Structural adaptations in compressed articular cartilage measured by diffusion tensor imaging.

OBJECTIVE: To demonstrate the use of diffusion tensor magnetic resonance micro-imaging to observe adaptations of collagen fibres to mechanical compression in articular cartilage. METHODS: Spin-echo and diffusion tensor images (156x156microm in-plane resolution, 2mm slice thickness) of bovine cartilage were obtained at a magnetic field of 7.0T in relaxed and compressed states. The parameters determined were: T2, maximum and mean diffusivity, direction of the maximum diffusion eigenvector and fractional anisotropy of diffusion. RESULTS: A correlation was found between the compressive strain applied to the cartilage and the change in both magnitude and direction of the maximum diffusivity. Compression resulted in a decrease in both the maximum and mean eigenvalues, particularly in the surface and transitional zones, while the change in orientation of the eigenvectors corresponding to maximum diffusion was greatest in the transitional region. In this region, the average orientation of the principal eigenvectors with respect to the normal to the articular surface increased by up to 40 degrees, indicating that the collagen fibre bundles were oriented more parallel to the surface when compressed. CONCLUSIONS: Diffusion tensor imaging can be used to monitor the changes in the direction of the collagen fibres due to compression. It may form the basis of a new non-invasive approach to functional evaluation of cartilage, with potential applications in the diagnosis and treatment of osteoarthritis.

Anisotropy of collagen fibre alignment in bovine cartilage: comparison of polarised light microscopy and spatially resolved diffusion-tensor measurements.

OBJECTIVE: To compare collagen fibre alignment angles obtained from polarised light microscopy (PLM) and diffusion-tensor imaging (DTI) in bovine articular cartilage. METHODS: Five samples of bovine articular cartilage from five different animals were studied using magnetic resonance imaging and PLM techniques. T(2)-weighted, diffusion-tensor (DT), and PLM images were acquired for each sample and average depth profiles of the PLM and DTI angles, as well as the banding patterns observed in T(2)-weighted magnetic resonance (MR) images, were compared. Statistical properties of the distributions of the DTI and PLM angles were examined. RESULTS: The samples exhibited a range of alignment morphologies. In the samples with the "conventional" three-zone alignment pattern, a correlation between the PLM and DTI alignment zones and the banding in T(2)-weighted MR images was observed. The shapes of the depth profiles of the PLM and DTI alignment angles were qualitatively similar for each sample. Three samples showed good quantitative correlation between the DT and PLM alignment angles. The correlation between the diffusion and PLM alignment angles was best in the regions of low degree of disorder of fibre alignment. CONCLUSIONS: This study provides the first quantitative comparison of DTI of cartilage with the more established PLM techniques. The correlation between alignment angles derived from PLM and DTI data was evident across a wide range of alignment morphologies. The results support the use of DTI for the quantitative measurement of collagen fibre alignment. The microscopic-scale (~10 microm) dispersion of fibre alignment angles appears to be an important factor for understanding the extent of quantitative correlation between PLM and DTI results.

Diffusion tensor imaging of articular cartilage as a measure of tissue microstructure.

OBJECTIVE: To use diffusion tensor MR micro-imaging to observe differences in magnitude and anisotropy of water diffusion between 'healthy' cartilage and cartilage enzymatically degraded to simulate arthritic damage. METHODS: Diffusion tensor images (156 x 156 microm in-plane resolution, 2mm slice thickness) of bovine cartilage were obtained at either 4.7 or 7.0 T using pulsed field gradient spin echo sequences. The parameters determined were: maximum and mean diffusivity, direction of the maximum diffusion eigenvector with respect to the normal to the articular surface and fractional anisotropy (FA) of diffusion. RESULTS: Both maximum and mean diffusion eigenvalues were found to decrease, respectively, from approximately 1.95 x 10(-9) and 1.80 x 10(-9) m2 s(-1) at the articular surface to approximately 1.08 x 10(-9) and 0.79 x 10(-9) m2 s(-1) in the deep zone. A systematic change was observed in the direction of the eigenvector corresponding to maximum diffusivity, reflecting the expected change in orientation of the collagen macrofibrillar bundles. Degradation with trypsin to remove proteoglycans resulted in a 10-15% increase in apparent diffusion coefficient of water in the cartilage, with no apparent change in FA. CONCLUSIONS: These methods have the potential to be used to probe local changes in tissue microstructure and the hydrodynamic status of cartilage during development of osteoarthritis.

