JPH09117425A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To rise the S/N ratios of input signals despite a simple constitution and at a low cost. SOLUTION: When an FID signal having resonance frequency fr is inputted in a receiving coil 31, a signal of frequency f1 which is different by fi from fr is received from a transmitter 36 and the signal of frequency fr is converted to a signal of f1 by a frequency converter 32. An IF filter 33 is a bandpass filter having steep and narrow bandpass characteristics at the center frequency of fr, and by using the same, a band limited IF signal is sampled at a sampling frequency of approximately two fold of the limited band and is digitized by an A/D converter 34.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR imaging apparatus for imaging by utilizing a nuclear magnetic resonance phenomenon, and more particularly to improvement of a receiving apparatus thereof.

[0002]

2. Description of the Related Art In a conventional MR imaging apparatus, a resonance signal receiver is constructed as shown in FIG.
The frequency of the ID signal) is down-converted at least twice (called a double superheterodyne method), A / D-converted at a sampling frequency higher than the upper limit frequency of the frequency band of the obtained signal, and then subjected to a two-dimensional Fourier transform. The image is then reconstructed.

That is, as shown in FIG. 4, the FID signal received by the receiving coil 31 has a resonance frequency fr, but the frequency converter 32 mixes this with the signal of the frequency f1 to obtain the frequency fi ( = Fr-
f1) signal is converted. Further, the frequency converter 37 again mixes it with the signal of the frequency f2 to convert it into a signal of the frequency fj (= fr- (f1 + f2), passes through the anti-aliasing filter 38, and
After being sent to the D converter 34 for sampling and conversion into digital data, it is sent to the digital signal processing device 35 for processing such as Fourier transform.

By these two down conversions,
The center frequency of the original FID signal is dropped to fj,
The spectrum 41 in FIG. 5 is obtained. The anti-aliasing filter 38 has a bandpass filter characteristic 42 centered on this frequency fj, and thereby limits the frequency band of the FID signal. The sampling frequency fs is higher than the upper limit of the limited frequency band. In FIG. 5, fn is the Nyquist frequency (fn =
fs / 2).

[0005]

However, in the case where the frequency of the resonance signal is down-converted twice as described above, there is a problem that the analog circuit becomes complicated and the quality of the signal deteriorates. Furthermore, since the signal converted into the frequency band near DC by the two down conversions is A / D converted, the signal near DC is converted to 1
/ F noise (flicker noise proportional to 1 / frequency)
Etc. are detected, and the S / N ratio deteriorates. Also, A
An anti-aliasing filter 38 has to be used before the / D converter 34, but this filter 38 is required to have a wide band and a very steep band pass filter characteristic, which is difficult to realize. There is also.

In view of the above, the present invention provides an MR imaging apparatus including a receiving apparatus which avoids the difficulty of the conventional filter characteristics, simplifies an analog circuit, and further, does not cause signal deterioration. With the goal.

[0007]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises a magnetic field generator, a high frequency transmitter, a receiver, and a sequence controller for controlling them. The receiving device of, a frequency converter for converting the frequency of the received signal into an intermediate frequency of a frequency of several MHz or more, a bandpass filter having a narrow band and steep characteristics, through which the signal of this intermediate frequency is passed, It is characterized by an A / D converter that samples the signal that has passed through the bandpass filter at a sampling frequency that is about twice the band of the signal and performs A / D conversion.

The received resonance signal is once down-converted to a relatively high intermediate frequency, then sampled and A / D converted. Therefore, 1 / f noise near DC can be avoided. The sampling frequency is assumed to be a low frequency that is about twice the band of the received resonance signal. When a signal with a relatively high intermediate frequency is sampled at such a low sampling frequency, from the Nyquist theorem, The above-mentioned signal band appears due to aliasing in the frequency range below the Nyquist frequency of 2, which has the same result as the conventional two-time down conversion.

[0009]

Next, embodiments of the present invention will be described in detail with reference to the drawings. The MR imaging apparatus according to the present invention is configured as a whole as shown in FIG. 3, and the receiver portion thereof is configured as shown in FIG. As shown in FIG. 3, the MR imaging apparatus includes a magnetic field generator 11 and an RF transmitter 12.
And a receiving device 13. Magnetic field generator 11
Generates a static magnetic field and a gradient magnetic field superimposed on the static magnetic field. A subject (not shown) is placed in this magnetic field, and the RF transmitter 12 irradiates the subject with an RF pulse to excite its nuclear spin. The NMR signal generated from the excited subject is received by the receiving device 13.
The sequence controller 14 controls the pulse waveform of the gradient magnetic field and the generation timing, controls the carrier frequency of the RF pulse, the envelope waveform and the generation timing thereof, and further gives a timing signal to the receiving device 13 to control it. The data obtained in the receiving device 13 is taken in by the computer 15 and processed for image reconstruction.

