Contrast Enhancement

The positive enhancers are strongly paramagnetic proton relaxation agents, the most widely used of which is gadolinium. The most common uses of gadolinium are in the detection of the integrity of the blood-brain barrier and defining the extent of abnormal tissue. Because retrobulbar fat also displays bright signal in T1-weighted images, the use of fat suppression sequences is crucial in demonstrating enhanced tissues within the orbit. In the diagnosis of orbital disorders, abnormalities of the blood-brain barrier are rarely of clinical value.18

FIGURE 10.1. Axial images of two young patients presenting with swelling of the medial canthal area. (A,B) Tl-weighted images; both lesions demonstrate the same signal intensity. (C,D) T2-weighted images; both lesions demonstrate the same signal intensity. There-

fore, based only on the precontrast MR characteristics, it is difficult to differentiate the solid (capillary hemangioma, A and C) from the cystic (dermoid cyst, B and D) lesions.

fore, based only on the precontrast MR characteristics, it is difficult to differentiate the solid (capillary hemangioma, A and C) from the cystic (dermoid cyst, B and D) lesions.

Unlike CT examination, which uses iodinated contrast materials, the enhancement of vascular structures in MR imaging is not uniform, and their visualization depends on the velocity of the flow. As a result, one cannot identify contrast MR images simply by looking at vessels.

In orbital lesions, the degree of contrast enhancement within an orbital lesion or abnormal enhancement of optic nerve and optic nerve sheath tumors may guide the differential diagnosis.1-3 Moderate to marked contrast enhancement is usually noticed in solid tumors as well as in acute inflammatory orbital lesions.1-4'19 Minimal contrast enhancement suggests a chronic or sclerosing orbital inflammation, a fibrotic tumor, or posttherapeutic scar tissue.1-4,19 No enhancement is documented in hemorrhagic process, dense scar tissue, collection of fluid, or the necrotic portion of neoplasms.5,20 Linear enhancement surrounding a nonenhancing, well-delineated lesion suggests that the tumor is cystic. Well-defined linear or enhancement signal a void within an enhancing lesion may suggest the presence of air, high blood flow vessels (artery or vein), fragments of cortical bone, or foreign bodies.1,2,4,19

MRI will give the clinician more information from the enhancement pattern of an orbital lesion than can be obtained by means of CT.

How To Reduce Acne Scarring

How To Reduce Acne Scarring

Acne is a name that is famous in its own right, but for all of the wrong reasons. Most teenagers know, and dread, the very word, as it so prevalently wrecks havoc on their faces throughout their adolescent years.

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