JPS62240040A - Magnetic resonance imaging apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 commonly used to selectively excite nuclear spins in a desired region of the sample by irradiating a sample placed in a static magnetic field or a gradient magnetic field with a high-frequency magnetic field6. , this excitation causes the nuclear spins to nutate by 90” from their equilibrium state. Ideally, all selectively excited nuclear spins should point in the same direction, that is, be in phase. However, in reality, it is known that the phase of the nuclear spin has an angle that varies oddly with respect to distance from the central plane of the selected region.

This phase rotation of the nuclear spins becomes a factor such as S/N deterioration of the measured nuclear magnetic resonance signal, a difference in the amount of one-phase encoding, and has an adverse 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. 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 has been determined by adjusting the intensity of the gradient magnetic field for region selection, which is applied together with a high-frequency magnetic field when excitation of nuclear spins, as described in Japanese Patent Application Laid-Open No. 55-20495. Approximately half of the value integrated over the magnetic field irradiation time is considered appropriate.

[Problems to be Solved by the Invention] However, in the above-mentioned prior art, the phase rotation of the nuclear spin depends on the pulse shape of the high-frequency magnetic field.

However, there is a problem in that the applied amount of the gradient magnetic field for one-phase correction is not necessarily optimal. Also,
There are also no reports on means for determining the optimal 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 the nuclear spin into a time shift of the nuclear magnetic resonance signal, measuring it, and calculating the optimum application amount of the gradient 6J & field for phase correction based on the measurement.

[Effect]

If the optimal amount of applied gradient magnetic field for phase correction can be calculated,
After excitation of the nuclear spins, if a gradient magnetic field is applied by the calculated amount, the phase of the excited nuclear spins will be optimally corrected as a whole, so the S/N of the measured nuclear magnetic resonance signal will be maximized. .

〔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 to implement the present invention. The sample is always exposed to a uniform and stable magnetic field Ho 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. X
Axial 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 phase dispersion of nuclear spins within a slice. The strength of each gradient magnetic field is determined by the power supplied from the gradient magnetic field power supply 6! ! 7. The high-frequency magnetic field irradiation coil 8 is for exciting nuclear spins in the sample, and the high-frequency magnetic field pulse irradiated from this is generated by a signal generated in the high-frequency magnetic field generation bag v112 being 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, a nuclear magnetic resonance signal is induced in the signal detection probe 9. After the induced signal is amplified to an appropriate amplitude by an 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. Each gradient magnetic field
A sequencer 14 controls the timing of generating a 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 4.6 First, in step 101, The spin dispersion time tl of the sequence is set equal to the spin convergence time t2. 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 in the sequence shown in FIG. This sequence first sets the nuclear spin to 90
'The sample is irradiated with a high-frequency magnetic field (hereinafter abbreviated as 90° pulse) that causes nutation. Next, a gradient magnetic field in the Z-axis direction is applied for a preset time t, 1. 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. 90'
After 1 hour of pulse irradiation, the nuclear spin becomes 180＠
A high-frequency magnetic field (hereinafter abbreviated as 180° pulse) that causes nutation is irradiated. 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 and the applied amount of the gradient magnetic field for phase convergence become equal. Therefore, when the gradient magnetic field strengths are equal and the dynamic characteristics can be ignored, tx:tz
becomes. However, in reality, it always has dynamic characteristics, so tl:
Not like tl. In step 103, considering that the spin echo peak appears at the time 2τ, the deviation ta between the measured spin echo peak appearance time and 2τ is
Using this tB, tl is corrected so that the time of appearance of the spin echo peak coincides with 2τ.

If the strengths of the gradient magnetic fields for phase dispersion and phase convergence are equal.

If the spin echo peak appearance time is before 2τ, it is corrected as t 1 = t 1 + t s, and if it is after 2τ, it is corrected as tt:tt ta. If the spin echo is measured again according to the sequence shown in FIG. 3 after correcting tl in this way, the peak of the echo will appear at the special training of 2τ.

In step 104, first, the application time t+s of the phase correction gradient magnetic field in the sequence of FIG. 4 is set to half the application time ta of the slicing gradient magnetic field. This time is approximate, so it does not necessarily have to be half of 4.
In 05, spin echoes are measured according to the sequence shown in FIG. In this case, the spin dispersion time t1 is from step 101 to
Using the time corrected in step 103, the application start time of the gradient magnetic field for spin convergence is also made equal to the sequence in FIG. 3.

In addition, the gradient magnetic field for slicing has an irradiation time t of the high-frequency magnetic field.
It is assumed that the voltage is applied only for a period of 4. The sequence shown in Fig. 3 is the sequence shown in Fig. 4 when the slicing gradient magnetic field 1 phase correction gradient magnetic field is not applied.
The deviation to of the pin appearance time of the spin echo measured in the sequence shown in the figure from 2τ is due to the phase rotation of the nuclear spin due to selective excitation. In step 106, the application time ts of the gradient magnetic field for phase correction is corrected based on the time lag tB of the appearance time of the 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 convergence, and the gradient magnetic field for 1 phase correction is 180°.
Assuming that it is applied before the pulse, if the spin echo peak appears before 2τ, then ts:t7s-ta
If the echo peak appears later than 2, it is corrected as ts = tδ - 76.If the gradient magnetic field for phase correction is applied after the 180@pulse, then the argument just stated becomes the opposite. Become.

As described above, the phase rotation of the nuclear spin due to selective excitation can be optimally corrected by the steps 101 and 6106.

〔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 one embodiment of the present invention, and FIG.
The figure is a block diagram of a magnetic resonance imaging apparatus according to the present invention, and FIGS. 3 and 4 are pulse sequences necessary for carrying out one embodiment of the present invention. τ...Eco hour, tz...Spin dispersion time, tz
...Spin convergence time, t3...Echo one hour deviation, number...Magnetic field application time for slicing, ta...Magnetic field application time for phase correction, ta...Echo one hour deviation.

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 changing the phase of the nuclear spins due to the excitation by applying a gradient magnetic field In a magnetic resonance imaging method equipped with a correction means, there is a means for converting and measuring information regarding the phase rotation of nuclear spins into a time shift of a magnetic resonance signal, and a method that minimizes the phase rotation of nuclear spins from the measured time shift. A magnetic resonance imaging apparatus comprising means for determining application conditions of a gradient magnetic field.