CN103278785B - The optimization method of radio-frequency pulse phase place in a kind of quick-speed spin echo pulse sequence - Google Patents

Abstract

The present invention proposes a method for optimizing the phase of radio frequency pulses in a fast spin echo pulse sequence. This method uses the difference in the effect of the "spoiler gradient" on the spin echo and the stimulated echo to separate the two, so that the spin echo and the stimulated echo More accurate coincidence of the stimulated echo; and using the phase relationship between the spin echo, the stimulated echo and the inversion pulse, the optimized inversion pulse phase can be obtained through simple calculation. The invention can reduce the influence of the unideal factors of the instrument hardware on the FSE sequence parameter optimization process, reduce the dependence degree of the process of finding the optimal parameter on the phase step precision, thereby improving the optimization efficiency.

Description

A Method for Optimizing the Phase of RF Pulses in Fast Spin Echo Pulse Sequences

technical field

The invention relates to a method for optimizing the phase of a radio frequency pulse in a fast spin echo pulse sequence, belonging to the technical field of magnetic resonance imaging.

Background technique

Magnetic resonance imaging (MRI) technology has become a very useful tool in medical diagnosis. Generally, in an MRI instrument, when the sample to be measured (such as human tissue) reaches equilibrium in a static magnetic field B ₀ (the B ₀ direction is defined as the Z-axis direction of the Cartesian coordinate system), the atomic nuclei (nuclear spin) in the sample are due to Polarized by B ₀ to produce a macroscopic magnetization vector M ₀ . M ₀ is rotated to the horizontal plane (XY plane) under the excitation of RF pulses, and then precesses around the Z axis. A receiving coil is placed around the tested sample, and it will induce the precession signal of the magnetization vector. After the magnetic resonance signal collected by the receiving coil is amplified and converted from analog to digital, it enters the computer for image reconstruction. Generally speaking, in order to perform imaging, an MRI instrument also needs to generate three orthogonal gradient magnetic fields in order to perform three-dimensional spatial positioning of magnetic resonance signals.

In clinical diagnosis, fast spin echo (FSE) sequence is one of the conventional magnetic resonance scanning sequences, and has been applied in most MRI systems. The FSE sequence is a multi-echo sequence (that is, the echo chain length ETL is not less than 2), and the phase between each echo is easily affected by factors other than the phase encoding gradient, resulting in artifacts and alternating light and dark stripes in the image. To eliminate these artifacts and streaks, one method is to adjust the phase of the RF excitation pulse in the imaging sequence so that the two components in the echo signal are in phase, that is, the spin echo signal and the stimulated echo signal are in phase; the other A similar method is to eliminate the stimulated echo signal by adjusting the area of the gradient pulse in the imaging sequence. The signal-to-noise ratio of the image obtained by the second method is worse than that obtained by the first method.

By adjusting the phase of the RF inversion pulse (hereinafter referred to as the inversion pulse) in the imaging sequence, so that the spin echo signal and the stimulated echo signal are in the same phase, the artifacts and streaks in the FSE image can be eliminated. However, this adjustment process is relatively complicated. In general, the time precision of a hardware system is better than its magnitude step precision. In the FSE sequence, the amplitude step accuracy of the gradient subsystem is often not enough to ensure that the peak points of the spin echo and the stimulated echo completely coincide, and the phase changes of adjacent sampling points in the echo signal are relatively large, generally close to +/ -π. This means that to adjust the spin echo and the stimulated echo to be in phase, it is necessary to step and invert the phase of the pulse with high precision to find the optimal parameters. Therefore, it often takes a long time to optimize the phase of the FSE sequence inversion pulse.

Contents of the invention

Aiming at the above problems, the present invention proposes a method for optimizing the phase of radio frequency pulses in a fast spin echo pulse sequence. This method separates the spin echo and the stimulated echo by using the difference in the effect of the "spoiler gradient" on the spin echo and the stimulated echo, so that the spin echo and the stimulated echo can be overlapped more accurately; The phase relationship of the inversion pulse can be optimized through a simple calculation process. The invention can reduce the influence of the unideal factors of the instrument hardware on the FSE sequence parameter optimization process, reduce the dependence degree of the process of finding the optimal parameter on the phase step precision, thereby improving the optimization efficiency.

To achieve the above object, the present invention takes the following technical solutions:

The method for optimizing the radio frequency pulse phase in the fast spin echo pulse sequence provided by the present invention comprises the following sequential steps:

a) Set the amplitude of the "phase encoding gradient" to 0, and set the amplitudes of the adjacent two groups of "spoiler gradients" to unequal values.

b) Adjust the starting time of the "readout gradient", so that the peak points of the echo signals of the respective cycles on the echo chain are located in the middle of the sampling window.

c) Set the amplitude of the "spoiler gradient" to an equal value. Keep the phase of the first inversion pulse as π/2, and set the phase of the other inversion pulses to 0, and Under the three conditions of , obtain the peak amplitude values A, A' and A'' of each echo signal on the echo chain except the first echo.

d) Calculate the phase of each inversion pulse according to A, A' and A'':

.

The invention can reduce the influence of the unideal factors of the instrument hardware on the FSE sequence parameter optimization process, reduce the dependence degree of the process of finding the optimal parameter on the phase step precision, thereby improving the optimization efficiency.

Description of drawings

Fig. 1 is a structural block diagram of the MRI system of the present invention;

Fig. 2 is a schematic diagram of a spin echo sequence according to the present invention;

Fig. 3 is a flowchart of an embodiment of the present invention.

illustration:

RF: radio frequency pulse; 90: excitation pulse; 180: inversion pulse; ACQ: sampling; Gs: layer selection gradient; Gc: spoiler gradient; Gp: phase encoding gradient; Gr: readout gradient; Echo: echo.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing and embodiment:

Fig. 1 is a structural block diagram of the MRI system of the present invention. In the MRI system, the magnet 101 has a cavity for placing a sample. A gradient coil 102 is placed around the cavity for generating gradient magnetic fields in the layer selection direction, phase encoding direction and reading direction, so as to spatially position the sample. A radio frequency transmitting coil 103 and a radio frequency receiving coil 104 are placed around the cavity, the transmitting coil is used to transmit radio frequency pulses to excite the magnetization vector of the sample, and the receiving coil is used to receive the magnetization vector precession signal. The gradient coil 102 is connected to a gradient power amplifier 112 , and the transmitting coil 103 and receiving coil 104 are respectively connected to a radio frequency power amplifier 113 and a preamplifier 114 .

Based on instructions given by the computer 130, the pulse sequence storage circuit 125 controls the gradient waveform generator 122 and the transmitter 123 according to the pulse sequence stored therein. The gradient waveform generator 122 outputs a gradient pulse signal with a predetermined timing and waveform, which is amplified by the gradient power amplifier 112 and then passed through the gradient coil 102 to generate a gradient magnetic field in the cavity of the magnet. The transmitter 123 outputs a radio frequency pulse signal with predetermined timing and envelope, the signal is amplified by the radio frequency power amplifier 113 , and then the nuclear spin in the sample is excited by the radio frequency transmitting coil 103 .

The radio frequency receiving coil 104 detects the precession signal of the magnetization vector, and the signal is amplified by the preamplifier 114 and then input to the receiver 124 . Under the control of the pulse sequence storage circuit 125, the receiver 124 performs wave detection and digital-to-analog conversion on the amplified signal to obtain a digital signal. The resulting digital signal is transmitted to computer 130 to reconstruct the image. The display/printer 126 is used to display/print scanned images.

Referring to FIG. 2 , for the sake of brevity, only three echo signals 201 , 202 and 203 are shown in the figure. In actual scanning, the echo chain length ETL is not limited to 3. Under the combined action of the slice selection gradient pulse 220 and the excitation radio frequency pulse 210, the magnetization vector (from the Z direction) in the selected slice in the sample is rotated to the XY plane. The magnetization vector precesses around the Z axis in the XY plane, and at the same time "dispersion" occurs. After a period of time, under the combined action of the slice selection gradient pulses 221, 222 and the inversion radio frequency pulse 211, the magnetization vector is reversed in the XY plane. Between 210 and 211, a readout direction gradient pulse 240 is applied. After a period of time (same as the "dephasing" process), the magnetization vectors "converge" in the XY plane, forming the echo 201 . After the magnetization vectors "converge" and then "deviate", the RF pulses 212, 213 are reversed so that the "divergent" magnetization vectors "converge" again. During the alternating process of "dispersion" and "convergence", echoes such as 202 and 203 are formed. During "convergence" phase gradient pulses 231, 233 and 235 are applied, which correspond to echo signals 201, 202 and 203, respectively. During the "dephasing" process, dephasing gradient pulses 232, 234 and 236 are applied, corresponding to 231, 233 and 235 respectively, with gradients of equal magnitude and opposite directions. In the process of collecting echo signals, read gradient pulses 241 , 242 and 243 are applied successively.

To cancel the free induction decay signal generated by 211 , spoiler gradients 223 , 224 are applied to the right of 222 . 223 , 224 are also applied to the left of 222 in order to satisfy the conditions for spin echo generation. When the amplitudes of 223 and 224 are not equal, the stimulated echo component in the echo signal is eliminated.

Referring to Fig. 1 and Fig. 2, the method for optimizing the RF pulse phase in the fast spin echo pulse sequence provided by the present invention comprises the following sequential steps:

a) Set the amplitudes of "phase encoding gradients" 231, 232, 233, 234, 235 and 236 to 0, and set the amplitudes of two adjacent groups of "spoiler gradients" 223 and 224 to unequal values; When the stimulated echo disappears.

b) Adjust the starting time of the "readout gradients" 241, 242 and 243, so that the peak points of the respective echo signals on the echo chain are located in the middle of the sampling window. If the time difference between the peak point of the second echo signal and the center of the sampling window is T ₂ , and the time difference between the peak point of the third echo signal and the center of the sampling window is T ₃ , the starting time of the "readout gradient" corresponding to the odd echo is advanced T ₃ , the starting time of the "readout gradient" corresponding to the even echo is advanced by T ₂ .

c) Set the amplitudes of the “spoiler gradients” 223 and 224 to be equal; at this time, the stimulated echo and the spin echo coincide exactly. Keep the phase α ₁ of the first inversion pulse 211 as π/2, and set the phases of other inversion pulses as , start the FSE pulse sequence pre-scan, record the echo signal peak amplitude (A ₂ , A ₃ ... _AN ) on the echo chain except the first echo; set the phase of the inversion pulse as , start the FSE pulse sequence pre-scan, record the amplitude of each echo signal peak point on the echo chain (A' ₂ , A' ₃ ... A' _N ); set the phase of the inversion pulse to , start the FSE pulse sequence pre-scan, and record the peak point amplitude of each echo signal on the echo chain (A'' ₂ , A'' ₃ ... A'' _N ).

d), the phase of the first inversion pulse 211 constant. Since the stimulated echo and the spin echo coincide exactly at this time, the phase relationship between them and the inversion pulse can be directly used to calculate the phases of other inversion pulses:

;

;

…

.

Claims (1)

1. an optimization method of radio frequency pulse phase in a fast spin echo pulse sequence, it is characterized in that the method may further comprise the steps: a), set the magnitude of the "phase encoding gradient" to 0, and set the magnitudes of the adjacent two groups of "spoiler gradients" to unequal values; b) Adjust the starting time of "reading gradient", so that the peak points of the respective echo signals on the echo chain are located in the middle of the sampling window; c), set the amplitude of the "spoiler gradient" to an equal value; keep the phase of the first inversion pulse as π/2, and set the phases of the other inversion pulses to three types of 0, π/2 and π Under the conditions, obtain the amplitude values A, A' and A" of the peak point amplitude values of the respective echo signals except the first echo on the echo chain; d), the phase of the first inversion pulse remains constant as π/2, and according to the peak point amplitude values A, A' and A" of the respective echo signals, the phase of each inversion pulse is calculated by the following formula: $\mathrm{<span\; class="notranslate">\; \&alpha;</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arctan</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}{\mathrm{<span\; class="notranslate">\; A</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; A</span>}}^{<span\; class="notranslate">\; \&prime;</span><span\; class="notranslate">\; \&prime;</span>}}{\mathrm{<span\; class="notranslate">\; A</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; A</span>}}^{<span\; class="notranslate">\; \&prime;</span><span\; class="notranslate">\; \&prime;</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; .</span>$