Frequency shifts of free water signals from compact bone: Simulations and measurements using a UTE-FID sequence.

PURPOSE: Free water in cortical bone is either contained in nearly cylindrical structures (mainly Haversian canals oriented parallel to the bone axis) or in more spherically shaped pores (lacunae). Those cavities have been reported to crucially influence bone quality and mechanical stability. Susceptibility differences between bone and water can lead to water frequency shifts dependent on the geometric characteristics. The purpose of this study is to calculate and measure the frequency distribution of the water signal in MRI in dependence of the microscopic bone geometry. METHODS: Finite element modeling and analytical approaches were performed to characterize the free water components of bone. The previously introduced UTE-FID technique providing spatially resolved FID-spectra was used to measure the frequency distribution pixel-wise for different orientations of the bone axis. RESULTS: The frequency difference between free water in spherical pores and in canals parallel to B0 amounts up to approximately 100 Hz at 3T. Simulated resonance frequencies showed good agreement with the findings in UTE-FID spectra. The intensity ratio of the two signal components (parallel canals and spherical pores) was found to vary between periosteal and endosteal regions. CONCLUSION: Spatially resolved UTE-FID examinations allow the determination of the frequency distribution of signals from free water in cortical bone. This frequency distribution indicates the composition of the signal contributions from nearly spherical cavities and cylindrical canals which allows for further characterization of bone structure and status.

Diagnostic value of water-fat-separated images and CT-like susceptibility-weighted images extracted from a single ultrashort echo time sequence for the evaluation of vertebral fractures and degenerative changes of the spine.

OBJECTIVES: To evaluate the performance of single-echo Dixon water-fat imaging and computed tomography (CT)-like imaging based on a single ultrashort echo time (sUTE) MR sequence for imaging of vertebral fractures as well as degenerative bone changes of the spine in comparison to conventional CT and MR sequences. METHODS: Thirty patients with suspected acute vertebral fractures were examined using a 3-T MRI, including an sUTE sequence as well as short-tau inversion recovery (STIR) and T1-weighted sequences. During postprocessing, water-fat separation was performed by solving the smoothness-constrained inverse water-fat problem based on a single-complex UTE image. By removing the unwanted low-frequency phase terms, additional MR-based susceptibility-weighted-like (SW-like) images with CT-like contrast were created. Two radiologists evaluated semi-quantitative and quantitative features of fractures and degenerative changes independently and separately on CT and MR images. RESULTS: In total, all 58 fractures were accurately detected of whom 24 were correctly classified as acute fractures with an edema detected on the water-fat-separated UTE images, using STIR and T1w sequences as standard of reference. For the morphological assessment of fractures and degenerative changes, the overall agreement between SW-like images and CT was substantial to excellent (e.g., Genant: κ 0.90 (95% confidence interval 0.54-1.00); AO/Magerl: κ 0.75 (95% confidence interval 0.43-1.00)). Overall inter-reader agreement for water-fat-separated UTE images and SW-like images was substantial to almost perfect. CONCLUSION: Detection and assessment of vertebral fractures and degenerative bone changes of the spine were feasible and accurate using water-fat-separated images as well as SW-like images, both derived from the same sUTE-Dixon sequence. KEY POINTS: • The detection of acute vertebral fractures was feasible using water-fat-separated images and CT-like images reconstructed from one sUTE sequence. • Assessment of the vertebral fractures using SW-like images with CT-like contrast was found to be comparable to conventional CT. • sUTE imaging of the spine can help reduce examination times and radiation exposure.

On the fat saturation effect in quantitative ultrashort TE MR imaging.

