JPH0318449B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a nuclear magnetic resonance imaging apparatus, particularly to an apparatus capable of high-speed imaging.

[Conventional technology]

Nuclear magnetic resonance (NMR) relies on the fact that when an atomic nucleus is placed in a uniform static magnetic field, it precesses around the magnetic field at a frequency proportional to the field strength.

This frequency is known as the Larmor frequency and is given by wo = γH ₀ . Here, γ is the gyromagnetic ratio of the atomic nucleus, and H ₀ is the strength of the magnetic field. When a static magnetic field is applied that varies in strength along a particular direction, atomic nuclei at different positions in that direction precess at different frequencies. When a radio frequency magnetic field pulse of sufficient strength is applied at the same time as a gradient magnetic field is applied to an object, only the atomic nuclei whose spins precess at the frequency of the radio frequency magnetic field pulse are rotated by 90° or 180°, and the other nuclei are rotated by 90° or 180°. Can be isolated from the atomic nucleus.

Recently, in British Patent No. 2079946,
It has been proposed to obtain a two-dimensional image of an object by a method known as "spin warp". Briefly, the spin warp method involves defining a thin slab of spin in an object and subjecting the object to a first gradient magnetic field parallel to this slab and a second gradient parallel to the slab and perpendicular to the first gradient magnetic field. exposure to a magnetic field and then reversing the first magnetic field gradient to generate a free nuclear magnetic induction signal;
FIS).

This FIS is caused by spins that are first dephased by a first gradient field and then rephased to form a spin echo. When sampled Nx times, the Fourier transform of the spin echo gives a projection of the spin density on a line parallel to the first gradient field. The second gradient magnetic field changes the phase of each spin in the direction of the second magnetic gradient field.
When this series of operations is repeated for a second gradient magnetic field of Nz values and the resulting output is Fourier transformed, an Nx×Nz array of density values is formed.

It will be appreciated that this method allows the formation of a two-dimensional image of a surface in an object.

[Problem that the invention seeks to solve]

In nuclear magnetic resonance imaging methods such as the conventional spin warp method described above, the time required to form a two-dimensional image of one tomographic plane is typically on the order of Nz seconds, so it is difficult to image a human body. However, there were problems in that the measurement time was not necessarily short enough, and it was not easy to image organs that pulsated, such as the heart.

The present invention was made to solve this problem, and an object of the present invention is to provide a nuclear magnetic resonance imaging apparatus having a sequence that can shorten the measurement time described above.

[Means for solving problems]

The nuclear magnetic resonance imaging apparatus according to the present invention includes a first section for exciting nuclear spins in a certain volume of an object in a static magnetic field under a first gradient magnetic field, and a second gradient magnetic field orthogonal to the first gradient magnetic field. The second period of applying , in order to generate a spin echo signal of the above nuclear spin.
A third section in which a 180° pulse is applied, a fourth section in which the spin echo signal is observed under a second gradient magnetic field, and a third section perpendicular to the first and second gradient magnetic fields before or after reading out the spin echo signal. 3 gradient magnetic fields are applied, and the nuclear spins are phase-modulated in the direction of the third gradient magnetic field. The pulse sequence is applied such that the third gradient magnetic field in the fifth interval has a particular absolute value of the time integral and the sign is alternately positive and negative every five intervals. It is prepared.

[Effect of means to solve problems]

In the nuclear magnetic resonance apparatus of the present invention, a plurality of spin echo signals are obtained using a plurality of 180° pulses, and in the fifth interval, an absolute value of the time integral value of the third gradient magnetic field is obtained in the direction of the third gradient magnetic field. Since the values are the same and the time integral value is phase modulated so that the signs are alternately positive and negative, the multiple spin echo signals required for image formation can be measured sequentially, eliminating the need for recovery time and shortening the measurement time. be shortened.

〔Example〕

A first embodiment of the present invention will be described below using the pulse sequence shown in FIG. Note that the configuration of the device for implementing such a sequence is the same as that of a conventional nuclear magnetic resonance imaging device, and for example, a static magnetic field _H , and transmit and receive high-frequency waves along the x-axis or y-axis perpendicular to the z-axis. The reception method uses cos components with a 90° phase difference and
QD (Quadrature) that measures both sin components
Detection) method is used to perform signal processing.

