Assessing dyspnea severity in chronic obstructive pulmonary disease using intercostal muscle ultrasound/photoacoustic multimodal imaging.

Objective: This study investigated the feasibility of ultrasound/photoacoustic imaging (US/PAI) for assessing dyspnea severity in patients with chronic obstructive pulmonary disease (COPD) by concurrently evaluating intercostal muscle oxygenation and morphology. Methods: In this retrospective study, participants who underwent US/PAI examination between January 2024 and September 2024 were stratified into three groups: Group 1 (non-COPD controls), Group 2 (COPD with modified Medical Research Council [mMRC] ≤1‌), and Group 3 (COPD with mMRC >1). US/PAI was performed on the right intercostal muscles. Gray-scale ultrasound measured muscle thickness, while PAI quantified oxygen saturation (SO₂). Diagnostic performance of thickness, SO₂, and combined diagnosis was evaluated using receiver operating characteristic (ROC) analysis and area under the curve (AUC). Results: In total, 103 participants were included: 20, 32, and 51 in Groups 1, 2, and 3, respectively. Intercostal muscle thickness measured by B-mode ultrasound decreased with severity (Group 1: 4.1 ± 0.5 mm; Group 2: 3.0 ± 0.8 mm; Group 3: 2.8 ± 0.8 mm), paralleled by a decline in SO₂ obtained from PAI (78.2 ± 0.8%, 68.8 ± 6.0%, 63.2 ± 7.2%). Between non-COPD and all COPD patients, SO₂ (cut-off 71.2%) achieved an AUC of 0.960, versus 0.926 for thickness (cut-off 3.5 mm). For Group 1 vs. Group 2, the respective AUCs were 0.939 (SO₂ cut-off 71.3%) and 0.890 (thickness cut-off 3.5 mm). For Group 2 vs. Group 3, AUCs were 0.761 (SO₂ cut-off 64.4%) and 0.608 (thickness cut-off 3.1 mm). Critically, the combination of thickness and SO₂ consistently yielded a higher diagnostic AUC than thickness alone across all group. Conclusion: The oxygenation and mass of intercostal muscles, measured by US/PAI, are negatively correlated with the dyspnea severity of COPD.