Which modalities share the 1.5 lp/mm resolution?

• A. Gamma camera
• B. MRI and CT
• C. Ultrasonography
• D. Screen-Film Radiography

Answer: (B)

Line-pair per millimeter measures how many alternating dark and light lines a system can distinguish in one millimeter. The higher the lp/mm, the finer the detail that can be resolved. CT and MRI sit at a similar, moderate resolution in routine practice—around 1 to 2 lp/mm, with 1.5 lp/mm serving as a representative value for typical CT and MRI protocols. For CT, resolution is limited by detector element size, focal spot blur, and how finely the image is sampled and reconstructed. For MRI, resolution is set by voxel size, which depends on the field of view, matrix, and gradient performance, as well as practical scan-time constraints. These shared physics-based limits explain why CT and MRI commonly converge near 1.5 lp/mm.

In contrast, gamma cameras are limited by collimation and detector blur, giving poorer spatial resolution; screen-film radiography achieves much higher resolution with film/screen systems; ultrasound resolution varies with transducer frequency and technique. Hence those modalities do not reliably sit at the same 1.5 lp/mm value.