JPH0723921A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To emphasize the image of blood flow by exciting RF pulses with different phases with the flow of blood (intraveneous floor) flowing in another slice face to be excited later from the slice face to be excited before. CONSTITUTION:In photographing a living body part where two blood vessels of which blood flow directions are opposite penetrate the parallel slice faces partially superposed in an almost vertical manner thereto, a transmission/ reception coil 3 and X-Z axis inclination magnetic power supply 7-9 are driven through the control of a sequencer 10, and an inclination magnetic field for slice Gs is applied with a specified strength. According to the strength the thickness of a slice face is determined, and after a specified time, the magnetic field Gs is impressed with a relatively week strength, and the thickness of another slice face is set to be relatively thick. From the nuclear spin of blood flow an MR signal of relatively strong due to spin echo is generated, and according to the MR signal reconstruction of image with emphasized blood flow of blood vessel is made.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus suitable for MR angiography for discriminating blood flow according to its flow direction.

[0002]

2. Description of the Related Art In recent years, MR angiography has attracted attention because blood flow can be identified by the direction of the flow without using a contrast agent. A so-called pre-saturation method is a typical example of the MR angiography.

The principle of this pre-saturation method will be described with reference to FIG. As shown in FIG. 8, two parallel slice planes A and B
On the other hand, two blood vessels b1 whose blood flow directions are opposite to each other,
Consider the case where b2 penetrates orthogonally.

First, the spin of atomic nuclei on the A-plane is sufficiently excited in advance by a 90 ° pulse to saturate it. As a result, the direction of the nuclear spin in the A plane is tilted 90 ° from the Z axis direction to the X axis direction in the rotating coordinate system. Thereafter, the B surface is similarly excited with a 90 ° pulse at a certain time interval to collect MR signals from the B surface. As a result, the blood flow flowing into the B surface through the blood vessel b1 is already saturated in the A surface, so the collected MR
The signal has a small signal value, and appears as a black defect image in the B-plane MR image. On the other hand, the blood flow flowing into the B surface through the blood vessel b2 appears in the MR image of the B surface. Therefore, the blood flow direction can be identified by checking whether or not it appears as a defect image in the MR image of the B surface.

However, since such a pre-saturation method discriminates the blood flow direction depending on whether or not there is a defect image, it is natural that the blood flow flowing from the A surface to the B surface to be defected is imaged and used for diagnosis. could not. In general, a venous flow having a low blood flow velocity and a relatively stable blood flow velocity is targeted for a defect. That is, surface A is set upstream of the vein and surface B is set downstream.
This is because, when an arterial flow whose blood flow velocity changes greatly according to pulsation is targeted for a defect, it seems that the arterial flow saturated on the A surface exists in the B surface when signal acquisition is performed on the B surface. , A
This is because it is difficult to set the time difference for exciting the plane B and the plane B according to the blood flow velocity that changes every moment.
Therefore, the deficient image is always the venous flow, and therefore it was not possible to image the venous flow while discriminating the blood flow direction.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to address the above-mentioned circumstances, and an object thereof is to provide a magnetic resonance imaging apparatus capable of imaging a venous flow while identifying the blood flow direction. Is to provide.

[0007]

A magnetic resonance imaging apparatus according to the present invention comprises means for selectively exciting at least two slice planes having different positions in the slice direction by using RF pulses having different phases while giving a predetermined time difference. After the selective excitation by the selective excitation means, means for collecting MR signals and means for reconstructing an image using the MR signals are provided.

[0008]

According to the magnetic resonance imaging apparatus of the present invention, the blood flow (venous flow) flowing from the pre-excited slice surface to another post-excited slice surface is excited by RF pulses having different phases. The MR signal generated from the nuclear spins of the blood flow becomes stronger than the MR signal generated from other portions, so that the blood flow is emphasized and imaged.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of the first embodiment. Inside the gantry 20, a static magnetic field magnet 1, an X-axis / Y-axis / Z-axis gradient magnetic field coil 2 and a transmission / reception coil 3 are provided. The transmission / reception coil 3 may be directly attached to the subject instead of being embedded in the gantry. The static magnetic field magnet 1 as the static magnetic field generator is configured by using, for example, a superconducting coil or a normal conducting coil. X-axis / Y-axis / Z-axis gradient magnetic field coil 2
Is a coil for generating an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, and a Z-axis gradient magnetic field Gz.

The transmission / reception coil 3 generates a radio frequency (RF) pulse as a selective excitation pulse for selecting a slice plane, and a magnetic resonance signal (MR) generated by magnetic resonance.
Signal). The selection of two parallel slice planes is accomplished by changing the frequency of the RF pulse when exciting each plane.

The subject P on the bed 13 is inserted into an imageable region (a spherical region where an imaging magnetic field is formed, and diagnosis is possible only in this region) in the gantry 20.

The static magnetic field magnet 1 is driven by the static magnetic field controller 4. The transmission / reception coil 3 is driven by the transmitter 5 when magnetic resonance is excited, and is coupled to the receiver 6 when detecting a magnetic resonance signal. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 is driven by an X-axis gradient magnetic field power source 7, a Y-axis gradient magnetic field power source 8 and a Z-axis gradient magnetic field power source 9.

The transmitting / receiving coil 3, the X-axis gradient magnetic field power source 7, the Y-axis gradient magnetic field power source 8 and the Z-axis gradient magnetic field power source 9 are driven by a sequencer 10 in a predetermined sequence,
Axis gradient magnetic field Gx, Y axis gradient magnetic field Gy, Z axis gradient magnetic field G
z, radio frequency (RF) pulses are generated in a predetermined pulse sequence described below. In this case, the X-axis gradient magnetic fields Gx, Y
The axis gradient magnetic field Gy and the Z axis gradient magnetic field Gz are mainly, for example, the phase encoding gradient magnetic field Ge and the reading gradient magnetic field G.
r and a gradient magnetic field Gs for slicing, respectively. The computer system 11 drives and controls the sequencer 10, captures the magnetic resonance signal received by the receiver 6 and performs predetermined signal processing to generate a tomographic image of the subject, and displays it on the display unit 12.

Next, the operation of the present embodiment thus constructed will be described. The pulse sequence shown in FIG. 2 is implemented by driving the transmitting / receiving coil 3, the X-axis gradient magnetic field power supply 7, the Y-axis gradient magnetic field power supply 8 and the Z-axis gradient magnetic field power supply 9 under the control of the sequencer 10. This pulse sequence is based on the spin echo method. Figure 3 here
As shown in FIG. 2, two blood vessels b1 and b2 whose blood flow directions are opposite to each other penetrate two slice planes A and B which are parallel and partially overlap each other at substantially right angles. Think
The blood vessel b1 is a vein and the blood vessel b2 is an artery.

First, the slicing gradient magnetic field Gs is applied with a predetermined intensity (this is referred to as "first intensity"). The thickness of the A surface is determined according to this strength. At this time, as shown in FIG. 4, the nuclear spins of atomic nuclei in the A plane are excited by a 90 ° pulse having a center frequency f0. The position of the plane A in the slice direction is uniquely determined by the center frequency f0 and the gradient magnetic field Gs. As a result, the direction of the nuclear spin in the A plane is tilted 90 ° from the Z axis direction to the plus side in the X axis direction in the rotating coordinate system.

After that, the slicing gradient magnetic field Gs is applied with a weaker intensity than the first intensity after a certain period of time. As a result, the thickness of the B surface is set to be thicker than that of the A surface. The B-side is set thicker than the A-side because it is a blood vessel b flowing from the A-side to the B-side.
A blood flow of unknown velocity of 1 exists at least in the B plane when exciting the B plane, that is, when exciting the B plane.
This is to ensure that it exists at any position on the surface. At this time, as shown in FIG. 5, nuclear spins of nuclei in the B plane are excited by a 180 ° pulse. This 180 ° pulse is formed with a center frequency f0 ± α. By adjusting this frequency difference α, the B surface is set at a position displaced in the slice direction while partially overlapping the A surface. As a result, the directions of the nuclear spins in the B-plane are changed from the state immediately before the application of the 180 ° pulse to 18 degrees around the Y axis of the rotating coordinate system.
It is rotated 0 °. The stationary part (mesh line part) existing in the overlapping part of A surface and B surface is already excited by 90 ° pulse, and the direction of nuclear spin is tilted 90 ° to the plus side in the X axis direction in the rotating coordinate system. Therefore, it is tilted 180 ° from that state to the minus side in the X-axis direction. 90 ° on side A
Blood flow in blood vessel b1 excited by pulse (dotted line)
Is flowing into the B surface when the 180 ° pulse is applied, and is excited by the 180 ° pulse and tilted 180 ° from the plus side to the minus side in the X-axis direction as in the stationary portion. The blood flow of the blood vessel b2 excited by the 90 ° pulse on the A surface flows out from the A surface to the side opposite to the B surface when the 180 ° pulse is applied. Blood flow flowing into the B surface along the blood vessel b2, stationary portion of the B surface other than the overlapping portion of the A surface and the B surface, and further flowing into the B surface along the blood vessel b1 without being excited by the 90 ° pulse. The blood flow is tilted 180 ° from the positive side to the negative side in the Z-axis direction in the rotating coordinate system.

Therefore, a 90 ° pulse is excited by a 90 ° pulse in the stationary portion existing in the overlapping portion of the A surface and the B surface and the B portion.
A relatively strong MR signal due to spin echo is generated from the nuclear spin of the blood flow of the blood vessel b1 excited by the 180 ° pulse on the surface. Further, the blood flow of the blood vessel b2 flowing out from the A surface on the side opposite to the B surface, the blood flow flowing into the B surface along the blood vessel b2, the stationary portion of the B surface other than the overlapping portion of the A surface and the B surface, The blood flow that has flowed into the B surface along the blood vessel b1 without being excited by the 90 ° pulse is excited by only one of the 90 ° pulse and the 180 ° pulse, and therefore these portions (the shaded portion in FIG. 5). From the nuclear spin of), only a relatively weak MR signal is generated, or no MR signal is generated.

