bluuebunny/crossref_metadata_2025 huggingface dataset:
Turbulent Separated and Reattached Flow Over a Curved Surface
Authors: Serpa J. M. , Lessmann R. C. , Hagist W. M.
Published: October 2009

An experimental investigation of two-dimensional separated and reattached turbulent flow has been carried out in a wind tunnel. The test surface consisted of a five to one, length to height, polynomial curve having zero slope and curvature at both ends. Data were taken at reference speeds of 9.1 m/s (30 f/s) and 15.2 m/s (50 f/s). Surface mean pressure distributions and fluctuations were measured. The pressure coefficient was found to agree with a potential flow prediction up to 40 percent of the chord. Significant pressure fluctuations were observed well upstream of separation. Velocity profiles, profile scaling parameters, and integral thickness variations were also measured. Intermittency measurements, i.e., the fraction of time that reversed flow existed at a place, showed that both the separation point and reattachment point wandered over about 30 percent of the chord. Nowhere in the separated region was the intermittency found to be 100 percent. Wall shear stress distributions were measured from upstream of separation to downstream of reattachment.
An Experimental Study of Liquid Entrainment by Expanding Gas
Authors: Tan M. J. , Delhaye J. M.
Published: October 2009

An experimental study of the hydrodynamic aspect of the expansion process in a hypothetical core disruptive accident (HCDA) by means of a shock-tube technique is described. The working fluid pair was water-air. Interface displacement data for shock strengths in the range 0.5–3.0 were obtained by means of high-speed photography. The displacement data are fitted into least-square polynomials in time, which are used to compute accelerations and entrainment velocities. The accelerations thus obtained, which are in the range 950 m/s2−15,000 m/s2, are uniform in time. The entrainment velocities for experiments performed with the circular driven section are consistently higher than those for the experiments performed with the square driven section at the same accelerations. The difference is attributed to the entrainment velocity for the latter case being time-dependent. Furthermore, an upper bound on the rate of entrainment in an HCDA is proposed.
The Effect of Water Droplets on the Relaxation Zone Developed Behind Strong Normal Shock Waves
Authors: Rakib Z. , Igra O. , Ben-Dor G.
Published: October 2009

The propagation of strong normal shock waves into quiescent argon gas seeded with water droplet is studied. For evaluating the effect of the water droplets on the post shock-wave flow field, i.e., the relaxation zone, the conservation equations appropriate to a steady, one-dimensional suspension flow are formulated and solved numerically. The solution, carried out for a range of shock Mach numbers 13 ≤ M ≤ 17 and a few different values for the water droplet concentration, indicates that the presence of the water droplets has a significant effect on the extent of the relaxation zone and on the values obtained for the flow properties throughout this zone.
Analytical Solution of Two Gradient-Diffusion Models Applied to Turbulent Couette Flow
Authors: Henry F. S. , Reynolds A. J.
Published: October 2009

Two widely used gradient-diffusion models of turbulence, when applied to fully-developed Couette flow, are shown to reduce to a set of equations that can be solved analytically. The solutions reveal that both models predict the turbulence kinetic energy to be constant across the entire central region in which the modeling is applied. The implications for the prediction of velocity and eddy viscosity are explored. It is found that the point at which the model equations are matched to the near-wall boundary conditions is an important parameter of the solution.
Solid Particle Dynamic Behavior Through Twisted Blade Rows
Authors: Hamed A.
Published: October 2009

Particle trajectory calculations provide the essential information required for predicting the pattern and intensity of turbomachinery erosion. Consequently, accurate flow field and blade geometry representation in the trajectory computational model are required to evaluate machine performance deterioration due to erosion. A model is presented that is simple and efficient yet versatile and general, applicable to axial, radial and mixed flow machines, and to inlets, nozzles, return passages and separators. Computations are presented for particle trajectories through a row of twisted vanes in the inlet flow field. The effect of the particle size on trajectory blade impacts, and on particle redistribution and separation are discussed.
Asymptotic Behavior in Slumping From a Cylindrical Tank
Authors: Bai Y. , Johnson W. , Low R. G. M. , Ghosh S. K.
Published: October 2009

When an oil tank collapses or ruptures any contained hazardous substance flows outwards and can damage nearby plant or people as well as lead to pollution of the local environment. In recent years, this and similar subjects have given rise to a new kind of engineering—spill prevention and control. However, theoretical background, backed by experiment, is lacking to work out reliable regulations. An intermediate-asymptotic analysis for late-stage spreading is carried out in this paper. This analysis reveals several characteristic features of the spill wave such as transition period and linear relationships between spreading area and time, and wave front velocity and the inverse of zone radius. Most of the latter results have been verified by model experiment. This paper also discusses the discrepancies between observations and the theory suggested in a recent UK Health and Safety Executive report. Finally, the present paper puts forward proper modeling rules for future work.
Forces Caused by the Radial Out-Flow Between Parallel Disks
Authors: Wark C. E. , Foss J. F.
Published: October 2009

