JP3018076B2 - Inspection equipment using nuclear magnetic resonance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inspection apparatus using magnetic resonance, and more particularly to a method of measuring a magnetic resonance signal for obtaining a phase distribution inside an inspection object at high speed.

[0002]

2. Description of the Related Art An inspection apparatus using magnetic resonance places an inspection object in a static magnetic field, applies a high-frequency pulse to the inspection object to generate a nuclear spin, and measures an echo signal generated thereby. The inspection is performed by reconstructing an image inside the inspection target. In such an inspection apparatus, it is important to improve the uniformity of the static magnetic field strength from the viewpoint of improving the accuracy of the inspection. Therefore, conventionally, in order to perform the uniformity processing of the static magnetic field strength (hereinafter, referred to as shimming), a multi-channel magnetic field generating mechanism usually called a shim coil is built in the static magnetic field generating magnet, and various types of these generated magnetic fields are generated. By superimposing the characteristic shim magnetic field on the static magnetic field generated by the static magnetic field coil, the static magnetic field intensity in the imaging region is made uniform.

However, in the ultra-high-speed imaging method or spectroscopic imaging, the S / N (signal-to-noise ratio) and the spectrum are reduced due to the non-uniformity of the static magnetic field intensity of about several ppm or less, which is not a problem in ordinary imaging. The resolution is significantly degraded. On the other hand, the static magnetic field strength distribution in the static magnetic field coil is distorted by the characteristics of the magnet itself, the influence of the surrounding magnetic material, and the magnetic permeability distribution of the test object itself. It is desirable to improve the uniformity of the intensity of the static magnetic field in a state in which the inspection target is contained in the test object. Therefore, based on the same conditions as the actual imaging, two nuclear magnetic resonance images containing different phase information are taken, and the phase information of the two images is compared to calculate the nonuniformity of the static magnetic field intensity distribution. And performing shimming so as to correct the non-uniformity. For example, a method of obtaining a phase image from two nuclear magnetic resonance images and further obtaining a static magnetic field intensity distribution (hereinafter, referred to as a phase method) is described in Journal of Magn.
etic Resonance, 77, pp. 40-52 (1988).

Here, the above-mentioned phase method will be briefly described. First, nuclear spins placed in a static magnetic field are precessing, and their frequency is proportional to the static magnetic field strength. Therefore, if there is a spatial inhomogeneity in the static magnetic field strength in the excitation region, the nuclear spins excited by the high-frequency pulse will precess at various frequencies immediately after that, and the phase coherency will be lost. To go. The nuclear magnetic resonance image obtained in such a state is provided with phase information that is proportional to the time when the nonuniformity of the static magnetic field intensity is sensed. Therefore, assuming that the static magnetic field strength of the pixel x, y based on the xy coordinates is E (x, y), the pixel values S ₁ , S _{2 of the two} images I ₁ , I ₂ having different times from excitation to signal measurement. Is given by Equations 1 and 2, respectively. In those equations, e ₁ , e ₂
Is the time during which the images I ₁ and I ₂ sense the non-uniformity of the static magnetic field strength, r is the density of the nuclei, and γ is the gyromagnetic ratio.

[0005]

S ₁ (x, y) = r (x, y) exp {γE (x, y) e ₁ } (1)

[0006]

S ₂ (x, y) = r (x, y) exp {γE (x, y) e ₂ } (2) From Equations 1 and 2, the static magnetic field strength E (x, y) is It is obtained by Expression 3.

[0007]

E (x, y) = 1 / γ (e ₂ −e ₁ ) · atan {imag (S ₂ (x, y) / S ₁ (x, y)) / real (S ₂ (x, y y) / S ₁ (x, y))} (3) where atan is the arc tangent and imag is (S
₂ (x, y) / S ₁ (x, y)), the real part is (S ₂ (x, y) / S
₁ (x, y)).

