JPH02224740A - Magnetic resonance image photographing device - Google Patents

Abstract

PURPOSE:To obtain image data of an arbitrary heat beat phase relative to motion of a heart by presuming the delay time from the reference wave of electrocardiographic signal of pulsation signal, generating trigger by this pulsation signal, changing over the amount of phase encoding, presuming the time obtained by subtracting the delay time from the trigger as the point of time of the reference wave of the electrocardiographic signal, and by determining desired heart beat phase point on the basis of the reference wave. CONSTITUTION:Ripple wave 41 is sensed by a ripple wave sensing circuit 35 with the phase delayed in an amount corresponding to the transmitting time, because a ripple wave sensing point is apart from the heat. The desired range of heat beat phases is entered into a computer 27 by an operational console 32 as specifying. This computer 27 generates control signals on the basis of the set value, and a sequence memory circuit 26 controls a slope magnetic field driver circuit 23 and a gate modulation circuit 28, and scan is starts with a trigger prepared in the sequence of cine scan on the basis of the ripple signals. Echo signals collected by the sequence of cine scan is amplified by a preamplifier 25, detected by a phase detector 30, and converted by an AD converter 31 into digital signals to be entered in the computer 27.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as MHI) that can realize cine scan, and particularly to an MHI that can obtain images at any phase of a heartbeat.

(Prior art) When an atomic nucleus is placed in a static magnetic field, it precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of nucleus. Magnetic resonance occurs when a high-frequency rotating magnetic field of the above-mentioned frequency is applied to an axis perpendicular to this static magnetic field, and a group of specific atomic nuclei having the above-mentioned constant undergoes a transition between levels due to the high-frequency magnetic field that satisfies the resonance condition, resulting in energy Transition to the higher level. After resonance, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

MHI is nuclear magnetic resonance (NM
This is a device that observes a phenomenon (referred to as R) and captures a tomographic image of a subject.

FIG. 5 shows a pulse sequence for applying a high frequency magnetic field and a gradient magnetic field used in the Fourier transform method in MRI. In the figure, (a) shows Gx, Gy, and Gz on the X+Y+Z axes, which are the read axis, warp axis, and slice axis, respectively.
FIG. 2 is a diagram showing a state in which a gradient magnetic field is applied and a high-frequency magnetic field is applied to the X-axis, and FIG. In period 1, spins in the slice plane perpendicular to two directions centered on z-0 are selectively excited by excitation pulse 1 and slice gradient 2. Period 2
The rephase gradient 3 is for restoring the phase of the spins disturbed by the slice gradient 2.

The day phase gradient 4 in the same period 2 is for giving a phase difference to the spins depending on the location so that the center of the SE signal 5 coincides with the temporal center of the data read period 4.

In period 2, a warb gradient 6 is further applied to shift the phase of the spin in proportion to the position in the y direction, and the warb gradient 6 is applied with the intensity, that is, the amount of phase encoding changed every cycle. Thereafter, an inversion pulse 7 is applied to align the magnetic moments, and the SE signal 5 that appears thereafter is observed. In period 4, a lead gradient 8 is applied to the X axis.

As a result, the phase difference given by the day phase gradient 4 is canceled out at the temporal center of the lead gradient 8 in the period 4, and an SE signal 5 appears. This sequence is called a view, and after the pulse repetition period TR, excitation pulse 1 is applied again to start the next view.

As a method for photographing the circulatory system such as the heart using MRI as described above, there is currently a method called cine scan that can obtain an image at any phase of the heartbeat of a subject. This method differs from the pulse sequence in Figure 5,
This method uses a high-speed scanning method to collect data, while changing the amount of phase encoding for each heartbeat cycle in synchronization with electrocardiographic signals. An example of a pulse sequence for the high-speed scanning method is shown in FIG. In the figure, parts equivalent to those in FIG. 5 are given the same reference numerals. Usually, it is excited at a flip angle α0 smaller than 90@, and the signal is obtained by reversing the slope of the read axis instead of using an inversion pulse, so the TR can be shortened and high-speed scanning is possible. ing.

In cine scan, data is collected multiple times within one heartbeat cycle using a high-speed scan method, and in synchronization with the electrocardiogram signal, the amount of phase encoding, that is, the warb gradient 6 in Fig. 6, is collected for each heartbeat cycle. Change the size. FIG. 7 is a conceptual diagram showing this. In the figure, (A) is a diagram of an electrocardiographic signal separately collected from a subject, and 10 is an R wave in the waveform. (B) is a diagram of the amount of phase encoding that is changed in synchronization with the electrocardiographic signal, and the magnitude of the warb gradient is switched for each heartbeat of the electrocardiographic signal. (C) is a diagram showing the timing of data collection in which a plurality of pieces of data are collected during one heartbeat. According to this method, for example, when performing a scan of 256 views, 256 views can be scanned during 256 heartbeats.
Data for the warb slope of the phase encode amount at different times is acquired. In order to obtain an image at an arbitrary heartbeat phase, interpolate the data of the desired heartbeat phase in each phase encode from the data of the same phase encode amount by a method such as proportionally dividing the data before and after the desired heartbeat phase. Just generate it. Regarding switching the phase encoding amount, observe the electrocardiogram signal in parallel with the scan,
The timing is determined by obtaining the electrocardiographic signal in (a) and extracting the trigger from the R wave 10 and the like.

(Problem to be Solved by the Invention) Incidentally, in scanning in MHI, in addition to the above-mentioned warb gradient 6, as shown in FIG. Although a gradient of 8 is used, noise is induced in the electrocardiographic signal line at the rising and falling points when these gradient magnetic fields change. In Figure 8, (a) is the R wave 10 of the original electrocardiogram waveform.
(b) shows the noise 11 generated due to the application of another gradient magnetic field. (c) is R wave 1 of the electrocardiogram waveform
This is a diagram of a waveform in which noise 11 is superimposed on 0, and the waveform of the electrocardiographic signal line is like this (c). Therefore, it is not possible to generate an appropriate trigger synchronized with heartbeat.

One method is to remove gradient noise through a filter, but this is not perfect as it may be difficult to separate the electrocardiogram signal from noise, such as in the case of a person with a small electrocardiogram signal.

The present invention has been made in view of the above points, and its purpose is to detect the movement of the heart without being affected by noise caused by gradient currents, and to obtain image data of an arbitrary heartbeat phase with respect to the movement of the heart. The aim is to realize MHI that can

(Means for Solving the Problems) The present invention solves the above-mentioned problems by providing an electrocardiographic signal detection means for extracting electrocardiographic signals from a subject and a gradient noise a pulsation detection means for extracting a heart pulsation signal of a subject in a manner that is not influenced by the heartbeat; a measurement means for determining an average delay time of the pulsation signal from a reference wave in the electrocardiographic signal; a trigger generating means for generating a trigger signal based on the signal; a means for sequentially switching the amount of phase encoding according to the trigger signal; and a position of the reference wave of the electrocardiographic signal based on the delay time of the output of the measuring means and the trigger signal. and a heartbeat phase point determining means to determine a heartbeat phase point to be measured based on the estimated time interval of the reference wave.

(Function) The delay time of the pulsation signal obtained from the pulsation detection means from the reference wave of the pulsation signal obtained from the pulsation detection means is estimated using the electrocardiogram signal from the electrocardiography signal detection means, and a trigger is generated based on the pulsation signal. The amount of phase encoding is switched based on this, and the time obtained by subtracting the delay time from the trigger is estimated as the reference wave point of the electrocardiographic signal, and the desired heartbeat phase point is determined based on the estimated reference wave of the electrocardiographic signal. Determine.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram of an MRI according to an embodiment of the present invention. In the figure, reference numeral 21 has a space (hole) into which the subject is inserted, and surrounding this space, a static magnetic field coil applies a constant static magnetic field to the subject and generates a gradient magnetic field. A gradient magnetic field coil (the gradient magnetic field coil is x
It is equipped with coils for three axes: , y, and z. ) and R which gives an RF pulse to excite the spin of the atomic nucleus within the object.
This is a magnet assembly in which a transmitting coil, a receiving coil for detecting an NMR signal from a subject, etc. are arranged. The static magnetic field coil, gradient magnetic field coil, RF transmitting coil, and receiving coil are connected to a static magnetic field power supply 22, a gradient magnetic field drive circuit 23, an RF power amplifier 24, and a preamplifier 25, respectively.

The sequence storage circuit 26 operates the gate modulation circuit 28 (modulates the RF output signal of the RF oscillation circuit 29 at a predetermined timing) in accordance with instructions from the computer 27 in an arbitrary view.
Then, based on the pulse sequence shown in FIG. 6, an RF pulse signal is applied from the RF power amplifier 24 to the RF transmitting coil.

Further, the sequence storage circuit 26 operates the gradient magnetic field drive circuit 23 using a sequence signal based on the pulse sequence shown in FIG. 6, and supplies gradient magnetic fields to the three axes of Xr)'+z, respectively. 30 is an RF oscillation circuit 29
This is a phase detector that detects the phase of the received signal output of the preamplifier 25 using the output of the preamplifier 25 as a reference signal.

This output signal is converted into a digital signal by the AD converter 31 and input to the computer 27.

32 is an operation console for inputting instructions and various setting values to the computer 27 for realizing various pulse sequences, and 33 is a display device for displaying images reconstructed by the computer 27. 34 is the magnet assembly 21
This is an electrocardiogram detection circuit that detects the electrocardiogram waveform captured from the electrodes attached to the subject.
is man-powered. Reference numeral 35 denotes a pulse wave detection circuit which extends outside the magnet assembly 21 and collects a pulse wave, which is the pulsation of the arterial wall due to the expansion and contraction of the heart, from the terminal end which is not affected by the gradient current, and its output is sent to the computer 27. and is used as a trigger signal to control the scan sequence.

Next, the operation of the apparatus of the embodiment configured as described above will be explained. In normal operation, the operation console 32 is operated to set the pulse sequence timing, RF pulse amplitude, pulse width, etc., and a signal based on the set values is input to the computer 27. The computer 27 generates a control signal based on the set value, and the sequence storage circuit 26 generates a control signal.
send to

The sequence storage circuit 26 controls the gradient magnetic field driving circuit 23 to generate a gradient magnetic field of a predetermined pulse sequence based on the above signal, and also controls the gate modulation circuit 28. The gate modulation circuit 28 modulates the RF multiplied signal oscillated and outputted by the RF oscillation circuit 29 into a signal having a set pulse width and amplitude, and supplies the modulated RF pulse to the RF power amplifier 24. This modulated RF pulse was amplified in the RF power amplifier 24 and excited in the static magnetic field generated by the static magnetic field power source 22 in the magnet assembly 21 together with the gradient magnetic field applied to each axis by the gradient magnetic field drive circuit 23. Make spin resonate. The signal produced by repeatedly inverting the lead slope is then received, amplified by preamplifier 25, and input to phase detector 30. Phase detector 3
0 is the human power N using the output of the RF oscillation circuit 29 as a reference signal.
The phase of the MR signal is detected and the output signal is sent to the AD converter 31.
send to The NMR signal converted into a digital signal by the AD converter 31 is subjected to predetermined processing according to the scan sequence in the computer 27 to reconstruct an image, and is displayed on the display device 33.

