Study On The Acoustic Field Of Multi-layered Tissue With Time-reversal Fcousing Phased Array Based On K-Wave

BackgroundPhased-array ultrasound,due to its electronic focusing characteristics,can realize the automatic deflection of the focus position and control the sound field in the focus field to optimize the distribution of the sound field.As for bone shielding,it can maximize the focus sound pressure gain.In recent years,Phased-array ultrasound has attracted extensive attention at home and abroad.However,one of the major problems in phased-array focused ultrasound treatment is how to accurately focus ultrasound on the target area of the lesion through the focusing algorithm to obtain the accurate delay,which will be the key to the application of high-intensity focused ultrasound phased array in the treatment of disease.Time reversal algorithm has been widely studied in the field of phased array ultrasound,such as ultrasonic imaging,underwater acoustic detection,ultrasonic rubble.In recent years,especially,the research on the application of time reversal algorithm in transcranial treatment of brain diseases is extremely hot.However,there are few relevant studies on the application of time reversal algorithm in the treatment of abdominal diseases.The characteristics and rules of the sound field of focusing and deflection focusing after the time reversal algorithm crosses multiple layers of abdominal tissues are not clear yet.At present,the treatment of most diseases by high-intensity focused ultrasound needs to pass through multiple layers of abdominal tissue,so it will be of great significance to study the sound field of phased array focused ultrasound algorithm through multiple layers of abdominal tissue.PurposeThe purpose of this study is to study the sound field characteristics of phased-array focused ultrasound based on time reversal algorithm in pure physical field and focusing through multi-layer abdominal tissue,and to explore the sound field distribution law of focus deflection after focusing through multi-layer abdominal tissue with time reversal algorithm.MethodsPhased-array focus ultrasonic abdomen multi-layer organization established by the simulation model,using the k-Wave acoustic calculation model of kit and k space pseudo spectrum method and time reversal algorithm,the phased array high intensity focused ultrasound in waters and the features of the multi-layer organization focusing on the three dimensional nonlinear acoustic field simulation,study time focusing acoustic field characteristics of the inversion algorithm,and compared with the traditional time-delay focusing(equal acoustic path)algorithm,the advantage of time reversal is obtained.Then,a phased array ultrasonic experimental research platform based on the time reversal focusing algorithm was built.Focusing and deflection focusing experiments based on the time reversal focusing algorithm were carried out in the water area and multi-layer biological tissue to further explore the focusing characteristics and sound field distribution law of the time reversal algorithm through multi-layer biological tissue.Results1.In the deflection simulation research of pure physical field,the geometric focus and the time reversal focus of the deflection focus can form an ideal sound field.When the focus deflected ±15mm at the acoustic axis,both the offsets of the focal position are 0.5mm,the sidelobe increased from 17.80% to 22.48% and 21.96%,respectively,and the sound pressure decreased from 49.36 MPa to 41.86 MPa and 44.48 MPa,respectively.However,when the radial geometric focus deflects to the right and to the left by 10 mm,the offset of the focus position is 0.25 mm,the change of the focus sound pressure and the deflection gate lobe are almost similar,the focus sound pressure decreases by about half,and the gate lobe increases from 7% to 14%.Compared with the time reversal focusing,the equal range focusing algorithm performs slightly worse in these three aspects,and the difference becomes more obvious with the increase of the deflection distance.2.When multiple layers of abdominal tissue are added to the acoustic channel for simulation study,the sound pressure will decrease significantly,the side lobe will increase,and the focus position will produce a larger deviation due to the entry of the tissue.When the focus is deflected ±15mm on the acoustic axis,the focal offset of the time reversal algorithm is only 0.75 mm and 0.5mm,the sidelobe increases from 19.03% to 24.60%and 23.48%,and the focal pressure decreases from 30.78 MPa to 24.81 MPa and 27.59 MPa.In the same sound path algorithm,the focus offset is2.50 mm and 2.25 mm,the sidelobe reaches 28.03% and 30.2%,respectively,and the focus sound pressure decreases from 27.23 MPa to 23.01 MPa and24.48 MPa.The radial deflection presents the same law as the axial deflection.It shows that the time reversal algorithm is less affected by the organization,and the focusing accuracy is obviously better than that of the same acoustic path focusing.3.Simulation,when the change of biological tissue in muscle or fat thickness in geometric focus focus,time reversal algorithm focusing accuracy basically remain unchanged,the error of the basic remain at around 0.25 mm,sidelobe change is small,just sound pressure due to the increase in tissue thickness and decreases,but relative to the path on the same thickness under its focus sound pressure is greater.Path algorithm such as the focus position will be increased with the increase of the