As a pulse sequence for photographing the images I ₁ and I ₂ , a spin echo method, a gradient echo method, or the like is used. In the case of the spin echo method, the phase coherency starts to be disturbed immediately after the excitation high-frequency pulse is applied, but by applying the inverted high-frequency pulse,
This coherency begins to recover. The time interval between the application time of the excitation high-frequency pulse and the application time of the inverted high-frequency pulse is represented by t ₁ , and the time interval between the application time of the inverted high-frequency pulse and the echo measurement time is represented by t ₂ , where t ₁ = t ₂ . The effect of the inhomogeneity of the static magnetic field strength on the phase is completely canceled. Therefore, when the spin echo method is used for measuring the static magnetic field intensity distribution, an asymmetric spin echo method in which t ₁ and t ₂ are different is used. In this case, the difference between t ₁ and t ₂ is e ₁ in Equation ₁ or e ₂ in Equation ₂ .

[0009]

As described above, when an image is photographed by an ultra-high-speed photographing method or the like, it is necessary to perform shimming to improve the uniformity of the static magnetic field intensity distribution. However, in the ultra-high-speed imaging method, a high-intensity gradient magnetic field is switched at high speed in order to perform signal measurement in a short time, so that an eddy current is generated on a conductor surface of the gradient magnetic field coil, and the static magnetic field intensity distribution is thereby reduced. There is a problem of being affected. In other words, a secondary magnetic field is generated by the eddy current generated by switching the high-intensity gradient magnetic field at high speed,
This is superimposed on the static magnetic field E (x, y).

The nonuniformity of the static magnetic field due to the influence of the eddy current cannot be obtained by the phase method using two images having different phase information. That is, in the imaging sequence for measuring two images, usually, only the echo generation time is different, and the readout gradient magnetic field waveforms are equal, so that the influence of the eddy current included in the echo is equal. Therefore, if the phases of the two images are subtracted from each other, the phase term affected by the eddy current is canceled out, so that it is impossible to determine the static magnetic field inhomogeneity due to the eddy current.

On the other hand, in order to solve the above-mentioned problem caused by eddy current, an apparatus for executing an imaging method in which a large amount of eddy current is generated often uses a gradient magnetic field coil with an active shield. This active coil has an additional coil in which the same current flows in the opposite direction to the current flowing in the main coil of the gradient magnetic field coil. Is to cancel.

[0012] However, since the effect of the active shield is not sufficient in practice, the photographing method such as the spin echo method in which the eddy current generation is small and the photographing method such as the echo planar method in which the eddy current generation is large are used. The static magnetic field strength distribution in the region is not equal. Therefore, using the static magnetic field strength distribution measured by the former imaging method, the non-uniformity of the static magnetic field intensity is obtained by the above-described phase method, and even if shimming is performed based on this, in that state, the latter imaging method When an image is taken, there is a problem that the effect of improving the uniformity of the static magnetic field cannot be sufficiently obtained because the amount of generated eddy current is different.

An object of the present invention is to accurately measure a static magnetic field intensity distribution or uniformity caused by an overcurrent.

[0014]

The above objects can be attained by the following means. First, a large eddy current is generated by high-speed switching of a high-intensity gradient magnetic field, which is mainly caused by a read-out gradient magnetic field. For example, the echo planar method, which is one of the ultra-high-speed imaging methods, uses a gradient magnetic field of the same intensity in the slice direction and the phase encoding direction as compared with the imaging methods such as the spin echo method, but in the lead-out direction. The gradient magnetic field having an intensity of 2 to 4 times is switched at a frequency of about 1 kHz.

In view of the above, according to the present invention, in capturing two static magnetic field intensity distribution images for measuring a static magnetic field inhomogeneity, a readout used in a static magnetic field intensity measuring sequence (pulse sequence) for capturing at least one of the images. By making the waveform of the gradient magnetic field almost the same as the waveform of the oscillating gradient magnetic field of the readout gradient magnetic field used in the actual ultra-high-speed imaging method, the static magnetic field intensity distribution image can be captured at the time of capturing the ultra-high-speed image. A similar eddy current is generated to measure the non-uniformity of the static magnetic field due to the eddy current at the time of actual ultra-high-speed image capturing, and the non-uniformity of the static magnetic field is corrected based on this.

