VCAD System Research Program, RIKEN
Kohei Okita

An ultrasonic imaging diagnostic system with which the inside of the body can be investigated using inaudible ultrasound has been widely used in clinical practice. There is a therapeutic method by which ultrasound stronger than that used in this ultrasonic imaging diagnostic system is focused on a target such as a tumor to necrotize tissues by heating, which is referred to as HIFU (High Intensity Focused Ultrasound). The main characteristic of HIFU is that treatment can be performed without incision, and the low impact on the body is of great advantage. The treatment of uterine fibroids and prostatic hyperplasia with therapeutic equipment using this HIFU has already been approved in other countries, and clinical studies on the treatment of liver tumors, etc., are being conduced and awaiting application for approval, but some problems remain. In the treatment of a deep-seated liver tumor, for example, the ultrasound transmitted from the HIFU system runs from the skin through organs such as fat, muscles, bones and liver to a focal point (right side of Figure 1). At this time, the ultrasound may be absorbed so that it decays when it passes through various organs, the ultrasound may be refracted so that it bends, and part of the ultrasound may be reflected. Thus, in treating deep-seated tumors with HIFU, the energy required to heat the target may be insufficient due to decay of the ultrasound, the focal point may be blurred because of reflection or refraction of the ultrasound, and the position of the focal point may deviate from the target. In order to control the HIFU system so that the ultrasound focuses on the target, it is therefore necessary to know how the ultrasound passes through the body, and using the biological information obtained by CT or MRI, we are trying to reproduce the behavior of ultrasound transmission in a biological body by simulation.[1]

So far, the result of a simulation of HIFU treatment for a liver tumor has been obtained as shown in Figure 1.[2] It is seen that the ultrasound sent from the HIFU system is transmitted in a complex way as shown in Figure 2, and focuses short of the target. In such a case, the normal part is heated instead of the site of tumor to be treated. In order to focus the ultrasound onto the target by controlling the ultrasound delivered by the system, a method called the time-reversal method[3] is therefore used. The timereversal method is to emit ultrasound from a sound source placed at the target point, to receive the ultrasound with the HIFU system, and to send the received signals by reversing the time. The result of simulation by controlling the HIFU system by this time-reversal method is shown in Figure 3, showing that the ultrasound focuses appropriately on the target. When ultrasound is transmitted in a complex way in the body as in this case, it is expected that it will be possible to treat a deep-seated tumor more precisely by controlling the HIFU system. Actually, since it is difficult to place a sound source at the target depth in the body, it is possible to perform highly precise HIFU treatment using the time-reversal method by determining the controlling parameter of the HIFU system by simulation in advance. Since a highly precise HIFU simulation is therefore required, we will increase the precision of a HIFU simulator by verifying the simulation result by comparison with experiment, and by introduction of a more advanced physical model. For realization of a highly precise and non-invasive HIFU treatment in the near future, we will contribute to the design and control of a HIFU system, and clinical studies for approval and examination of a preoperative treatment plan, etc., using the HIFU simulator.

Figure 1 : HIFU simulation of liver tumor using a numerical human body model The figure on the rights shows the distribution of tissues lying in the path of ultrasound transmission.

Figure 2 : Condition of ultrasound transmission (left) and the position of focal point (right) when the HIFU system is not controlled

Figure 3 : Condition of ultrasound transmission (left) and the position of focal point (right) when the HIFU system is controlled by the time-reversal method

References
1. Okita K., Ono K., Takagi S., Matsumoto Y., “Development of High Intensity Focused Ultrasound Simulator for Large Scale Computing,” Int. J. Numer. Meth. Fluids, Vol. 65, pp. 43-66, 2011.
2. Okita K., Ono K., Takagi S., Matsumoto Y., “Numerical Simulation of the Tissue Ablation in High Intensity Focused Ultrasound Therapy with Array Transducer,” Int. J. Numer. Meth. Fluids , Vol. 64, pp. 1395-1411, 2010.
3. Fink M., Montaldo G., Tanter M., “Time-reversal acoustics in biomedical engineering,” Annu. Rev. Biomed. Eng ., Vol. 5, pp. 456-497, 2003.