JPH0767442B2 - Nuclear magnetic resonance imaging method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Application of the Invention The present invention relates to nuclear magnetic resonance (Nuclear Magnetic Resonance,
This is related to an internal tomography apparatus utilizing a phenomenon (hereinafter referred to as NMR) and is used for medical diagnosis.

[Background of the Invention]

To perform internal tomography, a series of various magnetic fields are applied and NM
R signal needs to be measured. This series of sequences is generally called a pulse sequence. An example of this is shown in FIG. Among the noise contained in the signal measured here, NM
There is DC noise specific to the R signal. This is caused by a small amount of the high-frequency pulse signal irradiated to excite the hydrogen nuclei, which leaks to the detection system.

This signal is very weak and hard to detect in hardware. However, as shown below, when the image is reproduced by using the measurement signal, the value is greatly emphasized and appears as an abnormal luminance at the center of the image.

Now, _let S (t _x , G _y ) be the measurement signal and I (x, y) be the reproduced image. Where t _x is the observation time and G _y is the parameter in the pulse sequence. At this time, the following relationship holds.

Here, M is the number of sampling points in the time direction, N is the number of times of measurement, and the image center point density I (0,0) is determined by the following equation.

Here, E [•] indicates an average value.

Therefore, _if a uniform DC noise of ΔS is superimposed on S (t _x , G _y ), And the sensitivity is calculated by the following equation.

That is, the DC noise is multiplied by M · N and appears in the center of the image. Normally, M = 512 and N = 256, so M
・ N is emphasized to 131072, about 100,000 times.

A method for removing such abnormal concentration is disclosed in Japanese Patent Laid-Open No. 55-12
As shown in No. 1141, a device is used in which a pulse sequence is devised so that an image is taken by a procedure different from the sequence generally used in the world. In this method, since an extra pulse is applied for removal, it generally takes a long imaging time and the imaging procedure is complicated.

[Object of the Invention]

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide means for estimating and removing a DC noise component from a measurement signal with a pulse sequence generally used in the world.

[Outline of Invention]

The DC component is estimated by paying attention to the fact that the measurement signal sharply decreases at the lateral relaxation time constant T ₂ ^* due to the nonuniformity due to the gradient magnetic field, and the signal level attenuates to zero near both ends of the signal.

That is, the measurement signal S (t _x , G _y ) is Here, n (t _x , G _y ): white noise C: direct current noise, and assuming an expected value at the peripheral parts (both ends), E [S (t _x , G _y )] _{| tx |} = E [n (t _x , G _y )] _{| tx |} + E [C] _tx = C, and only DC noise can be specified.

More specifically, generating means for respectively generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, a measuring means for measuring a nuclear magnetic resonance signal from an inspection target excited by the high-frequency magnetic field, a generating means according to a predetermined pulse sequence. In a nuclear magnetic resonance imaging apparatus having a control means for controlling the, and a calculation means for reproducing the image of the inspection object by using the measurement signal data obtained by the measurement means, a part of the period between excitation and signal measurement Applied gradient magnetic field G _{y in} a specific direction
The signal level is considered to be approximately the same as the noise level for each measurement signal by repeating the pulse sequence that is executed while sequentially changing the value of as a parameter. The signal level is read at a plurality of different observation points in the range of both end portions that can be obtained, and a first average value that is an average value of the read plurality of signal levels is obtained. Obtaining an average value of a plurality of first average values obtained for all of the plurality of measurement signals to obtain a second average value, and setting the second average value to a plurality of values for obtaining the one image. Decrease this second average value from the nuclear magnetic resonance signal by regarding it as a direct current noise component peculiar to the entire measurement signal, and perform a two-dimensional Fourier transform on a plurality of measurement signals obtained by subtracting this second average value. Reproducing the image Te, and its constituent nuclear magnetic resonance imaging method comprising the steps of.

Example of Invention

Hereinafter, the present invention will be described in detail based on Examples. Second
The drawing is a block diagram showing the construction of an embodiment of the present invention. Sequence control unit that controls various pulses and magnetic fields generated to detect NMR signals from the subject
By 201, a transmitter 202 of a high frequency pulse generated to resonate a specific nuclide of the subject, and a static magnetic field that determines the resonance frequency of the NMR signal and a gradient magnetic field that can arbitrarily control the strength and direction Magnetic field control unit 203
And, after detecting the NMR signal generated from the subject, controls the receiver 205 that performs measurement, performs image reconstruction with the processing device 206 based on the measurement signal captured from the receiver 205, and is reconstructed. The image is displayed on the CRT display 207. Magnetic field drive
Reference numeral 204 generates a magnetic field necessary for measurement based on the control signal output from the magnetic field control unit 203.

A method for carrying out the present invention having the above-described structure will be described below with reference to FIGS. 2 and 3. FIG. 3 is an example of a float chart showing a DC noise removing procedure according to the present invention.
An example of reproducing an image using the two-dimensional Fourier transform method with a pulse sequence using the spin echo method will be described. Further, the procedure of performing the DC noise removal at the level of the measurement signal will be described, but it is of course possible to remove the DC intermediate noise and the reproduced image. In the method described here, noise removal processing is performed on the entire measurement signal. However, in the case of the intermediate calculation result, noise is applied to only one central point for the central 1-line data and the reproduced image. It may be removed.

The step 301 photographs according to the pulse sequence of FIG. 1 and measures the NMR signal 108 256 times. Each NMR signal is now 51
Two points shall be sampled.

Step 302 estimates the DC noise component from the measurement signal. That is, an average value of 128 points from both ends of the measurement signal is obtained for the previous measurement signal, and the average value is obtained, which is used as the estimated value of the DC noise.

Step 303 subtracts the estimated value obtained in the previous step from all the measurement data to remove DC noise.

Step 304 uses the measurement data after noise removal,
Image reproduction processing is performed.

Step 305 displays the obtained image on the CRT display.

〔The invention's effect〕

According to the present invention, it is possible to estimate and remove a DC noise component from a measurement signal obtained in a normal photographing procedure, improve the photographing efficiency, and make the photographing apparatus economical.

[Brief description of drawings]

FIG. 1 is an example of a pulse sequence generally used at present, FIG. 2 is a block diagram showing an embodiment of the present invention, and FIG. 3 is a flow chart showing a processing flow of the present invention. .

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Claims (1)

[Claims] 1. A generating means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field, a measuring means for measuring a nuclear magnetic resonance signal from an inspection object excited by the high-frequency magnetic field, and the above-mentioned generation in accordance with a predetermined pulse sequence. In a nuclear magnetic resonance imaging apparatus having a control means for controlling the means and a calculation means for reproducing the image of the inspection object by using the measurement signal data obtained by the measurement means, a) between excitation and signal measurement Perform multiple signal measurements by repeating a pulse sequence that is executed while sequentially changing the value of the gradient magnetic field G _{y in} a specific direction applied as a parameter during a part of the period. (B) For each of the above multiple signal measurements , For each of the measurement signals, the signal level can be considered to be approximately the same as the noise level. The signal level is read at a measurement point, and a first average value, which is the average value of the read signal levels, is obtained. C) A plurality of values obtained for all of the plurality of measurement signals for obtaining one image Of the above-mentioned first average value and obtain a second average value, d) DC noise specific to all of the plurality of measurement signals for obtaining the above-mentioned second average value. And subtracting the second average value from the nuclear magnetic resonance signal, and (e) reconstructing an image by two-dimensional Fourier transforming a plurality of measurement signals with the second average value subtracted. A nuclear magnetic resonance imaging method including a procedure.