Ophthalmologic ultrasound is a noninvasive technique that uses high-frequency sound waves to produce images of the structures of the eye. It is the simplest method of imaging an eye affliated with opacities such as a cataract or vitreous hemorrhaging. Ophthalmologic ultrasound usually employs frequencies of up to 10 million Hertz (10 MHz), but frequencies in the range of 50 to 100 MHz are used in ultrasound biomicroscopy of the eye. Humans cannot hear sounds that emit a frequency of greater than 20,000 Hertz. An ultrasound image is created by a transducer or probe that transforms electric energy to sound energy, which then penetrates the ocular tissue. The energy that is reflected off the tissue (i.e., neither absorbed as heat nor scattered within the tissue) produces the ultrasound image.
The purposes of ophthalmologic ultrasound are to study ocular anatomy and to diagnose pathology of the eye. There are many different types of ophthalmologic ultrasound. They include A-scans, B-scans, 3-D scans, duplex ultrasonography, and ultrasound biomicroscopy.
The A-scan ophthalmologic ultrasound is used to measure the axial length of the eye and the thickness of the lens of the eye. The most common use of an A-scan, along with keratometry, which measures the curvature of the anterior surface of the cornea, is to determine the power of the intraocular lens to be implanted following cataract extraction.
A B-scan ophthalmologic ultrasound gives images of the structures throughout the orbit. The B-scan is used by the ophthalmologist in some intraocular surgeries, such as in placement of a radioactive plaque to treat a retinal tumor, and in the extraction of a foreign body that has penetrated the globe. In cryotherapy, the clinical use of low temperatures, ophthalmologic ultrasound imaging helps guide the probe used to treat retinal tears in the presence of vitreous hemorrhaging. It is also used preoperatively in patients with dense cataracts to rule out pathology of the posterior pole, and to evaluate resorption of vitreous hemorrhages in diabetic retinopathy. B-scan ultrasonography can locate retinal and choroidal detachments and is used to assess drusen, or calcium deposits on the optic nerve and to locate intraocular tumors. The B-scan also can detect changes in structure of the posterior sclera, but because of its limited resolution, anterior scleral pathology is difficult to assess. The new-generation B-scans can assess optic nerve cupping, changes of the optic nerve seen in glaucoma.
Color doppler and duplex ophthalmologic ultrasonography are helpful in the assessment of glaucoma, and in diagnosis of ocular tumors and diseases of the anterior segment. Since they evaluate blood flow and resistance through the intraocular blood vessels, Doppler and duplex ultrasonography can be employed in the diagnosis of a central retinal artery or vein occlusion, and in the diagnosis of temporal arteritis. Temporal arteritis is an inflammation of the temporal artery which can affect vision. Restriction of blood flow through other ocular vessels affected in temporal arteritis can also be observed by duplex ultrasonography.
A 3-D ophthalmologic ultrasound gives the eye care practitioner a 3-D image of the eye, facilitating the diagnosis of a retinal detachment, intraocular tumors, or enlargement of the extraocular muscles. The 3-D ultrasound can be utilized prior to refractive surgery, to assess corneal thickness and irregularities in the corneal surface, and to determine with accuracy the depth of the anterior chamber before implantation of an intraocular lens.
Ultrasound biomicroscopy is employed to assess the normal spatial relationships among anterior segment structures of the eye such as the iris, ciliary processes, and the layers of the cornea. It is also used to assess pathology of the eye and adnexa. Applications of ultrasound biomicroscopy include: calculations of corneal thickness and endothelial cell count, assessment of the cornea after refractive surgery, angle assessment in pupillary block, and elucidation of the causes of glaucoma. Ultrasound biomicroscopy can image the position of implants such as an intraocular lens placed in the eye after cataract surgery, or a filtering bleb, placed intraocularly after glaucoma surgery. It can image tumors of the iris and ciliary body, detect anterior segment abnormalities, and isolate foreign bodies that penetrate the globe. With the higher resolution of ultrasound biomicroscopy, scleral pathology, such as scleritis, an inflammation of the sclera, is detectable.
Telesonography is a method of using ultrasound to diagnose medical conditions from a remote site. Ophthalmologic ultrasound images can be transmitted via the Internet with this technology.
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.
Special care is needed when performing an ophthalmologic ultrasound on a ruptured globe.
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.
The patient should be instructed not to rub the eyes for 20 to 30 minutes after an ophthalmologic ultrasound and warned that vision might be slightly compromised for the same time frame.
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.
The results of ophthalmologic ultrasounds are immediately available to the doctor. Abnormal results indicate an underlying problem and may require further testing and treatment.
Adnexa— Structures outside the orbit of the eye that include the lacrymal glands, the lacrymal ducts, the extraocular muscles and the eyelids.
Angle— Part of the eye through which fluid leaves the eye.
Anterior segment— The front part of the eye, that includes the sclera, the cornea, the tear film, the angle of the eye, the iris, and the ciliary body and its processes.
Cataract— Opacification (clouding) of the lens of the eye that occurs as a result of aging, disease, or trauma.
Choroid— Layer of the eye, rich in blood supply, that is found between the retina and the sclera.
Ciliary body processes— Structures of the eye that form the fluid of the anterior chamber and the vitreous.
Cornea— Transparent tissue on the front of the eye that focuses light into the eye through the pupil.
Extraocular muscles— The six muscles which are used to voluntarily move the eye.
Glaucoma— An ocular disease characterized by loss of visual field and damage to the optic nerve. It is often associated with increased intraocular pressure, but not in all cases.
Intraocular— Within the eyeball.
Lens— Intraocular structure in the eye that focuses light onto the retina.
Ophthalmologist— A medical doctor with residency training in medical and surgical management of eye disease.
Optic nerve— Large nerve in the back of the eye through which visual stimuli leave the orbit, to the occipital lobe where vision is processed.
Optometrist— An eye care doctor specifically trained in all aspects of vision and eye care. Optometrists are licensed in all states to diagnose and treat eye disease.
Orbit— The bony cavity of the skull that holds the eyeball.
Posterior pole— The posterior part of the eye that includes the retina and the vitreous.
Radiologist— A physician trained in radiology, the use of radiant energy, to diagnose and treat diseases.
Retina— The inner part of the eye where the photo-receptors are located.
Sclera— Tough white membrane covering the outer part of the eye, not covered by the cornea. It encircles the inside of the eye and is continuous with the optic nerve.
Vitreous— A nonvascular gelatinous material found behind the posterior capsule of the lens.
Health care team roles
A sonographer, a medical professional trained in sonography, can do an ophthalmologic ultrasound, but in an ophthalmic practice the ultrasound is done by an ophthalmic technician or the doctor. The ultra-sound image is always interpreted by a doctor, such as an ophthalmologist, an optometrist, or a radiologist.
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