Ординатура / Офтальмология / Английские материалы / Ultrasonography of the Eye and Orbit 2nd edition_Coleman, Silverman, Lizzi_2006

Figure 2.13. Several mechanical scan modalities have been used. The sector scan is most popular in that it is compact, most amenable to high scan rates, and provides approximate normality to the posterior retinal surface. The linear scan is the most simple conceptually and has the advantage that vectors do not diverge with range. The arc scan is complex in implementation but offers near normality to both the anterior and posterior surfaces of the eye.

B-MODE IMAGE QUALITY

Under ideal conditions, the pixel intensities in B-mode images correspond precisely to the acoustic reflectivity at each tissue point. In practice, these representations are constrained by the limited intensity ranges of display devices, including computer monitors, thermal printers, and video printers. Current digital display devices devote a maximum of 1 byte (8 bits) to each of three colors: red, green, and blue. This allows millions of colors to be displayed at once but only 256 levels of each color individually. Because shades of gray are composed of pixels with equal intensity values of red, green, and blue, only 256 shades of gray are available. This corresponds to a 24-dB dynamic range. Effective dynamic range can be increased by prior logarithmic amplification or other compression modes; however, it is often most expedient to use B-mode images for assessments of general anatomy and to obtain A-mode results along specifically chosen directions for quantitative reflectivity information.

Figure 2.14. High-frequency (50 MHz) scan of anterior segment produced using an arc-scan geometry. This scan geometry maintains near-normality relative to the anterior surfaces of the globe, allowing display of the full corneal contour.

The spatial resolution achievable in a B-mode system is limited both by acoustic constraints (frequency, focal length, aperture, and so forth) and by the pixel resolution of the display device. Let us consider a system with a 200-micron pulse length in which an image of the eye and orbit 5 cm in depth is displayed over 256 pixels in the axial direction. This means that each pixel represents 195 microns, just sufficient to represent the axial resolution to which we are entitled. However, if fewer pixels are used to represent the image, or a greater scan depth is displayed in the same number of pixels, the display resolution will be degraded. This effect is also important in implementation zoom functions in B-scanners. The simplest way to implement a zoom is to double the pixel size. This method, however, provides no actual increase in image detail. If, however, smaller pixels are derived from the stored data, then finer image detail can be achieved.

The resolution inherent in B-mode images can be limited by large pixel sizes, but these are usually not the limiting factors, and resolution is most often governed by the same considerations that determine A-mode resolution. Axial resolution is determined by the duration of the ultrasonic pulse; thus, excessively long pulses will cause apparent thickening of interfaces in the image and prevent detection of closely spaced surfaces.

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Figure 2.15. A wire phantom allows characterization of beam width as a function of range. Note that wire reflections in the region of the focus (dashed line) are less smeared than those in the near or far fields. Also note that even in the focal zone, the wire appears somewhat elongated in the scan direction because the beam width of focused beam is finite even at the focal point.

Lateral resolution is determined by the ultrasonic beam width. Wide beams exaggerate the apparent width of reflective structures in a manner that depends upon the scan pattern being used. To examine this effect, consider a sector scan across an array of long, thin wires aligned normally to the scan axis, as illustrated in Figure 2.15. Wires in the focal zone appear almost as points, because here the beam width reaches its minimum. Anterior or posterior to the focal plane, the apparent width of the wires is exaggerated, and they take on an arc-shaped appearance as a consequence of the sector scan geometry and the broad beam width, which causes detection of the wires across several adjacent vectors.

Figure 2.16. Tissue-mimicking phantoms are widely used to evaluate an ultrasound system's capacity to detect wire targets, reflective and cystic structures embedded in a scattering background. Evaluation of a 10-MHz transducer using a small-parts phantom (Radiation Measurements, Inc., Middleton, Wisconsin) is shown.

Wire targets are one type of “phantom” that can be used to characterize B-mode image quality. Several manufacturers offer ultrasound tissue phantoms suitable for transducers of specific frequency ranges. Although no eye phantoms are offered commercially, small parts phantoms (Figure 2.16) can be useful in determining a system's capacity to visualize cystic and echogenic targets of various sizes and contrast in relation to background.

