Sufficient Signal-to-noise Ratio Research Articles

Optimization of multiple spin–echo sequences for 3D polymer gel dosimetry

The overall performance of polymer gel dosimeters for three-dimensional radiation dosimetry is determined by the temporal and spatial stability of the gels, dose sensitivity and image quality with respect to both systematic and stochastic deviations. The dose resolution (DpΔ) is determined by the dose sensitivity and the signal-to-noise ratio (SNR) in the dose images. The dose sensitivity can be altered by changing the chemical composition of the polymer gel. The SNR is determined by the scanner and the imaging sequence.In the dose verification of conformal radiotherapy treatments the chosen number of slices may reach a number of 10–20. For these experiments, to obtain a sufficient SNR within a reasonable measurement time using a certain MR scanner, the imaging sequence should be optimized. A few other studies have emphasized the importance of optimizing the imaging sequence with respect to dose resolution (DpΔ) or SNR but do not give quantitative values for the optimal sequence parameters for scanning a polymer gel dosimeter in three dimensions.In this paper, it is proved that a multiple spin–echo sequence is preferable to a single spin–echo sequence. It is also shown that when using a multiple spin–echo sequence it is not the inter-echo time that should be optimized but the number of echoes. An algebraical expression is derived for the dose resolution in terms of sequence parameters. A mathematical formalism and look-up tables are provided that can be used to optimize both a single and a slice-selective multiple spin–echo sequence to acquire a set of dose images at various locations. The use of the optimization protocol is illustrated by some examples. The optimization protocol enables the user to derive the optimal sequence parameters to acquire a set of dose maps obtained by quantitative T2 imaging for each polymer gel dosimeter within the shortest time possible.

Signal-to-noise ratio of nondestructive photocurrent-detection readout in near-field photochromic memory: theoretical study

The signal-to-noise ratio (SNR) and possible data-transfer rate of nondestructive photocurrent-detection readout in near-field photochromic memory were calculated theoretically. The photocurrent-detection readout provided a much higher SNR and a higher data-transfer rate than those of conventional transmittance-detection readout. The SNR of the photocurrent-detection readout was dependent on the electric field applied to the medium and the potential barrier height between the photochromic recording layer and the photoabsorbing layer. A sufficient SNR was obtained under the conditions of an electric field of 2.5 V/m and a potential barrier height <0.4 eV in land area. Photocurrent-detection readout is a promising method to achieve high SNR, high data-transfer rate, and ultrahigh recording density with nondestructive-readout capability.

First spaceborne observation of an Earth‐reflected GPS signal

We present the first spaceborne observation of a Global Positioning System (GPS) signal reflected from the Earth's surface, specifically from the Pacific Ocean. This result is scaled to obtain the expected voltage signal‐to‐noise ratio (SNR) and altimetric accuracy for a generic GPS reflections altimetry mission and the current SAC‐C and CHAMP missions. Cross‐correlating a three‐parameter phase model with both a 1‐s and 4‐s segment of spaceborne imaging radar‐C (SIR‐C) calibration data, recorded before and after a Galapagos Islands imaging pass, results in beam‐limited signals having voltage SNRs of 10 and 334, respectively. Evidence for these results being reflected GPS signals includes: (1) The signals' temporal shapes agree closely with that predicted using a detailed scattering model, at two different observation geometries, and differ significantly from the expected direct signal shapes. (2) The signal in the 4‐s data has a measured coherence time of 1.0 ms, which agrees closely with that expected for a reflected signal and is completely inconsistent with the direct signal's coherence properties. (3) The 1‐ and 4‐s signals' voltage SNR is maximized by shifting the model frequency −2740 Hz and 497 Hz, respectively from that expected from their respective specular reflection points, or −2875 Hz and 690 Hz from the expected direct signal frequencies. These values agree with the −2900 Hz and 510 Hz Doppler frequency shifts expected from those points on the surface corresponding to the antenna's pointing direction, thus illustrating beam‐steering effects on the surface. (4) Plausible hypotheses for the detected waveform being a corrupted direct signal, including second‐order mismodeling effects, shuttle multipath effects, or a band‐pass cutoff of the GPS spread spectrum, are shown to be inconsistent with the data. Space‐based observations of reflected GPS signals, like the ones presented here, may enable a new class of ocean topography measurements unavailable from traditional altimeters, such as TOPEX/Poseidon, and perhaps surface wind vector measurements. Making such observations with sufficient SNR will require unusually large, high‐gain antennas. The measurement presented here is scaled to assess the expected SNR for those applications. Because this result lies in a nonlinear scaling regime, the correct scaling equations are presented, and the expected signal strength from a generic GPS reflections altimetry mission is derived to illustrate the most important contributions to the signal SNR. An SNR estimate is also derived for the SAC‐C and CHAMP missions, which are expected to make GPS reflection measurements in the near future. Finally, a qualitative wind speed determination is extracted from the observed signal.

Ambient seismic noise below the deep seafloor

Knowledge of ambient noise levels and of the physical mechanisms responsible for ambient noise is essential to optimize the return from a seismic acquisition system in petroleum exploration or reservoir monitoring. The ultimate goal of a seismic survey is to give the best image of the subsurface that the earth will allow for a given budget. Assuming that we have an acquisition system with sufficiently low system noise and that the processing can handle scattering and other “signal-generated” noise, our ability to image the subsurface will depend on the dynamic range between the earth's ambient noise and the clipping level of the receiver system. Active surveys can be run less expensively if we use the least source effort necessary to get sufficient signal-to-noise ratio (SNR) above the ambient noise to image the target horizons. Also if the characteristics of the ambient noise field are known, adaptive processing techniques can increase the useful SNR even further.