[0016]

[0017]

[0018]

Specifically, a static magnetic field generation for generating a static magnetic field
Means, gradient magnetic field generating means, high frequency magnetic field generating means,
Additional magnetic field for generating a magnetic field for adjusting the uniformity of the static magnetic field
Generating means, and a control function for executing control and calculation of each of the means.
Calculation means, wherein the control calculation means is configured to generate the static magnetic field.
The test object placed in the static magnetic field generated by the
After applying a field to excite, read out the oscillating gradient magnetic field.
Echo generated from the inspection object by applying a gradient magnetic field
Execute the shooting sequence to measure the signal, and
A nucleus for reconstructing the image of the inspection object based on a co-signal
In an inspection apparatus using magnetic resonance, the control operation means
Prior to execution of the shooting sequence,
One waveform of said oscillating gradient magnetic field at Cans,
First having a waveform having the same width, vibration frequency, and rise time
Of the inspection object by applying a readout gradient magnetic field of
A first sequence for photographing the image of
The oscillating gradient magnetic field, amplitude, oscillating frequency and
First rise-out to a gradient magnetic field waveform with the same
Second readout gradient magnetic field with a gradient magnetic field waveform added
Applying a field to capture a second image of the test object
Measurement sequence of static magnetic field intensity distribution consisting of the imaging sequence
The phase of each pixel of the first and second images.
Calculation is performed to obtain a static magnetic field strength distribution image, and the static magnetic field strength
Improving the inhomogeneity of the static magnetic field based on the degree distribution image
Magnetic field adjusting means for controlling the additional magnetic field generating means as described above
It is characterized by comprising.

Further, the second read-out gradient magnetic field has a substantial application amount of the phase encoding gradient magnetic field from the first echo signal generation of a plurality of echo signals generated by the oscillating gradient magnetic field of the imaging sequence. It is preferable to have a gradient magnetic field waveform obtained by adding the first readout gradient magnetic field to the oscillating gradient magnetic field applied until the generation of the zero encode echo signal which becomes zero. In this case,
It is preferable to use, as the first readout gradient magnetic field, one having at least one of an amplitude, a vibration frequency, and a rise time smaller than that of the vibration gradient magnetic field. In other words, captured image I ₁ with less conditions of occurrence of eddy currents, the influence of the eddy currents both readout gradient magnetic field used in the read-out of the imaging gradient magnetic field and image I ₁ used in ultra high speed imaging method occurs The image I ₂ is taken using the readout gradient magnetic field. As a result, the influence of the eddy current included in the strong static magnetic field distribution image obtained using the images I ₁ and I ₂ is equivalent to the eddy current perceived by the zero encode echo of the ultra-high-speed imaging method.

[0021]

When obtaining the static magnetic field intensity distribution, the operation shown in Expression 3 is performed using the images I ₁ and I ₂ , and this operation is to obtain the phase difference between the two images. Therefore, by using a readout gradient magnetic field waveform in which the influence of the eddy current having the phase difference is equivalent to the eddy current perceived by the zero encode echo of the ultra-high-speed imaging method, accurate eddy current including the influence of the eddy current is obtained. Shimming can be performed.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 shows an embodiment of a static magnetic field intensity distribution measuring sequence according to the present invention, and FIG.
FIG. 1 shows an overall configuration diagram of an embodiment of an inspection apparatus for implementing the present invention. FIG. 1A shows an example of a static magnetic field intensity distribution measurement sequence for capturing an image I _{2 according} to the present invention, to which an echo planar method of an ultra-high-speed imaging method used for actual inspection is applied. FIG. 1B shows an image I according to the present invention.
_This is a static magnetic field intensity distribution measurement sequence for imaging ₁ , and a gradient echo method using a readout gradient magnetic field that can ignore the effect of eddy currents on the nonuniformity of the static magnetic field is applied.

As shown in FIG. 2, the inspection apparatus according to the present invention comprises a static magnetic field generating coil 101 for generating a static magnetic field and a gradient magnetic field generating coil 102 for generating a gradient magnetic field in each of three orthogonal directions. The inspection target 103 is arranged in a magnetic field generated by these coils. The sequencer 104 sends a command to the gradient magnetic field power supply 105 and the high frequency oscillator 106 to apply the gradient magnetic field and the high frequency pulse to the inspection target 103. The high-frequency pulse passes through the high-frequency modulator 107 and the high-frequency amplifier 108,
9 is applied to the inspection target 103. Inspection object 10
3 is received by the receiver 110, the amplifier 111, the phase detector 112, the AD converter 1
The signal is sent to the CPU 114 through the CPU 13 where signal processing such as image reconstruction is performed. Then, the signal and the measurement condition can be stored in the storage medium 115 as necessary. The sequencer 104, the CPU 114, and the like mainly constitute a control operation unit according to the present invention.

The static magnetic field generating coil 101 is provided with a shim coil 116, and the shim coil 116 is composed of coils of a plurality of channels for generating magnetic fields having different characteristics. The shim power supply 117 supplies a current to the shim coil 116 in accordance with a command from the sequencer 104. This current value can be stored in the storage medium 115 and sent to the sequencer 114 by the CPU 114. Further, a memory may be built in the shim power supply 117 itself to store a current value. Since the primary coils (X, Y, Z channels) of the shim coil 116 may cause interference with the gradient magnetic field, the primary coil of the gradient magnetic field generating coil 102 and the shim coil 116 may be integrated. is there. Instead of using the primary coil of the shim coil 116, an offset of the gradient magnetic field can be used.

Next, with reference to FIGS. 1A and 1B, a method of measuring a static magnetic field intensity distribution which is a feature of the present invention will be described.
FIG. 3A shows a static magnetic field intensity distribution measurement sequence for capturing an image I _{2 according} to the present invention. As an example, an echo planar method of an ultra-high-speed imaging method used for an actual inspection is applied. In the figure, each time chart shows an RF pulse (excitation high frequency pulse) 201, a slice gradient magnetic field 202, a phase encoding gradient magnetic field 203, a readout gradient magnetic field 204, and an echo signal 205 in order from the top. FIG. 2B shows a static magnetic field intensity distribution measuring sequence for capturing the image I _{1 according} to the present invention. As an example, a readout gradient magnetic field having a low intensity that can ignore the influence of the eddy current on the nonuniform static magnetic field is used. Gradient echo method was applied. In the figure, each time chart shows an RF pulse (excitation high frequency pulse) 206 slice gradient magnetic field 207, phase encoding gradient magnetic field 208, readout gradient magnetic field 209, and echo signal 210 in order from the top.

First, the procedure of FIG. 1A for photographing the image I ₂ will be described. RF pulse 201 and slice gradient magnetic field 2
02 are simultaneously applied to excite only the magnetization in a specific slice region. After the excitation, the nuclear magnetization is affected by the non-uniformity of the static magnetic field and the like, and the phase angle varies. The degree of the phase angle variation is proportional to the time from excitation to detection of an echo signal. After a certain period of time, a readout gradient magnetic field 204 whose intensity changes periodically while reversing the polarity of the gradient magnetic field is applied in a direction perpendicular to the slice gradient magnetic field 202, and the position information in the readout gradient magnetic field direction is obtained. The read echo signals 205 are sequentially read. At this time, by applying a pulse-shaped phase encoding gradient magnetic field 203 in a direction perpendicular to both the slice gradient magnetic field 202 and the readout gradient magnetic field 204, the echo signal 20
5 is given position information in the direction of the phase encoding gradient magnetic field.
These echo signals 205 are affected by static magnetic field inhomogeneity and the like in proportion to the time after the excitation, so that the echo signals 20
The image obtained by Fourier-transforming 5 has phase information corresponding to each echo time.

In the ultra-high-speed imaging method shown in FIG. 1A, a high-intensity gradient magnetic field is switched at high speed in order to continuously perform signal measurement in a short time. Therefore, an eddy current is generated on the conductor surface of the gradient magnetic field coil. Then, the secondary magnetic field generated by the eddy current affects the uniformity of the static magnetic field intensity distribution. In particular, the readout gradient magnetic field of the ultra-high-speed imaging method such as the echo planar method has a higher intensity and a higher switching frequency than the spin echo method (for example, an intensity of 2 to 4 times, about 1 kHz).
z). Therefore, in the case of using the ultra-high-speed imaging method, it is necessary to perform shimming including the influence of the eddy current, and a static magnetic field intensity distribution image is obtained from an image captured using the same readout gradient magnetic field as in the ultra-high-speed imaging method.

However, further contrivance is required for the following reasons. That is, a case where the static magnetic field intensity distribution is obtained from the two images I ₁ and I ₂ obtained by making the phase information different by the above-described phase method will be considered. In this case, as in the conventional case, even if only the echo generation time is changed, if the waveform of the readout gradient magnetic field is equal between the _two images I ₁ and I ₂ , the influence of the eddy current included in the echo signal is also equal. Will be.
Therefore, if the phases of the two images I ₁ and I ₂ are subtracted from each other, the phase term due to the influence of the eddy current is canceled out, so that it is impossible to measure the non-uniformity of the static magnetic field intensity distribution due to the influence of the eddy current.

Therefore, as shown in FIG.
₁ is photographed using a pulse sequence of a gradient echo method in which no eddy current is generated or the eddy current is sufficiently small, and information on the inhomogeneity of the static magnetic field including the influence of the eddy current on the phase difference between the images I ₁ and I ₂ is obtained. We included it. That is, the RF pulse 206 and the slice gradient magnetic field 207 are simultaneously applied to excite only the magnetization in a specific slice region. After the excitation, the nuclear magnetization is affected by the non-uniformity of the static magnetic field and the like, and the phase angle varies. At the time t ₂₁ has elapsed from the application of the excitation RF pulse 206, it is applied in a direction perpendicular to the slice gradient magnetic field 207 the read-out gradient magnetic field 209, reads the echo signal 210 having position information of the readout gradient magnetic field direction. At this time, by applying a phase encoding gradient magnetic field 208 in a direction perpendicular to both the slice gradient magnetic field 207 and the readout gradient magnetic field 209, position information in the phase encoding gradient magnetic field direction is given to the echo signal 210. This echo signal 21
0 is affected by static magnetic field inhomogeneity or the like in proportion to the time after excitation, so that the image I ₁ obtained by Fourier transforming the echo signal 210 by the CPU 114 has phase information according to the echo time. However, the image I ₁ does not include phase information due to the non-uniformity of the static magnetic field due to the eddy current, since the readout gradient magnetic field, which has a sufficiently small influence of the eddy current, is used. The echo time difference between the echo signal 205 and the echo signal 210 is ε.
10 time to lead is t _21.

The two images I ₁ , thus obtained,
By using I ₂ and obtaining a static magnetic field intensity distribution image by the CPU 114 according to Equation 3 described in the section of the related art, the non-uniformity of the static magnetic field including the influence of the eddy current can be measured. Then, by controlling the shim power supply 17 via the sequencer 104 to adjust the characteristics of the magnetic field added to the static magnetic field so as to correct the nonuniformity of the static magnetic field, the static magnetic field is considered in consideration of the influence of the eddy current. Strength uniformity can be increased.

When obtaining the phase difference between the _two images I ₁ and I ₂ , it is preferable to make the fields of view, spatial resolution, and sampling intervals of those images equal, but this embodiment satisfies the conditions. Is difficult, two images I ₁ and I ₂ are compared by the following method. That is,
When the image I ₁ is photographed under conditions where the readout gradient magnetic field intensity is sufficiently small and the amount of eddy current generation can be neglected, an image with a large field of view can be obtained with a lower spatial resolution than the image I ₂ . Problem that this field is different, by cutting a portion of the image I ₁ can be solved by the same field of view. On the other hand, the spatial resolution issues, since static magnetic field intensity distribution image is always high spatial resolution is not required, or drop the spatial resolution by reducing the number of pixels of the image I _2, etc. reverse to the linear interpolation of the image I ₁ Apparent It can be solved by being combined spatial resolution of the upper to the image I _2. Or, by equalizing the size of the data area of the phase space before the Fourier transformation, it is possible to align the spatial resolution of the image I ₁ and the image I _2. Furthermore, since the spatial resolution becomes higher as the data area of the phase space becomes wider, the spatial resolution is lowered by discarding high frequency components on the phase space, or conversely, the data area on the phase space is reduced by using zero filling or the like. Can be solved by increasing the apparent spatial resolution.

