JP3322695B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nuclear magnetic resonance (hereinafter referred to as "N").
Abbreviated as "MR"), a technique for imaging a tomographic image at a high speed in a magnetic resonance imaging apparatus that obtains a tomographic image of a desired part of a subject, particularly when the intensity of a gradient magnetic field in a phase encoding direction greatly changes. The present invention relates to a magnetic resonance imaging apparatus capable of suppressing the deterioration of image quality.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus measures a nuclear spin (hereinafter, simply referred to as "spin") density distribution, relaxation time distribution, and the like at a desired inspection site in a subject by utilizing an NMR phenomenon. An arbitrary section of the subject is displayed as an image based on the measurement data. As shown in FIG. 1, the conventional magnetic resonance imaging apparatus includes a static magnetic field generating means (2) for applying a static magnetic field to the subject 1, a gradient magnetic field in the slice direction and a gradient magnetic field in the phase encoding direction, and A gradient magnetic field generating means (3) for applying a gradient magnetic field in the frequency encoding direction, and a high frequency pulse is repeatedly applied in a predetermined pulse sequence to cause nuclear magnetic resonance in nuclei of atoms constituting the living tissue of the subject 1. A transmission system 4, a reception system 5 for detecting a signal emitted by the above nuclear magnetic resonance, and a signal processing system 6 for performing an image reconstruction operation using the signal detected by the reception system 5; A sequence for measuring a signal emitted by resonance is repeatedly performed to obtain a tomographic image.

Conventionally, a two-dimensional Fourier imaging method is generally used as a method for taking a tomographic image in a magnetic resonance imaging apparatus. FIG. 8 shows a typical pulse sequence of a typical spin echo method among the two-dimensional Fourier imaging methods. In this pulse sequence, initially in a section P ₁ shown in FIG. 8 (f),
As shown in FIG. 3A, after applying a slice-direction gradient magnetic field 24 and applying a 90-degree pulse 21 to excite spins in a desired slice plane in the subject by 90 degrees, the echo time is represented by Te. and Te / 2 hours after the time that is to apply a slicing direction gradient magnetic field 24 and 180 degree pulse 22 in the section P _3. After the 90-degree pulse 21 is applied, each spin starts to rotate in the XY plane at a unique speed, so that a phase difference occurs between the spins with the passage of time. Here, when 180 degree pulse 22 in the section P ₃ is applied,
Each spin macroscopic magnetization is inverted symmetrically on X 'axis when fallen 180 degrees in the Y direction, and converging again thereafter in the interval P ₄ after time Te to continue the rotation at the same speed, FIG. 8
As shown in (e), an echo signal 23 is formed.

FIG. 9 shows a schematic diagram of a pulse sequence for multi-echo measurement. In this multi-echo measurement,
As shown in FIG. 9 (a), the 180 degree pulse 22 is applied more than once per Te time (22 _1, 22 _2, 22 _3, 22 _4), the time 2Te, 3Te, is converged spin even 4Te, As shown in FIG. 3E, a plurality of echo signals 23 ₁ , 23 ₂ ,
23 _3, 23 ₄ and measures the.

As described above, the echo signals 23 ₁ to 23 ₄
Is measured, but in order to form a tomographic image from these, the spatial distribution of the echo signal must be obtained. For this purpose, a linear gradient magnetic field is used, but a spatial magnetic field gradient is created by superimposing the gradient magnetic field on a uniform static magnetic field. At this time, since the spin rotation frequency is proportional to the magnetic field strength, the spin frequency of each spin is spatially different when the above-described gradient magnetic field is applied. Therefore, the position of each spin can be known by examining the rotation frequency. In order to know the position of each spin, in FIGS. 8 and 9, a gradient magnetic field 25 in the phase encoding direction and gradient magnetic fields 27 and 28 in the frequency encoding direction are used. Note that the gradient magnetic field 28 in the frequency encoding direction is a signal readout gradient magnetic field.

The pulse sequence described above is used as a basic unit, and the intensity of the gradient magnetic field 25 in the phase encoding direction is changed every time, and is repeated a predetermined number of times, for example, 256 times, at a constant repetition time (TR). By this way, the echo signals 23 ₁ to 23 ₄ which has been measured by two-dimensional inverse Fourier transform, the spatial distribution of the macroscopic magnetization is determined.

