Apparent Spin-spin Relaxation Research Articles

Elastic modulus of cadaveric acetabular labrum correlates with ultrashort echo time (UTE) apparent spin-spin (T2*) and spin-lattice (T1) relaxation times

IntroductionThe acetabulum labrum plays a critical role in hip function. Medical imaging techniques for measuring changes in labral properties due to its degeneration may help improve our knowledge of its role in hip osteoarthritis (OA). This study aimed to explore the correlation between the mechanical properties of the acetabulum labrum and ultrashort echo time (UTE) MRI properties.Material and methodAcetabular labrum specimens were dissected from 12 fresh-frozen human cadaveric hip joints (64.6 ± 11.6 years old at the time of death, 7 female). UTE Cones sequences were used to measure apparent spin-spin (T2*) and spin-lattice (T1) relaxation times using a knee coil at 3 T in a clinical MR scanner. The stiffness and the elastic modulus (E) of the specimens were measured before MRI scans using uniaxial tensile tests. Spearman's rank correlation coefficients of Tendons' UTE-T2* and -T1 with their mechanical properties were calculated.ResultsThe human labrum specimen elastic modulus showed a significant inverse correlation with UTE-T2* (R = −0.66, P < 0.01) and with UTE-T1 (R = −0.56, P = 0.05). The stiffness of the specimens showed significant inverse correlations with UTE-T2* (R = −0.53, P = 0.01) and UTE-T1 (R = −0.63, P = 0.02).ConclusionThis study highlighted the potential of UTE-MRI techniques for the mechanical assessment of the acetabular labrum. UTE-MRI may improve labrum degeneration detection and monitoring, which requires further investigations.

Quantitative Blood Oxygenation Level Dependent Magnetic Resonance Imaging for Estimating Intra-renal Oxygen Availability Demonstrates Kidneys Are Hypoxemic in Human CKD

Kidney blood oxygenation level dependent (BOLD) magnetic resonance imaging (MRI) has shown great promise in evaluating relative oxygen availability. This method is quite efficacious in evaluating acute responses to physiological and pharmacologic maneuvers. Its outcome parameter, R2∗ is defined as the apparent spin-spin relaxation rate measured in the presence of magnetic susceptibility differences and it is measured using gradient echo MRI. Although associations between R2∗ and renal function decline have been described, it remains uncertain to what extent R2∗ is a true reflection of tissue oxygenation. This is primarily because of not taking into account the confounding factors, especially fractional blood volume (fBV) in tissue. This case-control study included 7 healthy controls and 6 patients with diabetes and chronic kidney disease (CKD). Using data before and after administration of ferumoxytol, a blood pool MRI contrast media, the fBVs in kidney cortex and medulla were measured. This pilot study independently measured fBV in kidney cortex (0.23 ± 0.03 vs. 0.17 ± 0.03) and medulla (0.36 ± 0.08 vs. 0.25 ± 0.03) in a small number of healthy controls (n= 7) versus CKD (n= 6). These were then combined with BOLD MRI measurements to estimate oxygen saturation of hemoglobin (StO2) (0.87 ± 0.03 vs. 0.72 ± 0.10 in cortex; 0.82 ± 0.05 vs. 0.72 ± 0.06 in medulla) and partial pressure of oxygen in blood (bloodPO2) (55.4 ± 6.5 vs. 38.4 ± 7.6 mmHg in cortex; 48.4 ± 6.2 vs. 38.1 ± 4.5 mmHg in medulla) in control versus CKD. The results for the first time demonstrate that cortex is normoxemic in controls and moderately hypoxemic in CKD. In the medulla, it is mildly hypoxemic in controls and moderately hypoxemic in CKD. Whereas fBV, StO2, and bloodPO2 were strongly associated with estimated glomerular filtration rate (eGFR), R2∗ was not. Our results support the feasibility of quantitatively assessing oxygen availability using noninvasive quantitative BOLD MRI that could be translated to the clinic.

N-acetylaspartate concentration in psychotic disorders: T2-relaxation effects

We used proton magnetic resonance spectroscopy (1H MRS) to quantify N-acetylaspartate (NAA) concentrations and apparent spin-spin (T2) relaxation times, which also include diffusion attenuations, in first episode and chronic psychosis patients compared to matched healthy controls. 104 patients diagnosed with psychotic disorders and 50 matched controls were scanned at 4 Tesla. 1H MRS data were collected at four echo times (TE = 30, 90, 150 and 200 ms) using PRESS. NAA concentrations referenced to water and apparent T2 relaxation times of NAA and water were calculated. At TE = 30 ms, NAA concentration without T2 correction was significantly lower in chronic psychosis patients compared to age-matched healthy controls (p = 0.013, N = 61 and 26, respectively) but normal in first episode patients compared to matched controls (N = 43 and 24, respectively). After T2 correction, no significant differences remained between the groups. While diffusion attenuation was not independently accounted for, these findings indicate that T2 relaxation makes a major impact on NAA concentration measures in psychotic disorders. Thus, it may be neuronal microenvironment indexed by NAA and water T2 relaxation times and not neuronal integrity indexed by NAA concentration that underlies the widely reported NAA concentration reductions in psychotic disorders.

EPR-based oximetric imaging: a combination of single point-based spatial encoding and T1 weighting.

Spin-lattice relaxation rate (R1 )-based time-domain EPR oximetry is reported for in vivo applications using a paramagnetic probe, a trityl-based Oxo71. The R1 dependence of the trityl probe Oxo71 on partial oxygen pressure (pO2 ) was assessed using single-point imaging mode of spatial encoding combined with rapid repetition, similar to T1 -weighted MRI, for which R1 was determined from 22 repetition times ranging from 2.1 to 40.0 μs at 300 MHz. The pO2 maps of a phantom with 3 tubes containing 2 mM Oxo71 solutions equilibrated at 0%, 2%, and 5% oxygen were determined by R1 and apparent spin-spin relaxation rate ( R2*) simultaneously. The pO2 maps derived from R1 and R2* agreed with the known pO2 levels in the tubes of Oxo71. However, the histograms of pO2 revealed that R1 offers better pO2 resolution than R2* in low pO2 regions. The SDs of pixels at 2% pO2 (15.2 mmHg) were about 5 times lower in R1 -based estimation than R2*-based estimation (mean ± SD: 13.9 ± 1.77 mmHg and 18.3 ± 8.70 mmHg, respectively). The in vivo pO2 map obtained from R1 -based assessment displayed a homogeneous profile in low pO2 regions in tumor xenografts, consistent with previous reports on R2*-based oximetric imaging. The scan time to obtain the R1 map can be significantly reduced using 3 repetition times ranging from 4.0 to 12.0 μs. Using the single-point imaging modality, R1 -based oximetry imaging with useful spatial and oxygen resolutions for small animals was demonstrated.

Z-Spectrum analysis provides proton environment data (ZAPPED): a new two-pool technique for human gray and white matter.

A new technique – Z-spectrum Analysis Provides Proton Environment Data (ZAPPED) – was used to map cross-relaxing free and restricted protons in nine healthy subjects plus two brain tumor patients at 3T. First, MT data were acquired over a wide symmetric range of frequency offsets, and then a trio of quantitative biomarkers, i.e., the apparent spin-spin relaxation times (T2,f, T2,r) in both free and restricted proton pools as well as the restricted pool fraction Fr, were mapped by fitting the measured Z-spectra to a simple two-Lorentzian compartment model on a voxel-by-voxel basis. The mean restricted exchangeable proton fraction, Fr, was found to be 0.17 in gray matter (GM) and 0.28 in white matter (WM) in healthy subjects. Corresponding mean values for apparent spin-spin relaxation times were 785 µs (T2,f) and 17.7 µs (T2,r) in GM, 672 µs (T2,f) and 23.4 µs (T2,r) in WM. The percentages of Ff and Fr in GM are similar for all ages, whereas Fr shows a tendency to decrease with age in WM among healthy subjects. The patient ZAPPED images show higher contrast between tumor and normal tissues than traditional T2-weighted and T1-weighted images. The ZAPPED method provides a simple phenomenological approach to estimating fractions and apparent T2 values of free and restricted MT-active protons, and it may offer clinical useful information.

