| High intensity focused ultrasound has been a novel non-invasive or minimally invasive therapy from outside of human body recently.HIFU technique is beginning to emerge as a potential alternative for treatment of many solid tumors.The cancer cells were killed incompletely during HIFU therapy due to the poor understanding of acoustic characters,such as blood flow.This study aims at investigating the effect of blood vessel,which is surrounding with tumor or supplies tumor with nutrients,on the temperature distribution of focal region.Objective This study simulated the effect of blood perfusion,blood locations and diameters of blood vessel on the HIFU temperature field in the fat and liver tissue,and then studied the location where the effect of blood vessel is negligible on temperature field.The study is significant for improving the reliability and safety of HIFU therapy.Methods This study combined the Westervelt formula for the nonlinear propagation of ultrasound and the Pennes bio-heat transfer equation to simulate the effect of blood flow on the temperature field.Results1.Discussing the stability of numerical simulationThe condition of FDTD reliability is changed when there was a blood vessel in tissue.Results showed that when there was a blood vessel in tissue,the time step was less than the one required by single tissue.And the time step was different because different tissue owns different simulation parameters.2.Effect f blood perfusion on the HIFU temperature field formed in fat tissue(1)Whether the study considered the blood perfusion in fat tissue or not,the formed HIFU focal regions are almost the same.So blood perfusion was not directly addvessed in the following study if there is no special remark.(2)When the blood vessel with diameter of 2.0mm was located in the focal region,the temperature rise was lower than 60 ? both in and surrounding the blood vessel,and then the tissue would not be coagulated completly.When the distance between the nearest blood vessel wall and the acoustic axis was less than 3.0mm,the temperature profile was slightly asymmetric,and the boundary of focal region before the vessel is a wavelike form.When the distance is larger than 3.0mm,there was a negligible change in the temperature field,compared the temperature field with no blood vessel.(3)When the blood vessel was located on the acoustic axis symmetrically,the center temperature rise of the tissue was similarly decreased with incomplete coagulation while the diameter of blood vessel is defferent.The focal area temperature rise of left vessel wall was decreased,but the temperature rise of right vessel wall was first decreased and then increased as the diameter was increased.3.Effect of blood perfusion on the HIFU temperature field formed in liver tissue(1)When the input power and exposure time are kept constant,the teperature rise in fat is higher than the corresponding rise in liver.(2)Whether the study considered the blood perfusion in liver tissue or not,the formed HIFU focal regions are almost the same.So the blood perfusion was not directly addressed in the following study if there is no special remark.(3)When the blood vessel with diameter of 2.0mm was located in the focal region,the temperature rise was lower than 60 ? both in and surrounding the blood vessel,and then the tissue would not be coagulated completely.When the distance between the nearest blood vessel wall and the acoustic axis was less than2.0mm,the temperature profile was slightly asymmetric,but the peak temperature rise had a negligible change compared with when there was no blood vessel.When the distance is larger than 2.0mm,there was a negligible change in the temperature field,compared with when there was no blood vessel.(4)When the blood vessel was located on the acoustic axis symmetrically,the center temperature rise of the tissue was similarly decreased with incomplete coagulation while the diameter of blood vessel is defferent.The focal area and the temperature rise of blood vessel walls were decreased as the diameter was increased.4.Comparing the effect of blood flow on the temperature field in different tissuesWhen the blood vessel with diameter of 2.0mm was located in the focal region,the boundary of focal region before the vessel is a wavelike form in fat tissue,but not in liver tissue.When the exposure time was 8s in fat tissue,the focal area is 7.50mm x 1.50mm,the focal length was 33.5mm and the peak temperature rise was 55.52?in the absence of any blood vessels.When the exposure time was 10s in liver tissue,the focal area is 7.25mm X 1.50mm,the focal length was 33.5mm and the peak temperature rise was 56.88? in the absence of any blood vessels.Hence,the shape,size and location of the focal region are changed when the acoustic wave was focused on different tissues.Conclusions(1)In the fat tissue,when the distance between the nearest of blood vessel and acoustic axis is larger than 3.0mm,there was a negligible change in the temperature field,compared with no blood vessel.When the blood vessel was located on the acoustic axis symmetrically,the cancer cells may be killed incompletely during HIFU therapy inside the focal region but surrounding the blood vessel,and the focal area and the temperature rise of blood vessel walls were decreased as the diameter was increased.(2)In the liver tissue,when the distance between the nearest of blood vessel and acoustic axis is larger than 3.0mm,there was a negligible change in the temperature field,compared with no blood vessel.When the blood vessel was located on the acoustic axis symmetrically,the cancer cells may be killed incompletely during HIFU therapy inside the focal region but surrounding the blood vessel,and the focal area and the temperature rise of blood vessel walls were decreased as the diameter was increased. |
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