Quantitative Imaging of Bone Water with 3D UTE MRI

3D UTE sequences (Figure 1) were implemented on a 3T Signa TwinSpeed scanner (GE Healthcare Technologies, Milwaukee, WI). The basic 3D UTE sequence employed a short radio frequency (RF) rectangular pulse (duration = 26-52 μs) for signal excitation ^{21 (link)}. The z-gradient could be turned off to allow non-selective 2D UTE imaging for fast imaging of cortical bone. Both collagen-bound and pore water are detectable with the basic UTE sequences (Figure 1B). Furthermore, adiabatic inversion recovery prepared UTE (IR-UTE) sequences were developed for selective imaging of collagen bound water (Figure 1C). In the IR-UTE sequence, a Silver-Hoult adiabatic inversion pulse (duration = 8.64 ms, bandwidth = 1.5 kHz) was used to invert the longitudinal magnetization of pore water ^{18 (link),22 (link)}. The longitudinal magnetization of collagen-bound water which has a very short T₂* was not inverted but largely saturated by the adiabatic IR pulse ^{22 (link)}. After an inversion time (TI) during which the inverted pore water magnetization approached the null point, the UTE acquisition was initiated to selectively detect signal from collagen-bound water (Figure 1D). 2D UTE and IR-UTE sequences were used for fast T1 and T2* quantification, while 3D UTE and IR-UTE sequences were used for bound and pore water concentration quantification (details below).