JPH03228740A - Gradient magnetic field power source for nuclear magnetic resonance imaging system - Google Patents

Abstract

PURPOSE:To carry out the compensation with high precision without generating a phase strain over the whole frequency component region by compensating the strain of the gradient magnetic field wave shape due to the frequency responsiveness of a gradient magnetic field coil by correcting the input wave shape signal of an electric power amplifier. CONSTITUTION:The data obtained in an attenuation quantity detecting part 31 is proportional to the quantity including the variation portion due to the attenuation of the electric current pulse Ig due to the frequency response characteristic of an electric power amplifier 15 and a gradient magnetic field coil 3C and the attenuation portion due to the eddy current loss of the gradient magnetic field. The output data of a conversion part is sent into a calculation memory part 41, and converted to the correction data for reversely compensating the attenuation portion, and memorized in a memory part 39. A gradient magnetic field power source part 20 converts the rectangular-shaped wave pulses generated by a pulse sequencer 12 to the frequency component data by a Fourier transformation part 22, and multiplied by the correction data supplied from the memory part 39 in a correction part 23, and reversely converted to the analogue signal corrected in the reverse Fourier transformation part 24. The signal is electric-power-amplified in an electric power amplifying part 15, and the corrected electric current pulses Ig are supplied as the excitation current for the gradient magnetic field coil 3C, and a corrected gradient magnetic field is generated.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a nuclear magnetic resonance imaging system (hereinafter referred to as M
The present invention relates to a gradient magnetic field power source that supplies square-wave excitation current pulses to a gradient magnetic field coil disposed in a magnet (referred to as a Rni system), and particularly to a gradient magnetic field power source that has a gradient magnetic field distortion compensation function.

[Conventional technology]

FIG. 6 is a simplified block diagram showing the entire structure of the MR system.
A DC excitation current is supplied from a quiet magnetic field power supply 2 to the hollow part 1.
A highly uniform quiet magnetic field BO is generated within 0, and the gradient magnetic field is generated by superimposing the gradient magnetic field on the static magnetic field BO by supplying all square wave direct current pulses from the gradient magnetic field 1 source 6 at a predetermined timing to the gradient magnetic field coil 3C. Then, a high-frequency pulse is supplied from the transmitter 4 to the high-frequency coil 4C via the transmitter/receiver switch 4A. At this time, in the subject housed in the static magnetic field Bo, nuclear spins are excited by the high-frequency pulse in the slice plane selected by the gradient magnetic field, and the static magnetic field space 10 generates a magnetic resonance signal [
(hereinafter referred to as NMR signal) can be radiated as electromagnetic waves. The radiated NMR signal is detected by the high-frequency coil 4C and all antenna coils, is amplified and detected by the receiver 5 via the transmitter/receiver switch 4A, and is converted into an image by the computer 7 as the main controller via the A/D converter 6. By performing the reconstruction into signals, the entire tomographic image of the subject can be displayed on the display 8. Note that each part of the system is cooperatively controlled at a predetermined timing by a pulse sequence sent by the main control unit 7 via the interface 9 or the like.

FIG. 7 is a simplified block diagram of a conventional gradient magnetic field power supply, which includes a pulse sequencer 12 that receives signals from the main controller 7 and outputs a square wave pulse train based on a predetermined pulse sequence, and its output signal. A preamplifier 13 amplifies the output signal of the preamplifier 13, and a gradient magnetic field coil 3
Frequency correction circuit 1 that corrects based on the frequency response of C
4, a power amplifier 15 which amplifies the corrected output signal and outputs the same to the output current pulse E 9 to the gradient magnetic field coil 3C, and a current pulse Ljk current sensor 1716. The feedback circuit 16 functions to keep the current flowing through the gradient magnetic field coil 3C equal to the input signal of the total power amplifier 15, but the gradient magnetic field generated by the gradient coil 3C is distorted by eddy current loss occurring in surrounding metal objects. Therefore, it is not possible to compensate for the correspondence between the gradient magnetic field and the input signal of the power amplifier 15. The frequency component correction circuit 14 is provided to compensate for the distortion of the gradient magnetic field, and consists of a plurality of analog filters with different center frequencies, and a power amplifier 150 that adjusts the total gradient of the human input signal near each center frequency. The center frequency of the analog filter is selected at several points intermittently within the frequency component range that changes as the pulse current generator 9 changes distortion. Ru.

[Problem to be solved by the invention]

As mentioned above, in the conventional static magnetic field power supply, an eddy current flows through a metal object due to one gradient magnetic field, and the frequency response characteristics of the gradient magnetic field coil 3C, which change due to this, are compensated for at several discrete points in the frequency component band. Therefore, there is a problem in that it is not possible to uniformly compensate for the entire changing frequency component band. Furthermore, since the square wave pulse current waveform is compensated in real time by the analog circuit, phase distortion occurs in the compensation, resulting in the problem that highly accurate compensation cannot be achieved.

An object of the present invention is to perform highly accurate compensation in all frequency component bands without causing phase distortion.

[Means to solve the problem]

In order to solve the above problems, according to the present invention, a current pulse in the form of a square wave pulse is supplied via a power amplifier to a gradient magnetic field coil of a magnet including a static magnetic field coil, a gradient magnetic field coil, and a high frequency coil. In a device that generates a gradient magnetic field in a magnetic field space, a gradient magnetic field waveform detection sensor arranged in the static magnetic field space, and a detection waveform signal of this detection sensor and an input waveform signal of the power amplifier are each converted into frequency component data. an attenuation detection unit that calculates the difference between both frequency component data; and a correction data calculation that calculates correction data that inversely compensates for the attenuation based on the output data of this attenuation detection unit and stores the calculated data prior to imaging. a storage unit, and a frequency component correction unit that converts the original waveform signal of the current pulse output by the pulse sequencer at a predetermined timing into its frequency component data, corrects it using correction data stored in the calculation storage unit, and outputs the corrected signal. Then, this corrected frequency component data is converted into a full waveform signal and the power increase
and an inverse conversion unit that outputs the same output as the gradient magnetic field coil, and is configured to compensate for distortion of the gradient magnetic field waveform due to the frequency response of the gradient magnetic field coil by correcting the input waveform signal of the power amplifier. I'm sobbing.

[Effect]

According to the configuration of the present invention, the detection waveform signal detected by the gradient magnetic field detection sensor arranged at key points in the static magnetic field space and the input signal of the power amplifier at that time are respectively converted into frequency component data, and the frequency components of both are converted into frequency component data. The difference between the frequency response characteristics of the power amplifier and the gradient magnetic field coil, and the eddy Correction data including distortion of the gradient magnetic field due to current generation can be accurately grasped and stored in advance. Therefore, the square wave pulse signal as the original waveform of the current pulse emitted by the pulse sequencer is first converted into its frequency component data, multiplied by the correction data stored in advance in the frequency component correction section, and then converted back into an analog signal. By configuring the power amplifier to output it as a human input signal, the output current pulse of the power amplifier becomes a pulse waveform with correction data added mainly to the rising and falling parts, and in response to this, the gradient generated by the gradient magnetic field coil The magnetic field pulse waveform can be compensated into a square wave pulse shape that is faithful to the original waveform.

〔Example〕

The present invention will be explained below based on examples.

FIG. 1 is a block diagram showing a gradient magnetic field power source of a nuclear magnetic resonance imaging system according to an embodiment of the present invention, and the same parts as in the conventional apparatus are given the same reference numerals, and a detailed explanation will be omitted. In the figure, the gradient magnetic field power source is roughly divided into a gradient magnetic field power source section 20 and a correction circuit section 30. The correction circuit section 50 converts the detection signal of the gradient magnetic field waveform detection coil 32 arranged at important points in the static magnetic field space 10 of the magnet section 1 into a digital signal by the A/D converter 34 via the current integrator 33. Then, frequency component data is obtained by a 7-channel transformer 35. On the other hand, all of the input signals of the power amplifier 150 are converted into frequency component data by the Fourier transform unit 36 via an A/D converter (not shown). Then, the subtraction circuit 37 calculates the difference between both frequency component data. The circuit up to this point constitutes the attenuation amount detection section 31, and the data obtained by the attenuation amount detection section 31 includes the change due to the attenuation of the current pulse 9 due to the frequency response characteristics of the power amplifier 15 and the gradient magnetic field coil 3C, and the gradient magnetic field It is proportional to the travel, including the amount of attenuation due to eddy current loss. The output data of the conversion section is stored in the calculation section 38 and the storage section 3.
The data is sent to the arithmetic storage unit 41 consisting of 9, is converted into correction data for inversely compensating the amount of attenuation, and is stored in the storage unit 39 as correction data.

The gradient magnetic field power supply unit 20Fi converts square wave pulses emitted by the pulse sequencer 12 based on a predetermined pulse sequence into frequency component data in the Fourier transform unit 22 via an A/D converter (not shown in the figure), The correcting unit 25 combines the correction data from the storage unit 39 and inversely converts it into a corrected analog signal in the inverse Fourier transform unit 24, and the power amplifying unit 15 amplifies the power of this signal to generate the corrected current pulse 19t. -Q It is configured to be supplied as an excitation current to the magnetic field coil 3C to generate a corrected gradient magnetic field.

