Bone quantitative susceptibility mapping using a chemical species–specific signal model with ultrashort and conventional echo data

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Magnetic susceptibility is a fundamental tissue property that can be observed in MRI 1. Densely calcified tissues such as bone have strong diamagnetic susceptibility 2. Quantitative susceptibility mapping (QSM) 9 can provide quantitative, reproducible images of magnetic susceptibility sources to assess the health and disease of many tissues 3, but its application in bone has been limited. Given the importance of measuring bone mineral density for assessing bone fracture risks in postmenopausal women and the elderly (25), QSM may become a useful diagnostic tool for noninvasive imaging of bone health without the use of ionizing radiation.
Quantitative susceptibility mapping of the bone has been challenging because it requires complete measurements of phase everywhere within the region of interest (ROI), and cortical bone typically has very low signal at conventional echo times in gradient echo (GRE) imaging. Although water is abundant in cortical bone (∼15% by volume 26), it mostly exists in the “bound” form, that is, connected to the crystalline mineral structures or the collagen matrix. As a result, bound bone water has an ultrashort apparent transverse relaxation time ( JOURNAL/mrim/04.02/01445475-201801000-00012/math_12MM1/v/2017-12-21T175206Z/r/image-png ∼300 μs 27), resulting in no meaningful phase for QSM reconstruction on conventional MRI. Because of these limitations, previous work in musculoskeletal applications of QSM was either focused on cartilage, or used piece‐wise estimations of bone susceptibility 2. An additional problem arises from intermingling of fat and water protons in the bone marrow, necessitating the application of water–fat separation techniques for field mapping.
The purpose of this preliminary study was to investigate the feasibility of using QSM for measuring bone MRI signal and to highlight the inherent technical issues involved in this application. To that end, we measured bone MRI signal using an ultrashort echo time (UTE) pulse sequence with an echo time (TE) of ≪ 1 ms 35, and investigated chemical shift and effective transverse relaxation rate ( JOURNAL/mrim/04.02/01445475-201801000-00012/math_12MM2/v/2017-12-21T175206Z/r/image-png ) components to properly model bone MRI signal in both UTE and conventional GRE.

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