As shown in FIG. 1, the receiving device 13 includes a receiving coil 31, a frequency converter 32, and an intermediate frequency filter 3.
3, an A / D converter 34, a digital signal processing device 35, and an oscillator 36, and down-converts the intermediate frequency of one stage only once. That is, the resonance signal (F
The ID signal) is sent to the frequency converter 32. The frequency converter 32 outputs a signal having a frequency that is the difference between the frequencies of the two input signals, and for example, an analog multiplier that multiplies the FID signal and the signal of frequency f1 is used.

A signal of frequency f1 from an oscillator 36 is input to the frequency converter 32, and an FID signal having a center frequency fr as shown in FIG. 2 (a) is shown in FIG. 2 (b).
Is converted into a signal having a center frequency fi (= fr-f1) as shown in FIG. This intermediate frequency fi is, for example, several MHz
Use a frequency of from several tens of MHz. Although the center frequency is down-converted in this way, the spectrum of the frequency band of the FID signal itself does not change.

The signal of frequency fi is the intermediate frequency filter 3
3 and the band is limited. The filter 33 has a center frequency fi and a pass band of 1 as shown in FIG.
It has a narrow band steep band pass filter characteristic of 00 kHz to 200 kHz. The filter 33 having such characteristics is a crystal filter, a ceramic filter,
Alternatively, it can be easily realized with a SAW filter.

The band-limited signal is sampled and A / D-converted by the A / D converter 34 at a frequency fs much lower than the frequency fi, as shown in FIG. As a result of the sampling, a large number of spectra appear for each frequency fn by folding back from Nyquist's theorem as shown in (e) of FIG. 4, and the lowest one can be used. Where fn is the Nyquist frequency and is 1/2 the sampling frequency fs
Becomes The digital signal obtained as a result of this sampling and A / D conversion contains frequency components as shown in (e) of FIG. 2, but since frequency components below fn are used, the second down-conversion is performed as in the conventional case. It becomes the same as the case where the frequency is converted to fn or less.

Therefore, the sampling frequency fs must be higher than twice the pass band of the filter 33 so that the signal band falls within the band below the frequency fn. That is, the pass band of the filter 33 is 200 kH
If z, the sampling frequency fs is 400k
Must be higher than Hz, eg 500
It will be set to kHz.

From the A / D converter 34, a digital signal dropped to a band below the frequency fn is obtained.
Since it is the same as the A / D conversion output after performing the down-conversion, the digital signal processing device 35 handles this digital signal in the same manner as in the conventional case, performs signal processing, and finally performs two-dimensional Fourier transform. Reconstruct the image.

In this case, the filter 33 has a high frequency fi.
Since it suffices to have a bandpass filter characteristic with the center frequency as the center frequency, it is easy to easily obtain a sharp one in a narrow band by using a general technique such as a crystal filter or a ceramic filter. It is possible to avoid difficulties such as the anti-aliasing filter 38 which requires a wide band and a very steep band pass filter characteristic at a low frequency.

Further, since the signal having the intermediate frequency of the high frequency fi is sampled, 1 / f noise in the vicinity of DC can be avoided and the signal quality (S / N ratio) can be improved.

Further, since one down-conversion is required, only one frequency converter such as an analog multiplier is provided, the analog circuit is simplified, and the general filter is used as described above. It is possible to reduce the manufacturing cost.

[0019]

As described above, according to the MR imaging apparatus of the present invention, it is possible to obtain a high quality reconstructed image by reducing noise while simplifying the circuit configuration of the receiving apparatus to reduce the manufacturing cost. You can

[Brief description of the drawings]

FIG. 1 is a block diagram of a receiving device according to the present invention.

FIG. 2 is a graph showing a frequency spectrum of each part of FIG.

FIG. 3 is an overall block diagram of an MR imaging apparatus according to the present invention.

FIG. 4 is a block diagram of a receiving device according to a conventional example.

FIG. 5 is a graph showing a frequency spectrum in a conventional example.

[Explanation of symbols]

 11 magnetic field generator 12 RF transmitter 13 receiver 14 sequence controller 15 computer 31 receiver coil 32, 37 frequency converter 33 intermediate frequency filter 34 A / D converter 35 digital signal processor 36 oscillator

Claims (1)

[Claims] 1. A magnetic field generator, a high frequency transmitter, a receiver, and a sequence controller for controlling them, wherein the receiver has a frequency of a received signal of several MH.
A frequency converter for converting an intermediate frequency of about z or more, a band pass filter having a narrow band and steep characteristics, through which a signal of this intermediate frequency is passed, and a band of a received signal.
A / D that samples the signal that has passed through the bandpass filter at a sampling frequency that is about twice as low and performs A / D conversion
An MR imaging apparatus comprising a D converter.