**Core Concept**
Hounsfield units (HU) are a standardized scale used to quantify the attenuation or density of different tissues in medical imaging, particularly in computed tomography (CT) scans. This scale is essential for interpreting CT images and differentiating between various tissues, such as bone, soft tissues, and air-filled spaces.

**Why the Correct Answer is Right**
Hounsfield units are used to express the linear attenuation coefficient of a material, measured in Hounsfield units, relative to the attenuation of water at standard temperature and pressure. This scale allows for the accurate measurement of tissue density, which is critical in diagnosing various medical conditions, such as bone fractures, lung diseases, and liver abnormalities. The Hounsfield unit scale is calibrated to assign a value of 0 HU to water and +1000 HU to air, facilitating the interpretation of CT scans.

**Why Each Wrong Option is Incorrect**
**Option A:** Hounsfield units are not used in magnetic resonance imaging (MRI) as MRI uses a different scale to quantify tissue signal intensity.
**Option B:** Hounsfield units are not used in ultrasound imaging, which relies on the reflection of sound waves to produce images.
**Option C:** Hounsfield units are not used in positron emission tomography (PET) scans, which use a different scale to quantify the metabolic activity of tissues.

**Clinical Pearl / High-Yield Fact**
When interpreting CT scans, it is essential to remember that Hounsfield units are density-dependent, and a value of +1000 HU typically indicates air-filled spaces, while values between -30 and +50 HU are characteristic of soft tissues.

**Correct Answer: C. Computed Tomography (CT) scans.**