JPH10295667A - Mr imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a superior image of the blood vessel flowing the pulsing blood by measuring an electrocardiogram waveform by an electrocardiograph and monitoring the waveform so as to provide an electrocardiogram phase, and finding the blood flow velocity based on the relation between the electrocardiogram phase and the blood flow velocity so as to automatically set the optimal velocity encode intensity. SOLUTION: An electrocardiograph 58 measures the electrocardiogram waveform of a testee via an electrocardiogram sensor 59 and a computer 51 monitors the electrocardiogram waveform in real time. On the other hand, the relation between the heart beat period and the blood flow velocity is found beforehand so as to be stored in the computer 51. The flood flow velocity in timing of a cycling time of a pulse sequence is obtained so that the optimal velocity encode (VENC) intensity in compliance with it is automatically set. This constitution can provide a superior blood image by implementing a PC method which automatically sets to the optimal VENC intensity at all times in compliance to the changing flow velocity even in a case of pulsing the blood flow.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR imaging apparatus for producing an image by utilizing an MR phenomenon (nuclear magnetic resonance phenomenon), and particularly to a PC (Phase Contrast) for enhancing an image by emphasizing a signal from a blood flow portion. The present invention relates to an MR imaging apparatus suitable for performing the method.

[0002]

2. Description of the Related Art The PC method, which is known as a technique for enhancing and imaging a signal from a blood flow portion, is known as VENC (Veloc).
A bipolar pulse of a gradient magnetic field is used as a gradient magnetic field pulse for the (Enity Encode), and two pulse sequences are performed with their polarities changing in opposite directions, and the absolute value of the difference between the signals (vectors) obtained therefrom is obtained. It is. That is, data obtained from these signals are respectively subjected to two-dimensional Fourier transform to obtain image data of each pixel, and as post-processing, image vectors at the time of a positive VENC gradient magnetic field pulse and at the time of a negative VENC gradient magnetic field pulse are obtained. By calculating the absolute value of the difference, the signal in the stationary portion is canceled, and the signal from the blood flow portion is emphasized.

[0003] By obtaining the absolute value of the vector difference in this manner, it is possible to emphasize a signal from a specific speed portion included in a sampled signal. That is, the signal from the velocity portion that causes a phase difference of exactly 180 ° contained in the sampled signal is most emphasized, and the signal in the stationary portion is canceled out to zero.

[0004] Since the phase difference corresponds to the magnitude of the VENC pulse (the magnitude of the positive or negative part) and the velocity, conventionally, the VENC intensity is usually set according to the blood flow velocity that the operator wants to observe. Then, a phase difference of 180 ° is generated for the blood flow at the flow velocity, and an image in which the blood flow portion at the flow velocity is emphasized is obtained.

[0005]

However, since the blood flow velocity is not constant but pulsating and not constant, a problem occurs in data collection. That is, when the blood flow velocity is set V
The speed corresponding to the ENC intensity is only at a constant phase of the heartbeat. This is because the VENC intensity does not correspond to the blood flow velocity in the repetition time of the timing other than the phase. Therefore, there is a problem that a blood vessel in which a pulsating blood flow flows is not sufficiently imaged.

[0006] In view of the above, the present invention provides an MR imaging apparatus which is improved so that a VENC intensity setting operation is not required, and good imaging can be performed on a blood vessel in which a pulsating blood flow flows. Aim.

[0007]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
Means for generating a static magnetic field, gradient magnetic field means for generating a gradient magnetic field to be superimposed on the static magnetic field, RF transmitting means, RF
Receiving means, electrocardiographic measuring means for measuring the electrocardiographic waveform, and monitoring the electrocardiographic waveform to capture the electrocardiographic phase, and obtaining the electrocardiographic potential from the relationship between the electrocardiographic phase and the blood flow velocity separately determined. After determining the blood flow velocity corresponding to the phase and automatically setting the VENC intensity suitable for the blood flow velocity, the gradient magnetic field means, the RF transmission means and the RF reception means are controlled to control the slice selection gradient magnetic field pulse, Control means for performing a pulse sequence including a phase encoding gradient magnetic field pulse, a readout gradient magnetic field pulse, and a VENC gradient magnetic field pulse is provided.

[0008] An electrocardiographic waveform is measured by an electrocardiograph, and the waveform is monitored to detect a cardiac potential phase. On the other hand, the relationship between the cardiac potential phase and the blood flow velocity is separately obtained by an ultrasonic Doppler blood flow velocity measuring instrument, DBI method, or the like. Therefore, the blood flow velocity can be obtained from the cardiac potential phase, and the optimum VENC intensity for the blood flow velocity can be automatically set. By repeating the pulse sequence of the PC method in this manner, even if each repetition time is random with respect to the cardiac potential phase, that is, the blood flow velocity, it is optimal for the blood flow velocity at the timing of each repetition time. VENC intensity is automatically set.

[0009]

Next, embodiments of the present invention will be described in detail with reference to the drawings. The MR imaging apparatus according to the present invention is configured as shown in FIG. In FIG. 1, the magnet assembly 11 includes a main magnet for generating a static magnetic field, and a gradient coil for generating gradient magnetic fields Gx, Gy, Gz superimposed on the static magnetic field. A subject 61 placed on an examination table 62 is inserted into the space to which the static magnetic field and the gradient magnetic field are applied. The subject 61 is irradiated with an RF pulse and the NM generated by the subject 61 is emitted.
An RF coil 12 for receiving the R signal is attached. Further, an electrocardiographic sensor 59 is also attached to the subject 61.

A magnetic field control circuit 21 is provided as a circuit for supplying a gradient magnetic field current to the gradient magnetic field coil of the magnet assembly 11. A waveform signal from the waveform generation circuit 53 is sent to the magnetic field control circuit 21. In the waveform generating circuit 53, information on each pulse waveform of the gradient magnetic fields Gx, Gy, Gz is set in advance from the computer 51. At the timing instructed by the sequence controller 52, the gradient magnetic field Gx,
Waveform signals for each of Gy and Gz are generated and sent to the magnetic field control circuit 21, whereby pulsed gradient magnetic fields Gx, Gy and Gz having a predetermined waveform are generated.

R generated by the RF oscillation circuit 31
The F signal is sent to the amplitude modulation circuit 32, which becomes a carrier signal, and is amplitude-modulated according to the RF waveform signal sent from the waveform generation circuit 53. The RF signal after the amplitude modulation is amplified through the RF power amplifier 33 and then applied to the RF coil 12. The oscillation frequency of the RF oscillation circuit 31 is controlled by the computer 51 and the subject 61
Is matched to the resonance frequency of the body tissue. Information on the waveform of the modulation signal is given from the computer 51 to the waveform generation circuit 53 in advance. Waveform generation circuit 53
The timing of the RF oscillation circuit 31 is determined by the sequence controller 52.

NMR received by RF coil 12
The signal is sent to a phase detection circuit 42 via a preamplifier 41 and is subjected to phase detection. An RF signal from the RF oscillation circuit 31 is sent as a reference signal for the phase detection. The signal obtained by the phase detection is sampled at a predetermined sampling timing by the A / D converter 43 controlled by the sequence controller 52, and is converted into digital data. The data obtained from the A / D converter 43 is taken into the computer 51, and the computer 51 performs a two-dimensional Fourier transform to reproduce the image data of each pixel.

A display device 54, a keyboard 55, a mouse 56, a recording device 57, and an electrocardiograph 58 are connected to the computer 51. The display device 54 displays the reconstructed MR image and the like. Input and setting of an imaging sequence, imaging parameters, and the like are performed by a keyboard 55, a mouse 56, and the like.
The recording device 57 is composed of a magneto-optical disk device or the like, and records obtained data such as images. The electrocardiograph 58 measures the electrocardiographic waveform of the subject 61 via the electrocardiographic sensor 59. The measured electrocardiographic waveform becomes a waveform as shown in FIG. 3B, and this waveform signal is sent to the computer 51, and the computer 51 monitors the electrocardiographic waveform in real time.

In the PC method, a pulse sequence as shown in FIG. 2 is performed. Simultaneously with the application of the RF pulse 71, a gradient magnetic field pulse for selecting a slice (here, a Gz pulse)
72 is added to selectively excite one location in the Z direction. After that, a gradient magnetic field pulse (Gy pulse) 73 for phase encoding is added, and a gradient magnetic field pulse (Gx pulse) 74 for reading (and frequency encoding) that inverts is added.
The point at which the resonance signal 75 is generated is the same as in a normal imaging sequence. Further, in the PC method, a bipolar pulse inverting any (or all) of the gradient magnetic field from positive to negative or from negative to positive is added as a VENC pulse 76, and the same phase encoding sequence is performed twice, and each of the VENC pulses is performed. The direction in which the pulse 76 is reversed is reversed.

In FIG. 2, a VENC pulse 76 is applied to the readout gradient magnetic field Gx. In the pulse sequence shown on the left side and the pulse sequence shown on the right side, the size of the Gy pulse 73 for phase encoding is made the same so that the same amount of phase encoding is performed. The VENC pulse 76 is inverted from positive to negative, but in the other (right) pulse sequence, the direction of inversion of the bipolar gradient is reversed from negative to positive.

By adding the VENC pulse 76,
A phase shift occurs in a signal from a moving object. Here, since the VENC pulse 76 uses the gradient magnetic field Gx in the X direction, the phase of the component of the resonance signal 75 from the portion moving in the X direction is shifted. That is, VENC
Since the pulse 76 is composed of positive and negative pulses of the same magnitude, the effects of these pulses are offset in the stationary part, but the part moving in the X direction is the point of application of the positive pulse and the point of application of the negative pulse. Is moved to a different position in the X direction. Since the VENC pulse 76 uses Gx,
Since the magnetic field strengths are different at different positions in the directions, there is a difference in the amount of applied magnetic field between positive and negative. In other words, the phase shift caused by the first positive (or negative) pulse cannot be completely returned by the subsequent negative (or positive) pulse, or returns too much, resulting in a phase shift. The phase shift in this moving portion appears in the opposite direction in two pulse sequences (left and right sides in the figure) in which the inversion direction of the VENC pulse 76 is reversed.

