JPH0852122A - Method for impressing binominal pulse in magnetic resonance imaging and mri system - Google Patents

Abstract

PURPOSE:To enable prevention of reduction in the intensity of signals for objective components caused by distortion of the waveform at the rise and fall of binominal pulses. CONSTITUTION:Ef portion and Er portion formed at the rise and fall of a binominal pulse Ralpha, respectively, are reduced to a 1/4 sine waveform and simultaneously, the amplitude thereof is reversed, plus and minus through phase modulation of a carrier wave; this eliminates distortion in the waveform of the binominal pulse as the RF driven system will then be under no excessive load. Accordingly, inadvertent excitation of objective components may be prevented, keeping the intensity of the signal strength of the objective components from being reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a binomial pulse application method and an MRI apparatus in MR imaging (Magnetic Resonance Imaging). More specifically, the present invention relates to a method for applying a binomial pulse in MR imaging and an MRI apparatus capable of preventing a decrease in the signal intensity of a target component due to the disturbance of the waveform of the binomial pulse.

[0002]

2. Description of the Related Art At least two amplitudes are alternately inverted.
The binomial pulse is composed of two parts, the ratio of the absolute value of the time-amplitude product of each part becomes a binomial coefficient, and the sum of the time-amplitude product of each part becomes 0. The binomial pulse is fat suppression or MTC (Magnetization).
It is used in Transfer Contrast) and spectroscopy.

FIG. 5 shows an example of a conventional MR imaging pulse sequence using a binomial pulse. In this pulse sequence C, the binomial pulse Rα ′ is applied at time t0 ′ to excite only the non-target component. Next, at time t1, the dephase gradient magnetic field D is applied to the phase axis Gy to effectively set the signal from the excited non-target component to zero. Then, at time t2, the RF pulse Rβ having the flip angle β ° is applied and the slice gradient magnetic field S is applied to the slice axis Gz to excite the target component. Then, the encode gradient magnetic field IP is applied to the slice axis Gz and the phase axis Gy.
Further, before and after time t3, the echo G is sampled while applying the read gradient magnetic field R to the read axis Gx. IM is a rewind gradient magnetic field having the same magnitude as the phase encode gradient magnetic field IP but opposite polarity.
SP is a spoiler gradient magnetic field.

As shown in FIG. 6, the binomial pulse Rα '.
Divides the α ° excitation pulse into 1: -2: 1
1 pulse, and a portion b1 ′ of amplitude C and time width τ ′,
It is composed of three parts, that is, a portion b2 having an amplitude -C and a time width 2τ ', and a portion b3' having an amplitude C and a time width τ '. Flip angle α ° / of these three parts b1 ′, b2, b3 ′
Since the sum of 4, -α ° / 2 and α ° / 4 is 0, 0 ° is obtained for the target component (for example, free water proton) having T2 relaxation time sufficiently longer than the pulse irradiation time 4τ ′. Acts as a pulse. That is, the target component is not excited and only the non-target component is excited.

A conventional technique for obtaining an MTC by applying a binomial pulse is, for example, "Magnetic Transfer Time-of-Fl".
ight Magnetic Resonance Angiography ； G. Bruce Pike,
Bob S. Hu, Gary H. Glover and Dieter R. Enzmann;
MAGNETIC RESONANCE IN MEDICINE 25,372-379 (1992) ''
It is described in.

[0006]

Conventionally, the binomial pulse R is used.
A rectangular wave sequence is used as α ′. In the case of the rectangular wave series, the rising edge Ef and the falling edge Er are rectangular, as shown in FIG. However, the gate modulation circuit and RF
Due to the limited performance of the power amplifier, as shown in FIG. 7, the actual rising Ef and falling Er do not have a rectangular shape, but are accompanied by rounding and ringing. However, if there is such a waveform disturbance, the sum of the time-amplitude products of the respective parts b1 ′, b2, b3 ′ of the binomial pulse Rα ′ does not become 0, and the target component is undesirably excited. As a result, there is a problem that the signal strength of the target component decreases. Therefore, it is an object of the present invention to provide a method for applying a binomial pulse in MR imaging and an MRI apparatus capable of preventing a decrease in the signal intensity of a target component due to the disturbance of the waveform of the binomial pulse.

[0007]

According to a first aspect of the present invention, the present invention is composed of two or more parts whose amplitudes are alternately inverted, and the ratio of the absolute value of the time-amplitude product of each part is a binomial coefficient. In MR imaging in which a binomial pulse in which the sum of the time-amplitude product of each part is 0 is applied, at least one of the rising edge and the falling edge of the binomial pulse is smoothly changed, and the amplitude is changed by phase modulation of the carrier. And a method of applying a binomial pulse in MR imaging, which comprises inverting the sign.

According to a second aspect of the present invention, the present invention comprises two or more parts whose amplitudes are alternately inverted, and the time of each part is
The ratio of the absolute value of the amplitude product becomes the binomial coefficient, and the time of each part
In an MRI apparatus that applies a binomial pulse whose sum of amplitude products is 0, at least one of rising and falling has a waveform that changes smoothly, and the binomial pulse whose amplitude is inverted by a carrier phase modulation Provided is an MRI apparatus including a binomial pulse applying means for applying.

[0009]

In the binomial pulse applying method and the MRI apparatus of the present invention, the rising and / or the falling of the binomial pulse are smoothly changed, and the amplitude is inverted by the phase modulation of the carrier. If the rising and / or falling is changed smoothly, the gate modulation circuit and RF
Since the load on the power amplifier is reduced, the waveform is not blunted or ringing at the rising edge and / or the falling edge. Further, if the amplitude is inverted by the phase modulation of the carrier, the waveform is not blunted or ringing occurs when the polarity is inverted. Therefore, the binomial pulse having the waveform as planned can be actually obtained, and the sum of the time-amplitude products of the respective parts can be surely set to zero. Therefore, it is possible to prevent the target component from being unintentionally excited and to prevent the signal intensity of the target component from decreasing.

As the rising and / or falling waveforms, a sine waveform, an exponential function waveform, a Gaussian waveform, an exponential waveform, etc. can be considered. It is preferable to use a sine waveform because it is easy to calculate the time-amplitude product and to design the waveform.

[0011]

The present invention will be described in more detail with reference to the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 is a block diagram of an embodiment of the MRI apparatus of the present invention. In this MRI apparatus 100, the magnet assembly 1 has a space portion (hole) for inserting a subject therein, and a static magnetic field for applying a constant static magnetic field to the subject so as to surround this space portion. A coil, a gradient magnetic field coil for generating a gradient magnetic field (the gradient magnetic field coil includes a coil having a slice axis Gz, a phase axis Gy, and a readout axis Gx), and for exciting spins of atomic nuclei in a subject. The transmitting coil that gives the binomial pulse of 1 and the receiving coil that detects the NMR signal from the subject are arranged. The static magnetic field coil, the gradient magnetic field coil, the transmitting coil and the receiving coil are connected to the main magnetic field power source 2, the gradient magnetic field driving circuit 3, the RF power amplifier 4 and the preamplifier 5, respectively.

The sequence storage circuit 8 operates the gradient magnetic field drive circuit 3 based on the stored data acquisition pulse sequence in accordance with a command from the computer 7 to generate a gradient magnetic field from the gradient magnetic field coil of the magnet assembly 1. At the same time, the gate modulation circuit 9 is operated to modulate the high-frequency output signal of the RF oscillation circuit 10 into a pulse-shaped signal having a predetermined timing and a predetermined envelope, which is supplied to the RF power amplifier 4 as an RF pulse (including a binomial pulse). In addition, the power is amplified by the RF power amplifier 4 and then applied to the transmission coil of the magnet assembly 1 for transmission.

The preamplifier 5 includes a magnet assembly 1
The NMR signal from the subject detected by the receiving coil is amplified and input to the phase detector 12. The phase detector 12 is
The output of the RF oscillation circuit 10 is used as a reference signal, and the preamplifier 5
The NMR signal from is phase-detected and given to the A / D converter 11. The A / D converter 11 converts the analog signal after phase detection into a digital signal and inputs it to the computer 7.
The computer 7 performs an image reconstruction operation on the digital signal from the A / D converter 11 to generate an image of the target area (proton density image of the target component). This image is
It is displayed on the display device 6. Further, the computer 7 is responsible for overall control such as receiving information input from the console 13.

FIG. 2 illustrates a pulse sequence of MR imaging using the binomial pulse performed by the MRI apparatus 100. In this pulse sequence F, time t0
A binomial pulse Rα is applied to excite only the non-target component. Next, at time t1, the dephase gradient magnetic field D is applied to the phase axis Gy to effectively set the signal from the excited non-target component to zero. Then, at time t2, the RF pulse Rβ having the flip angle β ° is applied and the slice gradient magnetic field S is applied to the slice axis Gz to excite the target component.
Then, the encode gradient magnetic field IP is applied to the slice axis Gz and the phase axis Gy. Further, before and after time t3, the echo G is sampled while applying the read gradient magnetic field R to the read axis Gx. IM is a rewind gradient magnetic field having the same magnitude as the phase encode gradient magnetic field IP but opposite polarity. SP is a spoiler gradient magnetic field.

As shown in FIG. 3, the binomial pulse Rα
Divides the α ° excitation pulse into 1: -2: 1
One pulse is a portion b1 of a time width τ that rises to an amplitude C with a smooth rising Ef and a time width 2 with an amplitude -C.
It consists of three parts, a part b2 of τ ′ and a part b3 of a time width τ that falls from the amplitude C with a smooth fall Er. The flip angles of these three portions b1, b2, b3 are α ° / 4, −α ° / 2, α ° / 4, and the sum of them is 0, so that the pulse irradiation time is 2 (τ + τ ′). It acts as a 0 ° pulse for the target component (for example, free water proton) having a sufficiently long T2 relaxation time. That is, the target component is not excited and only the non-target component is excited. The positive / negative inversion of the amplitude from the part b1 to the part b2 and the positive / negative inversion of the amplitude from the part b2 to the part b3 are not the modulation of the envelope but the phase modulation of the carrier (0 ° to 180 °, 180 ° to 0 °). Done by.
This allows inversion without ringing.

As shown in FIG. 4, the portion b1 has a time width T
It is composed of a sine wave part of s and a rectangular wave part of a time width Tr (= τ-Ts). When the time-amplitude product S of the portion b1 is obtained with the maximum amplitude of the ¼ sine wave portion generally set to A, S = A · 2Ts / π + A · Tr. Since this may be equal to C · τ ′, C · τ ′ = A · 2Ts / π + A · Tr (1) It suffices to determine Ts, Tr, and A so that this expression (1) is satisfied and that Ts is a value larger than the time constant of the RF drive system including the gate modulation circuit 9 and the RF power amplifier 4. . When A = C, the equation (1) becomes τ ′ = 2Ts / π + Tr (2) Therefore, in this case, the maximum value of Ts is πτ '
/ 2, and the maximum value of Tr is τ '. If Ts =
If Tr = τ / 2, then τ = 2τ ′ / (1 + 2 / π) (3), which is about 1.5 times the time width τ ′ in the case of a rectangular wave. The same applies to the portion b3.

According to the MRI apparatus 100 described above, changes in the rising Ef and the falling Er of the binomial pulse Rα are smoothed and the load applied to the RF drive system is lightened, so that the waveform of the binomial pulse Rα is disturbed. It will not occur. Therefore, it is possible to prevent the target component from being unintentionally excited and to prevent the signal intensity of the target component from decreasing.

In the above embodiment, smooth waveforms are used for both the rising Ef and the falling Er of the binomial pulse Rα, but the rising Ef and the falling E are used.
Even if a smooth waveform is used for either one of r, some effect can be achieved. Further, in the above-described embodiment, the description has been made such that the 1 · 2 · 1 pulse is used as the binomial pulse Rα, but the 1 · 3 · 3 · 1 pulse or the like may be used. Also,
In the above-mentioned embodiment, three-dimensional FT (Fourier Transform)
Although it has been described that the data is collected by the pulse sequence to which the gradient echo method is applied, the data may be collected by other pulse sequences.

[0019]

According to the method of applying a binomial pulse in MR imaging and the MRI apparatus of the present invention, the change in the rising and / or the falling of the binomial pulse becomes smooth, and the load given to the RF drive system becomes light. Distortion of the waveform of the binomial pulse does not occur. Therefore, it is possible to prevent the target component from being unintentionally excited and to prevent the signal intensity of the target component from decreasing.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an MRI apparatus of the present invention.

FIG. 2 is a pulse sequence diagram of a three-dimensional FT gradient echo method using a binomial pulse having a waveform with smooth rising and falling edges.

FIG. 3 is a detailed waveform diagram of the binomial pulse of FIG.

FIG. 4 is an explanatory diagram of a rising portion of the binomial pulse in FIG.

FIG. 5 is a pulse sequence diagram of a conventional three-dimensional FT gradient echo method using a binomial pulse of a rectangular wave sequence.

6 is a detailed waveform diagram of the binomial pulse of FIG.

FIG. 7 is an explanatory diagram of the rising and falling disturbances of the binomial pulse in FIG.

[Explanation of symbols]

100 MRI apparatus 1 Magnet assembly 3 Gradient magnetic field drive circuit 4 RF power amplifier 7 Computer 8 Sequence storage circuit 9 Gate modulation circuit Rα Binomial pulse Ef Rise of binomial pulse waveform Er Fall of binomial pulse waveform G G gradient echo

Claims (2)

[Claims] 1. The method comprises two or more parts whose amplitudes are alternately inverted, and the ratio of the absolute value of the time-amplitude product of each part becomes a binomial coefficient, and the sum of the time-amplitude product of each part becomes 0. In the MR imaging in which the binomial pulse is applied, at least one of the rising edge and the falling edge of the binomial pulse is smoothly changed, and the amplitude is inverted by the phase modulation of the carrier. Pulse application method. 2. The method comprises two or more parts whose amplitudes are alternately inverted, and the ratio of the absolute value of the time-amplitude product of each part becomes a binomial coefficient, and the sum of the time-amplitude product of each part becomes 0. In the MRI apparatus for applying the above-mentioned binomial pulse, a binomial pulse applying means for applying the above-mentioned binomial pulse having a waveform in which at least one of rising and falling is smoothly changed and whose amplitude is inverted between positive and negative by phase modulation of a carrier. An MRI apparatus comprising: