05.11.2010
With a calibrated fiber-optic displacement sensor the particle velocity in HITU fields (HITU - High Intensity Therapeutic Ultrasound) was successfully measured. The sensors are highly resistant to strong ultrasound fields and allow measurements of particle velocity with high spatial and temporal resolution.
High intensity therapeutic ultrasound (HITU) is a therapeutic procedure that is used increasingly often due to its high success rate and its marginal side effects. In this procedure, ultrasound is focused in a very small elliptical volume (diameter Ø = 2 mm, length l = 2 cm), where tumor tissue (for example) is heated by absorption of ultrasound and thus necrosed. Whereas the narrow focus, on the one hand, allows for a locally sharply confined necrosis, it poses a challenge for measurement instrumentation, on the other hand. For example, pressures of p ˜ 50 MPa, temperatures of T ˜ 100 °C and cavitation effects can occur in the focus. Conventional hydrophones in general do not withstand such conditions. Fiber-optic (pressure) sensors, where the sound pressure changes the reflectivity at the end surface of an optical fiber and thus causes a detectable amplitude modulation of a laser beam, have already been used successfully to measure the pressure in HITU-fields. The relevant value for treatment planning, the sound intensity I = p · v (v: particle velocity), is commonly calculated as I = p2 / Z (Z: acoustic impedance of the medium). However, this implies the unrestricted validity of Euler’s equation p = Z · v, which is not given for many applications. A sensor that measures the particle displacement ? or its time derivative, the particle velocity v, is therefore a useful addition to the existing pressure sensors.
Figure 1: Some results from measurements with the fiber-optic displacement sensor in the focus of a HITU transducer (Sonic Concepts H-108MRA) at an acoustic power level of Pac = 25 W. Top left diagram shows a measured waveform of a HITU-burst (fundamental frequency f0 = 2.45 MHz, burst length 20 cycles) - the typical steepening of the waves by non-linearity is visible (lower left: magnified section). Right diagrams show the peak particle velocity versus the distance from the focus (bottom: measurement along the beam axis; top: measurement perpendicular to the beam axis, full symbols: positive peak, empty symbols: negative peak).
Hence, as a part of the iMERA+ project JRP7, a fiber-optic displacement sensor formerly developed at the PTB was modified and successfully used to measure the particle velocity in HITU fields. The sensor consists of a titanium-sputtered fiber end face, which follows the particle displacement and thus causes a phase modulation of a reflected laser beam, which is then detected in a heterodyne interferometer and demodulated in a discriminator. To perform reliable broadband measurements, the complex frequency response Mv(f) = U(f) / v(f) of the sensor has been determined by a secondary calibration.
Some exemplary results are shown in Figure 1. The spatial resolution (~ 125 microns, given by the diameter of the fiber), the temporal resolution or rather the detectable frequency bandwidth (> 100 MHz), and the robustness (withstood peak pressures: p+ ˜ 50 MPa, p- ˜ 13 MPa) indicate the sensor to be well-suited for the characterization of HITU fields.
Contact person:
Julian Haller, Dept. 1.6, WG 1.62, E-Mail: julian.haller@ptb.de