An experimental program, to determine the magnitude of the force on the impact disk for the radial out-flow between parallel disks, has been carried out for moderate Reynolds numbers. The present results are compared with a previously published, ad hoc, analytical formulation whose coefficients were established for low Reynolds numbers. Acceptable agreement exists for the low Reynolds number range of the present study; progressively significant disagreement is observed as the Reynolds number is increased. The present force data, when combined with previous observations and interpreted via appropriate theoretical considerations, reveal a complex blending of flow phenomena for the Reynolds number, diameter ratio, and plate separation values of the present study.
Effects of Free Stream Turbulence on Low Reynolds Number Boundary Layers
Authors: Castro I. P.
Published: October 2009

This paper documents some of the effects of free stream turbulence on the mean flow properties of turbulent boundary layers in zero pressure gradients. Attention is concentrated on flows for which the momentum thickness Reynolds number is less than about 2000. Direct Reynolds number effects are therefore significant and it is shown that such effects reduce as the level of free stream turbulence rises. A modification to Hancock’s [1] empirical correlation relating the fractional increase in skin friction at constant Reynolds number to a free stream turbulence parameter containing a dependence on both intensity and length scale is proposed. While this modification has the necessary characteristic of being a function of the free stream turbulence parameters as well as the Reynolds number, it is argued that the relative importance of intensity and length scale changes at low Reynolds numbers; the data are not inconsistent with this idea. The experiments cover the range 500 ⪝ Reθ ⪝ 2500, u′/Ue ⪝ 0.07, 0.8 ⪝ Le/δ ⪝ 2.9, where u′/Ue is the free stream turbulence intensity and Le/δ is the ratio of the dissipation length scale of the free stream turbulence to the 99 percent thickness of the boundary layer.
Dynamic Responses of Partially Cavitated Hydrofoil Cascade to Axial Gust in Bubbly Water
Authors: Nishiyama Tetsuo , Nishiyama Hideya
Published: October 2009

A method for analysis by singularity representations is presented for dynamic responses of partially cavitated hydrofoil cascade to axial gust in bubbly water. Conservation laws of mass and momentum are applied to take account of the unsteady parts of cavity area, cavity termination thickness, and cavity pressure for unsteady partial cavity model. Also, the cavitation compliance and mass flow gain factor obtained from the cavity model determined are considered in relations to cavity length, reduced frequency, and air volume ratio. Comparisons of the mean unsteady pressure rise and unsteady discharge difference are also made with the available former results.
An Analysis of the Motion of Pigs Through Gas Pipelines
Authors: Weingarten J. S. , Chapman A. J. , Walker W. F.
Published: October 2009

A one-dimensional, quasi-steady, model describing the motion of a pig moving in a gas pipeline is developed for the cases of a solid pig, which obstructs the cross section of the pipe, and one with a concentric hole through it. The resultant governing equations constitute a set of seven nonlinear differential equations. A numerical solution scheme, implemented by a computer program, is described. Results and discussion are presented for a set of typical cases.
Nonlinear Taylor Instability in a Cylindrical Vessel
Authors: Chang-Mateu I. , Bankoff S. G.
Published: October 2009

A new class of Taylor-unstable waves, consisting of ring waves appearing at the interface of two fluids of differing densities within a around pipe when the interface is accelerated in the direction of the denser fluid, is considered. The analysis employs a generalized coordinate method introduced by Dienes [16]. The characteristic “bubble-spike” configuration is obtained, although the singularity corresponding to infinite spike tip velocity occurs at a later time than for the one-dimensional plane wave. In addition, it has been shown previously [19] that condensation of vapor at the interface produces negligible correction to the growth rate.
Turbulent Air Flow Over Rough Surfaces: Mean Flow Parameters
Authors: Stukel J. J. , Hopke P. K. , Nourmohammadi K.
Published: October 2009

The objective of the study was to examine experimentally the turbulent flow structure near a repeated-rib geometry rough-walled surface as a function of the ratio of roughness height to pipe radius (K/R), the ratio of spacing between the elements to roughness height (P/K), and the axial position with the rib cycle. The centerline axial flow velocity was found to vary sinusoidally in the axial direction within a rib cycle for P/K>10 for K/R = 0.1437 and P/K>13 for K/R = 0.100. The roughness effect on the mean velocity profiles was characterized using a logarithmic equation which utilized an error in origin and a roughness function that was a function of the error in origin. The equation was found to be valid for P/K values of 7 or less for K/R = 0.1437 and 13 for K/R = 0.1 geometries.
Experimental Study of the Diversion Crossflow Caused by Subchannel Blockages. Part I: Experimental Procedures and Mass Flow Rates in the Channels
Authors: Genc¸ay S. , Tapucu A. , Troche N. , Merilo M.
Published: October 2009

