Which phantom is commonly referred to for testing noise in CT?

The water phantom is commonly used in computed tomography to assess noise performance because it effectively simulates the density of human tissue while providing a uniform medium for imaging. The properties of water make it an ideal choice for measuring the random fluctuations in pixel values, known as noise. These fluctuations can affect diagnostic quality, and using a water phantom helps quantify the signal-to-noise ratio by capturing how much noise is present in the images produced by the CT scanner.

In addition to its role in noise assessment, the water phantom is also valuable for calibrating CT scanners and ensuring their consistent performance over time. It allows technologists to measure how well the machine produces uniform images, which is critical for accurate diagnosis.

The other options refer to different types of phantoms that serve specific purposes, such as assessing spatial resolution or image uniformity, but they do not specifically focus on measuring noise. Hence, the water phantom's unique properties and applications make it the correct choice for testing noise in CT imaging.