FIG. 10 is a schematic diagram of a pulse sequence of the high-speed spin echo method. The fast spin echo method, as shown in FIG. 10 (a), a multi-echo as in the case 180 degree pulse measurement shown in FIG. 9 applied multiple times (22 _1, 22 _2, 22 _3, 22 ₄₎ Performed by Then, for each measurement of each echo signal 23 _{1-23 4} shown in FIG. 10 (e), as shown in FIG. (C), a predetermined gradient in the phase encoding direction (hereinafter referred to as "phase offset gradient") 26 _1, 26 _2, 26 _3, by applying 26 ₄ for a predetermined time, distributing the respective echo signals 23 ₁ to 23 ₄ in the phase direction of the raw data space (hereinafter referred to as "K-space"). This reduces the imaging time. At this time, as described in the multi-echo measurement shown in FIG. 9, for example, when four echoes are measured, the first echo 23 ₁ , the second echo 23 ₂ , the third echo 23 ₃ , and the Since the four data of four echoes 23 ₄ are measured as a pair, as shown in FIG.
_{1-23 4} data to a Ky direction, for example 8 division to occupy the same area, respectively on the K space, one echo signal may allocate to occupy one area. In this case, the arrangement order of the echo signals is determined by the number of echo signals to be used and the type of the echo signal that determines the image quality. The way to decide is the DC part of K space (K
The central region (y = 0) is a signal of a desired echo time for determining the image quality, and the adjacent region is an echo signal that is continuous with the above echo signal. Echo signals in the same order are arranged at one end and the other end in the Ky direction.

[0008] from the first echo in FIG 11 (a) in the case of obtaining the fourth echo 23 ₄ echo time highlighted using up to the fourth echo, showing the arrangement of data on the K space. here,
The number of data of each echo is almost the same. First, Ky =
0 as the desired echo in the DC portion and the low frequency region.
Allocate the data of echo 23 _4. Next, a region adjacent thereto, respectively the third echo 23 ₃ data further toward the high-frequency region, the second echo 23 _2, respectively, allocate first echo 23 ₁ data. As a result, the one end and the other end of the highest band become the first echo and the first echo, respectively, and become the echo signals in the same order. When the desired echo for determining the image quality is a signal having another echo time, barrel shift of FIG. 11A is performed in the Ky direction, and for example, as shown in FIG. 3 echo 23 ₃ may be to come into direct current part and a low frequency region of Ky = 0.

In the above case, in order to arrange the echo signals as shown in FIG.
In (a), Ky is determined in proportion to the integral value of the gradient magnetic field strength in the phase encoding direction from the application of the 90-degree pulse 21 to the measurement of an arbitrary echo. However, FIG.
Since the spin direction is inverted by the 180-degree pulse 22 in (a), it is necessary to calculate the integral value of the gradient magnetic field intensity in the phase encoding direction in consideration of the inverted direction.

Next, an application pattern of the gradient magnetic field in the phase encoding direction for arranging the echo signals as shown in FIG. 11A will be described with reference to FIGS. FIG. 12 shows the pulse sequence of the fast spin echo method shown in FIG.
Applying a high-frequency pulse and a gradient magnetic field in the phase encoding direction to obtain an image in which the division and measurement of four echo signals in one cycle are repeated four times, and the fourth echo of each cycle is enhanced as an echo signal for determining image quality. It shows a pattern. In order to measure four echo signals in each measurement and obtain an image in which the fourth echo is emphasized, as shown in FIG.
3 _1, 23 _2, 23 _3, it is necessary to arrange 23 _4.

For this purpose, in FIG. 12, in the first measurement shown in (c), the spin is tilted 90 degrees by the 90-degree pulse 21 in (a), and the gradient magnetic field 25 in the phase encoding direction shown in (b) is felt. At this time, the intensity of the gradient magnetic field 25 in the phase encoding direction is "-2". next,
The first 180-degree pulse 22 ₁ (a), the "-2"
The spin felt to be the magnitude of? Is inverted, which is equivalent to feeling the magnetic field strength of the magnitude of "+2". Next, (b)
By the first phase offset gradient magnetic field 26 ₁ shown in FIG. At this time, the integrated value of the gradient magnetic field in the phase encoding direction felt by the spin is “+8”. In this state, the first state shown in FIG.
Since the signal of the echo 23 ₁ is measured, K shown in FIG. 13
The data at the position “8” is measured in the Ky direction in the space.

Next, in FIG. 12, the second 18 of FIG.
The 0 degree pulse 22 _2, the intensity of the gradient magnetic field "+8"
Is reversed, which is equivalent to feeling the magnetic field strength of “−8”. Thereafter, the second intensity of the phase offset gradient 26 ₂ shown in (b) "+3"
I feel the size of. At this time, the integral value of the gradient magnetic field in the phase encoding direction felt by the spin is “−5”. Since the second echo 23 ₂ of the signal shown in FIG. 10 (e) in this state is measured, so that the data of the position of "-5" at Ky direction on the K space shown in FIG. 13 is measured .
Similarly, the third and fourth 18s shown in FIG.
The 0-degree pulses 22 ₃ and 22 ₄ are sequentially applied, and the third and fourth phase offset gradient magnetic fields 2 shown in FIG.
By sequentially applying the 6 _3, 26 _4, the third echo 23 ₃ and the fourth echo 23 ₄ shown in FIG. 10 (e) is "4" at Ky direction on the K space shown in FIG. 13, respectively, " -1 "
Is measured as position data. In this way, the first measurement is completed in FIG. 12C, but the second measurement, the third measurement, and the fourth measurement are continuously performed in the same manner as described above, and as shown in FIG. 1 to 16 in the Ky direction
(4 echoes × 4 measurements), all data are obtained.

