New MR sequences in daily practice: susceptibility weighted imaging. A pictorial essay

Abstract

BACKGROUND: Susceptibility-weighted imaging (SWI) is a relatively new magnetic resonance (MR) technique that exploits the magnetic susceptibility differences of various tissues, such as blood, iron and calcification, as a new source of contrast enhancement. This pictorial review is aimed at illustrating and discussing its main clinical applications. METHODS: SWI is based on high-resolution, three-dimensional (3D), fully velocity-compensated gradient-echo sequences using both magnitude and phase images. A phase mask obtained from the MR phase images is multiplied with magnitude images in order to increase the visualisation of the smaller veins and other sources of susceptibility effects, which are displayed at best after post-processing of the 3D dataset with the minimal intensity projection (minIP) algorithm. RESULTS: SWI is very useful in detecting cerebral microbleeds in ageing and occult low-flow vascular malformations, in characterising brain tumours and degenerative diseases of the brain, and in recognizing calcifications in various pathological conditions. The phase images are especially useful in differentiating between paramagnetic susceptibility effects of blood and diamagnetic effects of calcium. SWI can also be used to evaluate changes in iron content in different neurodegenerative disorders. CONCLUSION: SWI is useful in differentiating and characterising diverse brain disorders.

Fig. 1

Normal subject. a SWI magnitude image. b SWI, minimal intensity projection (minIP; 10 mm) image: cortical and subependymal veins are well displayed. c SWI, phase map: the distinction between the pars reticulata (arrow) and compacta (asterisk) of the substantia nigra is enhanced compared with conventional fast SE and GE sequences. The outer margins of the red nucleus are also better displayed (see d for comparison). d T2-weighted axial image

Fig. 2

Recent onset of altered mental status and cognitive impairment in a 67-year-old man. a Fluid attenuated inversion recovery (FLAIR) MRI: diffuse hyperintensity of the white matter, more pronounced in the right parietal region. b Diffusion-weighted imaging (DWI), apparent diffusion coefficient (ADC) map: the white-matter hyperintensity represents vasogenic oedema. c GE T2*-weighted image: small punctate hypointense foci in the right parietal cortex (open arrow). d SWI minIP: increased visualisation of markedly hypointense foci surrounding the white matter abnormalities which correspond to multiple cerebral microbleeds. On the basis of SWI imaging findings, CAA with diffuse inflammatory changes was suspected and steroid therapy was administered. e Follow-up MRI at 3 months showed significant reduction of the vasogenic oedema

Fig. 3

Patient with long-standing hypertension. a T2-weighted image shows a thin hypointense band in the right thalamus, which represents haemosiderin deposition (arrow). b GE T2*-weighted image shows multiple microbleeds, the largest one in the right thalamus (arrow). c SWI minIP: the number of identifiable microbleeds is increased compared with GE images. They can be recognised bilaterally in the basal ganglia and in the subcortical temporal white matter (arrows)

Fig. 4

Alzheimer’s disease. a Coronal reformatted T1-weighted MP-RAGE section depicts marked atrophy of the hippocampi. b Coronal reformatted SWI section shows a cerebral microbleed in the right frontal white matter (arrow)

Fig. 5

Left internal carotid artery (ICA) dissection with acute watershed infarct in a 42-year-old man with sudden onset of speech disturbance. a MR angiography, single partition at the level of the skull base showing dissection of the left ICA, which is characteristically enlarged with high-signal-intensity methaemoglobin (arrowheads) representing the intramural thrombus and a small eccentric lumen (arrow). b MR angiography, coronal MIP: narrowing of the ICA in its vertical segment just below the skull base (arrow). c DWI, axial image: acute watershed infarct in the left centrum semiovale. d SWI minIP: improved visualisation of the veins of the left cerebral hemisphere, which is related to increased oxygen extraction in the ischaemic penumbra and corresponds to the hypoperfusion deficit shown by the mean transit time (MTT) map. e Perfusion MRI, MTT map, showing delayed transit time in the left middle cerebral artery (MCA) territory. f Intra-arterial digital subtraction angiography (DSA), left carotid angiogram, confirming the ICA stenosis (arrow). g Intra-arterial DSA: the patient was treated using stent placement, which resulted in partial restoration of the vessel lumen. h Perfusion MRI, MTT map 24 h after stent placement: complete resolution of the perfusion deficit. i SWI minIP, normalisation of the venous drainage of the left hemisphere

Fig. 6

Epileptic seizures and sagittal sinus thrombosis in a 3-year-old child. a Sagittal T1-weighted image demonstrates hyperintensity of the posterior third of the sagittal sinus (open arrow). b MR venography confirms the thrombosis of the posterior third of the sagittal sinus ending at the torcular (open arrow). c DWI, b = 1,000 image demonstrates a right occipital acute ischaemic lesion. d SWI minIP shows a right thalamic haemorrhagic lesion in the vascular territory of the right deep internal cerebral vein (arrow)

Fig. 7

Brainstem haemorrhage in a 43-year-old patient. a CT shows an acute haemorrhagic lesion located in the medulla oblongata. b The SWI minIP image clearly depicts the brainstem haemorrhage, which is markedly hypointense. c, d SWI, axial and reformatted sagittal images, processed with a maximum intensity projection algorithm (MIP), show flow-related enhancement within the haemorrhagic lesion (arrow) and a hypertrophied right posterior inferior cerebellar artery (PICA; open arrow) which are consistent with a small ruptured brainstem AVM. e Cerebral angiography, selective injection of the right vertebral artery: brainstem AVM (arrow) fed by branches of the right PICA and early venous drainage into a cerebellar vein (arrowheads)

Fig. 8

Patient with headache and pulsatile tinnitus of the right ear. a T2-weighted image shows a dilated right temporal vein (arrows). b, c SWI axial and coronal reformatted minIP images show dilated right temporal veins (arrows), which arouse suspicion of a tentorial dural AV fistula. d Cerebral angiography, selective injection of the right external carotid artery: dural AV fistula of the right tentorium, fed by the right middle meningeal artery (open arrow), with early venous drainage into the tentorial and temporal veins (arrows)

Fig. 9

Multiple cavernous malformations in a 32-year-old man investigated for intraparenchymal haemorrhage. The patient turned out to be affected by familial cavernous angiomatosis. a T2-weighted image demonstrates a left frontal cavernous malformation surrounded by a thick ring of haemosiderin. b GE T2*-weighted image shows multiple punctate hypointense foci located in both hemispheres, corresponding to small cavernous malformations (arrows). c SWI minIP: there are supplemental cavernous malformations that are not detected by GE sequences (arrows). d SWI minIP, reformatted sagittal section: multiple small cavernous malformations located in the brainstem (arrows)

Fig. 10

Incidental discovery of a cerebellar developmental venous anomaly in a 48-year-old man investigated for right-sided sensorineural hearing loss. a T2-weighted image shows thin hypointense bands of flow void, in the right cerebellar hemisphere (open arrow). b SWI minIP demonstrates multiple thin medullary veins (arrows) that converge into a dilated collector vein (open arrow), consistent with a cerebellar venous angioma, better defined as developmental venous anomaly. c Post-contrast T1-weighted axial image shows the enhancing dilated veins (open arrow)

Fig. 11

Incidental discovery of a pontine telangiectasia in a 34-year-old woman investigated for migraine. a T2-weighted image barely shows the abnormality. SWI minIP axial (b) and sagittal reformatted section (c) show a markedly hypointense lesion with regular margins, located in the pons. d Post-contrast sagittal T1-weighted image. The capillary telangiectasia is characterised by mild enhancement with an arbor-like pattern

Fig. 12

Ataxia telangiectasia. A 31-year-old woman with a diagnosis of ataxia telangiectasia by the age of 7, investigated for dizziness and headache. a Sagittal T2-weighted image demonstrates severe atrophy of the cerebellar vermis. b GE T2*-weighted image reveals multiple hypointense foci located in both hemispheres. c SWI minIP: additional hypointense black spots can be identified, corresponding to haemosiderin deposits presumably due to gliovascular nodules with perivascular haemorrhage

Fig. 13

Diffuse axonal injury in a 14-year-old boy who had severe TBI following a motorcycle crash. Brain MR study was performed three days after the trauma, when the patient was in a severely comatose state (GCS 5). a FLAIR image identifies multiple hyperintense lesions in the splenium of the corpus callosum and in frontal subcortical and periventricular white matter. b GE T2*-weighted image: haemorrhagic shearing injuries are barely visible in the right fronto-opercular and parieto-occipital regions (arrows). c SWI minIP: additional microhaemorrhages are recognisable at the grey matter-white matter junction of the frontal lobes and in the right parieto-occipital white matter (black arrows). d SWI minIP, reformatted sagittal section shows microhaemorrhages in the corpus callosum (white arrows)

Fig. 14

Characterisation and grading of a glial tumour. A 30-year-old man with dizziness, long-standing behavioural changes and subsequent diagnosis of intra-axial cerebral tumour. a T2-weighted image shows a large frontal infiltrating glioma. b SWI, unprocessed image: absence of intratumoural susceptibility signals (due either to calcifications, haemorrhages or venous vasculature). c Contrast-enhanced SWI: detailed visualisation of the margins of a large anaplastic area (asterisk), which is in good correlation with enhancement characteristics (d), hypervolaemia on PWI relative cerebral blood volume (rCBV) map (e) and MR spectroscopy (f). d Post-contrast T1-weighted image. e PWI, rCBV map: area of increased cerebral blood volume (open arrow). f Single-voxel MR spectroscopy, showing inversion of the Cho/NAA ratio and the presence of a lactate peak. Targeted stereotactic biopsy in the supposed necrotic area confirmed the hypothesis of gliomatosis cerebri WHO grade III with several necrotic foci

Fig. 15

Internal architecture of a high-grade tumour. A 2-year-old child with headache, vomiting and right hemiparesis. a CT demonstrates a large heterogeneous left frontal tumour with calcified foci. b T2-weighted image: the mass is irregularly hyperintense with cystic changes and small hypointensities representing small vessels and calcifications. c Post-contrast T1-weighted image shows patchy enhancement of the lateral part of the lesion and some vascular structures. d CE-SWI minIP: calcifications appear as punctate hypointensities, unchanged after gadolinium injection (white arrows). Post-processing with minIP algorithm allows the simultaneous visualisation of arteries (hyperintense, black arrows) and veins (linear hypointensities, open arrow) around and inside the tumour. CE-SWI is superior to T1-weighted post-gadolinium sequences in demonstrating a diffuse BBB rupture in the lateral necrotic area (bright signal, asterisk). Histopathology revealed a primary neuroectodermal tumour (PNET) with extensive cystic and necrotic changes and increased vascularity