Milky way sign (MRI)

Extensive
immune reconstitution inflammatory syndrome in Fingolimod-associated PML: a case report with 7 Tesla MRI data. Overview on paraclinical findings and treatment decisions. The figure chronologically displays treatment decisions (first row), lymphocyte counts (second row), laboratory (third row), and MRI findings (fourth and fifth row). At baseline, only very few punctate milky way-like lesions were detectable (circle). After recovery of lymphocytes, new punctate milky way-like lesions developed (circle), and again ceased after corticosteroid pulse therapy (circle). The initial PML lesion in the left hemisphere was stable over time (see Fig. 2). Unfortunately, new MS-like lesions developed after discontinuation of fingolimod (white arrows). Anti-JCV antibody detection and quantification was performed at Unilabs Copenhagen, Denmark. JCV-DNA-PCR from CSF was performed at Institute of Virology, University Clinic of Duesseldorf.
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Extensive
immune reconstitution inflammatory syndrome in Fingolimod-associated PML: a case report with 7 Tesla MRI data. 3 T and 7 T MR imaging findings in early fingolimod-associated PML. The first row displays MRI images at baseline (a-c) and last follow up (d). At baseline, T2 weighted imaging (a) revealed a new C-shaped lesion with near microcystic appearance (zoom) that clearly infiltrates the short association fibres (“U fibres”). The lesion exhibited a patchy, irregular contrast enhancement (white arrow) on contrast enhanced T1 weighted images (b). The lesion was hyperintense on FLAIR (c, black arrow). The imaging pattern was strongly suggestive for PML. Thus, fingolimod was stopped immediately. Half year later, the size of the PML lesion remained unchanged (d, black arrow). The second and third row shows highly resolving 7 T MRI images. T2*w imaging with a resolution of 0.25 × 0.25mm2 delineates a small PML lesion (e). The PML lesion is T2*w hyperintense (black arrows), infiltrates the short association fibres (“U fibres”), and appears diffusely delineated against the white matter. Moreover, T2*w hypointense areas are visible within the surrounding cortex (black arrowhead). This finding is more pronounced on susceptibility weighted imaging (SWI, f, black arrowheads, “dark” signal). Unwrapped phase maps (g) showed positive phase changes (white arrowheads, “bright” signal) indicating paramagnetic effects. A minimal intensity projection map (MIP, h) of SWI illustrates SWI hypointense signal along white matter fibre tracts. Moreover, 7 T MRI differentiated between MS- and PML- associated lesions. On the one hand, 7 T T2* weighted MRI visualized a distinct central vein within MS-like lesions (J, white arrows). On top of that, numerous punctate contrast enhancing milky way-like lesions (I-L) were visible. Much of contrast enhancing lesions did not show a central vessel on T2*w images (i and j, black arrows). A very small vessel was faintly visible in other punctate lesions (j and k, black arrowheads). l demonstrates contrast-enhancement of the lesion displayed in K
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Radiopaedia • CC-by-nc-sa 3.0 • de

The Milky Way sign, also known less memorably merely as the punctate pattern, is an MRI feature described in progressive multifocal leukoencephalopathy (PML) and is particularly useful when distinguishing this from new lesions of multiple sclerosis .

Terminology

Although the term "Milky Way appearance" has primarily been reported in the context of progressive multifocal leukoencephalopathy, other pathologies share a similar appearance (see differential diagnosis below). Milky Way is capitalized as it refers to our own galaxy which is a proper noun.

Additionally, the term has also been used to denote the punctate enhancement pattern often seen in the same patients on post-contrast T1 weighted images .

This sign should not be confused with the similarly named starry sky appearance (MRI) seen in multiple biliary hamartomas and the starry sky appearance (ultrasound) seen in a variety of liver pathologies.

Pathology

The sign is believed to be the result of the accumulation of CD8-positive T-cells within the perivascular spaces with ensuing inflammation .

Radiographic features

MRI

The sign denotes multiple punctate regions of high T2 surrounding the main component of the new lesion, appearing reminiscent of the hazy band of whiteness (formed from the merging of the light of innumerable stars which are not individually visible) stretching across the dark background of the night sky representing our home galaxy, the Milky Way .

Differential diagnosis

Although the term Milky Way sign is mostly used in the context of progressive multifocal leukoencephalopathy a similar pattern has been reported in a number of other conditions including :