Furthermore, with respect to the static magnetic field H ₀ , gradient magnetic fields Gx, Gy, and Gz are applied in the x, y, and z directions perpendicular to each other, respectively.
A coil is provided for forming the. Devices of this type are described, for example, in the Journal of Physics E:
Scientific Instruments, 13, 947-955. The generation of pulses continues as shown below in the first section, second section, second section, .

First section 90° high frequency pulse RF (1) is applied together with the first gradient magnetic field Gz ⁽¹⁾ . The second and third gradient magnetic fields Gx and Gy are zero. As a result, nuclear spins in the tomographic plane of a certain thickness of the object in the static magnetic field are selectively excited depending on the frequency of the radio-frequency pulse. The thickness of the fault plane can be changed by changing the bandwidth of the radio frequency pulse or the amplitude of Gz ⁽¹⁾ .

Second section In the fourth section, the second gradient magnetic field Gx (2 ⁾ is applied in order to observe the spin echo signal S(4) in the presence of the second gradient magnetic field Gx ( ⁴⁾ .

In order to generate the third section spin echo signal S(4),
A 180° high frequency pulse RT (3) is applied together with the first gradient magnetic field Gz ⁽³⁾ . At this time, let A+B=C for the time integral values A, P, and C (shaded areas) of the gradient magnetic field Gz,
It is well known that the spin disturbance due to Gz is corrected.

(For example, PRLocher “ProtonNMR
tomography”Philips Technical Rev.41, 73~
88) Also, the phase of the 180° pulse is the same as the phase of the 90° pulse.
A known method is used to vary the signal by 90° to avoid signal attenuation due to imperfections in the 180° pulse.

(For example, S.Meidoom etal.Rev.Sci.Instr.29,
688, 1958) Fourth section Observe the spin echo signal S ( ^{4 ) in the presence of the second gradient magnetic field Cx (4)} . During the observation of the signal S(4), only the gradient magnetic field Gx ⁽⁴⁾ having a constant value is applied.
The maximum value of the absorption component of the above spin echo signal is observed τ time after the time when the maximum value of the 180° pulse is given. For the second gradient magnetic field Gx, the areas α and β of the shaded portion in the figure are equal.

5th section Spin echo signal S phase modulated in 7th section
In order to observe (7), a third gradient magnetic field Gy ⁽⁵⁾ for phase modulation is applied. Here, the magnitude of Gy ⁽⁵⁾ is γLy∫Cy ⁽⁵⁾ dt≦2π. However, Ly is the length of the measurement target in the y-axis direction.

Sixth section: In order to generate a spin echo signal S(7), a 180° high frequency pulse RF(6) is applied together with the first gradient magnetic field Gz ⁽⁶⁾ .

D=E for the area D and E of the hatched part shown in the diagram
shall be.

The pulse interval of each 180° is 2τ with respect to the pulse interval τ of the 90° pulse RF (1) and the 180° pulse RF (3).

7th section This is the same as the fourth section.

The amplitudes of Gx ⁽⁴⁾ and Gx ⁽⁷⁾ are the same.

In fact, the amplitudes of Gx ⁽⁴⁾ , Gx ⁽⁷⁾ , Gx ⁽¹⁰⁾ , ... are the same, and ∫2Gx ⁽²⁾ dt＝∫Gx ⁽⁴⁾ dt＝∫Gx ⁽⁷⁾ ＝∫Gx ^{(10 )}
dt=...

Eighth section: Apply the third gradient magnetic field Gy ⁽⁸⁾ for phase modulation.
At this time, Gy ⁽⁸⁾ and Cy ⁽⁵⁾ have the same absolute value of time integral,
And the sign is opposite. That is, ∫Gy ⁽⁸⁾ dt=∫y ⁽⁵⁾ dt.

Note that the integration in this text is performed for each time period during which each gradient magnetic field is applied.

9th section It is the same as the 6th section.

With respect to the areas F and G of the hatched part in the figure, F=G.

10th section It is the same as the 7th circle section.

Section 11 Same as section 5. That is, ∫y ⁽¹¹⁾ dt＝∫Gy ⁽⁵⁾ dt
The signs of the time integral values are the same.

After the 12th section Repeat the steps from the 6th section onwards to the 11th section sequentially.

With the above pulse sequence, n=0,...
..., the amount of phase modulation that the (n+1)th spin echo signal has for N/2 is (-1) ⁿ nγ∫Gy ⁽⁵⁾
It is expressed as dt・y.

Now, an example of a method for processing a received signal will be described. It goes without saying that the same results can be obtained by processing mathematically equivalent to this embodiment.

