Orbital tissue characteristics on MRI of major structures in orbit

Aqueous, vitreous and CSF: Hypointense (dark black) on T1 and hyper intense (bright white) on T2.

Lens: Hyper intense in relation to vitreous (grey) on T1 and low intense (grey) on T2.

Scelera, choroid and retina: All the coats appearing as one signal, intermediate signal (grey) on T1 and hypointense (dark) on T2.

Extraocular muscles: Intermediate intense (grey) on T1 and T2 but enhance with contrast on T1.

Optic nerve: Signal has similar intensity as cerebral white matter (grey) on T1 and T2. Normal optic nerve does not enhance with contrast.

Optic nerve sheath: Contains CSF, hence hypointense (dark) on T1 and hyperintense (bright white) on T2.

Orbital fat: Hyperintense (bright white) on T1 and intermediate (grey) on T2 and becomes dark on fat suppressed images.

Lacrimal gland: Intermediate signals as grey matter (grey) on T1 and T2 and gives mottled appearance of reduced intensity in orbital fat in lacrimal gland area. It enhances with contrast [Figure 12].

Cortical bone: Not well delineated as it contains little free water and appears dark (signal void) in T1 and T2. Hence, orbital apex and intracanalicular portion of optic nerve can be better visualized as compared to that of CT.

Bone marrow: Intense signal on T1 and gray on T2 due to high fat contents.

MRI versus CT in Orbit imaging

MRI: There is no exposure of ionizing radiation, it provides excellent soft tissue resolution of orbital content and is ideal for entire course of optic nerve and pituitary gland as cortical bone appears as hypo intense dark area as there is no water content in bone. Therefore, the entire optic nerve is imaged without any bony shadow interference. MRI can identify age of hematoma. Bone marrow lesions have better resolution. MRI uses safer contrast agent.

CT: It is relatively economical and ideal for boney pathologies, fractures and calcification in tumor mass. It is especially indicated in intraocular foreign bodies. It is ideal for unstable patients as the test duration is very short.

Disadvantages

MRI: It is costly, prolonged examination, not suitable for unstable patients who cannot lie still for long time. It is not possible for obese and claustrophobic patients. Test duration is long. Fat saturation can cause hindrance in interpretation. MRI is contraindicated with ferromagnetic intraocular foreign body. Air and bone, both gives picture of signal void hence difficult to interpret and not suitable for bony lesions and fractures.

CT: Exposure of ionizing radiation and lesser sensitive than MRI in imaging intracanalicular and intracranial optic nerve, cavernous sinus and diseases of white matter such as disseminated sclerosis. Also, the bony orbital wall can sometimes interfere with the interpretation of soft tissue orbital content.

What are the risks of an MRI?

Most sources agree, if done correctly, there are no known dangers to an MRI scan itself. The test is not painful and you cannot feel it. Since radiation is not used, the procedure can be repeated without problems. However, there is a small theoretical risk to a fetus in the first 12 weeks of a woman’s pregnancy, and therefore scans are not performed during this time.

The magnet used in an MRI is extremely powerful but there are no known harmful effects from it. The magnet may affect pacemakers, artificial limbs, and other medical devices that contain iron - even stopping a watch that is close to it. So, patients who have any type of metallic materials within their body must notify their physician or the MRI staff prior to the examination. Patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should not have MRI scanning.

Metallic chips, materials, surgical clips, or any foreign material like artificial joints, metallic bone plates, or prosthetic devices, etc. can significantly distort the MRI images. Metal parts in the eyes can damage the retina. An X-ray of the eyes may be done before the MRI and if metal is found, the MRI will not be done.

Iron pigments in tattoos or tattooed eyeliner can cause skin or eye irritation.

A person who is very overweight or obese may not fit into standard MRI machines.

An MRI can cause a burn with some medication patches. Be sure to tell your doctor or MRI staff if you are wearing a patch.

There is a slight risk of an allergic reaction if contrast material is used during the MRI as well as a slight risk of infection. But most reactions are mild and can be treated using medicine.

Diffusion Spectrum Imaging

Diffusion spectrum imaging may be described as the reference standard of diffusion imaging because it is the practical implementation of the principles derived earlier and is the diffusion imaging technique that has a sound basis in physical theory (12). Suitable for in vivo application, it provides a sufficiently dense q-space signal sample from which to derive a displacement distribution with the use of the Fourier transform. The technique was first described by Wedeen et al (13).

If established practice is followed, 515 diffusion-weighted images are acquired successively, each corresponding to a different q vector, that are placed on a cubic lattice within a sphere with a radius of five lattice units. The lattice units correspond to different b (or q) values, from b = 0 (which corresponds to the centerpoint of the sphere) to, typically, b = 12,000 sec/mm2 (which is a very high b value). The Fourier transform is computed over the q-space data. If the imaging matrix size is 128 × 128 × 30, the same number of Fourier transform operations will be necessary as the diffusion probability density function is computed for every brain location.

Traditionally, 515 images were considered necessary to obtain data of good quality, although the acquisition of that number of images is very time consuming. With improvements in MR imaging hardware and techniques in recent years, and in view of additional very recent experience, fewer sampling points seem to be necessary; the probability density function can be reconstructed with approximately 257 or even 129 images by sampling only one hemisphere in q-space. Of course, the signal-to-noise ratio and angular resolution may change accordingly. The time for imaging of both brain hemispheres thus can be reduced from approximately 45–60 minutes to as little as 10–20 minutes, an acquisition time that makes the technique feasible in a clinical setting (14).

With the application of the Fourier transform over q-space in every brain position, a 6D image of both position and displacement is obtained. Diffusion at each position is described by the displacement distribution or the probability density function, which provides a detailed description of diffusion and excellent resolution of the highly complex fiber organization, including fiber crossings. Since diffusion spectrum imaging is mostly used for fiber tractography, in which only directional information is needed, the probability density function is normally reduced to an orientation distribution function by summing the probabilities of diffusion in each direction

Diagram shows how an orientation distribution function (ODF) is computed and represented. Left: Image of a section through a schematized 3D displacement distribution. The value of the orientation distribution function was computed along two axes (yellow lines). Center: Histograms represent the displacement distribution along the two axes. The value of the orientation distribution function along those axes equals the area under the curve for each axis. In this example, the two areas under the curve are respectively small and large, indicating that there is much less diffusion in the one direction than in the other. Right: The sum of the areas under the curve is represented by a deformed sphere in which the lengths of the two radii (yellow lines) are short and long, corresponding to little diffusion and much diffusion, respectively. To compute the orientation distribution function, the area under the curve is computed for every direction.