Neurosurgery notes/Radiology/MRI/Susceptibility weighted imaging (SWI)

Susceptibility weighted imaging (SWI)

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MRI

Used

  • Assessment of acute ischemic stroke.
    • It provides crucial information about the thromboembolus and the neuroparenchyma at risk.

Physics Principles

  • SWI is a three-dimensional, fully velocity-compensated gradient echo sequence with high spatial resolution.
  • It uses both magnitude and raw phase data to create images with unique contrast.
  • Magnetic susceptibility effects, caused by
    • Substances like diamagnetic calcification (negative phase shift)
    • Paramagnetic deoxygenated hemoglobin or hemorrhage (positive phase shift)
  • Post-processing involves high-pass filtering of the phase image and combining it with the magnitude image to form the final SWI image.
  • Multi-echo SWI sequences allow simultaneous acquisition of Magnetic Resonance Angiography (MRA) and Magnetic Resonance Venography (MRV) data, improving differentiation between veins and thromboemboli without increasing scan time.
  • Susceptibility Weighted Imaging and Mapping (SWIM) is a newer quantitative form of SWI that allows accurate depiction of the thromboembolus and quantitative analysis.
 
Left handed system: Siemen Right handed system: GE/Philips
Left handed system: Siemen
Right handed system: GE/Philips
SWI sequence generates two sets of images: raw phase image (a) and magnitude image (c). The raw phase image contains background unwanted low frequency information and requires further high-pass filtering to produce a filtered phase image (b) that is of diagnostic value. The filtered phase image is then converted into a phase mask and combined with the magnitude image to form the final SWI image (d). An additional minimum intensity projection (mIP) image (e) is created by projecting the minimum signal over several slices, usually over four slices. SWI: susceptibility weighted imaging.
SWI sequence generates two sets of images: raw phase image (a) and magnitude image (c). The raw phase image contains background unwanted low frequency information and requires further high-pass filtering to produce a filtered phase image (b) that is of diagnostic value. The filtered phase image is then converted into a phase mask and combined with the magnitude image to form the final SWI image (d). An additional minimum intensity projection (mIP) image (e) is created by projecting the minimum signal over several slices, usually over four slices. SWI: susceptibility weighted imaging.

Clinical Applications in Acute Ischemic Stroke

Thromboembolus Detection and Characterisation
  • SWI allows direct visualization of the hypointense thromboembolus
    • Due to high iron and deoxyhemoglobin content.
    • SWI has superior sensitivity and better contrast resolution for detecting thromboemboli compared to conventional T2* gradient echo (GRE) sequences.
  • The susceptibility vessel sign
    • Hypointensity within an intracranial artery where the thrombosed vessel's diameter exceeds the contralateral normal vessel
    • Reflects the magnetic susceptibility effect of the thrombus, which is related to its histopathological composition (e.g., erythrocyte-rich thrombus).
      • Erythrocyte-rich thromboemboli are more sensitive to intravenous tissue plasminogen activator (IV-tPA) and associated with higher success rates for endovascular recanalization.
        • SWI and SWIM may help identify these subtypes.
acute left MCA stroke. SWI image (a) and phase image (b) demonstrates a thrombus in the superior M2 division of the left MCA, which is a CT equivalent of a MCA ‘dot sign’. DWI (c) shows acute left MCA territory infarct. Time-of-flight MRA confirms abrupt vessel occlusion of the distal superior M2 division of the left MCA. MCA: middle cerebral artery; SWI: susceptibility weighted imaging; CT: computed tomography; DWI: diffusion-weighted imaging; MRA: magnetic resonance angiography.
acute left MCA stroke. SWI image (a) and phase image (b) demonstrates a thrombus in the superior M2 division of the left MCA, which is a CT equivalent of a MCA ‘dot sign’. DWI (c) shows acute left MCA territory infarct. Time-of-flight MRA confirms abrupt vessel occlusion of the distal superior M2 division of the left MCA. MCA: middle cerebral artery; SWI: susceptibility weighted imaging; CT: computed tomography; DWI: diffusion-weighted imaging; MRA: magnetic resonance angiography.
Left PCA stroke. SWI image (a) and phase image (b) show a thrombus (arrow) in the P2 segment of the left PCA. Corresponding time-of-flight MRA (c) confirms abrupt vessel occlusion of the P2 segment of the left PCA. DWI (d) shows acute left PCA territory infarct. PCA: posterior cerebral artery; SWI: susceptibility weighted imaging; MRA: magnetic resonance angiography; DWI: diffusion-weighted imaging.
Left PCA stroke. SWI image (a) and phase image (b) show a thrombus (arrow) in the P2 segment of the left PCA. Corresponding time-of-flight MRA (c) confirms abrupt vessel occlusion of the P2 segment of the left PCA. DWI (d) shows acute left PCA territory infarct. PCA: posterior cerebral artery; SWI: susceptibility weighted imaging; MRA: magnetic resonance angiography; DWI: diffusion-weighted imaging.
  • SWI is more effective than MRA at locating the distal end of a thromboembolus, which is crucial for determining reperfusion success.
  • It can depict native vessel morphology and is well-suited for evaluating the intracranial vertebrobasilar circulation.
    • SWI is particularly sensitive for depicting intramural hematoma in intracranial vertebral artery or posterior inferior cerebellar artery (PICA) dissections.
      • Phase images and SWIM help differentiate hematoma (positive phase shift) from calcifications (negative phase shift).
  • SWI is sensitive in identifying fragmented thrombi and their location, providing critical information for neurointerventional planning as they predict worse outcomes.
 
(a) Labeled anatomical illustration of the anterior circulation of the Circle of Willis. (b) and (c) Collateral circulation as a modifiable variable in determining the final length of thromboembolus. (b) A short ‘original’ MCA thromboembolus in a patient with good antegrade and retrograde collateral circulation (red arrows) preventing blood stasis. (c) Conversely, in a patient with poor collateral circulation there is blood stasis on either side of the ‘original' thromboembolus promoting new thrombus formation. MCA: middle cerebral artery.
(a) Labeled anatomical illustration of the anterior circulation of the Circle of Willis. (b) and (c) Collateral circulation as a modifiable variable in determining the final length of thromboembolus. (b) A short ‘original’ MCA thromboembolus in a patient with good antegrade and retrograde collateral circulation (red arrows) preventing blood stasis. (c) Conversely, in a patient with poor collateral circulation there is blood stasis on either side of the ‘original' thromboembolus promoting new thrombus formation. MCA: middle cerebral artery.
  • For calcified thromboemboli, which are tPA-ineffective, SWI phase images and SWIM can help differentiate them from non-calcified thrombi based on phase shift.
      • Differentiation of Substances based on Phase Shift:
        • Diamagnetic substances,
          • Eg calcification
          • Weaken the external magnetic field, resulting in a negative phase shift for a left-handed MRI system.
          • On a left-handed MRI system (e.g., Siemens, Germany), diamagnetic calcium typically appears as low signal intensity on phase images
        • Paramagnetic substances
          • Eg deoxygenated haemoglobin and various stages of haemoglobin degradation (found in non-calcified thrombi)
          • Strengthen the external magnetic field, producing a positive phase shift for a left-handed system.
          • On a left-handed MRI system, paramagnetic haemoglobin products are displayed as high signal intensity on phase images
        • A convenient way to differentiate diamagnetic and paramagnetic phase shifts is to compare them to normal venous structures like the superior sagittal sinus or superficial cortical veins, as paramagnetic substances will show the same signal shift as seen in normal veins
      Acute left MCA territory stroke. Non-contrast enhanced CT of the head (a) shows a calcified thromboembolus (arrow) in the M4 cortical branch of the left MCA. SWI image (b) depicts the calcified thromboembolus (arrow) as a hypointense focus, and on the phase image (c) the thromboembolus (arrow) shows negative phase shift and also appears hypointense. (Images acquired on a left-handed MRI system). MCA: middle cerebral artery; CT: computed tomography; SWI: susceptibility weighted imaging.
      Acute left MCA territory stroke. Non-contrast enhanced CT of the head (a) shows a calcified thromboembolus (arrow) in the M4 cortical branch of the left MCA. SWI image (b) depicts the calcified thromboembolus (arrow) as a hypointense focus, and on the phase image (c) the thromboembolus (arrow) shows negative phase shift and also appears hypointense. (Images acquired on a left-handed MRI system). MCA: middle cerebral artery; CT: computed tomography; SWI: susceptibility weighted imaging.
Tissue Perfusion Assessment
  • Mechanism
    • A rough estimate of tissue perfusion through the depiction of asymmetric hypointense cortical veins in the ischemic territory, which indicate increased deoxyhemoglobin concentration.
      • These hypointense veins are hypothesised to represent the ischemic penumbra.
  • A SWI–diffusion-weighted imaging (DWI) mismatch (larger SWI-defined hypoperfused area relative to the DWI-positive infarct) is associated with a favourable outcome from reperfusion strategy.
  • SWIM enables quantitative analysis of deoxygenated haemoglobin levels in these veins.
Acute left MCA territory stroke. SWI (a) and phase (b) images demonstrate a thromboembolus in the superior M2 division of the left MCA. SWI image (c) at the ganglionic level shows asymmetric hypointense cortical veins in the left MCA territory (arrowheads) consistent with increased level of deoxyhemoglobin. Fluid-attenuated inversion recovery (FLAIR) image (d) shows hyperintense cortical vessel sign reflecting presence of slow flow in the cortical collateral vessels. DWI (e) shows acute cortical infarct in the left inferior parietal lobule. MCA: middle cerebral artery; SWI: susceptibility weighted imaging; FLAIR: fluid-attenuated inversion recovery; DWI: diffusion-weighted imaging.
Acute left MCA territory stroke. SWI (a) and phase (b) images demonstrate a thromboembolus in the superior M2 division of the left MCA. SWI image (c) at the ganglionic level shows asymmetric hypointense cortical veins in the left MCA territory (arrowheads) consistent with increased level of deoxyhemoglobin. Fluid-attenuated inversion recovery (FLAIR) image (d) shows hyperintense cortical vessel sign reflecting presence of slow flow in the cortical collateral vessels. DWI (e) shows acute cortical infarct in the left inferior parietal lobule. MCA: middle cerebral artery; SWI: susceptibility weighted imaging; FLAIR: fluid-attenuated inversion recovery; DWI: diffusion-weighted imaging.
 
Hemorrhagic Risk Assessment
  • SWI is sensitive in detecting cerebral microbleeds
    • These are linked to an increased risk of intracerebral hemorrhage following thrombolytic therapy.
  • While fewer than 5 microbleeds is generally considered safe, further research is needed to define a "high microbleed burden" for risk stratification.
acute left MCA territory stroke. SWI image (a) shows existing hypertensive pattern of cerebral microbleeds in the right basal ganglia. DWI (b) shows acute infarct in the left anterior limb of the internal capsule. Non-contrast CT of the head (c) performed 48 hours after administration of IV-tPA shows a small extra-ischemic zone hematoma in the left thalamus.
acute left MCA territory stroke. SWI image (a) shows existing hypertensive pattern of cerebral microbleeds in the right basal ganglia. DWI (b) shows acute infarct in the left anterior limb of the internal capsule. Non-contrast CT of the head (c) performed 48 hours after administration of IV-tPA shows a small extra-ischemic zone hematoma in the left thalamus.
Monitoring for Hemorrhagic Transformation
  • SWI has greater sensitivity than T2 GRE for detecting microhemorrhages and identifying hemorrhagic infarct patterns (HI1, HI2) after stroke.
  • It is crucial for early detection of hemorrhagic transformation, especially parenchymal hematomas (PH2), which are associated with greater morbidity and mortality.

Limitations

  • The susceptibility vessel sign on SWI is not entirely reliable for intracranial atherosclerotic stenosis due to potential confounding from vessel wall calcification, requiring reference to CT scans for clarification.
  • Changes in SWI signal intensity of a thromboembolus on follow-up studies do not reliably indicate vessel recanalization; vessel patency should be assessed by CT or MRA.

Reference