Magnetic Resonance Imaging
By Mutahir ShahResident M Phil VS Pakistan Institute of
Community OphthalmologyMagnetic Resonance Imaging
MRI:Magnetic resonance imaging (MRI) is a technique that uses a
magnetic field and radio waves to create detailed images of the
organs and tissues within your bodyUses a large magnetic field to
excite protons of water molecules.The energy given off as the
protons reequilibrate to their normal state is detected by
specialized receivers (coils) and that information is reconstructed
into a computer image. Obtain Multiplanar images without loss of
resolution.
Basic SequencesWeighting refers to two methods of measuring the
relaxation time of excited protons after magnetic field has been
switched off.T 1 and T 2 times sequences is used that is different
for different tissues In practice both types of scan are usually
performedThe time sequence based methods are T1 weighted and T 2
weighted
T 1 Weighted.Generally optimal for viewing normal
anatomyHypointense dark colors include Cerebrospinal fluid and
vitreous.Hyperintense structures include fats blood contrast agent
and Melanin. Air is signal void on T 1 and T2.Calcification appears
signal void fresh bleeding is signal void.We characterize the
lesion on the basis of post contrast . If it take contrast it is
malignant problem if does not take contrast it is benign.
T 2 weighted imagesImages in which water is shown as hyper
intense structure.Useful for viewing pathological changes because
edematous tissue (inflammation) will display a brighter signals
than the normal surroundings.CSF and Vitreous become
hyperintenseBlood vessels appear black on T 2 unless they are
occludedPathalogies appears more on T 2 Sensitivity for
calcification cant be picked by MRI.
MR scans. (A) T1-weighted coronal image through the globe in
which vitreous is hypointense (dark) and orbital fat is hyper
intense (bright); (B) T2-weighted axial image in which vitreous and
cerebrospinal fluid (CSF) are hyperintense; (C) T1-weighted midline
sagittal image through the brain in which the CSF in the third
ventricle is hypointense; (D) T2-weighted axial image through the
brain in which the CSF in the lateral ventricles is
hyperintense
The basic principles of MRI are listed below.T1 weightedFat
SuppressionGadoliniumT2 weightedProperties;Useful for Intraocular
structure such as optic nerve, EOM, and orbital veins.The strong
fat signal within the orbit gives poor resolution of lacrimal gland
and may also mask intraocular structure.T-1 weighted images with
bright intraconal fat signal suppressed in the orbit allowing for
better anatomic detail. Essential for all orbital MRIs.A
paramagnetic agent that distributes in the extracellular space and
does not cross the intact blood brain barrier .Gd is best for T 1
fat suppressed images. The lacrimal gland and EOM enhance on
Gd.Suboptimal Intraocular contrast.Demylinating lesions (MS) are
bright.InterpretationFat is Hyper intense and vitreous and
Intracranial ventricales are hypointense.Vitreous and fat are
dark.EOM are bighter after GadoliniumMost orbital masses are dark
on T1 and become bright with gadolinium enhancement.Fluid
containing structures such as vitreous and CSF are bright. Melanin
is dark
Tissues/LesionsExamples.
FatLipoma, LipsarcomaMucus /protienaceous materialDermoid cyst ,
mucocele, dacryocele, craniopharyngiomaMelaninMelanomaSubacute
blood (3-14 days Old)Lymphangioma with blood cyst , Hemorrhagic
choroidal detachementCertain fungal infections (iron
scavengers)Aspergillus
Tissues/Lesions that appear Bright (Hyperintens relative to
vitreous Before Gadolinium injection
Uses in OphthalmologyExcellent for defining the extent of
orbital/CNS masses.Poor bone definition (fractures).Excellent for
diagnosing Intracranial, cavernous sinus and orbital apex lesions,
many of which affect Neuro-Ophthalmic Pathway.For Suspected
neurogenic tumours (Meningioma, Glioma) Gadolinium is essential in
defining lesion extent.Brain MRI in Patients suspected with
Demylinating diseases.
Neuro Ophthalmic IndicationThe optic nerve is best visualized on
coronal STIR images in conjunction with coronal and axial T1 fat
saturation post-gadolinium images. Axial T1 images are useful for
displaying normal anatomy. MRI can detect lesions of the
intraorbital part of the optic nerve (e.g. neuritis, glioma) as
well as intracranial extension of optic nerve tumours. Optic nerve
sheath lesions (e.g. meningioma) are of similar signal intensity to
the nerve on T1- and T2-weighted images but enhance avidly with
gadolinium.
Sellar masses (e.g. pituitary tumors) are best visualized by
T1-weighted contrast-enhanced studies. Coronal images optimally
demonstrate the contents of the sella turcica as well as the
suprasellar and parasellar regions and are usually supplemented by
sagittal images.Cavernous sinus pathology is best demonstrated on
coronal images; contrast may be required. Intracranial lesions of
the visual pathways (e.g. inflammatory, demyelinating, neoplastic
and vascular). MRI allows further characterization of these lesions
as well as better anatomical localization.
Limitations of MRIBone appears black and is not directly imaged.
Recent haemorrhage is not detected, so MRI is inappropriate in
patients with suspected acute intracranial bleeding. It cannot be
used in patients with magnetic foreign objects (e.g. cardiac
pacemakers, intraocular foreign bodies and ferromagnetic aneurysm
clips). Substantial patient cooperation is required, including
remaining motionless; it is poorly tolerated by claustrophobic
patients as it involves lying in an enclosed space for many
minutes.
Rebdomyosarcoma
Choroidal Melanoma & Detachment
Optic Nerve meningioma extending intracranial
tram-track sign is composed of two enhancing areas of tumor
separated from each other by the negative defect of the optic
nerve.The sign helps distinguish between optic nerve sheath
meningioma and optic glioma. Optic glioma arises from glial cells
within the optic nerve and there is no clear separation between the
nerve and the tumor; hence the tram-track sign is not seen in optic
gliomas
