Yeeran

We have found that many users are relatively familiar with conventional ultrasound imaging modes, but still lack a clear understanding of some emerging high-end functions. Therefore, we have specially prepared an introduction to some of the latest ultrasound functions.

In the functional modalities of small animal research ultrasound, elastography has become an indispensable tool for evaluating tissue fibrosis, inflammatory infiltration, and early lesions. Unlike conventional B-mode ultrasound, which only observes morphological structure, elastography detects tissue stiffness, a mechanical property: when organs develop fibrosis, edema, necrosis, or tumor infiltration, tissue elasticity changes before morphology does, making it highly suitable for capturing early disease phenotypes and quantitatively evaluating intervention effects.

To date, elastography has developed along multiple technical routes. Among them, the two most widely used and most maturely implemented in scientific research on abdominal organs in mice and rats are strain elastography (SE) and shear wave elastography (SWE). This issue focuses on these two technologies, reviewing their development history, imaging principles, operational key points, and standardized data analysis protocols for research scenarios.

I. A Brief History of Elastography Technology

As early as the 1990s, scholars proposed the basic concept of “ultrasound elastography”: since lesions in biological tissues are accompanied by changes in mechanical properties, ultrasound could be used to visualize stiffness.

The first technology to achieve commercialization was pressure-based strain elastography, which gradually entered clinical use around 2000. It was initially used mainly for screening thyroid and breast lesions and was later gradually transferred to small animal ultrasound platforms. It has a low implementation threshold and controllable hardware costs, and was also the earliest elastography technology to become widespread in mouse and rat liver fibrosis models.

Strain elastography has one core shortcoming: it can only obtain relative strain values and cannot output absolute tissue stiffness. To solve this pain point, shear wave elastography technology was continuously developed and became mature and widely promoted around 2010. It uses ultrasound radiation force to actively excite shear waves and directly measures tissue Young's modulus (elastic modulus, in kPa). This parameter is positively correlated with tissue stiffness and is often used in research to evaluate tissue stiffness. Its quantitative capability has been greatly improved, and it has now become the mainstream approach for fibrosis mechanism research and drug efficacy evaluation.

II. Pressure-based (Strain) Elastography

Imaging Principle

The core logic of strain elastography is: external compression → tissue deformation → ultrasound tracking of deformation differences → conversion into strain distribution.

The operator applies slight, regular reciprocating pressure to the target tissue with the probe. Tissues with different stiffness deform to different degrees under compression: soft tissues deform more and have higher strain values; stiff fibrotic tissues deform less and have lower strain values. The device continuously acquires B-mode frames, uses a cross-correlation algorithm to track speckle displacement frame by frame, calculates the strain of each pixel, and overlays a color-coded map to form an elastogram.

Note: pressure-based elastography yields relative strain, not absolute stiffness. Its values are highly dependent on compression force and compression frequency, and can only be used for intra-group comparison within the same scan and under the same compression conditions.

Advantages and Limitations

Advantages: high imaging frame rate and good real-time performance; lower requirements for equipment hardware; intuitive color elastograms, suitable for rapid qualitative screening; stable imaging of superficial tissues.

Limitations: relies on manual compression, with substantial operator-dependent interference; no absolute modulus, making cross-group and cross-operator comparison difficult; compression transmission is markedly attenuated in deep abdominal organs (deep kidney, pancreas), with many artifacts; not suitable for target organs with large respiratory motion amplitude or deep location.

Applicable Scenarios in Small Animals

Suitable for initial elasticity screening of superficial subcapsular liver lesions and subcutaneous xenograft tumors in mice; suitable for observing lesion progression in the same animal using self pre/post control. Not recommended as a core quantitative endpoint indicator for cross-batch, multi-group statistical comparisons.

Figure 1: Pressure-based (strain) elastography

III. Shear Wave Elastography

Imaging Principle

Shear wave elastography does not require manual compression. The device uses focused ultrasound radiation force to instantly “push” within the tissue, exciting laterally propagating shear waves at the focal point. Shear wave propagation speed is directly related to tissue stiffness: the stiffer the tissue, the faster the shear wave propagates.

The device acquires the propagation timing of shear waves at high speed, calculates wave speed, first obtains the shear modulus G, and then converts it to Young's modulus E (in kPa), directly outputting the absolute tissue stiffness value while generating a color elastic modulus map. When soft tissue is approximated as incompressible and the Poisson's ratio is taken as 0.5, the conversion relationship is:

G = ρc²

E = 3ρc² = 3G

G: shear modulus; E: Young's modulus; ρ: tissue density; c: shear wave velocity.

This formula is also the core basis of SWE quantification, and this is the most fundamental difference between SWE and strain elastography.

