JPH03151935A - Mr imaging method by ve sequence - Google Patents

Abstract

PURPOSE:To obtain the best SNR by a method wherein a first echo signal is synchronous with an organism signal while a second echo signal suppresses a flow artifact with an application of an gradient magnetic field for phase correction and moreover, collection time is set as much as possible during a collection of these signals. CONSTITUTION:When a first echo signal is collected, an excitation pulse is applied based on a beat synchronization signal of a person to be inspected or a signal synchronous with blood of a peripheral blood tube and a spinning of a sliced surface is excited selectively and after a specified magnetic field is applied, the spinning inverted by a first inversion pulse is focused under a lead gradient. Then, when a second echo signal is collected, specified magnetic fields such as an gradient magnetic field for phase correction, a second inversion pulse and a slice gradient are applied, the spinning inverted by the second inversion pulse is focused under the lead gradient. Based on a flow velocity of a person to be inspected, the gradient magnetic field for phase correction is applied so that a phase change is down to zero with a second echo center, thereby reducing a flow artifact attributed to a flow part in a second echo image.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an MR imaging method using a VE (Variable Echo) sea fence. The present invention provides a method for improving SNR while reducing flow artifacts that occur when collecting echo signals.

(Prior Art) A nuclear magnetic resonance imaging diagnostic apparatus includes a magnet section that includes a static magnetic field coil that creates a --like static magnetic field and a gradient magnetic field coil that creates a magnetic field that has linear gradients in each of the x, y, and z directions. A transmitting/receiving unit (including a sea fence storage circuit, etc.) that applies an RF pulse to a subject placed in a magnetic field formed by the magnet unit and detects an NMR signal from the subject; the transmitting/receiving unit; It has a control image processing section that is centered around a computer that controls the operation of the magnet section, processes detected data, and displays images.

In the above configuration, during the VE sequence operation, the sea fence storage circuit stores the third
A signal based on the VE pulse sequence shown in the figure is generated. In FIG. 3, RF is a high frequency rotating magnetic field applied in a direction perpendicular to the static magnetic field. Gs is a gradient magnetic field applied to an axis called the slice axis. The slicing gradient applied to the slicing axis is used to excite only spins within a specific plane, and the rephase gradient is used to remove the phase difference that occurs during slicing.

The spoiler is intended to prevent artifacts due to angular errors in the inverted pulse. Gf is a gradient magnetic field that applies a required magnetic field of different magnitude each time to an axis called a phase encode axis. The hoop gradient is used to obtain phase information in the phase encode axis direction. Gr is a gradient magnetic field applied to an axis called the frequency encode axis. The lead gradient is used to observe the spin echo signal, and the day phase gradient is used to provide initial phase information in the frequency encode axis direction. The signals are a first echo and a second echo generated in response to the first and second inversion pulses. In this way, the VE sequence applies a reversal pulse several times between the first excitation pulse and the next excitation pulse, and adds a lead gradient each time to make echoes appear, thereby changing the density and spin width between each tissue. Image contrast is added by utilizing differences in signal strength based on relaxation time, and at least the second echo generation time TE2 is set to a time exceeding twice the first echo generation time TEI.

By the way, flow artifacts occur when echo signals are collected from a flow portion (blood, cerebral spinal fluid, etc.) and an image is obtained using this vE sequence.

Therefore, conventional methods for reducing flow artifacts include: ■ Applying excitation pulses based on the subject's heart rate synchronization signal or signals synchronized with the blood in peripheral blood vessels; ■ Applying a gradient magnetic field for phase correction to increase the flow rate. A method has been used in which the phase change based on the component is made zero at the echo center.

(Problems to be Solved by the Invention) However, in image capturing using a conventional VE sequence, the TE2 of the VE sequence is long, so the flow artifact in the second echo cannot be significantly reduced by method (2). In addition, since the method (2) requires a large gradient magnetic field, the TEI becomes long, making it impossible to take a long signal collection time, resulting in a problem of deterioration of the SNR.

The present invention has been made in view of the above points, and its purpose is to collect a first echo image and a second echo image obtained by collecting echo signals from a flowing part (blood, cerebral spinal fluid, etc.) using a VE sequence. An object of the present invention is to realize an imaging method that can obtain the best SNR with reduced flow artifacts.

