Quantitative ultrashort echo time (UTE) MRI of human cortical bone: correlation with porosity and biomechanical properties

Abstract

In this study we describe the use of ultrashort echo time (UTE) magnetic resonance imaging (MRI) to evaluate short and long T2* components as well as the water content of cortical bone. Fourteen human cadaveric distal femur and proximal tibia were sectioned to produce 44 rectangular slabs of cortical bone for quantitative UTE MR imaging, microcomputed tomography (µCT), and biomechanical testing. A two-dimensional (2D) UTE pulse sequence with a minimal nominal TE of 8 µseconds was used together with bicomponent analysis to quantify the bound and free water in cortical bone using a clinical 3T scanner. Total water concentration was measured using a 3D UTE sequence together with a reference water phantom. UTE MR measures of water content (total, free, and bound), T2* (short and long), and short and long T2* fractions were compared with porosity assessed with µCT, as well as elastic (modulus, yield stress, and strain) and failure (ultimate stress, failure strain, and energy) properties, using Pearson correlation. Porosity significantly correlated positively with total (R(2) = 0.23; p < 0.01) and free (R(2) = 0.31; p < 0.001) water content as well as long T2* fraction (R(2) = 0.25; p < 0.001), and negatively with short T2* fraction and short T2* (R(2) = 0.24; p < 0.01). Failure strain significantly correlated positively with short T2* (R(2) = 0.29; p < 0.001), ultimate stress significantly correlated negatively with total (R(2) = 0.25; p < 0.001) and bound (R(2) = 0.22; p < 0.01) water content, and failure energy significantly correlated positively with both short (R(2) = 0 30; p < 0.001) and long (R(2) = 0.17; p < 0.01) T2* values. These results suggest that UTE MR measures are sensitive to the structure and failure properties of human cortical bone, and may provide a novel way of evaluating cortical bone quality.

Figure 1

UTE pulse sequence diagrams: non-slice selective 2D UTE with a short rectangular hard pulse (32 μs in duration) excitation is followed by 2D radial ramp sampling (A), and 3D UTE with the same rectangular hard pulse excitation is followed by 3D radial ramp sampling (B).

Figure 2

Biomechanical testing device (A) and determination of properties from stress-strain data (B). In (A) the four-point bending test utilizes two loading jigs with a specific geometry (where a is the distance between inner and outer jigs, L is the width of outer jigs) compressing (to a depth of d) a rectangular sample (where b is the width and h is the height) while recording the force applied (F). Biomechanical measures were then obtained from the stress-strain curve (B). These included Young’s modulus (E), yield stress (σ_y), yield strain (ε_y), failure strain (ε_f), failure energy (W_f), and ultimate stress (σ_ult).

Figure 3

Data from cortical bone samples #1 (A, B) and #2 (E, F) imaged with μCT and UTE MR, respectively. Sample #1 shows a porosity of 1.8% (A). Sample #2 shows a porosity of 6.8% (E). Bi-component analysis shows a short T2* of 318 μs with a fraction of 67% and a long T2* of 2470 μs with a fraction of 33% for sample #1 (C). Sample #2 shows a short T2* of 237 μs with a fraction of 39% and a long T2* of 1904 μs with a fraction of 61% (G). (D) and (H) show the corresponding fitting residual signals. These are consistently less than 3% of the total signal, showing that the bi-component model shows a good fit of the UTE T2* signal decay.

Figure 4

Correlation between MR measures and μCT porosity. (A) Total water content, (B) free water content, and (E) long fraction all correlated positively (P<0.01) with porosity. (C) Short fraction, and (D) short T2* all correlated negatively (P<0.01) with porosity. (F) long T2* did not correlate significantly with porosity ( N=32 samples).

Figure 5

Significant (p<0.05) correlation between MR measures and biomechanical measures. (A) Short fraction, (D) short T2* and (G) free water content vs. failure strain. (B) Total water content, (E) long T2* and (H) bound water content vs. ultimate stress. (C) Total water content, (F) short T2* and (I) long T2* vs. failure energy (N=32 samples).