Differenzialdiagnose von Läsionen der weißen Hirnsubstanz in der Magnetresonanztomographie

Zusammenfassung

Signalveränderungen des Marklagers sind häufig und ihre Interpretation ist eine Aufgabe, von der die weitere klinische Versorgung oft entscheidend abhängt. Die diagnostische Herausforderung ist dabei zunächst, den Krankheitswert einer Signalalteration in der Magnetresonanztomographie (MRT) zu erkennen. Aufgrund zahlreicher altersbedingter Veränderungen ist die Identifizierung wirklich pathologischer Veränderungen und ihre Abgrenzung gegen Zufallsbefunde (unknown bright objects UBO) teilweise schwierig. Die nächste Aufgabe besteht in der Zuordnung zu einer pathogenetischen Hauptgruppe, also vaskulär mikroangiopathischen Veränderungen, demyelinisierenden Erkrankungen, Neoplasien, metabolischen oder entzündlichen Schäden. Danach kann versucht werden, die Krankheit in dieser Gruppe näher zu spezifizieren. Als empfindlich/sensibelste Sequenzen zum Nachweis von Marklagerläsionen haben sich die T₂-gewichteten Aufnahmen mit Liquorsignalunterdrückung (dark fluid, z. B. Flair) erwiesen. Sie sind die Basis der Diagnostik, da sie im Unterschied zu normalen T2-Sequenzen auch Läsionen an der Grenze zum Liquorraum erfassen. T₁-gewichtete Sequenzen vor Kontrastmittelgabe korrelieren gut mit der Schädigungsschwere und können nach Kontrastmittelgabe die Krankheitsaktivität anzeigen. Der Nachweis von Mikroblutungen mit der T₂*-gewichteten Bildgebung ist ein starkes Indiz für eine vaskuläre Erkrankung und korreliert mit einer erhöhten Blutungsneigung. Protonenspektroskopie, chemical shift imaging (CSI) und diffusionsgewichtete Bildgebung (DWI) haben bei bestimmten Fragestellungen einen hohen Stellenwert sowohl für die Differenzialdiagnostik wie für die Aktivitätsbeurteilung und sollten daher heute für die eingehende Analyse von Marklagerveränderungen zur Verfügung stehen.

Abstract

White matter signal changes in magnetic resonance imaging (MRI) are frequent and the correct clinical assessment is very important for the further clinical management of the patient. The first problem in the sequence of dealing with these patients is to determine, if a signal change is truly abnormal or a sign of normal aging. The abundance of unspecific signal changes in the brains of normal persons has lead to the creation of the term unknown bright objects (UBO) for these findings. If a white matter lesion is deemed truly abnormal, it should be categorised into one of the major categories such as microvascular, demyelinating, inflammatory, neoplastic or metabolic. Thereafter, a further classification can be attempted. The mainstay of the MRI diagnostics of white matter lesions are dark fluid T₂-weighted sequences such as fluid attenuated inversion recovery (FLAIR). In contrast to conventional T2, these images maintain a high sensitivity for white matter damage in the vicinity of the CSF spaces. T₁-weighted images prior to contrast administration have been shown to demonstrate the clinical significant lesion load whereas the contrast enhanced scan may aid in differential diagnosis and in the evaluation of lesion activity. T₂*-weighted imaging is useful to identify previous microbleeds, which usually occur in microvascular conditions and portend the risk of further bleeding. Proton spectroscopy, chemical shift imaging and diffusion weighted imaging have been shown to be very useful in some entities of white matter disease, both for differential diagnosis and the assessment of disease activity. They should hence be part of any advanced imaging protocol in patients with white matter disease.

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