To compare the gray-scale and contrast-enhanced ultrasound features between focal organizing pneumonia (FOP) and subpleural focal chronic nonspecific inflammation.

Twenty-four patients with FOP and 40 patients with focal chronic nonspecific inflammation pathologically diagnosed at Peking University Third Hospital from January 2017 to April 2022 were analyzed retrospectively. The conventional ultrasonic features of the two groups, as well as the arrival time of contrast agent, the phase of enhancement, the enhance mode, the degree of enhancement, the degree of uniformity of enhancement, and the presence or absence of non-enhanced areas were compared between the two groups.

On ultrasound images, the proportion of patients with air bronchogram sign (62.5%, 15/24) was significantly higher in the FOP group than in the focal chronic nonspecific inflammation group (14/40, 35%, P=0.032). The results of contrast-enhanced ultrasound showed that the two groups were supplied by the pulmonary artery. Compared with the focal chronic nonspecific inflammation group, FOP was mainly uniformly enhanced (20/24, 83.3%), while 55% of cases in the focal chronic nonspecific inflammation group was unevenly enhanced (22/40, 55.0%); the difference between the two groups was statistically different (P=0.003). Non-enhanced areas within the lesions were more common in the chronic nonspecific inflammation group (15/40, 37.5% vs 2/24, 8.3%, P=0.011).

FOP is more common with air bronchogram sign. Contrast-enhanced ultrasound shows homogeneous and diffuse enhancement, and few perfusion defects. These features may be helpful to the differential diagnosis of FOP and focal chronic nonspecific inflammation.