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Chapter 2: Surgical Anatomy :
Orbit and extraocular muscle imaging
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Two widely used techniques for imaging the orbit and the extraocular muscles are computed tomography (CT) and magnetic resonance imaging (MRI) (Figure 59). CT utilizes high resolution x-rays recorded in fine cuts. CT is superior to MRI for detecting calcium within a lesion or bony changes such as with an orbital fracture and in identifying early invasion or modeling from an adjacent lesion. Iodine-containing contrast material can aid the diagnostic capability of CT but possible side effects could be serious.
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B | Figure 59
A Axial CT
B Coronal CT
MRI provides high soft tissue contrast along with high spatial resolution achievable in multiple planes giving excellent anatomic detail of soft tissues. The MRI utilizes a strong magnetic field to align a portion of the nuclear spin that is ordinarily random. Using the aligned proton as a target, the field is exposed to pulsed energy that momentarily allows the proton to resume its usual position only to become realigned at the end of the pulse. The energy given off during realignment enables the signal which in turn produces the image. The pulse sequences are called T1 and T2. Fluid is dark on T1 weighted images, and fat is bright. Fat is darker and less distinct on T2 weighted images (Figure 60). Various techniques are available to suppress the fat with T1 images thus providing a better image of some orbital contents.
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B | Figure 60
A Normal T1 weighted coronal MRI
B Abnormal T1 weighted coronal MRI showing enlarged inferior recti (dark area).
For viewing the extraocular muscles, T1 weighted images of dark muscles are satisfactory. The views may be coronal, axial, or saggital (Figure 61). Thyroid myopathy with enlarged muscle bellies is seen well in a coronal view but can also be appreciated in the axial and saggital views. Enlarged muscles can also be seen in pseudo tumor which involves all of the muscle and tendon in contrast to thyroid myopathy that spares the tendon. Myositis is also readily observed as muscle enlargement. These subtle diagnostic differences can be detected on a complete MRI study.
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C | Figure 61
A Normal T2 weighted axial MRI
B T1 weighted axial MRI with diminished signal from fat. Note the enlarged muscle bellies.
C Normal T1 weighted saggital MRI
While coronal and saggital views are effective for finding muscle belly enlargement, the axial view is best for identifying a slipped or ‘lost’ muscle. Because of the anatomic characteristics of the rectus muscles, the medial rectus is the only muscle likely to undergo sufficient slippage to present a clinical challenge in finding the muscle.
Recent description of the muscle pulleys, particularly those related to the horizontal recti, has prompted Demer and associates to stress the value of imaging for the diagnosis and treatment planning of a variety of strabismus entities. These include: incomitant strabismus, ‘A’ and ‘V’ patterns, Brown syndrome, ‘heavy eye’ in high myopia, and others.
The strabismus surgeon now employs imaging on a selective basis. Some deterrents to routine use of imaging include: high cost, inability to use in young children, lack of need in many cases, and, of course, habit. It is likely, in the future, that techniques will improve and costs will come down making imaging a more frequently-utilized tool for strabismus management. Ultrasound in the A or B scan mode can be employed as an alternative method for orbital and extraocular muscle imaging. This technique is office based and less expensive but is more difficult to interpret.
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