In FIG. 12 (c), the intensity of the gradient magnetic field in the phase encoding direction is easily understood from "8", "-5",
Although the values are represented by integer values such as “4” and “−1”, the magnitude of the actual magnetic field strength is such that when the spin is shifted by one in the Ky direction on the K space, the spin is shifted to the imaging region (hereinafter “FOV”). ) Is made to feel a gradient magnetic field in the phase direction that makes one rotation at both ends. This is represented by the following equation. 2π = γΣ (GPn · Tn · FOV) (1) where γ is the gyromagnetic ratio, and Gpn is the intensity of the gradient magnetic field in the n-th phase encoding direction (however, one 180 ° pulse is used as the intensity felt by the spin). Tn is the application time of the gradient magnetic field in the n-th phase encoding direction, n is the number of gradient magnetic fields applied in the phase direction until the echo signal of the target echo number is measured, F
OV is the length of one side of the imaging area. Similarly, the m-th data from the central region of Ky = 0 in the K space is represented by the following expression: 2πm = γΣ (Gpn · Tn · FOV) (2) The application time Tn may be determined.

[0014]

[SUMMARY OF THE INVENTION However, in the imaging method in such a conventional magnetic resonance imaging apparatus, as it is apparent from the table of FIG. 12 (c), the intensity of the phase offset gradient 26 _{1-26 4} Is the first measurement and the second
In measurement, the size is equal for each echo,
When moving to the third measurement, it was applied as a greatly different one. In the third measurement, an abrupt change in the gradient magnetic field in the phase encoding direction causes an eddy current generated by the apparatus, hysteresis of the static magnetic field, or a history of the gradient magnetic field felt by spin to change. Sometimes a transient false image was measured. Further, in the fourth echo of the third measurement, the data at the position of "1" is measured in the Ky direction on the K space, but this is allocated to the central region of Ky = 0 as an echo for determining the image quality. The signal of the fourth echo includes the transient false image component, which degrades the image quality of the obtained tomographic image.

Accordingly, the present invention provides a magnetic resonance imaging apparatus which addresses such a problem and which can suppress deterioration in image quality of a tomographic image caused by a large change in the intensity of a gradient magnetic field in the phase encoding direction. The purpose is to:

[0016]

In order to achieve the above object, a magnetic resonance imaging apparatus according to the present invention comprises: a static magnetic field generating means for applying a static magnetic field to a subject; A gradient magnetic field generating means for applying a directional gradient magnetic field and a frequency encoding directional gradient magnetic field, and a high-frequency pulse is repeatedly applied in a multi-echo measurement pulse sequence to cause nuclear magnetic resonance in nuclei of atoms constituting the living tissue of the subject. And a signal processing system for performing an image reconstruction operation using the plurality of detected echo signals. In the resonance imaging apparatus, the intensity of the gradient magnetic field in the phase encoding direction and the frequency encoding The intensity of the phase offset gradient magnetic field applied immediately before the signal readout gradient magnetic field applied to the signal readout gradient is determined to have a predetermined magnitude, and the intensity of the phase offset gradient magnetic field is determined by the DC portion on the raw data space of the signal to be measured. and in the low frequency region in the vicinity it is to apply a phase encode direction gradient magnetic field pattern that does not change.

Further , the phase is generated by the gradient magnetic field generating means.
Gradient magnetic field in the encoding direction was changed to the first half of multiple measurements.
With the number of times, increasing the magnetic field strength with each advance
In the second half, the magnetic field strength decreases with each round.
It is good to apply so that

Further, the position is generated by the gradient magnetic field generating means.
Phase encoding direction gradient magnetic field, odd number of multiple measurements
Of the magnetic field strength at the time of the first measurement and the even measurement
Be sure to line up with each other and measure odd or even times.
And mark the values in such a way that
May be added.

Furthermore, the gradient magnetic field in the phase encoding direction can be converted by the gradient magnetic field generating means into an arbitrary pulse that does not use the data of the detected echo signal in a plurality of measurements.
The sequence may be applied at least once .

