Usefulness verification of handheld photoacoustic imaging system for evaluating hypodermal tissue

Knowing the elapsed time from subcutaneous hematoma generation can suggest appropriate treatment strategies. Today, ultrasound imaging, X-ray CT, and MRI are mainly used for diagnosis. However, they have difficulty in distinguishing hematoma from surrounding tissues because of their low image contrast and large device size. Furthermore, X-ray CT entails radio exposure; MRI is contraindicated for patients with embedded metal such as pacemakers. This study evaluated the feasibility of visualizing hematoma and evaluating its characterization using the handheld photoacoustic imaging system we developed. To evaluate the feasibility of estimating the bilirubin concentration, which increases as time passes from hematoma generation, simulated hematomas containing different bilirubin concentrations embedded into optical and acoustical transparent media were measured. Measurements of simulated hematoma using biological tissues were also conducted. The samples and linear ultrasound probe were set in degassed water. Pulses of laser light were guided to the sample surface by an optical fiber bundle close to the probe. Photoacoustic signals were obtained at 700-1030 nm wavelengths. Then photoacoustic spectra at high brightness areas in reconstructed images were calculated. Spectra differed by bilirubin concentrations. The photoacoustic signal ratio between two wavelengths was calculated. For both measurements, the feasibility of estimating bilirubin concentration using wavelengths of around 700 nm and around 900 nm were indicated. These analyses demonstrated the feasibility of visualizing subcutaneous hematoma and of evaluating its characterization using our handheld photoacoustic imaging system with multiple wavelengths.