PURPOSE: To investigate the effect of fat saturation (FatSat) on quantitative UTE imaging of variable knee tissues on a 3T scanner. METHODS: Three quantitative UTE imaging techniques, including the UTE multi-echo sequence for T2∗ measurement, the adiabatic T1ρ prepared UTE sequence for T1ρ measurement, and the magnetization transfer (MT)-prepared UTE sequence for MT ratio (MTR) and macromolecular proton fraction (MMF) measurements were used in this study. Twelve samples of cartilage and twelve samples of meniscus, as well as six whole knee cadaveric specimens, were imaged with the three above-mentioned UTE sequences with and without FatSat. The difference, correlation, and agreement between the UTE measurements with and without FatSat were calculated to investigate the effects of FatSat on quantification. RESULTS: Fat was well-suppressed using all three UTE sequences when FatSat was deployed. For the small sample study, the quantification difference ratio (QDR) values of all the measured biomarkers ranged from 0.7% to 12.6%, whereas for the whole knee joint specimen study, the QDR values ranged from 0.2% to 12.0%. Except for T1ρ in muscle and MMF in meniscus (p > 0.05), most of the measurements showed statistical differences for T1ρ , MTR, and MMF (p < 0.05) between FatSat and non-FatSat scans. Most of the measurements for T2∗ showed no significant differences (p > 0.05). Strong correlations were found for all the biomarkers between measurements with and without FatSat. CONCLUSION: The UTE biomarkers showed good correlation and agreement with some slight differences between the scans with and without FatSat.

Spatially resolved free-induction decay spectroscopy using a 3D ultra-short echo time multi-echo imaging sequence with systematic echo shifting and compensation of B0 field drifts.

PURPOSE: Biologically interesting signals can exhibit fast transverse relaxation and frequency shifts compared to free water. For spectral assignment, a ultra-short echo time (UTE) imaging sequence was modified to provide pixel-wise free-induction decay (FID) acquisition. METHODS: The UTE-FID approach presented relies on a multi-echo 3D spiral UTE sequence with six echoes per radiofrequency (RF) excitation (TEmin 0.05 ms, echo spacing 3 ms). A complex pixel-wise raw data set for FID spectroscopy is obtained by several multi-echo UTE measurements with systematic shifting of the readout by 0.25 or 0.5 ms, until the time domain is filled for 18 or 45 ms. B0 drifts are compensated by mapping and according phase correction. Autoregressive extrapolation of the signal is performed before Gaussian filtering. This method was applied to a phantom containing collagen-water solutions of different concentrations. To calculate the collagen content, a 19-peak collagen model was extracted from a non-selective FID spectrum (50% collagen solution). Proton-density-collagen-fraction (PDCF) was calculated for 10 collagen solutions (2%-50%). Furthermore, an in vivo UTE-FID spectrum of adipose tissue was recorded. RESULTS: UTE-FID signal patterns agreed well with the non-spatially selective pulse-acquire FID spectrum from a sphere filled with 50% collagen. Differentiation of collagen solution from distilled water in the PDCF map was possible from 4% collagen concentration for a UTE-FID sequence with 128 × 128 × 64 matrix (voxel size 1 × 1 × 2.85 mm3 ). The mean values of the PDCF correlate linearly with collagen concentration. CONCLUSION: The presented UTE-FID approach allows pixel-wise raw data acquisition similar to non-spatially selective pulse-acquire spectroscopy. Spatially resolved applications for assessment of spectra of rapidly decaying signals seem feasible.

Assessment of vertebral fractures and edema of the thoracolumbar spine based on water-fat and susceptibility-weighted images derived from a single ultra-short echo time scan.

PURPOSE: To develop a methodology to simultaneously perform single echo Dixon water-fat imaging and susceptibility-weighted imaging (SWI) based on a single echo time (TE) ultra-short echo time (UTE) (sUTE) scan to assess vertebral fractures and degenerative bone changes in the thoracolumbar spine. METHODS: A methodology was developed to solve the smoothness-constrained inverse water-fat problem to separate water and fat while removing unwanted low-frequency phase terms. Additionally, the corrected UTE phase was used for SWI. UTE imaging (TE: 0.14 ms, 3T MRI) was performed in the lumbar spine of nine patients with vertebral fractures and bone marrow edema (BME). All images were reviewed by two radiologists. Water- and fat-separated images were analyzed in comparison with short-tau inversion recovery (STIR) and with respect to BME visibility. The visibility of fracture lines and cortical outlining of the UTE magnitude images were analyzed in comparison with computed tomography. RESULTS: Unwanted phase components, dominated by the B1 phase, were removed from the UTE phase images. The rating of the diagnostic quality of BME visualization showed a high preference for the sUTE-Dixon water- and fat-separated images in comparison with STIR. The UTE magnitude images enabled better visualizing fracture lines compared with STIR and slightly better visibility of cortical outlining. With increasing SWI weighting osseous structures and fatty tissues were enhanced. CONCLUSION: The proposed sUTE-Dixon-SWI methodology allows the removal of unwanted low-frequency phases and enables water-fat separation and SWI processing from a single complex UTE image. The methodology can be used for the simultaneous assessment of vertebral fractures and BME of the thoracolumbar spine.

Trajectory correction based on the gradient impulse response function improves high-resolution UTE imaging of the musculoskeletal system.

PURPOSE: UTE sequences typically acquire data during the ramping up of the gradient fields, which makes UTE imaging prone to eddy current and system delay effects. The purpose of this work was to use a simple gradient impulse response function (GIRF) measurement to estimate the real readout gradient waveform and to demonstrate that precise knowledge of the gradient waveform is important in the context of high-resolution UTE musculoskeletal imaging. METHODS: The GIRF was measured using the standard hardware of a 3 Tesla scanner and applied on 3D radial UTE data (TE: 0.14 ms). Experiments were performed on a phantom, in vivo on a healthy knee, and in vivo on patients with spine fractures. UTE images were reconstructed twice, first using the GIRF-corrected gradient waveforms and second using nominal-corrected waveforms, correcting for the low-pass filter characteristic of the gradient chain. RESULTS: Images reconstructed with the nominal-corrected gradient waveforms exhibited blurring and showed edge artifacts. The blurring and the edge artifacts were reduced when the GIRF-corrected gradient waveforms were used, as shown in single-UTE phantom scans and in vivo dual-UTE gradient-echo scans in the knee. Further, the importance of the GIRF-based correction was indicated in UTE images of the lumbar spine, where thin bone structures disappeared when the nominal correction was employed. CONCLUSION: The presented GIRF-based trajectory correction method using standard scanner hardware can improve the quality of high-resolution UTE musculoskeletal imaging.

Simple compensation method for improved half-pulse excitation profile with rephasing gradient.

PURPOSE: To improve the slice profile quality obtained by RF half-pulse excitation for 2D-UTE applications. METHODS: The overall first-order and zero-order phase errors along the slice-selection direction were obtained with the help of an optimization task to minimize the out-of-slice signal contamination from the calibration 1-dimenisonal (1D) profile data. The time-phase-error evolution was approximated from the k-space readout data, which were acquired primarily for correction of the readout trajectories during data regridding to the rectilinear grids. The correction of the slice profile was achieved by rephasing gradient pulses applied immediately after the end of excitation. The total prescan calibration typically took less than 2 minutes. RESULTS: The improved image quality using the proposed calibration method was demonstrated both on phantoms and on ankle images obtained from healthy volunteers. It was demonstrated that calibration can be performed either as a separate water phantom measurement or directly as a prescan procedure. CONCLUSION: The slice-profile distortion from the half-pulse excitation could substantially affect the overall fidelity of 2D-UTE images. The presented experiments proved that the image quality could be substantially increased by application of the proposed slice-correction method.

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

The purpose of this study is to investigate the effect of extending the spiral sampling window on quantitative 3D ultrashort echo time (UTE) Cones imaging of major knee joint tissues including articular cartilage, menisci, tendons and ligaments at 3 T. Nine cadaveric human whole knee specimens were imaged on a 3 T clinical MRI scanner. A series of quantitative 3D UTE Cones imaging biomarkers including T2 *, T1 , adiabatic T1ρ , magnetization transfer ratio (MTR) and macromolecular fraction (MMF) were estimated using spiral sampling trajectories with various durations. Errors in UTE MRI biomarkers as a function of sampling time were evaluated using a nonstretched spiral trajectory as a reference standard. No significant differences were observed by increasing the spiral sampling window from 1116 to 2232 µs in the calculated T2 *, T1 , adiabatic T1ρ , MTR and MMF, as all P-values were over .05 as assessed by ANOVA with two-sided Dunnett's test. Although extending the sampling window results in signal loss for short T2 components, there was limited effect on the calculated quantitative biomarkers, with error percentages typically smaller than 5% in all the evaluated tissues. The total scan time can be reduced by up to 54% with quantification errors of less than 5% in any evaluated major tissue in the knee joint, suggesting that 3D UTE Cones MRI techniques can be greatly accelerated by using a longer spiral sampling window without causing additional quantitative bias.