(Second Embodiment) FIG. 3 shows another embodiment of the static magnetic field intensity distribution measuring sequence according to the features of the present invention. This embodiment can also be implemented using the inspection device shown in FIG. FIG. 3A shows an example of a static magnetic field intensity distribution measurement sequence for capturing an image I _{2 according} to the present invention, and an asymmetric spin echo method, which is basically not an ultra-high-speed imaging method, is applied. FIG. 3B shows a static magnetic field intensity distribution measurement sequence for capturing an image I _{1 according} to the present invention, and the same spin echo method as that in FIG. 3A is applied.

The present embodiment is basically different from the first embodiment in that a readout gradient magnetic field that includes (but has a small effect of) an eddy current is used for capturing the image I _1. is there. For this purpose, in the present embodiment, the readout gradient is set so that the eddy current component included in the static magnetic field intensity distribution image obtained by performing the phase calculation of both images is equal to the eddy current component generated in the ultra-high-speed image. It is characterized by setting the waveform of the magnetic field. The details will be described below.

FIG. 3A shows a static magnetic field intensity distribution measuring sequence for photographing an image I _{2 according} to the present invention, in which a spin echo method, which is not an ultra-high-speed photographing method, is applied. 301 and an inverted RF pulse 303;
5 shows a time chart of a first slice gradient magnetic field 302, a second slice gradient magnetic field 304, a phase encoding gradient magnetic field 305, a readout gradient magnetic field 306, and an echo signal 307. Similarly, FIG. 3B shows an image I _{1 according} to the present invention.
FIG. 3 shows a static magnetic field intensity distribution measurement sequence for capturing
The spin echo method is applied as in (a). Each time chart shows RF pulse 3 in order from the top in the figure.
08, an inverted RF pulse 310, a first slice gradient magnetic field 309 and a second slice gradient magnetic field 311, a phase encoding gradient magnetic field 312, a readout gradient magnetic field 313, and an echo signal 314.

The pulse sequence of the spin echo method shown in FIG. 3 except for the sequence of the readout gradient magnetic field,
The description is omitted because it is the same as that shown in FIG. That is, as shown in FIG. 3A, the read-out gradient magnetic field 306 used for capturing the image I ₂ includes a read-out gradient magnetic field of an oscillating gradient magnetic field used for an ultra-high-speed imaging method applied in an actual inspection. A waveform having the same amplitude, frequency, and rise time is applied. Then, in the pulse sequence of the ultra-high-speed imaging method, when the echo signal 307 ′ generated at the time of the nth inversion of the readout gradient magnetic field is an echo signal corresponding to the zero-encoded echo described with reference to FIG. ) Th echo signal 30
Obtaining an image I ₂ by Fourier-transforming the 7 ". For example, Figure 1
In the case where actual inspection imaging is performed by the echo planar method of (a), the phase encoding gradient magnetic field 2 shown in FIG.
03 'and the other phase encoding gradient magnetic field 203
Signal 205 'when the integrated value of the applied amount of
Is a zero-encoded echo. Other echo signals are not used for image reconstruction. Here, it is not limited to (n + 1), and when m is an integer of 1 or more (n + m)
The value of m is preferably as small as possible, for example, 1 or 2.

