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Benzene solutions of purified egg lecithin, with small amounts of water added, have been examined by 60 MHz and 100 MHz NMR spectroscopy, infrared spectrophotometry and phase contrast microscopy. The transverse relaxation times of the water, N-methyl and OH protons are dependent on water concentration. This dependence changes sharply for the water proton at a level of one water molecule per lecithin monohydrate molecule. These results do not fully agree with those reported by other workers. Four mathematical models are examined which could account for the behaviour of the water protons. Models which assume a constant transverse relaxation time for water protons above a level of one water molecule per lecithin molecule cannot predict the behaviour observed. It is sufficient to assume that water protons above this concentration have a single relaxation time which is a linear function of water concentration. The added water associates primarily with the phosphate in the lecithin head group. Above nine water molecules per lecithin monohydrate molecule free water is present in the system.

Experimental observation of ultrasonic free induction decay in glasses at low temperatures is presented. The variations of the decay time as a function of the input power have been observed and measured. The data are interpreted in the framework of the two-level systems (TLS) theory and we show that such an experiment provides us with a simple measurement of the transverse relaxation time T 2 of the TLS.

In a recent paper, Tenti and McCourt [J. Chem. Phys. 65, 623 (1976)] presented theoretical evidence for the existence of a minimum in the transverse relaxation time T2 of the nuclear magnetization in dilute gases. Here the transverse relaxation rate is reexamined and it is shown that in the low density regime it is impossible to define a T2. Moreover, the slowing down of the relaxation rate at low densities is due to a decreasing collision rate and not to a dynamical shift as reported by Tenti and McCourt. It is found that the transverse magnetization behaves as a damped oscillator below a certain critical density. Explicit calculations are presented for H2 gas and for this system the T2 description breaks down below 0.02 Torr.

Saturation recovery measurements of the electron spin–lattice relaxation times of the free radicals of melanin are reported. ’’Natural’’ melanin gently extracted from the choroid of bovine eyes was used. Over a wide range of experimental conditions, the relaxation time remained nearly unchanged; A characteristic value is 15 μsec. Progressive saturation measurements also were made and results varied widely among samples in the series, which presumably is a consequence of variations in the transverse relaxation time.

The influence of proton decoupling on the transverse relaxation time (T2) of 31P in 5′-AMP was examined. The 31P transverse relaxation time is an order of magnitude smaller when the protons are not decoupled. An expression was derived for scalar relaxation of the second kind for nucleus A in an AX2 system in which relaxation of X is in the slow relaxation limit. Values calculated using this expression were in good agreement with experimental T2 values indicating that transverse relaxation of 31P in 5′-AMP in the absence of proton decoupling is dominated by scalar relaxation of the second kind via scalar coupling to the two 5′ protons. With broadband proton decoupling, 31P spin-echo attenuation attributable to magnetization transfer is a potential problem. The 31P T2 measured in the presence of proton decoupling was compared with results obtained using the off-resonance rotating frame spin-lattice relaxation (T1ϱoff) experiment in the absence of proton decoupling, with broadband decoupling, and with coherent decoupling of the 5′ protons. The T1T2 ratio derived from the T1ϱoff experiments was in agreement with that obtained by measurement of T1 and of T2 at two different levels of broadband (both noise and square-wave modulated) proton-decoupling power and with coherent proton decoupling, revealing no complications in 31P T2 measurements in the presence of broadband proton decoupling for 5′-AMP (or probably other phosphates).

An acoustic hole burning experiment in a tetrasil glass is presented. A sharp discrepancy between experimental data and theoretical expectations based on the standard Bloch equations appears as soon as high input powers are involved. On the contrary, good agreement is obtained if the Bloch equations as modified in the Yamanoi and Eberly theory are used. The results of the experiment are the transverse relaxation time T20 and the correlation time τc of the random field due to spin-spin interactions.

A calculation shows that the dispersion of optical mixing of the type 2ω1 − ω2 provides a new means of determining directly both the longitudinal and transverse relaxation time associated with an inhomogeneously broadened optical transition of materials having very short relaxation times of a picosecond or less.

It is shown both theoretically and experimentally that the Carr-Purcell-Meiboom-Gill (CPMG) sequence is equivalent to a spin-locking sequence in the limit of short interpulse spacing (tcp). The increase apparent in the CPMG relaxation time observed in biological tissue as tcp decreases is explained in terms of spin locking. At an effective locking field frequency f 1 = 1 2 t cp the transverse relaxation time ( T 2 † ) measured with the CPMG sequence is indistinguishable from the rotating-frame spin-lattice relaxation time (T1ϱ). The origin of the variation in measured relaxation times in tissue is discussed in terms of low-frequency dipolar interactions of water.

The 1H NMR has been studied in the hydrogen bronzes H0.35MoO3 and H0.33WO3 prepared by galvanostatic cathodic reduction. Both systems reveal an extreme anisotropy of the hydrogen mobility due to quasi-one-dimensional behaviour. – In H0.35MoO3 the temperature dependent 1H wideline data give strong evidence that hydrogen is clustered in quasi-one-dimensional channels. Analysis of the temperature dependence of the second moment M2, the transverse relaxation time T2, and the spin-lattice relaxation time T1 yields essentially the same valueEA = 30 kJ/mol for the activation energy of the translational motion of hydrogen within the channels. In H0.33WO3 evidence for the quasi-one-dimensional behaviour is provided by the frequency dependence of T1 and the marked non-exponential recovery of the nuclear magnetization in the vicinity of the T1 minimum. The activation energy for the translational motion is EA = 3.6 kJ/mol only, indicating high mobility of hydrogen in this bronze.

The Bloch equation incorporating chemical exchange between two environments is solved and is expanded in a power series of the exchange lifetime τ. The expansion near the fast exchange region is given by a power series in T of which the first term corresponds to the fast exchange limit. The second term together with the first term defines the “boundary” intermediate exchange region. In the slow exchange region, the expansion is conducted in a power series of 1τ. The conditions of convergence of the expansion in both exchange regions are examined and as a result the chemical exchange is divided into five regions: fast limit, fast intermediate, pure intermediate, slow intermediate, and slow limit. In the “pure” intermediate region, a conventional simulation procedure is needed in order to determine the exchange rate. On the other hand, an analytical form is derived for the linewidth in the other regions. The linewidth as a function of exchange rate, transverse relaxation time, content of component species, and chemical shift difference between two environments is examined in the light of the proposed formula.

The measure of transverse relaxation time T2 of 7Li permits precise study of the diffusion of the nucleus, which has already been observed by 119Sn Mossbauer spectroscopy in three phosphides. The analysis of Hendrickson and Bray has been applied to the results obtained as a function of temperature. A discontinuity in the processes of diffusion is observed, and a possible explanation has been proposed.

Transient behavior of optical bistable system is studied theoretically for switching from the high light-transmitting state to the low one. For three-level system in solids, e.g., the ground, the exciton and the bi-exciton states in CuCl, the optical bistability is shown to be possible and to respond in the transverse relaxation time of an order of pico-second by adjusting the degree of the off-resonance of the incident field as well as the detuning of the Fabry-Perot cavity. Thus we will be able to dissolve the largest obstacle to application of the optical bistability to an optical device.

A method is described in which one determines the rate parameters for chemical exchange between two equally populated sites by measuring the dependance upon apparent transverse relaxation time of the NMR intensity lines. This method permits to obtain kinetics parameters when the coalescence temperature cannot be reached. It was tested by observing the low temperature spectra of well known exchanging systems. Then it was applied to five membered ring molecules (Tetrahydrofuran and pyrrolidin derivatives) where classical methods (TLS) cannot be used.

A nonlinear optical method to determine the transverse relaxation time of excitonic molecules by means of two-photon echo is proposed. In the case of two-photon transition such as excitonic molecule one needs three pulses to obtain an echo signal. When the wave-vector and frequency of the three successive pulses are denoted by k 1 ,ω 1 , k 2 ,ω 2 and k 3 ,ω 3 , the two-photon echo can be observed in the direction of 4k 2 -2k 1 -k 3 with frequency 4ω 2 -2ω 1 -ω 3 . Tuning both ω 1 and ω 2 to be two-photon resonant with excitonic molecule, we can satisfy the phase-matching condition rather easily for appropriate values of ω 3 due to the large dispersion of excitonic polaritons. From the correlation trace of the two-photon echo we can determine directly the transverse relaxation time of excitonic molecules.

An investigation of coherent transient effects in the generation and amplification in ultrashort pulses in Nd:YAG and ruby at ⋍ 100 K is reported for the first time. A theoretical description of the amplified pulse temporal shape and spectrum based on a numerical solution of Maxwell-Bloch equations is given. Intensity oscillations on the trailing edge of the pulse and a doublet structure of the spectrum that qualitatively agree with the computation results are observed. The transverse relaxation time T 2 in ruby is estimated from the oscillation damping.

For liquid methanol, the longitudinal relaxation of the OH and CH3 proton magnetic moments was found to depend on the rate of OH proton exchange between methanol molecules. This exchange dependence is explained quantiatively in terms of cross relaxation between the OH and CH3 protons resulting from the scalar interaction J(t)I·S. The good agreement obtained between experiment and theory indicates that measurement of the longitudinal relaxation rate can be used to obtain accurate values for the rate of proton exchange in methanol. In addition, for the range of rates where the longitudinal relaxation is affected, the spin—echo determination of T2, the transverse relaxation time, was found to be reliable for the CH3 proton but not for the OH proton.

A hole-burning effect is observed spectroscopically in a solution of cryptocyanine in methanol, when excited by a mode-locked ruby laser. The hole width is about 5 A. A transverse relaxation time of 5 psec is deduced and its significance is discussed.

Knight shift and some other properties of Cu nuclear magnetic resonance in Cu-Pd alloys were measured. It was observed that the shift is negative, and its magnitude is almost proportional to the susceptibility, having the value of -0.9% at 1 atomic percent of Cu. A deviation from the proportionality was observed in Cu dilute region and it is suggested that the charge screening radius is of short range. The transverse relaxation time increases with the concentration of Pd, and the resonance line width takes a maximum value at 25% Pd. The longitudinal relaxation time keeps a constant value, about 1/3 of that of pure Cu metal, through all the concentration measured.

SummaryObjectivePhysiological magnetic resonance imaging (MRI) under loading or knee malalignment conditions has not been thoroughly investigated. We assessed the influence of static loading and knee alignment on T2 (transverse relaxation time) mapping of the knee femoral cartilage of porcine knee joints using a non-metallic pressure device.MethodsTen porcine knee joints were harvested en bloc with intact capsules and surrounding muscles and imaged using a custom-made pressure device and 3.0-T MRI system. Sagittal T2 maps were obtained (1) at knee neutral alignment without external loading (no loading), (2) under mechanical compression of 140N (neutral loading), and (3) under the same loading conditions as in (2) with the knee at 10° varus alignment (varus loading). T2 values of deep, intermediate, and superficial zones of the medial and lateral femoral cartilages at the weight-bearing area were compared among these conditions using custom-made software. Cartilage contact pressure between the femoral and tibial cartilages, measured by a pressure-sensitive film, was correlated with cartilage T2 measurements.ResultsIn the medial cartilage, mean T2 values of the deep, intermediate, and superficial zones decreased by 1.4%, 13.0%, and 6.0% under neutral loading. They further decreased by 4.3%, 19.3%, and 17.2% under varus loading compared to no loading. In the lateral cartilage, these mean T2 values decreased by 3.9%, 7.7%, and 4.2% under neutral loading, but increased by 1.6%, 9.6%, and 7.2% under varus loading. There was a significant decrease in T2 values in the intermediate zone of the medial cartilage under both neutral and varus loading, and in the superficial zone of the medial cartilage under varus loading (P

Single-sided NMR devices, like NMR MOUSE (MObile Universal Surface Explorer), are designed to allow NMR surface measurements to be conducted on arbitrarily large samples, and are therefore valuable tools to evaluate non-invasively moisture content “inside” a planar object. In this work, we aim to perform “in situ” studies of moisture transport in building materials during drying experiments with NMR MOUSE. The local moisture content was monitored over time and mapped over sample thickness with a depth resolution of 0.5mm and a maximum penetration depth of 15mm. In addition, NMR MOUSE allows measuring transverse relaxation time T2. Experimental results show that moisture content is homogeneously distributed over the thickness, which indicates that moisture is transferred through capillary processes. Furthermore, transverse relaxation time distribution measurements revealed that the drainage of pore water occurs sequentially from larger pores to smaller pores.

Formation of transient induced grating in a medium, placed in a laser cavity, by two laser pulses arriving at an experimental sample noncoincidently and from different directions has been demonstrated. This technique was applied for measuring the transverse relaxation time in solution of the transition metal complex bis(-dimethylaminodithiobenzil)-nickel (BDN) in 1,2- dichloroethane.

Direct measurements of the proton transverse relaxation time ( T 2 ) at 61 MHz in hydrogen gas at 300 K are reported for densities between 0.2 and

An investigation was made of the temperature dependence of the two-photon absorption coefficient of mixed CdSxSe1?x single crystals. When the frequency of laser radiation (?= 1.78 eV) approached the frequency of an exciton transition terminating inside an allowed energy band, the temperature dependence of the investigated coefficient was found to be governed by the temperature dependence of the transverse relaxation time of the intermediate exciton state.

Electron spin echo detection of laser photogenerated tetramethylbenzidine cation in liquid sodium dodecylsulfate micellar solutions at room temperature is reported. The phase memory time is 180 ns which can be identified with the transverse relaxation time. Deuteration effects on the phase memory time indicate that the cation interacts more strongly with surfactant molecules than with water molecules.

Atomic relaxation in the presence of a strong coherent optical field is examined. Contrary to popular practice, we find that the Bloch equations do not apply when the applied field is strong enough to saturate the transition. The component of the induced polarization in phase with the field lives longer than the out-of-phase component. One important consequence is that pulse durations for adiabatic inversion of atomic levels need not be short relative to the transverse relaxation time of the atom if the pulse amplitude is sufficiently large.

Extremely narrow lines, observed by the double modulation ESR technique in systems with an inhomogeneously broadened spectrum, are explained in terms of higher-order processes in non-linear conditions. Under proper approximations the shape of these lines is accounted for by the longitudinal rather than the transverse relaxation time.

Fifty-three patients who had hepatic tumors (24 hepatomas, ten metastases, and 19 cavernous hemangiomas) underwent MR imaging using a 0.35-T superconducting imager. The transverse relaxation time (T2) was calculated from a pair of spin echo images (repetition time [TR] of 1600 msec) with echo delay times (TE) of 35 and 70 msec. The computed T2 value was obtained in a fashion similar to that used to obtain CT numbers with region-of-interest cursors. The mean T2 was 59 +/- 9 msec in hepatomas, 64 +/- 15 msec in metastases, and 100 +/- 30 msec in hemangiomas. The difference between the T2 of hemangioma and that of liver malignancies was statistically significant (P less than .001); however, differentiation between hepatoma and metastases was not possible. The T2 was shorter than 80 msec in all 24 hepatomas and in nine of ten metastases, and was longer than 80 msec in 16 of 19 hemangiomas. Forty-nine of 53 cases (92%) were correctly classified when the borderline of T2 between hemangioma and hepatic malignancies was set at 80 msec. MR with T2 calculation was valuable in differentiating between hemangioma and hepatic malignancies.

Deuteron magnetic resonance (DMR) measurements have been made on plasma-deposited samples of a-Ge: D, H; a-Si:D,F; and a-SiGe:D,F. The experiments were performed at 30.7 MHz with a Bruker CXP 200 spectrometer and a probe constructed to permit operation at high rf power down to temperatures near 4 K. The 50 to 100 mg samples comprise less than 1 mm flakes in Kel-F containers of 5 mm diameter.

We study by pulsed NMR techniques the effect of 4He impurities on spin—lattice and transverse relaxation times of solid 3He.1,2 Experiments are made in the temperature range 1.2 < T < 0.27°K, where the spin—lattice relaxation time T 1 is sensitive to the concentration x of 4He impurity in the sample. T 1 has been measured as a function of the temperature, the concentration x of 4He impurity, and the NMR frequency for several molar volumes in both bcc and hep phases.

Magnetic resonance (MR) imaging of the breast has shown promise in the diagnosis of breast lesions. In prior studies the time of MR examination in the menstrual cycle was not taken into account. If alterations in transverse relaxation time (T2) values of ductal tissue did occur during the menstrual cycle, they would have a significant bearing on the timing of MR studies of the breast. This study demonstrated no apparent alteration in T2 values during the menstrual cycle of normal young volunteers.

