5. Drag and other memory effects

Consider two images, N and N + 1, taken successively in a short space of time: the second (N + 1) may show, superimposed, a very attenuated "ghost" of the first (N), due to a parasitic phenomenon involving the trapping of electrical charges: the scintillator, photoconductor or photodiodes retain a residue of the previous signal in memory. This is known as the lag effect (the term afterglow refers to the same phenomenon, but limited to the scintillator). This memory is gradually erased (in a few tens or hundreds of milliseconds), but it can be a nuisance during a "dynamic" examination (fluoroscopy) and for certain high-dose modalities requiring a rapid sequence of several images: this is the case for angiography examinations. For the same reason, dragging is a nuisance when acquiring images for 3D reconstruction such as CT, CBCT or tomosynthesis, or for dual-energy radiography modalities....