JPS6382647A - Magnetic resonance imaging apparatus - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a magnetic resonance imaging apparatus, and particularly relates to correction of gain, phase, etc. in a receiving system thereof.

(Prior Art) In a magnetic resonance imaging (hereinafter abbreviated as MRI) device, high frequency waves are transmitted from a transmitting coil to a subject, and an MRI signal obtained from the subject is received by a receiving coil, amplified,
Imaging is performed through detection, filters, ADC (analog-digital conversion), FFT (fast Fourier transform), digital filters, and the like. This moment,
The transfer function of the receiving system is Δf centered around the center frequency fo.
The following characteristics are required in the frequency band of interest: That is, as shown in FIG. 3(A), it has a constant one-stroke width characteristic in the frequency band of interest fO±Δf;
As shown in Figure 3 (B), it has a linear phase characteristic in the above region, and as shown in Figure 3 (C), the phase difference θ of quadrature detection is 90' in the same region. be.

(Problems to be Solved by the Invention) In order to improve the above-mentioned three characteristics, conventional devices perform hardware or software processing of Ill constant system based only on the characteristics at the center frequency fo. Ta. However, according to the processing in such a conventional device, the center frequency fo
Appropriate characteristics can be obtained in the vicinity, but amplitude fluctuations and phase nonlinearity occur in other regions, and the phase difference of quadrature detection rarely becomes other than 90'.

Therefore, an object of the present invention is to eliminate the above-mentioned conventional drawbacks, correct frequency characteristics such as amplitude fluctuations and phase nonlinearity caused by processing in the receiving system, and maintain constant amplitude characteristics in the frequency band of interest. The present invention aims to provide a magnetic resonance imaging apparatus that can obtain ideal frequency characteristics such as linear phase characteristics.

[Structure of the Invention] (Means for Solving the Problems) The present invention receives an MRI signal from a subject with a receiving coil, and processes it in at least an amplified test wave section, an analog-digital conversion section, and a Fourier transform section. In a magnetic resonance imaging apparatus that performs imaging, a correction value for correcting the frequency characteristics of the received signal from the receiving coil to immediately before the Fourier transform unit is stored at least for the frequency band of interest; and a correction section that corrects the image data obtained via the receiving system based on the correction value.

(Function) In the present invention, for example, the frequency characteristics of the receiving system from the receiving coil to just before the Fourier transform section are obtained in advance as a whole process using a frequency sweep signal from the transmitting section, and a correction value for correcting this is stored in the large section. By correcting the gain, phase, etc. based on the correction values for the output from the receiving system during actual image data collection, one-stroke width fluctuations and phase fluctuations caused by processing in the receiving system can be corrected. Ideal frequency characteristics can be obtained by correcting nonlinearity and the like.

(Example) Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 9 is a block diagram of a receiving system of an MRI apparatus according to the present invention.

1 is a transmitter, which transmits high frequency (RF) when collecting image data.
In this embodiment, as a process before image data collection, a frequency sweep is sent from this transmitter 1 to measure the amplitude and phase characteristics of the receiving system. A signal is being emitted. 3 receives the MRI signal from the subject generated by the high frequency transmitted from the transmitting coil 2, and also receives the frequency sweep signal as pre-processing. After this receiving coil 3, a preamplifier 4. An amplification/detection section 5, an ADC 6, and an FFT 7 as a Fourier transform section are provided.

Reference numeral 9 denotes a storage unit that stores correction values of frequency characteristics measured by frequency sweep as preprocessing.

That is, by receiving the frequency sweep signal in the receiving system,
The transfer function of this receiving system is found. These are the hardware (and software) from reception coil 3 to just before FFT 7.
Therefore, coil 3. Unless some or all of the hardware (or software) such as the preamplifier 4 is changed, it can be considered to have a constant transfer function. Note that in some cases, accuracy may be improved by performing such measurements for each subject.

Then, a correction value calculation unit 8 provided before the storage unit 9 corrects the frequency characteristics of the transfer function obtained by preprocessing so that they become the frequency characteristics of an ideal receiving system. The calculation result is stored in the storage unit 9 in association with each frequency. Here, an example of the correction value is shown in Fig. 2 (A) and (B).
).

Shown in (C). Figure 2 (A>, (B), and (C) shows the correction values for the amplitude characteristic, two-phase characteristic, and the phase difference characteristic of quadrature detection, respectively. Gf, θf, and θf in each figure are for each of the above characteristics, respectively. Gf', θf t, and θf' indicate the actually measured characteristics.Here, ΔG, which is the difference between Gf and Gf', is the amplitude correction value, and θf and ΔO, which is the difference between θf' and θf', is the phase correction value, and ΔO, which is the difference between θf and θf', is the phase difference correction value. He will become a vassal.

