Systems and methods for suppressing Nyquist ghosts in diffusion-weighted magnetic resonance imaging

Abstract

The present invention discloses a system and method for suppressing Nyquist ghosts in diffusion-weighted magnetic resonance imaging. An exemplary method includes acquiring a plurality of k-space data sets using a plurality of sets of diffusion-weighted imaging pulse sequences, reconstructing magnetic resonance images respectively from each of the plurality of k-space data sets, and averaging the magnitudes of the magnetic resonance images to generate a magnetic resonance image of an average magnitude.

Description

Technical Field

[0001] The present disclosure relates to magnetic resonance imaging (MRI), and more particularly, to diffusion weighted imaging (DWI). Background Art

[0002] Diffusion weighted magnetic resonance imaging (DW-MRI) has been developed and plays a role in the diagnosis and investigation of tissue function in various organs (e.g., brain, cartilage, and liver) and various applications (e.g., pathology, oncology). Diffusion refers to the random movement of molecules in a system. In biological tissues, the diffusion of water molecules forms a pattern according to the tissue structure and properties. In pathological conditions such as acute stroke, the diffusion pattern may be disrupted, and the amount of diffusion may vary in the affected area. Therefore, tissue abnormalities can be detected by studying the changes in diffusion. A specialized magnetic resonance imaging (MRI) technique called diffusion weighted imaging (DWI) utilizes the diffusion of water molecules to visualize internal physiology. The image contrast in DWI reflects the differences in diffusion rates between tissues. DWI is particularly useful when conventional MRI sequences (e.g., T2-weighted imaging) do not show significant changes in the images. For example, in a pathological condition such as a stroke caused by local ischemia, the signal intensity on T2 does not change until at least 8 hours after the onset of the stroke. On the other hand, DWI can show changes in the brain as early as 30 minutes after the onset of the stroke and show significant signal differences within one hour of the onset.

[0003] Generally speaking, spin echo is the selected sequence for DWI. For example, a DWI sequence can include a pulsed gradient spin echo (PGSE) section, which is added as a preparation phase in front of an echo planar imaging (EPI) sequence. EPI is an ultrafast MRI technique in which the entire image can be formed by magnetic resonance (MR) signals generated by a single radio frequency (RF) excitation. The EPI pulse sequence consists of a series of echoes, and in k-space, each line of k-space along a zigzag trajectory generates an echo. In a zigzag acquisition, two adjacent echoes are acquired in opposite directions. If the forward echo and the reverse echo are not perfect mirrors of each other, artifacts will be introduced into the image during reconstruction. For example, the delay starting from the first echo will propagate to all subsequent echoes, resulting in a timing difference between the peaks of odd and even echoes. When performing Fourier transform, this phase error will cause the signal intensity to shift in the phase encoding direction across half of the image, which is called Nyquist ghosting. If there are N pixels on the field of view (FOV), this aliased ghost appears shifted by N / 2 pixels relative to the main image located at the correct position. Nyquist ghosting may be caused by many possible reasons, such as eddy currents induced in the coil and the magnet housing in response to rapid changes in gradient pulses, poor shimming, gradient coil heating, patient movement, etc.

[0004] To address the Nyquist ghosting in EPI, Qing-San Xiang et al. introduced a method called "Phase Labeling for Additional Coordinate Encoding" (PLACE). (See "Correction for Geometric Distortion and N / 2 Ghosting in EPI by Phase Labeling for Additional Coordinate Encoding (PLACE)", Magnetic Resonance in Medicine, Vol. 57: pp. 731-741 (2007), Qing-San Xiang et al.). Two sets of k-space data are acquired using two slightly different EPI pulse sequences. Compared with another EPI pulse sequence, one EPI pulse sequence adds a small gradient region in the phase-encoding direction, causing the odd and even echoes to be swapped, and thus the Nyquist ghosting has opposite polarities. The average of the two sets of k-space data is taken, and an MR image is reconstructed from the averaged k-space data. Alternatively, MR composite images are reconstructed separately from the two sets of k-space data, and the average is taken to generate a final averaged MR image. The Nyquist ghosting is suppressed in the MR image. However, the average of the complex values used in the PLACE method is not applicable to DWI because when the diffusion gradient is applied, due to the phase difference between the two acquisitions, it will significantly reduce the signal amplitude. An improved method for suppressing Nyquist ghosting in DWI is generally needed. Summary of the Invention

[0005] In one embodiment, the present disclosure provides a method for suppressing Nyquist ghosting in diffusion-weighted magnetic resonance imaging. The method includes acquiring a plurality of k-space data sets using a plurality of diffusion-weighted imaging pulse sequences, reconstructing a magnetic resonance image from each of the plurality of k-space data sets, and taking an average of the amplitudes of the magnetic resonance images to generate an average-amplitude magnetic resonance image.

