|Year : 2013 | Volume
| Issue : 1 | Page : 29-35
Interpretation of magnetic resonance imaging of orbit: Simplified for ophthalmologists (Part I)
Barun Kumar Nayak, Savari Desai, Shailendra Maheshwari
Department of Ophthalmology, P. D. Hinduja Hospital and Medical Research Center, Mumbai, India
|Date of Submission||16-Dec-2012|
|Date of Acceptance||17-Dec-2012|
|Date of Web Publication||22-Jan-2013|
Barun Kumar Nayak
Department of Ophthalmology, P.D. Hinduja Hospital and Medical Research Center, Mumbai- 400 016
Source of Support: None, Conflict of Interest: None
Magnetic resonance imaging (MRI) has become an indispensable diagnostic tool in the field of radiology. Due to better soft tissue contrast resolution and no ionizing radiation, it has become the modality of choice in most cases of orbital / ocular pathologies. Ophthalmologists tend to neglect interpreting orbital MRI, and rely mainly on the reports provided by the radiologists. This article deals with the basics of MRI and the appearance of normal orbital structures on MRI. This knowledge will help ophthalmologists interpret the MRI orbital pathologies which will be published in the subsequent issue.
Keywords: Magnetic resonance imaging (MRI), MRI orbit, T1 weighted image, T2 weighted image, MRI with contrast, fat suppression
|How to cite this article:|
Nayak BK, Desai S, Maheshwari S. Interpretation of magnetic resonance imaging of orbit: Simplified for ophthalmologists (Part I). J Clin Ophthalmol Res 2013;1:29-35
|How to cite this URL:|
Nayak BK, Desai S, Maheshwari S. Interpretation of magnetic resonance imaging of orbit: Simplified for ophthalmologists (Part I). J Clin Ophthalmol Res [serial online] 2013 [cited 2020 Jul 11];1:29-35. Available from: http://www.jcor.in/text.asp?2013/1/1/29/106283
Magnetic Resonance Imaging (MRI) is an indispensible diagnostic tool in the field of radiology. In the initial days, MRI was known as Nuclear Magnetic Resonance (NMR). Although the name has changed, the basic principles remain the same. General advances in techniques of MRI during recent years have had a positive effect on MRI of the eye and orbit.  An increasing number of clinical questions that were formally in the domain of computed tomography (CT) are now examined with MRI.  Lack of ionizing radiation exposure and a higher soft tissue contrast have made MRI the modality of choice in demonstrating ocular and orbital anatomy and pathology. Recent technical developments, including orbital surface coils, fat suppression techniques, fast gradient-echo pulse sequences, and MR contrast agents, allow this noninvasive modality to provide excellent spatial and contrast resolution of the orbital soft tissues with direct multiplanar imaging allowing us to study the lesion itself and the effect it has on the surrounding structures. 
As ophthalmologists, we have an astute understanding of ocular and orbital anatomy. However, when it comes to combining the knowledge we possess with the ability to interpret a MRI films, we end up relying entirely on the radiologist's report due to our lack of knowledge of the basics of MRI.
The purpose of this article is to provide essential knowledge to the ophthalmologists, which is necessary for the interpretation of MRI of orbit. It will be published in two parts. The first part, which appears in this issue, will deal with the basics of MRI, its operating parameters, technical terminologies used in its interpretation and normal appearance of orbital MRI. The second part, appearing in subsequent issue, will deal with the MRI pictures of common orbital lesions.
The principle of MRI ,,
Magnetic Resonance Imaging is based on the magnetic resonance properties of nuclear particles (specifically hydrogen) and its interaction with both a large external magnetic field and radiofrequency waves to produce highly detailed images of the human body.
Our body mass is made up of atoms. The nucleus of an atom consists of two particles, protons (positive charge) and neutrons (neutral charge). Orbiting the nucleus are electrons (negative charge) [Figure 1]. Each of these particles spins on its own axis. Due to the electric charge and spin of each of these particles, certain nuclei exhibit magnetic properties in the form of a local nuclear magnetic field and behave like a bar magnet.
|Figure 1: Structure of an atom. Nucleus contains protons (+ve charge), Neutrons (no charge), Electrons (-ve charge). Orbit around nucleus. Also all particles spin around the axes|
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All biological tissues are mainly composed of Carbon, Oxygen Hydrogen, and Nitrogen atoms. Hydrogen is the most abundant atom in the biological tissue, the majority of which is in water. Hydrogen nucleus contains a single proton [Figure 2], and has the largest and most significant magnetic field (magnetic moment). For these reasons, Hydrogen proton forms the basis of MRI.
