This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ULTRASOUND, Subject name: Radiology
Pulse echo principle is used by which modality? 
Select and explain. 
A. USG, B. CT, C. X-ray, D. MRI
A: USG
Answer: A: USG 
Explanation: Pulse Echo Technique is used in Ultrasound, which sho-duration Ultrasound pulses are transmitted into the region to be studied, and Echo signals resulting from scattering and reflection are detected and displayed. The depth of a reflective structure is inferred from the delay between pulse transmission and echo reception. CT scan is based on Linear Attenuation of X Rays. MRI is based on Gyromagnetic propey of H+ ions.