What causes the hyperintense T2-weighting and increased short T2 signal in the corticospinal tract?

The corticospinal tract (CST) appears hyperintense on both T₂-weighted images and myelin water maps. Here, an extended multiecho T₂ relaxation sequence with echoes out to 1120 ms was used to characterize the longer T₂ times present in the CST. The T₂ distribution from the CST was compared to other white matter structures in 14 healthy subjects. The intra-/extracellular T₂ peak of the CST was broadened relative to other white matter structures and often split into two distinct peaks. In the CST, it appeared that the intracellular and extracellular water environments had unique T₂ times, causing the intracellular water peak to be pushed down into the myelin water T₂ regime and the extracellular peak to be pushed up to longer T₂ times. The conventional myelin water T₂ limits of 5-40 ms resulted in an artificial increase in myelin water fraction (MWF), causing the CST to be bright on myelin water images. When the upper limit for MWF was decreased to 25 ms, the CST regions exhibited MWF values similar to those found for adjacent anterior and posterior regions. The CST has unique magnetic resonance characteristics, which should be taken into consideration when being examined, especially when compared to pathological tissue.     

Regional variations in MR relaxation of hip joint cartilage in subjects with and without femoralacetabular impingement

The objective of this study was to analyze regional variations of magnetic resonance (MR) relaxation times (T_1ρ and T₂) in hip joint cartilage of healthy volunteers and subjects with femoral acetabular impingement (FAI). Morphological and quantitative images of the hip joints of 12 healthy volunteers and 9 FAI patients were obtained using a 3 T MR scanner. Both femoral and acetabular cartilage layers in each joint were semi-automatically segmented on sagittal 3D high-resolution spoiled gradient echo (SPGR) images. These segmented regions of interest (ROIs) were automatically divided radially into twelve equal sub-regions (30⁰ intervals) based on the fitted center of the femur head. The mean value of T_1ρ/T₂ was calculated in each sub-region after superimposing the divided cartilage contours on the MR relaxation (T_1ρ/T₂) maps to quantify the relaxation times. T_1ρ and T₂ relaxation times of the femoral cartilage were significantly higher in FAI subjects compared to healthy controls (39.9 ± 3.3 msec in FAI vs. 35.4 ± 2.3 msec in controls for T_1ρ (P = 0.0020); 33.9 ± 3.1 msec in FAI vs. 31.1 ± 1.7 msec in controls for T₂ (P = 0.0160)). Sub-regional analysis showed significantly different T_1ρ and T₂ relaxation times in the anterior-superior region (R9) of the hip joint cartilage between subjects with FAI and healthy subjects, suggesting possible regional differences in cartilage matrix composition between these two groups. Receiver operating characteristic (ROC) analysis showed that sub-regional analysis in femoral cartilage was more sensitive in discriminating FAI joint cartilage from that of healthy joints than global analysis of the whole region (T_1ρ: area under the curve (AUC) = 0.981, P = 0.0001 for R9 sub-region; AUC = 0.901, P = 0.002 for whole region; T₂: AUC = 0.976, P = 0.0005 for R9 sub-region; AUC = 0.808, P = 0.0124 for whole region). The results of this study demonstrated regional variations in hip cartilage composition using MR relaxation times (T_1ρ and T₂) and suggested that analysis based on local regions was more sensitive than global measures in subjects with and without FAI.     

A comparative study at 3 T of sequence dependence of T2 quantitation in the knee

Objective

T₂ mapping has been used widely in detecting cartilage degeneration in osteoarthritis. Several scanning sequences have been developed in the determination of T₂ relaxation times of tissues. However, the derivation of these times may vary from sequence to sequence. This study seeks to evaluate the sequence-dependent differences in T₂ quantitation of cartilage, muscle, fat and bone marrow in the knee joint at 3 T.

Methods

Three commercial phantoms and 10 healthy volunteers were studied using 3 T MR. T₂ relaxation times of the phantoms, cartilage, muscle, subcutaneous fat and marrow were derived using spin echo (SE), multiecho SE (MESE), fast SE (FSE) with varying echo train length (ETL), spiral and spoiler gradient (SPGR) sequences. The differences between these times were then evaluated using Student's t test. In addition, the signal-to-noise ratio (SNR) efficiency and coefficient of variation of T₂ from each sequence were calculated.

Results

The average T₂ relaxation time was 36.38±5.76 ms in cartilage and 34.08±6.55 ms in muscle, ranging from 27 to 45 ms in both tissues. The times for subcutaneous fat and marrow were longer and more varying, ranging from 41 to 143 ms and from 42 to 160 ms, respectively. In FSE acquisition, relaxation time significantly increases as ETL increases (P<.05). In cartilage, the SE acquisition yields the lowest T₂ values (27.52±3.10 ms), which is significantly lower than those obtained from other sequences (P<.002). T₂ values obtained from spiral acquisition (38.27±6.45 ms) were higher than those obtained from MESE (34.35±5.62 ms) and SPGR acquisition (31.64±4.53 ms). These differences, however, were not significant (P>.05).

Conclusion

T₂ quantification can be a valuable tool for the diagnosis of degenerative disease. Several different sequences exist to quantify the relaxation times of tissues. Sequences range in scan time, SNR efficiency, reproducibility and two- or three-dimensional mapping. However, when choosing a sequence for quantitation, it is important to realize that several factors affect the measured T₂ relaxation time.     

MSE-MRI sequence optimisation for measurement of bi- and tri-exponential T2 relaxation in a phantom and fruit

The transverse relaxation signal from vegetal cells can be described by multi-exponential behaviour, reflecting different water compartments. This multi-exponential relaxation is rarely measured by conventional MRI imaging protocols; mono-exponential relaxation times are measured instead, thus limiting information about of the microstructure and water status in vegetal cells. In this study, an optimised multiple spin echo (MSE) MRI sequence was evaluated for assessment of multi-exponential transverse relaxation in fruit tissues. The sequence was designed for the acquisition of a maximum of 512 echoes. Non-selective refocusing RF pulses were used in combination with balanced crusher gradients for elimination of spurious echoes. The study was performed on a bi-compartmental phantom with known T₂ values and on apple and tomato fruit. T₂ decays measured in the phantom and fruit were analysed using bi- and tri-exponential fits, respectively. The MRI results were compared with low field non-spatially resolved NMR measurements performed on the same samples.     

Use of NMR Relaxometry to identify frankfurters of different meat sources

ABSTRACT

Frankfurters are categorised as ready to eat meat products and are prepared by using different meat sources including beef, chicken, turkey or pork or mixtures of them. Due to cost and feasibility, it is very common to adulterate beef-, chicken- and turkey-based frankfurters with pork meat. Since pork is not consumed by some part of the population due to religion, it is important to identify pork frankfurters based on easy quality control methods. In this study, NMR Relaxometry was used to differentiate four different frankfurter types based on their relaxation times. Relaxation measurements were conducted on three permanent field benchtop NMR spectrometers (13.52/20.34/22.35?MHz) and also using an FFC Relaxometer at a frequency range of 40?kHz–8.86?MHz. Physical properties of frankfurters such as moisture and solid fat contents, water activity (a_w) and hardness values were also measured. Results showed that among the permanent field systems, 22.34?MHz was the best to differentiate frankfurters based on both T₁ and T₂ relaxation times. FFC experiments also showed that the dependence of T₁ with respect to frequency follows a well fitted power law behaviour throughout the whole frequency range (R²?>?0.97) and the highest difference on T₁ was observed at the lowest frequency of 40?kHz. Thus the results of the study showed that NMR Relaxometry has the potential to discriminate frankfurters of different meat origin. Further studies are needed to detect the authenticity of the type of mixture in frankfurters.     

