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Related Concept Videos

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

1.2K
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
1.2K
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

1.5K
The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
1.5K
Prochirality02:05

Prochirality

4.7K
The concept of prochirality leads to the nomenclature of the individual faces of a molecule and plays a crucial role in the enantioselective reaction. It is a concept where two or more achiral molecules react to produce chiral products. A typical process is the reaction of an achiral ketone to generate a chiral alcohol. Here, the achiral reactant reacts with an achiral reducing agent, sodium borohydride, to generate an equimolar mixture of the chiral enantiomers of the product. For example, an...
4.7K
Stereoisomerism of Cyclic Compounds02:33

Stereoisomerism of Cyclic Compounds

10.7K
In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
10.7K
Conformations of Cyclohexane02:11

Conformations of Cyclohexane

14.8K
Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal...
14.8K
Chair Conformation of Cyclohexane02:02

Chair Conformation of Cyclohexane

17.5K
The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.
The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this...
17.5K

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Updated: Dec 12, 2025

Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy
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Atomic Scale Structural Studies of Macromolecular Assemblies by Solid-state Nuclear Magnetic Resonance Spectroscopy

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Unsupervised MR-to-CT Synthesis Using Structure-Constrained CycleGAN.

Heran Yang, Jian Sun, Aaron Carass

    IEEE Transactions on Medical Imaging
    |August 12, 2020
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    Summary
    This summary is machine-generated.

    This study introduces a structure-constrained CycleGAN for synthesizing CT images from MRI scans, improving accuracy and visual quality for cancer radiotherapy planning. The method ensures anatomical consistency, crucial for precise treatment.

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    Area of Science:

    • Medical Imaging
    • Artificial Intelligence
    • Radiotherapy

    Background:

    • Synthesizing CT images from MR images is vital for radiotherapy planning.
    • Existing CycleGAN methods lack structural consistency, leading to anatomical inaccuracies.
    • This poses a significant challenge for precise cancer treatment.

    Purpose of the Study:

    • To develop a structure-constrained CycleGAN for unsupervised MR-to-CT image synthesis.
    • To ensure anatomical consistency between input MR and synthetic CT images.
    • To improve the accuracy and visual quality of synthetic CT images for radiotherapy.

    Main Methods:

    • Proposed a structure-constrained CycleGAN incorporating a structure-consistency loss using modality-independent neighborhood descriptors.
    • Employed spectral normalization for training stability.
    • Integrated a self-attention module for modeling long-range spatial dependencies.
    • Utilized approximate affine pre-registration for unpaired training data.

    Main Results:

    • The proposed method significantly improved accuracy and visual quality in unsupervised MR-to-CT synthesis for brain and abdomen images.
    • Demonstrated superior performance compared to existing unsupervised synthesis methods.
    • Showcased the benefit of pre-registration for enhancing synthesis outcomes.

    Conclusions:

    • The structure-constrained CycleGAN effectively addresses the anatomical inconsistency issue in MR-to-CT synthesis.
    • The method offers a promising solution for generating high-quality synthetic CT images for radiotherapy planning.
    • Further improvements can be achieved with pre-registration of unpaired training data.