Susceptibility-weighted imaging (SWI) is an advanced MRI technique that enhances the detection of blood products and provides high-resolution images sensitive to variations in magnetic susceptibility within the brain. It is particularly effective in identifying small hemorrhages associated with traumatic brain injuries, coagulopathy, and vascular malformations, which are not visible using conventional imaging methods. Recent advancements have refined SWI for broader clinical applications in various neurological disorders in both adults and children.

1. PRESENTER–
NIJALINGAPPA
JJMMC
DAVANGERE

2. Susceptibility-weighted imaging (SWI) is a high-
spatial resolution 3D gradient-echo MR imaging technique
with phase post processing that accentuates the
paramagnetic properties
of blood products and is very sensitive in the detection of
intravascular venous deoxygenated blood as well as
extravascular blood products
Magnetic susceptibility is defined as the magnetic
response of a substance when it is placed in an external
magnetic field.
It was originally referred to as high resolution
blood oxygen level– dependent venography, but because
of its broader application than evaluating venous
structures, it is now referred to as SWI.

3. SWI exploits the loss of signal intensity created by
disturbance of a homogeneous magnetic
field, Briefly, these disturbances can be caused by various
paramagnetic, ferromagnetic, or diamagnetic substances
such as air/tissue or air/bone interfaces.
As spins encounter heterogeneity in the local
magnetic field, they precess at different rates and cause
overall signal-intensity loss in T2*- weighted (ie, gradient-
echo) images.
Sensitivity to susceptibility effects increase as one
progresses from fast spin-echo to routine spin-echo to
gradient-echo techniques, from T1- to T2- to T2*-
weighting, from short-to-long echo times, and from lower to
higher field strengths

4. Application of a magnetic field to the brain generates
an
induced field that depends on the applied magnetic field
and
on the magnetic susceptibility of molecules within the brain.
Magnetic susceptibility variations are higher at the
interface of 2 regions, and signal intensity changes are also
dependent on other factors, including
hematocrit, deoxyhemoglobin concentration, red blood cell
integrity, clot structure, molecular
diffusion, pH, temperature, field strength, voxel
size, previous contrast material use, blood flow, and vessel

5. SWI is not a new concept, but recent advances have
allowed
the technique to be refined, thereby expanding its
applicability
to study various conditions and pathologic states.
Haacke et al designed SWI, as first described in
Reichenbach et al, because it is a high-spatial-resolution
3D fast low-angle shot (FLASH) MR imaging technique that
is extremely sensitive to susceptibility changes.

6. The underlying contrast mechanism is primarily
associated with the magnetic susceptibility difference
between oxygenated and deoxygenated
hemoglobin, leading to a phase difference between regions
containing deoxygenated blood and surrounding
tissues, resulting in signal intensity cancellation.
The term “SWI” implies full use of magnitude and
phase information and is not simply a T2* imaging
approach.

7. The original sequence consisted of a strongly
susceptibility-weighted, low-bandwidth (78 Hz/pixel) 3D
FLASH sequence (TR/TE 57/40 ms, flip angle20°) first-
order flow compensated in all 3 orthogonal directions.
Thirty-two to 64 partitions of 2 mm could be acquired
by using a rectangular FOV (5/8 of 256 mm) and a matrix
size of 160X512,
The acquisition time varied from 5 to 10
minutes, depending on the desired coverage.

8. After the data are acquired, there is additional
postprocessing that accentuates the signal intensity loss
caused by any susceptibility effects.
This primarily involves the creation of a phase mask
to enhance the phase differences between paramagnetic
substances and surrounding tissue.
It should be noted that there are enhancements of
contrast just from the phase changes, even though there
may be no T2* effects and vice versa.

9. Creating Susceptibility-Weighted Filtered-Phase
Images
First, an HP filter is applied to remove the low-spatial
frequency components of the background field. This is
usually done by using a 64X64 low-pass filter divided into
the original phase image to create an HP-filter effect. The
end result is the HP filter.
A, Raw phase image.; B, HP-filtered phase image with a central filter size of
32x32;
C, Filtered-phase image with a central filter size of 64x64.

10. Combining Magnitude and Phase Images to Create Magnitude SWI
Data
Although HP-filtered phase images themselves are
quite illustrative of the susceptibility contrast, they are
influenced by
both the intratissue phase (fromBin) and the associated
extratissue
phase (from Bout), which can impair the differentiation of
adjacent tissues with different susceptibilities.
Hence, the ―phase mask‖ is designed to enhance
the contrast in the original magnitude image by
suppressing pixels having certain phase values.
Magnitude and phase data are brought together as a
final magnitude SWI dataset by multiplying a phase mask
image into the original magnitude image.

11. A, Unprocessed original SWI magnitude image.
B, HP-filtered phase image.
C,Processed SWI magnitude image (ie, after phase-mask multiplication).

12. After this processing, the data are often viewed by using a minimum
intensity projection (mIP) over 4 images. Clinically, 4 sections are the
current common display
mIP data over the original magnitude images (A) and over the processed SWI
images (B). The mIP is carried out over 7 sections (representing a 14-mm slab
thickness).
The images were collected at 3T. There is a dropout of signal intensity in the frontal
part of the brain, but otherwise the vessel visibility is much improved in B

13. There have been newer modifications to this
sequence, including echo-planar techniques, to decrease
the scanning time. The sequence, along with its automatic
postprocessing, is currently available for Siemens
(Erlangen, Germany) 1.5 and 3TMRimaging scanner
platforms; currently SWI is being used in approximately 5–
10 centers in the United States and several centers in
Europe and Asia.

14. As in adults, SWI has been found to be helpful in
children with various disorders, including traumatic brain
injury (TBI), coagulopathic or other hemorrhagic
disorders, cerebral vascular
malformations, venous shunt surgery or
thrombosis, infarction,
neoplasms, and degenerative/neurometabolic disorders
associated with intracranial calcifications or iron deposition.
The following sections discuss different pediatric
clinical applications of SWI.

15. Traumatic Brain Injury
Detection of large amounts of intracranial
hemorrhage
is of primary importance in the acute surgical management
of patients with TBI. However, identification of smaller
hemorrhages and their location now provides useful
information regarding mechanism of injury and potential
clinical outcome.
This is particularly helpful for evaluation of diffuse
axonal
injury (DAI), which is often associated with small hemorrhages
in the deep brain that are not routinely visible on CT or
conventional MR imaging sequences.

16. Susceptibility-weighted imaging
is, however, three to six times more sensitive than
T2*-weighted MRI for the detection of small
hemorrhages in diffuse axonal injury
SWI is particularly sensitive to the presence
of very small hemorrhages. Although larger
hemorrhages are visible on CT and conventional
MR imaging sequences, numerous small
hemorrhagic shearing lesions are only visible by
using the SWI technique.

17. Diffuse axonal injury. This 17-year-old boy had a severe TBI
in a motorcycle crash and had an initial GCS score of 5.
Intracranial pressure was normal. Axial T2 and SWI images at the
level of the centrum semiovale (A and B) and at the level of the
lateral ventricles (C and D) are depicted.
Small hemorrhagic shearing injuries in the left frontal
subcortical white matter (arrows) are more are visible on SWI (B).
At the lower level, SWI (D) shows additional small hemorrhagic
shearing foci (open arrows) in the left frontal white matter, right
subinsular region, and left splenium that are only partly visible on
T2-weighted images.

18. Only patients with involvement of 7 or more
regions were at risk for poor outcomes. More
recently authors have shown that the number and
volume of SWI hemorrhagic lesions correlated with
specific neuropsychological deficits.
These findings suggest that the SWI
technique can play an important role in more
accurately diagnosing the degree of injury as well
as providing valuable prognostic information that
may help guide the management and rehabilitation
of affected children.

