Focused ultrasound in ophthalmology

Abstract

The use of focused ultrasound to obtain diagnostically significant information about the eye goes back to the 1950s. This review describes the historical and technological development of ophthalmic ultrasound and its clinical application and impact. Ultrasound, like light, can be focused, which is crucial for formation of high-resolution, diagnostically useful images. Focused, single-element, mechanically scanned transducers are most common in ophthalmology. Specially designed transducers have been used to generate focused, high-intensity ultrasound that through thermal effects has been used to treat glaucoma (via ciliodestruction), tumors, and other pathologies. Linear and annular transducer arrays offer synthetic focusing in which precise timing of the excitation of independently addressable array elements allows formation of a converging wavefront to create a focus at one or more programmable depths. Most recently, linear array-based plane-wave ultrasound, in which the array emits an unfocused wavefront and focusing is performed solely on received data, has been demonstrated for imaging ocular anatomy and blood flow. While the history of ophthalmic ultrasound extends back over half-a-century, new and powerful technologic advances continue to be made, offering the prospect of novel diagnostic capabilities.

Keywords: Doppler imaging; high-intensity focused ultrasound (HIFU); ophthalmic ultrasound; ultrafast imaging; ultrasound biomicroscopy (UBM).

Figure 1

Ultrasound probes may be single-element (left), annular arrays (middle), or linear arrays (right). Notes: Single-element probes have a fixed focus, while annular arrays can be synthetically focused, allowing multiple foci. Both single-element and annular array transducers must be mechanically scanned. Linear arrays can both synthetically focus and scan electronically instead of mechanically.

Figure 2

Top: biometric immersion A-scan. Bottom: Immersion B-scan with axial length determination on an eye with dense cataract and axial high-myopia. Notes: Top: The arrow indicates position of anterior cornea, and the double arrow indicates anterior to posterior of the lens. Note the internal lens echoes consistent with the cataract. K-values (upper right) together with axial length are used for the determination of lens implant power. Bottom: Note the separate speed of sound values used for anterior chamber, lens, and vitreous. Abbreviations: AXL, axial length; ACD, anterior chamber depth; AC, anterior chamber; L, lens; V, vitreous; TL, total length.

Figure 3

Examples of an axial B-scan of a normal eye (top) and a combined B-/A-scan of an eye with a choroidal hemangioma temporal to the disc (bottom). Notes: The axial contact B-scan demonstrates the anterior chamber, iris/pupil, lens, and posterior pole, including the optic nerve. The hemangioma demonstrates sustained high-amplitude echoes on A-scan.

Figure 4

UBM images of the temporal aspect of the anterior segment (top) and a horizontal cross-section near the angle inferiorly (bottom). Notes: Temporally, the iris has a convex configuration and multiple ciliary body cysts are seen in the retroiridal space. The horizontal scan demonstrates a multiplicity of cystic structures throughout the ciliary body. Abbreviation: UBM, ultrasound biomicroscopy.

Figure 5

A 10 MHz B-scan (top) and UBM images (center and bottom) of an eye with choroidal detachment and ciliary body effusion over 360° following cataract surgery with IOL implantation. Note: IOL is seen in the center image. Abbreviations: IOL, intraocular lens; UBM, ultrasound biomicroscopy.

Figure 6

Top: insight-100 arc scan image of a 1-year post-LASIK cornea and pachymetric maps (bottom) derived from a series of scans along each clock-hour. Notes: Maps represent (top row, left to right) epithelial, stromal, and corneal thickness and (bottom row, left to right) flap depth, residual stromal thickness, and stromal component of flap thickness. Image courtesy Prof Dan Z. Reinstein, MD MA (Cantab) FRCSC DABO FRCOphth FEBO. Abbreviations: LASIK, laser-assisted in situ keratomileusis; min, minimum; max, maximum.

Figure 7

The 10 MHz (top) and 20 MHz (bottom) images of an eye with choroidal osteoma. Notes: In the 10 MHz image, the ON shadow should not be confused with the acoustic shadow of the brightly reflective and acoustically absorbent osteoma. Note the improved resolution in depiction of the tumor and posterior coats at 20 MHz. Abbreviation: ON, optic nerve.

Figure 8

Ultrafast plane-wave angiographic imaging of the posterior pole. Notes: B-scan (A) and high-resolution blood-flow image (B) of the posterior pole of a normal eye derived from coherently compounded plane-wave images (20 angles over ±10°) with data acquired continuously for 1 s at 20,000 planes/s (1,000 compound images/s) with an 18 MHz linear array. Abbreviations: Ch, choroid; CRA, central retinal artery; ON, optic nerve; PCA, posterior ciliary arteries; s, seconds.