Ultrasound modulated optical tomography in optical diffuse medium using acoustic radiation force

Ultrasound modulated optical tomography (UOT) is a hybrid technique which combines optical contrast with ultrasound (US) resolution to achieve deeper tissue imaging. However, the technique is currently limited due to the weak modulation signal strength and consequently a low Signal-to-Noise Ratio (SNR). One potential way to increase the SNR of UOT is to increase the ultrasound induced particle displacement, by either increasing the ultrasound amplitude or using the acoustic radiation force (ARF). In this thesis, I theoretically studied the relationship between the scatterers‘ displacements and the modulation signal strength and experimentally investigated the ARF in addition to investigating the detrimental effects of shear wave propagation on ultrasound modulated optical (UO) signals. A Monte Carlo simulation tool was developed to investigate how the UOT signal changes with increasing amplitude of ultrasound induced particle displacement in the simulation object. By combining a realistic ultrasound field with UOT simulation, the nonlinear effect of ultrasound on UOT signal was studied for the first time. An UOT experiment system, using a CCD camera and a single element transducer driven by an amplitude-modulated (AM) ultrasound signal to generate an oscillatory ARF, was tested on a tissue mimic phantom. The effect of AM ultrasound on UO signals was investigated for the first time. It was found that with longer CCD exposure times, larger ARF induced particle movements can be captured and the UO signal was increased. Next the effects of an ARF induced shear wave on UO signals are studied. The ARF induced shear waves can propagate transversely out of the focal region and may reduce spatial resolution. This is the first examination of the time evolution of the shear wave effect generated by a short ultrasound burst on the UO signal. The spatial resolution of the system was studied by scanning the phantoms. It was found that by adjusting the timing and length of CCD exposure, shear wave effects can be minimised and both the optical and mechanical properties of the phantom can be detected and distinguished