In this study, we present some examples of waveform engineering applications in frequency-domain photoacoustics (PA). An example of using linear frequency modulation (LFM) for PA spectroscopy is the capability of simultaneous probing/imaging with multiple wavelengths. Use of mismatched coded waveforms enables encoding the signal sources and, therefore, facilitates simultaneous probing and imaging. This method enables high frame rate functional imaging with reduced motion artifacts. Furthermore, it is shown than that phase of the PA cross-correlation induced with a LFM can yield the absolute absorption coefficient of the chromophores. This method is not affected by fluence attenuation or variation due to the absorption and scattering of the overlayer material.
This study explores wavelength-modulated differential photo-acoustic (WM-DPA) imaging for non-invasive early cancer detection via sensitive characterization of functional information such as hemoglobin oxygenation (sO2) levels. Well-known benchmarks of tumor formation such as angiogenesis and hypoxia can be addressed this way. While most conventional photo-acoustic imaging has almost entirely employed high-power pulsed lasers, frequency-domain photo-acoustic radar (FD-PAR) has seen significant development as an alternative technique. It employs a continuous wave laser source intensity-modulated and driven by frequency-swept waveforms. WM-DPA imaging utilizes chirp modulated laser beams at two distinct wavelengths for which absorption differences between oxy- and deoxygenated hemoglobin are minimum (isosbestic point, 805 nm) and maximum (680 nm) to simultaneously generate two signals detected using a standard commercial array transducer as well as a single-element transducer that scans the sample. Signal processing is performed using Lab View and Matlab software developed in-house.
Minute changes in total hemoglobin concentration (tHb) and oxygenation levels are detectable using this method since background absorption is suppressed due to the out-of-phase modulation of the laser sources while the difference between the two signals is amplified, thus allowing pre-malignant tumors to become identifiable. By regulating the signal amplitude ratio and phase shift the system can be tuned to applications like cancer screening, sO2 quantification and hypoxia monitoring in stroke patients. Experimental results presented demonstrate WM-DPA imaging of sheep blood phantoms in comparison to single-wavelength FD-PAR imaging. Future work includes the functional PA imaging of small animals in vivo.
This paper demonstrates the co-registration of ultrasound (US) and frequency domain photoacoustic radar (FD-PAR)
images with significant image improvement from applying image normalization, filtering and amplification techniques.
Achieving PA imaging functionality on a commercial Ultrasound instrument could accelerate clinical acceptance and
use. Experimental results presented demonstrate live animal testing and show enhancements in signal-to-noise ratio
(SNR), contrast and spatial resolution. The co-registered image produced from the US and phase PA images, provides
more information than both images independently.
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