body, an area of abnormally high activity suggests a fastgrowing malignancy (Fig. 9.23). After treatment of the tumor, the PET scan is useful to show if the lesion has become metabolically inactive or is still consuming glu-

9cose, thereby indicating continued activity [21]. PET can also provide images of blood flow or other biochemical functions, depending on the type of molecule that is radioactively tagged. Newer technologies are emerging that will enhance the value of PET scanning in the future [19].

PET scanning so far has shown more limited value for orbital lesions because of the high signal from the adjacent brain and the relatively low resolution of about 7 mm [29, 39].

Single-photon emission computed tomography (SPECT) is a technique similar to PET. However, the

Summary for the Clinician

■PET scanning is a modality that images tissues based on the concentration of specific atomic nuclei. Fluoride-labeled glucose is the most commonly used tracer and is concentrated in tissue with high glycolytic activity, such as tumors.

■The radioactive tracer is selectively taken up by certain tissues and, in the case of glucose, is concentrated in areas of high metabolic activity.

■Tissues with high metabolic activity image with a bright signal and can localize regions with suspected tumors.

Fig. 9.23 PET scan of a patient with a left orbital malignant melanoma (arrow) extending from an intraocular choroidal primary

radioactive substances used in SPECT are different, such as xenon-133, technetium-99, and iodine-123. These have longer decay times than those used in PET and emit single instead of double gamma rays. SPECT is better in providing information about blood flow and the distribution of radioactive substances in the body.

9.7Orbital Ultrasound

Orbital ultrasound (echography) has been used for over four decades to augment the clinical evaluation of patients with suspected orbital disease. Ophthalmic ultrasound was first introduced as a diagnostic tool by Mundt and Hughes in 1956. Beginning in the 1960s, Coleman and Bronson popularized the use of B-scan in ophthalmology, and around the same time Ossoinig developed the standardized A-scan instrument for the evaluation of intraocular and orbital disease [35]. These methods of ophthalmic ultrasound offer specific and comprehensive examination techniques for the detection, differentiation, and measurement of orbital and periorbital lesions [7–10, 33].

9.7.1Physics and Instrumentation

Ultrasound is the oscillation of particles at frequencies greater than 20 kHz (20,000 cycles/s). In ophthalmic ultrasound, frequencies generally range from 8 to 10 MHz (1 MHz = 1 million cycles/s). These relatively high frequencies provide short wavelengths that are necessary for the resolution of small orbital structures.

The velocity at which ultrasound travels is determined by the physical properties of the media through which it passes. Ultrasound instruments make distance measurements by taking into consideration the velocity of sound in specific media and the time it takes the sound waves to reach a given interface and then return to the probe. Short pulses of sound are emitted from a probe placed on the eye or lids. When the sound beam reaches an acoustic interface between two different tissues, an echo is produced that returns to the probe. Echoes are produced mainly through the phenomenon of scattering or reflection [10]. Scattering occurs at the surfaces of very small acoustic interfaces, such as clumps of tumor cells. Reflection occurs at the surfaces of large acoustic interfaces, like connective tissue septae and large blood vessels. The returned echoes are processed in the instrument for display as either an A-scan or B-scan echogram (Fig. 9.24). The one-dimensional standardized A-scan utilizes a small probe that emits a stationary, nonfocused sound beam at a frequency of 8MHz. The two-dimensional

B-scan employs a separate, larger probe that emits an oscillating, focused sound beam at a frequency in the range of 10 MHz.

Once an orbital mass is detected, the special examination techniques of topographic, quantitative, and kinetic echography are employed for differentiation (Table 9.5). These techniques incorporate the use of B-scan and A-scan as appropriate to ascertain a variety of acoustic data about the lesion.

9.7.1.1Topographic Echography

B-scan is the primary modality used to evaluate the topographic features of a lesion (location, shape, and extension) and to facilitate 3D thinking. The sound beam is directed through (transocular) or around the eye (paraocular) as appropriate, depending on the location of the lesion. Transocular approaches (transverse, longitudinal, and axial) are employed to display lesions behind the globe, whereas anterior lesions are better imaged with a paraocular approach [10]. The topographic examination serves to display a lesion in relationship to the globe and orbital bone as well as to the extraocular muscles or the optic nerve (Figs. 9.25–9.28).

9.7.1.2Quantitative Echography

Quantitative echography is employed to evaluate the strength of a lesion’s internal echoes (internal structure, internal reflectivity, and sound attenuation). These characteristics correlate with histopathologic features (e.g.,

the size and distribution of cell aggregates, the presence of connective tissue septae, large blood vessels, etc.) [10]. The sound beam incidence must be perpendicular to the lesion’s anterior and posterior surfaces. It is primarily carried out with A-scan using the tissue sensitivity gain setting. The amplitude of a lesion’s internal echoes is compared to the vitreous baseline (0% amplitude) and the peaks of the initial echo (100% amplitude). As an example, a cavernous hemangioma shows high reflectivity compared to a lymphangioma or glioma, which generallyshowlowreflectivity.(Figs.9.25and9.26,respectively). In Graves orbitopathy, the separation of muscle fascicles yields a highly reflective irregular pattern (Fig. 9.27).

The internal structure of a lesion is classified as either regular (similar texture) or irregular (dissimilar texture). This is done by observing the degree of uniformity in the echoes. Similar internal spike amplitude usually indicates homogeneous texture by histopathology. Conversely, irregular internal structure suggests heterogeneous texture by histopathology. Lesions with regular internal structure are further analyzed for their level of internal reflectivity, which refers to the strength of echoes; these correlate with the fine histologic texture of the lesion. The internal reflectivity is generally classified as low, 0–40%; medium, 40–60%; or high, 80–100%.

9.7.1.3Kinetic Echography

Kinetic echography is the dynamic assessment of motion (consistency and internal vascularity) and is one of the primary advantages of ultrasound in the evaluation of