Refractive index distribution and optical properties of the isolated human lens measured using magnetic resonance imaging (MRI).

Using a non-invasive MRI technique for measuring the refractive index distribution through the crystalline lens, refractive index maps were obtained through 20 intact isolated human lenses (7-82years). Focal length measurements, obtained by simulated light ray propagation through each index map were found to be in agreement with direct measurements performed on a scanning laser apparatus. With increasing age, the refractive index profiles became flatter in the central region, accompanied by steepening of the profile in the periphery. This appears to be an important mechanism underlying the observed changes in power and longitudinal aberration of the human lens.

Comparison of high-resolution MRI, optical microscopy and SEM for quantitation of trabecular architecture in the rat femur.

Magnetic resonance imaging (MRI) has been used to analyze trabecular bone architecture in femur heads taken from adult Wistar rats. The aim of this study was to validate the use of MRI in assessing trabecular structure and morphology by comparing standard measures of bone morphology in the rat femur obtained from high resolution MRI with those obtained by conventional optical microscopy and by scanning electron microscopy (SEM). MR images were obtained on a Bruker 4.7 T micro-imaging system using a three-dimensional spin echo sequence with spatial resolution of 23 microm in-plane and a slice thickness of 39 microm. Optical images were obtained by de-calcifying the bone in EDTA and then sectioning 5-microm-thick slices. SEM images were obtained from bone embedded in epoxy resin with surface preparation by diamond polishing. Values of standard bone morphological parameters were compared and correlation coefficients between the MRI and the optical- and SEM-derived measures of morphology were calculated. Partial volume effects in MRI were minimized in this study by the use of very thin slices, yielding better agreement with optical- and SEM-derived measures of trabecular bone morphology than have been obtained in previous studies. Correlations between the MRI and optical data were significantly lower than those between the MRI and SEM data. Effects of de-calcification were also investigated. The results indicate that comparison of MRI with thin (de-calcified) optical images may be inherently flawed due to the destructive de-calcification and sectioning process used to prepare samples for the optical imaging.

Assessment of occupational exposure to radiofrequency fields and radiation.

The use of personal monitors for the assessment of exposure to radiofrequency fields and radiation in potential future epidemiological studies of occupationally exposed populations has been investigated. Data loggers have been developed for use with a commercially available personal monitor and these allowed personal exposure records consisting of time-tagged measurements of electric and magnetic field strength to be accrued over extended periods of the working day. The instrumentation was worn by workers carrying out tasks representative of some of their typical daily activities at a variety of radio sites. The results indicated significant differences in the exposures of workers in various RF environments. A number of measures of exposure have been examined with a view to assessing possible exposure metrics for epidemiological studies. There was generally a good correlation between a given measure of electric field strength and the same measure of magnetic field strength.

Measuring optical properties of an eye lens using magnetic resonance imaging.

We compare the focal lengths of porcine lenses measured optically and by using a novel MRI technique. The geometric properties of the lenses were also measured and compared. The MRI technique exploits the dependence of both the lens refractive index and relaxation rates on the local protein concentration. By measuring the refractive index and corresponding values of R2 (=1/T2) for samples of lens homogenates with different protein concentrations, the dependence of refractive index on R2 was determined empirically. R2 maps, constructed from monoexponential fits to multiecho images of a slice through the lens containing the optical axis, were converted to refractive index maps using this relationship. A simulated ray trace through the refractive index map provided an estimate of lens focal length that was compared to a direct optical measurement of focal length using a laser ray-tracing method. It was found that the mean focal lengths estimated from the two techniques agreed within experimental uncertainty. The refractive index profile along the optical axis was found to be well described by a simple function of the form n=n0 + n1 x ra where r is the (normalized) lens radius.

