Pre-focal plane high-temperature regions induced by scanning focused ultrasound beams

A steady-state, three dimensional, bioheat transfer equation-based simulation program has been developed and used with single and multiple transducer arrangements to investigate the effects of spherical transducer characteristics (frequency and f-number) and scanning patterns on treatment temperature distributions. Results for uniformly perfused tissues show that: (1) a single circular scan induces radially non-uniform temperature distributions at the focal depth and undesirable pre-focal high temperature regions; and (2) multiple circular scans spaced by the focal diameter of the ultrasound beam and using feedback control can induce uniform temperature distributions in the focal plane, but still cause undesirable pre-focal plane high temperatures. These two results apply both to a single transducer scanned with its axis normal to the skin and to multiple tilted transducers with overlapping foci. The extent and magnitude of this pre-focal high-temperature region increases as frequency increases (from 0.5 to 2.0 MHz) and as the f-number increases (from 1.0 to 2.0). Finally, (3) when four tilted transducers with overlapping foci are used with multiple circular scans spaced by the focal diameter, if the transducer closest to the central axis of the scans is turned off (every transducer is turned off periodically) the pre-focal high-temperature region can be eliminated. From the results of this parametric study and practical considerations, a reasonable compromise of transducer choice for general use is one with a low f-number (about 1.0) and a frequency of about 1.0 MHz. For multiple tilted transducers with overlapping foci the choice of frequency is also 1.0 MHz and the f-number should be as low as possible.