Absolute doubly differential cross sections for electron bremsstrahlung from rare gas atoms at 28 and 50 keV

Absolute doubly differential bremsstrahlung cross sections from Xe, Kr, Ar, and Ne have been measured for electron bombarding energies of 28 and 50 keV. Bremsstrahlung photons have been detected at 90 degrees to the incident electron beam at energies ranging from 5 keV up to the kinematic end point. The results are compared with predictions of ordinary bremsstrahlung and of total bremsstrahlung that include polarizational bremsstrahlung from the target atom calculated in the stripping approximation. All previous absolute cross sections have been from thin-film solid targets and have not shown any polarizational bremsstrahlung contribution. The present results, the first from free atoms, provide definitive evidence for the contribution of polarizational bremsstrahlung to the photon spectrum from electron bremsstrahlung.     

Evaluation of a multiple spin- and gradient-echo (SAGE) EPI acquisition with SENSE acceleration: Applications for perfusion imaging in and outside the brain

Perfusion-based changes in MR signal intensity can occur in response to the introduction of exogenous contrast agents and endogenous tissue properties (e.g. blood oxygenation). MR measurements aimed at capturing these changes often implement single-shot echo planar imaging (ssEPI). In recent years ssEPI readouts have been combined with parallel imaging (PI) to allow fast dynamic multi-slice imaging as well as the incorporation of multiple echoes. A multiple spin- and gradient-echo (SAGE) EPI acquisition has recently been developed to allow measurement of transverse relaxation rate (R₂ and R₂^?) changes in dynamic susceptibility contrast (DSC)-MRI experiments in the brain. With SAGE EPI, the use of PI can influence image quality, temporal resolution, and achievable echo times. The effect of PI on dynamic SAGE measurements, however, has not been evaluated. In this work, a SAGE EPI acquisition utilizing SENSE PI and partial Fourier (PF) acceleration was developed and evaluated. Voxel-wise measures of R₂ and R₂^? in healthy brain were compared using SAGE EPI and conventional non-EPI multiple echo acquisitions with varying SENSE and PF acceleration. A conservative SENSE factor of 2 with PF factor of 0.73 was found to provide accurate measures of R₂ and R₂^? in white (WM) (r_R2 = [0.55–0.79], r_R2? = [0.47–0.71]) and gray (GM) matter (r_R2 = [0.26–0.59], r_R2? = [0.39–0.74]) across subjects. The combined use of SENSE and PF allowed the first dynamic SAGE EPI measurements in muscle, with a SENSE factor of 3 and PF factor of 0.6 providing reliable relaxation rate estimates when compared to multi-echo methods. Application of the optimized SAGE protocol in DSC-MRI of high-grade glioma patients provided T₁ leakage-corrected estimates of CBV and CBF as well as mean vessel diameter (mVD) and simultaneous measures of DCE-MRI parameters K^trans and v_e. Likewise, application of SAGE in a muscle reperfusion model allowed dynamic measures of R₂′, a parameter that has been shown to correlate with muscle oxy-hemoglobin saturation.     

Nonnegative solutions of a nonlinear recurrence

Orthonormal polynomials with weight ¦τ¦ exp(−τ⁴) have leading coefficients with recurrence properties which motivate the more general equations ξ_m(ξ_{m − 1} + ξ_m + ξ_{m + 1}) = γ_m², M = 1, 2,…, where ξ_o is a fixed nonnegative value and γ₁, γ₂,… are positive constants. For this broader problem, the existence of a nonnegative solution is proved and criteria are found for its uniqueness. Then, for the motivating problem, an asymptotic expansion of its unique nonnegative solution is obtained and a fast computational algorithm, with error estimates, is given.     

Magnetic resonance in the era of molecular imaging of cancer

Magnetic resonance imaging (MRI) has played an important role in the diagnosis and management of cancer since it was first developed, but other modalities also continue to advance and provide complementary information on the status of tumors. In the future, there will be a major continuing role for noninvasive imaging in order to obtain information on the location and extent of cancer, as well as assessments of tissue characteristics that can monitor and predict treatment response and guide patient management. Developments are currently being undertaken that aim to provide improved imaging methods for the detection and evaluation of tumors, for identifying important characteristics of tumors such as the expression levels of cell surface receptors that may dictate what types of therapy will be effective and for evaluating their response to treatments. Molecular imaging techniques based mainly on radionuclide imaging can depict numerous, specific, cellular and molecular markers of disease and have unique potential to address important clinical and research challenges. In this review, we consider what continuing and evolving roles will be played by MRI in this era of molecular imaging. We discuss some of the challenges for MRI of detecting imaging agents that report on molecular events, but highlight also the ability of MRI to assess other features such as cell density, blood flow and metabolism which are not specific hallmarks of cancer but which reflect molecular changes. We discuss the future role of MRI in cancer and describe the use of selected quantitative imaging techniques for characterizing tumors that can be translated to clinical applications, particularly in the context of evaluating novel treatments.     

UTILIZATION OF TOOLS FOR SUCCESSFUL. TROUBLESHOOTING OF STRUCTURAL PROBLEMS

A structural failure problem was solved using an integrated and iterative program of testing and analysis. The steps taken in solving the problem were: analytical calculations; operational testing; qualifications of analytical results; problem identification; design of corrective action; and confirmatory testing.     

Efficient room-temperature operation of Cr3+-sensitized,flashlamp-pumped, 2µm lasers

Utilizing the results of Cr³⁺ → Tm³⁺ transfer efficiency studies, we have demonstrated that yttrium aluminium garnet (YAG) is the preferred host for room-temperature, flashlamp-pumped solid-state lasers operating in the 2.0 µm spectral range. We report data on two different sensitizer-activator combinations in YAG and yttrium scandium gallium garnet (YSGG) laser materials: one is doped with Cr:Tm:Ho and operates on the Ho³⁺⁵I₇ →⁵I₈ transition at 2.097 µm; the other is doped only with Cr:Tm, which lases on the Tm³⁺³F₄ →³H₆ transition at 2.014 µm. We have achieved a slope efficiency of 5.1% with the Cr:Tm:Ho:YAG laser, which is the highest slope efficiency yet reported for a room-temperature, flashlamp-pumped, 2 µm solid-state laser. We have measured thresholds as low as 38 J and output energies >1.5 J for that system. We also report the first room-temperature operation of an efficient flashlamp-pumped Cr:Tm:YAG laser at 2.014 µm. Thresholds as low as 43 J, output energies exceeding 2 J, and slope efficiencies as high as 4.5% have been achieved. This is an order of magnitude higher than the efficiency previously reported for a 2.01 µm Cr:Tm:YAG laser operated at cryogenic temperatures. These two efficient 2 µm laser systems (Cr:Tm:Ho:YAG and Cr:Tm:YAG) are discussed in terms of their potential for Q-switched operation.     

Efficient room-temperature operation of Cr3+-sensitized, flashlamp-pumped, 2μm lasers

Utilizing the results of Cr³⁺ Tm³⁺ transfer efficiency studies, we have demonstrated that yttrium aluminium garnet (YAG) is the preferred host for room-temperature, flashlamp-pumped solid-state lasers operating in the 2.0 µm spectral range. We report data on two different sensitizer-activator combinations in YAG and yttrium scandium gallium garnet (YSGG) laser materials: one is doped with Cr:Tm:Ho and operates on the Ho^{3+ 5}I₇ ⁵I₈ transition at 2.097 µm; the other is doped only with Cr:Tm, which lases on the Tm^{3+ 3}F₄ ³H₆ transition at 2.014 µm. We have achieved a slope efficiency of 5.1% with the Cr:Tm:Ho:YAG laser, which is the highest slope efficiency yet reported for a room-temperature, flashlamp-pumped, 2 µm solid-state laser. We have measured thresholds as low as 38 J and output energies >1.5 J for that system. We also report the first room-temperature operation of an efficient flashlamp-pumped Cr:Tm:YAG laser at 2.014 µm. Thresholds as low as 43 J, output energies exceeding 2 J, and slope efficiencies as high as 4.5% have been achieved. This is an order of magnitude higher than the efficiency previously reported for a 2.01 µm Cr:Tm:YAG laser operated at cryogenic temperatures. These two efficient 2 µm laser systems (Cr:Tm:Ho:YAG and Cr:Tm:YAG) are discussed in terms of their potential for Q-switched operation.     

Simultaneous multielement detection in particle beam/hollow cathode-optical emission spectroscopy

Presented here is the development of a particle beam/hollow cathode-optical emission spectroscopy source that has been interfaced with a high resolution polychromator for use as a species-specific detector for chromatographic separations. Use of the high resolution JY RF-5000 polychromator allows simultaneous, multielement analysis; a necessary requirement for comprehensive speciation analysis. Parametric optimization was performed for the nebulization conditions, desolvation temperature, glow discharge current and pressure, and the source block temperature (vaporization) using nitrate salts containing lead, nickel, and silver. Peak area, height, and width were recorded for optical emission of Pb (I) 220.35 nm, Ni (I) 341.41 nm, and Ag (I) 338.28 nm in order to determine optimal peak characteristics under chromatographic separation conditions. Response curves for a multielement salt solution containing Pb, Ni, and Ag were obtained using the optimized conditions, with detection limits for triplicate injections of 2.2, 0.17, and 0.19 ng, respectively. The ability to monitor multiple elements simultaneously reveals the existence of interelement matrix effects that have not been noted previously in hollow cathode devices. The ability to monitor metals and non-metals is demonstrated towards the future application of this system as a tool for metallomic studies.     

Comparison of dual-echo DSC-MRI- and DCE-MRI-derived contrast agent kinetic parameters

The application of dynamic susceptibility contrast (DSC) MRI methods to assess brain tumors is often confounded by the extravasation of contrast agent (CA). Disruption of the blood-brain barrier allows CA to leak out of the vasculature leading to additional T(1), T(2) and T(2) relaxation effects in the extravascular space, thereby affecting the signal intensity time course in a complex manner. The goal of this study was to validate a dual-echo DSC-MRI approach that separates and quantifies the T(1) and T(2) contributions to the acquired signal and enables the estimation of the volume transfer constant, K(trans), and the volume fraction of the extravascular extracellular space, v(e). To test the validity of this approach, DSC-MRI- and dynamic contrast enhanced (DCE) MRI-derived K(trans) and v(e) estimates were spatially compared in both 9L and C6 rat brain tumor models. A high degree of correlation (concordance correlation coefficients >0.83, Pearson's r>0.84) and agreement was found between the DSC-MRI- and DCE-MRI-derived measurements. These results indicate that dual-echo DSC-MRI can be used to simultaneously extract reliable DCE-MRI kinetic parameters in brain tumors in addition to conventional blood volume and blood flow metrics.