JP3133083B2 - MR device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR apparatus for optimizing a flip angle of a pre-pulse applied to suppress an artifact due to an echo signal generated from saturation during MR image capturing.

[0002]

2. Description of the Related Art In recent years, techniques for suppressing the occurrence of artifacts (false images) in capturing tomographic images with an MRI apparatus (magnetic resonance imaging apparatus) have been actively developed.

[0003] Conventionally, a local saturation excitation method is often used to suppress an echo signal from a region that is a source of aliasing and body motion artifacts. In the local saturation excitation method, as shown in FIG. 6, a pre-pulse 1 (generally 90 degrees) is applied to an area where an echo signal is to be suppressed (hereinafter, referred to as a saturation area) to excite it, and immediately thereafter, a strong gradient magnetic field (spoiler) is used. 2) is applied to saturate this region. Therefore, even if the RF pulse 3 is applied, no echo signal is generated from this saturation region, so that artifacts due to this echo signal can be suppressed.

[0004]

However, in such a conventional local saturation excitation method, since the flip angle of the pre-pulse 1 is fixed at 90 degrees, the 90-degree RF pulse is applied after the pre-pulse 1 is applied. Until 3 is applied, longitudinal relaxation may occur and longitudinal magnetization may occur. For this reason, some echo signals may be generated from the saturation region to which the pre-pulse 1 is applied, and there is a problem that the artifact cannot be completely suppressed.

The present invention has been made to solve such a conventional problem, and an object of the present invention is to provide a method of applying a pre-pulse capable of suppressing an echo signal generated from a saturation region as much as possible. It is in. [Configuration of the Invention]

[0006]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a method for applying a pre-pulse to a saturation region to apply a pre-pulse to a saturation region before applying an excitation pulse, thereby reducing an echo signal from the saturation region. In an MR apparatus for performing imaging while suppressing, means for obtaining a relationship between parameters relating to the imaging and an echo signal from the saturation area, and obtaining a flip angle at which an echo signal from the saturation area is minimized based on the relationship. And a means for applying a pre-pulse at the determined flip angle.

[0007]

With the above arrangement, a relational expression between each sequence parameter and the echo signal can be obtained in advance. Then, a sequence parameter corresponding to the photographing condition is substituted into this relational expression, and the relational expression is made a function of only the flip angle α of the pre-pulse.

Thereafter, a differential operation is performed to obtain a flip angle α that minimizes the signal value, and a pre-pulse is applied at the flip angle α. As a result, the echo signal generated from the specific area (saturation area) becomes extremely small, and it is possible to prevent artifacts caused by this.

[0009]

FIG. 1 is a block diagram showing a hardware configuration of an MRI apparatus to which the method of the present invention is applied.

In FIG. 1, a magnet device 4 includes a static magnetic field coil 5 for forming a main magnetic field having a constant strength therein, a gradient magnetic field coil 6 for forming a gradient magnetic field in a slice direction, an encode direction, and a read direction. An excitation coil 8 for applying an RF pulse (including a pre-pulse) for exciting spins of atomic nuclei of the subject and a detection coil 7 for receiving an echo signal from inside the subject are provided. Static magnetic field control circuit 19
Controls the intensity of the static magnetic field generated by the static magnetic field coil 5.

A system controller 11 outputs a timing signal to a gradient magnetic field control circuit 12, a high frequency oscillator 13, and a gate circuit 14 so that an RF pulse and a gradient magnetic field are applied to a subject in a predetermined pulse sequence. It is. The gradient magnetic field control circuit 12 controls the gradient magnetic field in the slice direction, the encode direction, and the read direction at a timing given from the system controller 11.

The high-frequency oscillator 13 and the gate circuit 14 operate according to a timing signal given from the system controller 11, and the high-frequency signal output from the oscillator 13 is modulated by the gate circuit 14 to generate a high-frequency pulse. The power amplifier 15 amplifies the high-frequency pulse generated by the gate circuit 14 and supplies this to the excitation coil 8 as an RF pulse.

The preamplifier 18 amplifies the echo signal received by the detection coil 7, and includes a phase detection circuit 1
7 is for phase detection of the amplified echo signal,
The waveform memory 16 stores the phase detection output. The computer 9 takes in the contents stored in the waveform memory 16 to create an MR image and displays it on the display 10. The flip angle calculation unit 20 is a main part of the present invention, and calculates the flip angle α of the pre-pulse by performing the calculation described later. Next, the operation of the present embodiment will be described.

An echo signal I generated from a region where the echo signal is to be suppressed (hereinafter referred to as a saturation region) has a time τ from the application of a pre-pulse to the application of a 90-degree pulse, an echo time T _E , The repetition time T _R , the longitudinal relaxation time T ₁ of the subject, and the horizontal relaxation time T
₂ and changes depending on the number n of multi-slice shots and the flip angle α of the pre-pulse. That is, the following equation 1
Can be represented as [Number 1] I = I (τ, T R, T E, T 1, T 2, n, α)

In Equation 1, τ, T _R , T _E , T ₁ , T
_{Since 2} and n are known according to the imaging conditions, the signal value I ultimately depends only on the flip angle α of the pre-pulse. That is, if a flip angle α that minimizes the signal value I is obtained and a pre-pulse is applied at the flip angle α, an echo signal generated from the saturation region becomes minimum, and artifacts caused by this echo signal can be prevented. . Hereinafter, a procedure for obtaining Equation 1 as a specific equation will be described. Now, assuming that the longitudinal magnetization of the saturation region is M _Z and the transverse magnetization is M _X , the following equations 2 and 3 are given by Bloch's equation.

[0016]

(Equation 2)

[0017]

(Equation 3)

The number of images taken by multi-slice is n.
When the number of sheets is one, n pre-pulses α1 to αn are applied to the saturation region, so that the pulse sequence of the excitation pulse is as shown in FIG. 2, and α1 → (π / 2)
Excitation pulses are applied in the order of → π → α2 →... → αn.

Here, assuming that the longitudinal magnetization immediately before the application of the pre-pulse α1 is M _Z (0−), the longitudinal magnetization M _Z (0−) is longitudinally relaxed by the time τ after the pre-pulse α1 is applied, and becomes 90 ° R
Since F pulse is applied, the transverse magnetization M _X. The transverse magnetization M _X at the time of only transverse relaxation echo time T _E
(τ + _TE ) is obtained by solving the Bloch equation shown in Equations 2 and 3 under physical boundary conditions, as shown in Equation 4 below.

[0020]

(Equation 4)

Immediately before the application of the 90-degree pulse, the transverse magnetization M _X is almost zero due to the saturation effect.
Immediately after 90 ° pulse applied to the longitudinal magnetization M _Z becomes zero. After a lapse of time (T _E / 2), a 180-degree pulse is applied and the sign is inverted. The longitudinal magnetization M _Z (T _R / n) + immediately after the second pre-pulse α2 is applied is given by
Indicated by

[0022]

(Equation 5)

Thereafter, pre-pulses are sequentially applied at α3, α4,..., Αn and at every time (T _R / n). Here, the longitudinal magnetization immediately after the application of the i-th pre-pulse αi and (i
The relational expression with the longitudinal magnetization immediately after the (+1) -th pre-pulse α (i + 1) is applied is as shown in the following Expression 6.

[0024]

(Equation 6) Here, when the following equation 7 is defined, equation 8 is obtained. [Equation 7] M _Z (iT _R / n) ₊ = Mi (i is a natural number equal to or less than n)

[0025]

(Equation 8) When the following equation 9 is defined, equation 10 is obtained. [Equation 9] [Equation 10] (M _{i + 1} −K) = λ (M _i −K) Then, if Equation 10 is repeated and the order is lowered, Equation 11 is obtained. [Equation 11]

Since the magnitude M of the longitudinal magnetization immediately before the application of the pre-pulse α1 shown in Equation 4 corresponds to i = n in Equation 11, the magnitude M of the longitudinal magnetization is given by the following Equation 12. [Equation 12] M = M _n = λ ^n-1 (M ₁ −K) + K Here, in Equation 12, M ₁ is known from Equation 5. Then, by substituting Equation 12 into Equation 4 and omitting M ₀ , the signal value I from the saturation area is calculated by the following Equation 13.
Is required.

[0027]

(Equation 13) That is, Expression 13 is a specific arithmetic expression of Expression 1 described above.
Hereinafter, a procedure for obtaining the flip angle α based on Equation 13 will be described with reference to the flowchart shown in FIG.

First, a repetition time T _R , an echo time T _E , the number of slices n, and a time τ from the application of a pre-pulse to the application of a 90-degree RF pulse according to the imaging conditions are input to the theoretical formula shown in Expression 1 ( Step ST1). Then, enter the longitudinal relaxation time T ₁ of the inhibition zone (step ST
2). At this time, since the signal value I is a function of only the flip angle α, the signal value I (α) is differentiated by α. (At this time, since the transverse relaxation time T ₂ disappears from the equation, it is not necessary to substitute a numerical value. There is no real number α.
It is determined whether or not exists. [Equation 14]

That is, the flip angle α at which the signal value I (α) is minimized is obtained from Expression 14. If there is a real number α that satisfies Expression 14 (YES in step ST3), since α is both a minimum value and a minimum value, this α is set as a flip angle (step ST4).

On the other hand, when there is no real number α that satisfies Equation 14 (NO in step ST3), the signal value I (α) monotonically decreases, so that when α is 180 degrees, I becomes minimum. Therefore, the flip angle is set to 180 degrees (step ST5). Thus, the optimum flip angle is determined.

FIGS. 4 and 5 are characteristic diagrams showing the relationship between the flip angle α of the pre-pulse and the signal value I. FIG. 4 shows the time τ from the application of the pre-pulse to the application of the 90-degree pulse.
Is set to 16 [ms], and FIG.
s]. The solid line in each figure shows the experimental value, and the dotted line shows the numerical value obtained by the above-mentioned theoretical formula. 4 and 5, the optimum pre-pulse flip angle is 1
It turns out that it is about 00-140 degrees.

As described above, in this embodiment, the signal value I from the saturation region and the pre-pulse flip angle α
The flip angle that minimizes the signal value I is obtained from the relational expression, and the pre-pulse is applied at this angle. Therefore, an echo signal generated from the saturation region can be suppressed to a maximum, and an artifact generated due to the echo signal can be removed.

[0033]

As described above, in the present invention, the relational expression between the flip angle of the pre-pulse applied to the saturation region and the signal value is obtained, and the pre-pulse is applied at the flip angle that minimizes the signal value. . Therefore, it is possible to suppress the echo signal generated from the saturation region to the maximum, and it is possible to obtain an effect of preventing an artifact generated due to the echo signal.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an MR device to which the present invention is applied.

FIG. 2 is a diagram showing a pulse sequence of an excitation pulse in the present invention.

FIG. 3 is a flowchart showing a procedure for calculating a flip angle.

FIG. 4 is a characteristic diagram showing a relationship between a flip angle of a pre-pulse and a signal value. (Τ = 16ms)

FIG. 5 is a characteristic diagram showing a relationship between a flip angle of a pre-pulse and a signal value. (Τ = 25ms)

FIG. 6 is a diagram showing a conventional pulse sequence.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pre-pulse 2 Spoiler 3 90 degree pulse 8 Excitation coil 11 System controller 20 Flip angle calculation part

──────────────────────────────────────────────────続 き Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. An MR apparatus which applies a pre-pulse to a saturation region to apply a pre-pulse to a saturation region before applying an excitation pulse to suppress an echo signal from the saturation region and perform imaging, wherein: Means for determining a relationship with the echo signal from the saturation area; means for determining a flip angle at which the echo signal from the saturation area is minimized based on the relationship; and means for applying a pre-pulse at the determined flip angle. An MR apparatus comprising: