Photon-Counting Detector CT for Liver Lesion Detection-Optimal Virtual Monoenergetic Energy for Different Simulated Patient Sizes and Radiation Doses

OBJECTIVES The aim of this study was to evaluate the optimal energy level of virtual monoenergetic images (VMIs) from photon-counting detector computed tomography (CT) for the detection of liver lesions as a function of phantom size and radiation dose. MATERIALS AND METHODS An anthropomorphic abdominal phantom with liver parenchyma and lesions was imaged on a dual-source photon-counting detector CT at 120 kVp. Five hypoattenuating lesions with a lesion-to-background contrast difference of -30 HU and -45 HU and 3 hyperattenuating lesions with +30 HU and +90 HU were used. The lesion diameter was 5-10 mm. Rings of fat-equivalent material were added to emulate medium- or large-sized patients. The medium size was imaged at a volume CT dose index of 5, 2.5, and 1.25 mGy and the large size at 5 and 2.5 mGy, respectively. Each setup was imaged 10 times. For each setup, VMIs from 40 to 80 keV at 5 keV increments were reconstructed with quantum iterative reconstruction at a strength level of 4 (QIR-4). Lesion detectability was measured as area under the receiver operating curve (AUC) using a channelized Hotelling model observer with 10 dense differences of Gaussian channels. RESULTS Overall, highest detectability was found at 65 and 70 keV for both hypoattenuating and hyperattenuating lesions in the medium and large phantom independent of radiation dose (AUC range, 0.91-1.0 for the medium and 0.94-0.99 for the large phantom, respectively). The lowest detectability was found at 40 keV irrespective of the radiation dose and phantom size (AUC range, 0.78-0.99). A more pronounced reduction in detectability was apparent at 40-50 keV as compared with 65-75 keV when radiation dose was decreased. At equal radiation dose, detection as a function of VMI energy differed stronger for the large size as compared with the medium-sized phantom (12% vs 6%). CONCLUSIONS Detectability of hypoattenuating and hyperattenuating liver lesions differed between VMI energies for different phantom sizes and radiation doses. Virtual monoenergetic images at 65 and 70 keV yielded highest detectability independent of phantom size and radiation dose

Solid-state NMR enhanced by dynamic nuclear polarization as a novel tool for ribosome structural biology

The impact of Nuclear Magnetic Resonance (NMR) on studies of large macromolecular complexes hinges on improvements in sensitivity and resolution. Dynamic nuclear polarization (DNP) in the solid state can offer improved sensitivity, provided sample preparation is optimized to preserve spectral resolution. For a few nanomoles of intact ribosomes and an 800kDa ribosomal complex we demonstrate that the combination of DNP and magic-angle spinning NMR (MAS-NMR) allows one to overcome current sensitivity limitations so that homo- and heteronuclear 13C and 15N NMR correlation spectra can be recorded. Ribosome particles, directly pelleted and frozen into an NMR rotor, yield DNP signal enhancements on the order of ~25-fold and spectra that exhibit narrow linewidths, suitable for obtaining site-specific information. We anticipate that the same approach is applicable to other high molecular weight complexe

Broadband excitation in solid-state NMR of paramagnetic samples using Delays Alternating with Nutation for Tailored Excitation ('Para-DANTE')

This Letter shows that interleaved sequences of short pulses in the manner of 'Delays Alternating with Nutation for Tailored Excitation' (DANTE) with N = 1,2,3 . . . equidistant pulses per rotor period extending over K rotor periods can be used to excite, invert or refocus a large number of spinning sidebands of spin-1/2 nuclei in paramagnetic samples where hyperfine couplings lead to very broad spectra that extend over more than 1 MHz. The breadth of the response is maintained for rf-field amplitudes as low as 30 kHz since it results from cumulative effects of individual pulses with very short durations

Solid-state NMR enhanced by dynamic nuclear polarization as a novel tool for ribosome structural biology

The impact of Nuclear Magnetic Resonance (NMR) on studies of large macromolecular complexes hinges on improvements in sensitivity and resolution. Dynamic nuclear polarization (DNP) in the solid state can offer improved sensitivity, provided sample preparation is optimized to preserve spectral resolution. For a few nanomoles of intact ribosomes and an 800 kDa ribosomal complex we demonstrate that the combination of DNP and magic-angle spinning NMR (MAS-NMR) allows one to overcome current sensitivity limitations so that homo- and heteronuclear C-13 and N-15 NMR correlation spectra can be recorded. Ribosome particles, directly pelleted and frozen into an NMR rotor, yield DNP signal enhancements on the order of similar to 25-fold and spectra that exhibit narrow linewidths, suitable for obtaining site-specific information. We anticipate that the same approach is applicable to other high molecular weight complexes

Extending Timescales and Narrowing Linewidths in NMR

Among the different fields of research in nuclear magnetic resonance (NMR) which are currently investigated in the Laboratory of Biomolecular Magnetic Resonance (LRMB), two subjects that are closely related to each other are presented in this article. On the one hand, we show how to populate long-lived states (LLS) that have long lifetimes T_LLS which allow one to go beyond the usual limits imposed by the longitudinal relaxation time T_1. This makes it possible to extend NMR experiments to longer time-scales. As an application, we demonstrate the extension of the timescale of diffusion measurements by NMR spectroscopy. On the other hand, we review our work on long-lived coherences (LLC), a particular type of coherence between two spin states that oscillates with the frequency of the scalar coupling constant J_IS and decays with a time constant T_LLC. Again, this time constant T_LLC can be much longer than the transverse relaxation time T_2. By extending the coherence lifetimes, we can narrow the linewidths to an unprecedented extent. J-couplings and residual dipolar couplings (RDCs) in weakly-oriented phases can be measured with the highest precision

Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13C and 29Si NMR Enhanced by Dynamic Nuclear Polarization

We show that dynamic nuclear polarization (DNP) can be used to enhance NMR signals of13C and 29Si nuclei located in mesoporous organic/inorganic hybrid materials, at several hundreds of nanometers from stable radicals (TOTAPOL) trapped in the surrounding frozen disordered water. The approach is demonstrated using mesoporous silica nanoparticles (MSN), functionalized with 3-(N-phenylureido)propyl (PUP) groups, filled with the surfactant cetyltrimethylammonium bromide (CTAB). The DNP-enhanced proton magnetization is transported into the mesopores via 1H–1H spin diffusion and transferred to rare spins by cross-polarization, yielding signal enhancements εon/off of around 8. When the CTAB molecules are extracted, so that the radicals can enter the mesopores, the enhancements increase to εon/off ≈ 30 for both nuclei. A quantitative analysis of the signal enhancements in MSN with and without surfactant is based on a one-dimensional proton spin diffusion model. The effect of solvent deuteration is also investigated

Improving Sensitivity in Solid State NMR:Surfaces, Quadrupolar Nuclei and Dynamic Nuclear Polarization

Sensitivity in Nuclear Magnetic Resonance (NMR), especially in solid-state NMR, has always been a challenging and important issue and thus a motivation for new developments. The magnetic field B0, the gyromagnetic ratios of the observed nuclei, as well as the preparation of the samples are important for optimizing the sensitivity. Often only a small amount of sample is available and the nuclei of interest have low gyromagnetic ratios, which makes solid-state NMR extremely time consuming and, in some cases, practically impossible. In addition, strong interactions can cause extensive line-broadening, thus making it difficult to excite or detect high resolution spectra. In this thesis two main aspects concerning sensitivity in solid-state NMR are addressed. The first part reveals the difficulties of the excitation and detection of quadrupolar nuclei, in particular of the highly abundant nucleus nitrogen-14, and presents an indirect detection method combined with a more efficient excitation and reconversion scheme using Delays Alternating with Nutations for Tailored Excitation (DANTE). The second part presents a new experimental set-up for Dynamic Nuclear Polarization (DNP), which has evolved into a very successful method at the Ecole Polytechnique Fédérale de Lausanne. It is shown in this work that the DNP method combined with solid-state NMR allows one to enhance NMR signal intensities by one or two orders of magnitude, thus opening the doors towards a great variety of new exciting applications. In solids spinning at the magic angle, the indirect detection of single-quantum (SQ) and double-quantum (DQ) 14N spectra (I = 1) via spy nuclei S = 1/2 such as protons can be achieved in the manner of heteronuclear single- or multiple-quantum correlation (HSQC or HMQC) spectroscopy. The coherence transfer is achieved by recoupling the heteronuclear dipolar interactions, i.e., with symmetry-based sequences. The line broadening caused by the homogeneous decay of the transverse terms of the spy nuclei S can be reduced by implementing suitable dipolar decoupling schemes during the evolution period. The lack of efficiency of the excitation and reconversion can be overcome by replacing the conventional rectangular pulses applied to the nitrogen-14 nuclei by a train of short rotor-synchronized pulses in the manner of DANTE. These pulse trains excite a large number of crystallites in the sample uniformly and allow one to efficiently excite magnetization over large bandwidths, limited only by the quality factor of the probe. When combining N interleaved DANTE schemes, obtained by applying N pulses per rotor period, leading to so-called "DANTE-N" sequences, one obtains methods that greatly improve the uniformity of the excitation and lead to higher signal intensity in less rotor periods than basic DANTE. Applications to nitrogen-14 with fast magic angle spinning (typically νrot ≥ 60 kHz), using either direct or indirect detection, are backed up by simulations that provide insight into the properties of basic and interleaved DANTE sequences. DNP exploits the polarization transfer, driven by microwave irradiation, from unpaired electrons, usually carried by polarization agents such as TEMPO or TOTAPOL, to the target nuclei. This transfer results in enhancements of the signal-to-noise ratios and thus opens many new possibilities for challenging samples. In this thesis the experimental requirements are demonstrated and important parameters like the temperature dependence are investigated. Various sample preparation methods are proposed, e.g., by attaching the radical directly to the molecules under investigation, instead of preparing a solution that should forma glass upon freezing. The combination of indirect detection of 14N with DNP improves the signal-to-noise ratio dramatically and shortens the recovery delay between subsequent experiments. Besides samples of biological interest, it was also shown that the NMR signals of molecules on surfaces of various materials can be greatly enhanced by DNP. A characterization of the distribution of surface binding modes and interactions in a series of functionalized materials could be demonstrated. The bonding topology of functional groups in materials obtained via a sol-gel process and in materials prepared by post-grafting reactions could be identified and compared. Furthermore, the remarkable gain in time afforded by DNP allows the facile acquisition of two-dimensional correlation spectra. The surfaces were characterized by transferring the polarization from protons to other nuclei such as 13C, 29Si and 27Al by means of cross-polarization

Procédé pour spectroscopie RMN d'échantillons solides

A method for performing magnetic resonance spectroscopy on solid sample containing nuclei of interest with spin quantum number I, comprising: subjecting the sample to a static magnetic field, spinning the sample at the magic angle and broad-band excitation of transverse magnetization of the nuclei of interest, is characterized in that broad-band excitation is carried out by applying a first train of rotor-synchronized rf-pulses with a carrier frequency to the nuclei of interest with a pulse duration 0.1 µs 1. With the inventive method uniform excitation of a great number of spinning sidebands or families of sidebands that arise from large first-order quadrupole or hyperfine interactions is enabled and signal intensity can be improved