**Core Concept**
Piezoelectric crystals are materials that generate an electric charge in response to mechanical stress, such as pressure or vibration. In medical imaging, these crystals are used to produce high-frequency sound waves that can be used to create images of the body.

**Why the Correct Answer is Right**
Ultrasonography relies on the principle of piezoelectricity to produce and detect sound waves. When an electric current is applied to the piezoelectric crystals, they vibrate, producing high-frequency sound waves that are directed into the body. These sound waves are then reflected off internal structures and detected by the same crystals, which convert the vibrations back into electrical signals that are used to create images. The use of piezoelectric crystals allows for the production of high-frequency sound waves, which are necessary for high-resolution imaging.

**Why Each Wrong Option is Incorrect**
**Option B:** X-ray diffraction is a technique used to determine the structure of materials by analyzing the diffraction patterns produced when X-rays interact with the material. It does not rely on piezoelectric crystals.

**Option C:** NMR (Nuclear Magnetic Resonance) imaging, also known as MRI (Magnetic Resonance Imaging), uses powerful magnetic fields and radio waves to produce detailed images of the body. It does not involve the use of piezoelectric crystals.

**Option D:** Xeroradiography is a type of digital radiography that uses a photoconductive material to produce images. It does not rely on piezoelectric crystals.

**Clinical Pearl / High-Yield Fact**
Piezoelectric crystals are also used in other medical devices, such as ultrasound-guided injections and biopsies, where high-frequency sound waves are used to guide the needle into the desired location.

**✓ Correct Answer: A. Ultrasonography**