Just as in A-mode operation, absorption limits the resolution attainable with B-mode systems. High-resolution images of the posterior segment can be obtained at 20 MHz (14), but only a thin layer of the retro-ocular orbit can be penetrated at this high frequency. Deeper orbital penetration requires lower frequencies (i.e., 5 to 10 MHz).

REAL-TIME IMAGING

During scanning, B-mode images are generated at a rate equal to the number of scans per second performed by the probe. Early mechanical sector scanners provided perhaps four scans per second, but modern scanners can offer scans at 30 Hz or higher. This, essentially, offers real-time evaluation of ocular tissues. Real-time imaging has particular value in evaluation of vitreous membranes, retinal detachment, and vitreous hemorrhage. It can also be useful in evaluation of tumors by allowing visualization of vascular pulsatility. Real-time examinations can be captured using cine-loop (where available) or by attaching a video recording device to the analog output of the B-scanner.

B-MODE ARTIFACTS

B-mode images are susceptible to artifacts resulting from ultrasonic and electronic sources. The most commonly

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encountered artifacts are listed in Table 2.1 and are described later.

TABLE 2.1. Types of B-mode Artifacts

Figure 2.18. Dislocated hypermature cataractous lens in an eye with vitreous hemorrhage and total retinal detachment. The lens material is highly acoustically absorptive, resulting in an acoustic shadow trailing from the lens.

Multiple acoustic reflections constitute another source of artifacts, introducing duplication of tissue contours, as shown in Figure 2.19. In this immersion scan, ultrasonic echoes from the cornea and lens implant return to the transducer, where they are partially reflected back toward

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the eye. These echoes are then reflected by the cornea and arrive for a second time at the transducer after the transit time determined by the transducer-cornea separation. The multiply-reflected echoes appear in both A-mode signals and B-mode images, where they usually appear as phantom surfaces within the vitreous or in posterior regions. Recognition of multiple reflections is straightforward: changing the transducer-cornea standoff distance alters the location of the artifacts with relation to the other structures of the eye. These artifacts can be eliminated by making this standoff distance equal to the maximum tissue depth to be examined. Reduplication artifacts also occur in contact scans, although they are less common.

Figure 2.19. Artifactual duplication of cornea (solid arrow) and lens implant (dashed arrow) in central vitreous.

Electronic artifacts can assume several forms. “Snow” can appear on B-mode images, if amplifier gain is high and electronic noise is not rejected prior to display. Saturation can cause heterogeneous structures, such as orbital fat, to appear as uniformly bright areas. Recognition of these artifacts is aided by careful monitoring of A-mode signals.

DIGITAL IMAGE PROCESSING

Digital storage of B-mode images confers great advantages in postprocessing. This digital representation allows application of various digital image processing methods to enhance images. Most instruments include a set of simple operations, such as brightness and contrast adjustment and, possibly, a zoom function. An entire literature exists regarding digital image enhancement (15), and these techniques are readily applied to ultrasound B-mode images stored in a generic format, such as TIFF or JPEG. Examples of relatively useful and straightforward operations include modification of the pixel brightness intensity curve, thresholding, blurring, and median filtering, among others (Figure 2.20). Such operations can be performed with available software, such as Photoshop or NIH-Image.

In addition, a color scale can be substituted (pseudocolor) for the usual gray scale in representing pixel brightness. The use of color provides increased contrast in comparison with gray-scale. Color display is the default display mode in optical coherence tomography, for example. Color display, however, has not achieved widespread acceptance in B-mode ultrasound imaging because color scales are essentially arbitrary and can conceal as much as they reveal, if not used judiciously.

THREE-DIMENSIONAL IMAGING

Three-dimensional (3-D) imaging is made possible by digital storage of images. To form a 3-D image representation,

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an ordered series of B-mode images must be acquired and stored (16, 17, 18). Various 3-D scan geometries are illustrated in Figure 2.21. Conceptually, the simplest way to do this is to move the transducer in a linear fashion to sweep out a rectilinear B-mode image, move the transducer incrementally at right angles to the sweep direction, and repeat. This way, scans are stacked like a pack of cards. There are other 3-D scan modes. For instance, a mechanical sector probe can be moved linearly at right angles to the scan plane, or a sector probe can itself be sectored to sweep out a fan-shaped region. One commercially available 3-D ophthalmic scanner (Ophthalmic Techologies, Inc., Toronto, Ontario) rotates a mechanical sector probe along its axis to sweep out a cone-shaped region. Three-dimensional image data are rendered (Figure 2.22) with special purpose software. In addition to this general methodology, 3-D data can be acquired freehand using a probe in which sensors monitor the probe position and orientation (19). From this information, the position of each pixel in space can be computed and, using appropriate interpolation methods, a 3-D image can be generated. The 3-D renderings can be rotated, translated, zoomed, and sectioned, allowing additional information to be extracted from the data. In addition, 3-D allows quantitative information to be determined, such as surface areas and volumes, that may be useful for following tumors and other volume occupying pathologies. It should be understood, however, that 3-D image reconstruction is still subject to the same principles as 2-D B-mode imaging. For instance, if a structure fails to provide a high amplitude echo as a result of oblique presentation in a single B-mode image,

this will not be improved by taking a series of parallel B-mode slices at the same oblique incidence.

Figure 2.20. Image processing enhancement of digitized B-mode images can be performed using a variety of proprietary and public domain software. This figure shows application of sequential image processing operations to a B-mode image of an eye with orbital mass. Top to bottom: Operations degradation by addition of noise, Gaussian smoothing, thresholding, median filtering.

Figure 2.21. Three-dimensional imaging can be performed using a variety of scanning geometries, including serial rectilinear (left), sequential sector (center), and meridional rotational (right), among others.

Figure 2.22. Examples of rendered 3-D ultrasound images. Upper left: This surface-rendered image was derived from a series of parallel scans of an eggshell fragment resting on the retina. Produced by Silverman and Coleman in the early 1980s, it is, to our knowledge, the first opthalmic 3-D ultrasound image. Upper right: Early wire mesh surface rendering with hidden surface removal of a small choroidal melanoma, with computed values of tumor dimensions. Lower left: Shaded surface rendering of a large choroidal melanoma with secondary retinal detachment. Lower right: Volume rendered image of total retinal detachment. (see color image)

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VERY HIGH FREQUENCY ULTRASOUND/ULTRASOUND BIOMICROSCOPY

The terms ultrasound biomicroscopy (UBM) and very high frequency ultrasound (VHFU) have generally been taken to refer to use of frequencies of 25 MHz or higher. B-mode VHFU images do not differ fundamentally from those generated using conventional 10-MHz transducers, but the application of these high frequencies impacts upon what can be imaged and certain technical aspects of instrument design.

A number of technologic advances in the early 1990s made VHFU possible, including new transducer materials (both polymer and crystalline) and new, reasonably priced, broadband electronics, including high-speed digitizers. Pavlin and Foster (20, 21, 22) described clinical findings with a 50-MHz ultrasound biomicroscope that was later developed into a commercial instrument, the ultrasound biomicroscope, or UBM (Paradigm Instruments, Salt Lake City, Utah). Our laboratory independently developed a series of scan platforms for ophthalmic imaging at 50 MHz that included features, such as 3-D acquisition of radiofrequency data and wide-angle scanning incorporating the entire anterior segment (12,13,16).

Because of the effect of attenuation, VHFU cannot be used for imaging of the posterior segment but can generate superb images of anterior segment anatomy and pathology, such as corneal scars (including effects of refractive surgery), tumors and cysts of the iris and ciliary body, ciliary body detachment, glaucoma syndromes (e.g., pupillary block), and hypotony. At a frequency of 50 MHz, we can expect a fivefold improvement in resolution compared to that of 10-MHz images, with axial and lateral resolutions of about 30 and 60 microns achievable (depending upon specifics of transducer pulse length and focal ratio).

To image the anterior segment, VHFU scans must be performed using an immersion technique, with the attenuating eyelid absent from the acoustic path. This can be accomplished using an eye cup or by forming a water-bath with a disposable surgical drape, usually in combination with a lid speculum. The Artemis-2 system (Ultralink, LLC, St. Petersburg, Florida) uses a disposable eyepiece consisting of a viscoelastic foam ring that forms a seal around the eye. Acting like a reverse swimming goggle, normal saline is introduced into the eyepiece to establish acoustic coupling. This system has the additional advantage of allowing optical visualization of the eye during scanning by use of a coaxial video camera.

In addition to the UBM and the Artemis, other manufacturers have introduced cost-effective VHFU instruments with handheld 35-MHz probes.