Download Susceptibility weigthed imaging: an established tool on brain MRI

Susceptibility weigthed imaging: an established tool on
brain MRI routine protocols.
Poster No.:
C-1160
Congress:
ECR 2015
Type:
Educational Exhibit
Authors:
P. Garcia Barquin, M. Millor Muruzábal, M. Páramo, L. R. Zalazar,
M. R. Garcia de Eulate, J. L. Zubieta, P. Domínguez; Pamplona/
ES
Keywords:
Calcifications / Calculi, Blood, Imaging sequences, MR,
Neuroradiology brain, CNS
DOI:
10.1594/ecr2015/C-1160
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Page 1 of 31
Learning objectives
1. Learning objectives
The purpose of our educational exhibit is to:
· Describe the basic physics and technical aspects of susceptibility weighted imaging
(SWI).
· Show the normal SWI appearance, some artefacts and the main limitations.
· Review the most important clinical applications of this sequence on intracranial
pathology with illustrated examples.
Background
2. Background
Susceptibility weighted imaging (SWI) is emerging as a useful technique in a wide
variety of intracranial pathologies. The SWI sequences demonstrate high sensitivity to
magnetic susceptibility differences of various tissues in particular blood, calcification and
haemosiderin.
Technical aspects of susceptibility sequences.
SWI is a fully velocity-compensated, three-dimensional, gradient-echo sequence that
uses both phase and magnitude data to achieve exquisite sensitivity to tissue magnetic
susceptibility effects. The phase and magnitude data are acquired separately. The
combination of magnitude and phase data produces an enhanced contrast magnitude
image that is particularly sensitive to some molecules. Minimum intensity projections
image (mIP) can further demonstrate the continuity of tortuous vascular structures thus
differentiating them from focal lesions. Fig. 2 on page 4
Susceptibility sequences include T2 * and SWI that differs because this one is three
dimensional gradient echo sequence and has a high sensitivity.
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To understand the SWI sequences we have to review some basic concepts. The first
one is the magnetic susceptibility which is defined as the ability of magnetization of a
material in response to an applied magnetic field.
In this way there are
- Diamagnetic materials: that have only weak local magnetic effects (no unpaired
electrons).
- Paramagnetic materials: that generates magnetic fields that additively are going to
combine with the external magnetic field.
The second basic concept is the susceptibility effects of haemoglobin products.
Haemoglobin is the transporter of oxygen in the blood. It has four globin subunits that
each has a haeme molecule. The haeme molecule is composed of an iron atom (Fe2+)
and a porphyrin ring.
When oxygen binds to the iron atom the molecule is term OXYHAEMOGLOBIN,
which is diamagnetic and has a weak local magnetic field. DEOXYHAEMOGLOBIN is
formed when oxygen dissociates from the iron atom. It is paramagnetic and causes
alterations in local magnetic field. DEOXYHAEMOGLOBIN can be further oxidized to
METHAEMOGLOBIN that appears bright in all MRI sequences and has few susceptibility
effects. Finally, HAEMOSIDERIN is a heavily iron laden protein and is strongly
paramagnetic and produce alteration in local magnetic field. Fig. 3 on page 4
An interesting observation is seen in patients undergoing general anesthesia for MRI.
The cortical veins appear attenuated. This might be due to an increased rate of oxygen.
Fig. 4 on page 5
Calcium presents diamagnetic susceptibility effects. Although it induces less phase
effects than blood products, it still leads a dephasing of signal that is detected on SWI.
Calcium is commonly found in the brain and may be physiologic or present in a wide
variety of conditions.
Iron is paramagnetic in nature and produces strong susceptibility effects. Iron
accumulation increases with age and is also observed in various neurodegenerative
diseases. Fig. 5 on page 6
The most common SWI artifact is at air tissue interface manifests as concentric
hypointensities, limiting the investigation of areas next to paranasal sinuses and temporal
bones. Fig. 6 on page 7
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Another limitation is the sequence acquisition time that ranges from 5 to 8 minutes,
leading to increased incidence of movement's artefacts. We also have to be aware of the
possible limitation of this technique in differentiating calcium and blood.
Images for this section:
Fig. 2: Technical aspects and imaging parameters of our SWI protocol.
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Fig. 3: Susceptibility effects of haemoglobin products.
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Fig. 4: (A) This is the normal appearance of a SWI sequence with small cortical veins that
are seen as linear hypointensities due to the signal loss from the deoxyhaemoglobin. (B)
In patients undergoing general anesthesia with oxygen administration the cortical veins
appear attenuated due to the presence of oxyhaemoglobin.
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Fig. 5: Susceptibility effects of calcium and iron.
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Fig. 6: (A and B). Artefacts that occur at air-tissue interfaces are the most frequent and
are a limiting factor in the evaluation of areas next to the paranasal sinuses and the
temporal bones (yellow arrows).
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Findings and procedure details
3. Findings and procedure details.
1. Cerebral microbleeds (CMB)
Cerebral microbleeds are observed in various conditions.
Cerebral amyloid angiopathy:
It consists of deposition of amyloid protein within the small and medium sized cerebral
arteries which is likely responsible for increased vessel fragility with consequent micro
and macro haemorrhages demonstrated on SWI with cortical and subcortical distribution.
Fig. 7 on page 12
Chronic hyperintensive encephalopathy (HE):
Chronic hyperintensive encephalopathy HE is also characterized by multiple cerebral
microbleeds (CMB) which normally are silent. They are usually discovered both in deep
basal ganglia and subcortical white matter. Fig. 8 on page 12
2. Vascular malformations
Arteriovenous malformations are easily displayed with conventional MRI and MR
angiography because of their characteristic high flow.
But venous malformations can't be adequate visualized without contrast as they consist
of slow flow and could be entirely missed by conventional imaging techniques. On the
contrary, SWI are well suited for the visualization of small vessels.
Cavernomas:
They are composed of dilated disorganized vascular channels. Individuals with
cavernous malformations can present with epilepsy focal neurological deficits or acute
intracranial haemorrhages. Fig. 9 on page 13
Developmental venous anomalies
Abnormal veins that drain normal brain parenchyma. SWI demonstrates the slow flow
and their characteristic stellate appearance. Fig. 10 on page 14
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Telangiectasia.
Telangiectasia is typically a small lesion. It is a low-flow vascular malformation with low
signal intensity on SWI. Telangiectasias are found primarily in the pons and may occur
sporadically. SWI is a useful adjunct to conventional MRI in diagnosing telangiectasia.
Fig. 11 on page 15
3. Acute stroke
SWI has been demonstrated to be very useful in the acute phase of stroke.
It is very sensitive in the detection of cerebral microbleeds whose early identification is
believed to predict haemorragic transformation after thrombolytic treatment. Fig. 12 on
page 16
SWI is also capable of identifying the acute intravascular clot in the main and distal
branches of the cerebral arteries. Fig. 13 on page 17
SWI has been demonstrated to be useful in the assessment of tissue viability. An
improved visualization of draining veins, which is related to increased oxygen extraction,
within areas of impaired perfusion allows the identification of penumbra brain tissue with
SWI.
4. Traumatic brain injuries
Diffuse axonal injury (DAI) is a form of traumatic brain injury, caused by shearing stress
primarily in the white matter. The extension of axonal injury correlates with poor outcome.
However, lesions are usually poorly visualized with conventional methods while MRI
conventional appearance of hemorrhage is variable, due to the multiple parameters of
haemoglobin and blood cells. SWI sequences are very sensitive in the detection of DAI
lesions. Fig. 14 on page 18
5. Intracranial tumors
Primary brain tumors
The growth of solid tumors such as gliomas, is dependent on the angiogenesis of
pathological vessels. SWI can provide an assessment of the angioarchitecture of brain
tumors, together with the identification of haemorrhage and calcification. Fig. 15 on page
19
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Secondary brain tumors
SWI is able to demonstrate the presence of haemorrhage on metastases. SWI also can
differentiate the melanin that doesn't have susceptibility effect from the haemorrhage.
Fig. 16 on page 20
6. Neurodegenerative disorder
Iron deposition increases in the brain as a function of age, in the form of ferritine. Typical
sites include the globus pallidum, substantia nigra and red and dentate nuclei.
Abnormally elevated iron levels are evident in many neurodegenerative disorders like:
Parkinson's disease, Alzheimer's disease, Huntington's disease and amyotrophic lateral
sclerosis. Fig. 17 on page 21
7. Calcium related diseases
Calcium is commonly found in the brain and may be physiologic or present in a number
of conditions like toxoplasmosis, tuberous sclerosis and Sturge Weber syndrome. Fig.
18 on page 22 , Fig. 19 on page 23 and Fig. 20 on page 24 .
8. Subarachnoid haemorrhage
Non traumatic subarachnoid haemorrhage is most commonly due to rupture of an
aneurysm. SWI can demonstrate the subarachnoid haemorrhage and the parenchymal
and intraventricular extension. Fig. 21 on page 25 and Fig. 22 on page 26 .
9. Pneumoencephalus
Some studies suggest that SWI might be suitable for monitoring neurosurgical patients
recovering from pneumoencephalus that can be easily detected with SWI. Fig. 23 on
page 27 .
10. Localization of the subthalamic nucleus
An accurate localization of the subthalamic nucleus can be achieved in the SWI maps at
3.0 Teslas, allowing safe direct targeting for placement of electrodes in the treatment of
Parkinson's disease. Fig. 24 on page 28 .
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Images for this section:
Fig. 7: These images correspond to a woman with progressive cognitive decline. (A)
Axial FLAIR image demonstrates a cerebral atrophy and periventricular white matter
changes (yellow arrows). (C and D) Multiple hypointensities with cortical and subcortical
distribution are shown on SWI images keeping with cerebral amyloid angiopathy.
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Fig. 8: Patient with chronic hypertension and high arterial blood levels. Note the presence
of periventricular white matter changes on axial FLAIR sequence (A) and the multiple
microbleeds on SWI images (B and C) at the basal ganglia, the thalamus, and the
protuberance (yellow arrows). These imaging findings and the clinical history suggest the
diagnosis of chronic hypertensive encephalopathy.
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Fig. 9: (A-F). Nodular lesion on the right side of the pons, looking in "salt and
pepper" markedly hypointense on T1- weighted image and SWI, hypointense on
diffusion, isointense on T2 and FLAIR sequences, and without any enhancement after
administration of intravenous paramagnetic contrast, suggestive of a cavernoma of the
pons.
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Fig. 10: Notable anomaly of venous development in the posterior region of the left frontal
lobe with enhancement after contrast (A and B) and stellate appearance and hypointense
on SWI (C and D).
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Fig. 11: On the left side of the pons a focus of pathological enhancement (yellow
arrow) (B) without any signal alteration on FLAIR sequence (A) and hypointense on
SWI sequence (yellow arrow) is identified (C). It is highly suggestive of corresponding
to a capillary telangiectasia at this location, usually an incidental finding without clinical
relevance. An area of gliosis affecting right cerebellar hemisphere associated with
hemorrhagic component on SWI is also observed.
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Fig. 12: On the axial FLAIR sequence an area of signal alteration affecting the tail of the
caudate nucleus is observed (A). A peripheral area of restriction of free water on diffusion
image (B) suggests an acute infarction. A large area of hypointensity on SWI support the
presence of a hemorrhagic transformation in this location (C).
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Fig. 13: Patient with multiple areas of acute infarction on the right temporal lobe,
significantly restricting in DWI sequence (A).The T2* sequence demonstrates an
extensive hypointense area in the portion M1 of the right middle cerebral artery supporting
with a thrombus (yellow arrow)(B). In the study of 3D-ToF-MRA a complete occlusion of
the right middle cerebral artery is confirmed (yellow arrow)( (C).
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Fig. 14: Patient with a history of traffic accident and a left craniectomy. In the parenchyma
of the left front-parietal region, areas of signal hyperintensity of FLAIR image are
observed in connection with parenchymal damage (A). Multiple foci with radial distribution
extending from the lateral ventricle into the cortex are demonstrated on SWI, suggest
diffuse axonal injury (B and C).
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Fig. 15: Axial FLAIR sequence showing a mass in the left frontal lobe (A). A necrotic
heterogeneous mass, with peripheral contrast enhancement on the right frontal lobe
is demonstrated (B). Note that the tumor neovascularization and the presence of
hemorrhage are more accurately identified on SWI sequence (C).
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Fig. 16: 63 years old men with lung cancer.Axial FLAIR image shows the presence of
multiple lesions with edema (A). They present enhancement after contrast administration
suggesting metastases (B). SWI image confirms the haemorrhagic transformation of the
lesions (C).
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Fig. 17: A 55 years old man with cognitive decline. Areas of signal hyperintensity in
both coronal T1 and axial FLAIR sequences affecting the basal ganglia and the posterior
region of the thalamus (yellow arrows) (A and B). Hypointense foci is identified in putamen
on SWI sequence, primarily affecting the lenticular nuclei and the posterior thalamus
(yellow arrows) (C).These findings are remarkably pathological to the age of the patient
and suggest firstly Wilson disease . They could also be related to calcium deposition in the
context of Fahr disease or other diseases of calcium and phosphorus metabolism. Finally
it also could be related to a neurodegeneration brain disorder with iron accumulation .
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Fig. 18: Presence of multiple residual lesions many calcified, secondary to congenital
toxoplasmosis in both CT (A, B and C) and SWI sequences (D, E and F).
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Fig. 19: Presence of multiple foci of hyperintensity on FLAIR in both cortical hemispheres
relative to its known tuberous sclerosis ("tubers")(A). Multiple irregularities are also
observed in the wall of both ventricles in relation to the presence of subependymomas
mostly calcified observed on SWI sequence (B and C).
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Fig. 20: Findings consistent with meningeal angiomatosis in a patient with Sturge
Weber disease. Increased density with giriform morphology affecting rights occipital and
temporal lobes in relation to calcification, evident on CT, axial T2 and SWI. (A, B and C)
Page 25 of 31
Fig. 21: 55 years old woman, moved to the emergency department with symptoms of
disorientation in time and space , with unsteady gait and right lingual bite. (A, B and C).
Axial CT. Signs of subarachnoid bleeding especially in the frontal interhemispheric region
extending to both Sylvian fissures, bilateral front-temporal sulci and peripontin cisterns.
Ventricular signs of contamination with low levels in both region associated dorsal horn.
The temporal horns are dilated, signs of incipient hydrocephalus. Extensive subarachnoid
hemorrhagic pollution displayed on FLAIR sequence (D). Hypointense grooves marked
by the presence of deoxyhemoglobin in the SWI sequences. SWI sequence artifactual
by patient movement (E). MPR reconstruction images of the 3D TOF MR angiogram
demonstrates an aneurysm of the anterior communicating artery (yellow arrow).
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Fig. 22: Same patient as the previous figure . MRI scan performed 2 months later.
Extensive deposition of hemosiderin in the subarachnoid spaces , in relation to diffuse
siderosis (A, B and C).
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Fig. 23: Two examples of patients with postsurgical pneumoencephalus. (A and B).
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Fig. 24: Patient with Parkinson's disease with 11 years of evolution with a good response
to medication. In the last two years, he has presented a clear motor deterioration. He is
currently with very high levels of dopamine and presents motor fluctuations . The patient
is a candidate for implantation of neuroestimulator in the subthalamic nucleus. The STN
(the subthalamic nuclei), SN ( substantia nigra), and RN (red nuclei) are most sharply
delineated on the SWI images (A). Sagittal T1 weighted images after the stereotactic
procedure of placing electrodes into the STN (B and C).
Page 29 of 31
Conclusion
4. Conclusion
SWI is a fully velocity three dimensional gradient-echo sequence with exquisite sensitivity
to paramagnetic susceptibility effects.
It can better demonstrate haemorrhage, venous abdnormalities and mineralisation than
conventional sequences.
SWI should be include in the routine imaging protocols of trauma and vascular
abnormalities.
Further investigation is still needed into its extensive application.
Personal information
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