JP2739131B2 - MRI apparatus capable of synchronized imaging of biological signals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as MR) that obtains a tomographic image of a subject using a nuclear magnetic resonance (NMR) phenomenon.
In particular, the present invention relates to an MRI apparatus suitable for synchronous imaging (for example, electrocardiographic synchronization, respiratory synchronization, etc.) for performing imaging in synchronization with various biological signals of a subject.

[Conventional technology]

An MRI apparatus utilizes an NMR phenomenon to produce a nuclear spin at a desired examination site in a subject (hereinafter simply referred to as a spin).
The density distribution, relaxation time distribution, and the like of the subject are measured, and the cross section of the subject is displayed as an image from the measured data.

In this apparatus, as shown in FIG. 3, a subject 1 is placed in a static magnetic field generator 10 for generating a static magnetic field of about 0.02 to 2 Tesla.
Is placed. At this time, the spin in the subject is the strength of the static magnetic field.
Precession is performed with the direction of the static magnetic field as an axis at a frequency determined by H ₀ . This frequency is called the Larmor frequency. The Larmor frequency ν ₀ is Here, each of the types of nuclei represented by H ₀ : static magnetic field strength ν: gyromagnetic ratio has a unique value.
In addition, when the nucleus speed of Larmor precession and ω _0, is given by _{ω 0 = 2πν ω 0 = ν} · H 0 for a relationship of ₀ ... (2).

Here, when a high-frequency magnetic field (electromagnetic wave) having a frequency ₀ equal to the Larmor frequency ν ₀ of the nucleus to be measured by the high-frequency transmission coil 20a is applied, spins are excited and transit to a high energy state. When the high-frequency magnetic field is terminated, the spin returns to the original low energy state with a time constant corresponding to each state. The electromagnetic waves emitted at this time are received by the high-frequency receiving coil 20b, amplified by the amplifier 21,
After waveform shaping, the data is digitized by an A / D converter 23 (hereinafter, referred to as an ADC) and sent to a central processing unit 11 (hereinafter, referred to as a CPU). The CPU 11 performs a reconstructing operation based on the data, and the operated data is displayed on the display 28 as a tomographic image of the subject 1. The high-frequency magnetic field is obtained by amplifying a signal transmitted from the sequencer 12 controlled by the CPU 11 by the high-frequency transmission coil power supply 3 and transmitting the amplified signal to the high-frequency transmission coil 20a.

The MRI apparatus includes, in addition to the static magnetic field and the high-frequency magnetic field, a gradient coil group 24 for generating a gradient magnetic field for obtaining positional information in a space. These gradient magnetic field coils are supplied with current from a gradient magnetic field coil power supply 25 operated by a signal from the sequencer 12, and generate a gradient magnetic field.

Here, the imaging principle of the MRI apparatus will be described. As shown in FIG. 4 (a), the nucleus placed in the static magnetic field H ₀ in the Z direction behaves like a single bar magnet in classical physics, and the Larmor frequency ν ₀ described above. Performs a precession about the Z axis. This frequency is given by equation (2) and is proportional to the strength of the static magnetic field. Γ in the equations (1) and (2) is called a gyromagnetic ratio and has a value specific to an atomic nucleus. In general, the number of nuclei to be measured is enormous, and since each is rotating at an arbitrary phase, the components in the XY plane cancel each other out as a whole, and Z
Macroscopic magnetization of only the directional component remains. Applying a high frequency magnetic field H ₁ of frequency equal to the Larmor frequency [nu ₀ in this state in the X direction (FIG. 4 (b)), the macroscopic magnetization begins falling in the Y direction. This fall angle is proportional to the product of the amplitude and application time of the H _1, the H ₁ when fall 90 ° with respect to pulse application time points 90 degree pulse, an H ₁ when fall 180 degrees is referred to as a 180 ° pulse.

