Quantel Aviso Ultrasound
The immersion B-scan/vector A/B-scan technique prevents corneal compression, and the two-dimensional B-scan display helps guide the superimposed vector A-scan directly to the fovea for accurate measurements.
An immersion echogram through the posterior fundus is obtained using a horizontal axial B-scan approach. The goal is to center the cornea and lens echoes in the echogram while simultaneously displaying the optic nerve void near or slightly above the center.
The A-scan vector is then adjusted to pass through the middle of the cornea as well as the anterior and posterior lens echoes. Such alignment assures that the vector will intersect the retina in the region of the fovea. This technique is fundamental when the macula lies on the sloping wall of the staphyloma.
Image above – The B-scan is oriented horizontally with the vector A-scan going through the center of the cornea (C), anterior lens (L1), and posterior lens (L2).
With this alignment, the vector A-scan will intersect the retina at the approximate center of the macula, just below the void of the optic nerve. This technique has the advantage of allowing the operator to direct the axial length measurement to the region of the fovea, thereby providing the refractive, rather than the anatomic, axial length. For an eye with a mature cataract or a high axial myope with a peripapillary posterior staphyloma, this is the preferred biometry technique by ultrasound.
The immersion B-scan/vector A-scan technique is still limited by the resolution of a 10-MHz sound wave and variations in retinal thickness around the fovea. However, it does allow for direct visualization of the area being measured and, as a result, has even better consistency than the immersion A-scan technique.
The main drawbacks are that the equipment is more expensive, a higher level of skill is required, and the measurements take longer to complete.
How to learn more about A-scan techniques:
We highly recommend the book A-scan Axial Length Measurements by Sandra Frazier Byrne.
There is an excellent, national certification program in Ophthalmic Biometry available for your technicians:
