Effects of inversion time on inversion recovery prepared ultrashort echo time (IR-UTE) imaging of bound and pore water in cortical bone

Abstract

Water is present in cortical bone in different binding states. In this study we aimed to investigate the effects of inversion time (TI) on the signal from bound and pore water in cortical bone using an adiabatic inversion recovery prepared ultrashort echo time (IR-UTE) sequence on a clinical 3 T scanner. In total ten bovine midshaft samples and four human tibial midshaft samples were harvested for this study. Each cortical sample was imaged with the UTE and IR-UTE sequences with a TR of 300 ms and a series of TI values ranging from 10 to 240 ms. Five healthy volunteers were also imaged with the same sequence. Single- and bi-component models were utilized to calculate the T2 * and relative fractions of short and long T2 * components. Bi-component behavior of the signal from cortical bone was seen with the IR-UTE sequence, except with a TI of around 80 ms, where the short T2 * component alone were seen and a mono-exponential decay pattern was observed. In vivo imaging with the IR-UTE sequence provided high contrast-to-noise images with direct visualization of bound water and reduced signal from long T2 muscle and fat. Our preliminary results demonstrate that selective nulling of the pore water component can be achieved with the IR-UTE sequence with an appropriate TI, allowing selective imaging of the bound water component in cortical bone in vivo using clinical MR scanners.

Keywords: IR-UTE; UTE; bound water; cortical bone; pore water.

Figure 1

Two dimensional UTE sequences developed for FID sampling which employs a short rectangular pulse excitation followed by FID data acquisition with all gradients turned off (A), non-slice selective imaging which employs a short rectangular pulse excitation followed by 2D radial ramp sampling (B), and slice-selective imaging which employs a half-pulse excitation followed by 2D radial ramp sampling (C). The UTE sequences can be further combined with an adiabatic inversion recovery preparation pulse for IR-UTE imaging of bound and pore water components in cortical bone. (D) M_Z plotted against TI for bound water (BW) and pore water (FW). This shows the IR-UTE mechanism for imaging of bound and pore water components in cortical bone. With a shorter TI (TI₁), pore water magnetization is negative while the partially saturated bound water magnetization is positive. With an appropriate TI (TI₂), pore water is nulled and the signal is entirely from bound water. With a longer TI (TI₃), both bound and pore water magnetization are positive.

Figure 2

Real, imaginary and magnitude of complex FIDs of a bovine cortical bone sample acquired with IR-UTE with a TR of 300 ms and a TI of 20 ms (A), and UTE with a TR of 300 ms (C), and the corresponding phase plot (B, D) as a function of time after excitation. Nulling and rebounding of pore water signal was observed in IR-UTE FID magnitude (A) and phase (B) plots, but not in UTE FID magnitude (C) and phase (D) plots.

Figure 3

Normalized magnitude signal decay curve for IR-UTE images of a cadaveric human bone sample with a TR of 300 ms, a TI of 20 ms and TEs ranging from 8 µs to 5 ms (A), and selected magnitude and corresponding phase images with a TE of 8 us (B, F), 0.4 ms (C, G), 0.6 ms (D, H) and 1.4 ms (E, I). The relative longitudinal magnetizations of bound water (BM) and pore water (FW) at different TEs are also shown in (A), where the positive signal from bound water dominates the IR-UTE signal at short TEs. This signal decays quickly with increasing TEs, leaving the negative signal from pore water which decays more slowly dominating the IR-UTE signal at longer TEs. In subfigures (F) to (I) the signals were of opposite phase before and after TE of 0.5 ms (i.e., ϕ = 0.27 for TE of 8 µs, ϕ = 0.74 for TE of 0.4 ms, while ϕ = −2.47 for TE of 0.6 ms and ϕ = −2.03 for TE of 1.4 ms), consistent with a transition from positive to negative net magnetization at TE ~0.5 ms, the null point.

Figure 4

IR-UTE imaging of a bovine cortical bone acquired with a TR of 300 ms, TI of 20 ms, and TEs of 0.01 (A), 0.1 (B), 0.2 (C), 0.4 (D), 0.6 (E), 0.8 (F), 1.0 (G), 1.2 (H), 1.6 (I), 2.0 (J), 3.0 (K), 4.0 ms (L), as well as the corresponding single-component (M) and bi-component fitting (N). The signal decayed to a minimum at about 0.8 ms (F), increased again to a peak at about 1.6 ms (I), then decayed again with increasing TE (J–L). The single component fit is poor with residual signal more than 10% (M), but the bi-component fit is excellent (N), with 85.9% of the signal from the bound water component and 14.1% from the pore water component.

Figure 5

Bi-component fitting of IR-UTE (A–I) and UTE(J) images of a bovine cortical bone sample with TIs of 10 ms (A), 20 ms (B), 30 ms (C), 40 ms (D), 80 ms (E), 120 ms (F), 160 ms (G), 200 ms (H), 240 ms (I). UTE signal shows bi-component decay with 20% pore water and 80% bound water (J), while a single component was observed with a TI of 80 ms (E). Bi-component decay was observed with other TIs lower and higher than 80 ms due to imperfect nulling of the pore water components.

Figure 6

IR-UTE assessed bound water fractions as a function of TI for 4 bovine cortical bone samples (A–D). Reduced bound water fraction was observed at lower and higher TIs than 80 ms.

Figure 7

Single-component fitting (A) and bi-component fitting (C) of UTE images of a human cortical bone sample acquired with a TR of 300 ms and TEs ranging from 0.008 to 7 ms. The corresponding fitting residuals are shown in (B) and (D). The single-component model fits poorly while the bi-component fits well suggesting the existence of two water components in cortical bone with distinct T2* relaxation times. A single-component model fits well for the IR-UTE signal decays with a TI of 80 ms (E), with the fitting residuals (F) less than 2% consistent with nulling of the pore water component.

Figure 8

Bound water fraction of IR-UTE imaging of a human cortical bone sample acquired with a TR of 300 ms and a series of TIs ranging from 20 ms to 140 ms. Increased short T2* fraction is seen with TIs of around 80 ms.

Figure 9

In vivo imaging of the tibial midshaft of a 39 year old healthy female volunteer with UTE (A), IR-UTE with a TI of 80 ms (B), 110 ms (C) and 140 ms (D). A TR of 300 ms was used for all sequences. Increased contrast was achieved in imaging the tibia (thick arrow) with a TR of 300 ms and a TI of 110 ms. Surrounding pads (arrow heads) are also visible.

Figure 10

IR-UTE imaging of the tibial midshaft of a 39 year old healthy female volunteer with a TR of 300 ms, a TI of 110 ms, and TEs of 8 µs (A), 0.1 ms (B), 0.2 ms (C), 0.4 ms (D), 0.6 ms (E), 1.0 ms (F), 1.5 ms (G) and 2.5 ms (H) as well as single-component fitting of the bone signal decay (I), which accounted for 99.88% of the signal variance, consistent with only bound water with a T2* of 347 ± 10 µs being detected by the IR-UTE sequence. Residual signals from muscle and fat were better observed at longer TEs due to fast signal decay from cortical bone, which dominated the IR-UTE signal at shorter TEs.