JP3041688B2 - High spatial resolution magnetic resonance imaging system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique of a magnetic resonance imaging apparatus for taking a tomographic image of a subject using magnetic resonance, and in particular, as an excitation high-frequency pulse for exciting a subject.
It belongs to micro-imaging using a burst pulse in which a plurality of sub-pulses are discretely arranged on a time axis.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus (MRI apparatus) is an apparatus that captures a tomographic image of a subject using magnetic resonance and measures the characteristics of the subject based on the captured tomographic image. That is, a subject is placed in a static magnetic field, and a high-frequency pulse (RF pulse) is applied thereto to excite nuclei of a specific substance constituting the subject by resonance, and a magnetic resonance signal (echo signal) generated from the subject by the excitation. ) Is received, and the received echo signal is analyzed to obtain a distribution image related to the specific substance of the subject. In addition, at the time of excitation and at the time of readout for receiving an echo signal,
By selectively applying three gradient magnetic fields in the axial direction, spatial position information of the imaging region of the subject is added to the echo signal.

[0003] After the echo signal is received by a receiving probe arranged in the vicinity of the subject, a filtering process and a detection process are performed to extract a desired signal component. The filtering process is a process of extracting a frequency component corresponding to a specific substance from a received echo signal, and is performed before or after the detection process. Since the center frequency used as the reference for detection is the magnetic resonance frequency of the nucleus of a specific substance determined in relation to the static magnetic field strength, the filter processing is performed by a band pass with a certain bandwidth centered on the center frequency of detection. Filter processing. Generally, the frequency band of this filter is
The frequency is set from 20 kHz to 500 kHz around the center frequency, and a frequency component outside the band is removed.

[0004] The reason for performing this filter processing is that, besides the magnetic resonance signal generated from the subject in the received signal, the subject and the M
This is because thermal noise generated from the RI device is mixed in and the amount of the noise is reduced. The thermal noise is also called white noise, and the frequency characteristic is uniform over all frequencies. Therefore, as the pass band of the filter becomes narrower, the amount of noise decreases, and the ratio of effective signal component to noise component (SN)
Ratio) is improved. The S / N ratio of the image is inversely proportional to the square root of the filter's pass bandwidth, with the other conditions being the same. For example, if the pass band of the filter is reduced to 1/16, the SN ratio of the image is improved by a factor of four. For this reason, the pass bandwidth of the filter is set to a necessary minimum. However, if the setting is too narrow, the necessary frequency information is lost and shading occurs on the image.

Here, the minimum necessary pass band width of the filter is determined by the visual field of the image and the readout gradient magnetic field strength. For example, the visual field of the image is 102.4 mm, and the readout gradient magnetic field strength is 0.03 Tesla / m. The magnetic resonance frequency of the hydrogen nucleus at 1 Tesla is about 43 MH
Because of z, the difference in magnetic resonance frequency at both ends of the 102.4 mm field of view is approximately 133 kHz, as shown in the following equation, which is the minimum necessary pass bandwidth of the filter.

[0006] 0.03 x 0.1024 x 43,000,
000 = 132,096 (1) On the other hand, in the MRI apparatus, by using a burst pulse (hereinafter, appropriately referred to as an RF burst) as an excitation RF pulse, an image having a higher resolution than the spatial resolution determined by the readout gradient magnetic field is obtained. A method for obtaining the same is known (Japanese Patent Application No. 8-7496).
No. 0). That is, a burst pulse composed of a plurality of sub-pulses discretely arranged on the time axis is, for example, sin
When a Fourier transform is performed on an RF burst amplitude-modulated by the c-function, it becomes a square periodic wave having a specific width on the frequency axis. Here, the interval between the sub-pulses is u [second], and sin
Assuming that the cycle of the c function is T [seconds], the cycle of the square periodic wave on the frequency axis is 1 / u [Hz] and the width is 1 / T [Hz]. Since the frequency band of the square periodic wave is allocated to the coordinates of the object in the real space, if the strength of the gradient magnetic field applied simultaneously with the excitation RF pulse is unchanged during the RF pulse application period, the frequency band on the frequency axis Represents the excitation profile of the nucleus in the real space, that is, the absolute value of the transverse magnetization. Thus, for example, using an excitation RF burst set at T = 16u, the excited and unexcited regions are:
It will appear periodically at a ratio of 1:15. And
When the same excitation is performed while shifting the carrier frequency of the sub-pulse by 1/16 u [Hz], the period of the region to be excited is shifted by 1/16. The whole area in the cross section can be excited. As a result, as compared with the case of using a single excitation RF pulse, the pixel size of the re Doauto direction 1
Since it can be reduced to one sixth, it is possible to capture an image having a higher spatial resolution than the spatial resolution determined by the readout gradient magnetic field strength.

