Diagnostic Imaging for Cornea Diseases
Disorders of the cornea and anterior (front) segment of the eye are diagnosed with the help of state-of-the-art imaging technologies at the New York Eye and Ear Infirmary of Mount Sinai (NYEE) Ocular Imaging Center. Staffing the center is a team of highly skilled and trained ophthalmologists and technicians who help customize treatment plans for each patient by matching their clinical findings to the best available therapy. Among the advanced imaging technologies we routinely use are:
Corneal topography is a computer-assisted diagnostic tool that creates a three-dimensional map of the surface curvature of the cornea. The three-dimensional map is a valuable aid to the examining physician and can assist in the diagnosis and treatment of a number of conditions.
Corneal topography is most commonly used for the following purposes:
- Refractive surgery: To screen candidates for a normal corneal shape, patterns, and ruling out abnormal or keratoconic patterns, which are not good candidates for laser refractive surgery.
- Post surgery astigmatism: Post keratoplasty corneal astigmatism can be studied with the topographer and selective suture removal or other interventions can be managed and monitored with the technology.
- Effect of corneal and ocular surface disorders on corneal shape: Disorders such as pterygia, limbal dermoid, and localized corneal scars can cause changes in the corneal topography. Thus monitoring is very useful in understanding the progression of the disease and assessing treatment.
- Keratoconus: As a diagnostic early screening tool for keratoconus. Early keratoconus and keratoconus suspects look normal on slit lamp examination. Therefore corneal topography has become the gold standard in screening keratoconus suspects and documenting progression. In cases with established keratoconus, the role of topography is paramount for monitoring progression and evaluating the need for collagen cross-linking therapy.
- Surgical planning in cataract cases with astigmatism: Corneal topography can help assist in preoperative planning for toric IOL's and preoperative planning in femtosecond laser assisted cataract surgery (FLACS) to reduce or eliminate post operative astigmatism.
- Other uses: Contact lens fitting and intrastromal ring placement in keratoconus.
Ultrasound Biomicroscopy (UBM)
UBM provides high resolution imaging of the anterior part of the eye. The procedure is similar to ultrasound for the heart (cardiac echography) or an examination of a baby prior to birth. The ultrasound probe is moved slowly over the surface of the eye, and images are recorded. UBM can be used for imaging much of the anatomy of the anterior segment, as well as associated pathologies, including angle closure glaucoma, ciliary body cysts, neoplasms, and angle trauma. Two forms of glaucoma which have benefited the most from ultrasound biomicroscopy imaging are angle-closure glaucoma and pigment dispersion syndrome.
Corneal pachymetry measures the thickness of the cornea using ultrasound. Since this information is important to refractive surgery, corneal pachymetry can help determine if an individual is a candidate for laser vision correction. It is also used as a metric in accurately measuring intraocular pressure. Moreover, corneal pachymetry can be used to screen for a range of ocular diseases where corneal swelling or thinning may occur, including, Fuchs’ dystrophy, post-cataract surgery edema, and LASIK-induced ectasia.
Anterior Segment OCT
Optical Coherence Tomography (OCT) is a non-invasive diagnostic technique that renders an in vivo cross sectional view of the cornea or retina. OCT utilizes a concept known as inferometry to create a cross-sectional map of the cornea or retina that is accurate to within at least 10-15 microns. Anterior segment OCT utilizes higher wavelength light than traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration. In this fashion, images of the anterior segment (cornea, anterior chamber, iris, and angle) can be visualized.
Specular microscopy is a noninvasive photographic technique that allows you to visualize and analyze the corneal endothelium. Modern specular microscopes analyze the size, shape, and population of the endothelial cells. The instrument projects light onto the cornea and captures the image that is reflected from the optical interface between the corneal endothelium and the aqueous humor. The reflected image is analyzed by the instrument and displayed as a specular photomicrograph. In clinical practice, specular microscopy is the most accurate way to examine the corneal endothelium.
In vivo (taking place in a living organism) confocal microscopy (IVCM) is an emerging noninvasive imaging and diagnostic tool, which enables morphological and quantitative analysis of ocular surface microstructure, both in healthy and diseased eyes. The IVCM has the advantage of imaging through moderately opaque tissues (scarring or edema of the cornea) and also allows for observation of the dynamic process in the cornea, i.e., inflammatory reactions, monitoring in infectious keratitis, wound healing after refractive surgery, and assessing toxicity of medications. It is also commonly used as a diagnostic tool in dry eye disease, post-surgical wound healing, and cornea deposits from congenital and degenerative diseases.
Biometry is the method of applying mathematics to biology. The development of better instrumentation for measuring the eye's axial length (AL) and the use of more precise mathematical formulas to perform the appropriate calculations have significantly improved the accuracy with which the surgeon determines the IOL power to be used in cataract surgery. Axial length measurements can be performed using optical or ultrasound methods, which can further be done by direct contact or immersion.
Optical methods offer the benefits of non-contact procedures along with greater accuracy, but can be inaccurate in dense cataracts. Contact ultrasound measurements were the gold standard in the past, but are likely to have more subjective errors due to corneal compression and examiner skill. Immersion ultrasound avoids the corneal compression but offers less control over alignment, hence can give rise to varied results. Therefore, offering a combination of these techniques can often provide better results.