A method of further increasing treatment depth in dual thermal therapy

Photothermal therapy (PTT) is conducted by converting laser radiation into thermal energy due to the absorption of the photons in tissue. PTT has attracted much attention as a selective and non-invasive treatment. However, the difficulty of treating deep-lying lesions, which is due to optical scattering in biological tissues, is a major limitation of PTT. To alleviate this problem, we previously proposed dual thermal therapy (DTT) in which ultrasound and laser energies are transmitted simultaneously into the target lesion to add local temperatures increased by both energies. In DTT, the focus of ultrasound is located in the target lesion. It was demonstrated that DTT is capable of increasing treatment depth, compared to PTT. In this paper, we propose a method of further increasing treatment depth in DTT. Unlike the conventional DTT, the proposed method finds and uses an optimal ultrasound focal point to maximize the treatment depth of DTT; the focus of ultrasound is placed in the posterior portion of the target lesion during DTT and the focal depth is determined based on the results of ultrasound field simulation. By doing so, it is possible to avoid high optical scattering of the coagulation produced in the anterior segment of the target lesion. The performance of the proposed method was evaluated using a tissue-mimicking phantom. The experimental results showed that the coagulation area produced by the proposed method had a maximum size of 4 mm in the depth direction, whereas the size was about 2 mm in the case of the conventional DTT.