JPH01107746A - Magnetic resonance imaging - Google Patents

Abstract

PURPOSE:To suppress artifacts due to fluid such as of a blood stream and improve the quality of image, by saturating only spin in a region other than a tomographic section without disturbing the phase of spin in the tomographic section prior to obtaining a magnetic resonance signal. CONSTITUTION:A measured object in a high frequency coil is irradiated with a first high frequency magnetic pulse RF1 with a selectivity of a fall angle beta, and a slice magnetic field GS1 of a small magnetic field intensity is applied in a direction perpendicular to a tomographic section. A slice magnetic field GS2 with a polarity opposite to that the slice magnetic field GS1 is applied. A slice magnetic field GS3 with an inverted polarity is applied, and a slice magnetic field GS4 of a large magnetic field intensity is applied together with irradiation of a second high frequency magnetic field pulse RF2 with a selectivity of a fall angle alpha, and further a slice magnetic field GS5 with an inverted polarity is applied. The fall angle of spin only in a desired tomographic section increases, and the phase of magnetism in the lateral direction is disturbed and saturated. A phase encode magnetic field GP, data read magnetic fields GR1 and GR2 are applied, and a magnetic resonance signal M consisting of a spin echo signal is obtained after echo time TE. After n-times repetition of the above- mentioned while the phase encode magnetic field GP is being changed, n-magnetic resonance signals M obtained are subjected to Fourier transformation to obtain an image in the desired tomographic section.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to magnetic resonance imaging that suppresses artifacts caused by fluid, and in particular to magnetic resonance imaging that prevents a decrease in the intensity of magnetic resonance signals and provides high-quality images. It concerns the video method.

[Prior Art] Conventionally, magnets that generate a static magnetic field, high-frequency coils that generate high-frequency magnetic field pulses in a direction perpendicular to the static magnetic field, and orthogonal three-axis gradient magnetic field coils that generate a gradient magnetic field in a direction parallel to the static magnetic field have been used. A magnetic field that applies a static magnetic field, a high-frequency magnetic field pulse, and a gradient magnetic field to the object to be measured, acquires a predetermined number of magnetic resonance signals induced from the object to be measured, and uses Fourier transform to obtain an image of a desired tomographic plane. Resonance imaging is well known. It is also known that if fluid such as blood flow flows into the tomographic plane being imaged, artifacts will occur in the image of the tomographic plane, so it is necessary to prevent imaging due to the spin of the fluid in front and behind the tomographic plane, excluding the tomographic plane. It is being

FIG. 3 shows, for example, Masai (D, Matthae).
i) et al.
c Re5oncein Medicine)
(1987, Vol. 4), p. 303, is a pulse sequence diagram showing a conventional magnetic resonance imaging method that suppresses artifacts caused by blood flow.

In the figure, RF is a high frequency magnetic field pulse having a predetermined frequency band and flip angle (inclination angle), and M is a magnetic resonance signal such as a spin echo signal induced from the object to be measured.

Gs is a gradient magnetic field in the static magnetic field direction (for example, Z-axis direction), that is, a slice magnetic field, and cP is a direction perpendicular to the slice magnetic field aS (
e.g. Y-axis direction) gradient magnetic field, i.e., a phase encoding magnetic field,
GR is a gradient magnetic field in a direction (for example, the X-axis direction) orthogonal to the slice magnetic field Gs and the phase encode magnetic field Gp, that is, a data read magnetic field for frequency encoding.

Next, the conventional magnetic resonance imaging method shown in FIG. 3 will be explained.

First, a non-selective high-frequency magnetic field pulse RF1 having an inclination angle β (for example, 60°) and a wide frequency band is irradiated onto an object to be measured (for example, a human body) within a high-frequency coil (not shown). Subsequently, in order to disturb the phase of the transverse magnetization generated by this high-frequency magnetic field pulse RFI, a gradient magnetic field, ie, a slice magnetic field GsL, is applied in a direction perpendicular to the tomographic plane to be imaged (here, in the Z direction). As a result, all the nuclear spins within the radio frequency coil are out of phase and saturated.

Next, a selective high-frequency magnetic field pulse RF2 having an inclination angle α (for example, 40°) is applied while applying a slice magnetic field Gs2 that specifies the tomographic plane. Subsequently, high frequency magnetic field pulse R
In order to correct the phase disturbance in the thickness direction of the tomographic plane due to F2, a slice magnetic field GS3 whose polarity is reversed is applied.