The effect of water molecular self-diffusion on quantitative high-resolution MRI polymer gel dosimetry.

In polymer gel dosimetry, magnetic resonance imaging (MRI) is used to determine the spin-spin relaxation rate (R2) which in turn can be correlated with absorbed dose to provide a map of the spatial distribution of the absorbed dose in the irradiated dosimeter. High accuracy, precision and reproducibility of these dose maps are essential. Moreover, for dose verification around brachytherapy sources used for intravascular brachytherapy, a high spatial resolution is required (typically 0.01-0.1 mm). To achieve these microscopic resolutions, strong imaging gradients are applied. The Brownian motion of water molecules in the presence of these strong magnetic field gradients causes an attenuation of the MR signal. When using a multiple spin-echo sequence, this may result in a significant deviation in the measured R2. The diffusion-related change in R2 at high resolutions was investigated experimentally and correlated with predictions that were obtained numerically and algebraically. Diffusion weighting is determined by the self-diffusion coefficient D, and imaging parameters, quantified by the b-factor. The b-factor was calculated for a multiple spin-echo sequence for different gradient strengths and gradient pulse durations. The variations in R2 that were observed when changing the matrix size and slice thickness are explained. It is shown that a linear correlation between the matrix size and the variation in R2 is based on the diffusion weighting caused by the read-out gradients and slice selective gradients. In conclusion, the essence of taking into account molecular self-diffusion to quantify variations in the measured dose-R2 response when using high-resolution MRI in polymer gel dosimetry is emphasized.

Anisotropic water transport in the human eye lens studied by diffusion tensor NMR micro-imaging.

We report in vitro measurements of effective diffusion tensors characterising the anisotropic transport of water in human eye lenses ranging in age from 13 to 86 years. The measurements were obtained by means of a pulsed field gradient spin echo (PFGSE) magnetic resonance imaging (MRI) technique at a spatial resolution of 218 x 218 x 1000 microm(3). The results show that water diffusion is both spatially inhomogeneous and highly anisotropic on the timescale of the measurements (approximately 15 msec). Diffusion parallel to the long axes of the lens fibre cells is relatively unrestricted, whereas that between cells is substantially inhibited by the cell membranes, particularly in the inner cortex region of the lens. The data confirm the presence of a diffusion barrier surrounding the lens nucleus, which inhibits transport of water and other small molecules into and out of the nucleus. The results shed light on factors that may influence the onset of presbyopia and senile cataract. They also have implications for delivery of drugs to the lens nucleus.

Age-related changes in refractive index distribution and power of the human lens as measured by magnetic resonance micro-imaging in vitro.

We report a new technique for non-invasively mapping the refractive index distribution through the eye lens using magnetic resonance micro-imaging. The technique is applied to map the refractive index distribution throughout the sagittal plane of 18 human eye lenses ranging in age from 14 to 82 years in vitro. The results are compared with standard models for the human eye lens. They confirm that the refractive index distribution, when plotted as a function of normalised lens radius, is a function of lens age and differs both between the equatorial and axial directions and between the anterior and posterior halves of the optical axis. The refractive index of the lens nucleus exhibits a significant reduction with age amounting to 3.4+/-0.6 x 10(-4) years(-1). The contribution of the gradient index (GRIN) to the lens power decreases by 0.286+/-0.067 D/year, accounting almost entirely for the estimated overall change in lens power with age for these lenses, which were probably in their most accommodated state. The results provide experimental verification of hypothesised changes in the GRIN that have previously been invoked as contributing to presbyopia and support the hypothesis that changes in the GRIN are sufficient to offset effects of increasing curvature of human lenses with age in their unaccommodated state.

The interpretation of multi-exponential water proton transverse relaxation in the human and porcine eye lens.

