bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of Laminar Flow and Mass Transport in Crossing Linked Microchannels for Micro Fabrication
Authors: Shao Zhanjie , Ren Carolyn L. , Schneider Gerry
Published: May 2009

There has been tremendous interest in developing micro technologies towards the integration and automation of Biochips or Lab-on-a-Chip devices due to their wide range of applications in environmental, chemical and biomedical engineering fields. The laminar flow nature in microfluidic devices offers opportunities to microfabricate the desired structures inside microchannels and pattern culturing medium inside microchannels. However, no analysis tools are available to provide optimized configurations for control the flow for microfabrication. Therefore, the goal of this study is to develop a numerical model to study transport phenomena in a cross-linked microchannels aiming to explore an optimized configuration for the microfabrication of specific desired features inside microchannel networks through investigating the effects of controlling parameters on the multistream flow. In this study, electroosmotic flow with induced pressure-driven flow will be employed. This model consists of a set of equations describing the applied potential field, flow field and concentration field in such geometries. The effects of various operational parameters are investigated based on the simultaneous solution to this model, to explore optimized configurations for flow and mass transport control in crossing linked microchannels.
Constructal Design of Forced Convection Cooled Microchannel Heat Sinks and Heat Exchangers
Authors: Muzychka Y. S.
Published: May 2009

Heat transfer from arrays of circular and non-circular ducts subject to finite volume and constant pressure drop constraints is examined. It is shown that the optimal duct dimension is independent of the array structure and hence represents an optimal construction element. Solutions are presented for the optimal duct dimensions and maximum heat transfer per unit volume for the parallel plate channel, rectangular channel, elliptic duct, circular duct, polygonal ducts, and triangular ducts. Approximate analytical results show that the optimal shape is the isosceles right triangle and square duct due to their ability to provide the most efficient packing in a fixed volume. Whereas a more exact analysis reveals that the parallel plate channel array is in fact the superior system. An approximate relationship is developed which is very nearly a universal solution for any duct shape in terms of the Bejan number and duct aspect ratio. Finally, validation of the relationships is provided using exact results from the open literature.
Development of a Silicon-Based Wettability Controlled Membrane for Microscale Two-Phase Systems
Authors: Alyousef Yousef , Yao Shi-Chune
Published: May 2009

Surface structure modification and wettability control are very crucial for the advancement in microfluidic systems. The wettability control of the surface is achieved by depositing a Self-Assembled Monolayer (SAM) on the surface. The technology of SAM has been advanced by coating the selective interior regions of silicon wafer. This was accomplished using a mask wafer with open slots attached to define the areas that need to be coated. Besides the capability of controlling the wettability, structure modification such as controlling hole size leads to the magnification of the influence of surface tension force. As the size of the hole gets smaller, the surface tension could become a dominant force. This paper utilizes a unique approach to improve the water and air management at the cathode of a micro Direct Methanol Fuel Cell (DMFC). Both structure modification and local surface wettability control are utilized. A two-inch silicon wafer is formed of alternate strips of hydrophobic and hydrophilic zones with arrays of holes of different sizes. Water will be guided along the hydrophilic wetting zones with large hole openings; while the air goes into the cathode from the hydrophobic dry areas with smaller holes. This process ensures that the cathode is not flooded by water while air passes easily. The excess water at the cathode could be pumped back by a micro-pump to the anode so that the size of water storage will be minimized. Therefore, most of the storage reservoir space is used for the pure methanol, which enables the achievement of a high power density of the system. In this study, a silicon-based membrane is built accordingly to observe the water and air management. Images of the CCD camera showed clearly that the water drained from the big holes without blocking the air passages.
Time- and Frequency- Domain Analysis of Transient Electroosmotic Flow Induced by Sinusoidal AC Electric Field in a Curved Microtube
Authors: Luo Win-Jet , Chen Jia-Kun , Yang Ruey-Jen
Published: May 2009

A backwards-Euler time-stepping numerical method is applied to simulate the transient response of electroosmotic flow in a curved microtube. The velocity responses of the flow fields induced by applied sinusoidal AC electric fields of different frequencies are investigated. The transient response of the system is fundamentally important since both the amplitude and the time duration of the transient response must be maintained within tolerable or prescribed limits. When a sinusoidal AC electric field is applied, the transient response of the output velocity oscillates in the time-domain. However, after a certain settling time, the output velocity attains a sustained oscillation with the same amplitude as the driving field. In this study, the transient response of the electroosmotic flow is characterized by the time taken by the velocity response to reach the first peak, the peak of the sustained oscillation, the maximum overshoot, the settling time, and the bandwidth of the sustained oscillations in the time-domain. Meanwhile, the performance of the system is identified by plotting the output velocity response and the output velocity phase-shift against the frequency of the applied signal. A finite time is required for the momentum to diffuse fully from the walls to the center of the curved microtube cross-section. As the applied frequency is increased, the maximum overshoot and the bandwidth and peak of the sustained oscillations gradually decrease since insufficient time exists for the momentum to diffuse fully to the center of the microtube. Additionally, the phase-shift between the applied electric field and the output velocity response gradually increases as the frequency of the applied signal is increased.
An Integrated Microfluidic Dielectrophoretic (DEP) Cell Fractionation System
Authors: Li Youlan , Dalton Colin , Said Humud , Kaler Karan V. I. S.
Published: May 2009

