Can ultrashort-TE (UTE) MRI sequences on a 3-T clinical scanner detect signal directly from collagen protons: freeze-dry and D2O exchange studies of cortical bone and Achilles tendon specimens

Ultrashort-TE (UTE) sequences can obtain signal directly from short-T-2, 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 D2O-H2O exchange studies. Experiments were performed on bovine cortical bone and human Achilles tendon specimens, which were either subject to freeze-drying for over 66h or D2O-H2O exchange for 6days. Specimens were imaged using two- and three-dimensional UTE with Cones trajectory techniques with a minimum TE of 8s at 3T. UTE images before treatment showed high signal from all specimens with bi-component T-2* behavior. Bovine cortical bone showed a shorter T-2* component of 0.36ms and a longer T-2* component of 2.30ms with fractions of 78.2% and 21.8% by volume, respectively. Achilles tendon showed a shorter T-2* component of 1.22ms and a longer T-2* component of 15.1ms with fractions of 81.1% and 18.9% by volume, respectively. Imaging after freeze-drying or D2O-H2O 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 (c) 2016 John Wiley & Sons, Ltd.