What is Ultrasound Biomicroscopy?
What is Ultrasound Biomicroscopy?
The ultrasonic visualization of living tissue at microscopic resolution is referred to in the literature as “ultrasound backscatter microscopy” or “ultrasound biomicroscopy” (UBM) or “high-frequency ultrasound” or “very high-frequency ultrasound.” Introduced in 1990 by Princess Margaret Hospital, Canada, Ultrasound Biomicroscopy (UBM) is very much similar to conventional ultrasound imaging. However, it uses high frequency ultrasound transducers 50 MHz (vs US 10 MHz). Ultrasound biomicroscopy provides images in living eyes without affecting the internal relationships of the structures imaged and has proven valuable in clinical practice and research.
How does Ultrasound Biomicroscopy work?
The ultrasound biomicroscope works on the principle of an ultrasound but at a higher frequency. The normal B scan probe works at a resolution of 10 to 12 Mhz while the UBM probe works at a frequency of 35 - 50 Mhz or higher. The basic parts of a UBM are the same as that of a standard ultrasound and consist of a hand piece with transducer, a computer console which has the required hardware and software specific for the purpose, a monitor, a printer and a foot switch. The UBM software has special measuring features for measuring thickness of tissues or measuring angles.
What types of disorders can be discovered using ultrasound biomicroscopy?
Ultrasound biomicroscopy is a way to see the eye in great detail at the microscopic level. Any disorder that can be penetrated by high frequency sound and falls within the measureable wavelength of the ultrasound can be analyzed.
Ultrasound Biomicroscopy (UBM) can help detect / diagnose the following types of eye disorders:
UBM helps to study the angle in great detail. The exact configuration of the iris, ciliary body & processes can be defined. These structures can be seen in the presence of an opaque media. The angle can be quantified and the values can be followed up after treatment
UBM is helpful in the study of anterior uveitis. The presence of pars planitis, supra-ciliary effusion, cyclitic membranes and ciliary body detachments can be visualized on UBM.
Anterior segment trauma is usually associated with hyphema. In presence of hyphema it is difficult to visualize the iris and lens. UBM is helpful to study the position of the lens the status of the iris, ciliary body and the configuration of the angle. Angle recession and Cyclodialysis cleft can be evaluated on UBM.
4. Opaque media:
In presence of dense Conjunctival and Adnexal Disease, ultrasound biomicroscopy can provide valuable information in the differential diagnoses of tumors such as iris tumors, ciliar body tumors, anterior segment tumors and cysts.
Do I have to have anesthesia?
UBM is done with the patient in the supine position and the eye is open. Depending on the patient’s age and level of cooperation, the UBM study can be done under either general anesthesia or topical anesthesia. Since the piezoelectric crystal of the transducer is open it should not come in direct contact with the eye to prevent injury to the cornea. There is a special cup which fits in between the eyelids, keeping them open. The eye cup is filled with saline or sterile methylcellulose. The crystal of the transducer is placed in saline approximately 2 mm. from the eye surface. This distance of 2 mm prevents injury to the cornea and also helps as a fluid standoff. The eye is scanned in each clock hour from the center of the cornea to the ora serrata. As for the sound, it must be noted that ophthalmologic ultrasound usually employs frequencies of up to 10 million Hertz (10 MHz), but frequencies in the range of 50 to 100 MHz are used in ultrasound biomicroscopy of the eye. Humans cannot hear sounds that emit a frequency of greater than 20,000 Hertz. In order that an ultrasound image can be formed, a transducer or probe transforms electric energy to sound energy, which then penetrates the ocular tissue. The energy is not absorbed by the tissue as heat, nor is it scattered within the tissue, but is reflected off the tissue, forming the ultrasound image.
How far in my eye can it see?
The most important limitation of UBM is depth. UBM cannot visualize structures deeper more that 4 mm from the surface. The UBM has a low penetration because of high frequency and should be kept at least 11mm from the cornea.
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