Increased signal intensity of subarachnoid space on FLAIR MRI

 It is an inversion recovery sequence where
180° pulse is applied first to convert the
longitudinal magnetization to the opposite
direction.
 If left alone it will return to the parallel direction
(recovers).
 To obtain a signal, 90 ° RF pulse has to be
applied at specific time point which permits
signal of the brain to be loud and signal of the
CSF to be nulled.

 Fluid Attenuation Inversion Recovery
(FLAIR) is widely used as a routine protocol
during the MRI of the brain. It was first
described by Hajnal et al. in 1992.

 FLAIR imaging has two important characters:
1. Suppression of CSF signal:
So It is sensitive to pathology within or near CSF.
Factors that affect the T1-weighted relaxation time
of CSF may interfere with its suppression .
2. Long TE:
It is has T2 weighting effect with sensitivity to
conditions which produces T2 prolongation

Subarachnoid haemorrhage.
Post-traumatic brain injury.
Meningitis.
Leptomeningeal metastasis.
Ischemic stroke.
Moyamoya disease.
Neurocutaneous melanosis.
Following Gadolinium administration in renal dysfunction.
Fat containing tumors.
Dural vascular malformations.
Dural venous sinus thrombosis.
Hyperperfusion syndrome.
Intracranial space occupying lesions.

Inhaled oxygen.
Cerebrospinal fluid (CSF) flow-related artifact.
Inhomogeneity in the amplitude of initial inversion recovery.
Head motion.
Vascular pulsation.
Chemical shift artifact.
Cross-talk.
Truncation artifact.
Magnetic susceptibility artifact.
Overlapping of imaging planes.

 SAH will result in shortening of T1 relaxation
time of CSF with consequent loss of
suppressive effect of FLAIR on CSF.
 On CT scan the density is related to the
hematocrite value of blood.
 Consequently minimal hemorrhage will be
easier to detect by FLAIR than by CT scan.
 Also the beam hardening effect of bone on
CT scan can lead to missing SAH.

 Meningitis is the commonest infection of the
central nervous system.
 Elevated CSF protein is also a feature resulting
in:
1. Shortening of the T1 relaxation time.
2. Alteration of the point at which CSF is nulled.
3. T2 prolongation of CSF relaxation time.
Consequently there will be elevation of the signal
intensity coming from the subarachnoid space
on FLAIR MRI

 The survival of patients with malignant
leptomeningeal metastases is between 1 and 2
months without treatment. With palliative
treatment, survival can be up to 6–10 months.
 The golden standard in diagnosis is
pathological examination of CSF obtained by
lumber puncture.
 FLAIR imaging comes as a non-invasive
diagnostic tool in clinically suspected disease.

 The appearance of leptomeningeal
metastasis, whether on enhanced T1 WIs or
FLAIR, is non-specific even with relevant
clinical history.
 The leptomeningeal infection may also
complicate patients with malignant disease
elsewhere.
 Thus whenever lumber puncture is possible
it should be done for cytological examination
to keep the patient safe from the
unnescessary use of chemotheraputics

The sensitivity of MRI in diagnosis of
leptomeningeal metastasis depends partly
on the nature of the primary tumor.
1. Soft tissue tumor:
The nodules can adhere to leptomeninges.
MRI can easy detects.
2. Hematopoietic tumor:
Lumber puncture is more important.
Consequently neither MRI nor lumber puncture
can alone withstand for diagnosis.

 The FLAIR imaging can offer many
diagnostic values in cases of infarcts.
1. Hyperintense artery sign.
2. Indicator of collateral flow.
3. The hyperintense sign can serve like
perfusion in prediction of penumbra.

(Azizyan, Sanossian,
Mogensen, & Liebeskind,
2011)

 Is a chronic vascular steno-occlusive
disease of unknown etiology.
 Vascular hyperintensity has been described
in Moyamoya disease and is often referred
to as the ivy sign.
 Because of the chronic progressive nature of
Moyamoya disease, there is time for
collateralization to develop from the
leptomeninges as well as from the arteries at
the base of the brain

 Primary melanocytic neoplasms of the CNS
are rare and arise from leptomeningeal
melanocytes.
 The diagnosis is mainly pathological
however MRI finding together with the
clinical information are helpful.

 On MR imaging, most tumours have a
1. Low T2-weighted signal.
2. High T1 signal with postcontrast
enhancement .
3. High FLAIR signal intensity possibly related
to the T1 shortening effect of melanin and
T2 prolongation effect of high protein
contents

 Most of dural sinus fistulae and
arteriovenous shunts are acquired mostly
secondary to previous dural sinus
thrombosis.
 The sulcal hyperintensity is multifactorial.
1. Hyperintense vessel sign.
2. Venous engorgement with consequent leak
of protein contents into the CSF.
3. Subarachnoid hemorrhage.

 Is related to abrupt increase in the cerebral
blood flow secondary to restoration of
significant carotid artery stenosis by stenting
or endarterectomy.
 This results in injury to the blood brain
barrier with consequent extravasation of
contrast medium to the subarachnoid space.

 This was the differential diagnosis of
pathological causes of hyperintense signal
of SAS on FLAIR MRI.
 Of same degree of importance is to identify
the non-pathological conditions before
proceeding to diagnosis of serious condition
and consequently prevent hazardous non-
indicated management.

 When MRI is done under general anesthesia
especially in children supplemental oxygen
may be applied.
 CSF hyperintensity was observed in patients
who were receiving 100% FiO2 and was
eliminated or attenuated in all patients after
lowering the FiO2 level to 30%.

 This hyperintensity is likely related to the T1
shortening effect of O2 because of its
paramagnetic effect.
 The oxygen gains access to the CSF
through the walls of the arteries and
arterioles so it is usually seen in the cerebral
convexities and cisterns but not in ventricles
(poorer in vascular network).

100%O230%O2
(Frigon, Shaw, Heckbert,
Weinberger, & Jardine, 2004)

 It occurs at areas with high CSF flow. It is
common at basal cisterns, third and forth
ventricles and at ventricular foramina.
 It is rare at lateral ventricles and cerebral
convexities as CSF flow is minimal.
 This artifactual hyperintensity tends to be
inhomogeneous, and usually differs from
section to section.

 The most important cause of these high
signals is inflow of un-inverted, or only
partially inverted CSF into the slice during
the period between the initial slice-selective
180° pulse and the subsequent 90° pulse;
that is, during the inversion time (TI).

 This artifact reduces the sensitivity of FLAIR
MRI in diagnosis. So many techniques are
applied to reduce this effect.
1. Widening of the inversion pulse.
2. Use of non slice-selective initial 180° pulse.
3. Use of non-slice selective inversion pulse
with a k-space reordered by inversion time
for each slice position (KRISP) scheme.

4. 3D Application:
5. High magnet (3T).
6. Use of cardiac gating.

(Lummel, Schoepf,
Burke, Brueckmann, &
Linn, 2011)

 In parts of the body remote from the center
of the transmitter coil, the amplitude of the
RF field of an inversion pulse may be
reduced such that the magnetization of
tissues and fluids in these regions is only
partly inverted due to reduction of the RF flip
angle.

(Hajnal, Oatridge, Herlihy, &
Bydder, 2001)

 Both techniques (SS FSE and multisection
FSE) still may be compromised by head
motion artifact.
 This artifact is like CSF flow related artifact.
When the patient moves his head he brings
un-inverted CSF protons to the examined
section during the inversion time between
the 180° and the 90°vpukses.
 The location however differs. Here the
artifact is seen at brain convexities and not
at cisterns and foramina.

 One of the obstacles of MRI is the
examination of non-cooperative patient who
moves during examination.
 To overcome this problem, technologists
developed a single shot fast spin echo
sequence (SS FSE) which is a fast variant
from the FLAIR sequence that acquires the
image data for each section in 0.1 to 0.2
seconds rather than full study (20-30
sections at once).

Tha and Terae et al., 2009
 It results from synchrony
between the vascular
pulsations and phase
encoding steps.
 The artifact reproduces the
size, shape, and alignment
of the responsible vessel
along the phase-encoding
direction of the image

 The chemical shift phenomenon refers to the
signal intensity alterations that result from an
inherent difference in the resonant
frequencies of precessing protons.
 It is most evident between the signal of water
and lipid.
 This inherent difference results in spatial
misregistration, known as chemical shift
artifact.

 It appears as a bright band at side of
summation of water and lipid signal at water
fat interface and dark band on the opposite
side.
 Using wider band width solves this problem.
 This effect is higher with higher field
magnets . This depends on the fact that the
precession frequency of protons is
proportionally related to the magnetic field
strength.

 If the inter-slice gap is narrow the Fourier
transform, will not be able to discriminate
between adjacent RF pulses when.
 The interference between adjacent slices
introduces an artifact known as cross-talk.
 Widening the interslice gap solves the
problem.

 It occurs in relation to metallic implants
which produce magnetic field effect when put
in the magnet.
 The new magnetic field results in marked
distortion of the magnetic field with
consequent signal loss due to signal
misregistration. Adjacent high signal is
perceived due to displacement and
overcrowding of the adjacent signal.

 When examining
two decussating
planes the CSF
within the region of
overlap
experiences two
inversion pulses.
 The second
inversion pulse
counteracts the
first, resulting in
non-nulling.
Tha and Terae et al., 2009

 FLAIR MRI is a commonly used technique
during imaging of the brain.
 Careful assessment of the FLAIR images is
very important because a subtle change in
the CSF signal can be alarming for
underlying serious pathology. Hyperintense
artery sign is an example.
 Post-contrast FLAIR is an important
technique, that if added to the post-contrast
protocol, can augment the diagnostic
efficacy.

 Despite of the major contribution of FLAIR in
the diagnosis it is complicated by many
drawbacks related to the associated
artifacts.
 Understanding these artifacts can prevent
over-diagnosis of many serious conditions.
 Knowledge of how to alleviate these
artifacts is also important so that, full benefit
from FLAIR images is maintained.

Increased signal intensity of subarachnoid space on FLAIR MRI

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Increased signal intensity of subarachnoid space on FLAIR MRI

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