see: Ultrasound in the Management of Xerostomia (Dry Mouth) Following Salivary Gland Irradiation and Salivary ultrasound standardized diagnostic approach and report
Shear Wave Elastography - Acoustic Radiation Force Impulse Imaging (ARFI) - Quantitative Elastography
Benefit of elasticity imaging: "many soft tissues can share similar ultrasonic echogenicities but may have different mechanical properties" and all elasticity methods introduce a mechnical excitation and monitor the resulting tissue responese" (Shiina 2015)
Definitions:
Ultrasound elastography (Sonoelastography) employs ultrasound in an analogous manner to that of assessing an organ by palpating it to determine its stiffness.
Sonoelastography assesses the physical character of salivary gland parenchyma (as a correlate with degree of fibrosis) by determining the speed of an acoustic signal passing through it. The greater the speed to the signal through the parenchyma, the greater the degree of stiffness.
This technique in its rudimentary form (operator dependent and semiquantitative) is accomplished by analyzing changes to the signal of a structure as it is compressed with increasing pressure applied to the ultrasound probe (Li 2021). Alternatively, quantitative assessment may be performed by introducing a 'push-pulse' (aka "ARFI = Acoustic Radiation Force Impulse) of a focused ultrasound beam that permits analysis of the shear waves it generates as it passes through the tissue under analysis (Ferraioli 2022).
Two basic concepts used for Ultrasound Elastography (Elbeblawy 2020):
1. Quasistatic = strain-based: "examination of the strain or defomation of a tissue due to a force"
According to Varghese (2009) "In quasi-static elastography, radiofrequency echo signals acquired before and after a small (about 1%) of applied deformation are corelated to estimate tissue displacements". Elastographic imaging is done with pre-and post-deformation signals estimated to quantify with report of Young's modulus to discriminate tissue chacteristics based on the hypothesis that soft tissues deforms more than stiffer tissues.
2. Dyamic or shear wave-based: "analysis of the propagation speed of a shear wave"
Shear wave: "a transverse wave that occurs in an elastic medium when it is subjected to a periodic shear" (Britannica 2022). "Shear" is further identified as 'the change of shape, without change of volume, of a layer of the substance, produced by a pair of equal forces acting on opposite directison along the two faces of the layer". Shear waves are considered transverse as distinguised from compressional waves which are longitudinal.
Young's Modulus (E) characterizes the stiffness of tissue that correlates with a clinicians perceptions of the firmness determined in palpating tissue (Gennsion 2013). It is a measue of the elasticity - the compressive stiffness when a force is applied lengthwise to quantify the relationship between stress (
conversion of the propagating speed of a shear wave to Young's modulus in kilopascals (kPA) can be done with the formula:
E=3ρVs2 E= tissue elasticity (kPa) / ρ= density of tissue in kg/m3 / Vs = the shear wave speed (Ferraioli and Barr 2022)
Elastography: Process wherein an external force is applied to tissue understudy to determine resulting movement - may be done by static, 'quasistatic', and dynamic methods - with 'strain' identifying the displacement generated to create a map termed an 'elastogram' (Gennsion 2013).
The origin of the term ‘Elastography’ was attributed by Varghese (2009) to Ophir (1991) who employed the term “Elastography” to refer to an ultrasound based technique wherein computation of shifts in echo arrival times were determined following quasi-static tissue deformation. Varghese identified that “Elastography” has become a more general term applied to other methods (including ultrasound) that image tissue stiffness and include application of MRI, CT, OCT, and other perturbation techniques to deform tissue.
Dynamic elastography requires a complex system to generate shear waves (via ultrasound radiation pressure) and the measure the displacements induced by the shear waves (ultrafast or stroboscopic ultrasound) (Gennsion 2013).
Quantitative asssessment of targetted tissue characteristics through shear wave elastography employs the transmission of sucessive pulses of acoustic power through the tissue (Sebag 2010).
ARFI (Acoustic Radiation Force Impulse Imaging) - use of a secondary ultrasound pulse (aka 'push pulse' = ARFI) to generate laterally propagating shear waves.
Shear Wave Velocity measurement of periods of time between generation of shear waves and their crossing of a region of interest (ROI) allows for calcultation of shear wave velocity (SWV in mm/sec) - with the correlate that the stiffer a region in of tissue - the faster the propopagation of the shear wave through the ROI (Badea 2013).
