Histology by magnetic resonance microscopy.

Magnetic resonance microscopy (MRM) has advanced from a technical challenge to a practical tool in a wide range of basic sciences. This article focuses on the use of MRM as a tool for histological studies. The technical challenges of limited signal to noise have been overcome by improved radio-frequency (rf) coil design and 3DFT encoding with large arrays. Resolution limits imposed by motion in in vivo studies have been overcome by improved physiologic monitoring and control and projection encoding. Integration of technologies now permits routine studies in vivo down to 50 microns. MRM has also been applied to in vitro studies of fixed tissues where absence of motion allows studies down to 10 microns. The nondestructive nature of the technique allows repeated studies of the same sample, retrospective studies through any arbitrary plane, registered studies using different contrast mechanisms, and examination of valuable specimens. The many and unique proton contrasts provided by MRM, i.e., T1, T2, and diffusion weighting, permit direct examination of the state of water in tissues, something not possible with other microscopic techniques. Finally, the inherent three-dimensional nature of MRM allows acquisition of perfectly registered isotropic 3D arrays that, when displayed with appropriate visualization tools, provide new perspectives to histologic examination. The technology of MRM continues to develop rapidly. New pulse sequences are reducing acquisition times. New computer architectures allow larger arrays. A new class of superconducting rf probe has increased the signal to noise ratio by 10 times. These developments promise routine use of MRM in histology studies with resolution to 1 micron in the near future.