In vivo NMR T2 relaxation of experimental brain tumors in the cat: A multiparameter tissue characterization

Experimental gliomas (F98) were inoculated in cat brain for the systematic study of their in vivo T₂ relaxation time behavior. With a CPMG multi-echo imaging sequence, a train of 16 echoes was evaluated to obtain the transverse relaxation time and the magnetization M(0) at time t = 0. The magnetization decay curves were analyzed for biexponentiality. All tissues showed monoexponential T₂, only that of the ventricular fluid and part of the vital tumor tissue were biexponential. Based on these NMR relaxation parameters the tissues were characterized, their correct assignment being assured by comparison with histological slices. T₂ of normal grey and white matter was 74 ± 6 and 72 ± 6 msec, respectively. These two tissue types were distinguished through M(0) which for white matter was only 0.88 of the intensity of grey matter in full agreement with water content, determined from tissue specimens. At the time of maximal tumor growth and edema spread a tissue differentiation was possible in NMR relaxation parameter images. Separation of the three tissue groups of normal tissue, tumor and edema was based on T₂ with T_2(normal) < T_2(tumor) < T_2(edema). Using M(0) as a second parameter the differentiation was supported, in particular between white matter and tumor or edema. Animals were studied at 1–4 wk after tumor implantation to study tumor development. The magnetization M(0) of both tumor and peritumoral edema went through a maximum between the second and third week of tumor growth. T₂ of edema was maximal at the same time with 133 ± 4 msec, while the relaxation time of tumor continued to increase during the whole growth period, reaching values of 114 ± 12 msec at the fourth week. Thus, a complete characterization of pathological tissues with NMR relaxometry must include a detailed study of the developmental changes of these tissues to assure correct experimental conditions for the goal of optimal contrast between normal and pathological regions in the NMR images.     

Mössbauer study of 1/8 anomaly in La2−x Ba x CuO4

The measurements of Mössbauer effect, magnetic susceptibility and muon spin relaxation have been carried out for the high-T _c superconductor La_2?x Ba _x CuO₄. The intensity of Mössbauer doublet spectrum of the sample of x～1/8 begins to decrease rapidly at a certain temperature T _m, which we define as a magnetic transition temperature T _Möss. This temperature almost agrees with T _μSR determined from muon spin relaxation. The quadrupole doublet disappears at low temperature below T _m but a clearly splitted spectrum is not observed even at 4.2 K, which indicates a peculiar magnetic state with a wide distribution of internal magnetic field. Around x～1/8, the superconducting critical temperature T _c and T _m are competed each other. In conclusion, superconductivity disappears around 1/8 hole concentration and a peculiar magnetic state such as spin density wave appears.     

NMR T1–T2 correlation analysis of molecular absorption inside a hardened cement paste containing silanised silica fume

ABSTRACT

The influence of silanised silica fume addition on the pore size distribution and wettability of white cement paste was investigated using T₁–T₂ correlation nuclear magnetic resonance (NMR) relaxometry. Surface silanisation of silica fume particles was achieved by the hydrolysis reaction of APTES (3-Aminopropyltriethoxysilane) and condensation of the silanol functional groups on the surface. The methods used for characterisation of the silanised silica fume particles were scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). By adding silanised silica fume to the cement paste, the accessibility of water molecules to the porous system becomes restricted, leading to a lower permeability in comparison with the unmodified cement paste. Differential scanning calorimetry (DSC) measurements on the cement pastes saturated with Octamethylcyclotetrasiloxane confirm also that the size of inter-C–S–H and capillary pores is not influenced by the addition of silica fume in a detectable manner.     

Assessment of interstitial water content of articular cartilage with T1 relaxation

The interstitial water content typically increases in the early degeneration of articular cartilage. Previously, T₂ relaxation has been related to water content, yet it is known to be strongly affected by the collagen orientation. Articular cartilage plugs from the bovine patella, femur and tibia (N=20) were mapped for T₁ and T₂ at 9.4 T to test the ability of T₁ relaxation to reflect cartilage water content. As a reference, water and proteoglycan (PG) contents were determined. Significant (P<.01) linear associations were demonstrated between the relaxation rates and tissue water content (R₁: r=−.81, R₂: r=−.60) and PG content (R₁: r=.75). After adjustment for the tissue water content, partial correlation analysis did not show significant associations between the relaxation rates and tissue PG content. After the effect of PGs was removed, significant (P<.05) linear correlation between the relaxation rates and tissue water content (R₁: r=−.48, R₂: r=−.50) was observed. Thus, the spin-lattice relaxation rate is proposed to provide a biomarker for water content in articular cartilage.     

Activation Energies for the Longitudinal Relaxation Rates of the Water in Normal and Neoplastic Lung

Variable temperature water T₁ relaxation times have been measured on some normal and neoplastic human lung tissues. The corresponding activation energies have been derived and a short discussion of the experimental results is given.     

Contributions of Spin–Lattice Relaxation to the Echo Decay of Planar Cu in High-Temperature Superconductors

Measurement ofT_2G, the Gaussian component of the spin-echo envelope of planar Cu nuclei in high-temperature superconductors, gives important information about the real part of the Cu electron spin susceptibility. In the traditional picture of the planar Cu echo decay, the internuclear coupling is assumed to remain static with respect to spin–lattice relaxation and mutual exchange fluctuations. In some circumstances, however, this assumption breaks down. We calculate the internuclear corrections arising from spin–lattice relaxation to the conventional theory ofT_2Gand show thatT_2Gcan be easily corrected for these effects. We argue that mutual exchanges due to the perpendicular indirect couplings are suppressed in these materials. For YBa₂Cu₄O₈, we find a correction on the order of 10% inT_2Gand using the corrected values we find that the isotope ratio⁶³T_2G/⁶⁵T_2Gagrees with theory.     

Whole-body T2* mapping at 1.5 T

This study investigated the feasibility of an MRI protocol providing whole-body T2* maps at 1.5 T. Seven healthy volunteers (mean age=30.1+/-3.7, three women and four men) and two patients (both male, 53 and 46 years old) affected by transfusion-dependent anemias participated in the study. Coronally oriented images of five subsequent body levels were acquired using a fat-suppressed multiecho 2D gradient-echo sequence (12 echo times ranging from 4.8 to 76.3 ms were selected) and afterwards composed. Parametrical T2* maps of the whole body were reconstructed on a pixel-by-pixel basis. For both, healthy volunteers and patients, representative T2* values were computed from extended regions of interest (ROIs). Good-quality whole-body T2* maps were computed in all volunteers and patients. In healthy volunteers, T2* values were assessed in the cerebral white (58.5+/-4.2 ms) and gray (81.4+/-5.5 ms) matter, liver (34.3+/-7.0 ms), spleen (63.5+/-3.3 ms), kidneys (65.4+/-10.3 ms) and skeletal muscles (~30 ms). The liver presented faster relaxation rates in males as compared to females. One patient (serum ferritin concentration=927 microg/dl) showed shortened T2* values in liver (3.6+/-5.5 ms), spleen (3.1+/-4.8 ms), kidneys (11.1+/-7.1 ms) and muscles (25.1+/-3.4 ms). The second patient (serum ferritin concentration=346 microg/dl) presented reduced T2* values in liver (3.9+/-7.3 ms), spleen (20.1+/-9.8 ms) and kidneys (24.6+/-7.7 ms). The presented technique may find clinical application in the assessment of the iron burden in the entire body, and in monitoring of chelation therapies in patients treated with frequent blood transfusions.     

Reproducibility of myelin water fraction analysis: a comparison of region of interest and voxel-based analysis methods

This study compared region of interest (ROI) and voxel-based analysis (VBA) methods to determine the optimal method of myelin water fraction (MWF) analysis. Twenty healthy controls were scanned twice using a multi-echo T₂ relaxation sequence and ROIs were drawn in white and grey matter. MWF was defined as the fractional signal from 15 to 40 ms in the T₂ distribution. For ROI analysis, the mean intensity of voxels within an ROI was fit using non-negative least squares. For VBA, MWF was obtained for each voxel and the mean and median values within an ROI were calculated. There was a slightly higher correlation between Scan 1 and 2 for the VBA method (R²=0.98) relative to the ROI method (R²=0.95), and the VBA mean square difference between scans was 300% lower, indicating VBA was the most consistent between scans. For the VBA method, mean MWF was found to be more reproducible than median MWF. As the VBA method is more reproducible and gives more options for visualization and analysis of MWF, it is recommended over the ROI method of MWF analysis.     

