and retrobulbar edema are the main Þndings in posterior scleritis (Fig. 3.27) [118Ð121]. Occasionally, retinal or choroidal detachment also may be detected (Fig. 3.28). The combination of both A scan and B scan techniques simultaneously produces the most useful results in distinguishing posterior scleritis from orbital, choroidal, and retinal diseases, which clinically may mimic posterior scleritis [119].

A-Scan Ultrasonography

The A scan technique shows one-dimensional timeÐamplitude representations of echoes received along the beam path. The echoes appear as vertical deßections rising from a horizontal zero line. In the axial A echogram of the normal eye, high echoes are produced by the two corneal surfaces, two lens surfaces, and vitreoretinal interface. The vitreoretinal interface echo is followed by a complex of echoes representing retina, choroid, sclera, and retrobulbar fat. Posterior scleral thickening due to inßammation can be detected by high-amplitude continuous spikes.

Fig. 3.29 Computrized tomogram demonstrating opaciÞcation of the left maxillary sinus in this patient with WegenerÕs granulomatosis

B-Scan Ultrasonography

The B scan technique combines transducer scanning and electronic processing to produce two-dimensional cross-section images of the eye along any desired scan plane. The echoes are presented as dots instead of spikes. In the axial B echogram of the normal eye, echoes are produced by the anterior and posterior surface of the cornea separated by a sonolucent interval representing the corneal stroma, by the anterior surface of the iris, by the posterior surface of the iris merging with the anterior lens surface, and by the vitreoretinal interface. In the normal eye, echoes from the retina cannot be separated from echoes from the choroid and the sclera. Posterior scleral thickening due to inßammation can be detected by multiple reßections of the sound beam. The echoes remain after sound beam attenuation, indicating high internal reßectivity caused by multiple, relatively ßat interfaces. If retrobulbar edema surrounds the optic nerve, the ÒTÓ sign, which consists of squaring off of the normally rounded optic

nerve shadow with extension of the edema along the adjacent sclera, is seen.

High-Frequency Ultrasound Biomicroscopy

High-frequency (50Ð100 MHz) ultrasound biomicroscopy (UBM) produces high-resolution subsurface images of the eye at microscopic resolution which are four to six times greater than that of conventional ultrasound. UBM provides information on structural changes in the anterior episclera and sclera, as well as the underlying choroid, retina, and vitreous in the anterior part of the posterior segment.

3.2.8.2 Optical Coherence Tomography

Polarization-sensitive optical coherence tomography (central wavelength: 1,310 nm; A-line rate: 20 kHz) evaluates the three-dimensional structure of the anterior eye segment with the phase retardation associated with the anterior segment birefringence of the eyes. In normal sclera, a striking polarization change is observed

Fig. 3.30 Magnetic resonance imaging (MRI) scan demonstrating retrobulbar Òbright zonesÓ characteristic of inßammatory pseudotumor

in the cumulative-phase retardation images, whereas in necrotizing scleritis phase retardation is low, implying diffuse destruction of the collagen tissue.

3.2.8.3 Computer Tomography Scanning

Routine ocular computerized tomography scans consist of multiple axial ÒcutsÓ at different levels of the orbit. Eye, orbital walls, extraocular muscles, and paranasal sinuses are, therefore, sectioned longitudinally in the horizontal plane [122]. The ability of the X-ray CT to delineate small, soft tissues of different densities makes it a useful diagnostic tool to detect extraocular muscle or lacrimal gland enlargement, sinus tissue involvement (Fig. 3.29), or posterior scleral thickening [119, 123], which are important features for the differential diagnosis of posterior scleritis from orbital inßammatory diseases and orbital neoplasms [123Ð125]. Radiopaque medium may be injected during the scan to enhance the detection of scleral thickening. The primary disadvantages of CT scanning are poor contrast between some soft tissues,

Fig. 3.31 Magnetic resonance imaging scan in a patient ultimately shown to have systemic lupus erythematosus. Note the T2 bright spots indicative of microinfarcts compatible with a microangiitis

radiation hazard (orbital CT scan: 2Ð3 rads, depending on the slice thickness and the number of cuts made; this is similar to an orbital series of skull X-rays), and lack of scanning in the sagittal plane.

3.2.8.4 Magnetic Resonance Imaging

The most frequent atomic nucleus in living tissue is the hydrogen nucleus. The hydrogen nucleus is composed of a single proton, which has a positive electrical charge and spins on its axis. A magnetic Þeld is generated around this rotating electrical charge. The magnetic resonance imaging (MRI) technique is based on two different tissue properties: the density of hydrogen nuclei present in the tissue and the spin-relaxation rates [122]. MRI provides superior soft tissue contrast compared to CT scanning.

Differentiation of localized inßammatory pseudotumor from posterior scleritis in patients with proptosis (Fig. 3.30) or choroidal tumors from posterior scleritis in patients with a subreti-