CN111351813A - Method for measuring apparent diffusion coefficient based on non-uniform field magnetic resonance system - Google Patents
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- 238000009792 diffusion process Methods 0.000 title claims abstract description 39
- 238000000034 method Methods 0.000 title claims abstract description 21
- 238000005481 NMR spectroscopy Methods 0.000 claims description 31
- 230000005284 excitation Effects 0.000 claims description 21
- 238000005259 measurement Methods 0.000 claims description 14
- 238000007781 pre-processing Methods 0.000 claims description 9
- 238000005070 sampling Methods 0.000 claims description 7
- 238000012935 Averaging Methods 0.000 claims description 4
- 238000012545 processing Methods 0.000 claims description 3
- 238000003491 array Methods 0.000 claims description 2
- 238000000691 measurement method Methods 0.000 claims 6
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- 238000005728 strengthening Methods 0.000 abstract description 2
- 238000000685 Carr-Purcell-Meiboom-Gill pulse sequence Methods 0.000 description 28
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- 238000002595 magnetic resonance imaging Methods 0.000 description 5
- 238000001208 nuclear magnetic resonance pulse sequence Methods 0.000 description 5
- 238000010586 diagram Methods 0.000 description 4
- 238000003384 imaging method Methods 0.000 description 4
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- 230000009286 beneficial effect Effects 0.000 description 1
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- 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
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- 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
Abstract
The invention discloses an apparent diffusion coefficient measuring method based on a non-uniform field magnetic resonance system, which is based on the non-uniform field magnetic resonance system and comprises a non-uniform field magnet, a magnetic resonance spectrometer, a radio frequency power amplifier, a radio frequency coil and the like. The method does not need a complex diffusion strengthening sequence, has simple algorithm and low requirement on the system, and can reduce the system cost. Meanwhile, the method has stable algorithm, is not easily influenced by flowing liquid, and is also suitable for substances with smaller T1/T2.
Description
Technical Field
The invention relates to the technical field of nuclear magnetic resonance, in particular to an apparent diffusion coefficient measuring method based on a non-uniform field magnetic resonance system.
Background
The nuclear magnetic resonance technique is a technique for imaging or detecting the composition and structure of a substance by utilizing the nuclear magnetic resonance phenomenon of hydrogen protons. Nuclei in the human body containing a single proton, such as hydrogen nuclei, have a spin motion. The spin motion of the charged nuclei is physically similar to that of individual small magnets whose directional distribution is random without the influence of external conditions. When a human body is placed in an external magnetic field, the small magnets will realign with the lines of the external magnetic field. At this time, the nuclear magnetic resonance phenomenon is a phenomenon in which nuclei are excited by a radio frequency pulse of a specific frequency to deflect spins (small magnets) of the nuclei to generate resonance. After the emission of the radio frequency pulse is stopped, the excited atomic nuclei (small resonant magnets) are gradually restored to the state before excitation, electromagnetic wave signals are released in the restoration process, and magnetic resonance images or composition and structure information of substances can be obtained after the nuclear magnetic resonance signals are received and processed by special equipment.
Molecules in a substance all have some degree of diffusive motion, with random direction, called thermal or brownian motion of the molecules. If the diffusion movement of water molecules is not constrained, we call free diffusion. In the human body, diffusion of water molecules such as cerebrospinal fluid and urine is relatively less restricted and is considered to be free diffusion. In fact, the diffusion movement of water molecules in biological tissues is limited to different degrees due to the constraint of surrounding media, which is called limiting diffusion, and the diffusion movement of water molecules in general tissues belongs to limiting diffusion. The apparent diffusion coefficient is a physical quantity that describes the ability of water molecules to diffuse in tissue. After the magnetic resonance signal is excited, the diffusion movement of water molecules in the direction of the gradient magnetic field causes the attenuation of the magnetic resonance signal, and if the water molecules are more freely diffused in the direction of the gradient magnetic field, the larger the diffusion distance is during the application of the gradient magnetic field, the larger the magnetic field change is experienced, and the more obvious the attenuation of the tissue signal is. Therefore, the apparent diffusion coefficient of the object can be measured by the nuclear magnetic resonance technology, so that the microstructure characteristics and the change of the object are indirectly reflected.
In the magnetic resonance imaging technology, the apparent diffusion coefficient is widely used as an important clinical diagnosis index. The measurement is generally carried out by diffusion-weighted imaging techniques (DWI), such as SE-EPI sequences, i.e. spin echo Sequences (SE) for diffusion gradient encoding and planar echo sequences (EPI) for signal readout. In inhomogeneous magnetic field magnetic resonance systems, similar diffusion weighted imaging techniques are introduced for measuring the apparent diffusion coefficient of a substance. Several typical pulse sequences for measuring apparent diffusion coefficients are shown in figure 1.
Fig. 1a) is a SE-CPMG sequence, i.e. diffusion gradient encoding based on spin echo, followed by signal readout with an ultra-fast CPMG sequence.
Fig. 1b) shows a DSE-CPMG sequence, namely, diffusion gradient coding is carried out based on a double echo sequence, and signal reading is carried out by using an ultra-fast CPMG sequence, and the method can reduce the influence caused by low-speed liquid flow.
Fig. 1c) is a STE-CPMG sequence, namely diffusion gradient coding is carried out based on a stimulated echo sequence, the method can reduce the influence of T1 recovery, and when the T1/T2 of the detected object is small, the measurement ADC can improve the measurement accuracy by using the sequence.
In the prior art, the ADC measurement pulse sequence is composed of a diffusion gradient coding module and a signal readout module. In the inhomogeneous field nuclear magnetic resonance system, the gradient magnetic field is very large, which is usually 2-3 orders of magnitude higher than that of the conventional MRI system, and the gradient magnetic field cannot be controlled to change in the signal readout stage (the DWI technique in the MRI system can control the gradient field to decrease in the signal readout stage). Therefore, an ultra-fast signal readout module is required to reduce the influence of diffusion effect in the signal readout process. For example, the echo spacing used in the reference (Rata, d.g., et al (2006). "Self-differentiation measuring by a mobile-single-sized NMR sensor with improved magnetic field gradient." JMagn resonance 180 (2): 229-. Thus, the requirements on spectrometer equipment, radio frequency power amplifiers and radio frequency coils of the nuclear magnetic resonance system are very high.
In inhomogeneous field nuclear magnetic resonance systems, the signal is typically acquired using a CPMG sequence. The strong gradient field existing in the inhomogeneous field can play a role of frequency coding in the CPMG sequence, and simultaneously has a role of diffusion coding all the time. That is, if the echo interval of the CPMG sequence is larger, the signal is lower. The effect of diffusion Effects on CPMG signals is analyzed in the references [ M.D. hurliman.Difusion and relaxation Effects in General Stray Field NMR experiments.J.Magn.Reson 148, 367-:
where γ is the magnetic spin ratio, D is the ADC coefficient of the material, G is the gradient field magnitude, and τ is the echo spacing of the CPMG sequence. From this equation, it can be seen that the CPMG signal is related to the ADC coefficient, gradient field and echo spacing of the material. Therefore, the ADC coefficient can be estimated by collecting CPMG signals of different echo times.
Disclosure of Invention
The invention aims to provide an apparent diffusion coefficient measuring method based on a non-uniform field magnetic resonance system, which is stable in algorithm, not easy to be influenced by flowing liquid and also suitable for substances with smaller T1/T2.
In order to achieve the purpose, the invention is realized by adopting the following technical scheme:
the invention discloses an apparent diffusion coefficient measuring method based on a non-uniform field magnetic resonance system, which comprises the following steps:
s100, collecting M groups of echo signals in a non-uniform field nuclear magnetic resonance system, wherein the echo signals are four-dimensional arrays S (M, n, a, p),
wherein the first dimension is an echo spacing vector τ, length M,
the second dimension is the echo train length, which is N,
the third dimension is the average degree a,
the fourth dimension is the number of sampling points of single-time read data, and the number is P;
s200, data preprocessing, namely converting the signal S (m, n, a, p) into a two-dimensional array of S' (m, n):
s210, Fourier transform is carried out on the fourth dimension of the signal S to obtain frequency domain data, the low-frequency part is reserved and averaged,
s220, averaging the third dimension,
s230, taking logarithms for all data;
s300, estimating an equivalent time constant T (m):
fitting S' (m, n) line by line according to the following formula
Where τ (m) is the mth element in the echo spacing vector τ, C1Is an unknown constant;
s400, ADC coefficient D estimation:
fitting was performed according to the following formula
Where γ is the magnetic spin ratio, G is the magnitude of the gradient magnetic field, C2Are unknown constants.
Preferably, in step S100, in the inhomogeneous field nuclear magnetic resonance system, an excitation pulse, a refocusing pulse, a constant gradient field are applied,
the flip angle of the excitation pulse is theta, and then a plurality of refocusing pulses follow the excitation pulse, wherein the flip angle of the refocusing pulses is 2 theta;
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 tau/2, and the time interval from the first refocusing pulse to the first sampling window is tau/2; the time interval between the echo focusing pulses is the echo interval, and N echo signals are acquired by one-time excitation.
Preferably, the constant gradient field is the natural gradient field of the magnet.
Preferably, the echo signals are acquired a plurality of times and an average value is calculated.
Preferably, the echo interval is τ.
Preferably, M measurements are performed by varying the echo interval τ, and M sets of echo signals are acquired.
Preferably, the inhomogeneous field nuclear magnetic resonance system comprises a console, a nuclear magnetic resonance spectrometer, a magnet and a radio frequency system,
the console is connected with the nuclear magnetic resonance spectrometer, sends instructions to control parameter selection and ROI positioning of a measurement sequence, receives magnetic resonance signals collected by the spectrometer and completes real-time data processing;
the magnet is designed as a permanent magnet;
the radio frequency system mainly comprises a radio frequency power amplifier, a preamplifier, a receiving and transmitting change-over switch and a radio frequency coil, wherein the radio frequency coil can transmit an excitation signal and receive a magnetic resonance signal through the receiving and transmitting change-over switch.
Preferably, the magnet is a single-sided permanent magnet.
The invention has the beneficial effects that:
1. the invention is based on a non-uniform field nuclear magnetic resonance system, which comprises a non-uniform field magnet, a nuclear magnetic resonance spectrometer, a radio frequency power amplifier, a radio frequency coil and the like, and an ADC coefficient is fitted from a plurality of groups of signals by acquiring signals through a plurality of CPMG sequences with different echo intervals.
2. The invention does not need complex diffusion strengthening sequence, has simple algorithm and low requirement on the system, and can reduce the system cost.
3. The method has stable algorithm, is not easily influenced by flowing liquid, and is also suitable for substances with smaller T1/T2.
Drawings
FIG. 1 is a schematic diagram of a prior art ADC measurement pulse sequence based on a non-uniform field NMR system;
FIG. 2 is a schematic diagram of a non-uniform field NMR system for measuring apparent diffusion coefficients;
FIG. 3 is a schematic diagram of an apparent diffusion coefficient measurement sequence based on a non-uniform field nuclear magnetic resonance system;
FIG. 4 shows CPMG measurement data with different echo intervals using pure water as the test substance;
fig. 5 shows equivalent time constants measured by CPMG sequence with different echo intervals using pure water as a test substance.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings.
