JP2884243B2 - Nuclear magnetic resonance imaging system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nuclear magnetic resonance imaging diagnostic apparatus, and more specifically, to collect first and second echo signals from a flow portion (blood, cerebrospinal fluid, etc.). An object of the present invention is to provide a nuclear magnetic resonance imaging apparatus capable of improving the SNR while reducing the flow artifacts that sometimes occur.

(Prior Art) A nuclear magnetic resonance imaging apparatus includes a magnet unit including a static magnetic field coil that generates a uniform static magnetic field and a gradient magnetic field coil that generates a magnetic field having a linear gradient in each of x, y, and z directions. A transmitting / receiving unit (including a sequence storage circuit, etc.) for applying an RF pulse to an object placed in a magnetic field formed by the magnet unit and detecting an NMR signal from the object, the transmitting / receiving unit and the magnet It has a control image processing unit centered on a computer that controls the operation of the unit and processes the detected data to display an image.

In the above configuration, at the time of VE Sequence operation,
In accordance with a command from the computer, the sequence storage circuit generates a signal based on the VE pulse sequence shown in FIG. 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 a slice axis. The slice gradient applied to the slice axis is for exciting only spins in a specific plane, and the rephase gradient is for removing a phase difference generated at the time of slicing. The spoiler is for preventing an artifact due to an angle error of the inversion pulse. Gf is a gradient magnetic field for applying a required magnetic field having a different magnitude to an axis called a phase encoding axis each time. The warp gradient is for providing phase information in the phase encode axis direction. Gr is a gradient magnetic field applied to an axis called a frequency encoding axis. The read gradient is for observing the spin echo signal, and the dephase gradient is for providing initial phase information in the frequency encode axis direction. The signals are a first echo and a second echo generated with the first inverted pulse and the second inverted pulse. In this way, the VE sequence initially applies several inversion pulses during the time from one excitation pulse to the next excitation pulse, and adds a read gradient each time to generate an echo, so that the density and spin laterality between tissues can be seen. An image contrast is provided by using a difference in signal intensity based on the relaxation time, and at least the second echo generation time TE2 is set to a time exceeding twice the first echo generation time TE1.

By the way, a flow artifact occurs when an echo signal is collected from a flow portion (blood, cerebrospinal fluid, etc.) and an image is obtained by the VE sequence. Therefore, conventionally, as a method of reducing flow artifact, a method of applying an excitation pulse based on a heartbeat synchronization signal of a subject or a signal synchronized with blood of a peripheral blood vessel is applied to a flow velocity component by applying a gradient correction magnetic field for phase correction. There has been a method of making the phase change based on zero at the echo center.

(Problems to be Solved by the Invention) However, in the conventional image capturing using the VE sequence, since the TE2 of the VE sequence is long, the flow artifact in the second echo cannot be reduced much by the above method. In the other method, since a large gradient magnetic field is required, TE1 becomes long, so that it is not possible to take a long signal collection time, and there is a problem that SNR is deteriorated.

The present invention has been made in view of the above points, and an object thereof is to provide a first echo image and a second echo image obtained by collecting echo signals from a flow portion (blood, cerebrospinal fluid, etc.) by a VE sequence. It is another object of the present invention to realize a nuclear magnetic resonance imaging diagnostic apparatus capable of performing imaging with reduced flow artifacts and capable of obtaining the best SNR.

(Means for Solving the Problems) In order to achieve the above-mentioned object, a method according to the present invention applies a slice gradient and an excitation pulse to a subject placed in a static magnetic field. First echo collecting means for applying a slice gradient and applying an inversion pulse to the first to collect a first echo signal under the read gradient;
After that, a slice gradient is applied and an inversion pulse is applied, and a second echo signal is collected under the read gradient.
A nuclear magnetic resonance imaging diagnostic apparatus having at least an echo collecting means, and having a function of imaging by a VE sequence in which the second echo generation time is set to a time exceeding twice the first echo generation time. The one echo collecting means takes the timing of applying the excitation pulse based on the heartbeat synchronization signal of the subject or the signal synchronized with the blood in the peripheral blood vessel, and obtains a first echo signal collection time (2 × Td1) = T
A first echo signal is collected during E1 / 2-TW / 2, and the second echo collecting means generates a flow velocity component of the subject represented by a phase θ = γ∫G (t) · r (t) dt. Is based on the second
A gradient correction magnetic field that becomes zero at the echo center TE2 is applied, and the second echo signal collection time (2 × Td2) = (TE2−
The second echo signal is collected during TE1) / 2−TW / 2.

