All articles starting with S

Spectral computed tomography

Spectral computed tomography is an X-ray imaging method that measures attenuation in separate energy ranges, producing iodine maps, virtual non-contrast images, and uric acid stone differentiation.

Spectral counting is a label-free quantification method in shotgun proteomics that estimates relative protein abundance by counting identified tandem mass spectra, formalized in 2004 by Liu, Sadygov, and Yates.

Spectral CT

Spectral CT, including dual-energy CT and photon-counting CT, is computed tomography that acquires X-ray attenuation data at two or more energy levels, enabling material decomposition and tissue characterization.

Spectral decomposition (seismic)

Spectral decomposition in seismology is a family of time-frequency analysis methods that breaks a seismic trace into its frequency components, revealing bed thickness variations, discontinuities, and fluid effects.

Spectral density

The spectral density, or spectral power density, describes how the power or energy of a signal or random process is distributed over frequency, revealing oscillations and noise hidden in the time domain.

Spectral density estimation

Spectral density estimation is the set of statistical methods for estimating a time series' power spectral density, showing how its variance is distributed across frequency.

Spectral diffusion

Spectral diffusion is the random change over time of a spectral line's resonance frequency caused by environmental fluctuations, appearing as random spectral jumps in optical and spin resonance spectroscopy.

Spectral dimension

The spectral dimension is a scale-dependent measure of effective dimensionality, defined through diffusion; in quantum gravity it typically flows from 4 to 2 at small scales.

Spectral element method

The spectral element method is a numerical technique for solving partial differential equations, combining high-order polynomial basis functions with a finite-element mesh for exponential convergence.

Spectral embedding

Spectral embedding is a family of nonlinear dimensionality reduction methods that represent data points or graph nodes in low-dimensional space using eigenvectors of a graph Laplacian built from pairwise similarities.

Spectral fingerprinting

Spectral fingerprinting is an analytical chemistry method that identifies a substance by matching its infrared, Raman, terahertz, NMR, or mass spectrum against reference spectra, yielding a match score rather than a quantitative composition.

Spectral flow cytometry

Spectral flow cytometry is a single-cell analysis technique that measures the full fluorescence spectrum of each cell, separating dyes by spectral unmixing in panels of up to 50 colors.

Spectral gap (Markov chain)

The spectral gap of a Markov chain, for a reversible chain equal to 1 minus the second-largest eigenvalue, measures how fast the chain converges to its stationary distribution.

Spectral graph theory

Spectral graph theory is the branch of graph theory that studies graphs through eigenvalues and eigenvectors of their adjacency, degree, and Laplacian matrices, linking algebra to properties like connectivity.

Spectral hole burning

Spectral hole burning is a spectroscopy technique in which a narrow-band laser bleaches a dip into an inhomogeneously broadened absorption line, resolving features 1,000 to 100,000 times finer than conventional methods.

Spectral index (cosmology)

The scalar spectral index, ns, is the exponent describing how the amplitude of primordial density fluctuations depends on scale, measured near 0.965 and seen as key evidence for cosmic inflation.

Spectral interferometry

Spectral interferometry is an optical measurement technique that records the interference spectrum of two light beams, retrieving spectral phase to measure distance, film thickness, and dispersion.

Spectral karyotyping

Spectral karyotyping (SKY) is a cytogenetic technique that paints all 24 human chromosomes in distinct colors in one experiment, revealing translocations and complex rearrangements invisible to banding analysis.

Spectral kurtosis

Spectral kurtosis is a statistical signal-processing measure that shows how a signal's impulsiveness varies across frequency bands, used chiefly to select demodulation bands for bearing and gear fault diagnosis.

Spectral library search

Spectral library search is a proteomics method that identifies peptides by matching experimental MS/MS spectra against a reference library, hundreds of times faster than sequence database search.

Spectral line

A spectral line is a narrow band of electromagnetic frequencies emitted or absorbed by atoms, appearing as a bright or dark feature in a spectrum.

Spectral line shape

The spectral line shape, also called the line profile, describes how a spectral line's intensity is distributed around its center, broadened mainly by Doppler and pressure effects.

Spectral mammography

Spectral mammography is an X-ray breast imaging technique that measures energy-dependent attenuation to characterize breast tissue, including contrast-enhanced spectral mammography, which uses iodinated contrast to show enhancing lesions.

Spectral method

A spectral method is a numerical technique for solving partial differential equations using global basis functions like Fourier modes, achieving exponential convergence for smooth solutions.

Spectral mixture analysis

Spectral mixture analysis (SMA) is a remote sensing technique that decomposes each pixel's spectrum into fractional abundances of endmember materials such as vegetation, soil, and impervious surface.

Spectral norm

The spectral norm of a matrix, also called the matrix 2-norm, is the operator norm induced by the Euclidean vector norm, equal to the matrix's largest singular value.

Spectral proper orthogonal decomposition

Spectral proper orthogonal decomposition (SPOD) is a modal decomposition method that extracts energy-ranked coherent structures from statistically stationary flow data, yielding orthogonal modes at each frequency.

Spectral radius

The spectral radius of a graph is the largest absolute value among the eigenvalues of its adjacency matrix, central to spectral graph theory and epidemic modeling.

Spectral sequence

A spectral sequence is a sequence of differential modules used to compute homology and cohomology groups page by page, introduced by Jean Leray in 1946.