JPH07148137A - Mr imaging device - Google Patents

Abstract

PURPOSE:To detect change with time of two-dimensional distribution of eddy current magnetic fields as well as provide an image of the two-dimensional distribution of eddy current magnetic fields by means of one-time sequence of imaging, by repeating a specified gradient echo sequence while changing a Cy pulse. CONSTITUTION:A current for gradient magnetic fields to be applied to a magnet assembly 11 is controlled by means of a magnetic field control circuit 21, to thereby generate gradient magnetic fields Gz, Gy and Gx. An gradient magnetic field pulse for generating an eddy current magnetic field is applied, and subsequently a single excitation RF pulse is applied together with the gradient magnetic field pulse Gz for selecting slices. Next, the gradient magnetic field pulse Gy for encoding phases is applied and a gradient echo sequence which generates a gradient echo signal by inverting the gradient magnetic field pulse Gx for reading is repeated many without changing the Gy pulse in a predetermined period of time. In the predetermined period of time other than mentioned above, the gradient echo sequence is repeated while changing the Gy pulse. Thus, an 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 MR imaging apparatus for imaging by utilizing an NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus having a function of imaging the state of an eddy current magnetic field.

[0002]

2. Description of the Related Art In an MR imaging apparatus, since a gradient magnetic field is applied in a pulsed manner, it is possible to avoid generation of a magnetic field based on an eddy current flowing through a peripheral conductor (especially an assembly of a static magnetic field magnet). Absent. Such an eddy current magnetic field affects the phase of spins, which causes the phase change as designed in the pulse sequence to not occur. Therefore, conventionally, the eddy current magnetic field is measured to obtain a pulse waveform that eliminates the influence thereof, and the waveform of the gradient magnetic field pulse is adjusted accordingly.

As a method of imaging (measuring) the state of this eddy current magnetic field using the MR imaging device itself,
Conventionally, the polarity of the gradient magnetic field given in advance in two imaging sequences is reversed by using the phase change of the spin due to the eddy current magnetic field after the generation of the gradient magnetic field pulse, and the phase change is obtained from the difference of the obtained data. A method of mapping on a two-dimensional image is known. For example, a method using the spin echo method as an imaging sequence (US Pat. No. 4,
910, 460) and STEAM (Te
mporal and Spatial Analysis of Fields Generated by
Currents in Superconducting Magnets: Optical of C
orrection and Quantitative Characterization of Mag
net / Gradient System, MAGNETIC RESONANCE IN MEDIC
IN: 20, 268-284 (1991)) is known.

[0004]

However, in the eddy current magnetic field imaging sequence using the conventional spin echo method, the phase change due to the eddy current magnetic field is canceled before and after the 180 ° pulse, so that the TE (echo time) used is used. However, there is a problem that the accurate mapping cannot be performed because the map obtained by) is different. Further, although the method using STEAM was conceived to solve this problem, this eddy current magnetic field imaging sequence requires the use of a phantom with a long T1 value, which is a major limitation. Furthermore, these conventional eddy current magnetic field imaging sequences have a common drawback that the imaging sequence must be repeated many times by changing the generation timing of the gradient magnetic field pulse in order to capture the temporal change of the eddy current magnetic field. .

In view of the above, the present invention can accurately image the distribution of an eddy current magnetic field on a two-dimensional plane without using a phantom having a long T1 value, and further, the eddy current can be imaged in one imaging sequence. It is an object of the present invention to provide an MR imaging apparatus improved so as to detect a temporal change in a two-dimensional distribution of a magnetic field.

[0006]

In order to achieve the above object, in the MR imaging apparatus according to the present invention,
A gradient magnetic field pulse for generating an eddy current magnetic field is generated, then one excitation RF pulse is added together with a slice selection gradient magnetic field pulse, and then a phase encoding gradient magnetic field pulse is added and the readout gradient magnetic field pulse is inverted. A gradient echo sequence for generating a gradient echo signal is repeated many times without changing the gradient magnetic field pulse waveform for phase encoding. It consists of a gradient magnetic field pulse for generating an eddy current magnetic field and a number of subsequent gradient echo sequences. The feature is that the sequence is repeated while changing the gradient magnetic field pulse waveform for phase encoding.

