Analysis of water‐macromolecule proton magnetization transfer in articular cartilage

Abstract

These studies were designed to establish which structural elements of cartilage are responsible for proton magnetization transfer between water (Hf) and macromolecules (Hr) observed in MRI studies on articular cartilage. Saturation transfer techniques were used to monitor magnetization transfer in vitro on samples of the two major constituents of cartilage: collagen and proteoglycan. Articular cartilage samples were also evaluated in vitro before and after the removal of the proteoglycan fraction. Isolated hydrated collagen exhibited a significant proton magnetization transfer rate with water. In contrast, proteoglycans exhibited no proton magnetization transfer. Articular cartilage, in vitro, exhibited a high degree of magnetization transfer with water protons consistent with previous MRI studies in vivo. Enzymatic removal of proteoglycan from the cartilage did not alter the magnetization transfer rate between Hr and Hf. These data demonstrate that the structure and concentration of the collagen matrix are the predominant determinants of the magnetization transfer process in articular cartilage with little or no contribution from proteoglycans. This specificity of the magnetization transfer effect may prove useful in the noninvasive evaluation of cartilage composition and structure in vivo.

Detection of thrombus size and protein content by ex vivo magnetization transfer and diffusion weighted MRI

“…In vivo applications of MTC in human diseases other than atherosclerosis [50,51] have also reported a linear correlation between MT and tissue collagen concentration. The importance of collagen in creating an MT effect is evident by the fact that although this single macromolecule comprises only ~ 40% of the macromolecules in cartilage, it is responsible for nearly all of the MT effect observed [39,41]. Moreover, several other mechanisms contribute to the rate of MT, including the mobility, the hydration state and the number of hydroxyl and amide groups [40], as well as the concentration of the macromolecules and/or water [51].…”

“…These conditions were previously optimized in our lab using phantoms and carotid endarterectomy specimens [37]. MT employs an off-resonance pulse that selectively saturates the water molecules bound to macromolecules compared to those in the free water pool, resulting in images with reduced signal intensity that is proportional to the amount of macromolecules [39-41]. Finally, 2D DW images were acquired with: TR = 1 s, TE = 25 ms, Δ = 12.6 ms, δ = 5 ms, averages = 32, slice thickness = 1 mm, matrix = 128 × 128, and FOV = 6.5 mm (spatial resolution = 50 × 50 μm), scan time = 6.5 h. The apparent diffusion coefficient (ADC) was calculated from seven b-values = 0, 196, 442, 637, 867, 1770 and 2409 s/mm 2 .…”

Magnetization transfer in MRI: a review

“…37 Other areas of application for MT that are less well established include breast, 38 knee 39 and cartilage. 40 Within cartilage, it may be possible using Gd-DTPA to separate the effect of proteoglycan degradation, from the effect of collagen disruption, which is the major contributor to MT in this tissue (Deb. Burstein, MIT, private communication).…”

Magnetic resonance imaging of normal and osteoarthritic trabecular bone structure in the human knee

“…It is nonionizing, offers multiplanar capabilities, has high spatial resolution, and provides superior depiction of soft tissue detail. In addition to evaluation of articular cartilage volume, thickness, and degeneration (9)(10)(11)(12)(13)(14), recent advances have made it possible to use MRI to assess bony and soft tissue changes. In a guinea pig model, results showed that MR images of trabecular bone accurately reflected the degree of osteopenia and the development of subchondral sclerosis and osteophytes (15).…”