What test is commonly performed pre-operatively to determine axial length?

The A-scan is a specialized type of ultrasound that is specifically designed to measure the axial length of the eye, making it a critical test in the pre-operative assessment for cataract surgery and the calculation of intraocular lens (IOL) power. An A-scan provides a precise measurement by emitting ultrasonic waves that reflect off the various structures within the eye, including the cornea, lens, and retina. The time it takes for the waves to return is then used to calculate the distance to these structures, thereby determining the eye's overall axial length.

This measurement is essential because the axial length is a significant factor when selecting the appropriate IOL to ensure optimal visual outcomes following surgery. While other imaging techniques exist, the A-scan is the most commonly utilized tool for this specific purpose due to its precision, ease of use, and the reliability of the resulting measurements.

In contrast, the other options like X-rays and MRI are not suitable for axial length measurement because they do not provide the necessary resolution or are not designed for that purpose in ocular examinations. Regular ultrasound can provide some structural information but lacks the specialized focus and accuracy of the A-scan. Therefore, the A-scan is indeed the correct choice for determining axial length pre-operatively