When controlling the scan sequence in synchronization with the electrocardiogram signal, the operator must check the electrocardiogram detection circuit 3 before starting the scan.
The electrocardiographic signal and pulse wave signal of the subject detected by the pulse wave detection circuit 35 and inputted into the computer 27 are observed on the display device 33. FIG. 2 shows the relationship between the electrocardiogram signal and the pulse wave signal. In the figure, (a) is a diagram of an electrocardiogram signal that is the output signal of the electrocardiogram detection circuit 34, and (b) is a diagram of the trigger 40 by the R wave generated based on the R wave 10 of the electrocardiogram signal in (a). FIG. 5C is a diagram showing a pulse wave 41 detected by the pulse wave detection circuit 35. FIG. Since the heart and the pulse wave detection point are far apart, the pulse wave 41 is detected with a phase delay corresponding to the transmission time from when blood is pumped out from the heart to when it reaches the pulse wave detection point. Although this phase delay time is approximately constant for each device and patient, it is expected that there will be some variation as shown in the figure, such as il+i2+i3+... The operator continues measuring the electrocardiogram signal and pulse wave signal during a constant heart rate, and the delay time i1+i
2+i3+ ... average value T6 and heartbeat cycle average value T
Find c, input it into the calculator 27, and store it.

Once Ta and T- are obtained, it is not necessary to collect electrocardiographic signals thereafter.

The operator specifies the desired heartbeat phase or phase range by inputting it into the calculator 27 using the operation console 32. This specification may be made by specifying a ratio such as % of the interval between 10 R waves. The computer 27 generates a control signal based on the set value input through the operation console 32 and sends it to the sequence storage circuit 26. The sequence storage circuit 26 stores the gradient magnetic field drive circuit 23 and the gate modulation circuit 2 based on the above-mentioned signals.
8, and a scan is started by a trigger generated based on a pulse wave signal in a cine scan sequence. The phase designated at this time is also sent to the sequence storage circuit 26 and incorporated into the sequence.

The echo signals collected in the cine scan sequence are amplified by a preamplifier 25, detected by a phase detector 30,
It is converted into a digital signal by the AD converter 31 and sent to the computer 2.
7 is input. At this time, the switching timing of the phase encode amount uses a trigger generated from a pulse wave signal that is not affected by gradient noise.

By doing this, as shown in Fig. 3, the timing of switching the phase encode amount is known from the trigger signal generated by the pulse wave signal, and the timing of the R wave that should be originally is determined by the pulse wave signal. Since the average delay time Td from the R wave 10 is stored, it can be calculated from the trigger signal. It is possible to obtain data at each heartbeat phase.

In FIG. 3, (a) is a diagram showing the trigger 42 by the pulse wave generated based on the pulse wave signal, (b) is a diagram showing the phase encode switched by the trigger signal of (a), and (c) is a diagram showing the trigger 42 by the pulse wave generated based on the pulse wave signal. R wave 1 of the electrocardiogram signal measured in Figure 2
The R wave 4 is estimated by advancing the time based on the pulse wave trigger 42 by the average value Td of the delay time of the pulse wave signal from 0.
3 is a diagram showing a certain temporal position. Therefore, the data at the desired heartbeat phase is the estimated R of (c)
It is obtained at a heartbeat phase point at a specified percentage of the wave 43 interval.

Here, when the period of the heartbeat is not constant, the heartbeat phase data for each phase encode may not be aligned, as shown in FIG. 4. In FIG. 4, parts equivalent to those in FIG. 3 are given the same reference numerals. (A) is a diagram showing a trigger 42 based on a pulse wave, and shows a case where the intervals are uneven.

(B) is a diagram of the phase encode that is switched based on the pulse wave trigger 42, and the phase encode amount a.

The duration of phase encoding of b and c is τ1. τ
2. τ. (C) shows an R wave 43 which is estimated to be at an earlier point in time than the delay time Td from the pulse wave trigger 42.
This figure is similar to FIG. 3, except that the period of the pulse wave is asymmetric. (D) is a diagram showing the position of a desired heartbeat phase point that is n% of the estimated R wave 43 interval. The position of each heartbeat phase point is 01°θ2. If θ, then θ1. θ2. θ is τ and Xn% from the estimated position of the R wave 43, respectively.

