KR20110075859A - Method for correcting phase error in magnetic resonance imaging system - Google Patents

Abstract

PURPOSE: A method for correcting a phase error of a MRI system is provided to correct a phase error of an echo signal by controlling a flow rate encoding gradient field. CONSTITUTION: A pulse sequence except for a flow rate encoding gradient pulse and a phase encoding gradient pulse is applied. An echo signal is obtained through a receiving coil. A reference peak position of the obtained echo signal is detected and stored. The pulse sequence is applied to a gradient coil. The flow rate encoding gradient pulse is applied only to a frequency-axis gradient coil. The echo signal is obtained through the receiving coil. The peak position of the obtained echo signal is detected. The flow rate encoding gradient pulse is automatically controlled toward a direction for minimizing a phase error.

Description

Phase error correction method of MRI system {METHOD FOR CORRECTING PHASE ERROR IN MAGNETIC RESONANCE IMAGING SYSTEM}

The present invention relates to a magnetic resonance imaging (MRI) system, and more particularly, to a phase error correction method of an MRI system for correcting a phase of an echo signal in a phase contrast degree angiography technique.

Magnetic Resonance Imaging (MRI) is a relatively safe imaging technique because it not only provides noninvasive imaging information but also has no radiation exposure compared to CT or general x-ray imaging.

Such MRI has advantages of relatively free imaging conditions, easy re-photographing, and excellent contrast in soft tissues and various diagnostic information images.

Phase Contrast Magnetic Resonance Angiography (MRA) allows blood vessels to be photographed without the addition of contrast agents, so there are no side effects caused by the administration of contrast agents at the time of imaging. Because it gives the rate information, it contributes to the accurate diagnosis of vascular disease.

Phase contrast magnetic resonance angiography is often used as a diagnostic imaging technique in hospitals, but it is also widely used as an imaging technique for blood flow-related research in universities and research institutes.

Basically, the phase contrast technique uses a flow-induced phase shift phenomenon to distinguish moving blood flow from stationary tissue. This phase accumulation due to the gradient is proportional to the velocity of the blood flow and the gradient area, and if the directions of the velocity encoding gradient pulses are opposite to each other, the phase shift due to the second gradient becomes negative. .

Observing the phase change of the spin due to the velocity-encoded gradient pulses, dephasing at the stationary spin can be eliminated by placing another velocity-encoded gradient pulse in the opposite direction, while spinning against the flowing blood flow. Since the position shifts between the dephasing gradient and the rephasing gradient, the second velocity encoding gradient pulse cannot cause accurate rephasing.

1 and 2 are diagrams illustrating examples of pulse sequences for two-dimensional and three-dimensional scans, which are generally executed in an MRI system.

First, the MRI system includes a pair of magnets for forming a magnetic field in the photographing space, an RF antenna for generating an RF pulse to the photographing space, and a pair of x, y for forming a gradient magnetic field for selecting a photographing area of the subject in the photographing space. , a z-axis gradient coil, and a receiving coil for receiving an echo signal emitted from the subject.

Here, the pair of x-axis, y-axis, and z-axis gradient coils form a gradient magnetic field according to the pulse sequence generated according to the position and angle of the section to be photographed. Therefore, in the present invention, instead of a plurality of gradient coils as x-axis, y-axis, and z-axis gradient coils, a slice axis gradient (GS) coil, frequency, for convenience according to a function of a pulse sequence output through the gradient coil. It is referred to as a frequency encoding gradient (GF) coil and a phase encoding gradient (GP) coil. That is, the slice axis gradient magnetic field may be formed by the x-axis gradient coil, but may also be formed by the y-axis gradient coil or the z-axis gradient coil in some cases.

In the two-dimensional pulse sequence of FIG. 1, an imaging region for the subject is excited by an RF pulse RF and a cross section selection pulse of the GS coil for cross section selection.

After the cross section selection pulse is applied, a Velocity Encoding Gradient pulse is applied to the GS coil, the GF coil, and the GP coil. The phase contrast image may be obtained by the velocity encoding gradient pulse.

After the Velocity Encoding Gradient pulse is applied to the GP coil, a pulse for phase encoding of a signal of a selected cross section is applied.

After a phase encoding pulse is applied to the GP coil, a reading pulse for applying an echo signal is applied to the GF coil. An echo signal is sampled through the receiving coil while frequency encoding is performed by the leading pulse.

When the echo signal is obtained after the leading pulse is applied, a rewinding pulse is applied to the GP coil to restore the phase of the spin to its original state.

One echo signal is obtained through this one cycle procedure. If the resolution of the image is 256 * 256, this procedure must be repeated 256 times to obtain the screen image. Tr (repeat time) represents the repetition time for obtaining one echo signal.

Here, the pulse sequence of FIG. 1 is repeatedly output until one screen is completed, but the amplitude of the pulse and the pulse output through the GP coil is changed every time.

In the case of Figure 2 is a pulse sequence for obtaining a three-dimensional image, unlike in Figure 1 in the pulse sequence applied to the GS coil is applied a slice encoding pulse ⓘ for three-dimensional encoding between the pulse and the pulse, At the same time as the pulse, a rewinding pulse that returns the phase of the spin to its original state is applied.

General Phase Contrast In the case of angiography, a differential velocity signal is obtained by applying a velocity encoded gradient pulse to a general gradient pulse sequence as shown in FIGS. 1 and 2.

However, in the pulse sequences of FIGS. 1 and 2 which are basic pulse sequences, the phase of the echo signal due to the eddy current is greatly distorted due to the large gradient magnetic field rapidly changing in polarity.

In order to minimize the effects of eddy currents, eddy current compensation schemes are being developed, but the gradient magnetic field occurring in the actual system has an output error according to its polarity. Although the actual eddy current has decreased greatly by continuous research and development, a small amount of eddy current is still generated, and it is inevitable that the amount of eddy current is changed by the flux encoding gradient magnetic field which changes according to the blood flow rate to be measured. Therefore, if the phase error caused by the velocity encoding gradient magnetic field is eliminated, more accurate blood flow velocity can be measured.

Flow rate encoding gradient magnetic field (㉯) is composed of a gradient magnetic field of the same width as the anode (+,-), as shown in Figs. However, the shape of the actual gradient magnetic field is deformed (ⓐ, ⓑ) as shown in FIGS. 3A and 3B due to the output error and the distortion of the gradient magnetic field due to the eddy current. As a result, a phase error of the echo signal obtained by the reading pulse is generated. For example, when the area of the magnetic flux coding gradient magnetic field of the positive pole becomes larger, the phase of the echo signal is faster than the reference phase as shown in FIG. 3A, and a phase error occurs. If it becomes larger, the phase of the echo signal (echo) is later than the reference (reference) as shown in Figure 3b will cause a phase error.

Basically, the phase contrast angiography technique shows the phase difference between the basic image to which the velocity encoding gradient magnetic field is not applied and the image to which the velocity encoding gradient magnetic field is applied as shown in FIG. 1, or the phase signal by applying only the velocity encoding gradient magnetic field. Obtain and reconstruct the image. However, in both methods, if the area according to the anode of the velocity coding gradient magnetic field is deformed or distorted, the amount of phase error affects the signal. As a result, there is a problem that the accuracy of the blood flow rate to be measured decreases due to the phase error.

The present invention is to provide a phase error correction method of the MRI system to adjust the velocity encoding gradient magnetic field to correct the phase error of the echo signal according to the deformation of the velocity encoding gradient magnetic field in the phase contrast degree angiography technique.

Technical problems to be achieved by the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned above may be clearly understood by those skilled in the art from the following description. There will be.

In accordance with one aspect of the present invention, there is provided a method for correcting a phase error, the method including: applying a pulse sequence excluding a velocity encoded gradient pulse and a phase encoded gradient pulse to a plurality of gradient coils; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and detecting and storing a reference peak position of the obtained echo signal; Applying a pulse sequence determined to the gradient coil after acquiring a reference peak position of the echo signal, but applying a flow rate encoded gradient pulse only to the frequency axis gradient coil; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence and detecting a peak position of the obtained actual echo signal; And automatically tuning the flow rate encoded gradient pulse in a direction to minimize the phase error by comparing the peak position of the obtained echo signal and the stored reference peak position with each other.

Specifically, the peak position of the echo signal may be detected through the intensity of the echo signal and the position of the pixel on which the echo signal is displayed.

Further, when automatically tuning the flow rate encoded gradient pulse, only the magnitude of the flow rate encoded gradient pulse of -pole is adjusted, and if the peak position of the obtained echo signal is faster than the reference peak position, the magnitude of the flow rate encoded gradient pulse of + pole When the peak position of the obtained echo signal is later than the reference peak position, the size of the flow rate encoded gradient pulse of the pole is adjusted.

According to another aspect of the present invention, there is provided a method of correcting a phase error, the method including: applying a pulse sequence except for a velocity coded gradient pulse and a phase coded gradient pulse to a plurality of gradient coils; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and acquiring and storing reference phase data using the obtained echo signal; Applying a predetermined pulse sequence to the gradient coil after acquiring the reference phase data, and applying a flow rate encoded gradient pulse of a predetermined magnitude to the frequency axis gradient coil; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and acquiring actual phase data using the obtained echo signal; Acquiring an echo signal while varying a magnitude of the velocity encoding gradient pulse according to a predetermined value, and obtaining actual phase data of the obtained echo signal; And comparing the obtained actual phase data with the stored reference phase data and selecting a flow rate encoded gradient magnetic field having the phase data most similar to the reference phase data to automatically tune the flow rate encoded gradient pulses. It is done.

Specifically, the plurality of gradient coils are sequentially changed to the frequency axis, and phase errors are respectively corrected for the changed frequency axis.

The acquiring of the reference phase data may include: obtaining a profile of the subject by performing fast Fourier transform (FFT) on the obtained echo signal; And obtaining reference phase data by obtaining a phase of each pixel using the complex data of the profile and accumulating the obtained phases. The acquiring of the actual phase data includes: Fast Fourier transform (FFT) transforming the signal to obtain a profile of the subject; And obtaining actual phase data by obtaining a phase of each pixel using the complex data of the profile and accumulating the obtained phases.

Further, when varying the flow rate encoded gradient pulses, only the magnitude of the flow rate encoded gradient pulses of the -pole is tuned.

As described above, in the present invention, the fine contrast of the velocity encoding gradient magnetic field in the direction of minimizing the phase error of the echo signal in the phase contrast degree angiography technique can improve the image quality and at the same time the phase contrast degree. There is an advantage that can increase the accuracy of blood flow rate measurement in angiography.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention. Like elements in the figures are denoted by the same reference numerals wherever possible. In addition, detailed descriptions of well-known functions and configurations that may unnecessarily obscure the subject matter of the present invention will be omitted.

Figure 4 is a schematic diagram showing an MRI system applied to the present invention, the MRI system is a magnetic field part 100, gradient coil driver 210, RF coil driver 230, data acquisition unit 250, sequence control unit ( 270, a data processor 310, a display 330, and a command input unit 350.

The magnetic field part 100 includes a magnet part 110, a gradient coil part 130, an RF coil part 150, and a receiving coil part 170.

The magnet unit 110, the gradient coil unit 130, and the RF coil unit 150 are each a pair of magnets, a pair of gradient coils, and a pair of RF coils, which face each other up and down based on a shooting space, respectively. consist of.

The magnet unit 110, the gradient coil unit 130, and the RF coil unit 150 substantially have a disk shape, and share the same central axis. The photographing object 1 (subject) placed on the cradle 10 is moved to the photographing space of the magnetic field part 100 by a conveying means (not shown).

The magnet unit 110 generates a static magnetic field in the photographing space of the magnetic field unit 100. The direction of the static magnetic field produces a vertical magnetic field that is substantially orthogonal to the body axis direction of the subject 1. The magnet unit 110 may be formed of a permanent magnet, a superconducting magnet, or a resistive magnet.

The gradient coil unit 130 generates three gradient magnetic fields which are used to cause the static magnetic field strength to undergo a gradient along three axes perpendicular to each other, that is, the slice axis, the frequency axis, and the phase axis. To generate the gradient magnetic field, the gradient coil unit 130 includes three gradient coils 131, 133, and 135.

Assuming that the axes x, y, z defined as orthogonal to each other in the space where the static field is generated, any axis can be regarded as a slice axis. One of the two remaining axes is considered as the frequency axis and the other as the phase axis. In addition, the slice axis, the frequency axis, and the phase axis may be inclined to have a predetermined inclination with respect to each of the x, y, and z axes while maintaining mutual orthogonality. In this embodiment, the body axis direction of the subject 1 is regarded as the z axis direction.

The RF coil unit 150 applies an RF pulse of a predetermined frequency to the subject.

The receiving coil unit 170 receives an electromagnetic wave, ie, a magnetic resonance signal, generated by a spin in the subject 1 placed in a static field space excited by the application of an RF pulse. The signal received by the receiving coil unit 170 is transmitted to the data obtaining unit 250.

The gradient coil driver 210 is connected to the gradient coil unit 130 and generates a gradient magnetic field by applying a driving signal to the gradient coil unit 130. The gradient coil driver 210 includes three driving circuits related to the three gradient coils 131, 133, and 135 included in the gradient coil part 130, the GS coil driver 211, the GF coil driver 213, and the GP coil. The drive unit 215 is included. Here, the gradient magnetic field corresponding to the direction of the slice axis is called a slice selection gradient (GS), and the gradient magnetic field corresponding to the direction of the frequency axis is called a frequency encoding gradient (GF) or a leading encoding gradient field. The gradient magnetic field corresponding to the direction of the phase axis may be referred to as a phase encoding gradient magnetic field (GP).

The RF coil drive unit 230 is connected to the RF coil unit 150. The RF coil driver 230 transmits a driving signal to the RF coil unit 150 to apply an RF pulse. The RF coil unit 150 forms a high frequency magnetic field used to excite the spin in the imaging object 1 in the static magnetic field space. Forming a high frequency magnetic field is called transmitting an RF excitation signal, and an RF excitation signal is called an RF pulse.

The electromagnetic wave generated by the excited spin, that is, the magnetic resonance (MR) signal, is received by the receiving coil unit 170. The data acquisition unit 250 is connected to the reception coil unit 170.

The data acquirer 250 acquires the echo (or MR received signal) received by the receiving coil unit 170 in the form of digital data and transmits the echo to the data processor 310. The MR signal acquired by the data obtaining unit 250 through the receiving coil unit 170 is a signal defined in a frequency domain, for example, Fourier space. A gradient magnetic field whose direction corresponds to the phase axis direction and the frequency axis direction is applied to encode the distribution of the source of the MR signal along the two axes. For example, if the Fourier space is adopted as the frequency domain, the MR signal is provided as a signal defined in the two-dimensional Fourier space.

The sequence controller 270 is connected to the gradient coil driver 210 and the RF coil driver 230, respectively. The sequence controller 270 may be implemented as a single computer that calculates and controls a pulse sequence and includes a memory. A program describing a command to be provided to the sequence controller 270 and various kinds of data are stored in a memory.

The data processor 310 performs signal calculation and control different from the pulse sequence calculation and control included in the sequence controller 270, and may be configured as another computer 300 including a memory. The memory stores a program describing a command provided to the data processor 310 and various kinds of data.

The data processor 310 is connected to the sequence controller 270. The data processor 310 is higher than the sequence controller 270 and centrally manages various controls extended by the sequence controller 270. The specific procedure is implemented by the data processor 310 executing a predetermined program stored in the memory.

The data processor 310 stores data acquired by the data acquirer 250 in a memory. The data space associated with the k space is defined in memory. The data processor 310 reconstructs the scanned subject image by performing an inverse frequency transform, for example, a two-dimensional inverse Fourier transform, on the data defined in the k-space.

The display unit 330 is connected to the data processor 310. The display unit 330 is realized by a graphic display or the like.

The display unit 330 displays the reconstructed image transmitted from the data processor 310 and various kinds of information.

In addition, a command input unit 350 is connected to the data processing unit 310. The command input unit 350 is realized by a keyboard or the like including a GUI screen.

The command input unit 350 is operated by a user through a GUI screen, and various types of commands or information recorded in the pulse sequence database PSD are transmitted to the data processing unit 310. The user manipulates the MRI system interactively through the display unit 330 and the command input unit 350 operating under the control of the data processor 310.

Looking at the process of correcting the phase of the echo signal of the present invention using the MRI system configured as described above are as follows.

5A to 5C are diagrams for explaining a phase error correction method according to an embodiment of the present invention.

Meanwhile, as shown in FIG. 1, the flow rate encoding gradient pulses are applied to the slice axis gradient coil 131, the frequency axis gradient coil 133, and the phase axis gradient coil 135 at the same time.

As such, the velocity encoding gradient pulse has an eddy current due to a sudden polarity change to the anodes (+,-), and the shape of the velocity encoding gradient magnetic field formed in the actual photographing space by the generated eddy current is shown in FIG. 3A. And as shown in FIG. 3B. As the area of the velocity encoding gradient magnetic field is deformed, a phase error of an echo signal obtained by a reading pulse occurs. For example, when the area of the positive velocity flux-encoded magnetic field increases, the phase of the echo signal is increased as shown in FIG. 3A. The phase is slowed down.

Accordingly, the present invention looks at a tuning method for preventing the actual velocity encoding gradient field from being deformed due to the eddy current generated by the rapid polarity change of the velocity encoding gradient pulse, thereby changing the phase of the echo signal.

First, the test coil 1 is installed in the photographing space of the magnetic field part 100, and then the gradient coil driver 210 is inclined to encode the flow rate code by three gradient coil parts 130 including the frequency axis gradient coil 133. Pulse sequence except pulse and phase coded gradient pulses is applied. That is, as shown in FIG. 5A, the gradient coil driver 210 applies an end face selection pulse to the slice axis gradient coil 131 and then applies an echo signal to the frequency axis gradient coil 133 without applying the flow rate encoding gradient pulse. Only reading pulses to obtain are applied.

Accordingly, an echo signal is sampled through the receiving coil unit 170 while frequency encoding is performed by the reading pulse.

The sampled echo signal is signal-processed through the data acquisition unit 250 and the data processing unit 310 and then displayed on the display unit 330 as shown in FIG. 5A. Here, the data processor 310 detects a peak using the signal strength of the echo signal, and determines the peak position of the echo signal through the pixel position at which the peak of the echo signal is displayed on the display unit 330. do.

The data processor 310 stores the peak position of the obtained echo signal as a reference position in the memory.

Subsequently, the gradient coil driver 210 applies a cross-sectional selection pulse to the slice axis gradient coil 131 as shown in FIG. 5B, and then applies a flow rate encoding gradient pulse to the frequency axis gradient coil 133 and echoes. The reading pulses (㉱) are sequentially applied to obtain signals. Here, since the present invention is not a process of making an image, a phase encoding gradient pulse is not applied to the phase axis gradient coil 135.

Accordingly, an echo signal is sampled through the receiving coil unit 170 while frequency encoding is performed by the reading pulse.

The sampled echo signal is signal-processed through the data acquisition unit 250 and the data processing unit 310 and then displayed on the display unit 330 as shown in FIG. 5B.

)를 계산한다. Accordingly, the data processor 310 confirms the peak position of the echo signal obtained by applying the velocity encoding gradient magnetic field, and applies the reference position and the velocity encoding gradient magnetic field of the echo signal obtained above. Between the peak positions (echo) of the actual echo signals obtained by

Calculate

Here, when the actual position of the echo signal obtained by applying the velocity encoding gradient magnetic field is on the left side (when the phase is faster) than the reference position, the data processor 310 inclines the frequency axis as shown in FIG. 5C. Among the flow rate encoded gradient pulses applied to the coil 133, the amplitude of the pulse ㉯-1 corresponding to the + pole is adjusted to be smaller. Of course, if the actual position of the echo signal is out of phase than the reference position, the amplitude of the pulse (㉯-2) corresponding to the minus pole of the velocity-encoded gradient pulses may be adjusted, but in principle the area is increased. A large increase is not good in terms of eddy current generation.

On the other hand, when the actual position of the echo signal obtained by applying the velocity encoding gradient magnetic field is on the right side (when the phase is later) than the reference position, of the velocity encoding gradient pulses applied to the frequency axis gradient coil 133- Adjust the amplitude of the pulse (# -2) that corresponds to the pole to be smaller or the amplitude of the pulse that corresponds to the + pole of the velocity-encoded gradient pulses.

The adjusted pulse sequence is stored in the memory 315 of the data processor 310. Here, the adjustment of the velocity encoding gradient pulse may be manually changed and set by the user through a GUI screen for changing the pulse sequence displayed on the display unit 330. The GUI screen for changing the pulse sequence may be linked with automatic correction software, and such automatic correction software may be stored in the memory 315 of the data processor 310.

That is, the data processor 310 moves the position of the echo signal while automatically tuning the specified velocity encoding gradient magnetic field by a predetermined size in order to make the peak position of the echo signal changed by the velocity encoding gradient magnetic field equal to the reference position.

This tuning process changes the velocity encoding gradient magnetic field to a specified amount of change, and performs the automatic correction while repeatedly comparing the peak position of the obtained echo signal with the reference position.

5A to 5C are repeated for each of the frequency axis gradient coils in the x-axis, y-axis, and z-axis. The auto-correction values obtained for each of these axes are stored, and the actual velocity encoding gradient magnetic field is stored. When it is applied to the relevant axis, the corresponding correction value is applied and applied. That is, the method as shown in Figs. 5A to 5C is performed once with the x axis as the frequency axis, the method as shown in Figs. 5A to 5C is executed once with the y axis as the frequency axis, and the frequency axis is shown as Fig. 5A. To the method shown in FIG. 5C once.

6A to 6E are diagrams illustrating a phase error correction method according to another embodiment of the present invention.

6A and 6B illustrate a method of generating reference data for phase error correction, and FIGS. 6C through 6E illustrate actual data generation and correction methods for phase error correction.

6A to 6E may be performed in the second order for more precise and accurate correction of the peak position of the echo signal after performing the phase error correction as the first method in the same manner as FIGS. 5A to 5C. Of course, the correction method as shown in Figs. 5A to 5C and the correction method as shown in Figs. 6A to 6E may be executed independently of each other.

In addition, instead of applying all of the velocity encoding gradient pulses simultaneously on three axes (x-axis, y-axis, and z-axis), the method as shown in FIGS. The method as shown in Figs. 6A to 6E is performed once with the axis as the frequency axis, and the method as shown in Figs. 6A to 6E is executed once with the z axis as the frequency axis.

As a result, the flow rate encoded gradient pulses are tuned only in the frequency encoding direction, i.e., the direction in which the echo signal is obtained. If you do this for each axis, you will know the tuning value of the velocity coded pulses for all axes.

First, the test subject 1 is installed in the photographing space of the magnetic field part 100, and then an imaging region for the subject 1 as shown in FIG. 6A is used to select an RF pulse and a cross section. It is excited by the cross section selection pulse of the slice axis gradient coil 131.

After the cross section selection pulse of the slice axis gradient coil 131 is applied, a reading pulse for obtaining an echo signal is successively applied to the frequency axis gradient coil 133. Here, since the present invention is not a process of making an image, a phase encoding gradient pulse is not applied to the phase axis gradient coil 135.

The echo signal is sampled through the receiving coil while frequency encoding is performed by the reading pulse. The frequency axis gradient coil may be any one of an x axis, a y axis, and a z axis gradient coil.

That is, the gradient coil driver 210 applies a pulse sequence except for the velocity encoded gradient pulse and the phase encoded gradient pulse to the three gradient coil units 130 including the frequency axis gradient coil 133. That is, as shown in FIG. 6A, the gradient coil driver 210 applies the section selection pulse to the slice axis gradient coil 131 and then applies the echo signal to the frequency axis gradient coil 133 without applying the flow rate encoding gradient pulse. Only reading pulses to obtain are applied. Accordingly, an echo signal is sampled through the receiving coil unit 170 while frequency encoding is performed by the reading pulse.

The sampled echo signal is transmitted to the data processor 310 through the data acquirer 250.

The data processor 310 performs a fast fourier transform (FFT) on the obtained echo signal as shown in FIG. 6B to obtain a profile of the test subject 1 (phantom).

The data processor 310 obtains reference phase data by acquiring a phase corresponding to each pixel using complex data of the profile, accumulating the obtained phases, and obtaining the obtained reference phase data. Is stored in the memory 315 and converted into a graph for display.

Subsequently, the gradient coil driver 210 applies a section selection pulse to the slice axis gradient coil 131 as shown in FIG. 6C, and then applies a flow rate encoding gradient pulse to the frequency axis gradient coil 133 and echoes the echo. The reading pulses (㉱) are sequentially applied to obtain signals. Here, since the present invention is not a process of making an image, a phase encoding gradient pulse is not applied to the phase axis gradient coil 135.

Accordingly, an echo signal is sampled through the receiving coil unit 170 while frequency encoding is performed by the reading pulse.

The sampled echo signal is transmitted to the data processor 310 through the data acquirer 250.

The data processor 310 performs a fast fourier transform (FFT) on the obtained echo signal to obtain a profile of the test subject 1 (phantom) as shown in FIG. 6D.

