Investigations on electromagnetic-acoustics method for biomedical imaging and therapy applications
Date of Issue2016-04-22
School of Electrical and Electronic Engineering
Centre for Integrated Circuits and Systems
Nanyang Technological Univeristy
After several decades of evolution, single-wave imaging modalities, including ultrasound, magnetic resonance and optical imaging etc., have made tremendous success both in clinical diagnostics and in facilitating scientific studies. The inherent physical limits of single-wave imaging modalities, however, restricted their capability for attaining the best overall contrast and resolution. To overcome these limits, the concept of multi-wave imaging had been proposed to combine excellent contrast from one wave and good resolution from the other. Different interactions among various waves have been explored for multi-wave imaging: using electromagnetic waves for generating ultrasonic waves (thermoacoustics, magneto-acoustics), modulating/tagging electromagnetic waves by ultrasonic waves (acousto-optic imaging) and using fast wave to track slow waves (supersonic shear wave imaging), in which the definition of fast and slow waves are relative: any wave that can sample another wave above Nyquist rate could be regarded as the fast wave whereas the waves being sampled is the slow wave. Promising results had been demonstrated and some of these techniques already enjoyed impressive commercial success. Among these multiwave imaging techniques, using electromagnetic waves for generating ultrasonic waves, generally termed as electromagnetic-acoustics, has attracted tremendous attention owing to its potential in achieving high contrast, fine resolution and good penetration depth. The fusing of electromagnetic waves and ultrasonic waves provides the flexibility in tailoring the parameters in two different worlds, enabling its excellent system scalability for different applications. Yet, there is still a lot of headroom in electromagnetic-acoustic imaging for better imaging performances in terms of deeper penetration and higher contrast. This call for more research efforts and this thesis is the result of my humble explorations in this exciting field. After a brief introduction on the background of electromagnetic-acoustic imaging method and prior achievements that significantly advanced the field, this thesis sets out to expound our solution that uses near field low frequency magnetic field for thermoacoustic imaging towards deeper penetration, which is dubbed as magnetically mediated thermoacoustic imaging (MMTI). Various aspects of this method are covered in details. We then move on to elaborate the proposed modulation method for tackling the background issue that limits the contrast in thermoacoustic imaging. Specifically, background-free detection of magnetic nanoparticles by MMTI and thermally modulated photoacoustic imaging for contrast enhancement are demonstrated. Then, we proceed to investigate from application's perspective the potential of using thermoacoustic for temperature monitoring and for guiding subsequent close-loop temperature control in various hyperthermia therapy applications. This includes photoacoustics guided close-loop temperature control of nanoparticle hyperthermia and a self temperature regulated photothermal system based on a single continuous wave laser diode. Last, we present our theoretical and experimental efforts in unifying thermoacoustic wave with magneto-acoustic wave, which encompasses the simultaneous generation of these two waves and of more significance, their nonlinear mixing effects in water. Such mixing effects may open up the possibility of extracting elastic contrast using electromagnetic-acoustic technique. In summary, this thesis investigates the electromagnetic-acoustic sensing technique for biomedical imaging applications towards deeper penetration and better contrast. Close-loop temperature control empowered by electromagnetic-acoustic sensing are explored for various hyperthermia therapy applications. The concepts, methods and special techniques involved may be conducive for triggering better ideas to drive future developments inside and beyond this field.