JPH08266505A - Mr imaging system - Google Patents

Abstract

PURPOSE: To perform fast image pickup while correcting the influence of motion by generating a navigator echo for phase measurement in a time area in which the time integral capacity of a gradient magnetic field for phase encoding goes to zero and reconfiguring an image by correcting data by finding the phase of data of the navigator echo. CONSTITUTION: A signal obtained by phase detection is sampled by an A/D converter 43 controlled by a sequence controller 52, and converted to digital data. The data obtained from the A/D converter 43 is fetched in a computer 51, and processing to reconfigure the image from collected digital data is performed. In such a case, the navigator echo for phase measurement is generated to find the phase of data of the navigator echo in the time area in which the time integral capacity of the gradient magnetic field for phase encoding goes to zero, then, the image is reconfigured by correcting the data for image reconfiguration by the phase.

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 performing imaging using the NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus for performing high-speed imaging (data acquisition necessary for image reconstruction). .

[0002]

2. Description of the Related Art Conventionally, a high-speed spin echo method, a high-speed gradient spin echo method, an echo planar method, a spiral scan method and the like are known as high-speed imaging methods for MR imaging devices. With these high speed imaging methods,
A plurality of echo signals are generated by one excitation to reduce the number of repetitions, thereby shortening the data acquisition time.
As a method for correcting the movement of the subject, a gradient moment nulling method is known as a fast spin echo method (Japanese Patent Laid-Open No. 22922/1994).

[0003]

However, in the conventional high-speed imaging method, when the subject moves during the repetition of the pulse sequence, a special periodicity is generated in the K space (raw data space) and reconstruction is performed. However, there is a problem that an artifact is generated in the captured image. According to the conventional method of gradient moment nulling, the action of the gradient magnetic field differs depending on the movement of the subject in the gradient direction of the gradient magnetic field (the direction in which the magnetic field strength is inclined, not the direction of the magnetic field). Therefore, it corrects that the amount of phase change does not become as designed, and it is possible to correct the effect of motion while the gradient magnetic field is being applied, but it corrects the effect of motion between repetitions. You can't do it.

In view of the above, an object of the present invention is to provide an MR imaging apparatus improved so that a high-speed imaging method can be performed while correcting the influence of motion during repetition of a pulse sequence. .

[0005]

In order to achieve the above object, in the MR imaging apparatus according to the present invention,
An RF applying means for generating an excitation pulse, a means for applying a gradient magnetic field pulse for slice selection, a means for applying a gradient magnetic field pulse for phase encoding, a means for applying a gradient magnetic field pulse for reading, and an echo signal for receiving. , Means for obtaining data by performing A / D conversion by sampling after phase detection, and controlling the RF applying means to apply one excitation pulse and at the same time controlling the gradient magnetic field pulse applying means for slice selection After the slice selection gradient magnetic field pulse is applied, the readout gradient magnetic field pulse application means is controlled to apply the readout gradient magnetic field pulse and the phase encoding gradient magnetic field pulse application means is controlled to perform the phase encoding gradient magnetic field pulse. Is applied to generate multiple echoes that collect data for image reconstruction and phase encoding. The navigator echo for phase measurement is generated in the time domain where the time integration amount of the gradient magnetic field for use is 0, the phase of the data of the navigator echo is obtained, and the data for image reconstruction is corrected by this phase to reconstruct the image. It is characterized by having a calculation / control means for

The time domain in which the navigator echo for phase measurement is generated is the excitation pulse and the first refocusing pulse in a pulse sequence for obtaining a plurality of spin echoes by adding a plurality of refocusing pulses after one excitation pulse. It can also be a time domain between and.

Further, in the above time domain, the readout gradient magnetic field pulse may be inverted to refocus and generate the navigator echo.

[0008]

When the subject moves during the repetition, a phase change appears in the generated echo. This phase change appears the same for multiple echoes if they occur within one repetition time. The navigator echo has a time-integrated value of the phase encoding gradient magnetic field of 0.
Is generated in the time domain. In other words, the navigator echo is always applied with the same slice selection gradient magnetic field pulse and readout gradient magnetic field pulse in every repetition, and if the phase of this navigator echo is observed, it is This means that the phase change of the echo that collects the data for image reconstruction is captured. Therefore, if the phase of the data of this navigator echo is obtained and the data for image reconstruction is corrected by this, the phase error between repetitions due to the movement of the subject can be corrected. An image with no artifacts can be obtained by performing image reconstruction using the data of 1.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In the MR imaging apparatus according to the embodiment of the present invention, a pulse sequence as shown in FIG. 1, for example, is repeated. That is,
FIG. 1 shows a time chart of one repetition time (TR). An MR imaging apparatus that performs such a pulse sequence is configured as shown in FIG.

First, the structure of this MR imaging apparatus will be described. Referring to FIG.
Includes a main magnet for generating a static magnetic field and a gradient magnetic field coil for generating a gradient magnetic field that is superimposed on the static magnetic field. The gradient magnetic field is generated by the gradient magnetic field coil in X, Y,
It is generated as the magnetic field strengths are respectively inclined in the three Z directions. By selecting one of the gradient magnetic fields in the three-axis directions or by combining them, an arbitrary gradient magnetic field in the three-axis directions is created, and these gradient magnetic fields Gs for slice selection and readout (and frequency encoding) described later The gradient magnetic field Gr and the phase encoding gradient magnetic field Gp are used.

A subject (not shown) is placed in the space to which the static magnetic field and the gradient magnetic field are applied. For this subject,
An RF coil 12 is attached for irradiating the subject with RF pulses and for receiving the NMR signal generated in the subject.

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. The waveform signal from the waveform generation circuit 53 is sent to the magnetic field control circuit 21. Information about each pulse waveform of the gradient magnetic fields Gs, Gp, and Gr is set in the waveform generation circuit 53 from the computer 51 in advance. A waveform signal is generated from the waveform generation circuit 53 at a timing instructed by the sequence controller 52 and is sent to the magnetic field control circuit 21, whereby the gradient magnetic fields Gs, G having a pulse shape as shown in FIG.
p and Gr will be generated respectively.

The RF signal generated by the RF oscillation circuit 31 is sent to the amplitude modulation circuit 32, which becomes a carrier signal,
Amplitude modulation is performed according to the waveform signal sent from the waveform generation circuit 53. The RF signal after the amplitude modulation is applied to the RF coil 12 after being amplified by the RF power amplifier 33. The oscillation frequency of the RF oscillation circuit 31 is controlled by the computer 51. Information about the waveform of the above-mentioned modulated signal is given from the computer 51 to the waveform generating circuit 53 in advance. The timing of the waveform generation circuit 53 and the RF oscillation circuit 31 is determined by the sequence controller 52.

NMR received by RF coil 12
The signal passes through the preamplifier 41 and is sent to the phase detection circuit 42 for phase detection. The RF signal from the RF oscillation circuit 31 is sent as a reference signal for this 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 converted into digital data. The data obtained from the A / D converter 43 is taken into the computer 51.
The computer 51 performs processing such as reconstructing an image from the collected digital data. In addition, the computer 51, as described above, responds to the pulse sequences that make up various imaging sequences.
2 and necessary data is set in the waveform generation circuit 53, and the RF oscillation circuit 31 is controlled to determine its frequency,
Further, the preamplifier 41 and the phase detection circuit 42 are controlled to determine their gains, and the A / D converter 43 is further controlled.

In such an MR imaging apparatus, a computer 51 and a sequence controller 52
Under the control of, the pulse sequence as shown in FIG. 1 is repeated. In the example shown in FIG. 1, first, one 90 ° pulse (excitation pulse) is applied, and at the same time, a pulse for the slice selection gradient magnetic field Gs is added, and then a plurality of (here, four) 180 ° pulses ( The refocus pulse) is sequentially added together with the Gs pulse.

The Gr pulse for reading is added between the 90 ° pulse and the first 180 ° pulse to reverse the polarity, and then added after each 180 ° pulse, thereby aligning the phases and performing frequency encoding. , Gradient echo and four spin echoes are generated. This gradient echo will be called a navigator echo. In the example shown in this FIG.
The number of spin echoes has been obtained, and these are sampled and A / D converted to collect data. Therefore, a pulse of the gradient magnetic field for phase encoding Gp is added.
By setting the magnitude of this Gp pulse to be different for each of the first to fourth spin echoes, data of four different phase encoding steps are obtained during one iteration. As a result, for example, in order to reconstruct an image of 256 matrices, it is sufficient to repeat 256/4 = 64 times. It should be noted that after each occurrence of the first to fourth spin echoes, Gp added before the first to fourth spin echoes
A Gp pulse for rewinding is added to return the phase encoding amount of the pulse to zero after the echo is generated.

Then, for example, the Gp pulse before the first spin echo is changed in a region having a large absolute value, and the Gp pulse before the second spin echo has an absolute value larger than that before the first spin echo. The Gp pulse before the third spin echo is changed in a small region, and the Gp pulse before the fourth spin echo is changed in a region having the smallest absolute value. Therefore, a smaller phase encode amount is given to these first to fourth spin echoes as the number increases. Therefore, the data obtained from the first spin echo is shown in FIG.
As shown in (2), it is arranged at the end in the phase direction (Kp direction) of the K space (raw data space).
The data obtained from the fourth spin echo are sequentially arranged from the end side in the phase direction of the K space to the center side as shown in FIG. The data obtained from the fourth spin echo is arranged in the center of the K space.

As described above, in addition to the first to fourth spin echoes for collecting the data arranged on the K space for image reconstruction, as described above, for the phase encoding before the first 180 ° pulse. In the time region in which the time integration amount of the gradient magnetic field Gp is 0, the gradient magnetic field Gr for reading is inverted to cause a gradient echo (navigator echo).
Is generated, and a sampling pulse is also applied to this to perform A / D conversion to collect data. Then, the phase θ of the navigator echo data is calculated as

[Equation 1] Calculate with. Here, Rn is a 0 ° component of the navigator echo data, and In is a 90 ° component. This phase θ
Is calculated for all data or representative data in the time axis direction of the navigator echo data. Using the phase θ thus obtained,

[Equation 2] As described above, the phase of the data collected from each spin echo is corrected. In this equation 2, R and I are 0 ° and 9 before correction.
The 0 ° component and R ′ and I ′ are the corrected 0 ° and 90 ° components. Two-dimensional Fourier transform or the like is performed using the corrected data to reconstruct the image. The computer 51 performs the calculation for obtaining these phases, the phase correction calculation, and the image reconstruction calculation.

Normally, in the pulse sequence of the fast spin echo method, processing is performed so that the phase error between repetitions of the same spin echo within one repetition time is corrected, but there is movement of the subject. In some cases, a phase error occurs even for the same spin echo between repetitions. The navigator echo as described above is generated in order to measure this phase error. No Gp pulse for phase encoding is applied to this navigator echo. That is, this navigator echo is generated in a time region where the time integral value of Gp becomes zero. In each repetition, the Gs pulse and the Gr pulse always have the same waveform. Therefore, when there is no movement in the subject, the navigator echo data obtained at each iteration is always the same, and if there is movement in the subject,
A phase change due to the influence appears in this navigator echo. The phase change in this navigator echo is
This is the same as the phase change due to the influence of the movement of the subject during the repetition in other spin echoes. Therefore, the phase of this navigator echo is calculated as described above, and by correcting the raw data for image reconstruction by that phase, it is possible to remove the influence of the motion of the subject, and to eliminate the artifacts of the reconstructed image. Can be excluded.

In this case, since the time domain before the first 180 ° pulse is not used for data collection,
This free time is used to generate a navigator echo for phase measurement. Therefore, there is no waste of time, and there is no demerit such that the imaging time is extended or the number of slices is reduced.

The above description is for one embodiment, and various modifications such as the number of spin echoes and the arrangement relationship of raw data on the K space are not limited to the above. can do. Further, in the above description, the embodiment in which the present invention is applied to the fast spin echo method is described, but the present invention is not limited to the fast spin echo method, but the echo planar method, the fast gradient echo method, the spiral scan method, etc. Of course, it can be applied to the imaging sequence of.

[0022]

According to the MR imaging apparatus of the present invention, it is possible to obtain a reconstructed image free from artifacts due to the movement of the subject. Since the navigator echo for phase measurement is generated by using the free time that is not used for data acquisition, there is no waste of time, and there are disadvantages such as extended imaging time and reduced slice count. There is no.

[Brief description of drawings]

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

FIG. 2 is a block diagram of an MR imaging apparatus according to the same embodiment.

FIG. 3 is a diagram showing a K space in the same embodiment.

[Explanation 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

Claims (1)

[Claims] 1. RF applying means for generating an excitation pulse,
A unit for applying a slice selection gradient magnetic field pulse, a unit for applying a phase encoding gradient magnetic field pulse, a unit for applying a readout gradient magnetic field pulse, and a unit for receiving an echo signal, performing phase detection and then sampling A / After the D-converting means for obtaining the data and the RF applying means for applying one excitation pulse, the slice selecting gradient magnetic field pulse applying means for controlling the slice selecting gradient magnetic field pulse Controlling the readout gradient magnetic field pulse applying means to apply the readout gradient magnetic field pulse and controlling the phase encoding gradient magnetic field pulse applying means to apply the phase encoding gradient magnetic field pulse for image reconstruction When multiple echoes for collecting data are generated and the time integration amount of the phase encoding gradient magnetic field becomes 0 A navigator echo for phase measurement is generated in a region, the phase of the data of the navigator echo is obtained, and the arithmetic / control means for correcting the data for image reconstruction by this phase and performing image reconstruction is characterized. MR imaging device.