[0006] In another embodiment, the present disclosure provides an MRI system. The MRI system includes a gradient coil configured to generate an encoding gradient, a radio frequency (RF) coil configured to generate RF pulses, and a processor connected to the gradient coil and the RF coil. The processor is configured to instruct the gradient coil and the RF coil to generate a plurality of diffusion-weighted imaging pulse sequences to acquire a plurality of k-space data sets, reconstruct a magnetic resonance image from each of the plurality of k-space data sets, and take an average of the amplitudes of the magnetic resonance images to generate an average-amplitude magnetic resonance image.

[0007] In another embodiment, the present disclosure provides a method for suppressing Nyquist ghosting in diffusion-weighted magnetic resonance imaging. The method includes acquiring a first k-space data set using a first set of diffusion-weighted imaging pulse sequences, and acquiring a second k-space data set using a second set of diffusion-weighted imaging pulse sequences. The odd and even echoes of the second k-space data set are swapped relative to the first k-space data set. The method further includes reconstructing a first magnetic resonance image from the first k-space data set, reconstructing a second magnetic resonance image from the second k-space data set, and averaging the magnitudes of the first magnetic resonance image and the second magnetic resonance image to generate a magnetic resonance image of the average magnitude. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Aspects of the present disclosure may be better understood by reading the following detailed description and referring to the drawings, in which:

[0009] Figure 1 is a schematic diagram of a magnetic resonance imaging (MRI) system according to an exemplary embodiment;

[0010] Figure 2 is a schematic diagram of a pulse sequence of DWI with suppressed Nyquist ghosting according to an exemplary embodiment;

[0011] Figure 3A is according to PLACE according to Figure 2 of the pulse sequence and the k-space data acquisition trajectory of its combination;

[0012] Figure 3B is according to an exemplary embodiment from according to Figure 3A acquired k-space data and the MR image reconstructed from its combination;

[0013] Figure 4 is a flowchart of a method for suppressing Nyquist ghosting of DWI according to an exemplary embodiment; and

[0014] Figure 5 shows a comparison of a diffusion-weighted image obtained by the method disclosed herein with a diffusion-weighted image obtained by a conventional method according to an exemplary embodiment.

[0015] The drawings illustrate specific aspects of the described components, systems, and methods for suppressing Nyquist ghosting in DW-MRI. Together with the following description, the drawings illustrate and explain the structural principles, methods, and principles described herein. In the drawings, for clarity, the thickness and dimensions of the components may be enlarged or otherwise modified. Well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the described components, systems, and methods. DETAILED DESCRIPTION

[0016] The following describes one or more specific embodiments of the present disclosure to provide a thorough understanding. These described embodiments are merely examples of systems and methods for suppressing Nyquist ghosts in DW-MRI. Those skilled in the art will understand that specific details described in the embodiments can be modified during implementation without departing from the essence of the present disclosure.

[0017] When introducing elements of various embodiments of the present disclosure, the words "a", "an", and "the" are intended to mean that there is one or more of these elements. Terms such as "first", "second", etc. do not denote any order, quantity, or importance, but are used to distinguish one element from another. Terms such as "comprising", "including", and "having" are intended to be inclusive and mean that additional elements may exist in addition to the listed elements. As used herein, terms such as "connected to", "coupled to", etc., an object (e.g., material, element, structure, member, etc.) can be connected to or coupled to another object, regardless of whether the one object is directly connected or coupled to the other object, or whether there is one or more intervening objects between the one object and the other object. In addition, it should be understood that references to "one embodiment" or "an embodiment" of the present disclosure are not to be construed as excluding the existence of additional embodiments that also incorporate the recited features.

