Photoacoustic Imaging For Finger Joint Detection

Arthritic disease is a common chronic disease.There are more than 100 million arthritis patients in China,and the number is increasing.The clinical manifestations are joint redness,swelling,heat,pain,dysfunction,and joint deformities.Severe cases lead to joint disabilities and affect the life quality of patients.Currently there lack a non-invasive,safe,and economical technology to routinely diagnose arthritis.Therefore,developing a high resolution,high sensitivity,non-invasive and low-cost imaging method has become an urgent need for early diagnosis of arthritis.There are still challenges for clinical imaging modalities to image small blood vessels and measure their functional parameters in finger joint.The most commonly used imaging methods are digital radiography（DR）,X-ray computed tomography（CT）,ultrasound imaging,and magnetic resonance imaging（MRI）.Although X-ray imaging has a high sensitivity to bone,it is not sensitive to changes in the soft tissue inside the joint.It is suitable for the diagnosis of advanced arthritis and cannot be used to detect early arthritis.Moreover,since X-ray has radioactive radiation damage to the human body,it cannot be used as a routine inspection method.MRI is one of the rapidly developed imaging techniques in recent years.It is mainly used to observe soft tissue and bone marrow lesions and can effectively detect early arthritis in diagnosis.However,it is expensive,and time-consuming,so it is not suitable for daily monitoring;Ultrasound imaging uses information generated by the interaction between the physical characteristics of ultrasound and the acoustic impedance characteristics of different tissues.Ultrasound imaging is mainly used to assess the thickness of the joint cavity in diagnosis of arthritis.However,in the early stage of arthritis,the thickness of the joint cavity does not change significantly.Therefore,the application of ultrasound imaging in the early detection of arthritis is limited.Photoacoustic imaging is a new type of biomedical imaging method that is non-invasive and non-ionizing.Photoacoustic imaging combines the advantages of high contrast and ultrasound penetration in optical imaging to provide non-invasive,high contrast,and high resolution structural and functional imaging.In this study,we developed a multispectral functional photoacoustic tomography（PAT）system for noninvasively imaging of the human finger joints.The system uses two optical fiber bundles to deliver light.A cylindrically focused ultrasound transducer with a central frequency of 10M and a focal length of 25.4 mm is employed to collect wideband photoacoustic signals.The system comprises a 7.5 MHz side-view focused transducer for ultrasound imaging.We use delay&sum algorithm to reconstruct the 2D photoacoustic images,and use back projection algorithm to reconstruct the ultrasonic images.We carry out some phantom experiments to evaluate the resolutions of the system.In addition,we employed a multispectral strategy that utilized excitation wavelengths of 780 nm and 810 nm to statistically quantify the relative oxygen saturation（SO₂）values of vessels inhuman finger joints.The resolutions of the system are measured as 80μm and 350μm in lateral and axial,respectively.The images show rich detail of the blood vessels and SO₂ values of blood vessels.