Infrared image inside a phantom heated by ultrasound irradiation

Abstract

The present work describes a method for acquiring thermal images generated by physiotherapeutic ultrasonic beams along a phantom of fat tissue made of PVCP. Five tests were performed in three ultrasound equipments, transducers operating at 1 and 3 MHz and nominal intensity 1.0 W.cm -2 , totalizing thirty infrared thermal images that were then evaluated on a PC-like microcomputer. The method seems to be useful for calibration and inspection of ultrasound equipments.Keywords --infrared camera, phantom,therapeutic ultrasound.Therapeutic heating by ultrasound (US) is a common clinical practice. Warming biological tissues with intensity both therapeutic and safe is however a complex task as the irradiation distribution inside the tissue varies and depends on differences between US equipments. It is already known that equipments respond differently according to the effective radiation area (ERA) [cm 2 ] and the output power [W], which are related to the intensity emitted [W.cm -2 ] and therapeutic heating. Depending on each equipment response the final temperature may be lower or higher than the expected, therefore it is important a previous evaluation.The literature describes different methods of studying US heating: Calorimeters with thermocouples [1,2], which only provide the temperature at few points. Thermochromic test objects [3,4] and thermochromic crystal liquid sheets [5], which provide qualitative information. The method here proposed consists of employing an infrared camera to measure the temperature inside a phantom submitted to physiotherapeutic US irradiation.The experimental setup proposed employed cylindrical phantom made of PVCP (plasticized polyvinyl chloride) cut along its axis in two halves further joined with US coupling gel. A therapeutical transducer (models Avatar III, KLD) was statically coupled to the top cylinder basis aligned with its axis. US continuous irradiation was applied with nominal intensity of 1.0 W.cm -2 for 120s, taking care to avoid reverberation by coupling an absorbing plate (APTFLEX) to the opposite basis. After irradiation the phantom was split in two and an infrared camera (InfraCam TM, Flir Systems) with temperature range selected between 25 to 45°C recorded the thermal image exposed.The PVCP phantom was constructed in the Ultrasound Laboratory of Biomedical Engineering Program/UFRJ, with US speed of 1480 m.s -1 and attenuation coefficient of 0.3 dB.cm -1 .MHz -1 (fat tissue), dimensions: 10-cm height and 5.3-cm diameter. The environment illumination was common fluorescent, and room temperature was 25± 2.0°C.