Susceptibility-Weighted Imaging (SWI): Technical Aspects and Applications in Brain MRI for Neurodegenerative Disorders

Abstract

Susceptibility-weighted imaging (SWI) is a magnetic resonance imaging (MRI) sequence sensitive to substances that alter the local magnetic field, such as calcium and iron, allowing phase information to distinguish between them. SWI is a 3D gradient-echo sequence with high spatial resolution that leverages both phase and magnitude effects. The interaction of paramagnetic (such as hemosiderin and deoxyhemoglobin), diamagnetic (including calcifications and minerals), and ferromagnetic substances with the local magnetic field distorts it, leading to signal changes. Neurodegenerative diseases are typically characterized by the progressive loss of neurons and their supporting cells within the neurovascular unit. This cellular decline is associated with a corresponding deterioration of both cognitive and motor abilities. Many neurodegenerative disorders are associated with increased iron accumulation or microhemorrhages in various brain regions, making SWI a valuable diagnostic tool in clinical practice. Suggestive SWI findings are known in Parkinson's disease, Lewy body dementia, atypical parkinsonian syndromes, multiple sclerosis, cerebral amyloid angiopathy, amyotrophic lateral sclerosis, hereditary ataxias, Huntington's disease, neurodegeneration with brain iron accumulation, and chronic traumatic encephalopathy. This review will assist radiologists in understanding the technical framework of SWI sequences for a correct interpretation of currently established MRI findings and for its potential future clinical applications.

Keywords: Lewy body dementia; Parkinson disease; brain diseases; cerebral amyloid angiopathy; magnetic resonance imaging; multiple sclerosis; neurology; quantitative susceptibility mapping; radiology; susceptibility-weighted imaging.

Figure 1

The SWI pipeline involves several steps. Initially, the phase image undergoes unwrapping to correct for phase discontinuities. Subsequently, a high-pass filter is applied to the unwrapped phase to eliminate phase variations not associated with tissue properties. The filtered phase data is then transformed into a phase mask, typically using a sigmoid function. Finally, the magnitude image is multiplied by this phase mask to generate the SWI. To enhance vein visualization, a minimum intensity projection technique can be employed. Modified from Rimkus et al. [18] under the terms and conditions of the Creative Commons Attribution (CC BY) 4.0 license (https://creativecommons.org/licenses/by/4.0/ (accessed on 1 March 2025)).

Figure 2

SWI outputs from the Siemens MAGNETOM Aera (Erlangen, Germany) 1.5T MRI scanner at the level of the midbrain. (A) Magnitude Image: displays the intensity of the signal, showing general tissue contrast. (B) Phase Image: highlights variations in magnetic susceptibility, useful for detecting iron deposition and microhemorrhages. (C) SWI-Processed Image: combines magnitude and phase data to enhance the visualization of microvascular structures and pathological features. (D) Minimum Intensity Projection: a multislice projection image that accentuates low-signal structures such as veins and calcifications for improved spatial visualization.

Figure 3

SWI sequence outputs from the Siemens MAGNETOM Aera (Erlangen, Germany) 1.5T MRI scanner in a patient with hemosiderin deposits surrounding a surgical cavity in the left cerebral hemisphere. (A) Magnitude Image: displays areas of hypointense signal indicative of hemosiderin deposits. (B) Phase Image: shows the hemosiderin deposits as hyperintense structures, highlighting their magnetic susceptibility effects. (C) SWI-Processed Image: combines magnitude and phase data to enhance visualization of susceptibility artifacts, such as the hemosiderin rim around the surgical cavity. (D) Minimum Intensity Projection: highlights the distribution of low-signal susceptibility-related structures, improving the visualization of hemosiderin deposits and venous anatomy.

Figure 4

SWI sequence from the Siemens MAGNETOM Aera (Erlangen, Germany) 1.5T MRI scanner in a patient with calcification of the falx cerebri. (A) Magnitude Image: displays the calcification as a hypointense structure. (B) Phase Image: the calcification appears hypointense, consistent with its diamagnetic properties. (C) SWI-Processed Image and (D) Minimum Intensity Projection enhance the contrast of calcified structures and highlight low-signal calcification.

Figure 5

Susceptibility-weighted angiography (SWAN) sequence outputs from the GE Optima MR450w (Chicago, Illinois) 1.5T MRI scanner in a patient with diffuse axonal injury and a left frontal parenchymal contusion. (A,C) Magnitude Images: show multiple hypointense foci scattered throughout the brain parenchyma, consistent with microbleeds due to diffuse axonal injury, along with a larger hypointense area in the left frontal region indicative of a post-traumatic contusion. (B,D) Phase Images: the microbleeds and contusion appear hypointense, as is typical on GE systems. These findings are characteristic of traumatic brain injury, highlighting both diffuse and focal patterns of injury.

Figure 6

(A) SWI sequence acquired with a 3T scannerat the level of the mesencephalon, demonstrating the characteristic “swallow-tail sign” (white arrow). (B) Loss of the “swallow-tail sign” in a patient with Parkinson’s disease; the black arrow indicates the absence of the normal hyperintense signal in the substantia nigra. Modified from Lee et al. [37] under the terms and conditions of the Creative Commons Attribution (CC BY) 4.0 license (https://creativecommons.org/licenses/by/4.0/ (accessed on 1 March 2025). Image B has been magnified by 350% compared to the original. Accumulations of Lewy bodies are a pathological hallmark observed in both dementia with Lewy bodies and PD. The distinction between PD and dementia with Lewy bodies remains a subject of debate, with some researchers proposing they represent a spectrum of the same disease process. Dementia with Lewy bodies is diagnosed when cognitive impairment is the primary symptom or appears within 12 months of motor symptom onset. Conversely, PD is the diagnosis when motor features are dominant, though symptoms overlap and evolution occur over time in both conditions [38]. In this scenario, SWI can be used to differentiate AD from dementia with Lewy bodies based on the presence or absence of the swallow-tail sign, typically preserved in AD but not in dementia with Lewy bodies [36]. The abnormal findings observed in nigrosome-1 imaging lack the specificity needed to differentiate PD from atypical parkinsonian syndromes, including multiple system atrophy (MSA, parkinsonian and cerebellar subtypes), progressive supranuclear palsy (PSP), and corticobasal degeneration [39] where other MRI abnormalities may help in the differential diagnosis [1]. Compared to PD and PSP, MSA-parkinsonian type is characterized by greater iron deposition and atrophy within the posterolateral putamen in SWI sequences [40]. The distribution of iron deposition in SWI differs between PSP and PD. In PSP, iron accumulation is most prominent within several deep brain nuclei, including the putamen, red nucleus, substantia nigra pars reticulata, and cerebellar dentate nucleus, making it a distinguishing feature among atypical parkinsonian syndromes [41]. Finally, nigrosome-1 imaging is normal in both essential tremor and drug-induced parkinsonism, thus highlighting the role of SWI in the differential diagnosis with PD [1].

Figure 7

In oval-shaped lesions on fluid-attenuated inversion recovery (FLAIR) sequence (arrow in (A)), the central vein is visible as a hypointense line on SWI (arrow in (B)) and SWI-phase (arrow in (C)), creating a coffee-bean appearance. Conversely, in round-shaped lesions on FLAIR image (arrow in (D)), the central vein appears as a dark point on SWI (arrow in (E)) and frequently forms a target or doughnut shape on SWI-phase (arrow in (F)). Paramagnetic rim lesions are visible in both SWI and SWI-phase images. The FLAIR image (G) depicts a cavitated multiple sclerosis plaque, with a paramagnetic rim highlighted in both the SWI (arrow in (H)) and SWI-phase (arrow in (I)) images. Modified from Rimkus et al. [18] under the terms and conditions of the Creative Commons Attribution (CC BY) 4.0 license (https://creativecommons.org/licenses/by/4.0/ (accessed on 1 March 2025)).

Figure 8

Multiple small lobar cortical and subcortical microbleeds (white arrows) on 3D-SWI acquired with a 1.5T scanner reconstructed both in axial (A,B) and sagittal (C) planes. Hemosiderin deposits from previous right parieto-temporal lobar hemorrhage (empty black arrows in (B,C)). Sagittal SWI reveals a coating along surface of the sulcus (cortical superficial siderosis, white circle in (C)). Note the absence of microbleeds in the basal ganglia in (B), which assists in the differential diagnosis with hypertensive microangiopathy. The combination of these findings allows for the diagnosis of probable cerebral amyloid angiopathy according to the Boston 2.0 criteria.

Figure 9

(A) Example of SWI acquisition showing artifacts affecting the region near the ethmoid air cells and the left orbit (white arrows). (B) Example of SWI acquisition with motion artifacts in a neonate.