Temperature dependence of the transverse relaxation time T2 of Fe57 NMR signal in DyFe0.008 Co0.002 O3 is investigated both theoretically and experimentally in the vicinity cf the first order Morin-type phase transition. Anomalous increase of the relaxation time at approaching the Morin point is shown to be due to dynamic coupling between the electronic and nuclear spin subsystems.

In recent years, brain water content has been studied in various types of neurologic conditions by using proton nuclear magnetic resonance (proton NMR). Thus, it is possible to illustrate the brain water status on the basis of such parameters as proton density, longitudinal relaxation time (T1), and transverse relaxation time (T2). However, the literature dealing with basic studies of various types of information obtained with NMR is very small [1, 3, 6, 7]. Since little is known about the immature brain [2, 4, 5], we have measured the water content and the relaxation times (T1 and T2) in cerebral gray and white matters over time. The age range of the animals used extended from the 20th day of gestation to 8 weeks after birth.

Measurements of nuclear magnetic relaxation times for protons of water in living skeletal frog muscle show the transverse relaxation time, T 2 , increases when a muscle contracts isometrically. This result and other experimental data suggest that a fraction of the intracellular water molecules have restricted rotational freedom and that this fraction decreases when contraction occurs.

Temperature-controlled measurements of the transverse relaxation time were conducted with the NMR-MOUSE (nuclear magnetic resonance mobile universal surface explorer) at 50°C to investigate the heterogeneity of rubber sheets induced by thermo-oxidative aging. With temperature stabilization the relative error ofT2 decreased by a factor of 4 on the short time scale. Without temperature stabilization it may reach 30% in the long-time range because of temperature changes in the laboratory. Two different types of mobile temperature control units operating with the NMR-MOUSE are described. These temperature control units are small and portable like the NMR-MOUSE.

Cardiovascular magnetic resonance imaging (CMR) has become an indispensable clinical tool for the assessment of morphology, function and structure of the heart muscle. By exploiting quantification of the effective transverse relaxation time (T2*) CMR also affords myocardial tissue characterization and probing of cardiac physiology, both being in the focus of ongoing research. These developments are fueled by the move to ultrahigh magnetic field strengths, which permits enhanced sensitivity and spatial resolution that help to overcome limitations of current clinical MR systems with the goal to contribute to a better understanding of myocardial (patho)physiology in vivo. In this context, the aim of this report is to introduce myocardial T2* mapping at ultrahigh magnetic fields as a promising technique to non-invasively assess myocardial (patho)physiology. For this purpose the basic principles of T2* assessment, the biophysical mechanisms determining T2* and (pre)clinical applications of myocardial T2* mapping are presented. Technological challenges and solutions for T2* sensitized CMR at ultrahigh magnetic field strengths are discussed followed by a review of acquisition techniques and post processing approaches. Preliminary results derived from myocardial T2* mapping in healthy subjects and cardiac patients at 7.0 Tesla are presented. A concluding section discusses remaining questions and challenges and provides an outlook on future developments and potential clinical applications.

In this contribution, we report the results of NMR studies of the behaviour of water in a hydrating Sorel cement paste with a composition close to the stoichiometric optimum. Both the transverse spin-relaxation behaviour and water self-diffusion were studied in two separate experiments performed on samples on the basis of the same formulation. While there is a very strong initial decrease in the transverse relaxation time of the water in the paste, the diffusion coefficient is found to decrease mainly at later times of the hydration process where the decrease of the transverse relaxation time has already strongly slowed down. After about 6 h of the hardening process, the signal intensity available for a pulsed gradient diffusometry experiment is not sufficient any more for reliable measurements of the diffusion coefficients

Nuclear Magnetic Resonance (NMR) spectrometry has proved to be a good technique for determining the petrophysical properties of reservoir rocks; such as porosity and pore size distribution. We investigated how pore water rich in divalent ions affect the NMR signal from chalk with two different depositional textures. We compared two cases. The first experiments on outcrop chalk with high salinity brines showed that saturation with divalent ions (Mg2+,Ca2+and SO42-) cause major shifts in the T2 distribution curve, probably due to precipitation in the pore space. In a second set of experiments, fluid samples where precipitation takes place were found to show shifts in the T2 relaxation curve due to the creation of crystals. We were able to identify how differences in the rock texture and precipitants within the pore space may affect the transverse relaxation time by altering the surface-to-volume ratio of the pore space. The results of this work could benefit the ongoing study on the optimization of the water composition for Enhanced Oil Recovery (EOR) methods and shed light on how it can affect the mechanical and physical properties of the rock.

