JPH05253204A - Mri photographing device - Google Patents

Abstract

PURPOSE:To generate a good restructured image by easily performing the system adjustment which otherwise requires to complicated operations. CONSTITUTION:Peak position of a plurality of echo signals due to inversion of the lead-out gradient magnetic field 28 is sensed in the condition that the phase encode gradient magnetic field is not impressed equivalently, and dislocation times t1, t2,... tn from the regular peak position are determined, and with these values the start timing for each AD sampling in data collection is controlled to generate accurate collection of echo signals. Thus the data collection is made, wherein the peak position of the echo signals lies always in the center of AD sampling, without being influenced by the waveform of the lead-out gradient magnetic field, so that no complicated system adjustment is required, and an accurately restructured image can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI (nuclear magnetic resonance) imaging apparatus, and more particularly to an MRI imaging apparatus for measuring image data in a subject at an ultrahigh speed.

[0002]

2. Description of the Related Art As an ultra-high-speed imaging method using an MRI apparatus, Journal of Physics, C: Solid
State Physics 10, L55, 1977 (J.
Phys. C: Solid State Phys. ，
10, L55, 1977), the echo planner method first proposed and its modification are proposed. This method continuously generates an echo signal by applying a readout gradient magnetic field whose polarity is reversed at high speed from nuclear magnetization excited by a high-frequency pulse, and collects data necessary for image reconstruction in a few + ms. , Which enables ultra-high-speed shooting. As the read-out gradient magnetic field, a method using a rectangular wave (pulse waveform) and a method using a sine wave have been proposed.

In order to perform accurate image reconstruction in such ultra-high-speed imaging, it is necessary to collect data in which the peak positions of a plurality of echo signals are accurately matched, but in an actual device, the gradient magnetic field generating coil is used. Since it is impossible to generate an ideal read-out gradient magnetic field due to the characteristics and the characteristics of the driving power supply that drives the characteristics, the peak position of the echo signal deviates from the normal position.

Therefore, conventionally, it is necessary to finely adjust the amplitude of the readout gradient magnetic field and the application timing in order to match the peak positions of a plurality of echo signals, and these adjustments require a great deal of labor.

As a method for solving this problem, at least one of the switching timing of the read-out gradient magnetic field, the positive and negative amplitude values, and the offset is disclosed in Japanese Patent Laid-Open No. 64-86959. However, this method is difficult to apply when a readout gradient magnetic field is applied in a sinusoidal shape.

[0006]

In the conventional ultra-high speed MRI imaging apparatus, an ideal readout gradient magnetic field cannot be obtained due to waveform distortion of the readout gradient magnetic field and the characteristics of this driving power source. There is a problem that a complicated and labor-intensive system adjustment for accurately adjusting the peak position of P is required.

The present invention provides an MRI imaging apparatus capable of easily performing the above-mentioned system adjustment without being influenced by the applied waveform of the read-out gradient magnetic field in the ultra-high-speed MRI imaging apparatus and thereby obtaining a good reconstructed image. The purpose is to

[0008]

DISCLOSURE OF THE INVENTION The object of the present invention is to achieve ultra-high speed M.
The RI imaging apparatus detects and stores the peak positions of a plurality of echo signals obtained when the phase encode gradient magnetic field is not applied or when the phase encode gradient magnetic field is canceled, and based on the detected and stored peak positions. This is achieved by controlling the data collection start timing of the data collection means based on the deviation time from the regular peak position.

[0009]

[Function] When the pulse sequence of the ultra-high-speed imaging method is executed in a state in which the phase encode gradient magnetic field is not applied or the phase encode gradient magnetic field is canceled, a plurality of echoes of similar shape are generated due to positive and negative fluctuations of the readout gradient magnetic field. The signal is obtained. If the peak positions of the echo signals are detected and stored, and the start time of data sampling in data collection is automatically adjusted based on the detected and stored peak position information, the data centered on the peak positions of the echo signals Sampling will always be possible and an accurate reconstructed image will be obtained.

[0010]

The present invention will be described in detail below with reference to the drawings of the embodiments.

FIG. 1 is a view showing the arrangement of an MRI apparatus according to an embodiment of the present invention. In FIG. 1, the static magnetic field magnet 1 generates a uniform static magnetic field. The gradient magnetic field coil 2 is driven by a drive circuit 6 controlled by a sequencer 8 and a power supply 7, and applies a desired gradient magnetic field to the subject 11 on the bed 12 in three orthogonal directions (x, y, z directions). Occur. A high-frequency signal is transmitted / received to / from the subject 11 by the probe 3 from the transmitting / receiving unit 4 controlled by the sequencer 8, and a high-frequency magnetic field is applied and a magnetic resonance signal is received. The signal received by the probe 3 is amplified and detected by the transmission / reception unit 4, and the sequencer 8
It is digitized by the AD converter 5 controlled by and is sent to the computer 9. The computer 9 controls the sequencer 8 and performs Fourier transform of the data input from the AD converter 5 to reconstruct an image and display the image on the display 10.

FIG. 2 is a diagram showing an embodiment of a pulse sequence for ultra-high speed imaging executed in the present invention. Here, gradient magnetic fields (x, y, z) that are orthogonal to each other in three directions
In S, the magnetic field applied for slicing is denoted by Gs, the magnetic field applied for phase encoding is denoted by Ge, and the magnetic field applied for readout is denoted by Gr. This pulse sequence is controlled and executed by the sequencer 8 via the computer 9.

First, a high frequency pulse (RF) 21 and a slice gradient magnetic field (Gs) 22 are applied to selectively excite a region of interest to be measured. Next, the read-out gradient magnetic field (Gr) 25 is applied in the direction parallel to the slice plane while rapidly inverting the amplitude, and at the same time, the phase-encoding gradient magnetic fields (Ge) 23 and 24 are applied.
It is applied in a pulsed manner at every inversion of the readout gradient magnetic field (Gr) in the direction orthogonal to r). According to such a method, an echo signal is continuously generated every time the total amount (area) of the amplitude of the readout gradient magnetic field (Gr) and the application time becomes positive or negative, and as shown by AD. Data can be collected one after another, enabling ultra-high-speed shooting.

In the data collection shown by AD here,
The AD converter 5 is controlled by the sequencer 8 so that the sampling start timing of each AD conversion that AD-converts a plurality of echo signals becomes a correct timing. This method will be described in detail below.

In the present invention, before executing the pulse sequence shown in FIG. 2, a pulse sequence in which only the phase encode gradient magnetic field Ge of FIG. 2 is not applied or the phase encode gradient magnetic field Ge shown in FIG. The pulse sequence in which the canceling gradient magnetic fields such as the phase encoding gradient magnetic fields 26 and 27 are sequentially added is executed in advance. In this case, the phase encoding gradient magnetic field Ge
, The gradient magnetic field pulses 23 and 26 are applied so as to have the same area and opposite signs.
Similarly, 27 and 27 are applied. This is equivalent to the case where no phase encoding gradient magnetic field Ge is applied. FIG. 3 shows the relationship between the readout gradient magnetic field, the echo signal, and the sampling of the AD converter in this case. Figure 3
, The amplitude of the echo signal is attenuated by the time constant of the transverse relaxation time, and a peak occurs at the time when the area of the positive waveform and the area of the negative waveform of the readout gradient magnetic field (Gr) 28 become equal. If the read-out gradient magnetic field (Gr) 28 is an ideal rectangular wave, the peak position of the echo signal will be an intermediate position between the positive and negative periods. The waveform becomes distorted from the ideal waveform due to the characteristics of the power supply. As a result, the peak positions of the echo signals are t1, t2, ...
As indicated by tn, the position deviates from the normal position and the peak position changes for each echo signal. In order to perform correct image reconstruction, it is necessary to perform sampling so that the peak position of this echo signal becomes the center of AD sampling. Therefore, as shown by AD1 in FIG.
Correct sampling is impossible if the sampling start times s1, s2, ... Sn are set at equal intervals. In order to solve this, the peak position of the echo signal is detected, and the deviation time t1, t2 from the normal peak position is detected.
··· Tn is obtained, and the start time of each AD sampling is shifted by this time, and ss1 and ss shown in AD2 of FIG.
Correct sampling can be performed by AD sampling such as 2 ... ssn.

