JP3848005B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus that measures magnetic field fluctuations (distortion of gradient magnetic field waveform) based on eddy currents generated by a pulsed gradient magnetic field with high accuracy and at high speed.
[0002]
[Prior art]
A magnetic resonance imaging apparatus (hereinafter abbreviated as an MRI apparatus) applies a uniform static magnetic field by a static magnetic field magnet to an examination space including a subject to create polarization magnetization, and applies a high frequency electromagnetic field and a gradient magnetic field in pulses. In this apparatus, a magnetic resonance phenomenon is caused in a nucleus in the body of a subject, and a tomographic image is reconstructed by computer processing after receiving a magnetic resonance signal (NMR signal) generated by the magnetic resonance phenomenon. Since the gradient magnetic field is generated in a pulse shape by the gradient magnetic field coil and the gradient magnetic field amplifier, if there is an electric conductor around the gradient magnetic field coil, an eddy current is generated in the conductor when the gradient magnetic field is switched. Examples of the electrical conductor include a heat shield of a static magnetic field magnet. Currently, in the case of a superconducting magnet used in an MRI apparatus of 0.5 Tesla or more, the innermost layer has a liquid helium layer, and the outer layer has a plurality of metal layers such as a liquid nitrogen layer. Since the temperature, material, and size of these metal layers are not the same, the strength and time constant of the eddy current has a plurality of components. Typical time constants span a wide range from a few ms to a few s. The fluctuating magnetic field generated by the eddy current (hereinafter abbreviated as an eddy magnetic field) distorts the gradient magnetic field waveform indicated by the controller of the MRI apparatus, resulting in image artifacts.
[0003]
Several techniques have been reported for reducing the intensity of the generated eddy magnetic field and shaping the gradient magnetic field waveform in accordance with the eddy magnetic field. A typical example is the use of an active shield type gradient coil (ASGC) in the former, and eddy field compensation in the latter. In principle, the strength of the eddy magnetic field can be greatly reduced by the ASGC, but it is inevitable that a minute eddy magnetic field is generated due to the manufacturing error of the ASGC, the discrete arrangement of the coil wire, and the like. In ultra-high-speed imaging methods that have been remarkably developed in recent years, such as the echo planar method, artifacts may occur in an image due to the presence of this slight eddy magnetic field. For this reason, ASGC is not used alone but is often used in combination with eddy magnetic field compensation.
[0004]
In order to accurately perform this eddy magnetic field compensation, it is necessary to measure in advance the strength and time constant of the eddy magnetic field. Various methods using NMR signals have been proposed as means for measuring eddy magnetic fields with high sensitivity as described below. However, as will be described later, none of the methods can accurately measure the time constant of the eddy magnetic field over a wide range from several ms to several s in a short time.
[0005]
As a first eddy magnetic field measurement method, there is a method described in Japanese Patent Publication No. 3-49453. A sample for NMR imaging (for example, a bag filled with water) is placed away from the center of the magnetic field, and a gradient magnetic field (a gradient magnetic field applied for modification before the gradient magnetic field pulse used in data collection) is used. The FID signal of the sample is acquired after applying the pre-pulse because of the pulse), and the waveform of the eddy magnetic field is measured from the phase change. FIG. 12 shows a pulse sequence related to the acquisition method. Geddy represents a prepulse, eddyL and eddyS represent eddy magnetic fields induced by the prepulse Geddy, eddyL represents a component having a long time constant TL, and eddyS represents a component having a short time constant TS. TL corresponds to several s, and TS corresponds to several ms to several tens of ms. ADC1 and ADC2 represent the sampling period of the FID signal, and are typically about 100 ms.
[0006]
Since the FID signal is from the entire NMR sample, the sample must be small and must be located away from the center of the magnetic field. In order to measure the short time constant eddy magnetic field eddyS, only the sampling period ADC1 starting immediately after the application of the pre-pulse Geddy is sufficient. However, in order to measure the long time constant eddy magnetic field eddyL, it is necessary to collect the FID signal in the sampling period ADC2 having a predetermined delay time TD after the application of the pre-pulse Geddy. In this way, by collecting the FID signal several times while changing the interval (delay time TD) between the pre-pulse and the data collection period, the long time constant eddy magnetic field eddyL can be measured.
[0007]
FIG. 13 illustrates the principle. The graph of FIG. 13 represents the time change of the phase shift due to the eddy magnetic field in the NMR signal or MRI image. It is assumed that there are two component eddy magnetic fields of a short time constant and a long time constant as indicated by 131 and 132 in FIG. The sampling period ADC in FIG. 12 corresponds to each period in which arrows are attached to both ends in the “no readout gradient magnetic field” row in FIG. By repeating FID signal collection (ADC1 to ADCn), it is possible to measure until a component having a long time constant is attenuated. However, in this case, the prepulse application interval TG needs to be about 10 times the long time constant TL so that the eddy magnetic field due to the previous prepulse is sufficiently attenuated.
[0008]
Therefore, this method has a demerit that the measurement time becomes long. The measurement time is estimated under the following conditions. As the number N of excitations for collecting the FID signal, it is assumed here that N = 10 because data must be collected several times with the delay time TD changed. Assuming that TL is about 1 s, TG is about 10 s. Moreover, at least two places are required as positions for arranging the NMR samples. This is because the spatial component of the eddy magnetic field includes a zero-order component and a higher-order component in addition to the first-order component. Since the zero-order component is derived from the center shift (manufacturing error, assembly error) of ASGC, it changes for each ASGC. Higher order components are always generated due to the difference in radius between the ASGC and the magnet (vortex generating source). In eddy compensation, since only the primary component must be measured, it is necessary to change the position at which the NMR sample is arranged a plurality of times (M times: at least twice, here twice). Therefore, the time for measuring the NMR sample is at least TG × N × M (about 200 s), and the measurement time is considerably long.
[0009]
As an improved example of the first eddy magnetic field measurement method, there is a method described in Japanese Patent Laid-Open No. 4-189344. In this method, it is not necessary to move the NMR sample, and eddy magnetic fields at a plurality of sample points can be measured simultaneously. However, even in this method, when measuring a long time constant, it is still necessary to change the delay time TD, and the time required for data collection is the same as that of Japanese Patent Publication No. 3-49453.
[0010]
As a second eddy magnetic field measurement method, a method for measuring the spatial distribution of an eddy magnetic field is proposed in Japanese Patent Laid-Open No. 2-177940. In this method, an image is obtained by imaging by a spin echo (SE) method or a field echo (FE) method, and then a gradient magnetic field pre-pulse is applied to perform an imaging sequence of SE method or FE method. The spatial distribution of eddy magnetic field strength is obtained from the phase difference of each image. FIG. 14 shows a pulse sequence in the case of the two-dimensional FE method. In the figure, Gr is a readout gradient magnetic field, and Ge is a phase encoding gradient magnetic field. The obtained phase difference image value is substantially proportional to the integral value of the eddy magnetic field intensity from the application of the pre-pulse Geddy to the echo time TE.
[0011]
This prior art document describes that the time constant of the eddy magnetic field is measured, but does not disclose a method for measuring both the time constant and the intensity. In this second method, the interval between the pre-pulse and the echo convergence time (the pre-pulse application end time and the time until the excitation RF pulse is applied + echo time) is changed, or the pre-pulse application time is changed. This is because it is necessary to measure several of the above phase images. The sampling period ADC in FIG. 14 corresponds to the point in the row “with readout gradient magnetic field” in FIG. One of the points corresponds to one imaging of the FE method. However, in this case, an eddy magnetic field having a time constant shorter than the echo time cannot be measured. Therefore, an eddy magnetic field with a time constant of several ms cannot be measured. On the other hand, even for a vortex magnetic field having a longer time constant, the value is determined by fitting a plurality of phases with a complex function having eddy magnetic field strength and time constant as parameters, so the accuracy is not high.
[0012]
As an improved method of the second eddy magnetic field measurement method, there is a third eddy magnetic field measurement method that repeats the FE imaging sequence multiple times per pre-pulse ("New Efficient Eddy-Field-Mapping Procedure (FAME)", J. Schiff, H. Rotem, S. Stokar, and N. Kaplan, Journal of Magnetic resonance, Series B 104, 73-76 (1994)). As a result, it is possible to measure the time constant and intensity of the eddy magnetic field having a time constant longer than the execution time of the imaging sequence of the FE method. However, this method cannot measure an eddy magnetic field having a time constant shorter than the echo time, and the measurement accuracy of the eddy magnetic field having a time constant longer than the echo time and shorter than the execution time of the imaging sequence of the FE method is not high.
[0013]
As described above, in the conventional eddy magnetic field measurement method, the time constant over a wide time constant range of several ms to several s cannot be measured in a short time, and the value measured over a long time is also accurate. There was a problem of being bad.
[0014]
Furthermore, as described above, the presence of a coupling component of the eddy magnetic field (hereinafter referred to as coupling vortex) has become a problem due to the performance improvement of ASGC and the highly accurate measurement and compensation of the eddy magnetic field. For example, when an x-channel gradient magnetic field coil is pulse-driven, in addition to an eddy magnetic field whose intensity changes in the x direction, an eddy magnetic field component whose intensity changes in the y direction or the z direction is also generated. However, conventionally, since the eddy magnetic field could not be measured with high accuracy and in a short time, the influence of such a coupling vortex was ignored. For this reason, highly accurate eddy compensation could not be performed.
[0015]
[Problems to be solved by the invention]
As described above, in the conventional eddy magnetic field measurement method, the time constant over a wide time constant range of several ms to several s cannot be measured in a short time, and the value measured over a long time is also accurate. There was a problem of being bad. Furthermore, there is a problem that the coupling vortex cannot be compensated.
[0016]
An object of the present invention is to provide a magnetic resonance imaging apparatus capable of accurately measuring the time constant and intensity of a vortex magnetic field having a time constant over a wide time constant range of several ms to several s in a short time.
[0018]
[Means for Solving the Problems]
In order to solve the above problems and achieve the object, the present invention uses the following means.
[0019]
According to one aspect of the present invention, a magnetic resonance imaging apparatus applies a static magnetic field to a phantom disposed in an imaging space, applies a pulsed gradient magnetic field once, and then applies a slice selective gradient magnetic field. By selecting at least two slices and exciting the phantom multiple times for a single application of the pulsed gradient magnetic field, the signals from each excitation can be obtained without applying the pulsed gradient magnetic field multiple times. The signal acquisition measurement is performed at least twice by changing the intensity of the pulsed gradient magnetic field, and the time constant and intensity of the eddy magnetic field from the difference of the time change curves of the phases of the at least two acquired signal sets. And a means for obtaining.
According to another aspect of the present invention, a magnetic resonance imaging apparatus applies a static magnetic field to a phantom disposed in an imaging space, applies a pulsed gradient magnetic field once, and then applies a slice selective gradient magnetic field. At least two slices, and exciting the phantom a plurality of times for one application of the pulsed gradient magnetic field, so that the signal generated by each excitation without applying the pulsed gradient magnetic field multiple times. And at least two of the above signal acquisition measurements by changing the carrier frequency of the high frequency pulse used for exciting the phantom without changing the intensity of the pulsed gradient magnetic field and the intensity of the slice selective gradient magnetic field. Means for determining the time constant and the intensity of the eddy magnetic field from the difference between the time change curves of the phases of the at least two acquired signal sets.
According to yet another aspect of the present invention, at least two slices are applied by applying a static magnetic field to a phantom disposed in an imaging space, applying a pulsed gradient magnetic field once, and then applying a slice selective gradient magnetic field. And collecting the signals by each excitation without applying the pulsed gradient magnetic field multiple times by exciting the phantom multiple times with respect to one application of the pulsed gradient magnetic field. The signal acquisition measurement is performed at least twice by changing the intensity of the slice selective gradient magnetic field without changing the intensity of the pulsed gradient magnetic field and the carrier frequency of the high-frequency pulse used for exciting the phantom, Means for determining the time constant and strength of the eddy magnetic field from the difference between the phase change curves of the phases of the two acquired signal sets.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of an MRI apparatus according to the present invention will be described below with reference to the drawings.
[0032]
First Embodiment FIG. 1 is a block diagram showing a schematic configuration of an entire MRI apparatus to which the first embodiment is applied. In the gantry 20, a static magnetic field magnet 1, an x-axis, a y-axis, a z-axis gradient magnetic field coil 2, and a transmission / reception coil 3 are provided. The static magnetic field magnet 1 as a static magnetic field generator is configured using, for example, a superconducting coil, a normal conducting coil, or a permanent magnet. The x-axis, y-axis, and z-axis gradient magnetic field coils 2 are coils for generating an x-axis gradient magnetic field Gx, a y-axis gradient magnetic field Gy, and a z-axis gradient magnetic field Gz. The transmission / reception coil 3 generates radio frequency (RF) pulses and is used to detect echo signals generated by magnetic resonance. The subject P on the bed 13 is inserted into an imageable region in the gantry 20 (a spherical region where an imaging magnetic field is formed, and diagnosis is possible only in this region). Note that transmission of RF pulses and reception of echo signals may be performed by separate transmission coils and reception coils.
[0033]
The static magnetic field magnet 1 is driven by a static magnetic field control device 4. The transmitter / receiver coil 3 is driven by the transmitter 5 when magnetic resonance is excited, and is coupled to the receiver 6 when an echo signal is detected. The x-axis, y-axis, and z-axis gradient magnetic field coils 2 are driven by an x-axis gradient magnetic field amplifier 7, a y-axis gradient magnetic field amplifier 8, and a z-axis gradient magnetic field amplifier 9.
[0034]
The x-axis gradient magnetic field amplifier 7, the y-axis gradient magnetic field amplifier 8, the z-axis gradient magnetic field amplifier 9, and the transmitter 5 are driven by the sequence controller 10 according to a predetermined sequence, and the x-axis gradient magnetic field Gx, the y-axis gradient magnetic field Gy, and the z-axis A gradient magnetic field Gz and a radio frequency (RF) pulse are generated in a predetermined pulse sequence described later. In this case, the x-axis gradient magnetic field Gx, the y-axis gradient magnetic field Gy, and the z-axis gradient magnetic field Gz are mainly used, for example, as a read gradient magnetic field Gr, a phase encode gradient magnetic field Ge, and a slice gradient magnetic field Gs, respectively. The host computer 11 drives and controls the sequence controller 10, captures an echo signal as an echo signal received by the receiver 6, and performs predetermined signal processing to generate a tomographic image of the subject, and the display unit 12. Is displayed.
[0035]
FIG. 2 shows a pulse sequence for performing eddy magnetic field measurement according to the first embodiment. In the eddy magnetic field measurement of the first embodiment, a relatively small NMR sample is placed in a region where the eddy magnetic field is to be measured, and after applying a gradient magnetic field prepulse to the sample, an RF pulse is applied to excite the sample multiple times. The first step of continuously collecting the FID signal, the second step of obtaining the time change curve of the phase of the FID signal at each excitation, and the step of obtaining the time constant and intensity of the eddy magnetic field from the time change curve The pulse sequence shown in FIG. 2 relates to the first step. Geddy is a gradient magnetic field pre-pulse, and ADC1 and ADC2 are sampling periods of each FID signal. In FIG. 2, the FID signal is collected twice, but it may be more. In FIG. 2, eddyL represents an eddy magnetic field having a long time constant, and eddyS represents an eddy magnetic field having a short time constant. The time constant and intensity of eddyS having a time constant of about several ms to several tens of ms can be measured based only on the FID signal collected in the first FID signal collection period ADC1. The time constant and intensity of the longer time constant eddyL can be measured by combining the FID signals collected by the second and subsequent FID signal collection ADC2... ADCn.
[0036]
The time when the k-th data of the n-th FID signal collection period ADCn is collected is assumed to be t _{n, k} . In the second step, the phase of the phase is obtained by obtaining the phase of the signal and plotting the phase against t _{n, k} . Subsequently, in the third step, this time change curve is applied to a function of the strength of the eddy magnetic field and the time constant. The phase of the signal at the sample position r is expressed by the following equation.
[0037]
φ (r, t)
= Γατ _L G _pre · r (Exp (t _a / τ _L ) −Exp (t _b / τ _L ))
* Exp (−t _n / τ _L ) (1-Exp (− (t _{n, k} −t _n ) / τ _L ))
Here, τ _L and α are the time constant and intensity of the vortex, G _pre is the pre-pulse intensity (vector), t _a and t _b are the pre-pulse application start time and end time, and t _n is for generating the nth FID. RF pulse application time, γ is the magnetic rotation ratio, and the pre-pulse is assumed to be a rectangular pulse.