We report results of 1H NMR transverse relaxation experiments on human and porcine eye lenses. Several authors have reported that transverse relaxation is not mono-exponential when observed by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and have interpreted the results by postulating the presence of "pools" of water molecules in different binding environments that do not exchange rapidly on the NMR timescale. We have compared CPMG data for intact lenses with results for lens homogenates and have combined a CPMG spectroscopic pulse train with NMR micro-imaging to study the nature of the transverse relaxation process in human and porcine lenses. Fast exchange of water protons with the lens proteins (crystallins) leads to an enhanced transverse relaxation rate that varies linearly with protein concentration. At the resolution of NMR micro-imaging the transverse relaxation process is mono-exponential. The results show that the multi-exponential CPMG data observed spectroscopically for whole lenses reflect spatial variations in crystallin content through the lens rather than the presence of distinct "bound" and "free" water pools.

Diffusion limited evaporation rates in hydrogel contact lenses.

PURPOSE: To systematically measure evaporative water loss for a range of commercial hydrogel contact lens types, correlate the results with measured water self-diffusion coefficients and determine the best way to present and quantify the in vitro evaporation data. METHODS: Evaporative water loss was measured gravimetrically for the four commercial contact lens types: Acuvue (powers: -0.50 D, -0.75 D), Newvues (powers: -0.50 D, -0.75 D, -1.00 D), CIBAsoft (powers: -0.50 D, -0.75 D, -1.00 D) and SeeQuence 2 (power: -0.75 D), with water contents of 60%, 55%, 38%, and 38%, respectively. Water proton self-diffusion coefficients were measured for these materials using standard nuclear magnetic resonance (NMR) pulsed field gradient techniques. RESULTS: The initial rate of evaporation was linear for 5-10 minutes then decreased in all cases. No significant differences were observed in this initial rate between the lenses studied. However, the evaporative rate decreased more slowly with time for the higher water content lenses. The diffusion coefficients were found to be highly dependent on the equilibrium water content, regardless of material composition. CONCLUSIONS: The most effective way to present/quantify this type of evaporative data is to plot the time-course of the mass evaporation rate. We have found that the decrease in evaporative rate with time is a diffusion-limited effect whereby lenses with higher water content can maintain a higher rate of diffusion of water (to the evaporative surface) due to the presence of higher proportions of mobile water molecules. The NMR diffusion data corroborate these findings.

Test liquids for quantitative MRI measurements of self-diffusion coefficient in vivo.

A range of liquids suitable as quality control test objects for measuring the accuracy of clinical MRI diffusion sequences (both apparent diffusion coefficient and tensor) has been identified and characterized. The self-diffusion coefficients for 15 liquids (3 cyclic alkanes: cyclohexane to cyclooctane, 9 n-alkanes: n-octane to n-hexadecane, and 3 n-alcohols: ethanol to 1-propanol were measured at 15-30 degrees C using an NMR spectrometer. Values at 22 degrees C range from 0.36 to 2.2 10(-9) m(2)s(-1). Typical 95% confidence limits are +/-2%. Temperature coefficients are 1.7-3.2% degrees C. T1 and T2 values at 1.5 T and proton density are given. n-tridecane has a diffusion coefficient close to that of normal white matter. The longer n-alkanes may be useful T2 standards. Measurements from a spin-echo MRI sequence agreed to within 2%.

Age-related changes in the kinetics of water transport in normal human lenses.

Magnetic resonance microscopy (MRM) has been used to study the kinetics of water transport in human eye lenses. Fresh lenses obtained from the Queensland Eye Bank were incubated at 34.5 degrees C in artificial aqueous humour (AAH) containing nutrients and metabolites similar to those that are present in vivo. MR images were acquired over approximately a 20 hr period following replacement of H(2)O based AAH with deuterium oxide (D(2)O) based AAH. NMR signal intensity from the lenses decreased with time corresponding to a decrease in concentration of H(2)O within the lenses. A statistically significant correlation (P<0.001) was found between the rate of NMR signal loss from the lens nuclei and increasing age of the lenses. The results show that as lenses age, there is a reduction in the rate at which water and presumably also water soluble low molecular weight metabolites, can enter the cells of the lens nucleus via the epithelium and cortex. A decrease in the rate of transport of water, nutrients and anti-oxidants (e.g. glutathione) would be expected to lead to progressive oxidative damage to lenses with age, and may ultimately contribute to presbyopia and senile nuclear cataract.

Fourier transform Raman spectroscopy of polyacrylamide gels (PAGs) for radiation dosimetry.

Polyacrylamide gels (PAGs) are used for magnetic resonance imaging radiation dosimetry. Fourier transform (FT) Raman spectroscopy studies were undertaken to investigate cross-linking changes during the copolymerization of polyacrylamide gels in the spectral range of 200-3500 cm(-1). Vibrational bands of 1285 cm(-1) and 1256 cm(-1) were assigned to acrylamide and bis-acrylamide single CH2 deltaCH2 binding modes. Bands were found to decrease in amplitude with increasing absorbed radiation dose as a result of copolymerization. Principal component regression was performed on FT-Raman spectra of PAG samples irradiated to 50 Gy. Two components were found to be sufficient to account for 98.7% of the variance in the data. Cross validation was used to establish the absorbed radiation dose of an unknown PAG sample from the FT-Raman spectra. The calculated correlation coefficient between measured and predictive samples was 0.997 with a standard error of estimate of 0.976 and a standard error of prediction of 1.140. Results demonstrate the potential of FT-Raman spectroscopy for ionizing radiation dosimetry using polyacrylamide gels.

Relaxivity of Gd-EOB-DTPA in the normal and biliary obstructed guinea pig.

After injection of Gd-EOB-DTPA, T1 and T2 were determined on a clinical MR scanner (1.5 T) in the liver and kidneys of sacrificed but intact guinea pigs with normal and obstructed biliary systems and in bile, urine, and blood collected postmortem. Tissue [Gd] was determined by radioassay of 153Gd and relaxivities (R1 and R2; units of s(-1) x mmol(-1) x kg) of Gd-EOB-DTPA calculated. Compared with R1 in 2% agarose gel (4.49 +/- 0.03), in normal animals R1 was increased in liver (9.3 +/- 0.5), similar in kidney cortex (4.1 +/- 0.5), but reduced in kidney medulla (2.5 +/- 0.4) and papilla (2.7 +/- 0.4). Chronic biliary obstruction did not change R1 in liver (9.7 +/- 4.3) but reduced R1 further in kidney tissues (1.0-0.4). In normal animals, R2 values of all tissues (9.5-18.4) were greater than R2 in gel (5.72 +/- 0.12). Biliary obstruction possibly elevated R2 in liver (40.1 +/- 63.5), severely depressed R2 in kidney cortex (-4.2 +/- 6.2) and medulla (-2.3 +/- 5.4), and reduced R2 in papilla (5.4 +/- 4.6). Obstruction had little effect on R1 and R2 in bile and urine. Water content, macromolecular binding, microviscosity, compartmentalization, and susceptibility effects can readily account for the R1 and R2 observed in liver and kidney. Negative R2 could be a result of several factors, including reduced endogenous magnetic field gradients due to "susceptibility matching" as [Gd] increased, changes in tissue T2 with period of ligation, or a physiological effect of EOB-DTPA. These results show that disease can alter both R1 and R2 from their values in normal tissues.

The effects of solutes on the freezing properties of and hydration forces in lipid lamellar phases.

Quantitative deuterium nuclear magnetic resonance is used to study the freezing behavior of the water in phosphatidylcholine lamellar phases, and the effect upon it of dimethylsulfoxide (DMSO), sorbitol, sucrose, and trehalose. When sufficient solute is present, an isotropic phase of concentrated aqueous solution may coexist with the lamellar phase at freezing temperatures. We determine the composition of both unfrozen phases as a function of temperature by using the intensity of the calibrated free induction decay signal (FID). The presence of DMSO or sorbitol increases the hydration of the lamellar phase at all freezing temperatures studied, and the size of the increase in hydration is comparable to that expected from their purely osmotic effect. Sucrose and trehalose increase the hydration of the lamellar phase, but, at concentrations of several molal, the increase is less than that which their purely osmotic effect would be expected to produce. A possible explanation is that very high volume fractions of sucrose and trehalose disrupt the water structure and thus reduce the repulsive hydration interaction between membranes. Because of their osmotic effect, all of the solutes studied reduced the intramembrane mechanical stresses produced in lamellar phases by freezing. Sucrose and trehalose at high concentrations produce a greater reduction than do the other solutes.

