Fast quantitative 3D ultrashort echo time MRI of cortical bone using extended cones sampling

Abstract

Purpose:To investigate the effect of stretching sampling window on quantitative 3D ultrashort TE (UTE) imaging of cortical bone at 3 T. Methods: Ten bovine cortical bone and 17 human tibial midshaft samples were imaged with a 3T clinical MRI scanner using an 8-channel knee coil. Quantitative 3D UTE imaging biomarkers, including T 1 , T * 2 , magnetization transfer ratio and magnetization transfer modeling, were performed using radial or spiral Cones sampling trajectories with various durations. Errors in UTE-MRI biomarkers as a function of sampling time were evaluated using radial sampling as a reference standard. Results: For both bovine and human cortical bone samples, no significant differences were observed for all UTE biomarkers (single-component T * 2 , bicomponent T * 2 and relative fractions, T 1 , magnetization transfer ratio, and magnetization transfer modeling of macromolecular fraction) for spiral sampling windows of 992 µs to 1600 µs compared with a radial sampling window of 688 µs. Conclusion: The total scan time can be reduced by 76% with quantification errors less than 5%. Quantitative UTE-MRI techniques can be greatly accelerated using longer sampling windows without significant quantification errors. K E Y W O R D Scortical bone, quantitative, sampling window, UTE imaging | INTRODUCTIONCortical bone accounts for approximately 80% of the skeleton mass. 1 Cortical bone microstructure plays a critical role in determining the risk of fracture and is altered by diseases such as osteoporosis, hyperparathyroidism, renal disease and diabetes, which often have diffuse effects on bone metabolism and remodeling. [2][3][4][5] With the rapidly growing prevalence of osteoporosis, fragility fractures have become a major public health concern, motivating the urgent need to develop noninvasive clinical biomarkers of cortical bone strength, elasticity, and toughness. 6,7 Cortical bone consists of approximately 40% mineral, 35% collagen, and 25% water by volume. 8 Previous NMR spectroscopy studies have demonstrated that the cortical bone MR signal has multiple components that differ by T 2 values. 9 The short T 2 signals (T 2 values vary from 12-400 μs) originate from collagen backbone and side-chain protons, as 226 | WAN et Al.

Correlations of cortical bone microstructural and mechanical properties with water proton fractions obtained from ultrashort echo time (UTE) MRI tricomponent T2* model

“…First, tricomponent UTE‐T2* fitting used twenty MRI mages with different TEs that required 35 minutes of scan time. Employing a lower number of images with optimized TEs or accelerated imaging techniques such as readout stretching would be required for future clinical studies. Second, the selected short TR (i.e., 28 ms) and low flip angle (i.e., 10°) may have introduced some T1 weighting and, therefore, errors in fraction calculations.…”

Age-related decrease in collagen proton fraction in tibial tendons estimated by magnetization transfer modeling of ultrashort echo time magnetic resonance imaging (UTE-MRI)

“…Although, motion was not substantial on the images obtained from most of our subjects, image registration could help the cases where motion occurred between series. Employing different accelerating techniques, such as stretching the readout trajectory, could be used to accelerate the technique and to limit the scans to 20 minutes with negligible resultant errors 42 . Secondly, the fat present in tendon was not taken into account for this study.…”

Fast quantitative three‐dimensional ultrashort echo time (UTE) Cones magnetic resonance imaging of major tissues in the knee joint using extended sprial sampling

“…Furthermore, radial trajectory was not included in this study due to the much increased scan time. Our prior study of cortical bone specimens showed very similar results (<3% difference) for T 1 , T 2 *, MTR and MMF derived from 3D UTE Cones imaging with radial and Cones 1.0 trajectories, respectively 23 . The differences are expected to be even smaller for knee joint tissues as they have longer T 2 *s than that of cortical bone.…”

“…By using extended spiral sampling trajectories, the total scan time for these techniques can be shortened considerably compared with conventional radial sampling in UTE sequences 22,23 . The concept of stretching the spiral trajectory in 3D UTE Cones imaging, which means decreasing the pitch of a spiral readout, thereby increasing the duration of the readout to reach the same |k_max|, results in increased readout time (sampling window), but a reduced number of readouts.…”

“…The concept of stretching the spiral trajectory in 3D UTE Cones imaging, which means decreasing the pitch of a spiral readout, thereby increasing the duration of the readout to reach the same |k_max|, results in increased readout time (sampling window), but a reduced number of readouts. Specifically, stretching the spiral trajectory leads to more efficiently sampled outer k‐space, shorter scan time and more uniform sampling density, which improves signal‐to‐noise ratio (SNR) efficiency 14,22 . However, the signal of short T 2 components may decay significantly during the process of RF excitation and data acquisition 5,24 .…”