In real-time ultrasound, the relationship between imaging depth and frame rate is best described as

• A. Increases frame rate
• B. No change
• C. Frame rate changes with depth
• D. Decreases frame rate

Answer: (D)

The main idea is that frame rate in real-time ultrasound is limited by how long it takes to acquire a complete image. Each frame is made up of many scan lines, and the time to get data for each line depends on how far the ultrasound pulse has to travel and return. When you image deeper, the round-trip travel time to the deepest point increases, so each line takes longer to acquire. With the same number of lines per frame, the total time to complete a frame grows, which reduces how many frames you can display per second. A useful way to think about it is that frame rate is roughly proportional to the speed of sound divided by twice the imaging depth (times the number of lines per frame). So, as depth increases, frame rate decreases. To keep frame rate higher, you can image at a shallower depth or use fewer scan lines per frame (or a narrower sector), since these reduce the time needed to complete each frame.