First, the phase of the (n+1)th spin echo signal at time t is γGxtx+(-1) ⁿ nγ∫Gy ⁽⁵⁾ d·y. Here, the origin of time is the time at which the maximum value of the (n+1)th spin echo signal (absorption component) is given (n=0, . . . , N/2).

Here, γLy∫Gy ⁽⁵⁾ dt=2π, and the above spin echo signal at time t is S
(tn), ignoring the proportionality constant, becomes.

Here, ρ(x, y) represents the spin density, ignoring the effect of transverse relaxation within the measurement time.

Next, sampling is performed so that γLxGxt≦2πm (m=-M/2, . . . , M/2-1), where the total number of time samplings of S(t, n) is M. For example, if the equality holds, Here, Lx is the length of the measurement target in the x-axis direction. In equation ⁽¹⁾ , if we set k=(-1) ⁿ n m=M/2,..., M/2-1, K=0, -1, 2, -3,..., (-1) ^N/2 N/2 That is, in equation (2), the received signal S(m , k) to 2
By performing dimensional Fourier transformation, the spin density ρ(x, y) of the measurement target can be found.

Note that regarding k, if * is a complex conjugate operator, then S(m, -k)=S(-m, k)*. In other words, for any m, k=0, -1,
From the measured values of 2, -3,..., (-1) ^N/2 N/2, the measured values of k=-N/2,..., N/2-1 can be calculated.

In fact, the cos component a(t) obtained by the QD method,
u(m,k)=a(t)+jb for sin component b(t)
(t), generally u(m, k)=e ^j 〓S(m,
k), but only the absolute value of the spin density after calculation has meaning, so for u(m, n), u(m, -k) = u(-m,
The calculation is the same even if you consider it as k) *.

because, Therefore, e ^j 〓ρ(x, y) can be found from u(m, k), and ρ(x, y) can be obtained by taking the absolute value.

As can be seen from the above, we obtained (N/2+1) spin echo signals, and from the measured values obtained by sampling each spin echo signal at M points, we calculated M×N
The two-dimensional spin density of the object to be measured can be determined by obtaining the measured value of and subjecting it to two-dimensional Fourier transformation.

In the above embodiment, the phase disturbance in the z direction of the spins after selective excitation is caused by the first gradient magnetic field.
Although Gz ⁽¹⁾ was corrected without reversing, the gradient magnetic field Gz ⁽¹⁾
A known method of inverting may be used. In this case, the areas B and C of Gz ⁽³⁾ may be made the same.

Further, although a Gaussian function-like high-frequency pulse is shown as the high-frequency pulse, it may also be a sinc function, a square wave, a composite wave thereof, or a combination thereof.

If all the 180° high-frequency pulses are square waves, the first gradient magnetic field Gz that is applied at the same time may not be necessary. An example in this case is shown in FIG. Figure 2
Area of the shaded area A in Gz, A=B in B
A known method for correcting phase disturbance in the Z direction is used. The rest is the same as the first embodiment, so the explanation will be omitted.

Furthermore, although the waveform of the applied gradient magnetic field has been described as a trapezoidal wave, a triangular wave, or something similar to these, it may also be a square wave or the like. In particular, for the third gradient magnetic field Gy for phase modulation, ∫Gy ⁽⁵⁾ dt=-
If ∫Gy ⁽⁸⁾ dt＝∫Gy ⁽¹¹⁾ dt＝... then any shape can be used.

Also, ∫2Gx ⁽²⁾ dt＝∫Gx ⁽⁴⁾ dt＝∫Gx ⁽⁷⁾ dt＝∫Gx ⁽¹⁰⁾
If dt=..., then the shape of Gx ⁽²⁾ may be arbitrary.

Furthermore, regarding the shape of the gradient magnetic field Gz, as long as the high-frequency pulse application has a constant value, the rising and falling waveforms may be arbitrary as long as they satisfy the conditions described in the main text. The shape of the gradient magnetic field Gx may be arbitrary under the above conditions as long as it is a constant value during signal observation.

Further, the phase of the 90° high frequency pulse and each 180° high frequency pulse may be the same without changing by 90°.

Furthermore, in the above embodiment, the third gradient magnetic field Gy for phase modulation was applied in the fifth section, the eighth section, etc. after reading out the spin echo signal, but the third gradient magnetic field Gy for phase modulation was applied before reading out the spin echo signal. A gradient magnetic field Gy of 3 may be applied. In this case, the 7th section,
Gy may be applied to the 10th section...

