Ultrasound is an indispensable tool in medical imaging and plays an essential role in ophthalmologic diagnoses. It is the most critical imaging technique in eyes with opaque media. This chapter discusses the basic techniques of ultrasound examination and the technique of ultrasound biomicroscopy (UBM), which uses higher-frequency ultrasound to produce images of much higher resolution at or near the anterior chamber.
General considerations and conventional ultrasound diagnoses
Mechanical waves and vibrations occur over a wide range of frequencies called the acoustic spectrum. This spectrum extends from the human audible range (20 Hz–20,000 Hz), with which we are all familiar, to the range of phonons (>1012 Hz) that comprise the vibrational states of matter. Ultrasonic waves exhibit frequencies above 20 kHz, which are inaudible.
The frequency most commonly used in ocular imaging is 10 MHz ( Fig. 43.1A ), which can provide an axial resolution of 100 μm. Recently, a 20 MHz probe has been introduced ( Fig. 43.1B ) allowing better detection of details at the posterior pole and orbit. Higher-frequency ultrasound using 50 MHz probe is now used to examine the anterior segment providing an axial resolution of 50 μm ( Fig. 43.1C ); however, the penalty to be paid is loss of penetration. All human tissues exhibit ultrasound attenuation coefficients that increase with frequency. The maximum penetration that can be achieved with a 10 MHz system is approximately 50 mm. For a 50 MHz system, penetration is only 5 mm.
Electrical impulses are converted to sound by a vibrating crystal (transducer). These sound waves are propagated through the tissues at various speeds and are reflected or scattered from interfaces between tissues of different acoustic impedance (a property related to the tissue density and the speed at which sound passes through it). After emitting a pulse, the transducer “waits” for the reflected waves (echoes) to return, strike the quartz crystal, and initiate the reverse process. The electrical impulses thus produced are electronically amplified and modified to produce the familiar A-scan and B-scan displays. The intensity of echoes displayed on the screen can be adjusting by the gain setting.
Two common types of ultrasound displays are used: the A-scan and the B-scan. The A-scan is a one-dimensional image in which are presented as vertical spikes from a baseline. The longer an impulse takes to return, the farther it is placed on the display. Time can be converted to distance if one knows the speed of sound in the tissue through which the sound is traveling. Each type of tissue has a characteristic speed at which the sound travels through it. The height of the spike on the graph relates to the intensity of the returned echo, which is proportional to the density of the tissue.
There are three types of A-scan used in ophthalmic ultrasound; biometric A-scan, standardized A-scan, and vector A-scan ( Fig. 43.2 ). Biometric A-scan is most commonly used for axial eye length measurement. Cataract extractions are preceded by an A-scan examination to determine the axial length of the globe (i.e., the distance between the corneal surface and the retinal surface). This measurement is used along with a measurement of corneal curvature (keratometry) to determine the power of lens implant required to produce a desired postoperative refraction. Various formulas have been developed that use these two measurements to produce accurate postoperative results. The measurements are entered into a computer, often part of the ultrasound equipment itself.
Standardized A-scan uses a determined standard decibel setting, also termed tissue sensitivity and is used in tissue differentiation, particularly in diagnosis of tumors.
Vector A-scan is generated along a vector superimposed over a B-scan image.
The B-scan is produced by a moving transducer. At each point along the path of the transducer movement, a pulse is sent out and received. The intensity of returning sound represented on the screen as brightness instead of height on a A-scan graph. This series of lines produces a two-dimensional cross-sectional representation of the object being imaged ( Fig. 43.3 ). This type of display is more straightforward to interpret than an A-scan and is used for most diagnostic work, such as determining the state of the retina behind an opaque cataract or for intraocular tumors imaging.
Technique of examination
The most commonly used B-scan technique involves a handheld contact probe. The moving transducer is covered by a membrane to prevent contact with the globe. A methylcellulose-based gel, used as a coupling agent, is applied to the probe face. The probe is placed opposite to the area being examined. The examination can be done with open lids, in which the probe can be placed on the globe itself, or the examination can be done through the lids. The use of contact B-scan is considered analogous to the use of an ophthalmoscope. The B-scan display represents a two-dimensional cross-section of the globe. In a manner much like ophthalmoscopy, the ultrasound beam can be swept through the eye, making correlations along the way and stopping the fields on specific areas or structure of particular interest.
The B-scan probe always has a mark near the tip that indicates the top of the screen display.
The examination can be done on the open eye or through closed lids. The latter is more convenient for the patient and allows a somewhat greater range of probe movement. The lens produces considerable sound absorption and should be avoided by placing the probe face in the limbal area. By convention, the probe mark is oriented superiorly for transverse vertical scans ( Fig. 43.4 ) and nasally for transverse horizontal scans. It is best to hold the probe horizontally or vertically unless there is some reason for an oblique position.
The optic nerve image is located first. It provides a landmark that aids in determining the position of any pathologic condition. A routine examination of the posterior segment involves placing the probe at the nasal, temporal, superior, and inferior limbus and shifting the probe toward the fornix at each of these positions, thus producing a circumferential or transverse section through the globe quadrants. If any pathologic condition is detected, the probe is placed diametrically opposite to the lesion, with the mark oriented toward the center of the cornea ( Fig. 43.5 ), thus producing a radial section of the specific meridian. The shape, location, and other B-scan features of the abnormality are then delineated. Having the patient move the eye while the examiner holds the probe steady produces valuable information on the mobility of intraocular structures. An A-scan through the area of greatest interest complement the B-scan examination to further define internal tissue characteristics and provide more accurate measurements.
Pathologic conditions in the anterior segment of the eye cannot be well defined by the contact technique.
Anterior segment is better evaluated with a high-frequency transducer, however, a water-bath technique using the 10 MHz probe ( Fig. 43.6 ) can be a valuable alternative when higher frequency probes are not available. A water bath allows one to hold the probe farther from the eye and significantly improves resolution in this region. A miniature water-bath can be produced by using a biometry eyecup placed between the lids and filled to the brim with 1% methylcellulose. The probe is placed on the surface of the biometry eyecup in the methylcellulose, and imaging is performed. This technique provides a clear image of the anterior segment, and can detect lesions, such as ciliary body tumors ( Fig. 43.7 ).
It is possible to obtain useful diagnostic information from the biometry unit. This ability depends largely on the sophistication of the A-scan display. Machines with no display should be avoided because, in addition to the lack of diagnostic capabilities, it is impossible to monitor the accuracy of axial length readings. An ultrasound equipment with a classic A-scan display provides the most helpful information for diagnostics.
The most common problem encountered in routine cataract work is the patient with an opaque lens precluding a view of the fundus. While performing biometry on these patients, one should watch for any abnormal echoes between the lens echo and the echo from the retina. Artifacts can occur, but the presence of any persistent echo in this region should alert the examiner for the need for further assessment before surgery. A B-scan examination is indicated for any eye in which the posterior pole cannot be visualized.
Some typical ocular problems that can be diagnosed on B-scan examinations are discussed in the following paragraphs. It is important to remember that ultrasound is a nonspecific examination technique that can be used on any problem within the penetration range of the instrument.
Frequently, a major diagnostic question in an eye that we cannot see into is whether the retina is detached. The typical B-scan appearance of a total retinal detachment is that of a funnel-shaped, highly reflective, continuous membrane that inserts into the optic disc ( Fig. 43.8 ). In case of localized retinal detachment, the retina may not extend to the optic disc.
Choroidal detachments have a typical appearance on B-scan ultrasound, shown as a thick, smooth, hyperreflective membrane. Choroidal detachment presents minimal or no aftermovement on kinetic examination. Because the choroid is tethered to the sclera at the exit of the vortex veins, large choroidal detachments appear as smooth lobular elevations that insert sharply in the posterior segment at a short distance from the optic nerve. Choroidal detachment and suprachoroidal hemorrhage represent two distinct entities. Choroidal detachment, also termed choroidal effusion, describe an abnormal collection of exudative fluid that expands the suprachoroidal space, which appears anechoic on B-scan ( Fig. 43.9 ). Suprachoroidal hemorrhage is defined as blood within the suprachoroidal space, represented by typical opacities on B-scan ( Fig. 43.10 ).
Ultrasound is an indispensable tool for the diagnosis and follow-up of intraocular tumors. Differential diagnosis is performed by reference to the shape of the tumor and the pattern of intratumor reflectivity, which can vary depending on the internal structure of the tumor.
Choroidal melanoma is the most common type of primary intraocular tumor. They are generally dome-shaped or collar button (mushroom)-shaped ( Fig. 43.11 ) and have low to medium internal reflectivity. The collar button shape occurs when the tumor breaks through the Bruch’s membrane, which is a dense barrier at the surface of the choroid.
The typical low to medium internal reflectivity of choroidal melanomas permits the echographic differentiation of this type of tumor from other choroidal lesions, such as choroidal nevus ( Fig. 43.12 ) and choroidal hemangioma ( Fig. 43.13 ), which exhibit high internal reflectivity.