Abstract
Ultrashort-TE (UTE) sequences can obtain signal directly from short-T2 , collagen-rich tissues. It is generally accepted that bound and free water can be detected with UTE techniques, but the ability to detect protons directly on the collagen molecule remains controversial. In this study, we investigated the potential of UTE sequences on a 3-T clinical scanner to detect collagen protons via freeze-drying and D2 O-H2 O exchange studies. Experiments were performed on bovine cortical bone and human Achilles tendon specimens, which were either subject to freeze-drying for over 66 h or D2 O-H2 O exchange for 6 days. Specimens were imaged using two- and three-dimensional UTE with Cones trajectory techniques with a minimum TE of 8 μs at 3 T. UTE images before treatment showed high signal from all specimens with bi-component T2 * behavior. Bovine cortical bone showed a shorter T2 * component of 0.36 ms and a longer T2 * component of 2.30 ms with fractions of 78.2% and 21.8% by volume, respectively. Achilles tendon showed a shorter T2 * component of 1.22 ms and a longer T2 * component of 15.1 ms with fractions of 81.1% and 18.9% by volume, respectively. Imaging after freeze-drying or D2 O-H2 O exchange resulted in either the absence or near-absence of signal. These results indicate that bound and free water are the sole sources of UTE signal in bovine cortical bone and human Achilles tendon samples on a clinical 3-T scanner. Protons on the native collagen molecule are not directly visible when imaged using UTE sequences. Copyright © 2016 John Wiley & Sons, Ltd.
Highlights
Collagen is the most abundant protein in the human body and is the main component of connective tissue [1]
The purpose of this study is to further investigate whether UTE sequences can detect signal from collagen protons via freeze–dry and D2O exchange studies of cortical bone and Achilles tendon specimens at 3 T
The image signal-to-noise ratios (SNRs) of cortical bone and Achilles tendon acquired with the 2D UTE sequence with TE = 8 μs were 327 and 193, respectively
Summary
Collagen is the most abundant protein in the human body and is the main component of connective tissue [1]. Alterations of collagen structure, amount and type are of clinical interest in most biological systems, including musculoskeletal, neurological, cardiovascular and respiratory [3,4,5,6]. The highly anisotropic structure of collagen causes short transverse relaxation times, which result in little or no signal when imaged using conventional MR sequences. Ultrashort-TE (UTE) sequences, with TEs that range from 0.008 to 0.50 ms, have been increasingly used on clinical MRI systems to directly image collagen-rich short-T2 tissues or tissue components, such as cortical bone, calcified cartilage, menisci, ligaments and tendons [3,7,8]
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