System and method for reducing specific absorption rate in magnetic resonance imaging

Abstract

A method for imaging a subject using a magnetic resonance imaging (MRI) system includes determining a customized radio frequency (RF) pulse sequence having a plurality of refocusing pulses. In the method, a target peak RF pulse value is determined, and a transfer function is also determined that converts a first refocusing pulse of the plurality of refocusing pulses to a modified refocusing pulse having the target peak RF pulse value. A modified RF pulse sequence is generated based on the transfer function and the plurality of refocusing pulses. Finally, magnetic resonance (MR) signals are acquired from the subject based on the modified RF pulse sequence, and a medical image of the subject is generated based on the acquired MR signals.

Description

Background Art

[0001] Embodiments disclosed herein relate to magnetic resonance imaging (MRI) systems and methods, and more particularly, to techniques for reducing the specific absorption rate (SAR) in MRI technology.

[0002] MRI is a widely used medical imaging technique that provides detailed anatomical and functional information of the human body. MRI systems use powerful magnetic fields and radio frequency (RF) pulses to generate signals from body tissues, enabling the creation of high-resolution images.

[0003] However, one of the challenges associated with MRI is that the patient may be exposed to radio frequency energy, resulting in a phenomenon known as the specific absorption rate (SAR). SAR is a measure of the rate at which RF energy is absorbed by the patient's body during an MRI scan. Excessively high SAR levels can lead to tissue heating and potential adverse effects. Therefore, the International Electrotechnical Commission (IEC) has set specific limits for SAR in MRI. For example, for a duration of 6 minutes, the SAR limit is 4 W / kg for the body and 3.2 W / kg for the head.

[0004] There are many ways to reduce SAR in MRI. One way is to reduce the flip angle, which is the angle at which the protons tilt in the magnetic field. Another way is to reduce the number of slices imaged. In addition, SAR can be reduced by using lower radio frequency power. Although these techniques have shown some success in SAR reduction, they often compromise image quality, acquisition time, or diagnostic accuracy.

[0005] Therefore, there is a need for an improved method to effectively reduce SAR without compromising the overall performance of the MRI system. Summary of the invention

[0006] According to one embodiment of the present technology, a method for imaging a subject using a magnetic resonance imaging (MRI) system is provided. The method includes determining a customized radio frequency (RF) pulse sequence having multiple refocusing pulses and determining a target peak RF pulse value. The method also includes determining a transfer function to convert a first refocusing pulse in the multiple refocusing pulses into a modified refocusing pulse having a target peak RF pulse value, and generating a modified RF pulse sequence based on the transfer function and the multiple refocusing pulses. The method also includes acquiring magnetic resonance (MR) signals from the subject based on the modified RF pulse sequence, and generating a medical image of the subject based on the acquired MR signals.

[0007] According to another embodiment of the present technology, a magnetic resonance imaging (MRI) system is provided. The MRI system includes a magnet configured to generate a polarizing magnetic field around at least a portion of a subject arranged in the MRI system. A gradient coil assembly is arranged in the MRI system, and the gradient coil assembly includes a readout gradient coil, a phase gradient coil, and a slice selection gradient coil, and the gradient coil assembly is configured to apply at least one gradient field to the polarizing magnetic field. The MRI system also includes: a radio frequency (RF) system, and the RF system is configured to apply an RF field to the subject and receive magnetic resonance signals from the subject. The MRI system also includes a processing system. The processing system is configured to determine a customized radio frequency (RF) pulse sequence having multiple refocusing pulses, and determine a target peak RF pulse value. The processing system is also configured to determine a transfer function to convert a first refocusing pulse in the multiple refocusing pulses into a modified refocusing pulse having a target peak RF pulse value, and generate a modified RF pulse sequence based on the transfer function and the multiple refocusing pulses. The processing system is also configured to acquire magnetic resonance (MR) signals from the subject based on the modified RF pulse sequence, and generate a medical image of the subject based on the acquired MR signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other features, aspects and advantages of the present invention will be better understood when the following detailed description is read with reference to the accompanying drawings, in which like characters refer to like parts throughout the several views, and in which:

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

[0010] Figure 2 According to the embodiments of the present invention, Figure 1 A flowchart of an exemplary method specifically implemented in a system;

[0011] Figure 3 a graphical representation of a graph of a customized RF pulse sequence and corresponding slice gradient pulses according to an embodiment of the present technology;

