# Ultrasound-Guided Detection and Segmentation of Photoacoustic Signals from Bone Tissue In Vivo
## 3 Results and Discussion
This time multiplied by the speed of sound, \(c\) also represents twice the distance between the transducer and the bone surface, as marked by \(2d\) in Figure 5b. In Figure 5d, the dashed line indicates the time \(t_{1}\) of onset of the FAS of the US signal, which is also the demarcation point that we would define in this case. The typical PA signals received by the transducer are shown in Figure 5f. To segment the signal in Figure 5f, we reasonably assume that the PA signal of the cortical bone and the transmitted ultrasonic signal have the same propagation path. Our objective is to exclude the cortical bone signals and all the signals arriving after them, so that we could distinguish the PA signal of the trabecular portions from the others. First, by using the US reflection mode, we can get the thickness \(d\) of the overlying soft tissue and get the FAS of the skin. Second, in the US transmission mode, the US transducer in the left side was closed to the skin, so that the time-of-flight \(t_{1}\) of the FAS is equal to the FAS of skin in the PA transmission mode, as shown in Figure 5f. With the known FAS (\(t_{1}\)) from the skin and the thickness of the overlying soft tissue \(d\), the FAS from the cortical bone can be calculated as \(t_{2}=t_{1}-d/c\). Since the cortical bone of calcaneus bone is very thin, the thickness of the cortical bone is ignored. Then, the signal in the region of interest (ROI), which is generated by the trabecular bone, can be segmented by the time-of-flight \(t_{2}\). As a result, we successfully localized the ROI of the PA signal to ensure that it only came from the trabecular bone. Figure 4: Photoacoustic simulation results. (**a**) Simulation results of light distribution in bone. (**b**) Simulation results at t = 1 \(\upmu\)s in the PA mode. The red and cyan colors indicate the signals from TB and ST, respectively, which represent the initial pressures. (**c**) Simulation results at t = 10 \(\upmu\)s in the PA mode. The compression wave and Lamb wave are marked by the arrows. TB, trabecular bone; ST, soft tissue; RT, receiving transducer. To validate the simulation results, we conducted an in vivo study on a female volunteer. The PA and US signals were acquired using the three setups illustrated in Figure 6a,c,e and the signal profiles are presented in Figure 6b,d,f, respectively. Figure 6f shows a typical  
Figure 5: Simulation results of the US and PA signals generated by bone. The bone in the figure is aligned at the position where the signal originated. (**a**) Setup of the US reflection mode simulation; \(d\) is the distance between the transducer and bone surface, which can be considered as the thickness of the overlying soft tissue. The transducer placed at the left side of the bone is used as both the US transmitter and receiver. (**b**) Simulation results of the US reflection signal received by the ultrasound transducer in the reflection mode. (**c**) Setup of the 2D numerical simulation for the US transmission mode. (**d**) Simulation results of US signal received by the ultrasound transducer in the transmission mode. \(t_{1}\) is the time of the FAS of the US signal, which is the US guidance signal. (**e**) Setup of the 2D numerical simulation for the PA transmission mode. (**f**) Simulation results of PA signal received by the ultrasound transducer in the transmission mode. \(t_{2}=t_{1}-d/c\). The dashed box indicates the region of interest (ROI). FAS: first arriving signal. PA signal obtained from the heel of the volunteer. Here, the PA signal mainly originated from two portions: first is the TB, and second is the overlying soft tissue layer and the cortical bone. This was similar in the simulation, as we had done some processing to segment the PA signal. The US signal in Figure 6b provided the distance \(d\) to calculate the skin thickness near the laser side. The US signal in Figure 6d still provided the \(t_{1}\) time to locate the skin position near the laser side.