JPH03280935A - Magnetic resonance image device - Google Patents

Abstract

PURPOSE:To perform correct and quick sequence adjustment by providing a control means automatically adjusting the preset pulse sequence before performing the pulse sequence of the preset high speed imaging. CONSTITUTION:The uniform static magnetic field and the gradient magnetic field in the same direction as it and having linear slant magnetic field distributions in three directions perpendicular to each other are applied to a checked person 5. A high-frequency signal is sent to a probe 7 from a transmission section 8, and the high-frequency magnetic field is applied to the checked person 5. The magnetic resonance signal received by the probe 7 is orthogonally phase-detected by a reception section 9 then sent to a data collection section 11, and it is A/D-converted and sent to an electronic computer 12. The electronic computer 12 performs image re-configuration processing based on the magnetic resonance signal sent from the data collection section 11 to obtain image data. The image thus obtained is displayed on an image display 14.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus that collects nuclear spin density image data within a subject at high speed.

(Prior art) As is well known, magnetic resonance imaging is a method that uses a high-frequency magnetic field that rotates at a specific frequency when a population of nuclear spins with a unique magnetic moment is placed in a static magnetic field. This is a method that uses the phenomenon of resonant absorption of energy to visualize chemical and physical microscopic information about substances. This magnetic resonance imaging method uses ultrasonic diagnostic equipment and X-ray CT.
It takes much longer to collect data than other medical image diagnostic devices such as Therefore, there are problems in that artifacts occur due to movements such as respiration of the subject, and it is difficult to visualize a moving heart or vascular system.

Furthermore, since the imaging time is long, the pain caused to the subject is great.

Therefore, as a method to obtain image data at high speed in magnetic resonance imaging, the echo planar method by Mansfield is used.
The multiple echo Fourier method (ultrafast Fourier method) by Hutchison et al. has been proposed. By using these methods, it is possible to collect image data in tens of milliseconds or less. Figure 2 is an echo planer.
1 shows an example of a pulse sequence for image data acquisition using the method. As a high-frequency magnetic field RF, an α° high-frequency pulse for selective excitation is applied, and at the same time, a gradient magnetic field Gs for slicing is applied to selectively excite the magnetization in the slice plane, and then the magnetization is read out in a direction parallel to the slice plane. A gradient magnetic field Or is applied by switching positive and negative alternately at high speed, and at the same time, a phase encoding gradient magnetic field Ge is statically applied in a direction orthogonal to Gs and Gr. On the other hand, Fig. 3 shows an example of a pulse sequence of the multiple echo Fourier method, and in the echo planar method shown in Fig. 2, the phase encoding gradient magnetic field Ge is pulsed every time the readout gradient Gr is switched. The difference is that it is applied to By applying these pulse sequences,
An echo signal is observed every time the reading gradient magnetic field is switched.

From these signal trains, it is possible to collect all the data necessary for image reconstruction within the time that the magnetization in the selected slice plane relaxes due to the transverse magnetization relaxation phenomenon due to the excitation of a single nuclear spin. .

In order to obtain a correct image using such a high-speed imaging method, the reading gradient magnetic field Gr and the phase encoding gradient magnetic field Ge shown in FIG. 2 or 3 must be controlled by switching quickly at a predetermined timing. It won't happen. In the high-speed imaging method described above, it is necessary to apply a high magnetic field at high speed for the readout gradient magnetic field Gr, and it is necessary to apply a very weak magnetic field with high precision for the phase encoding gradient magnetic field Ge. be. Therefore, the response characteristics of the gradient magnetic field power source, the characteristics of the system response function of the gradient magnetic field generation control system due to the inductance of the gradient magnetic field generating coil, and the influence of eddy currents induced in nearby metal objects cannot be ignored.

In the high-speed imaging method, FIGS.
In a state where the phase encoding gradient magnetic field Ge shown in the figure is not applied, it is necessary to align the phases of each spin in the slice plane at a predetermined sampling position. Unless the readout gradient magnetic field Gr is adjusted so that the positions where the phases of these spins align are at the predetermined sampling positions, it will not be possible to correctly obtain data on each predetermined grid point on the phase space. Images reconstructed from such data produce artifacts, making it impossible to accurately obtain image information useful for disease diagnosis. In Fig. 4, the spins are aligned in phase when the areas Sia and 5i of the waveform of the gradient magnetic field for readout are aligned.
-x b (1" 1 * 2 + 3 ... n) occurs at the same time, so the readout gradient magnetic field Ge
If the switching waveform of is an ideal rectangular waveform as shown by the solid line, the positions where the phases of the nuclear spins are aligned are TP, , Tp
This is the center position of each positive and negative period, such as □, . . . -, TP. However, in reality, as mentioned above, the characteristics of the system response function of the gradient magnetic field generation control system due to the response characteristics of the gradient magnetic field power supply, the inductance of the gradient magnetic field generation coil, etc., and the eddy currents induced in nearby metal objects. The waveform of the applied gradient magnetic field deteriorates as shown by the broken line in FIG. 4 due to the influence of . Furthermore, the echo signal is affected by the linearity of the gradient magnetic field drive power source, offset, etc. Due to these causes, the position where the phases of the nuclear spins are aligned is Δτ8. in FIG. Δτ2. ...
,Δτ. Predetermined positions T□, TP□, ..., T
, will deviate from n.

Furthermore, the echo signal may disappear.

Conventionally, it was necessary to manually adjust the timing, amplitude, or offset of the gradient magnetic field switching time of a predetermined pulse sequence so that the position where the phases of the nuclear spins are aligned in this echo signal becomes the predetermined sampling position. , required a lot of effort.

Also, regarding the phase encoding gradient magnetic field Ge, the phase encoding amount applied to each echo signal is a predetermined magnitude, and one phase encoding amount is a predetermined amount for an echo signal whose predetermined phase encoding color should be zero. must have a zero value. Conventionally, these adjustments also required a great deal of effort as described above.

(Problems to be Solved by the Invention) As described above, the high-speed imaging method requires complicated adjustments in order to obtain a correct reconstructed image due to waveform deterioration of the gradient magnetic field, positive and negative amplitude differences, offset, etc. There was a problem. Furthermore, if the subject has multiple magnetic resonance frequencies due to chemical shift or the like, the echo signal will have a complex waveform, making it difficult to accurately perform the sequence adjustment.

The present invention does not require complicated sequence adjustment in high-speed imaging, and can easily and accurately adjust the sequence even when imaging a subject with multiple magnetic resonance frequencies, thereby producing good images. The object of the present invention is to provide a magnetic resonance imaging device that can be used to obtain magnetic resonance images.

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above problems, the magnetic resonance imaging apparatus according to the present invention, in adjusting the pulse sequence of the high-speed imaging, sets the readout gradient magnetic field Gr to the predetermined value. Phase encoding in a high-speed imaging pulse sequence A pulse sequence without applying a gradient magnetic field Ge is executed to collect multiple echo signal data, and their values are determined from the peak position of the echo signal or the phase value at the peak position. The switching time of the predetermined gradient magnetic field application function is calculated using a system response function of the gradient magnetic field generation system that is determined in advance so that the value becomes a predetermined value.
Means for calculating and changing parameter setting values such as positive and negative amplitude values and offset; A gradient magnetic field pulse is applied, and in order to make the encode step amount of each echo signal match the reference encode step amount, the amplitude value of the gradient magnetic field pulse for each encoding is set and switched so that the amplitude of the peak of each echo signal is maximized. By adjusting the time, waveform shape, etc., or by executing the predetermined high-speed imaging pulse sequence, multiple echo signal data are collected, and the amplitude of the peak of the echo signal, which should originally have a phase encoding amount of zero, is maximized. Software or hardware automatically provides means for applying the phase encoding gradient magnetic field Ge at an appropriate point in the pulse sequence between nuclear spin excitation and the start of data collection so that By providing a means for performing this, the problem of complicated sequence adjustment in high-speed imaging is solved.

(Function) According to the present invention, by automatically performing the predetermined high-speed imaging sequence adjustment using software or hardware, it is possible to accurately and quickly adjust the sequence, and the high-speed imaging sequence adjusted using the above-mentioned sequence adjustment is possible. By performing this, image information useful for disease diagnosis can be obtained accurately and quickly.

(Embodiment) FIG. 1 is a block diagram showing the configuration of a magnetic resonance diagnostic apparatus according to an embodiment of the present invention. In the figure, a static magnetic field magnet 1 and a gradient magnetic field generating coil 3 are driven by an excitation power source 2 and a gradient magnetic field generating coil power source 4, respectively.

As a result, a --like static magnetic field and a gradient magnetic field having a linear gradient magnetic field distribution in three mutually orthogonal directions in the same direction as the static magnetic field are applied to the subject 5. A high frequency signal is sent from the transmitter 8 to the probe 7, and a high frequency magnetic field is applied to the subject 5. Here, the probe 7 may be used for both transmission and reception, or may be provided separately for transmission and reception. The magnetic resonance signal received by the probe 7 is subjected to quadrature phase detection in the receiving section 9, and then transferred to the data collecting section 11 where it is A/D converted.

It is sent to the electronic computer 12. As described above, the excitation power source 2, the gradient magnetic field generating coil power source 4, the transmitting section 8, the receiving section 9, and the data collecting section 11 are all controlled by the system controller 10. The system controller 10 and computer 12 are controlled by a console 13, and the computer 12 performs image reconstruction processing based on the magnetic resonance signals sent from the data acquisition section 11 to obtain image data. The obtained image is displayed on the image display 14.

The pulse sequence for collecting image data within a slice plane within the subject 5 according to the present invention is as follows.

A high-speed imaging pulse sequence such as the echo planar method shown in FIG. 2 or the multiple echo Fourier method shown in FIG. 3 is used.

This pulse sequence is controlled by system controller 10.

Here, in the first embodiment of adjusting the reading gradient magnetic field related to the present invention, when obtaining image data by executing the pulse sequence shown in FIG. 2 or 3,
A pulse sequence is executed in which a high-frequency magnetic field RF, a slicing gradient magnetic field Gs, and a reading gradient magnetic field Or are applied as shown in FIG. 2 in advance, but the phase encoding gradient magnetic field Ge is not applied. However, if the object to be examined is a substance that has multiple magnetic resonance frequencies due to chemical shifts, etc., before imaging, it may be necessary to carry out the procedure using a phantom containing a substance that has a single predetermined magnetic resonance frequency, or Alternatively, a pulse sequence is added and executed to saturate each spin that causes magnetic resonance at a frequency other than a predetermined magnetic resonance frequency due to a chemical shift or the like within the subject. In this way, in the waveform of each echo signal of a single magnetic resonance frequency, the peak position is equivalent to the position where the phases of the spins in the slice plane are most aligned. Here, the peak of the echo signal indicates the maximum magnitude of the norm (absolute value) of the signal. Therefore, the peak positions (positions on the time axis) of these echo signals are detected. Detection of this peak position may be performed by software processing within the computer 12, or may be performed by hardware processing within the data collection section 11 or the receiving section 9.

In FIG. 4, it is assumed that the deviation of the peak position of the detected echo signal from the normal position (predetermined sampling position II) is Δτi (1" 1 + 2y...on). Now, read out. The positive amplitude of the gradient magnetic field Gr is Ga
The negative amplitude of y is expressed as G-, and the timing of inversion accompanying switching is assumed to be τ1. As described above, the position where the peak of the echo signal occurs is the time when the area Sta of the waveform of the readout gradient magnetic field Gr becomes equal to Si-□b (i=1.2...tn). This Sia”S l-x
To satisfy condition b, tl, G+ and G- or
Just adjust the offset. As a specific example, G ten, G
- are all the same for each echo signal, and a case will be described in which only the switching timing τl is adjusted.

However, in FIG. 4, it is assumed that the operation timing τ1 of the sequence controller Gr in the system controller 10 is fixed, and τ. , τ2. ..., τ. Adjust.

FIG. 5 shows a flowchart of the correction algorithm. First, the peak position TpL (1 = 1 + 2 + "' + n) of each echo signal collected as described above is detected. Here, the peak position of the signal is the norm value of the NMR signal detected by quadrature phase detection. means the sampling position where is the maximum.Next, the peak position deviation Δτt (i=1.2...on) between TPi and a predetermined peak position TP is determined.

Δτt=Tp−TPi (1=1 t 2 + ”
'+n) Here, for the first echo, τ. is adjusted so that TPi=TP. τ. The variable amount Δt□ is Δj1=α·Δτ1. However, α indicates a conversion coefficient to sequence controller data. Therefore, the operation timing τ of the sequence controller Gr in the system controller 10.

τ. Change +Δt. Accompanying this, the peak position shift △τi after the second echo signal, (1= 1 t 2m
...+n) is corrected according to the following formula.

Δτ■=Δτt+(-1,0)-1・Δ11/α(i=
2. ... + n) For the second and subsequent echo signals, τ, 1 (j = 2y...
・Variable n), Tpj=Tp (j=2y−+n
). At this time, j (j=2t...r n)
The variable amount △t j (j”2v...tn) is △tj=
α・(0,5・Δτj) (j” 2 +...tn) Therefore, the operation timing τj of the sequence controller Gr in the system controller 10 is set to τj+
Change to Δtj. As a result, the peak position shift Δτ*(i=j+1,...+
Correction of n) is performed according to the following formula.

Δτl=△τtechnique+(x,o)i−J・(2,0・Δtj
/α) (i=j+1...+n) The above processing is performed on a predetermined echo signal. Ideally, the deviation of Δτl can be eliminated with one control, but
If one control is incomplete, the same control may be repeated several times.

Next, FIG. 6 shows a first embodiment of adjusting the gradient magnetic field for phase encoding according to the present invention. In the pulse sequence of the multiple echo Fourier method, only the predetermined pulses to be adjusted are applied as gradient magnetic field pulses for phase encoding.

Furthermore, between the high-frequency excitation pulse and the start of data acquisition, a reference phase encoding gradient magnetic field pulse corresponding to the reference encoding step amount of each echo signal is added, and the phase encoding amount of the predetermined phase encoding gradient magnetic field is made equal to the reference encoding amount. In order to do this, the amplitude, waveform shape, switching time, etc. of the phase encoding gradient magnetic field pulse are adjusted so that the echo signal immediately after the predetermined phase encoding gradient magnetic field becomes maximum. The above control is performed using the multiple echo probe.
The amount of each phase encoding step is adjusted to a predetermined value by performing this on all the phase encoding gradient magnetic field pulses in the pulse sequence of the IJ method.6 As a second example, the pulse of the echo planar method as shown in FIG. In the sequence, a phase encoding gradient magnetic field pulse corresponding to the encoded amount of each echo signal is added between the high frequency excitation pulse and the start of data collection, and the phase encoding gradient magnetic field waveform is appropriately adjusted so that the predetermined echo signal is maximized. Adjust to. The amount of phase encoding to be added is changed sequentially, and the same adjustment is repeated for each echo signal. As a method of controlling the phase encode gradient magnetic field waveform, as shown in Fig. 7, the phase encode gradient magnetic field may be applied in a pulsed manner at the same timing as the switching time of the readout gradient magnetic field, or an appropriate shape may be applied depending on the case. You can use one.

As a third embodiment, as shown in FIG. 8, in a predetermined high-speed imaging pulse sequence, a phase encoding gradient magnetic field pulse is added in a pulsed manner between the high-frequency excitation pulse and the start of data acquisition. By varying the amplitude value or timing of switching time, adjustment is made so that the phase encode amount becomes zero at a predetermined position and a predetermined echo signal becomes maximum.

The present invention can be implemented in various modifications other than those described above.

In addition, in any of the above embodiments, the system response functions of the readout gradient magnetic field or the phase encode gradient magnetic field are determined in advance, and these system response functions are taken into consideration when adjusting the sequence.
You may also perform some adjustments.

The pulse sequence used in the present invention is not limited to the echo planar method or multiple echo Fourier method shown in FIG. Various modifications can be made to the pulse sequence for collecting signals.

〔Effect of the invention〕

According to the present invention, by providing a control means for automatically adjusting a predetermined high-speed imaging pulse sequence prior to executing the predetermined pulse sequence,
Sequence adjustments can be made accurately and quickly, and by using them to execute a WR adjusted pulse sequence, image information useful for disease diagnosis can be obtained accurately and quickly.

[Brief explanation of the drawing]

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 a diagram showing a pulse sequence of the echo planar method, which is a high-speed imaging method used in the present invention. FIG. 3 is a diagram showing a pulse sequence of the multiple echo Fourier method, which is a high-speed imaging method used in the present invention, and FIG. 4 is a diagram showing the readout gradient magnetic field waveform and signal according to an embodiment of the present invention. FIG. 5 is a flowchart of sequence adjustment according to an embodiment of the present invention; FIGS. 6, 7, and 8 are diagrams showing the application order of the pulse sequence used in the embodiment of the present invention. It is. 1... Static magnetic field magnet 2... Power source for excitation 3... Gradient magnetic field generating coil 4... Power source for gradient magnetic field generating coil 5... Subject 6... Bed 7... Probe 8... Transmitting section 9... Receiving section 10.
・System controller 11...data collection section 1
2...Electronic computer 13...Console 14...Image display

Claims (7)

[Claims] (1) In a magnetic resonance imaging device that applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a uniform static magnetic field according to a predetermined pulse sequence, and detects and images magnetic resonance signals from the subject, After exciting a predetermined slice plane by applying a high-frequency magnetic field and a slicing gradient magnetic field in a pulsed manner, the readout gradient magnetic field is switched between positive and negative alternately at high speed, and the readout gradient magnetic field is orthogonal to the readout gradient magnetic field within the slice plane. A gradient magnetic field for phase encoding with respect to the direction is excited by the high-frequency magnetic field by applying it in a pulsed manner when switching the readout gradient magnetic field, or by applying it steadily when the readout gradient magnetic field is applied. means for collecting all the data necessary for image reconstruction of the sliced plane within the time during which the transverse magnetization component of the nuclear spin in the sliced plane is relaxed by a relaxation phenomenon; and image reconstruction from the data obtained by the means. 1. A magnetic resonance imaging apparatus comprising means for configuring the magnetic resonance imaging apparatus, further comprising control means for automatically adjusting said predetermined pulse sequence. (2) The control means for automatically adjusting the predetermined pulse sequence executes a pulse sequence in which no gradient magnetic field for phase encoding is applied in the predetermined pulse sequence, and detects the peak of each of the plurality of collected echo signals. Detecting the position or the peak position of each of the plurality of echo signals and the phase at the peak position, and reading out so that the peak position or the peak position and the phase at the peak position become a predetermined value. 2. The magnetic resonance imaging apparatus according to claim 1, wherein at least one of a switching time, positive and negative amplitude values, an offset value, or a waveform shape of the gradient magnetic field application function is changed. (3) The control means for automatically adjusting the predetermined pulse sequence includes, in the predetermined pulse sequence, a control means for controlling each echo signal at an appropriate position between nuclear spin excitation and data collection with respect to the phase encoding gradient magnetic field. A phase encoding gradient magnetic field corresponding to a predetermined reference encoding step amount was applied with an opposite sign, and with respect to the phase encoding gradient magnetic field of the predetermined pulse sequence, only the phase encoding gradient magnetic field immediately before the predetermined echo signal was applied. Execute the pulse sequence, and set the switching time and amplitude value of the gradient magnetic field for phase encoding immediately before the predetermined echo signal so that the amplitude of the predetermined echo signal is maximized.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the control means for changing at least one waveform shape is performed for each echo signal. (4) In the control means for automatically adjusting the predetermined pulse sequence, in particular, in adjusting the phase encoding gradient magnetic field, the phase encoding gradient magnetic field is pulsed at an appropriate point between nuclear spin excitation and data collection. is applied, and by varying the amplitude or the timing of the switching time, the phase encode amount is adjusted to zero at a predetermined position, that is, the amplitude of a predetermined echo signal is maximized. The magnetic resonance imaging apparatus according to claim 1. (5) The echo signal peak position and phase detection method used when automatically adjusting the predetermined pulse sequence is the absolute value after quadrature phase detection of the echo signal obtained by executing the predetermined pulse sequence. Claim 2, 3 or 4, characterized in that it is a peak position of and a phase of an echo signal obtained by quadrature phase detection at the peak position.
The magnetic resonance imaging device described. (6) In the control means for automatically adjusting the predetermined pulse sequence, the switching time, positive and negative amplitude values, and offset values of the predetermined gradient magnetic field application function are utilized in advance by a system response function of the gradient magnetic field generation control system. 5. The magnetic resonance imaging apparatus according to claim 2, wherein the amount of change in any parameter of the waveform shape is determined. (7) The control means for automatically adjusting the predetermined pulse sequence may be performed using an echo signal observed from a phantom containing a substance having a predetermined single magnetic resonance frequency, or by using an echo signal within the subject. Claim 2, characterized in that the echo signal observed after saturating nuclear spins that cause magnetic resonance at a frequency other than a predetermined magnetic resonance frequency is used.
5. The magnetic resonance imaging apparatus according to 3 or 4.