Quantitative two‐dimensional ultrashort echo time magnetization transfer (2D UTE‐MT) imaging of cortical bone

Abstract

Purpose To investigate quantitative 2D ultrashort echo time magnetization transfer (UTE‐MT) imaging in ex vivo bovine cortical bone and in vivo human tibial cortical bone. Methods Data were acquired from five fresh bovine cortical bone samples and five healthy volunteer tibial cortical bones using a 2D UTE‐MT sequence on a clinical 3T scanner. The 2D UTE‐MT sequence used four or five MT powers with five frequency offsets. Results were analyzed with a two‐pool quantitative MT model, providing measurements of macromolecular fraction (f), macromolecular proton transverse relaxation times (T 2m ), proton exchange rates from water/macromolecular to the macromolecular/water pool (RM 0m /RM 0w ), and spin‐lattice relaxation rate of water pool (R 1w ). A sequential air‐drying study for a small bovine cortical bone chip was used to investigate whether above MT modeling parameters were sensitive to the water loss. Results Mean fresh bovine cortical bone values for f, T 2m , R 1w , RM 0m , and RM 0w were 59.9 ± 7.3%, 14.6 ± 0.3 μs, 9.9 ± 2.4 s −1 , 17.9 ± 3.6 s −1 , and 11.8 ± 2.0 s −1 , respectively. Mean in vivo human cortical bone values for f, T 2m , R 1w , RM 0m and RM 0w were 54.5 ± 4.9%, 15.4 ± 0.6 μs, 8.9 ± 1.1 s −1 , 11.5 ± 3.5 s −1 , and 9.5 ± 1.9 s −1 , respectively. The sequential air‐drying study shows that f, RM 0m , and R 1w were increased with longer drying time. Conclusion UTE‐MT two‐pool modeling provides novel and useful quantitative information for cortical bone. Magn Reson Med 79:1941–1949, 2018. © 2017 International Society for Magnetic Resonance in Medicine.