Magnetic resonance imaging method and apparatus

Abstract

A method for generating a magnetic resonance imaging (MRI) image of an object includes: applying a magnetic field B0 to the object; applying a sequence of electromagnetic pulses to the object; applying a magnetic field gradient other than the magnetic field B0, the magnetic field gradient including a plurality of first gradients and at least one second gradient; measuring signal echoes generated by the object in response to the plurality of electromagnetic pulses, the first magnetic field gradients, and the second magnetic field gradients; acquiring image data of a first spatial resolution from the signal echoes generated by the object in response to the electromagnetic pulses, the first magnetic field gradients, and the second magnetic field gradients; and combining the image data acquired from the signal echoes to generate at least one image of the object at a second spatial resolution higher than the first spatial resolution. The first gradients are completely revolved during the intervals between consecutive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral between consecutive electromagnetic pulses. The phase of at least one electromagnetic pulse in the electromagnetic pulse sequence differs from the phase of another electromagnetic pulse in the same sequence.

Description

Technical Field

[0001] This invention relates to magnetic resonance imaging (MRI), and more particularly to methods and apparatus that can provide high-resolution images. Background Technology

[0002] Magnetic resonance imaging (MRI), sometimes also called nuclear magnetic resonance imaging, involves placing the object to be scanned in a static magnetic field during an MRI scan. This field magnetizes the nuclei within the object, with the net magnetization aligned parallel to the magnetic field. Typically, the protons of hydrogen atoms are of interest, although MRI can be used to study nuclei of atoms other than hydrogen. The object is then exposed to an electromagnetic pulse, usually a radio frequency pulse, generated by an emitter. The frequency and duration of the pulse are chosen to perturb the net magnetization vector of the protons, specifically tilting it perpendicular to the static magnetic field. As the protons relax back to a lower energy state, their precession produces a magnetic flux that can be detected by a receiver. Based on this magnetic flux, the pixels that make up the image of the scanned object can be determined.

[0003] An additional magnetic field is typically applied to provide a gradient throughout the field. This causes protons at different locations within the field to precess at different rates, allowing for the selection and imaging of specific volumes or slices of an object.

[0004] Therefore, MRI is a non-invasive imaging modality that does not rely on ionizing radiation and is commonly used in healthcare to generate images of human tissues.

[0005] The signal generated by protons during an MRI scan is due to two relaxation processes. First, the component of the net magnetization vector of the protons, parallel to the static magnetic field applied to the object, increases. The rate at which this occurs is controlled by a constant T1. Second, in a process controlled by a constant T2, the component of the net magnetization vector of the protons, perpendicular to the static magnetic field applied to the object, decreases. The values ​​of T1 and T2 depend on the properties of the material constituting the object to be scanned.

[0006] Typically, radio frequency (RF) pulses are applied multiple times within a sequence. Therefore, a time-resolved sequence of RF pulses and magnetic field gradients is called a pulse sequence. In a sufficiently fast pulse sequence, where RF pulses are applied at intervals significantly smaller than T², the protons of the object do not have enough time to relax to their thermal equilibrium state between pulses. Instead, the protons reach a dynamic equilibrium called steady state. One such pulse sequence is refocusing steady-state free precession (SSFP), a popular method for generating images with a high signal-to-noise ratio (SNR) in a short acquisition time.

[0007] The goal is to provide high-resolution images of the subject in MRI scans. However, many methods for acquiring high-resolution images require strong radiofrequency pulses and strong magnetic field gradients, which can lead to increased hardware performance requirements, reduced SNR efficiency, and increased heating of the scanned subject. Summary of the Invention

[0008] To achieve this objective, a presently preferred embodiment of the present invention provides a method for generating a magnetic resonance imaging (MRI) image of an object, the method comprising: applying a magnetic field B0 to the object; applying an electromagnetic pulse sequence to the object; applying a magnetic field gradient other than the magnetic field B0, the magnetic field gradient including a plurality of first gradients and at least one second gradient; measuring signal echoes generated by the object in response to the plurality of electromagnetic pulses and the first and second magnetic field gradients; acquiring image data of a first spatial resolution from the signal echoes generated by the object in response to the electromagnetic pulses and the first and second magnetic field gradients; and combining the image data acquired from the signal echoes to generate at least one image of the object at a second spatial resolution higher than the first spatial resolution. The first gradients are completely rewound during the intervals between consecutive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral during the intervals between consecutive electromagnetic pulses. The phase of at least one electromagnetic pulse in the electromagnetic pulse sequence differs from the phase of another electromagnetic pulse in the electromagnetic pulse sequence.

[0009] In this way, the method provides an image of the object, where only certain specific locations within the object contribute to the signal echo. A first gradient allows the signal to be localized to specific locations within the object through a combination of at least one of slice selection, phase encoding, and frequency encoding. A second gradient provides periodic signal modulation, which depends on the non-resonant signal profile of protons within the object. Periodic spatial variations in magnetization can be generated in steady state and contain information about spatial frequencies above the Nyquist frequency. Therefore, a series of images with nominal spatial resolution can be acquired, each with a different periodic magnetization variation. These images can be combined to generate an image with a spatial resolution higher than the nominal spatial resolution.

[0010] Another embodiment of the present invention provides an MRI apparatus including a controller configured to control the MRI apparatus to perform the following operations: applying a magnetic field B0 to a subject; applying a sequence of electromagnetic pulses to the subject; applying a magnetic field gradient other than the magnetic field B0, the magnetic field gradient including a plurality of first gradients and at least one second gradient; and measuring signal echoes generated by the subject in response to the plurality of electromagnetic pulses and the first and second magnetic field gradients. The controller is further configured to: acquire image data of a first spatial resolution from the signal echoes generated by the subject in response to the electromagnetic pulses and the first and second magnetic field gradients; and combine the image data acquired from the signal echoes to generate at least one image of the subject at a second spatial resolution higher than the first spatial resolution. The first gradients are completely rewound during the intervals between consecutive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral between consecutive electromagnetic pulses. The phase of at least one electromagnetic pulse in the electromagnetic pulse sequence differs from the phase of another electromagnetic pulse in the electromagnetic pulse sequence.

[0011] When a linear magnetic field gradient is applied along a given axis, the linear change in the precession frequency results in a linear phase dispersion among protons along that axis. Since the first gradient has zero integral between successive electromagnetic pulses, these gradients do not produce any cumulative phase dispersion in a static object. The second gradient has a non-zero integral, thus producing a cumulative phase dispersion over the entire application of the successive electromagnetic pulses. The time integral of the second gradient between successive electromagnetic pulses is proportional to the slope of this linear phase dispersion and determines the periodicity Δx of the modulation pattern in space at steady state. This is related to the gradient time integral as follows: ∫G(t)dt=2π / YΔx, where G is the gradient intensity and γ is the gyromagnetic ratio. The smallest resolvable periodicity is where Δx equals the nominal voxel spacing along that axis, such that the pattern repeats once in each voxel.

[0012] MRI equipment typically includes a magnetic array adapted to generate a magnetic field B0 and a magnetic field gradient. MRI equipment typically includes a transmitter for emitting electromagnetic pulses. MRI equipment typically includes a receiver for measuring the signal echo generated by an object in response to the electromagnetic pulses.

[0013] Images can be merged voxel-by-voxel in the image domain. Image data can also be combined in the frequency domain. Artificial intelligence can be used to combine the data.

[0014] The first gradient can be completely rewound, such that their gradient time integral is zero between consecutive electromagnetic pulses.

[0015] Electromagnetic pulses can be radio frequency pulses. Sequences of electromagnetic pulses and magnetic field gradients can be steady-state free precession (SSFP) pulse sequences.

[0016] This method can be repeated to obtain images that are separated in space or time. In other words, images of other parts of an object can be acquired by adjusting the first gradient, the second gradient, or the phase of the electromagnetic pulse to provide an image of the desired location within the object, and imaging of the same part of the object can be repeated to provide multiple images of the same part separated by the passage of time.

[0017] The electromagnetic pulse sequence may include at least a first group of electromagnetic pulses, wherein the first pulse in the first group has a first phase. Furthermore, each subsequent pulse in the first group has a phase that increases with a first interval. This first interval can be fixed, such that the phase of subsequent electromagnetic pulses in the group increases linearly.

[0018] By linearly increasing the phase of the pulses between successive excitations, a portion of the signal modulation shift is induced in the resulting image. This process can be repeated to produce images with different minute shifts in the signal modulation produced by different fixed phase increments. These images can then be interleaved voxel-by-voxel before performing one-dimensional deconvolution using the estimated non-resonant contours.

[0019] An electromagnetic pulse sequence may include a second set of electromagnetic pulses, which may occur before or after the first set of electromagnetic pulses, wherein the first pulse in the second set has a second phase. Furthermore, each subsequent pulse in the second group has a phase that increases at a second interval different from the first interval. The electromagnetic pulse sequence may include a third group of electromagnetic pulses. The electromagnetic pulse sequence may also include other groups of electromagnetic pulses. The electromagnetic pulse sequence may include multiple groups of electromagnetic pulses as desired.

[0020] The first phase can be the same as the second phase, so that Images obtained from two sets of electromagnetic pulses can be combined to produce a single image. More than two images can be combined to create a single image.

[0021] The electromagnetic pulse sequence includes at least a first group of electromagnetic pulses, and the first pulse in the first group has a first phase. In this case, each subsequent pulse in the first group can have a phase that increases with a quadratic increment.

[0022] Increasing the phase of the consecutive electromagnetic pulses a second time can produce a pseudo-steady state, where the magnetization profile shifts slightly between consecutive electromagnetic pulses. This can then be used to generate at least two images with a first spatial resolution. These images can be reordered and combined into a single image with a spatial resolution greater than the first spatial resolution.

[0023] Image data from more than one signal echo can be measured during the intervals between subsequent pulses in an electromagnetic pulse sequence. This data can be fitted to a signal model to estimate quantitative characteristics of an object, such as T1, T2, or nuclear diffusion properties. Fourier transforms can be performed on data from several echoes within a single interval to provide estimates of the non-resonant profile.

[0024] Steady-state or pseudo-steady-state conditions can be interrupted by at least one other magnetic field gradient or electromagnetic pulse (such as an anti-phase pulse), causing the signal to be weighted by its quantitative characteristics. The recovery to steady-state or pseudo-steady-state conditions can be fitted to the signal model to estimate the quantitative characteristics of the object.

[0025] The first gradient may include at least one of a phase-encoded gradient, a slice-selection gradient, and a frequency-encoded gradient disposed along a first axis, and the gradient time integral of the second gradient may be non-zero along the first axis. The second gradient may have any shape that produces a non-zero time integral along any axis between consecutive electromagnetic pulses.

[0026] The first gradient can describe a non-Cartesian acquisition. This non-Cartesian acquisition may not have a fixed axis, provided that at least one of frequency encoding, phase encoding, or slice selection is used. The first or second gradient can generate a nonlinear magnetic field gradient.

[0027] Deconvolution of merged images can be performed based on spatial distribution. Image deconvolution can be performed in the frequency domain. Deconvolution can be initiated by estimating the inhomogeneity of the magnetic field B1 generated by the electromagnetic pulse. This B1 estimate can be derived from the data itself. Image data merging can be initiated by estimating the inhomogeneity of the static magnetic field B0. This B0 estimate can be derived from the data itself. Image data merging can be initiated by estimating the motion of objects during acquisition. This motion estimate can be derived from the data itself.

[0028] The advantages of these embodiments are set forth below, and further details and features of each of these embodiments are defined in the appended dependent claims and elsewhere in the following detailed description. Attached Figure Description

[0029] The various aspects of the teachings of the present invention and the arrangements embodying these teachings will be described below by way of illustrative examples with reference to the accompanying drawings, in which:

[0030] Figure 1 This is an example of a magnetic resonance imaging device according to the present invention;

[0031] Figure 2 and Figure 3 The magnetic field gradient applied by the MRI device is shown;

[0032] Figure 4The non-resonant frequency profiles of a set of protons in an MRI device are shown.