Now, a two-dimensional Fourier imaging method is a method generally used for imaging by an MRI apparatus at present. FIG. 5 shows a typical pulse sequence of a typical spin echo method among these methods. In this pulse sequence, a 90 ° pulse is applied first, and then a 180 ° pulse is applied at the time of Te / 2 where the echo time is Te. After the 90 ° pulse is applied, each spin starts to rotate in the XY plane at its own speed, and a phase difference occurs between the spins with the passage of time. Here, when a 180 ° pulse is applied, each spin reverses symmetrically to the X-axis as shown in FIG. 6, and thereafter spins converge again at time Te in order to continue rotating at the same speed to form an echo signal. .

Although the signal is measured as described above, the spatial distribution of the signal must be obtained in order to form a tomographic image. For this purpose, a linear gradient magnetic field is used. By superimposing a gradient magnetic field on a uniform static magnetic field, a spatial magnetic field gradient can be created. As described above, since the spin rotation frequency is proportional to the magnetic field strength, the spin frequency of each spin is spatially different when a gradient magnetic field is applied. Therefore,
By examining this frequency, the position of each spin can be known. For this purpose, a phase encoding gradient magnetic field and a frequency encoding gradient magnetic field shown in FIG. 5 are used.

The pulse sequence described above is used as a basic unit, and the intensity of the phase encoding gradient magnetic field is changed every time, and the repetition is repeated a predetermined number of times, for example, 256 times, at a constant repetition time TR. The spatial distribution of macroscopic magnetization is obtained by performing a two-dimensional inverse Fourier transform on the measurement signal thus obtained. In the above description, if the three types of gradient magnetic fields do not overlap each other,
Any of X, Y, and Z may be used, or a composite of them may be used.

As described above, the MRI apparatus can obtain an arbitrary cross-sectional image of the subject.However, since the MRI apparatus has a long relaxation time of the nuclei in the living tissue to be measured, the repetition time TR is long. It is necessary to take this, and the imaging time also becomes long. Accordingly, in imaging of a pulsating organ such as the heart or a part where there is movement due to respiration or the like, an image blur or an artifact due to the movement of the subject or the organ appears.

As a method used to solve these problems, there is a so-called biological signal synchronous imaging method such as an electrocardiographic synchronous imaging method and a respiratory synchronous imaging method. Among these synchronous imaging methods, the usefulness of the heart-motion synchronous imaging method is described in Circuit Utilization, 6
Volume 7, Issue 2 (1983), pp. 251 to 257 (Circulation 6
7 (2), (1983) pp 251-257).

[Problems to be solved by the invention]

As described above, the imaging time of the MRI apparatus is long (several minutes to several tens of minutes in normal imaging). However, as described above, arch-act does not occur, and the subject remains stationary. You need to get it. For this purpose, it is useful to notify the subject in advance of the required time to the subject, and to notify the subject of the time from the start of imaging to the end of imaging in the middle of imaging from the operator to the subject. Functions.

The total imaging time is calculated by (repetition time) x (number of phase encodings) x (laminated circuit). If the time when the actual measurement of data is completed is reduced from the total imaging time, the time until the end of imaging (hereinafter referred to as imaging) (Called the remaining time), which can be displayed. Therefore, the operator can notify the subject of the time. However, this display can be performed correctly only when the repetition time TR is constant, and a large error occurs in biological signal synchronous imaging in which TR is affected by the cycle of the biological signal of the subject. There was a problem. That is, the time displayed by the apparatus as the remaining imaging time is calculated based on the period of the biological signal of the subject at the start of imaging, so if the period of the biological signal of the subject changes during imaging, The cause is that the error is shifted.

An object of the present invention is to provide a means for correcting and displaying the remaining imaging time as needed according to a biological signal (heartbeat cycle or respiratory cycle) that changes with a change in the state of the subject (tension or relaxation that changes with time). An object of the present invention is to provide an MRI apparatus having the same.

