ANA LORENA ABELLORENATO HOFFMAN NUNESMAURICIO CASTILLOChapel Hill, NC

eEdE-72

The authors have no disclosures.

Disclosures

PHYSICAL PRINCIPLES

• Susceptibility-weighted imaging (SWI) is a “3D high-spatial resolution fully velocity corrected gradient echo MRI sequence.”

• It is based on the principle that compounds having paramagnetic, diamagnetic & ferromagnetic properties all interact & distort the local magnetic field thus altering the phase of local tissue which results in loss of signal.

Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain – A pictorial review. Neuroradiology. 2008.

Tong KA et al. Susceptibility-weighted MR imaging: a review of clinical applications in children. AJNR. 2008.

PHYSICAL PRINCIPLES•SWI uses 2 sources of data: Phase & Magnitude.•Both phase/magnitude data are acquired separately for processing to create a new SWI.

Magnitude + Filtered-phase

SWI

Sehgal V . Clinical applications of neuroimaging with susceptibility-weighted imaging. JMRI 2005

Postprocessing steps used to create an SWI image: Starts with a magnitude image (top left) and a phase image (top right). The phase image is filtered and a mask is created from this filtered image. The phase mask is then used to create the final SWI image (bottom left).

From: Sehgal V . Clinical applications of neuroimaging with susceptibility-weighted imaging. JMRI 2005

PHYSICAL PRINCIPLES

PHYSICAL PRINCIPLES

• Distinguishing between calcification & blood products is not possible on post processed SWI images; both demonstrate signal drop out & blooming.

• FILTERED PHASE images are able to distinguish between them as diamagnetic & paramagnetic compounds affect phase differently, appearing of opposite signal intensity.

Yamada et al. Intracranial calcification on gradient-echo phase image: depiction of diamagnetic susceptibility. Radiology. 1996

• Paramagnetic compounds include deoxyhemoglobin, ferritin & hemosiderin.

• Diamagnetic compounds include bone minerals & dystrophic calcifications.

• Greyscale inversion filtered phase images are not uniformly windowed or presented equally by all manufacturers, therefore care must be taken to ensure correct interpretation.

• A simple step to make sure that you always view the images in the same way is to look at venous structures & compare them with the lesions. The lesions with blood or ferritin (paramagnetic) will have the same signal as veins.

IMAGE INTERPRETATION

Mittal. S et al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications. AJNR 2009

IMAGE INTERPRETATION

Mittal. S et al. Susceptibility-Weighted Imaging: Technical Aspects and Clinical Applications. AJNR 2009

Magnitude Phase CT

Note that calcifications on the phase (center) image have different signal than blood in the sagittal sinus (arrow).

CLINICAL APPLICATIONS

DIFFERENTIATION BETWEEN MICROBLEED FROM MICROCALCIFICATIONS

• Both calcifications & iron accumulation in chronic hemorrhage are hypointense on T2-WI & show “blooming” in SWI. It is not possible to differentiate between them on conventional MR sequences & CT is usually required.

• SWI - phase represents an average magnetic field of protons in a voxel, which depends on the susceptibility of tissues.

• Calcium is diamagnetic in nature and the phase shift induced by it is opposite to that found with paramagnetic substances like deoxy-Hb, methemoglobin (Met-Hb), hemosiderin & ferritin.

Yamada et al. Intracranial calcification on gradient-echo phase image: depiction of diamagnetic susceptibility. Radiology. 1996

DIFFERENTIATION BETWEEN MICROBLEEDS AND MICROCALCIFICATIONS - AMYLOID ANGIOPATHY

A B.

SWI (A) shows subcortical black spots in parietal/occipital regions. One cannot be sure if they correspond to microhemorrhages or calcifications. In SWI-Phase (B) the spots have similar signal to that of the deep venous structures (black arrow) indicating that they correspond to blood.

DIFFERENTIATION BETWEEN MICROBLEEDS AND MICROCALCIFICATIONS - POST-RADIOTERAPHY

A B

High grade glioma after radiotherapy. SWI (A) cannot differentiate if the signal loss in the ventricular system & occipital lobes are due to choroid plexus microcalcifications or microhemorrhages. SWI-Phase (B) shows bright spots similar in signal to blood in the deep venous system (arrow) suggesting microhemorrhages instead of calcifications.

A B C

Patient with tuberous sclerosis. SWI (A) shows multiple black lesions in the brain & subependymal regions. Phase - SWI (B) shows dark lesions (opposite signal compared to deep veins, arrow), suggesting calcifications. NECT (C) confirms calcifications which are common in patients with this disease.

TUBEROUS SCLEROSIS

MANGANESE ACCUMULATION• Manganese is paramagnetic & shortens T1 relaxation time sufficiently to produce

pallidal T1 hyperintensity in liver disease. Usually SWI fails to show blooming. Magnetic susceptibility of paramagnetic manganese is much higher than the diamagnetic calcium.

• Association of manganese deposition & T1 hyperintensity is well known but causation is not well established.

Patient with portosystemic shunt.Axial non-enhenced T1WI (A) demonstrates high signal in the globus pallidi, compatible with manganese accumulation which is also seen on SWI (B). SWI-Phase image (C) shows a bright signal in the globus pallidi.

A. B. C.

Neurology India; 57(6):812-13, 2009.

VASCULAR MALFORMATIONS

• SWI-Phase offers improved sensitivity, revealing slow-flow vascular malformations that are not visible on conventional GRE images by virtue of identifying their deoxyhemoglobin contents.

Reichenbach JR. Small vessels in the human brain: MR venography with deoxyhemoglobin as an intrinsic contrast agent. Radiology. 1997.

VASCULAR MALFORMATIONS

SWI (A) & SWI-Phase (B) demonstrate a periventricular DVA. This sequences are useful when gadolinium-based contrast agents are contra-indicated. Note all veins have same signal intensity in B.

A B

VASCULAR MALFORMATIONS

A. B.

Cavernous malformation.SWI (A) shows a hypointense left parietal subcortical mass. SWI-Phase (B) shows that this lesion has the same signal as the deep venous system (arrow) corresponding to blood products.

• Many imaging characteristics have been suggested to predict glioma grade including heterogeneity, contrast enhancement, mass effect, necrosis, metabolic activity & high cerebral blood volume. In human glioma cells, levels of ferritin & transferrin receptors detected during immunohistochemical analysis correlate with tumor grade.

• SWI-Phase helps to detect calcifications and/or blood, therefore, improving accuracy in interpretation of brain tumors & aiding in grading.

Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain – A pictorial review. Neuroradiology. 2008 .

TUMOR CHARACTERIZATION

A B C

Glioblastoma. A. Post contrast T1W1, B. SWI and C. SWI-Phase show a tumor in the deep white matter & corpus callosum with gadolinium enhancement. The respectively dark & bright spots in B/C (black arrows) are compatible with blood which is more common in glioblastoma (GB) than in lower grade tumors & lymphomas. Note that intratumoral hemorrhage has same signal as the deep medullary veins (white arrow).

TUMOR CHARACTERIZATION

A B C

Glioblastoma. A. Axial FLAIR, B. SWI & C. SWI-Phase show an intraventricular mass. The respectively dark & bright spots in B/C (black arrows) are compatible with blood & enlarged blood vessels which are more prevalent in GB than in lower grade gliomas & lymphomas. Note that intratumoral hemorrhage has same signal as deep medullary veins (white arrow).

TUMOR CHARACTERIZATION

A B

Meningiomas, patient with NF 2 .Post Contrast T1WI (A) shows a multiple enhancing lesions. SWI-Phase (B) shows dark spots (black arrows) that correspond to calcifications in the frontal lesion as well as in the intraventricular one. Note that the signal of calcifications is different from that of the deep veins (white arrow).

TUMOR CHARACTERIZATION

Schwannomas, patient with NF 2 .Post contrast T1WI (A) demonstrates bilateral vestibular schwannomas. SWI-Phase (B) shows bright intratumoral spots compatible with microbleeds (black arrow) which are more frequently found in vestibular schwannomas than in meningiomas. Note the their signal is similar to the cerebellar veins (white arrow).

