Microscopy in magnetic resonance imaging

The notion of using magnetic resonance imaging (MRI) as a microscopic tool for obtaining three-dimensional images of living and nonliving material was envisioned early in the history of this technology. In this review we begin with a short discussion highlighting how imaging time is drastically lengthened as image resolution is increased. Obtaining sufficient signal and contrast is crucial in any imaging experiment and of particular importance in microscopic resolution MRI because of the intrinsically low sensitivity of the magnetic resonance phenomenon. In Section 2, we consider sources and ways to deal with signal and noise. In Sections 3 and 4, sample characteristics and how they can be taken advantage of with different MR imaging methods to provide high information content images are discussed. q-Space (displacement) imaging discussed in Section 5 is somewhat peculiar to microscopic MRI, while the notion of multimodal imaging is gaining acceptance at all scales. Section 6 is devoted to recent applications across a broad spectrum, but emphasizing biological samples. We conclude with a brief discussion of prospects for achieving higher resolution images in reasonable time spans