After a lapse of a predetermined time from exciting the B surface with a 180 ° pulse, the read gradient magnetic field Gr is applied, and the MR signal collecting state is started. As described above, from the nuclear spin of the blood flow of the blood vessel b1 excited by the 90 ° pulse in the A plane and further excited by the 180 ° pulse in the B plane at the stationary portion existing in the overlapping portion of the A plane and the B plane, A relatively strong MR signal is detected. A relatively weak MR signal is detected or not detected from the nuclear spins of the other portions.

When image reconstruction is performed by this MR signal, the blood flow (venous flow) of the blood vessel b1 excited in the stationary portion and the A and B surfaces existing in the overlapping portion of the A and B surfaces is emphasized. And imaged. Therefore, the tissue image of the stationary portion and the blood vessel images of the blood vessels b1 and b2 existing in the overlapping portion of the A surface and the B surface are confirmed. Further, the blood flow direction is identified such that the blood vessel b1 is the blood flow flowing from the A surface to the B surface and the blood vessel b2 is the blood flow flowing from the B surface to the A surface.

In the above description, the surface A and the surface B are partially overlapped to emphasize the still portion in the overlapped portion to form an image. However, it is necessary to emphasize the stationary portion to form an image. If not, the A surface and the B surface may be separately set without overlapping, and only the blood flow flowing from the A surface to the B surface may be emphasized for imaging. in this case,
To separate the A side and the B side without partially overlapping,
90 ° pulse applied to surface A and 180 ° applied to surface B
It is well known that this can be easily achieved by adjusting the difference component α between the center frequencies of the pulses.

In the above description, the application to the spin echo method has been described as an example, but the present invention is not limited to the spin echo method and may be applied to other acquisition methods. The present invention uses the flow compensation method (Flow-Com) based on the spin echo method.
When applied to the (pensation method), the pulse sequence is as shown in FIG. When the present invention is applied to the gradient field echo method (Gradient-Field-Echo method), the pulse sequence is as shown in FIG. In this case, blood flow in both directions can be imaged by collecting MR signals in response to the first excitation, and one direction can be obtained by collecting MR signals in response to the second excitation with the slice plane shifted. Only the blood flow of can be imaged. The present invention is not limited to the above-described embodiments, but can be implemented in other various modifications.

[0022]

As described above, the magnetic resonance imaging apparatus according to the present invention comprises means for selectively exciting at least two slice planes having different positions in the slice direction while applying a time difference using RF pulses having different phases, After selective excitation by the selective excitation means, means for collecting MR signals and means for reconstructing an image using the MR signals are provided.

Therefore, according to the present invention, the blood flow (venous flow) flowing from the pre-excited slice plane to another slice plane that is post-excited is excited by RF pulses having different phases. The MR signal generated from the nuclear spins of the magnetic field is stronger than the MR signals generated from other portions, and therefore, a magnetic resonance imaging apparatus capable of enhancing and imaging the blood flow can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a magnetic resonance imaging apparatus according to the present invention.

FIG. 2 is a diagram showing a pulse sequence of the present embodiment.

FIG. 3 is a diagram showing the principle of the present embodiment.

FIG. 4 is a diagram showing a state in which a 90 ° pulse is applied.

FIG. 5 is a diagram showing a state in which a 180 ° pulse is applied subsequent to a 90 ° pulse.

FIG. 6 is a diagram showing a pulse sequence when the present invention is applied to a flow compensation method.

FIG. 7 is a diagram showing a pulse sequence when the present invention is applied to a gradient field echo method.

FIG. 8 is a diagram showing the principle of a conventional pre-saturation method.

[Explanation of symbols]

1 ... Static magnetic field magnet, 2 ... X-axis / Y-axis / Z-axis gradient magnetic field coil, 3 ... Transmitting / receiving coil, 4 ... Static magnetic field control device, 5 ... Transmitter, 6 ... Receiver, 7 ... X-axis gradient magnetic field amplifier, 8 ... Y-axis gradient magnetic field amplifier, 9 ... Z-axis gradient magnetic field amplifier, 10 ... Sequencer, 11 ... Computer system, 12 ... Display part.

Claims (3)

[Claims] 1. A means for selectively exciting at least two slice planes having different positions in the slice direction while giving a predetermined time difference by using RF pulses having different phases, and MR signals after selective excitation by the selective excitation means. A magnetic resonance imaging apparatus comprising: a means for collecting and a means for reconstructing an image using the MR signal. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the selective excitation means partially overlaps the at least two slice planes. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the selective excitation means excites the at least two slice planes with RF pulses having a 90 ° phase difference.