[0033] Figure 5 The small shift in the non-resonant frequency profile is shown when a group of electromagnetic pulses with different linear phase increments is applied;

[0034] Figures 6 and 7 are example images from an MRI device. Figure 6 illustrates a simulation approach, while Figure 7 is an example using actual acquired data. Detailed Implementation

[0035] Figure 1 This is a block diagram of a magnetic resonance imaging (MRI) device 100 according to the present invention. The MRI device 100 includes a chamber 110 that houses an object to be scanned and is surrounded by a magnetic array 120 configured to generate a magnetic field within the chamber 110. The function of the magnetic array 120 is controlled by a controller 130. The controller 130 also controls a transmitter 140 configured to propagate radio frequency pulses into the chamber 110. A receiver 150 is configured to detect electromagnetic signals from the object in the chamber 110 and provide these signals to the controller 130.

[0036] Figure 2 This is a diagram illustrating the operation of an MRI device 100 when used for image acquisition with a refocusing steady-state free precession (SSFP) pulse sequence. Figure 2 The pulse and magnetic field applied during a repetition time TR are shown.

[0037] In the refocusing SSFP sequence, radio frequency pulses 210 and 220 are repeatedly applied, along with a magnetic field gradient that is completely rewound during the repetition time TR. This means that the time integral of the magnetic field gradient applied to the object during the interval between the two radio frequency pulses 210 and 220 is zero.

[0038] At the beginning of the repetition time TR, a first radio frequency pulse 210 is applied to the object. The first radio frequency pulse 210 has an intensity α and a phase. The first radio frequency pulse 210 excites the nuclei in the object to be scanned, causing the net magnetization vector of the nuclei in the object to rotate into the transverse plane by a flip angle α. The nucleus can be the nucleus of a hydrogen atom. The magnetization then precesses around the main magnetic field B0 at a rate proportional to the local non-resonant frequency. The magnetization of the object relaxes with time constants T1 and T2.

[0039] Figure 2The magnetic field gradients applied during the repetition time as part of slice selection (SS), phase encoding (PE), and frequency encoding (FE) are illustrated. The magnetic field gradients applied for slice selections 231, 232, 233, and 234 produce corresponding gradients in the precession frequency of the object, allowing selection of a given slice of the object for scanning. In particular, the magnetic field gradients applied for slice selections 231, 232, 233, and 234 can vary in intensity to select the desired slice width. Therefore, slice selection allows positioning along a first axis within the object.

[0040] The magnetic field gradients applied to phase codes (PEs) 241 and 242 cause phase dispersion in nuclear precession. The magnetic field gradients applied to phase codes 241 and 242 can also vary in intensity to allow positioning along a second axis perpendicular to the first axis within the object. In some embodiments, phase codes are used on both axes, with a second phase code PE2 supplementing or replacing the slice selection.

[0041] The magnetic field gradient applied to frequency codes (FE) 251, 252, 253 causes frequency dispersion of the precession of atomic nuclei along a third axis perpendicular to the first and second axes, so as to locate along the third axis within the object.

[0042] Thus, when the receiver 150 detects a signal, the signal associated with the region within the slice can be identified by a unique combination of their frequency and phase codes, and is typically used to generate pixels for use in the resulting image of the slice by applying a Fourier transform.

[0043] The signal is detected during each TR period, centered on the echo time (TE), which is the time between the application of the RF pulse and the center of the signal sensed in the receiver 150. TE is typically equal to half of TR.

[0044] Figure 3 This diagram illustrates the operation of the MRI apparatus 100 when used for image acquisition according to the invention. In operation, data acquisition is employed similar to that used for refocusing SSFP acquisition. Specifically, multiple radio frequency pulses 310, 320 are applied to the object to be scanned while it is located in chamber 110. In each TR, magnetic field gradients 331, 332, 333, 334, 341, 342, 351, 352, 353 are applied to the object to provide slice selection, phase encoding, and frequency encoding. An additional magnetic field gradient with a non-zero time integral is applied in a specific direction. Figure 3 In the example shown, an additional magnetic field gradient 343 is applied in the same direction as the phase encoding. An additional magnetic field gradient 343 is applied in each TR.

[0045] For a given material in the object to be scanned, the steady-state magnetization depends strongly on the non-resonant frequency Δf, which is a deviation from the frequency generated by an ideal uniform magnetic field. Figure 4 The dependence of signal amplitude on the non-resonant frequency is shown. Under the constraint of a low flip angle, such as α = 1°, the amplitude of the non-resonant profile approximates a damped comb function with a period of 1 / TR. At a phase of... After the first RF pulse, the phase of the next RF pulse is... By linearly increasing the phase of the continuous radio frequency excitation pulses, the non-resonant profile shifts along the frequency dimension in the final image.

[0046] The non-resonant profile and the additional magnetic field gradient 343 result in periodic modulation of the spatial magnetization along the direction of the additional magnetic field gradient, where the peak position in the periodic modulation depends on the value of the phase increment of the RF pulse sequence. Thus, as the phase of each RF pulse in the group increases... At that time, the position of the peak in the periodic modulation is slightly offset relative to the object.

[0047] Figure 5 This shows when the linear phase increment between radio frequency pulse groups from Change to Peak shifts occur in the periodic modulation. Therefore, the images derived from each group correspond to different, minute shifts in that periodic modulation. The images from each group can then be merged to generate a super-resolution image.

[0048] Figure 6A , Figure 6B , Figure 6C and Figure 6D It shows Figure 3 An example application of the method shown is in which an MRI device 100 is used to create a cross-sectional scan of the brain. Figure 6A The image data is shown in the image. Figure 6B It shows Figure 6A The basic magnetization profile in the image. If the phase increment between successive radio frequency excitations is changed, the modulation pattern shifts in the resulting image, as shown. Figure 6C As shown, this therefore contains different information about high spatial frequencies. Seventeen sets of image data were simulated with different phase increments, including... Figure 6A The images shown are then merged to generate... Figure 6D The super-resolution image shown.

[0049] Figure 7A A single 2D SSFP acquisition with a nominal spatial resolution of 0.3 × 1.8 mm is shown, using a flip angle α = 0.4° and including, for example,... Figure 3 The additional magnetic field gradient shown makes the periodicity of the modulation pattern equal to 1.8 mm in the vertical direction. For example... Figure 7B As shown, super-resolution in one dimension is achieved by interlacing six of these images on a volumetric basis, where the spatial modulation pattern has been shifted in six equidistant steps, and an image of 0.3 × 0.3 mm is produced.

[0050] When merging images, the use of a low flip angle allows for simple deconvolution operations because most signals are generated within narrow peaks in non-resonant contours, such as in... Figure 6B As can be seen in the image, and in the minimal signal cancellation from magnetization. Therefore, using only the amplitude of the complex image, voxel interlacing, and deconvolution operations works effectively. However, if combined with a deconvolution operation, the method can also be used for higher flip angles, which demonstrates the complex integral of a wider non-resonant profile, which can be achieved in... Figure 2 This can further improve the SNR efficiency of the proposed super-resolution method. However, using a low RF pulse flip angle can help reduce tissue heating.

[0051] Despite Figure 3 In this context, the additional magnetic field gradient is positive in the phase encoding direction, but it can take any shape as long as it has the required non-zero time integral, and it can be applied equally in any direction. For example, the additional magnetic field gradient can be applied in the slice selection direction or the second phase encoding direction, and it can also be applied in the frequency encoding direction.

