JP2695594B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus for obtaining a tomographic image of a subject by utilizing a magnetic resonance (NMR) phenomenon, and an MR for obtaining a tomographic image at high speed.
I device.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus utilizes a magnetic resonance phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter simply referred to as "spins") at a desired inspection site in a subject, From the measurement data, a tomographic image of the subject can be displayed.

A magnetic resonance imaging apparatus capable of obtaining such a tomographic image is operated based on a preset sequence of timing for generating an electromagnetic wave or a gradient magnetic field, and by repeating this sequence several times. Has become.

Various sequences are known as this sequence, and among them, the so-called high-speed spin echo method is effective because the tomographic image information can be obtained relatively quickly.

The characteristics of this high-speed spin echo method are:
A method for measuring tomographic image information by sequentially changing the magnitude of a so-called phase direction gradient magnetic field and operating one sequence a predetermined number of times, and at least two or more echoes are obtained by the operation of the one sequence. The signal can be obtained.

FIG. 4 shows an example of a conventional fast spin echo method (a type of multi-spin echo method). In the figure, in the first measurement (first measurement), first,
The 90 ° pulse 28 causes the spin to fall to 90 °, and the 180 ° pulse 29a causes the spin to fall to 180 °, and further, the slice gradient magnetic field 30 and the frequency direction gradient magnetic field 33.34 are applied, and the phase encode gradient magnetic field The first echo is obtained by applying 32a (whose intensity is set to 8). And
Further, a reverse polarity phase encoding gradient magnetic field 32a '(its strength is set to -8) is applied.

Then, the second 180 ° pulse 29b
The spin is inverted by the application of the magnetic field, and the second echo is obtained by applying the phase encode gradient magnetic field 32b (its strength is set to 6). Then, a phase-encoding gradient magnetic field 32b '(the magnitude of its intensity is -6) is applied.

After that, the same applies to the 180 ° pulse 29c,
Application of 29d, phase encoding gradient magnetic fields 32c, 32
A third echo and a fourth echo are obtained by applying c'and 32d, 32d '(the magnitude of the intensity is sequentially 4, -4, then 2, -2). Here, each tomographic image information of the first echo to the fourth echo that is sequentially obtained corresponds to the magnitude (8, 6, 4, 2) of the corresponding phase direction gradient magnetic field of each echo of the memory in the K space. It is stored along the kx (frequency direction) direction in ky (phase direction).

The change in the strength of the phase direction gradient magnetic field at the first measurement is shown in FIG. Also, the second
The change in the strength of the gradient magnetic field in the phase direction at the measurement position is also shown in the same figure.

Similarly, the second measurement (second measurement), the third measurement (third measurement), and the fourth measurement (fourth measurement) are sequentially performed, and the first measurement obtained by each measurement is performed. The tomographic image information of the echo or the fourth echo is stored in the memory on the K space.

FIG. 6 is an explanatory diagram showing a K space memory in which all the tomographic image information is stored by the above-described procedure. In the figure, the horizontal axis is the phase direction (kx) axis, and the vertical axis is the frequency direction (ky) axis.

Next, as a pulse sequence for a magnetic resonance imaging apparatus, there is a three-dimensional measuring method as a well-known method. This is such that detailed tomographic information can be obtained in the slice direction by applying a gradient magnetic field called slice encoding in the slice direction to the ordinary two-dimensional measurement method. FIG. 8 shows a pulse sequence diagram when the three-dimensional measurement method is used for the spin echo method. 2 in FIG.
The pulse sequence of the dimension measurement method is shown. In the two-dimensional measurement method, the pulse sequence of FIG. 7 is repeated a predetermined number of times, and the magnitude of the phase encode gradient magnetic field is sequentially changed at each repetition. In the three-dimensional measurement of FIG. 8, the magnitude of the slice encode gradient magnetic field is further changed. That is, first, the magnitude of the slice encode and phase encode gradient magnetic fields is set to a predetermined magnitude and measured. Next, the magnitude of the phase encode gradient magnetic field is not changed, and the magnitude of the slice encode is sequentially changed and measurement is performed. After the slice encode is changed a predetermined number of times, the magnitude of the phase encode is changed by 1, and the magnitude of the slice encode gradient magnetic field is sequentially changed.
Similarly, the magnitudes of the phase encode and slice encode gradient magnetic fields are changed to obtain tomographic information. The obtained data is arranged in the memory corresponding to the three-dimensional K space as shown in FIG. By first performing Fourier transform in the kz (slice) direction and then Fourier transform in the kx (frequency) direction and the ky (phase) direction, the tomographic information at a plurality of slice positions can be displayed.

[0013]

The technique of the conventional three-dimensional measuring method and its problems will be described with reference to FIGS. 7 and 8. FIG. 8 shows a pulse sequence of the three-dimensional measurement method, and FIG. 7 shows a pulse sequence of the two-dimensional measurement method.
For these measurements, one sequence shown in the drawing is repeated a predetermined number of times in a time called a repetition time TR. First, in the two-dimensional measurement, the number of repetitions is the phase encoding gradient magnetic field 3
The number of changes is 2 (denoted as PRJ). Therefore, the imaging time is the repetition time TR and the phase encoding gradient magnetic field 32.
Is proportional to the product of the number of changes PRJ and PRJ. On the other hand, in the three-dimensional measurement shown in FIG. 8, it is also proportional to the number of changes of the slice encode gradient magnetic field 31. That is, in the three-dimensional measurement, the repetition time TR and the phase encode gradient magnetic field 32
Is proportional to the product of the number of changes in the slice encoding gradient magnetic field 31 and the number of changes in the slice encoding gradient magnetic field 31. Thus, in three-dimensional measurement, instead of obtaining a plurality of tomographic images, it is necessary to sequentially change the applied slice encode gradient magnetic field and repeat the pulse sequence. Long.

An object of the present invention is to solve the above-mentioned problems, and it is an object of the present invention to provide an MRI apparatus capable of obtaining a tomographic image at high speed.

[0015]

According to the present invention, a 180 ° high frequency pulse signal is added in a time series after a 90 ° high frequency pulse signal is applied, and after each 180 ° high frequency pulse signal, the phase encoding directions are opposite to each other. In an MRI apparatus for obtaining a multi-order echo signal by applying a polar phase encode gradient magnetic field and a frequency encode gradient magnetic field, the measurement of the multi-order echo signal is performed including the application of slice encode gradient magnetic fields having mutually opposite polarities in the slice directions. And a K space memory for storing the multi-order echo signal, and the K space memory is addressed by the magnitude of the slice encode gradient magnetic field, the magnitude of the phase encode gradient magnetic field, and the magnitude of the frequency encode gradient magnetic field. Each of the multi-order echo signals is a slice encode gradient magnet when the echo signal is obtained. The magnitude and the phase encoding gradient magnetic field magnitude and address determined by the magnitude of the frequency encoding gradient magnetic field, it was decided to store.

[0016]

According to the magnetic resonance imaging apparatus configured as described above, the measurement data in the slice direction can measure two or more data in one operation of the sequence as compared with the conventional method, so that it can be performed in a short time. Imaging becomes possible. From this, the time for restraining the subject is shortened and the throughput is increased. Further, since the three-dimensional measurement is performed, detailed tomographic information can be obtained in the slice direction.

[0017]

An embodiment of the present invention will be described below with reference to FIG. FIG. 3 is a block explanatory diagram of the overall configuration showing a magnetic resonance imaging apparatus to which the present invention is applied. A magnetic resonance imaging apparatus to which the present invention is applied will be described with reference to FIG.
This magnetic resonance imaging apparatus is roughly classified into a central processing unit (CPU) 1, a sequencer 2, a transmission system 3, and
A static magnetic field generating magnet 4, a receiving system 5, and a signal processing system 6 are provided.

The central processing unit (CPU) 1 controls each of the sequencer 2, the transmission system 3, the reception system 5 and the signal processing system 6 according to a predetermined program. The sequencer 2 operates based on a control command from the central processing unit 1 and transmits various commands necessary for collecting data of a tomographic image of the subject 7, a transmission system 3, a gradient magnetic field generation system 21 of a static magnetic field generation magnet 4, It is sent to the receiving system 5.

