B-mode ultrasound (ocular B-scan) is a diagnostic imaging test that displays the reflection intensity of ultrasound as a two-dimensional image. Ultrasound waves emitted from the probe reflect at tissue interfaces, and the reflection intensity (brightness) and reflection time (tissue depth) are mapped in two dimensions to obtain cross-sectional images of the intraocular and orbital structures.
In ophthalmology, it is established as a test used when the interior of the eye cannot be observed due to opaque media and for diagnosing orbital lesions. When there is corneal opacity, advanced cataract, severe vitreous hemorrhage, or other opacities of the transparent media, observation of the interior of the eye with a slit lamp or ophthalmoscope becomes impossible. In such situations, B-scan is the only imaging test that can provide structural information about the inside of the eye.
In 1956, Mundt & Hughes reported the application of ultrasound to ophthalmology, which later developed into A-mode and B-mode. Today, it is widely used to evaluate the vitreous, retina, choroid, and orbital structures, and is an indispensable examination tool in outpatient ophthalmology and pre- and postoperative management.
This is mainly necessary when the transparent media (cornea, lens, vitreous) are opaque and the interior of the eye cannot be directly visualized, or when there are lesions that can only be evaluated with B-scan. Typical indications include confirming the presence or absence of retinal detachment or tumors in cases of severe vitreous hemorrhage, mature cataract, or corneal opacity, localizing intraocular foreign bodies, and diagnosing orbital tumors. Even when the interior of the eye can be directly visualized, it is useful for detailed evaluation of orbital lesions.
The examination is generally performed according to the following steps.
The transpalpebral approach is standard, with less patient invasiveness and is the first choice in many situations. It is essential not to press on the eyeball with the probe, and head fixation is important. In the supine position, it is desirable to stabilize the head with a pillow; when examining in a sitting position, head fixation by an assistant from behind is indispensable.
| Scanning Method | Characteristics | Advantages | Disadvantages |
|---|---|---|---|
| Linear scan (electronic scanning) | Electronically switches multiple transducers to move the ultrasound beam | Provides uniform images without defects | Probe is wide and expensive |
| Sector scan (mechanical scanning) | Mechanically scans the transducer at high speed | Probe tip is small, good maneuverability, and inexpensive | Image defects occur near the periphery of the anterior segment |
For evaluation of the posterior segment and orbit, sector scan is commonly used in daily practice, while linear scan is suitable for detailed evaluation of the anterior segment.
With the standard transpalpebral approach (placing the probe over the closed eyelid), topical anesthesia is not required and there is almost no pain. Only when the probe is placed directly on the cornea, topical anesthesia such as 0.4% oxybuprocaine is administered. The examination takes only a few minutes. Coupling gel is used and can be wiped off after the examination.
In a normal eye, the lens, retina, choroid, and sclera are visualized as a single layer within the eye. Outside the eye, relatively uniform tissue patterns are seen, and the optic nerve is identified as a low-echoic tubular structure.
Ultrasound brightness reflects differences in acoustic impedance of tissues. High-echoic echoes originate from interfaces with large acoustic impedance differences (e.g., sclera, calcifications, intraocular foreign bodies), while low-echoic echoes originate from fluids (e.g., normal vitreous, aqueous humor, cyst fluid).
When interpreting ocular B-mode ultrasound, focus on the following five points.
In eyes with vitreous hemorrhage, B-mode ultrasound may show membranous hyperechoic echoes. Differentiating whether this is a detached retina or a vitreous membrane (e.g., fibrous membrane associated with posterior vitreous detachment) greatly influences treatment strategy. Dynamic B-scan has been reported to have a sensitivity of approximately 96% and specificity of approximately 98% for detecting retinal tears in acute posterior vitreous detachment cases 2). Evaluate using a combination of the following three diagnostic methods.
Morphological Diagnosis
Evaluation: Check whether the membranous echo is connected to the optic disc.
Features of detached retina: Continuous with the optic disc. The membranous echo is continuous, smooth, and curved. The membrane thickness is uniform.
Features of vitreous membrane: Connection with the optic disc is unclear. May show intermittent and irregular morphology.
Quantitative Diagnosis (Gain Reduction Method)
Evaluation: Observe the order of disappearance of membrane echoes while gradually decreasing the amplification sensitivity dial (gain).
Characteristics of detached retina: Shows strong reflection, so echoes persist even when gain is reduced.
Characteristics of vitreous membrane: Reflection is weak, so when gain is reduced, it disappears before retinal echoes.
Dynamic Diagnosis (Eye Movement)
Evaluation: Observe the movement of membrane echoes while the patient moves their eyes.
Characteristics of detached retina: Shows regular, smooth, and continuous movement with eye movement. Movement stops when eye movement ceases.
Characteristics of vitreous membrane: Shows irregular, coarse, and discontinuous movement with eye movement. After eye movement stops, a slow undulating movement remains (after-motion).
Differentiate using a combination of three methods. ① Morphological diagnosis: Detached retina appears as a continuous membrane connected to the optic disc, smooth and uniform in thickness. ② Gain reduction method: When gain is gradually reduced, the vitreous membrane disappears first, so the echo that remains until the end corresponds to the retina. ③ Eye movement: Detached retina moves regularly and smoothly, stopping when the eye is still. Vitreous membrane shows residual undulating movement (after-motion) even after eye movement stops.
B-scan is an important examination method that provides structural information of the intraocular and orbital regions not only in eyes with opaque media but also in situations where ophthalmoscopy is difficult, and is used in the following clinical scenarios.
Based on B-scan findings, the following treatments or further evaluations are selected.
| B-scan finding | Corresponding treatment/evaluation |
|---|---|
| Rhegmatogenous retinal detachment | Vitrectomy (vitreous removal, laser, gas tamponade such as SF6) or scleral buckling |
| Tractional retinal detachment (diabetic) | Vitrectomy (removal of traction membranes, silicone oil tamponade, etc.) |
| Intraocular tumor (suspected choroidal melanoma) | Detailed examination (fluorescein angiography, MRI, detailed ultrasound) → radiotherapy (proton beam, brachytherapy) or enucleation |
| Suspected orbital tumor | Perform MRI/CT detailed examination, consider need for biopsy |
| Intraocular foreign body | After confirming location and material of foreign body, consider removal surgery (surgical method differs depending on magnetic or non-magnetic) |
| Vitreous hemorrhage only (no retinal detachment) | Investigate cause and observe; if hemorrhage does not resolve, perform vitrectomy |
The principle of ultrasound B-mode examination is as follows.
| Frequency range | Resolution | Penetration | Main applications |
|---|---|---|---|
| 30 MHz or higher | High | Shallow | Detailed observation of the anterior segment (cornea, iris, lens) |
| 10–20 MHz | Moderate | Moderate | Standard observation of the posterior segment (vitreous, retina, choroid) |
| 5–10 MHz | Somewhat low | Deep | Evaluation of orbital lesions |
The higher the frequency, the shorter the wavelength, which improves resolution, but attenuation in tissue increases, reducing ultrasound penetration (depth). A 10 MHz probe has a calculated resolution of approximately 0.2 mm. For standard evaluation of the posterior segment, probes around 10 MHz are widely used, while high-frequency probes in the 50–80 MHz range are used for ultrasound biomicroscopy (UBM) of the anterior segment.
Color Doppler ultrasound is a technique that superimposes color-coded blood flow velocity information on a conventional B-mode image. It allows quantitative evaluation of blood flow velocity and resistive index (RI) in orbital vessels (ophthalmic artery, central retinal artery, short posterior ciliary arteries, and ophthalmic veins). In glaucoma, changes in ophthalmic artery Doppler waveform parameters have been reported to correlate with the severity of glaucomatous optic neuropathy, and its application in evaluating ischemic optic neuropathy and orbital vascular lesions is being studied.
Using real-time B-mode ultrasound during vitreous surgery can help assess the status of traction membrane peeling, silicone oil fill, and track intraocular foreign bodies. It is gaining attention as an auxiliary tool in cases such as open globe injuries where fundus observation under a microscope is difficult.
Deep learning is increasingly applied to automated analysis of B-mode ultrasound images. An InceptionV3-Xception fusion model has been reported to achieve an accuracy of 0.97 and AUC of 0.999 for classifying retinal detachment, vitreous hemorrhage, and intraocular tumors 5). In the future, it is expected to reduce operator dependency and standardize diagnosis.