On the other hand, the readout gradient magnetic field 313 used for photographing the image I ₁ is the same as the waveform of the readout gradient magnetic field 306 of FIG. Have been. Then, the echo signal 314 read out according to this is Fourier-transformed to obtain an image I ₁ . Here, t _{1 1,} t ₁₂ in FIG. 3 (b) may optionally be determined. The echo signal 30
The difference between 7 ″ and the echo time of the echo signal 314 is ε. Then, e ₁ and e ₂ in Equation 3 are respectively e ₁ =
| T ₁₁ −t ₁₂ |, e ₂ = | t ₁₁ − (t ₁₂ + ε) |

Incidentally, in order to make the eddy current components generated in the sequences of FIGS. 3A and 3B as equal as possible, it is desirable to use a readout gradient magnetic field having a waveform that can be described by the same function if possible. That is, if the readout gradient magnetic field used in the pulse sequence of the ultra-high-speed imaging method is a rectangular pulse, the rectangular readout gradient magnetic field is also used for capturing a static magnetic field intensity distribution image. If the readout gradient magnetic field used in the pulse sequence of the ultra-high-speed imaging method is a sine wave pulse, the sine wave waveform readout gradient magnetic field is also used for capturing a static magnetic field intensity distribution image. In the case where ultra-high-speed imaging is performed by spiral scanning in which the trajectory of the scanning of the gradient magnetic field on the phase space is spiral, a similar readout gradient magnetic field is used for capturing the static magnetic field intensity distribution image.

In short, in the present embodiment, one obtained by adding one readout gradient magnetic field 313 used for capturing the image I ₁ to the readout gradient magnetic field used for actual inspection / photography is used for capturing the image I _2. It is characterized by the following. As a result, the eddy current sensed by the (n + 1) th echo signal 307 ″ becomes the sum of the eddy current sensed by the zero-encoded echo of the ultra-high-speed imaging method in the actual inspection and the eddy current sensed by the echo signal 314. Therefore, the image I
The ₁ and phase difference of each pixel of I _2, so that the zero encoding echo ultrafast imaging method used in the actual inspection includes substantially equal eddy current components and eddy current component sensitive. As a result, the non-uniformity of the static magnetic field can be measured based on the zero-encoded echo most affected by the non-uniformity of the static magnetic field, so that the measurement accuracy of the non-uniformity of the static magnetic field including the influence of the eddy current can be improved.

In this embodiment, the case where the readout gradient magnetic field 313 is made equal to the amplitude, frequency, and rise time of the readout gradient magnetic field 306 has been described, but the present invention is not limited to this. In short, it is sufficient that at least one gradient magnetic field having the same waveform as that of the readout gradient magnetic field 313 is included in the readout gradient magnetic field 306. For example, the waveform of the readout gradient magnetic field for generating the first echo signal is referred to as the readout gradient magnetic field 3.
13 or the waveform of the readout gradient magnetic field 306 for generating the (n + 1) th echo signal is the same as the readout gradient magnetic field 313.

(Third Embodiment) FIG. 4 shows another embodiment of the static magnetic field intensity distribution measuring sequence according to the features of the present invention. This embodiment can also be implemented using the inspection device shown in FIG. FIG. 4A is a static magnetic field intensity distribution measuring sequence for capturing the image I _{2 according} to the present invention, and FIG. 4B is a static magnetic field intensity distribution measuring sequence for capturing the image I _{1 according} to the present invention. There is a gradient echo method applied to each sequence. FIG.
In (a) and (b), 401 and 406 are R
F pulse, 402 and 407 are slice gradients, 403 and 408 are phase encode gradients, 404 and 409 are readout gradients, respectively.
405 and 410 are echo signals, respectively.

In the pulse sequence of the ultra-high-speed imaging method used for the actual inspection, when the echo signal 405 'generated at the time of the nth inversion of the readout gradient magnetic field is a zero-encoded echo, the echo signal 405 shown in FIG. The (n + m) -th (n + 1-th in the figure) echo signal 4
Obtaining an image I ₂ a 05 "by Fourier transform. Other echo signals are not used for image reconstruction. Also, to obtain the image I ₁ by Fourier transform of the echo signal 410 in Figure 4 (b).
Echo time difference of the echo signal 405 "and echo signal 410 is epsilon, the time to read the echo signal 410 from the RF pulse application is t _21. As for every single waveform of the readout gradient magnetic field 404,409 is Since this is the same as the second embodiment, the description is omitted.

The method of obtaining a static magnetic field intensity distribution image from the obtained images I ₁ and I ₂ is as described in the section of the prior art. However, the static magnetic field intensity distribution image obtained in the present embodiment includes not only the original static magnetic field intensity distribution but also an eddy current component generated by switching of the gradient magnetic field, as in the third embodiment. Is different from the conventional example.

