Tight junctions are well-developed between adjacent endothelial cells of blood vessels in the central nervous system, and play a central role in establishing the blood-brain barrier (BBB). Claudin-5 is a major cell adhesion molecule of tight junctions in brain endothelial cells. To examine its possible involvement in the BBB, claudin-5–deficient mice were generated. In the brains of these mice, the development and morphology of blood vessels were not altered, showing no bleeding or edema. However, tracer experiments and magnetic resonance imaging revealed that in these mice, the BBB against small molecules (<800 D), but not larger molecules, was selectively affected. This unexpected finding (i.e., the size-selective loosening of the BBB) not only provides new insight into the basic molecular physiology of BBB but also opens a new way to deliver potential drugs across the BBB into the central nervous system.

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 2 H, DQF spectra can be observed only in ordered structures. Thus, the observation of 2 H DQF spectra is an indication of order. The same is true for pairs of equivalent 1 H 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 2 H 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 2 H, 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 1 H DQF imaging technique was demonstrated on a phantom of rat tail where only the four tendons were detected at short creation times. 1 H 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.

The one-dimensional 2 H double quantum filtered (DQF) spectroscopic imaging technique was used to study the orientation of collagen fibers in articular cartilage. The method detects only water molecules in anisotropic environments, which in cartilage is caused by their interaction with the collagen fibers. A large quadrupolar splitting was observed in the calcified zone and a smaller splitting in the radial zone. In the transitional zone the splitting was not resolved and a small splitting was again detected in the superficial zone. From measurements performed at two orientations of the plug relative to the magnetic field it was deduced that in the calcified and radial zones the fibers are oriented perpendicular to the bone, bending at the transitional zone and flattening at the superficial zone. The effect of load applied to the cartilage-bone plug was monitored by the same technique. At low loads there is a small decrease in the quadrupolar splitting in the calcified zone, a marked decrease in the radial zone, and an increase of the splitting accompanied by a thickening of the superficial zone. Under high loads, while the thickening and the splitting of the superficial zone further increase, the splitting in the radial and calcified zones completely Key words: DQF MRI; articular cartilage; compression; collagen orientationArticular cartilage is a dense connective tissue that coats the ends of bones in their joints. It is mainly composed of water (ϳ75%) and of a solid matrix of collagen fibrils (ϳ15%) and proteoglycans (PG) (ϳ10%). The fibrous, triple helix collagen molecules define the tissue's shape and provide its tensile strength. The PG are composed of a central protein core with many glycosaminoglycan (GAG) sulfated sidechains. These are highly negatively charged and thus attract high concentrations of positive ions and water molecules. Scanning electron microscopy (SEM) has shown that the collagen fibers rise vertically from the bone through the radial zone, then bend and flatten, forming the superficial tangential zone (1-3). This structure, together with the large osmotic pressure in the tissue, is responsible for the remarkable compressive strength of the tissue.In conventional MR images, articular cartilage has a laminated appearance (4 -14). The number of laminae, their relative thickness and intensity, vary from study to study and from sample to sample and are strongly dependent on the orientation of the tissue in the magnetic field. In collagen-containing tissues, it has been shown that the transverse relaxation rate is dominated by the residual dipolar interaction (6,8,15), which is a result of the anisotropic motion of the water molecules. Thus, T 2 depends on the orientation of the collagen fibers with respect to the magnetic field.The main function of cartilage is to withstand pressure. Direct visualization of the orientation of the collagen fibers in articular cartilage at rest and under applied load is obtained by SEM (2,3). MRI investigations of articular cartilage under various degrees of...

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The imaging of connective tissues such as cartilage and tendons using standard MRI techniques is hampered by their low signal relative to the surrounding tissues. 1H double-quantum filtered (DQF) MRI is an imaging method that detects molecules associated with ordered structures, while the signal from isotropic fluids is filtered out, thus creating a new type of contrast. The technique is demonstrated on an intact rat tail, where the image of the tendons is highlighted. Although the signal-to-noise ratio is inferior to that in gradient-echo MRI, the contrast between the tendons and the surrounding tissues is significantly better in the DQF MRI. It is demonstrated how, by adjusting the parameters of the DQF imaging pulse sequence, one can modify the contrast and enhance the images of specific compartments within an organ. A comparison with 2H DQF imaging of the same tissue is also given.

A technique is described for displaying distinct tissue layers of large blood vessel walls as well as measuring their mechanical strain. The technique is based on deuterium double-quantum-filtered (DQF) spectroscopic imaging. The effectiveness of the double-quantum filtration in suppressing the signal of bulk water is demonstrated on a phantom consisting of rat tail tendon fibers. Only intrafibrillar water is displayed, excluding all other signals of water molecules that reorient isotropically. One-and twodimensional spectroscopic imaging of bovine aorta and coronary arteries show the characteristic DQF spectrum of each of the tissue layers. This property is used to obtain separate images of the outer layer, the tunica adventitia, or the intermediate layer, the tunica media, or both. To visualize the effect of elongation, the average residual quadrupole splitting <⌬ q > is calculated for each pixel. Two-dimensional deuterium quadrupolar splitting images are obtained for a fully relaxed and a 55% elongated sample of bovine coronary artery. These images indicate that the strong effect of strain is associated with water molecules in the tunica adventitia whereas the DQF NMR signal of water in the tunica media is apparently strain-insensitive. After appropriate calibration, these average quadrupolar splitting images can be interpreted as strain maps.The mechanical properties of blood vessel walls play a central roll in cardiovascular function. At normal blood pressure the length of the vessel is as much as 40% longer and its circumference is about 30% greater than in the unstressed condition. Standard MRI scanning techniques, such as magnetic resonance angiography, provide information about the content of the blood vessels-the blood. But the major site of cardiovascular diseases is in the artery walls. Thus there is a real need to search for new imaging methods able to focus on the blood vessel wall and capable of estimating its biochemical as well as its mechanical conditions. In the present study, we introduce a spectroscopic MRI method for displaying distinct tissue layers of the blood vessel wall as well as measuring its mechanical strain.To distinguish between the tissue and its surroundings, the measuring technique should be based on some unique property specific to the blood vessel wall. As the tissue consists of various fibrous proteins, collagen, elastin and muscles, a plausible technique would involve the imaging of the protons of the proteins backbone. However, the rigidity of the fibers renders this approach impractical. A more promising approach is to monitor the highly abundant water molecules to explore chemical and structural changes inside the tissue. This approach is reasonable because the NMR relaxation of water molecules signal is chiefly determined by their coupling with the structural elements of the tissue. However, the strong signal of the highly abundant water molecules not associated with the blood vessel walls greatly reduces the dynamic contrast range. Double-quantum filtering pro...