Potential for power deposition conformability using reflected-scanned planar ultrasound

Specialized heating devices for the simultaneous delivery of hyperthermia and ionizing radiation are being developed by several investigators in a effort to increase thermal radiosensitization in clinical treatment. One particular device is the SURLAS (Scanning Ultrasound Reflector-Linear Array System), which was designed specifically to operate concomitantly with medical linear accelerators. The technical feasibility of the SURLAS has been demonstrated, and a design optimization study has been performed. The main objective of this paper is to demonstrate the potential for power deposition conformability of the SURLAS. This has been done using a thermographic technique which provides qualitative, high spatial-resolution measurements of power deposition distributions. The technique consists of normally insonating one surface of a 1 cm layer of a Polyurethane phantom while the temperature field on the opposite air-exposed surface is recorded using an infrared camera during the first few minutes after power insult. The thermal fields measured in this way are good qualitative estimates of relative power deposition. To demonstrate conformability, a region of 10 cm (the length of the array) by 12 cm (the scanning distance) on the air-exposed phantom surface was divided into 24 sectors (24 subregions with independent power control). Each sector was 2.5 x 2 cm across and along the scanning direction respectively. Several sector insonation patterns were synthesized in an open-loop fashion by properly adjusting power levels to 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 crystal 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°C in 60 s at a scanning speed of 2.4 cm/s. The results show that measured relative power deposition patterns agreed well with programmed insonation patterns demonstrating that the SURLAS possesses great potential for power conformability, and thus, for temperature feedback power deposition control.