JPH0614903A - Mr imaging device - Google Patents

Abstract

PURPOSE:To facilitate high highly precise regulation of bipolar gradient by applying bipolar gradient so that the polarity is reversed every line, and collecting repeated data. CONSTITUTION:In a pulse sequence of one repeat period, RF signal is successively applied as one 90 deg.-pulse and two 180 deg.-pulse. After the second 180 deg.-pulse, a phase encoding Gy pulse 3 is given, and A/D converting sampling pulse is generated for the data sampling of spin echoes generated thereafter. After the first 180 deg.-pulse and before the second 180 deg.-pulse, pulses 1, 2 of bipolar gradient are given. The pulses 1, 2 are reversed only in polarity and coincident in wave height value. The polarities of the pulses 1, 2, of bipolar gradient are reversed every line.

Description

Detailed Description of the Invention

[0001]

This invention relates to nuclear magnetic resonance (NM).
The present invention relates to an MR imaging device that performs imaging using R).

[0002]

2. Description of the Related Art In an MR imaging apparatus, a subject is inserted into a static magnetic field, and gradient magnetic field pulses in each of the three-dimensional directions are used for selective excitation, frequency encoding, and phase encoding while controlling their waveforms. By adding a high-frequency pulse having a frequency corresponding to the resonance frequency to the subject to excite the subject and repeating a pulse sequence of receiving a resonance signal generated thereafter, data for imaging is obtained. Perform an imaging scan to collect.

In such an MR imaging apparatus, various pulse sequences have been devised, and in the high speed spin echo method and the phase contrast angiography method among them, a part of the pulse sequence is as shown in FIG. Bipolar glazing is used. This is a rephasing in which a dephasing pulse 1 that disturbs the phase of the magnetization vector is applied at a certain timing in one gradient magnetic field (Gy in FIG. 7), and then the disordered phase is returned to the original at a certain timing. The application pulse 2 is applied. In order to completely cancel the phase change due to the first pulse 1 with the next pulse 2, it is necessary that both time integrations are completely equal, and the time integration is completely 0 in the whole (bipolar gradient). Therefore, in the imaging sequence using these bipolar gradients, strict performance is required for the hardware system, and if the bipolar gradient is not adjusted accurately, it causes a false image.

[0004]

However, in reality, each pulse 1 and pulse 2 of the bipolar gradient is shown in FIG. 8 due to the positive and negative amplifier characteristics of the gradient magnetic field power supply and the eddy current magnetic field generated with the application of the gradient magnetic field. The waveform is as shown in, and the time integration of the bipolar gradient is completely zero.
However, the phase may not be completely returned by the rephasing pulse 2. In that case, the fast spin echo method appears as a blurred image, and the phase contrast angiography method causes the blood vessels not to be clearly extracted.

The error of the positive / negative pulse of this bipolar gradient should be about 0.1% or less according to the experiment and theory, but many frequencies are mixed in the raw data, and this error is detected from the raw data. It is extremely difficult to do. In fact, the peak value of raw data may be reduced by this error, but as a result of the experiment, the change of the signal peak value was within the range of the variation of the device, and the error could not be calculated from the peak value. .

In view of the above, the present invention is improved in that the error of the positive and negative pulses of the bipolar gradient is displayed in a form that can be easily recognized by human eyes so that the bipolar gradient can be easily adjusted with high accuracy. It is an object of the present invention to provide an MR imaging device.

[0007]

In order to achieve the above-mentioned object, in the MR imaging apparatus according to the present invention, the bipolar gradient in the gradient magnetic field for phase encoding is alternately reversed in sequence for each line. The feature is that the sequence of each line is repeated while applying the data.

[0008]

By applying the bipolar gradient as the gradient magnetic field for phase encoding, the error is added as an error in the amount of phase encoding. When the polarity of the bipolar gradient is alternately inverted for each line sequence, this error component is added to the original phase encoding amount alternately with positive and negative polarities. In this way, when the image with the error added is subjected to the two-dimensional Fourier transform and the image is reconstructed, the image with the aliasing artifact added to the original image is obtained. This artifact reflects only the above error, and if the bipolar gradient is adjusted so that the artifact disappears on the screen, the highly accurate adjustment of the bipolar gradient is completed.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. 1 is a time chart showing a pulse sequence used in an MR imaging apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a configuration of the MR imaging apparatus according to the embodiment.

In this embodiment, the present invention is applied to the fast spin echo method. What is the fast spin echo method?
After selective excitation with 90 ° pulse, 180 ° pulse is sequentially applied multiple times, and spin echoes are generated multiple times while changing the phase encoding amount at each of them, and multiple data having different phase encoding amounts are repeated in one repetition cycle. This means that multiple data items with different phase encoding amounts can be obtained during one repetition period, so the number of repetition periods can be small, and the data required to reconstruct one image can be obtained in an extremely short time. It is a pulse sequence that can be collected in.

In this case, a plurality of phase-encoding gradient magnetic field pulses having different peak values are given during one repetition period, but the gradient magnetic field pulse given later is not affected by the gradient magnetic field pulse given earlier. It is necessary to give a gradient magnetic field pulse having a polarity opposite to that of the preceding gradient magnetic field pulse to restore the phase disturbance due to the preceding gradient magnetic field pulse. That is, after a 180 ° pulse, a gradient magnetic field pulse for phase encoding is applied to perform phase encoding before a spin echo signal is generated, and then a gradient magnetic field pulse having an opposite polarity that cancels the gradient magnetic field pulse is applied, and then It is necessary to give a 180 ° pulse of 1 to generate the next spin echo signal.

When such a bipolar gradient is applied, if the effect of the previous gradient magnetic field pulse is not completely canceled by the next gradient magnetic field pulse, the error is added to the phase encoding amount for the next spin echo. , Cause artifacts.

Therefore, in this embodiment, the adjustment pulse sequence as shown in FIG. 1 is repeated. FIG. 1 shows a pulse sequence having one repetition period. In FIG. 1, an RF signal is sequentially applied as one 90 ° pulse and two 180 ° pulses, a Gz pulse for selective excitation is given simultaneously with these RF pulses, and position information in the X direction is frequency-encoded. Gx for aligning the spin phases with
The Gy pulse 3 is given to phase-encode the position information in the Y direction. The Gz pulse is a gradient magnetic field pulse in the Z direction, the Gy pulse is a gradient magnetic field pulse in the Y direction, and the Gx pulse is a gradient magnetic field pulse in the X direction. In this embodiment, the Z direction is the slice selection direction (direction perpendicular to the slice plane), the X direction is the frequency encoding direction, and the Y direction.
Although the direction is the phase encoding direction, the slice selection direction, the frequency encoding direction, and the phase encoding direction are not limited to these directions, and can be any direction.

In FIG. 1, a Gy pulse 3 for phase encoding is given after the second 180 ° pulse, and a sampling pulse for A / D conversion is generated for data sampling of the spin echo generated thereafter. .
And the second 180 ° after the first 180 ° pulse
Bipolar gradient pulses 1 and 2 are given before the pulse. Only the polarities of the bipolar gradient pulse 1 and the pulse 2 are opposite to each other, but the crest values are the same.

In the pulse sequence shown in FIG. 1, the magnitude of the phase encoding Gy pulse 3 is, for example, -128, -127, ..., -1, 0, 1 ,.
As shown in 27 (or in the reverse order), the process is repeated while sequentially changing it in 256 steps. In this way, data for 256 lines is collected, but the polarities of the bipolar gradient pulses 1 and 2 are set in opposite directions for each repetition, that is, for each line. When the pulses 1 and 2 are applied in the order of negative and positive in the first line as shown by the solid line in FIG. 1, the polarities are alternately changed so that they are applied in the order of positive and negative in the next line. The crest values of the pulses 1 and 2 may be sequentially changed according to the crest value of the phase encoding pulse 3, or may be constant at any repetition cycle regardless of the change of the crest value of the phase encoding pulse 3. Good.

Next, the hardware of the MR imaging apparatus that performs such a pulse sequence will be described with reference to FIG. The above-mentioned gradient magnetic fields Gz in three directions,
Gy and Gx are generated by passing a current through the Gz coil 12, the Gy coil 13, and the Gx coil 14 arranged in the main magnet 11 as shown by arrows. The main magnet 11 is for generating a static magnetic field in which the magnetic flux is oriented in the Z direction. These Gz coils 12, Gy
A current is applied to the coil 13 and the Gx coil 14 from the gradient magnetic field power supply 22. These current waveforms are provided by the waveform generator 21.

An examinee (not shown) is inserted into the space to which the static magnetic field and the gradient magnetic field are applied, and a transmitting antenna and a receiving antenna (not shown) are attached to the examinee. An RF excitation pulse is supplied from the transmission power amplifier 26 to the transmission antenna. This RF excitation pulse is used by the modulation circuit 25.
In the above, the RF signal from the signal generator 23 is modulated by the signal from the waveform generator 24. The NMR signal received by the reception antenna is sent to the detection circuit 28 through the preamplifier 27, the signal from the signal generator 23 is phase-detected as a reference signal, and further, is sampled by the A / D converter 29 and converted into digital data. Being a computer 2
It is taken into 0.

The computer 20 processes this data to reconstruct an image and obtain an MR image. The computer 20 controls the waveform of each gradient magnetic field generated from the waveform generator 21 and its timing, controls the RF pulse waveform from the waveform generator 24 and its timing, and further controls the signal generator 23. Then, the entire sequence including the imaging sequence is controlled by generating a signal of the resonance frequency.

The data collected by repeating the above pulse sequence is two-dimensionally Fourier transformed by the computer 20 to reconstruct an image. When this image is displayed on an appropriate image display device (not shown), FIG.
Then, in addition to the original image 4, the folding artifact 5 appears. This aliasing artifact 5 is due to an error between the pulse 1 and the pulse 2 of the bipolar gradient, and it does not occur unless there is this error. So, this turnback artifact 5
The amplitude of the design value of the pulse 2 of the bipolar gradient is adjusted so that is as thin as possible on the screen. That is,
This folding artifact 5 sensitively reflects only the error between the positive and negative pulses 1 and 2 of the bipolar gradient, and is easily adjusted by imaging as the artifact 5 which is easy for a person to visually recognize on the screen. You can

Further details will be described with reference to FIGS. 4 and 5. For convenience of explanation, FIG. 4 shows positive and negative pulses 1, 2 of bipolar gradient from the pulse sequence of FIG.
Only the points necessary for detecting the error of are extracted. In FIG. 4, bipolar gradient pulses 1 and 2 to be adjusted are added to the pulse sequence of the normal gradient echo method. Theoretically, their waveforms are designed so that the area sum of pulse 1 and pulse 2 becomes 0, but in reality, the area sum does not become 0 due to the reason described above, and the error component Join the gradient magnetic field pulse 3 for phase encoding.

The phase encoding gradient magnetic field pulse 3 changes by one step from the negative direction to the positive direction with each repetition of the pulse sequence.
If 2 does not change at each iteration, then the same amount of error will be added at every step of the amount of phase encoding. Since this error amount is about several percent at most, it is not reflected in the image at all.

However, here, as described above, the polarities of the pulses 1 and 2 are inverted each time the pulse sequence is repeated. Therefore, the amount of error is positive / negative with respect to the amount of phase encoding at each repetition of the pulse sequence.
It will be added as a negative value. As a result, in the raw data space, as shown in FIG. 5, an error in the vertical direction (phase encoding direction ky) is alternately added line by line, and the data cannot be obtained at the position shown by the solid line. Where it must be, data is obtained at positions vertically shifted alternately for each line as shown by a dotted line. When data including such an error is transformed into an image by performing a two-dimensional Fourier transform, due to the Fourier transform,
L / 2 (L is the field size in the phase encoding direction)
A wraparound artifact is generated at (Fig. 3).

FIG. 6 shows a pulse sequence according to a second embodiment in which the present invention is applied to the phase contrast angiography method. In this phase contrast angiography method, for example, a 90 ° pulse as shown in FIG. 6 is applied simultaneously with a Gz pulse for slice selection, and a Gx pulse for encoding X direction position information into a frequency and aligning a phase, and a Yx pulse. In the gradient echo method having a pulse sequence of applying a Gy pulse for encoding directional position information into a phase, FE is applied to each of the X, Y, and Z tilt axes.
G (Flow Encoding Grading pu
lse) is applied to cause a phase shift of blood flow. This FEG is a bipolar gradient because it causes no phase shift except moving objects. For example, a
The phase with respect to the direction of blood flow is different between when the FEG consisting of positive and negative order pulses is used as shown in FIG. 2 and when the FEG consisting of negative and positive order pulses is used as shown in b. Since the polarities of the shifts are different, the direction of blood flow can be determined.

In this case, if the phase shifts due to the positive and negative pulses of the bipolar gradient do not completely cancel each other out,
A phase shift may occur in a part other than the moving object, and subtraction between images when FEG is added and when FEG is not added may cause a portion other than the blood vessel to remain unerased, which may not be useful for diagnosis. Therefore, it is necessary to adjust the bipolar gradient, but here, for each repetition of the pulse sequence, FEG consisting of positive and negative order pulses as shown by a and negative and positive order as shown by b. The FEG and the FEG pulse are alternately applied as a Gy pulse to collect data. In this case, the peak value of FEG was changed according to the amount of phase encoding.

Then, an image is reconstructed from the collected data, and an image in which the aliasing artifact 5 appears in the original image 4 as shown in FIG. 3 is displayed. While looking at this image, the crest value of the earlier one of the positive and negative pulses of the FEG is changed with respect to the latter crest value so that the aliasing artifact 5 does not occur. This gives F
When the FEG design value is adjusted so that the peak value of the first of the two pulses of the EG becomes 99.65% of the peak value of the latter, it is possible to prevent the aliasing artifact 5 from occurring. It was And the FEG adjusted in this way
When the phase contrast angiography method was performed by using, a good image in which only blood vessels were extracted was obtained.

The above-described embodiment is an application of the present invention to the fast spin echo method and the phase contrast angiography method. However, the present invention is not limited to these pulse sequences, but of pulse sequences using a bipolar gradient. Anything can be applied, in which they are effective in adjusting the bipolar gradient.

[0027]

As described above, according to the MR imaging apparatus of the present invention, the error of the bipolar gradient can be imaged in a form that can be easily recognized by a person, and the adjustment accuracy of the bipolar gradient can be improved. Can be facilitated and contribute to the improvement of image quality.

[Brief description of drawings]

FIG. 1 is a time chart showing a pulse sequence in an embodiment of the present invention.

FIG. 2 is a block diagram showing the configuration of the same embodiment.

FIG. 3 is a diagram showing a display image of the embodiment.

FIG. 4 is a time chart showing a pulse sequence in which only essential parts are extracted for explaining the operation of the embodiment.

FIG. 5 is a diagram of a raw data space for explaining the operation of the embodiment.

FIG. 6 is a time chart showing a pulse sequence in the second embodiment.

FIG. 7 is a time chart showing a theoretical waveform of a bipolar gradient.

FIG. 8 is a time chart showing an actual waveform of a bipolar gradient.

[Explanation of symbols]

1, 2 Positive and negative pulse of bipolar gradient 3 Gradient magnetic field pulse for phase encoding 4 Original image 5 Folding artifact RF RF high frequency excitation pulse Gz Z-direction gradient magnetic field Gy Y-direction gradient magnetic field Gx X-direction gradient magnetic field 11 Main magnet 12-14 gradient magnetic field Generation coil 20 Computer 21 Gradient magnetic field waveform generator 22 Gradient magnetic field power supply 23 Signal generator 24 RF excitation waveform generator 25 Modulation circuit 26 Transmission power amplifier 27 Preamplifier 28 Detection circuit 29 A / D converter

Claims (1)

[Claims] 1. A means for applying three-dimensional gradient magnetic field pulses in each direction for selective excitation, frequency encoding, and phase encoding, respectively, means for generating high-frequency pulses for exciting nuclear spins, and the phase encoding. Means for applying a bipolar gradient in the gradient magnetic field so as to have opposite polarities alternately for each line sequence, means for receiving the generated resonance signal, and means for image reconstruction processing of the data obtained from the reception signal An MR imaging apparatus comprising: