CN110068783A - A kind of Chemical Exchange saturation transfer imaging method for eliminating fatty artifact - Google Patents
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Abstract
The invention discloses a kind of Chemical Exchange saturation transfer imaging methods for eliminating fatty artifact, this method includes Image Acquisition and image reconstruction step, and Image Acquisition is the mould figure and phase diagram that magnetic resonance imaging is obtained under conditions of without saturation radio frequency irradiation and under conditions of the irradiation of the saturation radio frequency of different frequency using the gradin-echo of more echoes；Its image reconstruction: based on multimodal fat model to the data reconstruction water figure that acquires under the irradiation without saturation radio frequency, fat figure, field figure and
Description
Technical Field
The invention relates to the technical field of magnetic resonance imaging, in particular to a chemical exchange saturation transfer imaging method for eliminating fat artifacts.
Background
The chemical exchange magnetization transfer imaging technology is a molecular imaging technology in the field of magnetic resonance imaging technology. When exchangeable protons in exogenous or endogenous molecules are frequency-selective radio-frequency saturated, the saturation effect can be transferred to the protons of the free water by chemical exchange, ultimately imaging the molecules containing the exchangeable protons indirectly based on changes in the water signal. The chemical exchange magnetic transfer imaging technology is used as a molecular level imaging technology and provides molecular level histopathological information for disease diagnosis, such as the concentration of imaging target molecules, the pH value of tissues, local temperature and the like. This technique is gaining increasing attention from researchers due to the great potential it exhibits in the monitoring of brain tumors and in the assessment of stroke. At present, the technology is mainly applied to brain tissues, and more researches show that the technology also has great potential in body part (such as breast, prostate and liver) imaging, however, fat existing in body parts can cause serious artifacts to the technical images.
Currently, researchers have applied conventional lipid suppression techniques to body chemistry exchange magnetization transfer imaging, including binomial hard pulse based water excitation techniques and single peak fat signal model based Dixon water-fat reconstruction techniques. In the theoretical assumptions of these techniques, not only the contributions of the multiple spectral peaks present in the fat signal are ignored, but also the saturation effects of saturation illumination on the individual peaks of fat in chemical exchange magnetization transfer imaging are not taken into account. These intrinsic defects allow for chemical exchange magnetization transfer imaging of the body to still be affected by fat artifacts.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a chemical exchange saturation transfer imaging method for accurately eliminating fat artifacts. The method not only considers the contribution of a plurality of spectral peaks in the water-fat reconstruction model to the total fat signal, but also considers the saturation effect of saturated radio frequency irradiation on each peak of fat in chemical exchange magnetization transfer imaging. The signal amplitudes of the individual peaks of fat after saturated irradiation are updated by numerical calculations based on the Bloch equation. The numerical calculation also takes into account information local to the tissue, such as the lateral decay rateAnd main magnetic field inhomogeneity. Based on the numerical calculation, the fat signal model for water-fat reconstruction will be adaptively updated on a voxel-by-voxel basis. Finally, water-fat reconstruction will produce the water and field maps required for chemical exchange saturation transfer imaging, where the field maps will further be used to correct for the effects of field inhomogeneity on the Z-spectrum.
In order to achieve the purpose, the invention adopts the following technical scheme:
a chemical exchange saturation transfer imaging method capable of eliminating fat artifacts comprises two parts of data acquisition and image reconstruction.
The data acquisition part comprises two steps of chemical exchange saturation transfer imaging data acquisition and reference data acquisition.
The image acquisition part comprises the following steps:
step 1: the acquisition of chemical exchange saturation transfer imaging data is to adopt a gradient echo sequence of multi-echo of unipolar readout gradient to acquire a mode image and a phase image of magnetic resonance imaging under the irradiation of saturated radio frequency of different frequencies.
Step 2: the reference data acquisition is to acquire a mode image and a phase image of magnetic resonance imaging by using a multi-echo gradient echo sequence of unipolar readout gradients under the irradiation without saturated radio frequency.
The image reconstruction section includes the steps of:
step 1: generating a complex value image by the acquired mode image and the phase image;
step 2: reconstructing image data (reference data) obtained under the irradiation of unsaturated radio frequency to obtain water map, fat map, field map anda drawing; the reconstruction will follow the water-fat signal model:
where ρ iswAnd ρ is the total signal amplitude of water and fat, respectively; omegapAnd αPIs the resonance frequency and relative signal amplitude of the p-th fat peakTEnThe echo time of the nth echo; this model assumes that each voxel has the same lateral decay rateΔB0The strength of the local magnetic field deviation (field map); gamma is the gyromagnetic ratio of hydrogen nuclei;initial phase of precession of water and fat;
water-fat signal modelThe chemical shift and initial relative amplitude of each peak of fat in the model are acquired in advance and used as prior conditions; these a priori conditions are used directly for modeling the fat signal in equation (1) since no saturation pulse exerts a saturation effect on each fat peak. Reconstructing the obtained water map is used as a reference map required for chemical exchange saturation transfer imaging; reconstructing the obtained field map andthe map will be used as a known parameter in the next step; the field map obtained by reconstruction in this step will also be used for calibration of magnetic field inhomogeneity in chemical exchange saturation transfer imaging.
And step 3: updating longitudinal magnetization vectors of fat peaks irradiated by saturated radio frequency with different frequencies one by using a numerical method; carrying out numerical calculation to enable signals of each peak of simulated fat to evolve under the action of saturation pulses related to chemical exchange saturation transfer imaging; the signal evolves as follows for the Bloch equation:
whereinMagnetization vector intensity for equilibrium state of the p-th fat peak;andmagnetization vector intensities in x, y, z directions of the p-th fat peak, respectively; r1Andlongitudinal relaxation rate and transverse decay rate respectively; omega1(ii) the amplitude of the saturated RF profile, assuming without loss of generality that the saturated RF pulse is applied in the x-axis direction; Δ ωpIs the difference between the center frequency of the saturated RF pulse and the resonant frequency of each fat peak, and the local magnetic field deviation (Δ B)0) It also has an effect:
Δωp＝σpγB0+γΔB0-ωRF(3)
wherein sigmapIs the chemical shift of the p-th peak, γ is the gyromagnetic ratio of the hydrogen nuclei;
as can be seen from equations (2) and (3), under saturated rf irradiation at a certain frequency, the numerical calculation for updating each fat peak will depend on the following parameters: r1，ΔB0(ii) a In which the local parameters are spatially relatedΔB0The calculation results from step 2 will be used; assuming that all fat peaks have the same R1The parameter is considered to be independent of spatial position R1The specific value thereof is found from the literature; the intensity of the saturated rf illumination may be seen as a function of time, obtained in advance according to the particular saturated rf used in the sequence, and used as a known parameter. The intensity of the saturated radio frequency radiation is varied in the corresponding space of the image, and the acquired B acquired additionally1The field will calibrate it. The differential equation in the formula (2) describes the evolution process of the magnetization vector of each peak of the fat along with the time under the saturated radio frequency irradiation, and the initial condition on which the numerical solution depends is the relative amplitude of each peak of the fat in the original state used in the step 2; the magnetization vector for the transverse direction has been corrupted by a dephasing gradient, and the longitudinal axis component of the magnetization vector known from numerical calculations will be used to update the relative signal amplitude intensities of the fat peaks.
And 4, step 4: reconstructing image data (data of a saturation marker) acquired under the irradiation of saturated radio frequency; the reconstruction will follow the model for the water-fat signal in equation (1) with the relevant parameters as followsRequiring updating: updating the relative signal amplitude intensity of all fat peaks one by one according to the numerical calculation result in the step 3; from points in image space in step 2The accurate estimate of (a) will be used as a known parameter in the model of the water-fat signal in this step; reconstruction will obtain water, fat and field maps;
and 5: the original Z spectrum was extracted from the resulting water map and based on the field map (Δ B) reconstructed in step 20) Correcting the field inhomogeneity of the magnetic field; the corrected Z spectra will be used for asymmetry analysis to extract the magnetization transfer intensity of each target molecule.
The step 2 in the image reconstruction part comprises the following steps:
step 2.1: by means of R2Performing initial reconstruction on the image by using IDEAL algorithm to obtain a water map, a fat map, a local magnetic field map andrough estimation of the graph.
Step 2.2: r is to be2Water, fat and fat maps generated by the IDEAL algorithmThe estimates of the graph will be passed as initial values to a modular-based least squares fit; the objective function of the least squares fit based on the modulus values is as follows:
whereinSn，refThe measured signal of the nth echo of the reference scan;
step 2.3: water map, fat map, local magnetic field map generated by least squares fitting based on modulus values andthe accurate estimation of the map is used as a determined parameter to be transmitted to least square fitting based on complex values, and the least square fitting based on the complex values can accurately estimate the field map; the objective function of the least squares fit based on complex values is as follows:
whereinSn,refThe measured signal of the nth echo of the reference scan;
the step 4 in the image reconstruction part specifically includes the following steps:
step 4.1: performing initial reconstruction on the image by using an IDEAL algorithm to obtain rough estimation values of a field map, a water map and a fat map;
step 4.2: further accurate estimation is carried out on the water map and the fat map by using the water map and the fat map obtained in the last step as initial values and by using least square fitting based on the module values; the objective function of the least-squares fit of the modulus values is as follows:
wherein theta ismag＝{ρw,ρf}，Sn,labThe measured signal of the nth echo scanned for the saturated mark.
Due to the adoption of the technical scheme, the invention has the following advantages: by adaptively updating the relative amplitude of each fat peak after being irradiated by saturated radio frequency, the fat model can more truly express the composition of fat signals, thereby leading the water-fat reconstruction to be more accurate. Accurate water-fat reconstruction ensures that no fat signal is interfered in a water map, fat artifacts occurring in chemical exchange saturation transfer imaging are effectively eliminated, and the method can improve the accuracy of the chemical exchange saturation transfer imaging.
Drawings
FIG. 1 is a schematic diagram of an imaging sequence;
FIG. 2 is a flow chart of the image reconstruction portion of the present invention;
FIG. 3 is a graph of fat fraction;
FIG. 4 is a Z spectrum corresponding to voxels with high fat fraction;
FIG. 5 is a graph of the saturation transfer intensity of proton chemical exchange for amide bond.
Detailed Description
The present invention will be described in further detail with reference to the following specific examples and the accompanying drawings. The procedures, conditions, experimental methods and the like for carrying out the present invention are general knowledge and common general knowledge in the art except for the contents specifically mentioned below, and the present invention is not particularly limited.
Examples
This example is mammary data of a healthy normal volunteer, which is derived from Siemens 3.0T magnetic resonance imaging system. The chemical exchange saturation transfer imaging sequence used is shown in figure 1.
The image acquisition part comprises the following steps:
step 1: the chemical exchange saturation transfer imaging data acquisition adopts unipolar reading under the irradiation of saturated radio frequency of different frequenciesGradient multi-echo gradient echo sequence acquires a mode map and a phase map of magnetic resonance imaging. Irradiation parameters: from-6 ppm to +6ppm, a total of 31 frequency shifts. The saturation pulse duration is 200ms, the saturation pulse number is 6, the image acquisition parameters TR/TE 1/delta TE are 11.0/2.3/1.5ms, the echo number is 5, the flip angle is 8 degrees, and the FOV is 320 multiplied by 320mm2Matrix size 128 × 128, voxel size 2.5 × 2.5mm2The layer thickness is 6 mm.
Step 2: the reference data acquisition is to acquire a mode image and a phase image of magnetic resonance imaging by using a multi-echo gradient echo sequence of unipolar readout gradients under the irradiation without saturated radio frequency. The image acquisition related parameters are in accordance with those described in the previous step.
The image reconstruction section includes the steps of:
step 1: generating a complex value image by the acquired mode image and the phase image;
step 2: reconstructing image data (reference data) acquired under the irradiation of unsaturated radio frequency to obtain a water map, a fat map, a field map anda drawing; the reconstruction will follow the water-fat signal model:
where ρ iswAnd ρfTotal signal amplitudes for water and fat, respectively; omegapAnd αPIs the resonance frequency and relative signal amplitude of the p-th fat peakTEnThe echo time of the nth echo; this model assumes that each voxel has the same lateral decay rateΔB0For local magnetic field biasPoor intensity (field pattern); gamma is the gyromagnetic ratio of hydrogen nuclei;initial phase of precession of water and fat;
the chemical shifts of the fat peaks used in this example were {0.60, -0.39, -1.94, -2.60, -3.40, -3.80} ppm, and the relative amplitudes were {0.048,0.039,0.004,0.128,0.693,0.087 }; these a priori conditions are used directly for modeling the fat signal in equation (1) since no saturation pulse exerts a saturation effect on each fat peak. Reconstructing the obtained water map is used as a reference map required for chemical exchange saturation transfer imaging; reconstructing the obtained field map andthe map will be used as a known parameter in the next step; the field map obtained by reconstruction in this step will also be used for calibration of magnetic field inhomogeneity in chemical exchange saturation transfer imaging.