The detection of the peak position of the echo signal and the calculation of the shift time are performed by software processing in the computer 9 and stored in the memory. The same thing may be performed by other hardware means. When the sequence of FIG. 2 described above is executed with the shift time stored in this way, the AD converter 5 is controlled by the computer 9 and the sequencer 8 and the sampling start timing is correctly controlled.

Here, various modifications can be made as the method of calculating the deviation time. For example, an arbitrary number of echo signals may be selected from a plurality of echo signals, and the average value of the deviation times of the selected echo signals may be used as the deviation time of each echo signal. The shift time obtained by interpolation from the echo signal may be used as the shift time of each echo signal.

FIG. 4 is a diagram showing another embodiment of the pulse sequence for ultrahigh-speed imaging executed in the present invention. The pulse sequence of FIG. 2 is different from that of FIG. 2 only in that the applied waveform of the readout gradient magnetic field (Gr) 31 is sinusoidal. The above-described control method of the AD sampling start timing can be applied to this sequence as it is. In this case, if a sine wave is used for the read-out gradient magnetic field, the sine wave has no harmonic component of the waveform as compared with the rectangular wave, and there is almost no waveform distortion due to the gradient magnetic field generating coil or its driving power source Can occur. Therefore, in FIG. 3, since the read-out gradient magnetic field (Gr) 28 is changed from a distorted rectangular wave to a sine wave close to an ideal, the time shifts t1 and t2 from the normal position of the peak position of the echo signal are obtained.
.. tn becomes a small amount, and even in the case of a rectangular wave, there is no case where the amount of time lag is large and the sampling start timing of AD cannot be controlled, and the sampling start timing of AD is easily controlled.

FIG. 5 is a diagram showing another embodiment of the pulse sequence for ultrahigh-speed imaging executed in the present invention. First, 90 ° high frequency pulse 41 and slice gradient magnetic field 4
3 is applied to selectively excite the region to be photographed, and then 180
A high frequency pulse 42 and a slice gradient magnetic field 44 are applied to invert the magnetization. After that, the sequence is the same as the phase encoding gradient magnetic field Ge and the readout gradient magnetic field Gr shown in FIG. 18 in this sequence
Since the magnetization is inverted by the application of the 0 ° high frequency pulse, the effect of non-uniformity of the static magnetic field is reduced, and there is an effect that accurate image capturing can be performed. The above-described control method of the AD sampling start timing can be applied to this sequence as it is. Although a rectangular wave is used as the readout gradient magnetic field Gr in FIG. 5, the same effect and the same AD sampling start timing can be controlled by using a sine wave as the readout gradient magnetic field Gr as described above. ..

FIG. 6 is a diagram showing another embodiment for measuring the deviation time of the plurality of echo signals described above. As shown in FIG. 6, a sample 51 having a small thickness (about one picture element) in the direction of the phase encoding gradient magnetic field is used as the subject. For example, the pulse sequence shown in FIG. 2 is directly executed for this sample 51. In this case, the phase encode gradient magnetic field is applied to the regular pulse without any operation,
Since the sample is thin in the phase encode gradient magnetic field direction, the sample is not affected by the phase encode gradient magnetic field in the thickness direction, which is equivalent to a state in which the phase encode gradient magnetic field is not applied. Therefore, as described with reference to FIG. 3, the peak position of the echo signal and the deviation time from the normal peak position can be obtained and stored. This stored shift time may be used to control the AD sampling start timing when imaging is performed while changing the subject. This method has an effect that it is not necessary to add any operation to the pulse sequence.

[0021]

As described above in detail, according to the present invention, the peak position of the echo signal obtained by the sequence executed in the state where the phase encode gradient magnetic field is not applied or the phase encode gradient magnetic field is canceled And the regular peak position are memorized, and the sampling start timing of AD conversion of a predetermined sequence is controlled by the memorized time lag. Therefore, the peak position of the echo signal is always the center of AD sampling. Data collection is performed, complicated system adjustment in the ultra-high speed MRI imaging apparatus is unnecessary, and an accurate reconstructed image can be obtained. Moreover, the adjustment can be performed without being affected by the applied waveform of the readout gradient magnetic field.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an MRI apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a pulse sequence executed in the present invention.

FIG. 3 is a readout gradient magnetic field, an echo signal, and an AD without a phase encode gradient magnetic field of a pulse sequence executed in the present invention or in a canceled state.
It is a figure which shows the relationship of the sampling of.

