Use of Ultrasound in Ophthalmology

Usman Raja

This article aims to provide a concise overview of the principles of ultrasound imaging and its clinical applications within ophthalmology.

What is Ultrasound?

Ultrasound refers to sound waves that transmit mechanical energy through vibration of particles at frequencies that are typically above the upper limit of human hearing, typically exceeding 20,000 Hertz (Hz) (1).

The use of ultrasound in medical imaging dates back to the 1950’s when early experiments were conducted using modified radar equipment (2-4).  It was first used in obstetrics and then expanded to other medical specialties (5). Rapid advances in technology have now established ultrasound as an essential tool across medicine. It is now routinely used in trauma assessment, examining the structure and function of the heart (echocardiograms) and monitoring foetal development (5).

Ultrasound waves between 2 and 15 MHz are typically used in medical imaging (6). Diagnostic ultrasound involves the use of a transducer (a hand-held probe) that is placed in contact with the patient (7). The transducer generates sound waves via the piezoelectric effect whereby an applied voltage causes crystals found in the transducer to vibrate producing ultrasound waves. The transducer also acts as a receiver and these waves are converted into electrical signals to create a visual image on a display screen (6).

There are four primary ultrasound modes used in medical imaging (7,8):

A-mode (Amplitude mode): a one-dimensional technique whereby a single transducer scans a line through the body displaying echoes as vertical spikes on graph which represent tissue depth.

B- mode (Brightness mode): produces a two-dimensional black and white when a patient is scanned. These can be displayed in different planes. It is the most commonly used imaging mode.

M-mode (Motion mode): Displays movement of anatomical structures over time, allowing assessment of motion.

Doppler: Utilises the Doppler effect to assess and visualise blood flow.

Ultrasound in Ophthalmology

The first documented use of ultrasound in ophthalmology was in 1956 and since then its use has become increasingly expanded for the evaluation of patients with conditions such as acute vision loss and ocular trauma (9.10). A and B-modes are typically used. Its advantages include rapid acquisition, non-invasive assessment and the absence of ionising radiation.

A-mode Ultrasound

These scans are used to measure the axial length of the orbit for intraocular tumour thickness and characterisation and determine the power for an intraocular lens implant (11).

Axial length measurement can be performed using either contact or immersion techniques. In a contact examination, topical anaesthetic drops are applied to the eye and the probe is then brought into contact with the central cornea. Immersion examinations involve the use of a scleral shell which keeps the eyelid open and forms fluid over the eye in which the probe is placed on (11,12).

Whilst A-mode ultrasound remains relevant, Optical Coherence Tomography (OCT) has now replaced ultrasound for evaluating the anterior segment and retina due to its superior spatial resolution (11).

B-mode Ultrasound

B-mode ultrasound has numerous applications in ophthalmology with the first ophthalmic-specific device developed in 1958 (13).

It is particularly valuable in patients presenting with acute or sub-acute vision loss when intraocular structures are obscured.

Retinal and vitreous detachment can be visualised on ultrasound with detachments typically seen as a ‘hyperechoic flap’ (14,15). As well as this, vitreous haemorrhages appear as heterogeneous mater in the globe (16).

Ultrasound can also be used in the context of ocular trauma where it can assist in differentiating underlying pathology. Foreign bodies can be identified within the globe, however if there is a suspicion of globe rupture, the examination should be discontinued (17,18). Applying pressure to the cornea using the probe may risk expulsion of intraocular content in the context of globe rupture (18).

Retrobulbar haematomas can also be detected as an echogenic mass posterior to the globe. In trauma scenarios, this is important to identify as they could rapidly progress and increase intraocular pressure (14).

Conclusion

Ultrasound is a valuable first-line imaging modality in ophthalmology particularly in acute settings. However, it should be recognised that other modalities, such as Computed Tomography (CT), may provide superior image clarity and anatomical detail.

References

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