In another example of the magnetic resonance imaging apparatus, a static magnetic field generating means for applying a static magnetic field to the subject, and applying a slice direction gradient magnetic field, a phase encoding direction gradient magnetic field, and a frequency encoding direction gradient magnetic field to the subject. Gradient magnetic field generation means, a transmission system that repeatedly applies a high-frequency pulse in a pulse sequence of multi-echo measurement to cause nuclear magnetic resonance in nuclei of atoms constituting the living tissue of the subject, and the above-described nuclear magnetic resonance In a magnetic resonance imaging apparatus including a receiving system that detects a plurality of emitted echo signals and a signal processing system that performs an image reconstruction operation using the detected plurality of echo signals, application of a gradient magnetic field in a slice direction is performed. Simultaneously apply a 90-degree pulse to excite spins in the slice cross section of the subject by 90 degrees and diffuse to the excited spins There is applied a high-frequency pulse and the slice direction gradient magnetic field simultaneously after a predetermined has occurred time, generates an echo signal after a predetermined time, performed the application of the phase encoding direction gradient magnetic field after application of the high frequency pulse, this for each echo signal produced A gradient magnetic field whose phase encoding direction is reversed before and after the generation of the echo signal is applied, and a pattern that does not change in the DC portion in the raw data space of the signal to be measured and the low frequency region in the vicinity thereof is applied. it is intended to apply a phase encode direction gradient magnetic field.

Then, the gradient magnetic field generated in the phase encoding direction applied after the application of the high-frequency pulse by the gradient magnetic field generating means is not repeated in the first half of the plurality of measurements.
As the field progresses, the magnetic field strength increases and the number of
Now, apply it so that the magnetic field strength decreases with each advance
You may. Further, the high gradient magnetic field is generated by the gradient magnetic field generating means.
Phase encoding direction gradient applied after application of frequency pulse
The magnetic field is measured during the odd number of measurements and the even number
If the value of the magnetic field strength is arranged alternately between positive and negative at the time of measurement
In both cases, the above values are in ascending order every odd or even number of measurements.
Or you may apply so that it may progress from a large order. Further
Then, the gradient magnetic field generating means
Multiple gradient magnetic fields in the phase encoding direction to be applied after application
Do not use the data of the detected echo signal during the measurement
Any pulse sequence may be applied at least once.
No.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a block diagram showing the overall configuration of a magnetic resonance imaging apparatus according to the present invention. This magnetic resonance imaging apparatus obtains a tomographic image of a subject by utilizing a nuclear magnetic resonance (NMR) phenomenon . As shown in FIG. 1, a static magnetic field generating magnetic circuit 2, a gradient magnetic field generating system 3, Transmission system 4, reception system 5, signal processing system 6, sequencer 7
And a central processing unit (CPU) 8.

The static magnetic field generating magnetic circuit 2 generates a uniform static magnetic field around the subject 1 in the direction of its body axis or in a direction perpendicular to the body axis. A permanent magnet type, normal conduction type, or superconducting type magnetic field generating means is arranged in the space provided. The gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 wound in three directions of X, Y, and Z, and a gradient magnetic field power supply 10 for driving each coil. By driving the gradient magnetic field power supply 10 of the coil, the gradient magnetic field Gs in the slice direction, the gradient magnetic field Gp in the phase encoding direction, and the gradient magnetic field Gf in the frequency encoding direction in the three axes of X, Y and Z are applied to the subject 1. Has become. The slice plane with respect to the subject 1 can be set by the method of applying the gradient magnetic field.

The transmission system 4 converts a high-frequency pulse into a pulse sequence for multi-echo measurement in order to cause a nuclear magnetic resonance in an atomic nucleus of an atom constituting a living tissue of the subject 1 by a high-frequency magnetic field pulse transmitted from a sequencer 7 described later. The high-frequency oscillator 11 and the modulator 12
, A high-frequency amplifier 13 and a high-frequency coil 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 in accordance with a command from the sequencer 7, and this high-frequency pulse is modulated by the high-frequency amplifier 13a. After being amplified by the above, the electromagnetic wave is applied to the high-frequency coil 14a disposed close to the subject 1, so that the subject 1 is irradiated with electromagnetic waves.

The receiving system 5 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of an atomic nucleus of a living tissue of the subject 1, and includes a high-frequency coil 14b on the receiving side, an amplifier 15, a quadrature phase detector. 16 and an A / D converter 17, and an electromagnetic wave (NMR signal) of the response of the subject 1 due to the electromagnetic wave emitted from the high-frequency coil 14 a on the transmission side is disposed in the high-frequency coil 14 disposed close to the subject 1.
b, is input to the A / D converter 17 via the amplifier 15 and the quadrature detector 16 and is converted into a digital quantity, and is sampled by the quadrature detector 16 at a timing according to a command from the sequencer 7. The collected data is two-series data, and the signal is sent to the signal processing system 6.

The signal processing system 6 includes a CPU 8, a recording device such as a magnetic disk 18 and a magnetic tape 19, and a CRT.
And the like, and the CPU 8 performs processing such as Fourier transform, correction coefficient calculation, and image reconstruction.
The signal intensity distribution at an arbitrary cross section or the distribution obtained by performing an appropriate operation on a plurality of signals is imaged and displayed on the display 20.
Is displayed as a tomographic image.

The sequencer 7 serves as control means for repeatedly applying a high-frequency magnetic field pulse for causing nuclear magnetic resonance to the nuclei of the atoms constituting the living tissue of the subject 1 in a predetermined pulse sequence. And sends various commands necessary for data acquisition of tomographic images of the subject 1 to the transmission system 4, the gradient magnetic field generation system 3, and the reception system 5.

Here, in the imaging method in the magnetic resonance imaging apparatus according to the present invention, the gradient magnetic field generation system 3 applies the gradient magnetic field immediately before the intensity of the gradient magnetic field in the phase encoding direction and the signal readout gradient magnetic field applied in the frequency encoding direction. The intensity of the phase offset gradient magnetic field is determined to be a predetermined magnitude, and the intensity of the phase offset gradient magnetic field is determined in the DC portion on the raw data space (K space) of the echo signal to be measured and the low frequency region in the vicinity thereof. Applying a gradient magnetic field in the phase encode direction that does not change
Than it is.

[0029]

FIG. 2 is an explanatory view showing an application pattern of a gradient magnetic field in the phase encoding direction showing a specific embodiment of the imaging method according to the present invention. FIG. 2 shows that, in the pulse sequence of the high-speed spin echo method, measurement of four echo signals in one cycle, for example, divided into 16 in the phase direction is repeated four times in the same manner as shown in FIG. 7 shows an application pattern of a high-frequency pulse and a gradient magnetic field in a phase encoding direction for obtaining an image in which the fourth echo is emphasized as an echo signal for determining image quality. In this embodiment, the gradient magnetic field generation system 3 increases the magnetic field strength in the phase encoding direction gradient magnetic field each time the first half of the plurality of measurements is performed, and the second half of the measurement. Is characterized in that the application is performed such that the magnetic field intensity is reduced as the number of times is increased.

[0031] That is, as shown in FIG. 2 (c), the strength of the phase encoding direction gradient magnetic field 25, in the first measurement of the first half and "-2", times as a "-1" in the second measurement progresses
The voltage is increased every time, and is set to “1” in the third measurement, which is shifted to the latter half, and is set to “0” in the fourth measurement, and is reduced and applied as the number of times advances. In this case, as apparent from comparison with the conventional one shown in FIG. 12, a gradient magnetic field application pattern in which the third measurement and the fourth measurement in FIG. 12 are interchanged. Thus, in the fourth echo of the third measurement,
The data at the position “2” is measured in the Ky direction on the K space, and the data can be moved from the central region of Ky = 0 shown in FIG. 6 to the high frequency region. Therefore, it is possible to remove a transient false image component from the DC portion and the low frequency region on the K space that determines the image quality, and it is possible to suppress the image quality deterioration.

FIG. 3 is an explanatory view showing an application pattern of a gradient magnetic field in a phase encoding direction showing a second embodiment of the imaging method according to the present invention. FIG. 3 shows an application pattern similar to that shown in FIG. 2 described above. In this embodiment, the gradient magnetic field in the phase encoding direction is changed by the gradient magnetic field generation system 3 into an odd number out of a plurality of measurements. It is characterized in that the value of the magnetic field strength is arranged alternately between positive and negative at the time of the measurement and the even number of measurements, and is applied in such a manner that the value advances in the order of small or large at each of the odd or even number of measurements. And

That is, as shown in FIG. 3C, the gradient magnetic field 25 in the phase encoding direction is set to "-2" in the first measurement.
And “0” in the second measurement, and “−1” in the third measurement.
Further, the value is set to "1" in the fourth measurement, the magnetic field strength is set to a negative value at the time of the odd number of measurements, and the value is set to be positive and negative alternately as the positive value at the time of the even number of measurements, and at every odd number of times, The negative value is sequentially increased from a small value, and even in the even-numbered measurement, the positive value is sequentially increased from a small value for each measurement.
Then, focusing on the magnitude of the phase offset gradient 26 _{1-26 4} is applied as shown in FIG. 3 (b), the same figure (c)
As shown in the figure, the first measurement and the third measurement are changed to the same value for each measurement, and the second measurement is performed for the even measurement.
The measurement and the fourth measurement have the same value. In this case, as is apparent from comparison with the conventional one shown in FIG. 12, a gradient magnetic field application pattern in which the second measurement and the third measurement in FIG. 12 are interchanged.

