A CT image is formed by projecting back what onto a matrix?

The correct choice is based on the principle of how computed tomography images are constructed. In CT imaging, a series of X-ray measurements are taken from various angles around the patient. These measurements quantify how much the X-ray beam has been attenuated as it passes through different tissues in the body.

Attenuation refers to the reduction in the intensity of the X-ray beam due to the interaction with matter, which varies depending on the density and composition of the tissues. The data collected from these measurements—specifically the attenuation values—are then processed using algorithms such as filtered back projection to reconstruct the image. This involves mapping the attenuation coefficients onto a matrix to form a cross-sectional image of the body.

Averaging values does not accurately describe the process, as the image reconstruction does not involve simple averaging of intensity. Contrast values are relevant, but they derive from the attenuation properties of the tissues rather than being the primary data projected onto the matrix. Brightness values pertain more to the displayed image characteristics rather than the fundamental data used in image formation. Therefore, attenuation values are indeed what are projected back onto the matrix to create the CT image.