The data processor 310 obtains phase data by acquiring a phase corresponding to each pixel by using complex data of the profile, accumulating the obtained phase, and obtaining the obtained actual phase data. Store in memory. Here, the phase data is shown to increase with a predetermined slope as the phase of each pixel of the profile accumulates, and the horizontal portions positioned before and after the phase data mean noise.

또는

)가 있을 수 있다.The deviation between the reference phase data obtained by FIGS. 6A and 6B and the actual phase data obtained by FIGS. 6C and 6D

or

There may be).

Subsequently, the data processor 310 repeatedly performs the process of FIGS. 6C and 6D while changing the magnitude of the magnetic flux coding gradient magnetic field (㉯-2) of the -pole gradually according to the preset value stored in the memory. . Here, only the magnitude of the velocity encoding gradient pulse of the + pole or the-pole may be changed when the flux encoding gradient magnetic field is changed, but the anode may be mixed.

The data processor 310 acquires phase data according to the magnitude of the flux encoding gradient magnetic field while repeating the processes of FIGS. 6C and 6D while changing the area of the flux encoding gradient magnetic field as shown in FIG. 6E, and stores the obtained phase data as a memory. Store in

Subsequently, the data processor 310 compares the slopes of the actual phase data obtained by applying the velocity encoding gradient magnetic fields of various sizes and the stored reference phase data, and generates a velocity encoding gradient magnetic field having the phase data most similar to the reference phase data. Once selected, it will be stored in memory.

Such a tuning process changes the flux encoding gradient magnetic field by a predetermined amount of change, and performs the automatic correction while repeatedly repeating the process of comparing the phase data of the obtained echo signal with the reference phase data. That is, the scan to which the velocity encoding gradient magnetic field is applied is applied while changing the velocity encoding gradient magnetic field of a predetermined size little by little each time. When the actual phase data is almost the same as the reference phase data or the error is minimal, the set velocity coding gradient pulse is stored as the final pulse sequence.

In this manner, the phase error when the x axis is the frequency encoding axis is corrected.

Subsequently, the same method as in FIGS. 6A to 6E is repeated using the y-axis as the frequency encoding axis, thereby correcting a phase error when the y-axis is the frequency encoding axis.

6A to 6E are repeated by using the z-axis as the frequency encoding axis, thereby correcting a phase error when the z-axis is the frequency encoding axis.

That is, each axis is sequentially set as a frequency encoding axis to correct phase error at each axis.

Therefore, in the present invention, if the phase data is calculated using the echo signal obtained by applying the velocity encoded gradient magnetic field after calculating the reference phase data using the echo signal obtained without the velocity encoded gradient magnetic field, the phase error caused by the velocity encoded gradient magnetic field is calculated. You can get it. Fine tuning of the flux-encoded gradient magnetic field in the direction of minimizing the phase error thus obtained can improve image quality and improve the accuracy of blood flow velocity measurement in the phase contrast angiography technique.

According to the present invention, if there is no phase error externally and internally in the process of acquiring the signal, and if the object to be photographed is a stationary object having no flow velocity, the echo signal is applied both before and after the application of the velocity encoding gradient magnetic field of opposite polarity and the same area. The principle was that the peak positions should remain the same.

The present invention has been described with reference to the preferred embodiments, and those skilled in the art to which the present invention pertains to the detailed description of the present invention and other forms of embodiments within the essential technical scope of the present invention. Could be. Here, the essential technical scope of the present invention is shown in the claims, and all differences within the equivalent range will be construed as being included in the present invention.

1 is a diagram illustrating an example of a pulse sequence for a two-dimensional scan that is generally executed in the MRI system.

2 is a diagram illustrating an example of a pulse sequence for a three-dimensional scan that is generally executed in the MRI system.

3A and 3B are diagrams for explaining a phase error of an echo signal due to a general gradient magnetic field.

4 is a schematic diagram showing an MRI system applied to the present invention.

5A to 5C are diagrams for explaining a phase error correction method according to an embodiment of the present invention.

6A to 6E are diagrams for explaining a phase error correction method according to another embodiment of the present invention.

6A and 6B illustrate a method of generating reference data for phase error correction according to an embodiment of the present invention.

6C to 6E illustrate actual data generation and correction methods for phase error correction according to an embodiment of the present invention.

