JPH0439855B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic resonance imaging apparatus that utilizes nuclear magnetic resonance phenomena.

[Conventional technology]

In magnetic resonance imaging, a method is generally used to selectively excite nuclear spins in a desired region of the sample by irradiating a sample placed in a static magnetic field and a gradient magnetic field with a high-frequency magnetic field.
Typically, this excitation causes the nuclear spin to nutate 90° from equilibrium. Ideally, all selectively excited nuclear spins point in the same direction,
In other words, it is desirable that the phases be aligned. However, in reality, it is known that the phase of the nuclear spin has an angle that changes in an odd function with respect to the distance from the central plane of the selected region.

This phase rotation of the nuclear spins causes S/N deterioration of the measured nuclear magnetic resonance signal, error in the phase encode amount, etc., and also has a negative effect on the reconstructed image. It is known that the phase rotation of nuclear spins can be corrected by applying a gradient magnetic field having the property of returning the phase after excitation, and this is widely used.
The phase of the nuclear spins returned by this phase correction gradient magnetic field is proportional to the applied amount of the gradient magnetic field, that is, the value obtained by integrating the magnetic field strength due to the gradient magnetic field felt by each excited nuclear spin over the application time. Therefore, in order to correct the phase rotation of the nuclear spins to a minimum, it is necessary to apply an optimal amount of gradient magnetic field.

Conventionally, the amount of applied gradient magnetic field for phase correction was as described in Japanese Patent Application Laid-Open No. 55-20495.
Approximately half of the value obtained by integrating the intensity of the gradient magnetic field for region selection applied together with the radio-frequency magnetic field during the excitation of nuclear spins over the radio-frequency magnetic field irradiation time is considered appropriate.

[Problem that the invention seeks to solve]

However, the above conventional technology does not take into account the fact that the phase rotation of nuclear spins depends on the pulse shape of the high-frequency magnetic field, and there is a problem that the amount of applied gradient magnetic field for phase correction is not necessarily optimal. . Furthermore, there is no report on means for determining the optimum application amount.

An object of the present invention is to provide a magnetic resonance imaging apparatus that can measure the phase rotation of nuclear spins and determine the optimal application amount of a gradient magnetic field for phase correction based on the measurement.

[Means for solving problems]

The above object is achieved by converting the phase rotation of nuclear spins into a time shift of a nuclear magnetic resonance signal, measuring it, and calculating the optimal application amount of a gradient magnetic field for phase correction based on the measurement.

[Effect]

If the optimal amount of applied gradient magnetic field for phase correction can be calculated, the phase of the excited nuclear spin can be optimally corrected as a whole by applying the calculated amount of gradient magnetic field after excitation of the nuclear spins. , the S/N of the measured nuclear magnetic resonance signal becomes maximum.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 2 is a block diagram of a magnetic resonance imaging apparatus used in practicing the present invention. The sample is constantly exposed to a uniform and stable magnetic field H ₀ generated by the static magnetic field coil 1. The strength of the magnetic field generated from this coil is determined by the magnitude of the power supplied from the static magnetic field power supply 2. The X-axis gradient magnetic field coil 3, the Y-axis gradient magnetic field coil 4, and the Z-axis gradient magnetic field coil 5 generate gradient magnetic fields orthogonal to the magnetic field, and play the role of selecting a slice and correcting the phase dispersion of nuclear spins within the slice. . The strength of each gradient magnetic field is changed by controlling the electric power supplied from the gradient magnetic field power source 6 by the gradient magnetic field control device 7. The high-frequency magnetic field irradiation coil 8 is used to excite nuclear spins in the sample, and the high-frequency magnetic field pulse irradiated from the coil 8 is a signal generated by the high-frequency magnetic field generator 12 and amplified to an appropriate amplitude by the amplifier 10. Occurs when given. The shape of the applied pulse determines the shape of the slice, and the amplitude determines the nutation angle of the nuclear spins. When the excited nuclear spins undergo free induction decay motion, the signal detection probe 9
A nuclear magnetic resonance signal is induced. After the induced signal is amplified to an appropriate amplitude by the amplifier 11,
The signal generated by the high-frequency magnetic field generator 12 is used as a reference wave and is orthogonally detected by the orthogonal detector 13 . The detected signal is A/D converted by an A/D converter 15, and then taken into a central processing unit 16 and processed. A sequencer 14 controls the timing of generating each gradient magnetic field and high-frequency magnetic field and the timing of starting A/D conversion, and the sequencer 14 is further controlled by a central processing unit 16.
The dice display 17 is used to display various information.

FIG. 1 is a diagram showing the flow of the procedure for implementing the present invention, and hereinafter, the specific processing procedure will be explained with reference to FIGS. 3 and 4.

First, in step 101, the spin dispersion time t ₁ of the sequence shown in FIG. 3 is set to be equal to the spin convergence time t ₂ . Since this time is an approximate value, it does not necessarily have to be equal to _t2 .