In this research, the hydrodynamic behavior of two laterally interconnected channels with blockages in one of them has been studied experimentally. For blockages of different shapes and severities, the mass flow rates as well as the pressures in the channels upstream and downstream of the blockage were determined. The experiments were conducted on a test sections which consists of two-square channels separated by an intermediate plate with slots of different geometric parameters. Two types of blockages have been considered: plate and smooth. The shape of the smooth blockage was a cosine. In the region upstream of the blockage, the diversion cross-flow takes place over a relatively short distance. Downstream of the blockage, the recovery of the diverted flow by the blocked channel is a slow process and the rate of this recovery worsens with increasing blockage severity. For a given blockage rate, the diversion crossflow caused by a smooth blockage is smaller than that of a plate blockage.
Collective Effects on the Growth of Vapor Bubbles in a Superheated Liquid
Authors: Chahine G. L. , Liu H. L.
Published: October 2009

The problem of the growth of a spherical isolated bubble in a superheated liquid has been extensively studied. However, very little work has been done for the case of a cloud of bubbles. The collective behavior of the bubbles departs considerably from that of a single isolated bubble, due to the cumulative modification of the pressure field from all other bubbles. This paper presents a theoretical study on bubble interaction in a superheated liquid during the growth stage. The solution is sought in terms of matched asymptotic expansions in powers of ε, the ratio between rb0, a characteristic bubble radius and l0, the interbubble distance. Numerical results show a significant inhibition of the bubble growth rate due to the presence of interacting bubbles. In addition, the temperature at the bubble wall decreases at a slower rate. Consequently, the overall heat exchange during the bubble growth is reduced.
An LDA Study of the Backward-Facing Step Flow, Including the Effects of Velocity Bias
Authors: Adams E. W. , Eaton J. K.
Published: October 2009

The subsonic, backward-facing step flow was studied experimentally to provide structural information and test data for modelers. A single-component laser-Doppler anemometer was the primary measurement instrument. The effects of velocity bias on the LDA results were examined in detail. It was concluded that velocity bias was small (< 4 percent) in the uncorrected measurements and nonexistent when a periodic sampling strategy was employed. The measurements show that the pressure gradient at reattachment is lower in the present experiment than in most previous work, due to the thick boundary layer at separation. Turbulence levels in the separated shear layer are also reduced by the thick upstream boundary layer. Scaling of the streamwise coordinate on the reattachment length produced the best agreement with previous data. Agreement of turbulence quantities was particularly good downstream of reattachment.
Time-Dependent Laminar Backward-Facing Step Flow in a Two-Dimensional Duct
Authors: Durst F. , Pereira J. C. F.
Published: October 2009

This paper presents results of numerical studies of the impulsively starting backward-facing step flow with the step being mounted in a plane, two-dimensional duct. Results are presented for Reynolds numbers of Re = 10; 368 and 648 and for the last two Reynolds numbers comparisons are given between experimental and numerical results obtained for the final steady state flow conditions. In the computational scheme, the convective terms in the momentum equations are approximated by a 13-point quadratic upstream weighted finite-difference scheme and a fully implicit first order forward differencing scheme is used to discretize the temporal derivatives. The computations show that for the higher Reynolds numbers, the flow starts to separate on the lower and upper corners of the step yielding two disconnected recirculating flow regions for some time after the flow has been impulsively started. As time progresses, these two separated flow regions connect up and a single recirculating flow region emerges. This separated flow region stays attached to the step, grows in size and approaches, for the time t → ∞, the dimensions measured and predicted for the separation region for steady laminar backward-facing flow. For the Reynolds number Re = 10 the separation starts at the bottom of the backward-facing step and the separation region enlarges with time until the steady state flow pattern is reached. At the channel wall opposite to the step and for Reynolds number Re = 368, a separated flow region is observed and it is shown to occur for some finite time period of the developing, impulsively started backward-facing step flow. Its dimensions change with time and reduce to zero before the steady state flow pattern is reached. For the higher Reynolds number Re = 648, the secondary separated flow region opposite to the wall is also present and it is shown to remain present for t → ∞. Two kinds of the inlet conditions were considered; the inlet mean flow was assumed to be constant in a first study and was assumed to increase with time in a second one. The predicted flow field for t → ∞ turned out to be identical for both cases. They were also identical to the flow field predicted for steady, backward-facing step flow using the same numerical grid as for the time-dependent predictions.
The Role of Eigensolutions in Nonlinear Inverse Cavity-Flow Theory
Authors: Parkin B. R.
Published: October 2009

The method of Levi Civita is applied to an isolated fully cavitating body at zero cavitation number and adapted to the solution of the inverse problem in which one prescribes the pressure distribution on the wetted surface and then calculates the shape. The novel feature of this work is the finding that the exact theory admits the existence of a “point drag” function or eigensolution. While this fact is of no particular importance in the classical direct problem, we already know from the linearized theory that the eigensolution plays an important role. In the present discussion, the basic properties of the exact “point-drag” solution are explored under the simplest of conditions. In this way, complications which arise from non-zero cavitation numbers, free surface effects, or cascade interactions are avoided. The effects of this simple eigensolution on hydrodynamic forces and cavity shape are discussed. Finally, we give a tentative example of how this eigensolution might be used in the design process.
Experimental and Numerical Study of the Response of an Axial Compressor to Distorted Inlet Flow
Authors: Billet G. , Huard J. , Chevalier P. , Laval P.
Published: October 2009

