Investigation of ultrasonic properties of MAGIC gels for pulse-echo gel dosimetry

Abstract

Ultrasound has been proposed as an alternative 3D method to read out dosimetric gels. The ultrasonic properties of MAGIC gel are investigated in relation to producing a dosimetry system based on a pulse-echo technique. The sound speed, attenuation coefficient, and density of MAGIC gel were measured over a range of temperatures and doses. A nonlinear relationship between ultrasonic attenuation and dose was observed, with the attenuation below 10 Gy being approximately constant. The sound speed was 1550.3 ± 1.5 m s -1 at 25 °C with a dose sensitivity of 0.14 ± 0.03 m s-1 Gy-1; both properties changed with temperature and were 1535.4 ± 0.8 m s-1 and -0.08 ± 0.01 m s -1 Gy-1 respectively at 15 °C. The density also varied with temperature and dose, and was 1028.9 ± 0.3 kg m-3 with a dose sensitivity of 0.178 ± 0.004 kg m-3 Gy-1 at 26 °C. The characteristic acoustic impedance of MAGIC gel was calculated to be 1.596 × 106 kg m2 s-1 at 0 Gy and 25 °C. The dose sensitivities measured indicate that changes to the characteristic acoustic impedance caused by irradiation are small, and therefore a reflector that has a similar characteristic acoustic impedance is required to maximise sensitivity of the pulse-echo technique. A suitable material has been developed and the reflection characteristics of the interface between MAGIC gel and the reflector are also temperature dependent. It is concluded that temperature management will be central to the development of any practical pulse-echo dosimetry system. © 2010 IOP Publishing Ltd.