Advantages and Limitations

Advantages: no manual compression required, significantly reducing operator-induced error; outputs absolute modulus in kPa, supporting statistical comparison between groups and across time points; fixed ROIs can be set for batch measurements; deep organs (rat kidney, liver) can also be stably sampled, making it the first choice for abdominal organ fibrosis models.

Limitations: extremely high requirements for probe stability and animal respiratory suppression; shear waves are strongly attenuated by calcifications, gas, and large vessels, producing signal void blind zones; imaging frame rate is lower than SE; different device algorithms and calibration parameters cause systematic bias in measurements of the same sample on different machine models.

Applicable Scenarios in Small Animals

Classic abdominal disease models such as liver fibrosis, renal interstitial fibrosis, chronic pancreatitis, and obstructive nephropathy in mice and rats; quantitative evaluation of drug intervention efficacy; early injury detection (when no structural abnormalities are seen on B-mode ultrasound, stiffness may already be increased); use as a core quantitative indicator in paper publication.

Figure 2: Shear wave elastography

IV. Standardized Data Analysis

Pre-analysis Quality Control (Must Be Completed Before Data Analysis)

1. 1. Image screening: exclude invalid frames with respiratory motion artifacts, probe sliding, abdominal gas obscuration, missing shear wave signal, or unstable compression; for SWE, preferentially select images with uniform elastogram filling and no large black signal void areas; for SE, retain cycles with stable compression rhythm and avoid extreme frames at compression peaks/troughs.

2. 2. ROI delineation principles: unify ROI shape, size, and position, fix them in the parenchymal region of the target organ, and avoid the capsule, large vessels, calcifications, and necrotic areas. For small animal experiments, it is recommended to specify the ROI size in the methods section of papers, for example, a ROI of 0.8 × 0.8 mm in the mouse renal cortex.

3. 3. Baseline matching: maintain consistent anesthesia depth and body temperature across all animals. Excessive anesthesia changes tissue tension and directly affects elasticity measurements.

Data Analysis for Pressure-based Strain Elastography (SE)

SE output parameters: strain value, strain ratio (lesion strain / surrounding normal parenchymal strain, SR).

It is not recommended to directly use absolute strain values for cross-group comparison; preferentially use the strain ratio SR, which to some extent offsets compression differences;

Continuously acquire 5 stable elasticity cycles from the same animal, delineate the ROI in each cycle to obtain strain values, remove outliers, and calculate the mean;

Statistical approach: suitable for paired t-test (self-control); not recommended for direct use in demonstrating results from multiple independent groups, and should only be used as an auxiliary indicator.

Interpretation logic: strain decreases in fibrotic stiff areas, and the strain ratio decreases.

Data Analysis for Shear Wave Elastography (SWE)

SWE output parameters: mean Young's modulus, modulus standard deviation, shear wave velocity.

Standard acquisition protocol: continuously acquire 3–5 qualified elastograms in the same target area, place the ROI at the same position in each image to obtain a set of modulus values, remove abnormal outliers, and calculate the mean for statistics;

Focus on two indicators: ROI Mean (mean stiffness, core endpoint) and ROI SD (elasticity heterogeneity; SD increases when fibrosis is uneven, and can be used as a secondary indicator);

Statistics: mean values can be directly used for multi-group comparison (one-way ANOVA, etc.), and are the most suitable quantitative indicator in elastography for publishing papers;

Note: SWE algorithms differ among ultrasound brands. Do not change equipment models throughout the experiment; if modulus values from references are cited, the equipment model and probe parameters should be specified.

V. How to Choose Between SE and SWE?

If you only want rapid qualitative assessment, superficial lesions, or rough self pre/post observation → pressure-based strain elastography (SE)

If you need quantification, fibrosis models, drug efficacy evaluation, primary indicators for papers, or deep abdominal organs (liver and kidney) → shear wave elastography (SWE)

Elastography upgrades ultrasound from “observing morphology” to “measuring mechanics,” providing a unique evaluation dimension for early lesions in abdominal organs of mice and rats. Pressure-based strain elastography is easy to learn but limited by operator-dependent factors, and is more suitable as a qualitative auxiliary method; shear wave elastography relies on absolute modulus quantification and is currently the gold-standard functional modality for fibrosis-related phenotype research.

In actual experimental design, everyone can reasonably combine these methods according to model type, experimental purpose, and statistical needs, while strictly controlling scanning and data analysis standards to reduce data dispersion and ensure reproducibility.

Going forward, we will continue to deepen our work in the field of small animal ultrasound research and continue to share practical content on various high-end ultrasound function modules, modality principles, standardized acquisition protocols, key points of data analysis quality control, and analysis of difficult experimental issues, continuously improving the entire small animal ultrasound research system, helping researchers standardize experimental operations, avoid experimental errors, and produce high-quality research results. You are welcome to keep following and learning.