what is the halo sign?
<answer> It refers to a zone of ground-glass attenuation surrounding a pulmonary nodule or mass on CT images. <context> The computed tomographic (CT) halo sign, also known as the halo sign, refers to a zone of ground-glass attenuation surrounding a pulmonary nodule or mass on CT images (1) (Figure). The presence of a halo of ground-glass attenuation is usually associated with hemorrhagic nodules (2).