The values of τ2Xn% and τ3Xn%, and as is clear from the figure, since τ2 is small, θ2, which is the position of τ2Xn%,
is not included in phase encode b, but is included in phase encode a
, and heartbeat phase data at phase encode b cannot be obtained. In this case, when the scan for all phase encode amounts is completed, it is checked whether data of all phase encode amounts are complete for designated heartbeat phase n%. As shown in FIG. 4, if data is missing, overscanning is performed until data is obtained for that phase encode amount. In this way, data for all phase encode amounts can be aligned for a desired heartbeat phase.

As explained above, according to this embodiment, cine scan can be performed in a method that is not affected by gradient noise, so it is possible to detect the timing of switching the phase encode amount incorrectly.At this time, the gradient noise can be removed. There is no need for a filter etc. to collect the heart rate synchronized signal.Also, since the time difference between the R wave of the electrocardiogram signal and the trigger used for scanning is measured in advance, it does not matter which method is used to collect the heart rate synchronized signal. ,
A desired heartbeat phase image can be obtained.

Note that the present invention is not limited to the above embodiments. In the above embodiment, a pulse wave signal is used as a signal synchronized with heartbeat, but a heart sound may be used instead of a pulse wave signal. This embodiment is the same as the previous embodiment except that a trigger is generated from the heart sounds collected at this time.

(Effects of the Invention) As described above in detail, according to the present invention, heart movement is detected without being affected by noise caused by gradient current, and image data of an arbitrary heartbeat phase with respect to the heart movement is obtained. The practical effect is great.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an MHI according to an embodiment of the present invention, and FIG.
The figure shows the relationship between the electrocardiogram signal and the pulse wave signal, Figure 3 shows the position of the R wave signal estimated from the pulse wave, and Figure 4 shows the heartbeat phase point in the case of pulse wave asymmetry. The figure shown in Fig. 5 is a normal M
A diagram of the pulse sequence of HI, Figure 6 is a diagram showing an example of the pulse sequence of the high-speed scan method, Figure 7 is an explanatory diagram of data collection by conventional cine scan, and Figure 8 is a diagram showing gradient noise in the electrocardiogram waveform. It is a diagram of superimposed electrocardiographic waveforms. 1...Excitation pulse 2...Slice gradient 5...
SE multiplication signal 6... Warp gradient 8... Lead gradient 10... R wave 11... Noise 21...
Magnet assembly 22... Static magnetic field power supply 23...
・Gradient magnetic field drive circuit 24...RFm force amplifier 2
5... Preamplifier 26... Sequence storage circuit 2
7... Computer 29... RF oscillation circuit 30.
・・Phase detector 32・・Operation console 33・
... Display device 34 ... Electrocardiogram detection circuit 35 ...
・Pulse wave detection circuit 41... Pulse wave 42... Pulse wave trigger 43... Estimated R wave Patent applicant Yokogawa Medical Systems Co., Ltd. Figure 6

Claims (1)

[Claims] In a magnetic resonance imaging apparatus capable of realizing cine scan, an electrocardiographic signal detection means for extracting an electrocardiographic signal from a subject, and a pulse detection means for extracting a pulse signal of the subject's heart using a method that is not affected by gradient noise. means for determining the average delay time of the pulsation signal from a reference wave in the electrocardiographic signal; trigger generation means for generating a trigger signal based on the pulsation signal; means for sequentially switching the encoding amount; a calculation means for estimating the position of the reference wave of the electrocardiographic signal based on the delay time of the output of the measuring means and the trigger signal; and measuring based on the time interval of the estimated reference wave. 1. A magnetic resonance imaging apparatus, comprising: a heartbeat phase point determining means for determining a heartbeat phase point to be detected.