In ordered systems, where the molecular motion is anisotropic, quadrupolar
and dipolar interactions are not averaged to zero. In such cases, double qu
antum (DQ) coherences can be formed. This review deals mainly with the effe
ct of anisotropic motion of water molecules and sodium ions in intact biolo
gical tissues on H-2, H-1 and Na-23 NMR spectroscopy and its application to
NMR imaging (MRI).
Double quantum filtered (DQF) spectra of water molecules and sodium ions we
re detected in a variety of ordered biological tissues. In collagen-contain
ing tissues such as ligaments, tendons, cartilage, skin, blood vessels and
nerves, the DQ coherences are formed as a result of the interaction with th
e collagen fibers. In red blood cells and presumably also in nerve axons it
stems from the interaction with the cytoskeleton.
For Na-23, an I = 3/2 nucleus, the DQ coherences can also be formed in isot
ropic media. By a judicial choice of the pulse angle in the DQ pulse sequen
ce only the DQ coherences arising from anisotropic motion are detected. For
1 = 1 nuclei such as 2H, DQF spectra can be observed only in ordered struc
tures. Thus, the observation of H-2 DQF spectra is an indication of order.
The same is true for pairs of equivalent H-1 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 resol
ved from different tissues by performing the measurements at different crea
tion times. In this way, the H-2 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 pe
rpendicular to the axon's long axis, a result from which the axon diameter
could be deduced. In blood vessel walls, this characteristic enabled the di
fferent layers of the vessel to be viewed and studied under strain.
For H-2, a DQF spectroscopic imaging sequence was used to study the orienta
tion of the collagen fibers in the different zones of articular cartilage a
nd bone plug. The effect of pressure on the fibers and their return to equi
librium 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 H-1 DQF imaging technique was demonstrated on a phant
om of rat tail where only the four tendons were detected at short creation
times. 'H DQF imaging and spectroscopy followed the healing of a rabbit's r
uptured Achilles tendon and the results were far more sensitive to the proc
ess than conventional imaging. Finally, the method was implemented on a com
mercial whole body MRI spectrometer. Images of human wrist and ankle showed
a positive contrast for the tendons and ligaments, indicating the potentia
l of the method fur clinical imaging. Copyright (C) 2001 John Wiley sr Sons