POTENTIAL FOR POWER DEPOSITION CONFORMABILITY USING REFLECTED-SCANNEDPLANAR ULTRASOUND

Specialized heating devices for the simultaneous delivery of hyperther mia and ionizing radiation are being developed by several investigator s in a effort to increase thermal radiosensitization in clinical treat ment. One particular device is the SURLAS (Scanning Ultrasound Reflect or-Linear Array System), which was designed specifically to operate co ncomitantly with medical linear accelerators. The technical feasibilit y of the SURLAS has been demonstrated, and a design optimization study has been performed. The main objective of this paper is to demonstrat e the potential for power deposition conformability of the SURLAS. Thi s has been done using a thermographic technique which provides qualita tive, high spatial-resolution measurements of power deposition distrib utions. The technique consists of normally insonating one surface of a 1 cm layer of a Polyurethane phantom while the temperature held on th e opposite air-exposed surface is recorded using an infrared camera du ring the first few minutes after power insult. The thermal fields meas ured in this way are good qualitative estimates of relative power depo sition. To demonstrate conformability, a region of 10 cm (the length o f the array) by 12 cm (the scanning distance) on the air-exposed phant om surface was divided into 24 sectors (24 subregions with independent power control). Each sector was 2.5 x 2 cm across and along the scann ing direction respectively. Several sector insonation patterns were sy nthesized in an open-loop fashion by properly adjusting power levels t o each of the elements of an array as a function of reflector position as the reflector was scanned continuously in a reciprocating fashion at a constant speed. The array was made of a single piezoelectric crys tal with resonant frequency of 2.2 MHz and electrically segmented into four 2.5 x 2.5 cm elements. The reflector was made of a 0.5 mm thick Brass plate. Sufficient power was supplied to the array to induce peak temperature elevations of about 10 degrees C in 60 s at a scanning sp eed of 2.4 cm/s. The results show that measured relative power deposit ion patterns agreed well with programmed insonation patterns demonstra ting that the SURLAS possesses great potential for power conformabilit y, and thus, for temperature feedback power deposition control.