**Core Concept**
Periventricular leucomalacia (PVL) is a form of white matter brain injury that occurs in preterm infants, often due to perinatal asphyxia. PVL is characterized by the necrosis of periventricular white matter, which can lead to cerebral palsy and other long-term neurological sequelae.

**Why the Correct Answer is Right**
The imaging modality used to diagnose PVL is typically ultrasound, which is non-invasive and can be performed at the bedside. Ultrasound is particularly useful in preterm infants due to its ability to detect changes in brain tissue architecture and identify areas of white matter injury. The characteristic findings of PVL on ultrasound include echogenicity changes, ventricular dilatation, and periventricular cysts.

**Why Each Wrong Option is Incorrect**
**Option A:** Magnetic Resonance Imaging (MRI) is sensitive for detecting white matter injury in preterm infants, but it is not typically used as the initial imaging modality due to its invasiveness and higher cost.
**Option B:** Computed Tomography (CT) scans are not typically used to diagnose PVL in preterm infants due to the high radiation exposure and limited ability to detect white matter injury.
**Option C:** Mammography is not relevant to the diagnosis of PVL in preterm infants, as it is used to detect breast abnormalities in adults.
**Option D:** PET scan is not typically used to diagnose PVL, as it is more useful for detecting metabolic changes in the brain.

**Clinical Pearl / High-Yield Fact**
PVL is often associated with other perinatal risk factors, including maternal hypertension, chorioamnionitis, and intrauterine growth restriction. Early detection of PVL on ultrasound is crucial for timely intervention and management of these high-risk infants.

**Correct Answer:** C. Ultrasound.