Figure 4 Schematic diagram illustrating a perfusion defect of At-PI in thyroid nodules. The left image depicts the US image in the double-image condition, the right image represents the At-PI image. The white arrow indicates the area of filling defect. (A) The US image reveals a solid hypoechoic lesion in the right lobe of the thyroid gland, characterized by clear borders, a regular shape, and uneven echoes. There are thick and strong echoes present within the lesion, accompanied by attenuation in the posterior region. The nodules are classified as C-TIRADS category 4a. The At-PI image shows a perfusion defect, which, when compared to the US image, highlights the presence of thick calcification and a significant sound shadow behind it. The pathology indicates nodular goiter with calcification and fibrosis. (B) The US image again shows the solid right lobe of the thyroid gland, where the hypoechoic lesions exhibit clear borders, irregular shapes, and uneven echoes. The nodules are classified as C-TIRADS category 4b. The At-PI image displays a combination of local abundant perfusion and defect, with the maximum diameter of the lesion measuring approximately 3.23 cm; the pathology is consistent with papillary thyroid carcinoma (PTC). Figure was created using PowerPoint version 2016 (Microsoft Corporation, Washington, USA).