Introduction
Ophthalmic ultrasonography is a useful diagnostic technique for intraocular and orbital evaluation, especially in the setting of opaque media. It involves pulse-echo technology, in which high frequency sound waves are emitted from a handheld transducer probe. Returning echoes are processed and displayed on video monitors or oscilloscopes.
Two modes of display are common:
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A-scan mode (time-amplitude), used predominantly for interpretation of tissue reflectivity—the returning echoes form a graph-like image seen as vertical deflections from a baseline.
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B-scan mode (intensity modulation), used predominantly for anatomical information—it shows cross-sectional images of the globe and orbit.
Both types of sonographic display are complementary. This chapter focuses on B-scan information.
Developed in the mid-1950s with water-immersion techniques, B-scan ultrasonography initially required a laboratory setting. In the early 1970s, contact devices utilizing methylcellulose or a similar sound-coupling agent were introduced, and this rapidly increased the availability and popularity of B-scan ultrasonography. Subsequent improvements in image quality and scanning rates made interpretation easier for the examiner.
Devices
Commercially available instruments for ocular and orbital contact B-scan ultrasonography usually employ 10-MHz (megahertz; megacycles per second) handheld transducer probes. A motor within the handpiece moves the ultrasonic source in a rapid sector scan to create cross-sectional B-scan images. These devices have resolution capacities of approximately 0.15 mm axially and 0.3 mm laterally. Most contact B-scan machines are freestanding and relatively mobile; they consist of a detachable transducer probe, a signal-processing box, and a display screen. Self-contained processing probes, which are capable of integrating with independent computer laptops or desktop units, are also available. ( )
Technique of Examination
The handheld ultrasonic probe is placed gently against the eyelid or sclera by using a sound-coupling agent, such as methylcellulose or, preferably, heat-sensitive ophthalmic gel. The ultrasonographer can move the probe systematically to scan the globe and orbit. Avoiding the lens system of the globe is important to prevent image artifacts ( Fig. 6.5.1 ).
Concepts of B-Scan Interpretation
Interpretation of a B-scan ultrasonogram depends on three concepts:
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Real time.
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Gray scale.
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Three-dimensional analysis.
Real Time
Ultrasound B-scan images are visualized at approximately 32 frames/sec, allowing motion of the globe and vitreous to be detected. Characteristic real-time movements are useful for identifying tissues. Detached retinal movement, for example, appears as a slow undulation, whereas vitreous movements are usually more rapid. Real-time ultrasonic information is often critical to surgical decisions.
Gray Scale
A variable gray-scale format displays the returning echoes as intensity-modulated dots. Strong echoes, such as those seen from sclera or detached retina, are displayed brightly at high instrument gain and remain visible even when the gain is reduced. Weaker echoes, such as those from vitreous hemorrhage, are seen as lighter shades of gray that disappear when gain is reduced. Because comparison of echo strengths is critical to tissue analysis, the examiner must ensure that all the returning echoes are captured and displayed. Perpendicularity to the object of regard ensures adequate comparable interpretive signals.
Three-Dimensional Analysis
Developing a mental three-dimensional image or anatomical map from multiple two-dimensional B-scan images is the most difficult concept to master. The examiner must learn to create a mental topographical map of the eye or orbit from as many imaging views as required. Three-dimensional understanding of ultrasound images is especially critical in the preoperative evaluation of complex retinal detachments and intraocular or orbital tumors.
Real Time
Ultrasound B-scan images are visualized at approximately 32 frames/sec, allowing motion of the globe and vitreous to be detected. Characteristic real-time movements are useful for identifying tissues. Detached retinal movement, for example, appears as a slow undulation, whereas vitreous movements are usually more rapid. Real-time ultrasonic information is often critical to surgical decisions.
Gray Scale
A variable gray-scale format displays the returning echoes as intensity-modulated dots. Strong echoes, such as those seen from sclera or detached retina, are displayed brightly at high instrument gain and remain visible even when the gain is reduced. Weaker echoes, such as those from vitreous hemorrhage, are seen as lighter shades of gray that disappear when gain is reduced. Because comparison of echo strengths is critical to tissue analysis, the examiner must ensure that all the returning echoes are captured and displayed. Perpendicularity to the object of regard ensures adequate comparable interpretive signals.
Three-Dimensional Analysis
Developing a mental three-dimensional image or anatomical map from multiple two-dimensional B-scan images is the most difficult concept to master. The examiner must learn to create a mental topographical map of the eye or orbit from as many imaging views as required. Three-dimensional understanding of ultrasound images is especially critical in the preoperative evaluation of complex retinal detachments and intraocular or orbital tumors.
Display Presentation and Documentation
Posterior B-scan images displayed on a screen are presented horizontally. Areas closest to the probe are imaged to the left of the screen, and those farthest away are imaged to the right. The top of the screen correlates with a manufacturer’s mark located on the examining probe that represents the initial transducer sweep for each sector scan. Registration of the screen is critical to understanding and interpreting examinations. Movement of the probe from one position to another changes the registration, making instant re-evaluation by the examiner an absolute necessity. Contact B-scan ultrasonography is a dynamic examination. Individual “frozen” cross-sectional images used for documentation should not alone be used for interpretation. ( )
Normal Vitreous Cavity
The normal vitreous space is almost clear echogenically. Occasional small dots or linear echoes can be seen at the highest gain settings (90 decibels [dB]), but they fade rapidly as the gain is reduced. Real-time scanning during eye movement usually shows some motion of these fine echoes as well as the position of the vitreous face.
Vitreous Hemorrhage
Intravitreal hemorrhage produces easily detectable diffuse dots and blob-like vitreous echoes that correlate with the amount of blood present. Reduction of gain to 70 dB results in rapid fading of all but the densest areas of reflectivity. Real-time evaluation usually shows a characteristic rapid motion during command voluntary eye movement.
Retinal Detachment
Detached retina appears as a highly reflective sheet-like tissue within the vitreous space ( Fig. 6.5.2 ). Small detachments often appear dome-like on imaging. The appearance of total retinal detachment, which anatomically is cone shaped, varies, depending on the position of the examining probe. Axial images are funnel shaped with attachment to the optic nerve head. Coronal images show a circular cross-section of the cone.
