WO2021184471A1 - Procédé de mesure de coefficient de diffusion apparent basé sur un système de résonance magnétique à champ non uniforme - Google Patents

Procédé de mesure de coefficient de diffusion apparent basé sur un système de résonance magnétique à champ non uniforme Download PDF

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WO2021184471A1
WO2021184471A1 PCT/CN2020/084743 CN2020084743W WO2021184471A1 WO 2021184471 A1 WO2021184471 A1 WO 2021184471A1 CN 2020084743 W CN2020084743 W CN 2020084743W WO 2021184471 A1 WO2021184471 A1 WO 2021184471A1
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magnetic resonance
echo
signals
radio frequency
nuclear magnetic
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PCT/CN2020/084743
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Chinese (zh)
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吴子岳
罗海
陈潇
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无锡鸣石峻致医疗科技有限公司
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
    • G01N24/08—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects by using nuclear magnetic resonance
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N13/00—Investigating surface or boundary effects, e.g. wetting power; Investigating diffusion effects; Analysing materials by determining surface, boundary, or diffusion effects

Definitions

  • the invention relates to the technical field of nuclear magnetic resonance, and in particular to a method for measuring the apparent diffusion coefficient based on a non-uniform field magnetic resonance system.
  • Nuclear magnetic resonance technology is a technology that uses the nuclear magnetic resonance phenomenon of hydrogen protons to image or detect the composition and structure of substances.
  • a nucleus containing a single number of protons in the human body such as a hydrogen nucleus, has a spin motion of the protons.
  • the spin motion of charged atomic nuclei is physically similar to a single small magnet, and the directional distribution of these small magnets is random without the influence of external conditions.
  • these small magnets will be rearranged according to the magnetic field lines of the external magnetic field.
  • a radio frequency pulse of a specific frequency is used to excite the atomic nuclei, and the spins (small magnets) of these atomic nuclei are deflected and resonance occurs.
  • Molecules in matter have a certain degree of diffusion motion, and the direction is random, which is called thermal motion of molecules or Brownian motion. If the diffusion motion of water molecules is not restricted in any way, we call it free diffusion. In the human body, the diffusion and movement of water molecules such as cerebrospinal fluid and urine are relatively small and are regarded as free diffusion. In fact, due to the restriction of the surrounding medium, the diffusion of water molecules in biological tissues will be restricted to varying degrees, which is called restricted diffusion. The diffusion of water molecules in general tissues belongs to restricted diffusion. The apparent diffusion coefficient is a physical quantity that describes the ability of water molecules to diffuse in tissues. After the magnetic resonance signal is excited, the diffusion movement of water molecules in the direction of the gradient magnetic field will cause the attenuation of the magnetic resonance signal.
  • the apparent diffusion coefficient of an object can be measured by nuclear magnetic resonance technology, thereby indirectly reflecting the characteristics of the object's microstructure and its changes.
  • the apparent diffusion coefficient is widely used as an important clinical diagnostic index.
  • measurement is performed by diffusion-weighted imaging technology (DWI), such as SE-EPI sequence, that is, spin echo sequence (SE) for diffusion gradient encoding, and plane echo sequence (EPI) for signal readout.
  • DWI diffusion-weighted imaging technology
  • SE-EPI sequence spin echo sequence
  • EPI plane echo sequence
  • Figure 1a shows the SE-CPMG sequence, which is based on the spin echo for diffusion gradient encoding, and then uses the ultra-fast CPMG sequence for signal readout.
  • Figure 1b shows the DSE-CPMG sequence, which is based on the double-echo sequence for diffusion gradient encoding, and also uses the ultra-fast CPMG sequence for signal readout. This method can reduce the impact of low-speed liquid flow.
  • Figure 1c is the STE-CPMG sequence, which is based on the stimulated echo sequence for diffusion gradient coding. This method can reduce the impact of T1 recovery.
  • the ADC can be used to measure the ADC to improve the accuracy of the measurement. Spend.
  • the ADC measurement pulse sequence is composed of a diffusion gradient encoding module and a signal readout module.
  • the gradient magnetic field is very large, usually 2 to 3 orders of magnitude higher than the gradient field of the conventional MRI system, and it is impossible to control the gradient magnetic field to change during the signal readout stage (DWI technology in the MRI system, In the signal readout stage, the gradient field can be controlled to decrease). Therefore, it is necessary to adopt an ultra-fast signal readout module to reduce the influence of the diffusion effect in the process of reading the signal.
  • reference documents Ra, DG, et al. (2006). "Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient. "J Magn Reson 180(2): 229-235.) used in The echo interval is 40us. This places very high requirements on the spectrometer equipment, radio frequency power amplifier and radio frequency coil of the nuclear magnetic resonance system.
