Increased Signal Intensity of Subarachnoid Space on FLAIR MRI

DR Wafik Ebrahim, MDLecturer of Radiology

Faculty of medicine Alazhar University

FLAIR

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.

FLAIR

Inversion Recovery

FLAIR

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 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

Pathological conditions: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.

Non-pathological conditionsInhaled 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.

Subarachnoid haemorrhage 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.

Acute SAH

Tha and Terae et al., 2009

Subacute SAH

Tha and Terae et al., 2009

Post-traumatic brain injury

Kim et al., 2014)

Meningitis

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

Meningitis .

Meningitis .

(Stuckey, 2007)

Leptomeningeal metastasis:

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.

Leptomeningeal metastasis:

(Stuckey, 2007)

Leptomeningeal metastasisPost-contrast T1 and FLAIR

Tha and Terae et al., 2009

Leptomeningeal metastasis: 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

Leptomeningeal metastasis:

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.

Cerebral infarct: 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.

Cerebral infarct:

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

Cerebral infarct:

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

Cerebral infarct:

Tha and Terae et al., 2009

Cerebral infarct:

Tha and Terae et al., 2009

Moyamoya disease:

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

Moyamoya disease:

Tha and Terae et al., 2009

Neurocutaneous melanosis

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.

Neurocutaneous melanosis

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

Neurocutaneous melanosis

Tha and Terae et al., 2009

Following Gadolinium administration in renal dysfunction:

Tha and Terae et al., 2009

Fat containing tumors:

Tha and Terae et al., 2009

Dural sinus malformation and thrombosis

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.

Dural sinus malformation and thrombosis

(Oppenheim et al., 2005)

Hyperperfusion syndrome:

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.

Hyperperfusion syndrome :

(Cho et al., 2014)

Intracranial space occupying lesions:

Tha and Terae et al., 2009

Non-pathological conditions : 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.

Inhaled oxygen:

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%.

Inhaled oxygen: 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).

Inhaled oxygen:100%

O2

30% O

2

(Frigon, Shaw, Heckbert, Weinberger, & Jardine, 2004)

CSF flow related artifact: 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.

CSF flow related artifact: 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).

CSF flow related artifact: 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.

CSF flow related artifact:

(Herlihy et al., 2001)

CSF flow related artifact:

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

CSF flow related artifact:

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

Inhomogeneity in the amplitude of the initial inversion recovery: 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.

Inhomogeneity in the amplitude of the initial inversion recovery:

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

Inhomogeneity in the amplitude of the initial inversion recovery:

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

Head motion artifact: 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.

Head motion artifact: 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).

Head motion artifact:

(Cianfoni et al., 2006)

Head motion artifact:

(Cianfoni et al., 2006)

Head motion artifact

(Cianfoni et al., 2006)

Head motion artifact

(Cianfoni et al., 2006)

Vascular pulsation artifact:

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

Chemical shift artifact: 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.

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.

Chemical shift artifact:

Tha and Terae et al., 2009

Cross talk artifact:

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.

Cross talk artifact:

Tha and Terae et al., 2009

Magnetic Susceptibility Artifact 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.

Magnetic Susceptibility Artifact

Tha and Terae et al., 2009

Overlapping of imaging planes: 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

Summary 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.

Summary 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.