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A Photoacoustic Spectrum Feature Extraction Method to Characterize the Hydroxyapatite Degradation Process in Cortical Bone

Ultrasonic imaging (USI) is becoming an effective modality for the non-invasive assessment of bone tissue structure, but it may not be sensitive to the early degradation of hydroxyapatite (HAP). In this study, we proposed a photoacoustic spectrum feature extraction method to characterize the HAP degradation process in cortical bone. Meanwhile, we designed a multimodal imaging system that USI combined with photoacoustic imaging (PAI). The parameters namely photoacoustic spectrum integral coefficient (PASIC) and ultrasonic spectrum integral coefficient (USSIC) were proposed for estimating HAP degradation. An FPGA-based multi-modal sequence control module (MMSCM) was constructed to control the laser pulse excitation and Verasonics Vantage for co-imaging, which effectively reduces the phase cancellation effect and improves the SNR in PAI. A time-division multiplexing imaging strategy (TDMIS) was proposed to realize 96-channel 10-fps multimodal imaging with real-time spectrum analysis. For the in-vitro P AI-USI imaging experiment, the bovine cortical bone plates were decalcified 30 minutes with a JYBL-I solution to simulate HAP loss in osteoporosis. The cortical bone responded to the 680 nm wavelength laser pulses and 2.5 MHz single-pulse ultrasonic excitations were analyzed in the time and frequency domains. The results show that both PAI and USI well present the bone structure, while PAI shows more sensitivity in presenting the decalcifying process. The spectrum analysis shows that the photoacoustic signals were mainly distributed from 0.5 MHz to 4.0 MHz. The ultrasonic signals were narrowly concentrated around the center excitation frequency of 2.5 MHz. PASIC is more sensitive to HAP reduction than USSIC, especially in the early stages. PAI combined with USI multimodal imaging might have a high potential in diagnosing HAP-associated bone disease.