Phase inversion in emulsions studied by low field NMR

Phase inversion phenomenon is studied using two novel low field NMR-based methods. The first method monitors the variation of the apparent spin-spin relaxation time (T2*) of the emulsions. It is based on the difference of the spin-spin relaxation time of water molecules inside a water droplet and within an aqueous bulk phase. The second method uses a diffusion measurement to distinguish between the water signal from droplets and the bulk. The relative distribution of water in droplets and in bulk can be measured. Moreover, droplet size distribution of water can be determined based on this method. Both methods were tested on the phase inversion of a model emulsion. A semi-batch process was used to perform the emulsification in situ. The information obtained through these methods is correlated to optical microscopy and conductivity measurements. When the phase inversion from w/o to o/w is induced by adding a hydrophilic surfactant to the nonionic Span® 80 the inversion seems to pass a double emulsion state before reaching the o/w state. This is an important observation to a mechanical understanding of the phase inversion.

Multiparametric fat–water separation method for fast chemical‐shift imaging guidance of thermal therapies

A k-means-based classification algorithm is investigated to assess suitability for rapidly separating and classifying fat/water spectral peaks from a fast chemical shift imaging technique for magnetic resonance temperature imaging. Algorithm testing is performed in simulated mathematical phantoms and agar gel phantoms containing mixed fat/water regions. Proton resonance frequencies (PRFs), apparent spin-spin relaxation (T2*) times, and T1-weighted (T1-W) amplitude values were calculated for each voxel using a single-peak autoregressive moving average (ARMA) signal model. These parameters were then used as criteria for k-means sorting, with the results used to determine PRF ranges of each chemical species cluster for further classification. To detect the presence of secondary chemical species, spectral parameters were recalculated when needed using a two-peak ARMA signal model during the subsequent classification steps. Mathematical phantom simulations involved the modulation of signal-to-noise ratios (SNR), maximum PRF shift (MPS) values, analysis window sizes, and frequency expansion factor sizes in order to characterize the algorithm performance across a variety of conditions. In agar, images were collected on a 1.5T clinical MR scanner using acquisition parameters close to simulation, and algorithm performance was assessed by comparing classification results to manually segmented maps of the fat/water regions. Performance was characterized quantitatively using the Dice Similarity Coefficient (DSC), sensitivity, and specificity. The simulated mathematical phantom experiments demonstrated good fat/water separation depending on conditions, specifically high SNR, moderate MPS value, small analysis window size, and low but nonzero frequency expansion factor size. Physical phantom results demonstrated good identification for both water (0.997 ± 0.001, 0.999 ± 0.001, and 0.986 ± 0.001 for DSC, sensitivity, and specificity, respectively) and fat (0.763 ± 0.006, 0.980 ± 0.004, and 0.941 ± 0.002 for DSC, sensitivity, and specificity, respectively). Temperature uncertainties, based on PRF uncertainties from a 5 × 5-voxel ROI, were 0.342 and 0.351°C for pure and mixed fat/water regions, respectively. Algorithm speed was tested using 25 × 25-voxel and whole image ROIs containing both fat and water, resulting in average processing times per acquisition of 2.00 ± 0.07 s and 146 ± 1 s, respectively, using uncompiled MATLAB scripts running on a shared CPU server with eight Intel Xeon(TM) E5640 quad-core processors (2.66 GHz, 12 MB cache) and 12 GB RAM. Results from both the mathematical and physical phantom suggest the k-means-based classification algorithm could be useful for rapid, dynamic imaging in an ROI for thermal interventions. Successful separation of fat/water information would aid in reducing errors from the nontemperature sensitive fat PRF, as well as potentially facilitate using fat as an internal reference for PRF shift thermometry when appropriate. Additionally, the T1-W or R2* signals may be used for monitoring temperature in surrounding adipose tissue.

Autoregressive moving average modeling for spectral parameter estimation from a multigradient echo chemical shift acquisition

The authors investigated the performance of the iterative Steiglitz-McBride (SM) algorithm on an autoregressive moving average (ARMA) model of signals from a fast, sparsely sampled, multiecho, chemical shift imaging (CSI) acquisition using simulation, phantom, ex vivo, and in vivo experiments with a focus on its potential usage in magnetic resonance (MR)-guided interventions. The ARMA signal model facilitated a rapid calculation of the chemical shift, apparent spin-spin relaxation time (T2*), and complex amplitudes of a multipeak system from a limited number of echoes (< or equal 16). Numerical simulations of one- and two-peak systems were used to assess the accuracy and uncertainty in the calculated spectral parameters as a function of acquisition and tissue parameters. The measured uncertainties from simulation were compared to the theoretical Cramer-Rao lower bound (CRLB) for the acquisition. Measurements made in phantoms were used to validate the T2* estimates and to validate uncertainty estimates made from the CRLB. We demonstrated application to real-time MR-guided interventions ex vivo by using the technique to monitor a percutaneous ethanol injection into a bovine liver and in vivo to monitor a laser-induced thermal therapy treatment in a canine brain. Simulation results showed that the chemical shift and amplitude uncertainties reached their respective CRLB at a signal-to-noise ratio (SNR) > or =5 for echo train lengths (ETLs) > or =4 using a fixed echo spacing of 3.3 ms. T2* estimates from the signal model possessed higher uncertainties but reached the CRLB at larger SNRs and/or ETLs. Highly accurate estimates for the chemical shift (<0.01 ppm) and amplitude (<1.0%) were obtained with > or =4 echoes and for T2*(<1.0%) with > or =7 echoes. We conclude that, over a reasonable range of SNR, the SM algorithm is a robust estimator of spectral parameters from fast CSI acquisitions that acquire < or =16 echoes for one- and two-peak systems. Preliminary ex vivo and in vivo experiments corroborated the results from simulation experiments and further indicate the potential of this technique for MR-guided interventional procedures with high spatiotemporal resolution approximately 1.6 x 1.6 x 4 mm3 in < or =5 s.

Reproducibility of measuring renal oxygenation by blood oxygen level-dependent MRI in rat

Objective To evaluate the reproducibility of measuring renal oxygenation in rat using blood oxygen level-dependent MR imaging(BOLD MRI).Methods Five intact male Wistar rats were included in this study and their body weights were measured before MRI scans.BOLD MRI were performed ou day 1(d1),day 5(d5),day 10(d10),day 30(d30)and day 50(d50)on a 3.0 T MR scanner to measure the apparent spin-spin relaxation(R2*).On d30 and d50,the differences of R2*(△R2*)were calculated between before and 10 min after furosemide injection through the tail vein.The reproducibility of the baseline R2* of 5 times repeated scans were verified by the repeated-measure ANOVA test.The R2* and △R2* between pre-and post-furosemide iniection on d30 and d50 were measured on the codex and the medulla and a paired t test was run to analyze their responses to furosemide and the reproducibility of △R2*.Results The average body weight on d1,d5,d10,d3 and d50 was(150.4±3.7)g、(170.2±7.0)g、(201.0±5.8)g、(306.2±17.0)g and(352.0±12.2)g,respectively,with statistical difference(F=422.103.P＜0.01).The R2* showed no statistical difference in the cortex and medulla among the five scans(P＞0.05).On d30.the R2* of the cortex and medulla was(25.2±1.2)and(32.8±2.2)Hz before and significantly decreased to(21.1±2.2)and(25.9±3.0)Hz after furosemide administration,respectively(P＜0.01).On d50,the R2* of the cortex and medulla was(25.9±0.8)and(34.3±3.9)Hz before and significantly decreased to(20.2±1.5)and(27.0±3.2)Hz after furosemide administration.respectively(P＜0.01).The △R2* on the codex and medulla was(4.1±1.7)and(6.9 4-2.8)Hz on d30 and(5.8±1.1)and(7.3±2.8)Hz on d50,respectively,but there is no significant difference between eortex and medulla(P＞0.05).Conclusion The baseline R2* in the cortex and medulla were reproducible over 50 days and they were not body weighted.On 3.0 T MR scanners.R2* in the codex and medulla decreased significantly after administration of furosemide. Key words: Kidney codex; Kidney medulla; Magnetic resonance imaging; Animals,laboratory

Effect of Diabetes on Renal Medullary Oxygenation During Water Diuresis

To study the effect of water diuresis on renal medullary and cortical oxygenation in patients with diabetes using blood oxygenation level--dependent magnetic resonance imaging (BOLD MRI). Nine mild diabetic subjects (48 +/- 2.7 years of age, six women) and nine nondiabetic subjects of similar age and sex, all without known vascular or renal disease, were studied noninvasively by MRI before and during water diuresis. Water diuresis induced an increase in medullary oxygenation in control subjects, producing a decrease in R2* (apparent spin-spin relaxation time) of 1.89 +/- 0.27 (P < 0.01), but no significant change in the group of diabetic subjects. These findings in middle-aged diabetic subjects, which resembled those previously described in elderly subjects >65 years of age, suggest early impairment of adaptive vasodilatation within the renal medulla in diabetes.

Nuclear-spin relaxation of59Cocorrelated with the spin-state transitions inLaCoO3

The ${}^{59}\mathrm{C}\mathrm{o}\ensuremath{-}\mathrm{N}\mathrm{M}\mathrm{R}$ on single crystals of ${\mathrm{LaCoO}}_{3}$ has been studied in order to investigate the spin-state transition through the nuclear-spin relaxation in this compound. Below $T\ensuremath{\sim}35\mathrm{K},$ the spin-lattice and spin-spin relaxation rates show a thermal-activation-type temperature dependence with an energy gap of \ensuremath{\sim}180 K. This value is close to the energy difference between the low spin state $(S=0)$ and the intermediate spin state $(S=1)$ predicted in the two-stage spin-state transition model with the intermediate spin state. The apparent spin-spin relaxation rates depend on the resonance peaks split due to the nuclear-electric-quadrupole interaction, which is explained by an indirect nuclear spin-spin interaction through the interacting paramagnetic electron spins. This observation is consistent with the ferromagnetic short-range correlation among the electron spins on ${\mathrm{Co}}^{3+}$ ions confirmed by neutron-scattering experiments.

Activity and mobility of subtilisin in low water organic media: hydration is more important than solvent dielectric

The relationship between hydration, catalytic activity and protein dynamics was investigated for subtilisin Carlsberg in organic solvents with low water content. The organic media were cyclohexane, dichloromethane or acetonitrile, with controlled thermodynamic water activity ( a w). Catalytic rate profiles showed the same dependence on a w for the three different solvents. The structural mobility of the enzyme in air and organic media was probed by proton solid-state NMR relaxation measurements. Both spin-lattice relaxation time ( T 1 ) and line width at half height (apparent spin-spin relaxation time ( T 2)) were determined for protein which was exchanged and hydrated with D 2O. We found NMR relaxation was much more dependent on a w than medium identity (despite very different dielectrics) showing that enzyme hydration is the primary determinant of mobility. Results suggest that initial hydration up to a w 0.22 causes rigidification of part of the protein structure. As a w is increased further, enzyme mobility is found to increase. Above a w 0.44, a large increase in the proportion of more mobile protons coincides with a steep rise in catalytic activity for the enzyme in each of the solvents studied.

Estimating relaxation times with a very simple ratemeter-type NQR spectrometer

A simple procedure for estimating nuclear quadrupolar apparent spin-spin relaxation time, T;, using a super-regenerative spectrometer was presented earlier [l]. The basic element in the spectrometer incorporated a valve oscillator. The aim of the present study was to examine whether an extremely simple form of the Ratemeter-type spectrometer f2] using a mosfet oscillator in place of the valve oscillator could be used in a similar way to estimate the T; of 35Cl in sodium chlorate. This system and procedure have been found to be quite efficacious for this purpose and it is therefore suggested that they could be used to supplement Ts values obtained with more sophisticated instruments; moreover, the spectrometer can be built easily by students for their studies.

Dipalmitoylphoshatidylcholine-palmitic acid phase diagram studied by 13C nuclear magnetic resonance

The phase diagram of dipalmitoylphosphatidylcholine (DPPC) and palmitic acid mixtures in excess D 2O was studied by 13C-NMR. Phase boundaries were determined from plots of apparent spin-spin relaxation time T 2 ∗ (for both choline methyl and fatty acid chain carbons) versus temperature. The peritectic transition in the 1–10 mol% region, whose existence has been theoretically inferred from the Gibbs phase rule but which was undetectable by differential thermal analysis (DTA) (S.E. Schullery et al. Biochemistry, 20 (1981) 6818–6824) was located by NMR at 41.6°C. A second, nearby peritectic line at 44°C, which had been shown by DTA to extend from about 3–25 mol% palmitic acid, was seen by NMR only above 10 mol%. The palmitic acid/DPPC complex (2:1), with a sharp melting point at 64°C, reported in earlier studies, was also seen by NMR. A phase diagram including both NMR and DTA results is presented. Important general conclusions from this study are: (i) NMR and scanning thermal analysis are complementary techniques for phase studies; each can see transitions that are invisible to the other. (ii) The case for the applicability of the Gibbs phase rule to lipid bilayer systems has been strengthened by the observance of two predicted, close-spaced boundaries. (iii) Low concentrations of fatty acids and related molecules can not be assumed to disperse as simple ideal solutes in the bilayer matrix.

