A brief description of MRI methods indication and limitation in eye

1. By
Mutahir Shah
Resident M Phil VS
Pakistan Institute of Community
Ophthalmology
Magnetic Resonance Imaging

2. 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 body
 Uses 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.

3. Basic Sequences
 Weighting 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
performed
 The time sequence based methods are T1
weighted and T 2 weighted

4. T 1 Weighted.
 Generally optimal for viewing normal anatomy
 Hypointense 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.

5. T 2 weighted images
 Images 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 hyperintense
 Blood vessels appear black on T 2 unless they
are occluded
 Pathalogies appears more on T 2
 Sensitivity for calcification can’t be picked by MRI.

6. 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

7.  The basic principles of MRI are listed below.
T1 weighted Fat
Suppression
Gadolinium T2 weighted
Proper
ties;
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.
Interpr
etation
Fat is Hyper
intense and
vitreous and
Intracranial
Vitreous and fat
are dark.
EOM are bighter
after Gadolinium
Most orbital
masses are dark
on T1 and
become bright
Fluid
containing
structures
such as

8. Tissues/Lesions Examples.
Fat Lipoma, Lipsarcoma
Mucus /protienaceous material Dermoid cyst , mucocele, dacryocele,
craniopharyngioma
Melanin Melanoma
Subacute blood (3-14 days Old) Lymphangioma with blood cyst ,
Hemorrhagic choroidal detachement
Certain fungal infections (iron
scavengers)
Aspergillus
Tissues/Lesions that appear Bright (Hyperintens relative to vitreous
Before Gadolinium injection

9. Uses in Ophthalmology
 Excellent 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.

10. Neuro Ophthalmic Indication
 The 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.

11.  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.

12. Limitations of MRI
 Bone 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.

13. Rebdomyosarcoma

14. Choroidal Melanoma & Detachment

16. Optic Nerve meningioma extending
intracranial

17.  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