JPH0636791B2 - NMR signal phase correction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Use of the Invention] The present invention relates to nuclear magnetic resonance (Nuclear Magnetic Resonance,
In the following, it relates to an internal tomography apparatus utilizing a phenomenon (hereinafter referred to as NMR), and particularly relates to an NMR signal phase correction method, which can be used for medical diagnosis and treatment.

[Background of the Invention]

NMR, which was independently established by Bloch and Purce 11 in 1946, has developed rapidly since then, and has become an indispensable analytical tool in the field of physical chemistry including structural analysis of substances.

This NMR is a magnetic interaction between the nuclear magnetic moment and the outside world, and its characteristic is that the energy is X
It is extremely low, about 10 ^{-9 of the} line. For this reason,
It attracts attention as a non-invasive biometric technique that hardly changes the state of the system to be measured.

In the 1970s, a new study on treating NMR as imaging was started by Lauterber. The information obtained is the proton spin density of the particular nuclide of the analyte,
This is an image of relaxation time and the like, and it is possible to obtain in-vivo information different from that of X-ray CT and ultrasonic images.

The signal detected in the above imaging device is NMR
It is called a signal and is generated when the nuclear spin returns from the energy level excited by the resonance signal to the ground state energy level.

The NMR signal is represented by the following equation.

S (t) = ke ^iφ ∫M (t, x) exp (-iω ₀ t) dx (1) where k: proportional constant (real number) φ: phase rotation angle due to the characteristics of the detection system, etc.

M (t, x): magnetic field strength ω ₀ : Larmor frequency φ corresponding to a static magnetic field The value of φ is determined by the characteristics of the device and is generally an unknown value. The value of φ is generally not zero and causes a phase shift of the signal.

Conventionally, focusing on the fact that the absorption mode spectrum can be obtained by Fourier-transforming the measured signal of equation (1),
The optimum phase correction angle is determined visually, or the correction angle is calculated by calculating the value that maximizes the integral value of the spectrum.

In either method, it is necessary to Fourier transform the measurement signal, which requires processing time. Further, the method of visually determining has a problem that it takes time to obtain it because human intervention is involved.

[Object of the Invention]

An object of the present invention is to provide a method for obtaining a phase correction angle at high speed without performing Fourier transform processing.

[Outline of Invention]

The NMR signal S (t) of the equation (1) is usually quadrature detected (Quadratur).
e phase detection), and detected as a complex signal S (t) of the following equation.

Here, k: proportional constant (real number) A (x): magnetic field strength (amplitude) T ₂ : spin-spin relaxation time θ (x): initial phase angle at each point in the cross section.

 φ: Phase rotation angle due to the characteristics of the detection system, etc.

ω _h (x): Rotational frequency generated by the gradient magnetic field.

Therefore, the signal at t = 0 is given by the following equation.

_{S (0) = S A (} 0) + iS B (0) = k∫A (x) [θ (x) + φ) + isin (θ (x) + φ) ] gives dx theta initial values of (x) = 0 When'll, the S (0) = k [cosφ + isinφ] _{∫A (x) dx = S A} (0) + iS B (0) ... (5) (5) the phase rotation angle θ from the formula, It is given by (−πφ <π). Here, S _ga (·) is a function that gives the sign of the argument.

(6) As is clear from equation value S (0) when the t = 0 of the measurement signal = with _{S A (0) + iS B} (0), it is possible to determine the phase rotation angle θ .

When correction is performed at the angle given by equation (6), equation (5) yields And the real part of the signal is maximum. This signal is ∫A (x)
dx, that is, the integral of the magnetic field strength,
This is equivalent to maximizing the integral of the result of Fourier transform of the measurement signal.

