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Susceptibility Weighted Imaging (SWI)

Susceptibility Weighted Imaging (SWI)



SWI , high susceptibility for blood products, iron depositions, and calcifications ...

SWI , high susceptibility for blood products, iron depositions, and calcifications
makes susceptibility-weighted imaging an important additional sequence for the diagnostic
workup of pediatric brain pathologic abnormalities. Compared with conventional MRI
sequences, susceptibility-weighted imaging may show lesions in better detail or with higher



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    Susceptibility Weighted Imaging (SWI) Susceptibility Weighted Imaging (SWI) Presentation Transcript

    • Susceptibility-weighted imaging (SWI) is a high-spatial resolution 3D gradient-echo MR imaging techniquewith phase post processing that accentuates theparamagnetic propertiesof blood products and is very sensitive in the detection ofintravascular venous deoxygenated blood as well asextravascular blood products Magnetic susceptibility is defined as the magneticresponse of a substance when it is placed in an externalmagnetic field. It was originally referred to as high resolutionblood oxygen level– dependent venography, but becauseof its broader application than evaluating venousstructures, it is now referred to as SWI.
    • SWI exploits the loss of signal intensity created bydisturbance of a homogeneous magneticfield, Briefly, these disturbances can be caused by variousparamagnetic, ferromagnetic, or diamagnetic substancessuch as air/tissue or air/bone interfaces. As spins encounter heterogeneity in the localmagnetic field, they precess at different rates and causeoverall signal-intensity loss in T2*- weighted (ie, gradient-echo) images. Sensitivity to susceptibility effects increase as oneprogresses from fast spin-echo to routine spin-echo togradient-echo techniques, from T1- to T2- to T2*-weighting, from short-to-long echo times, and from lower tohigher field strengths
    • Application of a magnetic field to the brain generatesaninduced field that depends on the applied magnetic fieldandon the magnetic susceptibility of molecules within the brain. Magnetic susceptibility variations are higher at theinterface of 2 regions, and signal intensity changes are alsodependent on other factors, includinghematocrit, deoxyhemoglobin concentration, red blood cellintegrity, clot structure, moleculardiffusion, pH, temperature, field strength, voxelsize, previous contrast material use, blood flow, and vessel
    • SWI is not a new concept, but recent advances haveallowedthe technique to be refined, thereby expanding itsapplicabilityto study various conditions and pathologic states. Haacke et al designed SWI, as first described inReichenbach et al, because it is a high-spatial-resolution3D fast low-angle shot (FLASH) MR imaging technique thatis extremely sensitive to susceptibility changes.
    • The underlying contrast mechanism is primarilyassociated with the magnetic susceptibility differencebetween oxygenated and deoxygenatedhemoglobin, leading to a phase difference between regionscontaining deoxygenated blood and surroundingtissues, resulting in signal intensity cancellation. The term “SWI” implies full use of magnitude andphase information and is not simply a T2* imagingapproach.
    • The original sequence consisted of a stronglysusceptibility-weighted, low-bandwidth (78 Hz/pixel) 3DFLASH 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 acquiredby using a rectangular FOV (5/8 of 256 mm) and a matrixsize of 160X512, The acquisition time varied from 5 to 10minutes, depending on the desired coverage.
    • After the data are acquired, there is additionalpostprocessing that accentuates the signal intensity losscaused by any susceptibility effects. This primarily involves the creation of a phase maskto enhance the phase differences between paramagneticsubstances and surrounding tissue. It should be noted that there are enhancements ofcontrast just from the phase changes, even though theremay be no T2* effects and vice versa.
    • 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 of32x32;C, Filtered-phase image with a central filter size of 64x64.
    • Combining Magnitude and Phase Images to Create Magnitude SWI Data Although HP-filtered phase images themselves arequite illustrative of the susceptibility contrast, they areinfluenced byboth the intratissue phase (fromBin) and the associatedextratissuephase (from Bout), which can impair the differentiation ofadjacent tissues with different susceptibilities. Hence, the ―phase mask‖ is designed to enhancethe contrast in the original magnitude image bysuppressing pixels having certain phase values. Magnitude and phase data are brought together as afinal magnitude SWI dataset by multiplying a phase maskimage into the original magnitude image.
    • A, Unprocessed original SWI magnitude image.B, HP-filtered phase image.C,Processed SWI magnitude image (ie, after phase-mask multiplication).
    • 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 displaymIP data over the original magnitude images (A) and over the processed SWIimages (B). The mIP is carried out over 7 sections (representing a 14-mm slabthickness).The images were collected at 3T. There is a dropout of signal intensity in the frontalpart of the brain, but otherwise the vessel visibility is much improved in B
    • There have been newer modifications to thissequence, including echo-planar techniques, to decreasethe scanning time. The sequence, along with its automaticpostprocessing, is currently available for Siemens(Erlangen, Germany) 1.5 and 3TMRimaging scannerplatforms; currently SWI is being used in approximately 5–10 centers in the United States and several centers inEurope and Asia.
    • As in adults, SWI has been found to be helpful inchildren with various disorders, including traumatic braininjury (TBI), coagulopathic or other hemorrhagicdisorders, cerebral vascularmalformations, venous shunt surgery orthrombosis, infarction,neoplasms, and degenerative/neurometabolic disordersassociated with intracranial calcifications or iron deposition. The following sections discuss different pediatricclinical applications of SWI.
    • Traumatic Brain Injury Detection of large amounts of intracranialhemorrhageis of primary importance in the acute surgical managementof patients with TBI. However, identification of smallerhemorrhages and their location now provides usefulinformation regarding mechanism of injury and potentialclinical outcome. This is particularly helpful for evaluation of diffuseaxonalinjury (DAI), which is often associated with small hemorrhagesin the deep brain that are not routinely visible on CT orconventional MR imaging sequences.
    • Susceptibility-weighted imagingis, however, three to six times more sensitive thanT2*-weighted MRI for the detection of smallhemorrhages in diffuse axonal injury SWI is particularly sensitive to the presenceof very small hemorrhages. Although largerhemorrhages are visible on CT and conventionalMR imaging sequences, numerous smallhemorrhagic shearing lesions are only visible byusing the SWI technique.
    • Diffuse axonal injury. This 17-year-old boy had a severe TBIin a motorcycle crash and had an initial GCS score of 5.Intracranial pressure was normal. Axial T2 and SWI images at thelevel of the centrum semiovale (A and B) and at the level of thelateral ventricles (C and D) are depicted. Small hemorrhagic shearing injuries in the left frontalsubcortical white matter (arrows) are more are visible on SWI (B).At the lower level, SWI (D) shows additional small hemorrhagicshearing foci (open arrows) in the left frontal white matter, rightsubinsular region, and left splenium that are only partly visible onT2-weighted images.
    • Only patients with involvement of 7 or moreregions were at risk for poor outcomes. Morerecently authors have shown that the number andvolume of SWI hemorrhagic lesions correlated withspecific neuropsychological deficits. These findings suggest that the SWItechnique can play an important role in moreaccurately diagnosing the degree of injury as wellas providing valuable prognostic information thatmay help guide the management and rehabilitationof affected children.
    • SWI is also useful in detecting other types ofhemorrhagic lesions, such as those seen withcoagulopathy, vasculitis, or some infections. In the settingof coagulopathy, either thrombosis (to be discussed later)or hemorrhage can occur, and the extent of bleeding maybe underestimated. An example of this is shown in Fig below, in which a6-month old infant presented with severe leukemia and hadprofound neurologic deficits and, even several yearslater, was profoundly incapacitated. Initially, it wasuncertain as to why such severe deficits were present, butSWI revealed numerous extensive small hemorrhagesthroughout the brain that were not visible on other imagingsequences.
    • Leukemic hemorrhage. This 6-month-old boy presented with fever, leukocytosisthrombocytopenia, anemia, and hepatosplenomegaly. Axial FLAIR (A), T2-weighted (B), and SWI (C) images show a largehematoma in the left frontal lobe (asterisks). A few small hemorrhages withsurrounding edema were also visible in the right subcortical white matter (openarrows) 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 bilateralhemispheric white matter are only visible on SWI
    • Certain types of vascular malformations withslow flow have been shown to be better visualizedwith SWI, including cavernomas andtelangiectasias. These types of low-flowmalformations have been designated ―occult‖because they are usually not visible onconventional angiography, unlike high-flowarteriovenous malformations.
    • Cavernomas that have previously bled are usuallydetectable on routine MR imaging due to the prominent signalintensity of hemorrhagic products. Cavernomas may alsosubsequently develop dystrophic calcifications that can bedetected on CT. However, if these lesions are intact and havenot 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 forslow-flow malformations Large and small cavernomas areeasily detected by susceptibility-weighted imaging because ofthe ―blooming‖ effect of blood degradation products within thecavernomas
    • Small cavernomas may be missed byconventional MRI. Their identificationis, however, important for differentiating familialand sporadic cavernomatosis. Susceptibility-weighted imaging is highly sensitive in detectingmicrocavernomas Susceptibility-weighted imaging is alsohelpful in identifying developmental venousanomalies, which are anatomic variants of thevenous drainage but may be associated withvarious vascular malformations
    • Multiple cavernous angiomas. This 7-year-old boy was admitted forevaluation of ataxia, dizziness, blurred vision, and alternating facial droop. MR imaging shows a large pontine hemorrhage (arrow) the onsagittal T1-weighted image (A) and axial SWI (B).Axial SWI at a higher level(C) shows numerous additional scattered smallhypointense lesions, consistent with multiple cavernomas. The pontine hemorrhage and cavernoma were subsequentlysurgically resected. He has been clinically stable.
    • Cavernoma/telangiectasia. This 24-year-old student was being evaluated for partial complexseizures, and anMR image revealed an incidental lesion in the left pons.Contrast-enhanced coronal GRE T1-weighted image (A) depicts asmall mildly enhancing lesion in the left pons (arrow). The lesion wasnot visible on T2-weighted images (not shown) but appears on axialSWI (B) as a markedly hypointense area. The ill-defined blush iscommonly seen in low-flow vascular malformations, and the markedhypointensity of the lesion on the SWI sequence suggests that thisrepresents an occult vascular malformation. Although there was nopathologic confirmation, given the location and size, it likely represents
    • Telangiectasias are smaller and less common thancavernomas and can occur in mixedcavernoma/telangiectasia lesions. These may occursporadically or may be associated with syndromes(eg, hereditary hemorrhagic telangiectasia) or may occuras a result of endothelial injury, such as radiation-inducedvascular injury, particularly in children who have receivedcranial irradiation Although the occasional lesion is usually clinicallybenign, more extensive lesions may result in subtleneurologic symptoms for which no explanation may befound with routine imaging. However, SWI can detect thesetypes of lesions
    • Radiation telangiectasias. This 11-year-old boy was diagnosed with medulloblastomaand had undergone surgical resection, radiation, and chemotherapy. He presented 8years later with intermittent pulsating parieto-occipital headaches and complete lossof hearing in the left ear. Multiple serial MR imaging studies during the course ofseveral years (not shown) showed a few scattered small faintly hypointense foci in thewhite matter but no other significant abnormalities. On his most recent MR imaging study, an axial T2-weighted image (A) againshows a few small hypointense foci in the white matter (arrows), similar to those inprior studies. However, axial SWI (B) shows numerous hypointense foci throughoutthe white matter, suggestive of radiation-induced telangiectasias.
    • SWS is thought to be a disorder of a primitivepersistent plexus resulting in a pial angioma. This isassociated with loss of normal cortical venousdrainage, which could be congenital or resulting fromprogressive obliteration. The resulting abnormal venous drainage occursthrough the deep venous system. Although this provides aninitial compensatory mechanism that may portend a betterprognosis, the deep venous circulation may be sluggish orassociated with stasis, particularly if there is dural sinusstenosis or occlusion The pial angioma is often easily seen with contrast-enhanced imaging. However, the deep veins are onlyoccasionally visible, usually with conventional angiography.In typical patients with SWS, chronic venous ischemiaeventually results in hemiatrophy and cortical ―tram-track‖
    • Susceptibility-weighted imaging is helpful to identifyand evaluate Sturge-Weber syndrome by revealing theabnormal venous Vasculature. The venous prominence isrelated to venous stasis or the subsequent lower bloodoxygenation due to increased oxygen extraction. Susceptibility- weighted imaging is also highlysensitive in detecting cortical ―tram-track‖ calcificationssecondary to chronic venous ischemia. The SWI technique remarkably demonstratesnumerous deep medullary veins that are not well visualizedby any other MR imaging sequence, particularly in the earlymild cases of SWS. The increased visibility of veins is not only consistentwith shunt surgery but may also imply increased oxygenextraction in the underlying brain parenchyma, particularlyif there is associated deep venous stasis.
