JP2677601B2 - 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 that collects image data in a subject at high speed.

(Prior Art) As is well known, magnetic resonance imaging is a high-frequency magnetic field that rotates at a specific frequency when a group of nuclei with unique magnetic moments is placed in a uniform static magnetic field. This is a technique for visualizing chemical and physical microscopic information of a substance by utilizing development in which the energy of is absorbed resonantly.

In this magnetic resonance imaging, data acquisition time is much longer than in other medical image diagnostic apparatuses such as an ultrasonic diagnostic apparatus and an X-ray CT. Therefore, there are problems that the image quality is deteriorated due to the movement of the subject such as breathing, and it is difficult to visualize the moving heart or vascular system. Further, since the photographing time becomes long, the pain to the subject is great.

Therefore, as a method for reconstructing an image at high speed in magnetic resonance imaging, the echo planar method by Mansfield et al., And the fast Fourier method by Hutcison et al. Have been proposed.

Fig. 3 shows a pulse sequence for image data acquisition by the echo planar method. 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 Gs for slice is applied to slice surface. After selectively exciting the magnetization in the inside of the slice, a 180 ° high-frequency pulse is applied, and then the read gradient magnetic field Gr is applied by switching it multiple times at high speed in the direction perpendicular to the slice plane. A phase encoding gradient magnetic field Ge is statically applied in a direction orthogonal to Gs and the readout gradient magnetic field Gr.

On the other hand, the fast Fourier method uses a gradient magnetic field Ge for phase encoding.
Is different from the echo planar method in that is applied in a pulsed manner each time the readout gradient magnetic field Gr is reversed.

According to these methods, by switching the readout gradient magnetic field at high speed within a time period during which the magnetization in the slice plane excited by the 90 ° high-frequency pulse is relaxed by the relaxation phenomenon of the transverse magnetization, a plurality of magnetic resonance-based Echo signals (multi-echo signals) can be generated and all the data necessary for imaging the slice plane can be collected, enabling high speed imaging.

In order to perform correct image reconstruction in such high-speed imaging, the readout gradient magnetic field Gr in FIG. 3 is used.
And the gradient magnetic field Ge for phase encoding must be rapidly switched at a predetermined timing. In particular, it is necessary to apply a high magnetic field at a high speed to the read gradient magnetic field Gr, and its response time cannot be ignored. In the state in which the phase encoding gradient magnetic field in FIG. 3 is not applied, the phases of the spins in the slice plane must be aligned at a predetermined sampling position. The readout gradient magnetic field Gr is adjusted so that the position where the phases of the spins are aligned becomes the predetermined sampling position.
If is not adjusted, the data on each predetermined grid point cannot be obtained correctly in the phase space, and the correct reconstructed image cannot be obtained. In FIG. 2, the phases of spins are aligned such that the positive waveform area S _ia of the read gradient magnetic field Gr and the negative waveform surface S _i-1b (i = 1, 2, ..., N) become equal. Since it occurs at the time, if the switching waveform of the read gradient magnetic field Gr is an ideal rectangular wave as shown by the solid line, the positions where the spin phases are aligned are positive and negative like T _P1 , T _p2 , ... T _pn. It becomes the middle position of the period.

However, in reality, the switching waveform of the read gradient magnetic field Gr is not an ideal waveform due to the inductance of the gradient magnetic field generating coil, the influence of the eddy current induced in the metal body near the gradient magnetic field generating coil, etc. Instead, as shown by the broken line in FIG. 2, the waveform has a gradual rise and fall. Furthermore, the read gradient magnetic field Gr has a difference between the positive and negative amplitude values G _{r +} and G _r− , an offset, etc. due to the driving power source of the gradient magnetic field generation coil. Gradient magnetic field Gr for such readout
The position where the spin phases are aligned is Δτ ₁ , Δτ in the second position due to the waveform rounding, the positive / negative amplitude difference, and the offset.
₂ , ... Δτ _n , which is deviated from the regular position T _P1 , T _p2 , ... T _pn . Alternatively, the multi-echo may disappear.

Therefore, conventionally, each timing of switching of the readout gradient magnetic field, positive / negative amplitude, or offset is manually adjusted so that the position where the phases of the spins are aligned becomes a predetermined sampling position. It required a great deal of effort to adjust.

(Problems to be Solved by the Invention) As described above, in the conventional high-speed imaging method, in order to perform the correct image reconstruction for the waveform deterioration of the readout gradient magnetic field, the positive / negative amplitude difference, the offset, and the like, in the slice plane, There is a problem that complicated system adjustment is required to make the phases of the spins at the same sampling position. Further, when the subject has a plurality of magnetic resonance frequencies due to chemical shift or the like, the echo signal has a complicated waveform, so that it is difficult to accurately perform the system adjustment.

INDUSTRIAL APPLICABILITY The present invention does not require complicated system adjustment in high-speed imaging, and enables easy and accurate system adjustment even in a subject having a plurality of magnetic resonance frequencies, and a good reconstructed image can be obtained. An object is to provide a magnetic resonance imaging apparatus.

[Configuration of the invention]

(Means for Solving the Problems) In an echo signal composed of a single magnetic resonance frequency,
The position where the phases of spins are aligned in the slice plane corresponds to the peak position of the echo signal. Therefore, in the present invention, 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 in one direction within the slice plane. By continuously switching and applying the readout gradient magnetic field, and by applying the phase encoding gradient magnetic field in the slice plane in the direction orthogonal to the readout gradient magnetic field, the magnetic resonance necessary for image reconstruction of the slice plane is obtained. In a magnetic resonance imaging apparatus that collects and samples signals, and performs image reconstruction based on the sampling data, if the subject is composed of multiple substances having different magnetic resonance frequencies, a single magnetic field resonance frequency is used in advance. Using a phantom containing a substance having, or a spin that causes magnetic resonance at frequencies other than a predetermined magnetic resonance frequency in the subject. Similarly to the above, after detecting the peak position of a plurality of echo signals obtained in the state where the phase encoding gradient magnetic field is not applied, or controlling the readout gradient magnetic field so that this peak position becomes a predetermined sampling position, or Alternatively, with respect to the timing of the high-frequency magnetic field applied to the subject, the peak position of the echo signal corresponds to one peak position of the plurality of echo signals when neither the read gradient magnetic field nor the phase encoding gradient magnetic field is applied. It is adjusted to one predetermined sampling position, the high-frequency magnetic field and the readout gradient magnetic field are applied at the predetermined timing, the peak position of the echo signal near the predetermined one sampling position is detected, and the predetermined position is determined. Gradient magnetic field for reading so that the peak position of the echo signal comes to one sampling position of Control, following the process for the peak position of the all of the echo signal by controlling the repeating all of the read gradient field, without performing complicated system adjustment in the high-speed imaging, to obtain a good reconstruction image.

(Operation) According to the present invention, when a sequence for high-speed imaging is executed in a subject made of a plurality of substances having different magnetic resonance frequencies without applying a phase-encoding gradient magnetic field, the readout gradient magnetic field is switched. Since an echo signal having a phase waveform that depends only on the lateral relaxation time T ₂ is obtained every inversion by, the peak position of the echo signal obtained from a substance having a predetermined magnetic resonance frequency can be easily detected. Based on the peak position of the echo signal detected in this way, feedback is applied to the drive source of the gradient magnetic field generation coil to automatically adjust the switching timing of the readout gradient magnetic field, positive and negative amplitude values, offset, etc. The peak position of the signal can be set to a predetermined sampling position, and the phase of the spins can be aligned at the predetermined sampling position by switching the readout gradient magnetic field, so that the adjustment of the pulse sequence can be completed quickly and artifacts can be prevented. It is possible to obtain a correct reconstructed image by high-speed imaging without any image.

Embodiment FIG. 1 is a diagram showing a configuration of a magnetic resonance imaging apparatus according to one embodiment of the present invention.

In the figure, the static magnetic field magnet 1 and the gradient magnetic field generating coil 3 are driven by an excitation power supply 2 and a drive circuit 4 which are controlled by a system controller 10, respectively, so that the static magnetic field magnet 1 and the gradient magnetic field generation coil 3 are directed to a subject 5 (eg, human body) on a bed 6. Such a static magnetic field, and a perpendicular x, in the desired cross section (slice plane) of interest
A gradient magnetic field whose magnetic field strength changes in two y directions and in a z direction perpendicular thereto 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, the gradient magnetic field applied in the x direction is the read gradient magnetic field Gr, y
The gradient magnetic field Ge for phase encoding
It will be described as.

Under the control of the system controller 10, a high-frequency magnetic field generated from the probe 7 by a high-frequency signal from the transmission unit 8 is further applied to the subject 5. In the present embodiment, the probe 7 is used both as a transmission coil for generating a high frequency magnetic field and as a reception coil for receiving magnetic resonance signals relating to various atomic nuclei in the subject 5, but the transmission and reception coils are used. You may provide separately.

The magnetic resonance signal (echo signal) received by the probe 7 is amplified and detected by the receiving unit 9 and then sent to the data collecting unit 11 under the control of the system controller 10. The data collecting unit 11 collects the magnetic resonance signals taken out via the receiving unit 9 under the control of the system controller 10, samples them by an A / D converter, digitizes them, and then sends them to the electronic computer 12. .

The electronic computer 12 is controlled by the console 13 and performs image reconstruction processing by Fourier transform on the sampling data of the echo signal input from the data acquisition unit 11 to obtain image data. The electronic computer 12 also controls the system controller 10. Image data obtained by the computer 12 is supplied to the image display 14,
The image is displayed.