* Explanation of symbols for the main parts of the drawings

100: magnetic field 110: magnet portion

130: gradient coil unit 131: slice axis gradient coil

133: frequency axis gradient coil 135: phase axis gradient coil

150: RF coil unit 170: receiving coil unit

210: gradient coil driver 230: RF coil driver

250: data acquisition unit 270: sequence control unit

310: data processing unit 330: display unit

350: command input unit

Claims (12)

Applying a pulse sequence excluding the flow rate coded gradient pulse and the phase coded gradient pulse to the plurality of gradient coils; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and detecting and storing a reference peak position of the obtained echo signal; Applying a pulse sequence determined to the gradient coil after acquiring a reference peak position of the echo signal, but applying a flow rate encoded gradient pulse only to the frequency axis gradient coil; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence and detecting a peak position of the obtained actual echo signal; And And automatically tuning the flow rate encoded gradient pulse in a direction to minimize the phase error by comparing the peak position of the obtained echo signal and the stored reference peak position with each other. 2. The method of claim 1, And a phase error correction method of sequentially changing the plurality of gradient coils to a frequency axis and correcting phase errors with respect to the changed frequency axis. The method of claim 1, The peak position of the echo signal, the phase error correction method of the MRI system detects through the intensity of the echo signal and the position of the pixel on which the echo signal is displayed. The method of claim 1, After correcting the phase error using the peak position of the echo signal, Applying a pulse sequence excluding the flow rate coded gradient pulse and the phase coded gradient pulse to the plurality of gradient coils; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and acquiring and storing reference phase data using the obtained echo signal; Applying a predetermined pulse sequence to the gradient coil after acquiring the reference phase data, but applying only the velocity encoded gradient pulse to the frequency axis gradient coil; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and acquiring actual phase data using the obtained echo signal; And automatically tuning the flow rate encoded gradient pulse in a direction to minimize phase error by comparing the obtained actual phase data and stored reference phase data with each other. 2. The method of claim 1, A method of correcting a phase error of an MRI system that automatically tunes only the magnitude of a flow rate encoded gradient pulse of a -pole when automatically tuning the flow rate encoded gradient pulse. The method of claim 1, If the peak position of the obtained echo signal is faster than the reference peak position, the magnitude of the velocity encoding gradient pulse of + pole is adjusted, and if the peak position of the acquired echo signal is later than the reference peak position, the velocity encoding gradient pulse of -pole is obtained. Phase error correction method of MRI system to tune the size of. The method of claim 6, The phase is delayed when the + -pole velocity-velocity gradient pulses are tuned small, and when the -pole velocity-coded gradient pulses are tuned small, the phase is accelerated. Applying a pulse sequence excluding the flow rate coded gradient pulse and the phase coded gradient pulse to the plurality of gradient coils; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and acquiring and storing reference phase data using the obtained echo signal; Applying a predetermined pulse sequence to the gradient coil after acquiring the reference phase data, and applying a flow rate encoded gradient pulse of a predetermined magnitude to the frequency axis gradient coil; Acquiring an echo signal through a receiving coil according to the application of the pulse sequence, and acquiring actual phase data using the obtained echo signal; Acquiring an echo signal while varying a magnitude of the velocity encoding gradient pulse according to a predetermined value, and obtaining actual phase data of the obtained echo signal; And Comparing the obtained actual phase data with the stored reference phase data and selecting a flow rate encoded gradient magnetic field having the phase data most similar to the reference phase data to automatically tune the flow rate encoded gradient pulses; Phase error correction method. The method of claim 8, And a phase error correction method of sequentially changing the plurality of gradient coils to a frequency axis and correcting phase errors with respect to the changed frequency axis. The method of claim 8, The acquiring of the reference phase data may include: obtaining a profile of the subject by performing fast Fourier transform (FFT) on the obtained echo signal; And obtaining reference phase data by obtaining a phase of each pixel using the complex data of the profile and accumulating the obtained phases. The method of claim 8, The acquiring of the actual phase data may include: obtaining a profile of the subject by performing fast Fourier transform (FFT) on the obtained echo signal; And obtaining actual phase data by obtaining a phase of each pixel using the complex data of the profile and accumulating the obtained phases. The method of claim 8, The phase error correction method of the MRI system for tuning only the magnitude of the flow rate-coded gradient pulse of the -pole when varying the flow rate-coded gradient pulse.

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