Effective Depth Of Measurement Research Articles

Three-Dimensional Structure Reconstruction System of Nasal Cavity Based on a Short-Source Acoustic Tube

The nasal cavity three-dimensional structure reconstruction is significant for the diagnosis and treatment of nasal and nasopharynx diseases. Currently, most systems for nasal cavity three-dimensional structure reconstruction are based on a long-source acoustic tube. However, the long-source acoustic tube has the weaknesses of low portability and high cost. To address these issues, a nasal cavity three-dimensional structure reconstruction system based on a short-source acoustic tube was designed in this research. The designed system comprises the lower computer of a nasal acoustic signal acquisition device and the upper computer of nasal acoustic signal analysis software. The lower computer of a nasal acoustic signal acquisition device consists of a signal processing unit, a detection unit, a microcontroller, and a USB communication interface. It is used to collect the original acoustic signal in the nasal cavity. The upper computer of nasal acoustic signal analysis software consists of the nasal cavity acoustic signal preprocessing method and the nasal cavity three-dimensional structure hierarchical reconstruction method. The nasal cavity acoustic signal preprocessing method is used to eliminate the DC offset component and suppress the high-frequency noise. Then, the nasal cavity three-dimensional structure hierarchical reconstruction method is used to establish the relationship between the cross-sectional area of the nasal cavity and the depth of the nasal cavity. In order to verify the effectiveness of the system designed in this research, the straight tube model and the simulated nasal cavity model were selected as test subjects, and the depth–cross-sectional area was selected as the evaluation indicator of the accuracy of the nasal cavity three-dimensional structure reconstruction result. The experimental results show that the root mean squared error and the mean relative error of the depth–cross-sectional area were reduced from 0.6284 cm2 to 0.0201 cm2 and 47.6467% to 1.8248%, respectively, under the straight tube model by using the nasal cavity acoustic signal preprocessing method. The root mean squared error and the mean relative error of the depth–cross-sectional area were reduced from 4.7730 cm2 to 0.1510 cm2 and 107.8128% to 3.3096%, respectively, under the simulated nasal cavity model. Meanwhile, this system can increase the effective measurement depth of the nasal cavity from 7 cm to 13 cm based on the preprocessing method. The results demonstrated that the designed system can not only reconstruct and visualize a nasal cavity three-dimensional structure with high precision but also generate comprehensive test reports. Therefore, this system can provide a basis for further auxiliary diagnosis of the disease and facilitate doctors in explaining the physiological structure of the nasal cavity to patients.

Estimation of root zone soil moisture at point scale based on soil water measurements from cosmic-ray neutron sensing in a karst catchment

The cosmic-ray neutron sensing (CRNS) is an emerging method for continuously monitoring soil water content (SWC) at an intermediate scale. However, when multiple hydrologic units are present within its footprint, the potential application of CRNS in water resources management is restricted. Here we propose a new strategy to predict point-scale SWC in root zone established on CRNS-based soil moisture and improved relative difference method. A total of 768 days of soil moisture data were collected by CRNS at the intermediate scale and EC-TM sensors at the point scale in a karst catchment. The original and improved mean relative difference methods predicted point-scale SWCs within and without the effective measuring depth, respectively. The mean effective measuring depth was 13.16 cm, ranging from 10.13 to 19.23 cm. Both land use type and soil structure played essential roles in regulating point scale SWC in the soil profile. Point-scale SWC in root zone can be predicted accurately (P < 0.001) based on SWC data derived from the CRNS system. The prediction accuracy of point scale SWC can be improved by increasing the averaging time of the soil moisture values. Our results demonstrated that the proposed strategy was reliable for CRNS to predict SWC beyond the effective measurement depth. This study provides a good perspective for effectively managing of water resources in areas with complex hydrological processes.