Furthermore, the third gradient magnetic field Gy for phase modulation may be a pulse train, or it may be applied multiple times before and after reading out the spin echo signal, and the time integral of the gradient magnetic field applied multiple times may be It goes without saying that a similar effect can be obtained if we consider the sum of ∫Gy ⁽⁵⁾ as explained in the text.

Note that when we want to obtain information about the longitudinal relaxation time (T ₁ ), we add a process in which the nuclear spin is reversed for a time approximately equal to the average longitudinal relaxation time of the nuclear spin at the beginning of the first step (first section). do it. A 180° pulse or adiabatic high speed method is known as a process for reversing this spin.

(Described in "Pulsed and Fourier Transform NMR" by Farabetzker, Yoshioka Shoten.) In this case, the first gradient magnetic field may be applied together with the 180° pulse.

Furthermore, when it is desired to obtain information regarding the transverse relaxation time (T ₂ ), it is sufficient to wait for a time approximately equal to the average transverse relaxation time of the sample after the first section, and then execute the next step.

〔Effect of the invention〕

As described above, the nuclear magnetic resonance imaging apparatus according to the present invention includes a first section that excites nuclear spins in a certain volume of an object in a static magnetic field under a first gradient magnetic field, and a second section that excites nuclear spins in a certain volume of an object in a static magnetic field under a first gradient magnetic field; a second section in which a gradient magnetic field of 2 is applied; a third section in which a 180° pulse is applied to generate a spin echo signal of the nuclear spin;
a fourth section in which the spin echo signal is observed under a gradient magnetic field; a third section perpendicular to the first and second gradient magnetic fields before or after reading out the spin echo signal;
a fifth section in which a gradient magnetic field of
A series of sections consisting of sections are sequentially executed, and the third gradient magnetic field in the fifth section is applied so that the absolute value of the time integral is equal and the sign is alternately positive and negative for each of the five sections. It is equipped with a pulse sequence that reduces measurement time, reduces the effects of body movements such as pulsation, peristalsis, and breathing on the chest, especially the heart and abdomen, and has good spatial and temporal resolution. It has the effect of producing images. It also facilitates imaging of blood flow, lymph flow, etc. Furthermore, since the power supply for phase modulation can be of small capacity, the power supply becomes inexpensive.
Furthermore, the total measurement time when obtaining multi-slice images can also be shortened.

[Brief explanation of drawings]

FIG. 1 is a pulse sequence diagram showing nuclear magnetic resonance imaging according to a first embodiment of the invention, and FIG. 2 is a pulse sequence diagram showing nuclear magnetic resonance imaging according to a second embodiment of the invention.

Claims (1)

[Claims] 1. A first section in which nuclear spins in a certain volume of an object in a static magnetic field are excited under a first gradient magnetic field, and a second gradient magnetic field orthogonal to the first gradient magnetic field is applied. Second
a third section in which a 180° pulse is applied to generate the nuclear spin Epin echo signal, a fourth section in which the spin echo signal is observed under a second gradient magnetic field, and before the spin echo signal is read out. Alternatively, a fifth section is provided in which a third gradient magnetic field orthogonal to the first and second gradient magnetic fields is applied to phase-modulate the nuclear spins in the direction of the third gradient magnetic field, and following these sections, A series of intervals consisting of a third interval, a fourth interval, and a fifth interval are executed sequentially, and the third gradient magnetic field in the fifth interval has the same absolute value of time integral, and the sign is the same for each of the five intervals. A nuclear magnetic resonance imaging device equipped with a pulse sequence that alternately applies positive and negative pulses. 2. Claim 1, characterized in that in the fifth section, the third gradient magnetic field consists of a pulse train.
The nuclear magnetic resonance imaging apparatus described in Section 1. 3 As a received signal, a cos component with a 90° phase difference and a
Claim 1, characterized in that signal processing is performed using a QD method that measures both sin components.
The nuclear magnetic resonance imaging apparatus according to item 1 or 2. 4. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein a step of reversing the nuclear spin is performed prior to the first section. 5. The nuclear magnetic resonance imaging apparatus according to claim 4, wherein a 180° pulse is used as a method for inverting nuclear spins prior to the first section. 6. The nuclear magnetic resonance imaging apparatus according to claim 4, wherein an adiabatic fast-pass method is used as a method for reversing the nuclear spin prior to the first section.