|Figure 2: Hydrogen atom has single proton and behaves like a dipole magnet|
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In a natural state, the magnetic moments of the hydrogen nuclei in the body are randomly oriented [Figure 3]. When these nuclei are placed in a strong static magnetic field, such as that produced by an MRI unit, majority align in a parallel (low energy state) and the remaining few in an anti parallel (high energy state) direction in relation to the magnetic field [Figure 4]a. Subsequently, radio frequency (RF) wave is applied to this steady state, the nuclei absorb energy and some of the atoms change from the parallel to anti-parallel direction [Figure 4]b,[Figure 5] and [Figure 6] and they are said to be in a state of "excitation". When the RF wave is turned off, the hydrogen nuclei gradually return to the pre-excited state by giving energy to the environment, the process is known as "relaxation". The relaxing energy is captured by the receiver RF coil. Computer then localizes, quantifies, and transforms into diagnostic images based on the degree, rate, direction and pattern of emission of energy by relaxing protons.
|Figure 3: Hydrogen atoms are in abundance in tissues and are arranged in haphazard manner, hence there is no resultant magnetic affect|
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|Figure 4: (a) When the strong magnetic fi eld (A) is applied in MRI console, the hydrogen atoms get aligned along the main MRI magnetic field (A): Majority are parallel (low energy state) and the remaining few are anti parallel (high energy state). (b) When RF waves are applied to this stable state, some of hydrogen atoms shift from low energy state to high energy state|
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|Figure 5: T1 Axial (Please note the hypointense (dark) signal from the vitreous, also note the gray matter is gray and white matter is white in the brain tissue|
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|Figure 6: T2 axial fat suppressed: note the hyperintense (bright white) signal from vitreous and also note how in brain tissue grey matter is white and white matter is grey|
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The images are dependent upon the proton density and relaxation dynamics which are different for different tissues based on their physical and chemical properties. As previously mentioned, hydrogen exists throughout the body in water, which in turn comprises up to 70% of body weight. Hydrogen is also present in fat and most other tissues in the body. The energy emitted by relaxing hydrogen atoms has a wide variable pattern based on the presence of hydrogen atoms in different tissues. For example, the hydrogen attached to carbon in fat and to oxygen in water results in difference in the relaxation pattern which helps in the tissue identification. It is also dependent upon the character of tissue such as viscosity, temperature, concentration and molecular environment. This forms the basis of MRI.
Now we will be describing certain terminologies and concepts whose understanding is necessary for interpretation of MRI of orbit by ophthalmologists. 
The Tesla unit: The tesla is a unit of magnetic field strength. One tesla is approximately 16,000 times greater in magnitude than the earth's magnetic field. Usually the MRI machines have the magnetic field strength between 0.5 and 3 tesla. As a general rule with increasing tesla strength of MRI machine, the image quality increases and the time taken to perform the imaging decreases.
Gradient coils, RF coils, and Surface coils: Various gradient coils are located in the main body of the MRI unit and are utilized in obtaining MRI images in different planes (axial, coronal, sagittal, and oblique). RF coils are also located within the gradient coils to transmit and receive RF signals. To obtain a better image quality of orbital MRI, additional surface orbital coil or head coil may be used.  These coils have the advantage of giving better spatial resolution with thinner slices due to improved signal to noise ratio (SNR), however, their main disadvantage is of limited depth of view.
Spatial resolution: This refers to the discrimination between the two points on the MRI images. Spatial resolution improves by reducing the field of view, slice thickness and pixel size, some of them may affect adversely the SNR.
Contrast resolution: It is the relative difference between the two adjoining anatomical structures on MRI images which may be normal or pathological. The rate and characteristics of relaxation is different from different tissues, which result in a good contrast resolution leading to identification of the various tissues. Contrast resolution is affected by certain intrinsic properties as well as extrinsic parameters. Intrinsic properties are fixed and cannot be altered but their proper knowledge can be used for tissue identification, whereas, extrinsic parameters can be altered to improve the contrast resolution. White areas on MRI are called hypertense and dark areas are called hypointense with the different shades of gray scale in between depending upon the relaxation characteristics of atoms. We will be describing the intrinsic and extrinsic parameters in short to give a clear understanding.
- T1 relaxation time: This is also known as spin-lattice relaxation time wherein the energized nuclei give energy to environment and attain the equilibrium of pre-excitation phase (low- energy state). The contrast depends upon the tissue characteristics as well as the environment around it. T1 relaxation time ranges from few hundred milliseconds to few seconds. Tissues with shorter T1 relaxation time will appear whiter than tissues with longer T1 relaxation time.