The operation of the gradient magnetic field power supply according to the embodiment will be explained below using the schematic characteristic diagrams shown in FIGS. 2 to 5. Second
The figure schematically shows the waveform of the gradient magnetic field before correction.
In contrast to the ideal waveform 51 which is a square wave, the actual gradient magnetic field waveform 52 is affected by its frequency response and eddy current and is distorted as shown in the figure. FIG. 3 is a distribution waveform diagram of the frequency component data transformed by the Fourier transform unit 35, and shows the lower limit frequency f1 of the frequency band to be corrected. upper limit frequency f,
The intensity of the frequency component is attenuated in the vicinity, resulting in a psychic waveform 53. FIG. 4 shows an intensity ratio distribution curve with peak values near the lower limit f1 and upper limit f of the correction frequency band, which is an intensity ratio showing the intensity of the frequency component of the correction data held in the storage unit 39, with the uncorrected level being 1. 54 is shown. FIG. 5 is a schematic characteristic diagram showing the waveform of the corrected current pulse 9 and the waveform of the gradient magnetic field generated by this current. By correcting the waveform 55 to the waveform 55, it is possible to compensate the waveform 56 of the generated gradient magnetic field to a waveform close to the ideal waveform 51 shown in FIG. Therefore, if the lower and upper limits of the frequency band are determined in advance to be the frequencies necessary to maintain the accuracy of the positional information of the slice plane that is included in the NMR signal, it is possible to accurately compensate the frequency components for the entire frequency band. This makes it possible to visualize tomographic images of slice planes with high accuracy.

Note that the collection of correction data by the correction circuit section 30 is performed a plurality of times by changing the position of the detection coil 32 prior to the actual imaging operation, and the average data is stored in the storage section 39. Then, during actual imaging, the pulse sequencer 12
The stored data total correction unit 23
By reading out the original waveform and correcting it, it is possible to compensate for phase shift distortion.

〔Effect of the invention〕

As described above, this invention detects the difference in frequency components between the gradient magnetic field and the original waveform on the input side of the power amplifier in the detection section, converts it into correction data that inversely compensates for the difference in frequency components, and stores it. , all frequency components of the square wave pulses emitted by the pulse sequencer during imaging are corrected by the correction data in the correction section, the corrected frequency components are inversely converted to analog pulses, the power is amplified by the power amplifier, and the current pulses are sent to the gradient magnetic field coil. and configured to generate a corrected gradient magnetic field. As a result, the detection unit can detect the frequency component attenuation of the gradient magnetic field caused by the frequency response of the power amplifier and the gradient magnetic field coil and the eddy current over the entire frequency range, and based on this, the correction data can be calculated and stored. , it is possible to correct the output current pulse waveform of the static magnetic field power supply unit over the entire required frequency component band during imaging, which cannot be achieved with the conventional gradient magnetic field power supply correction circuit that compensates for individual frequency component bands. High-fidelity compensation becomes possible, and a gradient magnetic field power supply having the ability to bring the gradient magnetic field close to the ideal waveform of a square wave pulse is obtained, and the quality of the positional information of the slice plane included in the NMR signal is improved. This makes it possible to provide an MR system that can visualize tomographic images with high precision.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a gradient magnetic field power source of a nuclear magnetic resonance imaging system which is an embodiment of the present invention, and FIGS. 2, 5, 4, and 5 are gradient magnetic field Characteristic diagrams are shown schematically showing the state of characteristic data of different parts of the power supply. Figure 6 is a block diagram showing a simplified overall configuration of a nuclear magnetic resonance imaging system. Figure 7 is a diagram of a conventional gradient magnetic field power supply. FIG. 2 is a block diagram showing a simplified configuration. 1... Magnet part, 2C... Static magnetic field coil, 3C
...Gradient magnetic field coil, 4C...High frequency coil, 2.
... Static magnetic field power supply, 3 ... Gradient magnetic field power supply, 7 ... Main controller, 10 ... Static magnetic field space, 12 ... Pulse sequencer, 14 ... Frequency correction circuit, 15 ... Electric power Amplifier, 17... Current sensor/su, 20... Gradient magnetic field power supply section, 22.5536...
Correction unit (multiplier) 24... inverse Fourier transform unit, 30.
...Correction circuit section, 31...Attenuation amount detection section, 32...
Detection coil, 53...Current integrator, 37...Subtractor, 41...Calculation storage unit, 38...Calculation unit, 39...
・Memory section. -2ri figure 2 vJ3 figure 1 same waste te 2 vJ4 figure ￥J5121 figure 6

Claims (1)

[Claims] 1) In a device that generates a gradient magnetic field in a static magnetic field space by supplying a square wave pulse-like current pulse to a gradient magnetic field coil of a magnet including a static magnetic field coil, a gradient magnetic field coil, and a high-frequency coil through a power amplifier, a gradient magnetic field waveform detection sensor arranged in the static magnetic field space; and an attenuation amount for converting the detection waveform signal of the detection sensor and the input waveform signal of the power amplifier into frequency component data and calculating the difference between both frequency component data. A detection unit, a correction data calculation storage unit that calculates correction data for inversely compensating the attenuation amount based on the output data of the attenuation amount detection unit and stores the calculated data prior to imaging, and a pulse sequencer that outputs the data at a predetermined timing. a frequency component correction section that converts the original waveform signal of the current pulse into its frequency component data, corrects it using correction data stored in the calculation storage section, and outputs the corrected frequency component data; and converts the corrected frequency component data into a waveform signal. and an inverse converter that outputs the same signal as the power amplifier, and is configured to compensate for distortion of the gradient magnetic field waveform due to the frequency response of the gradient magnetic field coil by correcting the input waveform signal of the power amplifier. A gradient magnetic field power source for a nuclear magnetic resonance imaging system, characterized in that: