JPH0489034A - MR imaging device - Google Patents

Abstract

PURPOSE:To obtain a good image with a high S/N ratio without causing an increase of the examination time by controlling the amplification degree of the receiving means of an NMR signal in response to the phase encoded quantity. CONSTITUTION:The attenuation quantity of a programmable attenuator 37 is controlled in response to the phase encoded quantity, the attenuation quantity is increased for the pulse sequence with the less phase encoded quantity, and the attenuation quantity is decreased for the pulse sequence with the larger phase encoded quantity. The dynamic range of an A/D converter 38 is invariably utilized effectively, and the quantization noise with the large phase encoded quantity is reduced in particular. The magnitude of the collected data is made constant regardless of the phase encoded quantity. The quantization noise contained in the data obtained in the pulse sequence with the large phase encoded quantity can be reduced, thus the precision of the data corresponding to the high-frequency component on the image is improved without averaging the data.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an MR imaging apparatus based on the Fourier transform method that reconstructs images using nuclear magnetic resonance phenomena.

[Conventional technology]

The nuclear magnetic resonance (NMR) phenomenon is a phenomenon in which when a high-frequency pulse is applied to a subject placed in a static magnetic field, the frequency of the high-frequency pulse is proportional to the strength of the magnetic field, causing the precession of specific nuclear spins. This is a phenomenon in which the nuclear spin undergoes an energy level transition when it matches the frequency of , and returns to its original level after relaxation time, at which time energy is released as a resonance signal. An MR imaging apparatus using the Fourier transform method utilizes this nuclear magnetic resonance phenomenon and reconstructs an image by Fourier transforming the received signal. In other words, in an MR imaging device using the Fourier transform method, a slice plane is selectively excited using a gradient magnetic field in one direction of three orthogonal axes, and the position information of these two axes is converted into frequency and information using gradient magnetic fields in the other two axes. The position information of these two axes is encoded as a phase shift amount into a resonance signal, and the received resonance signal is subjected to Fourier transform to decode the position information and reconstruct the image of the slice plane. In this case, the NM received by this MR imaging device
The R signal is very weak and alone cannot provide enough S/
Unable to obtain N ratio. Therefore, in general, the excitation/reception sequence is repeated multiple times under the same phase encoding conditions, and the collected data is integrated to improve the S/N ratio. [Problems to be Solved by the Invention] However, most of the time for restraining a subject before obtaining an image in an MR imaging device (hereinafter referred to as examination time) is due to the excitation/reception sequence. This is the time it takes to collect data repeatedly, and if the sequence is repeated multiple times under the same phase encoding conditions as in the past, there is a problem in that the data collection time becomes longer and the inspection time increases. . In view of the above, the present invention provides an MR imaging device that can increase the S/N ratio of reconstructed images while shortening examination time by collecting data without repeating sequences under the same phase encoding condition. The purpose is to

[Means to solve the problem]

In order to achieve the above object, the MR imaging apparatus according to the present invention includes means for generating a static magnetic field, means for generating a gradient magnetic field for slice selection, means for generating a gradient magnetic field for frequency encoding, and a phase means for generating a gradient magnetic field for encoding; means for exciting an object by applying an RF pulse; variable amplification receiving means for receiving and detecting NMR signals generated from the object; and NMR from the receiving means. A that converts a signal into a digital signal, /'
It is characterized in that it includes a D conversion means and a means for controlling the amplification degree of the NMR signal receiving means in accordance with the amount of phase encoding.

[For production]

The amplification degree of the NMR signal receiving means is controlled according to the amount of phase encoding, and the amplification degree is made smaller as the amount of phase encoding is smaller, and increased as the amount of phase encoding is larger. Generally, the larger the amount of phase encoding, the smaller the NMR signal becomes. Therefore, by controlling the amplification degree of the receiving means as described above, the magnitude of the input signal to the A/D converting means is made constant for each pulse sequence with a different amount of phase encoding,
The dynamic range of the A/D conversion means can always be used effectively. Therefore, even in the case of a pulse sequence with a large amount of phase encoding and a small NMR signal, quantization noise can be reduced, making it unnecessary to average the data obtained by repeating the pulse sequence with the same phase encoding condition multiple times. . As a result, it is possible to increase the S/N ratio of the image and obtain a reconstructed image with good imaging conditions without increasing the inspection time.

【Example】

Hereinafter, an MR imaging apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, a transmitting coil 12 and a receiving coil 1 are attached to a subject 11.
3 are attached to the main magnet 15, and the gradient coil 1 is attached so as to be superimposed on the static magnetic field formed by the main magnet 15.
4 is placed in a gradient magnetic field formed by 4. The gradient coil 14 is arranged in each direction of three orthogonal axes (S axis, R axis,
They are configured to be able to independently generate gradient magnetic fields whose magnetic field strengths are tilted toward the P-axis. The gradient magnetic fields of the three orthogonal axes are respectively a slice selection gradient magnetic field Gs, a reading gradient magnetic field Gr, and a phase encoding gradient magnetic field Gp. Current is supplied to the gradient coil 14 from power sources 21, 22, and 23 for gradient magnetic fields Gs, Gr, and Gp, and gradient magnetic fields in each direction are formed. The gradient magnetic field power supplies 21 to 21 are connected to each other so that each gradient magnetic field having a predetermined waveform is formed by the gradient coil 14.
The supplied current waveform of 23 is controlled by a gradient magnetic field control device 24. On the other hand, the high-frequency signal sent from the high-frequency power supply 33 to the transmitting coil 12 is converted into a sine wave signal as a carrier from the synthesizer 34 in the frequency converter 32, and the high-frequency signal is sent from the RF waveform generator 31. It is AM modulated with a 5-inch waveform. After the subject 11 is irradiated with an RF pulse from the transmitting coil 12 to excite its nuclear spins, the NMR signal generated with a delay of an echo time is received by the receiving coil 13 . This received NMR signal is amplified by a preamplifier 35, detected by a phase detector 36, then attenuated by a programmable attenuator 37, and then converted to digital data by an A/D converter 38 to be sent to the host. It is taken into the computer 41. This phase detector 36 is connected to the synthesizer 3
This circuit outputs the difference in frequency between the two signals by mixing the reference signal sent from 4 and the received signal. The sequence controller 42 is the host computer 41
Under the control of (corresponding to the resonant frequency),
Information regarding the amount of attenuation is sent to the programmable attenuator 37, and the sampling timing of the A/D converter 38 is controlled. A console 43 having a display device and an input device such as a keyboard device is connected to the host computer 41 . The data taken into the host computer 41 is Fourier transformed to reconstruct an image, and the image is displayed on the display device of the console 43. In such MR imaging equipment, a pulse sequence based on the spin echo method, which is a typical pulse sequence as shown in FIG. 2, is performed. That is, the sequence controller 42 or the RF waveform generator 31 is controlled by a command from the host computer 41.
A waveform for the ° pulse is generated, and the carrier signal from the synthesizer 34 is amplitude-modulated in the frequency converter 32 using this waveform. This 90° pulse is the high frequency power source 33
After being amplified by
1 is irradiated with a 90° pulse. At the same time, a command is given to the gradient magnetic field control device 24 to cause the gradient coil 14 to generate pulses of the gradient magnetic field Gs for slice selection, so that only a specific slice plane of the subject 11 is selectively excited. . Next, a pulse of the gradient magnetic field Gr for reading (for frequency encoding) and a pulse of the gradient magnetic field Gp for phase encoding are applied. Then 180°
A gradient magnetic field Gs pulse for slice selection is irradiated together with the pulse, and an NMR signal is generated after an echo time by applying a gradient magnetic field Gr for reading. This N
The MR signal is received by the receiving coil 13 and the preamplifier 35
, a phase detector 36, and a programmable attenuator 37 before being sent to an A/D converter 38. This A/D converter 3
8, the sampling of the NMR signal and the A/D are performed by a sampling pulse generated in accordance with the NMR signal.
Conversion is performed, and the resulting digital data is imported into host computer 41. Such an excitation/reception pulse sequence is repeated while changing the magnitude of the phase encoding gradient magnetic field Gp pulse. Therefore, assuming that n pieces of digital data are obtained as a result of sampling and A/D conversion in each pulse sequence, when these data are arranged, the result will be as shown in FIGS. 3A and 3H. In Fig. 3 A and B, the phase encode amounts are expressed as -2, -2, O1+1, +2, etc., and the phase encode amounts are shown on the vertical axis, which can be obtained with one pulse sequence. The horizontal axis represents the time series of n pieces of data. In this case, the intensity of the NMR signal is given by the following formula. s (t, cp) = k S 5 , ) (r, p) − ε 1rrJGr
dt+1ipfGpdt drdp(k is constant, ρ(
(r, p) are spin densities, γ is gyromagnetic ratio) Therefore, the amount of NMR signal changes depending on the amount of applied gradient magnetic field cp for phase encoding, and fGpdt → 0
The maximum value is reached when the phase encode amount is zero. Therefore, if the programmable attenuator 37 in FIG. 1 is always controlled to a constant amount of attenuation,
The data is as shown in FIG. 3A, and the size of the data changes depending on the amount of phase encoding. That is, it is the largest at the center in the vertical axis direction and is smallest at both ends of the vertical axis where the phase encode amount has the maximum positive and negative values. If the input signal to the A/D converter 38 is small, the dynamic range cannot be used effectively, resulting in an increase in quantization noise. Therefore, in this embodiment, the attenuation amount of the programmable attenuator 37 is controlled according to the phase encode amount, and the attenuation amount is increased for pulse sequences with a small phase encode amount, and the attenuation amount is increased for pulse sequences with a large phase encode amount. I'm keeping it small. That is, the value of k in the above equation is changed so that the input signal of the A/D converter 38 always has the maximum value in each pulse sequence having a different amount of phase encoding. As a result, the dynamic range of the A/D converter 38 is always effectively utilized, and quantization noise is reduced, especially when the amount of phase encoding is large. Therefore, the collected data becomes as shown in FIG. 3B, and the size of the data is constant regardless of the amount of phase encoding. If the data collected in this way (Figure 3B) is subjected to two-dimensional Fourier transform, the image can be reconstructed, but A/
Since the signal amount is converted by the D converter 38, it is necessary to return it to the original value. That is, two-dimensional Fourier transform is performed while performing appropriate scaling processing on the data by multiplying by 1/k as shown below. In this way, the quantization noise contained in the data obtained from pulse sequences with a large amount of phase encoding can be reduced, improving the accuracy of data corresponding to high frequency components on the image without averaging the data. It turns out. Since the data is not averaged, the number of repetitions of the pulse sequence is reduced, and the S/N ratio of the image can be increased without increasing the inspection time. In general, the high-frequency components of reconstructed images from an MR imaging device are believed to affect the focus characteristics of the image, so by improving the data accuracy of the high-frequency components in this way, reconstructed images with good imaging conditions can be obtained. This will have a huge impact.

【Effect of the invention】

According to the MR imaging device of the present invention, quantization noise can be minimized by always making effective use of the dynamic range of the A/D converter, so it is possible to repeat the pulse sequence multiple times under the same phase encoding condition. Even the high-frequency components of an image can be accurately reproduced without the need for Therefore, S/N can be improved without increasing inspection time.
A good image with a high ratio can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a time chart showing a pulse sequence, and FIG. 3 is a diagram showing a data arrangement. 11... Subject, 12... Transmission coil, 13...
Receiving coil, 14... Gradient coil, 15... Main magnet, 21... Gradient magnetic field power supply for slice selection, 22
...Gradient magnetic field power supply for readout, 23... Gradient magnetic field power supply for phase encoding, 24... Gradient magnetic field control device, 3
1... RF waveform generator, 32... Frequency converter, 3
3...High frequency power supply, 34...Synthesizer, 35.
... Preamplifier, 36... Phase detector, 37... Programmable attenuator, 38... A/D converter,
41...Host computer, 42...Siegens controller, 43...Console.

Claims (1)

[Claims] (1) A means for generating a static magnetic field, a means for generating a gradient magnetic field for slice selection, a means for generating a gradient magnetic field for frequency encoding, a means for generating a gradient magnetic field for phase encoding, and a means for generating a gradient magnetic field for phase encoding; means for applying an RF pulse for excitation, variable amplification receiving means for receiving and detecting the NMR signal generated from the subject, and A/D conversion means for converting the NMR signal from the receiving means into a digital signal; Above NMR
An MR imaging apparatus comprising: means for controlling the amplification degree of the signal receiving means according to the amount of phase encoding.