Fast volumetric imaging of bound and pore water in cortical bone using three‐dimensional ultrashort‐TE (UTE) and inversion recovery UTE sequences

We report the three‐dimensional ultrashort‐TE (3D UTE) and adiabatic inversion recovery UTE (IR‐UTE) sequences employing a radial trajectory with conical view ordering for bi‐component T₂* analysis of bound water (T₂*^BW) and pore water (T₂*^PW) in cortical bone. An interleaved dual‐echo 3D UTE acquisition scheme was developed for fast bi‐component analysis of bound and pore water in cortical bone. A 3D IR‐UTE acquisition scheme employing multiple spokes per IR was developed for bound water imaging. Two‐dimensional UTE (2D UTE) and IR‐UTE sequences were employed for comparison. The sequences were applied to bovine bone samples (n = 6) and volunteers (n = 6) using a 3‐T scanner. Bi‐component fitting of 3D UTE images of bovine samples showed a mean T₂*^BW of 0.26 ± 0.04 ms and T₂*^PW of 4.16 ± 0.35 ms, with fractions of 21.5 ± 3.6% and 78.5 ± 3.6%, respectively. The 3D IR‐UTE signal showed a single‐component decay with a mean T₂*^BW of 0.29 ± 0.05 ms, suggesting selective imaging of bound water. Similar results were achieved with the 2D UTE and IR‐UTE sequences. Bi‐component fitting of 3D UTE images of the tibial midshafts of healthy volunteers showed a mean T₂*^BW of 0.32 ± 0.08 ms and T₂*^PW of 5.78 ± 1.24 ms, with fractions of 34.2 ± 7.4% and 65.8 ± 7.4%, respectively. Single‐component fitting of 3D IR‐UTE images showed a mean T₂*^BW of 0.35 ± 0.09 ms. The 3D UTE and 3D IR‐UTE techniques allow fast volumetric mapping of bound and pore water in cortical bone. Copyright © 2016 John Wiley & Sons, Ltd.     

Volumetric imaging of myelin in vivo using 3D inversion recovery‐prepared ultrashort echo time cones magnetic resonance imaging

Direct myelin imaging is promising for characterization of multiple sclerosis (MS) brains at diagnosis and in response to therapy. In this study, a 3D inversion recovery‐prepared ultrashort echo time cones (IR‐UTE‐Cones) sequence was used for both morphological and quantitative imaging of myelin on a clinical 3 T scanner. Myelin powder phantoms with different myelin concentrations were imaged with the 3D UTE‐Cones sequence and it showed a strong correlation between concentrations and UTE‐Cones signals, demonstrating the ability of the UTE‐Cones sequence to directly image myelin in the brain. Quantitative myelin imaging with multi‐echo IR‐UTE‐Cones sequences show similar T₂* values for a D₂O‐exchanged myelin phantom (T₂* = 0.33 ± 0.04 ms), ex vivo brain specimens (T₂* = 0.20 ± 0.04 ms) and in vivo healthy volunteers (T₂* = 0.254 ± 0.023 ms), further confirming the feasibility of 3D IR‐UTE‐Cones sequences for direct myelin imaging in vivo. In ex vivo MS brain study, signal loss is observed in MS lesions, which was confirmed with histology. For the in vivo study, the lesions in MS patients also show myelin signal loss using the proposed direct myelin imaging method, demonstrating the clinical potential for MS diagnosis. Furthermore, the measured IR‐UTE‐Cones signal intensities show a significant difference between normal‐appearing white matter in MS patients and normal white matter in volunteers, which cannot be found in clinical used T₂‐FLAIR sequences. Thus, the proposed 3D IR‐UTE‐Cones sequence showed clinical potential for MS diagnosis with the capability of direct myelin detection of the whole brain.     

Evaluation of bound and pore water in cortical bone using ultrashort‐TE MRI

Bone water exists in different states with the majority bound to the organic matrix and to mineral, and a smaller fraction in ‘free’ form in the pores of cortical bone. In this study, we aimed to develop and evaluate ultrashort‐TE (UTE) MRI techniques for the assessment of T₂*, T₁ and concentration of collagen‐bound and pore water in cortical bone using a 3‐T clinical whole‐body scanner. UTE MRI, together with an isotope study using tritiated and distilled water (THO–H₂O) exchange, as well as gravimetric analysis, were performed on ten sectioned bovine bone samples. In addition, 32 human cortical bone samples were prepared for comparison between the pore water concentration measured with UTE MRI and the cortical porosity derived from micro‐computed tomography (μCT). A short T₂* of 0.27 ± 0.03 ms and T₁ of 116 ± 6 ms were observed for collagen‐bound water in bovine bone. A longer T₂* of 1.84 ± 0.52 ms and T₁ of 527 ± 28 ms were observed for pore water in bovine bone. UTE MRI measurements showed a pore water concentration of 4.7–5.3% by volume and collagen‐bound water concentration of 15.7–17.9% in bovine bone. THO–H₂O exchange studies showed a pore water concentration of 5.9 ± 0.6% and collagen‐bound water concentration of 18.1 ± 2.1% in bovine bone. Gravimetric analysis showed a pore water concentration of 6.3 ± 0.8% and collagen‐bound water concentration of 19.2 ± 3.6% in bovine bone. A mineral water concentration of 9.5 ± 0.6% was derived in bovine bone with the THO–H₂O exchange study. UTE‐measured pore water concentration is highly correlated (R² = 0.72, p < 0.0001) with μCT porosity in the human cortical bone study. Both bovine and human bone studies suggest that UTE sequences could reliably measure collagen‐bound and pore water concentration in cortical bone using a clinical scanner. Copyright © 2015 John Wiley & Sons, Ltd.     

Magic angle effect on adiabatic T1ρ imaging of the Achilles tendon using 3D ultrashort echo time cones trajectory

The protons in collagen‐rich musculoskeletal (MSK) tissues such as the Achilles tendon are subject to strong dipolar interactions which are modulated by the term (3cos²θ‐1) where θ is the angle between the fiber orientation and the static magnetic field B₀. The purpose of this study was to investigate the magic angle effect in three‐dimensional ultrashort echo time Cones Adiabatic T_1ρ (3D UTE Cones‐AdiabT_1ρ) imaging of the Achilles tendon using a clinical 3 T scanner. The magic angle effect was investigated by Cones‐AdiabT_1ρ imaging of five cadaveric human Achilles tendon samples at five angular orientations ranging from 0° to 90° relative to the B₀ field. Conventional Cones continuous wave T_1ρ (Cones‐CW‐T_1ρ) and Cones T₂* (Cones‐T₂*) sequences were also applied for comparison. On average, Cones‐AdiabT_1ρ increased 3.6‐fold from 13.6 ± 1.5 ms at 0° to 48.4 ± 5.4 ms at 55°, Cones‐CW‐T_1ρ increased 6.1‐fold from 7.0 ± 1.1 ms at 0° to 42.6 ± 5.2 ms at 55°, and Cones‐T2* increased 12.3‐fold from 2.9 ± 0.5 ms at 0° to 35.8 ± 6.4 ms at 55°. Although Cones‐AdiabT_1ρ is still subject to significant angular dependence, it shows a much‐reduced magic angle effect compared to Cones‐CW‐T_1ρ and Cones‐T₂*, and may be used as a novel and potentially more effective approach for quantitative evaluation of the Achilles tendon and other MSK tissues.     

Bi‐exponential 23Na T2* component analysis in the human brain

The purpose of this study was to measure the sodium transverse relaxation time T₂* in the healthy human brain. Five healthy subjects were scanned with 18 echo times (TEs) as short as 0.17 ms. T₂* values were fitted on a voxel‐by‐voxel basis using a bi‐exponential model. Data were also analysed using a continuous distribution fit with a region of interest‐based inverse Laplace transform. Average T₂* values were 3.4 ± 0.2 ms and 23.5 ± 1.8 ms in white matter (WM) for the short and long components, respectively, and 3.9 ± 0.5 ms and 26.3 ± 2.6 ms in grey matter (GM) for the short and long components, respectively, using the bi‐exponential model. Continuous distribution fits yielded results of 3.1 ± 0.3 ms and 18.8 ± 3.2 ms in WM for the short and long components, respectively, and 2.9 ± 0.4 ms and 17.2 ± 2 ms in GM for the short and long components, respectively. ²³Na T₂* values of the brain for the short and long components for various anatomical locations using ultra‐short TEs are presented for the first time.     

Can ultrashort‐TE (UTE) MRI sequences on a 3‐T clinical scanner detect signal directly from collagen protons: freeze–dry and D2O exchange studies of cortical bone and Achilles tendon specimens

Ultrashort‐TE (UTE) sequences can obtain signal directly from short‐T ₂ , collagen‐rich tissues. It is generally accepted that bound and free water can be detected with UTE techniques, but the ability to detect protons directly on the collagen molecule remains controversial. In this study, we investigated the potential of UTE sequences on a 3‐T clinical scanner to detect collagen protons via freeze–drying and D ₂ O–H ₂ O exchange studies. Experiments were performed on bovine cortical bone and human Achilles tendon specimens, which were either subject to freeze–drying for over 66 h or D ₂ O–H ₂ O exchange for 6 days. Specimens were imaged using two‐ and three‐dimensional UTE with Cones trajectory techniques with a minimum TE of 8 μs at 3 T. UTE images before treatment showed high signal from all specimens with bi‐component T ₂ * behavior. Bovine cortical bone showed a shorter T ₂ * component of 0.36 ms and a longer T ₂ * component of 2.30 ms with fractions of 78.2% and 21.8% by volume, respectively. Achilles tendon showed a shorter T ₂ * component of 1.22 ms and a longer T ₂ * component of 15.1 ms with fractions of 81.1% and 18.9% by volume, respectively. Imaging after freeze–drying or D ₂ O–H ₂ O exchange resulted in either the absence or near‐absence of signal. These results indicate that bound and free water are the sole sources of UTE signal in bovine cortical bone and human Achilles tendon samples on a clinical 3‐T scanner. Protons on the native collagen molecule are not directly visible when imaged using UTE sequences. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of normal cadaveric Achilles tendon and enthesis with ultrashort echo time (UTE) magnetic resonance imaging and indentation testing

Entheses are regions where tendons and ligaments attach to bone, and are the primary target in seronegative and other diseases of the musculoskeletal (MSK) system. MRI has been widely used for visualizing features of inflammatory and degenerative MSK disease; however, normal tendons and entheses have short transverse relaxation times (T₂), and show little or no signal with conventional clinical MRI pulse sequences, making it difficult to investigate their MR properties. In this study we examined the normal MR morphology of the cadaveric Achilles tendon and enthesis at 3 T using novel three‐dimensional ultrashort echo time (3D UTE) Cones sequences, and at 11.7 T using conventional MRI sequences. We also studied the MR properties of the Achilles tendon and enthesis including T₂*, T₁, and magnetization transfer ratio (MTR). In addition, MT modeling of macromolecular proton fractions was investigated using 3D UTE Cones sequences at 3 T. Indentation testing was performed to investigate the mechanical properties of the tendons and entheses, and this was followed by histological examination. In total five specimens (<50 years) were investigated. On average, tendons and entheses respectively had T₂* values of 0.93 ± 0.48 ms and 2.77 ± 0.79 ms, T₁ values of 644 ± 22 ms and 780 ± 55 ms, MTRs of 0.373 ± 0.03 and 0.244 ± 0.009 with an MT power of 1000° and frequency offset of 2 kHz, and macromolecular proton fractions of 18.0 ± 2.2% and 13.9 ± 1.9%. Compared with the tendon, the enthesis generally had a longer T₂*, a longer T₁, a lower MTR, and a lower macromolecular proton fraction as well as both a higher Young's modulus and stiffness. Results from this study are likely to provide a useful baseline for identifying deviations from the normal in seronegative arthritis and other disease of the entheses.     

Quantitative three‐dimensional ultrashort echo time cones imaging of the knee joint with motion correction

Knee degeneration involves all the major tissues in the joint. However, conventional MRI sequences can only detect signals from long T2 tissues such as the superficial cartilage, with little signal from the deep cartilage, menisci, ligaments, tendons and bone. It is highly desirable to develop new sequences that can detect signal from all major tissues in the knee. We aimed to develop a comprehensive quantitative three‐dimensional ultrashort echo time (3D UTE) cones imaging protocol for a truly “whole joint” evaluation of knee degeneration. The protocol included 3D UTE cones actual flip angle imaging (3D UTE‐Cones‐AFI) for T₁ mapping, multiecho UTE‐Cones with fat suppression for T₂* mapping, UTE‐Cones with adiabatic T_1ρ (AdiabT_1ρ) preparation for AdiabT_1ρ mapping, and UTE‐Cones magnetization transfer (UTE‐Cones‐MT) for MT ratio (MTR) and modeling of macromolecular proton fraction (f). An elastix registration technique was used to compensate for motion during scans. Quantitative data analyses were performed on the registered data. Three knee specimens and 15 volunteers were evaluated at 3 T. The elastix motion correction algorithm worked well in correcting motion artifacts associated with relatively long scan times. Much improved curve fitting was achieved for all UTE‐Cones biomarkers with greatly reduced root mean square errors. The averaged T₁, T₂*, AdiabT_1ρ, MTR and f for knee joint tissues of 15 healthy volunteers were reported. The 3D UTE‐Cones quantitative imaging techniques (ie, T₁, T₂*, AdiabT_1ρ, MTR and MT modeling) together with elastix motion correction provide robust volumetric measurement of relaxation times, MTR and f of both short and long T₂ tissues in the knee joint.     

Three‐dimensional ultrashort echo time cones T1ρ (3D UTE‐cones‐T1ρ) imaging

We report a novel three‐dimensional (3D) ultrashort echo time (UTE) sequence employing Cones trajectory and T_1ρ preparation (UTE‐Cones‐T_1ρ) for quantitative T_1ρ assessment of short T₂ tissues in the musculoskeletal system. A basic 3D UTE‐Cones sequence was combined with a spin‐locking preparation pulse for T_1ρ contrast. A relatively short TR was used to decrease the scan time, which required T₁ measurement and compensation using 3D UTE‐Cones data acquisitions with variable TRs. Another strategy to reduce the total scan time was to acquire multiple Cones spokes (N_sp) after each T_1ρ preparation and fat saturation. Four spin‐locking times (TSL = 0–20 ms) were acquired over 12 min, plus another 7 min for T₁ measurement. The 3D UTE‐Cones‐T_1ρ sequence was compared with a two‐dimensional (2D) spiral‐T_1ρ sequence for the imaging of a spherical CuSO₄ phantom and ex vivo meniscus and tendon specimens, as well as the knee and ankle joints of healthy volunteers, using a clinical 3‐T scanner. The CuSO₄ phantom showed a T_1ρ value of 76.5 ± 1.6 ms with the 2D spiral‐T_1ρ sequence, as well as 85.7 ± 3.6 and 89.2 ± 1.4 ms for the 3D UTE‐Cones‐T_1ρ sequences with N_sp of 1 and 5, respectively. The 3D UTE‐Cones‐T_1ρ sequence provided shorter T_1ρ values for the bovine meniscus sample relative to the 2D spiral‐T_1ρ sequence (10–12 ms versus 16 ms, respectively). The cadaveric human Achilles tendon sample could only be imaged with the 3D UTE‐Cones‐T_1ρ sequence (T_1ρ = 4.0 ± 0.9 ms), with the 2D spiral‐T_1ρ sequence demonstrating near‐zero signal intensity. Human studies yielded T_1ρ values of 36.1 ± 2.9, 18.3 ± 3.9 and 3.1 ± 0.4 ms for articular cartilage, meniscus and the Achilles tendon, respectively. The 3D UTE‐Cones‐T_1ρ sequence allows volumetric T_1ρ measurement of short T₂ tissues in vivo.     

Characterization of trabecular bone density with ultra‐short echo‐time MRI at 1.5, 3.0 and 7.0 T – comparison with micro‐computed tomography

The goal of this study was to test the potential of ultra‐short echo‐time (UTE) MRI at 1.5, 3.0 and 7.0 T for depiction of trabecular bone structure (of the wrist bones), to evaluate whether T₂* relaxation times of bone water and parametric maps of T₂* of trabecular bone could be obtained at all three field strengths, and to compare the T₂* relaxation times with structural parameters obtained from micro‐computed tomography (micro‐CT) as a reference standard. Ex vivo carpal bones of six wrists were excised en bloc and underwent MRI at 1.5, 3.0 and 7.0 T in a whole‐body MR imager using the head coil. A three‐dimensional radial fat‐suppressed UTE sequence was applied with subsequent acquisitions, with six different echo times T_E of 150, 300, 600, 1200, 3500 and 7000 µs. The T₂* relaxation time and pixel‐wise computed T₂* parametric maps were compared with a micro‐computed‐tomography reference standard providing trabecular bone structural parameters including porosity (defined as the bone‐free fraction within a region of interest), trabecular thickness, trabecular separation, trabecular number and fractal dimension (D_k). T₂* relaxation curves and parametric maps could be computed from datasets acquired at all field strengths. Mean T₂* relaxation times of trabecular bone were 4580 ± 1040 µs at 1.5 T, 2420 ± 560 µs at 3.0 T and 1220 ± 300 µs at 7.0 T, when averaged over all carpal bones. A positive correlation of T₂* with trabecular bone porosity and trabecular separation, and a negative correlation of T₂* relaxation time with trabecular thickness, trabecular number and fractal dimension, was detected (p < 0.01 for all field strengths and micro‐CT parameters). We conclude that UTE MRI may be useful to characterize the structure of trabecular bone, comparable to micro‐CT. Copyright © 2014 John Wiley & Sons, Ltd.     

Ultrashort echo time spectroscopic imaging (UTESI): an efficient method for quantifying bound and free water

Biological tissues usually contain distinct water compartments with different transverse relaxation times. In this study, two‐dimensional, multi‐slice, ultrashort echo time spectroscopic imaging (UTESI) was used with bi‐component analysis to detect bound and free water components in musculoskeletal tissues. Feasibility studies were performed using numerical simulation. Imaging was performed on bovine cortical bone, human cadaveric menisci and the Achilles' tendons of volunteers. The simulation study demonstrated that UTESI, together with bi‐component analysis, could reliably quantify both T₂* and fractions of the short and long T₂* components. The in vitro and in vivo studies each took less than 14 min. The bound water components showed a short T₂* of ~0.3 ms for bovine bone, ~1.8 ms for meniscus and ~0.6 ms for the Achilles' tendon. The free water components showed about an order of magnitude longer T₂* values, with ~2 ms for bovine bone, ~14 ms for meniscus and ~8 ms for the Achilles' tendon. Bound water fractions of up to ~76% for bovine bone, 50% for meniscus and ~75% for the Achilles' tendon were measured. The corresponding free water components were up to ~24% for bovine bone, 50% for meniscus and ~25% for the Achilles' tendon by volume. These results demonstrate that UTESI, combined with bi‐component analysis, can quantify the bound and free water components in musculoskeletal tissues in clinically realistic times. Copyright © 2011 John Wiley & Sons, Ltd.     

Separation of collagen‐bound and porous bone water transverse relaxation in mice: proposal of a multi‐step approach

The separation and quantification of collagen‐bound water (CBW) and pore water (PW) components of the cortical bone signal are important because of their different contribution to bone mechanical properties. Ultrashort TE (UTE) imaging can be used to exploit the transverse relaxation from CBW and PW, allowing their quantification. We tested, for the first time, the feasibility of UTE measurements in mice for the separation and quantification of the transverse relaxation of CBW and PW in vivo using three different approaches for T₂* determination. UTE sequences were acquired at 4.7 T in six mice with 10 different TEs (50–5000 μs). The transverse relaxation time T₂* of CBW (T₂*_cbw) and PW (T₂*_pw) and the CBW fraction (bwf) were computed using a mono‐exponential (i), a standard bi‐exponential (ii) and a new multi‐step bi‐exponential (iii) approach. Regions of interest were drawn at multiple levels of the femur and vertebral body cortical bone for each mouse. The sum of the normalized squared residuals (Res) and the homogeneity of variance were tested to compare the different methods. In the femur, approach (i) yielded mean T₂* ± standard deviation (SD) of 657 ± 234 μs. With approach (ii), T₂*_cbw, T₂*_pw and bwf were 464 ± 153 μs, 15 777 ± 10 864 μs and 57.6 ± 9.9%, respectively. For approach (iii), T₂*_cbw, T₂*_pw and bwf were 387 ± 108 μs, 7534 ± 2765 μs and 42.5 ± 6.2%, respectively. Similar values were obtained from vertebral bodies. Res with approach (ii) was lower than with the two other approaches (p < 0.007), but T₂*_pw and bwf variance was lower with approach (iii) than with approach (ii) (p < 0.048). We demonstrated that the separation and quantification of cortical bone water components with UTE sequences is feasible in vivo in mouse models. The direct bi‐exponential approach exhibited the best approximation to the measured signal curve with the lowest residuals; however, the newly proposed multi‐step algorithm resulted in substantially lower variability of the computed parameters. Copyright © 2016 John Wiley & Sons, Ltd.     