In the present application:
NMR: nuclear Magnetic Resonance technology
MRI: magnetic Resonance Imaging
K-space: k-space, the frequency domain space of the magnetic resonance signal
DWI: diffusion Weighted Imaging, Diffusion Weighted Imaging or Diffusion Weighted Imaging
T1: time constant for growing of longitudinal magnetization after RF-pulse, longitudinal magnetization vector recovery Time constant
T2: time constant for decay of transverse magnetization after RF-pulse
TR: repetition Time, Repetition Time or Repetition period
ADC: apparent diffusion coefficient of Apparent diffusion coefficient
EPI: echo planar imaging, planar Echo imaging technique
CPMG: a NMR pulse sequence and by magnetic resonance diagnostics (Carr, Purcell, Meiboom, Gill), nuclear magnetic resonance sequences named by Carr, Purcell, Meiboom, Gill, et al
SE-EPI: spin echo-echo planar imaging, Spin echo-planar 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
As shown in fig. 2, the inhomogeneous field nmr system for measuring the apparent diffusion coefficient is mainly composed of four parts: a console, a nuclear magnetic resonance spectrometer, a magnet and a radio frequency system;
the console is connected with the spectrometer, sends instructions to control parameter selection and ROI positioning of a measurement sequence, receives magnetic resonance signals acquired by the spectrometer, completes real-time data processing,
the magnets are typically of a permanent magnet design, such as a single-sided permanent magnet, still having a highly inhomogeneous magnetic field within the ROI,
the radio frequency system mainly comprises a radio frequency power amplifier, a preamplifier, a receiving and transmitting change-over switch and a radio frequency coil, wherein the radio frequency coil can transmit an excitation signal and receive a magnetic resonance signal through the receiving and transmitting change-over switch.
ADC coefficient measurement sequence:
FIG. 3 is a schematic diagram of an apparent diffusion coefficient measurement sequence of a non-uniform field NMR system, which uses a typical theta-2 theta … … RF pulse sequence: the flip angle of the first excitation pulse is theta, and then a plurality of refocusing pulses follow the first excitation pulse, wherein the flip angle is 2 theta; the phase difference between the first excitation pulse and the first refocusing pulse is 90 degrees, the time interval between the first excitation pulse and the first refocusing pulse is tau/2, and the time interval from the first refocusing pulse to the first sampling window is tau/2; the time interval between the refocusing pulses is τ, called the echo interval. The constant gradient field is the natural gradient field of the magnet and does not need to be controlled. And acquiring N echo signals by one-time excitation. It is also often necessary to acquire the signal multiple times, and to boost the signal-to-noise ratio by averaging the signals.
In order to estimate the ADC coefficients, M sets of echo signals need to be acquired with varying echo intervals τ.
ADC coefficient estimation method
The acquired signal is expressed as a 4-dimensional array S (M, n, a, p), the first dimension corresponds to different echo intervals, namely corresponds to an echo interval vector tau, and the length is M; the second dimension is the length of the echo chain, and the length is N; the third dimension is the average number of times A; the fourth dimension is the number of sampling points of the single read data, which is P. The ADC coefficient estimation based on the four-dimensional array mainly comprises the following 3 steps,
the data preprocessing comprises the following steps:
preprocessing step 1, performing Fourier transform on a fourth dimension of a signal S to obtain frequency domain data; only the low frequency part is retained and averaged;
a pretreatment step 2, averaging the third dimension;
a preprocessing step 3, taking logarithms of all data;
after data preprocessing, the signal S (m, n, a, p) is converted into a two-dimensional array of S' (m, n).
Equivalent time constant estimation:
fitting S' (m, n) line by line according to the following formula
Where τ (m) is the mth element in the echo spacing vector τ, C1Are unknown constants. Fitting the equivalent time constant T (m)
The ADC coefficient D is estimated by the equivalent time constant: fitting was performed according to the following formula
Where γ is the gyromagnetic ratio, G is the magnitude of the gradient magnetic field, for a predetermined known quantity, C2Are unknown constants.
The estimated parameter D is the ADC coefficient.
The experimental results are as follows:
the pure water is detected on the nuclear magnetic resonance system designed by the scheme of the invention. The main parameters comprise that the ROI area B0 has the field of 0.07T, the gradient field is 180Gauss/cm, the time interval of the CPMG sequence is six groups including 640us, 740us, 840us, 940us, 1040us and 1140us, the echo chain length of the CPMG sequence is 100, the average is 32 times, and the number of echo sampling points is 64.
Fig. 4 shows the measured CPMG data after the preprocessing step 1 and the preprocessing step 2, which shows that the attenuation degrees of the signals acquired by the CPMG sequences with different echo time intervals are different, and this difference is the manifestation of the diffusion effect described in the formula (1).