Effects of local forearm skin heating on skin properties

This study investigated impacts of local skin heating on skin properties and tested whether skin changes depended on heat-induced hyperaemia. It was reasoned that heat-induced vasodilation impacts accompanying interstitial fluid changes. Forearm skin was locally heated from a baseline of 35°C to 40-42°C in 30 young adults (15 females, 15 males, 24.9±2.1years) and non-heated in 10 others (5 females, 5 males, 25.2±1.3years. Skin blood flow (SBF) was continuously measured using a laser Doppler method and skin tissue dielectric constant (TDC), stratum corneum capacitance (SCC) and transepidermal water loss (TEWL) were measured before and after maintained heat for 12min. TDC values were determined to effective measurement depths of 1.5mm (TDC15) and 2.5mm (TDC25). Results showed a large heat-induced hyperaemia, with SBF increasing on average 8.8-fold from its baseline of 35°C. Heating also caused significant increases in TDC, SCC and TEWL that, compared to preheating, increased approximately 1.1-fold, 3.1-fold and 4.5-fold. None of these skin changes correlated with the magnitude of the SBF hyperaemic response. Absence of this correlation may indicate that in young healthy adults, increased capillary filtration due to heat-induced arteriolar vasodilation is rapidly accommodated by postcapillary reabsorption, enhanced lymphatic activity and TEWL processes. An alternate explanation is that heating caused increased red cell oscillations that were detected as part of the laser Doppler increase without representing increased capillary flux. The major determinant of the Increases in TDC, SCC and TEWL is likely a consequence of heat-induced eccrine gland activation. Studies of older persons or those with depressed function are warranted.

Design of Bio-Impedance Electrode Topologies for Specific Depth Sensing in Skin Layer.

Bio-impedance analysis provides non-invasive estimation of body composition. Recently, applications based on bio-impedance measurement in skin tissue such as skin cancer diagnosis and skin composition monitoring have been studied. For scanning the electrical properties along the skin depth, the relationship between the electrode topologies and the depth sensitivity should be clarified. This work reports a systematic analysis on designing line electrode topologies to measure the bio-impedance of the skin layer at specific depth using a finite element method (FEM). Four electrodes consisting of two outer current electrodes and two inner voltage electrodes in the form of Wenner-Schlumberger array were employed on the top of a collagen layer as a skin model. The numerical results demonstrate a change in the effective depth of measurement depending on the electrode topologies, which also have a good agreement with an analytic solution. This study suggests a decision guideline for designing the electrode topologies to achieve target depth sensitivity in bio-impedance measurement of skin tissue.Clinical Relevance-This establishes the effect of electrode topologies on depth sensitivity in bio-impedance measurements in skin layer.

Heat-related changes in skin tissue dielectric constant (TDC).

The impact of 20min of whole-body heating (WBH) on the tissue dielectric constant (TDC) of forearm and hand skin was evaluated in 24 young adults. TDC was measured in triplicate at 300MHz using an open-ended transmission line method in which the effective measurement depth was about 2mm. TDC measurements are an effective way to assess and track localized oedema and lymphoedema. The underlying hypothesis was that heat-induced eccrine gland activation would increase TDC values via an increase in fluid within the TDC measurement volume. The goal was to test this concept and to determine the magnitude of the change when environmental temperatures were elevated to near 42°C and to estimate TDC recovery time. The practical aspect of this research is motivated by the fact that patients in whom such measurements are made may arrive at the clinic in various states of sweat gland activation. Thus, knowledge of the effect of such activation on measured TDC values permits better understanding of possible relationships between such activation and TDC values. Results showed that increasing environmental temperature from 23·3±1·6 to 41·5±1·3°C increased forearm and thenar eminence skin temperatures to 37·8±0·5 and 37·9±0·4°C, respectively. These changes were associated with increases in TDC at arm from 30·7±4·6 to 36·3±5·7 (18·2%) and at hand from 34·7±4·9 to 45·1±5·5 (30%). Based on calculated TDC recovery rates, it is concluded that temperature-related TDC variability can be minimized using a wait time of at least 15min after bandage removal prior to TDC measurements in affected limbs.

Proton beam electron return effect: Monte Carlo simulations and experimental verification

Proton therapy (PT) is expected to benefit from integration with magnetic resonance (MR) imaging. However, the magnetic field distorts the dose distribution and enhances the dose at tissue-air interfaces by the electron return effect (ERE). The objectives were (a) to provide experimental evidence for the ERE in proton beams and (b) to systematically characterise the dependence of the dose enhancement ratio (DER) on magnetic field strength, orientation, proton energy and voxel size by computer simulations.EBT3 films were irradiated with 200 MeV protons with and without a 0.92 T transverse field of a permanent magnet to determine the DER at effective measurement depths of 0.156 and 0.467 mm from an air interface. High-resolution Monte Carlo simulations were performed to reproduce the irradiation experiments and to calculate the DER for proton energies between 50–200 MeV and magnetic field strengths between 0.35–3 T as function of distance from the air interface. Voxel sizes of 0.05, 0.5 and 1 mm were analysed.DERs of (2.2 ± 0.4)% and (0.5 ± 0.6)% were measured at 0.156 and 0.467 mm from the air interface, respectively. Measurements and simulations agreed within 0.15%. For a 200 MeV proton beam, the maximum DER in 0.05 mm voxels increased with magnetic field strength from 2.6% to 8.2% between 0.35 and 1.5 T, respectively. For a 1.0 T magnetic field, maximum DER increased from 3.2% to 7.6% between 50 and 200 MeV, respectively. Voxel sizes of 0.5 and 1 mm resulted in maximum DER values of 2.6% and 1.4%, respectively.The ERE for proton beams in transverse magnetic fields is measurable. The local dose enhancement is significant, well predictable, decreases rapidly with distance from the air interface, and is negligible beyond 1 mm depth. Its impact on air-filled ionisation chambers and porous tissues (e.g. lung) needs to be considered.

P153] A comparison of different dosimetry systems for surface and buildup region dose measurements

Purpose In this study, the surface doses obtained by optically stimulated luminescence (OSL), thermoluminescence (TLD) and markus parallel plane ion chamber were compared with TPS results and the availability of the systems in surface dosimetry was investigated. Methods The percentage depth dose (PDD) measurements of buildup region were made by TLD, OSL and markus pp ion chamber, at the surface, 1, 2, 5, 10 and 15 mm water equivalent depth at 100 cm source-detector distance for 5×5, 10×10, and 20×20 cm 2 field sizes.DD values were measured at the depth of 0.07 mm, which was suggested for surface dose measurements by ICRU Report 39. The effective measurement depth of each dosimeter was taken into account. The water equivalent thickness (WET) was considered to make an accurate comparison. Interpolations and extrapolations were performed for all systems to obtain the doses at the same depths. Under the same conditions, surface doses were calculated by TPS for different field sizes and were recorded. The buildup region doses measured by different dosimetry systems and calculated by TPS were compared for different field sizes. Results The overdoses occured in the buildup region were corrected for Markus chamber according to Gerbi’s method. The surface doses using 6 MV photon beams for 5×5, 10×10, and 20×20 cm 2 field sizes at 0.07 mm were found to be 14%, 24.75%, 13.17%, 7.74%; 19.69%, 31.78%, 21.63%, 11.04%; 30.87%, 43.80%, 28.71%, 13.61% for Markus chamber, TLD, OSL and TPS, respectively. Conclusions The lowest build-up region PDD values in all field sizes were obtained with the TPS. At 0.07 mm water equivalent depth, while the OSL and Markus pp. ion chamber results are close to each other, TLD results are higher in all field sizes. However, it is seen that the TLD gives closer results to Markus pp. ion chamber than OSL after 3 mm depth in all field sizes.

Do High-Frequency Radars Measure the Wave-Induced Stokes Drift?