Furthermore, from the time when the slice magnetic field GS3 is applied, a phase encoding magnetic field Gp is generated in one direction (here, the Y direction) within the tomographic plane.
At the same time, a phase encoding magnetic field Cp is applied within the tomographic plane.
A data read magnetic field GRI whose polarity is reversed is applied in a direction perpendicular to (in this case, the X direction). Note that the amplitude of the phase encode magnetic field GP changes by a constant amount as indicated by a broken line every time data (magnetic resonance signal M) is collected.

Next, a data read magnetic field GR2 whose polarity is inverted from that of the data read magnetic field GRI is applied so that the peak of the magnetic resonance signal M occurs after a predetermined echo time TE from the peak of the high frequency magnetic field pulse RF2.

Then, the above sequence is repeated N times while changing the amplitude of the phase encoding magnetic field cp to obtain N magnetic resonance signals M.
is acquired and subjected to two-dimensional Fourier transformation by a computer to construct an image of the desired tomographic plane.

-m, once the nuclear spins (for example, protons) absorb the energy of the high-frequency magnetic field pulse and are saturated, a predetermined time is required before they can absorb the energy of the high-frequency magnetic field pulse again. The standard for measuring the length of this time is the longitudinal relaxation time T1, and if the high-frequency magnetic field pulse RFI is irradiated at intervals shorter than the longitudinal relaxation time T during data collection,
The nuclear spin becomes saturated and cannot contribute to the magnetic resonance signal M.

According to conventional magnetic resonance imaging, the blood longitudinal relaxation time T,
Since is longer than the ** phase time T of the organ, the spin of the blood flowing into the high-frequency coil will be saturated by the irradiation with the high-frequency magnetic field pulse RF1 by the time it reaches the tomographic plane. Therefore, the longitudinal relaxation time T.

Only the magnetic resonance signal M of a short organ can be acquired, making high-speed imaging possible.

[Problems to be Solved by the Invention] As described above, in the conventional magnetic resonance imaging method, the spin in the high-frequency coil is saturated and an image of a desired tomographic plane is constructed based on the difference in longitudinal relaxation time T1. Therefore, the spins within the tomographic plane are also affected by the phase disturbance of the transverse magnetization caused by the radio frequency magnetic field pulse RFI and the slice magnetic field CS, resulting in a problem that the intensity of the magnetic resonance signal M decreases and the image quality deteriorates.

This invention was made to solve the above-mentioned problems, and it is a magnetic resonance system that does not cause phase disturbance of spins within the tomographic plane, prevents a decrease in the strength of magnetic resonance signals, and prevents deterioration of image quality. The purpose is to obtain the video method.

[Means for Solving the Problems] The magnetic resonance imaging method according to the present invention includes a first step of aligning the phases of spins in the front and rear regions including the tomographic plane, and A second step is provided in which the inclination angle of the spins of the chisel is increased and the spins in the front and rear regions excluding the fault plane are saturated.

[Operation] In this invention, the spin of the fluid entering the fault plane is saturated and the desired I! Acquire only the magnetic resonance signal from the yrNi surface and prevent the magnetic resonance signal from decreasing.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
The figure is a pulse sequence diagram showing one embodiment of the present invention, and RF, M, Gs, Gp and GR are the same as described above.

Next, while referring to the flowchart in FIG.
An embodiment of the present invention shown in the figure will be described.

First suite S1 First, a first high-frequency magnetic field pulse RF + with selectivity of an inclination angle β (for example, 35°) is irradiated to the object to be measured in the high-frequency coil, and at the same time, a small magnetic field intensity is applied in the direction perpendicular to the tomographic plane. Apply a slice magnetic field GSI. Slice magnetic field G at this time
The strength of S+ is a slice magnetic field with a large magnetic field strength (described later)
It is about 1,710. As a result, the spins of the regions before and after the ItIr layer surface are tilted by β. - Subsequently, after irradiation with the first high-frequency magnetic field pulse RF+, a slice magnetic field GSz whose polarity is reversed from that of the slice magnetic field GSI is applied in order to equalize the phase disturbance of the transverse magnetization caused by the slice magnetic field GS+. As a result, all the spins in the regions before and after the tomographic plane are aligned in phase.

Second step S2 First, a slice magnetic field G5ff with reversed polarity is applied, and then a slice magnetic field GS4 with a large magnetic field strength is applied while irradiating a second high-frequency magnetic field pulse RF2 with selectivity of an inclination angle α (for example, 35°). is applied, and furthermore, slice magnetic fields G and Ss whose polarities are reversed are applied.

As a result, the inclination angle of the spins only within the desired fault plane increases, and the spins in the regions before and after the fault plane, excluding the fault plane, are given the inclination angle β by the first high-frequency magnetic field pulse RF, so the spins in the lateral direction The phase of magnetization is disturbed and saturation occurs.

At this time, the second high-frequency magnetic field pulse RF 2 reaches a peak at the midpoint of the slice magnetic field Gsa, but the slice magnetic field GS
The phase disturbance in the first half of 4 is corrected by the slice magnetic field Gsz with reversed polarity, and the phase disturbance in the second half is corrected by the slice magnetic field GSs with reversed polarity.

Third step S3 Thereafter, in the same manner as described above, the phase encode magnetic field GP, the data read magnetic fields GRI and GR2 are applied, and the echo time T
After E, a magnetic resonance signal M consisting of a spin echo signal is obtained.

The sequence consisting of the above steps 31 to S3 is repeated N times while changing the phase encoding magnetic field cP, and the acquired N magnetic resonance signals M are Fourier transformed to obtain an image of a desired tomographic plane.

Here, even if blood flows from the area excluding the fault plane,
Since it is saturated by each high frequency magnetic field pulse RF and RF2, it does not contribute to the magnetic resonance signal M, and
Spins within the fault plane cause magnetic resonance signals M due to phase disturbances, etc.
It also does not reduce the

Furthermore, since it does not require the conventional broadband high-power high-frequency magnetic field pulse RFI, it is possible to reduce power consumption.

In the above embodiment, a two-dimensional Fourier transform method has been described, but a three-dimensional Fourier transform method can also produce similar effects.

Further, although the inclination angles α and β were each set to 35°, it goes without saying that they may be set to arbitrary angles depending on the organ to be measured.

Moreover, although the case where the magnetic resonance signal M is a spin echo signal due to nuclear magnetic resonance has been explained as an example, the same effect can be obtained even if it is applied to a case where electron spin resonance is applied.

Furthermore, although the case where one image is constructed by repeating the sequence consisting of the first step 81 to the third step S3 N times has been described, it may also be applied to multi-slices; in this case, By appropriately controlling the intensity of the slice magnetic field GS and the frequency of the second high-frequency magnetic field pulse RF2 during irradiation of the first high-frequency magnetic field pulse RF, the first high-frequency magnetic field pulse RF,
It is possible to collect magnetic resonance signals M of other tomographic planes within the region that is not irradiated.

[Effects of the Invention] As described above, according to the present invention, before acquiring a magnetic resonance signal, only the spins in a region other than the fault plane are saturated without disturbing the phase of the spins in the fault plane. Therefore, artifacts caused by fluids such as blood can be suppressed, and magnetic resonance imaging with improved image quality can be achieved.

[Brief Description of the Drawings] Fig. 1 is a pulse sequence diagram showing an embodiment of the present invention, Fig. 2 is a flow chart diagram showing an embodiment of the invention, and Fig. 3 is a diagram showing conventional magnetic resonance imaging. FIG. 3 is a pulse sequence diagram. M...Magnetic resonance signal RFl...First high frequency magnetic field pulse RF,...Second high frequency magnetic field pulse GS1...Slice magnetic field with small magnetic field strength GS4...Slice magnetic field GS2 with large magnetic field strength, GS
s, GSs... slice magnetic field S1 with reversed polarity
...First step/S2...Second step In the drawings, the same reference numerals indicate the same or corresponding parts. Figure 1 Figure 2

Claims (3)

[Claims] (1) In a magnetic resonance imaging method in which a desired tomographic plane is imaged by acquiring magnetic resonance signals, before acquiring the magnetic resonance signals, a first step is performed to align the phases of spins in regions before and after the tomographic plane, including the tomographic plane. and a second step of increasing the inclination angle of spins only within the tomographic plane and saturating the spins in regions before and after the tomographic plane, excluding the tomographic plane. (2) The first step includes a section in which a selective first high-frequency magnetic field pulse is irradiated and a slicing magnetic field with a small magnetic field strength is applied, and a slicing magnetic field whose polarity is reversed following the slicing magnetic field with a small magnetic field strength. The second step is a section in which a selective second high-frequency magnetic field pulse is applied and a slicing magnetic field with a large magnetic field strength is applied.
2. The magnetic resonance imaging method according to claim 1, further comprising a section in which a slice magnetic field whose polarity is reversed is applied before and after the slice magnetic field having a large magnetic field strength. (3) The magnetic resonance imaging method according to claim 2, wherein the slicing magnetic field having a small magnetic field strength is about 1/10 of the slicing magnetic field having a large magnetic field strength.