Ophthalmic ultrasonography is a diagnostic imaging method that uses ultrasound to noninvasively visualize structures within the eyeball and orbit. Because the eye is close to the body surface and filled with fluid, it is well suited for ultrasound examination.
High-frequency sound waves above 20 kHz are generated by a piezoelectric element inside the probe (transducer) and reflected at tissue boundaries. The intensity and arrival time of the reflected waves (echoes) are used to image the position and characteristics of tissues. Sound waves travel faster in solids than in liquids. Scattering, reflection, and refraction of sound waves occur at tissue boundaries with different acoustic impedance or density.
Based on echo intensity, each area in the image is represented as follows:
Behind a high-density lesion, acoustic shadowing may occur, resulting in an anechoic area.
A-mode (amplitude mode) displays reflected waves as waveforms (spikes) and numerically evaluates distances and reflectivity between tissues. B-mode (brightness mode) displays the intensity of reflected waves as brightness changes on the screen, obtaining two-dimensional tomographic images. For details, see the section “Types and Principles of Examination”.
Ultrasound examinations used in ophthalmology include A-mode, B-mode, and ultrasound biomicroscopy. It is desirable to use them appropriately depending on the examination site.
A-mode
Principle: A single ultrasound beam is transmitted, and the reflected wave is displayed as a waveform (spike). The horizontal axis represents time (distance), and the vertical axis represents echo intensity.
Frequency: 8 MHz
Main uses: Axial length measurement, corneal thickness measurement, evaluation of tissue characteristics inside tumors
B-mode
Principle: The intensity of the reflected wave is represented by changes in brightness, and by moving the probe, a two-dimensional tomographic image is constructed.
Frequency: 10 MHz (typically 5–20 MHz)
Main uses: Morphological diagnosis of intraocular and orbital lesions, detection of retinal detachment, measurement of tumors
Ultrasound Biomicroscopy
Principle: High-frequency ultrasound (30–60 MHz) provides high-resolution imaging of the anterior segment. Resolution is high but penetration depth is shallow.
Main uses: Morphological evaluation of the ciliary body, quantitative assessment of the anterior chamber angle, measurement of anterior chamber depth
General ultrasound diagnostic devices use transducers of 5–20 MHz. Two-dimensional images obtained from B-mode can be reconstructed into 3D images using computer graphics, allowing three-dimensional assessment of lesion size and boundaries.
Ultrasound examination is particularly necessary in the following situations.
| Situation | Specific Examples |
|---|---|
| Opaque media | Mature cataract, vitreous hemorrhage, corneal opacity |
| Detailed examination of intraocular lesions | Intraocular tumors, retinal detachment, lens dislocation |
| Biometry | Axial length measurement (IOL power calculation) |
When the fundus cannot be visualized due to opacities of the transparent media such as the cornea, lens, or vitreous, B-mode ultrasound is extremely useful. The examination is minimally invasive, the equipment is compact, and it can be easily used in an outpatient setting.
Even when the fundus is completely invisible due to vitreous hemorrhage, B-mode ultrasound can assess the presence or absence of retinal detachment and posterior vitreous detachment, making it indispensable as a preoperative test1). In the follow-up of diabetic retinopathy, it is also a useful diagnostic tool for evaluating the retinal condition when vitreous hemorrhage or other media opacities are present4).
In preoperative evaluation for cataract surgery, when optical biometry is not possible due to mature or dense cataracts, ultrasound biometry (A-mode and/or B-mode) is recommended 6). Although there is no significant difference between optical and ultrasound measurements, optical methods have the advantage of being non-contact, rapid, and accurate 6).
It is also essential for the diagnosis and follow-up of intraocular tumors such as choroidal melanoma, and combined A-mode and B-mode examination has an accuracy of over 95% in diagnosing choroidal melanoma with a thickness of 3 mm or more.
The most common indication is axial length measurement before cataract surgery. It is also necessary for excluding retinal detachment when the fundus cannot be observed due to vitreous hemorrhage or mature cataract, measuring and monitoring intraocular tumors, and detecting intraocular foreign bodies.
A-mode is mainly used for axial length measurement.
The segmented sound velocity method (lens: 1,641 m/s, anterior chamber and vitreous: 1,532 m/s) is considered to have less measurement error than the equivalent sound velocity method (phakic eye: 1,550 m/s). Compared to optical biometry, ultrasound A-mode measurements are displayed 0.2–0.3 mm shorter.
The ultrasound biomicroscope uses high frequencies of 30–60 MHz. It enables detailed morphological evaluation of the anterior segment including the ciliary body, and provides quantitative assessment with better objectivity and reproducibility than gonioscopy. It can assess the angle even when corneal transparency is reduced due to elevated intraocular pressure. However, due to the high frequency, its penetration depth is shallow, making it unsuitable for intraocular or orbital examination.
In normal eyes, the vitreous appears completely anechoic (negative image). If any echo (positive image) is observed within the vitreous, pathological changes should be suspected. Normally, the retina, choroid, and sclera are not separated and are observed as a single layer lining the inner wall of the eyeball.
For detecting retinal tears in fundus-obscuring vitreous hemorrhage associated with posterior vitreous detachment, the sensitivity of B-mode ultrasonography has been reported to vary widely, from 44% to 100% 1). If a retinal tear is suspected, ultrasonography should be repeated within 1 to 2 weeks after the initial evaluation 1).
Even if B-mode ultrasonography is negative in patients with vitreous hemorrhage obscuring the entire retina, weekly follow-up is recommended 1).
| Finding | Retinal detachment | PVD |
|---|---|---|
| A-mode spike | High | Moderate |
| Mobility | Regular, smooth | Irregular, rough |
| Continuity with optic nerve | Present | Absent |
In retinal detachment, the membrane echo is continuous with the optic disc, the spike wave on A-mode is high, and the movement after eye movement is regular and smooth. In posterior vitreous detachment, there is no continuity with the optic nerve, the spike wave is lower, the movement is irregular, and undulating movement persists even after eye movement stops. Lowering the gain is also useful for differentiation, as the vitreous membrane echo disappears earlier than the retinal echo.
B-mode is useful for detecting, measuring, and monitoring intraocular tumors such as choroidal malignant melanoma.
The ultrasound findings of various tumors are as follows:
In the classification of stage 5 retinopathy of prematurity (ROP), evaluation of retinal detachment using B-mode ultrasonography is required 3).
In cases where mature cataracts or other opacities make posterior observation difficult, B-mode is considered appropriate for detecting intraocular masses, retinal detachment, and posterior staphyloma 2).
B-mode ultrasound plays a central role in evaluating choroidal melanoma. Since basal diameter and lesion thickness correlate with metastasis and mortality, imaging-based measurement and follow-up are important.
Ramos-Dávila et al. (2025) performed morphological classification using B-mode ultrasound in 1,021 cases of uveal melanoma at the Mayo Clinic 5). They were classified into dome-shaped (739 cases, 72.4%), mushroom-shaped (119 cases, 11.7%), multilobulated (85 cases, 8.3%), and minimally elevated (77 cases, 7.5%). In multivariate analysis adjusted for tumor size, the multilobulated type had a 2.08-fold risk of metastasis (p = 0.003) and a 2.38-fold risk of death (p < 0.001).
This study shows that morphological evaluation of melanoma by B-mode ultrasound is also important as a prognostic factor 5).
Axial length measurement by A-mode ultrasound can have a measurement error of about 0.3 mm even for experienced examiners. A 1 mm error in axial length results in a refractive error of approximately 3.4 D in short eyes, 2.9 D in average eyes, and 1.6 D in long eyes. Therefore, measurement error should be kept within 0.2 mm.
To improve measurement accuracy, the following methods are recommended.
In silicone oil-filled eyes, optical biometry is considered more accurate than ultrasound for IOL power calculation 6).
The following artifacts may occur in B-mode examination.
Ultrasound examination is theoretically less affected by vitreous opacities, but in eyes filled with silicone oil or gas after vitrectomy, good images cannot be obtained due to changes in sound velocity and penetration depth.
In emergency departments, the usefulness of point-of-care ultrasound (POCUS) is gaining attention. Ophthalmic emergencies account for approximately 3% of emergency department visits, but since ophthalmologists are not always on site, the importance of ultrasound examinations by emergency physicians is increasing.
When using B-mode for retinal detachment evaluation, a technique based on the mnemonic “CASE” has been proposed.
In recent years, advances in anterior segment OCT have led to the replacement of some indications for ultrasound biomicroscopy. However, ultrasound biomicroscopy still maintains an advantage in observing the posterior iris and ciliary body, and the two are complementary.
Anterior segment OCT can non-invasively evaluate the anterior segment surface with high resolution. In contrast, ultrasound biomicroscopy is better for evaluating deep structures such as the posterior iris, ciliary body, and posterior chamber 7).
Vishwakarma et al. (2023) reported a case where the combined use of AS-OCT and ultrasound biomicroscopy was useful for diagnosing and evaluating subconjunctival mycosis, which was difficult to differentiate from nodular scleritis 7).
