The A-Scan Biometer

The use of optical biometry is limited when measuring eyes with a mature cataract. A-scan biometry is needed in these cases. The A-scan biometer measures the axial length using ultrasound. The transducer is located at the tip of the probe where electric pulses are transformed into sound waves. The transducer emits the ultrasound beam. Ultrasound consists of high-frequency sound waves over 20,000 cycles/s, which is the highest frequency audible to the human ear. The ultrasound beam is formed of ultrasound waves that display different characteristics depending on the ultrasound frequency, wavelength, velocity, and direction. The different characteristics of the commonly used ultrasound biometers are presented. The A-scan biometer with its transducer produces ultrasound impulses. These impulses propagate within the eye and are reflected, creating an A-scan pattern. The A-scan identifies the anterior and posterior surfaces of the cornea, the anterior and posterior surfaces of the lens, and the anterior surface of the retina.