  • CPMG sequences are usually used to acquire signals.
  • the strong gradient field existing in the non-uniform field can not only play the role of frequency encoding in the CPMG sequence, but also always have the role of diffusion encoding. That is to say, if the echo interval of the CPMG sequence is larger, the signal is lower.
  • the influence of the diffusion effect on the CPMG signal is analyzed, which can be described by the following formula:
  • ⁇ is the magnetic rotation ratio
  • D is the ADC coefficient of the substance
  • G is the size of the gradient magnetic field
  • ⁇ is the echo interval of the CPMG sequence.
  • the invention aims to provide a method for measuring the apparent diffusion coefficient based on a non-uniform field magnetic resonance system, which has a stable algorithm and is not easily affected by flowing liquids, and is also suitable for substances with small T1/T2.
  • the invention discloses a method for measuring the apparent diffusion coefficient based on a non-uniform field magnetic resonance system, which comprises the following steps:
  • the echo signals are a four-dimensional array S(m, n, a, p),
  • the first dimension is the echo interval vector ⁇ , the length is M,
  • the second dimension is the length of the echo chain, the length is N,
  • the third dimension is the average number of times A
  • the fourth dimension is the number of sampling points of a single read data, the number is P;
  • ⁇ (m) is the m-th element in the echo interval vector ⁇
  • C 1 is an unknown constant
  • ⁇ is the magnetic rotation ratio
  • G is the magnitude of the gradient magnetic field
  • C 2 is an unknown constant
  • step S100 in the non-uniform field nuclear magnetic resonance system, an excitation pulse, a refocusing pulse, and a constant gradient field are applied,
  • the flip angle of the excitation pulse is ⁇ , followed by a number of convergent pulses, and the flip angle of the convergent pulse is 2 ⁇ ;
  • the phase difference between the excitation pulse and the first refocusing pulse is 90 degrees, the time interval between the excitation pulse and the first refocusing pulse is ⁇ /2, the first refocusing pulse to the first sampling window The time interval is ⁇ /2; the time interval between refocusing pulses is the echo interval, and N echo signals are collected for one excitation.
  • the constant gradient field is the natural gradient field of the magnet.
  • the echo signals are collected multiple times and the average value is calculated.
  • the echo interval is ⁇ .
  • the echo interval ⁇ is changed, M measurements are performed, and M groups of echo signals are collected.
  • the non-uniform field nuclear magnetic resonance system includes a console, a nuclear magnetic resonance spectrometer, a magnet, and a radio frequency system,
  • the console is connected with the nuclear magnetic resonance spectrometer, sending instructions to control the parameter selection of the measurement sequence, ROI positioning, receiving the magnetic resonance signals collected by the spectrometer, and completing real-time data processing;
  • the magnet is a permanent magnet design
  • the radio frequency system mainly includes a radio frequency power amplifier, a preamplifier, a transceiving switch, and a radio frequency coil.
  • the radio frequency coil can transmit excitation signals and receive magnetic resonance signals through the transceiving switch.
  • the magnet is a unilateral permanent magnet.
  • the present invention is based on a non-uniform field nuclear magnetic resonance system, including non-uniform field magnets, nuclear magnetic resonance spectrometers, radio frequency power amplifiers and radio frequency coils, etc., through multiple CPMG sequences with different echo intervals to collect signals from multiple sets of signals Fit the ADC coefficients.
  • the present invention does not require a complicated diffusion enhancement sequence, the algorithm is simple, and the system requirements are low, and the system cost can be reduced.
  • the method of the present invention has a stable algorithm, is not easily affected by flowing liquid, and is also suitable for substances with a small T1/T2.
  • Figure 1 is a schematic diagram of an ADC measurement pulse sequence based on a non-uniform field nuclear magnetic resonance system in the prior art
  • Figure 2 is a schematic diagram of a non-uniform field nuclear magnetic resonance system for measuring apparent diffusion coefficient
  • Figure 3 is a schematic diagram of the apparent diffusion coefficient measurement sequence based on a non-uniform field nuclear magnetic resonance system
  • Figure 4 shows CPMG measurement data with different echo intervals with pure water as the test substance
  • Figure 5 shows the equivalent time constant measured by CPMG sequences with different echo intervals using pure water as the test substance.
  • K-space K-space, the frequency domain space of magnetic resonance signals
  • DWI Diffusion Weighted Imaging, Diffusion Weighted Imaging or Diffusion Weighted Imaging
  • T1 Time constant for regrowth of longitudinal magnetization after RF-pulse, longitudinal magnetization vector recovery time constant
  • T2 Time constant for decay of transverse magnetization after RF-pulse, transverse magnetization vector decay time constant
  • TR Repetition Time, repetition time or repetition period
  • ADC Apparent diffusion coefficient, apparent diffusion coefficient
  • EPI Echo planar imaging, planar echo imaging technology
  • CPMG a NMR pulse sequence named by several scientists (Carr, Purcell, Meiboom, Gill), NMR sequence named by Carr, Purcell, Meiboom, Gill, etc.
  • SE-EPI Spin echo-echo planar imaging, spin echo-plane echo sequence
  • SE-CPMG Spin echo-CPMG sequence, spin echo-CPMG sequence
  • DSE-CPMG Dual spin echo-CPMG sequence, dual spin echo-CPMG sequence
  • STE-CPMG Stimulated echo-CPMG sequence, stimulated echo-CPMG sequence
  • the non-uniform field nuclear magnetic resonance system used to measure the apparent diffusion coefficient is mainly composed of four parts: the console, the nuclear magnetic resonance spectrometer, the magnet and the radio frequency system;
  • the console is connected with the spectrometer, sends instructions to control the parameter selection of the measurement sequence, ROI positioning, and receives the magnetic resonance signals collected by the spectrometer to complete real-time data processing.
  • Magnets are generally designed as permanent magnets. For example, a unilateral permanent magnet still has a highly non-uniform magnetic field in the ROI.
  • the radio frequency system mainly includes a radio frequency power amplifier, a preamplifier, a transceiver switch and a radio frequency coil.
  • the radio frequency coil can transmit excitation signals and receive magnetic resonance signals through the transceiver switch.
  • Figure 3 shows a schematic diagram of the apparent diffusion coefficient measurement sequence of a non-uniform field NMR system, that is, a typical ⁇ -2 ⁇ -2 ⁇ -2 ⁇ ; RF pulse sequence: the first excitation pulse flip angle is ⁇ , followed by Several refocusing pulses with a flip angle of 2 ⁇ ; the phase difference between the first excitation pulse and the first refocusing pulse is 90 degrees, and the time interval between the first excitation pulse and the first refocusing pulse is ⁇ /2, The time interval between the first refocusing pulse and the first sampling window is ⁇ /2; the time interval between the refocusing pulses is ⁇ , which is called the echo interval.
  • the constant gradient field is the natural gradient field of the magnet and does not need to be controlled. Collect N echo signals in one excitation. Usually it is necessary to acquire the signal multiple times, and improve the signal-to-noise ratio by averaging the signal.
  • the collected signal is represented as a 4-dimensional array S(m, n, a, p), the first dimension corresponds to different echo intervals, that is, the corresponding echo interval vector ⁇ , the length is M; the second dimension is the echo chain Length, the length is N; the third dimension is the average number of times A; the fourth dimension is the number of sampling points of a single read data, which is P.
  • the ADC coefficient estimation based on the four-dimensional array mainly includes the following 3 steps,
  • Data preprocessing includes:
  • Preprocessing step 1 Perform Fourier transform on the fourth dimension of the signal S to obtain frequency domain data; only the low frequency part is retained and averaged;
  • Preprocessing step 2 average the third dimension
  • Preprocessing step 3 take the logarithm of all data
  • the signal S (m, n, a, p) is converted into a two-dimensional array of S'(m, n).
  • ⁇ is the magnetic rotation ratio
  • G is the magnitude of the gradient magnetic field, which is a known quantity determined in advance
  • C 2 is an unknown constant.
  • the estimated parameter D is the ADC coefficient.
  • the pure water is detected on the nuclear magnetic resonance system designed in the scheme of the present invention.
  • the main parameters include the B0 field of the ROI area is 0.07T, the gradient field is 180Gauss/cm, the time interval of the CPMG sequence is 640us, 740us, 840us, 940us, 1040us and 1140us in six groups, the echo chain length of the CPMG sequence is 100, average 32 times, the number of echo sampling points is 64.
  • Figure 4 shows the CPMG measurement data after the preprocessing step 1 and the preprocessing step 2 of the present invention. It can be seen that the attenuation degree of the CPMG sequence acquisition signal with different echo time intervals is different. This difference is described by formula (1) The manifestation of the diffusion effect.
  • Fig. 5 is the result after the preprocessing step and the equivalent time constant estimation step of the present invention. It can be seen that the equivalent time constant has a good linear relationship with ⁇ 2.
  • the ADC coefficient of pure water can be estimated to be 1.93e-3mm2/s, which is close to the theoretical value.

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Abstract

Procédé de mesure de coefficient de diffusion apparent basé sur un système de résonance magnétique à champ non uniforme, qui est basé sur un système de résonance magnétique nucléaire à champ non uniforme, comprenant un aimant de champ non uniforme, un spectromètre de résonance magnétique nucléaire, un amplificateur de puissance radiofréquence, une bobine radiofréquence, etc. Des signaux sont collectés au moyen d'une pluralité de séquences CPMG dotées de différents intervalles d'écho, et un coefficient ADC est ajusté à partir de multiples ensembles de signaux. Le procédé ne nécessite pas une séquence d'amélioration de diffusion complexe, et l'algorithme est simple. De plus, les exigences du système sont faibles, et les coûts du système peuvent être réduits. Dans le même temps, l'algorithme du procédé est stable et peu affecté par l'écoulement de liquide, et est également approprié pour des substances qui ont de petits T1/T2.
PCT/CN2020/084743 2020-03-17 2020-04-14 Procédé de mesure de coefficient de diffusion apparent basé sur un système de résonance magnétique à champ non uniforme WO2021184471A1 (fr)

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