(Means for Solving the Problem) In order to achieve the above object, the method according to the present invention applies a slice gradient and an excitation pulse to a subject placed in a static magnetic field, and then a first step of applying a slicing gradient and applying an inversion pulse to collect a first echo signal under the lead gradient; and after this first step applying a slicing gradient and an inversion pulse; In an imaging method using a VE sequence, the method comprises at least a second step of collecting a second echo signal under a lead gradient, and the second echo generation time is set to a time more than twice the first echo generation time. At the time of step 1, the timing of applying the excitation pulse is determined based on the subject's heartbeat synchronization signal or the signal synchronized with the blood of the peripheral blood vessel, and the first echo signal collection time (
The first echo signal is collected during 2xTd 1 >=TE1/2TW/2, and during the second procedure, the phase θ=γ
A gradient magnetic field for phase correction is applied such that the phase based on the flow velocity component of the object expressed by f G(t)·r(t)dt becomes zero at the second echo center TE2, and the second echo signal collection time ( 2×Td2>= (TE2−TE L )
The second echo signal is collected during /2-TW/2.

However, TW: inversion pulse application time, TEL, first echo center time, TE2: second echo center time (effect) When collecting the first echo signal, a heart rate synchronized signal of the subject or a signal synchronized with the blood of peripheral blood vessels. Since the excitation pulse is applied based on , flow artifacts due to flow portions in the first echo image are reduced. Also, the second
When collecting echo signals, a gradient magnetic field for phase correction is applied so that the phase change based on the flow velocity component of the subject becomes zero at the second echo center, so flow artifacts due to the flow part in the second echo image are reduced. Ru. Furthermore, since the time for collecting the first echo signal and the second echo signal is set as long as possible, the best SNR can be obtained.

(Example) The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of a nuclear magnetic resonance imaging diagnostic apparatus embodying the present invention. In FIG. , a static magnetic field coil that applies a constant static magnetic field to the subject, a gradient magnetic field coil that generates a gradient magnetic field, an RF transmitter coil that provides an RF pulse to excite nuclear spins within the subject, and an NMR signal from the subject. This is a magnet assembly in which the receiving coil for detection is arranged.

Static magnetic field coil - Gradient magnetic field coil - RP transmitter coil.

The receiving coils are connected to a static magnetic field power supply 2, a gradient magnetic field drive circuit 3, an RF power amplifier 4, and a preamplifier 5, respectively. The gated controller 14 receives a signal from the living body and sends a trigger signal for system synchronization to the Sea Fence control circuit 6. The sea fence control circuit 6 operates the gate modulation circuit 8 (modulates the RF output signal of the RF oscillation circuit 9 at a predetermined timing) in accordance with the command from the total nR 7,
An RF pulse signal is applied from the RFt force amplifier 4 to the RP transmit coil. Further, the sea fence control circuit 6 controls the gradient magnetic field drive circuit 3 and the AD converter 11 according to the command from the total IL device 7 and the sea fence signal based on FIG.
It is also designed to operate.

A phase detector 10 detects the phase of the received signal output of the preamplifier 5 using the output of the RF oscillation circuit 9 as a reference signal. This power signal is converted into a digital signal by the AD converter 11 and input to the calculation R7. 12 is an operation console for inputting instructions and various setting values for realizing various pulse sea fences to the computer 7; 13 is a calculation 8;
This is a display device that displays the image reconstructed in step 17.

FIG. 2 is a diagram representing the pulse sea fence of the present invention.
The difference between this figure and FIG. 3 is that (1) the lead gradient intensities Grl, Gf2 and data collection times Tdl, Ta2 are set to satisfy the following formula: Tdl=TE1/2-TS-TW/2 Td2= (TE2-TEI)/2-TS-TW/G
rl=N/ (2y・FOV・Td1)Gr2=N/
(2y-FOV・Ta2) where TS: Spoiler application time TW; Reversal pulse application time N: Data sampling number FOV, imaging field of view (2) Signal synchronized with the subject's heartbeat synchronization signal or blood in peripheral blood vessels At the point (3) Gs-Gf-Gr where an excitation pulse is applied based on Gradient magnetic field indicated by the dotted line in Figure 2)
is applied.

