Abstract
Bone possesses a highly complex hierarchical structure comprised of mineral (~45% by volume), organic matrix (~35%) and water (~20%). Water exists in bone in two forms: as bound water (BW), which is bound to bone mineral and organic matrix, or as pore water (PW), which resides in Haversian canals as well as in lacunae and canaliculi. Magnetic resonance (MR) imaging has been increasingly used for assessment of cortical and trabecular bone. However, bone appears as a signal void on conventional MR sequences because of its short T2*. Ultrashort echo time (UTE) sequences with echo times (TEs) 100–1,000 times shorter than those of conventional sequences allow direct imaging of BW and PW in bone. A series of quantitative UTE MRI techniques has been developed for bone evaluation. UTE and adiabatic inversion recovery prepared UTE (IR-UTE) sequences have been developed to quantify BW and PW. UTE magnetization transfer (UTE-MT) sequences have been developed to quantify collagen backbone protons, and UTE quantitative susceptibility mapping (UTE-QSM) sequences have been developed to assess bone mineral.
Highlights
Osteoporosis (OP) is a metabolic bone disease which affects more than 10 million people in the United States and leads to over two million fractures every year [1]
While previous Ultrashort echo time (UTE) technical developments were focused on reducing TE in an effort to continue improving the detection of signal from bone, the most recent UTE technical developments have been centered on improving the selective quantifications of bound water (BW), pore water (PW), and other bone components, including their relaxation times (e.g., T1 and T2∗), fractions, and volume concentrations
Total water (TW) content of cortical bone can be estimated by comparing the UTE magnetic resonance imaging (MRI) signal of bone with that of an external reference with known proton density [37,38,39,40,41,42,43], though the resulting estimated content must be corrected for the difference in T2∗ and T1 values of bone and the external reference [44]
Summary
Osteoporosis (OP) is a metabolic bone disease which affects more than 10 million people in the United States and leads to over two million fractures every year [1]. The lack of direct signal obtained from bone makes it impossible to quantify the MR relaxation times (e.g., T1 and T2∗), magnetization transfer ratio (MTR) and volume concentration of various bone compartments To address this shortcoming and take advantage of both MRI’s safety profile and its excellent assessment of soft tissues such as tendon [27] and muscle, a benefit not available in x-ray-based techniques, a number of advanced MRI techniques have recently been developed to evaluate bone more effectively [14, 28,29,30]. The following discussion describes UTE MRI techniques which have been developed for quantitative imaging of cortical and trabecular bone in order to estimate different components of bone and predict its microstructural and mechanical properties.
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