New architectural parameters derived from micro-MRI for the prediction of trabecular bone strength

This article reviews recent progress in magnetic resonance microimaging of cancellous bone in vitro and in vivo from the perspective of the authors' laboratory. It is shown that in particular in vivo the key technical prerequisites to satisfy are: (i) achieving sufficient signal-to-noise ratio (SNR) to allow for adequate spatial resolution; (ii) the image processing algorithms have to be robust enough to provide accurate structural information in the limited spatial resolution regime, i.e., in the presence of inevitable partial volume blurring and noise. The practical lower limit of voxel size in vivo was found to be about 6 x 10(-3) mm3 in the radius, and about 10(-4) mm3 for small specimens in vitro with state-of-the-art equipment and scan times of 10 and 30 minutes, resp., and SNR approximately 10. Finally, data are presented highlighting the potential of these methods for predicting the bone's elastic modulus in vitro and fracture risk in vivo.

0.1-nm narrow bandwidth transmission of a 2.5-Gb/s spectrum-sliced incoherent light channel using an all-optical bandwidth expansion technique at the receiver

We introduce a new all-optical technique to transmit a spectrum-sliced incoherent channel with its optical bandwidth smaller than the conventional limit. As a demonstration, we reduce the optical bandwidth to only 0.1 nm for the 2.5-Gb/s incoherent channel transmission. Even though the signal-to-noise ratio (SNR) is poor during the transmission, sufficient SNR can be obtained through the intrachannel four-wave mixing at the receiver. With this slim bandwidth transmission technique, the maximum number of spectrum-sliced wavelength-division-multiplexed channels can be increased greatly and the dispersion penalty can be reduced simultaneously.

Dual-energy digital mammography utilizing stimulated phosphor computed radiography

Dual-energy subtraction is a radiographic technique for the acquisition of a material selective image by the weighted subtraction of low- and high-energy digital X-ray images. This is achieved by exploiting the energy dependence of the X-ray attenuation components in the image. This can allow the removal of background morphology to enhance the presentation of otherwise obscured details. The detection of microcalcifications in a mammogram by dual-energy techniques has previously been investigated. These investigations indicated that, using dual-energy techniques, small microcalcifications could be extracted from the background breast morphology with sufficient signal to noise ratio (SNR) to be full visualized. The authors present the extension of a theoretical dual-energy model to incorporate practical considerations and then compare the results with experimentally derived data using a commercially available computed radiography system. In particular the extended model now takes into account the energy dependent detective quantum efficiency of a system. This is thought to be a major factor in reducing the efficiency of dual-energy mammography. The theoretical model predicts that dual-exposure dual-energy mammography, utilizing HRIII image plates, could not provide a detail SNR of five for calcifications smaller than 470 mu m. The experimental results verify this and indicate that dual-energy subtraction mammography, utilizing computed radiography, is currently not a viable technique for the detection of clinically significant microcalcifications. Further advances in X-ray image detector efficiency will be required if the full potential of this technique is to be achieved.

Use of consumable acoustic waveguide rods in continuous casting of steel

Electromagnetic acoustic transducers (EMATs) can emit and receive ultrasound on a conducting sample without contact, but are usually kept within 3 mm lift-off from the sample surface, to achieve a sufficient signal-to-noise ratio (SNR). There are scenarios under which EMATs must scan a sample at high speed, with the EMAT-sample separation varying by more than the standard lift-off range, such as for detection of gauge corner cracks in rail. A new EMAT has been designed that allows the low weight and flexible EMAT coil to skim over the sample surface, while the heavier and bulkier magnet behind the coil has a lift-off that can vary over 10 mm whilst still achieving a reasonable SNR. In experiments conducted with the EMATs mounted on a train, scanning a rail, they were demonstrated as being sufficiently robust, with an SNR sufficient for defect detection.

Fast signal acquisition by an adaptive antenna moving in a multipath field

An adaptive antenna algorithm is described which acquires a signal of unknown direction in the presence of uncorrelated interference and coherent multipath by using a sequential power search procedure on a small number of stored vector samples of the field. A set of candidate weighting vectors is generated, one of which, when used to filter the antenna circuit, will usually produce the desired signal at a sufficient signal-tonoise ratio for communications. The system is tested by simulation on a mobile receiver at 1 GHz in heavy multipath conditions with several interference sources and the effects of motion and bandwidth are assessed.

Remote sensing of precipitation by a satellite-borne microwave remote sensor

The problem of remote sensing of precipitation by a satellite-borne microwave rain scatterometer is discussed. A downward-looking scanning pencil-beam antenna system is used. The combination of the range-gate method and low side lobe level is used to separate echoes from precipitation layers in the main lobe from ground clutter in the side lobes. Various parameters of the satellite-borne microwave rain scatterometer are calculated and characteristics of systems at 10 and 34.45 GHz are considered. The transmitter peak power needed to observe precipitation with sufficient signal-to noise ratio is calculated by means of the radar equation. The signal (i.e. the received power from the resolution volume of the precipitation) and the received power due to the ground clutter are calculated and the signal-to-clutter ratio is obtained by applying the radar equation. An airborne microwave rain scatterometer is proposed for preliminary experiments.