In vivo carotid plaque MRI using quantitative T2* measurements with ultrasmall superparamagnetic iron oxide particles: a dose-response study to statin therapy

This study investigates T₂* quantification in carotid plaques before and after the administration of ultrasmall superparamagnetic iron oxide particles (USPIOs) in a cohort of patients receiving statin therapy. Phantom studies were performed using gels with varying concentrations of USPIOs. In the phantom study, 12 gels were prepared with a range of freely distributed concentrations of USPIO nanoparticles (0–0.05 mg/mL). Relative signal intensity measurements were obtained from a T₂*‐weighted sequence as well as quantitative T₂* (qT₂*) measurements. In the patient study, 40 patients with >40% carotid stenosis were randomised to low‐ and high‐dose statin therapy (10 and 80 mg of atorvastatin). Pre‐ and post‐ (36 h) USPIO‐enhanced MRI were performed at baseline, and at 6 and 12 weeks. A linear mixed‐effects model was applied to account for the inherent correlation of multiple‐plaque measurements from the same patient and to assess dose–response differences to statin therapy. In the phantom study, the T₂*‐weighted sequence demonstrated an initial increase (T₁ effect), followed by a decrease (T₂* effect), in relative signal intensity with increasing concentrations of USPIO. The qT₂* values decreased exponentially with increasing concentrations of USPIO. In the patient study, there was a highly significant difference in post‐USPIO T₂* measurements in plaques between the low‐ and high‐dose statin groups. This was observed for both the difference in qT₂* measurements (post‐USPIO minus pre‐USPIO) (p < 0.001) and for qT₂* post‐USPIO only (p < 0.001). The post‐USPIO qT₂* values were as follows: baseline: low dose, 13.6 ± 5.5 ms; high dose, 12.9 ± 6.2 ms; 6 weeks: low dose, 13.3 ± 6.7 ms; high dose, 14.3 ± 7.7 ms; 12 weeks: low dose, 14.0 ± 7.6 ms; high dose, 18.3 ± 11.2 ms. It can be concluded that qT₂* measurements provide an alternative method of quantifying USPIO uptake. These results also demonstrate that changes in USPIO uptake can be measured using post‐USPIO imaging only. Copyright © 2010 John Wiley & Sons, Ltd.     

Characterization of the ultrashort‐TE (UTE) MR collagen signal

Although current cardiovascular MR (CMR) techniques for the detection of myocardial fibrosis have shown promise, they nevertheless depend on gadolinium‐based contrast agents and are not specific to collagen. In particular, the diagnosis of diffuse myocardial fibrosis, a precursor of heart failure, would benefit from a non‐invasive imaging technique that can detect collagen directly. Such a method could potentially replace the need for endomyocardial biopsy, the gold standard for the diagnosis of the disease. The objective of this study was to measure the MR properties of collagen using ultrashort TE (UTE), a technique that can detect short T₂* species. Experiments were performed in collagen solutions. Via a model of bi‐exponential T₂* with oscillation, a linear relationship (slope = 0.40 ± 0.01, R² = 0.99696) was determined between the UTE collagen signal fraction associated with these properties and the measured collagen concentration in solution. The UTE signal of protons in the collagen molecule was characterized as having a mean T₂* of 0.75 ± 0.05 ms and a mean chemical shift of ?3.56 ± 0.01 ppm relative to water at 7 T. The results indicated that collagen can be detected and quantified using UTE. A knowledge of the collagen signal properties could potentially be beneficial for the endogenous detection of myocardial fibrosis. Copyright © 2015 John Wiley & Sons, Ltd.     

In vivo observation of quadrupolar splitting in 39K magnetic resonance spectroscopy of human muscle tissue

The purpose of this work was to explore the origin of oscillations of the T^*₂ decay curve of ³⁹K observed in studies of ³⁹K magnetic resonance imaging of the human thigh. In addition to their magnetic dipole moment, spin‐³/₂ nuclei possess an electric quadrupole moment. Its interaction with non‐vanishing electrical field gradients leads to oscillations in the free induction decay and to splitting of the resonance. All measurements were performed on a 7T whole‐body MRI scanner (MAGNETOM 7T, Siemens AG, Erlangen, Germany) with customer‐built coils. According to the theory of quadrupolar splitting, a model with three Lorentzian‐shaped peaks is appropriate for ³⁹K NMR spectra of the thigh and calf. The frequency shifts of the satellites depend on the angle between the calf and the static magnetic field. When the leg is oriented parallel to the static magnetic field, the satellites are shifted by about 200 Hz. In the thigh, rank‐2 double quantum coherences arising from anisotropic quadrupolar interaction are observed by double‐quantum filtration with magic‐angle excitation. In addition to the spectra, an image of the thigh with a nominal resolution of (16 × 16 × 32) mm³ was acquired with this filtering technique in 1:17 h. From the line width of the resonances, ³⁹K transverse relaxation time constants T^*_{2, fast} = (0.51 ± 0.01) ms and T^*_{2, slow} = (6.21 ± 0.05) ms for the head were determined. In the thigh, the left and right satellite, both corresponding to the short component of the transverse relaxation time constant, take the following values: T^*_{2, fast} = (1.56 ± 0.03) ms and T^*_{2, fast} = (1.42 ± 0.03) ms. The centre line, which corresponds to the slow component, is T^*₂, _slow = (9.67 ± 0.04) ms. The acquisition time of the spectra was approximately 10 min. Our results agree well with a non‐vanishing electrical field gradient interacting with ³⁹K nuclei in the intracellular space of muscle tissue. Copyright © 2016 John Wiley & Sons, Ltd.     

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

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. Copyright © 2014 John Wiley & Sons, Ltd.     

Measuring renal tissue relaxation times at 7 T

As developments in RF coils and RF management strategies make performing ultra‐high‐field renal imaging feasible, understanding the relaxation times of the tissue becomes increasingly important for tissue characterization, sequence optimization and quantitative functional renal imaging, such as renal perfusion imaging using arterial spin labeling. By using a magnetization‐prepared single‐breath‐hold fast spin echo imaging method, human renal T₁ and T₂ imaging studies were successfully performed at 7 T with 11 healthy volunteers (eight males, 45 ± 17 years, and three females, 29 ± 7 years, mean ± standard deviation, S.D.) while addressing challenges of B₁⁺ inhomogeneity and short‐term specific absorption rate limits. At 7 T, measured renal T₁ values for the renal cortex and medulla (mean ± S.D.) from five healthy volunteers who participated in both 3 T and two‐session 7 T studies were 1661 ± 68 ms and 2094 ± 67 ms, and T₂ values were 108 ± 7 ms and 126 ± 6 ms. For comparison, similar measurements were made at 3 T, where renal cortex and medulla T₁ values of 1261 ± 86 ms and 1676 ± 94 ms and T₂ values of 121 ± 5 ms and 138 ± 7 ms were obtained. Measurements at 3 T and 7 T were significantly different for both T₁ and T₂ values in both renal tissues. Reproducibility studies at 7 T demonstrated that T₁ and T₂ estimations were robust, with group mean percentage differences of less than 4%. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantitative blood oxygenation level‐dependent (BOLD) response of the left ventricular myocardium to hyperoxic respiratory challenge at 1.5 and 3.0 T

The aim of this study was to quantify the response of the myocardial transverse relaxation times (ΔT₂*) to hyperoxic respiratory challenge (HRC) at different field strengths in an intra‐individual comparison of healthy volunteers and in a patient with coronary artery disease. Blood oxygenation level‐dependent (BOLD) cardiovascular MR (CMR) data were acquired in 10 healthy volunteers (five women, five men; mean age, 29 ± 3 years; range, 22–35 years) at 1.5 and 3.0 T. Medical air (21% O₂), pure oxygen and carbogen (95% O₂, 5% CO₂) were administered in a block‐design temporal pattern to induce normoxia, hyperoxia and hyperoxic hypercapnia, respectively. Average T₂* times were derived from measurements by two independent and blind readers in 16 standard myocardial segments on three short‐axis slices per patient. Inter‐ and intra‐reader correlations of T₂* measurements were good [intra‐class correlation coefficient (ICC) = 0.75 and ICC = 0.79, both p < 0.001]. During normoxia, the mean T₂* times were 29.9 ± 6.1 ms at 1.5 T and 27.1 ± 6.6 ms at 3.0 T. Both hyperoxic gases induced significant (all p < 0.01) T₂* increases (?T₂* hyperoxia: 1.5 T, 12.7%; 3.0 T, 11.2%; hyperoxic hypercapnia: 1.5 T, 13.1%; 3.0 T, 17.7%). Analysis of variance (ANOVA) results indicated a significant (both p < 0.001) effect of the inhaled gases on the T₂* times at both 1.5 T (F = 17.74) and 3.0 T (F = 39.99). With regard to the patient imaged at 1.5 T, HRC induced significant T₂* increases during hyperoxia and hyperoxic hypercapnia in normal myocardial segments, whereas the T₂* response was not significant in ischemic segments (p > 0.23). The myocardial ?T₂* response to HRC can reliably be imaged and quantified with BOLD CMR at both 1.5 and 3.0 T. During HRC, hyperoxia and hyperoxic hypercapnia induce a significant increase in T₂*, with ?T₂* being largest at 3.0 T and during hyperoxic hypercapnia in normal myocardial segments. Copyright © 2014 John Wiley & Sons, Ltd.     

Diffusion tensor imaging and T2 relaxometry of bilateral lumbar nerve roots: feasibility of in‐plane imaging

Lower back pain is a common problem frequently encountered without specific biomarkers that correlate well with an individual patient's pain generators. MRI quantification of diffusion and T₂ relaxation properties may provide novel insight into the mechanical and inflammatory changes that occur in the lumbosacral nerve roots in patients with lower back pain. Accurate imaging of the spinal nerve roots is difficult because of their small caliber and oblique course in all three planes. Two‐dimensional in‐plane imaging of the lumbosacral nerve roots requires oblique coronal imaging with large field of view (FOV) in both dimensions, resulting in severe geometric distortions using single‐shot echo planar imaging (EPI) techniques. The present work describes initial success using a reduced‐FOV single‐shot spin‐echo EPI acquisition to obtain in‐plane diffusion tensor imaging (DTI) and T₂ mapping of the bilateral lumbar nerve roots at the L4 level of healthy subjects, minimizing partial volume effects, breathing artifacts and geometric distortions. A significant variation in DTI and T₂ mapping metrics is also reported along the course of the normal nerve root. The fractional anisotropy is statistically significantly lower in the dorsal root ganglia (0.287 ± 0.068) than in more distal regions in the spinal nerve (0.402 ± 0.040) (p < 10^–5). The T₂ relaxation value is statistically significantly higher in the dorsal root ganglia (78.0 ± 11.9 ms) than in more distal regions in the spinal nerve (59.5 ± 7.4 ms) (p < 10^–5). The quantification of nerve root DTI and T₂ properties using the proposed methodology may identify the specific site of any degenerative and inflammatory changes along the nerve roots of patients with lower back pain. Copyright © 2012 John Wiley & Sons, Ltd.     

Adiabatic multi‐echo 31P spectroscopic imaging (AMESING) at 7 T for the measurement of transverse relaxation times and regaining of sensitivity in tissues with short T2* values

An adiabatic multi‐echo spectroscopic imaging (AMESING) sequence, used for ³¹P MRSI, with spherical k‐space sampling and compensated phase‐encoding gradients, was implemented on a whole‐body 7‐T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T₂* and high T₂, this can theoretically lead to a potential maximum signal‐to‐noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst‐angle excitation. However, with T₂ values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst‐angle excitation should be feasible. The multi‐echo sequence enables the determination of localized T₂ values, and was validated with ³¹P three‐dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T₂ values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T₂ value of γ‐ATP was 25 ± 6 ms. A T₂ value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T₂ values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T₂* values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity. Copyright © 2013 John Wiley & Sons, Ltd.