[0007]

When one gradient magnetic field pulse for generating an eddy current magnetic field is applied, an eddy current magnetic field is generated and gradually attenuates with the passage of time. Therefore, when a gradient echo sequence is repeated many times after applying one gradient magnetic field pulse for generating an eddy current magnetic field, each time of the eddy current magnetic field that changes with time in each gradient echo sequence. An echo signal affected by the eddy current magnetic field at is obtained. In these gradient echo sequences, the gradient magnetic field pulse waveform for phase encoding cannot be changed, so at each time point of the time-varying eddy current magnetic field,
Data for one line can be collected. Then, the sequence consisting of this eddy current magnetic field generating gradient magnetic field pulse and a number of subsequent gradient echo sequences is repeated while changing the phase encoding gradient magnetic field pulse waveform. The data for each line can be collected one after another. Therefore, if the data of each line at the same certain point in time is arranged and subjected to a two-dimensional Fourier transform, an image showing the two-dimensional distribution of the eddy current magnetic field at that point can be obtained. It will be obtained at each time point.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described in detail with reference to the drawings. An MR imaging apparatus according to an embodiment of the present invention is constructed as shown in FIG. In FIG. 1, the magnet assembly 11 includes a main magnet for generating a static magnetic field,
A gradient magnetic field coil that generates a gradient magnetic field that is superimposed on the static magnetic field is included. The gradient magnetic field is generated by the gradient magnetic field coil.
It is generated as the magnetic field strength is inclined in the directions of the three axes of X, Y, and Z. One of the gradient magnetic fields in the three-axis directions is selected, or a combination thereof is used as a gradient magnetic field for slice selection, a gradient magnetic field for reading (and frequency encoding), and a gradient magnetic field for phase encoding, which will be described later. Here, as described later, the gradient magnetic field Gz in the Z direction is used as a slice selection gradient magnetic field, the gradient magnetic field Gx in the X direction is used as a read gradient magnetic field, and the gradient magnetic field Gy in the Y direction is used as a phase encoding gradient magnetic field.

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 for irradiating a subject with an excitation RF pulse and receiving an NMR signal generated in the subject.
Is attached.

The gradient magnetic field current applied to the gradient magnetic field coil of the magnet assembly 11 is controlled by the magnetic field control circuit 21 so that each of the gradient magnetic fields Gz, Gy, Gx in the form of a pulse having a waveform as shown in FIG. 2 is generated. To be
The RF signal from the RF oscillation circuit 31 is amplitude-modulated by the amplitude modulation circuit 32, passes through the RF power amplifier 33, and then the RF coil 1
Added to 2. The waveform of each gradient magnetic field and the amplitude modulation waveform or their timings are determined by the sequence controller 52
Is determined by

The echo signal received by the RF coil 12 is sent to the phase detection circuit 42 via the preamplifier 41 and is subjected to 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. This data is taken into the computer 51. The computer 51 controls the sequence controller 52 while performing processing such as reconstructing an image from the collected digital data.

In such an MR imaging apparatus, a computer 51 and a sequence controller 52
The pulse sequence as shown in FIG. In the pulse sequence shown in FIG. 2, B,
The gradient echo sequence is performed in each period of C, ..., In the period A preceding these imaging sequences, the gradient magnetic field pulse 6 for generating the eddy current magnetic field is previously generated.
1 is applied. As the gradient magnetic field pulse 61 for generating the eddy current magnetic field, a gradient magnetic field Gy or Gx is used here.

In the gradient echo sequence in each period of B, C, ..., First, the excitation RF pulse 71 having the flip angle α ° is applied, and at the same time, the Gz pulse 72 is applied, and Z
A predetermined slice plane perpendicular to the direction is selectively excited. Thereafter, a Gy pulse 73 is added to perform phase encoding in the Y direction, and a Gx pulse 74 to be inverted is further added to generate an echo signal 75 and frequency encoding is performed in the X direction. Note that Gx pulses 76 and G added after that
The y pulse 77 and the Gx pulse 78 are for the spoiler. One such gradient echo sequence is B,
It repeats at high speed in each period of C, .... B, C, ...
The waveform of the Gy pulse 73 in each period is the same,
The same amount of phase encoding is applied.

By applying the gradient magnetic field pulse 61 for generating an eddy current magnetic field, an eddy current magnetic field 62 is generated as shown in FIG. This eddy current magnetic field 62 gradually attenuates with time. It takes about 3 seconds until the eddy current magnetic field 62 disappears completely.
Since it takes 4 seconds, the gradient echo sequence is repeated until it disappears. Raw data obtained from the echo signal generated in each period of B, C, D, E, ...
The phase is shifted by the eddy current magnetic field 62 at each echo time between each excitation RF pulse 71 and the generation of the echo signal 75. That is, the phase of the raw data reflects the eddy current magnetic field 62 at each time point. Here, since the waveform of the Gy pulse 73 is the same, the raw data of one certain line is obtained at each time point.

The eddy current magnetic field generating gradient magnetic field pulse 61 in the period A and the gradient echo sequence in the periods B, C, D, E, ...
The sequence as shown in (1) is repeated by changing only the waveform of the Gy pulse 73. As a result, raw data for each time point is obtained for each line. Of the raw data collected in this way, the same data at the same time (for period B, only for period B, for period C, only for period C, etc.) are collected and arranged by line, and two-dimensional Fourier transform is performed. An image affected by the current magnetic field 62 can be obtained at each time point.

Here, in order to extract only the influence of the eddy current magnetic field 62, after performing a certain phase encoding sequence (consisting of periods A, B, C, D, E, ...), the same phase encoding sequence is performed. , The polarity of the gradient magnetic field pulse 61 for generating the eddy current magnetic field in the period A is reversed. Then, the raw data at each time point obtained in this sequence has the same amount as the raw data in the previous sequence, but has undergone a phase shift in the opposite direction. In the pair of sequences in which the polarities of the eddy current magnetic field generating gradient magnetic field pulse 61 are opposite, when the raw data at each time point is simply subtracted, the resulting raw data is the eddy current magnetic field 62 at each time point. It only represents.

Since the eddy current magnetic field 62 is generated by the gradient magnetic field pulse 61 described above, the eddy current magnetic field 62 has a gradient, for example, in the X direction as shown by the solid line in FIG. When the polarity of the gradient magnetic field pulse 61 is reversed, the eddy current magnetic field 62 is inclined in the opposite direction as shown by the dotted line. Since the angle of the phase shift caused by the eddy current magnetic field 62 corresponds to the magnitude and the polarity, the magnitude of the eddy current magnetic field 62 differs at each position as shown in FIG.
The phase shift angle varies depending on the position. For example, assuming that the position X1 is the center and the eddy current magnetic field 62 is 0 at that position, the eddy current magnetic field 62 of one polarity and the eddy current magnetic field 62 of the opposite polarity both cause a phase difference as shown in FIG. The shift angle is 0 °. At the position X2, if the eddy current magnetic field 62 of one polarity causes a phase shift of + 45 ° as shown by the solid line in FIG. 5 (b), the eddy current magnetic field 62 of opposite polarity indicates the dotted line in FIG. 5 (b). As shown, a -45 ° phase shift occurs. Further, at the position X3, as shown by the solid line and the dotted line in FIG.
The 90 ° phase shift is + 180 ° and −180 ° at the position X4 as shown by the solid line and the dotted line in FIG. 5D, so that the eddy current magnetic field 62 increases as the distance from the center increases. The phase shift amount advances by 360 ° or more by the same amount in the plus and minus directions.

Therefore, when the raw data is subtracted as described above, the phase shift amount can be converted into the signal strength. In addition, the actual raw data includes influences other than the gradient magnetic field pulse 61 for generating the eddy current magnetic field, for example, nonuniformity of the static magnetic field and Gz pulses 72 and G for imaging.
Although the influence of the eddy current magnetic field by the y pulse 73 and the Gx pulse 74 is included, it is considered that these do not change in the two sequences in which the polarities of the gradient magnetic field pulse 61 are reversed, and thus they are completely canceled by the above subtraction,
Only the component affected by the eddy current magnetic field 62 by the gradient magnetic field pulse 61 can be extracted.