[0038]
Since a plurality of data different in n and k are collected, the time constant and intensity of the eddy magnetic field can be obtained by applying the above phase to the data. An existing method such as a least square method can be used for the fitting. If the pre-pulse is not a rectangular pulse, the above equation needs to be modified, but this is easy for those skilled in the art.
[0039]
As described above, according to the present embodiment, the gradient magnetic field prepulse Geddy is applied only once, and a plurality of signals can be collected. Therefore, the method described in Japanese Patent Publication No. 3-49453 shown in FIG. Compared with the measurement of the constant eddy magnetic field eddyS and the measurement of the long time constant eddy magnetic field eddyL, the prepulse Geddy is applied, and the measurement becomes faster. In the description of the prior art, the measurement time of the method described in Japanese Patent Publication No. 3-49453 is estimated as 200 s, but in this embodiment, 20 s (TG = 10 s, N = 1, M = 2), and 10 minutes. Can be shortened to 1. In this case, similarly to the method described in Japanese Patent Publication No. 3-49453, the position of the NMR sample is changed at least twice in this embodiment. Usually, the position where the NMR sample is placed is determined so as to shift the phase in the direction in which the prepulse is applied. One time out of these two times, the NMR sample may be placed at the center of the magnetic field.
[0040]
As a better modification of the present embodiment, the first step and the second step of the present embodiment are performed twice by changing the intensity of the prepulse, and the third step is performed after taking the difference between the two phase change curves. There is a way to do it. This can effectively remove the influence when there is a phase shift due to non-uniformity of the static magnetic field or the like.
[0041]
Hereinafter, other embodiments of the apparatus according to the present invention will be described. In the description of the other embodiments, the same parts as those in the first embodiment are denoted by the same reference numerals, and the detailed description thereof is omitted.
[0042]
Second Embodiment Since the block diagram of the second embodiment is the same as the block diagram of the first embodiment, the illustration is omitted. FIG. 3 shows a pulse sequence related to the first step of this embodiment. The first embodiment is different from the first embodiment in that an almost uniform and large volume phantom including the center of the magnetic field is used as a sample for outputting the NMR signal, and when the phantom is excited to collect the FID signal, The slice selective gradient magnetic field Gs is also applied simultaneously with the application. Usually, the gradient magnetic field pre-pulse Geddy and the slice selection gradient magnetic field Gs are set so as to be gradient magnetic fields in the same direction. Therefore, in this case, the pre-pulse is applied in the slice direction. In the first embodiment, there is no limitation on the application direction of the pre-pulse. In the first step of continuously collecting FID signals, signals of the same slice are always collected. Compared with the first embodiment, this embodiment has the advantage that the time for moving the NMR sample is not required, and the time can be shortened accordingly. Unlike the first embodiment, the slice gradient magnetic field is applied at the time of excitation because a large phantom is used so that the excitation cross-sectional position is specified.
[0043]
Also in this embodiment, as in the modification of the first embodiment, by changing the prepulse intensity and performing the first step twice, static magnetic field inhomogeneity, eddy magnetic field due to other imaging gradient magnetic field pulses, etc. It is possible to cancel the phase shift. However, the shooting time will be doubled.
[0044]
Therefore, two other modifications of the present embodiment that can remove the influence of static magnetic field inhomogeneity without extending the imaging time will be described. In the modification shown in FIG. 4, two acquired data sets ADC1 (+) to ADCn (+) having different slice gradient magnetic field strengths Gs while keeping the carrier frequency Cf of the RF pulse for generating the NMR signal and the intensity of the prepulse the same. And a difference in phase between ADC1 (−) to ADCn (−). In the modification shown in FIG. 5, two acquired data sets ADC1 (+) to ADCn (+) and ADC1 having different carrier frequencies Cf of RF pulses for generating an NMR signal while maintaining the same slice gradient magnetic field strength and prepulse strength are used. A phase difference is performed between (−) to ADCn (−). 4 and 5 are both obtained by acquiring NMR signals from two slices, but the way of exciting the slices is different. These two modifications are effective only when there is not much difference in the static magnetic field inhomogeneity among the two slices to be excited, but there is an advantage that the imaging time is not extended. Moreover, the modification shown in FIG. 5 is a more preferable embodiment than the modification of FIG. 4 in that the influence of the eddy magnetic field caused by other imaging gradient magnetic field pulses can be removed.
