The NCRP rule of thumb for HVL states that at 70 kVp, it should be greater than:

The National Council on Radiation Protection and Measurements (NCRP) provides guidelines for Half-Value Layer (HVL), which is critical for evaluating the effectiveness of protective shielding in radiation environments. At 70 kVp, the recommended HVL value is indeed greater than 2.00 mm of aluminum (Al).

This value is significant because the HVL indicates the amount of material required to reduce the intensity of the radiation beam by half. A HVL greater than 2.00 mm Al at 70 kVp reflects adequate beam quality and ensures that the radiation dose to patients is minimized while still maintaining the quality needed for diagnostic imaging. This standard helps to ensure that equipment and procedures align with safety regulations and best practices in fluoroscopy, making it essential for professionals to understand and adhere to these guidelines for both patient and personnel protection.

In the context of the other options, values less than 2.00 mm Al would not meet the recommended safety standards as they could indicate insufficient beam filtration, leading to higher patient exposure to unnecessary radiation. The emphasis on the 2.00 mm Al benchmark underscores the importance of maintaining appropriate filtration to achieve a balance between image quality and patient safety.