Reference numeral 10 denotes a correction unit which, based on the correction values from the storage unit 9, calculates the ideal gain shown in FIG. This is corrected so that the characteristics are 2. Phase characteristics and phase difference characteristics.As a result, image data obtained with an ideal receiving system can be obtained.

Reference numeral 11 denotes an image reconstruction section, which reconstructs the cross section of the subject based on the data from the correction section 10 and images it.

According to such an MRI apparatus, before collecting image data, the transmitter 1 first outputs a frequency sweep signal, and the transmitter coil 2 transmits this signal. This frequency sweep signal is then received by the receiving coil 3, and the same processing as the actual image data processing is then performed in the receiving system. Then, by inputting the data after this processing to the correction value calculation unit 8, the transfer function of the reception system from the reception coil 3 to just before the FFT 7 is calculated, and the difference between this and the ideal characteristics is calculated as the correction value. and stores it in the storage unit 9.

This kind of preprocessing is considered to have a constant transfer function unless the hardware or software of the receiving system is changed as described above, so it is sufficient to perform it only - times, but in order to further improve the accuracy, may be performed for each subject.

After the correction values are once stored in the storage unit 9 through such pre-processing, image data from the subject is actually collected, and after being processed by the FFT 7, the correction value is stored in the storage unit 9 in the correction unit 10.
Based on the correction value from , correction is performed so that the fixed one-line width characteristic, the linear phase characteristic, and the phase difference of quadrature detection become 90°. As a result, it is possible to collect the same data as the image data obtained with the receiving system having ideal characteristics as shown in Fig. 3 (A>, (B), and (C)), and the amplitude fluctuation is 1. Phase nonlinearity and quadrature detection phase differences other than 90' can be reliably corrected within the frequency band of interest.

Note that the present invention is not limited to the above embodiments, and various modifications can be made within the scope of the gist of the present invention. .

It is not necessary to provide all of the above-mentioned three characteristics as correction values related to the frequency characteristics, and it is sufficient to have one or more of them. Further, the correction unit 9 is not necessarily provided after the FFT 7 to perform correction in frequency space, but may be provided before the FFT 7 to perform correction in time space.

[Effects of the Invention] As explained above, according to the present invention, the frequency characteristics of the receiving system are measured in advance, and the frequency characteristics of interest are corrected based on correction values that idealize the measured frequency characteristics. It is possible to obtain ideal characteristics such as constant amplitude and linear phase in the band.

[Brief explanation of the drawing]

Figure 1 is a block diagram of the magnetic resonance imaging apparatus according to the present invention, and Figures 2 (A), (B), and (C) are corrections for the amplitude characteristics of the receiving system, the two-phase characteristics, and the phase difference characteristics of quadrature detection, respectively. Characteristic diagram showing the values, Figure 3 (A>, (B), (C
) are characteristic diagrams showing the amplitude characteristic of an ideal receiving system, 1-phase iQr characteristic, and the phase difference characteristic of direct detection, respectively. DESCRIPTION OF SYMBOLS 1... Transmission part, 2... Transmission coil, 3... Receiving coil 4... Preamplifier, 5... Amplification detection part. 6...Analog-digital conversion section. 7... Fourier transform section, 9... Record n section. 10... Correction section. Agent Patent Attorney Masayoshi Misawa (C) Figure 2

Claims (5)

[Claims] (1) The high frequency waves emitted from the transmitting section are transmitted to the subject via the transmitting coil, the MRI signal from the subject is received by the receiving coil, and this is transmitted to at least the amplified test wave section, the analog-to-digital converter, and the Fourier A magnetic resonance imaging apparatus that performs processing and image formation in a conversion unit includes a storage unit that stores correction values for correcting the frequency characteristics of the reception system from the reception coil to immediately before the Fourier transformation unit, at least for a frequency band of interest; A magnetic resonance imaging apparatus comprising: a correction section that corrects image data obtained through the reception system based on correction values in a storage section. (2) The magnetic resonance imaging apparatus according to claim 1, wherein the storage unit includes, as the correction value, a correction value that makes the amplitude constant in the frequency band of interest. (3) The magnetic resonance imaging apparatus according to claim 1 or 2, wherein the storage unit includes, as the correction value, a correction value that compensates for phase nonlinearity in the frequency band of interest. (4) The magnetic resonance imaging apparatus according to any one of claims 1 to 3, wherein the storage unit includes, as the correction value, a correction value that compensates for a shift in phase difference of orthogonal detection. (5) The correction value recorded in the storage unit is determined according to the frequency characteristics obtained by processing the frequency sweep signal output from the transmitting unit in the receiving system. 2. The magnetic resonance imaging apparatus according to item 1.