Significance: One key pathological characteristic of seronegative spondyloarthropathy (SpA) is inflammation at the insertion of tendons and ligaments into the bone (enthesitis).
Aim: We explore the potential of the emerging photoacoustic (PA) imaging in diagnosis of SpA and review its feasibility in detecting SpA-associated Achilles tendon enthesitis.
Approach: A light-emitting diode (LED)-based PA and ultrasound combined system was employed. The PA images, both along the long and the short axes of each Achilles tendon insertion region, were acquired at 850-nm wavelength, which is sensitive in depicting increased blood volume (i.e., hyperemia). To assess the hyperemia indicating enthesis inflammation, two parameters were quantified in the imaged tendons, including the average intensity and the density of the color pixels in the pseudo-color PA images. Ten SpA patients, all of which met Assessment of SpA International Society (ASAS) criteria for SpA and were found to have Achilles enthesitis by clinical exam according to a board-certified rheumatologist, were included in the study.
Results: The PA and Doppler ultrasound imaging of Achilles enthesitis resulting from these 10 SpA patients were compared to those from 10 healthy volunteers, leading to statistically significant differences (p < 0.05) in the applied t-tests.
Conclusions: This preliminary clinical study suggests that the LED-based PA imaging holds a promise for sensitive and objective assessment of SpA enthesitis in an outpatient setting of the rheumatology clinic.
Our previous research has demonstrated that photoacoustic (PA) imaging is capable of evaluating the pathological condition in human peripheral joints affected by inflammatory arthritis. In this work, we tested the performance of a PA imaging system based on the LED light source and its performance for arthritis imaging. The LED-based PA imaging system not only has less cost but also has smaller footprint and, hence, is more portable and convenient for use in rheumatology clinic. 2D B-scan PA and US images of each metacarpophalangeal (MCP) joint were acquired along the sagittal sections. Along the same sections, US Doppler images were also acquired. Images from 12 joints with clinically active arthritis (i.e., positive on Doppler US), 5 joints with subclinically active arthritis (i.e., negative on Doppler US), and 12 joints of normal volunteers were compared. The blood volume in each joint reflecting hyperemia was quantified by counting the density of the color pixels in each pseudo-color PA image. T-tests were conducted to evaluate whether PA imaging can differentiate the three groups. The results from this study suggest that LED-based PA imaging is capable of detecting hyperemia as an important biomarker of joint inflammation. In addition, PA imaging could differentiate the subclinically active arthritis group and the normal group while Doppler US could not, suggesting that PA imaging has higher sensitivity to mildly hyperemia when compared to Doppler US. The imaging technique presented may contribute to rheumatology clinic by providing a new tool for early diagnosis and treatment evaluation of joint inflammation.
Light-emitting diode (LED) light sources have recently been introduced to photoacoustic imaging (PAI). The LEDs enable a smaller footprint for PAI systems when compared to laser sources, thereby improving system portability and allowing for improved access. An LED-based PAI system has been employed to identify inflammatory arthritis in human hand joints. B-mode ultrasound (US), Doppler, and PAIs were obtained from 12 joints with clinically active arthritis, five joints with subclinically active arthritis, and 12 normal joints. The quantitative assessment of hyperemia in joints by PAI demonstrated statistically significant differences among the three conditions. The imaging results from the subclinically active arthritis joints also suggested that the LED-based PAI has a higher sensitivity to angiogenic microvascularity compared to US Doppler imaging. This initial clinical study on arthritis patients validates that PAI can be a potential imaging modality for the diagnosis of inflammatory arthritis.
Presenting highly sensitive functional information in subsurface tissue with spatial resolution comparable to ultrasound imaging, the emerging photoacoustic (PA) imaging may shed new lights to early diagnosis and treatment monitoring of human inflammatory arthritis. This paper will introduce our recent development of LED-based PA imaging and its application to human inflammatory arthritis. Facilitated by the high pulse repetition rate of the LED arrays, extensive averaging of PA signal can be performed, which boosts the signal-to-noise ratio of the LED-based PA imaging system to levels comparable to laser-based PA imaging systems. In the experiments on arthritis patients and normal volunteers, each target finger joint is scanned using the LED-based PA imaging system which is integrated with a B-scan ultrasound (US) facilitating dual imaging modalities simultaneously. 2D PA and US of a sagittal section in the joint can be acquired in a real-time fashion with a frame rate up to 30 Hz; while a series of 2D images acquired along the cross sections of the joint can be reconstructed into a 3D image for analyzing the volumetric biomarkers of joint inflammation. In this initial study on human subjects, we have confirmed the feasibility of LED-based PA imaging in detecting and characterizing arthritic joints by evaluating the hemodynamic changes associated with soft-tissue inflammation. PA imaging findings are compared to the results from Doppler US acquired using a commercial US unit. This study demonstrates that the LED-based PA imaging can be developed into a point-of-care diagnostic tool for rheumatology and radiology clinics.
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