Film vs digital spatial resolution: screen-film resolves more, but digital is what you need to study

Screen-film resolves more line pairs per millimeter. But film isn’t on either ARRT® outline, so what you need for the registry is what limits a digital image. Line pairs per millimeter (lp/mm) is how spatial resolution is measured. Higher means finer detail.

System	Spatial resolution	Source
Very fast screens	7 lp/mm	Bushong
Screen-film, typical table	8 to 15 lp/mm	Bushong
General digital radiography	About 2.5 to 5 lp/mm	Bontrager

You may see film quoted as 10 lp/mm. That number isn’t in Bushong’s table. Bushong gives about 10 lp/mm as what the unaided eye can resolve.

Why did departments still move to digital?

Limiting resolution isn’t the whole picture. Bushong says every digital system has better contrast resolution than screen-film, and that digital performs better at low spatial frequencies. Its dynamic range is much wider. AAPM Report 116 adds that digital display is separate from acquisition, so a wide range of exposures can produce adequate images. Bontrager adds that digital receptors have the potential to lower dose greatly and reduce repeats. That isn’t automatic: Sherer says CR in general takes significantly higher mAs and patient dose than non-CR, while listing lower dose among DR’s advantages over CR, and AAPM Report 116 says dose tends to creep up after a department leaves screen-film. We found no source for the “40 to 50 times” dose figure you may see quoted, so we don’t repeat it.

What limits digital spatial resolution?

The 2027 outline lists pixel size and pitch, detector element (DEL) size, pitch and fill factor, matrix size, Nyquist frequency and MTF. The 2022 outline, in force through February 28, 2027, has sampling frequency (CR) where 2027 has Nyquist frequency.

• Pixel size: Bushong says no digital system can resolve an object smaller than its pixel.
• Nyquist frequency: 1 ÷ (2 × pixel pitch). A 0.125 mm pitch gives 4 lp/mm.
• Fill factor: the share of each element that detects x-rays, about 80% in Bushong. Smaller pixels lower it, so more exposure may be needed for the same signal.

Can technique change spatial resolution?

Partly. It isn’t fixed hardware alone. ARRT’s factor table marks OID, SID, focal spot size, motion and angle for spatial resolution. So a small focal spot, long SID, short OID and no motion all help, up to the detector’s limit. More mAs won’t sharpen an image.

What about CR?

Computed radiography is on the 2022 outline, which applies through February 28, 2027. The 2027 outline drops the term, though its quality control section still lists plate calibration and plate uniformity. The Image Production guide shows what else moves.

Next, read the Digital Image Processing chapter and the Radiographic Quality chapter, then drill in the Image Acquisition set.