hypodensities

A hypodensity is an area on a medical image that appears darker or less attenuating than the surrounding tissues. The term is most commonly used in computed tomography (CT) and magnetic resonance imaging (MRI) to describe lesions, cysts, infarcts, abscesses, or other structures that have lower radiodensity or signal intensity compared with normal tissue. In CT, hypodensities are measured in Hounsfield units and typically indicate fluid‑filled or fatty content, whereas in MRI, they correspond to low signal on T1‑weighted images and high signal on T2‑weighted images for fluid‑like structures.

Clinically, hypodensities have a broad differential diagnosis. In acute ischemic stroke, the affected brain tissue may appear hypodense within minutes to hours, reflecting cytotoxic edema. Pathologic cysts, such as arachnoid cysts or cystic neoplasms, produce well‑defined hypodense areas. Infections, including abscesses, manifest as rim‑enhancing hypodensities after contrast administration. On the chest, pulmonary emboli and certain tumors can produce focal hypodense patterns. Dental and maxillofacial imaging uses hypodensity to identify cystic lesions and cystic‐awareness.

Identification of a hypodensity requires careful correlation with clinical information and additional imaging sequences. Contrast enhancement patterns, diffusion characteristics on MRI, and serial imaging are often used to distinguish benign from malignant processes. In many settings, biopsies or surgical exploration confirm the underlying pathology.

Because hypodensities can represent a spectrum of benign to malignant conditions, radiologists typically provide a descriptive report that includes size, location, margins, and any associated findings, along with a suggested differential diagnosis to guide clinical decision‑making.