Which of the following imaging types does not use ionizing radiation?

Ultrasound is the correct choice because it utilizes high-frequency sound waves to create images of the internal structures of the body, rather than using ionizing radiation. This method allows for real-time imaging without exposing patients to the risks associated with radiation exposure.

In contrast, CT scans, X-rays, and fluoroscopy all involve ionizing radiation, which can potentially increase the risk of cancer and other radiation-related issues over time. While these imaging modalities are essential for various diagnostic purposes, they operate on principles that involve the emission of radiation to capture images, making ultrasound a preferable option for certain situations, especially in pregnant women and infants where minimizing radiation exposure is critical.