Pearl

Another important role of CT is in the detection of tumor-related alterations in bony anatomy, as when os-teolytic or hyperostotic changes have been induced (■ Fig. 20.6).

Advantages of CT as compared with MRI

The advantages of CT scanning, as compared with MRI imaging, are summarized as follows:

■ Shorter examination times (2 to 5 min for CT scanning, 20 to 45 min for an MRI)

■ Facilitated monitoring of rapidly changing findings

■ Better assessment of bone structure

■ Faster and safer detection of intracranial hemorrhages

Sphenoid Wing Meningioma

Fig. 20.6. a Sphenoid wing meningioma: tumor-induced hyperostosis (arrow) of the greater sphenoid wing (bone window), transaxial-plane orientation. b Sphenoid wing meningioma: soft tissue components of the tumor (arrow) in the lateral orbit, producing an exophthalmos (soft tissue window), transaxial-plane orientation.

Fig. 20.6. a Sphenoid wing meningioma: tumor-induced hyperostosis (arrow) of the greater sphenoid wing (bone window), transaxial-plane orientation. b Sphenoid wing meningioma: soft tissue components of the tumor (arrow) in the lateral orbit, producing an exophthalmos (soft tissue window), transaxial-plane orientation.

Most Important Indications for CT Scanning

■ Skull/brain trauma

■ Stroke: differential diagnosis of hemorrhage/infarction

■ Foreign-body detection

■ Space-occupying lesions with bony involvement or soft tissue calcification

Due to its better soft tissue differentiation, elective MRI has increasingly replaced CT scanning of the brain and orbits. Avoidance of radiation exposure has also played a big role in these decisions.

Magnetic Resonance Imaging | Definition

Magnetic resonance imaging is a tomographic process that permits the tomographic imaging in any chosen plane within the body. This is done with the help of a strong magnetic field parallel to the long axis of the patient's body and an additional, freely variable, location-dependent magnetic field (a gradient field). In place of X-rays, the MRI uses nonionizing, radiofre-quency energy.

With few exceptions, MRI today is the method of choice for the elective study of the soft tissue structures of the orbit, the optic nerve, and the intracranial portions of the visual system. In some places, the availability of MRI scanning remains somewhat limited, as compared with the availability of CT scanners. More important, though, are the following contraindications.

Contraindications to MRI Note

Absolute contraindications (patient endangerment):

■ Cardiac pacemakers

■ Incorporated ferromagnetic foreign bodies/ implants

■ Shrapnel wounds

■ Aneurysm clips of uncertain origin

Relative contraindications (no patient endangerment, but with comparatively poorer imaging quality):

■ Cosmetics, such as mascara or eyeliner, with metallic content

■ Nonferromagnetic metal implants, such as the metallic plates used in plastic surgical reconstructions of the face or orbit

■ Cochlear implants (loss of function)

■ Claustrophobia

■ Inadequate patient cooperation

The minimal requirements for patient cooperation are that he/she must be able to remain immobile with no head or eye movement for the time needed to complete one measuring sequence (about 3 min).

Fig. 20.8. a Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: Tjw contrast-enhanced study of a pituitary macroadenoma (superiorarrow) in a transaxial orientation, showing contact between the tumor and the optic chiasm (inferior arrow). b Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: coronal tomographic plane illustrating the relationship of the mass to the distal (supraclinoid) carotid (arrow). (Continuation see next page)

Fig. 20.7. a Tomographic orientation for magnetic resonance imaging (MRI) study of the orbits: parasagittal Trweighted (^w) image with a linear mark parallel to the optic nerve. b Normal optic chiasm with the typical "suspenders' shape" (black arrow), main trunk of the middle cerebral artery (superior white arrow). Posterior cerebral artery (inferior white arrow), transaxial T2w image

Fig. 20.8. a Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: Tjw contrast-enhanced study of a pituitary macroadenoma (superiorarrow) in a transaxial orientation, showing contact between the tumor and the optic chiasm (inferior arrow). b Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: coronal tomographic plane illustrating the relationship of the mass to the distal (supraclinoid) carotid (arrow). (Continuation see next page)

Advantages of MRI

If the prerequisite conditions noted above have been met, MRI scanning offers significant advantages as compared with CT scanning. Chief among them is the improved differentiation between retrobulbar soft tissues (fat, muscle, and optic nerve), between normal components of the brain (gray and white substances) and between differing forms of pathological change (infarction, hemorrhage, inflammation, and neoplasms). Additional advantages of MRI imaging lie in the use of high-frequency radio waves with no ionizing radiation at all, which is particularly important for protecting the ocular lens. Another, perhaps more important, advantage is the freedom with which data can be acquired and displayed, to study whatever tomographic section is desired without having to move the patient (■ Fig. 20.7). Also, the contrast material used in MRI scanning, usually containing a gadolinium-DTPA complex, is better tolerated than the iodinated CT contrast dye, lowering the risk of an allergic reaction substantially. Another important advantage of MRI scanning is the ease with which the sellar region (■ Fig. 20.8), the posterior fossa, and the course of the afferent visual pathways can all be clearly delineated, without the interference of image artifact caused by the dense bony structures, as is frequently encountered during CT scanning of these spaces.

Mri Optic Nerve

Fig. 20.8. (Continued) c Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: sagittal orientation of the tomographic plane to image the connection between the suprasellar tumor and the intrasellar remnants of the pituitary gland. d Illustration of multiplanar tomography in MR scanning: optic nerve sheath meningioma (arrow) as shown in an oblique sagittal tomographic plane (rotated about a vertical axis to an angle of about 23° away from the true sagittal plane) paralleling the course of the optic nerve

Fig. 20.8. (Continued) c Illustration of multiplanar tomography in MR scanning in the case of a pituitary macroadenoma: sagittal orientation of the tomographic plane to image the connection between the suprasellar tumor and the intrasellar remnants of the pituitary gland. d Illustration of multiplanar tomography in MR scanning: optic nerve sheath meningioma (arrow) as shown in an oblique sagittal tomographic plane (rotated about a vertical axis to an angle of about 23° away from the true sagittal plane) paralleling the course of the optic nerve

Indications for MRI

In routine diagnostic settings, a plane that is parallel to that of the optic nerves and with a thickness 2 to 3 mm provides the most useful information (■ Fig. 20.7). If additional study of intracranial contents is desired, an orientation of the plane of interest that is parallel to the line connecting the anterior and posterior commissures of the corpus cal-losum is ideal. A thickness of 3 to 5 mm is best. The total time for the procedure will vary from 20 to 60 min.

The most important advantage of MRI scanning lies in the fact that the signal strength in the image is not determined by measures of radiodensity, as in CT scanning, but is instead determined by tissue-specific parameters, the T1 and T2 relaxation times. This produces a high-resolution image with excellent tissue identification (■ Table 20.1). In the so-called T1-weighted (T1w) images, the cerebrospinal fluid (CSF) appears hypointense (dark), while fat, subacute hemorrhages and gadolinium-containing contrast materials appear hyperintense (bright) (■ Figs. 20.9 a and 20.10 a).

Table 20.1. Characteristic signal intensities seen on magnetic resonance imaging of the brain
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