Ultrasonic characterization of human cancellous bone in vitro using three different apparent backscatter parameters in the frequency range 0.6-15.0 mhz

Ultrasonic techniques based on measurements of apparent backscatter may provide a useful means for diagnosing bone diseases such as osteoporosis. The term "apparent" means that the backscattered signals are not compensated for the frequency-dependent effects of attenuation and diffraction. We performed in vitro apparent backscatter measurements on 23 specimens of human cancellous bone prepared from the left and right femoral heads of seven donors. A mechanical scanning system was used to obtain backscattered signals from each specimen at several sites. Scans were performed using five different ultrasonic transducers with center frequencies of 1, 2.25, 5, 7.5, and 10 MHz. The -6 dB bandwidths of these transducers covered a frequency range of 0.6-15.0 MHz. The backscattered signals were analyzed to determine three ultrasonic parameters: apparent integrated backscatter (AIB), frequency slope of apparent backscatter (FSAB), and time slope of apparent backscatter (TSAB). Linear regression analysis was used to examine the correlation of these ultrasonic parameters with five measured physical characteristics of the specimens: mass density, X-ray bone mineral density, Young's modulus, yield strength, and ultimate strength. A total of 75 such correlations were examined (3 ultrasonic parameters x 5 specimen characteristics x 5 transducers). Good correlations were observed for AIB using the 5 MHz (r = 0.70 - 0.89) and 7.5 MHz (r = 0.75-0.93) transducers; for FSAB using the 2.25 MHz (r = 0.70 - 0.88), 5 MHz (r = 0.79 - 0.94), and 7.5 MHz (r = 0.80 - 0.92) transducers; and for TSAB using the 5 MHz (r = 0.68 - 0.89), 7.5 MHz (r = 0.75 - 0.89), and 10 MHz (r = 0.75 - 0.92) transducers.     

Ultrasonic attenuation in human calcaneus from 0.2 to 1.7 MHz

Ultrasonic attenuation has been demonstrated to be a useful measurement in the diagnosis of osteoporosis. Most studies have employed ultrasound in a range of frequencies from about 200 kHz-300 kHz to 600 kHz-1 MHz, and many have assumed a linear dependence of attenuation on frequency. In order to investigate the attenuation properties of human calcaneus at higher frequencies, 16 defatted human calcanea were interrogated in vitro using two matched pairs of transducers with center frequencies of 500 kHz and 2.25 MHz. The linear dependence of attenuation on frequency seems to extend up to at least 1.7 MHz. The correlation between attenuation coefficient and frequency from 400 kHz to 1.7 MHz was r = 0.999 (95% confidence interval, CI, = 0.998-1.00). The measurements suggest that some deviations from linear frequency dependence of attenuation may occur at lower frequencies (below 400 kHz), however.     

Autocorrelation and cepstral methods for measurement of tibial cortical thickness

Cortical thickness of the tibia is related to stress fracture risk and overall skeletal status. Two methods are proposed for estimating tibial cortical thickness based on power spectra of ultrasonic echoes containing reflections from front and back surfaces of the cortex. The locations of the peaks in the autocorrelation function and the cepstrum are related to cortical thickness. Data were acquired on plastic plates in order to validate the methodology. These data indicate high correlations between estimated and true thickness with correlation coefficients r=0.99, (95% confidence interval: 0.993-1.00) for the autocorrelation method and r=0.99 (95% CI: 0.996-1.00) for the cepstral method. Data on six tibia samples in vitro indicate correlation coefficients of r=0.92 (95% CI: 0.72-1.00) for the autocorrelation method and r=0.85 (95% CI: 0.62-0.94) for the cepstral method. Estimates of precisions of the two methods were 0.3/spl plusmn/0.1 mm (autocorrelation method) and 0.5/spl plusmn/0.2 mm (cepstral method). One measurement in a human volunteer in vivo demonstrated clinical feasibility of the measurement and good agreement with cortical thickness assessed using peripheral quantitative computed tomography (QCT). This technology offers the promise of an inexpensive, fast, portable, simple, nonionizing technique for assessing skeletal status.     

US backscatter and attenuation 30 to 50 MHz and MR T2 at 3 Tesla for differentiation of atherosclerotic artery constituents in vitro

This study compares quantitative characterization of atherosclerotic artery constituents by high resolution estimates of ultrasonic attenuation, ultrasonic attenuation-compensated backscatter, and magnetic resonance transverse relaxation time. Atherosclerotic human arteries were studied in vitro at 37°C. Backscattered radio frequency signals were acquired with a 50 MHz backscatter acoustic microscope. Ultrasonic parametric images were constructed from the integrated (30 to 50 MHz) backscatter and attenuation obtained using FFT methods with diffraction correction and a multinarrow-band attenuation algorithm. Parametric magnetic resonance images were constructed from calculated values of the transverse relaxation time T2 determined from an 8 echo-single-slice sequence at 3 Tesla. In a total of 54 regions of interest, average values of integrated attenuation, integrated backscatter compensated for the attenuation between the artery surface and the scattering volume, and the transverse relaxation time were correlated with local tissue composition as assessed by histology. Results show that ultrasound and magnetic resonance techniques offer complementary approaches for characterization of plaque composition     

Absolute backscatter coefficient over a wide range of frequenciesin a tissue-mimicking phantom containing two populations of scatterers

The acquisition and interpretation of in vivo ultrasonic measurements in tissue encounter problems associated with limited access to the region of interest, intermixed scattering structures with different characteristic dimensions, and system-dependent effects. This work addresses these problems by adapting and testing a technique for measuring the absolute attenuation and the absolute backscatter coefficient (effective backscatter cross section per unit volume of material), as a function of frequency, in a single-transducer backscatter configuration. The frequency-dependent attenuation and backscatter coefficients of a tissue-mimicking gelatin phantom containing a random distribution of two populations of scatterers were measured, Three transducers with different center frequencies and focusing characteristics were used in order to verify that system-dependent effects were removed by the technique and to investigate the change in the measured parameters across a broad range of frequencies (2 to 60 MHz). A spherical autocorrelation model was applied to measurements of the backscatter coefficient in order to estimate the size of scatterers. Measurements demonstrate that the backscatter and attenuation properties of a mixture of two distinct intermixed scatterer-size populations change as a function of the frequency range across which the model is applied. Comparison of both the magnitude and the frequency dependence of the experimental results with the theoretical prediction of the backscatter coefficient showed good agreement     

Motorcycle protective clothing: protection from injury or just the weather?

Background

Apart from helmets, little is known about the effectiveness of motorcycle protective clothing in reducing injuries in crashes. The study aimed to quantify the association between usage of motorcycle clothing and injury in crashes.

Methods and findings

Cross-sectional analytic study. Crashed motorcyclists (n = 212, 71% of identified eligible cases) were recruited through hospitals and motorcycle repair services. Data was obtained through structured face-to-face interviews. The main outcome was hospitalization and motorcycle crash-related injury. Poisson regression was used to estimate relative risk (RR) and 95% confidence intervals for injury adjusting for potential confounders.

Results

Motorcyclists were significantly less likely to be admitted to hospital if they crashed wearing motorcycle jackets (RR = 0.79, 95% CI: 0.69–0.91), pants (RR = 0.49, 95% CI: 0.25–0.94), or gloves (RR = 0.41, 95% CI: 0.26–0.66). When garments included fitted body armour there was a significantly reduced risk of injury to the upper body (RR = 0.77, 95% CI: 0.66–0.89), hands and wrists (RR = 0.55, 95% CI: 0.38–0.81), legs (RR = 0.60, 95% CI: 0.40–0.90), feet and ankles (RR = 0.54, 95% CI: 0.35–0.83). Non-motorcycle boots were also associated with a reduced risk of injury compared to shoes or joggers (RR = 0.46, 95% CI: 0.28–0.75). No association between use of body armour and risk of fracture injuries was detected. A substantial proportion of motorcycle designed gloves (25.7%), jackets (29.7%) and pants (28.1%) were assessed to have failed due to material damage in the crash.

Conclusions

Motorcycle protective clothing is associated with reduced risk and severity of crash related injury and hospitalization, particularly when fitted with body armour. The proportion of clothing items that failed under crash conditions indicates a need for improved quality control. While mandating usage of protective clothing is not recommended, consideration could be given to providing incentives for usage of protective clothing, such as tax exemptions for safety gear, health insurance premium reductions and rebates.     