As a result, the transient false image component is superimposed on each measurement data, and its value differs between the odd-number measurement and the even-number measurement. Then, in the K space, a false image component is superimposed in the Ky direction as a wave having a high frequency and a Fourier-transformed image of the wave causes a false image at an end of the image. That is, by moving and collecting the false image component to the end of the image that is not important, the false image component can be removed from the vicinity of the center of the image, and the deterioration of the image quality can be suppressed. In FIG. 3, the gradient magnetic field 25 in the phase encoding direction is applied as a negative value during odd-numbered measurements and as a positive value during even-numbered measurements. However, the present embodiment is not limited to this. Instead, the positive / negative relationship may be reversed. Alternatively, the voltage may be applied such that the value of the magnetic field strength is sequentially decreased from the largest value at each of the odd number of measurements and the even number of measurements.

FIG. 4 is an explanatory view showing an application pattern of a gradient magnetic field in a phase encoding direction showing a third embodiment of the imaging method according to the present invention. In this embodiment, as is apparent from comparison with the above-described one shown in FIG. 3, a gradient magnetic field application pattern in which the second measurement and the fourth measurement in FIG. 3 are interchanged is used. That is, the phase encoding direction gradient magnetic field 25 is set to a negative value during the odd number of measurements, and
In even-numbered measurements, positive and negative values are arranged alternately as positive values, and in each odd-numbered measurement, negative values are sequentially increased from small ones, and in even-numbered measurements, positive values are increased in each measurement. The voltage is applied in such a manner that the voltage is sequentially reduced from the voltage. In this case, the same effects as in the embodiment of FIG. 3 can be obtained.

FIG. 5 is an explanatory view showing an application pattern of a gradient magnetic field in a phase encoding direction, showing a fourth embodiment of the imaging method according to the present invention. This embodiment is arranged so as not to vertically symmetrical in the vicinity of the echo signals 23 ₁ to 23 4 echo 23 ₄ Ky direction of a region to Ky = 0 in the arrangement of ₄ on K space shown in FIG. 6 9 shows an application pattern of a gradient magnetic field in the case. FIG 5 is, as apparent in comparison with those shown in prior art FIG. 12, changes the third echo and exertion phase offset gradient 26 _3, 26 ₄ at the time of measurement of the fourth echo at each time of the measurement It was done. As a result, similarly to the embodiment shown in FIG. 2, in the fourth echo of the third measurement, the data at the position of “2” is measured in the Ky direction on the K space, and the data is calculated from the central region of Ky = 0. It can be moved to a high frequency range. Therefore, it is possible to remove a transient false image component from the DC portion and the low frequency region on the K space that determines the image quality, and it is possible to suppress the image quality deterioration. Further in FIG. 6, in a region adjacent to the fourth echo 23 ₄ as desired echoes to determine the image quality, to place the third echo 23 ₃ continuous with the fourth echo 23 ₄ example continues downward, the two- When two echo areas are combined, they may be vertically symmetric with respect to Ky = 0.

FIG. 7 is an explanatory view showing an application pattern of a gradient magnetic field in the phase encoding direction showing a fifth embodiment of the imaging method according to the present invention. In this embodiment, the gradient magnetic field in the phase encoding direction is
Using the data of the detected echo signal from multiple measurements
It is characterized in that any arbitrary pulse sequence is applied at least once . That is, as shown in FIG. 7C, between the second measurement and the third measurement in which the phase offset gradient magnetic fields 26 _{1 to} 26 ₄ greatly change, “dummy measurement” in which the data of the detected echo signal is not used. Is applied at least once. The application pattern of the pulse sequence of the dummy measurement may be, for example, the same pattern as the third measurement immediately after the dummy measurement, or may be an application pattern with a shorter time, or may be the application pattern of the second measurement immediately before the third measurement and the pattern immediately after the second measurement. May be an intermediate application pattern to the application pattern of the third measurement. As a result, a transient false image component is generated in the DC portion in the K space in the pulse sequence of the dummy measurement in the same operation state as the conventional example shown in FIG. 12, but the data is not used in the dummy measurement. And will not appear in subsequent measurements. Therefore, it is possible to remove a transient false image component from the DC portion and the low frequency region on the K space that determines the image quality, and it is possible to suppress the image quality deterioration. Note that, when reconstructing a tomographic image, the signal obtained in the pulse sequence of the dummy measurement is superimposed on the signal obtained in the third measurement by averaging the signal obtained in the third measurement immediately thereafter. You may.

[0038]

In the above description, the measurement of four echo signals in one cycle is repeated four times (4 echoes × 4 echoes).
4 measurement) and the case where the fourth echo in each cycle is emphasized as an echo signal for determining image quality,
The present invention is not limited to this, and may perform measurement of five or more times with another number of echoes.

The present invention provides a high-speed imaging pulse other than the high-speed spin echo method by the excitation method such as 90 degrees-180 degrees-180 degrees of spin described in the first to fifth embodiments. It can be applied to the sequence method. FIG. 14 shows that a 90-degree high-frequency pulse (90-degree pulse) 21 is applied together with the application of the slice-direction gradient magnetic field 24b, and
Of the desired slice cross section is excited by 90 degrees in a predetermined direction in a plane orthogonal to the static magnetic field, and after a predetermined time after diffusion occurs in the excited spin, for example, after Te / 2, the high frequency pulse 22b ₁ and the slice are sliced. by applying a magnetic field gradient 24 simultaneously, thereby to generate a spin echo signal 23b ₁ after Te, 90 degree pulse 21 and a high echo signal generator
Between the frequency pulse 22b ₁ and the phase encoding direction inclination
Without applying a magnetic field, the gradient magnetic field
Application is a method to perform after the high frequency pulse 22b ₁ applied and an echo signal generated in the spin high-frequency pulses 22b ₁ ~22b ₄ is inverted 180 degrees out of phase while oriented in a spin 90 degrees excitation 2 shows a pulse sequence that repeats the above. This method is called a CPMG method, and it is known that the S / N ratio of the measurement data of the second and lower echoes is improved as compared with the above-described high-speed spin echo method, and in particular, the attenuation of the measurement data of the even-numbered echo can be reduced. However, the invention is also applicable to this method.

FIG. 15 shows a sixth embodiment of the present invention in which the present invention is applied to the pulse sequence of FIG. 14, and the phase encoding direction gradient magnetic field when the echoes shown in FIG. 2 (c) are encoded in the same order. Is shown. FIG. 15B illustrates the application of the gradient magnetic field in the phase encoding direction in the first measurement. The object of the present invention can also be achieved by the sixth embodiment. Note that the respective effects are also obtained when the second to fifth embodiments are applied to the pulse sequence of FIG.

[0042]

The present invention has been configured as described above.
In a magnetic resonance imaging apparatus for executing an imaging method for measuring a tomographic image by a pulse sequence of multi-echo measurement, a gradient magnetic field generating unit immediately before a signal read gradient applied in a phase encoding direction and a frequency encoding direction by a gradient magnetic field generating means. And the magnitude of the phase offset gradient magnetic field applied to each of them is determined to a predetermined magnitude, and the intensity of the phase offset gradient magnetic field varies in the DC portion on the raw data space of the echo signal to be measured and in the low frequency region near the DC portion. By applying a gradient magnetic field in the phase encoding direction that does not cause the pattern to be arranged in the DC data portion in the raw data space and in a region other than the low frequency region in the vicinity thereof.
By measuring such data, a transient false image component can be removed from the DC portion and the low frequency region on the raw data space that determines the image quality of the tomographic image. Therefore, it is possible to suppress image deterioration caused by a large change in the intensity of the gradient magnetic field in the phase encoding direction.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an overall configuration of a magnetic resonance imaging apparatus to which an imaging method according to the present invention and a conventional example is applied;

FIG. 2 is an explanatory view showing an application pattern of a gradient magnetic field in a phase encoding direction, showing a specific embodiment of an imaging method according to the present invention;

FIG. 3 is an explanatory diagram showing an application pattern of a gradient magnetic field in a phase encoding direction, showing a second embodiment of the imaging method according to the present invention;

FIG. 4 is an explanatory view showing an application pattern of a gradient magnetic field in a phase encoding direction, showing a third embodiment of the imaging method according to the present invention;

FIG. 5 is an explanatory diagram showing an application pattern of a gradient magnetic field in a phase encoding direction, showing a fourth embodiment of the imaging method according to the present invention;

FIG. 6 is an explanatory diagram showing the arrangement order of each echo signal on the K space in the state of FIG. 5;

FIG. 7 is an explanatory diagram showing an application pattern of a gradient magnetic field in a phase encoding direction, showing a fifth embodiment of the imaging method according to the present invention;

FIG. 8 is an explanatory view schematically showing a pulse sequence of a conventional spin echo method,

FIG. 9 is an explanatory diagram schematically showing a pulse sequence of multi-echo measurement,

FIG. 10 is an explanatory diagram schematically showing a pulse sequence of a high-speed spin echo method in a conventional example,

FIG. 11 is an explanatory diagram showing the arrangement order of each echo signal on the K space in the state of FIG. 10;

FIG. 12 is an explanatory view schematically showing an application pattern of a gradient magnetic field in a phase encoding direction in a high-speed spin echo method in a conventional example.

FIG. 13 is an explanatory diagram showing the arrangement order of each echo signal on the K space in the state of FIG. 12;

FIG. 14 is an explanatory view schematically showing a pulse sequence of the CPMG method;

FIG. 15 is a diagram illustrating a sixth embodiment of the imaging method according to the present invention, and is a diagram illustrating an application pattern of a gradient magnetic field in the phase encoding direction when the present invention is applied to the above-described CPMG method.

