JPH0723922A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To improve the contrast between a saturated blood flow part and a non-saturated blood flow part so as to improve the distinguishing performance for both parts by providing means for enabling at lest two parallel saturation faces to be selectively saturated while giving a time difference by use of saturation RF pulses with different phases. CONSTITUTION:In photographing the living body part in which a blood vessel is vertical to a slice face, an inclination magnetic field for phase encoding Ge is applied with a specified strength corresponding to the predetermined thickness of a slice face in a time t1, and the slice face is selectively excited through an RF pulses with the central frequency fo to enable it to be saturated. In a time t2 after a certain time has passed the magnetic field Ge is applied with a specified strength to enable another slice face to be saturated. After the saturated time, a pulse sequence is carried out by the conventional spin echo method. A third slice face being vertical to the two faces and including a blood vessel is excited in order by 90 degrees pulse and 180 degrees pulse, and generated MR signals are collected to perform reconstruction of image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a tagging method (Tagging method).
Method) for identifying the flow of blood flow by a magnetic resonance imaging method.

[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. Various signal acquisition methods have been studied for this MR angiography, and one of them is a method called a tagging method.

The principle of this tagging method will be described with reference to FIG. Consider a case where a blood vessel b penetrates orthogonally to the slice planes A of two orthogonal slice planes A and B as shown in FIG. 7A.

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. An MR image of the B plane generated by reconstructing this MR signal is shown in FIG. MR signals collected from the static part of the A surface indicated by the diagonal lines and the blood flow part of the blood vessel b (hereinafter referred to as the "tag part") that has moved during the time from the saturation of the A surface to the signal acquisition on the B surface. Has a small signal value and appears as a black defect image in the MR image of the B surface. On the other hand, the parts other than these, which are not saturated, appear in the MR image of the B surface. Therefore, the blood flow direction of the blood vessel b and its flow velocity can be identified by observing the positional relationship between the stationary portion and the tag portion that both appear as a defect image.

However, such a tagging method has the following problems. In order to clearly identify the blood flow direction, it is important to keep the stationary part and the tag part sufficiently apart. Considering that not only arterial flow with high blood flow velocity but also venous flow with low blood flow velocity exists in plane B, in order to clearly identify the blood flow direction of this venous flow, the venous flow tag It is necessary to set a sufficient time difference between the A surface and the B surface so that the portion is sufficiently separated from the stationary portion. However, this sufficient time difference is sufficiently restored to the nuclear spin saturated on the A-plane (T1 relaxation)
Give grace to do. Therefore, the contrast between the saturated portion and the other portions is reduced, which makes it difficult to identify 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 improve the contrast between a saturated portion and other portions to discriminate the blood flow direction. To provide a magnetic resonance imaging apparatus having a tagging method capable of improving

[0007]

A magnetic resonance imaging apparatus according to the present invention comprises means for selectively saturating at least two parallel saturated planes while giving a predetermined time difference using saturated RF pulses having different phases, After the saturation by the saturation means, means for collecting an MR signal from the signal collection surface by exciting the signal collection surface substantially orthogonal to the saturation surface with an RF pulse at a certain time, and using the MR signal And means for reconstructing an image.

[0008]

According to the magnetic resonance imaging apparatus of the present invention, the blood flow is saturated on one saturation plane and then again on the other saturation plane. It is much smaller than that of the non-saturated part, so that the contrast between the saturated blood flow part and the non-saturated part can be enhanced.

[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 signals received by the receiver 6 and performs predetermined signal processing to generate an MR image of the subject, and displays the MR image on the display unit 12.

Next, the operation of the present embodiment thus constructed will be described. Here, as shown in FIG. 2, the signal acquisition plane C is set substantially orthogonal to the two slice planes A and B parallel to the XZ plane, that is, parallel to the XZ plane, and the blood vessel b is also set to these slice planes. Consider a case where the holes A and B penetrate substantially orthogonally. The blood flow in the blood vessel b is assumed to flow from the A surface to the B surface.

FIG. 3 is a diagram showing a pulse sequence when the present embodiment is applied to the spin echo method. Of course, it may be applied to a signal acquisition method other than the spin echo method. Figure 4
3A to 3C sequentially show the saturated states at times t1, t2, and t3 in FIG. FIG. 5 is a diagram showing changes in magnetization of each part with time. In FIG. 5, the solid line indicates the magnetization change of the blood flow of the blood vessel b saturated on the A surface, the broken line indicates the magnetization change of the stationary portion saturated on the A surface, and the alternate long and short dash line is saturated only on the B surface. The change in magnetization of a part is shown.

First, at time t1, the phase-encoding gradient magnetic field Ge is applied with a predetermined intensity (this is referred to as "first intensity") according to the expected thickness of the A-plane. At this time, the plane A is selectively excited by the RF pulse (hereinafter referred to as the "first saturation pulse") at the center frequency f0 and saturated (see FIG. 4).
(See (a)). The position of the A surface in the X axis direction is determined by the center frequency f0 and the gradient magnetic field Gs. The first saturation pulse has a deep Flip angle, that is, 90 ° to 180
RF pulses of ° are used. Immediately after the application of the first saturation pulse is finished, the portion saturated on the A surface starts to recover due to T1 relaxation. The blood flow portion of the blood vessel b saturated on the A surface is hereinafter referred to as a tag portion.