In step 102, nuclear magnetic resonance signals are measured according to the sequence shown in FIG. In this sequence, the sample is first irradiated with a high-frequency magnetic field (hereinafter abbreviated as 90° pulse) that nutates nuclear spins by 90°. Next, a gradient magnetic field in the Z-axis direction is applied for a preset time _t1 . This gradient magnetic field has the effect of dispersing the phase of nuclear spins nutated by 90° from the equilibrium state due to 90° pulse irradiation. After the irradiation of the 90° pulse, and after a period of time τ, a high-frequency magnetic field (hereinafter abbreviated as 180° pulse) that nutates the nuclear spins by 180° is applied. Subsequently, a gradient magnetic field is applied to converge the phase of the dispersed nuclear spins and generate a nuclear magnetic resonance signal (in this case, generally referred to as a spin echo). The phase of the nuclear spins converges and the peak of the echo appears when the applied amount of the gradient magnetic field for phase dispersion becomes equal to the applied amount of the gradient magnetic field for phase convergence. Therefore, when the gradient magnetic field strengths are equal and the dynamic characteristics can be ignored, t ₁ =t ₂ . However, in reality, it always has dynamic characteristics, so t ₁ = t ₂ does not hold true. In step 103, considering that the spin echo peak appears at the time 2τ, the deviation t ₃ between the measured spin echo peak appearance time and 2τ is determined.
Next, using this t ₃ , t ₁ is corrected so that the appearance time of the peak peak of the spin echo coincides with 2τ. if,
If the intensities of the gradient-like fields for phase dispersion and phase convergence are equal, if the appearance time of the spin echo peak is before 2τ, it is corrected as t ₁ = t ₁ + t ₃ , and if it is after 2τ, it is corrected as t Correct it as ₁ = t ₁ − _{t 3} . If we measure the spin echo again using the sequence shown in Figure 3 after correcting t ₁ in this way, the peak of the echo will appear at the temporal position of 2τ.

In step 104, first, the application time _t5 of the phase correction gradient magnetic field in the sequence of FIG. 4 is set to half the application time _t4 of the slicing gradient magnetic field. This time is approximate and does not necessarily have to be half of _t4 . The switch 105 measures spin echoes in the sequence shown in FIG. In this case, the time corrected in steps 101 to 103 is used as the spin dispersion time _t1 . The application start time of the gradient magnetic field for spin convergence is also the third
Make it equal to the sequence in the figure. Further, it is assumed that the slicing gradient magnetic field is applied only during the irradiation time _t4 of the high-frequency magnetic field. The sequence shown in Figure 3 is the sequence shown in Figure 4 when neither the slicing gradient magnetic field nor the phase correction gradient magnetic field is applied. The deviation t ₆ is due to the phase rotation of the nuclear spin due to selective excitation. In step 106, the application time _t5 of the gradient magnetic field for phase correction is corrected based on the time lag _t6 of the appearance time of this spin echo peak. If the absolute value of the gradient magnetic field strength for phase correction is equal to the absolute value of the gradient magnetic field strength for phase focusing, and if the gradient magnetic field for phase correction is applied before the 180° pulse, then the peak of the spin echo will be before 2τ. When the echo peak appears after 2τ, it is corrected as t ₅ = t ₅ − t ₆ , and when the echo peak appears after 2τ, it is corrected as t ₅ = t ₅ − _{t 6} . Gradient magnetic field for phase correction
If it were applied after the 180° pulse, the argument just stated would be reversed.

As described above, the phase rotation of the nuclear spin due to selective excitation can be optimally corrected by the steps 101 to 106.

〔Effect of the invention〕

According to the present invention, it is possible to optimally correct the phase rotation of selectively excited nuclear spins, thereby preventing S/N deterioration of the measured nuclear magnetic resonance signal and occurrence of errors in the phase encode amount. This has the effect of preventing deterioration in the quality of the reconstructed image.

[Brief explanation of drawings]

FIG. 1 is a flowchart showing an embodiment of the present invention, FIG. 2 is a block diagram of a magnetic resonance imaging apparatus according to the present invention, and FIGS. This is the required pulse sequence. τ...Echo time, t ₁ ...Spin dispersion time, t ₂
...Spin convergence time, t ₃ ...Echo time shift, t ₄
...Magnetic field application time for slicing, t ₅ ...Magnetic field application time for phase correction, t ₆ ...Echo time shift.

Claims (1)

[Claims] 1. Means for selectively exciting nuclear spins in a desired region of the sample by irradiating a sample placed in a static magnetic field and a gradient magnetic field with a high-frequency magnetic field, and correcting the phase rotation of the nuclear spins due to the excitation by applying a gradient magnetic field. In the magnetic resonance imaging method, the method includes a means for converting information regarding the phase rotation of nuclear spins into a time shift of a magnetic resonance signal and measuring the time shift, and a method that minimizes the phase rotation of the nuclear spins from the measured time shift. 1. A magnetic resonance imaging apparatus comprising means for determining application conditions of a gradient magnetic field.