Chapter 105 – Contact B-Scan Ultrasonography
YALE L. FISHER
ANTONIO P. CIARDELLA
NICOLE E. GROSS
• Diagnostic technique useful in the evaluation of intraocular and orbital contents.
• It uses high-frequency sound waves emitted and received by a handheld transducer probe.
• Images are processed and displayed on a video monitor.
• Adequate interpretation for diagnoses of posterior segment disease depends on three concepts: real time, gray scale, and three-dimensional analysis.
Ophthalmic ultrasonography is one of the most useful diagnostic techniques 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.
In ophthalmology, two modes of display are common:
• 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.
• 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 were introduced. They utilized methylcellulose, or a similar sound-coupling agent, and rapidly increased B-scan availability and popularity. Subsequent improvements in image quality and scanning rates made interpretation easier for the examiner.       
Commercially available contact instruments for ocular and orbital B-scan ultrasonography usually employ 10?MHz (megacycles/s) transducer probes enclosed in a handheld container. A small motor within the handpiece moves the ultrasonic probe in a rapid sector scan to create cross-sectional B-scan images. In general, these devices have resolution capacities of approximately 0.4?mm axially and 1?mm laterally. Higher resolution ophthalmic instruments are available (20–50?MHz), but limited signal penetration renders them ineffective in the examination of posterior portions of the globe and orbit. 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.
TECHNIQUE OF EXAMINATION
The handheld ultrasonic probe is placed gently against the eyelid or sclera using a sound-coupling agent such as methylcellulose. The ultrasonographer can move the probe systematically to scan the globe and orbit. Lateral or medial displacement of the probe can be used to avoid the lens system and thus prevent image artifacts.
CONCEPTS OF B-SCAN INTERPRETATION
Interpretation of a B scan for accurate diagnoses of posterior segment disease depends on three concepts:
• Real time
• Gray scale
• Three-dimensional analysis
Ultrasound B-scan images can be visualized at approximately 32 frames/s, allowing motion of the globe and vitreous to be easily detected. Characteristic real-time movements can often identify imaged tissues such as detached retina or mobile vitreous, thus increasing diagnostic capability. Real-time ultrasonic information frequently aids in vitreoretinal surgery.
A variable gray-scale format is used to display the returning echoes as a two-dimensional image. 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 a vitreous hemorrhage, are seen as a lighter shade of gray that disappears when the gain is reduced. Comparing the echo strengths during ultrasonic examination is the basis for qualitative tissue analysis. This can enhance diagnostic accuracy provided that the strongest possible echoes from each tissue type are being evaluated. Diagnostic accuracy is achieved by ensuring that the probe remains perpendicular to the tissues of interest at all times.
Developing a mental three-dimensional image or topographic anatomical map from multiple two-dimensional B-scan images is the most difficult concept to master. It is essential because it
provides the vital architectural information that is the basis for B-scan diagnosis. Three-dimensional understanding of ultrasound images is especially critical in the preoperative evaluation of complex retinal detachments and also intraocular or orbital tumors.
DISPLAY PRESENTATION AND DOCUMENTATION
B-scan images displayed on a screen are presented horizontally. Areas that are 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 mark located on the examining probe that represents the initial transducer position for each sector scan.
Although contact B-scan ultrasonography is a dynamic examination, documentation and preservation of individual, “frozen” cross-sectional images are possible by photographic techniques. These illustrative images should not be used for interpretation.
Normal Vitreous Cavity
The normal vitreous space is almost clear of echogenic tissue. Occasional small dots or linear echoes can be seen at the highest gain settings (90?dB), but they fade rapidly as the gain is reduced. Real-time scanning during voluntary eye movement usually shows some motion of these fine echoes.
Intravitreal hemorrhage produces easily detectable diffuse dots and blob-like vitreal 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, staccato motion with eye movement. This occurs because vitreous hemorrhage induces a general vitreous gel liquefaction and separation from the retina.
Detached retina appears as a highly reflective sheet-like tissue within the vitreous space ( Fig. 105-1 ). Small detachments often appear dome-like on imaging. The appearance of total retinal detachment, which anatomically is cone shaped, varies depending upon the position of the examining probe. Axial images are funnel shaped with attachment to the optic nerve head. Coronal images show a cross section of the cone, that is, a circular image.
Figure 105-1 Contact B-scan image of a retinal detachment. This axial section of a total retinal detachment reveals a highly reflective sheet-like membrane in the vitreous space, detached from the posterior eye wall (arrows) and attached only to the optic nerve head.
Real-time evaluation varies; recent detachments have a characteristic undulating movement, which is slower than that of the vitreous gel. Long-standing detachments appear stiffer with less motion because of proliferation of scar tissue on the retinal surface.
Ultrasound evaluation of intraocular tumors requires not only topographic localization but also interpretation of acoustic gray-scale characteristics. Malignant choroidal melanomas, for example, have the most characteristic appearance. They are mostly dome or mushroom shaped, and on gray scale their anterior borders are strongly reflective, whereas the progressively deeper portions of the tumor are less reflective. This is due to cellular homogeneity that provides a false hollowing appearance. Tissues, such as orbital fat, localized behind these tumors are often shadowed (they appear less reflective) because of the absorption of sound by the tumor.
DIGITAL AND THREE-DIMENSIONAL CONTACT ULTRASOUND
A series of advances in electronics such as digital techniques and the development of high-capacity storage devices allows documentation of contact ultrasonography to become more than static photographs. Real-time kinetic movie recordings and playback of a B-scan examination with simultaneous amplitude information have made possible the recall of complete examinations. This is invaluable when comparison at a later date is of essence, for example, in tumor evaluation. Also, three-dimensional ultrasound systems allow the examiner to view horizontal, vertical, and diagonal aspects of the pathology at the same time in less than 2 seconds ( Fig. 105-2 ). This is done by collecting a series of two-dimensional scans in a preset fashion and transforming them into a volumetric image that can be processed, using multiplanar reconstruction. This new technique is particularly useful to inexperienced examiners when first learning three-dimensional thinking, and it is also helpful because it provides accurate linear and volume measurements within the context of the contact technique.
Figure 105-2 Three-dimensional contact B-scan image of a retinal detachment. Axial and sagittal views of a total retinal detachment (RD) are seen simultaneously. Notice the appreciation of a wide cone-shaped funnel. Also distinguishable are the large hole in the retina and vitreous hemorrhage (VH).
Contact B-scan ultrasound provides a convenient, noninvasive means for the evaluation of intraocular structures in situations where clinical examination is not possible because of opaque ocular media; it also allows a dynamic examination of the vitreoretinal relationship. Newly developed three-dimensional and digital contact techniques expand teaching capability and clinical availability of contact ultrasonography to a larger audience. Ultrasound studies should be used in conjunction with detailed clinical examination and other investigational modalities.
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