A practice capsule for ophthalmologists
✍️the most commonly obtained ancillary test in neuro-ophthalmology.
✍️provides excellent contrast resolution between soft tissues
✍️multiplanar imaging can be done without repositioning the patient.
✍️Various sequences are obtained that allow visualization of different tissues and the appearance of the tissues varies based on the sequence used.
✅ Magnetic Resonance Imaging( T1-weighted images )
✍️provide good anatomical details.
✍️Fat looks bright (hyperintense)
✍️Vitreous and CSF look black (hypointense)
✍️Subacute blood appears white (hyperintense).
✍️Brain gray matter is gray, and brain white matter is lighter.
✍️Most cerebral parenchymal lesions appear dark (hypointense), but they may appear white (hyperintense) in 2 conditions
👉If gadolinium contrast is given
👉if there is a breakdown in the blood–brain barrier
✍️The T1-weighted midline sagittal view
👉the first sequence systematically obtained on MRI
👉very helpful in neuro-ophthalmology
👉provides information on
• cerebellar tonsillar herniation (Chiari malformation)
• the pituitary gland
• the chiasm
• the superior sagittal venous sinus
✍️Regular T1 sequence does not allow good evaluation of the orbits due to
👉the orbits are filled with fat (hyper intense)
👉administration of contrast results in enhancement of normal extraocular muscles and abnormal intraorbital structures( brighter ) therefore cannot be distinguished from the white orbital fat on a regular T1 sequence.
✍️A T1 sequence with fat suppression transforms the bright signal of the fat into a black signal and allows for very good orbital studies before and after contrast administration
✍️The orbits are best studied with a combination of axial and coronal views. Thin cuts are necessary.
✍️Orbital MRI should include
👉Axial T1 sequence
👉Axial T1 sequence with fat suppression
👉Axial T1 sequence with fat suppression after injection of contrast
👉Coronal T1 sequence with fat suppression
👉Coronal T1 sequence with fat suppression after injection of contrast
🌘Magnetic Resonance Imaging ( T2-weighted images )
✍️ideal for screening for brain parenchymal abnormalities.
✍️On T2- weighted images
👉fat looks darker (hypointense)
👉Vitreous and CSF look bright (hyperintense)
👉Brain gray matter is lighter than brain white matter.
👉most cerebral parenchymal lesions appear bright (hyperintense), especially inflammatory or ischemic lesions.
✍️a regular T2 sequence shows
☝️the ventricles, and the subarachnoid space as very bright ( the ventricles are filled with CSF) and not allowing for good evaluation of periventricular lesions, such as the white matter lesions from multiple sclerosis.
🌘fluid-attenuated inversion recovery (FLAIR)
✍️allows transformation of the bright CSF signal into black signal, while maintaining the other characteristics of a T2-weighted image.
✍️FLAIR images are therefore nothing but black CSF-T2-weighted images
🌘Magnetic Resonance Imaging Gradient echo (Specific T2 sequences )
✍️allow better visualization of blood products, such as hemosiderin.
✍️These sequences are particularly helpful when evaluating patients with
👉cerebral vascular malformations
🌘Magnetic Resonance Imaging ( Diffusion-weighted images )
✍️ideal in detecting acute cerebral ischemia.
✍️They show restricted diffusion within hours of acute cerebral ischemia (while CT and other MRI sequences are often still normal)
✍️should be obtained in all patients with acute neurologic deficits or when cerebral ischemia is suspected
✍️The appearance of lesions varies on diffusion-weighted images depending on the time elapsed since the infarction.
👉Old ischemic lesions do not show restricted diffusion
👉acute ischemic lesions appear hyperintense on diffusion-weighted images
It is important to know which type of MRI to order.
✍️with suspicion of cerebral infarction
👉MRI without contrast
• sagittal T1-weighted sequence
• axial T1- weighted sequence
• axial FLAIR sequence
• T2-weighted sequence
• gradient echo to detect blood
👉MRI with diffusion weighted images (shows very acute infarctions)
👉MRA of the head (circle of Willis) and neck (extracranial cervical arteries).
✍️In the case of optic neuropathy, MRI of the brain and orbits with contrast should be ordered.
☝️sagittal T1-weighted sequence
☝️axial T1-weighted sequence
☝️axial FLAIR sequence
☝️T1- weighted sequence with contrast and fat suppression
• axial and coronal T1-weighted sequence with and without fat suppression
• axial and coronal T1-weighted sequence with fat suppression and with contrast
✍️In the case of chiasmal syndrome
☝️T1, T2(axial, coronal, and sagittal ) views of the sella turcica and pituitary gland with contrast should also be ordered.
✍️In the case of cranial nerve deficit,
☝️thin axial and coronal cuts in T1, T2, and T1 with contrast, covering the anatomical course of the cranial nerve in question.
The following are good indications for Magnetic Resonance Imaging ( MRI )
✍️Suspected optic nerve tumor
✍️Wooden foreign body
✍️Orbital apex or cavernous sinus syndrome