Enhanced pulsed thermoacoustic imaging by noncoherent pulse compression

Abstract

Microwave-induced thermoacoustic imaging (TAI) is a hybrid imaging technique that combines electromagnetic radiation and ultrasonic waves to achieve high imaging contrast and submillimeter spatial resolution. These characteristics make TAI a good candidate to detect material anomalies that change the material electric properties without a noticeable variation in material density. Conventional pulsed TAI systems work by sending a single short pulse to the imaged target and then detecting the generated pressure signal; therefore, a very high peak power microwave pulse or data averaging is needed to produce images with a high signal-to-noise ratio (SNR). In this paper, we propose to enhance the SNR of pulsed TAI systems by using non-coherent pulse compression. In this approach, a predefined pulse coded signal is used to illuminate the imaged sample and the received pressure signal is cross correlated with a template that is related to the power profile of the excitation signal. The proposed approach can be easily deployed to pulsed TAI systems without the need for major system modifications to the RF source because it only requires a timing circuit to control the triggering time of the RF pulses. In this paper, we demonstrate experimentally that the proposed approach highly improves the SNR of TAI signals and images and can be used to reduce the acquisition time by lowering the number of data averaging or reduce the required peak power from RF sources.

Development of an Accurate and Robust Air-Coupled Ultrasonic Time-of-Flight Measurement Technique

“…Figure 8 d). A higher correlation peak-to-peak ratio may be achieved by generating a Dirac-like acoustic pulse [ 61 ] or by using pulse compression techniques [ 21 , 22 , 29 , 57 ] that generate a single peak in the correlation output.…”

“…In many cases, the transducers are coupled directly [ 8 , 16 , 17 ] or with a couplant [ 18 , 19 ] to the specimen surface to reduce amplitude losses from reflection at the transducer-specimen interface. To speed up the measurements and avoid the use of couplant, the specimens are immersed in liquids [ 20 , 21 ] or air [ 22 , 23 , 24 ]. Especially in air, the increase in measurement flexibility comes with challenges transmitting sufficient acoustic energy into the air and further into the specimen.…”

“…If the transducer does not allow signal sensing, an additional receiver is required close to the transducer. However, this must be secluded from the acoustic axis [ 21 , 33 ], as most sensing devices would interfere with the generated signal. One method to circumvent this challenge is measuring the signal with and without specimen in a transmission arrangement.…”