Multiquantum filters and order in tissues.

Abstract

In ordered systems, where the molecular motion is anisotropic, quadrupolar and dipolar interactions are not averaged to zero. In such cases, double quantum (DQ) coherences can be formed. This review deals mainly with the effect of anisotropic motion of water molecules and sodium ions in intact biological tissues on (2)H, (1)H and (23)Na NMR spectroscopy and its application to NMR imaging (MRI). Double quantum filtered (DQF) spectra of water molecules and sodium ions were detected in a variety of ordered biological tissues. In collagen-containing tissues such as ligaments, tendons, cartilage, skin, blood vessels and nerves, the DQ coherences are formed as a result of the interaction with the collagen fibers. In red blood cells and presumably also in nerve axons it stems from the interaction with the cytoskeleton. For (23)Na, an I = 3/2 nucleus, the DQ coherences can also be formed in isotropic media. By a judicial choice of the pulse angle in the DQ pulse sequence only the DQ coherences arising from anisotropic motion are detected. For I = 1 nuclei such as 2H, DQF spectra can be observed only in ordered structures. Thus, the observation of 2H DQF spectra is an indication of order. The same is true for pairs of equivalent 1H nuclei. The dependence of the DQF signal on the creation time of the double quantum coherences is characteristic to each tissue and allows signals to be resolved from different tissues by performing the measurements at different creation times. In this way, the 2H DQF signals of the different compartments of sciatic nerve were resolved and water diffusion in each compartment was studied independently. In the axon, the diffusion was heavily restricted perpendicular to the axon's long axis, a result from which the axon diameter could be deduced. In blood vessel walls, this characteristic enabled the different layers of the vessel to be viewed and studied under strain. For 2H, a DQF spectroscopic imaging sequence was used to study the orientation of the collagen fibers in the different zones of articular cartilage and bone plug. The effect of pressure on the fibers and their return to equilibrium was studied as well. In blood vessels, a DQF image was obtained and strain maps of the different layers were calculated. The efficiency of the 1H DQF imaging technique was demonstrated on a phantom of rat tail where only the four tendons were detected at short creation times. 1H DQF imaging and spectroscopy followed the healing of a rabbit's ruptured Achilles tendon and the results were far more sensitive to the process than conventional imaging. Finally, the method was implemented on a commercial whole body MRI spectrometer. Images of human wrist and ankle showed a positive contrast for the tendons and ligaments, indicating the potential of the method for clinical imaging. (c) 2001 John Wiley & Sons, Ltd.