[Means for solving the problem]

The above object is to provide means for applying a static magnetic field to a subject, and a predetermined gradient magnetic field in a slice direction, a frequency encoding gradient magnetic field, and a high-frequency pulse for causing nuclear magnetic resonance to occur in nuclei of atoms constituting a tissue of the subject. Means for repeatedly applying a pulse sequence, means for detecting a nuclear magnetic resonance signal, means for detecting a biological signal periodically emitted by the subject, and means for repeatedly activating the pulse sequence in synchronization with the biological signal In an MRI apparatus capable of performing a biological signal synchronous imaging including: a means for sequentially measuring the time at which a predetermined number of biological signals generated from a certain imaging time are generated for each predetermined predetermined imaging number as the imaging progresses, and A solution is provided by providing means for calculating the remaining time until the completion of all imaging by using the processing means, and means for displaying the remaining imaging time calculated by the calculating means. It is.

[Action]

The biological signal periodically emitted from the subject is, as described above,
Although the cycle changes depending on the mental state, physical fatigue state, and the like of the subject, it is possible to measure the cycle. The principle of the present invention is to measure the time at which a predetermined number of biosignals generated from a certain imaging time point are generated, thereby predicting the remaining imaging time. According to the method of estimating the remaining imaging time based on the occurrence times of a predetermined number of biosignals that are retroactive to the imaging time, the entire imaging time is predicted based on the biosignal period at the start of imaging, and subtracted during the imaging. It is possible to display the remaining imaging time more accurately than the method of going.

As described above, the total imaging time of the MRI apparatus is proportional to (the number of phase encodings) × (the number of times of integration). [Average period of biological signal]. Incidentally, the number of phase encodings and the number of integrations are predetermined numbers at the start of imaging, for example, the number of phase encodings is a fixed value such as 256, the number of integrations being four. The number of integrations is the number of additions performed to improve S / N by adding a plurality of NMR signals calculated by the same phase encoding.However, when performing synchronous imaging with a biological signal, the motion of an organ is related. Therefore, the number of integrations in the same phase encoding requires the number of biological signals equal to the number of integrations. The same applies to phase encoding. Therefore, it means that (the number of phase encodings) × (the number of integrations) = (the number of biological signals). The number of phase encodings and the number of integrations are not corrected, and also represent the number of times the NMR signal is measured.

Accordingly, the measuring means of the present invention measures the time required to measure a predetermined number of already measured NMR signals connected to a certain imaging point. This is synonymous with measuring the occurrence times of a predetermined number of biosignals that are traced back from a certain imaging time. This operation is performed for a predetermined number of NM as the imaging progresses.
Repeat every R signal measurement. Using this measured time,
The calculation means is the NMR required from the time of imaging to the completion of imaging.
The remaining imaging time is calculated from the ratio of the number of NMR signals measured by the measuring means to the number of signal measurements. The calculating means performs the calculation every time a signal is input from the measuring means.

Then, the display displays the remaining imaging time. By seeing the remaining imaging time displayed on the display by the operator or the subject, it is possible to know how long the imaging is completed.

Further, the measuring means measures the average period of the biological signal each time a predetermined pulse sequence number is applied, and obtains the remaining imaging time by the calculating means each time, and displays it. The obtained remaining imaging time is finally corrected.

〔Example〕

Hereinafter, a preferred embodiment will be described with reference to an example in which the present invention is applied to ECG gating.

