Numerical andin vitroevaluation of temperature fluctuations during reflected-scanned planar ultrasound hyperthermia

Abstract

Temperature fluctuations inside a target volume during reflected-scanned planar ultrasound hyperthermia were investigated numerically and in vitro. The numerical approach consisted of integrating an ultrasonic power deposition model for a scanning ultrasound reflector linear array system (SURLAS) designed for simultaneous thermoradiotherapy, and a three-dimensional transient version of Pennes' bioheat transfer equation. The in vitro approach consisted of delivering hyperthermia to a fixed-perfused canine kidney phantom using a SURLAS prototype. Both approaches allowed the study of temperature fluctuations for several important clinically relevant parameters: scan time, scan distance, perfusion rate and skin cooling. The simulation results showed that the largest temperature fluctuations were located at the opposite ends of the scan window where the scanning reflector comes to a sudden and complete stop and reverses direction. The smallest fluctuations were located at the centre of the scan window. For a given scan distance, the magnitude of the temperature fluctuations increased linearly with increasing scan time, and increased almost linearly as a function of blood perfusion rate. For a scan window of 10 cm x 10 cm and a blood perfusion rate of 5 kg/m3 s, the simulated temperature fluctuations were within +/- 0.5 degree C from the average temperature for scan times less than or equal to 20 s. The in vitro results agreed well with the numerical findings. The measured temperature fluctuations were less than 1.0 degree C for flow rates into the renal artery of less than 200 ml/min and scan times less than 20 s.

Present and future technology for simultaneous superficial thermoradiotherapy of breast cancer

“…The reflecting surfaces of a scanning double-faced V-shaped reflector make 45° angles with respect to the sound propagation coming from the (high and low frequency) arrays at the side ends of the applicator head (Figure 1 shows the windows for the arrays, not the arrays), thus both sound beams are deflected in parallel toward the treatment volume. As the reflector scans back and forth, the sound energy is distributed over the treatment volume producing therapeutic time-averaged temperature distributions with acceptable temperature fluctuations [56, 57]. Each array has several individually powered elements.…”

SURLAS: A new clinical grade ultrasound system for sequential or concomitant thermoradiotherapy of superficial tumors: Applicator description

“…The reciprocating motion of the reflector in-between the arrays spreads the sound energy over the area scanned. 22,26,30 Control of power deposition over the surface target area (surface conformability) is achieved by adjusting the power input into the transducer elements of the arrays as a function of the position of the scanning reflector. The arrays operate at significantly different frequencies so that the mixing of beams of different frequencies and intensities can be used to control the depth of energy penetration (penetration depth controllability).…”

“…Clinically, scanning periods around 10 s are necessary to minimize temperature fluctuations in the heated tissue due to scanning as previously reported. 30 Therefore, the magnetic sensors are expected to function without failure at clinical scanning speeds. The sensors were also tested while irradiated with a 18 MV photon beam and 20 MeV electron beam.…”

“…It easily provides a speed of 3.5 cm/s for the current scanning mechanism that allows scan cycles of 10 s as required by design. 30 The torque versus speed curve for the motor around this speed is nearly constant, which allows varying the scanning speed without negatively impacting the scanning mechanism performance. During extensive testing, the mounted motor was operated without problems even at speeds three times as fast as those expected for clinical use.…”

“…The basic design of the new system, which was named SURLAS, for Scanning Ultrasound Reflector-Linear Arrays System, was first reported in the literature in 1995. 22 This and other papers reported on theoretical studies and experiments performed on various SURLAS laboratory prototypes to demonstrate several key features of the SURLAS concept such as lateral conformability, 26,27 penetration depth control using dual frequency insonation, 28,29 induced small temperature fluctuations due to scanning, 30 and compatibility with electron beam delivery. 17,22 In this paper we describe the first clinical grade SURLAS applicator hardware, empha-sizing several important aspects, and its basic (static) acoustic characteristics.…”