Application of Mouse and Rat Ultrasound in Abdominal Organs (Part 1)

In the previous six installments of this series, we have systematically established two core research frameworks—cardiovascular ultrasound and multimodal tumor ultrasound—for mice and rats. These comprehensively cover the fundamental imaging principles of small animal ultrasound, advanced multimodal techniques (CEUS/USR/SWE/ultrasound cavitation), practical protocols for animal models, key points for data interpretation, and a full-process standardized quality control system, laying a solid theoretical and practical foundation for standardized and normalized scientific applications of small animal ultrasound.

 

Beyond cardiovascular and tumor models, abdominal organs—including the liver, kidneys, spleen, pancreas, gastrointestinal tract, and major abdominal vessels—serve as core research substrates for toxicological evaluation, metabolic diseases, inflammatory organ injury, organ fibrosis, abdominal tumors, and drug-induced organ damage. Compared with cardiac and tumor ultrasound, abdominal organs in mice and rats present unique challenges: extremely small size, close anatomical proximity, significant respiratory motion interference, and prominent gas-induced artifacts from the gastrointestinal tract. In practical research, common pain points include non‑standardized scanning planes, lack of established imaging baselines, ambiguous differentiation between normal and pathological boundaries, and an absence of quantitative indicator frameworks—all of which greatly compromise the reproducibility and rigor of experimental data.

 

In response, this series launches a new dedicated educational segment on abdominal organ ultrasound in mice and rats. This installment (Part 1) focuses on the foundational imaging system for rodent abdominal organs, systematically elucidating the anatomical characteristics of core organs, standardized scanning planes, and normal baseline imaging findings, while also establishing a reference indicator framework for conventional and multimodal ultrasound. This provides a standardized prerequisite for Part 2, which will address pathological phenotyping, pharmacodynamic evaluation, and experimental quality control in abdominal disease models.

 

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I. Scientific Research Value and Technical Advantages of Abdominal Ultrasound in Mice and Rats

Currently, animal studies involving abdominal organs largely rely on conventional methods such as terminal anatomical weighing, histopathological sectioning, and serum biochemical assays. However, these techniques have inherent limitations: serum biochemical markers reflect only overall systemic injury levels and cannot precisely localize regional lesions or the extent of organ-specific damage; histopathological examination is terminal and destructive, capturing static data at a single time point without enabling longitudinal follow‑up in the same animal; and traditional anatomical observation reveals only gross morphological changes, failing to characterize subtle in‑vivo alterations in hemodynamics, microcirculatory perfusion, or tissue mechanical properties.

 

High‑frequency, high‑resolution small animal ultrasound offers core advantages including non‑invasiveness, absence of ionizing radiation, repeatability, longitudinal dynamic follow‑up, multidimensional precise quantification, and excellent soft‑tissue resolution. It enables integrated in‑vivo evaluation of abdominal organs encompassing "macroscopic morphological structure + hemodynamic parameters + microcirculatory perfusion levels + tissue stiffness characteristics + fine microvascular architecture." This versatile platform can be adapted to diverse research scenarios, including drug toxicity screening, mechanistic investigation of organ injury, validation of metabolic disease models, assessment of progressive organ fibrosis, and dynamic monitoring of abdominal pathologies.

 

Combined with the full multimodal technology suite detailed in earlier installments—including two‑dimensional ultrasound, color Doppler flow imaging (CDFI), contrast‑enhanced ultrasound (CEUS), shear‑wave elastography (SWE), and ultrasound super‑resolution microvascular imaging (URM1)—this approach can precisely capture subtle pathological changes that conventional methods fail to detect, such as subclinical early‑stage injury, microcirculatory dysfunction, and tissue mechanical remodeling, thereby effectively enhancing the experimental rigor and scientific innovation of abdominal organ‑related research projects.