[0018] Generally referring to the accompanying drawings, the present disclosure describes systems and methods for suppressing Nyquist ghosts in DWI. Multiple sets of k-space data are acquired by using multiple sets of slightly different DWI pulse sequences. Each set of DWI pulse sequences includes a PGSE portion appended as a preparation phase in front of the EPI sequence. By slightly changing the gradient region in the phase-encoding direction, odd and even echoes are swapped between multiple sets of DWI sequences. That is, the positive echoes in one set of DWI sequences become negative echoes in another set, and vice versa. MR images are reconstructed separately from each of the multiple sets of k-space data. The amplitudes of the MR images are averaged to generate an average MR image. In this way, Nyquist ghosts are suppressed in the average MR image. Since the amplitude average is used, the signal intensity is not reduced, and thus a good signal-to-noise ratio (SNR) can be achieved.

[0019] Now refer to Figure 1, shows a schematic diagram of an exemplary MRI system 100 according to an exemplary embodiment. The operation of the MRI system 100 is controlled by an operator workstation 110, which includes an input device 114, a control panel 116, and a display 118. The input device 114 can be a joystick, keyboard, mouse, trackball, touch-activated screen, voice control, or any similar or equivalent input device. The control panel 116 can include a keyboard, touch-activated screen, voice control, buttons, sliders, or any similar or equivalent control device. The operator workstation 110 is coupled to and communicates with a computer system 120, which enables the operator to control the generation and viewing of images on the display 118. The computer system 120 includes a plurality of components that communicate with each other via electrical and / or data connections 122. The computer system connection 122 can be a direct wired connection, fiber optic connection, wireless communication link, etc. The computer system 120 can include a central processing unit (CPU) 124, a memory 126, and an image processor 128. In some embodiments, the image processor 128 can be replaced by image processing functions implemented in the CPU 124. The computer system 120 can be connected to an archival media device, permanent or backup memory, or a network. The computer system 120 is coupled to and communicates with a separate MRI system controller 130.

[0020] The MRI system controller 130 includes a set of components that communicate with each other via electrical and / or data connections 132. The MRI system controller connection 132 can be a direct wired connection, fiber optic connection, wireless communication link, etc. The MRI system controller 130 can include a CPU 131, a pulse generator / sequencer 133 that communicates with the operator workstation 110, a transceiver (or RF transceiver) 135, a memory 137, and an array processor 139. In some embodiments, the pulse generator / sequencer 133 can be integrated into the resonance assembly 140 of the MRI system 100. The MRI system controller 130 can receive commands from the operator workstation 110 to indicate the MRI scan sequence to be performed during an MRI scan. The MRI system controller 130 is also coupled to and communicates with a gradient driver system 150, which is coupled to a gradient coil assembly 142 to generate magnetic field gradients during an MRI scan.

[0021] The pulse generator / sequencer 133 may also receive data from the physiological acquisition controller 155, which receives signals from a plurality of different sensors (such as electrocardiogram (ECG) signals from electrodes attached to a patient), and these sensors are connected to an object or patient 170 undergoing an MRI scan. And finally, the pulse generator / sequencer 133 is coupled to and communicates with the scan room interface system 145, which receives signals from various sensors associated with the state of the resonance assembly 140. The scan room interface system 145 is also coupled to and communicates with the patient positioning system 147, which sends and receives signals to control the movement of the patient table to the desired position for an MRI scan.

[0022] The MRI system controller 130 provides gradient waveforms to the gradient driver system 150, which includes G X 、G Y and G Z amplifiers, etc. Each G X 、G Y and G Z gradient amplifier excites the corresponding gradient coil in the gradient coil assembly 142 to generate a magnetic field gradient for spatially encoding MR signals during an MRI scan. The gradient coil assembly 142 is included within the resonance assembly 140, which also includes a superconducting magnet having a superconducting coil 144 that provides a static uniform longitudinal magnetic field B 0 through the cylindrical imaging volume 146 during operation, and the cylindrical imaging volume is surrounded by the resonance assembly 140. The resonance assembly 140 also includes an RF body coil 148 that provides a transverse magnetic field B 1 during operation, and this transverse magnetic field B_1 is substantially perpendicular to B 0 throughout the open cylindrical imaging volume 146. The resonance assembly 140 may also include an RF surface coil 149 for imaging different anatomical structures of a patient undergoing an MRI scan. The RF body coil 148 and the RF surface coil 149 can be configured to operate in a transmit and receive mode, a transmit mode, or a receive mode.

[0023] The object or patient 170 undergoing an MRI scan can be positioned within the cylindrical imaging volume 146 of the resonance assembly 140. The transceiver 135 in the MRI system controller 130 generates RF excitation pulses amplified by the RF amplifier 162 and provides them to the RF body coil 148 through the transmit / receive switch (T / R switch) 164.

[0024] As described above, the RF body coil 148 and the RF surface coil 149 can be used to transmit RF excitation pulses and / or receive the resulting MR signals from a patient undergoing an MRI scan. The resulting MR signals emitted by the excited nuclei within the patient undergoing the MRI scan can be sensed and received by the RF body coil 148 or the RF surface coil 149 and sent back to the preamplifier 166 through the T / R switch 164. The T / R switch 164 can be controlled by a signal from the pulse generator / sequencer 133 to electrically connect the RF amplifier 162 to the RF body coil 148 during the transmit mode and connect the preamplifier 166 to the RF body coil 148 during the receive mode. The T / R switch 164 can also enable the RF surface coil 149 to be used in the transmit mode or the receive mode.

[0025] In some embodiments, the resulting MR signals sensed and received by the RF body coil 148 or the RF surface coil 149 and amplified by the preamplifier 166 are stored in the memory 137 as a raw k-space data array for post-processing. The MR scan is complete when the raw k-space data array corresponding to the received MR signals has been acquired and temporarily stored in the memory 137 until the data is subsequently transformed to create an image.

[0026] In some embodiments, the MR signals sensed and received by the RF body coil 148 or the RF surface coil 149 and amplified by the preamplifier 166 are demodulated, filtered, and digitized in the receive portion of the transceiver 135 and transmitted to the memory 137 in the MRI system controller 130. For each image to be reconstructed, the data is rearranged into a separate k-space data array, and each of these separate k-space data arrays is input to the array processor 139, which operates to Fourier transform the data into an array of image data.

[0027] The array processor 139 uses a transform method, most commonly the Fourier transform, to create an image from the received MR signals. These images are transferred to the computer system 120, where they are stored in the memory 126. In response to commands received from the operator workstation 110, the image data can be archived in long-term memory or can be further processed by the image processor 128 and transferred to the operator workstation 110 for presentation on the display 118.

[0028] In various embodiments, the components of the computer system 120 and the MRI system controller 130 can be implemented on the same computer system or on multiple computer systems. It should be understood that Figure 1 the illustrated MRI system 100 is for illustration. A suitable MRI system can include more, fewer, and / or different components.

[0029] ReferenceFigure 2 , which shows a schematic diagram of a pulse sequence for suppressing Nyquist ghosts in DWI according to an exemplary embodiment. Each set of DWI pulse sequences includes a PGSE section 202 added as a preparation phase in front of the EPI sequence 204. The PGSE section 202 consists of a 90° RF pulse 212 (applied with a slice selection (SS) gradient 222) and a 180° RF pulse 214 (applied with an SS gradient 224), where two diffusion gradients 232 and 234 are placed on either side of the 180° RF pulse 214. The RF pulses 212 and 214 can be generated by a transmit coil (e.g., Figure 1 the RF body coil 148 in Figure 1 ). The gradients 222, 224, 232, and 234 can be generated by gradient coils (e.g.,

[0030] the gradient coil assembly 142 in

[0031] S(b) = S 0 e -bD (1),

[0032] where S(b) is the signal received with a specific diffusion gradient pair, and S 0 is the signal intensity without any diffusion gradients, D is the diffusion or apparent diffusion coefficient (ADC), and b is the diffusion weighting degree for a specific diffusion gradient pair. The value of b can be controlled by manipulating the intensity, duration, and spacing of the diffusion gradient pair 232 and 234. Specifically, the value of b is given by:

[0033]

[0034] Where γ is the gyromagnetic ratio of the hydrogen proton, a constant, G is the amplitude of the diffusion gradient, δ is the duration of the diffusion gradient, and Δ is the time between the application of two diffusion gradients. In clinical applications, typically the sequence is arranged to provide multiple images with a range of diffusion directions and b-values, and sometimes an ADC map can be calculated. For example, several DW images with different b-values can be obtained by changing the configuration of the diffusion gradient pairs. At higher b-values, the effect of diffusion is more pronounced in the image and in tissues with high diffusion, as shown by the low-signal regions in the image, while tissues with restricted diffusion are shown as high-signal regions.

