JPH074348B2 - NMR imaging method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an internal tomography apparatus using a nuclear magnetic resonance (NMR) phenomenon, and more particularly to a technique for imaging blood flow velocity in the body.

[Background of the Invention]

The principle of blood flow imaging is to apply a gradient magnetic field that does not affect a stationary object but only a moving object in the flow direction, and try to measure by adding different phase information depending on the flow velocity. To do. A gradient magnetic field G is applied at time τ ₁ in the direction of blood flow, and then an inversion gradient magnetic field (−G) is applied at an appropriate time τ ₂ . The reversal gradient magnetic field is a magnetic field whose sign is reversed without changing its magnitude.

Since a stationary object has no motion, at time τ ₁ and τ ₂ , we feel magnetic fields with the same magnitude and opposite signs, the influences of which are mutually canceled and the same state as when no gradient magnetic field is applied. Become. On the other hand, the blood part feels different magnetic fields at times τ ₁ and τ ₂ due to movement, and its influence is not canceled and changes the phase of spin.

Usually, the slice vertical direction is called the Z direction, the readout gradient magnetic field direction that is the image horizontal direction is called the x direction, and the phase encoding direction that is the image vertical direction is called the y direction. Hereinafter, description will be made in the x, y, and z directions.

The combination of the two sets of gradient magnetic fields described above is called a flow encode pulse and is always used in blood flow measurement, but its effect differs depending on whether the blood flow direction is the z direction or the x direction. The cause will be described in the imaging pulse sequence in FIG. 1. In the figure, there are three types called flow encode pulses. There are two types for G _{Z and} one type for G _X. G _Z
Has one set of pulses 102 and one set of pulses 105. There is only one pulse 105, but 180 ° pulse at the same time
Since 104 is applied, it has an equivalent function. In accordance with the principle described above, the gradient magnetic fields of the pulses 102 and 105 generate two phase rotations.

On the other hand, the pulses 103 and 107 relating to G _X are flow encode pulses in the x direction. However, this is different from the case of the z direction, which is the gradient magnetic field during reading of the measurement signal 108. Therefore, the blood flow in the x direction changes frequency during measurement. Focusing on the individual spins, the movements are complicated, but considering the blood flow in the entire blood vessel, the phase turns around a phase proportional to the flow velocity similar to the z direction.

Conventionally, the following announcements have been made regarding blood flow measurement in the x direction. The first method is to J. DGNorris "Phase Encoding NMR Flow Imaging" (Phase Enc
oded NMR Flow Imaging). The flow encode similar to the z direction is applied outside the measurement. Change the condition 8
If you take a picture twice and Fourier transform in that direction, the velocity resolution is 8
An image of the points is obtained.

Also, as a second method, V.I. Jay. VJWedeen's NMR Velocity Imaging by Phase Displa
y) This is a method of directly using the pulses 103 and 107 related to G _X , but to see the speed change,
The position of 107 is shifted so that the phase changes linearly with respect to the position. Since the phase cyclically changes every 2π, it looks like a striped pattern when imaged. In a stationary object, the stripes are beautiful vertical stripes, but when there is motion, the stripes are distorted according to their size. From this, the flow velocity can be seen as a pattern.

Both methods are qualitative imaging methods and have a problem in quantitativeness.

Moreover, in the first method, it is necessary to increase the number of images to be taken in order to increase the speed resolution, and thus, it takes a long time to take an NMR.
Image songs are a big problem. Further, in the second method, if there is movement in the z direction of the object, it cannot be distinguished from the x direction, so it is limited to the case of only movement in the xy plane.
There is a big problem in practical use.

[Object of the Invention]

An object of the present invention is to provide a practical and quantitative means capable of measuring the blood flow velocity in the x direction.

[Outline of Invention]

Similar to the measurement of the blood flow velocity in the z direction, the relationship between the blood flow velocity in the x direction and the phase angle is obtained. As a result, when the blood flows only in the x direction, the blood flow velocity can be obtained from the phase of the reproduced image. However, in general, there is often a flow in the z direction (vertical direction of the slice). Therefore, paying attention to the fact that two sets of flow encode pulses are included in the z direction, G _Z is set so that the respective phase changes are reversed. Thus, if G _Z is controlled well, the phase can be made almost zero regardless of the flow velocity in the z direction. When shooting is performed in such a sequence, x from the phase of the reproduced image
The flow velocity in the direction can be obtained.

In the sequence of FIG. 1, the relationship between the flow velocity and the phase when there is movement in the x direction is as follows.

θ = γ G _X vt _P t _I [rad] (1) where γ: nuclear magnetic rotation ratio G _X : gradient of gradient magnetic field in x direction v: flow velocity t _P : G _X basic time t _I : G _X application interval In order to remove the phase rotation due to the influence of the z direction,
Instead of applying G _Z with the polarity reversed, a normal pulse 10
5 may be applied, and a flow encode pulse whose phase rotation is reversed may be added again.

Although the blood flow is described as the movement, it is of course possible to measure the moving organ such as the heart.

