JPH08299301A - Magnetic resonance(mr) imaging device - Google Patents

Abstract

PURPOSE: To dispense with a high-speed A/D converter even the application of only a single FIR system to be enough for an intended purpose. CONSTITUTION: The phase of a local oscillation signal from DDS 34 is alternately selected between 0 degree and 90 degrees at every sampling period Ts of an A/D converter 29, and data obtained from the A/D converter 29 is filtered with a single FIR system 30. In addition, every data is alternately assigned through a multiplexer 32 for separation into I data and Q data. Regarding the Q data, a time delay is corrected through a multiplier 33.

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 an MR imaging apparatus having a digitized receiving system.

[0002]

2. Description of the Related Art In an MR imaging apparatus, it is necessary to receive a resonance signal, perform phase detection and A / D conversion, and further subject the digital data to image filtering processing.
In recent years, receiving systems that perform these processes have been digitized. Normally, the received signal is sampled at a frequency four times the signal band (four times oversampling), and the I channel (0 ° phase side signal channel) and the Q channel (90 ° phase side signal channel) are sampled. FIR (Finite Impulse Respon)
se) The system is configured to perform digital phase detection and image filtering processing. By digitizing the receiving system in this way, it becomes possible to remove the artifacts caused by the analog element due to drift or the like that is unavoidable in the analog receiving system.

[0003]

However, in a conventional digitized receiving system, a high-speed A / D converter is required because oversampling by 4 times the signal band is required, and an FIR system is also required. It is also necessary to have two systems. Therefore, there is a problem that the configuration is complicated and expensive.

In view of the above, the present invention does not require a high-speed A / D converter and requires only one FIR system.
Accordingly, it is an object of the present invention to provide an MR imaging device in which the digitized receiving system has a simple structure and is inexpensive.

[0005]

In order to achieve the above object, in the MR imaging apparatus according to the present invention,
A magnetic field generator, a high frequency transmitter, a receiver, a sequence controller that controls these, and a data processor that processes data obtained from the receiver are provided, and the receiver is a band of a received signal. The phase is 0 ° and 9 for each cycle determined by twice the sampling frequency
A local oscillator that alternately switches to 0 ° and a frequency conversion that converts the carrier frequency of the received signal to a frequency that is 0.5 times the band of the received signal by mixing the local oscillation signal and the received signal. , An A / D converter for A / D converting the output signal of the frequency converter for each sampling cycle, an FIR system for filtering output data from the A / D converter, and an FIR system for the FIR system. It is characterized in that the output data is alternately distributed for each one, and a multiplier for performing multiplication for time delay correction on one of the distributed data.

[0006]

The received signal is A / D converted after the carrier frequency is converted to a frequency 0.5 times the signal band and then sampled by so-called double oversampling. Since the phase of the local oscillation signal is alternately switched between 0 ° and 90 ° in each sampling cycle, the data obtained by sampling and A / D conversion has I data and Q data arranged alternately. It has become a problem. For data including such I data and Q data,
Filtering is done with one FIR system. Therefore, it is not necessary to separately provide the FIR system for I data and Q data, and only one FIR system is required. The output data of the FIR system is alternately distributed one by one, so that the I data and the Q data are separated. Since the Q data is multiplied for the time delay correction, it is corrected that every other Q data appears.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. An MR imaging apparatus according to an embodiment of the present invention includes a receiver as shown in FIG. 1, and the DDS 34 shown in FIG. 1 is configured as shown in FIG. The overall configuration of the MR imaging device is as shown in FIG.

For convenience of explanation, the overall structure will be described. As shown in FIG. 3, a magnetic field generator 11, an RF transmitter 12, and a receiver 13 are provided. The magnetic field generator 11 is for generating 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. An 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,
Processing for image reconstruction is performed.

The receiving device 13 shown in FIG. 3 is constructed as shown in FIG. In FIG. 1, the resonance signal received by the receiving coil 21 is first amplified by the preamplifier 22, passes through the bandpass filter 23, and is passed through the mixer 24.
Sent to The frequency characteristic of the received signal is as shown in FIG. 4A, and the center frequency is the frequency Fr equal to the resonance frequency, but the information necessary for image reconstruction has a certain bandwidth centered on the frequency Fr. It is assumed to be included in BW. The frequency characteristic of the bandpass filter 23 is such that only a high frequency signal having the center frequency Fr and the width BW is passed. Then, it is converted into a two-stage intermediate frequency by the heterodyne method.

First, in the mixer 24, the signal having the frequency Fa is mixed to be converted into the intermediate frequency Fi (Fa = Fr + Fi). The frequency characteristic of this signal is as shown in FIG. Here, the intermediate frequency Fi is an appropriate frequency of 2 MHz or higher. Then, it is sent to the next mixer 26 through the bandpass filter 25 having the frequency characteristic that allows only the signal of the center frequency Fi and the width BW to pass, and is mixed with the signal of the frequency Fb (Fb = Fi + Fof), It is converted into a signal having a frequency characteristic as shown in FIG. The center frequency of this signal is the offset frequency Fof, and its bandwidth is B
W, and Fof = BW / 2.

