Which ultrasound display is a one-dimensional amplitude versus depth plot commonly used in ophthalmology?

• A. C-mode
• B. A-mode
• C. B-mode
• D. M-mode

Answer: (B)

The display type that provides a one-dimensional plot of amplitude versus depth is A-mode. In this mode, the ultrasound pulse travels into the eye, and echoes from tissue interfaces appear as vertical spikes along a depth axis. The spacing between spikes corresponds to physical distances inside the eye, since travel time is converted to depth using the speed of sound in ocular tissues (about 1,530 m/s). The height of each spike reflects the reflectivity of that interface.

This makes A-mode ideal for precise ocular measurements, such as axial length, corneal thickness, and other dimensions needed for eye surgery planning. The other display types build two- or three-dimensional images (B-mode is a grayscale 2D image, M-mode shows motion over time along one line, and C-mode is a different cross-sectional mapping), which are not used for direct, one-dimensional distance measurements in the eye.