Reduction of residual dipolar interaction in cartilage by spin‐lock technique

Abstract

The influence of radiofrequency (RF) spin-lock pulse on the laminar appearance of articular cartilage in MR images was investigated. Spin-lock MRI experiments were performed on bovine cartilage plugs on a 4.7 Tesla small-bore MRI scanner, and on human knee cartilage in vivo on a 1.5 Tesla clinical scanner. When the normal to the surface of cartilage was parallel to B 0 , a typical laminar appearence was exhibited in T 2 -weighted images of cartilage plugs, but was absent in T 1 -weighted images of the same plugs. At the "magic angle" orientation (when the normal to the surface of cartilage was 54.7°w ith respect to B 0 ), neither the T 2 nor the T 1 images demonstrated laminae. At the same time, T 1 values were greater than T 2 at both orientations throughout the cartilage. T 1 dispersion (i.e., the dependence of the relaxation rate on the spin-lock frequency 1 ) was observed, which reached a steady-state value of close to 2 kHz in both parallel and magic-angle orientations. These results suggest that residual dipolar interaction from motionally-restricted water and relaxation processes, Articular cartilage is a soft connective tissue that covers the ends of bones and acts as a shock-absorbing tissue (1). It is responsible for load distribution, and provides a lowfriction surface. It is composed of water (65-80%), proteoglycans (PG; 5%), collagen (15-20%), and noncollagenous proteins (2-5). The PGs are highly charged macromolecules with a central protein core to which glycosaminoglycans (GAG) are attached (6,7). The GAGs have a high concentration of negatively charged sulfate groups, which attract positive ions and water molecules to maintain electroneutrality. The strong water-binding affinity of PGs produces an osmotic pressure that must be balanced by restraining forces in the collagen fibril network. The collagen in cartilage is primarily type II, and is present in the form of triple helical fibrils. These fibrils form a highly organized anisotropic fibrous meshwork that lends the cartilage its extremely high tensile strength and resistance to shear forces. These special properties are essentially influenced by the size, spatial arrangement, and linkage of the collagen fibers. Among all tissues, collagen fibers appear to be very specific, naturally weakly hydrated, and highly structured.Cartilage is subdivided into three structural zones on the basis of the collagen fiber orientation, which changes depthwise. The fibrils are densely packed and oriented parallel to the articular surface in the superficial zone, randomly in the transitional zone, and perpendicular to the articular surface in the radial zone, which makes up the major part of the aricular cartilage. A schematic of the orientation of collagen fibers in cartilage is shown in Fig. 1 (8,9). The deepest region of the articular cartilage is the calcified region, which separates the articular cartilage from the subchondral bone. The tide-mark region represents a histologically demonstrable region separating the radial and calcified cartilage zones. Th...

Principal component analysis-T1ρ voxel based relaxometry of the articular cartilage: a comparison of biochemical patterns in osteoarthritis and anterior cruciate ligament subjects

“…Quant Imaging Med Surg 2016;6(6):623-633 qims.amegroups.com in the cartilage region [6,858]. The mean Z-score can be computed as:…”

“…While T 2 relaxation times is primarily affected by hydration and collagen structure due to dipolar interactions (5), the spin-lock techniques used in T 1ρ reduce dipolar interactions (6). Chemical exchange on proteoglycan and water protons was suggested to contribute to T 1ρ in cartilage, although T 1ρ changes in cartilage may be affected by hydration and collagen structure as well (7).…”

Sodium and T1ρ MRI for molecular and diagnostic imaging of articular cartilage

“…Several groups have investigated exchange mechanisms in a variety of cases using T 1r and Carr-PurcellMeiboom-Gill (CPMG) pulse sequences (201)(202)(203)(204)(205)(206) (214,215).…”

Multinuclear NMR and microscopic MRI studies of the articular cartilage nanostructure

“…In the inset, the same graph is presented with its y-axis drawn in a logarithmic scale. From Keinan-Adamsky et al (29) It should be mentioned that in most cases of articular cartilage imaging, the TE used to obtain T 2 maps is relatively long (usually >8 ms) (30)(31)(32)(33). This may be the reason why the bi-exponential nature of the decay curve in the deep radial zone was overlooked in most studies and T 2 values calculated in this region are relatively long.…”