[Explanation of symbols]

1 ... subject, 2 ... static magnetic field generating magnetic circuit, 3 ... gradient magnetic field generating system, 4 ... transmission system, 5 ... reception system, 6 ... signal processing system, 7 ... sequencer, 8 ... CPU, 23 ₁ ~ 23 ₄
... Echo signal, 24 ... Slice direction gradient magnetic field, 25
... Gradient magnetic fields in the phase encoding direction, 26 _{1 to} 26 ₄ ... Gradient magnetic fields in the phase offset direction, 27, 28 ... Gradient magnetic fields in the frequency encoding direction.

Claims (8)

(57) [Claims] 1. A static magnetic field generating means for applying a static magnetic field to a subject; a gradient magnetic field generating means for applying a slice direction gradient magnetic field, a phase encoding direction gradient magnetic field, and a frequency encoding direction gradient magnetic field to the subject; A transmission system that repeatedly applies a high-frequency pulse in a pulse sequence of multi-echo measurement to cause nuclear magnetic resonance in the nuclei of the atoms constituting the living tissue, and detects a plurality of echo signals emitted by the above-described nuclear magnetic resonance A magnetic resonance imaging apparatus comprising: a receiving system; and a signal processing system for performing an image reconstruction operation using the plurality of detected echo signals. The intensity of the phase offset gradient magnetic field applied immediately before the signal readout gradient magnetic field applied in the encoding direction A predetermined magnitude is determined, and a gradient magnetic field in a phase encoding direction is applied so that the intensity of the phase offset gradient magnetic field does not change in a DC portion in a raw data space of a signal to be measured and a low frequency region in the vicinity thereof. A magnetic resonance imaging apparatus. 2. The gradient magnetic field generating means increases the magnetic field intensity in the phase encoding direction gradient magnetic field every time the first half of a plurality of measurements is performed, and increases the number of times in the second half of the measurement. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic field intensity is applied so as to decrease the magnetic field intensity every time. 3. The gradient magnetic field generating means sets the gradient magnetic field in the phase encoding direction such that the values of the magnetic field strength are alternately arranged in the positive and negative directions at the time of odd number measurement and at the time of even number measurement among a plurality of measurements. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the application is performed such that the value advances in ascending or descending order at each of odd-numbered or even-numbered measurements. 4. The method according to claim 1, wherein the gradient magnetic field in the phase encoding direction is detected by the gradient magnetic field generation means during a plurality of measurements.
-Reduce any pulse sequence that does not use signal data
2. The magnetic resonance imaging apparatus according to claim 1, wherein the voltage is applied at least once . 5. A static magnetic field generating means for applying a static magnetic field to a subject, a gradient magnetic field generating means for applying a slice direction gradient magnetic field, a phase encoding direction gradient magnetic field, and a frequency encoding direction gradient magnetic field to the subject, A transmission system that repeatedly applies a high-frequency pulse in a pulse sequence of multi-echo measurement to cause nuclear magnetic resonance in the nuclei of the atoms constituting the living tissue, and detects a plurality of echo signals emitted by the above-described nuclear magnetic resonance A magnetic resonance imaging apparatus comprising: a receiving system; and a signal processing system for performing image reconstruction calculation using the detected plurality of echo signals. The spin in the slice cross-section is excited by 90 degrees, and a high-frequency pulse and a slice are excited a predetermined time after diffusion occurs in the excited spin. Simultaneously applying a scan magnetic field gradient, a predetermined time to generate an echo signal after said after application of the RF pulse performs application of phase encoding magnetic field gradient, the positive and negative reversal in the front and rear of the echo signal generated for each echo signal produced Sa
And it applies a phase encode direction gradient magnetic field that has, and applying a phase encode direction gradient magnetic field pattern that does not vary in the low frequency region of the DC portion and the vicinity thereof in the raw data space of the signal to be measured Magnetic resonance imaging device. 6. A gradient magnetic field applied in the phase encoding direction applied after the application of the high-frequency pulse by the gradient magnetic field generating means for each first half of a plurality of measurements.
In addition to increasing the magnetic field strength,
6. The magnetic resonance imaging apparatus according to claim 5, wherein the magnetic field intensity is applied so as to decrease the magnetic field strength as it proceeds . 7. The high-frequency wave generated by the gradient magnetic field generating means.
Phase encoding direction gradient magnetic field applied after pulse application
For odd-numbered and even-numbered measurements
When the values of the magnetic field strength are alternately
In addition, the above value is smaller or smaller in every odd or even number of measurements.
Is characterized in that the voltage is applied in the order of increasing order.
The magnetic resonance imaging apparatus according to claim 5. 8. The high-frequency wave generated by the gradient magnetic field generating means.
Phase encoding direction gradient magnetic field applied after pulse application
And the data of the detected echo signal from multiple measurements
Apply any unused pulse sequence at least once
6. The magnetic resonance image according to claim 5, wherein
Device.