JPH0871056A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE: To improve throughput of patients and to improve resolving power and resolution of an image by providing a frequency adjusting means for adjusting the frequency of a high frequency exciting pulse in accordance with the position in the slice direction of a specified crosssection while a subject to be inspected is moved. CONSTITUTION: A magnetic resonance imaging apparatus transmits a carrier wave of an RF pulse into a modulation tool 6 from a high frequency oscillator 4. Then, a transmitting coil 8 receives supply of electric current from a high frequency electric power amplifier 7 and applies an RF pulse for a frequency- adjusted slice selective excitation to a subject and after excitation is finished, on the process where a magnetized spin is saturated, a magnetic resonance signal (an echo) is induced in a receiving coil. In this case, an information on a slice. position of a top board from a bed controlling part 2 is taken in and the frequency of the carrier wave is changed by changing the increment of the frequency to the central frequency in accordance with the position and it is made possible thereby to excite repeatedly the same crosssection while the subject to be inspected is moved.

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 adopting a selective excitation method.

[0002]

2. Description of the Related Art Selective excitation for selectively exciting an imaging cross section (selective excitation plane) at a specific position and thickness by transmitting a high-frequency excitation pulse (hereinafter referred to as an RF pulse) whose frequency is adjusted while applying a gradient magnetic field. The method is widely used in the field of magnetic resonance imaging.

FIG. 4 is a principle diagram of selective excitation. In FIG. 4A, the selective excitation plane (imaging cross section) is shown in relation to the distance from the gradient magnetic field center (isocenter) O in the slice direction.
FIG. 11B shows a magnetic field strength change (magnetic field gradient) when an RF pulse is applied according to the distance from the gradient magnetic field center O, and FIG. 11C shows the target nuclide according to the distance from the gradient magnetic field center O. The change in the resonance frequency f is shown. Here, the distance from the center O of the gradient magnetic field to the center of the selective excitation surface (specific cross section) is Δx.
[M], the static magnetic field strength at the gradient magnetic field center O is B0
[T], the resonance frequency of the target nuclide at the gradient magnetic field center O is γ [Hz], and the gradient magnetic field strength in the slice direction during RF pulse application is Gs [T / m], the selective excitation plane during RF pulse application The magnetic field strength excluding the central high-frequency magnetic field is B
Since 0 + Gs · Δx [T], the center frequency f _selective of the RF pulse for specifying the selective excitation plane is f _selective = (γ / 2π) · (B0 + Gs · Δx)
[Hz] where γ = 2.675 × 10 ⁸ [rad · T ⁻¹ ·
S ⁻¹ ].

From f0 = (γ / 2π) B0, the deviation Δ of the central function of the RF pulse at the gradient magnetic field center O from f0
f is given by Δf = (γ / 2π) · Gs · Δx [Hz].

In the conventional selective excitation method, the subject is stationary, that is, Δx is fixed during the imaging period in which all the data necessary for image reconstruction of the selective excitation surface is collected. Therefore, during this imaging period, Δf for determining the selective excitation plane
Is also fixed of course. This is seen from the pulse sequence as follows. FIG. 5 shows a pulse sequence when the conventional selective excitation method is applied to a field echo method (also called a grungeent echo method) which is one of MR imaging methods. It is assumed here that the 2DFT imaging is performed. In the figure, in addition to the normal pulse sequence notation,
A change in Δf is also shown for the purpose of explanation. In the field echo method, it is necessary to repeat the excitation with the RF pulse the same number of times as the number of image pixels in the phase encoding direction.
As shown in the figure, the center frequency f0 + Δ of the RF pulse applied
It can be seen that f is fixed, and the same cross section is selectively excited with the subject stationary.