FIG. 5 shows the results of the preprocessing step and the equivalent time constant estimation step of the present invention, showing the equivalent time constant andτ2in a better linear relationship.
Further, the ADC coefficient of pure water is estimated to be 1.93e-3mm2/s, which is close to the theoretical value, by linear fitting and by the formula (3).
The present invention is capable of other embodiments, and various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the invention.
Claims (8)
1. An apparent diffusion coefficient measuring method based on a non-uniform field magnetic resonance system is characterized by comprising the following steps:
s100, collecting M groups of echo signals in a non-uniform field nuclear magnetic resonance system, wherein the echo signals are four-dimensional arrays S (M, n, a, p),
wherein the first dimension is an echo spacing vector τ, length M,
the second dimension is the echo train length, which is N,
the third dimension is the average degree a,
the fourth dimension is the number of sampling points of single-time read data, and the number is P;
s200, data preprocessing, namely converting the signal S (m, n, a, p) into a two-dimensional array of S' (m, n):
s210, Fourier transform is carried out on the fourth dimension of the signal S to obtain frequency domain data, the low-frequency part is reserved and averaged,
s220, averaging the third dimension,
s230, taking logarithms for all data;
s300, estimating an equivalent time constant T (m):
fitting S' (m, n) line by line according to the following formula
Where τ (m) is the mth element in the echo spacing vector τ, C1Is an unknown constant;
s400, ADC coefficient D estimation:
fitting was performed according to the following formula
Where γ is the magnetic spin ratio, G is the magnitude of the gradient magnetic field, C2Are unknown constants.
2. The measurement method according to claim 1, characterized in that: in step S100, in the inhomogeneous 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 theta, and then a plurality of refocusing pulses follow the excitation pulse, wherein the flip angle of the refocusing pulses is 2 theta;
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 tau/2, and the time interval from the first refocusing pulse to the first sampling window is tau/2; the time interval between the echo focusing pulses is the echo interval, and N echo signals are acquired by one-time excitation.
3. The measurement method according to claim 2, characterized in that: the constant gradient field is the natural gradient field of the magnet.
4. A measuring method according to claim 2 or 3, characterized in that: and collecting echo signals for multiple times, and calculating an average value.
5. The measurement method according to claim 4, characterized in that: the echo interval is τ.
6. The measurement method according to claim 5, characterized in that: and changing the echo interval tau, carrying out M times of measurement, and collecting M groups of echo signals.
7. The measurement method according to claim 6, characterized in that: the inhomogeneous field nuclear magnetic resonance system comprises a console, a nuclear magnetic resonance spectrometer, a magnet and a radio frequency system,
the console is connected with the nuclear magnetic resonance spectrometer, sends instructions to control parameter selection and ROI positioning of a measurement sequence, receives magnetic resonance signals collected by the spectrometer and completes real-time data processing;
the magnet is designed as a permanent magnet;
the radio frequency system mainly comprises a radio frequency power amplifier, a preamplifier, a receiving and transmitting change-over switch and a radio frequency coil, wherein the radio frequency coil can transmit an excitation signal and receive a magnetic resonance signal through the receiving and transmitting change-over switch.
8. The measurement method according to claim 7, characterized in that: the magnet is a single-side permanent magnet.
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| CN112304811A (en) * | 2020-10-22 | 2021-02-02 | 无锡鸣石峻致医疗科技有限公司 | Low-field nuclear magnetic resonance apparent diffusion coefficient measuring method insensitive to motion |
| CN112462311A (en) * | 2020-11-09 | 2021-03-09 | 无锡鸣石峻致医疗科技有限公司 | Method and device for correcting and measuring transverse magnetization vector decay time constant, computer equipment and non-uniform field magnetic resonance system |
| WO2022095142A1 (en) * | 2020-11-09 | 2022-05-12 | 无锡鸣石峻致医疗科技有限公司 | Adc-t2 two-dimensional spectrum measurement method and apparatus, computer device, and non-uniform field magnetic resonance system |
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