**Core Concept**
Diffusion-weighted imaging (DWI) is a type of magnetic resonance imaging (MRI) sequence that measures the diffusion of water molecules in the brain. It is particularly sensitive to changes in tissue water diffusion, making it an ideal tool for detecting early ischemic changes in cerebral infarcts.

**Why the Correct Answer is Right**
Diffusion-weighted MRI (DWI) is the most sensitive imaging modality for detecting acute cerebral infarcts. This is because DWI is highly sensitive to the restricted diffusion of water molecules in ischemic brain tissue. In the setting of acute ischemia, the breakdown of the normal cellular barrier leads to a decrease in water diffusion, resulting in hyperintensity on DWI images. This change can be detected as early as 30 minutes after the onset of ischemia, making DWI the ideal imaging modality for the earliest diagnosis of cerebral infarct.

**Why Each Wrong Option is Incorrect**
**Option A:** FLAIR MRI (Fluid-Attenuated Inversion Recovery) is a type of MRI sequence that is sensitive to changes in brain tissue, but it is not as sensitive as DWI for detecting acute ischemia. FLAIR images can appear hyperintense in the setting of ischemia, but this change may not be as early or as specific as DWI.

**Option B:** Non-contrast CT (Computed Tomography) is the initial imaging study of choice for acute stroke, but it may not detect early ischemic changes. CT scans may appear normal or show subtle changes in the setting of acute ischemia, making it less sensitive than DWI for early diagnosis.

**Option C:** CECT (Contrast-Enhanced CT) may show signs of ischemia, such as hypoattenuation or hyperattenuation, but it is not as sensitive as DWI for detecting early ischemic changes. CECT may also be less specific than DWI, as it can be affected by various factors, such as contrast enhancement and beam hardening.

**Clinical Pearl / High-Yield Fact**
The "penumbra" is a region of brain tissue surrounding the core of an acute cerebral infarct that is at risk of infarction but may still be salvageable with timely treatment. DWI is essential for identifying the penumbra and guiding treatment decisions in acute stroke.

**✓ Correct Answer: D. Diffusion-weighted MRI. Diffusion-weighted MRI is the best imaging study for the earliest diagnosis of cerebral infarct.**