pulse sequences to acquire navigator echo signals for measuring the position of the patient's brain during the acquisition of said image frames, each motion adjustment pulse sequence includes the step of producing non- selective rf excitation pulses that produce transverse magnetization throughout the patient's brain; and c) adjusting a scan parameter on the MRI system during the performance of the scan using the acquired position measurements such that an MRI system imaging coordinate is moved in response to patient head movement.

12. The method as recited in claim 11 in which the motion adjustment pulse sequence includes: producing two, orthogonal readout magnetic field gradients during the readout of the navigator echo signal such that the navigator echo signal samples a substantially circular path in k-space.

13. The method as recited in claim 12 in which the motion adjustment pulse sequence produces three navigator echo signals which sample k-space in three orthogonal planes.

14. The method as recited in claim 11 in which the scan parameter is the frequency of an rf signal employed during the performance of the imaging pulse sequence.

15. The method as recited in claim 11 in which the scan parameter is the orientation of magnetic field gradients employed during the performance of the imaging pulse sequence.