Ultrasonic attenuation and backscatter from flowing whole blood are dependent on shear rate and hematocrit between 10 and 50 MHz

Ultrasonic backscatter has recently been used extensively to investigate erythrocyte aggregation, which is an inherent hematological phenomenon in the blood circulation system. The size of rouleaux can be estimated by measuring certain parameters of signals backscattered from flowing blood. However, most measurements of backscatter from blood use a constant value for the attenuation coefficient to compensate for the loss of ultrasound energy. This correction may be inaccurate because the attenuation varies with the blood properties, which prompted us to explore the effects of hemodynamic properties on ultrasonic attenuation and backscatter to better understand the blood rheological behaviors. Experiments were performed on porcine whole blood in a Couette flow apparatus. Ultrasonic attenuation and the backscattering coefficient of blood were measured at various frequencies (from 10 to 50 MHz), hematocrits (from 0 to 60%), and shear rates (from 0.1 to 200 s?1). The results indicated that the attenuation and backscattering coefficients of blood are highly variable, depending in a complex manner on shear rate, hematocrit, and the measurement ultrasound frequency. The attenuation of blood decreased rapidly with increasing shear rates, eventually reaching a steady state asymptotically, and increased linearly with the hematocrit from 10 to 50 MHz at various shear rates, and also with the ultrasound frequency. The effect of erythrocyte aggregation means that the change in ultrasonic attenuation in blood with shear rate may be attributed to the absorption mechanism, which is enhanced by the increased blood viscosity at lower shear rates. Compensating the measured backscattering coefficients of blood for the shear-rate-dependent attenuation coefficient increased the accuracy of erythrocyte aggregation assessments. Together, the experimental results suggest that the shear-rate-dependent attenuation coefficient should be considered in future developments of ultrasonic technologies for characterizing blood rheology when the ultrasound frequency is higher than 20 MHz.     

Comparison of stroke volume measurements during hemodialysis using bioimpedance cardiography and echocardiography

Background: Fluid management remains a major challenge of hemodialysis (HD) care, with serious implications for morbidity and mortality. Intradialytic fluid management is typically guided by blood pressure, an indirect resultant of hemodynamics status. Direct measurements of hemodynamic parameters may improve cardiovascular outcomes by providing rational bases for intervention. We compare stroke volume (SV) measurements using a noninvasive, regional biompedance cardiography device (NiCaS) with Doppler echocardiography (Echo) in HD setting. Methods: Stroke volumes were simultaneously measured using the devices in 17 patients receiving maintenance HD. Measurements were made during 2 weekly HD treatments, and twice within each HD treatment during the first and last hour of each treatment, for a total of 64 SV measurements. Agreement between devices was assessed using linear regression, a Pearson's correlation coefficient, and a Bland‐Altman plot all adjusted for repeated measures within patients. Results: Echo and NiCaS SV mean and 95% CIs were 58.0 (50.1, 65.8) and 56.7 (49.4, 64.0) mL, respectively. NiCaS SV correlated strongly with Echo SV during the first and last hours of treatments (r = 0.93, P < 0.001 and r = 0.92, P < 0.001, respectively). Linear regression of NiCaS on Echo showed a slope of 0.97, 95% CI (0.91, 1.02) which did not differ from 1, P = 0.20. A Bland‐Altman plot and 4‐Quadrant plot confirmed that the 2 methods produced comparable measurements. Conclusion: NiCaS SV measurements are similar to and strongly correlated with Echo SV measurements. This suggests that noninvasive NiCaS technology may be a practical method for measuring SV during HD.     

Ultrasonic scattering from cancellous bone: a review

This paper reviews theory, measurements, and computer simulations of scattering from cancellous bone reported by many laboratories. Three theoretical models (binary mixture, Faran cylinder, and weak scattering) for scattering from cancellous bone have demonstrated some consistency with measurements of backscatter. Backscatter is moderately correlated with bone mineral density in human calcaneus in vitro (r(2) = 0.66 - 0.68). Backscatter varies approximately as frequency cubed and trabecular thickness cubed in human calcaneus and femur in vitro. Backscatter from human calcaneus and bovine tibia exhibits substantial anisotropy. So far, backscatter has demonstrated only modest clinical utility. Computer simulation models have helped to elucidate mechanisms underlying scattering from cancellous bones.     

Method to derive ocean absorption coefficients from remote-sensing reflectance

A method to derive in-water absorption coefficients from total remote-sensing reflectance (ratio of the upwelling radiance to the downwelling irradiance above the surface) analytically is presented. For measurements made in the Gulf of Mexico and Monterey Bay, with concentrations of chlorophyll-a ranging from 0.07 to 50 mg/m(3), comparisons are made for the total absorption coefficients derived with the suggested method and those derived with diffuse attenuation coefficients. For these coastal to open-ocean waters, including regions of upwelling and the Loop Current, the results are as follows: at 440 nm the difference between the two methods is 13.0% (r(2) = 0.96) for total absorption coefficients ranging from 0.02 to 2.0 m(-1); at 488 nm the difference is 14.5% (r(2) = 0.97); and at 550 nm the difference is 13.6% (r(2) = 0.96). The results indicate that the method presented works very well for retrieval of in-water absorption coefficients exclusively from remotely measured signals, and that this method has a wide range of potential applications in oceanic remote sensing.     