Limitations of in vitro contact lens dehydration/rehydration data in predicting on-eye dehydration.

PURPOSE: The purpose of this study was to use in vitro dehydration and rehydration data to model the predicted hydration changes that may occur as a soft contact lens loses and gains water on the eye between blinks. METHODS: Using a recent in vitro data set for four lens materials dehydrated and rehydrated in saline, we derived a mathematical model to describe dehydration and rehydration time courses. The model further combined the dehydration and rehydration data iteratively, as a function of blink frequency, and pre-lens break-up time. RESULTS: The model showed that reduced break-up times or decreased blink frequencies significantly affected dehydration rates and steady state dehydration for lenses of a variety of water contents. However, the model did not agree with the commonly accepted clinical belief that high water content lens materials dehydrate more than low water content materials. DISCUSSION: The discrepancy of the model with historical observations may be accounted for by one or more of the following factors, which were not accounted for in the present model: 1) temperature dependent dehydration (as a lens is taken from a room temperature vial and warmed when placed on the eye); 2) the colloid osmotic pressure, ionicity, pH, and chemical potential of tears (compared to saline); and 3) dehydration by other non-evaporative mechanisms.

Fast T1 imaging of dual gel samples for diffusion measurements in NMR dosimetry gels.

Diffusion of iron is one of the major problems limiting the usefulness of NMR gel dosimetry. This was studied in dual gel samples using a 4.7T micro-imaging MR scanner and a fast T1 imaging sequence which allowed the acquisition of a 64 x 128 x 8 data sets (phase encoding x frequency encoding x number of inversion times) in less than 15 min. The procedure enabled us to obtain relative relaxation times for any region of interest within the sample. After the two differently doped gels were brought into contact in the dual gel samples (diameter 12 mm), the diffusion could be observed on subsequent images as a function of time. An inverse square root function was used to fit the change of 1/T1 across the junction between the two gel phases. A diffusion constant of 0.014 +/- 0.003 cm2/h was determined for Fe3+ in a typical dosimetry gel (1.5% agarose, 50 mM H2SO4). This could be lowered by adding a chelating agent such as xylenol orange to the gel. It was also found that diffusion was slower in gelatine gels, however these gels tended not to set properly when H2SO4 was added as required for NMR dosimetry. From the present results we propose that a gel consisting of 1.5% agarose (for stability), 3% gelatine and 0.1 mM xylenol orange (to combat diffusion and allow a visual evaluation) is a suitable base for NMR dosimetry gels. The use of a fast T1 imaging sequence reduces acquisition times and therefore the potential impact of diffusion.

An investigation of the fluidity of concentration polarisation layers in crossflow membrane filtration of an oil-water emulsion using chemical shift selective flow imaging.

A chemical shift selective NMR flow imaging sequence using stimulated echoes for data acquisition is presented. The sequence was tested using a 20% (vol/vol) oil-water emulsion formed from a soluble cutting oil, which was passed through a simple flow phantom to yield two-dimensional velocity distribution maps of the oil droplets and of the water separately. It was then used to investigate the fluidity of concentration polarisation layers formed from the oil droplets during crossflow membrane filtration of a 5% (vol/vol) emulsion of the same cutting oil. A simple membrane filtration module was used for this purpose, with the feedstock emulsion fed into the lumen of a single tubular membrane at a trans-membrane pressure difference P approximately 70 kPa and crossflow Reynolds number, Re, in the range 100-1000. The results confirm a net axial flow rate < 7.5 microns/s (half digital resolution in the velocity dimension) for the oil polarisation layer. Under these conditions, the upper limit for oil flow tangential to the membrane in the polarised layer is less than 15% of the convective flow of oil towards the membrane.