FIG. 4 is a diagram showing another embodiment of a pulse sequence executed in the present invention.

FIG. 5 is a diagram showing another embodiment of a pulse sequence executed in the present invention.

FIG. 6 is a diagram showing another embodiment for measuring the deviation time of the echo signal executed in the present invention.

[Explanation of symbols]

1 ... Static magnetic field magnet, 2 ... Gradient magnetic field coil, 3 ... Probe,
4 ... Transceiver unit, 5 ... AD converter, 6 ... Driving circuit, 7 ... Power supply, 8 ... Sequencer, 9 ... Calculator, 10 ... Display device, 1
1 ... Subject, 12 ... Bed.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location 9118-2J G01N 24/08 Y

Claims (5)

[Claims] 1. A high-frequency magnetic field and a slice gradient magnetic field are applied to a subject placed in a static magnetic field to excite a predetermined slice plane, and then a read-out gradient magnetic field is applied in a rectangular wave shape or a sine wave shape, and at the same time, a read is performed. By applying a phase encode gradient magnetic field in a direction orthogonal to the out gradient magnetic field, a data collecting means for sampling and collecting a plurality of echo signals necessary for image reconstruction of the slice plane, and data collected by the data collecting means In an MRI apparatus including an image reconstructing means for performing a basic image reconstruction, peak positions of a plurality of echo signals obtained in a state in which the phase encode gradient magnetic field is not applied or the phase encode gradient magnetic field is canceled Detecting and storing the peak position detecting and storing means, and the peak position detecting and storing means. MRI imaging apparatus characterized by comprising a control means for have not a group to control the data sampling start timing of said data acquisition unit to the peak positions. 2. A 90 ° high-frequency magnetic field and a slice gradient magnetic field are applied to a subject placed in a static magnetic field to excite a predetermined slice plane, and further a 180 ° inverted high-frequency magnetic field and a slice gradient magnetic field are applied, By applying the read-out gradient magnetic field in the form of rectangular wave or sine wave, and by applying the phase-encoding gradient magnetic field in the direction orthogonal to the read-out gradient magnetic field, multiple echo signals necessary for image reconstruction of the slice plane are sampled. MRI including data collecting means for collecting and image reconstructing means for reconstructing an image based on the data collected by the data collecting means
In the imaging apparatus, peak position detection storage means for detecting and storing peak positions of a plurality of echo signals obtained in a state in which the phase encode gradient magnetic field is not applied or a state in which the phase encode gradient magnetic field is canceled, and the peak position detection storage And a control unit for controlling the data sampling start timing of the data collection unit based on the peak position detected and stored by the unit. 3. The peak position detection recording means detects peak positions of a selected arbitrary number of echo signals among a plurality of echo signals in a state in which a phase encode gradient magnetic field is not applied or a phase encode gradient magnetic field is canceled. Claim 1 or Claim 2 which detects and memorizes.
The described MRI imaging apparatus. 4. A high-frequency magnetic field and a slice gradient magnetic field are applied to a subject placed in a static magnetic field to excite a predetermined slice plane, and then a read-out gradient magnetic field is applied in a rectangular wave shape or a sine wave shape, and the read By applying a phase encode gradient magnetic field in a direction orthogonal to the out gradient magnetic field, a data collecting means for sampling and collecting a plurality of echo signals necessary for image reconstruction of the slice plane, and data collected by the data collecting means In an MRI apparatus equipped with an image reconstructing means for performing a basic image reconstruction, the sample was obtained in the state of applying a phase encode gradient magnetic field using a thin sample in which the phase encode gradient magnetic field is negligible in the phase encode direction. Peak position detection storage means for detecting and storing peak positions of a plurality of echo signals; Detected by means, MRI imaging apparatus characterized by and have not a group of the stored peak position with a control means for controlling the data sampling start timing of the data acquisition means. 5. The peak position detecting means selects one of a plurality of echo signals obtained in a state in which a phase encode gradient magnetic field is applied by using a sample thin enough to ignore the phase encode gradient magnetic field in the phase encode direction. The MRI apparatus according to claim 4, wherein the peak positions of the arbitrary number of echo signals are detected and stored.