Ординатура / Офтальмология / Английские материалы / Imaging of Orbital and Visual Pathway Pathology_Muller-Forell_2005

1.3.2

General Considerations of MRI Imaging Protocols

As MRI is only able to image tissue with mobile protons (see chapter 1.3.1 Physical and technical principles), the trace of mobile protons within pure bone is not sufficient to generate a perceptible MR signal, resulting in a hypointense appearance of compact bone; in other words, it looks black on MR images. Air, surrounding the head and filling the paranasal sinuses, does not contain any protons, and thus also appears black. This is the reason that those interfaces between compact bone and air cannot be displayed on MR, although they are seen on CT. Not only in the cranial region is bone normally surrounded by MR signal-generating structures, it normally contains fatty marrow, well seen on MRI. For example, an indirect definition of the bony contours is made by the mucosa of a paranasal sinus. The domain of MRI is soft tissue; its MR contrast is superior to that of CT. This excellent natural intrinsic contrast of MR, differentiating bone, fat, muscles, nerves, glands, components of the globe, and the intracranial nervous tissue (Table 1.2), is not restricted by artifacts from surrounding bone. Another advantage of MRI is the

capability of real multiplanar high resolution imaging without additional impairment of the patient. To optimize image resolution in orbital MRI, one should use phased-array surface coils (Breslau et al. 1995).

Although most diseases require the same examination protocol, every individual patient suffers from an individual disease, which explains why universally valid “recipes” may be right in one patient and inadequate in another. In general, two different planes (axial and coronal) at least should be done as a minimum; especially in the orbit, a third (or fourth, individually planned) may improve the examination result.

As a rule, in our institutions every MRI examination of the head includes a T2-weighted overview of the entire brain, whether the orbit or the pituitary is the target of interest, as, e.g., in optic nerve neuritis the main underlying disease may be multiple sclerosis. The different axis of orbital imaging can be planned directly according to the individual patient’s anatomy, and thus the external landmarks, known from CT are not necessary. A T1-weighted axial sequence without contrast is essential for orbital MRI examination. The orbital T2-weighted scans should be combined with fat suppression. Alternatively, a

short-tau inversion-recovery (STIR) sequence or a combined fatand water-suppression sequence can be used to prevent the confusing signal elevation of normal fat caused by the so-called J-coupling effect (Henkelman et al. 1992) of the fast (turbo) spinecho sequences. This undesirable high signal of fat in a T2-weighted sequence also covers discrete changes in the water content of the fat as, e.g., in the case of inflammation. The T1-weighted imaging after i.v. administration of gadolinium also has to be combined with a fat signal suppression to avoid the contrast canceling interference of contrast-enhanced structures and the naturally high signal of healthy fat. One possibility for examination of the orbit with MRI is to use a 3D volume acquisition scan and later postscan processing, i.e., multiplanar and curved (along the optic nerves) reformations, and also an equalization of primary nonsymmetrical patient positioning. However, one should realize that sequences used for 3D acquisition mostly have disadvantages, such as reduced internal contrast (fast spin-echo with long echo train) or susceptibility artifacts (gradient-echo sequences).

So-called proton density-weighted sequences (PD), which have a short TE (echo time) and a long TR (time repetition), are more sensitive to internal structural alterations such as demyelinization or edema than T2-weighted images, especially in turbo spin-echo with a long echo train. The T2-weighted turbo, or fast spin-echo sequence, allows an early or first echo to be generated in addition to the late T2weighted echo. This first echo is described as proton density weighted, but it is a mixture of most commonly the first four echoes of a longer echo train and, therefore, should be correctly designated as pseudoecho.Despite disadvantages compared with a conventional proton density spin-echo, this is more sensitive to changes in myelinization or tissue hydration than the T2-weighted images. On the one hand, strong T2weighted sequences are able to display the interfaces of nervous tissue to cerebrospinal fluid (CSF)-filled spaces with high resolution, but on the other hand, a T2-weighted sequence is not suitable for demonstrating small T2-hyperintense lesions bordering on these interfaces. This diagnostic restriction of T2-weighted images is caused by the fact that a hyperintense lesion would merge with the abutting, similarly hyperintense CSF-filled space and therefore can easily be overlooked. The fluid-attenuated inversion recovery (FLAIR) sequence resolves this problem by suppressing the signal of free water and in this way improving the detectability of hyperintense lesions bordering on CSF-filled spaces such as a ventricle or subarach-

noid space. The classic example of this problem is to demonstrate small subependymally located plaques of multiple sclerosis. However, it should be mentioned that the FLAIR sequence does not produce as good spatial resolution as, e.g., a T2-weighted TSE sequence and is liable to produce artifacts (Bakshi et al. 2000).

One of the quality limiting factors in orbital MRI is the acquisition time. The longer this time, the higher the risk of image artifacts caused by eye movements. To image the optic nerves, the scan plane should be orientated along the infraorbitomeatal line (Tamraz 1994). This orientation offers the highest probability of displaying almost the entire optic nerve path in one of the cuts. There is no guarantee, however, because the nerves often follow a slightly undulating course, making an image in a noncurved plane impossible. The slice thickness is best chosen between 2 and 3.5 mm with a minimal interslice gap.Thinner slices usually have a too low signal-to-noise ratio, thus looking less definite. The coronal planes can be adjusted orthogonal to the infraorbitomeatal line, or orthogonal to each long axis of the orbits. In special cases, two different scans are needed for this latter purpose, one for each orbit. Additional sagittal scans precisely along the long axis of each orbit can be performed. One should be aware that in parallel MRI scans along the extension of the optic nerves, a small inaccuracy in the symmetry of the scan plane, relative to the course of the optic nerves, results in a large deviation on the MRI, complicating a correct assessment and comparison of both sides. On the other hand, a smaller inaccuracy, in fact slightly more orthogonally oriented to the course of the nerves, will not cause a strong distortion. For coronal scans, a thickness of about 3 mm is sufficient.

The extension of the chiasm, adjoining prechiasmal optic nerves, and optic tracts are best seen on scans parallel to the plane along the chiasm and the posterior commissure (Tamraz 1994). Of course, coronally oriented scans also need to be performed.

For the region of the optic pathway from the lateral geniculate nucleus to the visual cortex, the scans should be adjusted along the canthomeatal plane,and should be orientated in the coronal and if possibly also in the sagittal direction. For structures embedded in nervous tissue such as the lateral geniculate nucleus and the optic radiation, it is worthwhile trying a high-resolution T1-weighted inversion recovery (IR) sequence that offers an excellent contrast between gray and white matter but needs a longer acquisition time (see Figs. 2.44, 2.47, 2.48, 6.3, 6.9, 6.11, 6.83, 6.179, 7.63).

Ophthalmologic Imaging Methods

For suspected vascular lesions, the protocols described above should be supplemented with MR angiography (see Figs. 6.61, 6.62, 6.186, 6.197, 7.53).

In special cases, diffusion-weighted MRI helps to distinguish reliably between liquor-isointense epidermoid and arachnoid cyst (Laing et al. 1999; Gizewski 2001) (see Fig. 7.65). Diffusion-weighted MRI also indicates an acute infarction very early on, just a few hours after onset (Schaefer 2001) (see Figs. 3.27, 7.73, 7.77, 7.78). Diffusion-weighted MRI might be able to discriminate cystic tumor and abscess (Kim et al 1998).

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CONTENTS

2.3Globe 39

2.4The Extraocular Muscles, Intra-

and Extraconal Space 41

2.4.1Lacrimal Gland 47

2.5Optic Nerve 50

2.5.1Intracanalicular Optic Nerve, Prechiasmal Area 52

2.7.1Optic Tract 54

2.7.2Optic Radiation 55

2.7.3Striate Cortex 56

In this chapter, Gray’s Anatomy (Williams 1995) serves as the basis for most of the anatomical descriptions made; if not, the references are given in full.

