Patent Publication Number: US-2002002331-A1

Title: Mr imaging with partial k-space acquisition using spiral scanning

Description:
BACKGROUND OF THE INVENTION  
       [0001] This invention relates to nuclear magnetic resonance imaging methods and systems and, more particularly, to acquisition of images using spiral scanning methods.  
       [0002] When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B 0 ), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field along a longitudinal z axis, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B 1 ) 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 t . A nuclear magnetic resonance (NMR) signal is emitted by the excited spins after the excitation signal B 1  is terminated, and may be received and processed to form an image.  
       [0003] When utilizing NMR signals to produce images, magnetic field gradients (G x  G y  and G z ) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used to sample a two or three dimensional region of k-space. The resulting set of received k-space signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.  
       [0004] Most magnetic resonance (MR) scans used to produce medical images require many minutes to acquire the necessary k-space data. Reducing this scan time is an important objective, since a shortened scan increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. Reduction of scan time is particularly important in cardiac imaging, for example, where it is highly desirable to acquire sufficient NMR data to reconstruct an image in a single breath hold.  
       [0005] Many different pulse sequences are known in the art for acquiring NMR signals from which an image may be reconstructed. Most of these pulse sequences sample k-space in a rectilinear pattern, but there is a class of pulse sequences which sample k-space in a spiral pattern. It is known that a spiral sampling pattern can be achieved by applying a sinusoidally varying readout magnetic field gradient during acquisition of each NMR signal and that spiral scanning methods can be used to rapidly acquire NMR data from which an image may be reconstructed. A spiral scanning method is also known wherein the sinusoidal readout gradient is shaped to more rapidly traverse the spiral sampling trajectory and, therefore, more rapidly sample k-space data. Scan time has been further reduced in the past by acquiring samples from little more than only one-half of k-space using interleaved spiral sampling trajectories. The missing k-space data are produced from a Hermitian approximation using the complex conjugate of the acquired k-space data or as described by D. C. Noll, et al., “Homodyne Detection in Magnetic Resonance Imaging”  IEEE Transactions on Medical Imaging,  vol. 10, No. 2, June 1991.  
       SUMMARY OF THE INVENTION  
       [0006] NMR image data, from which an image can be reconstructed, are rapidly acquired in a magnetic resonance imaging (MRI) system in which a pulse sequence is performed to acquire NMR data that sample k-space in a trajectory comprised of a spiral segment that extends from the center of k-space to the periphery of k-space, and a symmetric spiral tail segment that extends from the center of k-space to sample only a central region of k-space. The pulse sequence may be repeated to sample along a plurality of interleaved trajectories such that the central region of k-space is substantially completely sampled and the periphery of k-space is only partially sampled. An image is produced from the resulting incomplete k-space data set using a homodyne reconstruction method. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0007]FIG. 1 is a block diagram of an MRI system employing the invention;  
     [0008]FIG. 2 is a graphic representation of a preferred pulse sequence for practicing the invention; and  
     [0009]FIG. 3 is a graphic representation of the k-space sampling pattern performed by the pulse sequence of FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0010]FIG. 1 illustrates the major components of an MRI system that incorporates the invention. Operation of the system is controlled from an operator console  100  which includes a keyboard and control panel  102  and a display  104 . Console  100  communicates through a link  116  with a separate computer system  107  that enables an operator to control the production and display of images on a screen of display  104 . Computer system  107  includes a number of modules which communicate with each other through a backplane  105 . These include an image processor module  106 , a CPU module  108 , and a memory module  113  which is known in the art as a frame buffer for storing image data arrays. Computer system  107  is linked to a disk storage  111  and a tape drive  112  for storage of image data and programs, and communicates with a separate system control  122  through a high speed parallel link  115 .  
     [0011] System control  122  includes a set of modules coupled together by a backplane  118 . These include a CPU module  119  and a pulse generator module  121  which is coupled to operator console  100  through a serial link  125 . System control  122  receives commands from the system operator through link  125  which indicate the scan sequence to be performed. Pulse generator module  121  operates the system components to carry out the desired scan sequence, producing data that indicate the timing, strength and shape of the RF pulses to be produced, and the timing of and length of the data acquisition window. Pulse generator module  121  is coupled to a set of gradient amplifiers  127  to control the timing and shape of the gradient pulses to be produced during the scan. Pulse generator module  121  also receives patient data from a physiological acquisition controller  129  that receives signals from sensors attached to the patient, such as ECG (electrocardiogram) signals from electrodes or respiratory signals from a bellows. Pulse generator module  121  is also coupled to a scan room interface circuit  133  which receives signals from various sensors associated with the condition of the patient and the magnet system. A patient positioning system  134  receives commands through the scan room interface circuit  133  to move the patient to the desired position for the scan.  
     [0012] The gradient waveforms produced by pulse generator module  121  are applied to gradient amplifier system  127  comprised of G x , G y  and G z  amplifiers. Each gradient amplifier excites a corresponding gradient coil in a gradient coil assembly  139  to produce the magnetic field gradients used for position encoding acquired signals. Gradient coil assembly  139  forms part of a magnet assembly  141  which includes a polarizing magnet  140  and a whole-body RF (radio frequency) coil  152 . Gradient amplifiers  127  are limited in amplitude of peak current they can provide and in the rate at which they can change current in gradient coils  139 . As a result, the gradient field amplitude is limited, as is its slew rate.  
     [0013] A transceiver module  150  in system control  122  produces pulses which are amplified by an RF amplifier  151  and coupled to RF coil  152  by a transmit/receive switch  154 . The resulting signals radiated by the excited nuclei in the patient may be sensed by the same RF coil  152  and coupled through transmit/receive switch  154  to a preamplifier  153 . The amplified NMR signals are demodulated, filtered, and digitized in the receiver section of transceiver  150 . Transmit/receive switch  154  is controlled by a signal from pulse generator module  121  to electrically couple RF amplifier  151  to coil  152  for the transmit mode and to preamplifier  153  for the receive mode. Transmit/receive switch  154  also enables a separate RF coil (for example, a head coil or surface coil, not shown) to be used in either the transmit or receive mode.  
     [0014] The NMR signals picked up by RF coil  152  are digitized by transceiver module  150  and transferred to a memory module  160  in system control  122 . The receiver in transceiver module  150  preserves the phase of the acquired NMR signals in addition to signal magnitude. The down converted NMR signal is applied to an analog-to-digital (A/D) converter (not shown) which samples and digitizes the analog NMR signal. The samples are applied to a digital detector and signal processor (not shown) which produces 16-bit in-phase (I) values and 16-bit quadrature (Q) values corresponding to the received NMR signal. The resulting stream of digitized I and Q values of the received NMR signal are supplied through backplane  118  to memory module  160  where they are employed to reconstruct an image. For a more detailed description of the receiver, reference is made to Stormont et al. U.S. Pat. No. 4,992,736, issued Feb. 12, 1991, assigned to the instant assignee, and which is incorporated herein by reference.  
     [0015] When the scan is completed and an entire array of data has been acquired in memory module  160 , an array processor  161  operates to grid the data into an array when necessary, and Fourier transform the data into an array of image data which is conveyed through link  115  to computer system  107  where the data are stored in disk memory  111 . In response to commands received from operator console  100 , these image data may be archived on tape drive  112 , or may be further processed by image processor  106  and conveyed to operator console  100  for presentation on display  104 .  
     [0016] The MRI system of FIG. 1 is employed to acquire NMR data using the pulse sequence of FIG. 2. As explained above, this pulse sequence is performed under the direction of pulse generator module  121  which directs the system components to produce the indicated RF pulses and gradient waveforms.  
     [0017] As shown in FIG. 2, the preferred pulse sequence is a two-dimensional, gradient recalled echo pulse sequence including a selective RF excitation pulse  201  that is produced in the presence of a G z  slice select gradient pulse  203  to produce transverse magnetization in a selected slice of spins in the subject to be imaged.  
     [0018] A readout gradient dephasing pulse  207  is produced following the transverse excitation and is immediately followed by sinusoidal readout gradient waveforms  209  and  211  that produce time varying magnetic field gradients along the respective G x  and G y  gradient axes. An NMR echo signal peak is produced at an echo time TE, and the NMR echo signal is acquired during the interval indicated by dashed lines  213  and  215 .  
     [0019] Each of the time varying readout gradient waveforms  209  and  211  is comprised of two waveform segments: a spiral waveform segment  217  that is played out during the data acquisition window after echo time TE; and a symmetric spiral tail waveform segment  219  that is played out during the data acquisition window prior to the echo time TE. The spiral waveform segment  217  is derived from the k-space Archemedian spiral of the form:  
       k   x ( t )= a ( t ) cos [ a ( t )] 
       k   y ( t )= a ( t ) sin [ a ( t )]  (1)  
     [0020] The readout gradient waveform amplitude is related to the velocity at which the k-space spiral sampling trajectory is sampled by the following equations:  
       G   x ( t )=( dk   x ( t )/ dt )/γ 
       G   y ( t )=( dk   y ( t )/ dt )/γ.  (2)  
     [0021] The readout gradient waveform slew rate is related to the acceleration at which the k-space spiral sampling trajectory is sampled by the following equations:  
       dG   x ( t )/ dt =( d   2   k   x ( t )/ dt   2 )/γ 
       dG   y ( t )/ dt =( d   2   k   y ( t ) dt   2 )/γ  (3)  
     [0022] Given the amplitude and slew rate limitations of the gradient system hardware, the function a(t) in equation (1) is determined numerically by solving the differential equation that relates the maximum gradient slew rate and maximum gradient amplitude to the velocity and acceleration of k-space sampling.  
     [0023] One technique for estimating a low frequency field map is to use two different TE times. Another technique samples the symmetric spiral tail waveform  219 , which employs the same function a(t) and samples a spiral trajectory which is the k-space complement of the sampling trajectory of spiral waveform segments  217 . This symmetric spiral tail segment has the form:  
       k   x ( t )= a ( t−t ′) cos [ a ( t−t′ )] 
       k   y ( t )= a ( t−t′ ) sin [ a ( t−t′ )],  (4)  
     [0024] where t′ is a constant that determines the size of central k-space that is sampled by the symmetric spiral tail segment  219 .  
     [0025] The k-space sampling trajectory performed by the pulse sequence of FIG. 2 is shown in FIG. 3 to include a symmetric spiral tail segment indicated by dashed line  225  and a spiral segment indicated by solid line  227 . As the time varying readout gradients  209  and  211  (FIG. 2) are played out during the data acquisition period, sampling begins at a k-space location  229  and spirals inward along a trajectory  225 , reaching the center of k-space at the echo time TE. Spiral waveform segment  217  is then played out and sampling spirals outward along trajectory  227  toward the periphery of k-space. The sampling is completed at the periphery of k-space at location  231 . Rephasing readout gradient pulses  233  and  235  (FIG. 2) are applied after the data acquisition window to prepare the transverse magnetization for the next repetition of the pulse sequence. RF spoiling is employed to null transverse magnetization prior to execution of the next pulse sequence.  
     [0026] While it is possible to acquire an image in a single spiral pulse sequence, it is much more common to perform a plurality of spiral pulse sequences in which the spiral sampling trajectories are interleaved to uniformly sample k-space. If fewer spiral trajectories or “arms” are sampled, then each spiral trajectory must encircle, or “wrap” around the center of k-space more times to adequately sample k-space. Table A lists a number of spiral interleave combinations which produce good reconstructed images.  
                                   TABLE A                                       Total   Tail       Spiral       Total   Tail   Readout   Readout       Arms   Wraps   Samples   Samples   Time (μs)   Time (μs)                                                        21   3   316   30   1.264   .12       15   4   463   38   1.852   .152       11   5   610   46   2.44   .184       9   6   757   52   3.028   .208       7   7   905   60   3.62   .24       7   8   1053   68   4.212   .27       5   9   1348   82   5.392   .328                  
 