As the pulse sequence for collecting the image data of the slice plane in the subject 5 in the present invention, the echo planar method shown in FIG. 3 or the fast Fourier method shown in FIG. 6 is used. This pulse sequence is controlled by the system controller 10.

Here, in the first embodiment according to the present invention, the high-frequency magnetic field RF, the slicing gradient magnetic field Gs, and the reading gradient magnetic field Gr are applied as shown in FIG. 3 before executing the pulse sequence of FIG. However, a pulse sequence in which the phase encoding gradient magnetic field Ge is not applied is executed. However, when the subject is a substance having a plurality of magnetic resonance frequencies due to chemical shift or the like, a phantom containing a substance having a single magnetic resonance frequency is used before imaging. As described above, in the waveform of each echo signal of a substance having a single magnetic resonance frequency, its peak position is equivalent to the position where the phases of spins are aligned in the slice plane. Here, the peak of the echo signal indicates that the magnitude of the absolute value of the signal is maximum. As described above, when the phantom is used, the homogeneity of the static magnetic field is not disturbed by the subject itself, and it is obtained from a substance having a predetermined magnetic resonance frequency without being affected by the nonuniformity of the static magnetic field. It is possible to detect the peak position of the echo signal. Therefore, the peak positions (positions on the time axis) of these echo signals are detected. The detection of the peak position may be performed by software processing in the computer 12 or may be performed by hardware in the data collection unit 11 or the reception unit 9.

In FIG. 2, it is assumed that the deviation of the peak position of the detected echo signal from the normal position (predetermined sampling position) is Δτ _i (i = 1, 2, ... N). Now, it is assumed that the positive amplitude of the read gradient magnetic field Gr is G ₊ , the negative amplitude is G _−, and the inversion timing associated with switching is τ _i . As described above, the position at which the echo signal is leaked is determined by the area S _ia of the positive waveform and the area S S of the negative waveform of the read gradient magnetic field Gr.
_{It is a time when i-1b} (i = 1, 2, ... N) becomes equal. This S
_To satisfy the condition of _ia = S _i-1b , τ _i or G ₊ , G ₋ or the offset may be adjusted. Specific examples, G _+, G _-
Describes the case where all echo signals are the same and only the switching timing τ _i is adjusted.

The peak of the i-th echo signal has a deviation of ΔTi, and correction is performed by setting this as (τ _i + Δt _i ). Assuming that the peak of the i-th echo signal has a positive amplitude, −Δτ _i · G ₊ −Δτ _i · G ₊ = Δt _i · G ₋ + Δτ _i-1 · G ₋ , Δt _i = − (Δτ _{i · G + + Δτ i-} 1 · G -) / (G + + G -) to become. Therefore, the value of Δt _i is calculated by the calculator 12,
System controller 10 for controlling the read gradient magnetic field Gr
It suffices to control the operation timing of the internal pulse sequencer. Ideally, the deviation of Δτ _i can be eliminated by one control, but if one control is incomplete, similar control may be repeated several times.

The present invention can be modified in various ways other than the above-mentioned embodiments. As a second embodiment, when the object has a plurality of magnetic resonance frequencies due to chemical shift or the like, spins other than those magnetically resonating at a predetermined magnetic resonance frequency by a frequency selective excitation high frequency pulse as shown in FIG. After excitation, selective saturation is performed by dispersing the phases of spins excited by a gradient magnetic field or the like, and before the saturated spins are recovered, the phase encoding in FIG. 3 is performed in the same manner as described in the first embodiment. A pulse sequence that does not apply a gradient magnetic field is executed. Then, the peak position of the echo signal obtained at that time is detected, the information is transferred to the computer 12 as in the above embodiment, the deviation from the normal peak position is calculated, and the system controller 10 for controlling the read gradient magnetic field Gr is used. Controls the operation timing of the pulse sequencer.

In the present embodiment, it is not necessary to use a phantom having a single magnetic resonance frequency as in the first embodiment, and it is possible to correct the influence of the individual difference of the subject. In the embodiment shown in FIG. 4, the frequency selective excitation high frequency pulse is applied only once, but may be applied multiple times if necessary. At that time, the center frequency, band, and amplitude may be changed for each pulse. Furthermore, as a method for saturating spins other than a predetermined value, a spin selective saturation method other than this embodiment may be used.

As a third embodiment, as shown in FIG.
The 90 ° high frequency pulse and the 180 ° high frequency pulse are applied to the predetermined peak position of the first echo signal of the plurality of echo signals generated by the switching of the read gradient magnetic field. The sequence is adjusted such that the echo time 2τ, which is twice the interval, is adjusted. Subsequently, the peak position is detected as in the above embodiment. Then, the timing of the read gradient magnetic field Gr is adjusted so that the peak position of the first echo signal is at the predetermined position, as in the above embodiment. Hereinafter, the application timing of the 90 ° high frequency pulse and the slice gradient magnetic field is sequentially set to the second, third, ...
Echo time 2τ ′, 2 at a predetermined peak position of the echo signal of
τ ″, ... And the same operation is repeated. By thus setting the echo time to the peak position of each echo signal, even when the subject has a plurality of magnetic resonance frequencies, It is possible to correctly adjust the read gradient magnetic field from the peak position.In this embodiment, the timing of applying the 90 ° frequency pulse and the slice gradient magnetic field was controlled, but the timing of the 180 ° high frequency pulse was controlled. By doing so, the same adjustment may be performed.

Further, in the first, second, and third embodiments, the timing of switching the read gradient magnetic field Gr is controlled, but the positive and negative amplitudes G ₊ , G ₋ , the offset, etc. may be controlled. Two or more of one parameter may be controlled simultaneously.
In addition, the first and second embodiments have the same 180 ° as shown in FIG.
It can also be applied to sequences without high frequency pulses.

The pulse sequence used in the present invention is not limited to the echo planar method, and may be the sequence of the fast Fourier method as shown in FIG. 6 or a pulse sequence of applying other switching gradient magnetic field for switching or modulation.

〔The invention's effect〕

According to the present invention, the sequence of high-speed imaging is executed without applying the phase encoding gradient magnetic field, the peak position of the echo signal is detected, and the switching of the readout gradient magnetic field is performed based on the detected peak position of the echo signal. By controlling the timing, positive and negative amplitude values, offset, etc., and adjusting the sequence so that the peak position of the echo signal is correctly located at a predetermined position, it is possible to correct the sequence without complicated system adjustment in high-speed imaging. A reconstructed image can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention, and FIG. 2 is an echo signal obtained by a high-speed imaging sequence without applying a phase-encoding gradient magnetic field in the present invention and for reading. FIG. 3 is a diagram showing a relationship with a gradient magnetic field, FIG. 3 is a diagram showing a pulse sequence of an echo planar method which is one method of high speed imaging used in the present invention, and FIG. 4 is used in the second embodiment of the present invention. FIG. 5 is a diagram showing a pulse sequence, FIG. 5 is a diagram showing a pulse sequence used in the third embodiment of the present invention, and FIGS. 6 and 7 are diagrams showing pulse sequences for other high-speed imaging. 1 ... Static magnetic field magnet, 2 ... Excitation power supply 3 ... Gradient magnetic field generating coil, 7 ... Probe 8 ... Transmitting unit, 9 ... Receiving unit

Claims (2)

(57) [Claims] 1. A slice is prepared by applying a high-frequency magnetic field and a gradient magnetic field for slicing to a subject made of a plurality of substances having different magnetic resonance frequencies and placed in a uniform static magnetic field to excite a predetermined slice plane, and then slice the slice. An image of the slice plane is obtained by applying the read gradient magnetic field in one direction in the plane by continuously switching and applying the phase encode gradient magnetic field in the direction orthogonal to the read gradient magnetic field in the slice plane. Data collection means for collecting and sampling a plurality of echo signals based on magnetic resonance necessary for reconstruction,
In a magnetic resonance imaging apparatus provided with an image reconstruction means for performing image reconstruction based on the sampling data obtained by this means, a phantom containing a substance having at least one magnetic resonance frequency among the magnetic resonance frequencies, Using the phase encode gradient magnetic field to detect the peak position of the plurality of echo signals obtained in the state not applied, the switching timing of the gradient magnetic field for reading, positive and negative so that the peak position becomes a predetermined sampling position A magnetic resonance imaging apparatus having a control means for changing at least one of an amplitude value and an offset. 2. A slice is prepared by applying a high-frequency magnetic field and a gradient magnetic field for slicing to a subject made of a plurality of substances having different magnetic resonance frequencies and placed in a uniform static magnetic field to excite a predetermined slice plane, and then slice the slice. An image of the slice plane is obtained by applying the read gradient magnetic field in one direction in the plane by continuously switching and applying the phase encode gradient magnetic field in the direction orthogonal to the read gradient magnetic field in the slice plane. Data collection means for collecting and sampling a plurality of echo signals based on magnetic resonance necessary for reconstruction,
In a magnetic resonance imaging apparatus including an image reconstructing unit that reconstructs an image based on sampling data obtained by this unit, spins that magnetically resonate at a frequency other than a predetermined magnetic resonance frequency among the magnetic resonance frequencies After saturation, the peak position of the plurality of echo signals obtained without applying the phase encoding gradient magnetic field is detected, and the timing of switching the readout gradient magnetic field so that the peak position becomes a predetermined sampling position, A magnetic resonance imaging apparatus comprising a control means for changing at least one of a positive and negative amplitude value or an offset.