[0035] Immediately following the second diffusion gradient 234, k-space data acquisition is performed. This is a typical echo-planar sequence 204 that uses fast oscillating phase encoding (PE) and frequency encoding (FE) gradients that generate multiple gradient echoes. The PE and FE gradients can be generated by a gradient coil (e.g., Figure 1 the gradient coil assembly 142 in Figure 2 . The fast image acquisition can minimize the effect of overall motion on the DW image. As Figure 2 shown, a spiked EPI sequence is used for data acquisition. In spiked EPI, after the first large PE gradient 240, there are multiple small PE gradient "spikes" at each location where the FE gradient is reversed. For example, spike 241 is placed at the start of the negative FE (or readout) gradient 251; spike 242 is placed at the reversal of the negative readout gradient 251 to the positive readout gradient 252, spike 243 is placed at the reversal of the positive readout gradient 252 to the negative readout gradient 253, and so on. The spikes have a constant size, and each adds further phase encoding to the previous spike.

[0036] Refer to Figure 3A and, according to Figure 2 's pulse sequence shows a schematic diagram of the k-space data acquisition trajectory. In 312, k-space data is first acquired along the lowest line in the presence of the FE gradient 251. When spike 242 is applied, data is acquired along the second lowest line in the presence of the FE gradient 252, and so on. Each spike adds a constant phase encoding to the previous spike, which results in a regular path through k-space. The amplitudes of the FE gradients 251, 252, 253, 254, 255, and 256 are typically large such that appropriate values can be sampled quickly, and the entire data set can be collected within a single free induction decay (FID). Because the PGSE preparation phase requires a reasonable diffusion time, the echo time (TE) value of the DWI pulse sequence can be quite high, typically greater than 100 ms.

[0037] Refer back to Figure 2, as described above, multiple k-space data sets are acquired by using multiple sets of slightly different DWI pulse sequences. In the second set of DWI pulse sequences, the small PE gradient region is subtracted from the large PE gradient 240, as shown by the dashed line. The subtracted region is equal to a single spike of one PE step. Thus, the k-space data acquisition trajectory corresponding to this second DWI pulse sequence becomes Figure 3A 314 in

[0038] Although Figure 2 shows two DWI pulse sequences, it should be understood that more than two DWI pulse sequences can be applied. For example, a third DWI pulse sequence can add the small PE gradient region to the large PE gradient 240. The added area is equal to a single spike of one PE step, such that the k-space data acquisition trajectory moves down one PE step. Any suitable number of k-space data sets can be acquired based on applications such as 2, 3, 4, etc.

[0039] After acquiring multiple k-space data sets according to DWI pulse sequences (e.g., Figure 2 those shown), MR images are reconstructed from each k-space data set separately. In some embodiments, the reconstruction includes a Fourier transform from k-space to image space, as is known in the art. Then the magnitudes of the multiple MR images are averaged to generate an average MR image:

[0040]

[0041] where n is the number of multiple acquisitions, I 1 to I n are the MR magnitudes of each acquisition respectively, and I is the average MR magnitude. Figure 3A 316 in

[0042] On the other hand, Figure 3B 326 in schematically shows the combination of 322 and 324 in the actual space (or image space) according to the method described herein. Although the combination of two k-space data acquisitions is used for illustration herein, it should be understood that the combination is applicable to more than two acquisitions. In each acquisition, Nyquist ghosts appear in the signal region (e.g., region 332 in the circle) and outside the signal region (i.e., region 334 outside the circle). For example, in the first acquisition, the ghost signal may have a polarity opposite to that of the MR signal in the signal region (i.e., region 332), and thus the total amplitude in region 332 is |MR + ghost|. Outside the signal region (i.e., region 334), the amplitude in region 334 is the ghost. In the second acquisition, the odd and even echoes are exchanged relative to the first acquisition. Thus, the ghost signal now has the same polarity as the MR signal in the signal region (i.e., region 332), and thus the total amplitude in region 332 is |MR + ghost|. The amplitude outside the signal region (i.e., region 334) remains the ghost for the second acquisition.

[0043] Generally speaking, the amplitude of the MR signal is greater than the amplitude of the ghost signal (i.e., MR > ghost). Similarly, when the amplitudes of the first and second acquisitions are averaged, the ghost artifact is canceled in the signal region (i.e., region 332), and only the MR signal remains. Compared with the MR signal, the ghost artifact outside the signal region (i.e., region 334) may have a small amplitude and can be removed by adjusting the window center during post-processing.

