Transcript post-radioteraphy

eEdE-72
SWI- PHASE IMAGES:
PRACTICAL APPLICATIONS AND PITFALLS
Chapel Hill, NC
ANA LORENA ABELLO
RENATO HOFFMAN NUNES
MAURICIO CASTILLO
Disclosures
The authors have no disclosures.
PHYSICAL PRINCIPLES
• Susceptibility-weighted imaging (SWI) is a “3D highspatial resolution fully velocity corrected gradient echo
MRI sequence.”
• It is based on the principle that compounds having
paramagnetic, diamagnetic & ferromagnetic properties
all interact & distort the local magnetic field thus
altering the phase of local tissue which results in loss of
signal.
Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain – A pictorial review.
Neuroradiology. 2008.
Tong KA et al. Susceptibility-weighted MR imaging: a review of clinical applications in children. AJNR. 2008.
PHYSICAL PRINCIPLES
• SWI uses 2 sources of data: Phase & Magnitude.
• Both phase/magnitude data are acquired separately
for processing to create a new SWI.
Magnitude
+
Filtered-phase
SWI
Sehgal V . Clinical applications of neuroimaging with susceptibility-weighted imaging.
JMRI 2005
PHYSICAL PRINCIPLES
Postprocessing steps used to create an SWI image: Starts with a magnitude image (top left) and a phase
image (top right). The phase image is filtered and a mask is created from this filtered image. The phase
mask is then used to create the final SWI image (bottom left).
From: Sehgal V . Clinical applications of neuroimaging with susceptibility-weighted imaging. JMRI 2005
PHYSICAL PRINCIPLES
• Distinguishing between calcification & blood products is not
possible on post processed SWI images; both demonstrate
signal drop out & blooming.
• FILTERED PHASE images are able to distinguish between them
as diamagnetic & paramagnetic compounds affect phase
differently, appearing of opposite signal intensity.
• Paramagnetic compounds include deoxyhemoglobin, ferritin
& hemosiderin.
• Diamagnetic compounds include bone minerals & dystrophic
calcifications.
Yamada et al. Intracranial calcification on gradient-echo phase image: depiction of diamagnetic susceptibility. Radiology. 1996
IMAGE INTERPRETATION
• Greyscale inversion filtered phase images are
not uniformly windowed or presented equally by all
manufacturers, therefore care must be taken to ensure
correct interpretation.
• A simple step to make sure that you always view the
images in the same way is to look at venous structures &
compare them with the lesions. The lesions with blood
or ferritin (paramagnetic) will have the same signal as
veins.
Mittal. S et al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications. AJNR 2009
IMAGE INTERPRETATION
Magnitude
Phase
CT
Note that calcifications on the phase (center) image have different signal than blood in
the sagittal sinus (arrow).
Mittal. S et al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications. AJNR 2009
CLINICAL APPLICATIONS
DIFFERENTIATION BETWEEN MICROBLEED
FROM MICROCALCIFICATIONS
• Both calcifications & iron accumulation in chronic hemorrhage
are hypointense on T2-WI & show “blooming” in SWI. It is
not possible to differentiate between them on conventional
MR sequences & CT is usually required.
• SWI - phase represents an average magnetic field of protons
in a voxel, which depends on the susceptibility of tissues.
• Calcium is diamagnetic in nature and the phase shift induced
by it is opposite to that found with paramagnetic substances
like deoxy-Hb, methemoglobin (Met-Hb), hemosiderin &
ferritin.
Yamada et al. Intracranial calcification on gradient-echo phase image: depiction of diamagnetic susceptibility. Radiology. 1996
DIFFERENTIATION BETWEEN MICROBLEEDS AND
MICROCALCIFICATIONS - AMYLOID ANGIOPATHY
A
B.
SWI (A) shows subcortical black spots in parietal/occipital regions. One cannot be sure if they
correspond to microhemorrhages or calcifications. In SWI-Phase (B) the spots have similar
signal to that of the deep venous structures (black arrow) indicating that they correspond to
blood.
DIFFERENTIATION BETWEEN MICROBLEEDS AND
MICROCALCIFICATIONS - POST-RADIOTERAPHY
A
B
High grade glioma after radiotherapy.
SWI (A) cannot differentiate if the signal loss in the ventricular system & occipital lobes are
due to choroid plexus microcalcifications or microhemorrhages. SWI-Phase (B) shows
bright spots similar in signal to blood in the deep venous system (arrow) suggesting
microhemorrhages instead of calcifications.
TUBEROUS SCLEROSIS
A
B
C
Patient with tuberous sclerosis.
SWI (A) shows multiple black lesions in the brain & subependymal regions. Phase - SWI (B)
shows dark lesions (opposite signal compared to deep veins, arrow), suggesting calcifications.
NECT (C) confirms calcifications which are common in patients with this disease.
MANGANESE ACCUMULATION
• Manganese is paramagnetic & shortens T1 relaxation time sufficiently to produce
pallidal T1 hyperintensity in liver disease. Usually SWI fails to show blooming.
Magnetic susceptibility of paramagnetic manganese is much higher than the
diamagnetic calcium.
• Association of manganese deposition & T1 hyperintensity is well known but
causation is not well established.
A.
B.
C.
Patient with portosystemic shunt.
Axial non-enhenced T1WI (A) demonstrates high signal in the globus pallidi, compatible with manganese
accumulation which is also seen on SWI (B). SWI-Phase image (C) shows a bright signal in the globus pallidi.
Neurology India; 57(6):812-13, 2009.
VASCULAR MALFORMATIONS
• SWI-Phase offers improved sensitivity,
revealing slow-flow vascular malformations
that are not visible on conventional GRE
images by virtue of identifying their
deoxyhemoglobin contents.
Reichenbach JR. Small vessels in the human brain: MR venography with deoxyhemoglobin as an intrinsic contrast agent. Radiology. 1997.
VASCULAR MALFORMATIONS
A
B
SWI (A) & SWI-Phase (B) demonstrate a periventricular DVA. This sequences are
useful when gadolinium-based contrast agents are contra-indicated. Note all
veins have same signal intensity in B.
VASCULAR MALFORMATIONS
A.
B.
Cavernous malformation.
SWI (A) shows a hypointense left parietal subcortical mass. SWI-Phase (B) shows that this
lesion has the same signal as the deep venous system (arrow) corresponding to blood
products.
TUMOR CHARACTERIZATION
• Many imaging characteristics have been suggested to predict
glioma grade including heterogeneity, contrast enhancement,
mass effect, necrosis, metabolic activity & high cerebral blood
volume. In human glioma cells, levels of ferritin & transferrin
receptors detected during immunohistochemical analysis
correlate with tumor grade.
• SWI-Phase helps to detect calcifications and/or blood, therefore,
improving accuracy in interpretation of brain tumors & aiding in
grading.
Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain – A pictorial review. Neuroradiology. 2008 .
TUMOR CHARACTERIZATION
A
B
C
Glioblastoma. A. Post contrast T1W1, B. SWI and C. SWI-Phase show a tumor in the deep
white matter & corpus callosum with gadolinium enhancement. The respectively dark &
bright spots in B/C (black arrows) are compatible with blood which is more common in
glioblastoma (GB) than in lower grade tumors & lymphomas. Note that intratumoral
hemorrhage has same signal as the deep medullary veins (white arrow).
TUMOR CHARACTERIZATION
A
B
C
Glioblastoma. A. Axial FLAIR, B. SWI & C. SWI-Phase show an intraventricular mass. The
respectively dark & bright spots in B/C (black arrows) are compatible with blood & enlarged
blood vessels which are more prevalent in GB than in lower grade gliomas & lymphomas.
Note that intratumoral hemorrhage has same signal as deep medullary veins (white arrow).
TUMOR CHARACTERIZATION
A
B
Meningiomas, patient with NF 2 .
Post Contrast T1WI (A) shows a multiple enhancing lesions. SWI-Phase (B) shows dark spots (black
arrows) that correspond to calcifications in the frontal lesion as well as in the intraventricular one. Note
that the signal of calcifications is different from that of the deep veins (white arrow).
TUMOR CHARACTERIZATION
A
B
Schwannomas, patient with NF 2 .
Post contrast T1WI (A) demonstrates bilateral vestibular schwannomas. SWI-Phase (B)
shows bright intratumoral spots compatible with microbleeds (black arrow) which are more
frequently found in vestibular schwannomas than in meningiomas. Note the their signal is
similar to the cerebellar veins (white arrow).
TUMOR CHARACTERIZATION
A
B
C
Retinoblastomas.
A. SWI, B. SWI-Phase. Tumors show “blooming” effect (black arrows) in both globes and avid
enhancement in C. Note that signal of lesions is different to signal of blood in sinus (white
arrow).
TUMOR CHARACTERIZATION
A
B
C
D
Lung cancer metastasis.
A. FLAIR, B. post contrast T1WI, C. SWI and D. SWI-Phase show multiple hyperintense foci on
FLAIR without gadolinium enhancement suggesting blood components (circles in all images).
Presence of blood made the lesions more conspicuous & suggested hemorrhagic
metastases. Signal intensity of intratumoral blood is similar to that in deep veins (arrow).
VENOUS TROMBOSIS
• SWI aids the detection of cortical venous
thrombosis which are otherwise difficult to
detect in conventional spin-echo T2 & T1
images. However, phase images may not be
able to distinguish clotted from un-clotted
blood.
Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain – A pictorial review. Neuroradiology. 2008 .
VENOUS TROMBOSIS
A
B.
C
Cortical vein thrombosis.
Post contrast T1WI (A) shows filling defect is seen in a cortical vein compatible with thrombus
(white arrow). The thrombosed vein is better demonstrated in SWI (B) but on the phase
image (C) its signal is equivalent to blood in non-thrombosed veins (black arrow) a possible
pitfall.
THRAUMATIC AXONAL INJURY
•SWI is 3–6 times more sensitive than
conventional gradient-echo sequences in better
characterizing size, number, volume, &
distribution of hemorrhagic lesions in diffuse
axonal injury.
Mittal S. Susceptibility-weighted imaging: technical aspects and clinical applications, part 2. AJNR. 2009
THRAUMATIC AXONAL INJURY
A
B
C
Non-enhanced CT (A) shows a hypodense area (black arrow) in contiguity to a skull fracture in a
patient with head trauma. SWI (B) & SWI-Phase (C) reveal hemorrhagic in the lesion (black
arrow) & adjacent subarachnoid space compatible with adjacent subcortical hemorrhagic
axonal injury. Note that blood in lesion has similar signal to that in veins (white arrow).
SUBARACHNOID HEMORRHAGE
• SWI is better than CT in detecting SAH &
intraventricular hemorrhage. SWI is very
sensitive to small amounts of SAH. Aliasing on
phase images could be used to help
differentiate SAH from veins.
Wu Z. et al. Evaluation of traumatic subarachnoid hemorrhage using susceptibility-weighted imaging.
AJNR. 2010.
SUBARACHNOID HEMORRHAGE
A.
B.
C.
Axial FLAIR (A) shows bright signal in sulci of the right parietal & left frontal lobes. SWI (B)
reveals “blooming effect” in these areas (black arrows). SWI-Phase (C) confirms subarachnoid
hemorrhage demonstrating bright lesion within the sulci displaying similar signal to that in the
superior sagittal sinus (white arrow).
LIMITATIONS
• Aliasing
– A major limitation in phase imaging is aliasing. Although
filtered phase images are more sensitive to small amounts
of calcium than CT they may perform poorly & can be
confusing when larger amounts of calcification are
present. When the field is large enough that the phase
exceeds π (pi) radians, it will alias to -π radians & will
appear dark rather than bright. The net effect is that large
regions of calcifications and/or blood can be
inhomogeneous & have signal dropout making it difficult
to ascertain their nature.
Wu Z etal. Identification of calcification with MRI using susceptibility-weighted imaging: a case study. JMRI 2009;29: 177-82
ALIASING
A
B
C
Parafalcine hemorrhagic lesion.
While SWI (A) shows a low signal lesion, the SWI-Phase (B) reveals mixed signal inside the
lesion that prevents confirming its nature. Non-enhanced CT (C) shows no calcifications,
compatible with a purely hemorrhagic lesion, which was later diagnosed as a meningioma.
CONCLUSIONS
• There are some several conditions in which SWI-Phase is
a useful tool in clinical practice. The most relevant are the
differentiation of microcalcifications from microbleeds
related to small vascular lesions & to identify calcium or
bleeds in tumors.
• It is important to be aware of artifacts, the most common
one is aliasing that may limit visualization of large
calcifications & hematomas as they appear of
heterogeneous signal masking the underlying nature of
the lesion.
REFERENCES
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susceptibility weighted MR imaging of the brain - a pictorial review. Neuroradiology. 2008;50(2):105-16.
2. Tong KA, Ashwal S, Obenaus A, Nickerson JP, Kido D, Haacke EM. Susceptibility-weighted MR imaging: a review of
clinical applications in children. AJNR American journal of neuroradiology. 2008;29(1):9-17.
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4. Yamada N, Imakita S, Sakuma T, Takamiya M. Intracranial calcification on gradient-echo phase image: depiction of
diamagnetic susceptibility. Radiology. 1996;198(1):171-8.
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hyperintensity due to manganese deposition? Neurology India; 57(6):812-13, 2009.
8. Wu Z, Mittal S, Kish K et-al. Identification of calcification with MRI using susceptibility-weighted imaging: a case
study. J Magn Reson Imaging. 2009;29 (1): 177-82.