Therefore, if the absolute value of the difference between the signals (vectors) obtained from these two pulse sequences (the left and right sides in the figure) is taken, the beam moves in the X direction at such a speed as to produce a phase difference of 180 °. The signal from the moving part is the largest. In other words, the absolute value of the difference between the signal from the part moving at a certain speed and the phase difference between the signals from that part is one.
When the VENC pulse intensity (the area of the positive or negative portion) is set to 80 °, it becomes the largest. Therefore, the VENC pulse 76 may be set in accordance with the flow velocity of the blood vessel to be observed, and a 180 ° phase difference may be generated at the flow velocity.

Here, as shown in FIG. 3A, T
It is assumed that R (repetition time of the pulse sequence on the left or right side in FIG. 2) is continuously repeated irrespective of the heartbeat. At this time, since the computer 51 monitors the electrocardiographic waveform as shown in FIG. 3B in real time, for example, each of the TRs is delayed by the delay time from the R wave.
Is in which phase of the heartbeat cycle.

On the other hand, the relationship between the heartbeat cycle and the blood flow velocity is obtained in advance by an ultrasonic Doppler blood flow rate measuring instrument or the DBI method as shown in FIG. The DBI method (direct bolus imaging method) is a method for directly measuring the blood flow velocity using an MR imaging apparatus. When a slice plane perpendicular to the blood flow is excited, the excited blood flow is measured after the echo time. The state away from the slice plane is observed by imaging from a direction parallel to the slice plane (Koji Shimizu “Quantification of Blood Flow by MR Imaging” Image Information (M) Vol. 20, No. 2,
1988, pp. 90-94). The relationship between the heartbeat cycle and the blood flow rate is stored in the computer 51 in advance, and therefore, the blood flow rate at the timing of each TR is known from the data as shown in FIG. The optimum VENC intensity corresponding to the intensity can be automatically set as shown in FIG.

If the TR is sufficiently short, the left and right pulse sequences in FIG. 2 can be successively performed with the same amount of phase encoding. Otherwise, one of the left or right pulse sequences in FIG. Phase encoding amount
The sequence is performed with the VENC intensity corresponding to the flow velocity at that time, and the other sequence on the left or right side is performed again with the same phase encoding amount at TR at the timing when the flow velocity becomes the same later.

The present invention can, of course, be variously modified without departing from the spirit thereof. For example, in the above description, Gx is used as the VENC gradient magnetic field, but other gradient magnetic fields may be used. Although the pulse sequence in which the PC method is applied to the gradient echo method as shown in FIG. 2 has been described as an example, the same applies to the case where the PC method is applied to other pulse sequences.

[0022]

As described above, according to the MR imaging apparatus of the present invention, even when the blood flow is pulsating, the optimum VENC is always adjusted according to the fluctuating flow velocity.
An excellent blood vessel image can be obtained by performing the PC method in which the intensity is automatically set. Since the VENC intensity is set automatically,
The burden of the setting operation of the operator is reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a time chart showing a pulse sequence used in the embodiment.

FIG. 3 shows TR timing, electrocardiographic waveform, blood flow velocity, and VEN.
6 is a time chart showing a relationship with C intensity.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 Magnet assembly 12 RF coil 21 Magnetic field control circuit 31 RF oscillation circuit 32 Amplitude modulation circuit 33 RF power amplifier 41 Preamplifier 42 Phase detection circuit 43 A / D converter 51 Computer 52 Sequence controller 53 Waveform generation circuit 54 Display device 55 Keyboard 56 Mouse 57 Recording device 58 Electrocardiograph 59 Electrocardiographic sensor 61 Subject 62 Examination table 71 RF excitation pulse 72 Gradient magnetic field pulse for slice selection 73 Gradient magnetic field pulse for phase encoding 74 Gradient magnetic field pulse for reading 75 Resonance signal 76 VENC Gradient magnetic field pulse

Claims (1)

[Claims] 1. A means for generating a static magnetic field, a gradient magnetic field means for generating a gradient magnetic field to be superimposed on the static magnetic field, an RF transmitting means, an RF receiving means, and an electrocardiographic measuring means for measuring an electrocardiographic waveform And monitor the electrocardiographic waveform to capture the cardiac potential phase, determine the blood flow velocity corresponding to the cardiac potential phase from the relationship between the separately determined cardiac potential phase and the blood flow velocity, and determine the blood flow velocity After automatically setting the VENC intensity suitable for the above, the gradient magnetic field means, the RF transmitting means and the RF receiving means are controlled to control the slice selecting gradient magnetic field pulse, the phase encoding gradient magnetic field pulse, the readout gradient magnetic field pulse, and the VENC. Control means for performing a pulse sequence including a gradient magnetic field pulse for use in an MR imaging apparatus.