By the way, imaging can be performed at any position within the imageable region where the changes in the magnetic field strength in the three orthogonal directions are all linear. As a procedure for imaging a specific step surface by the conventional selective excitation method, the center frequency f0 of the fixed RF pulse +
The subject is transported to a certain position in the imageable area according to Δf, after which the image pickup is started, and after the image pickup is finished, the subject is carried out to the outside. This means longer imaging time and lower patient throughput. In other words, in the imaging time, the time taken to move from the time when the specific section starts to enter the imageable area to a certain position in the imageable area corresponding to f0 + Δf, and after the end of imaging, the specific section can be imaged from the certain position. At the time of being carried out, although imaging is possible, imaging is not performed. Therefore, it can be considered that the patient throughput is reduced by both times. In particular, in the case of multistage imaging, the intermittent movement gives a shock to the subject at the time of starting and stopping the movement, and the riding comfort is extremely deteriorated as the imaging speed increases.

[0007]

SUMMARY OF THE INVENTION The present invention is to provide a magnetic resonance imaging apparatus capable of improving patient throughput.

[0008]

SUMMARY OF THE INVENTION The present invention is a magnetic resonance imaging apparatus for selectively exciting a specific cross section of a subject by applying a high frequency excitation pulse together with a gradient magnetic field. And a frequency adjusting means for adjusting the frequency of the high-frequency excitation pulse according to the position in the slice direction.

[0009]

In the present invention, the frequency of the high-frequency excitation pulse is adjusted according to the position in the slice direction of the specific cross section without fixing the frequency of the high-frequency excitation pulse as in the conventional case. Therefore, conventionally, even if the change in magnetic field strength in the three orthogonal directions is entirely in the imageable region, the subject is moved so that the specific cross section comes to a position corresponding to the fixed frequency of the high-frequency excitation pulse. Although it was necessary, in the present invention,
By adjusting the frequency of the high-frequency excitation pulse, it is possible to capture an image while tracking the specific section from the stage where the specific section enters the imageable area to the point where the specific section exits the imageable area. Therefore, even if the time required for the imaging itself (the time required for collecting all the data necessary for imaging one cross section) is the same, the imaging can be performed even while the subject is moving, so that the patient throughput can be improved. . Further, since it is possible to take an image even when the subject is moving, it is expected that the imaging time is lengthened and the number of data samplings is increased, thereby improving the resolution and resolution of the image. Furthermore, since it is possible to capture images while continuously moving, the shock received by the subject is reduced (when the subject is a person, the riding comfort is improved).

[0010]

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 shows the configuration of the main part of the magnetic resonance imaging apparatus according to this embodiment. A gantry (not shown) having a cylindrical internal space for accommodating a subject is equipped with a static magnetic field magnet unit (not shown), a gradient magnetic field coil unit (not shown), a transmission coil 8, and a reception coil (not shown). The static magnetic field magnet unit including a permanent magnet, a normal conductive magnet or a superconductive magnet forms a static magnetic field inside the cylinder. The gradient magnetic field coil unit receives a current supply from a gradient magnetic field power supply and has a slice gradient magnetic field whose magnetic field strength changes along the slice direction, a phase encoding gradient magnetic field whose magnetic field strength changes along the phase encode direction, and a read direction. It is possible to independently generate the read gradient magnetic field in which the magnetic field strength changes along with. Data collection (imaging) is possible within the region where the magnetic field strength in these three directions changes linearly, and this region is called the imageable region. At the time of data collection, the subject is placed on the tabletop of the bed 3 and inserted into the imageable region together with the slide of the tabletop. The movement of the top of the bed 3 is controlled by the bed controller 2.

The transmission coil 8 receives current supply from the high frequency power amplifier 3 and applies a frequency-adjusted RF pulse for slice selective excitation to the subject. As a result, only the magnetized spins at the slice position corresponding to this frequency are selectively excited. This is a method called the selective excitation method.
A magnetic resonance signal (hereinafter referred to as an echo) is induced in the receiving coil in a process in which the magnetization spin is relaxed after completion of the excitation. This echo is sent to a signal processing circuit (not shown), where the image is reconstructed.

The high frequency oscillator 4 sends the carrier wave (frequency: f0 + Δf) of the RF pulse to the modulator 6. Also,
The high-frequency oscillator 4 takes in information on the slice position of the tabletop from the bed control unit 2, and according to the position, the center frequency f0
By changing the frequency increment Δf with respect to the carrier frequency (f 0 + Δf). By changing the frequency of the high-frequency excitation pulse according to the position of the slice in the slice direction, it is possible to repeatedly excite the same slice while continuously moving the subject.