thickness of the muscle to close to the transducer direction deviation,with the increase of the thickness of the fat is to stay away from the transducer direction deviation,its location accuracy errors lead to lower its focus gain relative time reversal,and relatively higher sidelobe and rising with the increase of tissue thickness rate is relatively faster;When the muscle thickness is 10 mm and 40 mm,the relative geometric focus offset to the transducer is 1.25 mm and 3mm,respectively.The sidelobe increases from21.1% to 24.0%,and the focal sound pressure decreases from 33.23 MPa to21.36 MPa.4.In the simulation of focusing through tissue,a good focusing can be achieved even after the radial left and right deflection is 5mm based on the time reversal algorithm.When the deflection distance reaches 10 mm,the gate lobe around the focal region starts to change more obviously.With the increase of the deflection distance,the sound pressure decreases faster in the radial direction than in the axial direction.When the deflection is±10mm in the axial direction,the geometric focus sound pressure decreases by 11.6% and 1.7%,respectively,while the radial left-right deviation is±10mm,the sound pressure decreases by 46.1% and 48.2%,respectively.Equal path algorithm also has similar law.In addition,when the focus on the acoustic axis is constantly deflected to 15 mm away from the transducer,the focal area size has a trend of increasing,the focal length of-6d B increases from 10.25 mm to 17.50 mm,and the focal length changes from 2.5mm to 2.75 mm.However,for radial deflection,the focal area size has no change.5.In the simulation study,the amplitude of sound pressure reduction is different in the direction of axial deflection,whether in the water or through the multi-layer abdominal tissue focusing.In terms of time reversal focusing,the focal pressure in the water decreases from 49.36 MPa at the geometric focus to 44.48 MPa and 41.84 MPa at the axial offset of ±15mm,and the focal pressure decreases from 30.78 MPa at the geometric focus to27.59 MPa and 24.81 MPa after the addition of tissue.The results are consistent between the two algorithms.6.In the experimental study,for the pure physical field,the maximum sound pressure gain position based on the time reversal algorithm is also5 mm in front of the geometric focus when the deflection is on the acoustic axis.When the deflection is ±15mm,the focal offset is 1.6mm and 1.8mm respectively,and the percentage of side lobe increases from 19.3% to31.8% and 33.9%,respectively.The focal sound pressures are respectively85% and 80% when the geometric focus is focused,and this result is consistent with the simulation results.7.In the experimental study,when the focus passed through the tissue,the sidelobe increased significantly with the increase of the deflection distance,the focus sound pressure decreased continuously,and the focus offset distance also increased,and the drop of sound pressure was more prominent in the radial deflection.At the geometric focus,the time reversal focus offset is only 1mm,and the sidelobe is 22.8%.However,when the focus deflection is ±15mm,the error of the focus position will be 2.2mm,and the sidelobe reaches 41.8%,and the focus pressure is 75% of that without the deflection.When the focus is deflected in radial direction,the offset distance of the focus reaches 1.5mm when the focus is deflected to the right by 8mm,the gate valve reaches 42.6%,and the sound pressure of the focus is 40% of that of the undeflected focus.ConclusionBoth the simulation and experimental results obtained the characteristics of the acoustic field in the water area and through the tissue focusing based on the time reversal algorithm,and the laws presented by the experimental results and the simulation results are consistent.Based on the simulation results and the experimental results,the acoustic field characteristics of time reversal focusing in water area and through tissue and the acoustic field variation rule of deflection focusing are clarified.1.Compared with the traditional equal-path focusing,time reversal has the advantages of high focusing accuracy,high focus pressure gain and small side lobe in focusing through multi-layer complex tissue and deflection focusing.The focal position of the algorithm is almost unaffected by the thickness of muscle and adipose tissue,but has a greater effect on the focal position of the traditional focusing algorithm of equal sound path.2.Based on time reversal focusing,whether in the water area or through the tissue,the focusing ability in the direction away from the transducer is weaker than that in the direction near the transducer,and the radial deflection ability is weaker than that in the axial direction when deflecting on the acoustic axis.3.When the acoustic axis is deflected in the direction away from the transducer,the size of the focal area has a tendency to increase continuously,while the size of the focal area remains unchanged in the radial deflection.4.The phased-array nonlinear time-domain sound field research based on k-Wave can accurately and quickly calculate the sound field information,which will greatly save computing resources and time.5.The time reversal algorithm has high requirements on the system and experimental platform when it is applied to focusing research.The complex conditions of the outside world and the performance of the whole phased array system should be fully considered to better achieve the focusing effect under ideal conditions.