JP2746987B2 - Magnetic resonance imaging - Google Patents

Description

The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus capable of obtaining a high-resolution image in ultra-high-speed imaging.

(Prior Art) In general, in a magnetic resonance imaging apparatus, a subject is placed in a uniform static magnetic field, and a high-frequency magnetic field and a gradient magnetic field for slicing, phase encoding, and reading are applied in a predetermined sequence. Then, magnetic resonance signal data from a region of interest in the subject is collected, and information on the region of interest is imaged.

Since the magnetic resonance imaging apparatus takes a long time to collect magnetic resonance signal data, the image is deteriorated due to movement of the subject due to respiration or the like, it is difficult to image a moving cardiovascular system, and the burden on the subject is increased. large. Therefore,
Ultrafast imaging methods such as the echo planar method by Mansfield et al. And the fast Fourier method by Hitchson et al. Have been proposed.

FIG. 6 shows a pulse sequence for imaging by the echo planar method, in which a 90 ° high-frequency pulse for selective excitation is applied as a high-frequency magnetic field RF, and at the same time, a gradient magnetic field for slice Gs is applied to reduce the magnetization in the slice plane. After selective excitation, a 180 ° high-frequency pulse is further applied, and then the readout gradient magnetic field Gr is rapidly applied in a direction parallel to the slice plane and pulse-wise applied. A gradient magnetic field Ge for phase encoding is statically applied in a direction orthogonal to the gradient magnetic field Gr.

As shown in FIG. 7, the fast Fourier method differs from the echo planar method in that the phase encoding gradient magnetic field Ge is applied in a pulsed manner every time the readout gradient magnetic field Gr is inverted.

According to these methods, the readout gradient magnetic field Gr is switched at a high speed within a time in which the magnetization in the slice plane excited by the 90 ° high-frequency pulse is relaxed by the transverse magnetization relaxation phenomenon, so that the magnetic resonance signal is obtained. A co-signal (multi-echo signal) is generated, and all data necessary for imaging a slice plane can be obtained in a short time of several tens of milliseconds. As a result, a good image with less deterioration due to movements such as breathing, heart wall movement, and blood flow can be obtained.

In these ultra-high-speed imaging methods, the readout gradient magnetic field Gr is switched and alternately inverted to be applied in a pulsed manner. Ideally, this pulse waveform is a rectangular wave, but it actually takes a certain amount of time to rise and fall, which is considered to affect the spatial resolution in the reading direction.

When the pulse waveform of the read gradient magnetic field is an ideal rectangular wave, the spatial resolution in the read direction is given by the minimum decomposition distance Δl shown in the following equation (1).

Δl = 2π / ΔT · γGr (1) γ: nuclear magnetic rotation ratio Gr: intensity of the readout gradient magnetic field ΔT: signal observation time When the pulse waveform of the readout gradient magnetic field is an ideal rectangular wave, the signal observation time ΔT Since Δ coincides with the pulse width, simply increasing the pulse width from equation (1) reduces Δl and increases the spatial resolution. However, when the pulse waveform of the read gradient magnetic field is not an ideal rectangular wave, and, for example, its rise and fall characteristics are determined by the slew rate, a method of determining the waveform of the read gradient magnetic field so that the spatial resolution is maximized. Is not considered.

(Problems to be Solved by the Invention) As described above, since there is no means for maximizing the spatial resolution in the reading direction in the conventional magnetic resonance imaging apparatus, ultra-high-speed imaging in which the rising characteristics of the reading gradient magnetic field is particularly important is important. However, there is a problem that the resolution is greatly deteriorated by the pulse waveform of the read gradient magnetic field.

In view of the foregoing, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of optimizing a pulse waveform of a read gradient magnetic field with respect to a spatial resolution in a read direction.

[Means for Solving the Problems] In the present invention, the rising and falling times in the pulse waveform of the readout gradient magnetic field are set to ΔT ₁ .ΔT ₂ , respectively, and the zero crossing point from the zero crossing point to the rising edge is set. when the time until the point was T _0, and wherein the set to _{ΔT 1 + ΔT 2 = T 0} /2.

(Operation) When the rise and fall characteristics of the pulse waveform of the read gradient magnetic field are determined by the slew rate, the rise and fall time Δ in the pulse waveform of the read gradient magnetic field are used.
Maximum T _1, and [Delta] T _2, by setting the relationship between the time T ₀ from the rising zero crossing point to the zero cross point of the fall in _{ΔT 1 + ΔT 2 = T 0} /2, the spatial resolution of the reading direction Becomes

Embodiment FIG. 1 is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to one embodiment of the present invention. In the figure, a static magnetic field magnet 1 and a gradient coil 3 are driven by an excitation power supply 2 and a drive circuit 4, respectively. Thus, the subject 5 has a uniform static magnetic field and x,
A gradient magnetic field having a linear gradient magnetic field distribution in three directions of y and z is applied.

In the present embodiment, a gradient magnetic field applied in the z direction is hereinafter referred to as a slice gradient magnetic field Gs, a gradient magnetic field applied in the y direction is referred to as a phase encoding gradient Ge, and a gradient magnetic field applied in the x direction is referred to as a read gradient magnetic field Gr. .

The probe 7 receives a high-frequency signal from the transmission unit 8 and applies a high-frequency magnetic field to the subject 5. Further, the probe 7 receives a magnetic resonance signal generated in the subject 5. The probe 7 may be provided for both transmission and reception or separately for transmission and reception.

The magnetic resonance signal received by the probe 7 is
After being amplified and detected by, the data is transferred to the data collection unit 11, is converted into a digital signal by an A / D converter in the data collection unit 11, becomes magnetic resonance signal data, and is sent to the electronic computer 12.