 

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II. Anatomy and Ultrasound Imaging Characteristics of Core Abdominal Organs in Mice and Rats

The abdominal viscera of mice and rats are anatomically delicate and spatially compact. In addition, substantial respiratory motion and pronounced gastrointestinal gas interference during experiments create imaging principles and scanning logic that differ markedly from those in large clinical animals or human ultrasound, making it impossible to directly apply standard clinical scanning protocols. This section systematically reviews the anatomical features and ultrasound imaging characteristics of five core abdominal organs—liver, kidneys, spleen, pancreas, and major abdominal vessels—to establish an anatomical foundation for subsequent standardized scanning and image interpretation.

 

2.1 Liver

In mice and rats, the liver occupies the largest proportion of the abdominal cavity, exhibits well‑defined lobation, and has uniformly textured parenchyma, making it the least challenging organ for imaging and the most broadly applicable across abdominal ultrasound studies. On normal ultrasound imaging, the liver parenchyma appears homogeneous with fine, uniform echotexture, devoid of focal hyperechoic, hypoechoic, or patchy abnormal lesions, and the intrahepatic biliary/vascular structures display natural courses with clear, well‑defined borders. In mice, the hepatic lobes are thin and highly mobile, rendering them susceptible to motion artifacts from respiratory interference; in rats, the hepatic structure is comparatively more stable, making it better suited for long‑term quantitative monitoring and longitudinal time‑series follow‑up experiments.

 

Fig. 1. Hepatic ultrasound

2.2 Kidneys

The kidneys of mice and rats present a regular bean-shaped morphology, with clearly distinguishable layered structures comprising the cortex, medulla, and renal pelvis. Under normal physiological conditions, the renal cortex exhibits homogeneous echogenicity, the medulla appears as regularly hypoechoic, and the renal pelvis shows no separation, no hydronephrosis, and no hyperechoic foci suggestive of calculi. The kidneys are richly vascularized with stable blood flow signals, making them a classic target organ for Doppler flow assessment and contrast‑enhanced ultrasound (CEUS) evaluation of microcirculatory perfusion. They are widely employed in various research models, including acute kidney injury, renal fibrosis, diabetic nephropathy, and drug‑induced nephrotoxicity evaluation.

 

Fig. 2. Renal ultrasound

 

2.3 Spleen

The splenic parenchyma has a dense structure, with relatively low and homogeneous echogenicity. Under normal physiological conditions, there is no splenomegaly, no regional echotexture heterogeneity, no capsular thickening, and no abnormal fluid accumulation. Splenic ultrasound is primarily applicable to research scenarios involving inflammation‑ and immunity‑related models, cirrhotic portal hypertension, splenic congestion, and immune organ injury. It can serve as an important auxiliary evaluation indicator for assessing systemic inflammatory levels and circulatory perfusion abnormalities.

 

Fig. 3. Splenic ultrasound

 

2.4 Pancreas

The murine pancreas is thin and diffusely distributed, with close proximity to the gastrointestinal tract, making it highly susceptible to interference from gastrointestinal gas shadowing and thus the most challenging abdominal organ to image. Under normal conditions, the pancreas displays homogeneous echogenicity with smooth, well-defined borders, and shows no tissue thickening or swelling, no surrounding anechoic areas indicative of inflammatory exudative fluid, and no pseudocyst formation. It serves as a core observational target in research on acute pancreatitis, glucolipid metabolic disorders, and mechanisms of pancreatic injury.

 

Fig. 4. Pancreas ultrasound

 

2.5 Major Abdominal Vessels

The core abdominal vessels for observation primarily include the abdominal aorta, inferior vena cava, and the portal venous system. These are key structures for evaluating abdominal hemodynamics, portal hypertensive pathology, vascular structural remodeling, and abnormal organ perfusion. Under normal physiological conditions, the vessel lumina are regular and patent, with complete血流 filling and stable, regular spectral waveforms, showing no pathological findings such as abnormal flow velocity, turbulent flow, vascular stenosis, or abnormal dilation.

 

Fig. 5. Ultrasound of the abdominal aorta

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III. Standardized Scanning Planes for Each Organ

 

3.1 Standard Liver Scanning Plane

For standardized liver imaging, the maximum longitudinal section and transverse section of the right hepatic lobe are uniformly selected as the fixed observation planes. Throughout the experiment, scanning depth, instrument gain, and focal position are consistently maintained to ensure comparability of data across groups and time points. Baseline criteria for normal liver imaging: the liver parenchyma exhibits homogeneous and fine echogenicity, with naturally coursing and regularly distributed intrahepatic vessels, and a smooth, continuous capsule; under CDFI mode, intrahepatic blood flow signals are evenly distributed with regular courses, showing no focal flow deficits or abnormal turbulent signals.

 

3.2 Standard Kidney Scanning Plane

For the kidneys, the coronal and longitudinal sections of both kidneys are uniformly selected as the standard planes, fully displaying all layered structures including the cortex, medulla, and renal pelvis. Baseline findings for normal kidneys: clear corticomedullary differentiation with distinct echogenic layers, no renal pelvic separation or hydronephrotic dilation, symmetrical morphology and full structure of both kidneys; Doppler flow imaging shows abundant and evenly distributed cortical blood flow, with no focal flow deficits or abnormal high-velocity turbulent signals.

 

3.3 Standard Spleen Scanning Plane

For the spleen, the maximum longitudinal section is selected as the standard observation plane, fully covering the entire splenic contour and capsular structure. Baseline criteria for normal spleen: bilateral splenic thickness and length are symmetric and consistent, the parenchyma exhibits uniformly low echogenicity, the capsule is smooth and continuous, with no splenomegaly, no regional echotexture heterogeneity, and no abnormal findings such as pleural or peritoneal effusion.

 

3.4 Standard Pancreas Scanning Plane

Pancreatic imaging requires avoidance of gastrointestinal gas interference, with the transverse pancreatic section in the upper abdomen selected as the standard scanning plane. Baseline findings for normal pancreas: the pancreatic parenchyma displays homogeneous echogenicity with smooth, well-defined borders, showing no tissue thickening, swelling or hypertrophy, no surrounding anechoic areas indicative of inflammatory exudate, no pancreatic duct dilation, and a stable, normal morphological structure.

 

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IV. Normal Evaluation Indicator System for Multimodal Abdominal Ultrasound

 

4.1 Application of B‑Mode Imaging

B‑mode gray‑scale imaging is the fundamental core modality for abdominal ultrasound examination in mice and rats, and serves as the essential prerequisite scanning method for all multimodal advanced imaging. It is primarily used for standardized baseline assessment of macroscopic morphology, anatomical structure, and parenchymal echotexture of abdominal organs. This modality enables precise acquisition of thickness, length, transverse diameter, and volumetric parameters of abdominal organs such as the liver, kidneys, spleen, and pancreas, facilitating dynamic quantitative monitoring of organ volume and morphological changes, and is applicable to longitudinal evaluation of macroscopic phenotypes including organ hypertrophy, atrophy, and structural remodeling.

 

At the level of parenchymal structural interpretation, B‑mode ultrasound can clearly identify capsular continuity, parenchymal echogenicity uniformity, and regularity of internal vascular/biliary courses, systematically screening for pathological abnormalities such as peritoneal effusion, punctate calcifications, solid nodules, cystic lesions, and focal structural disorganization. Under normal physiological conditions, all abdominal organs exhibit smooth and intact capsules, homogeneous and fine parenchymal echogenicity, and naturally symmetric vascular/biliary structures, with no mixed hyper‑ or hypo‑echoic areas, structural defects, or abnormal protrusions.

 

From a research application perspective, B‑mode ultrasound serves as the first‑line screening standard for distinguishing "physiological individual variation" from "pathological structural injury," enabling precise establishment of normal anatomical imaging baselines for each organ. These baselines provide fundamental reference controls for subsequent structural abnormality interpretation in models of drug‑induced organ injury, inflammatory lesions, fibrotic proliferation, and tumor infiltration, making B‑mode ultrasound a core foundational technique for pharmacodynamic evaluation, toxicological screening, and disease model validation in abdominal organ research.

 

4.2 Application of Doppler Flow Imaging

Doppler flow imaging (CDFI/PW) is a core technique for evaluating hemodynamic homeostasis and macrovascular perfusion characteristics in abdominal organs. It enables non‑invasive, dynamic, and repeatable detection of spectral Doppler parameters in feeding vessels of abdominal viscera in mice and rats, with key indicators including peak systolic velocity (PSV), end‑diastolic velocity (EDV), resistive index (RI), and pulsatility index (PI)—all classic hemodynamic parameters.

