Direct Attenuation Measurements Research Articles

Technical note: A PET/MR coil with an integrated, orbiting 511keV transmission source for PET/MR imaging validated in an animal study.

BackgroundMR‐based methods for attenuation correction (AC) in PET/MRI either neglect attenuation of bone, or use MR‐signal derived information about bone, which leads to a bias in quantification of tracer uptake in PET. In a previous study, we presented a PET/MRI specific MR coil with an integrated transmission source (TX) system allowing for direct measurement of attenuation. In phantom measurements, this system successfully reproduced the linear attenuation coefficient of water.PurposeThe purpose of this study is to validate the TX system in a clinical setting using animals and to show its applicability compared to standard clinical methods.MethodsAs test subject, a 15‐kg piglet was injected with 53 MBq of 18F‐NaF. The μ‐map obtained with the TX system and the reconstructed activity distribution were compared to four established AC methods: a Dixon sequence, an ultra‐short echo time (UTE) sequence, a CT scan, and a 511 keV transmission scan using a Siemens ECAT EXACT HR+ as the reference. The PET/MRI measurements were performed on a Siemens Biograph mMR to obtain the μ‐map using the TX system as well as the Dixon and UTE sequence directly followed by the CT and ECAT measurements.ResultsThe reconstructed activity distribution using the TX system for AC showed similar results compared to the reference (<5% difference in hot regions) and outperformed the MR‐based methods as implemented in the PET/MRI system (<10% difference in hot regions). However, the additional hardware of the TX system adds complexity to the acquisition process.ConclusionOur porcine study demonstrates the feasibility of post‐injection transmission scans using the developed TX system in a clinical setting. This makes it a useful tool for PET/MRI in cases where transmission information is needed for AC. Potential applications are studies using larger animals where state‐of‐the‐art atlas‐based or artificial intelligence AC methods are not available.

Source spectral studies using Lg wave in western Tibet

We investigate the spectral characteristics of Lg wave using 346 waveforms recorded at 24 seismic stations located across Karakoram Fault (KKF) in western Tibet. The tectonic structure of western Tibet is very complex and formed of several blocks, which are separated by distinct suture zones. It is predominated by KKF which is one of the major strike-slip faults in western Tibet. To understand the spatial variation in crustal properties, we subdivide the study region into two parts across the KKF. The analysis of the decay of the spectral amplitudes of Lg waves with epicentral distances allows for the evaluation of the crustal quality factor QLg, which is a direct measurement of attenuation. We have observed a frequency-dependent QLg with strong attenuation in the crust though no variation is observed across the KKF. This is consistent with the reported body wave attenuation study for this region. The corner frequency (fc) is observed to decrease linearly with distance in both sides of the KKF with an effective shift of 2.5 to 1 Hz at 400 km and 1 to 0.7 Hz near 1000 km respectively. The moment magnitude of each earthquake is computed using displacement spectrum and subsequently compared with the reported magnitude.

On the issue of combined active-passive radio sounding of precipitation on radio-occultation paths

The present study is a continuation of the earlier work on monitoring the atmospheric meteoparameters by the radio-occultation method. The possibility of identification of rains and detection of the rain-fall signature is investigated. It is shown that identification of rains and dry snowfalls with the use of activepassive (AP) sounding can be performed by direct measurements of attenuation by the Bouguer method and radiometric method. Radiophysical rainfall models for estimating attenuation and scattering of radio waves by rain are proposed. It is proposed to increase the informativity of the solution to the problem of precipitation identification through improvement of the new AP method for sounding precipitation on radio-occultation paths. It is shown that there exists a functional dependence between attenuation factor k (active sounding) and scattering factor α (passive sounding).

Dosimetric measurements of Onyx embolization material for stereotactic radiosurgery

Arteriovenous malformations are often treated with a combination of embolization and stereotactic radiosurgery. Concern has been expressed in the past regarding the dosimetric properties of materials used in embolization and the effects that the introduction of these materials into the brain may have on the quality of the radiosurgery plan. To quantify these effects, the authors have taken large volumes of Onyx 34 and Onyx 18 (ethylene-vinyl alcohol copolymer doped with tantalum) and measured the attenuation and interface effects of these embolization materials. The manufacturer provided large cured volumes (∼28 cc) of both Onyx materials. These samples were 8.5 cm in diameter with a nominal thickness of 5 mm. The samples were placed on a block tray above a stack of solid water with an Attix chamber at a depth of 5 cm within the stack. The Attix chamber was used to measure the attenuation. These measurements were made for both 6 and 16 MV beams. Placing the sample directly on the solid water stack and varying the thickness of solid water between the sample and the Attix chamber measured the interface effects. The computed tomography (CT) numbers for bulk material were measured in a phantom using a wide bore CT scanner. The transmission through the Onyx materials relative to solid water was approximately 98% and 97% for 16 and 6 MV beams, respectively. The interface effect shows an enhancement of approximately 2% and 1% downstream for 16 and 6 MV beams. CT numbers of approximately 2600-3000 were measured for both materials, which corresponded to an apparent relative electron density (RED) ρ(e) (w) to water of approximately 2.7-2.9 if calculated from the commissioning data of the CT scanner. We performed direct measurements of attenuation and interface effects of Onyx 34 and Onyx 18 embolization materials with large samples. The introduction of embolization materials affects the dose distribution of a MV therapeutic beam, but should be of negligible consequence for effective thicknesses of less than 8 mm. The measured interface effects are also small, particularly at 6 MV. Large areas of high-density artifacts and low-density artifacts can cause errors in dose calculations and need to be identified and resolved during planning.

Measurement of the linear attenuation coefficients of breast tissues by synchrotron radiation computed tomography

The measurement of the linear attenuation coefficients of breast tissues is of fundamental importance in the field of breast x-ray diagnostic imaging. Different groups have evaluated the linear attenuation coefficients of breast tissues by carrying out direct attenuation measurements in which the specimens were thin and selected as homogeneous as possible. Here, we use monochromatic and high-intensity synchrotron radiation computed tomography (SR CT) to evaluate the linear attenuation coefficients of surgical breast tissues in the energy range from 15 to 26.5 keV. X-ray detection is performed by a custom digital silicon micro-strip device, developed in the framework of the PICASSO INFN experiment. Twenty-three human surgical breast samples were selected for SR CT and histological study. Six of them underwent CT, both as fresh tissue and after formalin fixation, while the remaining 17 were imaged only as formalin-fixed tissues. Our results for fat and fibrous tissues are in good agreement with the published values. However, in contrast to the published data, our measurements show no significant differences between fibrous and tumor tissues. Moreover, our results for fresh and formalin-fixed tissues demonstrate a reduction of the linear attenuation coefficient for fibrous and tumor tissues after fixation.

ARIMA Modeling of Tropical Rain Attenuation on a Short 28-GHz Terrestrial Link

This letter reports the result of modeling of the tropical rain attenuation at 28 GHz adopting the auto-regressive integrated moving average (ARIMA) model. The result obtained is useful for the evaluation of transmission system design in radio communications at millimeter-wave frequencies in tropical areas. In this research, radio power measurement on a 56.4-m link at 28 GHz was carried out in Surabaya, Indonesia, with a data acquisition system that recorded a sample once every second. Approximation of the data by ARIMA(p,d,q) models for every rain event was carried out in order to obtain a valid time series model. In the validation, comparisons were made of the distributions of attenuation from the models against those from direct measurement of attenuation and from estimation based on the synthetic storm method. Comparisons were also made of the attenuation slope distributions against that from the measurement. Each of rain attenuation time series obtained from events in February 2009 was found to be well approached by the ARIMA model with various sets of parameters (p,d,q). The best model was found to be ARIMA (0, 1, 1) , as indicated by the Akaike Information Criteria (AIC) and Schwarz Bayesian Criteria (SBC) test results. A procedure for generating rain events based on the model is presented.

Use and effectiveness of hearing protection in construction

This paper presents an analysis of data on noise exposure and hearing protection device (HPD) use among construction workers drawn from several large datasets covering nine construction trades. A unique combination of one-minute dosimetry noise exposure level and simultaneous self-reported HPD use data were evaluated, as were occupational and non-occupational HPD use data collected by questionnaire as part of a longitudinal noise and hearing loss study among apprentices. Direct HPD attenuation measurements were also made on workers at their worksite. Workers reported using HPDs less than one-quarter of the time they were exposed above 85 dBA, the NIOSH Recommended Exposure Limit. Workers who reported always using HPDs in high noise were found to wear them only one-third of the time their exposures exceeded 85 dBA. Direct attenuation measurements indicated that workers achieved more than 50% of the rated attenuation of their HPD on average, but that the variability in achieved attenuation was large. Lastly, when the measured HPD attenuation level and use time data were combined, the effective protection afforded by HPDs was less than 3 dB on average, though there was substantial variation among the different trades. These results demonstrate the need for better hearing conservation programs in construction.

The Effectiveness of Hearing Protection Among Construction Workers

Effective hearing conservation programs in the construction industry are rare. Where programs are present, they often rely on workers' use of hearing protection devices (HPDs) rather than on exposure controls to reduce noise exposure levels. Dependence on HPDs for protection from high noise is problematic, as the protection provided by the HPD depends on both the HPD's attenuation level and the time the HPD is used. This article presents an analysis of data on noise exposure and hearing protection among construction workers drawn from several large datasets covering nine construction trades. A unique combination of 1-min dosimetry noise exposure levels and simultaneous self-reported use of HPDs was evaluated, as were occupational and nonoccupational HPD use data collected by questionnaire as part of a longitudinal study of noise exposure and hearing loss among apprentices. Direct measurements of HPD attenuation were also made on workers at their work site. The workers assessed in this study were found to use hearing protection less than one-quarter of the time that they were exposed above 85 dBA. Workers who reported “always” using HPDs in high noise on questionnaires were found to wear them only one-third of the time their exposures exceeded 85 dBA. Workers' self-reported use of HPDs during most noisy nonoccupational activities was also found to be low. Direct attenuation measurements found that workers were able to achieve more than 50% of the rated attenuation of their HPD on average, but that the variability in achieved attenuation was large. When the measured HPD attenuation levels and use time data were combined, the effective protection afforded by HPDs was less than 3 dB, a negligible amount given the high exposure levels associated with construction work. However, there was substantial variation in effective protection among the different trades assessed. These results demonstrate the need for better hearing conservation programs and expanded noise control efforts in the construction industry.

Critical Review of Through-Wall Attenuation of Mechanical Properties in RPV Steels

Abstract This paper reviews the current state of knowledge on attenuation of damage parameters in reactor pressure vessels (RPVs). There are two methods for evaluating attenuation of properties through a reactor pressure vessel wall. The first is by direct measurement of the change in mechanical properties from decommissioned RPV sections or from simulated RPV wall experiments. It is shown that, although this approach is appealing, issues associated with knowledge of the start-of-life properties throughout the vessel wall sample and material property data scatter have made past measurements non-definitive in establishing attenuation changes. There is a need for further data on the direct measurement of attenuation, and an experiment is described, that is planned in 2002 under IAEA sponsorship. An alternative method for evaluating embrittlement is the use of a neutron damage exposure parameter and attenuation model coupled with an embrittlement correlation developed from surveillance capsule testing. The significant change in neutron flux spectrum when neutrons are attenuated through the RPV wall defines the need for a suitable neutron exposure damage parameter. The best available neutron exposure damage parameter is dpa. It is shown that plant-specific calculation of dpa through the RPV wall is the best method to be used for the neutron exposure. However, it can lead to slightly less attenuated values for damage at ¼-T and ¾-T for the vessel, as compared to using the simple exponential model quoted in Regulatory Guide 1.99, Rev. 2. Finally it is concluded that when using a surveillance correlation model to predict the attenuation of mechanical properties through the RPV wall, the use of a mechanistically guided model appears to be more appropriate than the embrittlement correlation provided in Regulatory Guide 1.99, Revision 2.

