JPH0572811B2 - - Google Patents

Description

[Detailed description of the invention] [Object of the invention] (Industrial application field) The present invention is directed to nuclear magnetic resonance (NMR).
In particular, it involves a magnetic resonance imaging method that applies the phenomenon of magnetic resonance (magnetic resonance), in which a gradient magnetic field based on a two-dimensional Fourier transform method and high-frequency pulses of a 90°-180° pulse sequence are applied to a subject placed in a static magnetic field. The present invention relates to a magnetic resonance imaging method in which morphological information or functional information of a detection target region is obtained by applying a magnetic resonance signal and collecting induced echo signals.

(Prior Art) Nuclear magnetic resonance is a phenomenon in which atomic nuclei placed in a magnetic field resonate and absorb electromagnetic wave energy of a specific wavelength, and then emit this energy as electromagnetic waves. Devices that utilize this phenomenon to diagnose living organisms detect the electromagnetic waves emitted from the above-mentioned atomic nuclei, especially protons, process the detected signals, and calculate the density of the atomic nucleus (proton), the longitudinal relaxation time T. ₁ ,
Diagnostic information such as a tomographic image of the subject reflecting information such as transverse relaxation time T ₂ , flow, and chemical shift can be obtained.

Incidentally, as a magnetic resonance imaging method used to obtain this tomographic image, a spin echo method using high-frequency pulses in the 90°-180° series is often used. In this spin echo method, in order to eliminate the phase shift due to the inhomogeneity of the static magnetic field, the center of the 90° pulse and the peak value of the echo signal obtained is
A 180° pulse is applied. That is, between the timing time of the center of application of the 90° pulse t=0, the timing time of the center of application of the 180° pulse t=T ₁₈₀ , and the time of the peak value of the echo signal t= _TE , T ₁₈₀ =
A relationship of T _E /2 is required.

Therefore, there is a problem that the echo signal collection time cannot be increased and the S/N ratio cannot be improved. In other words, the earliest time the echo signal collection starts is after the end of the application of the 180° pulse with the pulse width _tw , so if the fall time of the slicing gradient magnetic field is α, the echo signal collection time is
T _aq is subject to the constraint of T _aq ≦T _E −t _w −2α, assuming that data is collected symmetrically with respect to the echo peak value. Therefore, under the same resolution condition,
The upper limit of T _aq is determined by the values of T _E , _tw and α. As a result, the noise of the echo signal is 1√ _aq
Since it is proportional to , the S/N ratio cannot be improved.

(Problem to be Solved by the Invention) As described above, in the conventional spin echo method, the 180° pulse is placed approximately midway between the timing time of the application center of the 90° pulse and the time of the peak value of the echo signal. Because of this necessity, it was not possible to secure sufficient signal acquisition time, and the S/N ratio could not be sufficiently improved.

The purpose of the present invention is to develop a multi-echo method that improves the S/N ratio, cancels out the effects of static magnetic field inhomogeneity, and compensates for the spin phase shift caused by the chemical shift of water and fat. The object of the present invention is to provide a suitable magnetic resonance imaging method.

[Structure of the invention]

(Means for Solving the Problems) The present invention provides a gradient magnetic field based on a two-dimensional Fourier transform method and a 90°-
Magnetic resonance imaging comprising the steps of respectively applying high-frequency pulses of a 180° pulse series, and collecting echo signals induced by the application of the gradient magnetic field and high-frequency pulses to obtain morphological information or functional information of the predetermined region. In the method, when the time t at the center of application of the 90° pulse is set to t=0, the time at the center of the application of the first 180° pulse is
Apply voltage so that it becomes TE′/2 (here, TE′ is
It is the value obtained by subtracting an integral multiple τc of the period in which the spin phases of water and fat are aligned, which is determined based on the amount of chemical shift between water and fat, from the first echo time TE1. ), and the second 180° pulse is applied such that the time Tπ2 at the center of its application becomes Tπ2 = (TE2 + TE1 - τc)/2 (here, TE2 is the second echo time). ) is a magnetic resonance imaging method characterized by:

(Function) In the magnetic resonance imaging method of the present invention, the first
The timing time of the application center of the 180° pulse is t, and the timing time of the application center of the 90° pulse is t.
= 0, by setting t = T _E ′/2, by bringing the timing time of the application center of the 180° pulse closer to the 90° pulse by τc/2,
The echo signal collection time of the first echo signal can be increased.