The modulator 6 amplitude-modulates the carrier wave from the high frequency oscillator 4 according to the waveform signal from the waveform generator 5.
The high frequency power amplifier 7 inputs the modulated wave from the modulator 6 as a gain signal and applies a pulse signal of frequency (f0 + Δf) to the transmission coil 8. The host computer 1 controls the bed control unit 2, the high frequency oscillator 4, the waveform generator 5, and the gradient magnetic field coil unit to execute a pulse sequence described below.

FIG. 2 shows a pulse sequence according to this embodiment. Note that the field echo method to which 2DFT imaging is applied is shown as an example. Here, in addition to the notation of a normal pulse sequence, a change in frequency increment Δf is also shown.

The top of the bed 3 has the subject placed thereon,
It moves continuously at a constant speed along the slice direction (usually defined parallel to the body axis of the subject) within the imageable region. In the present embodiment, the slice position of the specific cross section that continuously moves the frequency of the high-frequency excitation pulse (also simply referred to as RF pulse) from the time when the specific cross section of the subject enters the imageable area to the time when the specific cross section exits. By changing in accordance with the change of (1), the same cross section of the subject is tracked while the subject is moved and repeatedly excited, and all echoes necessary for reconstructing one image from the cross section are collected.

Therefore, the frequency (f0 + Δf) of the carrier wave from the high frequency oscillator 4 is adjusted according to the slice position of the tabletop from the bed control unit 2. Specifically, the frequency increment .DELTA.f with respect to the center frequency f0 of the carrier wave is adjusted (changed) in accordance with the position of the top plate while the slice gradient magnetic field is kept constant.

Here, the moving speed of the subject in the slice direction (the direction from left to right on the paper is positive) is V [m / m
s], the repetition time of the pulse sequence is TR [s],
When the resonance frequency of the target nuclide is γ [Hz] and the gradient magnetic field strength in the slice direction during RF pulse application is Gs [T / m], a specific cross section is moving in the imageable region,
In order to continue exciting the same cross section, the frequency increment Δf may be changed (increased here) by (γ · Gs · V · TR) / 2π [Hz] each time the RF pulse is applied.

However, during the data acquisition period, the moving speed (equivalent to the moving speed of the subject) V of the tabletop is controlled by the bed control unit 2 or the host computer 1 so that the specific cross section is within the imageable area. To be done. That is, assuming that the data acquisition time is fixed, the time from when the specific cross section enters the imageable area to when it exits is at least longer than the data acquisition time, in other words, after the specific cross section enters the imageable area. The moving speed V of the tabletop is adjusted by the bed control unit 2 or the host computer 1 so that all the data necessary for imaging a specific cross section is completed within the time until it exits.

FIG. 3 is a diagram showing the effect of shortening the image pickup time according to this embodiment in comparison with the conventional one. FIG. 3A shows the conventional image pickup time, and FIGS. 3B and 3C show this embodiment. The imaging time is shown. Here, the time from when the specific cross section enters the imageable area to when it exits is defined as the imaging time. The width of the imageable area in the slice direction is L [m], the maximum speed of the top plate is Vmax [m / s], and the time required for data acquisition is Tscan.
[S], for convenience of explanation, it is assumed that the specific cross section of the imaging target is sufficiently thinner than L and has no inertia. It should be noted that FIG. 7B shows L / Tscan ≦ Vmax (L / Vmax ≦ T
(scan) image pickup time, L / Tscan> V in the figure (c)
The imaging times in the case of max (L / Vmax> Tscan) are shown.

In the conventional case, since movement and image pickup cannot be performed at the same time, the image pickup time T is the time L / Vmax required for moving the tabletop.
And the total time Tscan required for data collection, that is, T = Tscan + L / Vmax [s]. The time required to move the tabletop is the sum of the time required for the specific cross section to move to a position corresponding to the frequency of the fixed RF pulse and the time for the specific cross section to leave the imageable area after the end of imaging. It's time.

