A region-of-interest (ROI) indicating an average Hounsfield unit (HU) of -697 mostly likely corresponds to which of the following?

In computed tomography, Hounsfield units (HU) are used to quantify radiodensity, with values based on the attenuation of X-ray beams as they pass through different tissues. A value of -697 HU is indicative of a low-density structure, which aligns with the properties of lung tissue.

Lung tissue, largely comprised of air spaces, is characterized by significantly negative Hounsfield unit values since the air has an HU of approximately -1000. The value of -697 suggests that the ROI includes structures like lung tissue, but possibly could also indicate the presence of fluid or other components that make the lung slightly less negative than pure air by including some water content or pathology.

In contrast, blood, water, and bone would present with very different HU values: blood is usually around +40 to +80 HU, water is defined as 0 HU, and bone has a much higher positivity, typically around +300 to +2000 HU depending on density and structure. Therefore, the negative HU value of -697 specifically corresponds to lung tissue, making it the correct identification for this ROI.