[0044] Referring to Figure 4 , a flowchart 400 of a method for suppressing Nyquist ghosts in DWI according to an exemplary embodiment is shown. The method can be performed by an MRI system (e.g., Figure 1 the MRI system 100 in ). At operation 402, multiple k-space data sets are acquired using multiple sets of slightly different DWI pulse sequences. The DWI sequence can be associated with Figure 2is similar to or identical with the DWI sequence shown. Each group of DWI pulse sequences includes a PGSE part and an EPI sequence following the PGSE part. The PGSE part includes a pair of diffusion gradients symmetric to the 180° RF pulse, a phase shift, and a rephasing gradient in the exact opposite direction. The first diffusion gradient introduces a phase shift to the protons according to their positions, while the second diffusion gradient reverses the change made by the first diffusion gradient. The phase of the fixed spins is not affected by the pair of diffusion gradients because any phase accumulation from the first diffusion gradient will be reversed by the second diffusion gradient. If the protons move, the second diffusion gradient will not be able to completely cancel the phase shift caused by the first diffusion gradient, and there will be signal attenuation. The degree of diffusion weighting can be controlled by manipulating the configuration of the pair of diffusion gradients (e.g., the strength, duration, and spacing of the pair of diffusion gradients).

[0045] The readout sequence following the PGSE part may include a spiky EPI sequence. In spiky EPI, after a large PE gradient, there are multiple small PE gradient "spikes" at each position where the FE gradient is reversed. The spikes have a constant size, and each adds further phase encoding to the previous spike, which results in a regular path in k-space. According to the k-space data acquisition, the even echoes and the odd echoes are in opposite directions. For example, if the odd echoes (or lines) are forward, the even echoes (or lines) are backward, and vice versa.

[0046] Between multiple acquisitions, the odd and even echoes in one group are swapped relative to the other group. Taking two sets of k-space data acquisitions as an example. The forward echoes (or lines) in the first acquisition become backward echoes (or lines) in the second acquisition, while the backward echoes (or lines) in the first acquisition become forward echoes (or lines) in the second acquisition.

[0047] At operation 404, after acquiring multiple sets of k-space data according to multiple groups of DWI pulse sequences, MR images are reconstructed from each k-space dataset respectively. In some embodiments, the reconstruction includes a Fourier transform from k-space to image space, as is known in the art.

[0048] At operation 406, the amplitudes of multiple MR images are averaged to generate an average amplitude MR image. The ghosting artifacts in the signal region are canceled by averaging the amplitudes of multiple acquisitions. Compared with the signal, the ghosting artifacts outside the signal region may have a small amplitude and can be removed, for example, by adjusting the window center during post-processing.

[0049] Reference Figure 5, according to an exemplary embodiment, the diffusion weighted images obtained by the methods disclosed herein are compared with the diffusion weighted images obtained by conventional methods. Using the conventional method, an image 512 is acquired on the dotted line, where a single set of DWI pulse sequences is used. The Nyquist ghost 513 is clearly visible in the image. As a comparison, an image 514 is acquired on the same dotted line using the average magnitude method described herein. It can be seen that the Nyquist ghost is substantially suppressed. Image 522 is an axial brain image of a volunteer acquired using the conventional method, where a single set of DWI pulse sequences is used. The Nyquist ghost 523 is clearly visible in the image. As a comparison, image 524 is an axial brain image of the same volunteer acquired using the average magnitude method described herein. It can be seen that the Nyquist ghost is substantially suppressed.

[0050] In addition to any previously indicated modifications, those skilled in the art can design many other variations and alternative arrangements without departing from the substance and scope of this description, and the appended claims are intended to cover such modifications and arrangements. Thus, although the information has been specifically and detailedly described above in connection with the currently considered most practical and preferred aspects, it will be apparent to those of ordinary skill in the art that many modifications can be made without departing from the principles and concepts set forth herein, including but not limited to form, function, mode of operation, and use. Similarly, as used herein, in all respects, the examples and embodiments are intended to be illustrative only and should not be construed in any way as restrictive.

