Clients are injected with a radioactive substance for which of the following medical procedures?

The use of radioactive substances is integral to Positron Emission Tomography (PET) scans. During this procedure, a small amount of a radioactive tracer is administered to the patient. This tracer emits positrons, which are detected by the PET scanner to produce detailed images of the body's internal processes. By visualizing metabolic activity, PET scans are particularly useful in identifying cancer, monitoring response to treatment, and assessing brain disorders.

In contrast, other imaging modalities mentioned do not utilize radioactive substances. Magnetic Resonance Imaging (MRI) relies on strong magnetic fields and radio waves to generate images of the body's organs and tissues. Ultrasound employs sound waves to create images, making it suitable for examining soft tissues without any radiation exposure. Computed Tomography (CT) scans utilize X-rays rather than radioactive tracers. Therefore, the specific application of radioactive substances in medical imaging points clearly to the PET scan as the correct answer.