This review of tendon/collagen structure shows that the orientational variation in MRI signals from tendon, which is referred to as the "magic angle" (MA) effect, is caused by irreducible separation of charges on the main chain of the collagen molecule. These charges are held apart in a vacuum by stereotactic restriction of protein folding due in large part to a high concentration of hydroxyproline ring residues in the amino acids of mammalian collagen. The elevated protein electrostatic energy is reduced in water by the large dielectric constant of the highly polar solvent ( ϳ 80). The water molecules serve as dielectric molecules that are bound by an energy that is nearly equivalent to the electrostatic energy between the neighboring positive and negative charge pairs in a vacuum. These highly immobilized water molecules and secondary molecules in the hydrogenbonded water network are confined to the transverse plane of the tendon. Orientational restriction causes residual dipole coupling, which is directly responsible for the frequency and phase shifts observed in orientational MRI (OMRI) described by the MA effect. Reference to a wide range of biophysical measurements shows that native hydration is a monolayer on collagen h m ϭ 1.6 g/g, which divides into two components consisting of primary hydration on polar surfaces h pp ϭ 0.8 g/g and secondary hydration h s ϭ 0.8 g/g bridging over hydrophobic surface regions. Primary hydration further divides into side-chain hydration h psc ϭ 0.54 g/g and main-chain hydration h pmc ϭ 0.263 g/g. The main-chain fraction consists of water that bridges between charges on the main chain and is responsible for almost all of the enthalpy of melting ⌬H ϭ 70 J/g-dry mass. Main-chain water bridges consist of one extremely immobilized Ramachandran water bridge per tripeptide h Ra ϭ 0.0658 g/g and one double water bridge per tripeptide h dwb ϭ 0.1974 g/g, with three water molecules that are sufficiently slowed to act as the spinlattice relaxation sink for the entire tendon. THIS REVIEW OF BIOPHYSICAL STUDIES of mammalian tendon extends beyond the MR literature to show how the molecular structure of the collagen molecule causes the highly exceptional variation of MR signal with tendon orientation in the magnetic field that is referred to as the "magic angle" (MA) effect (1-14). Tendon is a unique tissue for elucidating not only orientational effects but also the molecular sources of MR contrast in other tissues. The unique "Rosetta stone" character of tendon relates to four structural factors:1. The nearly monomolecular composition of tendon, in which the collagen content approaches 100% of the total dry mass in some instances. 2. The uniform parallel alignment of collagen molecules along the tendon long axis, which provides tensile strength for connecting muscles to bones and forming cables necessary for mechanical motion of the body. 3. The quasi-crystalline association of collagen molecules in tendon, which allows the application of x-ray crystallography methods to both synthetic c...

This review provides a formalism for understanding magic angle effects in clinical studies. It involves consideration of the fiber-to-field angle for linear structures such as tendons, ligaments, and peripheral nerves, disc-like and circular structures such as menisci and labra, as well as complex three-dimensional structures. There may be one or more fiber types with different orientations within each of these tissues. The orientation of these fibers to B 0 is crucial in determining their magic angle effect. Tissues may show a variety of appearances depending on their baseline T2, as well as the increase in T2 produced by the magic angle effect. The appearances are affected by TE, which affects both the general tissue signal level and the change in signal produced by the magic angle effect, fiber-to-slice orientation, and partial volume effects. Deliberate positioning of structures and tissues at particular orientations to B 0 can be used to increase the signal from tissues such as tendons and ligaments and so allow them to be imaged with conventional sequences. The technique can also be used to produce contrast between tissues with fibers that have different orientations to B 0 . IN PULSE SEQUENCES with a moderate or short TE, the signal intensity of tendons and ligaments depends on their orientation to the static magnetic field (B 0 ) (1-3). These highly ordered, collagen-rich tissues contain water that is bound to collagen. The protons within this water are subject to dipolar interactions whose strength depends on the orientation of the fibers to B 0 . These interactions usually result in rapid dephasing of the MR signal after excitation. As a consequence, tendons and ligaments typically produce little or no detectable MR signal and appear dark when imaged with conventional clinical pulse sequences. KeyThe dipolar interactions are modulated by the term 3cos 2 Ϫ 1, where is the angle the structures make with the magnetic field B 0 . When 3cos 2 Ϫ 1 ϭ 0 ( ϭ 55°, 125°, etc., approximately, the magic angle), dipolar interactions are minimized with the result that the T2 of these tissues is increased and signal intensity may become evident within them when they are imaged with conventional pulse sequences. The magnitude of the magic angle effect may be quite large. For example, Fullerton et al (1) have described an increase in T2 of Achilles tendon from 0.6 to 22 msec and Henkelman et al (2) have described an increase from 7 to 23 msec when the orientation of the tendon to B 0 was changed from 0°to 55°. With some commonly used pulse sequences this may correspond to the tendon signal intensity increasing from the bottom of the imaging gray scale to the top of it.In initial studies with clinical MRI the effect was seen principally in tendons that underwent a change of direction along their course such as the supraspinatus tendon near its insertion so that the fibers of one part of the tendon were oriented at or near 55°to B 0 . The high signal resulting from the increase in T2 in the part of the tendon at 55°to B 0...

A molecular model is proposed to explain water 1H NMR spin-lattice relaxation at different levels of hydration (NMR titration method) on collagen. A fast proton exchange model is used to identify and characterize protein hydration compartments at three distinct Gibbs free energy levels. The NMR titration method reveals a spectrum of water motions with three well-separated peaks in addition to bulk water that can be uniquely characterized by sequential dehydration. Categorical changes in water motion occur at critical hydration levels h (g water/g collagen) defined by integral multiples N = 1, 4 and 24 times the fundamental hydration value of one water bridge per every three amino acid residues as originally proposed by Ramachandran in 1968. Changes occur at (1) the Ramachandran single water bridge between a positive amide and negative carbonyl group at h1 = 0.0658 g/g, (2) the Berendsen single water chain per cleft at h2 = 0.264 g/g, and (3) full monolayer coverage with six water chains per cleft level at h3 = 1.584 g/g. The NMR titration method is verified by comparison of measured NMR relaxation compartments with molecular hydration compartments predicted from models of collagen structure. NMR titration studies of globular proteins using the hydration model may provide unique insight into the critical contributions of hydration to protein folding.

Significant variation occurs in the lesion size produced using the same ablation device and algorithm. These findings must be considered when planning ablation strategies.

Multiple diagnostic imaging modalities are available and beneficial for the evaluation of the diabetic foot. There is not yet "one best test" for sorting out the diagnostic dilemmas commonly encountered. The differentiation of cellulitis alone from underlying osteomyelitis and the early detection of abscesses remain important diagnostic goals. Equally important, differentiation of osteomyelitis and neuroarthropathy remains a difficult job. This is often compounded by postoperative diabetic foot states status after reconstruction. Diagnostic evaluation often involves multiple studies that are complementary and that include conventional radiography, computed tomography, nuclear medicine scintigraphy, magnetic resonance imaging, ultrasonography, and positron emission tomography.

Results validate the use of tendon dilatometry amplification factors of 10(6)-10(8) as an effective model to investigate protein molecule shape change response to solvent molecules. The tendon model for the first time allows direct study of protein hydration and functional response under physiological conditions.

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