Question no 1

2. High-intensity focused ultrasound (HIFU) therapy is an emerging therapeutic technique that uses ultrasound to destroy tissue using thermal and mechanical bioeffects. Consider a single- element bowl-shaped transducer used for HIFU therapy in the abdomen (to treat liver cancer, for example). For a given transducer size, the heating induced by the transducer can be optimised by careful selection of the transmit frequency. (a) (i) Briey explain why focused rather than unfocused transducers are used for HIFU therapy. (ii) List two characteristics of the ultrasound beam produced by a focused transducer that depend on frequency. (b) If the transducer is used in water where the acoustic absorption is negligible, use the concept of focussing gain to find an expression for the pressure at the focus of the transducer as a function of the pressure at the transducer surface F", the ultrasound frequency w in rad-s", the sound speed Co: and the bowl height h. Assume linear wave propagation. (c) In tissue, acoustic absorption will act to reduce the focal pressure. List the two dominant mechanisms that contribute to absorption in tissue. (d) If the transducer is used in so biological tissue where the absorption is linearly dependent on frequency, i.e., the absorption coefficient at = (run), where an has units of Np-m'1 -rad\" -s, extend the expression derived in part (b) to account for absorption. Assume plane wave relations can be used and the transducer focal distance is given by F. (e) Using your answer from part (d), again assuming the absorption coefficient a: = auto, show that the volume rate of heat deposition at the focus of the transducer is given by 3P2h2 ZamF Q = M. (1} 3 Pct) (f) Using Eq. (I), show that the frequency that maximises the volume rate of heat deposition is given by 3 = ZaOF' {U (2} (g) If the absorption in tissue is given by \"0.413 = 0.75 dB'chl 'MHz'l, calculate the absorption coefficient an in Np-m" -rad'1-s. (h) Assume the transducer has a radius of curvature (focal length) of 140 mm and an outer aperture diameter of 120 m. (i) Using the absorption value from part (g) and Eq. (2}, calculate the frequency in MHz that maximises the volume rate of heat deposition. (ii) Calculate the length and width of the focal zone assuming the sound speed in tissue is 1540 ms". (iii)State an advantage and a disadvantage of' using a focal zone of this size for HIFU therapy