However, TW: inversion pulse application time, TE1: first echo center time, TE2: second echo center time (operation) At the time of collecting the first echo signal, a heartbeat synchronization signal of the subject or a signal synchronized with blood in peripheral blood vessels , The flow artifact in the first echo image due to the flow portion is reduced. When collecting the second echo signal, a gradient magnetic field for phase correction is applied so that the phase change based on the flow velocity component of the subject is zero at the second echo center. Artifacts are reduced.
Further, 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) Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of a nuclear magnetic resonance imaging diagnostic apparatus according to one embodiment of the present invention. In the figure, reference numeral 1 denotes a space having a space for inserting a subject therein, and a static magnetic field coil for applying a constant static magnetic field to the subject and a gradient magnetic field for generating a gradient magnetic field surrounding the space. The magnet assembly includes a coil, an RF transmission coil for applying an RF pulse for exciting nuclear spins in the subject, a receiving coil for detecting an NMR signal from the subject, and the like. The static magnetic field coil, the gradient magnetic field coil, the RF transmitting coil, and the receiving coil are connected to a static magnetic field power supply 2, a gradient magnetic field driving circuit 3, an RF power amplifier 4, and a preamplifier 5, respectively. The gating controller 14 receives a signal from a living body and generates a trigger signal for system synchronization in the sequence control circuit 6.
The sequence control circuit 6 operates the gate modulation circuit 8 (modulates the RF output signal of the RF oscillation circuit 9 at a predetermined timing) according to a command from the computer 7, and applies the RF pulse signal from the RF power amplifier 4 to the RF transmission coil. I do. In addition, the sequence control circuit 6 also operates the gradient magnetic field drive circuit 3 and the AD exchanger 11 by a sequence signal based on FIG. 2 in accordance with a command from the computer 7.
Reference numeral 10 denotes a phase detector for detecting the output of the received signal 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 computer 7. Reference numeral 12 denotes an operation console for inputting instructions for realizing various pulse sequences and various setting values to the computer 7, and 13 denotes a display device for displaying an image reconstructed by the computer 7.

FIG. 2 is a diagram showing a pulse sequence of the present invention. The points different from FIG. 3 in the figure are as follows: (1) Read gradient intensities Gr1, Gr2 and data collection time Td
1, the point at which Td2 is set to satisfy the following equation Td1 = TE1 / 2−TS−TW / 2 Td2 = (TE2−TE1) / 2−TS−TW / 2 Gr1 = N / (2γ · FOV · Td1) Gr2 = N / (2γ · FOV.Td2) where TS; spoiler application time TW; reversal pulse application time N; data sampling number FOV; field of view (2) Heart rate synchronization signal of subject or blood of peripheral blood vessels The point where the excitation pulse is applied based on the synchronized signal. (3) The gradient for phase correction is applied to Gs, Gf, and Gr so that the phase change based on the flow velocity component of the subject becomes zero at the second echo center. This is a point where a magnetic field (a gradient magnetic field indicated by a dotted line in FIG. 2) is applied.

First, when collecting the first echo signal, the excitation pulse is applied based on the heartbeat synchronization signal of the subject or the signal synchronized with the blood of the peripheral blood vessel, and the spin on the slice surface is selectively excited, and the second After the predetermined magnetic field based on the drawing is applied, the spin inverted by the first inversion pulse converges under the read gradient. Next, when collecting the second echo signal, after applying a predetermined magnetic field such as a phase correction gradient magnetic field, a second inversion pulse, and a slice gradient shown by a dotted line in FIG. The spins inverted by are focused under a read gradient. The gradient magnetic field indicated by the dotted line in FIG. 2 shows the case where the time change of the position of the magnetization vector is assumed to be r (t) = r ₀ + vt, and the phase is compensated by the rectangular waveform of the opposite polarity.
Here, at the phase θ = γ∫G (t) · r (t) dt,
As a result of determining the gradient correction magnetic field such that the term θ _r = φ of r ₀ and the term θ _v = φ _{other of v} , the phase based on the primary component (velocity component) of the flow velocity of the subject is obtained. The change is made zero in the second echo center, and an image in which the flow artifact due to the flow portion is reduced in the second echo image is obtained.