[0045]
Third Embodiment A third embodiment of the present invention will be described. In the present embodiment, the phantom or the subject itself is arranged so as to include the eddy magnetic field region to be measured, and then the gradient magnetic field prepulse Geddy is applied, and the phantom or a part of the subject is excited. The second step of applying the phase encode pulse Ge, the third step of collecting the NMR signal, and the first step to the third step while changing the intensity of the phase encode pulses Ge1 and Ge2 (here) Then, a fourth step repeated twice), a fifth step for reconstructing a plurality of images (two images) from those NMR signals, a sixth step for obtaining a phase change curve of the plurality of images, The seventh step is to obtain the time constant and strength of the eddy magnetic field from the phase change curve. FIG. 6 shows a pulse sequence of the first to fourth steps when this embodiment is configured to measure a spatial two-dimensional distribution using an FID signal. It can be easily analogized that this embodiment is modified for spatial one-dimensional or spatial three-dimensional measurement.
[0046]
The fifth to seventh steps can be realized by known methods. For example, the chemical shift imaging reconstruction method can be applied to the fifth step, and the second step and the third step of the first embodiment can be applied to the sixth step and the seventh step.
[0047]
This embodiment is similar to the eddy magnetic field spatial distribution measurement method disclosed in Japanese Patent Laid-Open No. 4-189344, but differs in that a plurality of NMR signals are excited for one prepulse. Thereby, the measurement time can be significantly reduced. For example, in the case of the same eddy magnetic field and pre-pulse as in the first embodiment, in the method described in Japanese Patent Laid-Open No. 4-189344, the measurement time is TG (= 10 s) × 16 per 16 × 16 matrix image. × 16 × N (= 10 times: number of excitations per pre-pulse) = 7.1 hours, whereas in this embodiment, N = 1, so measurement can be performed in 43 minutes, which is 1/10 of that. is there. Also in this embodiment, as in the first embodiment and the second embodiment, the prepulse application intensity is changed and measured twice, and the time constant and intensity of the eddy magnetic field are calculated after taking the difference between the phase change curves. Spatial distribution may be measured.
[0048]
A better improvement in the first, second, and third embodiments is to use a transverse magnetization spoiler or RF spoiler in combination with a gradient magnetic field. This has the effect of preventing the mixing of NMR signals from one or more previous excitations when continuously excited, and improving the accuracy of FID signal measurement. For the same reason, it is desirable that the transverse relaxation time of the NMR sample is about the excitation interval TR or less.
[0049]
As described above, according to the first, second, and third embodiments of the present invention, the time constant and strength of the eddy magnetic field over a wide range of time constants from several ms to several s can be increased more quickly than in the past. Moreover, it becomes possible to measure with high accuracy.
[0050]
Fourth Embodiment Next, an embodiment in which a coupling vortex, which is a second problem in eddy magnetic field measurement / compensation, is measured and compensated with high accuracy will be described. In the fourth embodiment of the present invention, a first step for measuring a coupling vortex, a second step for generating a coupling waveform reflecting the strength and time constant of the coupling vortex, and a first step for generating a normal eddy magnetic field compensation waveform. 3 steps, and a fourth step of adding and outputting the coupling waveform and the vortex compensation waveform. FIG. 7 is a flowchart showing the flow of processing in the first step and the second step. In FIG. 7, using the first, second, and third embodiments of the present invention and a known eddy magnetic field measurement method, a pre-pulse in the first axis (here, x-axis) direction is applied (step S2). ), The eddy magnetic field in the x-axis direction is measured (step S4). After obtaining the time constant and strength of the eddy magnetic field, the eddy compensation of the x coil is performed by setting the parameters of the eddy compensator Ex of the x coil in consideration of the transfer function of the gradient magnetic field coil (step S6). The parameter modifies the impulse response (hereinafter referred to as H (s)) of the magnetic field to the gradient magnetic field waveform input as follows. That is, when the impulse response before the eddy magnetic field compensation is f (s), the parameter of the vortex compensator Ex is H (s) = f (s) · (1 / f (s)) = 1. It is determined. As a specific realization method, the method described in the aforementioned Japanese Patent Publication No. 3-49453 can be used. Subsequently, a pre-pulse in the direction of the second axis (here, y-axis) orthogonal to the first axis direction (x-axis direction) is applied (step S8), and the eddy magnetic field in the x-axis direction is measured similarly. (Step S10). This eddy magnetic field is a coupling vortex, and a parameter reflecting this time constant and strength is set in the coupling vortex compensator Eyx (step S12). Finally, the waveform outputs of the compensators Ex and Eyx are added and used as the input of the gradient magnetic field amplifier, thereby realizing a desired magnetic field waveform (x direction) compensated for eddy.