The raw data after being subtracted in this way are collected at each time point, arranged line by line and subjected to a two-dimensional Fourier transform to obtain an image at each time point. In this image, for example, the amount of phase shift becomes 0 °, 90 °, 180 °, 270 ° as it goes in the X direction.
... and so on, so the magnitude of the signal strength is repeated,
Therefore, the image becomes a striped pattern (each stripe extends in the Y direction) arranged in the X direction. A change in the strength of the eddy current magnetic field 62 means a change in the slope between the solid line and the dotted line in FIG. 4, and therefore a change in the frequency of the striped pattern on the image. Therefore, by observing each of these images, the eddy current magnetic field 62 is
It can be seen that the state becomes gradually smaller at each time of C, D, ....

It should be noted that instead of performing the subtraction of raw data as described above, an addition may be performed. In the addition, since the signals are added and the signal is increased in the case of (a) and (d) of FIG. 5, and the signals are decreased by the subtraction in the case of (c) of FIG. 5, the striped pattern of the image is obtained. This is because the phase of is only reversed. Further, the raw data after subtraction (or addition) can be subjected to image reconstruction processing by an absolute value or image reconstruction processing with a sign. The gradient magnetic field pulse 61 for generating an eddy current magnetic field need not be a single pulse as described above, but can have various waveforms. Further, the gradient magnetic field pulse 61 for generating an eddy current magnetic field is not given as a certain polarity and then as an opposite polarity as described above, but is set to 0 at the second time, that is, the period A is omitted at the second time B, Only the gradient echo sequence in the period of C, D, ... May be performed. Other,
It goes without saying that various modifications can be made without departing from the spirit of the present invention.

[0021]

According to the MR imaging apparatus of the present invention, the spatial distribution of the eddy current magnetic field can be imaged and
Even a temporal change in the eddy current magnetic field can be captured with a single imaging. Therefore, when the MR imaging apparatus is installed, it can be useful for measurement of the eddy current magnetic field and analysis / adjustment based on it.

[Brief description of drawings]

FIG. 1 is a block diagram of an MR imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a time chart showing a pulse sequence of the same embodiment.

FIG. 3 is a time chart showing a temporal change of an eddy current magnetic field.

FIG. 4 is a graph showing the magnitude of an eddy current magnetic field with respect to position.

FIG. 5 is a diagram showing a change in phase due to an eddy current magnetic field.

[Explanation of symbols]

Reference Signs List 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 61 eddy current magnetic field generation gradient magnetic field pulse 62 Eddy current magnetic field 71 Excitation RF pulse 72 Gradient magnetic field pulse for slice selection 73 Gradient magnetic field pulse for phase encoding 74 Gradient magnetic field pulse for reading 75 Echo signal 76-78 Spoiler pulse

Claims (1)

[Claims] 1. A means for applying an RF pulse to a subject, a means for applying a gradient magnetic field pulse for slice selection, a means for applying a gradient magnetic field pulse for phase encoding, and a gradient magnetic field pulse for reading. Means and the echo signal is received and the phase is detected, then this is sampled and A
A means for obtaining data by D / D conversion and a gradient magnetic field pulse for generating an eddy current magnetic field are generated, and then one excitation RF pulse is added together with a gradient magnetic field pulse for slice selection to generate a gradient magnetic field pulse for phase encoding. In addition, the gradient magnetic field pulse for eddy current magnetic field generation, in which a gradient echo sequence for generating a gradient echo signal by inverting the gradient magnetic field pulse for reading is repeated many times without changing the waveform of the gradient magnetic field pulse for phase encoding, And a control means for controlling the RF pulse applying means and each gradient magnetic field pulse applying means so as to repeat a sequence consisting of a number of subsequent gradient echo sequences while changing the gradient magnetic field pulse waveform for phase encoding. Characteristic MR imaging device.