Water in tendon: Orientational analysis of the free induction decay

Abstract

The orientation dependence of the free induction decay (FID) of 1 H NMR water signal in ex vivo bovine digital flexor tendon at the native level of hydration is reported. Residual dipolar coupling due to the overall tissue anisotropy produces a 6:1 change in the signal intensity as an angle between the long axis of a specimen and the external magnetic field is changed from the "magic angle" of 54.7°to 0°. The strength of residual dipolar interactions between water protons was estimated by orientational analysis of the signal intensity to be equal to 780 Hz. Apparent signal maxima are observed at orientations 8 -13°a way from 54.7°due to an inhomogeneous contribution to the decay. A small fraction of total water in tendon is detectable at all orientations and exhibits a shift in the precession frequency. It is hypothesized that this water fraction resides in the interconnecting gaps at the ends of collagen molecules. The gaps have a disordered environment that allows for a zero time average of dipolar interactions. Measured frequency and phase shifts are interpreted as signatures of the bulk magnetic susceptibility effect due to geometry of the cavity formed by adjacent gaps at the ends of the collagen molecules. The multiexponentiality of the FID decay is hypothesized to be due to the exchange between orientationally restricted water structured along the length of the collagen molecule and disordered water in the cavity. Key words: free induction decay; bulk magnetic susceptibility; residual dipolar coupling; anisotropy; collagen; tendon Water protons are a major source of signal in clinical MRI. The NMR water signal provides valuable information about tissues. It depends not only on water content but also on types of micro-and macroscopic order, the presence of barriers, the extent of participation in hydrogen bonds, and interactions with numerous macromolecules.No other macromolecule is as abundant in the human body as collagen. Collagen is present in different amounts, types, and spatial structures, and its water content varies throughout the body. In tendon, collagen accounts for up to 95% of the total protein content, or 80 -90% of its dry weight (1). Its triple helical structure is embedded into the extracellular matrix, which is composed of elastins, glycoproteins, and tendocytes. The composition and structure of tendon are optimized for the function of transmitting force created in the muscle to the bone, and making joint movement possible. Fibrils are assembled in an anatomically defined preferential direction from the nanometer scale and up (2). Packing of collagen in microfibrils, as revealed by scanning electron microscopy (SEM) (3), shows a pattern of alternating gaps and overlaps indicative of a staggered supramolecular assembly.The ordered hierarchy of collagen in fibrils precludes isotropic motion of almost all water present in tendon, which behaves quite differently from bulk water. In addition, the motion is slowed because water molecules are involved in the formation of hydrogen bonds and wate...

Investigations of the origin of phase differences seen with ultrashort TE imaging of short T2 meniscal tissue

“…As expected, the phase calculated using Eq. 10 using the low flip angle approximation agrees with both shaped and hard pulse at 10 , but not at 90 , while the phase calculated using Eq. 4 agrees with the hard pulse for either flip angle, but does not agree with the shaped pulse.…”

“…4 and 10. A plot of the phase evolution resulting from a hard RF pulses with flip angles of 10 and 90 is shown in Fig. 2.…”

“…2. Also shown is the phase of the final slice profile resulting from the UTE half pulses with flip angles of 10 and 90 . As expected, the phase calculated using Eq.…”

“…In this article, we investigate the origin of the surprisingly high phase contrast that is found in the meniscus with UTE sequences using TEs far shorter than those previously used for susceptibility weighted imaging. Krasnosselskaia et al studied the orientation dependence of the MR signal of a bovine tendon with respect to the main magnetic field B 0 (10). Besides the well-known line narrowing at 54 (magic angle effect), they found that rotating the tendon from a parallel to a perpendicular position resulted in a frequency shift of up to 3 ppm and attributed these frequency shifts to the inherent structure of the tendon fibers.…”

Magnetic resonance imaging of short T2 relaxation components in the musculoskeletal system

“…In addition to this effect, there is a change in tissue susceptibility with orientation to Bo for tendons and ligaments of about 3 ppm with fibers parallel to Bo at the water end of the proton spectrum and fibers at 90°to Bo at the fat end [4].…”