JPH0357436A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE:To effectively suppress a signal generated from a flow passing through a slice area and to effectively suppress an artifact generated by the flow by applying a series of plural divided pulses which are divided, as excitation pulses. CONSTITUTION:Plural divided high frequency pulses used as excitation pulses consisting of (n) pieces of pulses having magnitude of alpha=pi/2n, when an inclination angle of magnetization at the time when one piece of pulse is received is denoted as alpha, and that which satisfies the expression is enough. These divided pulses can be set to an arbitrary number, and can be selected suitably in accordance with whether a repeat time and an echo time are long or short. Also, as for magnitude or each divided pulse, when it satisfies the expression, it is allowable that magnitude of each divided pulse is different, but when a circuit constitution, etc., are taken into consideration, it is desirable to set it to the same magnitude. At the time of a speak of the divided pulse applied as an excitation pulse, it is desirable to apply a gradient magnetic field so that a phase shift of magnetization caused by a slice gradient magnetic field is compensated, for instance, the polarity is varied in a half cycle of the gradient magnetic field, etc.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to nuclear magnetic resonance (NMR)
Magnetic resonance technology that applies the arMagnetic Resonance phenomenon? The present invention relates to a magnetic resonance imaging method, and in particular to a magnetic resonance imaging method that can prevent artifacts caused by areas where blood flow or the like is flowing.

(Prior Art) Nuclear magnetic resonance is a phenomenon in which atomic nuclei placed in a magnetic field resonate and absorb electromagnetic wave energy of a specific wavelength, and then emit this energy as electromagnetic waves. There is a device that uses this phenomenon to diagnose living organisms. The above-mentioned nuclei, in particular,
The electromagnetic waves emitted from protons are detected and the detected signals are processed to produce a signal that reflects information such as nuclear (proton) density, longitudinal relaxation time T, horizontal and vertical sum time T2, flow, and chemical shift. Diagnostic information such as a tomographic image of the examiner can be obtained. By the way, in the normal Subin echo method for obtaining this tomographic image, the detection area is selectively excited with a high frequency pulse (906 pulse), and this 90" pulse and Applying a 180° pulse midway between the peak value of the echo signal obtained,
This cancels out the phase shift caused by the inhomogeneity of the static magnetic field. When taking a cross-sectional M image using such a spin echo method, if there is a part where blood flow passes through the detected part, artifacts may occur in the tomographic image taken of the detected part. This results in a loss of image quality.

The brisaturation method is commonly used as a method to suppress the generation of ghost-like artifacts in the encoding direction that occur from the flow passing through the detected region. That is, as the pulse sequence is shown in Fig. 4,
In this pre-saturation method, a 90° pulse and a 18
Before applying the 0' pulse, a 90° pulse (pre-pulse) is applied in advance to excite (saturate) the slice area, and when the 90° pulse, which is an excitation pulse for exciting the slice area, is applied, the blood flow increases. The magnetization is tilted by 180 degrees, suppressing signals from the blood flow. This pre-saturation method is effective for imaging with short echo time (TE) and repetition time (TR) (short Ta, show hT+t sequences). However, in the case of imaging to obtain a T2-weighted image (mouth TR, long TE sequence), the number of multi-slices is generally large, so this pre-saturation method has the problem of not being able to sufficiently suppress artifacts due to flow. Moreover, in this pre-saturation method, it is difficult to apply the 90' pulse applied in advance only to both ends outside the slice area, and therefore it is also applied to the boundary portion within the slice area. As a result, there is a problem that the image quality of the boundary area within the slice area is adversely affected.

(Problems to be Solved by the Invention) As described above, the brisaturation method used in the conventional spin echo method to suppress artifacts caused by the flow passing through the slice region is
This has a negative effect on the image quality of the boundary portion within the slice area, and also has the problem that artifacts due to flow cannot be sufficiently suppressed when imaging to obtain a T2-weighted image.

An object of the present invention is to be able to effectively suppress signals generated from a flow passing through a slice region, effectively suppress artifacts caused by the flow, and thereby obtain a tomographic image with good image quality. The purpose of the present invention is to provide a magnetic resonance imaging method that can be used.

[Structure of the invention]

(Means for Solving the Problems) The present invention applies an excitation pulse having a predetermined magnitude to selectively excite a detected region of a subject placed in a static magnetic field, and In a magnetic resonance imaging method that obtains morphological information or functional information of a detected region by detecting a magnetic resonance signal obtained by the excitation, the excitation pulse is divided into a plurality of divided pulses, and each divided pulse has a magnitude of the divided pulse. The magnetic resonance imaging system is characterized in that the sum of the pulses is divided into a predetermined size that can selectively excite the detected region, and the divided pulses are applied as the excitation pulses.

In the magnetic resonance imaging method of the present invention, the plurality of divided high-frequency pulses used as excitation pulses are as follows:
If α is the angle of inclination of magnetization when receiving 2 pulses, then it is sufficient that it consists of n pulses having a magnitude of α=π/2n and satisfies the following relational expression.

Σ α． Numa π/2 The number of divided pulses can be any number, and can be selected as appropriate depending on the length of the repetition time and the echo time.

Also, the size of each divided pulse may be different as long as it satisfies the above relational expression, but
Considering the circuit structure, etc., it is preferable to set them to the same size.

In the magnetic resonance imaging method of the present invention, at the peak of the divided pulse applied as an excitation pulse, for example, the half cycle of the gradient magnetic field is tilted so that the phase shift of magnetization due to the slice gradient magnetic field is compensated for, such as by changing the polarity. Preferably, a magnetic field is applied.

Further, in the magnetic resonance imaging method of the present invention, by applying a 180° pulse between each divided pulse, non-uniformity of the static magnetic field is canceled out, and signal degradation due to T2x attenuation can be prevented.

In this case, by applying a 180" pulse, the phase shift of magnetization due to the slice gradient magnetic field is compensated, so
There is no need to change the polarity of the slice gradient magnetic field when applying split pulses. (Function) In the magnetic resonance imaging method of the present invention, a series of divided pulses divided into a plurality of parts are applied as excitation pulses, so that the magnetization of a stationary region within the slice region is excited by all of these divided pulses. However, since the magnetization of the part where the blood flow passing through the slice region is excited by only a few of the divided pulses, the tilt is only shallow.

As a result, a sufficient signal is induced from the stationary part, whereas almost no signal is induced from the flowing part. Therefore, in the magnetic resonance imaging method of the present invention, it is possible to obtain a tomographic image in which undesirable flow artifacts are effectively suppressed without adversely affecting the tomographic image of the boundary region within the slice region.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Second
The figure is a schematic diagram showing the configuration of a magnetic resonance imaging apparatus used in an embodiment of the present invention. As shown in Figure 2,
This device consists of a gradient magnetic field generating coil for generating a gradient magnetic field to obtain positional information of a site where a magnetic resonance signal (echo signal) is induced, and a magnetic resonance signal that is induced while emitting a rotating high-frequency magnetic field. It has a transmitting and receiving coil ■ for detecting. This gradient magnetic field generating coil (■) generates a gradient magnetic field about the Z-axis, which is the axis in the height direction of the subject (P), and the X-axis and Y-axis, which are perpendicular to the Z-axis, respectively. X-axis gradient magnetic field generation coil (2a
). It consists of a Y-axis gradient magnetic field generation coil (2b) and a two-axis gradient magnetic field generation coil (2c). Each gradient magnetic field generating coil (2a), (2b), (2c) is connected to an X-axis gradient magnetic field power source (4a). They are connected to a Y-axis gradient magnetic field power supply (4b) and a Z-axis gradient magnetic field power supply (4c), respectively. Further, the transmitting/receiving coil (2) is connected to the transmitting circuit system (2) and the receiving circuit system (2). Furthermore, this device (1) includes a sequencer 0 that executes a pulse sequence, and each power source (4a).
．． (4b) = (4c) A computer system (2) is provided which controls all of the transmitting circuit system (2), the receiving circuit system (0), and the sequencer (0) and processes the detection signal. The signals processed by this computer system (2) are displayed on a display (9). This device (1) includes a static magnetic field coil (not shown) that generates a static magnetic field in the Z-axis direction toward the subject (P), and a power source (not shown) that supplies current to this static magnetic field coil. Also prepared. Next, the imaging method of this embodiment will be explained using FIG. 1. In this example, the excitation pulse consists of two pulses (hereinafter referred to as 45° pulses) that cause the spin system to rotate by approximately 45° (hereinafter referred to as 45° pulses). First, an excitation pulse consisting of these two 45″ pulses is applied to the slice area. At this time, a slicing gradient magnetic field Gs with a different polarity in the subsequent half cycle is applied so that the phase shift of magnetization is compensated by the slicing gradient magnetic field. Next, according to the usual spin echo method, an encoding gradient magnetic field GE, a read gradient magnetic field GR, and an 180° pulse are applied to detect an echo signal S. In this pulse sequence, the magnetization of the stationary site within the slice region is excited by two 45' pulses and collapses 906. In contrast, the magnetization of the blood flow passing through the slice region is excited by the first 45° pulse and collapses to 45@, but moves out of the slice region when the second 45" pulse is applied. In the end, only 451 collapses.Also, the second 4
Similarly, the blood flow magnetization that has undergone only a 5° panoresth is tilted only by 45°.

Therefore, when a 180 @ pulse is applied under such conditions, a sufficiently large echo signal is induced from a stationary region within the slice region, whereas an extremely small echo signal is induced from a flowing blood flow. Only certain signals are induced. the result,
Artifacts caused by blood flow are effectively suppressed in the obtained tomographic image.

Next, another embodiment of the present invention will be described using FIG. 3. In this embodiment, as the excitation pulse, a pulse having two 45° pulses similar to the above embodiment and a 180° pulse arranged between these 456 pulses is used. The 180° pulses between the 45° pulses compensate for static magnetic field inhomogeneities. By applying this 180° pulse, it is possible to prevent a drop in the echo signal due to T-attenuation, thereby further improving the image quality of the obtained tomographic image.

In this example, the above 45'' pulse, 180'''
A pulse and a 45° pulse are sequentially applied to the slice area. At this time, in this embodiment, 1
Since a pulse of 80° is applied, the phase shift of magnetization due to the non-uniformity of the slicing gradient magnetic field is also canceled out, so there is no need to compensate for the slicing gradient magnetic field as in the above embodiment, and it is simple. becomes. Next, according to the normal spin echo method, an encoding gradient magnetic field G8, a read gradient magnetic field G. and 180° pulses are applied to detect the echo signal S. In this embodiment as well, by applying a plurality of divided pulses as excitation pulses in the same manner as in the above embodiment,
Echo signals from flow penetrating the slice region are suppressed, and flow artifacts are effectively suppressed.

In the above embodiment, two 456 pulses for rotating the spin system by 90'' were used as excitation pulses, but the number of divided pulses used as excitation pulses in the present invention depends on the pulse sequence to be executed. Repetition time TR, echo time TI! From which relationship, for example, by applying a spin system such that the sum of the magnitudes of each divided pulse is approximately equal to the magnitude of the 90' pulse, the spin system becomes approximately 30'. This can be arbitrarily selected, such as using three pulses rotating by .degree.. It is clear that the present invention can be applied to field echo methods as well as spin echo methods.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to effectively suppress signals generated from a flow penetrating a slice region, and effectively suppress artifacts caused by the flow. A magnetic resonance imaging method capable of obtaining an image can be provided.

[Brief explanation of drawings]

FIG. 1 is a graph showing a pulse sequence according to an embodiment of the present invention, FIG. 2 is a schematic diagram showing the structure of a magnetic resonance imaging apparatus used in an embodiment of the present invention, and FIG. 3 is a graph showing another embodiment of the present invention. Graph showing an example pulse sequence. FIG. 4 is a graph showing a pulse sequence of the conventional pre-saturation method. DESCRIPTION OF SYMBOLS 1... Magnetic resonance imaging device, 2... Gradient magnetic field generation coil, 3... Transmission/reception coil, 4a... X-axis gradient magnetic field power supply, 4b... Y-axis gradient magnetic field power supply, 4c... Z-axis Gradient magnetic field power supply, 5...
Transmitting circuit system, 6... Receiving circuit system, 7... Sequencer, 8... Computer system.

Claims (1)

[Claims] An excitation pulse having a predetermined magnitude is applied to a subject placed in a static magnetic field to selectively excite the target region, and the induced magnetic resonance signal is detected to detect the target region. In a magnetic resonance imaging method for obtaining morphological information or functional information, the excitation pulse is composed of a plurality of divided pulses, and each divided pulse has a sum of the magnitudes of each divided pulse that can selectively excite a region to be detected. A magnetic resonance imaging method characterized in that the pulse is divided into a predetermined size and the divided pulse is applied as the excitation pulse.