JP3307114B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR (Magnetic Resonance) imaging apparatus which performs imaging by utilizing an NMR (nuclear magnetic resonance) phenomenon.

[0002]

2. Description of the Related Art An MR imaging apparatus uses the resonance phenomenon of nuclei to capture and image the spin relaxation time difference in each tissue in a living body, and is capable of obtaining an image with excellent contrast representing the relaxation time difference. Therefore, it has become extremely useful in the field of medical form diagnosis. As the NMR parameters, usually, a proton density ρ and two relaxation times T1 and T2 are mainly used.

Meanwhile, in this MR imaging apparatus, a contrast improvement method using an MT (magnetization transfer) effect has recently become known. These are the protons of free water in living tissue, the protons of macromolecules such as membranes and proteins, and the protons of water around and whose movement is restricted (herein referred to as bound water for convenience of explanation). The contrast of the image created by the magnitude of the MT is determined by the MTC (magnetization transfer).
contrast). The MT effect is expected to be useful not only for improving the contrast of an image but also for medical diagnosis as reflecting the tissue properties.

The MTC method includes an On Resonance method in which the frequency of a carrier signal of an excitation RF pulse added as an MTC pulse to an imaging sequence such as a gradient echo method or a spin echo method matches the resonance frequency of free water, Resona frequency is different from water resonance frequency
nce method. Conventionally, the carrier signal envelope (modulated wave) is represented by On Reson
In the ance method, a binomial pulse is used, and in the Off Resonance method, a Gaussian pulse is used.

[0005]

However, in the conventional MTC method, the signal from the brain parenchyma or the like is certainly reduced by the MT effect, but the signal from fat is not reduced. It has the disadvantage of being emphasized.

[0006] In order to solve this problem, an attempt has been made to use an imaging sequence for fat suppression in combination. Here, the imaging sequence for fat suppression is a method of suppressing a signal from fat by utilizing the property that the resonance frequencies of water and fat are different due to chemical shift, such as Dixon method and Fat-Saturation (CHESS) method. It has been known.

[0007] However, when such a fat suppression method is used together, the Dixon method limits the selection of TE (time until an echo is generated), and the Fat-Satura.
Tion causes an increase in TR (repetition time), and in any case, lacks versatility.

In view of the above, the present invention provides an MR imaging apparatus in which the MTC pulse and the fat suppression are simultaneously achieved while securing the degrees of freedom of TE and TR by devising the waveform (envelope) of the MTC pulse. The purpose is to provide.

[0009]

In order to achieve the above object, an MR imaging apparatus according to the present invention comprises:
Assuming that A (t) is obtained by multiplying a predetermined window function by an amplitude coefficient, a free function is performed so that the envelope becomes a function F (t) such that F (t) = A (t) · (1-cosωt) / ωt. A basic RF pulse obtained by amplitude-modulating a carrier signal having a frequency substantially equal to the resonance frequency of water is irradiated a plurality of times at predetermined time intervals so that the amplitude becomes a binomial expression in the order of irradiation. After that, an imaging sequence is performed.

[0010]

[Function] The envelope of the basic RF pulse is set to F (t) = A (t) · (1-cosωt) / ωt, and this is irradiated a plurality of times at predetermined time intervals. And
It is assumed that the amplitude of each basic RF pulse is a binomial expression such as 1-2-1 or 1-3-3-1 in irradiation order. Such a plurality of basic RF pulses are added as MTC pulses to the head of an imaging sequence such as a gradient echo method or a spin echo method. Then, according to the MTC pulse composed of a plurality of basic RF pulses, almost no excitation occurs near the frequency of the carrier signal of the RF pulse (that is, the center resonance frequency of water), and the amount of longitudinal magnetization near the center resonance frequency of fat. Is 0, and an excitation frequency characteristic is obtained such that excitation can be greatly increased in a region farther away from the frequency. Therefore, it is possible to excite only the bound water while setting the longitudinal magnetization amount of the fat portion to 0, and at the same time, not excite the free water. Therefore, if an imaging sequence is performed after the MTC pulse including a plurality of basic RF pulses, a sufficient MT effect can be obtained while suppressing a signal from fat, and an image having a large contrast due to the MT effect can be obtained. be able to.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In the MR imaging apparatus according to one embodiment of the present invention, an MTC pulse is generated by irradiating a basic RF pulse having an envelope waveform as shown in FIG. This MR imaging apparatus is configured as shown in FIG. The MTC pulse is given prior to a normal imaging sequence as shown in FIG.

First, the structure of the MR imaging apparatus will be described. Referring to FIG.
Includes a main magnet for generating a static magnetic field, and a gradient coil for generating a gradient magnetic field superimposed on the static magnetic field. Gradient magnetic fields are X, Y,
The magnetic field strengths are generated in such a manner that the magnetic field strengths incline in the three Z directions. By selecting one of these three-axis gradient magnetic fields or by combining them, an arbitrary three-axis gradient magnetic field is created, and these are the slice selection gradient magnetic field Gs and the readout and frequency encoding gradient magnetic fields described later. Gr and a phase encoding gradient magnetic field Gp.

An object (not shown) is arranged in the space to which the static magnetic field and the gradient magnetic field are applied. In this subject,
An RF coil 12 for irradiating the subject with an RF pulse and receiving an NMR signal generated by the subject is attached.

A magnetic field control circuit 21 is provided as a circuit for supplying a gradient magnetic field current to the gradient coil of the magnet assembly 11. A waveform signal from the waveform generation circuit 53 is sent to the magnetic field control circuit 21. In the waveform generation circuit 53, information on each pulse waveform of the gradient magnetic fields Gs, Gp, and Gr is set in advance from the computer 51. A waveform signal is generated from the waveform generating circuit 53 at the timing instructed by the sequence controller 52, and is sent to the magnetic field control circuit 21, whereby the pulsed gradient magnetic fields Gs and G having the waveform shown in FIG.
p and Gr are respectively generated.

The RF signal generated by the RF oscillation circuit 31 is sent to an amplitude modulation circuit 32, which becomes a carrier signal.
The amplitude is modulated according to the waveform signal sent from the waveform generation circuit 53. The RF signal after the amplitude modulation is amplified through the RF power amplifier 33 and then applied to the RF coil 12. The oscillation frequency of the RF oscillation circuit 31 is controlled by the computer 51. Information on the waveform of the modulation signal is given from the computer 51 to the waveform generation circuit 53 in advance. The timing of the waveform generation circuit 53 and the RF oscillation circuit 31 is determined by the sequence controller 52.

NMR received by RF coil 12
The signal is sent to a phase detection circuit 42 via a preamplifier 41 and is subjected to phase detection. An RF signal from the RF oscillation circuit 31 is sent as a reference signal for the phase detection. The signal obtained by the phase detection is sampled at a predetermined sampling timing by the A / D converter 43 controlled by the sequence controller 52, and is converted into digital data. The data obtained from the A / D converter 43 is taken into the computer 51.
The computer 51 performs a process of reconstructing an image from the collected digital data. In addition, the computer 51 operates as described above in accordance with the pulse sequences constituting various imaging sequences.
2 and the data necessary for the waveform generating circuit 53 are set, and the frequency is determined by controlling the RF oscillation circuit 31.
Further, it controls the preamplifier 41 and the phase detection circuit 42 to determine their gains and the like, and further controls the A / D converter 43.

In such an MR imaging apparatus, a computer 51 and a sequence controller 52
Under the above control, for example, an imaging sequence by the MTC method as shown in FIG. 4 is performed. In the example shown in FIG.
A gradient echo method is employed as a pulse sequence that constitutes an imaging sequence. That is, the pulse of the gradient magnetic field Gs for slice selection is applied at the same time as the excitation pulse (90 ° pulse) 61 is applied, and then the pulse of the gradient magnetic field Gr for readout and frequency encoding is applied with a certain polarity, and then the polarity is switched. , An echo signal 62 is generated. Before the generation of the echo signal 62, a pulse of the phase encoding gradient magnetic field Gp is applied. Such a pulse sequence is converted into a phase encoding gradient magnetic field Gp.
Is repeated by the number corresponding to the image matrix while changing the magnitude of the pulse little by little to perform the imaging sequence.

The MTC pulse 63 is added non-selectively at the beginning of each repetition period of such an imaging sequence.
The MTC pulse 63 is formed by irradiating a basic RF pulse having an envelope as shown in FIG. 1 a plurality of times at predetermined time intervals τ. Assuming that the center resonance frequency of free water is ω0, the RF oscillation circuit 3
The frequency of the carrier signal from 1 is set to ω0, and the waveform of the modulation signal supplied from the waveform generation circuit 53 to the amplitude modulation circuit 32 is as shown in FIG.

To obtain such a modulated signal waveform,
The computer 51 first obtains a function F (t) shown below. F (t) = A (t) · (1-cosωt) / ωt Here, the value of the parameter ω is determined according to the static magnetic field strength and the desired excitation frequency band and non-excitation frequency band. The function A (t) is obtained by multiplying an appropriate window function represented by, for example, a Hamming window by an amplitude coefficient. The computer 51 obtains waveform data from the function F (t) and supplies the waveform data to the waveform generation circuit 53.

The basic RF pulse is irradiated a plurality of times at a predetermined time interval τ.
The difference between the center resonance frequencies of water and fat (about 3.5 ppm) is a value close to the time interval τ ′ such that the phase difference becomes π for the first time, and the first peak is at the center resonance frequency of fat in the excitation frequency characteristics. Is a value that appears. Then, the amplitudes of the plurality of basic RF pulses are set to 1-2-1.
Alternatively, a binomial equation such as 1-3-3-1 is used.

In this embodiment, it is assumed that the above basic RF pulse is irradiated four times, and the amplitude of each is 1: -3:
3: -1 and these are set to MTC as shown in FIG.
The pulse 63 is used. FIG. 2 shows the excitation frequency characteristics in this case. The vertical axis indicates the normalized longitudinal magnetization amount, and the horizontal axis indicates the offset frequency from the carrier frequency of the basic RF pulse in ppm. As can be seen from the excitation frequency characteristics shown in FIG. 2, the longitudinal magnetization is almost 1 (that is, almost not excited) near the carrier frequency (the center resonance frequency of water), but the longitudinal magnetization becomes 0 as the frequency increases. Alternatively, a plurality of negative peaks appear.

At frequencies corresponding to these peaks, excitation is large, but when focusing on the first peak, the longitudinal magnetization is near 0 at about 3.5 ppm. The frequency having a frequency difference of about 3.5 ppm corresponds to the central resonance frequency of fat (because the time interval τ is adjusted as described above so that the first peak appears in such a frequency band). The fact that the longitudinal magnetization becomes 0 in this frequency band means that all the magnetizations are transverse magnetization, and the signal from fat is suppressed in the immediately following imaging sequence. As described above, some peaks are present in the frequency region where the frequency is farther than this, and the peak is greatly excited in the frequency region, so that the bound water is excited and a sufficient MT effect can be obtained.

In the above embodiment, a pulse sequence based on the gradient echo method has been exemplified as the imaging sequence, but any other type of imaging pulse sequence such as a spin echo method can be adopted.

[0024]

As described above, according to the MR imaging apparatus of the present invention, the fat suppression effect and the MT effect can be simultaneously obtained by devising the waveform of the basic RF pulse constituting the MTC pulse. Since the MTC pulse having the excitation frequency characteristic as described above is given, unlike the conventional case where MTC and fat suppression are performed separately, while securing the degrees of freedom of TE and TR,
In addition, it is possible to obtain an image in which the contrast due to the MT effect is sufficiently large while suppressing fat.

[Brief description of the drawings]

FIG. 1 is a graph showing an envelope waveform of a basic RF pulse constituting an MTC pulse according to an embodiment of the present invention.

FIG. 2 shows an M consisting of a basic RF pulse having the envelope of FIG.
4 is a graph showing an excitation frequency characteristic of a TC pulse.

FIG. 3 is a block diagram of the MR imaging apparatus according to the embodiment;

FIG. 4 is a time chart showing a pulse sequence of the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Magnet assembly 12 RF coil 21 Magnetic field control circuit 31 RF oscillation circuit 32 Amplitude modulation circuit 33 RF power amplifier 41 Preamplifier 42 Phase detection circuit 43 A / D converter 51 Computer 52 Sequence controller 53 Waveform generation circuit 61 Excitation pulse 62 Echo Signal 63 MTC pulse

Claims (1)

(57) [Claims] A means for generating a static magnetic field; a means for generating a gradient magnetic field for each of slice selection, reading, and phase encoding; a means for irradiating an RF signal;
Means for receiving signals, means for collecting data from the received NMR signals, and controlling these to change the gradient magnetic field for phase encoding to repeat the imaging sequence, and at the beginning of each imaging sequence, a carrier signal The frequency substantially coincides with the resonance frequency of free water, and its envelope is obtained by multiplying A (t) by a predetermined window function and an amplitude coefficient, and F (t) = A (t) · (1- cos ωt) / ωt A plurality of times, the amplitude of the basic RF pulse amplitude-modulated so as to be a function F (t) represented by The above-mentioned gradient magnetic field generating means and RF so that irradiation is added
Means for controlling signal irradiation means.