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

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 T1 , T2∗ , 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 T2∗ , bicomponent T2∗ and relative fractions, T1 , 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.

Correlations of quantitative MRI metrics with myelin basic protein (MBP) staining in a murine model of demyelination.

Myelin imaging in the central nervous system is essential for monitoring pathologies involving white matter alterations. Various quantitative MRI protocols relying on the modeling of the interactions of water protons with myelinated tissues have shown sensitivities in case of myelin disruption. Some extracted model parameters are more sensitive to demyelination, such as the bound pool fraction (f) in quantitative magnetization transfer imaging (qMTI), the radial diffusivity in diffusion tensor imaging (DTI), and the myelin water fraction (MWF) in myelin water imaging (MWI). A 3D ultrashort echo time (UTE) sequence within an appropriate water suppression condition (Diff-UTE) is also considered for the direct visualization of the myelin semi-solid matrix (Diff-UTE normalized signal; rSPF). In this paper, we aimed at assessing the sensitivities and correlations of the parameters mentioned above to an immuno-histological study of the myelin basic protein (MBP) in a murine model of demyelination at 7 T. We demonstrated a high sensitivity of the MRI metrics to demyelination, and strong Spearman correlations in the corpus callosum between histology, macromolecular proton fraction (ρ>0.87) and Diff-UTE signal (ρ>0.76), but moderate ones with radial diffusivity and MWF (|ρ|<0.70).

A radial sampling strategy for uniform k-space coverage with retrospective respiratory gating in 3D ultrashort-echo-time lung imaging.

The purpose of this work was to develop a 3D radial-sampling strategy which maintains uniform k-space sample density after retrospective respiratory gating, and demonstrate its feasibility in free-breathing ultrashort-echo-time lung MRI. A multi-shot, interleaved 3D radial sampling function was designed by segmenting a single-shot trajectory of projection views such that each interleaf samples k-space in an incoherent fashion. An optimal segmentation factor for the interleaved acquisition was derived based on an approximate model of respiratory patterns such that radial interleaves are evenly accepted during the retrospective gating. The optimality of the proposed sampling scheme was tested by numerical simulations and phantom experiments using human respiratory waveforms. Retrospectively, respiratory-gated, free-breathing lung MRI with the proposed sampling strategy was performed in healthy subjects. The simulation yielded the most uniform k-space sample density with the optimal segmentation factor, as evidenced by the smallest standard deviation of the number of neighboring samples as well as minimal side-lobe energy in the point spread function. The optimality of the proposed scheme was also confirmed by minimal image artifacts in phantom images. Human lung images showed that the proposed sampling scheme significantly reduced streak and ring artifacts compared with the conventional retrospective respiratory gating while suppressing motion-related blurring compared with full sampling without respiratory gating. In conclusion, the proposed 3D radial-sampling scheme can effectively suppress the image artifacts due to non-uniform k-space sample density in retrospectively respiratory-gated lung MRI by uniformly distributing gated radial views across the k-space.

Quantifying temperature-dependent T1 changes in cortical bone using ultrashort echo-time MRI.

PURPOSE: To demonstrate the feasibility of using ultrashort echo-time MRI to quantify T1 changes in cortical bone due to heating. METHODS: Variable flip-angle T1 mapping combined with 3D ultrashort echo-time imaging was used to measure T1 in cortical bone. A calibration experiment was performed to detect T1 changes with temperature in ex vivo cortical bone samples from a bovine femur. Ultrasound heating experiments were performed using an interstitial applicator in ex vivo bovine femur specimens, and heat-induced T1 changes were quantified. RESULTS: The calibration experiment demonstrated that T1 increases with temperature in cortical bone. We observed a linear relationship between temperature and T1 with a linear coefficient between 0.67 and 0.84 ms/°C over a range of 25-70°C. The ultrasound heating experiments showed increased T1 changes in the heated regions, and the relationship between the temperature changes and T1 changes was similar to that of the calibration. CONCLUSION: We demonstrated a temperature dependence of T1 in ex vivo cortical bone using a variable flip-angle ultrashort echo-time T1 mapping method.

High-resolution quantitative sodium imaging at 9.4 Tesla.

PURPOSE: Investigation of the feasibility to perform high-resolution quantitative sodium imaging at 9.4 Tesla (T). METHODS: A proton patch antenna was combined with a sodium birdcage coil to provide a proton signal without compromising the efficiency of the X-nucleus coil. Sodium density weighted images with a nominal resolution of 1 × 1 × 5 mm(3) were acquired within 30 min with an ultrashort echo time sequence. The methods used for signal calibration as well as for B0, B1, and off-resonance correction were verified on a phantom and five healthy volunteers. RESULTS: An actual voxel volume of roughly 40 µL could be achieved at 9.4T, while maintaining an acceptable signal-to-noise ratio (8 for brain tissue and 35 for cerebrospinal fluid). The measured mean sodium concentrations for gray and white matter were 36 ± 2 and 31 ± 1 mmol/L of wet tissue, which are comparable to values previously reported in the literature. CONCLUSION: The reduction of partial volume effects is essential for accurate measurement of the sodium concentration in the human brain. Ultrahigh field imaging is a viable tool to achieve this goal due to its increased sensitivity.

Influence of physical activity on T1 and T2* relaxation times of healthy Achilles tendons at 3T.

PURPOSE: To evaluate longitudinal (T1) and transverse (T2*) relaxation times at different Achilles tendon locations (insertion, mid-portion, and musculotendinous area) in a cohort of subjects with variable tendon straining activity, but without any signs of tendinopathy. MATERIALS AND METHODS: T1 and T2* relaxation times of the Achilles tendon were measured using an ultrashort echo time (UTE) imaging sequence with flip angle and echo time variation on a 3T whole-body clinical magnetic resonance (MR) scanner. Twenty-four tendons of 12 healthy volunteers and 18 tendons of nine healthy recreational long-distance runners (average weekly running distance of 47.4 ± 5.3 km) were enrolled. RESULTS: Mean T1 values of the whole tendon showed no marked group differences, whereas T2* relaxation times of runners were significantly longer (mean percentage increase 15.7 ± 4.9%; P = 0.0181) compared to controls. Regarding group differences for the investigated insertional, mid-portion, and musculotendinous area of the Achilles tendon, only the mid-portion of the Achilles tendon in the running group presented significantly longer T2* values (mean percentage increase 29.1 ± 23.0%; P = 0.0420) in comparison to the control group. CONCLUSION: Prolonged T2* values especially in the mid-portion of the Achilles tendon of runners are likely to reflect an adaptation of the tendon microstructure to repetitive tendon straining activity.

Short-term exercise-induced changes in hydration state of healthy Achilles tendons can be visualized by effects of off-resonant radiofrequency saturation in a three-dimensional ultrashort echo time MRI sequence applied at 3 Tesla.

PURPOSE: Off-resonant RF saturation influences signal intensity dependent on free and bound water fractions as well as the macromolecular content. The extent of interaction between these compartments can be evaluated by using the off-resonance saturation ratio (OSR). Combined with UTE sequences quantification of OSR even in tendinous tissues with extremely fast signal decay is possible. The aim of this prospective study was to investigate short-term exercise-induced effects of hydration state of the Achilles tendon by means of OSR and tendon volume. MATERIALS AND METHODS: Measurements of OSR and tendon volume before and after ankle-straining activity were performed in seven healthy male volunteers (median age 29 years) using a 3D UTE sequence with implemented off-resonance saturation pulse at 3T (off-resonance frequency 2/3 kHz) and by an automated contour detection in isotropic T2-weighted MR images with sub-millimeter resolution, respectively. Different tendon regions were evaluated. Reproducibility of OSR was measured in subsequent imaging sets. Root-mean-square-deviation (RMSD) and coefficient of variations (CV) were determined. RESULTS: RMSD of OSR in resting position were between 0.006 and 0.01 for different tendon regions and off-resonance frequencies (CV 2 to 3%). A significant increase (P < 0.05) of OSR after exercise was seen in all tendon regions except at the insertion (off-resonance frequency 3 kHz). Tendon volume was decreased significantly after ankle-straining activity (P = 0.003). CONCLUSION: The observed decreased tendon volume and increased OSR directly after exercise indicates a short-term change in tendinous proton compartments, most likely a loss of free water molecules within the tendon.