JPH0658377B2 - Magnetic resonance imager - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus for obtaining a flow velocity vector of a fluid flowing in a subject based on an image signal obtained from a magnetic resonance signal. Regarding

[Technical background of the invention and its problems]

As is well known, magnetic resonance imaging (MRI) shows that when a group of nuclei having an intrinsic spin and associated nuclear magnetic efficiency is placed in a static magnetic field of intensity H ₀ ,
Using the phenomenon of resonantly absorbing the energy of a high-frequency magnetic field rotating at an angular velocity determined by ω ₀ = Υ H ₀ (where Υ is the nuclear magnetic rotation ratio) in the plane perpendicular to the direction of the static magnetic field, the magnetic resonance This is a method of detecting the signal and visualizing the distribution of specific atomic nuclei in the subject, and it is technically almost complete for stationary parts such as the human head.

However, it is hard to say that the imaging of the cardiovascular system, the abdomen and the like where there is a flow or movement with MRI has reached the level of practical use, and has become a major research and development theme of MRI in recent years. In particular, although the visualization of blood flow velocity is highly expected to be clinically effective, it is limited to the visualization of blood flow velocity in the direction perpendicular to the fault plane, and the flow velocity vector The actual situation is that a practical method for obtaining the flow velocity distribution in the two orthogonal directions and also in the direction perpendicular to the fault plane has not been established yet.

[Object of the Invention]

The present invention has been made in view of the above circumstances, and provides a magnetic resonance imaging apparatus capable of obtaining a flow velocity vector of a fluid in a subject based on a magnetic resonance signal detected from the subject. With the goal.

[Outline of Invention]

In order to achieve this object, the present invention applies a uniform static magnetic field to the subject and applies a gradient magnetic field in a pulsed manner,
Further, by applying a high-frequency magnetic field, in a magnetic resonance imaging apparatus for detecting and imaging a magnetic resonance signal from a subject, the gradient magnetic field is changed with respect to at least one of blanking time, application time or intensity, The difference between each real part and each imaginary part between two image signals having a real part and an imaginary part obtained based on the magnetic resonance signals detected before and after the change of the gradient magnetic field is calculated, and these differences are calculated. It is characterized in that the flow velocity of the fluid in the subject is calculated using the difference.

〔The invention's effect〕

According to the present invention, the flow velocity of a fluid such as a blood flow in one or more directions to which a gradient magnetic field is applied in a subject, that is, a flow velocity vector can be obtained.

Example of Invention

Embodiments of the present invention will be described below with reference to the drawings. First
The figure shows a schematic configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

In FIG. 1, the static magnetic field coil 1 and the gradient magnetic field 3 are driven by power sources 2 and 4, respectively, and the subject 6 on the bed 5 is
A static magnetic field that is uniform to the human body (for example, a human body) and changes linearly in the direction perpendicular to the tomographic plane with respect to the two orthogonal x and y directions in the desired tomographic plane of interest and the z direction perpendicular thereto. A gradient magnetic field is applied. A high frequency magnetic field having a frequency corresponding to the position of the tomographic plane generated by the probe head 7 is further applied to the subject 6 by the high frequency signal from the transmitter 8, and the echo (magnetic resonance signal) is detected by the probe head 7. Then, it is received by the receiving unit 9.

The system controller 10 controls the entire magnetic resonance imaging apparatus, and the image processing operation apparatus 11 is controlled by the system controller 10 to perform imaging based on the magnetic resonance signal from the receiving unit 9 and, for example, the present invention. Calculate the velocity vector for blood flow.

FIG. 2 is a time chart for explaining a pulse sequence when imaging is performed by the Fourier imaging method in the embodiment, and (a) is 90 ° and 180 ° supplied from the transmitter 8 to the probe head 7. The timing of a transmission pulse and the reception timing of an echo (magnetic resonance signal) are shown, and (b) to (d) respectively show a slice magnetic field gradient Gz and a magnetic field gradient for imaging in which a magnetic field is applied to the subject 6 in a pulsed manner. The timing of Gy and Gx is shown.

