Focused Limited-Diffraction Beams for Ultrasound Therapy Applications

Limited-diffraction beams (LDBs) such as Bessel beams and X waves have been studied since late 1980s and early 1990s respectively. These beams have a large depth of field and were applied mainly for medical imaging. (For example, LDBs can maintain a narrow -6-dB beam width of about 2.54 mm in water from the surface of the transducer to more than 200 mm depth at a frequency of 2.5 MHz.) In these applications, LDBs were not focused in order to have a large depth of field. In this paper, both focused zeroth-order and higher-order LDBs were studied for ultrasound therapy applications. The advantage of using focused LDBs is that their focal areas and shapes can be systematically controlled to treat tumors of various sizes. In the study, Bessel beams of 2.5 MHz were produced with a 50 mm diameter transducer. The Bessel beams were focused at 50 mm depth and the sizes and shapes of the focal areas were controlled by the scaling parameter and the order of the Bessel beams. To increase the total transmitting power and reduce the number of elements of the transducer needed to produce the Bessel beams, the amplitudes of the lobes of the Bessel beams can be quantized at the surface of the transducer. Results show that with the focused Bessel beams, the sizes of the focal areas increase with the increase of the Bessel scaling parameter, and higher-order Bessel beams produce a perforated focus area.