B-Scan
B-scan ultrasonography commonly uses 10 MHz, with axial resolution of 100 µm, to provide 2-dimensional images of the eye and orbit. Combining data from 2 orthogonal scans at a given point yields 3-dimensional information: shape, location, and extent. Three types of B-scans—axial, transverse, and longitudinal—are obtained depending on the position of the probe on the eye and the orientation of the linear white marker on its surface (Fig 17-4). These scans are best performed with direct contact on an anesthetized ocular surface, facilitated by a coupling agent such as methylcellulose. This improves image resolution and allows the examiner to monitor the position of the patient’s eyes. The probe marker indicates the direction of the scan and corresponds to the top of the 2-dimensional B-scan image.
Axial scans
Axial scans are performed by placing the probe directly on the cornea with the patient looking straight ahead and the probe marker oriented vertically at 12 o’clock or horizontally with the probe marker oriented nasally (Fig 17-5). This allows visualization of the posterior pole and the optic nerve. The posterior sclera and underlying Tenon space can also be examined, as in cases of posterior scleritis. Attenuation of the signal by the cornea and lens limits the resolution of these scans.
Transverse scans
During B-scan ultrasonography, transverse scans cover the greatest area of the posterior segment of the eye. The probe is placed on the sclera, avoiding image degradation from the anterior segment, and is oriented parallel to the limbus, providing a circumferential scan of the opposing retina (ie, when imaging the nasal quadrant, the probe is placed on the temporal sclera with the patient adducting his or her eye; Fig 17-6). The farther the probe traverses posteriorly from the limbus, the more the anterior part of the eye is imaged (ie, with the patient looking just nasal to midline, the probe is touching the edge of the limbus, and the back of the 2-dimensional image is posterior to the equator). As the patient looks farther nasally, the probe slides posteriorly on the surface of the globe and the scan is directed more anteriorly (Fig 17-7). This maximizes visualization of that quadrant.
When the posterior segment cannot be visualized, 4 transverse scans (ie, superior, inferior, nasal, and temporal) in addition to the axial scan are typically performed as part of the screening B-scan. The nasal and temporal scans are known as the lateral transverse scans (see Fig 17-6). By convention, when the superior or inferior eye is imaged, the probe marker is oriented nasally. In all other positions, the probe marker is oriented superiorly. Figure 17-8 demonstrates the appropriate positioning of the probe and the orientation of the marker for the 4 primary transverse scans.
Longitudinal scans
Similar to transverse scans, longitudinal scans are performed with the probe placed on the sclera, with the marker oriented perpendicular to the limbus. These scans are performed when a lesion is identified on a screening B-scan and describe the anterior-posterior extent. The optic nerve should be visualized below the center on longitudinal scans (Fig 17-9). Longitudinal scans can also be used to visualize the macula (Fig 17-10).
Dynamic B-scan
B-scan ultrasonography is not a static process. Already, we have discussed the anterior-to-posterior excursion of the ultrasound probe to increase the area imaged during transverse scans. In addition, the patient can be asked to look up and down during lateral transverse scans and right and left during superior/inferior transverse scans to study movement of the vitreous/posterior hyaloid face and a detached retina. Furthermore, the gain of the scan can be adjusted to enhance visualization of particular structures (Table 17-5).
Figures 17-11, 17-12, and 17-13 highlight the differential diagnoses requiring ophthalmic ultrasonography and their diagnostic features.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.