First, when collecting the first echo signal, the excitation pulse is applied based on the subject's heartbeat synchronized signal or the signal synchronized with the blood of the peripheral blood vessel, and the spins on the slice plane are selectively excited, and the second After applying a predetermined magnetic field based on the figure, the spins reversed by the first reversal pulse are focused under the lead gradient. When collecting the second echo signal, the dotted line in Figure 2 After a predetermined magnetic field such as a phase correction gradient magnetic field, a second inversion pulse, and a slice gradient is applied, the spins inverted by the second inversion pulse are focused under the lead gradient.
The gradient magnetic field shown by the dotted line in FIG. !
! A case is shown in which phase compensation is performed using a polar rectangular waveform. Here, the phase θ=r j” G (t) ・r
(t) In dt, ro term θ, = φ, ■ term θ
,=φ. As a result of determining the gradient magnetic field for phase correction so that that
In the second echo image, an image in which flow artifacts due to flow portions are reduced is obtained.

Therefore, in this embodiment, it is possible to obtain an MR imaging method using a VE sequence that has the following effects. That is, (1) the best SNR can be obtained when collecting MR multiplied signals (2)
FIG. 3 is a diagram showing a rus sequence in which an image with reduced flow artifacts due to flow portions is obtained in the first echo image (3) and an image with reduced flow artifacts due to flow portions is obtained in the second echo image.

Note that the present invention is not limited to the above embodiments, and various modifications can be made within the scope of the claims.

For example, the time change in the position of the magnetization vector is r(t)=r
0+vtl'', and 4. If necessary, it is sufficient to take it to a higher order and change the gradient for phase compensation accordingly.Furthermore, there is no need to apply a spoiler.

(Effects of the Invention) As described above, according to the present invention, the first echo signal is synchronized with a biological signal, and the second echo signal is applied with a gradient magnetic field for phase correction to suppress flow artifacts. When collecting signals, the time for collecting the signals is set as long as possible, so the best SNR can be obtained.

[Brief explanation of the drawing]

Claims (3)

[Claims] (1) Apply a slice gradient and an excitation pulse to the subject placed in a static magnetic field, then apply a slice gradient and an inversion pulse, and generate the first echo under the lead gradient. a first step of collecting a signal; and a second step of applying a slice gradient and applying an inversion pulse after the first step and collecting a second echo signal under a lead gradient; In an imaging method using a VE sequence in which the second echo generation time is set to a time that is more than twice the first echo generation time, during the first step, the second echo generation time is set to a time that is more than twice the first echo generation time. The timing of applying the excitation pulse is determined based on the signal, and the first echo signal collection time (
The first echo signal is collected between 2×Td1)=TE1/2−TW/2, and during the second procedure, the phase θ=γ∫G(t)・r(
t) A gradient magnetic field for phase correction is applied such that the phase based on the flow velocity component of the object expressed as dt becomes zero at the second echo center TE2, and the second echo signal collection time (2×T
d2)=(TE2-TE1)/2-TW/2
An MR imaging method using a VE sequence characterized by collecting echo signals. However, TW: inversion pulse application time, TE1: first echo center time, TE2: second echo center time (2) Apply a slice gradient and an excitation pulse to the subject placed in a static magnetic field, then apply a slice gradient and an inversion pulse, and generate the first echo under the lead gradient. a first step of collecting a signal; and a second step of applying a slice gradient and applying an inversion pulse after the first step and collecting a second echo signal under a lead gradient; In an imaging method using a VE sequence in which the second echo generation time is set to a time exceeding twice the first echo generation time, a spoiler is provided after the inversion pulse in the first step and before and after the inversion pulse in the second step. In the first step, the application of the excitation pulse is timed based on the subject's heart rate synchronization signal or the signal synchronized with the blood in the peripheral blood vessel, and the first echo signal collection time (2×
The first echo signal is collected during Td1)=TE1/2-TS-TW/2, and during the second procedure, the phase θ=γ∫G
A gradient magnetic field for phase correction is applied such that the phase based on the flow velocity component of the object expressed as (t)・r(t)dt becomes zero at the second echo center TE2, and the second echo signal collection time (2× Td2)=(TE2-TE1)/2-TS-
An MR imaging method using a VE sequence, characterized in that a second echo signal is collected during TW/2. However, TW: inversion pulse application time, TS: spoiler application time, TE1: first echo center time, TE2: second echo center time (3) The gradient magnetic field for phase correction is one of the flow velocity of the subject.
V according to claim (1) or claim (2), characterized in that it is a gradient magnetic field for phase correction that makes the phase based on the next component (velocity component) zero at the second echo center TE2.
MR imaging method using E-sequence.