الفهرس 
Optical PrinciplesOnly 14 pages are availabe for public view
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Abstract
High frequency, high-resolution UBM provides visualization of anterior segment anatomy. UBM is based on 50- to 100 MHz transducers incorporated into a B-mode clinical scanner.
Anterior segment OCT is a new non-contact and noninvasive imaging technique that can produce high resolution cross-sectional images of the human eye.
The resolution of ultrasound measurement depends directly on the frequency or wavelength of the sound waves. In contrast, optical measurement and imaging techniques rely on the use of light rather than sound waves. For this reason optical diagnostics can be performed without physical contact to the eye, thereby minimizing patient discomfort during examination, in addition to producing higher spatial resolution. The principal disadvantage of optical techniques is that light is highly scattered or absorbed within most biological tissues and therefore optical imaging is limited to tissues which are optically accessible.
The UBM procedure is more time consuming and requires a highly skilled operator to obtain high-quality precision images. There is a risk for infection or corneal abrasion because of the contact nature of the examination, and it is contraindicated in suspected open-globe injuries.
The limitations of AS-OCT are that it cannot obtain clear images through opaque media and is obstructed by the eyelids, making imaging of the superior and inferior angles difficult. It also provides limited visualization of the ciliary body
Important information on the cornea can be obtained by UBM as well as AS-OCT. The acoustical examination by UBM is in competition with optical methods but has the advantage of imaging an opaque media. The slit-lamp-adapted OCT was able to determine the location and dimension of the corneal changes precisely.
Progression of keratoconus can be demonstrated by AS-OCT and UBM. Using UBM, the central and peripheral corneal thickness in patients with early stages of keratoconus can be calculated. The thinning and ectasia of the corneal stroma and the complications caused by intracorneal ring implantation is reported by AS-OCT.
Anterior and posterior chamber lenses are well imaged by UBM because of the highly reflective nature of their usual one-piece PMMA. Identification of capsular bag supporting ring and 1OL and haptics by UBM show typical schemes of reflection. IOL couldn’t be visualized by OCT in vivo, because the lenses consist of homogeneous and extremely translucent material with a smooth surface. The capsular bag, particularly if secondary cataract formation was present, could be demonstrated. Both AS-OCT and UBM could demonstrate the complications of cataract surgery.
The role of these techniques is very obvious in diagnosing glaucoma. UBM generates high-resolution images of the angle, which can be used in quantitative and qualititative analysis. However, this method requires trained and experienced technicians and is time consuming. Ultrasound can penetrate beyond the iris to visualize rare causes of secondary glaucoma in the posterior chamber. The post-iris structures cannot be clearly visualised with OCT, but OCT is easier to use and is more comfortable for the patient. OCT is a new imaging modality that has higher spatial resolution than ultrasound and was used to quantify changes in anterior segment morphology after Laser peripheral iridotomy. OCT offers a better alternative for bleb imaging compared with UBM after trabeculectomy because it provides a better approach for bleb assessment and requires no coupling medium or supine positioning. Unlike UBM, the position and orientation of the scan can be localised and visualised from the real time camera panel. However, the considerable cost of OCT may inhibit its widespread use for this purpose.
UBM is an important tool in planning for penetrating keratoplasty, anterior segment reconstruction, and IOL removal, repositioning, or implantation. It allows the surgeon to assess the need for anterior segment reconstruction at the time of keratoplasty and more intelligently explain the potential risks and benefits of the planned procedure to patients and their families.
UBM is not available intraoperatively because it requires a contact immersion technique and an experienced examiner. Thus AS-OCT is more feasible, and it should improve intraoperative evaluation of anterior segment structures.
Anterior segment imaging with UBM or OCT may allow for definitive diagnosis of certain tumours and, therefore, more conservative management. However, UBM is superior to OCT in diagnosing lesions behind the iris.
Finally, in traumatic cases, UBM has a valuable adjuvant role in the detection of intraocular foreign bodies. Eyes with angle recession, iridodialysis, cyclodialysis were imaged without complication. OCT can show peripheral anterior synechia after a perforating eye injuries.