[0012] Figure 4 is a graphical representation of applying a transfer function to a refocusing pulse and a corresponding slice gradient pulse in accordance with an embodiment of the present technique; and

[0013] Figure 5 A graphical representation of the comparison results of three RF pulse sequences. DETAILED DESCRIPTION

[0014] One or more specific embodiments will be described below. In order to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be understood that in the development of any such actual implementation, as in any engineering or design project, many implementation-specific decisions must be made to achieve the developer's specific goals, such as complying with system-related and business-related constraints that may vary from implementation to implementation. In addition, it should be understood that such development efforts may be complex and time-consuming, but are still routine tasks for design, fabrication, and manufacturing for ordinary technicians who benefit from this disclosure.

[0015] When introducing the elements of the various embodiments of the present embodiment, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of these elements. The terms "comprise", "comprising" and "having" are intended to be inclusive, and mean that there may be additional elements in addition to the listed elements. In addition, any numerical examples in the following discussion are intended to be non-limiting, and therefore additional numerical values, ranges and percentages are within the scope of the disclosed embodiments. In addition, the terms "circuit" and "circuitry" and "controller" may include a single component or multiple components, which are active and / or passive and are connected or otherwise coupled together to provide the functions.

[0016] In magnetic resonance imaging (MRI), an object or subject is placed in a magnet. When the object is in the magnetic field generated by the magnet, the magnetic moments of the nuclei (such as protons) attempt to align with the magnetic field, but precess around the magnetic field in a random order at the Larmor frequency of the nuclei. The magnetic field of the magnet is called B0 and extends in the longitudinal or z direction. During the acquisition of MR images, a magnetic field in the xy plane and close to the Larmor frequency (called the excitation field B1) is generated by a radio frequency (RF) coil and can be used to align the net magnetic moment M of the nuclei. z Rotation or "tilt" from the z direction toward the lateral or xy plane. After the excitation signal B1 is terminated, the nucleus emits a signal, which is called an MR signal. In order to use the MR signal to generate an image of the object, a magnetic field gradient pulse (G x , G y and G z ). Gradient pulses are used to scan through k-space, the inverse of spatial frequency or distance. There is a Fourier relationship between the acquired MR signals and the image of the object, so the image of the object can be derived by reconstructing the MR signals. The image of the object may include a two-dimensional (2D) or three-dimensional (3D) image.

[0017] Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings, in which Figure 11 is a schematic diagram of a magnetic resonance imaging (MRI) system 10. The operation of the system 10 can be controlled from an operator console 12, which includes an input device 13, a control panel 14, and a display screen 16. The input device 13 can be a mouse, a joystick, a keyboard, a trackball, a touch-activated screen, a light stick, a voice controller, and / or other input devices. The input device 13 can be used for interactive geometric shape designation. The console 12 communicates with a computer system 20 via a link 18, which enables an operator to control the generation and display of images on the display screen 16. The link 18 can be a wireless or wired connection. The computer system 20 may include modules that communicate with each other via a backplane 20A. The modules of the computer system 20 may include, for example, an image processor module 22, a central processing unit (CPU) module 24, and a memory module 26, which may include, for example, a frame buffer for storing an array of image data. The computer system 20 may be linked to an archival media device, a permanent or backup memory, or a network for storing image data and programs, and communicates with an MRI system control 32 via a high-speed signal link 34. The MRI system control 32 may be separate from or integrated with the computer system 20. The computer system 20 and the MRI system control 32 together form an "MRI controller" 33 or "controller."

[0018] In an exemplary embodiment, the MRI system control 32 includes modules connected by a backplane 32a. These modules include a CPU module 36 and a pulse generator module 38. The CPU module 36 is connected to the operator console 12 via a data link 40. The MRI system control 32 receives commands from the operator via the data link 40 to indicate the scan sequence to be performed. The CPU module 36 operates the system components to implement the desired scan sequence and generates data indicating the timing, intensity and shape of the generated RF pulses and the timing and length of the data acquisition window. The CPU module 36 is connected to the components operated by the MRI controller 32, including the pulse generator module 38 that controls the gradient amplifier 42, the physiological acquisition controller (PAC) 44, and the scan room interface circuit 46.

[0019] In one example, the CPU module 36 receives subject data from a physiological acquisition controller 44, which receives signals from sensors connected to the object, such as ECG signals received from electrodes attached to the patient. As used herein, an object or subject is a person (or patient), an animal, or a phantom. The CPU module 36 receives signals from sensors associated with the condition of the patient and the magnet system via a scan room interface circuit 46. The scan room interface circuit 46 also enables the MRI controller 33 to command a patient positioning system 48 to move the patient to a desired position for scanning.

[0020] The whole body RF coil 56 is used to transmit the waveform toward the subject's anatomical structure. The whole body RF coil 56 can be a body coil. The RF coil can also be a local coil, which can be placed closer to the subject's anatomical structure than the body coil. The RF coil 56 can also be a surface coil. An RF coil containing RF receiver channels can be used to receive signals from the subject's anatomical structure. A typical surface coil will have eight receiving channels; however, a different number of channels is possible. It is known to use a combination of both the body coil 56 and the surface coil to provide better image quality.

[0021] The pulse generator module 38 can operate the gradient amplifier 42 to achieve the desired timing and shape of the gradient pulses generated during the scan. The gradient waveform generated by the pulse generator module 38 can be applied to the gradient amplifier system 42 having Gx, Gy and Gz amplifiers. Each gradient amplifier excites a corresponding physical gradient coil in the gradient coil assembly 50 to generate a magnetic field gradient for spatially encoding the acquired signal. Specifically, Gx corresponds to a flow / frequency encoding gradient, Gy corresponds to a phase encoding gradient, and Gz corresponds to a slice selection gradient. The gradient coil assembly 50 can form a part of a magnet assembly 52, which also includes a polarizing magnet 54 (in operation, the polarizing magnet provides a longitudinal magnetic field B0 throughout a target space 55 surrounded by the magnet assembly 52) and a whole-body RF coil 56 (in operation, the coil provides a transverse magnetic field B1, which is approximately perpendicular to B0 throughout the target space 55). The transceiver module 58 in the MRI system control 32 generates pulses that can be amplified by an RF amplifier 60, which is coupled to the RF coil 56 through a transmit / receive switch 62. The resulting signals emitted by the excited nuclei in the subject's anatomy may be sensed by a receive coil (not shown) and provided to a preamplifier 64 via a transmit / receive switch 62. The amplified MR signals are demodulated, filtered, and digitized in the receiver portion of the transceiver 58. The transmit / receive switch 62 is controlled by a signal from the pulse generator module 38 to electrically connect the RF amplifier 60 to the coil 56 during a transmit mode and to connect the preamplifier 64 to the receive coil during a receive mode.

[0022] The MR signals generated by the excitation of the target are digitized by the transceiver module 58. The digitized signals are then processed by the MRI system control 32 via Fourier transform to generate k-space data, which is transmitted to the memory module 66 or other computer-readable medium via the MRI system control 32. "Computer-readable medium" may include, for example, a structure configured so that an electrical, optical, or magnetic state can be fixed in a manner that is perceptible and reproducible by a conventional computer (e.g., text or images printed on paper or displayed on a screen, a compact disk or other optical storage medium, "flash" memory, EEPROM, SDRAM or other electrical storage medium; a floppy disk or other magnetic disk, a tape or other magnetic storage medium).

[0023] The scan is complete when an array of raw k-space data is acquired in the computer readable medium 66. For each image to be reconstructed, the raw k-space data is rearranged into a separate k-space data array, and each of these k-space data arrays is input to the array processor 68, which operates to reconstruct the data into an array of image data using a reconstruction algorithm such as a Fourier transform. When complete k-space data is obtained, it represents the entire volume of the subject's body, and the k-space so obtained may be referred to as reference k-space. Similarly, when only partial k-space data is obtained, the image may be referred to as partial k-space. The image data is transmitted to the computer system 20 via the data link 34 and stored in the memory. In response to commands received from the operator console 12, the image data may be archived in a long-term storage device, or may be further processed by the image processor 22 and transmitted to the operator console 12, and presented on the display 16.

[0024] The MR signal is represented by a complex number, where each position at k-space is represented by a complex number, where the I and Q quadrature MR signals are real and imaginary components. A composite MR image can be reconstructed based on the I quadrature MR signal and the Q quadrature MR signal using a process such as Fourier transform of the k-space MR data. A composite MR image is an MR image having each pixel represented by a complex number, which also has a real component and an imaginary component. The magnitude M of the received MR signal can be determined as the square root of the sum of the squares of the I quadrature component and the Q quadrature component of the received MR signal, as shown in equation (3) below:

[0025]

[0026] And the phase φ of the received MR signal can also be determined as follows:

[0027]

[0028] Because radio frequencies (RF) are used in MR imaging, less energy is absorbed by the patient's body tissues. The measure of the rate at which energy is absorbed by the body is called the specific absorption rate (SAR), typically expressed in units of watts per kilogram (W / kg). Regulatory agencies such as the United States (US) Food and Drug Administration (FDA) and the International Electrotechnical Commission (IEC) have established SAR limits to ensure that exposure to RF energy during MRI scans remains within safe levels.

[0029] Two-dimensional (2D) fast spin echo (FSE) pulse sequences are a widely used technique in magnetic resonance (MR) imaging. 2D FSE can generate multiple contrasts, allowing visualization of various tissue characteristics. These contrasts include, for example, T1, T2, T1 FLAIR (fluid attenuated inversion recovery), T2 FLAIR, STIR (short time inversion recovery).

[0030] Typically, in MRI, a pulse sequence refers to the specific timing and order of RF pulses and gradient fields used to scan a patient or subject. By applying a specific combination of RF pulses and gradients to the RF coil and gradient coil, respectively, a 2D FSE pulse sequence can selectively manipulate the magnetization of different tissues, resulting in the generation of these various contrasts. However, because 2D FSE uses a series of RF pulses, the pulse sequence is SAR intensive for the patient. In order to reduce SAR, in one embodiment, FSE with customized RF pulses can be used. Compared to 2D FSE, the customized RF pulse uses a different RF pulse shape and peak value (denoted B1) and flip angle for the refocusing pulse. As will be appreciated by those skilled in the art, the refocusing pulse is an RF pulse applied after the excitation RF pulse to overcome the phase shift. In another embodiment, the variable rate selective excitation (VERSE) technique can also be used to reduce the RF power. The VERSE technique reduces the peak B1 value of the RF pulse by a specific scaling factor, and shapes the gradient waveform based on the scaling factor to excite the same slice.

[0031] In the present technology, a modified RF pulse sequence algorithm for reducing SAR is proposed. The modified RF pulse sequence algorithm generates a transfer function to convert the customized RF pulse and gradient waveform in the FSE technique into low-power RF and gradient segments. The transfer function will be generated for the excitation and refocusing pulses with a B1 value lower than the defined B1 value. The transfer function of the first refocusing pulse with a defined B1 value is applied to all other subsequent refocusing pulses of different shapes of the customized RF pulse. By applying the same transfer function to all different refocusing pulse shapes, the pulse widths of all refocusing pulses will be the same.

[0032] Figure 2FIG. 1 is a flow chart of an exemplary method 150 for reducing SAR in MRI imaging. The method 150 may be Figure 1 The MRI controller 33 is specifically implemented in the MRI controller 33. In one or more embodiments, the MRI controller is a special-purpose component configured to perform operations that cannot be performed by a general-purpose computer or device. The software embodying these processes can be stored by any non-transient tangible medium including a fixed disk, a floppy disk, a CD, a DVD, a flash drive, or a tape. In an exemplary embodiment, the method includes determining a customized radio frequency (RF) pulse sequence having a plurality of refocusing pulses at step 152.

[0033] In one embodiment, determining a customized RF pulse sequence may include designing a customized RF pulse to reduce the specific absorption rate (SAR) of the patient being scanned. This customization process results in different parameters of the subsequent refocusing pulses applied after the excitation pulse, such as variable shapes, variable pulse amplitudes, and / or variable flip angles. Customized RF pulses using refocusing pulses help reduce SAR compared to using a single-shaped RF pulse. Typically, the first refocusing customized RF pulse has a higher B1 and an optimal slice profile. Subsequent RF pulses may have a lower B1 and a compromise slice profile. Combining these pulses together gives an overall good management of SAR and image quality. For example, the first refocusing pulse after the excitation pulse may be designed to be stronger in the center of the slice, while the second pulse may be designed to be stronger in the middle of the edge of the slice, and the third refocusing pulse may be designed to be stronger at the outer edge of the slice. This design can reduce local SAR in a specific region of interest. Other refocusing pulses can be designed in a similar manner, i.e., the peak of the pulse can be adjusted according to the portion of the slice being focused. In addition, a variable flip angle sequence can redistribute RF energy deposition on the echo chain. For example, if the initial refocusing pulse has a lower flip angle, it results in lower SAR during the early part of the echo train when the echoes are weaker. As the echo train progresses, the flip angle increases and the SAR level adjusts accordingly.

[0034] In one embodiment, the factors on which the shape of the refocusing pulse depends include the bandwidth of the refocusing pulse, the slice width and the intensity of the gradient waveform. Generally speaking, the shape of the RF pulse determines the shape of the slice to be excited. Therefore, depending on which area of ​​the slice needs to be excited, the RF pulse can be customized. In a customized RF pulse sequence, the RF pulse is stretched in time, which means that the slice to be excited is also stretched in time. This requires a corresponding change in the gradient waveform. In one embodiment, the gradient waveform can be stretched in time by the same factor as the RF pulse.

[0035] Figure 3A graphical representation 200 of a graph of a custom RF pulse sequence 202 and corresponding slice gradient pulses 204 is shown. The custom RF pulse sequence is essentially a fast spin echo (FSE) with a custom RF pulse sequence. In both graphs 202 and 204, a horizontal axis 206 represents time and a vertical axis 208 represents the amplitude of the corresponding graph.

[0036] As can be seen from the graph 202, there are 90 0 An RF excitation pulse 210 is followed by a plurality of RF refocusing pulses 212, 214, 216, 218, 220, which have, at least for the first three pulses 212, 214, 216, parameters that are variable relative to each other. As previously described, the variable parameters of each of the refocusing pulses, such as amplitude, shape or flip angle, are customized depending on which region of the slice needs to be excited or depending on the weighting of different contrasts. Thus, it can be seen that the refocusing pulse 212 has a different shape compared to the shapes of the refocusing pulses 214-220, and the same is true for the remaining refocusing pulses. It should be noted that different shapes refer to different pulse amplitudes (B1 峰值 ) and / or different flip angles or pulse time periods. For example, refocusing pulses 212, 214, and 216 have pulse lengths equal to 120.775 0 ,109.959 0 ,111.212 0 of flip angles, and their amplitudes are also different, as can be seen from graph 202. In addition, it can be seen from graph 204 that the shape of the slice gradient pulse 222 also changes according to the shape of the refocusing pulse. Each of the gradient pulses 222 has lobes 224, 226 on either side of the pulse. These lobes are called crusher gradients and are used to shift the phase of spurious signals from outside the slice of interest. The amplitude of the crusher gradient, the duration and amplitude of the crusher outer gradient pulse 228 can be changed according to the shape of the refocusing pulse. The gradient amplitude depends on the slice thickness and the bandwidth (BW) of the RF pulse. In one embodiment, the amplitude of the slice selective gradient (G) can be determined by the following equation: BW=γ*G*slice thickness, where γ is the gyromagnetic ratio.

[0037] See again Figure 2 At step 154, the method includes determining a target peak RF pulse value (B1 目标_峰值 ). Target peak RF pulse value B1 目标_峰值 Essentially represents the required peak amplitude of the refocusing RF pulse. In one embodiment, based on the acceptable peak specific absorption rate value (SAR 峰值 ) to determine the target peak RF pulse value. In one embodiment, B1 目标_峰值 The values ​​are given as:

[0038]

[0039] where A is the area of ​​the patient's body tissue, ρ is the tissue mass, D is the duty cycle of the RF pulse, ω is the RF pulse frequency and σ is the conductivity of the tissue. It should be noted that there are 峰值 Confirm B1 目标_峰值 There are many other ways of and equation (1) is only one example. In addition, the relationship between the flip angle (α) and the RF pulse amplitude B1 can be given as:

[0040] α~γ.B1 / D(2)

[0041] Where γ is the gyromagnetic ratio of the particle, which is a constant (e.g., for hydrogen, γ = 42.58 MHz / T). Thus, in one embodiment, based on B1 目标_峰值 and D, the corresponding flip angle α can also be determined 目标_峰值 .

[0042] At step 156, the method includes determining a transfer function (TF) to convert a voltage having a peak amplitude B1 峰值 The first refocusing pulse 212 is converted into a pulse having a target peak RF pulse value B1 目标_峰值 In one embodiment, such a transfer function can be determined by first determining the area under the curve (AUC) of the original refocusing pulse 212, and then determining a modified RF pulse with the same AUC but with an amplitude equal to B1. 目标_峰值 In another embodiment, the transfer function may be determined based on an amplitude modulation algorithm such as piecewise linear modulation, polynomial modulation, or optimal control theory. Furthermore, in one embodiment, the transfer function may be determined based on the relationship between B1 and the gradient pulse amplitude and the relationship between B1, D, and SAR. In this embodiment, the algorithm may combine the values ​​of the gradient pulse amplitude and pulse width of the RF refocusing pulse and the gradient pulse to determine which values ​​of the RF pulse and the gradient pulse achieve the goal of limiting the value of B1 to the target B1. 目标_峰值 and other image parameters (e.g., image contrast).

[0043] In yet another embodiment, such a transfer function is simply (B1 目标_峰值 / B1 峰值 Based on this transfer function, the duty cycle D(D) of the modified RF pulse can be determined. m ). For example, if in Equation 1, A, ρ, ω, and σ are assumed to be constants, then the modified RF pulse D m The D can be determined as:

[0044]

[0045] Where TF = B1 目标_峰值 / B1 峰值 Based on D m and B1 目标_峰值 , the flip angle α of the modified RF pulse m Can be determined as:

[0046] α m ~γ.B1 目标_峰值 / D m (4)

[0047] Therefore, at step 158, the method 100 includes generating a modified RF pulse sequence based on the transfer function and the plurality of refocusing pulses. Specifically, to generate the modified RF pulse sequence, the same transfer function TF determined for the first refocusing pulse 212 is used for the remaining refocusing pulses 214-220 and the gradient pulses 222. This is possible because the first refocusing RF pulse 212 has a maximum peak value B1 compared to the subsequent refocusing pulses 214-220. Therefore, if the B1 value of the refocusing RF pulse 212 is limited to the target B1 value (B1 目标_峰值 ), the B1 value of the remaining refocusing pulses will automatically be lower than the target B1 value (B1 目标_峰值 ). In other words, the first refocusing pulse 212 contributes to the maximum SAR compared to the other refocusing pulses. It should be noted that since the transfer function for all refocusing pulses is the same, the pulse width D m This also remains the same for all refocusing pulses.

[0048] At step 160, the modified pulse sequence generated at step 158 is applied to the MR system to acquire MR signals from the subject. Once the MR signals are obtained, at step 162, a medical image of the subject is generated based on the MR using some reconstruction technique as previously explained.

[0049] Figure 4 A graphical representation 300 of applying a transfer function to a refocusing pulse and a corresponding slice gradient pulse is shown. Figure 4 In , rows 302, 304, and 306 correspond to Figure 3 308 shows the refocusing pulses, and column 310 shows a plurality of modified refocusing pulses after the transfer function is applied to the refocusing pulses in column 308. Column 312 represents the modified gradient pulses corresponding to the modified refocusing pulses in column 310, and column 314 represents the corresponding slice profile graph. As will be appreciated by those skilled in the art, the slice profile graph shows the magnetization of the spins in the corresponding slice.

[0050] It should be noted that each of the columns 302, 304, and 306 may show three curves representing the magnitude, real component, and imaginary component of the corresponding graph. In addition, the same transfer function applied to the refocusing pulse 212 in row 304 is applied to the remaining refocusing pulses 214 and 216 in rows 302, 306 and the corresponding gradient pulses. Figure 5 As shown in the graph 406, when 90 0 After the RF excitation pulse, the pulses shown in column 310 are applied one after the other in a modified RF pulse sequence. The same is true for the slice gradient pulses in column 312, i.e., Figure 5 As shown in the graph 406 of , these pulses are applied one after another. In addition, when the transfer function is applied to Figure 3 These modified gradient pulses are generated after the gradient pulse 222 in . As will be appreciated by those skilled in the art, applying a transfer function to a refocusing pulse or a gradient pulse means modifying the parameters of the refocusing pulse and the gradient pulse based on the transfer function as shown in Equations 3 and 4.

[0051] Figure 5 A graphical representation 400 of the comparison results of three RF pulse sequences is shown. Specifically, Figure 5 The curve 402 in FIG. Figure 3 406 is a diagram of a customized RF pulse sequence of pulse sequence 202 for reducing SAR in a subject. Graph 404 is a basic variable rate selective excitation (VERSE) pulse sequence. As previously explained, in the basic VERSE pulse sequence, the peak B1 value of the FSE pulse sequence using similarly shaped pulses (i.e., the same amplitude) for all refocusing pulses is reduced by a specific scaling factor to reduce SAR. In addition, graph 406 shows a modified RF pulse sequence obtained after applying a transfer function to the customized RF pulse sequence 402, as described in the previous paragraph with respect to Figure 4 As explained. In all graphs 402, 404 and 406, the lower parts 428, 430, 432 show the pulse sequence waveforms, whereas the upper parts 434, 436, 438 show the corresponding slice gradient waveforms. As can be seen in graphs 402 and 406, the amplitudes of the first refocusing pulse (410, 412), the second refocusing pulse (414, 416) and the third refocusing pulse (418, 420) are different, but the amplitudes of the first refocusing pulse (422), the second refocusing pulse (424) and the third refocusing pulse (426) of graph 404 are the same. Graphs 402, 404 and 406 also show 90 0 RF excitation pulses 426, 428, and 430. Modified excitation pulse 430 is generated after applying the transfer function to excitation pulse 426. Similarly, gradient waveform 438 is applied after applying the transfer function to gradient waveform 434.

[0052] Table 1 below compares the simulated SAR values ​​for each of the pulse sequences 428, 430, and 432. As can be seen from the table, the flip angles of the first refocusing pulse, the second refocusing pulse, and the third refocusing pulse are the same for all pulse sequences, namely, 120.775 and 120.775, respectively. 0 ,109.959 0 and 111.212 0 It can also be seen from the table that the SAR value of the custom RF pulse sequence 428 is 2.52 W / kg; the SAR value of the VERSE RF pulse sequence 430 is 1.89 W / kg, and the SAR value of the modified RF pulse sequence 432 is 1.15 W / kg. Therefore, compared with the custom RF pulse sequence and the VERSE RF pulse sequence, the modified RF pulse sequence proposed in this article has the least SAR.

[0053] Pulse Sequence (PS) 1st refocusing pulse flip angle (RPFA) 2ndRPFA 3rdRPFA SAR Customized RF PS 120.775 109.959 111.212 2.52 VERSE RF PS 120.775 109.959 111.212 1.89 Improved RF PS 120.775 109.959 111.212 1.15

[0054] Table 1

[0055] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any included methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to fall within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insignificant differences from the literal language of the claims.

Claims

1. A method for imaging a subject using a magnetic resonance imaging system, the method comprising: determining a customized RF pulse sequence having multiple refocusing pulses; determining a target peak RF pulse value; determining a transfer function to convert a first refocusing pulse of the plurality of refocusing pulses into a modified refocusing pulse having the target peak RF pulse value; generating a modified RF pulse sequence based on the transfer function and the plurality of refocusing pulses; acquiring MR signals from the subject based on the modified RF pulse sequence; as well as A medical image of the subject is generated based on the acquired MR signals.

2. The method according to claim 1, wherein the plurality of refocusing pulses have variable parameters, the variable parameters comprising at least one of a variable pulse amplitude or a variable flip angle or a variable shape. The method of claim 2 , wherein the variable parameter is determined based on a specific absorption rate value.

4. The method of claim 1, wherein the customized RF pulse sequence comprises a 90° pulse prior to the plurality of refocusing pulses. 0 RF excitation pulse.

5. The method of claim 1, wherein the same transfer function is applied to each of the plurality of refocusing pulses to generate a modified RF pulse sequence, wherein the modified RF pulse sequence comprises a modified excitation pulse followed by a plurality of modified refocusing pulses. The method of claim 5 , wherein the pulse width of each of the plurality of modified refocusing pulses is the same. 7 . The method of claim 1 , wherein the transfer function is applied to gradient pulses of the customized RF pulse sequence to generate modified gradient pulses for the modified RF pulse sequence. The method of claim 1 , wherein the target peak RF pulse value is determined based on a peak specific absorption rate value.

9. The method of claim 1, wherein the transfer function is determined based on an area under a curve of the first refocusing pulse and the target peak RF pulse value.

10. The method of claim 1, wherein the transfer function is determined based on a ratio of the target peak RF pulse value and an amplitude of the first refocusing pulse.

11. A magnetic resonance imaging system, comprising: a magnet configured to generate a polarized magnetic field around at least a portion of a subject disposed in the magnetic resonance imaging system; a gradient coil assembly, the gradient coil assembly comprising a readout gradient coil, a phase gradient coil, and a slice selection gradient coil, the gradient coil assembly being configured to apply at least one gradient field to the polarizing magnetic field; a radio frequency system configured to apply an RF field to the subject and receive magnetic resonance signals from the subject; A processing system programmed to perform the method according to any one of claims 1 to 10.

Images