In the second and third embodiments described above,
A description has been given of the case where the asymmetric spin echo method or the gradient echo method is applied to the static magnetic field intensity distribution measurement sequence. However, the present invention is not limited to this, and the image I ₂ may be captured using an ultra-high-speed imaging method such as an echo planar method. In short, what is necessary is just to use the same readout gradient magnetic field as the readout gradient magnetic field used in the actual ultra-high-speed imaging method. Regarding the phase method using the echo planar method, see JP-A-5-64633, Pr.
oceedings of 3rd Annual Meeting of Society of Magne
tic Resonance, p616 (1995).

[0045]

As described above, according to the present invention, more accurate shimming including the influence of overcurrent can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a static magnetic field intensity distribution measurement sequence according to a feature of the present invention.

FIG. 2 is an overall configuration diagram of an embodiment of an inspection device according to the present invention.

FIG. 3 is a diagram showing another embodiment of a static magnetic field intensity distribution measurement sequence according to the features of the present invention.

FIG. 4 is a diagram showing still another embodiment of a static magnetic field intensity distribution measurement sequence according to the features of the present invention.

[Explanation of symbols]

 Reference Signs List 101 Static magnetic field generating coil 102 Gradient magnetic field generating coil 103 Inspection object 104 Sequencer 105 Gradient magnetic field power source 106 High frequency transmitter 107 High frequency modulator 108 High frequency amplifier 109 High frequency transmitter 110 Receiver 111 Amplifier 112 Phase detector 113 AD converter 114 CPU 115 Storage media

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiromichi Shimizu 1-1-1 Uchikanda, Chiyoda-ku, Tokyo Inside Hitachi Medical Corporation (56) References JP-A-9-276243 (JP, A) JP-A Heisei 9-253070 (JP, A) JP-A-5-64633 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055

Claims (2)

(57) [Claims] 1. A static magnetic field generating means for generating a static magnetic field, a gradient magnetic field generating means, a high-frequency magnetic field generating means, an additional magnetic field generating means for generating a magnetic field for adjusting the uniformity of the static magnetic field, and each of the means Control arithmetic means for performing control and arithmetic operations, the control arithmetic means applies a high-frequency magnetic field to a test object placed in a static magnetic field generated by the static magnetic field generating means to excite, and then vibrates. A readout of a gradient magnetic field is performed.A photographing sequence for measuring an echo signal generated from the inspection object by applying a gradient magnetic field is executed, and nuclear magnetic resonance is used to reconstruct an image of the inspection object based on the measured echo signal. In the inspection apparatus, the control and calculation means is configured to determine the oscillating gradient magnetic field in the imaging sequence prior to the execution of the imaging sequence .
One waveform equals amplitude, vibration frequency and rise time
A first sequence for capturing a first image of the inspection object by applying a first readout gradient magnetic field having a sharp waveform
And the oscillating gradient magnetic field in the imaging sequence ;
Amplitude, a second for capturing a first readout gradient second lead-out second image of the gradient magnetic field is applied said object the magnetic field waveform were added to equal gradient waveform vibrating frequency and rise time Static magnetic field consisting of the imaging sequence
An intensity distribution measurement sequence is executed , a phase operation is performed for each pixel of the first and second images to obtain a static magnetic field intensity distribution image, and the static magnetic field unevenness is determined based on the static magnetic field intensity distribution image. An inspection apparatus using nuclear magnetic resonance, comprising: a static magnetic field adjusting means for controlling the additional magnetic field generating means so as to improve the performance once. Wherein said second readout gradient magnetic field, from the first of said echo signal generating a plurality of said echo signals generated by the oscillating gradient magnetic field in the imaging sequence, a substantial phase encoding gradient A gradient magnetic field waveform obtained by adding a first readout gradient magnetic field to an oscillating gradient magnetic field applied until the generation of a zero-encoded echo signal at which the applied amount becomes zero.
2. An inspection apparatus using nuclear magnetic resonance according to 1 .