The excitation power supply 2, the drive circuit 4, the transmission unit 8, and the reception unit 9 are all controlled by the system controller 10. The electronic computer 12 controls the system controller 10 based on a command input from the console 13. The electronic computer 12 performs a process such as Fourier exchange on the magnetic resonance signal data sent from the data collection unit 11, and
The image data is obtained by calculating the density distribution of desired nuclei in the device. The image data thus obtained is displayed as an image on the image display 14.

In this embodiment, the pulse sequence for imaging uses the ultra-high-speed imaging method shown in FIG. 6 or FIG.

FIG. 2 shows a pulse waveform of the read gradient magnetic field Gr. As shown in the figure, the rising waveform of the pulse of the read gradient magnetic field Ge is determined only by the slew rate, and changes linearly. In this case, the rise time is ΔT ₁ , the fall time is ΔT _2, and the time from the rising zero crossing point to the falling zero crossing point (this is when the pulse waveform of the gradient magnetic field Gr is an ideal rectangular wave, That is, if the signal observation time when the signal rises instantaneously) is T ₀ , the following equations (2) and (3) are obtained.

Gr = a (ΔT ₁ + ΔT ₂ ) (2) ΔT = T ₀ − (ΔT ₁ + ΔT ₂ ) (3) a: Proportional constant By substituting equations (2) and (3) into equation (1), The minimum resolution distance Δl representing the spatial resolution in the reading direction (x direction) is given by the following equation (4).

Δl = 2π / ΔT · γGr = 2π / γ (T ₀ −ΔT ₁ −ΔT ₂ ) ｛a (ΔT ₁ + ΔT ₂ )｝ (4) Therefore, to minimize Δl, ΔT ₁ + ΔT ₂ = T ₀ / 2 ... (5) FIG. 3 shows the denominator A = γ (T ₀ −Δ
T ₁ -ΔT ₂₎ shows the time change of _{{a (ΔT 1 + ΔT 2} )}, the maximum _{ΔT 1 + ΔT 2 = T 0} /2 ... (6). Therefore, by setting the waveform of the read gradient magnetic field Gr so as to satisfy the expression (6), the spatial resolution in the read direction can be maximized.

Next, a procedure for optimizing the pulse waveform of the phase encoding gradient magnetic field Ge with respect to the spatial resolution will be described.

FIG. 4 shows the pulse waveform of the phase encoding gradient magnetic field Ge in the pulse sequence of FIG.
Δte is the application time. In this case, the pulse waveform and amplitude of Ge are determined so that the application time Δte is minimized.
The phase shift by the phase encoding is obtained by the following equation (7).

The pulse waveform of the phase encoding gradient magnetic field Ge has a length Δte
Equation (7) becomes Δφe (t) γ (Δte / 2) · y (8).

On the other hand, as shown in FIG. 5, the range in the y direction of the region of interest (collection region of the magnetic resonance signal data) of the subject is ΔY
−ΔY / 2 → ΔY / 2) Then, since the sampling theorem must be satisfied at y = ΔY / 2, γG · (ΔY / 2) ≦ 2π / 2Δte ′ = 2πF (9) (F: maximum frequency [ = 1 / 2Δte ′], Δte ′ is a value obtained by converting the encode step into a rectangular pulse having a gradient strength G and an application time te ′). Thus, the phase change amount of the magnetic resonance signal at y in one encode step is Δφe (Y) = {γ · G · ΔY · (2y / ΔY) (nte ′)} / n = (2π / Δte ′) (2y / Δ2Y) (te ′) = (4π / ΔY) · y (10) ). From the equations (10) and (7), it is possible to obtain the Ge amplitude and the waveform such that Δte is as short as possible within the range allowed by the rise of the pulse waveform of the phase encoding gradient magnetic field Ge.

The present invention is not limited to the above embodiments. For example, in the embodiments, super-high-speed imaging has been described.
The present invention can be applied to any imaging method in which a read gradient magnetic field is applied in a pulsed manner. Others
The present invention can be implemented with various modifications without departing from the scope of the invention.

According to the present invention, it is possible to obtain a high-resolution image by optimizing the pulse waveform of the read gradient magnetic field with respect to the spatial resolution in the read direction.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to one embodiment of the present invention, FIG. 2 is a diagram showing a pulse waveform of a read gradient magnetic field in the embodiment, and FIG. FIG. 4 is a diagram showing a pulse waveform of a gradient magnetic field for phase encoding in the embodiment, FIG. 4 is a diagram used to determine an optimization condition of the pulse waveform, FIG. 6 and 7 are views showing a pulse sequence for ultra-high-speed imaging in a magnetic resonance imaging apparatus. 1 ... Static magnetic field magnet, 3 ... Gradient magnetic field generating coil, 5 ...
Subject, 6 probe, 8 transmission unit, 9 reception unit, 10 system controller, 11 data collection unit, 12 computer, 14 image display.

Claims (1)

(57) [Claims] 1. A high-frequency magnetic field and a gradient magnetic field for slicing are applied in a pulsed manner to a subject placed in a uniform static magnetic field to excite a predetermined slice plane, and then the read-out gradient magnetic field is changed to positive or negative. Switching is applied repeatedly in a pulsed manner, and a readout gradient magnetic field for phase encoding is applied either statically or in a pulsed manner at each inversion of the readout gradient magnetic field. In a magnetic resonance imaging apparatus for collecting and imaging resonance signals, the rise and fall times in the pulse waveform of the readout gradient magnetic field are ΔT ₁ and ΔT ₂ , respectively, and a zero crossing point from the zero crossing point of the rising edge. when the time until the point was T _0, the magnetic resonance imaging apparatus characterized by set to _{ΔT 1 + ΔT 2 = T 0} /2.