Unique applications of VHF digital ultrasound

Imaging and measurement of internal corneal anatomy is the next frontier for better understanding the cornea both before and after a refractive procedure. Accurate biometry is also an essential component for optimizing intraocular lens (IOL) surgery. Inserting a lens into an eye based on external measurements and expecting them to be stable over decades is unrealistic.

One solution

All of these applications and more can be provided by very high-frequency (VHF) digital ultrasound, as with the Artemis (ArcScan Inc, Morrison, Colorado, USA). One of the major advantages of ultrasound techniques over optical techniques is the ability to image the posterior chamber; optical instruments cannot image the posterior chamber because the optical path is blocked by the pigment epithelium of the iris.

As posterior chamber IOLs gain in popularity, accurate biometry of the posterior chamber becomes increasingly important. Traditionally, the size of posterior chamber IOLs has been chosen using a formula based on the horizontal white-to-white (W-to-W) diameter, which assumes a correlation exists between W-to-W and posterior chamber dimensions - the sulcus-to-sulcus (S-to-S) diameter in particular. However, multiple studies using ultrasound biomicroscopy have demonstrated that no such correlation exists,¹ and, therefore, the lens size might as well have been chosen according to shoe size!

Given that the vast majority of IOL complications are related to an error in lens sizing, (an oversized lens can cause angle closure leading to malignant glaucoma or the lens can chafe the iris leading to pigment dispersion; an undersized lens can cause cataract or damage to the zonules with dislocation of the IOL) it seems that using a direct measurement of S-to-S is the way forward for improving the safety of posterior chamber phakic IOL surgery. Indeed, a number of surgeons are now doing this with encouraging results.²

Layered corneal pachymetry

There are also numerous unique corneal applications for VHF digital ultrasound. Firstly, VHF digital ultrasound offers corneal pachymetric mapping with the best repeatability (1.68 µm) of any method of measuring corneal pachymetry.³ VHF digital ultrasound also enables layered pachymetric mapping and can distinguish individual corneal layers including the epithelium, stroma, stromal component of a LASIK flap and residual stromal bed. The ability to obtain a direct in vivo flap thickness measurement can improve the safety of LASIK retreatments. While flap thickness can also be measured by OCT, the repeatability is between 5–8 µm^4,5 compared with 1.68 µm using Artemis VHF digital ultrasound.³ However, the flap interface becomes very difficult to image with OCT later than about 3 months after LASIK; the flap boundary detection rate of an automated algorithm was 61% at 3 months and only 42% at 6 months.⁵