JPS61222440A - Magnetic resonance imaging apparatus - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance apparatus that collects image data on a plurality of tomographic planes within a subject using a multi-slice method.

[Technical background of the invention and its problems]

As is well known, a magnetic resonance imaging apparatus uses magnetic resonance to collect the distribution of specific atomic nuclei within a living body or other subject as image data.

Practical methods for collecting image data using magnetic resonance imaging equipment include a three-dimensional imaging method that images the entire distribution at once, and a method that images a specific two-dimensional tomographic plane by selecting it. . The latter method is common due to the data collection time and computer processing power. In this method, in order to obtain the distribution of all protonocytes in the region of interest, it is necessary to collect data on a large number of tomographic planes, but it would be very time-consuming if the data were collected sequentially. Therefore, a multi-slice method has been considered in which image data on another tomographic plane is collected by utilizing the recovery time of nuclear magnetization during the collection of image data on one tomographic plane. In the multi-slice method, a gradient magnetic field for slicing is applied to the subject, and then the static magnetic field or the frequency of the transmission radio frequency is switched at high speed.The latter method, in which the frequency of the transmission radio frequency is switched, is easier to produce. .

By the way, in magnetic resonance imaging apparatuses, the S/N of magnetic resonance signals detected from a subject is generally low. For example, in the projection reconstruction method, the data acquisition sequence is repeated, and the magnetic resonance signals obtained at each data acquisition step are A method is used in which the resonance signals are averaged for each tomographic plane. In this case, since the magnetic resonance signal is detected by phase-sensitive detection, there is a constant difference between the phase of the transmission radio frequency (ωg+iΔω) for exciting nuclear spins and the reference signal (ωa) for phase-sensitive detection. A phase relationship is required.

Conventionally, methods for obtaining a transmission high frequency with an angular frequency of ω+1Δω (i=1.2...n) include synthesis using a frequency synthesizer, and a reference high frequency signal of ω (same as the reference signal for phase sensitive detection). ) and a pair of shift frequency signals in a quadrature phase relationship with a frequency iΔω (this is called a shift frequency). It is being However, in these methods, when the deviation frequency 1Δω is changed to Δω, 2Δω, ...nΔω for each data acquisition step, the phase relationship between the transmitted high frequency and the standard high frequency signal (reference signal) cannot be maintained constant. There is no guarantee that it will sag. Therefore, in the above-mentioned averaging, it is necessary to perform phase correction on each signal of the magnetic resonance signals subjected to phase-sensitive detection and then add them, which requires a huge amount of calculation. In addition, especially in the Fourier imaging method, it is not possible to perform phase correction for each magnetic resonance signal, so there is a problem that a method of improving the S/N by averaging cannot be used.

[Purpose of the invention]

An object of the present invention is to easily determine the phase relationship between a transmitted high frequency and a reference signal for phase-sensitive detection in each data collection step when time-divisionally collecting video data on multiple tomographic planes within a subject. The object of the present invention is to provide a magnetic resonance imaging device that can be kept constant.

[Summary of the invention]

In order to achieve this object, the present invention uses a memory (RO) in which cosine and sine waveforms are respectively stored as digital values.
M), the contents of these ROMs are read out at each data acquisition step using a clock whose frequency corresponds to the deviation frequency with an angular frequency of 1Δω, and the digital value of the output is D/A converted, and the data is transmitted. It is characterized by sequentially generating a pair of shifted frequency signals with an angular frequency of 1Δω having an orthogonal phase relationship for generating high frequencies.

The pair of shift frequency signals obtained in this way are mixed with a reference high frequency signal having an angular frequency of ω0, which is the same as the reference signal for phase sensitive detection of the magnetic resonance signal from the subject, and the mixed output of the pair is further modulated. By combining these signals, a transmission high frequency wave consisting of a single sideband wave with an angular frequency of ω(1+iΔω) is obtained.

(Effects of the Invention) In the present invention, since the shift frequency of the transmission high frequency is changed depending on the read clock frequency of the memory, the initial phase of the shift frequency signal in each data acquisition step, that is, the initial phase of the transmission high frequency is changed from the memory. The reproducibility is extremely high.Thereby, the phase relationship between the transmission high frequency and the reference signal for phase-sensitive detection can always be kept constant.

Therefore, according to the present invention, for example, in the case of projection reconstruction method,
Since it becomes possible to perform averaging for each tomographic plane without performing complicated phase corrections on the magnetic resonance signals received in each data acquisition step, data processing time is significantly shortened, and calculation errors are introduced accordingly. is prevented.

In addition, with Fourier imaging, it is now possible to average magnetic resonance signals for each tomographic plane, which was previously considered impossible.
It is possible to improve the S/N ratio.

[Embodiments of the invention]

FIG. 1 is a block diagram showing the configuration of the main parts of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

In FIG. 1, data 1 has an angular frequency of 1Δω (+=1.2...
・n) is digital data corresponding to the deviation frequency, and is converted into an analog voltage by the D/A converter 2.

The output of this D/A converter 2 is converted by a voltage-frequency converter 3 into a signal with an angular frequency of N-iΔω. Memory (ROM) 7a, 7b will be described later.
This is the number of words for one period of cosine and sine wave data respectively stored in .

The output signal of the voltage-frequency converter 3 is supplied to one input terminal of a gate (AND gate) 4.

A gate pulse 5 is applied to the other input end of the gate 4 from a controller or the like (not shown) at a timing when the D/A converter 2 and the voltage-frequency converter 3 are stabilized in response to the input of the data 1. This gate pulse 5 is RO
The read start timing of M7a and M7b is determined. The output of the gate 4 is input as a read clock to an N-ary address counter 6 that controls read addresses of the ROMs 7a and 7b. As a result, the contents of ROM7a and 7b are
It is read out by a readout clock having an angular frequency of N-iΔω. As a result, the outputs of the D/A converters 8a and 8b that convert the outputs of the ROMs 7a and 7b into analog
Signals Δωt and 5iniωΔt, that is, a pair of shift frequency signals having an orthogonal phase relationship are obtained. Based on these pair of shifted frequency signals, a single sideband transmission high frequency signal is generated by a known SsB modulation circuit.

That is, the pair of shift frequency signals from the D/A converters 8a and 8b are supplied to one input terminal of each of the mixers 9a and 9b. A signal obtained by phase-splitting a reference high frequency signal having an angular frequency of ω0 from a high frequency oscillator 1o by a 90° splitter 11 is supplied to the other input terminal of each of the mixers 9a and 9b. . The outputs of the follower and mixers 9a and 9b each have cos iΔωt −cosωOt.

A pair of signals sin iΔωt−sinωot is obtained. Then, the output signals from these mixers 9a and 9b are combined by the 18° combiner 12, so that the C
A signal OS (ω0+1Δω), that is, a transmission high frequency signal consisting of a single sideband wave having a frequency equal to the sum of the shift frequency signal and the reference high frequency signal is obtained.

-〇- After this transmission high frequency signal is amplified by the amplifier 13, it is supplied to the probe head 14, whereby the subject is irradiated with the transmission high frequency signal.

On the other hand, the magnetic resonance signal from the subject is transmitted to the probe head 14.
After being further amplified by a preamplifier 15, it is subjected to phase-sensitive detection by a detector 16 using the reference high frequency signal from the high frequency oscillator 10 as a reference signal. This phase-sensitive detection output is digitized by an A/D converter 17, and guided to a computer (not shown) for processing.

FIG. 2 is a time chart for explaining the multi-slice data collection sequence using the Fourier imaging method in the above embodiment, in which (a) shows the transmitted high-frequency pulses at 90° and 180°, and (b) shows the transmitted high-frequency pulses from the subject. (C) is the timing for switching the read clock frequency of the ROMs 7a and 7b (the gate pulse 5
), (d), (e) and (f) are the slicing gradient magnetic field G7 and the imaging gradient magnetic field Gx.

The application timing of Gy is shown respectively.

In the above embodiment, the transmission high frequency of ωD+iΔω and ω8
The phase difference between the reference high frequency signal (reference signal) and the reference high frequency signal is supplied to the address counter 6 in the ROM 7a.

After the time from the time when the readout clock 7b is started (timing of the gate pulse 5), the time becomes iΔt. Therefore, if the time between this readout clock star and the transmitted high-frequency pulse is kept constant, it is possible to excite nuclear spins with a constant phase difference with respect to the reference signal, and as a result, the detected The magnetic resonance signal will also have a certain phase difference with respect to the reference signal. Therefore, the magnetic resonance signals obtained by repeating this data acquisition sequence always have the same phase for each tomographic plane in each data acquisition step, so it is possible to add and average these degrees for each tomographic plane. , it is possible to improve the S/N ratio.

Furthermore, in the projection reconstruction method, addition and averaging can be performed without performing the phase correction that is conventionally required, and a high S/N ratio can be obtained with a small amount of calculation.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the main parts of a magnetic resonance imaging apparatus according to an embodiment of the present invention, and FIG. 2 is a time chart 1 to 1 for explaining its operation. DESCRIPTION OF SYMBOLS 1... Deviation frequency data, 2... D,/A converter, 3... Voltage-frequency converter, 4... Game 1~.
5... Gate pulse, 6... Address counter, 7
a, 7b...ROM (cosine and sine waveforms),
8a, 8b・D/A converter, 9a, 9b・fi mixer 1
0...High frequency oscillator, 11...90° splitter,
12...180@combiner, 13...amplifier, 1
4... Probe head, 15... Preamplifier, 16
...Detector (phase sensitive detection means), 17...A/D
converter.

Claims (2)

[Claims] (1) Means for generating a pair of shift frequency signals having a quadrature phase relationship, means for mixing the pair of shift frequency signals and a reference high frequency signal, and modulating the pair of mixed outputs obtained by the means. means for generating a transmission radio frequency consisting of a single sideband wave having a frequency equal to the sum of the deviation frequency signal and the reference radio frequency signal; a means for irradiating the specimen;
The apparatus includes means for detecting a magnetic resonance signal from the subject, and means for phase-sensitive detection of the magnetic resonance signal using the reference high frequency signal as a reference signal, and sequentially changing the frequency of the shifted frequency signal to detect the magnetic resonance signal within the subject. In a magnetic resonance imaging device that collects image data on multiple tomographic planes in a time-sharing manner,
The deviation frequency signal generating means includes a memory that stores cosine and sine waveforms as digital values, and the contents of these memories are clocked at a frequency corresponding to the frequency of the deviation frequency signal to be generated by the signal generating means. reading means, and a digital value read out from the memory;
2. A magnetic resonance imaging apparatus comprising: means for performing /A conversion to obtain the pair of shifted frequency signals. (2) The means for reading the contents of the memory includes means for converting a digital value corresponding to the frequency of the deviation frequency signal into an analog voltage, and converting the analog voltage from voltage to frequency to read the contents of the memory. 2. A magnetic resonance imaging apparatus according to claim 1, further comprising means for obtaining a clock for outputting a clock.