Claims

1. A method for generating magnetic resonance imaging (MRI) images of an object, the method comprising: A magnetic field B0 is applied to the object; A sequence of radio frequency electromagnetic pulses is applied to the object; An additional magnetic field gradient, in addition to the magnetic field B0, is applied, the magnetic field gradient comprising a plurality of first gradients and at least one second gradient, wherein the plurality of first gradients allow a signal to be localized to a specific location in the object by a combination of at least one of slice selection, phase encoding and frequency encoding, and the at least one second gradient provides periodic signal modulation depending on the non-resonant signal profile of the magnetization in the object. Measure the signal echo generated by the object in response to a plurality of electromagnetic pulses and the first gradient and the second gradient; Image data of a first spatial resolution is acquired from the signal echo generated by the object in response to the electromagnetic pulse and the first and second gradients; and The image data acquired from the signal echo are combined to generate at least one image of the object at a second spatial resolution higher than the first spatial resolution. Wherein, the first gradient completely rewinds during the intervals between consecutive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral between consecutive electromagnetic pulses. Wherein, the phase of at least one electromagnetic pulse in the electromagnetic pulse sequence is different from the phase of another electromagnetic pulse in the electromagnetic pulse sequence.

2. The method for generating an MRI image of an object of claim 1, wherein, The electromagnetic pulse sequence includes at least a first group of electromagnetic pulses, wherein a first pulse in the first group has a first phase φ1, and each subsequent pulse in the first group has a phase that increases at a first interval.

3. The method for generating an MRI image of an object of claim 2, wherein, The electromagnetic pulse sequence includes a second group of electromagnetic pulses, wherein the first pulse in the second group has a second phase φ2, and each subsequent pulse in the second group has a phase that increases at a second interval different from the first interval.

4. The method for generating an MRI image of an object of claim 1, wherein, The electromagnetic pulse sequence includes at least a first group of electromagnetic pulses, wherein the first pulse in the first group has a first phase φ1, and each subsequent pulse in the first group has a phase that increases at a quadratic increment.

5. The method for generating an MRI image of an object according to any one of claims 1 to 4, wherein, Image data from more than one signal echo is measured during the intervals between subsequent pulses in the electromagnetic pulse sequence.

6. A magnetic resonance imaging (MRI) device, comprising a controller configured to control the MRI device to perform the following operations: Apply a magnetic field B0 to the object; A sequence of radio frequency electromagnetic pulses is applied to the object; An additional magnetic field gradient, besides the magnetic field B0, is applied, the magnetic field gradient comprising a plurality of first gradients and at least one second gradient, wherein, The plurality of first gradients allow the signal to be located at a specific location in the object by a combination of at least one of slice selection, phase encoding and frequency encoding, and the at least one second gradient provides periodic signal modulation that depends on the non-resonant signal profile of the magnetization in the object; as well as Measure the signal echo generated by the object in response to a plurality of electromagnetic pulses and the first gradient and the second gradient; The controller is also configured to: Image data of a first spatial resolution is acquired from the signal echo generated by the object in response to the electromagnetic pulse and the first gradient and the second gradient; as well as The image data acquired from the signal echo are combined to generate at least one image of the object at a second spatial resolution higher than the first spatial resolution. Wherein, the first gradient completely rewinds during the intervals between consecutive electromagnetic pulses, while the at least one second gradient has a non-zero gradient time integral between consecutive electromagnetic pulses. Wherein, the phase of at least one electromagnetic pulse in the electromagnetic pulse sequence is different from the phase of another electromagnetic pulse in the electromagnetic pulse sequence.

7. The MRI apparatus of claim 6, wherein, The electromagnetic pulse sequence includes at least a first group of electromagnetic pulses, wherein a first pulse in the first group has a first phase φ1, and each subsequent pulse in the first group has a phase that increases at a first interval.

8. The MRI apparatus of claim 7, wherein, The electromagnetic pulse sequence includes a second group of electromagnetic pulses, wherein the first pulse in the second group has a second phase φ2, and each subsequent pulse in the second group has a phase that increases at a second interval different from the first interval.

9. The MRI apparatus of claim 6, wherein, The electromagnetic pulse sequence includes at least a first group of electromagnetic pulses, wherein the first pulse in the first group has a first phase φ1, and each subsequent pulse in the first group has a phase that increases at a quadratic increment.

10. The MRI device according to any one of claims 6 to 9, wherein, Image data from more than one signal echo is measured during the intervals between subsequent pulses in the electromagnetic pulse sequence.

Images