**Core Concept**
The CT scanner room walls are coated with a specific material to minimize the effects of radiation exposure on patients, staff, and the environment. The material used is a type of radiation shielding that reduces the transmission of X-rays.

**Why the Correct Answer is Right**
The material used to coat the walls of the CT scanner room is Lead (Pb). Lead is an effective radiation shield due to its high atomic number and density, which allow it to absorb a significant amount of X-ray energy. This reduces the exposure of patients, staff, and the environment to ionizing radiation. The walls are typically coated with a lead-lined material to ensure maximum protection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a material that is commonly used for radiation shielding in CT scanner rooms.
**Option B:** This option is incorrect because it is a type of radiation detector, not a material used for radiation shielding.
**Option C:** This option is incorrect because it is a type of radiation-absorbing material, but it is not the most effective material for CT scanner room walls.

**Clinical Pearl / High-Yield Fact**
When working with ionizing radiation, it is essential to use lead-lined aprons and gloves to minimize exposure. A good rule of thumb is to wear personal protective equipment (PPE) whenever working with radiation-emitting equipment.

**Correct Answer: D. Lead.**