In this embodiment, the flow velocity vector of the blood flow in the subject 6 is obtained by using the pulse sequence shown in FIG. When the velocity gradient in the direction of the flow velocity vector is small, the image signal I (x, y) obtained by imaging by the Fourier image method using the sequence of FIG. Described in.

I (x, y) = ρ (x, y) exp (-iγGx Vx (Tx ² + Tx′Tx) −iγGy Vy (Ty ² + Ty′Ty) −iγGz Vz (Tz ² + Tz′Tz) + iΔφ (x, y)) (1) where ρ (x, y) is nuclear spin density ΥΥ is nuclear gyromagnetic ratio, Gx,
Gy, Gz are magnetic field gradients in the x, y, z directions, Tx, Ty, Tz are magnetic field application times in the magnetic field gradients Gx, Gy, Gz, Tx ', Ty', T
z ′ is the blanking period of the gradient magnetic field, Vx, Vy, Vz are the components of the flow velocity vector, and Δφ (x, y) is the phase generated by the static magnetic field, inhomogeneity of the high-frequency magnetic field, eddy current, sampling point shift, etc. It is a deviation. When the normal tomographic image is visualized, only the real part of the equation (1) is used, but since the information of the flow velocity vector is also included in the imaginary part of the equation (1), the image signal in the present invention is used. Compute the flow velocity vector using both the real and imaginary parts of.

Next, the procedure of flow velocity vector calculation in the present invention will be described with reference to the time chart of FIG. 2 and the flowchart of FIG.

First, in step 31 shown in FIG. 3, magnetic field gradients Gx, G
The applied time of the magnetic field in y, Gz is shown by the solid line in Fig. 2 Tx, T
Set to y and Tz respectively and visualize. The video signal obtained at that time is I ₀ (x, y).

Next, in steps 32, 33, and 34, the blanking times of the magnetic fields in the magnetic field gradients Gx, Gy, Gz are calculated from Tx ', Ty', Tz 'to Tx' + ΔTx ', Ty' + as shown by the broken lines in FIG.
ΔTy ′, Tz ′ + ΔTz ′ are sequentially changed, and the video signals obtained in each case are I ₁ (x, y), I ₂ (x, y), and I ₃ (x, y).

Then, in step 35, in step 31, the blanking time of the magnetic field in the magnetic field gradients Gx, Gy, Gz is Tx ′,
For Ty ′ and Tz ′ (before changing the blanking time)
The video signal I ₀ (x, y) obtained in step S32 and the magnetic field application time at the magnetic field gradients Gx, Gy, Gz are calculated as Tx ′ + ΔT in steps 32-34.
Video signals I ₁ (x, y), I ₂ (x, y), and I ₃ (x, x) obtained by changing x ′, Ty ′ + ΔTy ′, and Tz ′ + ΔTz ′, respectively.
Calculate the difference between y) and each real and imaginary part. The calculation results are shown below.

I ₁ (x, y) −I ₀ (x, y) = ρ (x, y) exp (-iγGx Vx (Tx ² + Tx′Tx) −iγGy Vy (Ty ² + Ty′Ty) −iγGz Vz ( Tz ² + Tz′Tz) + iΔφ (x, y)) × {exp (-iγGx Vx TxΔTx ′)-1} = I ₀ (x, y) {exp (-iγGx Vx TxΔTx ′)-1}… ( _{2) I 2 (x, y} ) -I 0 (x, y) = I 0 (x, y) {exp (-iγGy Vy TyΔTy ') - 1} ... (3) I 3 (x, y) -I ₀ (y, y) = I ₀ (x, y) {exp (-iγGz Vz TzΔTz ′)-1} (4) where γGx Vx TxΔTx ′ << 1 γGy Vy TyΔTy ′ << 1 γGz Vz TzΔTz When ′ << 1, it can be approximated by the first term of the expansion of the exponential function, so equations (2) to (4) are I ₁ (x, y) −I ₀ (x, y) = −iγGx Vx Tx ΔTx′I ₀ (x, y) ... (5) I ₂ (x, y) −I ₀ (x, y) = − iγGy Vy TyΔTy′I ₀ (x, y) ... (6) I ₃ (x , y) −I ₀ (x, y) = − iγGz Vz TzΔTz′I ₀ (x, y) (7) Then, the flow velocities Vx, Vy, Vz in the x, y, z directions are calculated as the flow velocity vector from the equations (5) to (7). (Five)~
In order to obtain Vx, Vy, Vz from the equation (7), it is first considered to divide both sides by I ₀ (x, y), but the value of the pixel included in I ₀ (x, y) is near zero. When it has a value of, not only the calculation accuracy deteriorates, but also when it is zero, calculation becomes impossible.

Therefore, in this embodiment, in step 36, (5)
~ Using the three differences shown in the equation (7) and I ₀ ^* (x, y) (* is a complex conjugate), the flow velocity vector is calculated as follows. That is, both sides of the equation (5) are multiplied by I ₀ ^* (x, y), and then |
By dividing by I ₀ (x, y) | ² , the value of the flow velocity at each point can be accurately obtained.

In equation (8), except Vx (x, y) is known, so I ₀ (x,
Vx (x, y) can be obtained from two video signals of y) and I ₁ (x, y). Hereinafter, Vy (x, y) and Vz (x, Y) can be similarly obtained by the calculations shown in the equations (9) and (10).

As described above, according to the present invention, it is possible to easily obtain the flow velocity vector of the blood flow or the like, which has been conventionally difficult in the magnetic resonance imaging apparatus.

In the above embodiment, the blanking time of the magnetic field in the magnetic field gradient was changed, and the flow velocity vector was calculated using two video signals before and after the blanking time changed, but instead of the blanking time, The flow velocity vector can also be calculated by changing the application time or intensity of the magnetic field in the magnetic field gradient and similarly using the image signals obtained before and after the application time or intensity changes. In addition, two or three of the blanking time, the application time, and the strength of the magnetic field in these magnetic field gradients are used.
The blanking time, the application time and the intensity may be changed at the same time. Further, in the embodiment, the magnetic field gradients Gx, Gy, Gz in the three directions of x, y, z are changed so that three components Vx, V of the flow velocity vector are obtained.
Although all y and Vz are obtained, any one or two of Vx, Vy and Vz may be obtained. In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a magnetic resonance imaging apparatus according to an embodiment of the present invention, FIG. 2 is a time chart for explaining a pulse sequence for imaging in the embodiment, and FIG.
The figure is a flow chart for explaining the procedure for calculating the flow velocity vector in the same embodiment. 1 ... Static magnetic field coil, 3 ... Gradient magnetic field coil, 6 ... Subject,
7 ... probe head, 8 ... transmitter, 9 ... receiver, 10 ...
System controller 11, image processing unit.

Claims (2)

[Claims] 1. A uniform static magnetic field is applied to a subject, a gradient magnetic field is applied in a pulsed manner, and a high-frequency magnetic field rotating in a plane perpendicular to the direction of the static magnetic field is applied. In a magnetic resonance imaging apparatus for detecting and imaging a magnetic resonance signal from a specimen, a means for changing at least one of a blanking time, an application time and an intensity of the gradient magnetic field, and a blanking of the gradient magnetic field by this means. For each real part and each imaginary part between two video signals having a real part and an imaginary part obtained based on magnetic resonance signals detected before and after at least one of time, application time and intensity changes, respectively. A magnetic resonance imaging apparatus comprising: a means for calculating a difference; and a means for calculating the flow velocity of the fluid in the subject using the difference calculated by the means. 2. The means for calculating the flow velocity calculates the flow velocity by using a difference between the real and imaginary parts of the two video signals and a complex conjugate of one of the two video signals. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a device for calculating.