Imaginary refractive-index effects on desert-aerosol extinction versus backscatter relationships at 351 nm: numerical computations and comparison with Raman lidar measurements

A numerical model is used to investigate the dependence at 351 nm of desert-aerosol extinction and backscatter coefficients on particle imaginary refractive index (mi). Three ranges (-0.005 < or = mi < or = -0.001, -0.01 < or = mi < or = -0.001, and -0.02 < or = mi < or = -0.001) are considered, showing that backscatter coefficients are reduced as /mi/ increases, whereas extinction coefficients are weakly dependent on mi. Numerical results are compared with extinction and backscatter coefficients retrieved by elastic Raman lidar measurements performed during Saharan dust storms over the Mediterranean Sea. The comparison indicates that a range of -0.01 to -0.001 can be representative of Saharan dust aerosols and that the nonsphericity of mineral particles must be considered.     

Characterization of aortic microstructure with ultrasound: implications for mechanisms of aortic function and dissection

Specific ultrasonic tissue characterization parameters were correlated with the three-dimensional architecture and material properties (density, compressibility, size, and orientation) of aortic elastic elements at the microscopic level. The medial layer of 10 samples of normal canine aorta were insonified in vitro utilizing acoustic microscopy from 30 to 44 MHz. The following quantitative indexes exhibited substantial anisotropic elastic behavior in radial (R), circumferential (C), and longitudinal (L) directions: backscatter coefficient (R:0.9/spl plusmn/0.2; C:0.008/spl plusmn/0.0008; L:0.0077/spl plusmn/0.0008 sr/sup -1/ cm/sup -1/); frequency dependence of backscatter (R:3.3; C:1.4; L:1.5); attenuation coefficients (R:105/spl plusmn/22; L:135/spl plusmn/13; C:131/spl plusmn/14 dB/cm). Thus, the ultrasonic indexes are anisotropic: equivalent in the C and L directions, but markedly different in the R direction. These data are indicative of an aortic microstructure that interacts with ultrasonic waves as thin sheet-like elastic layers instead of independent elastin fibers. This specific sheet-like organization of elastin microfibers may function to limit shear injury to concentric aortic lamellae and prevent aortic dissection. The marked anisotropic behavior of normal aortas suggests that ultrasound may be useful for nondestructive characterization of vascular integrity.     

Spatial variability in acoustic backscatter as an indicator of tissue homogenate production in pulsed cavitational ultrasound therapy

Spatial variability in acoustic backscatter is investigated as a potential feedback metric for assessment of lesion morphology during cavitation-mediated mechanical tissue disruption ("histotripsy"). A 750-kHz annular array was aligned confocally with a 4.5 MHz passive backscatter receiver during ex vivo insonation of porcine myocardium. Various exposure conditions were used to elicit a range of damage morphologies and backscatter characteristics [pulse duration = 14 micros, pulse repetition frequency (PRF) = 0.07-3.1 kHz, average I(SPPA) = 22-44 kW/cm2]. Variability in backscatter spatial localization was quantified by tracking the lag required to achieve peak correlation between sequential RF A-lines received. Mean spatial variability was observed to be significantly higher when damage morphology consisted of mechanically disrupted tissue homogenate versus mechanically intact coagulation necrosis (2.35 +/- 1.59 mm versus 0.067 +/- 0.054 mm, p < 0.025). Statistics from these variability distributions were used as the basis for selecting a threshold variability level to identify the onset of homogenate formation via an abrupt, sustained increase in spatially dynamic backscatter activity. Specific indices indicative of the state of the homogenization process were quantified as a function of acoustic input conditions. The prevalence of backscatter spatial variability was observed to scale with the amount of homogenate produced for various PRFs and acoustic intensities.     

Risk factors for fatal road traffic accidents in Udine, Italy.

In the Province of Udine, Northeast Italy, mortality from road accidents is 37% higher than in the country as a whole. To identify the major risk factors for fatal crashes in this area, we analyzed the Police reports of 10,320 road traffic accidents that occurred from 1991 to 1996. Logistic regression was used to evaluate the association of characteristics of drivers and accidents with accident severity. The risk of involvement in fatal rather than non-fatal accidents was lower among females than among males (odds ratio (OR) = 0.65; 95% confidence interval (95% CI), 0.53-0.80). Compared with subjects < 30 years of age, subjects aged > or = 65 had a significantly increased risk of fatal injury as pedestrians (OR = 10.87; 95% CI, 4.45-26.54), car drivers (OR = 1.85; 95% CI, 1.08-3.18), moped riders (OR = 3.53; 95% CI, 1.42-8.78), and bicycle riders (OR = 7.72; 95% CI, 2.56-23.29). In accidents that occurred from 1:00 to 5:00 h the risk of death was higher than from 6:00 to 11:00 h among pedestrians (OR = 8.88; 95% CI, 2.58-30.52), car drivers (OR = 4.95; 95% CI, 3.09-7.95), motorcycle riders (OR = 13.44; 95%CI, 2.54-71.05) and moped riders (OR = 8.76; 95% CI, 2.42-31.69). Risk of death among pedestrians, car drivers, moped, and bicycle riders was also significantly increased on roads outside the urban center. Driver's injury was strongly associated with lack of use of seat belts (OR = 13.27; 95% CI, 9.39-18.74, for fatal injury; OR = 2.49; 95% CI, 2.17-2.86, for non-fatal injury). Simple interventions focused on protecting the weakest road users and based on law enforcement, behavioral change and environmental modification might result in reducing the significant excess of road traffic accident mortality found in the study area.     

Calibration of the 1064 nm lidar channel using water phase and cirrus clouds

Calibration is essential to derive aerosol backscatter coefficients from elastic scattering lidar. Unlike the visible UV wavelengths where calibration is based on a molecular reference, calibration of the 1064 nm lidar channel requires other approaches, which depend on various assumptions. In this paper, we analyze two independent calibration methods which use (i) low-altitude water phase clouds and (ii) high cirrus clouds. In particular, we show that to achieve optimal performance, aerosol attenuation below the cloud base and cloud multiple scattering must be accounted for. When all important processes are considered, we find that these two independent methods can provide a consistent calibration constant with relative differences less than 15%. We apply these calibration techniques to demonstrate the stability of our lidar on a monthly scale, along with a natural reduction of the lidar efficiency on an annual scale. Furthermore, our calibration procedure allows us to derive consistent aerosol backscatter coefficients and angstrom coefficient profiles (532-1064 nm) along with column extinction-to-backscatter ratios which are in good agreement with sky radiometer inversions.     

The epidemiology of pregnancy-associated emergency department injury visits and their impact on birth outcomes

OBJECTIVE: Describe the demographics, injury types, mechanisms, and intents of emergency department (ED) injury visits by pregnant women and to quantify their risk of adverse birth outcomes. METHODS: Through a retrospective cohort study design, Utah ED, birth, and fetal death records were probabilistically linked to identify women seen in an ED with an injury during pregnancy among births and fetal deaths from 1999 to 2002. Logistic regression was used to assess the effect of having an injury-related ED visit on various adverse pregnancy outcomes. RESULTS: 7350 (3.9%) women experienced an injury-related ED visit during pregnancy. Motor vehicle occupant injuries were the leading mechanism of ED injury visits (22.4%). Controlling for known risks, pregnant women with an injury-related ED visit were more likely than non-injured pregnant women to experience preterm labor (OR=1.22, 95% CI=1.12-1.34), placental abruption (OR=1.33, 95% CI=1.08-1.65), and cesarean delivery (OR=1.27, 95% CI=1.19-1.36). Infants born to women who were injured during pregnancy were more likely to be born preterm (OR=1.23, 95% CI=1.12-1.34) and have low birth weight (OR=1.22, 95% CI=1.1-1.35). CONCLUSIONS: Most injured pregnant women are treated and released from the ED; however, significant increased risks remain for several maternal complications and birth outcomes.     

The dependence of ultrasonic backscatter on trabecular thickness in human calcaneus: theoretical and experimental results

Trabecular thickness within cancellous bone is an important determinant of osteoporotic fracture risk. Noninvasive assessment of trabecular thickness potentially could yield useful diagnostic information. Faran's theory of elastic scattering from a cylindrical object immersed in a fluid has been used to predict the dependence of ultrasonic backscatter on trabecular thickness. The theory predicts that, in the range of morphological and material properties expected for trabecular bone, the backscatter coefficient at 500 kHz should be approximately proportional to trabecular thickness to the power of 2.9. Experimental measurements of backscatter coefficient were performed on 43 human calcaneus samples in vitro. Mean trabecular thicknesses on the 43 samples were assessed using micro computed tomography (CT). A power law fit to the data showed that the backscatter coefficient empirically varied as trabecular thickness to the 2.8 power. The 95% confidence interval for this exponent was 1.7 to 3.9. The square of the correlation coefficient for the linear regression to the log transformed data was 0.40. This suggests that 40% of variations in backscatter may be attributed to variations in trabecular thickness. These results reinforce previous studies that offered validation for the Faran cylinder model for prediction of scattering properties of cancellous bone, and provide added evidence for the potential diagnostic utility of the backscatter measurement.     

The effect of phase cancellation on estimates of broadband ultrasound attenuation and backscatter coefficient in human calcaneus in vitro

Broadband ultrasound attenuation (BUA) is a clinically proven indicator of osteoporotic fracture risk. BUA measurements are typically performed in through- transmission with single-element phase sensitive (PS) receivers and therefore can be compromised by phase cancellation artifact. Phase-insensitive (PI) receivers suppress phase cancellation artifact. To study the effect of phase cancellation on BUA measurements, through-transmission measurements were performed on 16 human calcaneus samples in vitro using a two-dimensional receiver array that enabled PS and PI BUA estimation. The means plus or minus standard deviations for BUA measurements were 22.1 plusmn 15.8 dB/MHz (PS) and 17.6 plusmn 7.2 dB/MHz (PI), suggesting that, on the average, approximately 20% of PS BUA values in vitro can be attributed to phase cancellation artifact. Therefore, although cortical plates are often regarded as the primary source of phase cancellation artifact, the heterogeneity of cancellous bone in the calcaneal interior may also be a significant source. Backscatter coefficient estimates in human calcaneus that are based on PS attenuation compensation overestimate 1) average magnitude of backscatter coefficient at 500 kHz by a factor of about 1.6 plusmn 0.3 and 2) average exponent (n) of frequency dependence by about 0.34 plusmn 0.12 (where backscatter coefficient is fit to a power law form proportional to frequency to the nth power).     

Arsenic speciation in rice and soil containing related compounds of chemical warfare agents

Diphenylarsinic acid, phenylarsonic acid, methylphenylarsinic acid (MPAA), dimethylphenylarsine oxide (DMPAO), and methyldiphenylarsine oxide (MDPAO) in soil and rice were extracted, separated by reversed-phase chromatography, and quantified by ICPMS with a membrane desolvating system. For the extraction of arsenicals from rice grain and straw, 68% HNO3 provided better extraction efficiency than water, 50% methanol, or 2.0 mol L(-1) trifluoroacetic acid. For the extraction from soil, 68% HNO3 provided better extraction efficiency than H2O, 1 mol L(-1) H3PO4, or 1 mol L(-1) NaOH. The contaminated soil contained all five aromatic arsenicals along with inorganic arsenicals as main species (5.86 +/- 0.19 microg of As kg(-1): 60.8 +/- 2.0% of total extracted As). After pot experiments, rice straw contained mainly DMPAO (7.71 +/- 0.48 microg of As kg(-1): 60.5 +/- 3.7%), MDPAO (0.91 +/- 0.07 microg of As kg(-1): 7.2 +/- 0.5%), and inorganic As (2.85 +/- 0.20 microg of As kg(-1): 22.3 +/- 1.6%). On the other hand, rice grain contained mainly MPAA (1.17 +/- 0.04 microg of As kg(-1): 86.7 +/- 2.7%). The root uptake of each species from the soil and transport from straw to grains were significantly related to the calculated log K(ow) values.     

Spectral analysis of a sampling optical time-domain reflectometer

A spectral analysis is presented for the backscatter signal of optical time-domain reflectometers (OTDR). Periodic spatial fluctuations in the fiber attenuation produce modulation sidebands in the frequency spectrum of the backscatter signal. Applying the sampling technique to OTDR leads to a considerable improvement of the signal-to-noise ratio (S/N) due to spectrum compression, as a tradeoff with the measurement time. It gives a performance equivalent to digital averaging. Design examples are given at wavelengths of 0.85, 1.3, and 1.55 μm. A practical setup of a sampling OTDR is described