    • SWS. This 7-year-old boy was originally seen at 18 days of life anddiagnosed with SWS . His first MR imaging scan at 7 months (not shown)demonstrated intense contrast enhancement of the vascular structures of theright parietal and occipital regions. Follow-up MR imaging. A mildly enlarged deep subependymal veinalong the right lateral ventricle is visible (arrows) on a T2W (A), contrast-enhanced T1W (B), and SWI (C). The postcontrast T1W (B) also revealsadditional prominently enhancing right subcortical and deep medullary veins(open arrows). However, SWI (C) best delineates these prominent veinsrelated to abnormal venous drainage in this condition and also shows asubtly prominent left subependymal vein (arrowhead), indicating early
    • 13-year-old boy with Sturge-Weber syndrome.A–C, Axial contrast-enhanced T1-weighted image (A), phasesusceptibility-weighted image (B), and minimal-intensity-projection susceptibility-weighted image (C) show prominentleptomeningeal angiomatosis.On T1-weighted image (A), moderate cortical parietooccipitalatrophy is noted. On susceptibility-weighted images (BandC), marked cortical calcifications (arrows) are seen.Intracortical calcifications induce mild phase shift on phase
    • Compared with CT and T2*-weightedMRI, susceptibility-weighted imaging is more sensitive forthe detection of a hemorrhagic conversion of an infarction.Susceptibility- weighted imaging typically shows prominentsusceptibility-weighted imaging– hypointense medullaryveins in brain regions with diminished or critical perfusionbecause of an increased deoxygenated hemoglobinconcentration in that area (Fig. 8). These areas typically match hypoperfused areas onperfusion-weighted imaging. Susceptibility weightedimaging may evaluate hypoperfused viable brain regionswithout using IV contrast agent. The ischemic penumbracan be identified by correlating susceptibility-weightedimaging with diffusion-weighted imaging
    • Fig. 8—6-year-old boy with known sickle cell disease presenting with acuteleft-side hemiplegia and decreased responsiveness. A,ADC map shows restricted diffusion characterized by low ADCvalues within right insula and frontal operculum. Additional focal lesionswith restricted diffusion are seen in right frontopolar region as well as in leftanterior basal ganglia and left central region. B, Matching minimal-intensity-projection susceptibility-weightedimage shows prominent intramedullary veins matching area of diffusionabnormality. In addition,prominent sulcal veins are noted along lefthemisphere draining region of brain that exceeds region of diffusionabnormality.
    • • 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.
    • Venous Thrombosis. Sinovenous thromboses are increasingly beingdiagnosed in neonates, infants, and children, with the use of MRimaging and MRvenography, and have contributed to theevaluation and management of such patients SWI can be helpful in demonstrating unsuspectedthrombosis to a much better degree of resolution thanconventional imaging. Venous stasis likely results in greater prominence of theveins due to higher concentrations of intravasculardeoxyhemoglobin. SWI can also demonstrate the extent of parenchymalbrain hemorrhage that can occur after venous thrombosis thatleads to infarction.
    • Cortical vein thrombosis. This 16-year-old boy with acutelymphoblastic leukemia presented with emesis, headache, andimpaired speech. On the postcontrast coronal T1-GRE image (A), there isa large irregular acute hemorrhage in the left parietal lobe withsurrounding edema and adjacent sulcal effacement. Tubularfilling defects are also visible on either side of the superiorsagittal sinus (white arrows), consistent with thrombosed corticalveins. An axial FLAIR image (B) shows symmetric bilateraledema, adjacent to large hypointense cortical veins (blackarrows) on SWI (C).
    • Susceptibility-weighted imaging is highly sensitive forthe detection of germinal matrix hemorrhages, andresidual blood depositions may be noted many months oryears later. In neonatal periventricular hemorrhagicinfarctions, dilated or thrombosed intramedullary veins maybe noted. Susceptibility-weighted imaging may also identifyengorged intramedullary veins in hypoxic-ischemicencephalopathy (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.
    • prominent deep medullary veins has been seen inseveral cases of children with severe cerebral edemabefore brain death. As shown in FIG. SWI was helpful inthis patient with TBI, who was sedated and paralyzedbecause the presence of diffusely prominent medullaryveins suggested severe ischemic brain injury that was likelyirreversible and predictive of brain death. The etiology of the increased visibility of deep veinsremains uncertain but could reflect a combination ofincreased oxygen extraction, venous stasis, and/or possiblyvenous dilation secondary to release of substances suchas adenosine after cell death
    • Prominent veins in brain death. This 8-year-old boy was ejected in amotor vehicle crash and sustained severe TBIAnteroposterior view of nuclear medicine brain scan 5 days later (A)demonstrates decreased but not absent cerebral blood flow asevidenced by activity in the superior sagittal sinus. MR imaging wasobtained 11 days later. Axial SWI (B) shows scattered smallhypointense hemorrhages (arrows) in the subcortical white matter andcorpus callosum, consistent with shearing injuries. In addition, there areprominent deep medullary veins (open arrows) throughout the bilateralhemispheres.He subsequently met clinical criteria for brain death and was taken offventilatory support 2 weeks after admission.
    • Neurocysticercosis is the most common parasiticinfection ofthe CNS worldwide In the CNS, T solium is deposited in thecerebral parenchyma, meninges, spinal cord, and eyes andforms small 1- to 2-mm fluid-filled cysticerci cysts. After thelarvae die, the capsule thickens and the cysts degenerate. In thefinal stage, the small cystic lesionscompletely calcify On CT, tiny calcified nodules without mass effect aretypically noted Conventional GE and SWI magnitude imagesboth show calcifications and hemorrhage as areas ofhypointense signal intensity. In a recent article, it has been demonstrated the value ofSWI filtered-phase images as a means of differentiatingparamagnetic from diamagnetic substances, because the formerwill have a negative phase and the latter, a positive phase (for aright-handed system). In a patient with a history of neurocysticercosis, the SWIphase image associated with the SWI magnitude image shows a
    • Conventional GE magnitude images show both calcifications andhemorrhage as areas of hypointense signal intensity. A, This is equally true for the SWI magnitude image with a TE of 40ms at 1.5T. B, Usually, a CT scan is used to identify calcium. Here, the CTscan reveals many small calcifications. The SWI filtered-phase image is a means to differentiateparamagnetic from diamagnetic substances because the former will have anegative phase and the latter a positive phase (for a right-handed system). C, The phase image associated with the magnitude image in Ashows a 1-to-1 correspondence of the positive phase with the calcifications
    • Susceptibility-weighted imaging is helpful indetecting calcifications, hemorrhages, and abnormal tumorvascularity, yielding information about the tumor grade.Calcifications are typical of low-grade tumors, hemorrhagesand increased vascularity are typical of high-grade tumors. it can sometimes be difficult to distinguishhemorrhage from abnormal venous vasculature.However, pre- and postcontrast SWI studies can potentiallyresolve this dilemma because blood vessels will change insignal intensity after contrast, whereas areas ofhemorrhage will not.
    • SWI also has FLAIR-like contrast, where upon CSFis suppressed but parenchymal edema remainsBright, which can also be helpful for delineation of tumorarchitecture. Information provided by SWI can better definethe location of hemorrhage or neovascularity and may helpguide the surgical or medical management of suchpatients.
    • Increased vascularity in a tumor: This 8-year-old girl presented withheadache, nausea, ataxia, and vomiting. Axial postcontrast T1-weighted image (A) shows patchy irregularenhancement of the lesion. The mass is largely isointense on the T2-weightedimage (B), except for a circular dark region (open arrow) that likely reflects anarea of hemorrhage. On SWI (C), the hemorrhage (open arrow) is markedly hypointensedue to ―blooming‖ effect. There are also small irregular hypointense areas in theposteromedial tumor, suggestive of increased venous vascularity (arrow). Amildly enlarged subependymal vein (arrowhead) is seen on all MR images.Pathology revealed choroid plexus carcinoma.
    • Calcification is a very important indicator in diagnosis anddifferential diagnosis of braintumors. Calcification cannot be definitivelyidentified by MR imaging because its signals are variable onconventional spinecho T1- or T2- weighted images and cannot bedifferentiated from hemorrhage by GE images because both will causelocal magnetic field changes and appear as hypointensities. phase images can help differentiate calcification fromhemorrhage because calcification is diamagnetic, whereas hemorrhageis paramagnetic, resulting in opposite signal intensities on SWI phase a case of a histologically confirmedoligodendroglioma with intratumoral calcification identified by CT. Thehypointensity shown on SWI magnitude images cannot be identified aseither calcification or hemorrhage. On SWI filtered-phase images, the veins along the lateralventricle and sulci appear dark, which means the hyperintensitiesinside the tumor are diamagnetic (ie, they are calcifications, rather thanhemorrhage).
    • An oligodendroglioma in the right frontoinsular region. A, CT scan shows patchycalcification inside the tumor. B, SWI magnitude image shows hypointense signalintensity inside the tumor but cannot identify whether the hypointensity ishemorrhage or calcification. C, On the SWI phase image, calcification isidentified and matches the CT results well.
    • 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 andtemporal horns (asterisks) due to obstructive partly solid enhancinghydrocephalus is also observed. mass. A smallerPathology revealed a diffusely infiltrating low-grade (I-II) nonenhancingastrocytoma with pilocytic features. heterogeneousThe dark signal intensity on SWI was due to calcification component in the right dorsal pons isin this low-grade glioma. noted to have a round
    • Iron and calcium can be discriminated on the basis of theirparamagnetic-versus-diamagnetic behaviors on the SWIfilteredphase images. Generally, the veins will appear dark whenparallel to the field (and when perpendicular depending ontheresolution used) because the field increases. However, forsmall calcium deposits, the phase will appear brighter thanthebackground because the field decreases
    • Calcium deposition can be visualized in certain geneticneurometabolic diseases such as Fahr disease (Fig13), which is usually familial with autosomal dominant(variably expressive) inheritance. Calcium deposition isthought to be due to a disorder of neuronal calciummetabolism, associated with defective iron transport andfree radicals that result in tissue damage and dystrophiccalcification. Calcification typically occurs in the pallidalregions but can also involve the cerebellum andhemispheric white matter. Age at clinical presentation isvariable with development of parkinsonian-like symptoms.Previously CT provided a better diagnostic specificity forcerebral calcification than MR imaging, but SWI may bemore sensitive for early changes.
    • Fahr disease. This 15-year-old girl presented with tics, migraineheadaches, and seizures. There is no significant abnormality onthe T2-weighted (A) image. However, SWI (B) revealscorresponding marked symmetric hypointensity in theanteromedial globus pallidi (arrows) that corresponds to irregularcoarse calcification (arrows) on the CT image (C)—both of whichare much greater than expected for her age. Without the SWI,the underlying disease might not be suspected
    • A 46-year-old male patient presented with acute onset ofright hemiparesis for two hours prior to admission.Diffusion-weighted images showed a small area ofdiffusion restriction in the left temporal region [Figure1]A. On SWI [Figure 1]B, prominent hypointense signalwas seen in the vessels in the same region. On MRperfusion scan, cerebral blood flow (CBF) [Figure 1]Cand mean transit time (MTT) [Figure 1]D maps showed asimilar, matching, defect. A follow-up scan did not showany progression of the infarct size.
    • Figure 1 (A– D): A 46-year-old male with acute onset of righthemiparesis.Diffusion-weighted image (A) shows an area of restricteddiffusion (arrow) in the left temporal lobe. SWI (B), cerebralblood volume (CBV) map (C), and mean transit time (MTT)map (D) show matching areas of abnormality (arrows). Nomismatch is seen
    • Case 2: A 52-year-old male patient presented with transientweakness of the left side of the body. Routinesequences, including diffusion-weighted images [Figure 2]Adid not reveal any abnormality. On SWI images [Figure2]B, a hypointense signal with blooming was seen at thedistal right middle cerebral artery (MCA)bifurcation, indicating thrombosis (the susceptibility sign). Afew prominent hypointense vessels were seen along theleft cerebral hemisphere, indicating ischemia [Figure 2]C.On an MTT map [Figure 2]D, a perfusion defect was seenin the left MCA territory.
    • Figure 2 (A– D): A 52-year-old male with transient weakness ofthe left side of the body. Axial diffusion-weighted image (A) shows noabnormality. SWI image (B) shows a thrombus (arrow) at theright MCA bifurcation (the susceptibility sign). SWI image at ahigher level (C) shows prominent cortical vessels (arrow) onthe right side. Perfusion mean transit time (MTT) map (D)confirms a focal perfusion abnormality (arrow) in the sameregion
    • venous vessels in the ischemic region appearhypointense and prominent.[2],[3],[5] This focal concentrationof hypointense vessels can be potentially used as a markerfor ischemic regions in the brain. The same principle canbe used for identifying the ischemic penumbra (at-risktissue) in acute stroke. Thus, a diffusion - susceptibilitymismatch could potentially provide information similar tothat obtained from a diffusion - perfusion mismatch. [3] In case 1, we see that the affected area ondiffusion, SWI, and perfusion images are almost similar. Onrepeat diffusion imaging 24 hours later, no extension of theinfarct was seen. A diffusion-perfusion mismatch is not asaccurate in predicting the ischemic penumbra as wasoriginally thought. [6] It has been suggested that BOLD-related MRI imaging may provide a better estimation of thepotentially salvageable zone. [7] SWI-diffusion mismatchmay provide similar information, however, further studies
    • Acute thrombus contains deoxyhemoglobin.Therefore, acute arterial thrombosis can be identified onSWI images by what is known as the susceptibilitysign. [3],[4] This comprises of a focal hypointense signal inthe vessel and apparent enlargement of its diameter, due toblooming, related to its paramagnetic properties. Thesusceptibility sign is seen in cases 2. Case 2 also demonstrates the ability of SWI todetect ischemia even when diffusion-weighted images arenormal, as was eventually confirmed by perfusion imagingin our case. This can alert the clinician to the potential forsubsequent infarction and leads to appropriate preventivemeasures.
    • The signal intensity of veins depends on bloodoxygenation. Veins are hypointense compared with arteries. Themagnetic properties of intravascular deoxygenated hemoglobinand the resultant phase differences with adjacent tissues causesignal loss. Thus, veins draining hypoperfused brain regionsappear hypointense on susceptibility-weighted imaging . Veins are, however, typically less hypointense inintubated children because of the high oxygen concentrationsand the high carbon dioxide concentration, which increases theblood flow. The veins consequently appear nearly isointense tothe brain. To achieve good susceptibility-weighted imaging venouscontrast, the carbon dioxide level should be kept below 30–35mm Hg. To prevent misregistration of vessels versus focallesions, the thickness of the minimal intensity projection shouldbe adjusted to the brain size to inimize partial volumeeffects, particularly in neonates.
    • 1-day-old boy with severe perinatal hypoxicischemic injury.A and B, Initial axial minimal-intensity-projection susceptibility-weighted image (A) was obtained ateffective arterial blood oxygenation level of 80%, and follow-up axial minimal-intensity-projectionsusceptibility-weighted image (B) was obtained 3 days later with blood oxygenation level of 100%using Identical imaging parameters. Comparison of images confirms high signal variancerelated to differences in blood oxygenation. Venous blood oxygen level– dependent signal loss iseffectively suppressed by high blood oxygen concentrations. On initial susceptibility weighted minimal-intensity-projection image (A), all veins of superficial and deep venous system, includingintramedullary veins, are prominently hypointense. For correct interpretation of susceptibility-weighted images, radiologists should be aware ofeffective blood oxygenation to avoid misdiagnosis. Children who are examined under generalanesthesia are typically hyperoxygenated, resulting in higher signal of veins on susceptibility-weightedimaging.
    • • Widowing and greyscale inversion: Filtered phase images are not uniformly windowedor presented by all manufacturers and as such care mustbe taken to ensure correct interpretation. As simple step tomake sure that you always view the images in the sameway is to look at venous structures and make sure they areof low signal (if bright you should invert the grey scale).