Shear wave dispersion - a more recently developed imaging technology (2018 Sugimoto; 2020 Sugimoto) to assess the 'dispersion slope' of shear waves - related to tissue viscosity in liver disease. There is a frequency dependence of shear wave velocity that can be calculated to provide an indirect estimate of tissue viscocity. Shear wave velocity is modified by the stiffness (reflecting fibrosis) of the tissue through which it passes. Shear wave velocity through the same tissue is also modified by the freqency of the shear wave delivered - with 'tissue viscosity' impacting the relationship between ultrasound frequency and speed creating a 'frequency dispersion'. A shear wave viscosity value can be calculated by the gradient of the shear wave speed = the slope of the graph of speed vs frequency. This gradient estimates the viscous components of tissue in that the slope will be altered by the tissue visocity.
as per Sugimoto et al (2020): In perferctly elastic tissue, shear wave speed is constant regardless of the shear wave frequency. The greater the variability that changed induced in shear wave speed through successive modification of the push pulse (ARFI), the higher the viscosity level. Employing a liver model, these investigators conclude that shear wave speed is more useful than shear wave dispersion slope assessment to predict degree of fibrosis. They suggest shear wave dispersion analysis is more useful than shear wave speed in predicting degree of necroflammation. They conclude that the dispersion slope - reflecting tissue visocity - may provide additional useful information in analyzing diffuse liver disease.
Elasticity "is the ability of a body to resist a distorting influence and to return to its original size and shape when that influence or force is removed"
Viscosity: a measure of the resistance of a fluid to deformation at a given rate (syrup has a higher viscosity than water) (Wikipedia contributors date retrieved 30 October 2022 'Viscosity') "viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion"
Anisotropy (Britannica on-line 2024): "... in physics, the quality of exhibiting properties with different values when measured along axes in different directions"
"...the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement.' . (Wikipedia contributors, "Anisotropy," Wikipedia, 2025)
Normal Values for Salivary Shear Wave Elastography
A. As per Muntean et al (Muntean 2022)
These investigators identify that 'vendor-specific cuf-off values might be necessary' provide due to variability reported by different ultrasound vendors and equipement - related to bandwidth and shear wave vibration mean frequency.
through study of 40 healthy subjects offered analysis with curvelinear probe (their linear probe was not embedded with the Vi.PLUS) to endeavor to discriminate the mechanical properties of:
Elasticity which they reported to be related to shear wave speed - as a measure of fibrosis -
and
Viscosity which they reported to be realted to shear wave dispersion - influenced by inflamatory changes - "Pi.PLUS analysis of shear wave dispersion 'quantitatively quantified in Pa.s" [difference in shear wave velocity across frequencies]
difference in elasticity and viscosity as they describe:
"Vi.PLUS analyses the shear wave propagation speed at different frequencies providing information regarding shear wave dispersion inside tissues. The difference in shear wave velocity across frequencies is quantitatively quantified in Pa.s and displayed on a color coded map. Elasticity measurements are displayed as mean, median, minimum, maximum, and SD, while viscosity measurements are displayed as mean, median, and SD. Information regarding the depth and diameter of the Q-Box and the Stability Index values are also provided."
"The mean value of three valid measurements obtained in a homogenous area from three different frames was considered (quantified in kPa for elasticity and Pa.s for viscosity, respectively)."
Study:
Subjects fasted for three hours before examination done with "SuperSonic MACH® 30 ultrasound system (SuperSonic Imagine, Aix-en-Provence, France). The ShearWave Elastography Plane-wave Ultrasound (2D-SWE.PLUS) and Viscosity Plane-wave Ultrasound (Vi.PLUS) modes were employed, available on the C6-1X curvilinear transducer".
Salivary gland imaging: neck in hyperextension and head oriented 'oppositely' to the examiner with 'longitudinal' views of each gland both unstimulated then after oral lemon juice held in mouth for 10 seconds, swallowed and then measurements 30 seconds later. The transducer was held with a minimal pressure for at least 3 seconds to stabilize the image - subjects asked to hold their breath and avoid swallowing while taking measurements. The mean value of three valid measurements obtained in a homogenous are for three different frames.
Parotid gland: "parallel plane to the posterior border of the vertical mandibular ramus"
Submandibular gland: "parallel plane to the inferior border of the horizontal mandibular ramus"
Findings: [the mean viscosity and stiffness increased significantly following gustatory stimulation compared to [parotid gland] basal state"; for the SMG "the values were only slightly higher, with no statistically significant difference" thought to be related by these authors to the glands physiologic mechanism - dynamic MR sialography identified PG ducts were detectable immediately after citric acid stimulation - in contrast to SMG ducts which were slower to be detected (Pedersen 2018).
B. Need to Standardize MRI Analysis (ADC) Timed to a Reproducible Physiologic State of the Gland with Attention to Appropriate Region of Interest
Bruvo et al evaluated - in the analysis of specially adapted MRI imaging hardware and technique to standardize measurement of ADC (apparent diffusion coefficient), these investigators defined shortcomings to inter-study comparisons (Bruvo 2021).
- Analysis of Regions of Interest (ROI) are 'usually manually drawn' as a 'reader-based circular ROISs or as an outline of as much of the gland parenchyma as possible' to exclude large blood vessels. Problem: may not reflect tissue heterogeneity. Defining selection of ROI is needed in studies where ADC is used.