A model representing the response of fixed or rotating axial compressor blade-rows is coupled to a 3-D numerical simulation of the flow outside the blade rows. The code can be used to study nonuniform compressible 3-D flows through turbomachines. The fluid is assumed to be inviscid in the space outside the rows, while the viscous effects are taken into account inside. Numerical results are compared with experimental data obtained on a test stand with steady distorted inflow. This comparison shows that this numerical approach is capable of predicting the response of the compressor. This work is part of a larger project aimed at predicting the response of a compressor to a nonuniform inlet flow that is periodic in time, or fully unsteady.
Measurement of Intake Valve/Cylinder Boundary Flows Using a Multiple Orientation Hot-Wire Technique
Authors: Wagner T. C. , Kent J. C.
Published: October 2009

A new measurement method is utilized to determine velocity distributions on the intake-valve/cylinder boundary for different induction system designs. The velocity information is being used to calculate the angular momentum flux and to define local discharge coefficients around the valve periphery. The contribution of local flow direction (effective area) and local flow speed (velocity losses) to the global discharge coefficients is examined. The dependence of the discharge coefficient on the flow direction and mean velocity magnitude provides useful diagnostic data to relate intake port geometry and flow performance. The measurement technique is also described in detail. The directional response of a single hot-wire anemometer is utilized along with sequential sampling of the signal as the probe shaft is rotated through 360 deg. Within the range of velocity and flow direction required, the velocity magnitude and direction can be determined to within 2 percent and 2 deg, respectively.
Symmetric and Asymmetric Turbulent Flows in a Rectangular Duct With a Pair of Ribs
Authors: Liou T.-M. , Kao C.-F.
Published: October 2009

Laser-Doppler velocimetry (LDV) measurements are presented of mean velocity and turbulence intensity for turbulent flows past a pair of ribs in a rectangular duct of aspect ratio 2. The Reynolds number based on the duct hydraulic diameter was varied in the range of 2.0 × 103 to 7.6 × 104. The experiments cover ribs with rib height to duct height ratios from 0.13 to 0.33 and with rib width to height ratios from 1 to 10. The critical rib height above which and the critical Reynolds number below which the flow patterns become asymmetric were determined from the results. In addition, the effects of the rib width and boundary layer thickness on the formation and the size of the separation bubbles on the top surface of the ribs as well as on the reattachment length behind the ribs were documented. Furthermore, the degree of turbulence enhancement was compared between the asymmetric and the symmetric flows.
A comparison of AAPM TG‐108 PET/CT shielding recommendations to measurements in an oncology center
Authors: Nye Jonathon A. , Dubose Michael , Votaw John R.
Published: October 2009

Purpose:

The AAPM Task Group Report 108 provides methods for calculating photon shielding for PET scanner suites. Calculated shielding requirements may be conservative in some areas when assuming a static point source and absence of scanner self‐shielding. This study compares TG‐108 shielding calculations to measured data from an oncology PET/CT scanner.

Methods:

Optically stimulated luminescence dosimeters were place at eight positions at 1 m height on the walls surrounding a PET/CT scanner. Dosimeters were also placed at three positions in the ceiling along the axial length of the scanner. Each position contained three dosimeters covered with 0, 1.6, or 3.2 mm of lead to separate the x‐ray and 511 keV photon components. Dosimeters were switched out every month for a total collection period of 10 months. The dosimeter measurements were compared to TG‐108 estimates by dividing the measured dose by the TG‐108 calculated dose.

Results:

The PET gantry provided considerable shielding in the transaxial detector plane resulting in a low measured to calculated dose ratio of 0.26. The ratio increased to 0.48 at 45° between the transaxial and axial planes then to 0.95 at the head of the scanner. Dose ratios at the foot of the scanner were near unity at the walls but as high as 2 in the ceiling. The high ceiling dose was attributed to unshielded radiation contributions from the dose handling area.

Conclusions:

TG‐108 provides accurate dose estimates when there is no barrier between the patient and measurement location. Careful consideration of the shielding properties of the scanner can reduce the amount of shielding necessary to include in walls.
Report of AAPM Therapy Physics Committee Task Group 74: In‐air output ratio, , for megavoltage photon beams
Authors: Zhu Timothy C. , Ahnesjö Anders , Lam Kwok Leung , Li X. Allen , Ma Chang‐Ming Charlie , Palta Jatinder R. , Sharpe Michael B. , Thomadsen Bruce , Tailor Ramesh C.
Published: October 2009

The concept of in‐air output ratio was introduced to characterize how the incident photon fluence per monitor unit (or unit time for a Co‐60 unit) varies with collimator settings. However, there has been much confusion regarding the measurement technique to be used that has prevented the accurate and consistent determination of . The main thrust of the report is to devise a theoretical and measurement formalism that ensures interinstitutional consistency of . The in‐air output ratio, , is defined as the ratio of primary collision water kerma in free‐space, , per monitor unit between an arbitrary collimator setting and the reference collimator setting at the same location. Miniphantoms with sufficient lateral and longitudinal thicknesses to eliminate electron contamination and maintain transient electron equilibrium are recommended for the measurement of . The authors present a correction formalism to extrapolate the correct from the measured values using high‐ miniphantom. Miniphantoms made of high‐ material are used to measure for small fields (e.g., IMRT or stereotactic radiosurgery). This report presents a review of the components of , including headscatter, source‐obscuring, and monitor‐backscattering effects. A review of calculation methods (Monte Carlo and empirical) used to calculate for arbitrary shaped fields is presented. The authors discussed the use of in photon dose calculation algorithms, in particular, monitor unit calculation. Finally, a summary of data (from RPC and other institutions) is included for QA purposes.
The effect of angular and longitudinal tube current modulations on the estimation of organ and effective doses in x‐ray computed tomography
Authors: van Straten Marcel , Deak Paul , Shrimpton Paul C. , Kalender Willi A.
Published: October 2009

Purpose:

Tube current modulation (TCM) is one of the recent developments in multislice CT that has proven to reduce the patient radiation dose without affecting the image quality. Presently established methods and published coefficients for estimating organ doses from the dose measured free in air on the axis of rotation or in the CT dose index (CTDI) dosimetry phantoms do not take into account this relatively new development in CT scanner design and technology. Based on these organ dose coefficients effective dose estimates can be made. The estimates are not strictly valid for CT scanning protocols utilizing TCM. In this study, the authors investigated the need to take TCM into account when estimating organ and effective dose values.

Methods:

A whole‐body adult anthropomorphic phantom (Alderson Rando) was scanned with a multislice CT scanner (Somatom Definition, Siemens, Forchheim, Germany) utilizing TCM (CareDose4D). Tube voltage was 120 kV, beam collimation 19.2 mm, and pitch 1. A voxelized patient model was used to define the tissues and organs in the phantom. Tube current values as a function of tube angle were obtained from the raw data for each individual tube rotation of the scan. These values were used together with the Monte Carlo dosimetry tool IMPACTMC (VAMP GmbH, Erlangen, Germany) to calculate organ dose values both with and without account of TCM. Angular and longitudinal modulations were investigated separately. Finally, corresponding effective dose conversion coefficients were determined for both cases according to the updated 2007 recommendations of the ICRP.

Results:

TCM amplitude was greatest in the shoulder and pelvic regions. Consequently, dose distributions and organ dose values for particular cross sections changed considerably when taking angular modulation into account. The effective dose conversion coefficients were up to 11% lower for a single rotation in the shoulder region and 17% lower in the pelvis when taking angular TCM into account. In the head, neck, thorax, and upper abdominal regions, conversion coefficients changed similarly by only 5% or less. Conversion coefficients for estimating effective doses for scans of complete regions, e.g., chest or abdomen, were approximately 8% lower when taking angular and longitudinal TCMs into account.

Conclusions:

The authors conclude that for accurate organ and effective dose estimates in individual cross sections in the shoulder or pelvic regions, the angular tube current modulation should be taken into account. In general, using the average of the modulated tube current causes an overestimation of the effective dose.
Breast density analysis for whole breast ultrasound images
Authors: Chen Jeon‐Hor , Huang Chiun‐Sheng , Chien Kuang‐Che Chang , Takada Etsuo , Moon Woo Kyung , Wu Jeffery H. K. , Cho Nariya , Wang Yi‐Fa , Chang Ruey‐Feng
Published: October 2009

Breast density has been established as an independent risk factor associated with the development of breast cancer. The terms mammographic density and breast density are often used interchangeably, since most breast density studies are performed with projection mammography. It is known that increase in mammographic density is associated with an increased cancer risk. A sensitive method that allows for the measurement of small changes in breast density may provide useful information for risk management. Despite the efforts to develop quantitative breast density measurements from projection mammograms, the measurements show large variability as a result of projection imaging, differing body position, differing levels of compression, and variation of the x‐ray beam characteristics. This study used two separate computer‐aided methods, threshold‐based and proportion‐based evaluations, to analyze breast density on whole breast ultrasound (US) imaging and to compare with the grading results of three radiologists using projection mammography. Thirty‐two female subjects with 252 images per case were included in this study. Whole breast US images were obtained from an Aloka SSD‐5500 ultrasound machine with an ASU‐1004 transducer (Aloka, Japan). Before analyzing breast density, an adaptive speckle reduction filter was used for removing speckle noise, and a robust thresholding algorithm was used to divide breast tissue into fatty or fibroglandular classifications. Then, the proposed approaches were applied for analysis. In the threshold‐based method, a statistical model was employed to determine whether each pixel in the breast region belonged to fibroglandular or fatty tissue. The proportion‐based method was based on three‐dimensional information to calculate the volumetric proportion of fibroglandular tissue to the total breast tissue. The experimental cases were graded by the proposed analysis methods and compared with the ground standard density classification assigned by a majority voting of three experienced breast radiologists. For the threshold‐based method, 28 of 32 US test cases and for the proportion‐based density classifier, 27 of 32 US test cases were found to be in agreement with the radiologist “ground standard” mammographic interpretations, resulting in overall accuracies of 87.5% and 84.4%, respectively. Moreover, the concordance values of the proposed methods were between 0.0938 and 0.1563, which were less than the average interobserver concordance of 0.3958. The experiment result showed that the proposed methods could be a reference opinion and offer concordant and reliable quantification of breast density for the radiologist.
Localizing spherical fiducials in C‐arm based cone‐beam CT
Authors: Yaniv Ziv
Published: October 2009

Purpose:

C‐arm based cone‐beam CT (CBCT) imaging enables the in situ acquisition of three dimensional images. In the context of image‐guided interventions, this technology potentially reduces the complexity of a procedure's workflow. Instead of acquiring the preoperative volumetric images in a separate location and transferring the patient to the interventional suite, both imaging and intervention are carried out in the same location. A key component in image‐guided interventions is image to patient registration. The most common registration approach, in clinical use, is based on fiducial markers placed on the patient's skin which are then localized in the volumetric image and in the interventional environment. When using C‐arm CBCT, this registration approach is challenging as in many cases the small size of the volumetric reconstruction cannot include both the skin fiducials and the organ of interest.

Methods:

In this article the author shows that fiducial localization outside the reconstructed volume is possible if the projection images from which the reconstruction was obtained are available. By replacing direct fiducial localization in the volumetric images with localization in the projection images, the author obtains the fiducial coordinates in the volume's coordinate system even when the fiducials are outside the reconstructed region.

Results:

The approach was evaluated using two types of spherical fiducials, clinically used 4 mm diameter markers and a custom phantom embedded with 6 mm diameter markers that is part of a commercial navigation system. In all cases, the method localized all fiducials, including those that were outside the reconstructed volume. The method's mean (std) localization error as evaluated using fiducials that were directly localized in the CBCT reconstruction was 0.55(0.22) mm for the 4 mm markers and 0.51(0.18) mm for the 6 mm markers.

Conclusions:

Based on the evaluations the author concludes that the proposed localization approach is sufficiently accurate to augment or replace direct volumetric fiducial localization for thoracic‐abdominal interventions. This allows the physician to position fiducials in a more flexible manner, relaxing the requirement that both the organ of interest and skin surface be contained in the volumetric reconstruction.
Impact of JPEG 2000 compression on lesion detection in MR imaging
Authors: Paz Juan , Pérez Marlen , Schelkens Peter , Rodríguez José
Published: October 2009

Purpose:

Lossy compression algorithms enable the efficient transmission of large medical image datasets over bandwidth‐limited digital networks or facilitate the long‐term storage of the daily image production. Although JPEG 2000 has been adopted by DICOM as the standard for lossy‐to‐lossless compression, still a set of guidelines needs to be derived that allows for the usage of the lossy mode, potentially jeopardizing the accuracy of lesion detection and characterization, and so, of the resulting diagnosis task effectiveness in the medical diagnosis process. In this article the authors present and evaluate a generic methodology for the determination of the minimal bit rate that still ensures an accurate detection in magnetic resonance images of specific lesions. In this article, they demonstrate the methodology for two particular pathologies, i.e., multiple sclerosis and Virchow–Robin space enlargements.

Methods:

Involving qualified personnel, the minimal bit rate is estimated from ROC experiments initially simulated through mathematical observers that are designed with several objective metrics. The mathematical observer models included three variants of the Hotelling observer plus the nonprewhitening matched filter with eye model, while the objective measures are based on distance, correlation, singular value decomposition, and structural similarity.

Results:

The results indicate that the highest compression without seriously affecting the detection of the studied lesions is achieved at a bit rate of 0.125 bpp. At this value, the detection effectiveness exceeded 95% with less than 5% standard deviation, while only 4.4% of the outcomes were classified as false negatives by the experts and 11.6% as false positives.

Conclusions:

The optimal bit rate found assures that important information on the small investigated structures is still preserved for their accurate detection and their a posteriori characterization.
Time‐resolved in vivo luminescence dosimetry for online error detection in pulsed dose‐rate brachytherapy
Authors: Andersen Claus E. , Nielsen Søren Kynde , Lindegaard Jacob Christian , Tanderup Kari
Published: October 2009

Purpose:

The purpose of this study is to present and evaluate a dose‐verification protocol for pulsed dose‐rate (PDR) brachytherapy based on in vivo time‐resolved (1 s time resolution) fiber‐coupled luminescence dosimetry.

Methods:

Five cervix cancer patients undergoing PDR brachytherapy (Varian GammaMed Plus with) were monitored. The treatments comprised from 10 to 50 pulses (1 pulse/h) delivered by intracavitary/interstitial applicators (tandem‐ring systems and/or needles). For each patient, one or two dosimetry probes were placed directly in or close to the tumor region using stainless steel or titanium needles. Each dosimeter probe consisted of a small aluminum oxide crystal attached to an optical fiber cable (1 mm outer diameter) that could guide radioluminescence (RL) and optically stimulated luminescence (OSL) from the crystal to special readout instrumentation. Positioning uncertainty and hypothetical dose‐delivery errors (interchanged guide tubes or applicator movements from ±5 to ±15 mm) were simulated in software in order to assess the ability of the system to detect errors.

Results:

For three of the patients, the authors found no significant differences for comparisons between in vivo measurements and calculated reference values at the level of dose per dwell position, dose per applicator, or total dose per pulse. The standard deviations of the dose per pulse were less than 3%, indicating a stable dose delivery and a highly stable geometry of applicators and dosimeter probes during the treatments. For the two other patients, the authors noted significant deviations for three individual pulses and for one dosimeter probe. These deviations could have been due to applicator movement during the treatment and one incorrectly positioned dosimeter probe, respectively. Computer simulations showed that the likelihood of detecting a pair of interchanged guide tubes increased by a factor of 10 or more for the considered patients when going from integrating to time‐resolved dose verification. The likelihood of detecting a ±15 mm displacement error increased by a factor of 1.5 or more.

Conclusions:

In vivo fiber‐coupled RL/OSL dosimetry based on detectors placed in standard brachytherapy needles was demonstrated. The time‐resolved dose‐rate measurements were found to provide a good way to visualize the progression and stability of PDR brachytherapy dose delivery, and time‐resolved dose‐rate measurements provided an increased sensitivity for detection of dose‐delivery errors compared with time‐integrated dosimetry.
Longitudinal study using a diode phantom for helical tomotherapy IMRT QA
Authors: Geurts M. , Gonzalez J. , Serrano‐Ojeda P.
Published: October 2009

Purpose:

The Caribbean Radiation Oncology Center acquired a DELTA4® diode phantom for helical tomotherapy IMRT QA and presents the results of their first 264 clinical cases.

Methods:

The validation consisted of several case studies comparing existing ionization chamber and Gafchromic film IMRT QA results to diode phantom results, along with a longitudinal study analyzing the IMRT QA results against other machine QA procedures for a complete sample of IMRT patients.

Results:

The case studies resulted in a maximum observed difference of 0.7% between the diode phantom and the ionization chamber measurements in low dose‐gradient regions. Over the longitudinal study, every IMRT QA plan passed a gamma specification of a 3%/3 mm and 98% of the diodes yielded a value of less than 1. In addition, the mean 90% isodose absolute difference for all plans was 0.05% with a (1σ) standard deviation of 1.19%.

Conclusions:

The phantom measurements closely match the planned dose distributions in high and low dose‐gradient regions. In addition, a significant positive statistical correlation was determined between the IMRT QA, daily QA, and rotational variation output measurements. Together, these results signify high degree of accuracy of both the DELTA4 phantom as well as the TomoTherapy® Hi‐Art® system.
Treatment planning for volumetric modulated arc therapy
Authors: Bedford James L.
Published: October 2009

Purpose

: Volumetric modulated arc therapy (VMAT) is a specific type of intensity‐modulated radiation therapy (IMRT) in which the gantry speed, multileaf collimator (MLC) leaf position, and dose rate vary continuously during delivery. A treatment planning system for VMAT is presented.

Methods

: Arc control points are created uniformly throughout one or more arcs. An iterative least‐squares algorithm is used to generate a fluence profile at every control point. The control points are then grouped and all of the control points in a given group are used to approximate the fluence profiles. A direct‐aperture optimization is then used to improve the solution, taking into account the allowed range of leaf motion of the MLC. Dose is calculated using a fast convolution algorithm and the motion between control points is approximated by 100 interpolated dose calculation points. The method has been applied to five cases, consisting of lung, rectum, prostate and seminal vesicles, prostate and pelvic lymph nodes, and head and neck. The resulting plans have been compared with segmental (step‐and‐shoot) IMRT and delivered and verified on an Elekta Synergy to ensure practicality.

Results:

For the lung, prostate and seminal vesicles, and rectum cases, VMAT provides a plan of similar quality to segmental IMRT but with faster delivery by up to a factor of 4. For the prostate and pelvic nodes and head‐and‐neck cases, the critical structure doses are reduced with VMAT, both of these cases having a longer delivery time than IMRT. The plans in general verify successfully, although the agreement between planned and measured doses is not very close for the more complex cases, particularly the head‐and‐neck case.

Conclusions:

Depending upon the emphasis in the treatment planning, VMAT provides treatment plans which are higher in quality and/or faster to deliver than IMRT. The scheme described has been successfully introduced into clinical use.
Initial investigation on the use of MR spectroscopy and micro‐MRI of GAFCHROMIC® EBT radiotherapy film
Authors: Holly Rick , Keller Brian M. , Pignol Jean‐Philippe , Lemaire Claude , Peemoeller Hartwig
Published: October 2009

Purpose:

This article presents an initial investigation of the efficacy of using MRS and micro‐MRI as analysis techniques for irradiated GAFCHROMIC EBT® radiotherapy films.

Methods:

GAFCHROMIC® EBT radiotherapy film was irradiated with 6 MV x rays to known doses ranging from 5 to 1000 cGy. 24 h following irradiation MRS measurements were performed to access the degree of post‐irradiation polymer cross‐linking. 2D micro‐MRI experiments were also performed for film irradiations of 0 and 300 cGy.

Results:

Linear response of the MRS linewidth to dose in the range from 0 to 400 cGy was observed. Such linearity is not seen when analyzed under conventional light analysis. The sensitivity of the film, as measured by the slope of the curve between 0 and 400 cGy, is , demonstrating the sensitivity of the MRS technique used to analyze the film. The film saturates at a dose of approximately 900 cGy. Broadline MRS provides a quantitative measure of the degree of polymerization of the film.

Conclusions:

A quantitative measurement of the degree of polymerization of GAFCHROMIC® EBT film has been presented using MRS. The saturation of the film at approximately 900 cGy is corroborated by that observed with light analysis. Further MR spectroscopic experiments are needed to investigate the response of the film to dose, allowing for a better understanding of the relationship between polymer cross‐linking in the active layer.
Uncluttered single‐image visualization of the abdominal aortic vessel tree: Method and evaluation
Authors: Won Joong‐Ho , Rosenberg Jarrett , Rubin Geoffrey D. , Napel Sandy
Published: October 2009

Purpose:

The authors develop a method to visualize the abdominal aorta and its branches, obtained by CT or MR angiography, in a single 2D stylistic image without overlap among branches.

Methods:

The abdominal aortic vasculature is modeled as an articulated object whose underlying topology is a rooted tree. The inputs to the algorithm are the 3D centerlines of the abdominal aorta, its branches, and their associated diameter information. The visualization problem is formulated as an optimization problem that finds a spatial configuration of the bounding boxes of the centerlines most similar to the projection of the input into a given viewing direction (e.g., anteroposterior), while not introducing intersections among the boxes. The optimization algorithm minimizes a score function regarding the overlap of the bounding boxes and the deviation from the input. The output of the algorithm is used to produce a stylistic visualization, made of the 2D centerlines modulated by the associated diameter information, on a plane. The authors performed a preliminary evaluation by asking three radiologists to label 366 arterial branches from the 30 visualizations of five cases produced by the method. Each of the five patients was presented in six different variant images, selected from ten variants with the three lowest and three highest scores. For each label, they assigned confidence and distortion ratings (low/medium/high). They studied the association between the quantitative metrics measured from the visualization and the subjective ratings by the radiologists.

Results:

All resulting visualizations were free from branch overlaps. Labeling accuracies of the three readers were 93.4%, 94.5%, and 95.4%, respectively. For the total of 1098 samples, the distortion ratings were low: 77.39%, medium: 10.48%, and high: 12.12%. The confidence ratings were low: 5.56%, medium: 16.50%, and high: 77.94%. The association study shows that the proposed quantitative metrics can predict a reader's subjective ratings and suggests that the visualization with the lowest score should be selected for readers.

Conclusions:

The method for eliminating misleading false intersections in 2D projections of the abdominal aortic tree conserves the overall shape and does not diminish accurate identifiability of the branches.
Image‐based iterative compensation of motion artifacts in computed tomography
Authors: Schretter Colas , Rose Georg , Bertram Matthias
Published: October 2009

Purpose:

This article presents an iterative method for compensation of motion artifacts for slowly rotating computed tomography (CT) systems. Patient's motion introduces inconsistencies among projections and yields severe reconstruction artifacts for free‐breathing acquisitions. Streaks and doubling of structures can appear and the resolution is limited by strong blurring.

Methods:

The rationale of the proposed motion compensation method is to iteratively correct the reconstructed image by first decomposing the perceived motion in projection space, then reconstructing the motion artifacts in image space, and finally subtracting the artifacts from an initial image. The initial image is reconstructed from the acquired data and might contain motion blur artifacts but, nevertheless, is considered as a reference for estimating the reconstruction artifacts.

Results:

Qualitative and quantitative figures are shown for experiments based on numerically simulated projections of a sequence of clinical images resulting from a respiratory‐gated helical CT acquisition. The border of the diaphragm becomes progressively sharper and the contrast improves for small structures in the lungs.

Conclusions:

The originality of the technique stems from the fact that the patient motion is not explicitly estimated but the motion artifacts are reconstructed in image space. This approach could provide sharp static anatomical images on interventional C‐arm systems or on slowly rotating X‐ray equipments in radiotherapy.
Ionizing Radiation Exposure of the Population of the United States
Authors: Broga Dean
Published: October 2009

Ionizing Radiation Exposure of the Population of the United StatesNCRP Report No. 160, NCRP Publications, Bethesda, MD, 2009, 400 pp. $125.00. ISBN: 978‐0‐929600‐98‐7 (hardcover).