W. Wichmann, MD, PhD

Professor, Institute of Neuroradiology and Radiology, Klinik im Park AG, Seestrasse 220, 8027 Zürich, Switzerland

PD W. S. Müller-Forell, MD

Institute of Neuroradiology, Medical School, University of Mainz, Langenbeckstrasse 1, 55101 Mainz, Germany

2.1

Bony Orbit and Optic Canal

(Figs. 2.1c–2.6c, 2.9c–2.13c)

The orbits are bilateral bony cavities composed of multiple bones (frontal, temporal, zygomatic, lacrimal, sphenoidal, ethmoidal, maxillary, and palatinal bones) containing and protecting the eyeballs, extraocular muscles, lacrimal apparatus, fascial sheaths, nerves, ciliary ganglion, vessels, and the fat in which all is embedded. The orbits have a quasi-pyramidal shape with the base opening forward. As all walls converge to the apex, the medial walls are essentially parallel (Casper et al. 1993), although the long axes of the orbit diverge anterolaterally at an angle of about 45°. The orbits directly border on the paranasal sinuses, only separated by bony plates, and thus the osseous walls of the orbits are shared with the sinuses.

The floor of the orbit is also the roof of the maxillary sinus. This bony plate is notched by a small infraorbital groove, which runs from posterior to anterior, becoming a small canal, leading anteriorly to the infraorbital foramen and containing the infraorbital nerve (Figs. 2.12d, 6.167). This is the site where an inferior blow-out fracture can occur.

A very thin bony plate separates the orbit medially from the ethmoid air cells. This translucent paper-thin osseous plate (lamina papyracea) presents another weak point of the orbital walls. Not only can a medial blow-out fracture occur with resulting orbital emphysema, but the walls can be distorted and displaced medially by massively enlarged medial rectus muscles in Grave’s disease (Casper et al. 1993).

The anterior roof of the orbit is the floor of the frontal sinus, while the posterior orbital roof forms a part of the floor of the anterior cranial fossa. Anterolaterally of the superior orbital wall is a shallow groove for the lacrimal gland. The lateral orbital wall is relatively thick, the posteromedial region of the lateral orbital wall directly abuts the middle cranial fossa, and the anterolateral region of the lat-

eral wall adjoins the temporal fossa (Daniels et al. 1995).

The optic canal (or the optic foramen) in the posterior region of the superior orbital plate connects the superomedial corner of the orbital apex to the middle cranial fossa (Figs. 2.4c, 2.8c, 2.9c). The optic canal forms an angle of about 45° with the sagittal plane, carrying the optic nerve and the ophthalmic artery.

The gap between the posterior lateral and superior wall of the orbit is called the superior orbital fissure (Figs. 2.3b,d, 2.9b,d, 2.10b,d). This opening between the small and great wing of the sphenoid bone also communicates with the middle cranial fossa. The superior and the inferior ophthalmic veins, the oculomotor, trochlear, abducent nerves, branches of the ophthalmic division of the trigeminal nerve, and the carotid sympathetic plexus pass through the superior

orbital fissure. Posterior to the slightly widened inferomedial region of the superior orbital fissure lies the cavernous sinus. The superior orbital fissure is separated from the optic canal by a bony bridge, called the optic strut (Rhoton and Natori 1996; Daniels et al. 1995).

The inferior orbital fissure is located in the posterior orbital floor, posteromedially and inferiorly in communication with the pterygopalatine

fossa, anterolaterally with the infratemporal fossa (Figs. 2.2.b,d, 2.8–2.10). Small venous connections from the inferior ophthalmic vein are transmitted through the inferior orbital fissure to the pterygoid plexus, but it also contains branches of the internal maxillary artery, the zygomatic and infraorbital nerves. A thin layer of smooth muscle borders the superior region of the inferior orbital fissure.

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a

c

W. Wichmann and W. Müller-Forell

10.2

9.3

9.3

10.15

10.4

10.4

13.1

13.1

13.11

b

2.5