Description: Purpose The quantification of myocardial perfusion using a Look–Locker flow-sensitive alternating inversion recovery- arterial spin labeling experiment is considered. Due to the anatomy of the heart, a substantial but unintended partial inversion of the inflowing blood occurs during the slice-selective inversion. Both, the partial inversion as well as the Look–Locker pulse train, influence the myocardial perfusion quantification and are addressed in this work. Methods The mean relaxation time approximation is used to calculate the monoexponential relaxation time of the signal in perfused tissue under Look–Locker readout. The left ventricular blood serves as an approximation of the inflowing blood in the description of FAIR-ASL measurements with global and slice-selective inversion to correctly quantify the myocardial perfusion. Results The analysis shows that the myocardial perfusion can be overestimated if the T 1 -based quantification method is not adapted respecting the Look–Locker pulse train explicitly. Additionally, it turns out that without correction for the partial inversion of the blood pool during the slice-selective inversion the myocardial perfusion is underestimated. Conclusion It is shown that the Look–Locker readout as well as the nonideal slice-selective inversion experiment have a considerable influence and have to be included properly to correctly quantify myocardial perfusion. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.

Description: Environmental Science & Technology DOI: 10.1021/es303778t

Description: Performance studies of a microscope-camera system (MCS) and a laser ablation/ionisation mass spectrometer (LIMS) instrument (referred to here as a laser mass spectrometer or LMS) are presented. These two instruments were designed independently for in situ analysis of solids on planetary surfaces and will be combined to a single miniature instrument suite for in situ chemical and morphological analysis of surface materials on planetary bodies. LMS can perform sensitive chemical (elemental, isotope and molecular) analyses with spatial resolution close to micrometre-sized grains. It allows for studies with mass resolution (M/ΔM) up to 800 in ablation mode (elemental composition) and up to 1500 in desorption mode (molecular analysis). With an effective dynamic range of at least eight orders of magnitude, sensitive and quantitative measurements can be conducted of almost all elements and isotopes with a concentration larger than a few ng g −1 . Hence, in addition to the major element composition, which is important for the determination of mineralogical constituents of surface materials, trace elements can also be measured to provide information on mineral formation processes. Highly accurate isotope ratio measurements can be used to determine in situ geochronology of sample material and for investigations of various isotope fractionation processes. MCS can conduct optical imagery of mm-sized objects at several wavelengths with micrometre spatial resolution for the characterisation of morphological surface details, and to provide insight to surface mineralogy. Furthermore, MCS can help in the selection of sample surface areas for further mass spectrometric analysis of the chemical composition. Surface auto-fluorescence measurements and images in polarised light are additional capabilities of the MCS, to identify either fluorescing minerals or organic materials, if present on the analysed surface, for further investigation by LMS. The results obtained by investigations of NIST reference materials, amino-acid films and a natural graphite sample embedded in silicate rock are presented to illustrate the performance of the instruments and their potential to deliver chemical information for mineral and organic phases in their geological context This article is protected by copyright. All rights reserved.

Description: Off-resonant spinlock (SL) enables an NMR imaging technique that can detect dilute metabolites similar to chemical exchange saturation transfer. However, in clinical MR scanners, RF pulse widths are restricted due to recommended specific absorption rate limits. Therefore, trains of short RF pulses that provide effective saturation during the required irradiation period are commonly employed. Quantitative evaluation of spectra obtained by pulsed saturation schemes is harder to achieve, since the theory of continuous wave saturation cannot be applied directly. In this paper we demonstrate the general feasibility of quantifying proton exchange rates from data obtained in pulsed SL experiments on a clinical 3 T MR scanner. We also propose a theoretical treatment of pulsed SL in the presence of chemical exchange using an interleaved saturation–relaxation approach. We show that modeling magnetization transfer during the pauses between the RF pulses is crucial, especially in the case of exchange rates that are small with respect to the delay times. The dynamics is still governed by a monoexponential decay towards steady state, for which we give the effective rate constant. The derived analytical model agrees well with the full numerical simulation of the Bloch–McConnell equations for a broad range of values of the system parameters. Copyright © 2014 John Wiley & Sons, Ltd. Saturation of a two-pool-CEST system by pulsed spinlock can be modeled by an interleaved saturation–relaxation (ISAR) approach. Taking into account the chemical exchange during the pause is crucial, and leads to an analytical solution that agrees well with the full numerical simulation of the Bloch–McConnell equations for a broad range of values of the system parameters. This analytical solution allows the quantitative evaluation of Z -spectra obtained from creatine model solutions on a clinical 3 T scanner.