This signal is sent to an A / D converter 29 through a low-pass filter (filter that passes the frequency BW and below) 27 for preventing aliasing and a capacitor 28 for cutting DC components. In the A / D converter 29,
Frequency Fs from the sequence controller 14 (FIG. 3)
Is sampled by the sampling signal of and converted into digital data. This sampling frequency is twice the frequency of BW as shown in (c) of FIG.
So-called double oversampling is performed. Due to double oversampling, aliasing can be removed.

The output data of the A / D converter 29 is FI.
In the R system 30, the coefficient read out from the coefficient RAM 31 is multiplied by the coefficient, added, and digitally filtered. The multiplexer 32 alternately separates the data into I data and Q data one by one, and the Q data is multiplied by a predetermined coefficient in the multiplier 33. This multiplier 3
The time delay is corrected by the multiplication in 3.

The local oscillation signal of frequency Fb sent to the second-stage mixer 26 is a DDS (Dgital Direct Synthesizer).
Is generated by using the bandpass filter 35.
It is input to the mixer 26 via. And this DD
A signal for instructing the phase and frequency is given to S34 from the sequence controller 14 (FIG. 3), and the phase is switched between 0 ° and 90 ° at every sampling period Ts.

The DDS 34 is constructed as shown in FIG. 2, and the value set in the register 43 is added to the adder 44.
Then, the address value of the ROM 45 is generated by sequentially adding with, and the ROM 45 is read with this address value to take out the waveform data previously stored in the ROM 45 and obtain the analog waveform signal through the D / A converter 46. , Is. The phase register 41 and the frequency register 42 are set by the sequence controller 14, and these values are set in the register 43. By the sequence controller 14, the upper two bits of the phase register 41
The second bit is inverted every sampling period Ts, which causes the output signal to have a 0 ° phase, 9 ° for each Ts.
Alternately switched to 0 ° phase.

As described above, since the phase of the signal of the frequency Fb to the mixer 26 is alternately switched at the cycle Ts, the A / D converter 29 outputs I data and Q data at every sampling cycle Ts. The data will be output alternately. The data in such a sequence is filtered by the FIR system 30 and then separated into I data and Q data by the multiplexer 32. That is, 1
Since one FIR system 30 filters the I data and the Q data, only one FIR system is required, which simplifies the configuration.

Since the Q data separated by the multiplexer 32 has a time delay, it is corrected by multiplication in the multiplier 33. This time delay becomes 2Ts because Q data appears every other sampling period Ts. Therefore, the multiplication coefficient becomes | exp (2π · Fa · Ts) |.

This will be described in more detail. When the transfer function of the FIR system 30 which is a linear time discrete system is h (nT), its input x (nT) and output y
The relationship with (nT) is as follows in Equation 1,

[Equation 1] Is represented by Considering the Z transformation Y (z) = H (z) · X (z) of this, when the sampling sequence (input) is delayed by time kT, that is, when the input becomes x (nT−kT), Is the following Equation 2 in Z conversion,

[Equation 2] Equivalent to. Therefore, the following Equation 3

(Equation 3) And therefore in the time domain,

[Equation 4] Becomes From this, it can be seen that the time delay can be corrected by multiplying by the multiplication coefficient as described above.

The I data and Q data, which are the phase detection output data thus obtained, are sent to the computer 15 (FIG. 3), arranged in a predetermined raw data area, and two-dimensional Fourier transformed to form an image. Reconstructed.

The above description relates to one embodiment, and it is needless to say that the specific structure and the like can be variously modified without departing from the spirit of the present invention.
For example, the oscillator that generates a signal of frequency Fb that changes in phase for each sampling period input to the mixer 26 may have a configuration other than the DDS shown in FIG.

[0020]

As described in the above embodiments, according to the MR imaging apparatus of the present invention, since double oversampling is sufficient, it is possible to use a slow A / D converter and perform filtering. Since only one FIR system is required, the structure is simple and can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is a block diagram showing a receiver of an MR imaging apparatus according to an embodiment of the present invention.

2 is a block diagram showing a detailed configuration of a DDS 34 in the receiving apparatus of FIG.

FIG. 3 is a block diagram showing the overall configuration of the same embodiment.

FIG. 4 is a frequency characteristic diagram of a signal of each unit of the embodiment.

[Explanation of symbols]

 11 magnetic field generator 12 RF transmitter 13 receiver 14 sequence controller 15 computer 21 receiver coil 22 preamplifier 23, 25, 35 bandpass filter 24, 26 mixer 27 lowpass filter 28 DC cut capacitor 29 A / D converter 30 FIR system 31 coefficient RAM 32 multiplexer 33 multiplier 34 DDS 41 phase register 42 frequency register 43 register 44 adder 45 waveform ROM 46 D / A converter

Claims (1)

[Claims] 1. A magnetic field generator, a high-frequency transmitter, a receiver, a sequence controller for controlling them, and a data processor for processing data obtained from the receiver, wherein the receiver comprises: 2 of received signal band
A local oscillator whose phase alternates between 0 ° and 90 ° in each cycle determined by the double sampling frequency, and the carrier frequency of the received signal is changed by mixing the local oscillator signal and the received signal. Of the frequency converter for converting the frequency to a frequency of 0.5 times the band of A, the A / D converter for A / D converting the output signal of the frequency converter for each sampling period, and FIR system for filtering output data, and this F system
An MR imaging apparatus comprising: a multiplier for alternately allocating output data of the IR system for each one and multiplying one of the distributed data for time delay correction.