|Year : 2021 | Volume
| Issue : 1 | Page : 1-4
Imaging of the orbit: A concise review
Alsawi Yusuf Abdulmannan Yusuf1, Kamal Hashim Binnawi2, Howaida Alnour Makki3
1 Department of Radiology, University of Kordofan, ElObeid, North Kordofan, Sudan
2 Department of Ophthalmology, Sudan Eye Center, Khartoum, Sudan
3 Department of Ophthalmology, University of Kordofan, ElObeid, North Kordofan, Sudan
|Date of Submission||12-Sep-2021|
|Date of Decision||09-Sep-2022|
|Date of Acceptance||09-Sep-2022|
|Date of Web Publication||17-Mar-2023|
Dr. Alsawi Yusuf Abdulmannan Yusuf
University of Kordofan, El-Obeid, North Kordofan
Source of Support: None, Conflict of Interest: None
Imaging plays an important role in diagnosing most orbital conditions. Imaging modalities commonly employed in orbital imaging are ultrasound, computed tomography, and magnetic resonance imaging. Conventional radiography plays a small role in imaging the orbits, particularly in rural areas in low-income countries such as Sudan. In this review, we will classify orbital conditions according to the systemic surgical sieve. Ophthalmologists need to understand the role and nature of each imaging modality to use the most appropriate investigation for a particular patient or condition. When requesting an imaging study, the radiologist needs to get full clinical information, including the surgical history, previous investigations, and a well-formulated question regarding what the clinician expects and wants.
Keywords: Computed tomography, magnetic resonance, medical imaging, ultrasound
|How to cite this article:|
Yusuf AY, Binnawi KH, Makki HA. Imaging of the orbit: A concise review. Sudanese J Ophthalmol 2021;13:1-4
| Introduction|| |
The orbit is a complex anatomical structure within the orbital cavity of the skull. It comprises the eyeball, optic nerve and vessels, orbital fat, extraocular muscles, lacrimal apparatus, and surrounding bony orbital walls. For diagnostic imaging purposes, the retro-ocular orbital space is usually divided into intraconal and extraconal spaces, referring to the cone-shaped space surrounded by the extraocular muscles. Imaging modalities commonly employed in orbital imaging are ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI). Conventional radiography plays a small role in imaging the orbits, particularly in rural areas in low-income countries such as Sudan. In this review, we will classify orbital conditions according to the systemic surgical sieve. When requesting an imaging study, the radiologist needs to get full clinical information, including the surgical history, previous investigations, and a well-formulated question regarding what the clinician expects and wants.
| Imaging Modalities|| |
Ultrasonography is an imaging technique that employs high-frequency sound waves beyond the hearing limit of the human ear. Frequencies used in imaging the eye and orbit are usually in the range of 10–20 MHz. The technique does not involve ionizing radiation, hence can be used in pregnant patients safely. US can be used to image soft tissues in real-time, including the imaging of blood flow in vessels utilizing the Doppler effect. However, ossified bones and gas-containing organs reflect most of the sound beam, and consequently, these structures are not usually imaged by US. This technique is relatively affordable, quick, and readily available as a bedside portable machine. Dedicated ophthalmic US usually utilizes A – (Amplitude) as well as B – (brightness) scans. B-scan US is the technique currently used to produce two-dimensional gray-scale images of the orbit. Doppler US helps in the assessment of vascular lesions and mapping the vascularity of mass lesions. US biomicroscopy is a relatively new technique for the detailed imaging of the anterior segment of the eye using very high frequencies (35–100 MHz).
CT is an X-ray-based technique. A rotating X-ray tube is employed to produce a large number of axial images (slices or cuts) recorded by an array of electronic X-ray detectors. Powerful computers use back projection mathematical algorithms to construct an image formed of small picture elements (pixels) corresponding to the X-ray attenuation of the tissue's volume contained in that area (voxel). Modern machines can image a large volume of tissues in one rotation of the X-ray tube, using multiple row detectors to produce multiple thin slices, down to <1 mm thick, known as multidetector CT or multislice CT. The image data obtained from these axial slices can be reformatted using computer software to produce sagittal, coronal, oblique, or three-dimensional surface rendered images. CT images contain a huge amount of information displayed as a limited gray-scale image. Certain setting is used to best view different tissues, known as windows. A window is a range of X-ray attenuation values, selected from the large spectrum (from −2000 to +4000 Hounsfield Units or HU). Hence, in a bone window, bones are best viewed while other tissues are obscured. The orbit is best viewed with a soft-tissue window. Iodinated intravenous contrast media can be injected to enhance certain vascular, neoplastic, and inflammatory lesions. CT is a fast examination, compared with MR, and is the first investigation in the setting of traumatic and nontraumatic emergencies.,
Magnetic resonance imaging
MRI employs strong magnetic fields (0.35–3 Tesla, most commonly 1.5 T) to manipulate the magnetization of hydrogen atoms in different body tissues. It does not involve ionizing radiation. Tissue contrast is obtained by detecting the relaxation times of these tissues in the form of radio frequency signals, known as T1 and T2, usually depending on the hydrogen (water) content of the particular tissue. Various acquisition techniques are used, including those with suppression of the fat signals FS or short-tau inversion recovery, the suppression of water signals fluid-attenuated inversion recovery (FLAIR), measuring water diffusion and the restriction thereof in certain pathologies diffusion-weighted imaging and detecting local magnetic distortion (seen in areas of hemorrhage/blood products susceptibility-weighted imaging. MR is contraindicated in patients with ferromagnetic metallic implants, such as surgical clips and cardiac pacemakers, and cases of suspected metallic foreign bodies.