Selected States Magnetic Resonance Spectroscopy (SSMRS): Detection of Paramagnetic Element Dynamics

Abstract

Magnetic resonance (MR) response obtained in a strongly heterogeneous magnetic field with a linear gradient is analysed. It is shown that the employment of strong magnetic field gradients enables the MR spectroscopy to be accomplished in a system with selectively populated energy states (SSMRS). The method can be applied for measuring such physical quantities as the spin diffusion coefficient, D, spin-lattice, T_1m and spin-spin, T_2m, relaxation times and mobility, p_m, of paramagnetic elements in individual Zeeman energy states.

     

Effects of off-resonance irradiation, cross-relaxation, and chemical exchange on steady-state magnetization and effective spin-lattice relaxation times

In the presence of an off-resonance radiofrequency field, recovery of longitudinal magnetization to a steady state is not purely monoexponential. Under reasonable conditions with zero initial magnetization, recovery is nearly exponential and an effective relaxation rate constant R_1eff = 1/T_1eff can be obtained. Exact and approximate formulas for R_1eff and steady-state magnetization are derived from the Bloch equations for spins undergoing cross-relaxation and chemical exchange between two sites in the presence of an off-resonance radiofrequency field. The relaxation formulas require that the magnetization of one spin is constant, but not necessarily zero, while the other spin relaxes. Extension to three sites with one radiofrequency field is explained. The special cases of off-resonance effects alone and with cross-relaxation or chemical exchange, cross-relaxation alone, and chemical exchange alone are compared. The inaccuracy in saturation transfer measurements of exchange rate constants by published formulas is discussed for the creatine kinase reaction.     

Changes in Porcine Muscle Water Characteristics during Growth—An in Vitro Low-Field NMR Relaxation Study

This study investigates the effects of developmental stage and muscle type on the mobility and distribution of water within skeletal muscles, using low-field ¹H-NMR transverse relaxation measurements in vitro on four different porcine muscles (M. longissimus dorsi, M. semitendinosus, M. biceps femoris, M. vastus intermedius) from a total of 48 pigs slaughtered at various weight classes between 25 kg and 150 kg. Principal component analysis (PCA) revealed effects of both slaughter weight and muscle type on the transverse relaxation decay. Independent of developmental stage and muscle type, distributed exponential analysis of the NMR T₂ relaxation data imparted the existence of three distinct water populations, T_2b, T₂₁, and T₂₂, with relaxation times of approximately 1–10, 45–120, and 200–500 ms, respectively. The most profound change during muscle growth was a shift toward faster relaxation in the intermediate time constant, T₂₁. It decreased by approx. 24% in all four muscle types during the period from 25 to 150 kg live weight. Determination of dry matter, fat, and protein content in the muscles showed that the changes in relaxation time of the intermediate time constant, T₂₁, during growth should be ascribed mainly to a change in protein content, as the protein content explained 77% of the variation in the T₂₁ time constant. Partial least squares (PLS) regression revealed validated correlations in the region of 0.58 to 0.77 between NMR transverse relaxation data and muscle development for all the four muscle types, which indicates that NMR relaxation measurements may be used in the prediction of muscle developmental stage.     

T1–T2 Correlation Spectra Obtained Using a Fast Two-Dimensional Laplace Inversion

Spin relaxation is a sensitive probe of molecular structure and dynamics. Correlation of relaxation time constants, such as T₁ and T₂, conceptually similar to the conventional multidimensional spectroscopy, have been difficult to determine primarily due to the absense of an efficient multidimensional Laplace inversion program. We demonstrate the use of a novel computer algorithm for fast two-dimensional inverse Laplace transformation to obtain T₁–T₂ correlation functions. The algorithm efficiently performs a least-squares fit on two-dimensional data with a nonnegativity constraint. We use a regularization method to find a balance between the residual fitting errors and the known noise amplitude, thus producing a result that is found to be stable in the presence of noise. This algorithm can be extended to include functional forms other than exponential kernels. We demonstrate the performance of the algorithm at different signal-to-noise ratios and with different T₁–T₂ spectral characteristics using several brine-saturated rock samples.     