The transmission system 3 includes a high frequency oscillator 8 and a modulator 9
And an irradiation coil 11 as a high-frequency coil, and a modulator 9 amplitude-modulates a high-frequency pulse from a high-frequency oscillator 8 according to a command from the sequencer 2, and the amplitude-modulated high-frequency pulse is amplified via a high-frequency amplifier 10. By supplying to the irradiation coil 11, the subject 7 is irradiated with a predetermined pulsed electromagnetic wave.

The static magnetic field generating magnet 4 is for generating a uniform static magnetic field around the subject 7 in an arbitrary direction. Inside the static magnetic field generating magnet, in addition to the irradiation coil 11, a gradient magnetic field coil 13 for generating a gradient magnetic field and a receiving coil 14 of the receiving system 5 are provided. The gradient magnetic field generation system 21 has a structure capable of independently applying a gradient magnetic field in Cartesian coordinate axis directions orthogonal to each other.
And gradient magnetic field power supply 12 for supplying current to the gradient coil
And a sequencer 2 for controlling the gradient magnetic field power supply 12.

The receiving system 5 has a receiving coil 14 as a high frequency coil, an amplifier 15 connected to the receiving coil 14, a quadrature phase detector 16 and an A / D converter 17, and the NMR from the subject 7 is detected. When the receiving coil 14 detects the signal,
The signal is converted into a digital amount through the amplifier 15, the quadrature detector 16, and the A / D converter 17, and the quadrature detector 1 is operated at the timing instructed by the sequencer 2.
The data is converted into two series of collected data sampled by 6 and sent to the central processing unit 1.

The signal processing system 6 has an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 such as a CRT. When the data from the receiving system 5 is input to the central processing unit 1, The central processing unit 1 executes processing such as signal processing and image reconstruction, and the subject 7
While displaying the desired cross-sectional image of
The data is stored in the magnetic disk 20 or the like of the external storage device.

FIG. 1 is a schematic explanatory view of a pulse sequence according to an embodiment of the present invention. Although FIG. 1 particularly shows the slice encoding gradient magnetic field 31 and the phase encoding gradient magnetic field 32, the application pattern of other gradient magnetic fields is not specified in the present invention. First, the high frequency magnetic field 90 ° pulse 28 is applied, the spin is tilted 90 °, the 180 ° pulse 29A is applied, the slice encode gradient magnetic field 31A is applied with a magnitude of 2, and the phase encode gradient magnetic field 32 is applied with a magnitude of 8. , Obtain the first echo signal. Then, the slice echo gradient magnetic field 31A ′ is set to −2, and the phase encode gradient magnetic field 32 is set.
A'is applied -8. Further, a 180 ° pulse 29B is applied, a slice encode gradient magnetic field 31B 'is applied 1 and a phase encode gradient magnetic field 32B' is applied 8 to obtain a second echo signal. Similarly, 180 degree pulse 29C, 29D,
Slice encode gradient magnetic fields 31C, 31C ', 31
D, 31D ', phase encoding gradient magnetic fields 32C, 32
The application of C ', 32D, 32D' is repeated to obtain the third echo signal and the fourth echo signal. With the above, the first measurement (1
The measurement eye) ends. The lower column of FIG. 1 shows the magnitude of the slice encode gradient magnetic field 31 and the magnitude of the phase encode gradient magnetic field 32. In the first measurement, the magnitude of the phase encode gradient magnetic field 32 is 8, −8, and in the second measurement,
The magnitude of the phase encode gradient magnetic field 32 is 7, −7, and thereafter, the third, fourth, fifth, sixth, seventh, eighth, ... Arranges the measured data in the memory corresponding to the K space in FIG. In FIG. 2, kz indicates a slice direction, kx indicates a frequency direction, and ky indicates a phase encoding direction. kz and ky correspond to the magnitudes of the slice encode gradient magnetic field 31 and the phase encode gradient magnetic field 32, respectively. For example, the first echo signal of the first measurement in FIG. 1 is stored at the positions of kz = 2 and ky = 8 in FIG.

