Is PET inherently a 3D modality?

• A. False
• B. It is always two-dimensional
• C. It may involve both 2D and 3D acquisitions
• D. True

Answer: (D)

PET imaging yields a three-dimensional map of tracer distribution. Each detected positron annihilation event defines a line of response through three-dimensional space, and many such lines are used to reconstruct a 3D volume showing how the radiotracer is distributed in the body. While acquisition can be performed in different modes—historically 2D with septa or 3D without septa—the final reconstructed image is still a 3D representation. The 2D vs 3D acquisition distinction reflects detection geometry and sensitivity, not the fundamental dimensionality of the data. So, PET is inherently three-dimensional.