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Introduction to Combining MRI with PET
Published in George C. Kagadis, Nancy L. Ford, Dimitrios N. Karnabatidis, George K. Loudos, Handbook of Small Animal Imaging, 2018
Volkmar Schulz, Jakob Wehner, Yannick Berker
The B0 distortion map is measurable within the MRI using the phase images of two gradient echo (GRE) sequences with two different echo times TE,1 and TE,2 (TE extension: ΔTE = TE,2 − TE,1). A large phantom filling the FOV under investigation is used. In a given pixel, the phase advance between both measurements is given by Δϕ = 2πΔfΔTE, whereby Δf is proportional to ΔB0, the local B0 modification. Thus, via subtraction of both phase images and rescaling, the B0 distortion map can be reconstructed. However, the B0 map contains information only about the field homogeneity and not about the absolute B0 field at a given point. The absolute scale of B0 could be shifted and has to be measured separately by determining the resonance frequency using, for example, spectroscopic methods. A well-defined phantom material with a clear resonance spectrum (e.g., no peak splitting due to chemical shifts) is preferred to quantitatively measure any shifts. Tetramethylsilane (TMS), for example, is widely accepted in NMR spectroscopy as reference material since it provides single-peak spectra for 1H, 13C, and 29Si nuclei due to its highly symmetric structure and thus might be a suitable choice (Mohrig et al. 2006).
Asphalt Chemistry: An NMR Investigation of the Benzylic Hydrogens and Oxidation
Published in Arthur M. Usmani, Asphalt Science and Technology, 1997
R. W. Jennings, Jacqueline Fonnesbeck, Jennifer Smith, J.A.S. Pribanic
Carbon and hydrogen nuclei devoid of any surrounding electrons have specific resonance frequencies dictated by the static magnetic field, Ha, of the superconducting magnet used in the experiment. However, in reality the carbon and hydrogen nuclei are surrounded by electrons. These electrons shield the nuclei so that the resonance frequencies are shifted. The extent of the shift depends upon the electron density, which differs for the different carbon and hydrogen functionalities in a molecule. Because the electron density surrounding the different carbon and hydrogen types depends on the compound and the compound’s molecular structure, the shift in resonance frequency is referred to as the chemical shift. The chemical shift values are dependent on the strength of the magnetic field. To remove the field dependence of the chemical shift for a given carbon or hydrogen type, chemical shifts are referenced to a standard compound and the shifts are reported in the dimensionless units of parts per million (ppm). Tetramethylsilane (TMS) is the most commonly used chemical shift reference compound for both carbon and hydrogen nuclei. The chemical shifts for different carbon types range from 0 to 215 ppm relative to TMS and for different hydrogen types range from 0 to 15 ppm. Nuclei that are less shielded than the reference compound are assigned positive values of the chemical shift. Aromatic carbons and hydrogens are less shielded by electrons than aliphatic carbons and hydrogens and, thus, have larger positive chemical shift values.
Spectroscopy
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
There has to be a “zero point” in order to provide a well-defined position in the nmr for a given molecule that can be repeated and identified. A scale makes no sense unless there is a peak that all other peaks in the spectrum can be related to. Such a peak is due to an added compound that can be used as a reference standard. This standard must be taken as the zero point in all samples under all conditions. All absorption peaks are therefore reported with reference to this standard. The most common standard in nmr is the chemical tetramethylsilane (TMS), which is added to the sample. Where is the “zero point” in nmr?
Synthesis, spectroscopic profiling, biological evaluation, DFT, molecular docking and mathematical studies of 3,5-diethyl-2r,6c-diphenylpiperidin-4-one picrate
Published in Molecular Physics, 2023
S. Bharanidharan, S. Savithiri, G. Rajarajan, P. Sugumar, A. Nelson
All the solvents used were of spectral grade. The compound’s uncorrected melting point was determined using open capillaries. KBr was used to record IR spectra on a Thermo Nicolet AVATAR-330 FT-IR spectrometer (pellet form). Using DMSO-d6 as the compound’s solvent, 1H NMR spectra were captured at 400 MHz and 13C NMR spectra at 100 MHz on a BRUKER model. All NMR spectra employed tetramethylsilane (TMS) as an internal reference, and chemical shifts were recorded in units (parts per million) in relation to the standard. Singlet (s), doublet (d), doublet of doublet (dd), triplet (t), quartet (q), and multiplet are the several types of 1H NMR splitting patterns (m). Coupling constants are expressed in Hertz (Hz). The VarioMicro V2.2.0 CHN analyser was used to perform the microanalyses. An UV-Visible spectrophotometer made by Shimadzu (UV 1650 PC model) was used to measure the absorption spectrum. Using different solvents, fluorescence spectra were captured using a Varian RF-5310PC spectrofluorimeter.