What is ultrasound
- Sound pressure with a frequency greater than the upper limit of
- Although this limit varies from person to person, it is approximately
20 kilohertz (20,000 hertz) in healthy, young adults.
B-scan ultrasonography is an important noninvasive technique for the clinical assessment of various ocular and orbital diseases.
Principles of ultrasound:
• By definition, an ultrasound wave has a frequency greater than 20 kHz (20,000 oscillations/ second)
• As the frequency of USG increases, the wavelength decreases and wavelength of an ultrasound determines its depth of tissue penetration and resolution
Wavelength α Depth of penetration of the ultrasound
• So, Larger is the frequency of US = shorter is its wavelength = shallower is its penetration = better is the resolution of resultant echo graph.
That’s why USG probes used for Ocular USG are of higher frequency(10MHz)as it needs much less tissue penetration (an eyeball is 23.5 mm long on average) and higher resolution.
• In contrast, ultrasound probes used for purposes such as obstetrics, use lower frequencies (1-5Hz) for deeper penetration into the body, and, because the structures being imaged are larger, they do not require the same degree of resolution
In B-scan ultrasonography, an oscillating sound beam is emitted, passing through the eye and imaging a slice of tissue; the echoes of which are represented as a multitude of dots that together form an image on the screen.
The stronger the echo, the brighter the dot.
example, the dots that form the posterior vitreous hyaloid membrane are not as bright as the dots that form the retinal membrane.
This is very useful in differentiating a posterior vitreous detachment (a benign condition) from a more highly reflective retinal detachment (a blinding condition) because the retina is denser than vitreous.
Angle of incidence
• The angle of incidence of the probe is critical for both A-scan and Bscan ultrasonography.
• When the probe is held perpendicular to the area of interest, more of the echo is reflected directly back into the probe tip and sent to the display screen.
• When held oblique to the area imaged, part of the echo is reflected away from the probe tip and less is sent to the display screen.
• The more oblique the probe is held from the area of interest, the weaker the returning echo and, thus, the more compromised the displayed image.
On A-scan, the greater the perpendicularity, the more steeply rising the spike is from baseline and the higher the spike.
On B-scan, the greater the perpendicularity, the brighter the dots on the surface of the area of interest
.Because various parts of the eye and various pathologies are different in size and shape, understanding this concept and anticipating the best possible display for that eye are important. Perpendicularity to the area of interest should be maintained to achieve the strongest echo possible for that structure
• Ultrasound is absorbed by every medium through which it passes.
• The denser the medium, the greater the amount of absorption.
• This means that the density of the solid lid structure results in absorption of part of the sound wave when B-scan is performed through the closed eye, thereby compromising the image of the posterior segment
Likewise, when performing an ultrasound through a dense cataract as opposed to the normal crystalline lens, more of the sound is absorbed by the dense cataractous lens and less is able to pass through to the next medium, resulting in weaker echoes and images on both A-scan and B-scan. For this reason, the best images of the posterior segment are obtained when the probe is in contact with the sclera rather than the corneal surface, bypassing the crystalline lens or intraocular lens implant.
• Ophthalmic ultrasound instruments use what is known as a pulse-echo system, which consists of a series of emitted pulses of sound, each followed by a brief pause (microseconds) for the receiving of echoes and processing to the display screen.
• The amplification of the display can be altered by adjusting the gain, which is measured in decibels (dB). Adjusting the gain in no way changes the frequency or velocity of the sound wave but acts to change the sensitivity of the instrument’s display screen.
• When the gain is high, weaker signals are displayed, such as vitreous opacities and posterior vitreous detachments.
• When the gain is low, the weaker signals disappear, and only the stronger echoes, such as the retina, remain on the screen.
• Typically, all examinations begin on highest gain so that no weak signals are missed; then, the gain is reduced as necessary for good resolution of the stronger signals
Indications of B-Scan
• when direct visualization of intraocular structures is difficult or impossible.
• Situations that prevent normal examination lid problems (eg, severe edema, partial or total tarsorrhaphy), corneal opacities (eg, scars, severe edema), hyphema, hypopyon, miosis, pupillary membranes dense cataracts
vitreous opacities (eg, haemorrhage, inflammatory debris).
Source: Dr Gyanendra Lamichhane, Lumbini Eye Institute