A N.Q.R. study of phase transitions in the p-dihalobenzenes

Phase transitions in p C 6H 4Cl 2 and p C 6H 4I 2 were monitored by observing the frequencies and apparent spin-spin relaxation times of the 35Cl and 127I nuclear quadrupole resonances. Decreases in the apparent spin-spin relaxation times were found to accompany the onset of the transitions.

Measurements of N.Q.R. apparent spin-spin relaxation times in monochloroacetic acid

Measurements of T ★ 2 for monochloroacetic acid in the α,β and γ phases were made. Major variations in T ★ 2 values following the γ→β and β→α transitions were observed, leading the authors to conclude that these transitions were a source of defects and crystal strains. The results further emphasized the need for describing the history of samples when quoting T ★ 2 measurements. The observation of T ★ 2 variations in all three phases highlights the sensitivity of T ★ 2 measurements to changes in these defects and strains.

Temperature dependence of the apparent spin-spin relaxation times, T 2* measured with a super-regenerative oscillator on 35Cl containing molecules

The temperature dependence of the resonance frequencies v and apparent spin-spin relaxation times T 2★ is reported for SbCl 3, CrCl 3 , GaCl 3, HgCl 2, p-φCl 2, KCl 3, and NaClO 3, using two proven techniques and a Dean type super-regenerative oscillator. The line shapes of the lines are not directly measured, T 2★, the inverse Line width parameter being obtained, (a) by measuring the quench frequency, T 1....T n at a fixed signal voltage output V o as a function of sample mass m 1....m n and using ▪ under specific operating conditions, or (b) by measuring the voltage change V 1....V n at a fixed sample mass m as a function of the quench frequency change, t 1....T n , using ▪ T 2* from (a) or (b) are self consistent. The equations apply only to pure Gaussian or Lorentzian line shapes. Lines are rarely pure Gaussian or Lorentzian, but consideration of the factors contributing to line shape shows the shape to be Gaussian. T 2★ values calculated assuming a Gaussian line shape for HgCl 2 , NaClO 3, CrCl 3, SbCl 3 , Cu 2O and p-φCl 2Cl 2 at room temperature, reproduce literature results. In GaCl 3 a phase transition is observed in the v dependence near 100° K for two of the three 35Cl lines, and the 71Ga and 79Ga lines. In CrCl 3 a new line is found, and its frequency and T 2★ measured. The T 2★ behaviour of p-φCl 2 as a function of sample preparation supports the idea that crystal memory is reflected in T 2★ but not in the frequencies v.

137BaCl 2 · 2H 2O: Temperature study of the nuclear quadrupole resonance frequencies and apparent spin-spin relaxation times using a super-regenerative oscillator on 137Ba using the coherent mode

The 137Ba nuclear quadrupole resonance frequency υ and apparent spin-spin relaxation time T 2 * of 137BaCl · 2H 2O were measured using a self-quenching superregenerative oscillator between room temperature and liquid nitrogen temperature. The T 2 * were recorded using the spectrometer in the coherent mode.

Apparent spin-spin relaxation times, T 2★, of some nitrogen compounds using a super-regenerative nuclear quadrupole spectrometer

The T 2 ★ of hexainethylenetetramine, C 6H 12 N 4, and sodium nitrite, NaNO 2, at 297°K and 77°K, and of 4-methoxy analine, C 5H 9 NO, at 297°K are reported. Two different nitrogen super-regenerative N.Q.R. spectrometers were used. Two different techniques for obtaining T 2 ★ were used: one measured the shift in signal detection threshold quench frequency with changing sample mass; the other measured the change in signal intensity for a small change of quench frequency near threshold. The importance of quench frequency studies and spectrometer construction for signal detection are discussed.

Variations of the apparent spin-spin relaxation times of the nuclear quadrupole resonances in the p-dihalobenzenes

Variations of T 2 ★ of the n.q.r. signals of 35Cl, 81Br and 127I in their p-dihalobenzenes were observed to depend on the previous history of the sample. Zonerefined samples were cooled from the melt to room temperature, XXX20°C, in two ways (a) slowly over several hours and (b) quickly over a few minutes. Samples (a) gave higher T 2 ★ values because the fewer crystal defects produced in the slowcooling process gave a more homogeneous electric field gradient. The results gave new measurements of T 2 ★ , emphasized the need for describing past history when quoting linewidth measurements and showed the importance of T 2 ★ measurements in the study of phase transitions.