Magnetic Resonance Imaging

MRI is also very useful for detecting and evaluating orbital mass lesions because of its excellent tissue resolution.19-21 This technique allows one to generate cross-sectional images of the tissues without using x-rays. It is based on a physical phenomenon called the nuclear magnetic resonance effect on the atomic nuclei, primarily hydrogen atoms of water molecules within human tissues. When an external, static magnetic field is applied to the tissues, the random dis tribution of the atomic dipoles is distorted, and they tend to align in the direction of the field. When the applied radiofrequency pulse is discontinued, the macroscopic, magnetic field returns to its original state by emitting electromagnetic waves with preci-sional frequencies. The waves emitted during the reformation of the magnetic status (relaxation) are measurable and represent contrast values, corresponding to the brightness of the individual pixels, which, in turn, construct the images with the use of mathematical algorithms.

FIGURE 9.3. Bone windows: (A) normal axial CT, (B) normal coronal reconstruction, and (C) normal sagittal reconstruction.

FIGURE 9.4. Orbital mass with focal density. (B) The CT number, shown as 451.29, is consistent with calcification, which is evidence of a phlebolith in a lymphangioma.

There are two types of relaxation: longitudinal (T1) relaxation and transverse (T2) relaxation. During the T1 relaxation, the excess energy is transferred from the nuclei to the environment. The T2 form, which is also termed spin-spin relaxation, represents the decay of the signal vector perpendicular to the strong magnetic field. These signals, which are obtained from biological tissues, depend on the water concentration of tissues, which can be excited. The degree of this excitation depends on the relaxation characteristics of the protons in water molecules of the tissues being examined. The water reveals a high concentration of excitable protons and a slow relaxation. In contrast, protons bound to macromolecules would reveal a fast relaxation. The relaxation times are therefore determined by the composition of the MRI characteristics of different types of tissues. A summary of signal characteristics of orbital tissues listed in Table 9.3 and Figure 9.6 gives MRI nomenclature and parameters, respectively. Orbital MRI examinations are also performed and interpreted according to predetermined neuroradiology protocols (Box 9.3).20 MRI first became widely available in 1986 and gadolinium contrast enhancement in 1988.21

Currently MRI is most often done with a 1.5-tesla unit using either a head coil or a specially designed surface coil. Fast spin-echo sequences greatly reduce scan time over what is possible with traditional spin-echo techniques. Studies should include pre- and postcontrast Tl-weighted axial and coronal images of the orbits extending through the optic chiasm. Fat suppression is done on postcontrast images, to increase lesion conspicuity by removing high-signal fat. Following intravenous administration of contrast agents, such as gadopentate, di-meglumine (Gd-DPTA), a different take-up by the tissues is seen in MRI studies. The axial pre- and postcontrast images should also be interlaced to optimize imaging of the optic nerves (Figure 9.7A-C). This is done with 100% gap or an interlace technique. T2-weighted images should also be obtained routinely through the orbits and optic nerves (Figure 9.7D). Many protocols also include a routine brain scan consisting of precontrast Tl-weighted axial and sagittal images, diffusion axial, T2-weighted axial, FLAIR (fluid attenuation inversion recovery), and postcontrast Tl-weighted axial, sagittal, and coronal images of the brain. MRI is also very sensitive for detection of intra- and extraconal orbital lesions. The optic nerve and chiasm are also very well defined. Optic nerve lesions that are not detected with CT may be well delineated with MRI. MRI offers tissue resolution superior to that available from CT scanning, allowing better detection of

BOX 9.2. Indications for CT and MRI

CT

MRI

Orbitocranial trauma

Detection of orbital masses

Orbitocranial hemorrhage

Evaluation of orbital and ocular masses

Detection of orbital masses

Evaluation of sinuses

Evaluation of orbital bones

Evaluation of optic pathway

Evaluation of sinuses

Orbital changes secondary to ocular tumors

Detection of calcification

Orbitocranial hemorrhage

TABLE 9.2. Advantages and Disadvantages of CT and MRI.

Advantages

Disadvantages

CT Availability and fast examination time Evaluation of bony involvement

MRi Detects virtually all lesions of the orbit except trauma No ionizing radiation

Ionizing radiation Contrast reaction

Beam-hardening and other artifacts Motion and other artifacts Missile and thermal injuries

Incompatible with a number of medical devices and metal implants Longer scanning times

Overweight and claustrophobic patients cannot be accommodated

FIGURE 9.5. CT artifacts. (A) Dental fillings causing streak artifacts. (B) Artifact in the patient shown in (A) is reduced in bone windows. (C) Shotgun pellets causing streak artifacts. (D) Detector artifact. (E,F) Positioning artifacts. (G) Distortion due to patient motion.

TABLE 9.3. Commonly Encountered Signal Types in Orbital MRI.

Signal type

Tissue type

Tl-weighted

T2-weighted

Fat suppression

Globe Fat

Extraocular muscle Optic nerve Cerebrospinal fluid Bone Vessels

Hypo (dark gray) Hyper (white) Hypo (dark gray) Hyper (light gray) Hyper (dark gray) Void (black) Void (black)

Hyper (white) Hypo (white) Hypo (light gray) Hypo (light gray) Hyper (white) Void (black) Void (black)

Hypo (dark gray) Intermediate (gray) Hyper (white) Hyper (light gray) Hypo (dark gray) Void (black) Void (black)

SIgr» 1.5T SYStOCllX*r is tesla magnet Ex: 6159 exam number sei 3 sequence number

1(1 : 3 image number

OA* I0.5+C with iv contrast

Tulane Medical Center

DflTE 11 Feb 03 05:10;43 PH of me Hag = 1.0 lOFUP FL:

f se-xl /9Û fast spin echo c »0 fup angle TR;516 repet10n time 51« msec

TE:20.9/Ef effective echo time zoo msec EC : 1/1 20.8kHz bawd width f se-xl /9Û fast spin echo c »0 fup angle TR;516 repet10n time 51« msec

TE:20.9/Ef effective echo time zoo msec EC : 1/1 20.8kHz bawd width

H?BR AI N hgh fiêsolution brain

FOV: 16x16 field of view

3,0tHk/0 ,0sp/C TMR£e mm fhkk suces c no spacing gap <le loo* gap) 16 /03 ; 23 number and time of suces per aooisition

256X192/2 HEX matrw see & number of examinations window level

StF/NP/VB/TRF/Z512 extra parameters fat sat etc] W = 28 L =

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