Furthermore, by setting the timing time T〓 ₂ of the application center of the second 180° pulse with respect to the echo time T _E2 of the second echo signal as T〓 ₂ = (T _E2 +T _E1 −τc)/2, For all echo signals after the second echo obtained with this pulse sequence,
The effects of static magnetic field inhomogeneity are canceled out, and the Spen phase shift due to chemical shifts of water and fat is compensated.

(Example) Prior to describing the embodiment of the present invention, an invention to which the embodiment of the present invention is applied will be described. This invention has an improved S/N ratio compared to the conventional Spen echo method, and has already been filed by the present applicant as Japanese Patent Application No. 126513/1983.
This is what the applicant named the hybrid echo method. That is, as the pulse sequence is shown in Fig. 3, the timing time at the center of application of the 180° pulse is changed to 90° for half the period τc, which is an integral multiple of the period in which the phases of the spins are aligned due to the chemical shift of water and fat. It brings it closer to the pulse. the result,
In the conventional method, the echo signal collection time T _aq ′ can be extended to T _aq ′=T _E +τc−(t _w +2〓), and the timing time of the center of application of the 180° pulse is t=T _E /2. Compared to the echo signal collection time T _aq in the spin echo method, T _aq = T _E − (t _w +2〓), it can be made longer by τc time, and if the image resolution is constant, 1√ _aq ′ The noise in the image is reduced by _aq , and as a result, the S/N ratio can be improved. Here, T _E is the echo time until the echo signal, T _w is the pulse width of the 180° pulse,
α is the rise or fall time of the gradient magnetic field.

As a pulse sequence in which this excellent hybrid echo method is applied to the multi-echo method to obtain multiple echo signals, the present applicant has proposed a patent application filed in 1983-
214920 has been filed. That is, as shown in FIG. 4, this pulse sequence is 180 degrees below the second echo time with respect to the echo time T _En ' of the m-th echo signal (m is an integer of 2 or more) below the second echo signal. The timing time T〓 _n at the center of pulse application is set as T〓 _n =(T _En ′+T _E(n-1) ′)/2.

However, in this hybrid echo method, the timing time of the application center of the first 180° pulse is not set to T _E /2, and therefore the influence of the non-uniformity of the static magnetic field cannot be canceled out. Also,
Also in the above-mentioned pulse sequence in which this hybrid echo method is applied to the multi-echo method, the timing time of the application center of the second 180° pulse is placed in the center between the first echo signal and the second echo signal. Therefore, this drawback of the hybrid echo method is inherited, and the influence of static magnetic field inhomogeneity cannot be canceled out.

Furthermore, taking into account the difference in chemical shift between water and fat, the shift is made by an integer multiple τc of the period in which the phases of fat spin and water spin are aligned; Since it differs depending on the tissue, a slight decrease in signal is inevitable.

Therefore, the present invention solves this inconvenience and realizes a suitable pulse sequence for applying the hybrid echo method to the multi-echo method.
Embodiments of the present invention will be described below with reference to FIGS. 1 and 2. FIG. 2 is a schematic diagram showing the configuration of a magnetic resonance imaging apparatus used in an embodiment of the present invention. As shown in FIG. 2, this device 1 includes a gradient magnetic field generating coil 2 for generating a gradient magnetic field for obtaining positional information of a region where an echo signal is induced, and a gradient magnetic field generating coil 2 for generating a gradient magnetic field for obtaining position information of a region where an echo signal is induced. It has a transmitting/receiving coil 3 for detecting resonance signals. This gradient magnetic field generating coil 2
The axis in the height direction of the subject P is the Z axis, and this Z
If the axes perpendicular to the axis are the X axis and the Y axis,
An X-axis gradient magnetic field generation coil 2a and a Y-axis gradient magnetic field generation coil 2 generate gradient magnetic fields about these axes.
b, consists of a Z-axis gradient magnetic field generating coil 2c. Each gradient magnetic field generating coil 2a, 2b, 2c is
X-axis gradient magnetic field power supply 4a, Y-axis gradient magnetic field power supply 4b,
Each is connected to a Z-axis gradient magnetic field power supply 4c.
Further, the transmitting/receiving coil 3 is connected to a transmitting circuit system 5 and a receiving circuit system 6. Furthermore, this device 1 includes a sequencer 7 that executes a pulse sequence, a computer that controls all of the power supplies 4a, 4b, 4c, a transmitting circuit system 5, a receiving circuit system 6, and the sequencer 7, and also performs signal processing of detection signals. System 8 is provided. The signals processed by this computer system 8 are displayed on a display 9. This device 1 includes a static magnetic field coil (not shown) that generates a static magnetic field in the Z-axis direction toward the subject P.
It also includes a power source (not shown) that supplies current to the static magnetic field coil.