FIG. 2 is a block diagram showing the overall configuration of the NMR imaging apparatus according to the present invention. This NMR imaging apparatus includes a static magnetic field generating magnet 10, a central processing unit (CPU) 11, a sequencer 12, a transmitting system 13, a receiving system 14, a gradient magnetic field generating system 15, and a signal processing system 16. The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 1 in a body axis direction or a direction orthogonal to the body axis.
A permanent magnet type, a normal magnet type, or a superconducting magnet type magnetic field generating means is arranged in a space having a certain width around the periphery. The sequencer 12 operates under the control of the CPU 11 and issues various commands necessary for data acquisition of tomographic images of the subject 1 based on a certain timing, based on a transmission system 13, a gradient magnetic field generation system 15, and a reception system 14. To send to. The transmission system 13 includes a high-frequency oscillator 17, a modulator 18, a high-frequency amplifier 19
And a high-frequency coil 20a on the transmission side, and selectively excites the high-frequency pulse output from the high-frequency oscillator 17, that is, the aforementioned 90 ° pulse and 180 ° pulse in accordance with a command of the sequencer 12, based on a command from the CPU 11. The amplitude of the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19 so that the region to be overlapped partially overlaps with the high-frequency coil 20a disposed close to the subject 1. By supplying, the electromagnetic wave of a higher harmonic is irradiated to the said evacuated subject. After the two regions of the subject 1 are excited by the 90 ° pulse and the 180 ° pulse in which the frequency bands applied in a time series partially overlap, after a certain period of time when the high frequency pulse is cut, that is, the Te time Thereafter, a weak electromagnetic wave is emitted from the subject 1. The receiving system 14 includes a receiving high-frequency coil 20b, an amplifier 21, a quadrature phase detector 22, and an A / D converter 23 in order to receive the emitted electromagnetic waves. The weak electromagnetic wave emitted from the subject 1 is detected by the receiving high-frequency coil 20b disposed close to the subject 1 and the
1 and sent to the A / D converter 23 via the quadrature phase detector 22,
Converted to digital quantity. At this time, the signal processing timing is such that the output from the quadrature phase detector 22 is collected as two-sequence sampling and data in accordance with an instruction from the sequencer 12, and the collected data is sent to the signal processing system 16.

The gradient magnetic field generating system 15 includes a gradient magnetic field coil 24 arranged to generate gradients in three orthogonal X, Y, and Z axes.
And a gradient magnetic field power supply 25 for driving each coil. By driving the gradient magnetic field power supply 25 for each coil in accordance with an instruction from the sequencer 12, X, Y, Z
The gradient magnetic fields G _X , G _Y , and G _Z in the three axial directions are applied to the subject 1. By changing the method of applying the gradient magnetic field, the tomographic plane with respect to the subject 1 can be arbitrarily set.

The signal processing system 16 includes a recording device such as a CPU 11, a magnetic disk 26 and a magnetic tape 27, and a display 28 such as a CRT, and performs Fourier transform of an NMR signal obtained by the CPU 11,
A process such as phase correction is performed, a reconstructed image is created, and a signal intensity distribution of an arbitrary tomographic plane or a distribution obtained by performing an appropriate operation on a plurality of signal groups is imaged to display the image.
To be displayed. Note that the high-frequency coils 20 on the transmitting and receiving sides
The a, 20b and the gradient magnetic field coil 24 are arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 1.

In addition, this device includes an electrocardiograph to perform ECG-gated imaging.
30 and an electrocardiogram detection system 31 are provided. The electrocardiogram detection system 31 is a system that detects an electrocardiogram of the subject, and generally includes electrodes and cables attached to the chest and limbs of the subject 1. In addition, the electrocardiogram detection system 31 may include a transmitter for FM-modulating and transmitting a detection signal in addition to the electrodes and the cable.

The operation of each part during ECG-gated imaging will be described below with reference to FIG. The electrocardiogram detection system 31 captures the chest lead (or limb lead) of the subject 1 and transmits an electrocardiogram signal 32 to the electrocardiograph 30. The electrocardiograph 30 receives the electrocardiogram signal 32, generates an electrocardiogram gate signal 33 synchronized with a certain timing of the electrocardiogram signal, for example, the timing of the R wave, and inputs the same to the sequencer 12.
The sequencer 12 monitors the electrocardiographic gate signal 33, and activates the above-described pulse sequence at an appropriate delay time (arbitrarily set by the operator) from the rise thereof.
In accordance with this pulse sequence, the transmission system 13, the gradient magnetic field generation system 15, and the reception system 14 perform a series of operations to acquire an NMR signal synchronized with the heart beat.

Next, the remaining imaging time display system, which is a characteristic part of the present invention, will be described with reference to FIG. NM emitted from the subject 1 through a series of operations according to the above pulse sequence
The R signal is detected and A / D-converted by the reception system 14 and transmitted to a measurement data buffer memory (hereinafter, buffer memory) 29. As shown in FIG. 8, data groups transmitted and stored here are data in units of rows (hereinafter, referred to as data rows).
The content indicated by each data row is a series of NMR signals obtained by one signal detection. In addition, the data row stores the NMR signal in a predetermined number of samples (for example, 512 or 10
24), which is a set of digital values divided in the time direction. Accordingly, the number of data lines transmitted to the buffer memory is one for one slice / echo, but four for two slices / echo as shown in FIG. The description of FIG. 1 will be continued. The buffer memory 29 stores a number of data lines corresponding to the number of slices and the number of echoes at each repetition time TR, and stores a predetermined number of lines (for example, 128 lines).
Is reached, the data group is collectively transferred to the magnetic disk 26. On the other hand, the CPU 11 has a built-in timer and monitors the time until the predetermined number of rows of the buffer memory 29 is filled with data. . The timer controller 36 controls the remaining imaging time display 37 to display the remaining imaging time on the display 37 as a numerical value. Display 37
Is provided on the control panel, not shown.
The operator can easily recognize the remaining imaging time from the display.

Here, the calculation formula of the remaining imaging time performed by the CPU 11 is as follows.

T _n = ΔT _n × (K−n) (2) Here, the buffer memory represents the number of disk transfers of data from the start of imaging to the end of imaging by K, and the remaining imaging time at the n-th time is T It is represented by _n . ΔT _n
Indicates a time interval from the (n-1) -th data disk transfer to the n-th data disk transfer. K one method (2) is expressed by _{K = (M × N S ×} N E × N A) / L ... (3). Here, M number of phase encodes necessary for image pickup, N _S is the number of slices, N _E is the number of echoes, N _A is the number of integrations, L is shows the capacity of the buffer memory 29 in the number of data lines. That is, if the number of slices or the number of echoes increases, the number of disk transfers increases in proportion to the increase,
The error of the imaging time displayed accordingly can be reduced. This is because the accumulated error of the remaining time becomes an integral value of the change in the repetition time TR. For example, if the former fluctuates according to a linear function, the latter will increase according to a quadratic function. . That is, in order to improve the accuracy of the remaining time displayed, it is necessary to increase K in Expression (3). However, practically, 256 encodings, two integrations, one slice / echo imaging are performed about four times. It is considered sufficient if the remaining time can be corrected, and it is sufficient to set L to about 128 rows from the equation (3). The capacity of the buffer memory can be variously set depending on the system, and an optimum capacity may be selected from the viewpoint of cost.

The effect of this embodiment will be described with reference to FIG. In this example, 256 phase encoding, 2 times of integration 1 thrust / 1
It is assumed that the echo is imaged, and the subject heartbeat cycle changes from 1000 milliseconds at the start of the imaging to 1200 milliseconds in the 256th phase encoding. It should be noted that this change is proportional to the change in phase encoding for the sake of simplicity. Further, the remaining time correction according to the present invention is performed every 128 lines of NMR signal data. In this case, since two data rows are generated for one phase encoding, correction is made every 64 phase encodings.

FIG. 9 (a) shows a change in the cardiac cycle with respect to the phase encode number, and FIG. 9 (b) shows an error in the display time with respect to the phase encode number. In FIG. 9B, the case where the remaining time is not corrected is indicated by a broken line, and the case where the correction according to the present invention is performed is indicated by a solid line. As is apparent from the figure, when no correction is made, errors accumulate, and finally have a display error of 51.2 seconds, which is not practical. Stays in seconds error. An error of 3.2 seconds at the maximum is considered to be a sufficiently allowable error with respect to the total imaging time of 9 minutes and 23 seconds in this example.

The reason why such a large difference occurs as described above is that the accumulated error of the display time is an integral value of the periodic change, and an error according to the quadratic function is generated with respect to the periodic change according to the linear function. Accordingly, when the correction is performed with the 1/4 phase encoding number as shown in FIG. 9B, the error can be suppressed to 1/16. In this example, description has been made only for the case of 1 slice / echo, but when the number of slices or the number of echoes is increased, the number of data lines obtained per the same repetition time increases, so correction is performed. The number of phase encodes is reduced, and it is possible to suppress the error at the time of 1 slice / echo to 1 / (the number of slices × the number of echoes) ² .

Although the above embodiment has described the case where an electrocardiographic signal is used as a synchronization signal, a signal indicating a heart rate, a signal indicating pulmonary respiration, a blood flow signal, a heart sound signal, a pulse wave signal, a plethysmograph, and the like are used as the synchronization signal. It is also possible to use.

In addition, the remaining imaging time display is provided on the operation panel,
Although the case where the operator can recognize the remaining imaging time has been described, the display may be installed in a place where the subject can see it.

Further, in the above-described embodiment, an example in which the occurrence time of the biological signal is indirectly measured has been described. However, it is needless to say that the occurrence time may be directly measured.

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to this invention, the imaging remaining time at the time of biological signal synchronous imaging which fluctuates depending on the state of the subject can be displayed with much higher accuracy than in the related art.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an imaging remaining time display system according to the present invention, FIG. 2 is a block diagram showing an overall configuration of an MRI apparatus according to an embodiment of the present invention, and FIG. 3 is for explaining an outline of the MRI apparatus. FIG. 4 is a diagram showing the behavior of nuclear spins to show the principle of MRI imaging, FIG. 5 is a diagram showing a pulse sequence of the two-dimensional Fourier imaging method, and FIG.
The figure is a schematic diagram showing how spins form an echo signal,
FIG. 7 is a view showing the operation of the apparatus at the time of ECG-gated imaging, FIG. 8 is a view showing a data array of a measurement data buffer memory,
FIG. 9 is a diagram showing the effect of the present invention. 1 ... subject, 10 ... static magnetic field generator, 11 ... CPU, 12
…… Sequencer, 13 …… Transmission system, 14 …… Reception system, 15 ……
Gradient magnetic field generation system, 16 Signal processing system, 29 Measurement data buffer memory, 30 Electrocardiograph, 31 Electrocardiogram detection system
32: ECG signal, 33: ECG gate signal, 35: Remaining imaging time display system, 36: Timer controller, 37: Remaining time display.

Claims (2)

(57) [Claims] 1. A means for applying a static magnetic field to a subject, and applying nuclear magnetic resonance to a slice-direction gradient magnetic field, a frequency encoding gradient magnetic field, a phase encoding gradient magnetic field, and nuclei of atoms constituting the tissue of the subject. Means for repeatedly applying a high-frequency pulse to be generated in a predetermined pulse sequence, means for detecting a nuclear magnetic resonance signal, means for detecting a biological signal periodically emitted by the subject, and synchronously with the biological signal In the MRI apparatus capable of performing biological signal synchronous imaging including means for repeatedly starting the pulse sequence, a time at which a predetermined number of biological signals generated from a certain imaging time are generated for each predetermined predetermined imaging number with the progress of imaging. Means for repeatedly measuring, means for calculating the remaining time until the completion of all imaging using the measured values, and imaging residual calculated by the calculating means Biological signal synchronized imaging possible MRI apparatus comprising the means for displaying between. 2. The MR according to claim 1, wherein said display means is a numerical display.
I equipment.