Which structure is primarily examined using A-mode ultrasound?

A-mode ultrasound is primarily utilized for measuring distances and assessing anatomical structures in a linear fashion, making it particularly suited for determining the axial length of the eyeball. This technique provides a one-dimensional representation where the amplitude of the returning echoes is plotted against depth, enabling precise measurements that are crucial for calculations in refractive surgery and determining intraocular lens power.

The axial length of the eyeball is a critical measurement in ophthalmology because it helps in diagnosing and managing conditions such as myopia (nearsightedness) and hyperopia (farsightedness), as well as in planning for cataract surgeries and other procedures. Accurate measurements of the axial length contribute significantly to successful outcomes in lens placement.

In contrast, the other options involve structures and dimensions that are typically assessed using different modalities or measurements. The curvature of the cornea is best evaluated using corneal topography or Pentacam technology, while the width of the optic nerve and the thickness of the retina are usually examined through different imaging techniques such as B-mode ultrasound or optical coherence tomography (OCT).