| Orbital Trauma|| |
Trauma can be blunt or penetrating. Blunt trauma associated with fractures of the bony orbital wall, commonly the blowout of the inferior wall, can be seen in conventional X-rays [Figure 1]a, but is best seen with CT scanning in bone window [Figure 1]b. Although radio-opaque foreign bodies can be demonstrated with plain X-ray or US, CT is the modality of choice. Traumatic retinal detachment as well as vitreous hemorrhage are readily detected on real-time US scans [Figure 1]c.
|Figure 1: (a) x-ray of blowout fracture, Left inferior orbital wall. (b) CT blowout fracture right inferior orbital wall, (c) US of traumatic retinal detachment (Courtesy Dr Ian Bickle Radiopaedia.org rID 84895, Dr Laughlin Dawes Radiopaedia.org rID 35968, Dr G Balachandran Radiopaedia.org rID 5695)|
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Choroid melanoma [Figure 2]a has a characteristic MR appearance with high T1 and low T2 signals due to the presence of melanin. Intraconal lesions include optic nerve glioma [Figure 3]a, associated with neurofibromatosis type I, particularly when bilateral, optic sheath meningioma [Figure 3]b, with classic tram-track enhancing appearance, and metastatic lesions to the optic nerve [Figure 3]c. Contrast-enhanced thin-slice CT is usually the first investigation performed, although MR is more sensitive. Extraconal lesions include lymphoma [Figure 4]c, which is usually secondary non-Hodgkin type, metastatic secondaries from the breast, prostate, melanoma, lung, kidney and pediatric neuroblastoma, and lacrimal gland tumors [Figure 2]b and [Figure 2]c. Both CT and MR with contrast show these lesions, although the final histological diagnosis can only be confirmed with biopsy. Retinoblastoma on US appears as an echogenic soft-tissue mass with areas of calcification, necrosis, and hemorrhage. It appears on CT as a contrast-enhancing calcified mass [Figure 4]a. MR, however, is the modality of choice for diagnosis and staging., [Figure 4]b.
|Figure 2: (a) Contrast enhanced CT (CECT) showing malignant melanoma, (b) CECT of lacrimal carcinoma, (c) CE T1 weighted MR of lacrimall carcinoma. (Courtesy Dr Frank Gaillard Radiopaedia.org rID 4297, Dr Frank Gaillard Radiopaedia.org rID 9480, Dr R Mannava Radiopaedia.org rID 53133)|
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|Figure 3: (a) CT retinoblastoma, (b) MR (T1 with contrast) invasive retinoblastoma, (c) MR of lymphoma (T1 with contrast) (Courtesy Dr Frank Gaillard Radiopaedia.org rID 9764, Dr Ian Bickle Radiopaedia.org rID 42598, Radswiki Radiopaedia.org rID 11696)|
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|Figure 4: (a) Coronal CT of left orbital pseudotumor, (b) CT of thyroid eye disease, (c) CECT of orbital varix. (Courtesy Dr Frank Gaillard Radiopaedia.org rID 4749, Dr Frank Gaillard Radiopaedia.org rID 9365, Dr Frank Gaillard Radiopaedia.org rID 4881)|
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Orbital infections (cellulitis), orbital pseudotumor (idiopathic orbital inflammation) [Figure 5]a, and optic neuritis are the most common inflammatory processes encountered in the orbit. Infection can spread from the paranasal sinuses and can extend into the cranial fossa. CT shows diffuse swelling of soft tissues, gas-fluid level, and peripheral enhancement if abscess is formed. Optic neuritis is mostly due to multiple sclerosis, in which case MR may show demyelinating lesions other than the optic nerve, particularly in FLAIR sequences. Orbital pseudotumor is a diagnosis of exclusion as it has no specific radiological appearance.,
|Figure 5: (a) T2 weighted MR of optic nerve glioma, (b) Sagittal CT showing optic nerve meningioma, (c) Coronal MR of optic nerve metastasis from breast cancer. (Courtesy Dr Amr Farouk Radiopaedia.org rID 38567, Dr Frank Gaillard Radiopaedia.org rID 5560, Dr Chris O'Donnell Radiopaedia.org rID 49521)|
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Vascular lesions may develop within the orbit, including capillary hemangiomas, which are the most common vascular lesion, venous malformations and varices [Figure 5]c, lymphatic malformations and arteriovenous malformations, and fistulae. Contrast-enhanced CT or MR shows the lesions with high sensitivity, and has replaced angiography for diagnostic purposes. However, conventional angiography is the standard for diagnosis combined with endovascular interventional treatment.
| Endocrine Orbitopathy|| |
Thyroid orbitopathy is the most common endocrine eye disease. Radiological signs may precede thyroid hormone abnormalities and may be seen with reduced or elevated thyroid hormone levels. Extraocular muscles are involved in a predictable sequence, starting with the inferior and medial rectus and sparing the tendons and the fat planes [Figure 5]b. Changes are best seen and monitored on nonenhanced CT and best on coronal reformat images.
| Conclusion|| |
Modern imaging is essential in the diagnostic workup of orbital pathology, as well as in management planning and monitoring. US, CT, and MR provide an excellent array of imaging tools. The clinician needs to be aware of the advantages and indications of each modality. CT is the preferred modality for foreign bodies, fractures, or calcifications. MR is the modality of choice for orbital pathologies, with its superior inherent soft-tissue contrast.
Knowledge of the orbital anatomy and the various orbital compartments as they are depicted by imaging is crucial for the interpretation of radiological studies and coming up with a short differential diagnosis.
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Conflicts of interest
There are no conflicts of interest.
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