Radiometer‐based calibration of the relation between radar reflectivity and microwave attenuation

The use of higher and higher frequencies in telecommunication systems requires the investigation of the effects of troposphere on electromagnetic wave propagation, and particular attention must be paid to the attenuation induced by raindrops along the radio path. For this purpose the direct measurements approach, in many cases, is not feasible or convenient, and the use of indirect measuring technique is now a common practice. In this respect, meteorological radar operating at nonattenuating frequencies can be of great help owing to their ability in collecting data over wide areas in a short time. The drawback is the incomplete knowledge of the relation between radar reflectivity (Z) and the quantity of direct interest so that an external source of “calibration” is required. In this paper we show that radiometers are proper instruments to calibrate single parameter radar and specifically to derive, on event basis, the relation to convert Z into specific attenuation (α). For this purpose, brightness temperatures at 13 GHz collected by a radiometer along the 30.6° slant path toward the Olympus satellite, direct attenuation measurements, and radar reflectivity profiles have been jointly worked out. First, the choice of the average effective medium temperature to estimate attenuation from brightness measurements has been addressed. Second, the best α ‐Z relation on an event basis has been evaluated through a best fit procedure that forced the radar predicted attenuation to agree with radiometer “measured” ones. The results are very encouraging and confirm the possibility to make use of radiometers as an external source of radar calibration.

Shielding of pulsed laser radiation in light-erosion burner of metallic and dielectric targets

On irradiation of matrials by laser radiation (LR) with power density 10s-107 W/cm 2 a light-erosion burner arises, in which absorption and scattering of the initial IR occurs [1-3]. An important problem in investigation of the passage of LR in the light-erosion burner is measurement of the dynamics of the shielding properties of the light erosion burner and autovibration processes of shielding possible here [I, 4]. Previously measurements were performed of attenuation of LR in a burner; for example, Byk et al. [2] and Basov et al. [5] probed a laser plasma by an additional laser beam of low intensity in the direction perpendicular to the axis of the burner. However, the results of these investigations are difficult to interpret for determination of the attenuation of LR along the burner axis, which is the most essential for investigation of questions of self-shielding. Uglov et al., [i], Walters et al., [3], Dymshits et al. [6], and Rykalin et al. [7] performed direct measurements of attenuation of LR in a light erosion burner through a small aperture in the center of the irradiated metal target. However, the recording apparatus did not permit the authors of these investigations to perform a careful comparative analysis of the transmitted and initial LR, taking into account the complex shape of the laser pulse, and additional investigations are required on grounding of the passage of radiation through the narrow channel in the target, filled with products of erosion and with a changing geometry at high radiation intensities~

Determination of Rain Rate from a Spaceborne Radar Using Measurements of Total Attenuation

Several experimental and theoretical studies have shown that path-integrated rain rates can be determined by means of a direct measurement of attenuation. For ground-based radars, this is done by measuring the backscattering cross section of a fixed target in the presence and absence of rain along the radar beam. A ratio of the two measurements yields a factor proportional to the attenuation from which the average rain rate can be deduced. In this paper, we extend the technique to spaceborne radars by choosing the ground as a reference target. The technique is also generalized so that both the average and range-profiled rain rates can be determined. The accuracies of the resulting estimates are evaluated for a narrow-beam radar located on a low earth-orbiting satellite.

Prediction of attenuation due to rainfall on earth‐space links

A review is given of the current state of the art in the prediction of rain attenuation on slant paths. Different types of research effort are identified and discussed. A critical assessment of data analysis methods and theoretical models is made to identify areas where more effort is needed. Various types of data available are discussed (direct attenuation measurements, radiometric data, rain rate, and radar reflectivity) as well as the models and methods available to date to relate these to attenuation on a satellite path. Finally, prediction methods for worldwide use are reviewed. While great progress is being made in the physical modeling of the atmosphere, statistical prediction methods are still very general and coarse. In the future, the emphasis should be on the development of detailed models to predict the variability of attenuation statistics within a coverage area. In this connection there is a need for coordination in the comparison of experimental data. In Europe, projects of international cooperation for this purpose are being carried out.

Use of dual-polarization radar data for evaluation of attenuation on a satellite-to-earth path

Using conventional (single-polarization) radar data to assess attenuation due to rain on satellite-to-Earth paths requires the assumptions that the drops have a particular statistical distribution of sizes, and that the drops are of water. In the very localised regions of intense rain which influence such paths most severely, these assumptions can lead to large errors. This paper presents comparisons between direct measurements of attenuation and those computed using data from a dual-polarization radar technique. This technique can considerably reduce the errors of radar-derived estimates of attenuation. It also gives a clear distinction between ice and water.

