JPH04357937A - Nuclear magnetic resonance device - Google Patents

Abstract

PURPOSE:To prevent the artifact by detecting a magnetic resonance signal by two reference waves whose resonance frequencies are different from each other by 90 deg. and separating it into two signals, executing Fourier-transformation to the detected signal, eliminating a noise by correcting the signal on a Fourier space, and reconstituting its signal. CONSTITUTION:A tuner 3 selects an electromagnetic wave of a specific frequency from the electromagnetic wave generated in a transmitting system 4, and applies an excitating pulse to a transmitting/receiving coil 2 so as to tune with a specific nuclear specifies in a body 1 to be examined. This received nuclear magnetic resonance signal passes through an amplifier 5 and is inputted to phase detectors 6A, 6B, subjected to phase detection by a reference wave different from each other by 90 deg., outputted from a reference signal generator 7 and separated into two signals of a real number part and an imaginary number part. Subsequently, each separated signal passes through low-pass filters 9A, 9B, and A/D converters 10A, 10B and is digitized, and inputted to a noise correction processor 11. The noise correction processor 11 eliminates the noise of a low frequency component. This corrected signal is reconstituted with an imaging processor 12.

Description

[Detailed description of the invention]

[Object of the invention]

0002

[Field of Industrial Application] The present invention relates to a nuclear magnetic resonance apparatus that utilizes nuclear magnetic resonance phenomena to reconstruct an image that reflects the spin density or relaxation time distribution of a specific atomic nucleus existing in a liquid. It is something.

0003

2. Description of the Related Art In a nuclear magnetic resonance apparatus, a nuclear magnetic resonance signal is first detected when obtaining an image reflecting the spin density, specific relaxation number, etc. of a specific atomic nucleus within a subject. If we write this signal as F(t), F(t) is basically the following number 1
It can be expressed as

0004

[Math 1]

Here, p(ω), frequency spectrum of the signal (real value function) ω, angular frequency t, time However, the actually observed signal F'(t) has a low frequency in the F(t). , and can be expressed by the following Equations 2 and 3.

[0006]

[Math 2]

[0007]Here,

[0008]

[Math 3]

[0009] However, δ is a small positive number Δ(ω) is an offset component (real value function) Therefore,
For example, when a signal F'(t) having a real part as shown in FIG. 2 and an imaginary part as shown in FIG. 3 is Fourier transformed,
Originally, we wanted to obtain a noise-free frequency spectrum P(ω) (also called projection data) as shown in Figure 1, but since a low frequency offset β(t) is added, we need to obtain the noise-free frequency spectrum P(ω) as shown in Figure 4. So, the frequency spectrum P
'(ω) becomes an abnormal waveform with unevenness at its center. When image reconstruction is performed using the projection data of such an abnormal waveform as it is, as shown in Fig. 5,
A radial artifact Af passing through the center of the image occurs in the image.

0010

[Problem to be Solved by the Invention] The cause of the addition of low-frequency offsets that causes such artifacts Af is temporal fluctuations in hardware characteristics, or discretization of data when processed by a computer. However, the accuracy of AD conversion at that time may be poor. However, such a detected signal F′
It is extremely difficult to completely remove the low frequency offset noise β(t) of (t) purely using hardware.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a nuclear magnetic resonance apparatus that can display images free of artifacts caused by noise of low frequency components.

0012 [Structure of the invention]

[0013]

[Means for Solving the Problems] A summary of the present invention for achieving the above object is to detect a magnetic resonance signal induced by a magnetic resonance phenomenon using two reference waves whose resonant frequencies have phases different by 90 degrees from each other. a detection means for separating the detected signal into two signals; a correction means for performing Fourier transform on the detected signal and removing low frequency component noise by interpolating the signal in Fourier space; The present invention is characterized by comprising a reconstruction means for reconstructing the signal from which noise has been removed.

[0014]

[Operation] Even if the detected nuclear magnetic resonance signal contains low frequency offset noise, it can be automatically removed during the data processing process.

[0015]

[Embodiment] First, the principle of this invention will be explained.

The processing procedure for reconstructing an image based on the detected nuclear magnetic resonance signal F'(t) includes, for example, at least (1) Fourier transformation of the nuclear magnetic resonance signal F'(t) to obtain projection data; and filtering each projection data for each direction around the subject by, for example, a convolution method to obtain filtered projection data, and (
2) After filtering the nuclear magnetic resonance signal F'(t), it is necessary to obtain filtered projection data by Fourier transformation.

In the present invention, when the processing procedure (1) above is adopted, noise due to low frequency components in the projection data after Fourier transformation is removed, and the processing procedure (2) above is adopted. In this case, noise due to low frequency components in the nuclear magnetic resonance signal F'(t) before Fourier transformation is removed. And (2) above
In the case of the processing procedure, it is not possible to immediately remove noise due to low frequency components from the nuclear magnetic resonance signal F'(t), so the nuclear magnetic resonance signal F'(t), which gives uneven parts in the frequency spectrum obtained by Fourier transform, is Select only the predetermined signal in t), perform Fourier transform on it, find the correction amount Δc from the discrete value data on both sides of the uneven part in the frequency spectrum, and inversely convert this correction amount Δc. The quantity β(t) obtained by Fourier transformation is converted into nuclear magnetic resonance signal F'
By subtracting it from (t), a nuclear magnetic resonance signal F(t) without offset of low frequency components is obtained. This can be expressed using equation 4 as follows. That is,

[0018]

[Math 4]

However, k is an integer near the rotation angle ω=0.

[0020] Next, when expressed by number 5,

[0021]

[Math 5]

[0022] However, N is the number of points in the discrete Fourier transform.

Next, an embodiment of the present invention embodying the above principle will be described with reference to the drawings.

In FIG. 7, the subject 1 is exposed to a static magnetic field HO
The transmitter/receiver coil 2 is arranged between the transmitting and receiving coils 2 which are wound so as to induce a magnetic field orthogonal to the static magnetic field HO. The tuner 3 selects an electromagnetic wave of a specific frequency from the electromagnetic waves generated by the transmitting system 4, and selects an electromagnetic wave of a specific frequency from the electromagnetic waves generated in the transmitting system 4, and
An excitation pulse is applied to the transmitter/receiver coil 2 in synchronization with the transmitter/receiver coil 2. The amplifier 5 amplifies the nuclear magnetic resonance signal received by the transmitter/receiver coil 2 and outputs it to two phase detectors 6A and 6B. The reference signal generator 7 has a phase shifter 7A and a 90° phase converter 7B, and generates two types of reference waves having the same frequency as the nuclear magnetic resonance signal and having phases different from each other by 90°,
Each of the reference signals is configured to be output to phase detectors 6A and 6B. The two phase detectors 6A and 6B each phase-detect the nuclear magnetic resonance signal using a reference wave and separate it into two analog signals F'(t) (real part and imaginary part). The separated two signals F'(t) are amplified by amplifiers 8A and 8B, high-frequency components are removed by low-pass filters 9A and 9B, and then digitized by A/D converters 10A and 10B and sent to a noise correction processor. 11. The noise correction processor 11 receives the digitized (discrete value) signal F'.
It is an arithmetic device that removes the influence of low frequency components when obtaining filtered projection data from (t), and the processing procedure for signal F'(t) is the same as (1) shown in the explanation of the principle of this invention. If the procedure is as follows, the influence of low frequency components is removed according to the flow in FIG. , the influence of low frequency components is removed according to the flow of FIG. That is, as shown in FIG. 8, the signal F'(t) is Fourier transformed, and the data on both sides of the data giving unevenness in the projection data P'(ω) obtained (P'n and P shown in FIG. ′
n+4) by performing linear interpolation, the correction amount Δc
, and then the projection data P'(ω)
The projection data P(ω) is corrected by subtracting the correction amount Δc from the data giving the unevenness in the inside, and then the corrected projection data P(ω) is filtered by, for example, a convolution method. Obtain the projected projection data. In addition, in the case of the procedure (2) above, as shown in FIG.
(t), the data that gives unevenness to the projection data P'(ω) after Fourier transforming the signal F'(t), and the data on both sides thereof (Pn to Pn+ in FIG.
4) is selected and subjected to Fourier transform to obtain P'n to P'n+4. Next, a correction amount Δc is obtained by performing linear interpolation between P′n and P′n+4, and the signal F′(t) is corrected using the correction amount Δc according to equation 5, thereby reducing noise in the low frequency component. Find the signal F(t) without. This F(t) is filtered and the filtered F(t) is Fourier integrated to obtain filtered projection data.

The image processing device 12 inputs the filtered projection data obtained in the noise correction processor 11 according to any of the procedures described above, reconstructs an image based on this data, and reconstructs a specific atomic nucleus. For example, it is configured to output a video signal for an image reflecting the spin density, relaxation time, etc. of H'.

[0026] According to the configuration described in detail above, noise due to low frequency offset components is removed from the projection data that has been filtered, which is necessary for image reconstruction, so it is possible to reconstruct an image free of artifacts. .

Although one embodiment of the present invention has been described above in detail, the present invention is not limited to the above embodiment, and can be implemented with appropriate modifications within the scope of the gist of the invention. Needless to say.

The present invention can be applied to nuclear magnetic resonance apparatuses that employ various image reconstruction methods other than the back projection method, such as the successive approximation method and the Fourier transform method.

[0029]

According to the present invention, even if a detected nuclear magnetic resonance signal contains a low frequency offset, the noise due to this low frequency offset is automatically removed during the data processing process, thereby eliminating artifacts. A nuclear magnetic resonance apparatus that can display good images can be provided.

[Brief explanation of drawings]

[Figure 1] Frequency spectrum P of an ideal nuclear magnetic resonance signal
Explanatory diagram showing (ω)

[Figure 2] Explanatory diagram showing the real part signal obtained by phase detection of the actually measured nuclear magnetic resonance signal

[Figure 3] Explanatory diagram showing the imaginary part signal obtained by phase detection of the actually measured nuclear magnetic resonance signal

[Figure 4] Explanatory diagram showing the frequency spectrum P'(ω) obtained by Fourier transforming the signal after phase detection

[Fig. 5] Explanatory diagram showing an image with artifacts

[Fig. 6] Explanatory diagram showing calculation of correction amount Δc by linear interpolation

[Fig. 7] Block diagram showing one embodiment of the present invention

[Figure 8] Flow diagram showing the processing procedure of noise correction

[Figure 9] Flow diagram showing the processing procedure of noise correction

[Explanation of symbols]

1. Subject 2 Transmitting/receiving coil 3 Tuner 4 Transmission system 5　Amplifier 6A, 6B Phase detector 7A phase shifter 7B 90° phase converter 8A, 8B amplifier 9A, 9B Low pass filter 10A, 10B A/D converter 11 Noise correction processor 12 Imaging processing device

Claims (1)

[Claims] 1. A detection signal that detects a magnetic resonance signal induced by a magnetic resonance phenomenon with two reference waves whose resonance frequencies have phases different from each other by 90 degrees, and separates the detected signal into two signals. A correction means for Fourier transforming the signal and removing low frequency component noise by interpolation processing on the signal in Fourier space, and a reconstruction means for reconstructing the signal from which the low frequency component noise has been removed by the correction means. A nuclear magnetic resonance imaging device featuring: