Automatic rescaling in DR can affect longitudinal image comparisons by...

• A. Direct grayscale comparison over time remains entirely consistent.
• B. Direct grayscale comparison over time may be misleading.
• C. It has no impact on image comparisons.
• D. It standardizes grayscale relationships across exams.

Answer: (B)

Automatic rescaling changes how the grayscale values are mapped for each image to optimize contrast on that particular study. Because this processing is applied to the image based on its own histogram, the same tissue can end up with different gray values on different exams even if its actual attenuation hasn’t changed. Since longitudinal comparisons rely on consistent brightness/contrast, directly comparing grayscale values over time can be misleading. To mitigate this, use consistent window/level settings across visits or rely on measurements that are less dependent on display mapping, such as structured regional assessments or calibrated comparisons, rather than raw pixel values.