- T2 relaxation time: This is also known as spin-spin relaxation time wherein the decrease in signal from the excited nuclei is due to transfer of energy to adjacent unexcited nuclei. This occurs due to loss of phase coherence of the excited nuclei after sometime when the 90° RF pulse brings them initially into phase so that the magnetic moment is directed transversely to the main magnetic field. T2 relaxation time ranges from few milliseconds to few hundred milliseconds. Tissues with longer T2 relaxation time will appear brighter than that of shorter T2 relaxation time. With little alteration in the technical parameters another time constant relaxation (T2*) images can be taken, the detailed description is out of scope for this article. MRI images can be T1 weighted images or T2 weighted images. In general, anatomical details can be depicted very well in T1 weighted images whereas T2 weighted images are useful for identifying pathological conditions. T1 weighted images are identified by hypotense vitreous in eye, whereas grey matter appears as grey and white matter appears as white in brain. T2 images will show very intense white signal from vitreous whereas grey matter appears as white and white matter appears as grey in brain. CSF as well as any fluid will have same features as vitreous on T1 and T2. T2 relaxation time will always be shorter than T1 relaxation time for any given tissue.
- Proton density weighted images: This is not useful for orbital MRI. This is rarely used these days.
- Flow of blood in blood vessels: Normally, lumen of the blood vessels appears dark as the excited nuclei of blood are replaced by unexcited blood by the time RF signals are recorded in a particular tissue slice.
MRI versus CT
- Pulse sequence: The contrast can be manipulated by altering the power, amplitude, and frequency of successive RF pulses. Various pulse sequences have been used by various companies but recently fast sequence echo (FSE) Pulse is used most commonly as it reduces the imaging time considerably and useful for orbital imaging. Basic principle remains the same but different vendors have given it different names (SPGR, FLASH, GRASS, FISP, MPGR, etc).
- Fat suppression techniques: Orbit contains lot of fat and water, each of them resonates slightly differently, thereby, causing disturbance of spatial resolution and loss of anatomical details. Further the hyper intense signals from abundant fat in orbit obscure the signals from the other structures. Various techniques have been developed to suppress the fat signal to improve the visualization of other structures (normal as well as pathological). Some of them are STIR, CHESS, Dixon, and chopper methods.
- FLAIR (Fluid attenuation inversion recovery): This suppresses the signal from fluid. It is useful in studying the demyelinating conditions on T2 images wherein bright signal from CSF is suppressed for better visualization of pathological area.
- Contrast agents: Certain pathological conditions can show enhancement after injection of contrast agent which helps in their identification. Gadolinium diethylenetriamine pentaacetic acid (Gd- DTPA) is the paramagnetic agent approved by FDA for this purpose and is quite safe. It does not cross intact blood brain barrier. It causes shortening of T1 relaxing time on T1 weighted images; hence, it is always done on T1 weighted images with fat suppression. In normal tissue, the extra ocular muscles show enhancement, which provides a clue that it is post contrast image without reading the written details on the film.
- Diffusion weighted images (DWI): Basically this is used in the study of area with acute infarction. Not used for orbital images.
- MR angiography (MRA) and MR venography (MRV): By using certain techniques MRA and MRV is done to study the pathologies related to arteries or veins.
- Surface coils: By using different external orbital coils (small and large) or brain coils, orbital study can be done more precisely.
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.
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.
MRI - indication and contraindications
CT scan imaging has no absolute contraindications and can be done for all clinical case scenarios. However, a small amount of radiation is delivered with each CT scan to the patient, which may be considered as a relative contraindication, especially if it has to be repeated many times. With the advent of MRI, the patient is prevented from being exposed to the radiation dosage even though it may be small and insignificant.
- Palpable Orbital Mass
- Optic nerve lesions - Optic nerve glioma and meningioma
- Visual loss that is clinically compatible with an intracranial pre-or postchiasmal lesion
- Orbital Cellulitis/Abscess
- Intraocular lesions
- Eyelid/Ocular surface lesions
- Sino orbital Lesions
Before MRI is performed, the patient must remove all metallic objects present on them such as necklaces, rings, watches. Eye makeup should be removed as it often contains ferrous elements.
- Ocular, Orbital, Intracranial Metallic Foreign Body
- Cardiac Pacemakers or any device which generates electromagnetic field for
- Magnetic Intracranial aneurysm clips (older models)
- Cochlear implants
- Metallic Orbital Floor implants
Note: MRI safety for the fetus during pregnancy is not known, and hence, is said to be used with caution.
- Orbital floor implants (titanium mesh)
- Eyelid Gold Weight Implants
- Trauma cases with known fractures
- Presence of Calcification or Bony changes in suspected lesion
- Obese Patients (difficult to perform)
- Claustrophobic Patients (difficult to perform)
Choice of MRI or CT in orbital lesions 
Proptosis: For most of the non-vascular lesions, pseudotumor, orbital cellulitis, thyroid eye diseases orbital cysticercosis, MRI is modality of choice. However, certain facts must be kept in mind if we suspect calcification in lesion, bony erosion or hyperostosis then CT is a better option, but MRI will be a better option if we suspect extension of lesion in orbital apex, optic canal or intracranial extension. For vascular lesions such as arterio- venous malformations, orbital varix and carotico- cavernous fistulas, MRI and MRI with contrast are better options.
Retinoblastoma: MRI is the better choice. However, it will not pick up calcification.
Papilledema and optic neuritis: MRI is the better option.
Isolated 3 rd nerve plasy: MRI with MRA to detect posterior communicating artery aneurysm, but sometimes MRA can miss aneurysm less than 5 mm in size.
Multiple cranial nerve palsy: MRI is better choice. Sometimes T2* weighted images are required to study the intracranial course of the nerves.
Unexplained vision loss: MRI.
Optic atrophy: MRI.
Trauma: Bony fracture can be better depicted by CT but soft tissue damage can be better assessed by MRI. CT is indicated for intraocular and intra orbital foreign body.
Ferromagnetic foreign body is absolute contraindication for MRI; however, MRI is better for wooden foreign body.
Orbital structures are studied with the slice thickness of 3 or 4 mm and the inter slice distance of 0.5 to 1.0 mm. Sometimes we may have to go for thinner slices depending upon the type of the lesions to be studied. Please remember by reducing the slice thickness the test duration as well as the price goes up. Usually, intracranial cuts are 3 to 5 mm with inter slice distance of 0.5 to 2.5 mm.
Interpretation of orbital MRI: Following points must be kept in mind while reading the orbital MRI.
- T1 weighted images depict anatomical details.
- T2 weighted images are mainly used for pathologies.
- T1 weighted images are recognized by hypotense vitreous and CSF. Grey matter of brain will be hypointense as compared to that of white matter (Grey will be grey and white will be white).
- T2 weighted images are recognized by very bright (hyperintense) signal from vitreous and CSF. In brain, grey will be white and white will be grey.
- Axial scan [Figure 7]: It can be recognized by presence of both orbits in the scan with brain tissue behind. It has anterior end (upper side of film), posterior end (lower side of film), right side (viewer's left side), and left side (viewer's right side).
- Coronal scan [Figure 8], [Figure 9] and [Figure 10]: It can be recognized by presence of both orbits in the scan with brain tissue above. It has superior end (upper side of film), inferior end (lower side film), right side (viewer's left side), and left side (viewer's right side). In the anterior sagittal cuts, the eye ball will be in full size, but as we move backwards the eyeball size will go on reducing, and later, we will get no eyeball cuts but orbital apex structures and optic nerve with its sheath around.
- Sagittal scan [Figure 11]: It can be recognized by presence of only one orbit at a time with brain tissue above and behind. It has superior end (upper side of film), inferior end (lower side of film), anterior end (viewer's left side), and posterior end (viewer's right side).
- Contrast studies [Figure 12] are done on T1 weighted images with fat suppression. Contrast images can be recognized by hyperintense signals from extraocular muscles as compared to that of without contrast on T1 images. Mucous membrane lining of sinuses are black on T1 but will be white on T1 with contrast.
- Fat suppressed images [Figure 12]: This can be recognized by suppression of hyperintense signals from orbital fat and subcutaneous fat.
- T2 FLAIR : Grey and white matter of brain will be like T2 images but the CSF will be dark.
|Figure 10: T2 coronal near orbital apex: Note the optic nerve moderately intense signal from optic nerve surrounded by hypertintense ring due to presence of CSF (cerebro spinal fl uid) in the meninges|
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|Figure 12: T1 axial with contrast with fat suppression (the lateral rectus and the medial rectus both becoming hyperintense due to presence of contrast however, the optic nerve no. 3 not showing enhancement with contrast. Also note complete blockage of white signal from the orbital fat|
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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.
The technical details are always given on the various scans and should also be looked at. Please note some of the important structures in the orbital MRI [Figure 7],[Figure 8],[Figure 9],[Figure 10],[Figure 11] and [Figure 12]. Further details are not possible in this communication, and interested readers are advised to go through the references for full understanding and recognition of normal anatomical structures by looking at the labeled diagrams in various cuts. ,,, Various pathological conditions will be discussed in the subsequent issue of this journal.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]