Improving spatial resolution and contrast in ultrasound modulated optical tomography

Ultrasound imaging has benefited from non-linear approaches to improve image resolution and reduce the effects of side-lobes. A system for performing second harmonic ultrasound modulated optical tomography is demonstrated which incorporates both pulsed optical illumination and acoustic excitation. A pulse acoustic inversion scheme is employed which allows the second harmonic ultrasound modulated optical signal to be obtained while still maintaining a short pulse length of the acoustic excitation. For the experiments carried out the method demonstrates a reduction in the effective line spread function from 4mm for the fundamental to 2.4mm for the second harmonic. The first use of pulsed ultrasound modulated optical tomography in imaging fluorescent targets is also discussed. Simple experiments show that by changing the length of the acoustic pulse the image contrast can be optimized. The modulation depth of the detected signal is greatest when the length of the object along the acoustic axis is an odd number of half wavelengths and is weakest when the object is an integer multiple of an acoustic wavelength. Preliminary ultrasound modulated imaging results are also presented where the target generates light within the medium without the use of an external light source. Although signal to noise ratio is likely to be a major challenge, this result highlights a potentially useful application of ultrasound modulated optical tomography in bio-or chemi-luminescence imaging.