Anisotropic analysis of multi-component T2 and T1? relaxations in achilles tendon by NMR spectroscopy and microscopic MRI.
ABSTRACT: To study the anisotropic characteristics of both multi-component T2 and T1? relaxation times in tendon.T2 and T1? were measured in tendon by NMR spectroscopy at different orientations and by microscopic MRI at the magic angle. Several experimental issues in the multi-component relaxation measurements were investigated, including the effects of echo spacing, the resolution of MRI experiments, the influence of the specimen orientations, and the strengths of different spin-lock frequencies in T1? experiments.Both the values and fractions of T2 in tendon showed significant orientational dependence. The values and fractions of T1? strongly depended on both the specimen orientation and the spin-lock strength. The imaging resolution (35-280 ?m) had little influence in the T2 experiments. Both the echo spacings (0.6-3.0 ms) in the T2 experiment and the spin-lock strengths (0.5-5 kHz) in the T1? experiment affected the quantification of the multi-component relaxation. Up to three T2 and T1? components were resolved in tendon.Multi-component relaxations could be attributed to different populations of water in the tissue. The transitions between a mono-component and multi-component result call for the caution in interpreting the relaxation results.
Project description:BACKGROUND:In addition to the articular cartilage, osteoarthritis (OA) affects several other tissues such as tendons, ligaments, and subchondral bone. T1? relaxation study of these short T2 tissues may provide a more comprehensive evaluation of OA. PURPOSE:To develop a 3D spin-lattice relaxation in the rotating frame (T1? ) prepared zero echo time (ZTE)-based pointwise encoding time reduction with radial acquisition (3D-T1? -PETRA) sequence for relaxation mapping of semisolid short-T2 tissues on a clinical 3?T scanner. STUDY TYPE:Prospective. POPULATION:Phantom, two bovine whole knee joint and Achilles tendon specimens, 10 healthy volunteers with no known inflammation, trauma or pain in the knee or ankle. FIELD STRENGTH/SEQUENCE:A customized PETRA sequence to acquire fat-suppressed 3D T1? -weighted images tissues with semisolid short T2 / T2* relaxation times in the knee and ankle joints at 3?T. ASSESSMENT:Mono- and biexponential T1? relaxation components were assessed in the patellar tendon (PT), anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and Achilles tendon (AT). STATISTICAL TESTS:Kruskal-Wallis with post-hoc Dunn's test for multiple pairwise comparisons. RESULTS:Phantom and ex vivo studies showed the feasibility of T1? relaxation mapping using the proposed 3D-T1? -PETRA sequence. The in vivo study demonstrated an averaged mono-T1? relaxation of (median [IQR]) 15.9 [14.5] msec, 23.6 [9.4] msec, 17.4 [7.4] msec, and 5.8 [10.2] msec in the PT, ACL, PCL, and AT, respectively. The bicomponent analysis showed the short and long components (with their relative fractions) of 0.65 [1.0] msec (46.9 [15.3]%) and 37.3 [18.4] msec (53.1 [15.3]%) for PT, 1.7 [2.1] msec (42.5 [12.5]%) and 43.7 [17.8] msec (57.5 [12.5]%) for ACL, and 1.2 [1.9] msec (42.6 [14.0]%) and 27.7 [14.7] msec (57.3 [14.0]%) for PCL and 0.4 [0.02] msec (58.8 [13.3]%/) and 31.3 [10.8] msec (41.2 [13.3]%) for AT. Statistically significant (P???0.05) differences were observed in the mono- and biexponential relaxation between several regions. DATA CONCLUSION:The 3D-T1? -PETRA sequence allows volumetric, isotropic (0.78?×?0.78?×?0.78?mm), biexponential T1? assessment with corresponding fractions of the tissues with semisolid short T2 / T2* . LEVEL OF EVIDENCE:2 Technical Efficacy Stage: 1 J. Magn. Reson. Imaging 2019;50:1207-1218.
Project description:The objective of this study was to examine age-dependent changes in both T1-weighted and T2-weighted image contrasts and spin-echo T2 relaxation time in the human brain during healthy ageing.A total of 37 participants between the ages of 49 and 87 years old were scanned with a 3 Tesla system, using T1-weighted, T2 weighted and quantitative spin-echo T2 imaging. Contrast between image intensities and T2 values was calculated for various regions, including between individual hippocampal subfields.The T1 contrast-to-noise (CNR) and gray:white signal intensity ratio (GWR) did not change in the hippocampus, but it declined in the cingulate cortex with age. In contrast, T2 CNR and GWR declined in both brain regions. T2 relaxation time was almost constant in gray matter and most (but not all) hippocampal subfields, but increased substantially in white matter, pointing to an age effect on water relaxation in white matter.Changes in T1 and T2 MR characteristics influence the appearance of brain images in later life and should be considered in image analyses of aged subjects. It is speculated that alterations at the cell biology level, with concomitant alterations to the local magnetic environment, reduce dephasing and subsequently prolong spin-echo T2 through reduced diffusion effects in later life.
Project description:In highly organized tissues, such as cartilage, tendons and white matter, several quantitative MRI parameters exhibit dependence on the orientation of the tissue constituents with respect to the main imaging magnetic field (B0). In this study, we investigated the dependence of multiple relaxation parameters on the orientation of articular cartilage specimens in the B0. Bovine patellar cartilage-bone samples (n?=?4) were investigated ex vivo at 9.4 Tesla at seven different orientations, and the MRI results were compared with polarized light microscopy findings on specimen structure. Dependences of T2 and continuous wave (CW)-T1? relaxation times on cartilage orientation were confirmed. T2 (and T2*) had the highest sensitivity to orientation, followed by TRAFF2 and adiabatic T2?. The highest dependence was seen in the highly organized deep cartilage and the smallest in the least organized transitional layer. Increasing spin-lock amplitude decreased the orientation dependence of CW-T1?. T1 was found practically orientation-independent and was closely followed by adiabatic T1?. The results suggest that T1 and adiabatic T1? should be preferred for orientation-independent quantitative assessment of organized tissues such as articular cartilage. On the other hand, based on the literature, parameters with higher orientation anisotropy appear to be more sensitive to degenerative changes in cartilage.
Project description:Quantitative T2 -relaxation-based contrast has the potential to provide valuable clinical information. Practical T2 -mapping, however, is impaired either by prohibitively long acquisition times or by contamination of fast multiecho protocols by stimulated and indirect echoes. This work presents a novel postprocessing approach aiming to overcome the common penalties associated with multiecho protocols, and enabling rapid and accurate mapping of T2 relaxation values.Bloch simulations are used to estimate the actual echo-modulation curve (EMC) in a multi-spin-echo experiment. Simulations are repeated for a range of T2 values and transmit field scales, yielding a database of simulated EMCs, which is then used to identify the T2 value whose EMC most closely matches the experimentally measured data at each voxel.T2 maps of both phantom and in vivo scans were successfully reconstructed, closely matching maps produced from single spin-echo data. Results were consistent over the physiological range of T2 values and across different experimental settings.The proposed technique allows accurate T2 mapping in clinically feasible scan times, free of user- and scanner-dependent variations, while providing a comprehensive framework that can be extended to model other parameters (e.g., T1 , B1 (+) , B0 , diffusion) and support arbitrary acquisition schemes.
Project description:Rotating frame spin-lattice relaxation, with the characteristic time constant T1?, provides a means to access motion-restricted (slow) spin dynamics in MRI. As a result of their restricted motion, these spins are sometimes characterized by a short transverse relaxation time constant T2 and thus can be difficult to detect directly with conventional image acquisition techniques. Here, we introduce an approach for three-dimensional adiabatic T1? mapping based on a magnetization-prepared sweep imaging with Fourier transformation (MP-SWIFT) sequence, which captures signal from almost all water spin populations, including the extremely fast relaxing pool. A semi-analytical procedure for T1? mapping is described. Experiments on phantoms and musculoskeletal tissue specimens (tendon, articular and epiphyseal cartilages) were performed at 9.4?T for both the MP-SWIFT and fast spin echo (FSE) read outs. In the phantom with liquids having fast molecular tumbling and a single-valued T1? time constant, the measured T1? values obtained with MP-SWIFT and FSE were similar. Conversely, in normal musculoskeletal tissues, T1? values measured with MP-SWIFT were much shorter than the values obtained with FSE. Studies of biological tissue specimens demonstrated that T1?-weighted SWIFT provides higher contrast between normal and diseased tissues relative to conventional acquisitions. Adiabatic T1? mapping with SWIFT readout captures contributions from the otherwise undetected fast relaxing spins, allowing more informative T1? measurements of normal and diseased states.
Project description:PURPOSE:The optimization and analysis of spin ensemble trajectories in the hybrid state-a state in which the direction of the magnetization adiabatically follows the steady state while the magnitude remains in a transient state. METHODS:Numerical optimizations were performed to find spin ensemble trajectories that minimize the Cramér-Rao bound for T1 -encoding, T2 -encoding, and their weighted sum, respectively, followed by a comparison between the Cramér-Rao bounds obtained with our optimized spin-trajectories, Look-Locker sequences, and multi-spin-echo methods. Finally, we experimentally tested our optimized spin trajectories with in vivo scans of the human brain. RESULTS:After a nonrecurring inversion segment on the southern half of the Bloch sphere, all optimized spin trajectories pursue repetitive loops on the northern hemisphere in which the beginning of the first and the end of the last loop deviate from the others. The numerical results obtained in this work align well with intuitive insights gleaned directly from the governing equation. Our results suggest that hybrid-state sequences outperform traditional methods. Moreover, hybrid-state sequences that balance T1 - and T2 -encoding still result in near optimal signal-to-noise efficiency for each relaxation time. Thus, the second parameter can be encoded at virtually no extra cost. CONCLUSIONS:We provided new insights into the optimal encoding processes of spin relaxation times in order to guide the design of robust and efficient pulse sequences. We found that joint acquisitions of T1 and T2 in the hybrid state are substantially more efficient than sequential encoding techniques.
Project description:In vivo oximetry by pulsed electron paramagnetic resonance is based on measurements of changes in electron spin relaxation rates of probe molecules, such as the triarylmethyl radicals. A series of experiments was performed at frequencies between 250 MHz and 1.5 GHz to assist in the selection of an optimum frequency for oximetry. Electron spin relaxation rates for the triarylmethyl radical OX063 as a function of radical concentration, salt concentration, and resonance frequency were measured by electron spin echo 2-pulse decay and 3-pulse inversion recovery in the frequency range of 250 MHz-1.5 GHz. At constant OX063 concentration, 1/T1 decreases with increasing frequency because the tumbling dependent processes that dominate relaxation at 250 MHz are less effective at higher frequency. 1/T2 also decreases with increasing frequency because 1/T1 is a significant contribution to 1/T2 for trityl radicals in fluid solution. 1/T2-1/T1, the incomplete motional averaging contribution to 1/T2, increases with increasing frequency. At constant frequency, relaxation rates increase with increasing radical concentration due to contributions from collisions that are more effective for 1/T2 than 1/T1. The collisional contribution to relaxation increases as the concentration of counter-ions in solution increases, which is attributed to interactions of cations with the negatively charged radicals that decrease repulsion between trityl radicals. The Signal-to-Noise ratio (S/N) of field-swept echo-detected spectra of OX063 were measured in the frequency range of 400 MHz-1 GHz. S/N values, normalized by ?Q, increase as frequency increases. Adding salt to the radical solution decreased S/N because salt lowers the resonator Q. Changing the temperature from 19 to 37 °C caused little change in S/N at 700 MHz. Both slower relaxation rates and higher S/N at higher frequencies are advantageous for oximetry. The potential disadvantage of higher frequencies is the decreased depth of penetration into tissue.
Project description:In vivo spin spin relaxation time (T2) heterogeneity of hyperpolarized [13C,15N2]urea in the rat kidney was investigated. Selective quenching of the vascular hyperpolarized 13C signal with a macromolecular relaxation agent revealed that a long-T2 component of the [13C,15N2]urea signal originated from the renal extravascular space, thus allowing the vascular and renal filtrate contrast agent pools of the [13C,15N2]urea to be distinguished via multi-exponential analysis. The T2 response to induced diuresis and antidiuresis was performed with two imaging agents: hyperpolarized [13C,15N2]urea and a control agent hyperpolarized bis-1,1-(hydroxymethyl)-1-13C-cyclopropane-2H8. Large T2 increases in the inner-medullar and papilla were observed with the former agent and not the latter during antidiuresis. Therefore, [13C,15N2]urea relaxometry is sensitive to two steps of the renal urea handling process: glomerular filtration and the inner-medullary urea transporter (UT)-A1 and UT-A3 mediated urea concentrating process. Simple motion correction and subspace denoising algorithms are presented to aid in the multi exponential data analysis. Furthermore, a T2-edited, ultra long echo time sequence was developed for sub-2 mm3 resolution 3D encoding of urea by exploiting relaxation differences in the vascular and filtrate pools.
Project description:PURPOSE:Multi-echo spin-echo (MESE) protocol is the most effective tool for mapping T2 relaxation in vivo. Still, MESE extensive use of radiofrequency pulses causes magnetization transfer (MT)-related bias of the water signal, instigated by the presence of macromolecules (MMP). Here, we analyze the effects of MT on MESE signal, alongside their impact on quantitative T2 measurements. METHODS:Study used 3 models: in vitro urea phantom, ex vivo horse brain, and in vivo human brain. MT ratio (MTR) was measured between single-SE and MESE protocols under different scan settings including varying echo train lengths, number of slices, and inter-slice gap. MTR and T2 values were extracted for each model and protocol. RESULTS:MT interactions biased MESE signals, and in certain settings, the corresponding T2 values. T2 underestimation of up to 4.3% was found versus single-SE values in vitro and up to 13.8% ex vivo, correlating with the MMP content. T2 bias originated from intra-slice saturation of the MMP, rather than from indirect saturation in multi-slice acquisitions. MT-related signal attenuation was caused by slice crosstalk and/or partial T1 recovery, whereas smaller contribution was caused by MMP interactions. Inter-slice gap had a similar effect on in vivo MTR (21.2%), in comparison to increasing the number of slices (18.9%). CONCLUSIONS:MT influences MESE protocols either by uniformly attenuating the entire echo train or by cumulatively attenuating the signal along the train. Although both processes depend on scan settings and MMP content, only the latter will cause underestimation of T2 .