Which of the following is NOT a characteristic of a normal X-ray?

In the context of a normal X-ray, the correct characteristic that stands out is the darkening of lung tissue. In a normal chest X-ray, the lung fields should appear radiolucent, which means they will show up darker due to the presence of air in the lungs. This air absorbs less X-ray radiation, resulting in darker areas on the film.

The other characteristics mentioned in the options are typically observed in a normal chest X-ray. Both hemidiaphragms being rounded indicates normal diaphragm positioning and function. The right hemidiaphragm is often slightly higher than the left due to the presence of the liver beneath it, which is a normal anatomical variant. Finally, the trachea being midline suggests that there is no mediastinal shift, which is also an indicator of a normal chest.

Thus, darkening of lung tissue due to excess air or pathological conditions would not be considered a characteristic of a normal X-ray, making it the best choice for this question.