Magnetic resonance imaging device
Patent Information
- Application Number
- JP2023137290
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-08-25
- Publication Date
- 2025-03-07
Smart Images
Figure 2025031218000001_ABST
Abstract
Description
[Technical field]
[0001] The embodiments disclosed in this specification and in the drawings relate to a magnetic resonance imaging apparatus. [Background technology]
[0002] A magnetic resonance imaging device is an imaging device that excites the nuclear spins of a subject placed in a static magnetic field with a radio frequency (RF) signal at the Larmor frequency, and generates an image by reconstructing the magnetic resonance (MR) signals generated from the subject in response to the excitation.
[0003] One of the causes of image quality degradation in magnetic resonance images is known to be disturbance of the magnetic field caused by eddy currents. When a pulse current is applied to a gradient coil, the leakage magnetic field of the gradient coil causes eddy currents to flow in metals near the gradient coil, such as the heat shield plate of a static magnetic field coil. These eddy currents generate an eddy current magnetic field.
[0004] The eddy current magnetic field disturbs the static magnetic field or the gradient magnetic field, which causes deterioration of image quality. In particular, it is known that the fluctuation of the zero-order component of the magnetic field caused by the eddy current magnetic field causes the fluctuation of the magnetic resonance frequency.
[0005] Several methods have been proposed to suppress the fluctuation of the magnetic field caused by eddy current magnetic fields, i.e., the zero-order component of the eddy magnetic field. For example, a method is known in which the fluctuation of the magnetic resonance frequency at a certain point in time is calculated by accumulating the eddy magnetic fields caused by a series of gradient magnetic fields applied from that point in time until that point in time.
[0006] However, in the conventional technique described above, the imaging method in which imaging is synchronized with an external trigger signal such as an ECG (electrocardiogram) signal, it is not possible to accurately obtain the cumulative value of eddy magnetic fields, and it is not possible to sufficiently suppress degradation of image quality due to eddy magnetic fields. [Prior art documents] [Patent documents]
[0007] [Patent Document 1] JP 2019-017494 A Summary of the Invention [Problem to be solved by the invention]
[0008] One of the problems to be solved by the embodiments disclosed in this specification and the drawings is to estimate eddy magnetic fields with high accuracy and to sufficiently suppress deterioration of image quality caused by eddy magnetic fields even in an imaging method in which imaging is performed in synchronization with an external trigger signal. However, the problems to be solved by the embodiments disclosed in this specification and the drawings are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described below can also be positioned as other problems. [Means for solving the problem]
[0009] A magnetic resonance imaging apparatus according to an embodiment includes a time data generating unit, a gradient magnetic field generating unit, an estimating unit, a calculating unit, and a correcting unit. The time data generating unit continuously generates time data at a predetermined interval. The gradient magnetic field generating unit generates a series of gradient magnetic fields based on a predetermined pulse sequence in association with a series of first time data, which is the generated time data and corresponds to the generation time of each of the series of gradient magnetic fields. The estimating unit estimates an eddy magnetic field caused by the series of gradient magnetic fields, and a value of the eddy magnetic field at the second time, using the series of gradient magnetic fields, the series of first time data, and the generated time data, which is the generated time data and corresponds to a second time later than the generation time of each of the series of gradient magnetic fields. The calculating unit calculates a frequency fluctuation or a phase fluctuation caused by the eddy magnetic field of the magnetic resonance signal at the second time from the estimated value of the eddy magnetic field. The correcting unit corrects the frequency or phase of at least one of an RF transmission signal applied to a subject and a magnetic resonance signal emitted from the subject, using the calculated frequency fluctuation or the phase fluctuation. [Brief description of the drawings]
[0010] [Figure 1] 1 is a diagram showing an example of the configuration of a magnetic resonance imaging apparatus according to an embodiment; [Diagram 2] FIG. 2 is a block diagram showing an example of the configuration and functions of a sequence controller. [Diagram 3] 10 is a flowchart showing an example of a transmission process when an external trigger is not input. [Figure 4] FIG. 4 is a diagram showing an example of a pulse sequence together with time data. [Diagram 5] FIG. 1 is a schematic diagram showing the behavior in the time direction of each zero-order eddy magnetic field generated by a series of gradient magnetic fields, and the behavior of the fluctuations in the integrated eddy magnetic field obtained by integrating these zero-order eddy magnetic fields. [Figure 6] 6 is a timing chart showing an operation concept of a process for estimating an eddy magnetic field when a time of interest (second time) is in a transmission period of an RF transmission signal. [Figure 7] 10 is a flowchart showing an example of a reception process when an external trigger is not input. [Figure 8] 10 is a timing chart showing an operation concept of an estimation process of an eddy magnetic field when a time of interest (second time) is a period during which an MR signal is received. [Figure 9] FIG. 2 is a conceptual explanatory diagram of a first technique for correcting the frequency or phase of a received signal. [Figure 10] FIG. 11 is a conceptual explanatory diagram of a second technique for correcting the frequency or phase of a received signal. [Figure 11] 11 is a flowchart showing an example of a transmission process when an external trigger is input. [Figure 12] FIG. 4 is a conceptual diagram illustrating a transmission process when an external trigger is input. [Figure 13] 10 is a flowchart showing an example of a receiving process when an external trigger is input. [Figure 14] FIG. 4 is a conceptual diagram illustrating a receiving process when an external trigger is input. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] A magnetic resonance imaging apparatus according to an embodiment will be described with reference to the accompanying drawings. In the following embodiments, parts with the same reference numerals perform the same operations, and duplicated descriptions will be omitted as appropriate.
[0012] 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus 1 according to an embodiment. The magnetic resonance imaging apparatus 1 includes a magnet stand 100, a control cabinet 300, a console 400, a bed 500, and an RF (Radio Frequency) coil 20.
[0013] The magnetic gantry 100 has a static magnetic field magnet 10, a gradient magnetic field coil 11, and a WB (Whole Body) coil 12. These components are housed in a cylindrical housing. The bed 500 has a bed body 50 and a top board 51.
[0014] The control cabinet 300 includes gradient magnetic field power supplies 31 (for the X-axis 31x, for the Y-axis 31y, and for the Z-axis 31z), an RF receiver 32, an RF transmitter 33, and a sequence controller .
[0015] The console 400 includes a processing circuit 40, a memory circuit 41, a display 42, and an input I / F 43. The console 400 functions as a host computer.
[0016] The static magnetic field magnet 10 of the magnet gantry 100 has a roughly cylindrical shape and generates a static magnetic field in a bore in which a subject, for example, a patient, is transported. The bore is the space inside the cylinder of the magnet gantry 100. The static magnetic field magnet 10 incorporates a superconducting coil, which is cooled to an extremely low temperature by, for example, liquid helium. In the excitation mode, the static magnetic field magnet 10 applies a current supplied from a static magnetic field power supply (not shown) to the superconducting coil to generate a static magnetic field. After that, when the mode is switched to the persistent current mode, the static magnetic field power supply is disconnected. Once the mode is switched to the persistent current mode, the static magnetic field magnet 10 continues to generate a static magnetic field for a long period of time, for example, for more than one year.
[0017] In order to maintain an extremely low temperature state, the static magnetic field magnet 10 contains the liquid helium and the superconducting coil in a cylindrical heat shield plate made of an alloy such as aluminum.
[0018] The gradient magnetic field coil 11 also has a roughly cylindrical shape and is fixed inside the static magnetic field magnet 10. This gradient magnetic field coil 11 applies gradient magnetic fields in the X-axis, Y-axis, and Z-axis directions to the subject by currents supplied from gradient magnetic field power supplies (31x, 31y, 31z).
[0019] The bed body 50 of the bed 500 can move the top plate 51 in the vertical and horizontal directions. Before imaging, the subject placed on the top plate 51 is moved to a predetermined height. Then, during imaging, the top plate 51 is moved horizontally to move the subject into the bore.
[0020] The WB coil 12 is fixed in a generally cylindrical shape so as to surround the subject inside the gradient magnetic field coil 11. The WB coil 12 transmits RF pulses transmitted from an RF transmitter 33 toward the subject. It also receives magnetic resonance signals, i.e., MR (Magnetic Resonance) signals, emitted from the subject by excitation of hydrogen nuclei.
[0021] The magnetic resonance imaging apparatus 1 includes an RF coil 20 as shown in FIG. 1 in addition to the WB coil 12. The RF coil 20 is placed close to the body surface of the subject. There are several types of RF coils 20. For example, the RF coil 20 includes a body coil placed on the chest, abdomen, or legs of the subject as shown in FIG. 1, and a spine coil placed on the back of the subject. Most of the RF coils 20 are receive-only coils, but there are also types that perform both transmission and reception. The RF coil 20 is configured to be detachable from the tabletop 51 via a cable.
[0022] The RF transmitter 33 amplifies a transmission signal, i.e., an RF pulse, output from a sequence controller 34. The amplified RF pulse is transmitted to the WB coil 12 and applied to the subject. An MR signal is generated from the subject by the application of the RF pulse. The RF coil 20 or the WB coil 12 receives this MR signal.
[0023] The MR signal received by the RF coil 20 is input to the RF receiver 32 via cables provided on the tabletop 51 and the bed body 50, for example.
[0024] The RF receiver 32 amplifies the MR signal, which is an analog signal, and outputs it to the sequence controller .
[0025] The sequence controller 34, under the control of the console 400, generates a gradient magnetic field signal (or gradient magnetic field data) to be output to the gradient magnetic field power supply 31 and an RF pulse (hereinafter, sometimes referred to as an RF transmission signal or simply a transmission signal) to be output to the RF transmitter 33, based on various data related to a pulse sequence required for scanning the subject. In addition, the sequence controller 34 performs AD (Analog to Digital) conversion of the MR signal output from the RF receiver 32 to convert it into an MR signal of a digital signal, and further generates an MR signal of a baseband complex signal. The MR signal of a baseband complex signal is sometimes called raw data or k-space data.
[0026] Moreover, the sequence controller 34 of the embodiment performs processing for suppressing the frequency fluctuation (or phase fluctuation) of the magnetic resonance signal caused by the above-mentioned eddy magnetic field. Specific processing of the sequence controller 34 will be described later.
[0027] The console 400 controls the entire magnetic resonance imaging apparatus 1, such as setting and changing imaging conditions and imaging protocols including pulse sequences for imaging a subject, and issuing commands to start and end imaging, and also displays and stores MR images. The console 400 includes a storage circuitry 41, a display 42, an input interface (I / F) 43, and a processing circuitry 40. The storage circuitry 41 is a storage medium including a ROM (Read Only Memory), a RAM (Random Access Memory), and an external storage device such as a HDD (Hard Disk Drive) and an optical disk device. The storage circuitry 41 stores various information and data, and also stores various programs executed by a processor included in the processing circuitry 40.
[0028] The display 42 is a display device such as a liquid crystal display panel, a plasma display panel, an organic EL panel, etc. The input I / F 43 is, for example, a mouse, a keyboard, a trackball, a touch panel, etc., and includes various devices for an operator to input various information and data.
[0029] The processing circuit 40 is, for example, a circuit equipped with a CPU or a dedicated or general-purpose processor. The processor realizes various functions by executing various programs stored in the memory circuit 41. The processing circuit 40 may be configured with hardware such as an FPGA or an ASIC. Various functions can also be realized by such hardware. Moreover, the processing circuit 40 can realize various functions by combining software processing by a processor and a program with hardware processing.
[0030] Fig. 2 is a block diagram showing an example of the configuration and functions of the sequence controller 34. Fig. 2 is a block diagram that particularly focuses on the process for suppressing the frequency fluctuation (or phase fluctuation) of the magnetic resonance signal caused by the eddy magnetic field.
[0031] 2, the sequence controller 34 has a processing circuit 340. The processing circuit 340 realizes various functions shown in FIG.
[0032] The processing circuit 340 is a circuit equipped with a CPU and a dedicated or general-purpose processor. The processor executes various programs stored in a storage circuit (not shown) to realize various functions. The number of CPUs and processors is not particularly limited, and may be one or two or more. The processing circuit 340 may be configured with hardware such as one or more FPGAs (Field Programmable Gate Arrays) or ASICs (Application Specific Integrated Circuits). The various functions shown in FIG. 2 can also be realized by such hardware. The processing circuit 340 can also realize various functions by combining software processing by a processor and a program with hardware processing.