ABSTRACTHigh-frequency (HF) radars remotely measure ocean near-surface currents based on the Doppler shift of electromagnetic waves backscattered by surface gravity waves with half the electromagnetic wavelength, called Bragg waves. Since their phase velocity is affected not only by wave–current interactions with vertically sheared mean Eulerian currents but also by wave–wave interactions with all the other waves present at the sea surface, HF radars should measure a quantity related to the Stokes drift in addition to mean Eulerian currents. However, the literature is inconsistent—both theoretically and experimentally—on the specific expression and even on the existence of the Stokes drift contribution to the HF radar measurements. Three different expressions that have been proposed in the literature are reviewed and discussed in light of the relevant published experimental results: 1) the weighted depth-averaged Stokes drift, 2) the filtered surface Stokes drift, and 3) half of the surface Stokes drift. Effective measurement depths for these three expressions are derived for the Phillips wave spectrum. Recent experimental results tend to discard the second expression but are not inconsistent with the first and third expressions. The latter is physically appealing, since it is a quasi-Eulerian quantity that would be measured by a current meter at a fixed horizontal position but allowed to follow the free surface moving vertically up and down with the passage of the waves. A definitive answer will require further experimental investigations.

Spatial soil sampling design using apparent soil electrical conductivity measurements

Soil sampling is an important stage of digital soil mapping. The objective of this study was to characterize the spatial design of soil sampling using soil apparent electrical conductivity (ECa) and its optimized spatial sampling. For the characterization, it was used the Spatial Simulated Annealing (SSA) technique, incorporated on the software SANOS 0.1, and the method of response surface sampling design on the software ESAP 2.35. The ECa was measured at 1,887 points in an area of 6 ha located in the northwestern region of Spain. The EM38-DD equipment (Geonics Limited 2005) was used at 2 depths: vertical dipole (1.5 m effective measurement depth) and horizontal dipole (0.75 m effective measurement depth). Semivariogram showed trend for ECa in vertical dipole (ECa-V) and ECa in horizontal dipole (ECa-H). Software SANOS 0.1 and ESAP 2.35 were used to obtain the 40-point sampling scheme, using the 2 schemes (SANOS and ESAP). ECa-V estimation values at the 1,887 points were calculated with residual ordinary kriging. The sampling scheme obtained from ESAP was better than with SANOS.

Soil water content determination with cosmic-ray neutron sensor: Correcting aboveground hydrogen effects with thermal/fast neutron ratio

Summary The cosmic-ray neutron sensor (CRNS), which estimates field scale soil water content, bridges the gap between point measurement and remote sensing. The accuracy of CRNS measurements, however, is affected by additional hydrogen pools (e.g., vegetation, snow, and rainfall interception). The objectives of this study are to: (i) evaluate the accuracy of CRNS estimates in a farmland system using depth and horizontal weighted point measurements, (ii) introduce a novel method for estimating the amounts of hydrogen from biomass and snow cover in CRNS data, and (iii) propose a simple approach for correcting the influences of aboveground hydrogen pool (expressed as aboveground water equivalent, AWE) on CRNS measurements. A field experiment was conducted in northeast China to compare soil water content results from CRNS to in-situ data with time domain reflectometry (TDR) and neutron probe (NP) in the 0–40 cm soil layers. The biomass water equivalent (BWE) and snow water equivalent (SWE) were observed to have separate linear relationships with the thermal/fast neutron ratio, and the dynamics of BWE and SWE were estimated correctly in the crop seasons and snow-covered seasons, respectively. A simple approach, which considered the AWE, AWE at calibration, and the effective measurement depth of CRNS, was introduced to correct the errors caused by BWE and SWE. After correction, the correlation coefficients between soil water contents determined by CRNS and TDR were 0.79 and 0.77 during the 2014 and 2015 crop seasons, respectively, and CRNS measurements had RMSEs of 0.028, 0.030, and 0.039 m 3 m −3 in the 2014 and 2015 crop seasons and the snow-covered seasons, respectively. The experimental results also indicated that the accuracies of CRNS estimated BWE and SWE were affected by the distributions of aboveground hydrogen pools, which were related to the height of the CRNS device above ground surface.

Surface and Buildup Region Dose Measurements with Markus Parallel-Plate Ionization Chamber, GafChromic EBT3 Film, and MOSFET Detector for High-Energy Photon Beams

The aim of the study was to investigate surface and buildup region doses for 6 MV and 15 MV photon beams using a Markus parallel-plate ionization chamber, GafChromic EBT3 film, and MOSFET detector for different field sizes and beam angles. The measurements were made in a water equivalent solid phantom at the surface and in the buildup region of the 6 MV and 15 MV photon beams at 100 cm source-detector distance for 5 × 5, 10 × 10, and 20 × 20 cm2field sizes and 0°, 30°, 60°, and 80° beam angles. The surface doses using 6 MV photon beams for 10 × 10 cm2field size were found to be 20.3%, 18.8%, and 25.5% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface doses using 15 MV photon beams for 10 × 10 cm2field size were found to be 14.9%, 13.4%, and 16.4% for Markus chamber, EBT3 film, and MOSFET detector, respectively. The surface dose increased with field size for all dosimeters. As the angle of the incident radiation beam became more oblique, the surface dose increased. The effective measurement depths of dosimeters vary; thus, the results of the measurements could be different. This issue can lead to mistakes at surface and buildup dosimetry and must be taken into account.

Correction of systematic model forcing bias of CLM using assimilation of cosmic-ray Neutrons and land surface temperature: a study in the Heihe Catchment, China

Abstract. The recent development of the non-invasive cosmic-ray soil moisture sensing technique fills the gap between point-scale soil moisture measurements and regional-scale soil moisture measurements by remote sensing. A cosmic-ray probe measures soil moisture for a footprint with a diameter of ~ 600 m (at sea level) and with an effective measurement depth between 12 and 76 cm, depending on the soil humidity. In this study, it was tested whether neutron counts also allow correcting for a systematic error in the model forcings. A lack of water management data often causes systematic input errors to land surface models. Here, the assimilation procedure was tested for an irrigated corn field (Heihe Watershed Allied Telemetry Experimental Research – HiWATER, 2012) where no irrigation data were available as model input although for the area a significant amount of water was irrigated. In the study, the measured cosmic-ray neutron counts and Moderate-Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) products were jointly assimilated into the Community Land Model (CLM) with the local ensemble transform Kalman filter. Different data assimilation scenarios were evaluated, with assimilation of LST and/or cosmic-ray neutron counts, and possibly parameter estimation of leaf area index (LAI). The results show that the direct assimilation of cosmic-ray neutron counts can improve the soil moisture and evapotranspiration (ET) estimation significantly, correcting for lack of information on irrigation amounts. The joint assimilation of neutron counts and LST could improve further the ET estimation, but the information content of neutron counts exceeded the one of LST. Additional improvement was achieved by calibrating LAI, which after calibration was also closer to independent field measurements. It was concluded that assimilation of neutron counts was useful for ET and soil moisture estimation even if the model has a systematic bias like neglecting irrigation. However, also the assimilation of LST helped to correct the systematic model bias introduced by neglecting irrigation and LST could be used to update soil moisture with state augmentation.