 

Under normal physiological conditions, the feeding vessels of richly perfused organs such as the liver and kidneys show complete blood flow filling, smooth and regular spectral waveforms, and velocity and resistance parameters maintained within stable baseline ranges, with no significant turbulence, flow interruption, or flow deficits. By establishing standardized hemodynamic baselines, physiological blood flow fluctuations can be precisely distinguished from pathological flow derangements.

 

This modality is widely applicable in research, sensitively capturing early hemodynamic alterations in various abdominal diseases: in ischemic organ injury models, reduced peak velocity and hypoperfusion may be observed; in congestion and inflammatory edema models, elevated flow resistance and diastolic flow attenuation may occur; in portal hypertension and vascular remodeling models, disturbed flow velocity and abnormal spectral morphology may be present. Compared with static structural observation, Doppler flow imaging can detect functional perfusion abnormalities at an earlier stage, providing quantitative hemodynamic evidence for early disease mechanism research and evaluation of drug intervention efficacy.

 

4.3 Application of Contrast‑Enhanced Ultrasound (CEUS)

Contrast‑enhanced ultrasound (CEUS) is a microbubble‑based microcirculation‑specific imaging technique that overcomes the limitation of conventional Doppler, which can only assess macrovascular blood flow, and precisely focuses on microvascular perfusion levels and microcirculatory functional status in abdominal organs. It is a core advanced modality for evaluating microscopic blood supply and capturing early subclinical injury.

 

Under normal physiological baseline conditions, parenchymal abdominal organs such as the liver and kidneys exhibit uniform, synchronous contrast agent filling with an overall homogeneous enhancement trend. Core perfusion parameters—including time to peak (TTP), peak intensity (PI), and area under the curve (AUC)—remain stable and reproducible, with no focal perfusion defects, patchy hypoperfusion, or abnormally hyperperfused regions, indicating that the microcirculation is in a steady‑state equilibrium.

 

A substantial body of research has confirmed that early pathological injury in abdominal organs often first manifests as microcirculatory dysfunction. Even when B‑mode structure and macrovascular flow show no obvious abnormalities, CEUS can sensitively detect subtle changes such as regional perfusion heterogeneity, delayed filling, reduced peak intensity, and focal perfusion defects. This modality is broadly applicable to research scenarios including drug‑induced micro‑injury, early‑stage fibrotic microvascular remodeling, inflammatory perfusion derangement, and tumor neovascularization abnormalities. It can precisely differentiate normal microcirculation from pathological microvascular remodeling, providing high‑precision, high‑sensitivity quantitative microcirculatory data for early pharmacodynamic evaluation and disease mechanism exploration, substantially enhancing research innovation and data rigor.

 

4.4 Application of Shear‑Wave Elastography (SWE)

Shear‑wave elastography (SWE) is a dedicated non‑invasive technique for quantitative characterization of the mechanical microenvironment and tissue stiffness properties of abdominal organs. By measuring the propagation speed of shear waves within organ tissues, it accurately calculates tissue elastic modulus (kPa), achieving objective quantification of organ stiffness and completely eliminating the subjectivity inherent in conventional visual interpretation.

 

Under normal physiological conditions, abdominal organs such as the liver, kidneys, and spleen in mice and rats exhibit homogeneous tissue architecture, regular cellular arrangement, and absence of abnormal collagen deposition, resulting in soft overall texture and stable elasticity values. There are no regional differences in softness or stiffness within the organ, with elasticity values maintained within a stable baseline range, providing precise reference standards for subsequent pathological stiffness changes.

