Susceptibility-Weighted MR Imaging: A Reviewof Clinical Applications in Children

PRESENTER– NIJALINGAPPA

JJMMC DAVANGERE

IntroductionSusceptibility-weighted imaging (SWI) is a

high-spatial resolution 3D gradient-echo MR imaging technique with phase post processing that accentuates the paramagnetic propertiesof 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 resolutionblood oxygen level– dependent venography, but because of its broader application than evaluating venous structures, it is now referred to as SWI.

Principles of SWISWI 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 

SW imaging uses phase information in addition to T2*-based contrast to exploit the magnetic susceptibility differences of the blood and of iron and calcification in various tissues

Application of a magnetic field to the brain generates aninduced field that depends on the applied magnetic field andon 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 orientation

SWI is not a new concept, but recent advances have allowedthe technique to be refined, thereby expanding its applicabilityto 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.

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.

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.

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.

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.

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 byboth the intratissue phase (fromBin) and the associated extratissuephase (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.

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 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

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.

Pediatric Clinical Applications of SWI

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 vascularmalformations, venous shunt surgery or thrombosis, infarction,neoplasms, and degenerative/neurometabolic disordersassociated with intracranial calcifications or iron deposition.

The following sections discuss different pediatric clinical applications of SWI.

Detection of Hemorrhagic Lesions

Traumatic Brain Injury

Detection of large amounts of intracranial hemorrhageis 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 axonalinjury (DAI), which is often associated with small hemorrhages in the deep brain that are not routinely visible on CT or conventional MR imaging sequences.

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.

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.

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.

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.

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

Vascular Malformations

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.

Cavernomas

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

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

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.

Cavernoma/telangiectasia. This 24-year-old student was being evaluated for partial complex seizures, and anMR 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 a cavernoma that had not bled.

TelangiectasiasTelangiectasias 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

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.

Neurocutaneous Disorders: Sturge-Weber Syndrome(SWS)

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” calcification

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.

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 contralateral involvement

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 susceptibility-weighted image (B).

Hemorrhage, Stroke, and SinovenousThrombosis

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

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.

• 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.

Demonstration of Increased Veins Related to Venous Stasis or Increased Oxygen Extraction

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.

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).

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-degreehypoxic-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.

Brain Deathprominent 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

Prominent veins in brain death. This 8-year-old boy was ejected in a motor vehicle crash and sustained severe TBIAnteroposterior 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.

NeurocysticercosisNeurocysticercosis 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 lesionscompletely 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 1-to-1 correspondence of the positive phase with the calcifications in the CT

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 in the CT scan (B).

Delineation of NeoplasmsSusceptibility-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.

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.

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.

Intratumoral CalcificationCalcification 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).

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.

on SWI (A) that corresponds to coarse calcification (arrow) on the postbiopsy CT (B). Dilation of the temporal horns (asterisks) due to obstructive hydrocephalus is also observed. Pathology revealed a diffusely infiltrating low-grade (I-II) astrocytoma with pilocytic features.The dark signal intensity on SWI was due to calcificationin this low-grade glioma.

Calcification in tumor: This 12-year-old girl had a 2-month history of intermittent headaches, emesis, and lefthemiparesis. Her initial CT scan (not available) showed moderate hydrocephalus and a fourth-ventricular mass. Conventional MR imaging images (not shown) demonstrated apartly cystic and partly solid enhancing mass. A smaller nonenhancing heterogeneous component in the right dorsal pons is noted to have a round area of markedly hypointensesignal intensity (arrow)

Depiction of Calcification or Iron DepositionIron and calcium can be discriminated on the basis of theirparamagnetic-versus-diamagnetic behaviors on the SWI filteredphase images. Generally, the veins will appear dark whenparallel to the field (and when perpendicular depending on theresolution used) because the field increases. However, forsmall calcium deposits, the phase will appear brighter than thebackground because the field decreases

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.

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

SWI cerebral ischemiaA 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.

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

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.

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

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 are required to evaluate its usefulness

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.

Pitfalls in Susceptibility-WeightedImaging Interpretation

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.

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.

• 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).

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