SU-E-T-79: Study On the Effective Depth of Measurement for Gafchromic EBT2 and EBT3 Films

Purpose: To determine the effective depth of measurement for Gafchromic EBT2 and EBT3 filmsMethods: Absolute dose point measurements were made with EBT2 and EBT3 film on the surface of flat phantom and at various depths. The films were cut to an approx. 1–2 mm square size and placed on the beam central axis. Measurements with an Exradin A1SL ion chamber were performed for each irradiation. A dose of 1 Gy was delivered to Dmax=1.5 cm. A previously validated Monte Carlo (MC) model was used for simulation and comparison to measurements. The MC simulation data was collected at 0.1 mm steps along the beam central axis. Film measurements were converted to dose using a post-irradiation signal consisting of the red and blue color channels from an Epson Expression 10000XL scanner. Film analysis was performed using ImageJ software and MATLAB. Measurements were converted to PDD and then compared to MC data to determine an effective depth of film measurement. Results: EBT2 measurements showed a systematic upstream shift of 0.10 mm with respect to the MC data while EBT3 showed the shift to be slightly less at 0.07 mm. The error in film measurement was estimated to be 0.04 mm (2σ). All measurements were within one STD of the MC error of 0.33 mm. Conclusion: The PDD measurements of EBT2 and EBT3 were consistent amongst themselves and agreement with MC was excellent. The observed shift in the results and the measurement error are smaller than the error in MC data, a possible indication that the nominal depth of measurement can be considered as the effective depth of measurement. A study with more accurate geometry for comparison is needed to determine a conclusive depth of measurement.

Re-evaluation of correction factors of a primary standard graphite calorimeter in 60Co gamma ray beams as a basis for the appointment of the BEV absorbed dose rate to water reference value

The graphite calorimeter of the Federal Office of Metrology and Surveying (BEV-Bundesamt für Eich- und Vermessungswesen) was established in the 1980s as the primary standard for the absorbed dose to water for (60)Co gamma ray beams. To maintain the primary standard at an international level the graphite calorimeter and its corresponding components had to undergo a refurbishment and modernisation process. The correction factors of the graphite calorimeter were re-evaluated with Monte Carlo and experimental methods to obtain improved values. These are the correction for the effect of the gaps (1.0061), the scaling correction (0.9998), the correction for the difference in air attenuation (0.9971) and the corrections for the effective measurement depths in the graphite phantom for the graphite calorimeter (0.9886) and the CC01-105 ionisation chamber (0.9913). Consequently, it was necessary to change the reference value for the absorbed dose rate to water of the (60)Co teletherapy unit used for the calibration of secondary standard dosemeters.

Spatial variations in forearm skin tissue dielectric constant

Tissue dielectric constant (TDC) values measured at 300MHz via the open-ended coaxial line reflection method depend on the effective measurement depth and the anatomical site being evaluated. Measurements on the forearm have shown that the TDC values decrease with increasing measurement depth but the spatial variability of the TDC values among forearm anatomical positions is unknown. Our goal was to characterize the extent of such spatial variations. In 30 healthy seated women (27.4±6.5 years), TDC was measured on the forearm midline and 1.2cm medial and lateral to the midline at sites 4, 8 and 12cm distal to the antecubital crease. The midline and medial TDC values increased progressively from 4 to 8 to 12cm sites (P<0.001), with the largest spatial gradient along the midline. At a depth of 2.5mm, the TDC values increased from 26.3±2.8 to 27.4±3.4 to 28.4±3.7, with a maximum difference of 8.2±10.6%. For all sites, the TDC values were significantly (P<0.001) less for increasing depths. The findings reveal increased TDC values along the forearm from proximal to distal, most prominent at the midline and medial positions. Because many skin-related dermatological and biophysical studies utilize the forearm as a test target, such differences may be important to consider because TDC values in part are reflective of local tissue water (LTW). Although the variation in the TDC values among sites was less than 10%, such differences are of importance when evaluating LTW changes using the TDC method in patients with arm lymphedema that is present in variable arm anatomical locations.