19. SWI is also useful in detecting other types of
hemorrhagic lesions, such as those seen with
coagulopathy, vasculitis, or some infections. In the setting
of coagulopathy, either thrombosis (to be discussed later)
or hemorrhage can occur, and the extent of bleeding may
be underestimated.
An example of this is shown in Fig below, in which a
6-month old infant presented with severe leukemia and had
profound neurologic deficits and, even several years
later, was profoundly incapacitated. Initially, it was
uncertain as to why such severe deficits were present, but
SWI revealed numerous extensive small hemorrhages
throughout the brain that were not visible on other imaging
sequences.

20. Leukemic hemorrhage. This 6-month-old boy presented with fever, leukocytosis
thrombocytopenia, anemia, and hepatosplenomegaly.
Axial FLAIR (A), T2-weighted (B), and SWI (C) images show a large
hematoma in the left frontal lobe (asterisks). A few small hemorrhages with
surrounding edema were also visible in the right subcortical white matter (open
arrows) on the FLAIR and T2-weighted images.
There is also a small right convexity subdural collection with hemorrhage (arrows).
However, numerous additional hemorrhagic foci throughout the bilateral
hemispheric white matter are only visible on SWI

21. Certain types of vascular malformations with
slow flow have been shown to be better visualized
with SWI, including cavernomas and
telangiectasias. These types of low-flow
malformations have been designated ―occult‖
because they are usually not visible on
conventional angiography, unlike high-flow
arteriovenous malformations.

22. Cavernomas that have previously bled are usually
detectable on routine MR imaging due to the prominent signal
intensity of hemorrhagic products. Cavernomas may also
subsequently develop dystrophic calcifications that can be
detected on CT. However, if these lesions are intact and have
not bled, they may be almost invisible except for a faint or ill-
defined blush of enhancement after contrast administration.
susceptibility-weighted imaging is more sensitive for
slow-flow malformations Large and small cavernomas are
easily detected by susceptibility-weighted imaging because of
the ―blooming‖ effect of blood degradation products within the
cavernomas

23. Small cavernomas may be missed by
conventional MRI. Their identification
is, however, important for differentiating familial
and sporadic cavernomatosis. Susceptibility-
weighted imaging is highly sensitive in detecting
microcavernomas
Susceptibility-weighted imaging is also
helpful in identifying developmental venous
anomalies, which are anatomic variants of the
venous drainage but may be associated with
various vascular malformations

24. Multiple cavernous angiomas. This 7-year-old boy was admitted for
evaluation of ataxia, dizziness, blurred vision, and alternating facial droop.
MR imaging shows a large pontine hemorrhage (arrow) the on
sagittal T1-weighted image (A) and axial SWI (B).
Axial SWI at a higher level(C) shows numerous additional scattered small
hypointense lesions, consistent with multiple cavernomas.
The pontine hemorrhage and cavernoma were subsequently
surgically resected. He has been clinically stable.

25. Cavernoma/telangiectasia. This 24-year-old student was being evaluated for partial complex
seizures, and an
MR image revealed an incidental lesion in the left pons.
Contrast-enhanced coronal GRE T1-weighted image (A) depicts a
small mildly enhancing lesion in the left pons (arrow). The lesion was
not visible on T2-weighted images (not shown) but appears on axial
SWI (B) as a markedly hypointense area. The ill-defined blush is
commonly seen in low-flow vascular malformations, and the marked
hypointensity of the lesion on the SWI sequence suggests that this
represents an occult vascular malformation. Although there was no
pathologic confirmation, given the location and size, it likely represents

26. Telangiectasias are smaller and less common than
cavernomas and can occur in mixed
cavernoma/telangiectasia lesions. These may occur
sporadically or may be associated with syndromes
(eg, hereditary hemorrhagic telangiectasia) or may occur
as a result of endothelial injury, such as radiation-induced
vascular injury, particularly in children who have received
cranial irradiation
Although the occasional lesion is usually clinically
benign, more extensive lesions may result in subtle
neurologic symptoms for which no explanation may be
found with routine imaging. However, SWI can detect these
types of lesions

27. Radiation telangiectasias. This 11-year-old boy was diagnosed with medulloblastoma
and had undergone surgical resection, radiation, and chemotherapy. He presented 8
years later with intermittent pulsating parieto-occipital headaches and complete loss
of hearing in the left ear. Multiple serial MR imaging studies during the course of
several years (not shown) showed a few scattered small faintly hypointense foci in the
white matter but no other significant abnormalities.
On his most recent MR imaging study, an axial T2-weighted image (A) again
shows a few small hypointense foci in the white matter (arrows), similar to those in
prior studies. However, axial SWI (B) shows numerous hypointense foci throughout
the white matter, suggestive of radiation-induced telangiectasias.

28. SWS is thought to be a disorder of a primitive
persistent plexus resulting in a pial angioma. This is
associated with loss of normal cortical venous
drainage, which could be congenital or resulting from
progressive obliteration.
The resulting abnormal venous drainage occurs
through the deep venous system. Although this provides an
initial compensatory mechanism that may portend a better
prognosis, the deep venous circulation may be sluggish or
associated with stasis, particularly if there is dural sinus
stenosis or occlusion
The pial angioma is often easily seen with contrast-
enhanced imaging. However, the deep veins are only
occasionally visible, usually with conventional angiography.
In typical patients with SWS, chronic venous ischemia
eventually results in hemiatrophy and cortical ―tram-track‖

29. Susceptibility-weighted imaging is helpful to identify
and evaluate Sturge-Weber syndrome by revealing the
abnormal venous Vasculature. The venous prominence is
related to venous stasis or the subsequent lower blood
oxygenation due to increased oxygen extraction.
Susceptibility- weighted imaging is also highly
sensitive in detecting cortical ―tram-track‖ calcifications
secondary to chronic venous ischemia.
The SWI technique remarkably demonstrates
numerous deep medullary veins that are not well visualized
by any other MR imaging sequence, particularly in the early
mild cases of SWS.
The increased visibility of veins is not only consistent
with shunt surgery but may also imply increased oxygen
extraction in the underlying brain parenchyma, particularly
if there is associated deep venous stasis.

30. SWS. This 7-year-old boy was originally seen at 18 days of life and
diagnosed with SWS . His first MR imaging scan at 7 months (not shown)
demonstrated intense contrast enhancement of the vascular structures of the
right parietal and occipital regions.
Follow-up MR imaging. A mildly enlarged deep subependymal vein
along the right lateral ventricle is visible (arrows) on a T2W (A), contrast-
enhanced T1W (B), and SWI (C). The postcontrast T1W (B) also reveals
additional prominently enhancing right subcortical and deep medullary veins
(open arrows). However, SWI (C) best delineates these prominent veins
related to abnormal venous drainage in this condition and also shows a
subtly prominent left subependymal vein (arrowhead), indicating early

31. 13-year-old boy with Sturge-Weber syndrome.
A–C, Axial contrast-enhanced T1-weighted image (A), phase
susceptibility-weighted image (B), and minimal-intensity-
projection susceptibility-weighted image (C) show prominent
leptomeningeal angiomatosis.
On T1-weighted image (A), moderate cortical parietooccipital
atrophy is noted. On susceptibility-weighted images (B
andC), marked cortical calcifications (arrows) are seen.
Intracortical calcifications induce mild phase shift on phase

32. Compared with CT and T2*-weighted
MRI, susceptibility-weighted imaging is more sensitive for
the detection of a hemorrhagic conversion of an infarction.
Susceptibility- weighted imaging typically shows prominent
susceptibility-weighted imaging– hypointense medullary
veins in brain regions with diminished or critical perfusion
because of an increased deoxygenated hemoglobin
concentration in that area (Fig. 8).
These areas typically match hypoperfused areas on
perfusion-weighted imaging. Susceptibility weighted
imaging may evaluate hypoperfused viable brain regions
without using IV contrast agent. The ischemic penumbra
can be identified by correlating susceptibility-weighted
imaging with diffusion-weighted imaging

33. Fig. 8—6-year-old boy with known sickle cell disease presenting with acute
left-side hemiplegia and decreased responsiveness.
A,ADC map shows restricted diffusion characterized by low ADC
values within right insula and frontal operculum. Additional focal lesions
with restricted diffusion are seen in right frontopolar region as well as in left
anterior basal ganglia and left central region.
B, Matching minimal-intensity-projection susceptibility-weighted
image shows prominent intramedullary veins matching area of diffusion
abnormality. In addition,prominent sulcal veins are noted along left
hemisphere draining region of brain that exceeds region of diffusion
abnormality.

34. • C, Follow-up CT shows progressing demarcation of
ischemic lesion in right hemisphere and progression of
infarction in left hemisphere where sulcal veins appeared
prominently susceptibility-weighted-imaging hypointense
and widened. This susceptibility-weighted imaging and
diffusion-weighted imaging mismatch apparently reflects
ischemic penumbra. Susceptibility-weighted imaging may
consequently serve as fast map that renders relevant
hemodynamic information about brain tissue with critical
perfusion.

35. Venous Thrombosis.
Sinovenous thromboses are increasingly being
diagnosed in neonates, infants, and children, with the use of MR
imaging and MRvenography, and have contributed to the
evaluation and management of such patients
SWI can be helpful in demonstrating unsuspected
thrombosis to a much better degree of resolution than
conventional imaging.
Venous stasis likely results in greater prominence of the
veins due to higher concentrations of intravascular
deoxyhemoglobin.
SWI can also demonstrate the extent of parenchymal
brain hemorrhage that can occur after venous thrombosis that
leads to infarction.

36. Cortical vein thrombosis. This 16-year-old boy with acute
lymphoblastic leukemia presented with emesis, headache, and
impaired speech.
On the postcontrast coronal T1-GRE image (A), there is
a large irregular acute hemorrhage in the left parietal lobe with
surrounding edema and adjacent sulcal effacement. Tubular
filling defects are also visible on either side of the superior
sagittal sinus (white arrows), consistent with thrombosed cortical
veins. An axial FLAIR image (B) shows symmetric bilateral
edema, adjacent to large hypointense cortical veins (black
arrows) on SWI (C).

37. Susceptibility-weighted imaging is highly sensitive for
the detection of germinal matrix hemorrhages, and
residual blood depositions may be noted many months or
years later.
In neonatal periventricular hemorrhagic
infarctions, dilated or thrombosed intramedullary veins may
be noted. Susceptibility-weighted imaging may also identify
engorged intramedullary veins in hypoxic-ischemic
encephalopathy (Fig. 10).
Fig. 10—Term newborn boy with moderate-degree
hypoxic-ischemic brain injury. Axial minimal
intensity-
projection SWI shows multiple mildly prominent or
dilated hypointense intramedullary veins (arrow)
that drain toward subependymal deep venous
system of lateral ventricles.
This finding is typically seen in brain
edema and may occur in perinatal hypoxic-
ischemic injury. This finding may solidify diagnosis
of hypoxicischemic encephalopathy.

38. prominent deep medullary veins has been seen in
several cases of children with severe cerebral edema
before brain death. As shown in FIG. SWI was helpful in
this patient with TBI, who was sedated and paralyzed
because the presence of diffusely prominent medullary
veins suggested severe ischemic brain injury that was likely
irreversible and predictive of brain death.
The etiology of the increased visibility of deep veins
remains uncertain but could reflect a combination of
increased oxygen extraction, venous stasis, and/or possibly
venous dilation secondary to release of substances such
as adenosine after cell death

39. Prominent veins in brain death. This 8-year-old boy was ejected in a
motor vehicle crash and sustained severe TBI
Anteroposterior view of nuclear medicine brain scan 5 days later (A)
demonstrates decreased but not absent cerebral blood flow as
evidenced by activity in the superior sagittal sinus. MR imaging was
obtained 11 days later. Axial SWI (B) shows scattered small
hypointense hemorrhages (arrows) in the subcortical white matter and
corpus callosum, consistent with shearing injuries. In addition, there are
prominent deep medullary veins (open arrows) throughout the bilateral
hemispheres.
He subsequently met clinical criteria for brain death and was taken off
ventilatory support 2 weeks after admission.

40. Neurocysticercosis is the most common parasitic
infection of
the CNS worldwide In the CNS, T solium is deposited in the
cerebral parenchyma, meninges, spinal cord, and eyes and
forms small 1- to 2-mm fluid-filled cysticerci cysts. After the
larvae die, the capsule thickens and the cysts degenerate. In the
final stage, the small cystic lesions
completely calcify
On CT, tiny calcified nodules without mass effect are
typically noted Conventional GE and SWI magnitude images
both show calcifications and hemorrhage as areas of
hypointense signal intensity.
In a recent article, it has been demonstrated the value of
SWI filtered-phase images as a means of differentiating
paramagnetic from diamagnetic substances, because the former
will have a negative phase and the latter, a positive phase (for a
right-handed system).
In a patient with a history of neurocysticercosis, the SWI
phase image associated with the SWI magnitude image shows a

41. Conventional GE magnitude images show both calcifications and
hemorrhage as areas of hypointense signal intensity.
A, This is equally true for the SWI magnitude image with a TE of 40
ms at 1.5T. B, Usually, a CT scan is used to identify calcium. Here, the CT
scan reveals many small calcifications.
The SWI filtered-phase image is a means to differentiate
paramagnetic from diamagnetic substances because the former will have a
negative phase and the latter a positive phase (for a right-handed system).
C, The phase image associated with the magnitude image in A
shows a 1-to-1 correspondence of the positive phase with the calcifications

42. Susceptibility-weighted imaging is helpful in
detecting calcifications, hemorrhages, and abnormal tumor
vascularity, yielding information about the tumor grade.
Calcifications are typical of low-grade tumors, hemorrhages
and increased vascularity are typical of high-grade tumors.
it can sometimes be difficult to distinguish
hemorrhage from abnormal venous vasculature.
However, pre- and postcontrast SWI studies can potentially
resolve this dilemma because blood vessels will change in
signal intensity after contrast, whereas areas of
hemorrhage will not.

43. SWI also has FLAIR-like contrast, where upon CSF
is suppressed but parenchymal edema remains
Bright, which can also be helpful for delineation of tumor
architecture. Information provided by SWI can better define
the location of hemorrhage or neovascularity and may help
guide the surgical or medical management of such
patients.

44. Increased vascularity in a tumor: This 8-year-old girl presented with
headache, nausea, ataxia, and vomiting.
Axial postcontrast T1-weighted image (A) shows patchy irregular
enhancement of the lesion. The mass is largely isointense on the T2-weighted
image (B), except for a circular dark region (open arrow) that likely reflects an
area of hemorrhage.
On SWI (C), the hemorrhage (open arrow) is markedly hypointense
due to ―blooming‖ effect. There are also small irregular hypointense areas in the
posteromedial tumor, suggestive of increased venous vascularity (arrow). A
mildly enlarged subependymal vein (arrowhead) is seen on all MR images.
Pathology revealed choroid plexus carcinoma.

45. Calcification is a very important indicator in diagnosis and
differential diagnosis of braintumors. Calcification cannot be definitively
identified by MR imaging because its signals are variable on
conventional spinecho T1- or T2- weighted images and cannot be
differentiated from hemorrhage by GE images because both will cause
local magnetic field changes and appear as hypointensities.
phase images can help differentiate calcification from
hemorrhage because calcification is diamagnetic, whereas hemorrhage
is paramagnetic, resulting in opposite signal intensities
on SWI phase a case of a histologically confirmed
oligodendroglioma with intratumoral calcification identified by CT. The
hypointensity shown on SWI magnitude images cannot be identified as
either calcification or hemorrhage.
On SWI filtered-phase images, the veins along the lateral
ventricle and sulci appear dark, which means the hyperintensities
inside the tumor are diamagnetic (ie, they are calcifications, rather than
hemorrhage).

46. An oligodendroglioma in the right frontoinsular region. A, CT scan shows patchy
calcification inside the tumor. B, SWI magnitude image shows hypointense signal
intensity inside the tumor but cannot identify whether the hypointensity is
hemorrhage or calcification. C, On the SWI phase image, calcification is
identified and matches the CT results well.

47. Calcification in
tumor: This 12-year-
old girl had a 2-month
history of intermittent
headaches, emesis,
and left
hemiparesis. Her
initial CT scan (not
available) showed
moderate
hydrocephalus and a
fourth-ventricular
mass. Conventional
MR imaging images
(not shown)
on SWI (A) that corresponds to coarse calcification demonstrated a
(arrow) on the postbiopsy CT (B). Dilation of the partly cystic and
temporal horns (asterisks) due to obstructive partly solid enhancing
hydrocephalus is also observed. mass. A smaller
Pathology revealed a diffusely infiltrating low-grade (I-II) nonenhancing
astrocytoma with pilocytic features. heterogeneous
The dark signal intensity on SWI was due to calcification component in the
right dorsal pons is
in this low-grade glioma. noted to have a round

48. Iron and calcium can be discriminated on the basis of their
paramagnetic-versus-diamagnetic behaviors on the SWI
filtered
phase images. Generally, the veins will appear dark when
parallel to the field (and when perpendicular depending on
the
resolution used) because the field increases. However, for
small calcium deposits, the phase will appear brighter than
the
background because the field decreases

49. Calcium deposition can be visualized in certain genetic
neurometabolic diseases such as Fahr disease (Fig
13), which is usually familial with autosomal dominant
(variably expressive) inheritance. Calcium deposition is
thought to be due to a disorder of neuronal calcium
metabolism, associated with defective iron transport and
free radicals that result in tissue damage and dystrophic
calcification. Calcification typically occurs in the pallidal
regions but can also involve the cerebellum and
hemispheric white matter. Age at clinical presentation is
variable with development of parkinsonian-like symptoms.
Previously CT provided a better diagnostic specificity for
cerebral calcification than MR imaging, but SWI may be
more sensitive for early changes.

50. Fahr disease. This 15-year-old girl presented with tics, migraine
headaches, and seizures. There is no significant abnormality on
the T2-weighted (A) image. However, SWI (B) reveals
corresponding marked symmetric hypointensity in the
anteromedial globus pallidi (arrows) that corresponds to irregular
coarse calcification (arrows) on the CT image (C)—both of which
are much greater than expected for her age. Without the SWI,
the underlying disease might not be suspected

51. A 46-year-old male patient presented with acute onset of
right hemiparesis for two hours prior to admission.
Diffusion-weighted images showed a small area of
diffusion restriction in the left temporal region [Figure
1]A. On SWI [Figure 1]B, prominent hypointense signal
was seen in the vessels in the same region. On MR
perfusion scan, cerebral blood flow (CBF) [Figure 1]C
and mean transit time (MTT) [Figure 1]D maps showed a
similar, matching, defect. A follow-up scan did not show
any progression of the infarct size.

52. Figure 1 (A– D): A 46-year-old male with acute onset of right
hemiparesis.
Diffusion-weighted image (A) shows an area of restricted
diffusion (arrow) in the left temporal lobe. SWI (B), cerebral
blood volume (CBV) map (C), and mean transit time (MTT)
map (D) show matching areas of abnormality (arrows). No
mismatch is seen

53. Case 2:
A 52-year-old male patient presented with transient
weakness of the left side of the body. Routine
sequences, including diffusion-weighted images [Figure 2]A
did not reveal any abnormality. On SWI images [Figure
2]B, a hypointense signal with blooming was seen at the
distal right middle cerebral artery (MCA)
bifurcation, indicating thrombosis (the susceptibility sign). A
few prominent hypointense vessels were seen along the
left cerebral hemisphere, indicating ischemia [Figure 2]C.
On an MTT map [Figure 2]D, a perfusion defect was seen
in the left MCA territory.

54. Figure 2 (A– D): A 52-year-old male with transient weakness of
the left side of the body.
Axial diffusion-weighted image (A) shows no
abnormality. SWI image (B) shows a thrombus (arrow) at the
right MCA bifurcation (the susceptibility sign). SWI image at a
higher level (C) shows prominent cortical vessels (arrow) on
the right side. Perfusion mean transit time (MTT) map (D)
confirms a focal perfusion abnormality (arrow) in the same
region

55. venous vessels in the ischemic region appear
hypointense and prominent.[2],[3],[5] This focal concentration
of hypointense vessels can be potentially used as a marker
for ischemic regions in the brain. The same principle can
be used for identifying the ischemic penumbra (at-risk
tissue) in acute stroke. Thus, a diffusion - susceptibility
mismatch could potentially provide information similar to
that obtained from a diffusion - perfusion mismatch. [3]
In case 1, we see that the affected area on
diffusion, SWI, and perfusion images are almost similar. On
repeat diffusion imaging 24 hours later, no extension of the
infarct was seen. A diffusion-perfusion mismatch is not as
accurate in predicting the ischemic penumbra as was
originally thought. [6] It has been suggested that BOLD-
related MRI imaging may provide a better estimation of the
potentially salvageable zone. [7] SWI-diffusion mismatch
may provide similar information, however, further studies

56. Acute thrombus contains deoxyhemoglobin.
Therefore, acute arterial thrombosis can be identified on
SWI images by what is known as the susceptibility
sign. [3],[4] This comprises of a focal hypointense signal in
the vessel and apparent enlargement of its diameter, due to
blooming, related to its paramagnetic properties. The
susceptibility sign is seen in cases 2.
Case 2 also demonstrates the ability of SWI to
detect ischemia even when diffusion-weighted images are
normal, as was eventually confirmed by perfusion imaging
in our case. This can alert the clinician to the potential for
subsequent infarction and leads to appropriate preventive
measures.

57. The signal intensity of veins depends on blood
oxygenation. Veins are hypointense compared with arteries. The
magnetic properties of intravascular deoxygenated hemoglobin
and the resultant phase differences with adjacent tissues cause
signal loss. Thus, veins draining hypoperfused brain regions
appear hypointense on susceptibility-weighted imaging .
Veins are, however, typically less hypointense in
intubated children because of the high oxygen concentrations
and the high carbon dioxide concentration, which increases the
blood flow. The veins consequently appear nearly isointense to
the brain.
To achieve good susceptibility-weighted imaging venous
contrast, the carbon dioxide level should be kept below 30–35
mm Hg.
To prevent misregistration of vessels versus focal
lesions, the thickness of the minimal intensity projection should
be adjusted to the brain size to inimize partial volume
effects, particularly in neonates.

58. 1-day-old boy with severe perinatal hypoxicischemic injury.
A and B, Initial axial minimal-intensity-projection susceptibility-weighted image (A) was obtained at
effective arterial blood oxygenation level of 80%, and follow-up axial minimal-intensity-projection
susceptibility-weighted image (B) was obtained 3 days later with blood oxygenation level of 100%
using Identical imaging parameters. Comparison of images confirms high signal variance
related to differences in blood oxygenation. Venous blood oxygen level– dependent signal loss is
effectively suppressed by high blood oxygen concentrations. On initial susceptibility weighted minimal-
intensity-projection image (A), all veins of superficial and deep venous system, including
intramedullary veins, are prominently hypointense.
For correct interpretation of susceptibility-weighted images, radiologists should be aware of
effective blood oxygenation to avoid misdiagnosis. Children who are examined under general
anesthesia are typically hyperoxygenated, resulting in higher signal of veins on susceptibility-weighted
imaging.

59. • Widowing and greyscale inversion:
Filtered phase images are not uniformly windowed
or presented by all manufacturers and as such care must
be taken to ensure correct interpretation. As simple step to
make sure that you always view the images in the same
way is to look at venous structures and make sure they are
of low signal (if bright you should invert the grey scale).