     [0027] From FIG. 3, it should be apparent that because the symmetric tail segment  225  only samples the central region of k-space, this central region is sampled with twice the density as the surrounding k-space peripheral region. In a preferred embodiment the central region is sampled to provide the desired image resolution and SNR (signal-to-noise ratio) and the peripheral region is thus under-sampled. The missing peripheral k-space samples are produced by calculating the complex conjugate of the k-space data acquired by the outer portion of the spiral segments  227 . For example, if a signal sample  
     
       S=I+Q  
     
     [0028] is acquired at a point  240  on the spiral trajectory  227 , its complex conjugate signal  
     
       S*=I−Q  
     
     [0029] fills in for the missing peripheral k-space data at point  242 . A complete k-space data set is thus formed and used to reconstruct an image using the homodyne method described in the above-cited Noll et al publication. It has been found that the increased sampling of the center of k-space provided by the symmetric tail segments can be used to eliminate phase errors that are introduced at low spatial frequencies near the origin of k-space. MRI systems typically have slow variations in the magnetic fields which they produce, and these variations may cause image artifacts when complex symmetry near the origin of k-space is used in image reconstruction. These variations are confined to low spatial frequencies and the symmetric tail segments need not extend far from the center of k-space.  
     [0030] Use of the present invention reduces the spiral scan time by nearly 50%. A 50% reduction in scan time could be achieved by simply sampling one-half of k-space using spiral sampling trajectory  227  alone. However, unacceptable image artifacts may be produced. By adding the symmetric spiral tail trajectory  225  to the pulse sequence, such image artifacts are eliminated or substantially reduced with an increase in scan time of less than 10%, as shown in Table A.  
     [0031] Many variations are possible from the preferred embodiments described above. For example, a three-dimensional image may be acquired by adding phase encoding in the slice selected direction as indicated in FIG. 2 by dashed lines  250 . For each separate G z  phase encoding value k x , k y  space is sampled using one or more spiral trajectories as described above. The process is repeated for each G z  phase encoding value (e.g. 16 values) until a 3D k-space data set is acquired.  
     [0032] The invention may also be used with other spin echo pulse sequences. Other RF excitation methods for producing transverse magnetization may also be used. Such methods include spectral-spatial excitation. Also, one or more gradient axis may be flow compensated by the addition of gradient moment nulling pulses as described in Glover et al. U.S. Pat. No. 4,731,583, issued Mar. 15, 1988 and assigned to the instant assignee.  
     [0033] While only certain preferred features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.