Male–female differences in forearm skin tissue dielectric constant

Tissue dielectric constant (TDC) measurements at 300 MHz via the coaxial line reflection method are useful to evaluate local skin tissue water and its change, but virtually all available data relate to measurements on women. Because TDC values in part depend on skin thickness, we hypothesized that differences in male-female skin may be associated with male-female differences in TDC. To test this hypothesis, we compared TDC values in volar forearm skin of 60 young adult volunteers (30 men, 25.0 +/- 2.5 years, 30 women, 27.4 +/- 6.6 years) in the seated position using a probe with an effective measurement depth of 1.5 mm. Results showed that TDC values (mean +/- SD) for men were significantly greater than for women (33.2 +/- 4.0 versus 29.4 +/- 2.7, P<0.001) constituting an overall difference of about 13%. This finding suggests that when TDC measurements are used in research or clinical studies in which both men and women are included in a common study population, it would be prudent to consider this difference in both experimental design and data interpretation. This is especially true if absolute TDC values are of interest in contrast to changes in TDC values on the same subject subsequent to time passage or secondary to an intervention. Despite greater TDC values measured in men, calculations of the impact of a greater male skin thickness indicate that the greater TDC values of men may or may not reflect a greater relative local skin tissue water in men compared to women.

Evaluation of two‐dimensional bolus effect of immobilization/support devices on skin doses: A radiochromic EBT film dosimetry study in phantom

In this study, the authors have quantified the two-dimensional (2D) perspective of skin dose increase using EBT film dosimetry in phantom in the presence of patient immobilization devices during conventional and IMRT treatments. For 6 MV conventional photon field, the authors evaluated and quantified the 2D bolus effect on skin doses for six different common patient immobilization/support devices, including carbon fiber grid with Mylar sheet, Orfit carbon fiber base plate, balsa wood board, Styrofoam, perforated AquaPlast sheet, and alpha-cradle. For 6 and 15 MV IMRT fields, a stack of two film layers positioned above a solid phantom was exposed at the air interface or in the presence of a patient alpha-cradle. All the films were scanned and the pixel values were converted to doses based on an established calibration curve. The authors determined the 2D skin dose distributions, isodose curves, and cross-sectional profiles at the surface layers with or without the immobilization/support device. The authors also generated and compared the dose area histograms (DAHs) and dose area products from the 2D skin dose distributions. In contrast with 20% relative dose [(RD) dose relative to dmax on central axis] at 0.0153 cm in the film layer for 6 MV 10 x 10 cm2 open field, the average RDs at the same depth in the film layer were 71%, 69%, 55%, and 57% for Orfit, balsa wood, Styrofoam, and alpha-cradle, respectively. At the same depth, the RDs were 54% under a strut and 26% between neighboring struts of a carbon fiber grid with Mylar sheet, and between 34% and 56% for stretched perforated AquaPlast sheet. In the presence of the alpha-cradle for the 6 MV (15 MV) IMRT fields, the hot spot doses at the effective measurement depths of 0.0153 and 0.0459 cm were 140% and 150%, (83% and 89%), respectively, of the isocenter dose. The enhancement factor was defined as the ratio of a given DAH parameter (minimum dose received in a given area) with and without the support device. For 6 MV conventional 10 x 10 cm2 field, the enhancement factor was the highest (3.4) for the Orfit carbon fiber plate. As for the IMRT field, the enhancement factors varied with the size of the area of interest and were as high as 3.8 (4.3) at the hot spot of 5 cm2 area in the top film layer (0.0153 cm) for 6 MV (15 MV) beams. Significant 2D bolus effect on skin dose in the presence of patient support and immobilization devices was confirmed and quantified with EBT film dosimetry. Furthermore, the EBT film has potential application for in vivo monitoring of the 2D skin dose distributions during patient treatments.