From time t1 when the surface A is saturated, a fixed time T
At time t2 which is separated by D1, the gradient magnetic field Ge for phase encoding is applied with a predetermined intensity (this is referred to as "second intensity"). At this time, the B plane is selectively excited by the RF pulse (hereinafter referred to as the "second saturation pulse") at the center frequency f0 ± α at a position deviated from the A plane by a predetermined distance in a predetermined direction and saturated. An RF pulse having a small Flip angle, that is, 0 ° to 90 ° is used as the second saturation pulse.
Ideally 90 ° pulses are used. Therefore, the tag portion (dotted line portion) of the blood vessel b, whose saturation on the A surface is recovering with the passage of time TD1, is saturated again on the B surface. The blood flow other than the static part and the tag part of the blood vessel b included in the B surface is saturated by the second saturation pulse for the first time.
Recovery further progresses in the stationary part of the A side.

The second intensity is set weaker than the first intensity. Therefore, the B surface is formed thicker than the A surface.
The B-side is set thicker than the A-side, whether the blood vessel b is an artery (fast blood flow) or a vein (slow blood flow), the blood flow of the blood vessel b saturated on the A-side excites the B-side. This is to ensure that it exists at least in the B-plane when performing, that is, it exists in any position of the B-plane when exciting the B-plane. Therefore, the thickness of the B surface is determined based on the difference in the moving distance at the time TD1 between when the blood vessel b is assumed to be an artery and when the blood vessel b is assumed to be a vein.

If the B surface is displaced from the A surface, they may be adjacent to each other or partially overlap each other. The shift amount of the B surface with respect to the A surface can be easily changed by adjusting the frequency difference α between the first saturation pulse and the second saturation pulse.

Further, after the time t3, which is a time TD2 apart from the time t2 when the B surface is saturated, a pulse sequence by the normal spin echo method is executed. That is, A
The plane C, which is orthogonal to the plane B and the plane B and includes the blood vessel b, is selectively excited by the 90 ° pulse and the 180 ° pulse in this order, and the MR signal from the C plane is collected after the lapse of TE time from the 90 ° pulse. This signal acquisition is repeated while sequentially moving the encode position.

The plane C at the time t3 is in the state shown in FIG. 4 (c). That is, the tag portion has moved to a position deviating from the plane B in accordance with the blood flow velocity of the blood vessel b during the time TD2. Therefore, in the C-plane MR image obtained by reconstructing the MR signal, the tag portion appears separately from other portions with good contrast. Blood flow around the tag portion saturated only on the stationary portion of the A surface or B surface or on the B surface appears separately with a slight contrast from an unsaturated portion according to the degree of recovery.

By thus saturating the tag portion twice with RF pulses having different Flip angles, the tag portion can be clearly discriminated from other portions, whereby the blood flow direction can be recognized well. Further, it becomes possible to measure the blood flow velocity with high accuracy based on the moving distance from the A surface or B surface and the time difference between the saturation timing of each surface and the signal acquisition timing.

The setting of the saturated surface for the first time and the saturated surface for the second time is not limited to the above-mentioned embodiment, and various modes can be adopted. For example, as shown in FIG. 6A, after saturation on the A surface, two surfaces B and B ′ on both sides of the A surface are saturated at the same time or with a time difference, and then signal acquisition is performed. Of course, in this case, blood flow in both directions can be identified. Further, as shown in FIG. 6B, by using a technique called SPAMM that simultaneously excites / saturates a plurality of cross sections, two kinds of first saturation pulses are used to generate two A-planes and A ′.
The surfaces may be simultaneously saturated, and the two B-side and B′-sides may be simultaneously saturated by two kinds of second saturation pulses, and then signal acquisition may be performed. The average value can improve the accuracy of speed measurement and can measure the speed difference between both tag portions. Further, after the A surface and the B surface shown in FIG. 6B are sequentially saturated, the A ′ surface and the B ′ surface may be sequentially saturated. Further, as shown in FIG. 6C, the A ′ surface and the B ′ surface, which are oblique to the A surface and the B surface that are sequentially saturated, may be sequentially saturated.

Further, in the above description, the Flip angle of the first saturation pulse is deep and the second saturation pulse is shallow, but conversely,
The Flip angle of the first saturation pulse may be set shallow and the second saturation pulse may be set deep, and in this case, the blood flow around the tag part should obtain good contrast to the part that is not saturated at all. However, the discriminating ability of the tag portion is maintained.
The present invention is not limited to the above-described embodiments, but can be implemented in other various modifications.

[0025]

As described above, the magnetic resonance imaging apparatus according to the present invention is a means for selectively saturating at least two parallel saturated planes while giving a predetermined time difference using saturated RF pulses having different phases. After saturation by the saturation means, means for collecting a MR signal from the signal collection surface by exciting a signal collection surface substantially orthogonal to the saturation surface with an RF pulse at a certain time interval, and using the MR signal And means for reconstructing an image.

Therefore, according to the present invention, since the blood flow is saturated on one saturation plane and then again saturated on the other saturation plane, the MR signal of the saturated blood flow portion is not saturated on the other side. It is possible to provide a magnetic resonance imaging apparatus in which the contrast between the saturated blood flow portion and the non-saturated portion can be enhanced and the discriminating ability of both portions can be improved.

[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 the principle of the present embodiment.

FIG. 3 is a diagram showing a pulse sequence of the present embodiment.

FIG. 4 is a diagram showing a state of the C surface at each time in FIG.

FIG. 5 is a diagram showing a change in magnetization of each part of the C surface with time.

FIG. 6 is a diagram showing a modified example in which the settings of the first saturated surface and the second saturated surface are changed.

FIG. 7 is a diagram showing the principle of a conventional tagging 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 (1)

[Claims] 1. A means for selectively saturating at least two parallel saturated surfaces while giving a predetermined time difference using saturated RF pulses having different phases, and a predetermined time after the saturation by the saturation means. The apparatus further comprises means for exciting a signal collecting surface substantially orthogonal to the saturation surface with an RF pulse to collect an MR signal from the signal collecting surface, and means for reconstructing an image using the MR signal. Magnetic resonance imaging device.