Here, the phase direction ky direction is easy to understand +
Although represented by integer values of 1, -1, 0, etc., when the magnitude of the actual gradient magnetic field strength is deviated by one in the ky direction, the spin is at both ends of the imaging region (FOV). It feels a gradient magnetic field in the phase direction that makes one rotation. When it becomes an expression

(Equation 1) Becomes Here, γ is the gyromagnetic ratio, Gpn is the nth phase direction gradient magnetic field strength, tn is the application time of the nth phase direction gradient magnetic field, and N is the phase direction until the echo signal of the target echo number is measured. The number of gradient magnetic fields applied to (F.
O. V) is the length of one side of the imaging region. Similarly, the mth data from the center in K space is

[0025]

(Equation 2) The gradient magnetic field strength in the phase direction and the application time are determined so that Similarly, for the slice direction, if the thickness of the selected region is slub, from (Equation 1),

[0026]

(Equation 3) Becomes Here, γ is the gyromagnetic ratio, Gpn is the nth phase direction gradient magnetic field strength, tn is the application time of the nth phase direction gradient magnetic field, and N is the slice direction until the echo signal of the target echo number is measured. The number of gradient magnetic fields applied to the. Similarly, kz from the center in the K space (slice direction)
The mth data is from (Equation 2)

[0027]

(Equation 4) The gradient magnetic field strength in the slice direction and the application time are determined so that

In the above description, the number of slice encodes is 4,
Although the number of phase encodes is 16 and the number of measurement echoes is 4, the number of slice encodes, the number of phase encodes, and the number of measurement echoes can of course be the same. Furthermore,
There are various arrangements of echo signals in the kx-kz plane on the K space.

[0029]

According to the present invention, in three-dimensional measurement,
Since it is possible to obtain two or more pieces of measurement data in the slice direction in the one sequence, the imaging time can be shortened, the subject restraint time can be shortened, and the throughput can be increased. 3
Due to the dimension measurement, there is an effect that detailed tomographic information can be obtained in the slice direction.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory diagram of a pulse sequence according to an embodiment of the present invention.

FIG. 2 is a schematic explanatory diagram of an arrangement order of raw data according to an embodiment of the present invention.

FIG. 3 is a block diagram of the overall configuration showing a magnetic resonance imaging apparatus to which the present invention is applied.

FIG. 4 is a schematic explanatory diagram of a pulse sequence of a conventional fast spin echo method.

FIG. 5 is an explanatory diagram of an application pattern of a phase direction gradient magnetic field in a conventional spin echo method.

FIG. 6 is a schematic explanatory diagram of an arrangement order of raw data in a conventional high-speed spin echo method.

FIG. 7 is a schematic explanatory diagram of a pulse sequence of a spin echo method using a two-dimensional measurement method.

FIG. 8 is a schematic explanatory diagram of a pulse sequence of a spin echo method using a three-dimensional measurement method.

FIG. 9 is a schematic explanatory diagram of a memory corresponding to a K space using a three-dimensional measurement method.

[Explanation of symbols]

 2 sequencer 7 subject 8 high-frequency oscillator 12 gradient magnetic field power supply 13 gradient magnetic field coil 21 gradient magnetic field generation system 24 first echo signal 25 second echo signal 26 third echo signal 27 fourth echo signal

Claims (2)

(57) [Claims] 1. After applying a 90 ° high frequency pulse signal, 1
80 degree high frequency pulse signal is added in time series, and each 1
The value is different for each echo signal generated by the 80 ° high frequency pulse signal, and a phase encode gradient magnetic field having the same value in the phase encode direction but opposite polarity is applied to the single echo signal before and after the echo signal. MR for measuring multi-order echo signals by applying encoding gradient magnetic field
In the apparatus I, the measurement of each multi-order echo signal has a different value for each echo signal, and the single echo signal includes the application of slice-encoding gradient magnetic fields having the same value in the slice direction but opposite polarities. MRI characterized by performing
apparatus. 2. A K space memory having an address corresponding to the applied amount of each of the slice encoding gradient magnetic field, the phase encoding gradient magnetic field, and the frequency encoding gradient magnetic field is provided for storing the multi-order echo signal, and The MRI apparatus according to claim 1, wherein each of the multi-order echo signals is stored in a corresponding address of the K space memory.