The density of all patient-equivalent phantom films should be within what percent of the average density?

• A. +/- 5 percent
• B. +/- 8 percent
• C. +/- 12 percent
• D. +/- 10 percent

Answer: (D)

In radiographic QA, maintaining consistent film density across patient-equivalent phantom films is crucial for reliable image quality. The process involves taking several phantom films, measuring their densities, and comparing each film’s density to the average of the set.

The allowed variation is plus or minus 10 percent. This tolerance accounts for normal, unavoidable fluctuations in film processing (chemistry age, temperature, replenishment), film speed, and densitometer readings, while still being strict enough to detect meaningful drift that could affect diagnostic quality. If a film’s density deviates by more than 10% from the average, it signals a potential issue with the processor, chemicals, temperature, or exposure setup that should be investigated and corrected.

A smaller tolerance, like 5%, would be too tight given routine variability; 8% or 12% aren’t the standard referenced in typical QA guidelines, with 10% serving as a practical, widely used benchmark. For example, with an average density of 1.00, acceptable values range from about 0.90 to 1.10.