- Parotid glands show changes in blood perfusion and salivary section in response to circadian variation and gustatory stimulation. Timing of MRI ADC analysis with respect to meals and time of day important.
circadian salivary rhythms:
Dawes C. Circadian rhythms in human salivary flow rate and composition. J Physiol 1972;220:529-45
Dawes C. Circadian rhythms in the flow rate and composition of unstimulated and stimulated human submandibular saliva. J Physiol. 1975 Jan;244(2):535-48. doi: 10.1113/jphysiol.1975.sp010811. PMID: 1142126; PMCID: PMC1330775.
'the high amplitude of many of these rhythms [circadian]..must be taken into account when establishing the normal range of saliary values..."
Pedersen A, Sørensen CE, Proctor GB, Carpenter GH. Salivary functions in mastication, taste and textural perception, swallowing and initial digestion. Oral Dis 2018;24:1399-416.
Papagerakis S, Zheng L, Schnell S, Sartor MA, Somers E, Marder W, McAlpin B, Kim D, McHugh J, Papagerakis P. The circadian clock in oral health and diseases. J Dent Res 2014;93:27-35
References
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* Ferraioli G, Barr RG, Farrokh A, Radzina M, Cui XW, Dong Y, Rocher L, Cantisani V, Polito E, D'Onofrio M, Roccarina D, Yamashita Y, Dighe MK, Dietrich CF. How to perform shear wave elastography. Part I. Med Ultrason. 2022 Feb 16;24(1):95-106. doi: 10.11152/mu-3217. Epub 2021 May 4. PMID: 33945590.
(excellent section in article addressing salivary gland assessment with shear wave elastography)
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Wikipedia contribuotrs: Page name: Viscosity / Author: Wikipedia contributors /Publisher: Wikipedia, The Free Encyclopedia. / Date of last revision: 20 October 2022 16:36 UTC / Date retrieved: 30 October 2022 14:51 UTC /Permanent link: https://en.wikipedia.org/w/index.php?title=Viscosity&oldid=1117234919 / Primary contributors: revision history statistics / Page Version ID: 1117234919
Wikipedia contributors: Page name: Elasticity (physics) /Author: Wikipedia contributors / Publisher: Wikipedia, The Free Encyclopedia. / Date of last revision: 5 August 2022 13:53 UTC / Date retrieved: 30 October 2022 14:58 UTC /
Permanent link: https://en.wikipedia.org/w/index.php?title=Elasticity_(physics)&oldid=1102524257 / Primary contributors: revision history statistics / Page Version ID: 1102524257
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Muntean DD, Lenghel ML, Petea-Balea DR, Ciurea AI, Solomon C, Dudea SM. Functional Evaluation of Major Salivary Glands Using Viscosity PLUS and 2D Shear-Wave PLUS Elastography Techniques in Healthy Subjects-A Pilot Study. Diagnostics (Basel). 2022 Aug 13;12(8):1963. doi: 10.3390/diagnostics12081963. PMID: 36010313; PMCID: PMC9406548.
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three articles cited by Muntean to clarify difference between elasticity (related to shear wave speed, linked to tissue fibrosis) and viscosity (related to shear wave dispersion - influence by inflammatory changes)
1. Sugimoto, K.; Moriyasu, F.; Oshiro, H.; Takeuchi, H.; Yoshimasu, Y.; Kasai, Y.; Furuichi, Y.; Itoi, T. Viscoelasticity Measurement in Rat Livers Using Shear-Wave US Elastography. Ultrasound Med. Biol. 2018, 44, 2018–2024.
2. Popa, A.; Bende, F.; S, irli, R.; Popescu, A.; Bâldea, V.; Lupus, oru, R.; Cotrău, R.; Fofiu, R.; Foncea, C.; Sporea, I. Quantification of Liver Fibrosis, Steatosis, and Viscosity Using Multiparametric Ultrasound in Patients with Non-Alcoholic Liver Disease: A “Real-Life” Cohort Study. Diagnostics 2021, 11, 783
3. Sugimoto, K.; Moriyasu, F.; Oshiro, H.; Takeuchi, H.; Yoshimasu, Y.; Kasai, Y.; Itoi, T. Clinical utilization of shear wave dispersion imaging in diffuse liver disease. Ultrasonography 2020, 39, 3–10.
also cited by Muntean - identifying degree of stiffness decreased after lemon ice stimulation in parotid and submandibular glands
Tasdemir, B.; Goya, C.; Dostbil, Z.; Sengul, E.; Sezgin, I.; Hattapoglu, S. A comparison of acoustic radiation force impulse imaging and scintigraphy in the functional evaluation of the major salivary glands. Nucl. Med. Commun. 2015, 36, 1220–1226.
cited by Muntean: differences in duct visualization with MRI sialography - stimulation reveals parotid ducts more readily
Pedersen, A.M.L.; Sørensen, C.E.; Proctor, G.B.; Carpenter, G.; Ekström, J. Salivary secretion in health and disease. J. Oral Rehabilit. 2018, 45, 730–746.
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Shiina T. JSUM ultrasound elastography practice guidelines: basics and terminology. J Med Ultrason (2001). 2013 Oct;40(4):309-23. doi: 10.1007/s10396-013-0490-z. Epub 2013 Sep 19. PMID: 27277449.
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