On the other hand, in this embodiment, L / Vmax and Tscan
The imaging time is given by the following two types, depending on the magnitude relationship of. (When L / Tscan ≦ Vmax) In this case, the moving speed of the tabletop is adjusted to V = L / Tscan. Data collection starts when the specific cross section enters the imageable area, and all data collection is completed when the specific cross section exits the imageable area. Therefore, the imaging time T is given by T = Tscan [s]. (When L / Tscan> Vmax) In this case, the moving speed of the tabletop is set to V = Vmax. Data collection is started when the specific cross section enters the imageable area, and collection of all data is completed in the middle of the imageable area of the specific cross section. As described above, the imaging time is defined as the time from when the specific cross section enters the imageable area to when it exits. It takes time for the top plate to move out of the imageable area while moving at Vmax. Therefore, the imaging time T in this case is given by the time required for the tabletop to move within the width L of the imageable region, that is, T = L / Vmax [s].

As described above, in any case, the imaging time of the present embodiment is shorter than that of the conventional one, and the improvement of the patient throughput can be realized. Although the field echo method is shown as an example in the above-mentioned embodiments, the same is applicable to all other MR imaging methods using the selective excitation method. The present invention is not limited to the above-mentioned embodiments and can be modified in various ways.

[0023]

INDUSTRIAL APPLICABILITY The present invention relates to a magnetic resonance imaging apparatus which selectively excites a specific cross section of a subject by applying a high frequency excitation pulse together with a gradient magnetic field. A frequency adjusting means for adjusting the frequency of the high frequency excitation pulse according to the position is provided. That is, in the present invention, the frequency of the high-frequency excitation pulse is adjusted according to the position in the slice direction of the specific cross section without fixing the frequency of the high-frequency excitation pulse as in the conventional case. Therefore, conventionally, even if the change in magnetic field strength in the three orthogonal directions is entirely in the imageable region, the subject is moved so that the specific cross section comes to a position corresponding to the fixed frequency of the high-frequency excitation pulse. Although it was necessary, in the present invention, by adjusting the frequency of the high-frequency excitation pulse, the specific cross section is tracked from the stage where the specific cross section enters the imageable area to the time when the specific cross section exits the imageable area. However, it becomes possible to take an image. Therefore, the imaging itself (1
Even if the time required to collect all the data necessary for imaging the cross section) is the same, it is possible to take an image even while the subject is moving, so that the patient throughput can be improved. Further, since it is possible to take an image even when the subject is moving, it is expected that the imaging time is lengthened and the number of data samplings is increased, thereby improving the resolution and resolution of the image. Furthermore, since it is possible to capture images while continuously moving, the shock that the subject receives is a phenomenon (when the subject is a person, the riding comfort is improved).

[Brief description of drawings]

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

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

FIG. 3 is a diagram showing the effect of reducing the imaging time according to the embodiment of FIG.

FIG. 4 is a principle diagram of selective excitation.

FIG. 5 is a diagram showing a pulse sequence when a conventional selective excitation method is applied to a field echo method which is one of MR imaging methods.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Host computer, 2 ... Bed control part, 3 ... Bed, 4 ... High frequency oscillator, 5 ... Waveform generator, 6 ... Modulator, 7 ... High frequency power amplifier, 8 ... Transmission coil.

Claims (3)

[Claims] 1. A magnetic resonance imaging apparatus for selectively exciting a specific cross section of an object by applying a high frequency excitation pulse together with a gradient magnetic field, the object being moved according to the position in the slice direction of the specific cross section. A magnetic resonance imaging apparatus comprising a frequency adjusting means for adjusting the frequency of the high frequency excitation pulse. 2. The frequency adjusting means changes the frequency of the high-frequency excitation pulse according to the position in the slice direction of the specific cross section so as to repeatedly excite the same cross section while the subject is continuously moving. The magnetic resonance imaging apparatus according to claim 1, wherein: 3. The moving speed of the subject is controlled so that collection of all data necessary for imaging the specific cross section is completed within a time period from when the specific cross section enters the imageable area to when the specific cross section exits. The magnetic resonance imaging apparatus according to claim 2, further comprising a control unit.