**Core Concept**
Radiation exposure is a significant concern in medical imaging, particularly in procedures that involve ionizing radiation. Ionizing radiation has enough energy to remove tightly bound electrons from atoms, leading to DNA damage and potentially causing cancer.

**Why the Correct Answer is Right**
The correct answer is **B. Doppler USG**. Doppler ultrasound (USG) is a non-invasive imaging technique that uses high-frequency sound waves to produce images of internal structures. Unlike other imaging modalities, Doppler USG does not involve ionizing radiation, making it a safe option for patients. This is because sound waves have a lower energy level than ionizing radiation, and they do not cause DNA damage.

**Why Each Wrong Option is Incorrect**
**Option A:** MRI (Magnetic Resonance Imaging) uses a strong magnetic field and radio waves to produce images, but it also involves ionizing radiation in some cases, such as when using iodine-based contrast agents.

**Option C:** Digital subtraction angiography (DSA) uses X-rays to visualize blood vessels and involves ionizing radiation, making it a high-radiation procedure.

**Option D:** Tc 99 scan (Technetium-99m scan) involves the use of a radioactive isotope, which emits ionizing radiation, making it a high-radiation procedure.

**Clinical Pearl / High-Yield Fact**
Remember that non-ionizing radiation imaging modalities like ultrasound and MRI are preferred over ionizing radiation-based modalities like CT scans and X-rays when possible, especially in pregnant patients or children, to minimize radiation exposure.

**Correct Answer: B. Doppler USG**