We report the results of an experimental investigation of the longitudinal relaxation time T 1 of the electronic polarization of the ground state of potassium atoms in cells with an antirelaxation coating on the walls. Investigations were performed for a number of cells at temperature T=24°C. The maximum recorded relaxation times τ are 3.6, 4.3, and 5.2 s for cells with diameter D=50,75, and 100 nm, respectively. These are the longest longitudinal relaxation times ever recorded in coated cells. The transverse relaxation time was measured for these cells at the same temperature: T 2=0.72,1.1, and 1.4 s, respectively.

Objectives: Human pregnancies complicated by placental dysfunction may be characterized by a high hyperoxic Blood oxygen level-dependent (BOLD) MRI response. The pathophysiology behind this phenomenon remains to be established. The aim of this study was to evaluate whether it is associated with altered placental baseline conditions, including a lower oxygenation and altered tissue morphology, as estimated by the placental transverse relaxation time (T2*).Method: We included 49 normal pregnancies (controls) and 13 pregnancies complicated by placental dysfunction (cases), defined by a birth weight Results: In the control group, the relative Delta BOLD response increased during gestation from 5% in gestational week 20 to 20% in week 40. In the case group, the relative Delta BOLD response was significantly higher (mean Z-score 4.94; 95% CI 2.41, 7.47). The absolute Delta T2*, however, did not differ between controls and cases (p = 0.37), whereas the baseline T2* was lower among cases (mean Z-score - 3.13;95% CI -3.94, - 2.32). Furthermore, we demonstrated a strong negative linear correlation between the Log(10) DBOLD response and the baseline T2* (r = -0.88, p Conclusion: The high hyperoxic DBOLD response demonstrated in pregnancies complicated by placental dysfunction may simply reflect altered baseline conditions, as the absolute increase in placental oxygenation (Delta T2*) does not differ between groups. (c) 2017 Elsevier Ltd. All rights reserved.

Spin echo NMR and ac susceptibility measurements have been performed at 60 mK∼4.2 K in (Ce 1- x Gd x )Ru 2 . The sharp superconducting transition is confirmed for x ≦0.110. The NMR signals associated with the ferromagnetically ordered Gd atoms are observed down to x =0.105 in zero external field and to x =0.095 with applying field. The hyperfine field at Gd site changes as -206+88· x kOe. The intensity of the signal in zero field decreased rapidly in accordance with the appearance of superconductivity. The temperature, concentration and external field dependence of the transverse relaxation time T 2 show that T 1 is governed mainly by the Weger mechanism. The rapid increase of T 1 with lowering temperature for 0.105≦ x ≦0.110 indicates a possible appearance of small energy gap associated with superconducting state. The results support the coexistence of superconductivity and ferromagnetism.

We report solutions for the paramagnetic magnetization of a two-level spin system for terms varying at the second-harmonic frequency of the driving field and their use to determine the power dependence of the transverse relaxation time T 2 in saturated spin systems.

NMR on 3He atoms in the very small pores of Vycor glass reveals magnetic states which are temperature, pressure, and magnetic field dependent. The susceptibility is Curie-Weiss-like with a Θ that increases with pressure. The longitudinal relaxation time τ1 is a weak, monotonically increasing function of temperature. The transverse relaxation time τ2 is a strong function of temperature at low temperature, with a positive slope at 286 gauss and a negative slope at 71.5 gauss.

The transverse relaxation time measurement for Raman mode by Raman enhanced nonde-generate four wave mixing (RENFWM) has been made in the frequency domain. We get spectra of RENFWM that the nonresonant background is suppressed. The transverse relaxation time of Raman mode in benzene and methlybenzene is obtained as 1.5 ps and 2 ps respectively.

Background Quantification of the longitudinaland transverse relaxation time in the myocardium has shown to provide important information in cardiac diagnostics. Several methods are currently available for cardiac relaxation time mapping but they generally demand a long breath hold to measure either T1 or T2 in a single 2D slice. In order to make quantification of relaxation parameters in the myocardium clinically applicable, the methods must be improved in speed and coverage. In this work we propose a novel method for 3D interleaved T1 and T2 quantification of the whole left ventricular myocardium within one single breath hold.