Therefore, in this embodiment, it is possible to obtain an MR imaging method using a VE sequence having the following effects. That is, (1) the best SNR is obtained at the time of MR signal acquisition. (2) an image in which the flow artifact due to the flow portion is reduced in the first echo image is obtained. (3) the flow artifact due to the flow portion in the second echo image is obtained. A reduced image can be obtained. The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the claims. For example, the time change of the position of the oxidation vector does not need to be r (t) = r ₀ + vt, but may be increased to a higher order as needed, and the gradient for phase compensation may be changed accordingly. Further, the spoiler need not be applied.

(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 correction magnetic field to suppress flow artifacts. At the time of signal collection, the time for collecting the signal is set as long as possible, so that the best SNR can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a nuclear magnetic resonance imaging diagnostic apparatus according to one embodiment of the present invention, FIG. 2 is a diagram showing a pulse sequence according to one embodiment of the present invention, and FIG. 3 is a diagram showing a conventional pulse sequence. It is. 1 ... magnet assembly, 2 ... static magnetic field power supply, 3 ...
... Gradient magnetic field drive circuit, 4 ... RF power amplifier, 5 ... Preamplifier, 6 ... Sequence control circuit, 7 ... Calculator,
8: Gate modulation circuit, 9: RF oscillation circuit, 10: Phase detector, 11: AD converter, 12: Operation console, 13:
... Display device, 14 ... Gating controller

Claims (3)

(57) [Claims] 1. A slice gradient and an excitation pulse are applied to a subject placed in a static magnetic field, and then a slice gradient and an inversion pulse are applied to the subject under a read gradient. A first echo collecting means for collecting one echo signal; and a second echo collecting means for applying a slice gradient and applying an inversion pulse thereafter to collect a second echo signal under a read gradient. (2) In a nuclear magnetic resonance imaging diagnostic apparatus having a function of imaging by VE sequence in which an echo generation time is set to a time exceeding twice the first echo generation time, the first echo acquisition means may be a subject. The timing of applying the excitation pulse is set based on a heartbeat synchronization signal or a signal synchronized with blood in a peripheral blood vessel, and the first echo signal acquisition time (2 × Td1) = TE1 / 2−TS−TW / 2 One echo signal is collected, and the second echo collecting means has a phase θ = γ∫G (t) · r
(T) The second echo center TE2 provides a phase correction gradient magnetic field in which the phase based on the flow velocity component of the subject represented by dt becomes zero, and the second echo signal collection time (2 × Td
2) A nuclear magnetic resonance imaging apparatus characterized in that the second echo signal is collected during the period of = (TE2-TE1) / 2-TS-TW / 2. Here, TW: inversion pulse application time, TE1: first echo center time, TE2: second echo center time 2. Applying a slice gradient and applying an excitation pulse to a subject placed in a static magnetic field, then applying a slice gradient and applying an inversion pulse, and applying a slice gradient and an inversion pulse under the read gradient. A first echo collecting means for collecting one echo signal; and a second echo collecting means for applying a slice gradient and applying an inversion pulse thereafter to collect a second echo signal under a read gradient. (2) In a nuclear magnetic resonance imaging diagnostic apparatus having a function of imaging by VE sequence in which the time for generating two echoes is set to a time exceeding twice the time for generating the first echo, the first echo collecting means comprises: The second
The echo collecting means applies a spoiler before and after the inversion pulse, and the first echo collecting means further adjusts the timing of applying the excitation pulse based on a heartbeat synchronization signal of the subject or a signal synchronized with blood in peripheral blood vessels. And collects the first echo signal during a first echo signal collection time (2 × Td1) = TE1 / 2−TW / 2, and the second echo collection means has a phase θ = γ∫G (t)・ R
(T) The second echo center TE2 provides a phase correction gradient magnetic field in which the phase based on the flow velocity component of the subject represented by dt becomes zero, and the second echo signal collection time (2 × Td
2) A nuclear magnetic resonance imaging apparatus characterized in that the second echo signal is collected during the period of = (TE2-TE1) / 2-TW / 2. However, TW: inversion pulse application time, TS: spoiler application time, TE1: first echo center time, TE2: second echo center time 3. The phase correction gradient magnetic field, wherein the phase based on the primary component (velocity component) of the flow velocity of the subject is zero at the second echo center TE2. The nuclear magnetic resonance imaging apparatus according to claim 1 or 2, wherein: