Chapter 9

Core Messages

■Radiologic imaging is an important adjunct to the evaluation of any orbital disease and will contribute to establishing a likely diagnosis.

■Orbital imaging should not replace a careful physical examination to establish a differential diagnosis.

■Each imaging modality will contribute redundant data, but each also can provide unique information that may not be apparent with other imaging techniques.

■Computerized tomography (CT) utilizes X-rays to create a two-dimensional image in any plane; this is a uniparametric modality based only on tissue transparency to the passage of X-rays.

■Magnetic resonance imaging (MRI) is a multiparametric modality that utilizes atomic charac-

teristics of tissue protons and their behavior in an external magnetic field; the image therefore reflects biochemical differences between tissues based on the molecular environment in which the proton is situated.

■Positron emission tomography (PET) is a newer technique that images tissues based on biological activity, most specifically the metabolism of fluoridated glucose in actively metabolizing tissues, such as tumors.

■Orbital ultrasound (echography) can provide nonradiologic but complementary examination techniques for the detection, differentiation, and measurement of orbital and periorbital lesions.

Radiographic examination is an important component in the evaluation of any patient with suspected orbital disease. Such studies contribute to narrowing the differential diagnosis and often provide guidance in planning the most appropriate medical therapy or surgical approach. CT scanning and MRI have largely replaced older techniques, although specialized studies may still be necessary to define certain lesions. Newer technologies, such as PET, are adding to our repertoire of useful modalities. All of the available imaging techniques may provide some redundant information, but they each also provide some unique information not seen with other modalities. Orbital imaging should therefore never be used as a replacement for a careful and complete clinical examination and the creation of an initial differential diagnosis. This is then used to decide the most appropriate imaging studies that will confirm or rule out suspected lesions.

9.1Computerized Tomography

Computed tomography (CT) is an imaging technique that relies on the differential passage of X-rays through tissues, but unlike standard X-ray studies, CT can image soft tissues in addition to bone. Scans can be reconstructed in any plane through the body and contrast adjusted to maximize visualization of specific tissues. CT is the imaging modality of choice for showing details of bony structures or the location of foreign bodies but is less useful for differentiating details of the optic nerve or small lesions in the orbital apex. For these, MRI is superior.

CT utilizes an array of thin, collimated X-ray beams that pass through tissue along pathways of a complex intersecting matrix (Fig. 9.1). The cross-sectional area defined by any two intersecting beams is referred to as a pixel and is analogous to a single dot in a newspaper photograph. Because the X-ray beam has a certain thickness,

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Fig. 9.1 Simplified diagrammatic representation of computed tomographic scanning matrix. As X-rays pass through tissues, the beam is attenuated by reflection and absorption so that the exiting beam is weaker than the entering beam. The width and thickness of the intersecting beams define the size of the pixel and voxel, which in turn define the image resolution

the area of beam intersection defines a volumetric space, referred to as the voxel. The smaller the pixel size and the thinner the tissue slice are, the smaller will be the volume of the voxel and therefore the higher the resolution of the final image. As the X-ray beams traverse the body, they are weakened or attenuated according to the density of the tissues through which they pass. The degree of attenuation of each intersecting beam emerging from a volume of tissue allows calculation of the average attenuation

value for all the tissues included within the area of intersection of the beams, which is the voxel. This mean attenuation assigned to each voxel is proportional to the density of the tissues with respect to the passage of X-rays.

Attenuation values are designated in Hounsfield units, a 2,000-unit scale ranging from −1,000 to +1,000. By convention, the density of air is assigned a value of −1,000, the density of water is 0, and the density of bone is +1,000. The CT image contrast is based on these attenuation values, and the final CT image is seen in variations of gray scale. Tissues with low attenuation and therefore low tissue density (e.g., air) allow more X-rays to pass through and appear black or dark on the final image. Areas of high attenuation, and therefore high tissue density (e.g., bone), block the X-rays and appear white or lighter on the final image. Each tissue type in the orbit usually exhibits a characteristic density on CT (Table 9.1) and pathologic lesions may also show consistent density and homogeneity changes (Table 9.2).

For visualization by the human eye, this 2,000-unit scale is collapsed to 64 levels of gray between black and white. Because of this, tissues of different but similar densities may not be distinguishable on standard CT studies. For more specific anatomic detail, the CT image may be manipulated by setting “windows.” The window level refers to the Hounsfield unit on which a narrow range of units is centered. The window range is the inclusive number of Hounsfield units above and below this level that are expanded into the black-to-white scale for final imaging. Soft tissue windows are used to image normal anatomic

structures such as the eye, muscles, and optic nerve, but details of bone are not seen. Bone window settings give excellent visualization of bony detail, but soft tissue structures fade to low-contrast shades of gray (Fig. 9.2).

Iodinated intravenous contrast agents are frequently used to improve contrast by increasing the Hounsfield value of blood vessels or highly vascularized tissues. Such agents may help outline normal anatomy and can more clearly define pathologic processes compared with noncontrasted scans (Fig. 9.3a, b).

Early scanners were slow with poor resolution. Moderngeneration CT scanners utilize a spiral or helical technique with multiple detectors or a detector system that rotates continuously around the patient. This allows a continuous series of thin-section, high-resolution images that scan a

volume of tissue rather than individual slices. The data are reformatted automatically to display images as axial slices. Additional reconstructed images can be produced readily in the coronal, sagittal, and oblique planes [20, 23, 25]. Spiral scanning has several advantages. The scan time is much shorter than in conventional CT. Better resolution is achieved in all planes because more closely spaced scans can be obtained. CT angiography is also possible. The multislice CT scanner is an advanced spiral scanner that employs up to eight rows of detectors. This allows much faster data acquisition and larger scanned volumes.

For most orbital studies, a standard CT scan should include images in both the axial and coronal planes. Axial images allow the simultaneous view of both orbits, the ethmoid sinuses, the middle cranial fossa, and the

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Fig. 9.2 Bone window CT scan of a patient with fibrous dysplasia showing fine details in bony structures involving the sphenoid and ethmoid bones on the left side

temporal fossae. Coronal scanning has proved to be invaluable in evaluating the orbit and skull base. These views give better definition of structures oriented parallel to the axial plane, such as the orbital floor and roof. It also allows more accurate size comparison of structures such as the optic nerve and extraocular muscles. Both views are usually necessary to properly localize any pathology within the various orbital anatomic compartments and to characterize their relationship with other structures [3]. For evaluation of the cavernous sinus, optic canals, and

intraorbital optic nerves, thin 1.5-mm or overlapping 3-mm sections may be useful, but there is a certain sacrifice of low contrast and increased background noise. If bone erosion or remolding is suspected or for the detection of calcification, bone window images should be obtained.

Unless contraindicated because of iodine allergy, a contrast series should be included in all orbital scans. Only the rare orbit, such as a posttraumatic one, can adequately be studied with a noncontrasted study alone.

Summary for the Clinician

■CT utilizes the passage of X-rays through tissues as the basis for contrast differentiation.

■Tissues that are similar in their ability to transmit or block X-rays will appear similar on the final CT image and therefore may not be anatomically distinguishable.

■The attenuation values calculated for each voxel are compressed to only 64 gray levels so that nearly similar tissues will show identical imaging characteristics.

■Window settings are used to expand small segments of the Hounsfield scale so that tissues can be more readily distinguished.

■The clinician should use the clinically derived differential diagnosis to help in ordering the most appropriate type of scan and window settings.