Claims

1. A method for suppressing Nyquist ghosting in diffusion - weighted magnetic resonance imaging, the method comprises: acquiring a first k - space data set using a first set of diffusion - weighted imaging pulse sequences; acquiring a second k - space data set using a second set of diffusion - weighted imaging pulse sequences, wherein the odd and even echoes of the second k - space data set are swapped relative to the first k - space data set, and each of the first set of diffusion - weighted imaging pulse sequences and the second set of diffusion - weighted imaging pulse sequences includes an echo - planar imaging sequence and a pulsed - gradient spin - echo part appended in front of the echo - planar imaging sequence, the pulsed - gradient spin - echo part includes a first diffusion gradient and a second diffusion gradient, the first diffusion gradient introduces a phase shift, and the second diffusion gradient reverses the phase - shift change generated by the first diffusion gradient; reconstructing a first magnetic resonance image from the first k - space data set; reconstructing a second magnetic resonance image from the second k - space data set; and taking an average of the amplitudes of the first magnetic resonance image and the second magnetic resonance image to generate a magnetic resonance image with an average amplitude.

2. The method according to claim 1, wherein, the echo - planar imaging sequence comprises a spike echo - planar imaging sequence, which has phase - encoding gradient spikes at each frequency where the encoding gradient is reversed.

3. The method according to claim 2, wherein, the method comprises: acquiring the first k - space data set using the first set of diffusion - weighted imaging pulse sequences including a first spike echo - planar imaging sequence; and acquiring the second k - space data set using the second set of diffusion - weighted imaging pulse sequences including a second spike echo - planar imaging sequence; wherein the odd and even echoes of the second spike echo - planar imaging sequence are swapped relative to the first spike echo - planar imaging sequence.

4. The method according to claim 3, wherein, the method further comprises: acquiring a third k - space data set using a third set of diffusion - weighted imaging pulse sequences including a third spike echo - planar imaging sequence; and acquiring a fourth k - space data set using a fourth set of diffusion - weighted imaging pulse sequences including a fourth spike echo - planar imaging sequence; wherein the odd and even echoes of the third spike echo - planar imaging sequence are swapped relative to the second spike echo - planar imaging sequence, and the odd and even echoes of the fourth spike echo - planar imaging sequence are swapped relative to the third spike echo - planar imaging sequence.

5. A magnetic resonance imaging system, comprises: gradient coils configured to generate encoding gradients; radio - frequency coils configured to generate radio - frequency pulses; and a processor connected to the gradient coils and the radio - frequency coils, the processor being configured to: instruct the gradient coils and the radio - frequency coils to generate multiple sets of diffusion - weighted imaging pulse sequences to acquire multiple k - space data sets; reconstruct magnetic resonance images from each of the multiple k - space data sets respectively; and take an average of the amplitudes of the magnetic resonance images to generate a magnetic resonance image with an average amplitude; wherein, acquiring the multiple k - space data sets includes: Acquire a first k-space data set using a first set of diffusion-weighted imaging pulse sequences; and Acquire a second k-space data set using a second set of diffusion-weighted imaging pulse sequences; wherein odd and even echoes in the second k-space data set are swapped relative to the first k-space data set, and each of the multiple sets of diffusion-weighted imaging pulse sequences includes an echo planar imaging sequence and a pulsed gradient spin echo portion appended to the front of the echo planar imaging sequence, the pulsed gradient spin echo portion including a first diffusion gradient and a second diffusion gradient, the first diffusion gradient introducing a phase shift and the second diffusion gradient reversing the phase shift change produced by the first diffusion gradient.

6. The magnetic resonance imaging system according to claim 5, wherein, the echo planar imaging sequence comprises a spiked echo planar imaging sequence having phase encoding gradient spikes at each frequency where the encoding gradient is reversed.

7. The magnetic resonance imaging system according to claim 6, wherein, the acquisition of multiple k-space data sets includes: acquire a first k-space data set using a first set of diffusion-weighted imaging pulse sequences including a first spiked echo planar imaging sequence; and acquire a second k-space data set using a second set of diffusion-weighted imaging pulse sequences including a second spiked echo planar imaging sequence; wherein odd and even echoes of the second spiked echo planar imaging sequence are swapped relative to the first spiked echo planar imaging sequence.

8. The magnetic resonance imaging system according to claim 7, wherein, the acquisition of multiple k-space data sets further includes: acquire a third k-space data set using a third set of diffusion-weighted imaging pulse sequences including a third spiked echo planar imaging sequence; and acquire a fourth k-space data set using a fourth set of diffusion-weighted imaging pulse sequences including a fourth spiked echo planar imaging sequence; wherein odd and even echoes of the third spiked echo planar imaging sequence are swapped relative to the second spiked echo planar imaging sequence, and odd and even echoes of the fourth spiked echo planar imaging sequence are swapped relative to the third spiked echo planar imaging sequence.

Images