[0007]

As described above, in order to obtain an image having a high SN ratio by an imaging method using a single excitation RF pulse, it is necessary to increase the readout gradient magnetic field intensity. Although the capacity of the power supply must be increased, the use of the RF burst pulse proposed in Japanese Patent Application No. 8-74960 makes it possible to obtain an image with a low readout gradient magnetic field strength and a high spatial resolution.

However, according to the method using an RF burst pulse, since the excitation profile in the readout direction has a comb shape, the amount of excited nuclei in the subject is reduced, so that the imaging conditions such as the field of view and the spatial resolution are the same. Then, there is a problem that the SN ratio is lower than that of an image obtained by an imaging method using a single pulse as the excitation RF pulse. The reason for this is that using the same filter reduces the total amount of valid signals, even though the total amount of noise is the same.

The problem to be solved by the present invention is that the excitation R
Another object of the present invention is to further improve the S / N ratio in an imaging method using an RF burst pulse as an F pulse.

[0010]

According to the present invention, there is provided a radio frequency burst comprising a plurality of sub-pulses arranged at regular intervals on a time axis on a subject placed in a static magnetic field space. A magnetic resonance signal generated from the subject while applying a pulse and a gradient magnetic field to excite the subject, applying a readout gradient magnetic field in the same direction as the gradient magnetic field, and repeatedly reversing the polarity of the readout gradient magnetic field And a magnetic resonance imaging apparatus for extracting the magnetic resonance signal component from the received signal by a band-pass filter and obtaining a magnetic resonance image of the subject based on the extracted magnetic resonance signal. Is the high
Excitation profile on frequency axis of frequency burst pulse
It has a plurality of matching comb-shaped passbands .

That is, the high frequency burst pulse causes
When the object is excited, as described above,
Excitation only in the region and only the magnetic resonance signal in that region
Not born. Fourier transform of the magnetic resonance signal
Frequency distribution is a sub-pulse of a high-frequency burst pulse
And the period T of the amplitude modulation function,
Excitation profile with frequency components discrete as combs
become. Therefore, according to the comb-like frequency distribution,
By setting the frequency characteristics of the bandpass filter,
Noise in frequency band where no effective magnetic resonance signal component exists
Noise component can be removed, thus improving the SN ratio.
be able to.

In this case, it is preferable to modulate the amplitude of the high-frequency burst pulse, and the sinc function is desirable as the amplitude modulation function. However, the amplitude modulation function is not limited to the sinc function, but may be any amplitude modulation function that becomes a square periodic wave when the frequency of the amplitude-modulated high-frequency burst pulse is converted.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on embodiments. FIG. 1 is a block diagram of a characteristic portion of an MRI apparatus according to an embodiment to which the present invention is applied, FIG. 2 is an overall configuration diagram thereof, and FIG. 3 is an example of an imaging sequence. As shown in FIG. 2, the magnetic resonance imaging apparatus includes a magnet 101 for generating a static magnetic field and a coil 102 for generating a gradient magnetic field.
In the magnetic field formed by the above, the subject 103 is set and a tomographic image of a desired portion of the subject 103 is taken. In the present embodiment, the gradient magnetic field generating coil 102
It is composed of three coils that generate magnetic fields inclined in three orthogonal (x, y, z) directions. Sequencer 10
4 sends a command to the gradient magnetic field power supply 105 and the RF pulse generator 106 to generate a desired gradient magnetic field and RF burst pulse from the coil 102 and the probe 107, respectively. The RF burst pulse output from the RF pulse generator 106 is amplified by the RF power amplifier 115 and applied to the subject 103 through the probe 107. The echo signal generated from the subject 103 is the probe 1
07, filtered by the filter 117, and detected by the receiver 108. Center frequency (magnetic resonance frequency) used as a reference for detection and filter 1
The frequency characteristics of 17 are set by the sequencer 104. The signal detected by the receiver 108 is sent to a computer 109, where signal processing such as image reconstruction is performed. The result of the signal processing is displayed on the display 110.
Note that signals and measurement conditions can be stored in the storage medium 111 as needed. In addition, a shim coil 112 is provided to adjust the uniformity of the static magnetic field. The shim coil 112 includes a plurality of channels that generate various magnetic field patterns, and is supplied with current from a shim power supply 113. When adjusting the uniformity of the static magnetic field, the current flowing through each coil is controlled by the sequencer 104. In other words, the sequencer 104 sends a command to the shim power supply 113 to cause the shim coil 112 to generate an additional magnetic field that corrects the non-uniformity of the static magnetic field. In addition, the sequencer 10
The control unit 4 controls each device to operate at a timing and intensity programmed in advance. Of this program,
In particular, the portion describing the excitation RF burst pulse, the gradient magnetic field, the timing and intensity of signal reception is called an imaging sequence.

FIG. 3 shows an example of the photographing sequence. In this figure, the time sequence of the RF burst pulse, the gradient magnetic field Gx in the x-axis direction, the gradient magnetic field Gy in the y-axis direction, the gradient magnetic field Gz in the z-axis direction, and the echo signal Echo are shown in order from the top. In the figure, the horizontal axis represents time, and the vertical axis represents RF.
The concept of the intensity of a burst pulse, a gradient magnetic field, or an echo signal is shown. First, the first RF burst pulse 1-1 is applied simultaneously with the gradient magnetic field of Gx to excite the nuclei inside the subject. Next, when the Gz slice gradient magnetic field 34 and the 180-degree pulse 31 are simultaneously applied to the subject, the magnetic moment of the nucleus in a cross section having a specific width perpendicular to the direction of application of the slice gradient magnetic field (imaging cross section) is reversed. As a result, the phases of the magnetic moments of the nuclei outside the cross section become different. Next, a spin echo 6-1-1 is generated by applying the readout gradient magnetic field 2. Echo 6-1
After observing No. 1, the polarity of the readout gradient magnetic field 2 is repeatedly inverted, and the field echo 6-1-n (n = 2 to 2)
n) occurs. As described above, a plurality of echoes corresponding to the sub-pulses are regarded as one set, and n sets of echo groups are generated by repeating the readout gradient magnetic field reversal.
The number of echoes required for image reconstruction is measured.

Here, the RF burst pulse will be described. The RF burst pulse applied in the present embodiment is
As shown in FIG. 4A, a series of RF pulses composed of a plurality of sub-pulses 121 arranged at regular intervals on the time axis are amplitude-modulated by a sinc function 122 shown by a dotted line. . When such an RF burst pulse on the time axis is Fourier-transformed, as shown in FIG. 4B, a square periodic wave 123 having a specific width on the frequency axis is obtained.
Becomes Here, assuming that the interval between the sub-pulses 121 constituting the RF burst pulse on the time axis is u [second], the period of the square periodic wave 123 on the frequency axis is 1 / u [Hz]. In addition, the RF burst on the time axis is amplitude-modulated si
Assuming that the cycle of the nc function 122 is T [seconds], the width of the square periodic wave on the frequency axis is 1 / T [Hz]. Note that R
The function of amplitude-modulating the F burst pulse is sinc
The function is not limited to the function 122, and it is essential that the function be a square when Fourier-transformed.

By the way, in the magnetic resonance imaging apparatus, the frequency band to be measured is allocated to the coordinates in the real space by the gradient magnetic field, so that the intensity of the gradient magnetic field applied simultaneously with the RF burst pulse during the RF burst pulse application period If not changed, the excitation waveform on the frequency axis directly represents the excitation profile of the nucleus in real space, that is, the absolute value of the transverse magnetization. For example, FIG.
As shown in the figure, in the case of an RF burst pulse amplitude-modulated with T = 16u, an excited region and an unexcited region appear periodically at a ratio of 1:15. The Fourier transform of the RF burst pulse obtained by shifting the carrier frequency of the RF burst pulse 121 with T = 16u by 1/16 u [Hz] shows that the excited region is 16 minutes as shown in FIG. Is shifted by one cycle. In this way, when the RF burst pulse whose carrier frequency is shifted by 1/16 u [Hz] is Fourier-transformed, the excited region is further shifted by 1/16 period. Therefore, by sequentially applying the first to sixteenth RF burst pulses with the carrier frequency shifted by 1/16 u [Hz] to the subject, it is possible to excite almost all the nuclei in the photographing cross section of the subject to perform the photographing. it can. Then, each of the echo signals obtained for each photographing is subjected to two-dimensional inverse Fourier transform and then combined to obtain one tomographic image. For details of this synthesis method, refer to Japanese Patent Application No. Hei.
No. 74960.

For example, the image field of view in the application direction of the readout gradient magnetic field is 102.4 mm, the strength of the readout gradient magnetic field is 0.03 Tesla / m, the spatial resolution determined by the strength of the readout gradient magnetic field is 3.2 mm, and T = 16u, and the excited and unexcited regions are 1
A ratio of 15 is assumed. When the first RF burst pulse 1-1 is applied to the subject 103, the width of the square periodic wave of the excitation profile is 0.2 mm, and the pixel size (pixel size) of the captured image in the application direction of the readout gradient magnetic field is Under the condition of 3.2 mm, measurement is performed according to the imaging sequence shown in FIG. When a plurality of echoes 6-1-n obtained by this measurement are subjected to two-dimensional inverse Fourier transform to obtain an image, the positional relationship between one pixel of the image and the excitation profile is as shown in FIG. If an RF burst pulse composed of an infinite number of sub-pulses 121 is used,
Although the excitation profile is a perfect square periodic wave, the excitation profile is not a perfect square periodic wave because the imaging sequence of FIG. 3 uses a finite number of sub-pulses. As a result, slight leakage occurs in the adjacent pixels, but there is no practical problem. Therefore, the excitation profile will be described here as being able to be regarded as a substantially rectangular periodic wave. One square wave 123 in FIG. 6 corresponds to one pixel of the image, and is actually 0.2 excited.
(= 3.2 / 16) mm only. When the second RF burst pulse 1-2 whose carrier frequency is shifted by 1/16 u [Hz] is applied to the subject 103 and photographed, and two-dimensional inverse Fourier transform is performed, one pixel of the next image has the previous excitation region. Has information of only the 0.2 mm area next to.

A first portion for applying the first RF burst pulse 1-1 to the subject 103 to perform photographing, and a second portion for performing the second RF burst pulse 1-1.
In the second part in which the burst pulse 1-2 is applied to the subject 103 for photographing, the sequence is exactly the same except that the carrier frequency of the sub-pulse is different. Similarly, third to sixteenth RF burst pulses 1-3 to 1-16 not shown
Is applied to the subject 103 to obtain 16 pieces of image information, which are sequentially arranged every other pixel and synthesized. Thus, an image having a spatial resolution of 0.2 mm in the direction in which the readout gradient magnetic field is applied can be obtained. That is, the pixel size of the captured image in the direction in which the readout gradient magnetic field is applied can be improved from 3.2 mm to 0.2 mm. It should be noted that since the first to sixteenth RF burst pulses have different excitation regions, it is not necessary to provide a time zone for waiting for the recovery of longitudinal magnetization between the 16 imagings. In this manner, continuous imaging is performed using a plurality of RF burst pulses as excitation RF pulses, and by synthesizing the images, an image having a spatial resolution higher than the spatial resolution determined by the readout gradient magnetic field strength is obtained. Obtainable.

