Table 1.

A glossary of commonly used terms in elastography

Acoustic radiation force impulse (ARFI): Ultrasonic compression waves focused on a spot to create shear waves perpendicular to the ultrasound beam axis. pSWE and SWE use acoustic radiation force impulses (also called acoustic push pulses) to generate shear waves. ARFI may also refer to a particular technique commercialized by Siemens that includes both the wave generation method described above and a proprietary wave tracking method.

Attenuation: The loss in amplitude of waves through viscous tissue.

Compression wave: A type of wave in which oscillation motion is parallel to the direction of wave propagation; it is also known as P, primary, or longitudinal wave.

Complex shear modulus (G*): A measure of the overall resistance of a material to an applied shear stress; it has two components: an elastic component called the storage modulus, which is the real part of the complex shear modulus, and a viscous component called the loss viscous modulus, which is the imaginary part of the complex shear modulus.

Dynamic (shear) elastography: A type of elastography technique that relies on the production of shear waves and measurement of shear wave speed propagation.

Effective shear modulus (μ): A term used in the MR elastography literature to describe the resistance to deformation upon application of a shear stress; it is related to the shear wave speed by the following equation: μ = ρ c2, where μ is the effective shear modulus, ρ is the density of the tissue and c is the shear wave speed.

Elastic (Young’s) modulus (E): A measure of elasticity (spring-like behavior); it indicates the ability of a material to resist normal (perpendicular) deformation.

Elasticity: A characteristic of materials that tend to return to their initial shape after a deformation.

Elastogram: A graphical display (parametric map) indicating the spatial distribution of stiffness or stiffness-related parameter.

Elastography: A field of medical imaging that displays or measures the mechanical properties of soft tissues.

Inversion algorithm: Mathematical algorithms that allow calculation of mechanical properties from wave images.

Kilopascals (kPa): A unit of pressure (force per unit area); it is a measure of tissue stiffness in ultrasound elastography and MR elastography.

Loss (viscous) modulus (G″): The imaginary part of the complex shear modulus; it describes the dashpot-like, energy-absorbing and -damping behavior.

Point shear wave elastography (pSWE): An elastography technique that measures regional values of shear wave speed using acoustic radiation force impulse; the prototype is ARFI quantification.

Quasi-static elastography: A type of elastography technique that relies on measurement of tissue deformation that arise from cardiac or respiratory motion.

Shear stiffness: A term commonly used in the literature to describe the magnitude (absolute value) of the complex shear modulus.

Shear wave: A type of wave in which oscillation motion is perpendicular to the direction of wave propagation; it is also known as S, secondary, or transverse waves.

Shear wave elastography (SWE): A term used to describe elastography techniques that generate images of shear wave speed using excitations by acoustic radiation force; the prototype is supersonic shear imaging (SSI).

Static elastography: A type of elastography technique that relies on measurement of tissue deformation before and after manual compression with a transducer.

Stiffness: A qualitative property that has traditionally been assessed through palpation. The concept of “stiffness” refers to resistance to deformation in response to an applied force; it is defined mathematically by numerous moduli (elastic, shear, bulk) each one describing the resistance to deformation of a material in response to different types of stresses or applied pressures: tensile forces, shear forces, and volumetric compressive forces, respectively.

Storage (shear) modulus (G′): The real part of the complex shear modulus; it describes elasticity (spring-like, energy-storing behavior), i.e. the ability of a medium to resist shear (parallel to the surface) deformation without energy loss.

Transient elastography: A term used to designate techniques that rely on shear waves of short duration; the prototype is Fibroscan.

Viscoelasticity: A characteristic of materials that exhibit both viscous and elastic behavior when undergoing deformation.

Viscosity: A characteristic of materials that resists movement or deformation. It applies to both viscous fluid or viscous tissues.

Wave amplitude images: A graphic display of shear wave propagation often represented as a cine-loop.