Example of Invention

Hereinafter, the present invention will be specifically described with reference to examples. FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.
1 is a processing device that controls the entire system and performs image reconstruction based on measured data, 2 is a sequence control unit that controls various pulses generated to detect an NMR signal from a subject and a detected signal, Reference numeral 3 denotes an RF control unit that controls a high-frequency magnetic field generated to resonate a specific nuclide of the subject.
Is obtained by 3 and 4, and a selective excitation modulator 5 that modulates at least one of the frequency and amplitude of the high-frequency magnetic field generated in 3 and generates a signal for selecting a region by band-limiting its frequency component. A transmitter for supplying an electric current to a coil based on the received signal, 6 is a receiver for detecting an NMR signal generated from a subject and extracting an FID signal, and 7 is data for A / D converting the signal obtained in 6. The take-in unit, 8 is a gradient magnetic field control unit for generating an arbitrary gradient magnetic field according to the purpose, and 9 is a gradient magnetic field coil power source that can be controlled independently in the x-direction, the y-direction, and the z-direction. , 12 are coils for generating magnetic fields in the x-direction, y-direction, and z-direction, 13 is a static magnetic field control unit for generating a static magnetic field that determines the resonance frequency of the NMR signal, 14 is a power source for the coil, and 15 is , Processor 1 And displays the processed result to the CRT, or light pen, CRT console control unit for controlling the input device such as a keyboard, 16 a CRT Deisupurei for displaying the reconstructed image.

A method for carrying out the present invention having the above-described structure will be described with reference to FIGS.
It will be described below with reference to the drawings. Here, an example of reconstructing an image by the procedure shown in the flow chart of FIG. 2 by using projection reconstruction with a pulse sequence by the most general spin echo method will be described.

In step 201 in FIG. 2, the 90 ° pulse 301 in FIG. 3 generated by the RF control unit 3 is irradiated through the transmitter 5, and at the same time, the gradient magnetic field control unit 8 controls the coil 1
2 is used to apply a gradient magnetic field G _z 302 in the z direction. With this pulse, only the nuclear spins within a particular slice are
Fall 0 °.

At step 202, after τ time, the transmitter 5 transmits the 180 ° pulse 303 shown in FIG. 3, which is also generated by the RF controller 3.
And spin is inverted 180 °.

In step 203, after τ, the gradient magnetic field controller 8 applies the gradient magnetic fields G _x and G _y 304 in the _x and y directions to the coil 1.
Simultaneously with applying 0 and 11, the NMR signal 305 is measured through the receiver 6, and the data acquisition section 7 performs quadrature detection and sampling.

In step 204, the phase rotation angle is calculated according to the equation (6) from the measurement signal obtained as the complex number signal of the equation (5). If the angle is φ, the signal after phase correction Can be calculated by the following equation.

In step 205, normal image reconstruction processing is performed using the phase-corrected measurement data obtained in the previous stage. That is, the product operation with the filter function is performed.

Fourier transform is performed.

Perform a pattern procedure.

Reconstruction processing is performed according to the procedure.

The above steps 201 to 205 are repeated for the number of projection directions, or after one cycle, the next step 20
The timing of starting 1 is after a sufficiently long t _τ time when the influence of the previous magnetic field disappears.

The projection direction is set by changing the gradient magnetic field strengths G _x and G _y in step 203.

〔The invention's effect〕

According to the present invention, since the phase correction angle can be obtained without performing the Fourier transform, the processing time can be significantly reduced. Further, since the correction angle can be calculated at the same time when the data is taken in, real-time phase correction is possible.

[Brief description of drawings]

FIG. 1 is a system configuration block diagram of an embodiment for realizing image reconstruction of the present invention, FIG. 2 is a flow chart showing the processing procedure of the present invention, and FIG. 3 is a data measurement pulse sequence by the pulse echo method. It is the figure shown.

Front Page Continuation (56) References Japanese Patent Publication 3-40608 (JP, B2) Japanese Patent Publication 4-49419 (JP, B2) Japanese Publication 5-53495 (JP, B2)

Claims (1)

[Claims] 1. A means for generating a static magnetic field, a gradient magnetic field and a high frequency magnetic field, and N from an inspection object placed in the generated magnetic field.
In a nuclear magnetic resonance imaging apparatus having a detection means for detecting an MR signal and a processing means for reconstructing an image of an object from the detected signal, the phase shift of the NMR signal caused by the characteristics of the detection means, NMR
An NMR signal phase correction method characterized by performing correction so that the value of the real part of the measurement signal corresponding to time O of the signal becomes maximum and the imaginary part becomes zero.