Author_Institution :
Dept. of Electr. & Comput. Eng., Illinois Univ., Urbana, IL, USA
Abstract :
The monopole-source solution was used to calculate the three-dimensional complex acoustic pressure field for focused circular apertures (diameters: 1, 2, 4 cm) and transmit f-numbers (radius of curvature/diameter values: 0.7, 1, 1.3, 1.6, 2, 3, 4, 5) for various ultrasonic frequencies (1, 2, 3, 4, 5, 7, 9, 12 MHz) from which the three-dimensional temperature distribution was calculated using the bio-heat transfer equation in homogeneous, perfused media (attenuation=absorption=0.3 dB/cm-MHz; perfusion length: 1.0 cm). For the 184 cases, the acoustic field was normalized to the derated spatial peak, temporal average intensity (ISPTA.3) of 720 mW/cm2 , the maximum value allowed by the FDA 510(k) diagnostic ultrasound equipment approval process, from which the axial temperature increase profile and the maximum temperature increase (ΔTmax ) were determined. Also, from the normalized acoustic field, the soft-tissue thermal index (TIS) was determined according to the procedures of the Output Display Standard from the normalized acoustic field. In general, TIS and ΔTmax increase with increasing transmit f-number and TIS tracks ΔTmax which allows for As Low As Reasonably Achievable (ALARA) Principle implementation. TIS mostly underestimates ΔTmax (TIS⩽ΔTmax) for f-numbers⩽1 and mostly overestimates ΔTmax (TIS⩾ΔTmax) for f-numbers⩾2. For the lower transmit f-numbers ⩽2, ΔT max does not exceed a value of 0.30°C (0.0060⩽ΔTmax⩽0.30°C for these 115 cases) and the maximum value of TIS does not exceed a value of 0.40 (0.00010⩽TIS⩽ 0.40 for these 115 cases). Therefore, for f-numbers ⩽2, the TIS display would never be available to the system operator since the TIS value does not equal or exceed 1.0. For the higher transmit f-numbers⩾3, TIS is generally within a factor of 2 of ΔTmax for predicting ΔTmax. These results generally confirm the applicability of the TIS estimation procedure but question the FDA ISPTA.3 intensity limit of 720 mW/cm2
Keywords :
acoustic field; acoustic radiators; biomedical ultrasonics; biothermics; dosimetry; health hazards; standards; ultrasonic focusing; 1 cm; 1 to 12 MHz; 2 cm; 4 cm; As Low As Reasonably Achievable Principle implementation; FDA intensity limit; Output Display Standard; TIS estimation procedure; absorption; acoustic field; attenuation; axial temperature increase profile; bio-heat transfer equation; derated spatial peak; diagnostic ultrasound equipment; diagnostic ultrasound equipment approval process; diameter values; focused circular apertures; homogeneous perfused media; maximum temperature increase calculations; monopole-source solution; normalized acoustic field; output display standard TIS estimates; perfusion length; radius of curvature; soft-tissue thermal index; temporal average intensity; three-dimensional complex acoustic pressure field; three-dimensional temperature distribution; transmit f-numbers; ultrasonic frequencies; Acoustical engineering; Apertures; Biomedical acoustics; Computer displays; Distributed computing; Frequency; Laboratories; Manufacturing; Temperature distribution; Ultrasonic imaging;