Millimeter wave attenuation prediction using a piecewise uniform rain rate model

A piecewise uniform rain rate distribution model is introduced as a quasi-physical model of real rain along earth-space millimeter wave propagation paths. It permits calculation of the total attenuation from specific attenuation in a simple fashion. The model predications are verified by comparison with direct attenuation measurements for several frequencies, elevation angles, and locations. Also, coupled with the Rice-- Holmberg rain rate model, attenuation statistics are predicated from rainfall accumulation data.

Precipitation attenuation studies based on measurements of ATS-6 20/30-GHz beacon signals at Clarksburg, MD

COMSAT Laboratories participated in the millimeter wavelength propagation experiment under NASA contract NAS5- 20740, performing measurements of the 20/30-GHz ATS-6 satellite beacon signals and auxiliary measurements such as radiometric sky temperature and minute precipitation at Clarksburg, MD. These measurements were intended to broaden the data base required to advance the understanding of the propagation characteristics of the earth-satellite path at frequencies over 10 GHz. A correlation is established between direct measurement of attenuation of the signal radiated from the satellite and the indirect measurement of attenuation by auxiliary ground-based equipment. The indirectly derived statistics agreed reasonably well or can be reconciled with earlier published results and may therefore be used as a basis for estimating long-term cumulative attenuation statistics.

Survey of measurements of attenuation by rain and other hydrometeors

This paper surveys the most recently reported attenuation measurements at microwave and millimetric frequencies on terrestrial and satellite (slant) radio paths. Greatest attention is given to direct measurements of attenuation, but radar and radiometric techniques are also considered. Measurements relating fading to the various meteorological parameters are reviewed as well as those presenting statistical information for the system designer. From the summary of all available statistical measurements of greater than one year duration, the variation of fade depth (exceeded for 0.01% of the time) with path length is characterised for 11 and 18 GHz on terrestrial paths. The accuracy of radar and radiometric measurements on satellite paths is also discussed, and related where possible to direct measurements with satellite beacons.

X-Band Attenuation and Liquid Water Content Estimation by a Dual-Wavelength Radar

Abstract Direct measurements of attenuation due to liquid water content (LWC) between two points in the common beam of a dual-wavelength radar system will be possible using a real-time digital processor of radar signals presently under construction. Precise measurements of attenuation are possible since accurate absolute calibration of the radars is not required. A resolution of 0.65 dB km−1 is possible in contiguous kilometer sections along the beam axis with an average over 32 independent values of echo power. In terms of LWC this is a resolution of ∼2 mg m−3 of precipitation using a new relation M = 2.23A0.787where M is the liquid water content (gm m−2) and A the attenuation rate (dB km−1) along the common beam.

Sun Tracker Measurements of Attenuation by Rain at 16 and 30 GHz

This paper describes an instrument for measuring attenuation statistics on an earth-space path simultaneously at 16 and 30 GHz; the high attenuations result from heavy rain. The sun is used as a signal source during the time of day when the sun ordinarily is visible; a measuring range of more than 30 dB is achieved at both frequencies. The brightness temperature of the atmosphere also is measured both day and night. At night the antenna beam is stationary on the local meridian. Daytime brightness temperatures in conjunction with direct attenuation measurements are used to determine the equivalent absorber temperatures which are necessary for the reduction of night brightness temperatures to attenuation values. This paper discusses the measurements made during the first 12 months of operation and gives statistics of these measurements and an analysis of errors in the system.

An experimental investigation of the propagation of a sonic pulse along the surface of a semi-infinite medium

The waveform of a sonic pulse along the surface of a semi-infinite medium is investigated for short distances from the source. The velocities of the various modes are compared and used to deduce the elastic constants of the medium. The breadth of the Rayleigh wavelets is used to verify the wavelet theory and deduce the law of absorption of the medium. Direct attenuation measurements are somewhat scattered and do not agree exactly with wavelet breadth results.