Next, regarding the imaging method of this example,
This will be explained using FIG. In this pulse sequence, for the first echo signal, based on the hybrid echo method described above, the timing time T〓 ₁ of the application center of the 180° pulse is calculated as T〓 ₁ = T _E1 '/2 = (T _E1 −τc)/2, As a result, the echo signal collection time for the first echo signal can be extended compared to the conventional spin echo method. Here, τc is an integral multiple of the period in which the spin phases of water and fat are aligned, which is determined based on the amount of chemical shift between water and fat.
When the static magnetic field strength is H ₀ , the amount of chemical shift between water and fat is σ, and the gyromagnetic ratio is γ, it is expressed by the following formula.

τc=2π/γσH ₀ .

The chemical shift amount σ of water and fat is usually 3.3
~3.6ppm.

Next, for the second echo signal in the multi-echo method, the timing time T〓 ₂ at the center of application of the second 180° pulse is set as follows.

T〓 ₂ = (T _E2 + T _E1 − τc)/2.

That is, it is set earlier by τc/2 from the center of the time T _E1 of the peak value of the first echo signal and the time T _E2 of the peak value of the second echo signal. As a result, the influence of the non-uniformity of the static magnetic field of the first echo signal is canceled out.

Furthermore, for the third echo signal, the timing time T〓 ₃ of the application center of the third 180° pulse is
It is set at the center of the time of the peak value of the second echo signal and the time of the peak value of the third echo signal (not shown).

T〓 ₃ = (T _E3 + T _E2 )/2.

In this third echo signal, the influence of the static magnetic field inhomogeneity on the first echo signal is canceled out during acquisition of the second echo signal, so that the influence of the static magnetic field inhomogeneity no longer appears.

Also, for the m-th echo signal below the fourth echo signal, the 180° pulse is set at the center of the peak value of each echo signal, similar to the setting of the 180° pulse for the third echo signal. Therefore, the timing time T〓 _n of the application center of the 180° pulse for the m-th echo signal (m is an integer of 3 or more) below the third echo signal may be set so as to satisfy the following relational expression.

T〓 _n = (T _En +T _E(n-1) /2.

In this way, the pulse sequence of the present invention
Since no phase disturbance occurs in the echo signal due to non-uniformity of the static magnetic field, the S/N ratio can be improved. Furthermore, since there is no phase disturbance due to chemical shift between water and fat, the resolution of tomographic images of areas where water and fat coexist is also improved. In addition, the image accuracy of areas susceptible to susceptibility artifacts, such as the palate and cranium, is improved, and the effects of non-uniformity occurring at the boundaries of bones and soft tissues can also be offset.

〔Effect of the invention〕

As described above, according to the present invention, the S/N ratio is improved, and the influence of static magnetic field inhomogeneity is canceled out in the echo signals after the second echo. A magnetic resonance imaging method suitable for a multi-echo method in which spin phase shift is compensated for can be provided.

[Brief explanation of the drawing]

Fig. 1 is a graph showing a pulse sequence in an embodiment of the present invention, Fig. 2 is a schematic diagram showing the configuration of a magnetic resonance imaging apparatus used in an embodiment of the present invention, and Fig. 3 is a pulse sequence of a hybrid echo method. Figure 4 shows the hybrid
It is a graph showing a pulse sequence when an echo method is adapted to a multi-echo method. 1... Magnetic resonance imaging device, 2... Gradient magnetic field generation coil, 3... Transmission/reception coil, 4a... X-axis gradient magnetic field power supply, 4b... Y-axis gradient magnetic field power supply, 4c... Z-axis gradient magnetic field power supply, 5... Transmission circuit system, 6 ...Reception circuit system, 7. Sequencer, 8. Computer system.

Claims (1)

[Scope of Claims] 1. A step of respectively applying a gradient magnetic field based on a two-dimensional Fourier transform method and a high-frequency pulse of a 90°-180° pulse series to a predetermined region in a static magnetic field;
In the magnetic resonance imaging method comprising the step of collecting echo signals induced by application of the gradient magnetic field and high-frequency pulse to obtain morphological information or functional information of the predetermined region, time t at the center of application of the 90° pulse is set to t. = 0, the first
A 180° pulse is applied so that the time at the center of its application is TE'/2 (here, TE' is the water and fat calculated based on the amount of chemical shift between water and fat from the first echo time TE1). ), and the second 180° pulse is set at the time at the center of its application.
A magnetic resonance imaging method characterized in that Tπ2 is applied so that Tπ2 = (TE2 + TE1 - τc)/2 (here, TE2 is the second echo time).