TUMOR CHARACTERIZATION

A B

TUMOR CHARACTERIZATION

Retinoblastomas. A. SWI, B. SWI-Phase. Tumors show “blooming” effect (black arrows) in both globes and avid enhancement in C. Note that signal of lesions is different to signal of blood in sinus (white arrow).

A B C

Lung cancer metastasis. A. FLAIR, B. post contrast T1WI, C. SWI and D. SWI-Phase show multiple hyperintense foci on FLAIR without gadolinium enhancement suggesting blood components (circles in all images). Presence of blood made the lesions more conspicuous & suggested hemorrhagic metastases. Signal intensity of intratumoral blood is similar to that in deep veins (arrow).

A B C D

TUMOR CHARACTERIZATION

VENOUS TROMBOSIS

• SWI aids the detection of cortical venous thrombosis which are otherwise difficult to detect in conventional spin-echo T2 & T1 images. However, phase images may not be able to distinguish clotted from un-clotted blood.

Thomas B. et al. Clinical applications of susceptibility weighted MR imaging of the brain – A pictorial review. Neuroradiology. 2008 .

VENOUS TROMBOSIS

A B. C

Cortical vein thrombosis. Post contrast T1WI (A) shows filling defect is seen in a cortical vein compatible with thrombus (white arrow). The thrombosed vein is better demonstrated in SWI (B) but on the phase image (C) its signal is equivalent to blood in non-thrombosed veins (black arrow) a possible pitfall.

THRAUMATIC AXONAL INJURY

•SWI is 3–6 times more sensitive than conventional gradient-echo sequences in better characterizing size, number, volume, & distribution of hemorrhagic lesions in diffuse axonal injury.

Mittal S. Susceptibility-weighted imaging: technical aspects and clinical applications, part 2. AJNR. 2009

A B C

Non-enhanced CT (A) shows a hypodense area (black arrow) in contiguity to a skull fracture in a patient with head trauma. SWI (B) & SWI-Phase (C) reveal hemorrhagic in the lesion (black arrow) & adjacent subarachnoid space compatible with adjacent subcortical hemorrhagic axonal injury. Note that blood in lesion has similar signal to that in veins (white arrow).

THRAUMATIC AXONAL INJURY

SUBARACHNOID HEMORRHAGE

• SWI is better than CT in detecting SAH & intraventricular hemorrhage. SWI is very sensitive to small amounts of SAH. Aliasing on phase images could be used to help differentiate SAH from veins.

Wu Z. et al. Evaluation of traumatic subarachnoid hemorrhage using susceptibility-weighted imaging. AJNR. 2010.

SUBARACHNOID HEMORRHAGE

A. B. C.

Axial FLAIR (A) shows bright signal in sulci of the right parietal & left frontal lobes. SWI (B) reveals “blooming effect” in these areas (black arrows). SWI-Phase (C) confirms subarachnoid hemorrhage demonstrating bright lesion within the sulci displaying similar signal to that in the superior sagittal sinus (white arrow).

LIMITATIONS

• Aliasing– A major limitation in phase imaging is aliasing. Although filtered phase images

are more sensitive to small amounts of calcium than CT they may perform poorly & can be confusing when larger amounts of calcification are present. When the field is large enough that the phase exceeds π (pi) radians, it will alias to -π radians & will appear dark rather than bright. The net effect is that large regions of calcifications and/or blood can be inhomogeneous & have signal dropout making it difficult to ascertain their nature.

Wu Z etal. Identification of calcification with MRI using susceptibility-weighted imaging: a case study. JMRI 2009;29: 177-82

ALIASING

A B C

Parafalcine hemorrhagic lesion.While SWI (A) shows a low signal lesion, the SWI-Phase (B) reveals mixed signal inside the lesion that prevents confirming its nature. Non-enhanced CT (C) shows no calcifications, compatible with a purely hemorrhagic lesion, which was later diagnosed as a meningioma.

CONCLUSIONS• There are some several conditions in which SWI-Phase is

a useful tool in clinical practice. The most relevant are the differentiation of microcalcifications from microbleeds related to small vascular lesions & to identify calcium or bleeds in tumors.

• It is important to be aware of artifacts, the most common one is aliasing that may limit visualization of large calcifications & hematomas as they appear of heterogeneous signal masking the underlying nature of the lesion.

REFERENCES

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