Next, the filter 11 according to the characteristic portion of the present invention will be described.
7 will be described. The basic characteristic of the filter 117 is a band-pass filter having the characteristic shown in FIG.
This figure shows the frequency characteristic of the reciprocal of the filter attenuation ratio (input / output amplitude ratio), and a certain range of the input signal with respect to the center frequency (usually the magnetic resonance frequency at the center of the gradient magnetic field). Only frequency components are transmitted to the output side. It is desirable that this frequency characteristic be as close to a square as possible. Note that the range (pass band width) where the reciprocal of the attenuation ratio is high needs to be set to about 133 kHz in the case of the conventional example.

As the filter 117 of the present embodiment, a digital filter for performing a filtering process by a digital processor is applied. FIG. 1 shows an example of a block configuration diagram illustrating the concept of the filter 117. In FIG. 1,
The peripheral devices of the filter 117 are the same as those described with reference to FIG. As shown in FIG. 1, the filter 117 includes a plurality of (32 in the illustrated example) filters F _{1 to} F ₃₂ . The echo signals received by the probe 107 are respectively filtered by filters F _{1 to} F
₃₂ is entered. The center frequencies of the filters F _{1 to} F ₃₂ are different from each other by 4128 Hz, and the frequency bandwidths are set to 258 Hz. That is, when the first RF burst pulse 1-1 is applied to the subject 103, the filters F _{1 to} F ₃₂ are arranged such that the filters F _{1 to} F ₃₂ include one of the frequency bands excited in one pixel. The frequency and the band pass width are set to be sequentially shifted. Therefore, all filters F ₁ are added by the adder S.
When synthesizing the passband of to F _32, it is adapted to match the excitation profile in the field of view of the readout gradient magnetic field application direction. The center frequency of the filters F _{1 to} F ₃₂ is set by the sequencer 104 in accordance with the shift of the carrier frequency of the RF burst pulse applied at the time of excitation.

Here, the frequency characteristic of the filter 117 is
This will be described in detail with reference to FIG. FIG. 8A shows the first R
6 shows an excitation profile 51 when an F burst pulse 1-1 is applied to a subject 103 and an echo signal is measured.
Now, the visual field of the readout gradient magnetic field application direction is set to 1
Assuming that the spatial resolution determined by the intensity of the read-out gradient magnetic field is 3.2 mm and the number of pixels in the application direction of the read-out gradient magnetic field is 102.4 / 3.2.
= 32. In addition, when the nucleus related to the specific substance to be imaged is a hydrogen atom, the magnetic resonance frequency of the hydrogen nucleus at 1 Tesla is about 43 MHz, so the frequency band allocated to the pixel size is the same as described above.
4128 Hz. The frequency band excited in one 3.2 mm pixel is 1/16 of the frequency band, and is 4128/16 = 258 Hz.

Therefore, while the echo signal 6-1-1 corresponding to the RF burst pulse 1-1 is being measured, the frequency characteristics of the filters 117 (F _{1 to} F ₃₂ ) are changed by the sequencer 104 as shown in FIG. ). In other words, the frequency characteristics have a plurality of comb-shaped passbands in accordance with the excitation profile on the frequency axis of the RF burst pulse. In the present embodiment, there are 32 ranges where the reciprocal of the attenuation ratio is high (pass band), and the width of the range where the reciprocal of the attenuation ratio is high (pass band width) is set wider than 258 Hz. In other words, the frequency characteristic of the filter 117 is
Within the visual field (approximately 133 kHz across the center frequency), it is a substantially square periodic wave. Outside the visual field, the reciprocal of the attenuation ratio is small. When an RF burst pulse is applied to the subject, The frequency band excited in one pixel is set to be always included in one of the plurality of pass bandwidths of the filter 117.

By setting the frequency characteristics of the filter 117 in this way, it is possible to prevent a frequency component not containing a frequency component of a magnetic resonance signal (a frequency component containing only noise) from being input to the receiver 108. As a result, the S / N ratio of the image can be significantly improved as compared with the case where a conventional filter having a uniform frequency characteristic in the visual field is used. That is, as described above, since the SN ratio of an image is inversely proportional to the square root of the pass bandwidth of the filter 117,
By setting the frequency characteristics shown in FIG. 8B, the pass band of the filter is reduced to about 1/16.
The N ratio is improved about four times as compared with the related art. That is, although the total amount of the echo signal is the same as the conventional one, the pass band of the filter is narrowed, so that the total amount of noise is reduced.

Similarly, the second RF burst pulse 1-2
Is applied to the subject 103 and the echo signal is measured, the center frequency of each of the filters 117 (F _{1 to} F ₃₂ ) is shifted by 258 Hz to set the characteristics shown in FIG. 8C. When the filter frequency characteristics of FIGS. 8B and 8C are compared, they are shifted by one-sixteenth cycle. Similarly, the third
8D to 16E are applied to the subject 103, and the filter frequency characteristics during the measurement of the echo signal are changed for 16 minutes as shown in FIGS. Is shifted by one cycle. That is, the filter 117
The frequency characteristics of (F _{1 to} F ₃₂ ) are matched with the excitation profile on the frequency axis of the RF burst pulse, so that the characteristics have a plurality of comb-shaped pass bands.

The echo signal measured in this way is
Inverse dimensional Fourier transform yields 16 images,
By sequentially arranging and synthesizing every pixel, an image having a spatial resolution of 0.2 mm in the application direction of the readout gradient magnetic field can be obtained. The number of pixels in the direction of application of the read-out gradient magnetic field of this composite image is 32 × 16 = 5.
There are twelve, and the S / N ratio of the composite image is about four times higher than in the past.

As described above, according to the present embodiment, the frequency characteristic of the filter 117 is set so as to match the excitation profile in the visual field in the direction in which the readout gradient magnetic field is applied. Can be significantly improved as compared with the related art.

In FIG. 3, the application direction of the readout gradient magnetic field was selected in the x-axis direction, the application direction of the encoding gradient magnetic field was selected in the y-axis direction, and the application direction of the slice gradient magnetic field was selected in the z-axis direction. These can be chosen in any direction. For example, even if the application direction of the readout gradient magnetic field is selected in the y-axis direction, the application direction of the encode gradient magnetic field is selected in the x-axis direction, and the application direction of the slice gradient magnetic field is selected in the z-axis direction, the same cross section can be taken. Alternatively, another section can be taken by selecting the application direction of the readout gradient magnetic field in the z-axis direction, the application direction of the encode gradient magnetic field in the x-axis direction, and the application direction of the slice gradient magnetic field in the y-axis direction.

Further, in the above-described example, the case where T = 16u has been described. However, the present invention is not limited to this and can be set within the range of T ≧ 2u. In particular, as u / T is reduced, the SN ratio can be improved.

In the embodiment shown in FIG.
Although the filter processing according to the feature of the present invention is performed before the detection processing is performed, the detection processing is performed by the receiver 108 instead, as shown in FIG.
Filter processing according to the features of the present invention can be performed. That is, as shown in FIG.
The echo signal received by 7 is input to the receiver 108 via the filter 118. The echo signal detected by the receiver 108 is input to the computer 109 via the filter 119. The frequency characteristic of the filter 118 is
It is set in the same way as the conventional filter, and the frequency band is 133
kHz. As a result, frequency components outside the visual field are removed. After the echo signal processed by the filter 118 is detected by the receiver 108, the filter 11
9 is input. The filter 119 is formed having filters F _{1 to} F ₃₂ , similarly to FIG. 1, the center frequencies of the filters F _{1 to} F ₃₂ are set to be different by 4128 Hz, and the frequency band is set to 258 Hz. The center frequency is changed by the sequencer 104 according to the set and the shift of the carrier frequency of the RF burst pulse. Therefore, similarly to the embodiment of FIG. 1, there is an effect that the S / N ratio of an image can be remarkably improved as compared with the related art.

Although the present invention has been described based on the embodiments, the frequency characteristics of the filter may be similarly adjusted to the excitation profile in the visual field in the direction of application of the readout gradient magnetic field in the other embodiments. By setting, it is possible to significantly improve the SN ratio of the image as compared with the related art. For example, in FIG. 1 or FIG. 9, not only the filters 117 and 119 but also a plurality of receivers 108 may be arranged in parallel.

