Ophthalmologic Ultrasounds Health Article

Description

The images formed by ophthalmologic ultrasound must be resolvable. Resolution is the ability of the eye to distinguish between objects. Resolution can be linear, which determines how far apart two objects are from each other, or contrast, which determines the differences of shades of gray between objects. The higher the frequency employed, the greater the resolution, i.e. smaller objects can be discerned. A frequency of 10 Mhz gives a resolution of 150 micrometers, but resolution as small as 20 micrometers is possible with a 100 Mhz transducer.

An A-scan ophthalmologic ultrasound produces a one dimensional display of intraocular structures. It can employ either applanation or water immersion techniques. The applanation probe, or transducer, touches the cornea of the eye, while the immersion probe is mounted in a water bath surrounding the eye and never compresses the globe. Because the applanation probe applies more pressure to the eye, it can underestimate axial length. Since the probe of the water immersion unit is not in direct contact with the eye, and the sound waves must pass through water before reaching the back of the eye, it is more difficult to judge the layers of the internal eye with this technique, especially when a dense cataract is present.

The B-scan ophthalmologic ultrasound produces a two dimensional real time image. Usually an applanation probe is used, but a water bath technique may give better resolution, important in location of small foreign bodies. In B-scan ultrasound exams the probe is oriented perpendicular to the structure being examined. The images of B-scans are displayed on a video monitor, and can be recorded.

The 3-D ophthalmologic ultrasound produces its image as the probe passes over the eye at numerous angles, and then combines these slices of the eye to produce an image larger than that formed by the B-scan. A 3-D ultrasound can reproduce an image in less than 12 seconds, but it is not a real time image. The anterior segment cannot be imaged well by 3-D ultrasonography.

Doppler ultrasonography assesses blood flow in the eye. Duplex ultrasonography combines the B-scan with the Doppler ultrasonography. The color duplex ultrasound is superimposed with color, allowing the examiner to assess blood flow direction, identify blood vessels, and calculate velocity of blood flow. These techniques, when applied to the eye, assess blood flow through ocular blood vessels.

Ultrasound biomicroscopy uses higher frequencies and thus can image the structures of the eye with greater resolution than a B-scan ultrasound and gives the eye care practitioner a real-time image. Ultrasound biomicroscopy can penetrate the eye only up to 5 mm and thus cannot image the posterior pole. The average length of the eye is 25 mm.

Precautions

Special care is needed when performing an ophthalmologic ultrasound on a ruptured globe.

Preparation

Ophthalmologic ultrasounds are usually performed in the supine position (lying down) and in dim light.

Prior to using the applanation A-scan measurement, an anesthetic drop is instilled in the patient's eye and the patient looks at a target at the end of the probe which gently touches the cornea. An eye cup may keep the eye open or the probe may be held against the eyelid. With the water immersion technique a plastic bag with a hole large enough for the eye and lids to protrude, is placed around the eye.

Prior to a B-scan ultrasonography, an anesthetic is applied to the eye and the patient's eye is held open with an eye cup filled with methyl cellulose. A protective contact lens may be placed on the eye. The patient is given a target on the ceiling on which to fixate, with the eye not being examined. The probe is covered with a coupling gel, and then applied in various directions across the eye, perpendicular to the internal structures of interest. An eye cup, filled with the methyl cellulose, can be held over parts of the ocular adnexa, such as over a closed eye for examination of the lids, when structures external to the globe are examined.

Aftercare

The patient should be instructed not to rub the eyes for 20 to 30 minutes after an ophthalmologic ultrasound and warned that his vision might be slightly compromised for the same time frame.

Complications

There are no known complications from ophthalmologic ultrasound when used for these time periods, and at levels indicated for ultrasound of the orbit and when performed by trained personnel.

Results

The results of ophthalmologic ultrasounds are immediately available to the doctor. Abnormal results indicate an underlying problem and may require further testing and treatment.