Example of Invention

Hereinafter, the present invention will be described in detail based on Examples. Second
FIG. 1 is a block diagram of an embodiment of the present invention. From the sequence control unit 201 that controls various pulses and magnetic fields generated to detect an NMR signal from the subject, a transmitter 202 of a high-frequency pulse generated to resonate a specific nuclide of the subject, and resonance of the NMR signal. A magnetic field control unit 203 for generating a static magnetic field that determines the frequency and a gradient magnetic field that can arbitrarily control the strength and direction, and controls the receiver 205 that performs measurement after detecting the NMR signal generated from the subject. , The image is reconstructed by the processing unit 206 based on the measurement signal acquired from the receiver 205, and the reconstructed image is
Display on RT Display 207. The magnetic field drive unit 204 generates a magnetic field required for measurement based on the control signal output from the magnetic field control unit 203.

The procedure for carrying out the present invention in the above configuration will be described with reference to FIGS.
It will be described below with reference to the drawings. Here, a method of reversing the pulse 105 will be described as a cancel method for phase change in the z direction. FIG. 3 is a pulse sequence used for the present invention, and FIG. 4 is a processing procedure flow chart using the sequence of FIG. It will be described with reference to FIG.

Step 401: In accordance with the pulse sequence of FIG. 3, the phase encode gradient magnetic field (G _Y ) 306 is changed 256 times and an image is taken. The difference from the normal pulse sequence shown in FIG. 1 is the polarity of the gradient magnetic field (G _Z ) 305 and the application time. The polarity is reversed from the gradient magnetic field (G _Z ) 302 because the gradient magnetic field (G _Z )
_{This is because the} phase change due to the flow velocity of _{Z Z} ) 302 and the phase change due to the flow velocity of the gradient magnetic field (G _Z ) 305 are in opposite directions. Gradient magnetic field (G _Z ) 305 application time, high-frequency pulse width 3
When it is about 1.5 times 04, the phase becomes zero regardless of the flow velocity in the z direction.

Step 402: The signal 308 seen after quadrature detection undergoes a phase change due to the distortion of the device, so it is corrected. The distortion that affects the phase includes the deviation of the origin of the sample position of the NMR signal, the characteristics of the detection system, and the nonuniformity of the static magnetic field. This correction can be performed by using a known method [Sano et al .: Phase distortion correction technology in nuclear magnetic resonance imaging, 1985 IEICE General Conference]. Image reproduction is performed while performing these corrections.

Step 403: The NMR image obtained in the previous step becomes a complex signal shown by the following equation.

(X, y) = _R (x, y) + i _I (x, y) The phase angle can be calculated by the following equation.

This phase angle is independent of the flow velocity in the z direction and is proportional to the flow velocity in the x direction, as described above. The relational expression is as follows.

θ = G _X γ vt _P t _I where G _X : gradient magnetic field strength in the x direction γ: nuclear magnetic rotation ratio t _P : application time of pulse 303 t _I : application interval of pulse 303 and 307 Therefore, t _P cannot be smaller than a certain value. The phase angle θ is (−π) to (+
Since it is limited during π), the maximum measurement flow velocity is limited here.

A sequence for solving this is shown in FIG. The difference from FIG. 3 lies in the (−G _X ) pulse pair 501 and 503. This pulse pair is a flow encode pulse and has the opposite polarity to the pulse pair 502 and 504 of G _X. Therefore,
The phase rotation angle due to the flow velocity becomes opposite, and the phase angle becomes smaller for the same flow velocity, and as a result, the dynamic range can be widened.

It is also possible to extract the blood vessel portion by paying attention to the fact that the phase of the moving portion changes, and extracting the portion where the phase deviates from 0 °.

〔The invention's effect〕

According to the present invention, since only the flow velocity component in the x direction of a blood vessel flowing in an arbitrary direction can be taken out purely by one imaging, the quantitativeness is excellent and the measurement throughput can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a pulse sequence used in a normal NMR imaging apparatus, FIG. 2 is a block configuration diagram showing an embodiment of the present invention, and FIG. 3 is a pulse showing an example of an imaging procedure of the present invention. FIG. 4 is a flow chart showing a sequence, FIG. 4 is a flow chart showing a processing procedure for carrying out the present invention, and FIG. 5 is a pulse sequence showing an example of an imaging procedure for expanding the dynamic range.

Claims (4)

[Claims] 1. A plurality of gradient magnetic fields including a static magnetic field, a high-frequency magnetic field, and a gradient magnetic field for slice selection, which is applied so that a change in phase angle due to movement of an object is canceled and becomes substantially zero. Generated magnetic field is applied to an object to be imaged, at least one NMR signal is selectively detected from the object, and predetermined operation including image reconstruction operation is performed on the detected NMR signal. An NMR imaging method characterized by: 2. The NMR imaging method according to claim 1, wherein the gradient magnetic field for slice selection is applied at the time of applying a 90 ° pulse and a 180 ° pulse for generating a high frequency magnetic field. 3. The NMR imaging method according to claim 2, wherein the gradient magnetic field for slice selection is applied so as to have different polarities when the 90 ° pulse is applied and when the 180 ° pulse is applied. 4. The NMR imaging method according to claim 1, wherein the movement of the object is blood flow in a blood vessel.