Original Contributions to Understanding Nonlinear Photoacoustic Effects in Biological Tissue Models for Medical Imaging Applications in Nigerian Hospitals

Background: Photoacoustic imaging combines optical contrast with acoustic resolution for deep-tissue medical imaging, but nonlinear photoacoustic effects in biological tissue reduce image quality and quantitative accuracy. Understanding and modelling these nonlinearities represents both a scientific gap and a practical barrier to photoacoustic imaging adoption in Nigerian hospitals. Aim: This study made original contributions to understanding nonlinear photoacoustic effects in biological tissue-equivalent models, developing a theoretical model and experimental validation platform. Methods: A k-Wave numerical simulation framework extended with nonlinear acoustic propagation terms was developed to model photoacoustic wave generation and propagation in tissue-equivalent phantoms. Experimental photoacoustic signals were generated using a 5 ns pulsed Nd:YAG laser and detected by a calibrated focused ultrasonic transducer array. Biological tissue phantoms were formulated using agarose, intralipid, and blood-equivalent absorbers with varied optical and acoustic properties. Results: Nonlinear photoacoustic distortion was significant (above 8%) at fluences above 12 mJ/cm2 in blood-equivalent absorbers. The extended k-Wave model predicted nonlinear waveform distortion with less than 6% error. Harmonic photoacoustic imaging using the nonlinear component improved contrast-to-noise ratio by 9.4 dB compared with linear fundamental mode imaging. Conclusion: Harmonic photoacoustic imaging exploiting nonlinear effects offers superior image quality for medical imaging. The developed model provides a simulation tool for optimising photoacoustic imaging systems for Nigerian hospital deployment. Keywords: photoacoustic imaging, nonlinear effects, biological tissue, medical imaging, k-Wave.