SU-GG-T-190: In-Vivo Surface Dosimetry With An Optically Stimulated Luminescence Dosimeter

Purpose: We investigate the use of a new optically stimulated luminescencedosimeter (OSLD) for surface dosimetry.Method and Materials: OSLDs are small plastic disks (5mm diameter × 0.2mm thick) infused with aluminum oxide doped with carbon ( Al 2 O 3 : C ) . Their operation is similar to TLDs; the irradiated crystals yield a signal proportional to absorbed dose upon optical stimulation. The detectors are housed in a light‐tight plastic casing measuring 24 × 10 × 1‐mm. A dose‐response curve for 6MV photons from Varian 21Ex was established. OSLDs were next used to measure surface/buildup doses in a plastic water phantom. Three field sizes were considered: 10×10cm, 5×5cm and 2×2cm and results compared with a parallel plate ion chamber. Clinical performance of OSLD was next evaluated by measuring surface doses in head and neck patients undergoing 3DCRT/IMRT. All measurements were repeated with a MOSFETdosimeter currently used in our clinic. Results: The detectors are easy to use, require no preparation/annealing and can be read 10 minutes post‐irradiation. The effective depth of measurement for OSLD was found to be 0.4 mm inside the plastic casing. Surface/buildup region doses with OSLD were in excellent agreement with ion chamber data. When these detectors were used to measure patient surface dose, they compared favorably with MOSFET detectors. Due to smaller intrinsic buildup, OSLD measured surface doses were slightly lower with a mean dose ratio of 0.968 ± 0.011. Conclusion: A new commercially available Al 2 O 3 detector from Landauer is a convenient tool for measuring patient surface dosimetry. Detector performance compares well with existing dosimeters in radiotherapy. Unlike other dosimeters, however, the new detector shows no field size, energy or angular dependence.

SU‐FF‐T‐262: Indications of Effective Depth Reversal for Cylindrical Ion Chambers in Electron Beams

Purpose: Cylindrical ionization chamber measurements of electron beam depth dose are corrected for the effective depth of measurement and water/air stopping power ratio. The purpose of this presentation is to examine whether a unidirectional upstream shift from physical to effective depth of measurement as often used in data acquisition systems makes sense. Method and Materials: Ionization chamber measurements of depth dose corrected for effective depth by a unidirectional shift and water/air stopping power ratio at depth are compared with diode and EBT film measurements for standard electron beam cone sizes over the nominal energy range of 6 MeV to 20 MeV. The diode results are obtained with a NIST traceable electrometer and are used without correction for any change in water/silicon stopping power ratio vs depth. The EBT film density is converted to dose based on a characteristic curve from films exposed in plastic water at the nominal depth of dose maximum. EBT film results are used without additional corrections for any change in water/EBT film stopping power ratio vs depth. The EBT film density is evaluated with a pin‐hole densitometer as well as a VIDAR film scanner. Results: Good agreement of all measurements at depth from 80 to 10 percent suggest ignoring variation in stopping power ratio for both diode and EBT film has little impact on the results. Both diode and film suggest a slightly deeper depth of dose maximum and a more rapid drop in dose from Dmax towards the surface. Conclusion: Agreement between ionization chamber results and the high resolution solid detector results would improve if the shift in the effective depth of measurement of the ionization chamber is based on dose gradient and reverses as the gradient reverses with perhaps no shift at the zero gradient position. This may impact on calibration protocols.

Dosimetric evaluation of a 2D pixel ionization chamber for implementation in clinical routine

In this paper we present the results of a dosimetric evaluation of a 2D ionization chamber array with the objective of its implementation for quality assurance in clinical routine. The pixel ionization chamber MatriXX (Scanditronix Wellhofer, Germany) consists of 32 × 32 chambers with a distance of 7.6 mm between chamber centres. The effective depth of measurement under the surface of the detector was determined. The dose and energy dependence, the behaviour of the device during its initial phase and its time stability as well as the lateral response of a single chamber of the detector in cross-plane and diagonal directions were analysed. It could be shown, that the detector's response is linear with dose and energy independent. Taking the lateral response into account, two different dose profiles, for a pyramidal and an IMRT dose distribution, were applied to compare the data generated by a treatment planning system with measurements. From these investigations it can be concluded that the detector is a suitable device for quality assurance and 2D dose verifications.