What causes the appearance of patient tissue outside the collimated borders?

• A. Off-focus radiation
• B. Scatter radiation
• C. Patient movement
• D. Grid error

Answer: (A)

Off-focus (extrafocal) radiation is the idea here. It comes from parts of the x-ray tube other than the intended focal spot, such as areas on the anode or housing. These photons are emitted at a range of angles, so some travel outside the collimated beam and expose tissue that sits beyond the borders of the field. That faint exposure along the edges is what makes tissue appear outside the collimated borders on the image. It’s more noticeable when the beam restriction isn’t tight, when kVp is high, or when a larger focal spot is used. To reduce it, keep the beam tightly collimated and well aligned with the image receptor, use the smallest practical focal spot, and maintain the tube so extrafocal output is minimized. Scatter, motion, and grid errors don’t typically produce tissue outlines beyond the collimated edges in the same way extrafocal radiation does.