T2* measurements in human brain at 1.5, 3 and 7 T

Measurements have been carried out in six subjects at magnetic fields of 1.5, 3 and 7 T, with the aim of characterizing the variation of T2* with field strength in human brain. Accurate measurement of T2* in the presence of macroscopic magnetic field inhomogeneity is problematic due to signal decay resulting from through-slice dephasing. The approach employed here allowed the signal decay due to through-slice dephasing to be characterized and removed from data, thus facilitating an accurate measurement of T2* even at ultrahigh field. Using double inversion recovery turbo spin-echo images for tissue classification, an analysis of T2* relaxation times in cortical grey matter and white matter was carried out, along with an evaluation of the variation of T2* with field strength in the caudate nucleus and putamen. The results show an approximately linear increase in relaxation rate R2* with field strength for all tissues, leading to a greater range of relaxation times across tissue types at 7 T that can be exploited in high-resolution T2*-weighted imaging.     

Effect of physiological Heart Rate variability on quantitative T2 measurement with ECG-gated Fast Spin Echo (FSE) sequence and its retrospective correction

Object

Quantitative T₂ measurement is applied in cardiac Magnetic Resonance Imaging (MRI) for the diagnosis and follow-up of myocardial pathologies. Standard Electrocardiogram (ECG)-gated fast spin echo pulse sequences can be used clinically for T₂ assessment, with multiple breath-holds. However, heart rate is subject to physiological variability, which causes repetition time variations and affects the recovery of longitudinal magnetization between TR periods.

Materials and methods

The bias caused by heart rate variability on quantitative T₂ measurements is evaluated for fast spin echo pulse sequence. Its retrospective correction based on an effective TR is proposed. Heart rate variations during breath-holds are provided by the ECG recordings from healthy volunteers. T₂ measurements were performed on a phantom with known T₂ values, by synchronizing the sequence with the recorded ECG. Cardiac T₂ measurements were performed twice on six volunteers. The impact of T₁ on T₂ is also studied.

Results

Maximum error in T₂ is 26% for phantoms and 18% for myocardial measurement. It is reduced by the proposed compensation method to 20% for phantoms and 10% for in vivo measurements. Only approximate knowledge of T₁ is needed for T₂ correction.

Conclusion

Heart rate variability may cause a bias in T2 measurement with ECG-gated FSE. It needs to be taken into account to avoid a misleading diagnosis from the measurements.     

Variations in T(2)* and fat content of murine brown and white adipose tissues by chemical-shift MRI

Purpose

The purpose was to compare T₂* relaxation times and proton density fat-fraction (PDFF) values between brown (BAT) and white (WAT) adipose tissue in lean and ob/ob mice.

Materials and Methods

A group of lean male mice (n=6) and two groups of ob/ob male mice placed on similar 4-week (n=6) and 8-week (n=8) ad libitum diets were utilized. The animals were imaged at 3 T using a T₂*-corrected chemical-shift-based water–fat magnetic resonance imaging (MRI) method that provides simultaneous estimation of T₂* and PDFF on a voxel-wise basis. Regions of interest were drawn within the interscapular BAT and gonadal WAT depots on co-registered T₂* and PDFF maps. Measurements were assessed using analysis of variance, Bonferroni-adjusted t test for multigroup comparisons and the Tukey post hoc test.

Results

Significant differences (P<.01) in BAT T₂* and PDFF were observed between the lean and ob/ob groups. The ob/ob animals exhibited longer BAT T₂* and greater PDFF than lean animals. However, only BAT PDFF was significantly different (P<.01) between the two ob/ob groups. When comparing BAT to WAT within each group, T₂* and PDFF values were consistently lower in BAT than WAT (P<.01). The difference was most prominent in the lean animals. In both ob/ob groups, BAT exhibited very WAT-like appearances and properties on the MRI images.

Conclusion

T₂* and PDFF are lower in BAT than WAT. This is likely due to variations in tissue composition. The values were consistently lower in lean mice than in ob/ob mice, suggestive of the former's greater demand for BAT thermogenesis and reflective of leptin hormone deficiencies and diminished BAT metabolic activity in the latter.