Simulation Study On Blood Flow Effect Of In Tumor Tissue During High-intensity Focused Ultrasound Therapy

As a new minimally invasive or noninvasive treatment from outside of human body,High-intensity focused ultrasound(HIFU)has made breakthrough in the field of tumor treatment recently.But due to the noninvasive measurement and real-time monitoring of target temperature distribution failed to resolved,particularly in the treatment of liver tumor surrounding the larger blood vessels.The treatment target temperature become more complex when considering blood flow,which may result in some clinical problems such as unexpected tumor tissue residue or blood vessels injures.ObjectiveTo predict the blood flow effects of large vessel in liver tumor on temperature distribution during HIFU therapy,through the numerical simulation results of different vessel locations and different blood flow velocity can obtain accurate treatable focal region(TFR)above 60?,which will provide the theoretical data and reference basis for making clinical treatment schedule before HIFU clinical therapy.MethodsIn this study,a 3-D numerical model based on clinical examples of tumor adjacent to large vessel was established.The Westervelt acoustic propagation equation and the Pennes bio-heat transfer equation were used to calculate the temperature field of simulation by the finite difference time domain(FDTD)method.And according to the continuous wave excitation in HIFU treatment process and the characteristics of forming stable acoustic pressure field,improving the numerical simulation method by different space and time step to calculate the sound pressure field and temperature field.Results(1)When a 6mm diameter located at acoustic axis and on the front-end focal region,compared with the vessel located on the after end,the time formation the treatable focal region(TFR)above 60 ? is slightly earlier,and the volume of TFR is also larger.(2)Compared with no vessel condition,a 6mm blood vessel outside of a 12mm diameter spherical tumor located on the acoustic axis and on the after-end focal region,the effects of blood flow on the TFR is negligible.But when the area above 60? expanded to surrounding of the vessel,the cooling effect of blood flow frustrating the temperature diffusion.(3)When blood vessel in the acoustic axis,with respect to its cooling effects on TFR temperature distribution,the different target area's location inside the tumor are more significant under the same irradiation time.(4)When blood vessel located in the front of target area on acoustic axis and in close proximity to the tumor boundary,blood vessel diameter increased by 2 mm to 8 mm,the highest temperature of target area and the TFR showed a trend of increase with the increasing of blood vessel diameter,which are all larger than no blood vessels.(5)When a 6mm blood vessel located at acoustic axis and on the front-end focal region,the temperature of focal point in target area showed a trend of increase with the vessel moving from the transducer to target area,and slightly larger than the absence of blood vessel.When the right side of vessel into the internal target area,the TFR has a significant reduction and internal and surrounding vessel area's temperature falls below 60 ?,also the highest temperature point shifted from predicted focal point to near the vessel wall.(6)Under the condition of considering different blood flow velocity,when a 2mm diameter vessel within the target area,with or without blood flow velocity had a significant influence on the formation of TFR,but the shape of TFR does not change significantly when the velocity increasing on the basis of existing velocity.(7)When the target area is located in the center of the tumor,no matter whether consider flow velocity,the effects of a 6mm diameter vessel on TFR are negligible when it located in non-axial area outside of tumor.(8)When the distance between the focal point of target area and tumor boundary is 2mm,the temperature of normal liver tissue increased to above 60 ? when without vessel.Compared with no vessel situation,the surrounding tissue of blood vessel's temperature falls to below 60? due to blood flow.(9)When the distance between the focal point of therapeutic target and tumor boundary is 1mm,area above 60? normal liver tissue outside of tumor is larger than a distance of 2mm.At this time,the cooling effects are more significant and result in the area above 60 ? obviously reduce,the difference is obvious by the influence of presence of blood flow velocity.Conclusions1.Under the condition that blood vessel located on the acoustic axis and in the outside of target region,when vessel is located between the transducer and therapeutic target,the influence of blood vessel on TFR cannot be ignored,and the influence increase with the increasing of vessel diameters;but when the vessel is in the back-end therapeutic target its impact can be ignored.2.When blood vessel in non-axial and next to therapeutic target area,the shape,the size and the position of TFR were mainly related to the position of the blood vessel,but the blood flow velocity was not,also,the blood flow still has obvious cooling effects.3.When the large vessel located in target area,the effect of the blood flow velocity on treatable focal region was a non-ignorable factor.4.The different locations of therapeutic target area inside the tumor may result in significant differences during the formation of TFR.