 

Pathological processes including organ fibrosis, chronic inflammation, and tumor infiltration are all accompanied by significant tissue mechanical remodeling: abnormal collagen deposition leads to sustained increases in organ stiffness; inflammatory edema may cause localized decreases in tissue elasticity; and tumor proliferation and infiltration form focal high‑stiffness nodular regions. SWE can precisely capture these mechanical changes, dynamically monitor fibrotic stage progression, inflammatory sclerosis severity, and tumor aggressiveness, while also quantitatively evaluating the reversal of tissue stiffness following anti‑fibrotic, anti‑inflammatory, or anti‑tumor drug interventions. As an innovative entry point for mechanistic research, SWE complements traditional structural and hemodynamic indicators from the dimension of "mechanical microenvironment," serving as an important technical support for chronic abdominal disease mechanism studies and high‑impact paper data mining.

 

4.5 Application of Ultrasound Super‑Resolution Microvascular Imaging (URM)

Ultrasound super‑resolution microvascular imaging (URM) breaks through the optical diffraction limit of conventional ultrasound, elevating imaging resolution to the micrometer scale and enabling visualization and analysis of the fine microvascular network architecture in abdominal organs. It represents a cutting‑edge advanced technique for research on microcirculatory mechanisms, vascular remodeling, and micro‑lesion blood supply in abdominal organs. Compared with conventional CEUS, which can only provide global perfusion parameters, URM can directly visualize the spatial distribution, morphological structure, and organizational patterns of microvessels.

 

Under normal physiological baseline conditions, microvessels in abdominal organs such as the liver and kidneys of mice and rats are uniformly distributed, with regular courses and consistent diameters. Structural parameters including microvascular density, inter‑vascular spacing, and vascular fractal dimension remain stable and reproducible, with the overall microvascular network exhibiting symmetric and orderly arrangement and no remodeling features such as rarefaction, tortuosity, disorganization, or aberrant proliferation, thereby establishing standardized anatomical baselines for organ microvasculature.

 

In various abdominal disease models, subtle microvascular remodeling is an early core event in disease initiation: inflammatory injury triggers compensatory microvascular proliferation and disorganized distribution; fibrotic progression leads to microvascular rarefaction, tortuosity, and focal vascular dropout; and the tumor microenvironment induces disordered proliferation of aberrant neovessels. URM can precisely capture these micrometer‑scale vascular structural changes and quantitatively analyze alterations in microvascular density, vessel tortuosity, and fractal structural remodeling. It is capable of uncovering highly innovative microvascular mechanism data at early stages when macroscopic structure and conventional perfusion parameters have not yet shown differences, making it perfectly suited for mechanistic investigation and novel insight discovery in high‑impact research projects, and serving as a core tool for early micro‑injury and vascular remodeling mechanism research in abdominal organs.

 

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V. Summary of Part 1 and Preview of Part 2

 

As the first installment of this series on abdominal organ ultrasound, this article has systematically outlined the anatomical characteristics, standardized scanning planes, normal imaging baselines, and multimodal evaluation frameworks for core abdominal organs in mice and rats, effectively addressing fundamental research challenges in abdominal ultrasound experiments—namely, "ambiguous recognition of normal structures, lack of standardized imaging baselines, and non‑standardized scanning planes"—and establishing a unified standardized reference system for pathological phenotyping, injury quantification, pharmacodynamic evaluation, and full‑process quality control implementation in various abdominal disease models, which will be covered in Part 2.

 

In Part 2, we will focus on clinically and scientifically prevalent abdominal disease models, providing in‑depth analysis of typical ultrasound pathological phenotypes, dedicated core evaluation indicators, and multimodal combined testing protocols for models including fatty liver, hepatic fibrosis, drug‑induced liver injury, diabetic nephropathy, obstructive nephropathy, inflammatory splenomegaly, and acute pancreatitis. We will also detail full‑process quality control standards and artifact avoidance techniques for abdominal ultrasound, comprehensively covering all practical research scenarios in abdominal organ ultrasound.

 

Moving forward, we will continue to deepen our efforts in the field of small animal ultrasound research, regularly updating practical operational tips and technical analyses across abdominal, cardiovascular, and tumor ultrasound applications, and continuously refining the complete small animal ultrasound research system. Our goal is to assist researchers in standardizing experimental procedures and producing high‑quality scientific outputs. We welcome your continued attention and engagement in future installments.