JP3111419B2 - Nuclear magnetic resonance inspection system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a nuclear magnetic resonance phenomenon (M
R phenomenon), a nuclear magnetic resonance inspection apparatus for performing imaging and spectroscopy measurement,
The present invention relates to improvement of an MR signal receiving system.

[0002]

2. Description of the Related Art A nuclear magnetic resonance examination apparatus irradiates a subject (a human body) with an RF pulse obtained by amplitude-modulating an RF carrier signal having a frequency corresponding to the resonance frequency of the subject, thereby exciting the subject and applying the RF pulse to the subject. The resulting NMR signal is received, data is collected from the received signal, and the data is processed to reconstruct an image or perform spectroscopy measurements.

Conventionally, as a configuration for collecting data from the received signal, conventionally, the received signal is detected using a complex phase detection circuit using the above-mentioned carrier signal as a reference signal, and then sampled and held. /
The configuration of D conversion is adopted. In other words, the received signal has a distribution in a band with a high frequency around the resonance frequency, and this is converted into a signal in a band around DC by phase detection. It can be called frequency band conversion.

[0004]

However, as in the prior art, after the received signal is converted into a signal of a band centered on DC by phase detection, sampling and holding, A
When the configuration of performing the A / D conversion is employed, it is difficult to prevent the low frequency noise from being mixed into the signal before the A / D conversion. Therefore, the low frequency noise is included in the collected data. Problems such as the occurrence of artifacts on the image due to the inclusion of low frequency noise components in the image information occur.

[0005] In view of the above, the present invention can effectively prevent low frequency noise from being mixed into collected data, and can improve measurement accuracy such as improvement of image quality in MR imaging by reduction of artifacts. ,
An object of the present invention is to provide a nuclear magnetic resonance inspection apparatus.

[0006]

In order to achieve the above object, a nuclear magnetic resonance inspection apparatus according to the first aspect of the present invention comprises: a magnetic field generator for generating a static magnetic field and a gradient magnetic field;
An RF pulse transmitting device for irradiating an object placed in a magnetic field with an RF pulse obtained by amplitude-modulating an RF carrier signal having a frequency corresponding to a resonance frequency; a receiving device for receiving an NMR signal from the object; A signal having a frequency higher than the frequency by an offset frequency obtained by multiplying the frequency band of the NMR signal by a predetermined multiple is given as a reference signal,
A phase detection circuit that converts the received signal into a signal in a frequency band in which the offset frequency is the center frequency, and a frequency band in which the center frequency is the offset frequency and the output of the phase detection circuit is passed. A filter and a coupling capacitor having a bandpass frequency characteristic having the width of the above-mentioned NMR signal frequency band, a sampling circuit for sampling a signal passing through the filter and the coupling capacitor, and an A for converting the sampled signal into digital data. / D converter is provided.

[0007] The sampling frequency in the sampling circuit of the nuclear magnetic resonance inspection apparatus according to claim 1 is set to be 2
As described above, the frequency band can be set near the upper limit of the frequency band of the signal after the frequency conversion by the phase detection circuit.

[0008]

When an NMR signal from a subject is received, the received signal exists in a frequency band near a resonance frequency.
This received signal is frequency-converted by the phase detection circuit. The frequency of the reference signal of the phase detection circuit is a signal having a frequency higher than the resonance frequency by an offset frequency obtained by multiplying the frequency band of the NMR signal by a predetermined multiple, and the reception signal has such a frequency that the offset frequency becomes the center frequency. It is converted to a band signal. That is, instead of being converted from a direct current into a signal having a predetermined frequency band, the signal is converted into a signal in a band having a predetermined frequency offset. This frequency-converted signal is passed through a filter and a coupling capacitor. This filter has a bandpass frequency characteristic in which the center frequency is the offset frequency and the frequency bandwidth is the width of the NMR signal frequency band. Therefore, this filter functions as a filter for preventing alias, and can remove noise components. The DC component can be removed by the coupling capacitor. Thus NM
Since noise and DC components outside the R signal frequency bandwidth are removed and then sampled and A / D converted to obtain data, it is possible to effectively prevent low frequency noise components and DC components from being mixed into the data. .

When the sampling frequency in the sampling circuit is set near the upper limit of the frequency band of the signal after the frequency conversion by the phase detection circuit, a signal appearing as an alias can be sampled, and a high-speed and expensive A / D converter can be used. Eliminates the need to use

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. A nuclear magnetic resonance inspection apparatus according to one embodiment of the present invention is configured as shown in FIG. In FIG. 1, the main magnetic field magnet 10
Is for generating a static magnetic field. Usually, it consists of a superconducting magnet or the like. A gradient magnetic field coil 11 is provided to generate a gradient magnetic field superimposed on the static magnetic field. The gradient magnetic field coil 11 is composed of three sets of coils, each of which generates three gradient magnetic fields Gx, Gy, Gz whose magnetic field strengths incline in three axes of X, Y, and Z, respectively. By selecting one of these three axial gradient magnetic fields Gx, Gy, Gz, or by combining them,
A gradient magnetic field Gs for slice selection, a gradient magnetic field Gr for reading (and frequency encoding), and a gradient magnetic field Gp for phase encoding in arbitrary directions can be created.

A subject (not shown) is arranged in the space to which the static magnetic field and the gradient magnetic field are applied. In this subject,
An RF coil 12 for irradiating the subject with an RF pulse and receiving an NMR signal generated by the subject is attached.

The coils of each set of the gradient magnetic field coils 11 include:
A current having a predetermined waveform is supplied from the gradient magnetic field power amplifier 23. Under the control of the sequence controller 52, the digital gradient magnetic field waveform generator 21 outputs Gs, Gr, Gp
Is generated, and this is the D / A
The signal is converted into an analog signal by the converter 22 and input to the gradient power amplifier 23. Since the amplified analog signal is supplied to each set of coils of the gradient coil 11, Gs, Gr, and Gp pulses corresponding to a desired waveform output from the digital gradient magnetic field waveform generator 21 are generated. Will be. This makes it possible to generate Gs, Gr, and Gp pulses having a pulse waveform required for a pulse sequence such as a spin echo method or a gradient echo method.

The subject is irradiated with the RF pulse from the RF coil 12. Therefore, the RF from the RF generation circuit 34
The carrier signal is amplitude-modulated by the amplitude modulation circuit 33, the modulated output is amplified by the RF power amplifier 35, and then supplied to the RF coil 12. The RF generation circuit 34 is controlled by the sequence controller 52 and generates an RF carrier signal having a frequency f corresponding to the resonance frequency of the subject.
The amplitude modulation signal is obtained by converting a digital RF pulse waveform generated from the digital RF waveform generator 31 under the control of the sequence controller 52 into an analog signal by the D / A converter 32. The RF generation circuit 34 generates a signal having a frequency (f + If) higher than the frequency f by a predetermined frequency offset (frequency If) in addition to the RF carrier signal having the frequency f, and outputs the signal to the complex phase detection circuit 43. As a reference signal.

When the RF coil 12 irradiates the subject with an RF pulse, nuclear spins in the subject are excited, and an NMR signal is generated. This NMR signal is received by the RF coil 12, and is input to the complex phase detection circuit 43 via the preamplifier 41 and the high-pass filter 42. The high-pass filter 42 is a high-pass filter that passes only signals near the resonance frequency f, and removes noise components existing outside the band of the resonance frequency f.

In the complex phase detector 43, as described above, the frequency (f) higher than the frequency of the carrier signal of the RF pulse by a predetermined frequency offset is used as the reference signal.
+ If), a signal in a frequency band around the offset frequency If is obtained by complex phase detection using this signal as a reference signal. That is, the phase detection circuit 43 functions as a frequency converter. The output signal of the phase detection circuit 43 passes through a band-pass filter 44, further passes through a buffer amplifier 45, passes through a coupling capacitor 46, and is sent to a sample-and-hold circuit 47, where it is sampled and held. Converted to data.

Here, the band filter 44 is for preventing aliasing, and is a band-pass filter that passes only a signal in a predetermined frequency band centered on the frequency If, thereby removing noise outside the frequency band. Further, since the analog signal is sent to the sample and hold circuit 47 via the coupling capacitor 46, the DC component is cut.

In the sample and hold circuit 47, the offset frequency I of the signal output from the complex phase detection circuit 43
The input signal is sampled at a sampling frequency set slightly above the upper limit of the frequency band around f. As a result, although sampling is performed at a sampling frequency lower than the Nyquist frequency, it is possible to sample an alias component generated from a direct current of the input signal while satisfying the Nyquist theorem. Thus, while reducing the sampling frequency, it is possible to obtain the same effect as in the case of the 4 times oversampling frequency, and it is not necessary to increase the sampling frequency. Becomes unnecessary.

The data obtained by the A / D converter 48 is taken into the host computer 51, and subjected to Fourier transform processing for reconstructing an image. The host computer 51 controls the sequence controller 52 in accordance with not only such data processing but also pulse sequences constituting various imaging scans.

A specific example will be described with reference to FIG. Here, the frequency band of the received NMR signal is
It is assumed that the frequency is 125 KHz centered on the resonance frequency f, and the offset frequency If is set to 187.5 which is 1.5 times the band.
KHz. Normally (conventional), since a carrier signal matching the resonance frequency f is given to the complex phase detection circuit 43 as a reference signal, the detection output is output in a frequency band from DC to 62.5 KHz as shown in FIG. Therefore, sampling is performed at a sampling frequency of 125 KHz. In this case, since the detection output appears in the band from direct current, drifts of various devices appear as low-frequency components, which are mixed into data.

In this embodiment, the output signal of the complex phase detection circuit 43 has an offset frequency of 1 as shown in FIG.
This occurs in a frequency band of 125 KHz centered at 87.5 KHz. Thus, the frequency characteristic of the anti-aliasing bandpass filter 44 is such that the center frequency is 187.5 KH.
By having a band-pass characteristic that allows only a width of 125 KHz to pass through z, noise components outside that band can be obtained,
That is, frequency components lower than 125 KHz and 250 K
The frequency components higher than Hz are removed. Further, since the direct current component is removed by the coupling capacitor 46, low-frequency components that cannot be removed when a detection output having a frequency characteristic as shown in FIG. You can do it.

Thereafter, as shown in FIG. 2C, sampling is performed by a sample and hold circuit 47 having a sampling frequency of 250 KHz. Then, the sampling of the alias component of the input signal (dotted line) existing at the center frequency of 187.5 KHz and the frequency band of 125 KHz (appearing in the band from DC to 125 KHz as shown by the solid line) could be performed. Become. Since the center frequency of this alias component is 62.5 KHz,
By sampling this at 250 KHz, the same effect as in the case of a 4 times oversampling frequency can be obtained.

When the signal to be sampled is in a frequency band having a center frequency of 187.5 KHz and a width of 125 KHz, the sampling frequency is usually set to 500.
It is thought that it is necessary to set the frequency to KHz. However, if this is the case, a high-speed A / D converter 48 is required, resulting in a disadvantage in cost. By setting the sampling frequency at the upper limit of the frequency band of the output signal of the complex phase detection circuit 43 as in this embodiment, the alias component of the signal can be sampled, and the sampling frequency is lower than the Nyquist frequency. We can sample while satisfying the Nyquist theorem,
It is not necessary to use a high-speed A / D converter 48.

Therefore, a signal having an appropriate frequency offset is obtained from the complex phase detection circuit 43 as described above.
An unnecessary frequency band is removed by a band-pass filter, a DC component is removed by a coupling capacitor 46, and sampling and A / D conversion are performed in a frequency region having an appropriate frequency offset, so that an inexpensive A is obtained. In the case where NMR data without low-frequency noise can be collected using the / D converter 48 and imaging is performed,
It is possible to reconstruct an image of excellent image quality without artifacts due to low frequency noise.

It should be noted that the above description is for one embodiment, and the specific configuration and the like can be variously changed without departing from the spirit of the present invention.
For example, the specific value of the frequency can be another value.

[0025]

As described above, according to the nuclear magnetic resonance inspection apparatus of the present invention, a signal having an appropriate frequency offset is obtained as a detection output, and the signal is passed through a filter to obtain a low-frequency signal. Since the noise component is suppressed and the DC component is removed through the capacitor, data from which the low-frequency noise component has been effectively removed can be collected, and the measurement accuracy can be improved. In MR imaging, artifacts generated in an image due to low-frequency noise can be eliminated, and a reconstructed image with excellent image quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram of a nuclear magnetic resonance inspection apparatus according to one embodiment of the present invention.

FIG. 2 is a graph showing frequency characteristics.

[Explanation of symbols]

 Reference Signs List 10 Main magnetic field magnet 11 Gradient magnetic field coil 12 RF coil 21 Digital gradient magnetic field waveform generator 22, 32 D / A converter 23 Gradient magnetic field power amplifier 31 Digital RF waveform generator 33 Amplitude modulation circuit 34 RF generation circuit 35 RF power amplifier 41 Preamplifier 42 high-pass filter 43 complex phase detector 44 band-pass filter 45 buffer amplifier 46 coupling capacitor 47 sample-and-hold circuit 48 A / D converter 51 host computer 52 sequence controller

Claims (2)

(57) [Claims] 1. A magnetic field generator for generating a static magnetic field and a gradient magnetic field, and an RF pulse transmitter for irradiating an object placed in the magnetic field with an RF pulse obtained by amplitude-modulating an RF carrier signal having a frequency corresponding to a resonance frequency. Device and N from subject
A receiving device for receiving the MR signal;
A phase detection circuit for providing a signal having a frequency higher by an offset frequency obtained by multiplying the frequency band of the MR signal by a predetermined multiple as a reference signal, and converting the received signal into a signal having a frequency band having the offset frequency as a center frequency. And the center frequency is the offset frequency and the frequency bandwidth is N.
A filter and a coupling capacitor having a band-pass frequency characteristic having a width of an MR signal frequency band, a sampling circuit for sampling a signal passing through the filter and the coupling capacitor, and an A / D for converting the sampled signal into digital data A nuclear magnetic resonance inspection apparatus comprising a converter. 2. The nuclear magnetic resonance inspection apparatus according to claim 1, wherein the sampling frequency of the sampling circuit is set near the upper limit of the frequency band of the signal after frequency conversion by the phase detection circuit.