[0033] As is well known, when a gradient magnetic field pulse is applied to the gradient magnetic field coil 11 of the magnetic resonance imaging apparatus 1, an eddy magnetic field is generated. When a gradient magnetic field pulse is applied to the cylindrical gradient magnetic field coil 11, a gradient magnetic field is generated not only inside but also outside the gradient magnetic field coil 11. The magnetic field generated outside the gradient magnetic field coil 11 is called a leakage magnetic field. When this leakage magnetic field interlinks with the heat shield plate of the static magnetic field magnet 10, an eddy current flows in the heat shield plate because the heat shield plate is a conductor.
[0034] Eddy currents generate eddy magnetic fields. The generated eddy magnetic fields are superimposed on the gradient magnetic field and static magnetic field, disturbing the originally intended magnetic field environment and causing degradation of image quality. Therefore, in order to suppress the generation of eddy currents, an active shielded gradient coil (ASGC) is used, which is provided with a coil for canceling the leakage magnetic field on the outside of the gradient coil 11. The gradient coil 11 shown in FIG. 1 may also be configured as an active shielded gradient coil 11.
[0035] Although the eddy current is suppressed by the active shielding type gradient magnetic field coil 11, it is difficult in reality to completely eliminate the eddy current. Therefore, it is also difficult in reality to completely eliminate the eddy magnetic field, and it is important to suppress the influence of this eddy current magnetic field.
[0036] Eddy magnetic fields can be divided into magnetic fields that are independent of spatial position and magnetic fields that are dependent on spatial position. An eddy magnetic field that is independent of spatial position is called the zeroth-order component of the eddy magnetic field (or simply the zeroth-order eddy magnetic field). On the other hand, a component of the eddy magnetic field that changes as a linear or quadratic or higher function with respect to spatial position is called the first-order component of the eddy current magnetic field, etc.
[0037] The first-order component of the eddy magnetic field and the like are factors that cause the magnitude of the gradient magnetic field to fluctuate. Fluctuations in the magnitude of the gradient magnetic field cause problems such as errors in pixel positions. To correct the first-order component of the eddy magnetic field and the like, a method of correcting the waveform shape of the gradient magnetic field pulse is known.
[0038] On the other hand, the zeroth-order component of the eddy current magnetic field can be regarded as an offset amount relative to the static magnetic field strength, and is a factor that fluctuates the magnetic resonance frequency (i.e., the Larmor frequency). The zeroth-order component of the eddy current magnetic field shows the same value spatially, but fluctuates over time.
[0039] As a method for suppressing such fluctuations in the zeroth-order component of the eddy magnetic field, for example, a method is known in which the fluctuations in the magnetic resonance frequency at a certain point in time are calculated from a value obtained by previously accumulating the eddy magnetic fields caused by a series of gradient magnetic fields applied prior to that point in time (i.e., the integrated value of the eddy magnetic fields).
[0040] However, in the case of an imaging method in which imaging is performed in synchronization with an external trigger signal such as an ECG (electrocardiogram) signal, the start timing of a pulse sequence including the application timing of a gradient magnetic field changes depending on the occurrence timing of the external trigger signal. Therefore, the integrated value of the eddy magnetic field changes dynamically depending on the occurrence timing of the external trigger, and it becomes impossible to accurately obtain the integrated value of the eddy magnetic field. As a result, there is a problem that the amount of fluctuation in the magnetic resonance frequency cannot be accurately calculated, and the deterioration of image quality due to the fluctuation in the magnetic resonance frequency cannot be sufficiently suppressed.
[0041] As will be described below, the sequence controller 34 of the magnetic resonance imaging apparatus 1 of the embodiment has a function for solving such problems.
[0042] 2, the processing circuit 340 of the sequence controller 34 realizes each of the functions of a time data generation function F01, a gradient magnetic field data / signal generation function F02, an eddy magnetic field estimation function F03, a frequency / phase fluctuation calculation function F04, a transmission processing function F05, a reception processing function F06, a pulse sequence data setting function F07, a control function F08, and a reconstruction processing function F09. The transmission processing function F05 has, as its internal configuration, a transmission signal generation function F51 and a transmission signal correction function F52. Moreover, the reception processing function F06 has, as its internal configuration, a reception function F61 and a reception signal correction function F62. The time data generating function F01 continuously generates time data at a predetermined interval. For example, the time data generating function F01 generates time data with the start of imaging at 0 seconds so that the time data increases continuously at intervals shorter than 1 microsecond (for example, intervals on the order of nanoseconds). There are no particular limitations on the method of generating the time data, and for example, the processor may generate the time data sequentially, or the time data may be generated by counting up a clock signal of a predetermined frequency.
[0043] Based on a command from the console 400, the pulse sequence data setting function F07 sets parameter values related to the gradient magnetic field and RF pulses according to the type of pulse sequence to be applied to the subject as pulse sequence data in the gradient magnetic field data / signal generation function F02, the transmission processing function F05, and the reception processing function F06.
[0044] The gradient magnetic field data / signal generation function F02 generates a series of gradient magnetic field data or gradient magnetic field signals based on a predetermined pulse sequence in association with a series of time data (hereinafter referred to as a series of first time data) generated by the time data generation function F01 and corresponding to the generation times of each of the series of gradient magnetic fields based on the predetermined pulse sequence.
[0045] The series of generated gradient magnetic field signals are converted into a predetermined gradient magnetic field current by the gradient magnetic field power supply 31, and is applied to the gradient magnetic field coil and output to the eddy magnetic field estimation function F03.
[0046] The eddy magnetic field estimation function F03 estimates an eddy magnetic field caused by a series of gradient magnetic fields, and a value of the eddy magnetic field at a second time, using a series of gradient magnetic field data generated by the gradient magnetic field data / signal generation function F02, a series of first time data generated by the time data generation function F01, and second time data corresponding to a second time later than the generation times of each of the series of gradient magnetic fields. Here, as described later, the second time is a time corresponding to the generation time of an RF transmission signal or the sampling time of a received signal.
[0047] A frequency / phase variation calculation function F04 calculates a frequency variation or a phase variation of the magnetic resonance signal at a second time, which is caused by the eddy magnetic field, from the estimated value of the eddy magnetic field.
[0048] The transmission signal generating function F51 of the transmission processing function F05 generates an RF transmission signal in association with the above-mentioned second time data based on a predetermined pulse sequence (for example, based on pulse sequence data output from the pulse sequence data setting function F07). The RF transmission signal is an RF pulse having a predetermined magnetic resonance frequency and a predetermined envelope.
[0049] The RF transmission signal can be generated by using a DDS (Direct Digital Synthesizer). In the DDS, a signal corresponding to the magnetic resonance frequency is directly generated as digital data, and then the RF transmission signal is generated by D / A conversion. Instead of generating the RF transmission signal by such digital processing, the RF transmission signal may be generated by analog processing. For example, a baseband pulse signal having a predetermined envelope may be generated as an analog signal by D / A conversion or the like, and then the RF transmission signal having the magnetic resonance frequency may be generated by up-converting it by a local signal of a predetermined frequency.
[0050] The transmission signal generating function F51 can generate an RF transmission signal including at least one of an excitation pulse, a refocus pulse, and a labeling pulse as a relatively wideband RF pulse including magnetic resonance signals of both water and fat. The transmission signal generating function F51 can also generate an RF transmission signal that is narrower in band than each of the above RF pulses and includes at least one of a fat suppression pulse having a center frequency equal to the magnetic resonance frequency of fat, and a water excitation pulse having a center frequency equal to the magnetic resonance frequency of water.
[0051] The transmission signal correction function F52 of the transmission processing function F05 uses the estimated eddy magnetic field value to correct the frequency or phase of the RF transmission signal to generate an RF transmission signal so that fluctuations in the frequency or phase of the magnetic resonance signal caused by the eddy magnetic field are suppressed.
[0052] The frequency or phase of the RF transmission signal may be corrected for a baseband analog signal or for a local signal used for up-conversion. When the RF transmission signal is generated by a DDS, the frequency or phase may be corrected for the signal as digital data.
[0053] On the other hand, the reception processing function F06 uses the estimated eddy magnetic field value to frequency convert the magnetic resonance signal received by the RF coil 20 or the WB coil 12 to baseband, or corrects the frequency or phase of the magnetic resonance signal converted to baseband, so as to suppress fluctuations in the frequency or phase of the magnetic resonance signal caused by the eddy magnetic field.
[0054] More specifically, the receiving processing function F06 corrects the frequency of a local signal for frequency converting the magnetic resonance signal to baseband in the analog domain or digital domain so that fluctuations in the frequency or phase of the magnetic resonance signal caused by eddy magnetic fields are suppressed.
[0055] Alternatively, the reception processing function F06 may correct the frequency or phase of the magnetic resonance signal (i.e., k-space data) converted to baseband by digital calculation so that fluctuations in the frequency or phase of the magnetic resonance signal caused by eddy magnetic fields are suppressed.
[0056] The reconstruction processing function F09 reconstructs the frequency- or phase-corrected magnetic resonance signal (i.e., k-space data) by Fourier transform or the like to generate an MR image. The reconstruction processing function F09 may be realized by the processing circuit 40 of the console 400, instead of the sequence controller 34. The control function F08 performs the overall control of the sequence controller 34.
[0057] The more specific operation of each of the above-mentioned functions will be described below with reference to the flowcharts of Figures 3, 7, 11, and 13, as well as Figures 4, 5, 6, 8, 9, 10, 12, and 14.
[0058] The magnetic resonance imaging apparatus 1 of the embodiment has the same configuration regardless of whether an external trigger is input or not. However, in the following, from the viewpoint of ease of understanding, the operation when an external trigger is not input will be described first, and then the operation when an external trigger is input will be described.
[0059] 3 is a flowchart showing an example of a transmission process when an external trigger is not input. First, in step ST100, imaging conditions, a type of pulse sequence, and the like are set as pulse sequence data. The process of step ST100 is performed by a pulse sequence data setting function F07.
[0060] FIG. 4 is a diagram showing an example of a pulse sequence set by the pulse sequence data setting function F07, together with time data (FIG. 4(a)) generated by the time data generating function F01. FIG. 4 illustrates a pulse sequence of a standard spin echo method. FIG. 4(b) shows an RF pulse having an excitation pulse and a refocus pulse, FIG. 4(c) shows a slice selection gradient magnetic field pulse Gss, FIG. 4(d) shows a phase encoding gradient magnetic field pulse Gpe, FIG. 4(e) shows a readout gradient magnetic field pulse Gro, and FIG. 4(f) shows an MR signal received by the RF coil 20.
[0061] The RF pulse shown in Fig. 4(b) and each gradient magnetic field pulse shown in Fig. 4(c) to Fig. 4(e) are generated in association with the time data generated by the time data generating function F01. The MR signal shown in Fig. 4(f) is also sampled at a timing associated with the time data generated by the time data generating function F01.
[0062] There is no limitation on the type of pulse sequence that can be applied to the magnetic resonance imaging apparatus 1 of this embodiment, and any pulse sequence can be used in the magnetic resonance imaging apparatus 1 of this embodiment.
[0063] The criteria for starting the generation of time data by the time data generation function F01 are not particularly limited, but for example, the generation of time data by the time data generation function F01 may be started by using the start of imaging as a trigger, as in steps ST101 and ST102.
[0064] In step ST103, as illustrated in FIG. 4(c) to FIG. 4(e), a series of gradient magnetic fields based on a pulse sequence are generated in association with the generated series of first time data.
[0065] In the next step ST104, at a second time (transmission time of the next RF transmit signal) after the generation times of each of the series of gradient magnetic fields, an RF transmit signal based on the pulse sequence is generated in association with the second time data.
[0066] Then, in the next step ST105, an eddy magnetic field at a second time is estimated using the series of gradient magnetic fields, the series of first time data, and the series of second time data.
[0067] Figure 5 is a schematic diagram showing the behavior over time of each zero-order eddy magnetic field generated by a series of gradient magnetic fields, and the behavior of the fluctuation ΔB(t) of the integrated eddy magnetic field obtained by integrating these zero-order eddy magnetic fields.
[0068] 5(a), 5(c), and 5(e) each show a portion of the slice selection gradient magnetic field pulse Gss, the phase encoding gradient magnetic field pulse Gpe, and the readout gradient magnetic field pulse Gro of the pulse sequence shown in FIG.
[0069] The shape of the gradient magnetic field pulse is actually approximated as a trapezoidal wave rather than a rectangular wave. An eddy magnetic field is generated to cancel the magnetic field fluctuation at the rising and falling edges of the trapezoidal wave. As described above, the eddy magnetic field has a zero-order component that does not depend on the spatial position, and a component that changes as a linear function of the spatial position or a higher-order function of the second or higher order.
[0070] Fig. 5(b) is a diagram showing a waveform of the zero-order eddy magnetic field (zero-order component of the eddy current magnetic field) ΔBss generated in response to the slice selection gradient magnetic field pulse Gss. Similarly, Fig. 5(d) is a diagram showing a waveform of the zero-order eddy magnetic field ΔBpe generated in response to the phase encoding gradient magnetic field pulse Gpe, and Fig. 5(f) is a diagram showing a waveform of the zero-order eddy magnetic field ΔBro generated in response to the readout gradient magnetic field pulse Gro.
[0071] When each gradient magnetic field pulse rises, i.e., when the gradient magnetic field changes in the positive direction, a negative zero-order eddy magnetic field is generated to cancel out this change, and this zero-order eddy magnetic field decays according to a function that is approximated by an exponential function. On the other hand, when each gradient magnetic field pulse falls, i.e., when the gradient magnetic field changes in the negative direction, a positive zero-order eddy magnetic field is generated to cancel out this change, and this zero-order eddy magnetic field also decays according to a function that is approximated by an exponential function.
[0072] The time constant of the exponential function can be a large value, for example, from several msec to about 2000 msec. Therefore, as shown in Fig. 5(b), (d), and (f), the multiple zero-order eddy magnetic fields generated at the rising and falling edges of each gradient magnetic field pulse continue to attenuate even after the application of each gradient magnetic field pulse.
[0073] The time constant of the zero-order eddy magnetic field can be obtained in advance from actual measurement data, etc. Also, the magnitude of the zero-order eddy magnetic field can be estimated in advance from the magnitude of change in the rising and falling edges of the gradient magnetic field pulse.
[0074] Therefore, based on the type of pulse sequence set in step ST100 and its parameters, a plurality of zero-order eddy magnetic fields generated corresponding to the rising and falling edges of a series of gradient magnetic fields as shown in Fig. 5(b), (d), and (f) can be estimated as a function of time. Then, by integrating a plurality of zero-order eddy magnetic fields generated in the past from a certain time in each of the slice selection direction, phase encoding direction, and readout direction on the time axis, and further integrating the zero-order eddy magnetic fields integrated on the time axis in the slice selection direction, phase encoding direction, and readout direction, it is possible to estimate an integrated value of the zero-order eddy magnetic field ΔB(t) that changes from moment to moment as a function of time as shown in Fig. 5(g).
[0075] As described above, in the magnetic resonance imaging apparatus 1 of the embodiment, a series of gradient magnetic fields based on a pulse sequence is generated in association with a series of first time data. Here, the series of first time data refers to time data (for example, time data corresponding to the rising and falling edges of each gradient magnetic field pulse) associated with each of the slice selection gradient magnetic field pulse Gss, the phase encoding gradient magnetic field pulse Gpe, and the readout gradient magnetic field pulse Gro among the time data generated by the time data generating function F01.
[0076] Furthermore, a time of interest (i.e., the time at which the integrated value of the zero-order eddy magnetic field ΔB(t) is to be calculated) that is later than the generation time of the series of gradient magnetic fields is set as the second time. In this case, the values of the series of exponentially decaying eddy magnetic fields at the second time can be calculated based on the magnitude of each of the series of gradient magnetic fields and the time differences between the series of first time data and the second time data corresponding to the second time. Then, by integrating all the calculated values of the eddy magnetic fields at the second time as described above, the value of the zero-order eddy magnetic field ΔB at the second time, which is the time of interest, can be calculated.
[0077] In addition, since the waveform of the zero-order eddy magnetic field is approximated as an exponential function, the zero-order eddy magnetic field once generated does not become completely zero even after a long time has passed. Therefore, when calculating the integrated value of the zero-order eddy magnetic field ΔB(t), the multiple zero-order eddy magnetic fields generated by the gradient magnetic field pulses applied within the past Tpast seconds immediately preceding the second time are used as the integration targets.
[0078] FIG. 6 is a timing chart showing the operation concept of steps ST103 to ST105 when the time of interest (ie, the second time) is immediately before the transmission of the RF transmission signal or during the transmission period of the RF transmission signal.
[0079] Figures 6(a), 6(b), 6(c), and 6(d) are timing charts corresponding to Figures 4(a), 4(c), 4(d), and 4(e), respectively. Also, Figure 6(e) is a diagram corresponding to Figure 5(g), and is a diagram showing a schematic diagram of the time change of the integrated value of the zero-order eddy magnetic field ΔB(t).
[0080] FIG. 6(f) is a timing chart showing generation of RF pulses, and FIG. 6(f) shows an example in which an excitation pulse and a refocus pulse are repeatedly generated at a repetition time TR.
[0081] Fig. 6 shows a situation in which the time immediately before the generation of the third excitation pulse from the left shown in Fig. 6(f) is taken as the time of interest, i.e., the second time, and the zero-order eddy magnetic field at the second time is calculated. In this case, as described above, the zero-order eddy magnetic field at the second time is calculated using a series of gradient magnetic fields generated between the time going back a predetermined time Tpast from the second time and the second time, and a series of first times and second times associated with this series of gradient magnetic fields.
[0082] In the time data shown in Figure 6(a), each successively increasing time data is represented by a white vertical stripe, and among these time data, a series of first time data each corresponding to the generation time of a series of gradient magnetic fields (or the rise and fall times of a series of gradient magnetic fields) is represented by a black vertical stripe.
[0083] Returning to FIG. 3, the zero-order eddy magnetic field ΔB(t) estimated in step ST105 in FIG. 3 is ΔB(Ttoi) where the time of interest (second time) is Ttoi.
[0084] 3, the fluctuation in frequency or phase of the magnetic resonance signal at the second time is calculated from the estimated eddy magnetic field. For example, the frequency fluctuation Δf(Ttoi) of the magnetic resonance frequency at the second time (time of interest Ttoi) can be calculated from the estimated eddy magnetic field ΔB(Ttoi) using the following (Equation 1) to (Equation 4).
[0085] B(Ttoi)=B0+ΔB(Ttoi) (Equation 1) f(Ttoi)=f0+Δf(Ttoi) (Equation 2) f0=λ·B0 / (2π) (Eq. 3) Δf(Ttoi)=λ·ΔB(Ttoi) / (2π) (Equation 4) Here, B0 is the magnitude of the static magnetic field, f0 is the magnetic resonance frequency determined only by the static magnetic field B0, and λ is a constant called the gyromagnetic ratio.
[0086] It is also possible to calculate a phase variation Δφ(Ttoi) that has a one-to-one correspondence with the frequency variation Δf(Ttoi).
[0087] 3, the frequency or phase of the transmission signal is corrected so as to suppress fluctuations in the calculated frequency or phase of the magnetic resonance signal. This correction is performed by the transmission signal correction function F52 of the transmission processing function F05, as described above.
[0088] In step ST108, the corrected RF transmission signal (RF transmission pulse) is transmitted to the subject. The above is an example of the transmission process when no external trigger is input.
[0089] On the other hand, Fig. 7 is a flow chart showing an example of reception processing when an external trigger is not input. The processing from step ST100 to step ST103 is the same as in Fig. 3, so a description thereof will be omitted.
[0090] In step ST204, a second time, which is after the generation time of each of the series of gradient magnetic fields and is the reception time of the next reception signal, is determined based on the pulse sequence.
[0091] In the next step ST205, an eddy magnetic field at a second time is estimated using the series of gradient magnetic fields, the series of first time data, and second time data corresponding to the second time.
[0092] 8 is a timing chart showing the operation concept of step ST204 and step ST205. In the reception process, a time of interest (i.e., a second time) corresponds to a time when reception of an MR signal starts or a time during a reception period of an MR signal (i.e., a sampling period).
[0093] The reception process shown in Fig. 8(a) to Fig. 8(e) differs in that the time of interest (i.e., the second time) corresponds to the time when the MR signal starts to be received (or during the sampling period of the MR signal), and the transmission process shown in Fig. 6(a) to Fig. 6(e) differs in that the time of interest (i.e., the second time) corresponds to the transmission time of the RF transmission signal. However, apart from this difference, Fig. 8(a) to Fig. 8(e) and Fig. 6(a) to Fig. 6(e) are substantially the same, and the method of estimating the zero-order eddy magnetic field ΔB(Ttoi) at the time of interest (i.e., the second time) Ttoi is also substantially the same as the method described in the transmission process.
[0094] In step ST206 of FIG. 7, the fluctuation (Δ) in the frequency or phase of the magnetic resonance signal at the time of interest (second time) is calculated from the estimated zero-order eddy magnetic field ΔB(Ttoi). This calculation method is also the same as the method described in the transmission process, and can be calculated using (Equation 1) to (Equation 4).
[0095] In step ST207, the frequency or phase of the received signal is corrected so that the calculated fluctuation in the frequency or phase of the magnetic resonance signal is suppressed.
[0096] Fig. 9 is a conceptual diagram of a first method for correcting the frequency or phase of a received signal. In this first method, the reception processing function F06 shown in Fig. 2 is realized by an A / D conversion function 321, a DDS (Direct Digital Synthesizer) function 322, and a detection function 323, as shown in Fig. 9.
[0097] The reception processing function F06 illustrated in Fig. 9 corresponds to a so-called direct sampling method. In the direct sampling method, the MR signal amplified by the RF receiver 32 is directly sampled by the A / D conversion function 321 without frequency conversion, and directly converted from an analog signal to a digital signal. Furthermore, in the direct sampling method, the MR signal converted to a digital signal is detected using a reference signal having a carrier frequency component output from the DDS 322, and converted to a baseband digital MR signal. This baseband digital MR signal is, for example, a complex signal composed of an I signal and a Q signal, and as described above, may be called raw data or k-space data.
[0098] In the correction process of the first method, the frequency of the reference signal output from the DDS 322 is configured to change in accordance with the fluctuation of the eddy magnetic field. As a result, in the output of the detection function 323, the frequency fluctuation caused by the fluctuation of the eddy magnetic field is offset, and the influence of the eddy magnetic field can be suppressed.
[0099] 10 is a conceptual explanatory diagram of a second technique for correcting the frequency or phase of a received signal, in which the frequency or phase fluctuation of the received signal is corrected by digital calculation of k-space data.
[0100] Fig. 10(a) shows a schematic diagram of the MR signal as an analog signal. Fig. 10(b) shows the MR signal S(t) as a baseband digital signal. The MR signal before correction is represented as S(t).
[0101] Fig. 10(c) shows a phase variation Δφ(t) obtained by time-integrating a frequency variation Δf(t) of a received signal. Fig. 10(d) is a diagram showing a schematic diagram of a corrected MR signal (digital). If the corrected MR signal is represented as S'(t), the corrected MR signal is calculated, for example, by the following (Equation 5).
[0102] S'(t)=S(t)·exp[-jΔφ(t)] (Equation 5) Returning to FIG. 7, in step ST208 in FIG. 7, the corrected received signals (MR signals) are subjected to reconstruction processing to generate an MR image.
[0103] Up to this point, the transmission process and reception process when no external trigger is input have been explained. Below, the transmission process and reception process when an external trigger is input will be explained.
[0104] The external trigger is, for example, an ECG signal or a respiratory synchronization signal. Imaging using an ECG signal as an external trigger is called electrocardiogram-gated imaging, which is an imaging method for acquiring MR signals at a specific cardiac phase in synchronization with an ECG signal, for example, in cardiac imaging. Imaging using a respiratory synchronization signal as an external trigger is called respiratory synchronization imaging, which is an imaging method for acquiring MR signals in synchronization with a respiratory synchronization signal, for example, during a specific period of the expiratory phase out of the expiratory and inspiratory phases.
[0105] In the synchronous imaging method using an external trigger, imaging starts after a certain delay time Tdl from the time when the external trigger is input, and imaging is stopped once MR signals corresponding to a certain number of TR (pulse repetition periods) are acquired. After that, when the next external trigger is input, imaging is resumed after a delay time Tdl from that time, and this is repeated every time an external trigger is input to acquire MR signals required to generate an image.
[0106] In normal imaging without using an external trigger, the interval between adjacent excitation pulses (i.e., TR) is always constant. However, the period of an external trigger signal such as an ECG signal or a respiratory synchronization signal is not constant but fluctuates. For this reason, in a synchronous imaging method using an external trigger signal, the interval between an excitation pulse immediately before an external trigger signal is input and an excitation pulse immediately after the external trigger signal is input is not TR, but changes depending on the input timing of the external trigger. Similarly, the repetition interval of a gradient magnetic field in a specific application direction (e.g., readout direction) is not constant, and the interval between a gradient magnetic field pulse immediately before an external trigger signal is input and a gradient magnetic field pulse immediately after the external trigger signal is input changes depending on the input timing of the external trigger.
[0107] The magnetic resonance imaging apparatus 1 of the embodiment can accurately determine the eddy magnetic field at a time of interest, as will be described with reference to Figures 11 to 14, even in an imaging method in which periodicity is lost before and after the input of an external trigger.
[0108] The external trigger can be acquired, for example, by the control function F08 of the processing circuit 340. The control function F08 acquires the external trigger by associating it with trigger time data, which is time data generated by the time data generating function F01 and corresponds to the acquisition time of the external trigger.
[0109] FIG. 11 is a flowchart showing an example of a transmission process when an external trigger is input.
[0110] Steps ST100 to ST102 in FIG. 11 are the same processes as those performed when no external trigger is input, and so a description thereof will be omitted.
[0111] In step ST303, a series of gradient magnetic fields based on a pulse sequence are generated or regenerated in association with the generated series of first time data.
[0112] The magnetic resonance imaging apparatus 1 of the embodiment uses a method of generating a series of gradient magnetic fields based on a pulse sequence in association with time data. Therefore, data for generating a series of gradient magnetic fields (for example, data defining the waveform of a gradient magnetic field pulse) is created and stored in advance in association with time data, and this data is read out by checking it against time data that increases in real time, thereby generating a series of gradient magnetic fields in real time.
[0113] However, in the synchronous imaging using an external trigger, data for generating a gradient magnetic field created in advance cannot be used after the time when the external trigger is input. Therefore, in step ST303, when the external trigger is input (YES in step ST305), the gradient magnetic field corresponding to the time after the input of the external trigger is discarded (step ST306), and then a series of gradient magnetic fields based on the pulse sequence are "regenerated" in association with the series of generated first time data.
[0114] In step ST304, at a second time (transmission time of the next transmission signal) after the generation times of each of the series of gradient magnetic fields, an RF transmission signal based on the pulse sequence is generated or regenerated in association with second time data. As with the series of gradient magnetic fields, data for generating the RF transmission signal (for example, data defining the waveform of an RF pulse) is generated in advance in association with time data and stored, and this data is read out by checking against the time data that increases in real time, thereby generating the RF transmission signal in real time.
[0115] However, in synchronous imaging using an external trigger, data for generating an RF transmit signal created in advance cannot be used after the external trigger is input. Therefore, in step ST303, when an external trigger is input (YES in step ST305), an RF transmit signal corresponding to a time after the external trigger is input is discarded (step ST306), and then an RF transmit signal based on a pulse sequence is "regenerated."
[0116] Fig. 12 is a conceptual explanatory diagram of the transmission process when an external trigger is input. Fig. 12(b) shows the input timing of the external trigger. In the example shown in Fig. 12, an excitation pulse is generated and imaging is resumed at a second time point that is a delay time Tdl after the external trigger is input. Therefore, data for generating a gradient magnetic field and data for generating an RF transmission signal corresponding to the period from the input of the external trigger to the second time point are discarded as shown by the dashed line in Fig. 12.
[0117] On the other hand, after a delay time Tdl has elapsed since the external trigger was input, a new RF pulse and a new series of gradient magnetic fields are generated, as shown by the solid lines in FIG.
[0118] In step ST105 of FIG. 11, an eddy magnetic field at a second time is estimated using a series of gradient magnetic fields, a series of first time data, and a series of second time data. Here, in estimating the eddy magnetic field at the second time, which is a time of interest, the gradient magnetic fields discarded between the external trigger and the second time are not used, and eddy magnetic fields based on each of a series of gradient magnetic fields actually applied from the second time to the past are used. In addition, the attenuation amount of the eddy magnetic field according to a predetermined time constant is calculated using each time difference between a series of first time data associated with a series of gradient magnetic fields actually applied from the second time to the past and the second time data, which is the actual time of interest. Therefore, even in synchronous imaging using an external trigger, the eddy magnetic field at the second time (the generation time of the excitation pulse after the input of the external trigger in the example shown in FIG. 12) can be estimated with high accuracy.
[0119] Steps ST106 to ST108 in FIG. 11 are the same processes as those in FIG. 3, and therefore the description thereof will be omitted.
[0120] Fig. 13 is a flow chart showing an example of reception processing when an external trigger is input. Steps ST100 to ST102 in Fig. 13 are the same processing as when an external trigger is not input, and therefore description thereof will be omitted.
[0121] In step ST403 in Fig. 13, a series of gradient magnetic fields based on a pulse sequence are generated or regenerated in association with a series of generated first time data. In step ST403, similarly to step ST303 in Fig. 11, when an external trigger is input (YES in step ST405), the gradient magnetic fields corresponding to the time after the input of the external trigger are discarded (step ST406), and then a series of gradient magnetic fields based on a pulse sequence are "regenerated" in association with a series of generated first time data.
[0122] In step ST404, a second time, which is a time when the next reception signal is received and is after the generation time of each of the series of gradient magnetic fields, is determined or re-determined based on the pulse sequence. When an external trigger is input, the reception time after the input of the external trigger is determined by the input timing of the external trigger, so in step ST404, the second time, which is the time when the next reception signal is received, is "re-determined" based on the pulse sequence.
[0123] Fig. 14 is a conceptual explanatory diagram of the reception process when an external trigger is input. In the example shown in Fig. 14, an excitation pulse is generated after a delay time Tdl from the input of an external trigger, and imaging is resumed, as in Fig. 12. Then, a reception period of an MR signal (or a sampling period of an MR signal) starts at a time of interest (a second time) after a predetermined echo time TE from the excitation pulse.
[0124] On the other hand, data for generating a gradient magnetic field and data for generating an RF transmission signal corresponding to the period from when the external trigger is input until the delay time Tdl has elapsed are discarded, as shown by the dashed lines in Figure 14, and data relating to the reception time corresponding to this period is also discarded.
[0125] In step ST205, the eddy magnetic field at the second time is estimated using a series of gradient magnetic fields, a series of first time data, and a second time data corresponding to the second time. Here, the second time, which is the time of interest, is an actual time of interest that is newly determined after the application of the external trigger. In addition, the gradient magnetic field used for estimating the eddy magnetic field is an eddy magnetic field based on each of a series of gradient magnetic fields that have actually been applied from the second time to the past.
[0126] Therefore, even in synchronous imaging using an external trigger, the eddy magnetic field at the second time (in the example shown in FIG. 14, the sampling time of the received signal after the input of the external trigger) can be estimated with high accuracy.
[0127] According to the magnetic resonance imaging apparatus of each of the embodiments described above, even in an imaging method in which imaging is performed in synchronization with an external trigger signal, it is possible to estimate eddy magnetic fields with high accuracy and sufficiently suppress degradation of image quality caused by eddy magnetic fields.
[0128] In addition, the time data generation function, gradient magnetic field data / signal generation function, eddy magnetic field estimation function, and frequency / phase fluctuation calculation function in the description of the embodiments are examples of the time data generation unit, gradient magnetic field generation unit, estimation unit, and calculation unit, respectively, in the description of the claims. In addition, the transmission signal correction function and reception signal correction function in the description of the embodiments are examples of the correction unit in the description of the claims. In addition, the control function in the description of the embodiments is an example of the external trigger acquisition unit in the description of the claims. In addition, the reception processing function in the description of the embodiments is an example of the reception unit in the description of the claims.
[0129] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]
[0130] 1. Magnetic resonance imaging equipment 34 Sequence Controller 340 Processing Circuit F01 Time data generation function F02 Gradient magnetic field data / signal generation function F03 Eddy magnetic field estimation function F04 Frequency / phase fluctuation calculation function F05 Transmission processing function F06 Receiving processing function F51 Transmission signal generation function F52 Transmission signal correction function F61 Receiving function F62 Received signal correction function
Claims
1. a time data generating unit that continuously generates time data at predetermined intervals; A gradient magnetic field generating unit that generates a series of gradient magnetic fields based on a predetermined pulse sequence in association with a series of first time data that is the generated time data and corresponds to the generation time of each of the series of gradient magnetic fields; an estimation unit that estimates an eddy magnetic field caused by the series of gradient magnetic fields, the value of the eddy magnetic field at the second time, using the series of gradient magnetic fields, the series of first time data, and the generated second time data, the second time data corresponding to a second time later than the generation times of the series of gradient magnetic fields; a calculation unit that calculates a frequency fluctuation or a phase fluctuation of the magnetic resonance signal at the second time caused by the eddy magnetic field from the estimated value of the eddy magnetic field; a correction unit that corrects at least one of a frequency or a phase of an RF transmission signal to be applied to a subject and a magnetic resonance signal emitted from the subject, using the calculated frequency variation or phase variation; A magnetic resonance imaging apparatus comprising:
2. the estimation unit estimates a value of the eddy magnetic field at the second time by using a time difference between the series of first time data and the series of second time data.
2. A magnetic resonance imaging apparatus according to claim 1.
3. an external trigger acquisition unit that acquires an external trigger in association with trigger time data that is the generated time data and corresponds to an acquisition time of the external trigger; the respective time differences vary depending on when the external trigger is acquired; The estimation unit estimates a value of the eddy magnetic field at the second time by using the time difference that changes depending on the acquisition time of the external trigger.
3. A magnetic resonance imaging apparatus according to claim 2.
4. each of the eddy magnetic fields resulting from the series of gradient magnetic fields is a zero-order eddy magnetic field that exponentially decays with a predetermined time constant; the estimation unit estimates the eddy magnetic field at the second time by integrating the zero-order eddy magnetic fields estimated using the respective time differences at the second time.
3. A magnetic resonance imaging apparatus according to claim 2.
5. an RF signal generating unit that generates the RF transmission signal based on the predetermined pulse sequence in association with the second time data, 2. A magnetic resonance imaging apparatus according to claim 1.
6. the estimation unit estimates a value of the eddy magnetic field at a transmission time of the RF transmission signal by using a time difference between the series of first time data and the second time data corresponding to a generation time of the RF transmission signal; The RF signal generating unit corrects a frequency or a phase of the RF transmission signal using the estimated value of the eddy magnetic field so that a fluctuation in a frequency or a phase of a magnetic resonance signal caused by the eddy magnetic field is suppressed, and generates the RF transmission signal.
6. A magnetic resonance imaging apparatus according to claim 5.
7. The RF signal generating unit corrects a frequency or a phase of the RF transmission signal by using the estimated value of the eddy magnetic field at each time that advances by a predetermined increment during a transmission period of the RF transmission signal so that fluctuations in the frequency or the phase of the magnetic resonance signal caused by the eddy magnetic field are suppressed, thereby generating the RF transmission signal.
7. A magnetic resonance imaging apparatus according to claim 6.
8. The RF signal generating unit generates the RF transmission signal as a pulse including at least one of an excitation pulse, an inversion pulse, and a labeling pulse.
8. A magnetic resonance imaging apparatus according to claim 6 or 7.
9. The RF signal generating unit generates the RF transmission signal as a pulse including at least one of a fat saturation pulse and a water excitation pulse.
8. A magnetic resonance imaging apparatus according to claim 6 or 7.
10. A receiving unit that samples and receives the magnetic resonance signal in association with the second time data based on the predetermined pulse sequence.
2. A magnetic resonance imaging apparatus according to claim 1.
11. the estimation unit estimates a value of the eddy magnetic field at a reception time of the magnetic resonance signal by using a time difference between the series of first time data and the second time data corresponding to a reception time of the magnetic resonance signal; The receiving unit uses the estimated value of the eddy magnetic field to frequency-convert the magnetic resonance signal to a baseband, or corrects the frequency or phase of the magnetic resonance signal converted to the baseband, so that fluctuations in frequency or phase of the magnetic resonance signal caused by the eddy magnetic field are suppressed.
11. A magnetic resonance imaging apparatus according to claim 10.
12. The receiving unit corrects a local frequency for frequency converting the magnetic resonance signal to a baseband in an analog domain or a digital domain so that a fluctuation in frequency or phase of the magnetic resonance signal caused by the eddy magnetic field is suppressed.
12. A magnetic resonance imaging apparatus according to claim 11.
13. The receiving unit corrects the frequency or phase of the magnetic resonance signal converted to a baseband by digital calculation so that fluctuations in the frequency or phase of the magnetic resonance signal caused by the eddy magnetic field are suppressed.
12. A magnetic resonance imaging apparatus according to claim 11.
Citation Information
Patent Citations
Magnetic resonance imaging device
JP2019017494A