Penumbra Imaged with Magnetic Resonance Imaging in Humans

The advent of novel magnetic resonance imaging techniques in the 1990s opened the door for magnetic resonance imaging (MRI) of the ischemic penumbra in the acute stroke clinical setting. The first diffusion-weighted MRI study in a human was reported in 1992 (1). In the mid-1990s as studies of perfusion-weighted imaging findings appeared in the literature (2), a new concept for defining the penumbra with MRI using the diffusion-perfusion mismatch model emerged (3). Reports of the natural history of diffusion and perfusion lesions were followed by studies describing the effect of therapies on these imaging findings. In the early 2000s, it became apparent that the mismatch model faced a number of challenges in defining the penumbra, and a new era of research attempting to better define the penumbra was born. While there are a number of approaches to define the ischemic penumbra, for the purposes of this chapter, we will use the following definition, which is most practical and relevant to MRI of the penumbra-tissue that is at risk of infarction, but still salvageable (Fig. 1). This chapter will review the general principles of MRI of the penumbra, the results of natural history, and treatment studies and their impact on the MRI defined penumbra. The current role and future directions of MRI of the penumbra will then be discussed.