In this article we report on a planar miniaturized dielectrophoretic (DEP) microfluidic device developed for the purpose of continuous fractionation and purification of sample suspensions of microscopic particles or biological cells, employing specially shaped nonuniform (isomotive) electric fields. The device integrates three fully functional and distinct sub-units consisting of 1) sheath and sample injection ports, arranged to achieve hydrodynamic focusing of the cell stream; 2) the DEP fractionation region and 3) two sample collection ports. In the DEP fractionation region, the magnitude of the field induced DEP force acting on the particle is essentially constant and independent of the particle’s position and furthermore only dependent on the intrinsic polarization response of the particle, for identical sized particles. The operation and performance in terms of sample throughput, separation efficiency and repeatability of the device was evaluated using test microscopic sized dielectric particles and biological particles, including cancerous cell lines.
Fabrication and Testing of Microstructure Heat Exchangers for Thermal Applications
Authors: Brandner Juergen J. , Schubert Klaus
Published: May 2009

For a couple of years, microstructure heat exchangers have been numerically simulated, designed, manufactured and tested at the Institute for Micro Process Engineering of the Forschungszentrum Karlsruhe. The microstructure heat exchangers consist of single metal foils which are connected by a diffusion bonding process to form a nearly monolithic body. The number of integrated microchannels is in the order of several hundreds to several thousands. This internal numbering-up leads to a sufficiently low pressure drop. The devices have an extremely high heat transfer to volume ratio of about 30,000 m2 · m−3 which makes it possible to transfer thermal power in the range of several kilowatt within a volume of some centimeters cubed only.
Enhancement of the Concentration Gradients Generated in Microchambers Appended in Microfluidic Systems
Authors: Hu Yandong , Lee Jacky S. H. , Werner Carsten , Li Dongqing
Published: May 2009

Concentration gradient in a chamber appended to a microchannel is important to cell movement control and to the concentration gradient based assays on Lab-on-a-Chip devises. In this paper, the effects on the concentration field of the asymmetrical injection, the Peclet number, the mobility ratio of electrophoresis to electroosmosis, the chamber’s downstream position, and the chamber’s geometry parameters, are investigated. The most sensitive parameter is the asymmetrical injection, which can increase the concentration gradient twice as large as to that in the symmetrical injection. Furthermore, using heterogeneous surface patches is a very effective way to enhance the concentration gradient generated in the chamber. Different patches for certain chambers are investigated. Finally, experimental visualization of the concentration fields was conducted, and good agreements were found between the numerical simulation results and the experimental results of the concentration fields generated in a micro-chamber with/without a heterogeneous patch.
Fluid Flow Behaviour in Microneedle Arrays
Authors: San A. N. , Ben Mrad R. , Sullivan P.
Published: May 2009

This paper examines the geometries of basic straight microneedle arrays, slanted channel arrays with varying angles, and arrays with diverging and converging interior cross sections for the purpose of interstitial fluid extraction and transdermal drug delivery. Flow behaviour is analyzed under biometric pressure driven conditions including frictional losses, minor losses due to the array geometry, and losses due to electrokinetic effect in microchannels. This paper also presents design and fabrication details of preliminary work that will lead to a design for microneedle arrays.
Enhancement of Mixing for Electro-Kinetically Driven Side Channel Type Micromixer
Authors: Wu Chien-Hsien , Yang Ruey-Jen
Published: May 2009

Electro-osmotic flow (EOF) in microchannels is restricted to low Reynolds number regimes. Since the inertia forces are extremely weak in such regimes, turbulent conditions do not readily develop. Therefore, species mixing occurs primarily by diffusion, with the result that extended mixing channels are generally required. In this paper we present an investigation to predict the optimal applied voltage for the side channel type micromixer (SCTM) which is capable of continuous sample mixing for microfluidic applications. The device uses electrokinetical focusing which is an important EOF phenomenon. In this study, according to the conservation of mass, a simple theoretical model, based on the ‘flow-rate-ratio’ model and Kirchhoff’s law, is first proposed to predict the performance of the device. Computational fluid dynamics simulations are performed to investigate the effect of this model on the mixing efficiency. The results reveal that the mixing efficiency can be enhanced by using ‘flow-rate-ratio’ model and Kirchhoff’s law to predict the optimal applied voltage.
Evaporative Heat Transfer and Pressure Drop of CO2 in a Microchannel Tube
Authors: Jeong Siyoung , Cho Eunsang , Kim Hark-koo
Published: May 2009

Evaporation heat transfer and pressure drop characteristics of carbon dioxide were investigated in a multi-channel micro tube. The aluminum tube has 3 square channels with a hydraulic diameter of 2mm, a wall thickness of 1.5mm, and a length of 5m. The tube was heated directly by electric current. Experiments were conducted at heat fluxes ranging 4–16 kW/m2, mass fluxes from 150 to 750 kg/m2s, evaporative temperature from 0 to 10°C, and qualities from 0 to superheated state. The heat transfer coefficient measured was in the range of 6–15kW/m2K, and the pressure drop was 3–23kPa/m. For the qualities lower than 0.5, the heat transfer coefficient was found to increase with the quality, which is assumed to be the effect of convective boiling. For the qualities higher than 0.6, sudden drop in heat transfer coefficients was sometimes observed due to local dry-out. It was found that dry-out occurred at lower quality if mass flux was smaller. The average heat transfer coefficient was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of heat flux was the greatest. At given experimental conditions the pressure drop increased almost linearly with increasing quality. The total pressure drop was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of mass flux was the greatest. From the experimental results simple correlations for heat transfer coefficients and pressure drop were developed.
Convective Boiling in Silicon Microchannels With Localized Heating and Thermometry
Authors: Flynn Roger D. , Kramer Theresa A. , Koo Jae-Mo , Cheng Ching-Hsiang , Fogg David W. , Wang Evelyn N. , Goodson Kenneth E.
Published: May 2009

While silicon microchannel heat sinks are promising for high heat flux integrated circuits, they have not reached their potential because microscale convective boiling is poorly understood. Previous work integrated sensors and heaters into a silicon chip to provide distributed thermometry, but did not specifically examine hotspots or thoroughly treat experimental uncertainty. This work microfabricates a single channel in a thinned silicon beam, instrumented with doped sensors and aluminum heaters, to study the wall temperature and fluidic response to flow boiling induced by non-uniform heating. Uncertainty analysis shows a need for better measurements of the fabricated channel including channel cross section and surface roughness. Refined data from this work will suggest improvements to existing boiling flow models, which may then be implemented into a design tool for optimizing boiling flow microchannel heat sinks.
Micro Heat Exchanger for Gas Phase Reaction
Authors: Yu Sang-Phil , Lee Seung-Jae , Song Kwang-Sup
Published: May 2009

Each microstructured stainless steel foil was brazed in vacuum for stacking. Inner surface of micro channels was coated with Al2O3 layer to support Pt catalyst by sol-gel method. The stack was designed like a cross-flow type heat exchanger to perform the combination of exothermic and endothermic reactions simultaneously. It is expected to apply to the micro reformer which produces hydrogen for micro PEMFC (Proton Exchange Membrane Fuel Cell). As the first step in our study, we measured experimentally the heat transfer rate and the spatial temperature distribution of the stack. An then, the reaction of C3H8-air with heat transfer to cold air flow was performed in the stack. As a consequence, quantitative and qualitative thermal characteristics of the stack for reaction were investigated.
Visualization of Convective Boiling Heat Transfer in Single Micro-Conduits With Different Shapes of Cross Sections
Authors: Yen Tzu-Hsiang , Shoji Masahiro , Takemura Fumio , Suzuki Yuji , Kasagi Nobuhide
Published: May 2009

Visualization experiments of convective boiling in transparent single micro conduits with the same hydraulic diameter but different cross sections are carried out with simultaneous measurement of local heat transfer coefficients and pressure losses. Two different cross sections with the same similar hydraulic diameters are applied: A circular microtube of 210μm in diameter and a square microchannel of 214μm × 214μm cross section. ITO/Ag thin film of 100 nm is sputtered on the outer surface of the conduits for the direct joule heating. The convective boiling shows some periodic variation of different flow patterns in both square and circular conduits. These flow patterns include bubbly, plug, slug, annular and capillary flows. The capillary flow pattern is the independent liquid droplets moving in the flow direction and very rarely observed in conventional tubes. The reason of such variation of flow patterns is that confined spaces limit the bubble growth in radial direction. So the nucleation bubble grows in both upstream and downstream and makes the flow pattern varies radically. The square microchannel conduit has more simple flow pattern variation, more nucleation bubbles and larger local heat transfer coefficients at lower vapor quality. It is due to that corners of the square microchannel act as helps nucleation cavities. Corners also promotes the formation of liquid film and the contact line between liquid and wall, which can stabilize the flow field. Local heat transfer coefficients decrease with increasing local vapor qualities. Local heat transfer coefficients increase with increasing boiling number but have their maximum value when boiling number reaches critical value. Such peculiar heat transfer characteristics can also be explained by the visualization results.
Interactions of Wall Roughness and Electroosmotic Flows Inside Nanochannels
Authors: Kim Daejoong , Darve Eric
Published: May 2009

We are reporting electroosmotic flows in nanochannels having different surface roughness. Molecular dynamics simulation technique has been applied to understand microscopic or molecular aspects of solid-liquid interactions. The surface roughness in this study was modeled as a succession of expanding and contracting steps along the flow direction, determined by the electric field direction. Water and sodium ion density profiles for the smooth wall show strong layering of water molecules near the solid wall. For rough walls, the density profiles are very similar for all cases except where the steps are located. We observed that this disturbance grows with the amplitude of the roughness and decreases with the period of the roughness. To further investigate strong layering of the water molecules, the net electric dipole moment was computed for the smooth and rough walls. It shows ordering of water molecules near the wall. The rough wall result show water molecules are ordered more in the expanded region. The velocity profiles and the flow rate were calculated for all cases. From these, electric double layers are found to overlap. The maximum velocities and the flow rate decreased for the lower period and the higher amplitude of surface roughness.
Design of a Polymeric Mini-Channelled System for the Cooling of a High-Energy Particle Detector at FERMILAB
Authors: Chignoli L. , Cossali G. E. , Marengo M.
Published: May 2009

BTeV is a high-energy physics experiment, which is designed to proof several aspects of the so-called Standard Model. Precise measurements will reveal if the Standard Model contains breakdowns and therefore they will hint new matters for a more fundamental theory. One of BTeV’s main goals is to precisely measure CP violation in the beauty quark system. CP violation was first observed in strange quarks in 1963 and recently in beauty quarks at BaBar and Belle. CP violation causes particles and antiparticles to behave differently. The BTeV experiment was approved by FERMILAB and was currently being developed. In fact a very recent decision from the Department of Energy (February 2005) cancelled the project due to budget restrictions. A prototype of an innovative detector, called μ-strip detector, is under construction in a team leaded by an Italian group at INFN. The detector is based on copper strips deposited onto 300μm thick high resistivity bulk silicon. A hybrid electronic linked at the two terminals of the strips is positioned on the silicon layer. The system is inserted in a carbon fiber structure and then finally located around the particle beam. Even if the details of the electronic power dissipation and the chip geometry are not yet completely defined, the major constraints of the experiment (radiation hard structure, no mechanical vibration, high signal noise ratio with extremely low electrical charges, low atomic number of the components) have led the μ-strip team to make an effort in direction of a heat-sink based on a PEEK mini-tube system.
High Performance Micro-Channel Cross Flow Heat Exchangers
Authors: Kelly Kevin W. , McCandless Andrew , Marques Christoffe , Turner Ryan A. , Motakef Shariar
Published: May 2009

The performance of a micro-channel gas-liquid cross flow heat exchanger, manufactured by the LIGA technique is presented. Large heat transfer coefficients are achieved on the gas side by achieving gas-flow passage dimensions as low as 300 microns. Cross flow heat exchanger panels have been produced as large as 20 cm by 15 cm. These panels can be arranged in a variety of ways to produce heat exchangers capable of handling large thermal loads. Experimental results have shown that these heat exchangers are approximately one order of magnitude better, in terms of heat transfer per unit volume, than the commercially available tube-fin heat exchangers with characteristic cross flow channel dimensions that are typically three times larger.
Micro-Jet Arrays for Cooling of Electronic Equipment
Authors: Overholt Matthew R. , McCandless Andrew , Kelly Kevin W. , Becnel Charles J. , Motakef Shariar
Published: May 2009

A high heat flux cooling system using an array of micro-jets is presented. The Micro-Jet Cooling Array (MJCA) is based on an array of small diameter micro-jets, with jet diameters as low as 300 microns. The return flow in MJCA is arranged so that the effect of neighboring jets is essentially eliminated and the high heat fluxes achievable in a single jet are reproduced over large areas. MJCA has been machined using the LIGA process and has been extensively tested. Extremely high heat fluxes have been achieved with MJCA, making is suitable for a variety of high heat flux applications.
Evaluation of a lithium formate EPR dosimetry system for dose measurements around brachytherapy sources
Authors: Antonovic Laura , Gustafsson Håkan , Alm Carlsson Gudrun , Carlsson Tedgren Åsa
Published: May 2009

A dosimetry system using lithium formate monohydrate as detector material and electron paramagnetic resonance (EPR) spectroscopy for readout has been used to measure absorbed dose distributions around clinical sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam. irradiations were performed in a PMMA phantom using two different source models, the GammaMed Plus HDR and the microSelectron PDR‐v1 model. Measured absorbed doses to water in the PMMA phantom were converted to the corresponding absorbed doses to water in water phantoms of dimensions used by the treatment planning systems (TPSs) using correction factors explicitly derived for this experiment. Experimentally determined absorbed doses agreed with the absorbed doses to water calculated by the TPS to within ±2.9%. Relative standard uncertainties in the experimentally determined absorbed doses were estimated to be within the range of 1.7%–1.3% depending on the radial distance from the source, the type of source (HDR or PDR), and the particular absorbed doses used. This work shows that a lithium formate dosimetry system is well suited for measurements of absorbed dose to water around clinical HDR and PDR sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride (LiF) dosimeters, lithium formate monohydrate dosimeters are well suited to measure absorbed doses in situations where the energy dependence cannot easily be accounted for such as in multiple‐source irradiations to verify treatment plans. Their wide dynamic range and linear dose response over the dose interval of 0.2–1000 Gy make them suitable for measurements on sources of the strengths used in clinical applications. The dosimeter size needs, however, to be reduced for application to single‐source dosimetry.
An overview of the comprehensive proton therapy machine quality assurance procedures implemented at The University of Texas M. D. Anderson Cancer Center Proton Therapy Center–Houston
Authors: Arjomandy Bijan , Sahoo Narayan , Zhu X. Ronald , Zullo John R. , Wu Richard Y. , Zhu Mingping , Ding Xiaoning , Martin Craig , Ciangaru George , Gillin Michael T.
Published: May 2009

The number of proton and carbon ion therapy centers is increasing; however, since the publication of the International Commission on Radiation Units and Measurements report, there has been no dedicated report dealing with proton therapy quality assurance. The purpose of this article is to describe the quality assurance procedures performed on the passively scattered proton therapy beams at The University of Texas M. D. Anderson Cancer Center Proton Therapy Center in Houston. The majorities of these procedures are either adopted from procedures outlined in the American Association of Physicists in Medical Task Group (TG) 40 report or are a modified version of the TG 40 procedures. In addition, new procedures, which were designed specifically to be applicable to the synchrotron at the author's center, have been implemented. The authors’ procedures were developed and customized to ensure patient safety and accurate operation of synchrotron to within explicit limits. This article describes these procedures and can be used by others as a guideline for developing QA procedures based on particle accelerator specific parameters and local regulations pertinent to any new facility.
The imaging performance of compact powdered phosphor screens: Monte Carlo simulation for applications in mammography
Authors: Liaparinos P. F. , Kandarakis I. S.
Published: May 2009

In medical mammographic imaging systems, one type of detector configuration, often referred to as indirect detectors, is based on a scintillator layer (phosphor screen) that converts the x‐ray radiation into optical signal. The indirect detector performance may be optimized either by improving the structural parameters of the screen or by employing new phosphor materials with improved physical characteristics (e.g., x‐ray absorption efficiency, intrinsic conversion efficiency, emitted light spectrum). is a relatively new phosphor material that exhibits improved scintillating properties indicating a promising material for mammographic applications. In this article, a custom validated Monte Carlo program was used in order to examine the performance of compact powdered phosphor screens under diagnostic mammography conditions (x‐ray spectra: 28 kV Mo, 0.030 mm Mo and 32 kV W, 0.050 mm Rh). screens of coating weight in the range between 20 and were examined. The Monte Carlo code was based on a model using Mie‐scattering theory for the description of light propagation within the phosphor. The overall performance of powdered phosphor screens was investigated in terms of the (i) quantum detection efficiency, (ii) luminescence efficiency, (iii) compatibility with optical sensors, (iv) modulation transfer function, (v) the Swank factor, and (vi) zero‐frequency detective quantum efficiency. Results were compared to the traditional rare‐earth phosphor material. The increased packing density and therefore the light extinction properties of phosphor were found to improve the x‐ray absorption (approximately up to 21% and 16% at for Mo and W x‐ray spectra, respectively), the spatial resolution (approximately 2.6 and 2.4 cycles/mm at for Mo and W x‐ray spectra, respectively), as well as the zero‐frequency detective quantum efficiency (approximately up to 8% and 18% at for Mo and W x‐ray spectra, respectively) of the screens in comparison to the screens. Data obtained by the simulations indicate that certain optical properties of make this material a promising phosphor which, under appropriate conditions, could be considered for use in x‐ray mammography imagers.
Commissioning a passive‐scattering proton therapy nozzle for accurate SOBP delivery
Authors: Engelsman M. , Lu H.‐M. , Herrup D. , Bussiere M. , Kooy H. M.
Published: May 2009

Proton radiotherapy centers that currently use passively scattered proton beams do field specific calibrations for a non‐negligible fraction of treatment fields, which is time and resource consuming. Our improved understanding of the passive scattering mode of the IBA universal nozzle, especially of the current modulation function, allowed us to re‐commission our treatment control system for accurate delivery of SOBPs of any range and modulation, and to predict the output for each of these fields. We moved away from individual field calibrations to a state where continued quality assurance of SOBP field delivery is ensured by limited system‐wide measurements that only require one hour per week. This manuscript reports on a protocol for generation of desired SOBPs and prediction of dose output.
Study of the effective point of measurement for ion chambers in electron beams by Monte Carlo simulation
Authors: Wang L. L. W. , Rogers D. W. O.
Published: May 2009

In current dosimetry protocols for electron beams, for plane‐parallel chambers, the effective point of measurement is at the front face of the cavity, and, for cylindrical chambers, it is at a point shifted upstream from the cavity center. In this study, Monte Carlo simulations are employed to study the issue of effective point of measurement for both plane‐parallel chambers and cylindrical thimble chambers in electron beams. It is found that there are two ways of determining the position of the effective point of measurement: One is to match the calculated depth‐ionization curve obtained from a modeled chamber to a calculated depth‐dose curve; the other is to match the electron fluence spectrum in the chamber cavity to that in the phantom. For plane‐parallel chambers, the effective point of measurement determined by the first method is generally not at the front face of the chamber cavity, which is obtained by the second method, but shifted downstream toward the cavity center by an amount that could be larger than one‐half a millimeter. This should not be ignored when measuring depth‐dose curves in electron beams. For cylindrical chambers, these two methods also give different positions of the effective point of measurement: The first gives a shift of , which is in agreement with measurements for high‐energy beams and is the same as the value currently used in major dosimetry protocols; the latter gives a shift of , which is closer to the value predicted by a theoretical calculation assuming no‐scatter conditions. The results also show that the shift of is more appropriate if the cylindrical chamber is to be considered as a Spencer‐Attix cavity. In electron beams, since the water/air stopping‐power ratio changes with depth in a water phantom, the difference of the two shifts will lead to an incorrect evaluation of the water/air stopping‐power ratio at the point of measurement, thus resulting in a systematic error in determining the absorbed dose by cylindrical chambers. It is suggested that a shift of be used for electron beam calibrations with cylindrical chambers and a shift of be used for depth‐dose measurements.
Determination of point spread function in computed tomography accompanied with verification
Authors: Ohkubo Masaki , Wada Shinichi , Ida Satoshi , Kunii Masayuki , Kayugawa Akihiro , Matsumoto Toru , Nishizawa Kanae , Murao Kohei
Published: May 2009

A method for verifying the point spread function (PSF) measured by computed tomography has been previously reported [Med. Phys. 33, 2757–2764 (2006)]; however, this additional PSF verification following measurement is laborious. In the present study, the previously described verification method was expanded to PSF determination. First, an image was obtained by scanning a phantom. The image was then two‐dimensionally deconvolved with the object function corresponding to the phantom structure, thus allowing the PSF to be obtained. Deconvolution is implemented simply by division of spatial frequencies (corresponding to inverse filtering), in which two parameters are used as adjustable ones. Second, an image was simulated by convolving the object function with the obtained PSF, and the simulated image was compared to the above‐measured image of the phantom. The difference indicates the inaccuracy of the PSF obtained by deconvolution. As a criterion for evaluating the difference, the authors define the mean normalized standard deviation (SD) in the difference between simulated and measured images. The above two parameters for deconvolution can be adjusted by referring to the subsequent mean normalized SD (i.e., the PSF is determined so that the mean normalized SD is decreased). In this article, the parameters were varied in a fixed range with a constant increment to find the optimal parameter setting that minimizes the mean normalized SD. Using this method, PSF measurements were performed for various types of image reconstruction kernels (21 types) in four kinds of scanners. For the 16 types of kernels, the mean normalized SDs were less than 2.5%, indicating the accuracy of the determined PSFs. For the other five kernels, the mean normalized SDs ranged from 3.7% to 4.8%. This was because of a large amount of noise in the measured images, and the obtained PSFs would essentially be accurate. The method effectively determines the PSF, with an accompanying verification, after one scanning of a phantom.
Image‐guided respiratory‐gated lung stereotactic body radiotherapy: Which target definition is optimal?
Authors: Zhao Bo , Yang Yong , Li Tianfang , Li Xiang , Heron Dwight E. , Saiful Huq M.
Published: May 2009

In stereotactic body radiotherapy (SBRT), the respiratory tumor motion makes target definition very important to achieve optimal clinical results for treatment of early stage lung cancer. In this article, the authors quantitatively evaluated the influence of different target definition strategies on image‐guided respiratory‐gated SBRT for lung cancer. Twelve lung cancer patients with 4D CT estimated target motion of were selected for this retrospective study. An experienced physician contoured gross target volumes (GTVs) at each 4D CT phase for all patients. Three types of internal target volumes (ITVs) were generated based on the contoured GTVs:(1) : GTV contoured on deep expiration breath‐hold (BH) CT with an isotropic internal margin (IM) of ; (2) : GTV contoured at the end‐expiration (50%) phase with an isotropic IM of ; (3) : Composite volume of all GTVs within the gating window, defined as several phases around phase 50% with residual target motion of . Planning target volumes (PTVs) were generated by adding isotropic setup error margin to ITVs. Three treatment plans, namely, , , and , were created based on the three PTVs. Identical beam settings and planning constraints were used for all three plans for each patient. The prescription dose was in three fractions. The potential toxicities to the critical organs were quantified by mean lung dose (MLD), lung volume receiving (V20), mean heart dose (MHD), and spinal cord dose (SCD). It is shown that the tumor volume and dose coverage are comparable for and . On average, are 38% less than . Although for most patients encompasses the entire , up to (6%) of is outside . Compared to , prescribed percentage is about 2% higher for , and the average dose decreases in critical organs are for MLD, 1.02% for V20, for MHD and for maximum SCD. For the cases receiving high lung and heart dose with , the dose reduction is for MLD and for MHD with . Our preliminary results show that a patient‐specific ITV, defined as the composite volume of all GTVs within the gating window, may be used to define PTV in image‐guided respiratory‐gated SBRT. This approach potentially reduces the irradiated volume of normal tissue further without sacrificing target dose coverage and thus may minimize the risk of treatment‐related toxicities.
Temporal resolution improvement using PICCS in MDCT cardiac imaging
Authors: Chen Guang‐Hong , Tang Jie , Hsieh Jiang
Published: May 2009

The current paradigm for temporal resolution improvement is to add more source‐detector units and/or increase the gantry rotation speed. The purpose of this article is to present an innovative alternative method to potentially improve temporal resolution by approximately a factor of 2 for all MDCT scanners without requiring hardware modification. The central enabling technology is a most recently developed image reconstruction method: Prior image constrained compressed sensing (PICCS). Using the method, cardiac CT images can be accurately reconstructed using the projection data acquired in an angular range of about 120°, which is roughly 50% of the standard short‐scan angular range ( for an MDCT scanner). As a result, the temporal resolution of MDCT cardiac imaging can be universally improved by approximately a factor of 2. In order to validate the proposed method, two in vivo animal experiments were conducted using a state‐of‐the‐art 64‐slice CT scanner (GE Healthcare, Waukesha, WI) at different gantry rotation times and different heart rates. One animal was scanned at heart rate of 83 beats per minute (bpm) using gantry rotation time and the second animal was scanned at using gantry rotation time, respectively. Cardiac coronary CT imaging can be successfully performed at high heart rates using a single‐source MDCT scanner and projection data from a single heart beat with gantry rotation times of 400 and . Using the proposed PICCS method, the temporal resolution of cardiac CT imaging can be effectively improved by approximately a factor of 2 without modifying any scanner hardware. This potentially provides a new method for single‐source MDCT scanners to achieve reliable coronary CT imaging for patients at higher heart rates than the current heart rate limit of without using the well‐known multisegment FBP reconstruction algorithm. This method also enables dual‐source MDCT scanner to achieve higher temporal resolution without further hardware modifications.
Full‐field acoustomammography using an acousto‐optic sensor
Authors: Sandhu J. S. , Schmidt R. A. , La Rivière P. J.
Published: May 2009

In this Letter the authors introduce a wide‐field transmission ultrasound approach to breast imaging based on the use of a large area acousto‐optic (AO) sensor. Accompanied by a suitable acoustic source, such a detector could be mounted on a traditional mammography system and provide a mammographylike ultrasound projection image of the compressed breast in registration with the x‐ray mammogram. The authors call the approach acoustography. The hope is that this additional information could improve the sensitivity and specificity of screening mammography. The AO sensor converts ultrasound directly into a visual image by virtue of the acousto‐optic effect of the liquid crystal layer contained in the AO sensor. The image is captured with a digital video camera for processing, analysis, and storage. In this Letter, the authors perform a geometrical resolution analysis and also present images of a multimodality breast phantom imaged with both mammography and acoustography to demonstrate the feasibility of the approach. The geometric resolution analysis suggests that the technique could readily detect tumors of diameter of 3 mm using 8.5 MHz ultrasound, with smaller tumors detectable with higher frequency ultrasound, though depth penetration might then become a limiting factor. The preliminary phantom images show high contrast and compare favorably to digital mammograms of the same phantom. The authors have introduced and established, through phantom imaging, the feasibility of a full‐field transmission ultrasound detector for breast imaging based on the use of a large area AO sensor. Of course variations in attenuation of connective, glandular, and fatty tissues will lead to images with more cluttered anatomical background than those of the phantom imaged here. Acoustic coupling to the mammographically compressed breast, particularly at the margins, will also have to be addressed.
Energy spectrum control for modulated proton beams
Authors: Hsi Wen C. , Moyers Michael F. , Nichiporov Dmitri , Anferov Vladimir , Wolanski Mark , Allgower Chris E. , Farr Jonathan B. , Mascia Anthony E. , Schreuder Andries N.
Published: May 2009

In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than . To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.
Code status discussions and goals of care among hospitalised adults
Authors: Kaldjian L C , Erekson Z D , Haberle T H , Curtis A E , Shinkunas L A , Cannon K T , Forman-Hoffman V L
Published: May 2009

Background and objective:

Code status discussions may fail to address patients’ treatment-related goals and their knowledge of cardiopulmonary resuscitation (CPR). This study aimed to investigate patients’ resuscitation preferences, knowledge of CPR and goals of care.

Design, setting, patients and measurements:

135 adults were interviewed within 48 h of admission to a general medical service in an academic medical centre, querying code status preferences, knowledge about CPR and its outcome probabilities and goals of care. Medical records were reviewed for clinical information and code status documentation.

Results:

41 (30.4%) patients had discussed CPR with their doctor, 116 (85.9%) patients preferred full code status and 11 (8.1%) patients expressed code status preferences different from the code status documented in their medical record. When queried about seven possible goals of care, patients affirmed an average of 4.9 goals; their single most important goals were broadly distributed, ranging from being cured (n = 36; 26.7%) to being comfortable (n = 8; 5.9%). Patients’ mean estimate of survival to discharge after CPR was 60.4%. Most patients believed it was helpful to discuss goals of care (n = 95; 70.4%) and the chances of surviving inhospital CPR (n = 112; 83.0%). Some patients expressed a desire to change their code status after receiving information about survival following inhospital CPR (n = 11; 8.1%) or after discussing goals of care (n = 2; 1.5%).

Conclusions:

Doctors need to address patients’ knowledge about CPR and take steps to avoid discrepancies between treatment orders and patients’ preferences. Addressing CPR outcome probabilities and goals of care during code status discussions may improve patients’ knowledge and influence their preferences.
A renewed, ethical defense of placebo-controlled trials of new treatments for major depression and anxiety disorders: Figure 1
Authors: Dunlop B W , Banja J
Published: May 2009

The use of placebo as a control condition in clinical trials of major depressive disorder and anxiety disorders continues to be an area of ethical concern. Typically, opponents of placebo controls argue that they violate the beneficent-based, “best proven diagnostic and therapeutic method” that the original Helsinki Declaration of 1964 famously asserted participants are owed. A more consequentialist, oppositional argument is that participants receiving placebo might suffer enormously by being deprived of their usual medication(s). Nevertheless, recent findings of potential for suicidality in young people treated with antidepressants, along with meta-analyses suggesting that antidepressants add no significant clinical benefit over placebos, warrant a re-evaluation of the arguments against placebo. Furthermore, the nature of placebo treatment in short-term clinical trials is often not well understood, and lack of understanding can foster opposition to it. This paper will show how scientific justifications for placebo use are morally relevant. The fundamental ethical importance of placebo controls is discussed in relation to several aspects of clinical trials, including detection of adverse events, accurate assessment of clinical benefit, advancing understanding of the heterogeneity of depression and anxiety disorders and respecting informed consent requirements. Prohibiting the use of placebo controls is morally concerning in that such prohibitions allow for the possibility of serious adverse public health consequences. Moral worries that research participants receiving placebo are being unduly jeopardised will be shown to be exaggerated, especially in relation to the net benefits for end-users to be gained from the quality of data resulting from using placebo controls.
Enhancement and human nature: the case of Sandel
Authors: Lewens T
Published: May 2009

If we assume that “enhancement” names all efforts to boost human mental and physical capacities beyond the normal upper range found in our species, then enhancement covers such a broad range of interventions that it becomes implausible to think that there is any generic ethical case to be made either for or against it. Michael Sandel has recently made such a generic case, which focuses on the importance of respecting the “giftedness” of human nature. Sandel succeeds in diagnosing an important worry we may have about the use of some enhancements by some parents, but his arguments are better understood as opposing “procrustean parenting” rather than enhancement in general.