What is a plain/projection radiograph?

• A. 3D volumetric scan
• B. Color image with depth
• C. 2D image of 3D anatomy; tissue volume projected onto receptor
• D. Digital ultrasound image

Answer: (C)

Plain or projection radiography creates a two-dimensional image by passing X-rays through the body and recording the transmitted photons on a flat receptor. Because the body is three-dimensional, the result is a shadow of the internal structures projected onto that flat detector. Dense or thick tissues (like bone) absorb more X-rays and appear lighter, while less dense tissues absorb fewer and appear darker. Since everything along the same line of sight overlaps, depth information is lost and structures can obscure one another. This is different from 3D imaging like CT, or from modalities that produce color or depth information.