[0051]
As an example, FIG. 8 shows how the waveform signal of the gradient magnetic field and the magnetic field are changed when a coupling vortex exists between the x coil and the y coil and only the x coil is driven. The sequence controller generates an x coil waveform output as shown in 8-1 in order to realize a rectangular x gradient magnetic field pulse. The x-coil vortex compensator Ex generates a waveform as shown in 8-3 so as to compensate the eddy magnetic field of the x-coil. Further, the coupling compensator Exy generates a waveform for compensating for the coupling vortex as shown in 7-4. On the other hand, since the waveform output of the y coil is zero as shown in 8-2, the output signal of the vortex compensator Ey shown in 8-6 and the output signal of the coupling compensator Eyx shown in 8-5 are also zero. It remains. When the output signals of the compensators Ex and Eyx are added and input to the gradient coil amplifier of the x coil, as shown in 8-7, the x gradient magnetic field waveform is squared from the distorted waveform (broken line) before eddy compensation. Shaped into a shape pulse (solid line). At the same time, as shown in 8-8, the gradient magnetic field waveform in the y direction becomes zero (solid line) by canceling the vortex waveform (broken line) before compensation.
[0052]
FIG. 9 shows a configuration for realizing compensation of both the normal vortex and the coupling vortex as described above. The vortex compensator Ex, Ey, Ez and the coupling vortex compensator Exy, Eyx, Exz, Ezx, Eyz, Ezy are configured by an analog circuit such as a differential circuit, a digital circuit such as a digital filter, or software. Such a circuit configuration can also be realized by using a known circuit (for example, JP-A-6-86766).
[0053]
FIG. 10 shows another configuration of the present embodiment. The difference from FIG. 9 is that the waveform output from the controller is added to the output signal of the coupling vortex compensator and then input to the vortex compensator. When configured in this way, the time constant and intensity of the coupling vortex magnetic field waveform can be set as they are in the coupling vortex compensator. On the other hand, in the case of FIG. 9, it is necessary to set after converting the time constant and the intensity in consideration of the transfer function of each gradient coil system.
[0054]
As a modification of this embodiment, the first step of measuring the coupling vortex can be replaced with a spatial two-dimensional eddy magnetic field measurement method. FIG. 11 shows the flow of processing in this case. A pre-pulse in the direction of the first axis (here, x-axis) is applied (step S22), and the second axis orthogonal to the first axis is applied. An eddy magnetic field in a plane (xy plane) defined by an axis (here, y-axis) is measured (step S24). In accordance with the time constant and strength of the eddy magnetic field, the parameters of the eddy compensator Ex of the x coil (step S26) and the parameters of the coupling vortex compensator Eyx are set (step S28).
[0055]
The present embodiment includes a step of obtaining the circuit constants and parameters of the eddy compensator from the measurement results (time constant and intensity) of the eddy magnetic field in consideration of the transfer function of each gradient magnetic field coil system. The transfer function may be obtained in advance, and the circuit constant and the filter coefficient may be obtained by calculation. In addition, the transfer function is not calculated, the circuit constants and filter coefficients are changed several times, the actual eddy magnetic field is measured, and the circuit constants and filter coefficients at which the eddy magnetic field becomes zero are obtained by existing optimization methods. May be.
[0056]
【The invention's effect】
As described above, according to the present invention, the spatial distribution of strength and time constant can be measured with high accuracy and high speed with respect to eddy magnetic fields in a wide range of time constant. In addition, measurement and compensation of coupling vortices can be realized, and deterioration in image quality due to eddy currents can be prevented.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a first embodiment of a magnetic resonance imaging apparatus according to the present invention.
