Simulation of bidirectional ultrasound hyperthermia treatments of neck tumours.

The temperature distributions produced in neck tumours by using either a single, scanned transducer (a unidirectional scan) or two separate transducers whose axis are perpendicular (a bidirectional scan) were simulated. The three-dimensional neck model included separate anatomical regions for the normal neck muscle tissue, the tumour, the spinal column and the trachea (no large blood vessels). The effects of variations in the transducer frequency and f number, the tumour size and location, and the normal and tumour blood perfusion rates were studies. The best simulated temperature distributions were produced by bidirectionally scanned, 2 MHz, f number 2.0 ultrasound transducers whose powers were modulated as a function of position. The simulated temperature distributions from such modulated bidirectional scans were significantly better than those of both unidirectional and unmodulated bidirectional scans. The 1-MHz transducers generally produced hot spots at the tissue-spine and/or tissue-trachea interface. The 3-MHz transducers eliminated those deep hot spots but created other hot spots close to the skin surface, and did not adequately heat the deeper regions of the tumour. These results from the simplified computer simulations may be used to guide the construction of improved ultrasound hyperthermia systems for the treatment of neck tumours.