[0031]

As described above, according to the present invention,
In an imaging method using an RF burst pulse capable of obtaining an image with a higher spatial resolution than the spatial resolution determined by the readout gradient magnetic field, the SN ratio of the image can be further improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a filter according to an embodiment of the present invention.

FIG. 2 is an overall configuration diagram of a magnetic resonance imaging apparatus according to an embodiment to which the present invention is applied.

FIG. 3 is an example of a photographing sequence applicable to the present invention.

FIG. 4 is a diagram illustrating an example of an RF burst pulse amplitude-modulated by a sinc function.

FIG. 5 shows another example of an RF burst pulse amplitude-modulated by a sinc function.
[z] is a diagram for explaining a case where a shift has occurred.

FIG. 6 is a diagram illustrating an excitation state inside a subject excited by the RF burst pulse of FIG. 5;

FIG. 7 is a diagram showing frequency characteristics of a conventional band-pass filter.

FIG. 8 is a diagram showing one embodiment of the frequency characteristic of the band-pass filter of the present invention.

FIG. 9 is a configuration diagram of another embodiment of a band-pass filter according to a characteristic portion of the present invention.

[Explanation of symbols]

1-1, 1-2 RF burst pulse 2 Gradient magnetic field in the x-axis direction 33 Gradient magnetic field in the y-axis direction 34 Gradient magnetic field in the z-axis direction 6-1-1 to 6-1-n, 6-2-1 to 6 -2-n Echo signal 31 180 degree pulse 101 Magnet generating static magnetic field 102 Coil generating gradient magnetic field 103 Subject 104 Sequencer 105 Gradient magnetic field power supply 106 High frequency magnetic field generator 107 Probe 115 RF amplifier 108 Receiver 109 Calculator 117 Filter 119 filter

──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Keiji Tsukada 1-280 Higashi Koigakubo, Kokubunji-shi, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Hiromichi Shimizu 1-1-1 Uchikanda Uchikanda, Chiyoda-ku, Tokyo Stock (56) References JP-A-9-262219 (JP, A) JP-A-3-77536 (JP, A) JP-A-59-91345 (JP, A) JP-A-63-105748 (JP) , A) JP-A-63-117743 (JP, A) JP-A-9-154830 (JP, A) JP-A-8-508667 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) A61B 5/055 JICST file (JOIS)

Claims (3)

(57) [Claims] 1. A high-frequency burst pulse composed of a plurality of sub-pulses discretely arranged on a time axis and a gradient magnetic field are applied to a subject placed in a static magnetic field space to excite the subject, Applying a readout gradient magnetic field in the same direction as the gradient magnetic field, receiving a magnetic resonance signal generated from the subject while repeatedly inverting the polarity of the readout gradient magnetic field, and banding the magnetic resonance signal component from the received signal. A magnetic resonance imaging apparatus for extracting a magnetic resonance image of the object based on the extracted magnetic resonance signal component by using a high-frequency burst pulse;
Comb-shaped pattern matching the excitation profile on the frequency axis of
A magnetic resonance imaging apparatus having a number of pass bands . 2. A high frequency burst pulse composed of a plurality of sub-pulses discretely arranged on a time axis and a gradient magnetic field are applied to a subject placed in a static magnetic field space to excite the subject, Applying a readout gradient magnetic field in the same direction as the gradient magnetic field, receiving a magnetic resonance signal generated from the subject while repeatedly inverting the polarity of the readout gradient magnetic field, and banding the magnetic resonance signal component from the received signal. Magnetic resonance imaging for extracting the magnetic resonance image by repeating the operation of extracting the magnetic resonance image of the subject based on the extracted magnetic resonance signal components while shifting the carrier frequency of the high frequency burst pulse In the apparatus, the band-pass filter includes the high-frequency burst pulse.
Comb-shaped pattern matching the excitation profile on the frequency axis of
A number of passbands, the center frequency of said plurality of passbands
Is set in accordance with the shift of the carrier frequency of the high-frequency burst pulse . 3. The high-frequency band-pass filter according to claim 1, wherein
A plurality of filters corresponding to the shift of the carrier frequency of the burst pulse.
3. The magnetic resonance imaging apparatus according to 2.