FIG. 2 is a diagram showing a pulse sequence according to the first embodiment of the present invention.
FIG. 3 is a diagram showing a pulse sequence according to a second embodiment of the present invention.
FIG. 4 is a view showing a pulse sequence of a first modification of the second embodiment.
FIG. 5 is a view showing a pulse sequence of a second modification of the second embodiment.
FIG. 6 is a diagram showing a pulse sequence according to a third embodiment of the present invention.
FIG. 7 is a diagram illustrating a flow of eddy magnetic field measurement and vortex compensator parameter setting according to a fourth embodiment of the present invention.
FIG. 8 is a view showing a relationship between a waveform signal and a magnetic field waveform in the fourth embodiment.
FIG. 9 is a diagram illustrating a configuration example of a fourth embodiment.
FIG. 10 is a diagram showing another configuration example of the fourth embodiment.
FIG. 11 is a diagram showing a flow of processing in a modification of the fourth embodiment.
FIG. 12 is a diagram showing a pulse sequence in the prior art.
13 is a diagram showing the relationship between the FID collection period of FIG. 12 and the time constant of phase shift.
FIG. 14 is a diagram showing another conventional pulse sequence.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet 2 ... X-axis, y-axis, z-axis gradient magnetic field coil 3 ... Transmission / reception coil 4 ... Static magnetic field control apparatus 5 ... Receiver 6 ... Transmitter 7 ... X-axis gradient magnetic field amplifier 8 ... Y-axis gradient magnetic field amplifier 9 ... z-axis gradient magnetic field amplifier 10 ... sequence controller 11 ... host computer 12 ... display unit

Claims (4)

A static magnetic field is applied to the phantom arranged in the imaging space, and a pulsed gradient magnetic field is applied once, then a slice selection gradient magnetic field is applied to select at least two slices, and the pulsed gradient magnetic field Means for collecting signals by each excitation without applying the pulsed gradient magnetic field multiple times by exciting the phantom multiple times for one application;
Means for performing the signal acquisition measurement at least twice by changing the intensity of the pulsed gradient magnetic field, and obtaining the time constant and the intensity of the eddy magnetic field from the difference between the phase change curves of the phases of the at least two acquisition signal sets; ,
A magnetic resonance imaging apparatus comprising: A static magnetic field is applied to the phantom arranged in the imaging space, and a pulsed gradient magnetic field is applied once, then a slice selection gradient magnetic field is applied to select at least two slices, and the pulsed gradient magnetic field Means for collecting signals by each excitation without applying the pulsed gradient magnetic field multiple times by exciting the phantom multiple times for one application;
Without changing the intensity of the pulsed gradient magnetic field and the intensity of the slice selective gradient magnetic field, the signal collection measurement is performed at least twice by changing the carrier frequency of the high-frequency pulse used for excitation of the phantom, and the at least 2 Means for determining the time constant and strength of the eddy magnetic field from the difference of the time-varying curves of the phases of the two acquired signal sets;
A magnetic resonance imaging apparatus comprising: A static magnetic field is applied to the phantom arranged in the imaging space, and a pulsed gradient magnetic field is applied once, then a slice selection gradient magnetic field is applied to select at least two slices, and the pulsed gradient magnetic field Means for collecting signals by each excitation without applying the pulsed gradient magnetic field multiple times by exciting the phantom multiple times for one application;
Without changing the intensity of the pulsed gradient magnetic field and the carrier frequency of the high-frequency pulse used for exciting the phantom, the signal acquisition measurement is performed at least twice while changing the intensity of the slice selective gradient magnetic field, and the at least 2 Means for determining the time constant and strength of the eddy magnetic field from the difference of the time-varying curves of the phases of the two acquired signal sets;
A magnetic resonance imaging apparatus comprising:   After the excitation of the phantom, applying a phase encoding gradient magnetic field and repeating the signal acquisition by each excitation, changing the phase encoding amount, and obtaining the time constant and intensity of the eddy magnetic field from the difference of the time change curve of the phase of the acquired signal The magnetic resonance imaging apparatus according to claim 1, further comprising a unit.

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