MRI in Ophthalmic practice ✍️✍️✍️✍️✍️✍️
✍️ Tissue exposed to a short electromagnetic pulse with rearrangement of its hydrogen nuclei.
✍️ When the pulse subsides, the nuclei return to their normal resting state and reradiating some energy they have absorbed.
✍️ Sensitive receivers pick up this electromagnetic echo.
✍️ T1 and T2 times are two complex parameters that depend on proton density, tissue components, and their magnetic properties
✍️ T1 relaxation is measured using a time constant called T1 in millisecond And defined as the time when 63% of the longitudinal magnetization has recovered
✍️ T2 relaxation is measured using a time constant called T2 in millisecond and defined as the time when 63% of the transverse magnetization has decayed
✍️ T1-weighted imaging
• differentiate anatomical structures mainly on the basis of T1 values
• Tissues with high fat content (white matter) appear bright and compartments filled with water (CSF) appears dark. This is good for demonstrating anatomy.
✍️ T2-weighted imaging
• differentiate anatomical structures mainly on the basis of T2 values
• Compartments filled with water (CSF ) appear bright and tissues with high fat content (white matter) appear dark.
• This is good for demonstrating pathology since most (not all) lesions are associated with an increase in water content.
✅ Indications of MRI in Ophthalmic practice
✍️ Orbital masses or tumours.
✍️ Optic nerve tumours such as glioma or meningioma.
✍️ Intracranial extension of orbital tumours.
✍️ Suspected compressive optic neuropathy.
✍️ In retrobulbar neuritis, the presence of multiple white matter plaques is predictive of the development of clinical multiple sclerosis (MS).
✍️ Suspected lesions of the chiasm such as pituitary tumours.
✍️ Intracranial aneurysms.
✅ Method of MRI in Ophthalmic practice
✍️ Conventional sequences are T1 and T2 weighted.
✍️ Protocols are determined by the examining radiologist, based on the clinical situation.
✍️ Orbital imaging uses specialized fat suppression techniques, which is useful for optic nerve visualization, usually masked by the high signals from orbital fat.
✍️ Diffusion weighted, or diffusion tensor, MRI sequences (image Brownian motion within tissues) were initially found to be useful in acute strokes.
✍️ More recently, the technique has been shown to be useful for intracranial abscesses and distinguishing an epidermoid from an arachnoid cyst.
✍️ IV paramagnetic gadolinium is used as contrast
✍️ Gadolinium-enhanced scans are useful in the detection of blood–brain barrier abnormalities, inflammatory changes, and increased vascularity.
✍️ In tumour staging around the skull base and orbits, CT and MRI are often complementary.
✅ Interpretation of MRI in Ophthalmic practice
✍️ Always review your own scans in conjunction with the radiology team.
✍️ It is also important to consider the quality of the scan ( adequate slices, appropriate use of contrast or processing especially when unexpectedly normal
✅ MRA
✍️ MRA is a non-invasive method of imaging the intra and extracranial carotid and vertebrobasilar circulations.
✍️ The principle of the computerized image construction is based on the haemodynamic properties of flowing blood, rather than on vessel anatomy.
✍️ Demonstrates abnormalities such as stenosis, occlusion, AVMs, and aneurysms.
✍️ MRA is usually a static evaluation however, time-resolved MRA can be useful, as it highlights the separate arterial and venous supplies to an intracranial AVM.
✍️ Disadvantages
• Cannot detect aneurysms <5mm in diameter
• long acquisition time
• suboptimal detection of intravascular calcifications.
