VCAD System Research Program, RIKEN
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.
So far, the result of a simulation of HIFU treatment for a liver tumor has been obtained as shown in Figure 1. 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 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
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