With the source-to-image distance (SID) of 80 cm (31 inches), the actual central ray must be within what distance of the indicated image field center?

With the source-to-image distance (SID) of 80 cm (31 inches), the actual central ray must be within what distance of the indicated image field center?

• A. 0.8 cm (0.3 inches)
• B. 1.6 cm (0.6 inches)
• C. 4.0 cm (1.6 inches)
• D. 8.0 cm (3.1 inches)

Answer: (A)

The key idea is that the central ray must align with the image field center within a small, fixed tolerance that scales with SID. The standard tolerance used here is about 1% of the SID. So for an SID of 80 cm, 1% is 0.8 cm (0.3 inches). This means the central ray should pass within 0.8 cm of the indicated image field center to ensure the beam stays centered on the intended area and avoids clipping the field edges. The other distances represent larger misalignments (2% would be 1.6 cm, and so on) that could compromise image quality.

The key idea is that the central ray must align with the image field center within a small, fixed tolerance that scales with SID. The standard tolerance used here is about 1% of the SID. So for an SID of 80 cm, 1% is 0.8 cm (0.3 inches). This means the central ray should pass within 0.8 cm of the indicated image field center to ensure the beam stays centered on the intended area and avoids clipping the field edges. The other distances represent larger misalignments (2% would be 1.6 cm, and so on) that could compromise image quality.