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probe face directly over the center of the cornea (Fig. 2A). It is the easiest orientation to identify because the resulting B-scan image includes the lens and is bisected by the optic nerve. However, the sound attenuation from the lens often limits the diagnostic use of this view. Longitudinal and transverse B-scans require the probe to be placed on the conjuctiva overlying the sclera and directed through the vitreous bypassing the lens. Longitudinal B-scans produce a cross-section of the eye along a specific clock hour, displaying the anterior portion of the eye at the top of the screen and the optic nerve at the bottom (Fig. 2B). Transverse B-scans produce a lateral cross-section of the eye that results in an image with the superior or nasal aspect displayed at the top of the screen and the inferior or temporal aspect displayed at the bottom of the screen (Fig. 2C).
SPECIAL TECHNIQUES
Ultrasound Biomicroscopy
Ultrasound biomicroscopy is a ultrasound instrument introduced by Pavlin and colleagues24 that uses frequencies from 35 to 80 MHz for the acoustic evaluation of anterior segment of the eye. Details of this instrument and technique are described in the article by Pavlin and colleagues, elsewhere in this issue.
Immersion B-scan
Immersion B-scan refers to the use of balanced salt solution between the probe and the surface of the eye. The vessel holding the balanced salt solution, usually a bottomless cup that is fitted for the eye, is placed in a fixed position. The mobility of the probe is significantly limited, which prohibits the sound beams from reaching posterior structures in the desired perpendicular manner. For this reason, immersion B-scan is not routinely used for the evaluation of posterior segment structures. Immersion B-scan is valuable in the evaluation of pathology located near the ora serrata (anterior limit of the retina), an area that is too anterior to image with contact B-scan and too posterior to image with ultrasound biomicroscopy.
Color Doppler Ultrasonography
Color Doppler imaging simultaneously allows for two-dimensional B-scan imaging of structure and evaluation of blood flow. Conventional duplex scanning of ocular and orbital structures produces a waveform graph of Doppler information on one screen and a B-scan scan image on a separate screen. The small vessel diameters found in intraocular and orbital vasculature are too small to be
imaged with B-scan, and Doppler spectra are obtained without knowing the exact location of the vessels.25 Color Doppler imaging allows realtime blood flow information to be color encoded and superimposed on the gray-scale B-scan image.26 Doppler shifts are usually displayed at the red end of the spectrum when flow is moving toward the transducer and at the blue end of the spectrum when flow is moving away. Color Doppler imaging has proved to be effective in the display of various pathologic ocular conditions, including the detection of ocular and orbital tumor vasculature,27,28 carotid disease,26 central retinal artery and vein occlusions,25,29 and nonarteric ischemic optic neropathy.30
Three-Dimensional Ultrasonography
Three-dimensional ultrasonography is a system developed by Ophthalmic Technologies, Inc. (Toronto, Ontario, Canada) that reconstructs multiple consecutive two-dimensional B-scan images to create a three-dimensional block. The probe is held in fixed, transscleral orientation and serial images are rapidly obtained as the transducer rotates 200 .31 Software transforms the data into a three-dimensional image that is able to be sectioned in longitudinal, transverse, oblique, and coronal views. Three-dimensional ultrasound has been shown to be useful in clinical settings, including estimating the volume of intraocular lesions 32,33 and evaluating the retrobulbar optic nerve.34,35
REFERENCES
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20.Char DH, Ljung BM, Miller T, et al. Primary intraocular lymphoma (ocular reticulum cell sarcoma) diagnosis and management. Ophthalmology 1988; 95(5):625–30.
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Ultrasound
Biomicroscopy
Charles J. Pavlin, MD, FRCSa,b,*, E. Rand Simpson, MD, FRCSb, F. Stuart Foster, PhDc
KEYWORDS
Ultrasound Ultrasound biomicroscopy
High frequency ultrasound Ocular tumors Imaging
Ultrasound is an indispensable tool in medical imaging and has an important role in ophthalmologic diagnoses. Conventional B-scan examinations produce two-dimensional cross-sectional views of the eye and orbit. This method of imaging is the most important examination technique for intraocular lesions, particularly in the presence of anterior segment opacities; however, there are limitations to conventional ultrasound. In our attempts to gain a greater understanding of the mechanisms of ocular disease, we have a never ending need for higher resolution. In the same way that optical microscopy has allowed improved understanding of basic processes, improved imaging resolution allows us to see and understand disease mechanisms that were not previously apparent. The use of high frequencies in the 20 to 100 MHz range for ocular imaging has greatly improved the resolution of ocular ultrasound. The basic physics and techniques of using higher frequency ultrasound to image living structures were developed in the laboratories of Stuart Foster at the University of Toronto. The authors subsequently applied these techniques to ocular imaging and named this process ultrasound biomicroscopy1–5, that is, the imaging of living structures at microscopic resolution.
A broad clinical experience in normal patients and ocular disease has been gained over the years since its development. This article
summarizes the theoretic basis for this technology and illustrates the clinical application of this tool in clinical ophthalmology and ophthalmic research.
THEORETIC CONSIDERATIONS
AND DEVELOPMENT OF THE
ULTRASOUND BIOMICROSCOPE
Mechanical waves and vibrations occur over a wide range of frequencies called the acoustic spectrum. This spectrum extends from the audible range (10 to 20,000 Hz), with which we are all familiar, to the range of phonons (>1012 (10 to the 12th power) Hz) which comprise the vibrational states of matter. This article considers the development and application of ultrasound imaging systems that use very high frequency ultrasound in the range of 20 to 100 MHz, which provides subsurface detail with resolution approaching that of optical microscopy.
Resolution
The limits in current clinical imaging have been set as a result of the need to provide the maximum resolution consistent with the need for the ultrasound beam to penetrate the globe of the eye and orbit. In an ultrasound imaging system, the lateral resolution can be related via diffraction theory to the full width of the ultrasound beam at half
aOphthalmology Department, Mt Sinai Hospital, Suite 410, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada
bDepartment of Ocular Oncology, Princess Margaret Hospital, Toronto, Ontario M5G 2M9, Canada
cDepartment of Medical Biophysics, University of Toronto, Ontario Cancer Institute, Princess Margaret Hospital, Toronto, Ontario M5G 2M9, Canada
* Corresponding author. Mt Sinai Hospital, Ophthalmology, Suite 410, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada.
E-mail address: cpavlin@rogers.com (C.J. Pavlin).
Ultrasound Clin 3 (2008) 185–194 doi:10.1016/j.cult.2008.04.001
1556-858X/08/$ – see front matter ª 2008 Elsevier Inc. All rights reserved.
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