The Sankara Nethralaya Atlas of Ophthalmic Ultrasound and Ultrasound Biomicroscopy Muna Bhende, B Shantha, Harshali Kamat, Vikas Khetan, Tandava Krishna
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1Getting Started2

Principles of Ophthalmic Ultrasound1

The acoustic spectrum extends from the audible range of 10–20,000 Hz to the vibrational states of matter with frequencies of >1012 Hz. While abdominal ultrasound uses frequencies of 1–2 MHz, ophthalmic ultrasound incorporates frequencies of 8 MHz and above to produce images of the eye and orbit. These structures are more easily accessible from the surface, which permits the use of a higher frequency probe. Frequencies of 8–12 MHz comprise conventional ophthalmic ultrasound, while high frequency systems utilize a 20 MHz probe. Probe frequencies of 35–100 MHz comprise ultrasound biomicroscopy.
The physical principles are similar for all ultrasound systems and require an understanding of a few terms:
  • Pulse echo system: The basic unit which includes a piezoelectric transducer to generate the ultrasonic wave, a receiver which processes the returning waves and a display screen
  • Acoustic impedance: The difference between the strength of the returning echoes from tissue boundaries with abrupt changes in acoustic properties. These surfaces are also known as acoustic interfaces. The greater the difference between the acoustic impedance of adjacent structures, the stronger is the returning echo
  • Angle of incidence: This refers to the angle of the sound beam relative to the area of interest. The more perpendicular the beam, the stronger is the returning echo
  • Resolution: The ability to distinguish between adjacent echoes, both axial and lateral. This is enhanced by the use of a focused sound beam
  • Amplification: This is an important part of the signal processing that occurs before the returning sound beam is displayed on the monitor. Three types of amplification are commonly used:
    1. Linear: Limited range of echo densities, but can show minor differences within this range
    2. Logarithmic: Wider range of echo densities, but does not show very minor differences between echo intensity
    3. S curve: A method that tries to combine the benefits of both the above to enable better tissue differentiation
  • Gain: This is the procedure of increasing or decreasing the amplitude of echoes that are displayed on the screen
  • Time gain compensation (TGC): The technique used to enhance the returning echoes from deeper structures by reducing those from the structures closer to the surface. This is typically utilized in studying the orbit
  • Display of signals: The ultrasound signal that is received can be displayed in 3 ways: A mode, B mode or a combination of the above. Other modifications include the three-dimensional ultrasound which uses a rotating transducer rather than the oscillating one used in the conventional ultrasound system and a combination of color Doppler with the B scan4
  • A scan: One-dimensional time—A mplitude display in which the echoes, represented by spikes from the baseline, are spaced depending on their distance from each other and the probe. The amplification is linear in the A scan used for axial biometry, logarithmic when combined with the B scan and S-shaped in the standardized A scan for tissue differentiation
  • B scan: Two-dimensional B rightness display where the strength of the returning echo is displayed as a dot on the screen, the brightness being directly proportional to the echo strength5
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Figures 1.1A and B: B scan images of an eye with vitreous hemorrhage. Note the clear vitreous cavity when the scanning is done at a lower gain. Figure B shows that at higher gain there are significant intragel echoes (arrowhead) and an incomplete posterior vitreous detachment (PVD).
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Figures 1.2A and B: B scan images of an eye with vitreous hemorrhage (arrow) and a retained IOFB (arrowhead). This set of images demonstrates the usefulness of reducing the gain to better delineate the IOFB (arrowhead) as seen in Figure B.
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Figures 1.3A and B: B scan images of an eye with retinoblastoma. Note the high reflective nature of the tumor. Clumps of very high reflectivity are seen within the mass that are suggestive of calcification. By reducing the gain in Figure B, the calcification (arrowhead) and shadowing is seen much better.
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Figures 1.4A and B: B scan images of an eye with a large choroidal coloboma. Note that at the depth setting of 3 cm, the posterior ectatic area of the coloboma (arrowhead) is not well seen (Fig. A). In Figure B, the depth has been increased to better study the posterior globe contour. Note also the TGC scales at the bottom of the images. In Figure A, the TGC setting is raised to image the vitreous echoes. Figure B shows how, by dampening the TGC anteriorly, one is able to enhance the returning echoes from the orbit.
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Figures 1.5A to C: These images are of an eye with retinoblastoma and optic nerve involvement. Figure A shows the large exophytic tumor with calcification (arrowhead). Figure B includes an A vector as well as a deeper probe penetration to better visualize the optic nerve(ON). Note that the TGC setting is such that the tumor as well as the enlarged optic nerve shadow is seen. In Figure C, the depth penetration has been further increased, and with a change in the TGC settings, the vitreous echoes have been dampened so as to get an excellent view of the thickened optic nerve.
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Figures 1.6A and B: B scan images of similar sized choroidal melanomas. Figure A is an image taken with a 10 MHz probe, whereas Figure B is a 20 MHz image. Note the improved resolution with the 20 MHz probe where the shallow retinal detachment (arrow) is well seen as well as the small extrascleral nodule (arrowhead)(Figure B courtesy Dr Vincenzina Mazzeo, Italy).
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Figures 1.7: Ultrasound biomicroscopy with a 35 MHZ probe shows that anteriorly located structures till the pars plana can be imaged better than with the 10 or 12 MHz probe. In this case, a subconjunctival cysticercus cyst.
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Figure 1.8: An ultrasound biomicroscopy image of proliferation from the sclerotomy site (arrows) using a 50 MHz probe. This image is of still higher resolution; however, note the corresponding decrease in depth penetration.