And step 3: updating longitudinal magnetization vectors of fat peaks irradiated by saturated radio frequency with different frequencies one by using a numerical method; carrying out numerical calculation to enable signals of each peak of simulated fat to evolve under the action of saturation pulses related to chemical exchange saturation transfer imaging; the signal evolves as follows for the Bloch equation:
whereinMagnetization vector intensity for equilibrium state of the p-th fat peak;andthe magnetization vector in the x, y, z directions of the p-th fat peak, respectively, is strongDegree; r1Andlongitudinal relaxation rate and transverse decay rate respectively; omega1(t) is the amplitude of the saturated rf profile, assuming without loss of generality that the saturated rf pulse is applied in the x-axis direction; Δ ωpIs the difference between the center frequency of the saturated RF pulse and the resonant frequency of each fat peak, and the local magnetic field deviation (Δ B)0) It also has an effect:
Δωp＝σpγB0+γΔB0-ωRF(3)
wherein sigmapIs the chemical shift of the p-th peak, γ is the gyromagnetic ratio of the hydrogen nuclei;
as can be seen from equations (2) and (3), under saturated rf irradiation at a certain frequency, the numerical calculation for updating each fat peak will depend on the following parameters: r1，ΔB0(ii) a In which the local parameters are spatially relatedΔB0The calculation results from step 2 will be used; assuming that all fat peaks have the same R1The parameter is considered to be independent of spatial position R1In this example, it is assumed that all fat peaks have the same R1((2.72s-1) (ii) a The intensity of the saturated rf illumination may be seen as a function of time, obtained in advance according to the particular saturated rf used in the sequence, and used as a known parameter. The intensity of the saturated radio frequency irradiation is changed in the space corresponding to the image, and B acquired by using a double flip angle method1The field calibrates it. The differential equation in the formula (2) describes the evolution process of the magnetization vector of each peak of the fat along with the time under the saturated radio frequency irradiation, and the initial condition on which the numerical solution depends is the relative amplitude of each peak of the fat in the original state used in the step 2; for transverse directionThe magnetization vector of (a) has been corrupted by a dephasing gradient, and the component of the longitudinal axis of the magnetization vector known from numerical calculations will be used to update the relative signal amplitude intensities of the fat peaks.
And 4, step 4: reconstructing image data (data of a saturation marker) acquired under the irradiation of saturated radio frequency; the reconstruction will follow the model for the water-fat signal in equation (1), where the relevant parameters are updated as required: updating the relative signal amplitude intensity of all fat peaks one by one according to the numerical calculation result in the step 3; from points in image space in step 2The accurate estimate of (a) will be used as a known parameter in the model of the water-fat signal in this step; reconstruction will obtain water, fat and field maps;
and 5: the original Z spectrum was extracted from the resulting water map and based on the field map (Δ B) reconstructed in step 20) Correcting the field inhomogeneity of the magnetic field; the corrected Z spectra will be used for asymmetry analysis to extract the magnetization transfer intensity of each target molecule. The corrected Z spectrum will be used for asymmetry analysis to extract the signal intensity of the amide bond at 3.5 ppm.
The step 2 in the image reconstruction part comprises the following steps:
step 1: by means of R2Performing initial reconstruction on the image by using IDEAL algorithm to obtain a water map, a fat map, a local magnetic field map andrough estimation of the graph.
Step 2: r is to be2Water, fat and fat maps generated by the IDEAL algorithmThe estimates of the graph will be passed as initial values to a modular-based least squares fit; the objective function of the least squares fit based on the modulus values is as follows:
whereinSn,refThe measured signal of the nth echo of the reference scan;
and step 3: water map, fat map, local magnetic field map generated by least squares fitting based on modulus values andthe accurate estimation of the map is used as a determined parameter to be transmitted to least square fitting based on complex values, and the least square fitting based on the complex values can accurately estimate the field map; the objective function of the least squares fit based on complex values is as follows:
whereinSn,refIs the measured signal of the nth echo of the reference scan.
The step 4 in the image reconstruction part comprises the following steps:
step 1: performing initial reconstruction on the image by using an IDEAL algorithm to obtain rough estimation values of a field map, a water map and a fat map;
step 2: further accurate estimation is carried out on the water map and the fat map based on the least-product fitting of the module values by using the water map and the fat map obtained in the last step as initial values; the objective function of the least-squares fit of the modulus values is as follows:
wherein theta ismag＝{ρw,ρf}，Sn,labThe measured signal of the nth echo scanned for the saturation mark;
reconstructing a water map and a fat map from the image data acquired in the reconstruction step 2 under the irradiation of the saturation-free radio frequency by using the formula PDFF (rho)f/(ρf+ρw) Fat fraction plots were calculated (fig. 3). Based on the fat score map, a point with high fat content inside the gland was selected and the corresponding Z-spectrum was plotted (FIG. 4). The Z spectrum showed no fat artifact at-3.5 ppm. The final calculated map of the intensity of the chemical exchange saturation transfer signal for the amide bond is shown in fig. 5, which is overlaid on the model obtained from the reference scan. In the chemo-exchange saturation transfer signal intensity plot, there was no significant difference in signal intensity between fat-free and high-fat glandular regions, indicating that fat artifacts have been eliminated.
The protection of the present invention is not limited to the above embodiments. Variations and advantages that may occur to those skilled in the art may be incorporated into the invention without departing from the spirit and scope of the inventive concept, and the scope of the appended claims is intended to be protected.
Claims (1)
1. A chemical exchange saturation transfer imaging method for eliminating fat artifacts is characterized by comprising the following specific steps:
step 1: image acquisition
1.1, acquiring chemical exchange saturation transfer imaging data under the irradiation condition of saturated radio frequency with different frequencies, and acquiring a mode diagram and a phase diagram of magnetic resonance imaging by adopting a gradient echo sequence of multi-echo of unipolar readout gradient;
1.2, acquiring reference data under the condition of no irradiation of saturated radio frequency, and acquiring a mode image and a phase image of magnetic resonance imaging by adopting a multi-echo gradient echo sequence of unipolar readout gradient;
step 2: image reconstruction
2.1 generating a complex value image by the acquired mode image and the phase image;
2.2 reconstructing the image data, i.e. reference data, obtained under the irradiation of the unsaturated radio frequency to obtain a water map, a fat map, a field map anda drawing; the reconstruction will follow the water-fat signal model:
where ρ iswAnd ρfTotal signal amplitudes for water and fat, respectively; omegapAnd αPIs the resonance frequency and relative signal amplitude of the p-th fat peakTEnThe echo time of the nth echo; this model assumes that each voxel has the same lateral decay rateΔB0Is the intensity of local magnetic field deviation, namely a field pattern; gamma is the gyromagnetic ratio of hydrogen nuclei;initial phase of precession of water and fat; the chemical shift and initial relative amplitude of each peak of fat in the step are directly used for modeling fat signals in formula (1);
2.3 updating the longitudinal magnetization vector of each fat peak after the saturated radio frequency irradiation of different frequencies by using a numerical method; carrying out numerical calculation on the evolution of signals simulating each peak of fat under the action of saturation pulses in chemical exchange saturation transfer imaging; the signal evolves as follows for the Bloch equation:
whereinMagnetization vector intensity for equilibrium state of the p-th fat peak;andmagnetization vector intensities in x, y, z directions of the p-th fat peak, respectively; r1Andlongitudinal relaxation rate and transverse decay rate respectively; omega1(t) is the amplitude of the saturated rf profile, assuming that the saturated rf pulse is applied in the x-axis direction; Δ ωpIs the difference between the center frequency of the saturated RF pulse and the resonant frequency of each fat peak, and the local magnetic field deviation Delta B0The effect on this is:
Δωp＝σpγB0+γΔB0-ωRF(3)
wherein sigmapIs the chemical shift of the p-th peak, γ is the gyromagnetic ratio of the hydrogen nuclei;
parameters in formula (2) and formula (3)And Δ B0The value of (d) will take the calculation from step 2.2; assuming all fat peaks have the same R1The specific value thereof is found from the literature; the intensity of the saturated radio frequency irradiation is regarded as a function changing along with time, and is extracted from the data of the saturated radio frequency specifically used in the sequence; the intensity of the saturated radio frequency radiation is varied in the corresponding space of the image, and the acquired B acquired additionally1The field will calibrate it; in formula (2)The differential equation delineates the evolution process of the magnetization vector of each peak of the fat along with the time under the irradiation of saturated radio frequency, and the initial condition on which the numerical solution depends is the relative amplitude of each peak of the fat in the original state used in the step 2.2; the magnetization vector for the transverse direction has been destroyed by the dephasing gradient, and the longitudinal axis component of the magnetization vector known from numerical calculation will be used to update the relative signal amplitude intensity of the fat peak;
2.4, reconstructing image data acquired under the irradiation of saturated radio frequency, namely data of a saturated mark; the reconstruction will follow the model for the lipid signal in equation (1), where the model for the lipid signal is updated as required: updating the relative signal amplitude intensity of all fat peaks one by one according to the numerical calculation result in the step 2.3; from points in image space in step 2.2The accurate estimate of (a) will be used as a known parameter in the model of the water-fat signal in this step; the reconstruction will yield a water map, a fat map and a field map;
2.5 extracting the original Z spectrum from the obtained water map and from the field map Δ B reconstructed in step 2.20Correcting the field inhomogeneity of the magnetic field; the corrected Z spectrum is used for asymmetric analysis to extract the magnetization transfer intensity of each target molecule; wherein,
step 2.2, reconstructing image data, namely reference data, acquired under the irradiation of unsaturated radio frequency, specifically comprises the following steps:
2.2.1 Using R2Performing initial reconstruction on the image by using IDEAL algorithm to obtain a water map, a fat map, a local magnetic field map anda rough estimate of the graph;
2.2.2 reaction of R2Water, fat and fat maps generated by the IDEAL algorithmThe estimates of the map are delivered as initial values to a modular-based least squaresFitting; the objective function of the least squares fit based on the modulus values is as follows:
whereinSn，refThe measured signal of the nth echo of the reference scan;
2.2.3 Water, fat, local magnetic field maps generated by least squares fitting based on the modulus valuesThe accurate estimation of the map is used as a determined parameter to be transmitted to least square fitting based on complex values, and the least square fitting based on the complex values can accurately estimate the field map; the objective function of the least squares fit based on complex values is as follows:
whereinSn，refThe measured signal of the nth echo of the reference scan;
the step 2.4 specifically comprises the following steps:
2.4.1 adopting IDEAL algorithm to carry out preliminary reconstruction on the image to obtain rough estimation values of a field map, a water map and a fat map;
2.4.2 further accurate estimation is carried out on the water map and the fat map by using the water map and the fat map obtained in the previous step as initial values and by using least square fitting based on module values; the objective function of the least-squares fit of the modulus values is as follows:
wherein theta ismag＝{ρw，ρf}，Sn，labThe measured signal of the nth echo scanned for the saturated mark.
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| CN110992435A (en) * | 2019-11-06 | 2020-04-10 | 上海东软医疗科技有限公司 | Image reconstruction method and device, and imaging data processing method and device |
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| WO2023108325A1 (en) * | 2021-12-13 | 2023-06-22 | 中国科学院深圳先进技术研究院 | Fat quantitative imaging method and apparatus, and device and storage medium thereof |
| WO2024235016A1 (en) * | 2023-05-15 | 2024-11-21 | 浙江大学 | Cest lipid artifact elimination method, medium, device, and magnetic resonance imaging apparatus |
| CN117765286A (en) * | 2024-02-22 | 2024-03-26 | 清华大学 | method and device for representing water phase state of cement-based material |
| CN117765286B (en) * | 2024-02-22 | 2024-05-07 | 清华大学 | Method and device for representing water phase state of cement-based material |
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