Blood-Brain-Barrier Imaging in Brain Tumors: Concepts and Methods

Malignant gliomas are often very heterogeneous tumors with complex vasculature, frequently exhibiting angiogenesis and increased vascular permeability. In vivo measurement of the tumor vessel permeability can serve as a potential imaging biomarker to assess tumor grade and aggressiveness. It can also be used to study the response of tumors to various therapies, especially antiangiogenic therapy. Central to the concept of permeability is a thorough knowledge of the BBB and its role in brain tumors and angiogenesis. Much work has been done in the past to understand the structural/molecular composition of the BBB and the role it plays in various pathologic processes, including brain tumors. Various imaging techniques have also been used to evaluate BBB leakiness in brain tumors because higher tumor vascular leakiness is known to be associated with higher grade and malignant potential of the tumor and hence poor patient prognosis. These imaging techniques range from routine postcontrast T1-weighted images to measurement of vascular permeability using various quantitative or semiquantitative indices based on multicompartment pharmacokinetic models. The purpose of this article is to discuss BBB anatomy; various clinically available imaging techniques to evaluate tumor vascular leakiness (perfusion imaging), including their advantages and limitations; as well as a brief discussion of the clinical utility of measuring vascular permeability in brain tumors. We will also discuss the various permeability-related indices along with the pharmacokinetic models to simplify the “nomenclature soup.”