A bibliometric analysis of ultrasound-based evaluation in renal transplantation

Introduction

Renal transplantation is the preferred renal replacement therapy for patients with end-stage renal disease, offering significant improvements in quality of life and long-term survival (1). Compared to long-term dialysis, transplantation confers additional benefits, including better metabolic regulation, reduced cardiovascular risk, and lower socioeconomic burden (2). With advancements in surgical techniques and continuous optimization of immunosuppressive therapies, short-term transplant outcomes have improved markedly (3). However, long-term graft survival remains challenging due to complications such as acute rejection, chronic allograft injury, immunosuppression-related adverse effects, and recurrence of primary diseases (3,4). Particularly during the chronic phase, renal graft dysfunction may progress insidiously, ultimately leading to graft failure and the need for dialysis or re-transplantation if not detected and managed early (5). Therefore, accurate assessment and early identification of graft function decline remain central issues in post-transplant care. In addition to traditional biochemical markers and histopathological evaluation, emerging non-invasive monitoring tools are being explored to enable earlier detection and prolong graft survival.

Imaging plays a crucial role in post-transplant monitoring, with ultrasound being the first-line modality due to its non-invasive nature, availability, repeatability, and real-time capabilities (6). Conventional grayscale ultrasound provides structural assessment of the graft, including size, morphology, parenchymal echotexture, and collecting system dilation. Color Doppler imaging enables dynamic evaluation of graft vascularity, aiding in the detection of vascular complications such as stenosis, thrombosis, and hypoperfusion. The resistive index (RI), a commonly used hemodynamic parameter, has been shown in some studies to correlate with acute rejection or graft injury (7). However, conventional ultrasound alone has limited value in identifying early functional impairment and fibrotic changes (8,9). To address this, novel ultrasound techniques such as contrast-enhanced ultrasound (CEUS) and shear wave elastography (SWE) have been introduced, offering new perspectives for assessing graft microcirculation and tissue stiffness non-invasively (10,11).

Bibliometrics, as a valuable tool for analyzing scientific output and knowledge structure (12), has been widely applied in medicine and interdisciplinary research in recent years (13-15). By quantitatively examining publication volume, research topics, author collaboration, institutional distribution, and citation metrics, bibliometric analysis can identify research hotspots, developmental trends, and frontiers within a given field (16). It also facilitates the recognition of prolific authors and core journals, mapping of international collaboration networks, and strategic guidance for researchers. Given the expanding research on ultrasound in renal transplantation, conducting a comprehensive bibliometric study is essential to delineate the current landscape, uncover knowledge gaps, and inform future directions.

Therefore, this study performed a bibliometric analysis of literature on ultrasound-based evaluation of renal allografts, aiming to summarize publication trends, research hotspots, and emerging directions in this field, with the goal of supporting clinical practice and guiding future research development. We present this article in accordance with the BIBLIO reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2025-656/rc).

Methods

Data sources and search strategy

The data for this study were retrieved from the Web of Science (WoS) and PubMed databases (Figure 1). WoS served as the primary source for collecting core literature for bibliometric and quantitative analyses, while PubMed was used to identify clinical trials and explore trends in clinical research. To improve the precision of the search, a strategy combining title and author keywords fields was adopted.

Figure 1 Publications screening flow chart. AK, author keyword; MeSH, medical subject headings; TI, title; WoS, Web of Science.

All documents were manually screened based on their titles and abstracts, with full-text reviews conducted when necessary to ensure accuracy. The inclusion criteria were as follows:

• The study population involved renal transplants;
• The study investigated ultrasound or related techniques, such as Doppler ultrasound, CEUS, various types of elastography, or intravascular ultrasound;
• The study addressed the role of ultrasound in the management of renal grafts, including functional assessment and the identification of postoperative complications (e.g., arterial stenosis, venous thrombosis, etc.).

Bibliometric analysis

A comprehensive bibliometric analysis was conducted using VOSviewer and the R package bibliometrix. The analysis covered multiple dimensions, including authorship, institutional affiliations, countries/regions, journals, keywords, and cited references. Given the relatively limited number of publications in this research domain, a threshold of ≥3 publications was applied for analyses related to publication frequency. All other analyses were performed using the default settings of the respective tools. To enhance visual clarity and graphical presentation, origin software was additionally employed for advanced data visualization and figure refinement.

Results

The final analysis included a total of 268 publications (including 28 case reports) authored by 1,634 individuals, affiliated with 106 institutions across 43 countries or regions, and published in 106 different journals.

Figure 2 illustrates the annual publication output and total citation trends from 2010 to 2025, reflecting the academic influence of research on ultrasound-based evaluation in renal transplantation. Since 2020, the total number of citations has remained at a relatively high level, indicating sustained research interest and growing academic attention in recent years.

Figure 2 Annual number of published articles and total citations in this field.

National/regional output and international cooperation

An analysis of publication volume and citation frequency by country (Figure 3A) showed that the United States (n=58, 21.6%), China (n=48, 17.9%), and Germany (n=35, 13.1%) ranked among the top three both in the number of publications and total citations, underscoring their significant academic influence in this field. Notably, although France and Italy produced fewer publications, their relatively high citation counts suggest that their research outputs carry substantial academic value.

Figure 3 Visual analysis of countries and institutions. (A) Country scientific production in this field; (B) global collaborations in the field.

Figure 3B depicts the international collaboration network among countries. The size of each circle represents the number of publications, while the number and thickness of the connecting lines indicate the frequency and strength of collaborations. It is evident that leading contributing countries have established collaborative links, which may promote academic advancement and lay the groundwork for future multi-center international studies.

Journals and co-citation journals

Figure 4A presents journals with more than six publications in the field of ultrasound evaluation of renal transplants, along with their corresponding citation counts. Notably, Transplantation Proceedings had the highest number of publications (n=20, 7.5%), whereas Clinical Hemorheology and Microcirculation recorded the highest number of citations (n=266, 8.7%). According to Bradford’s Law, eight journals were identified as core sources in this field: Transplantation Proceedings (n=20, 7.5%), Experimental and Clinical Transplantation (n=13, 4.9%), Clinical Hemorheology and Microcirculation (n=12, 4.5%), Journal of Ultrasound in Medicine (n=12, 4.5%), Clinical Transplantation (n=10, 3.7%), Nephrology Dialysis Transplantation (n=9, 3.4%), Journal of Clinical Ultrasound (n=8, 3.0%), and Pediatric Transplantation (n=8, 3.0%). These journals serve as essential resources for researchers seeking current advances in this domain.

Figure 4 Visual analysis of journals. (A) Journals with high number of articles in this field; (B) journals that publish highly cited articles in this field.

Figure 4B illustrates the journal co-citation network within this research field. In the VOSviewer-based visualization, circle size represents the number of citations, and color denotes different clusters. Four distinct clusters were identified. The green cluster, representing clinical medicine, is dominated by Transplantation and American Journal of Transplantation. The red cluster comprises radiology and imaging journals, led by Radiology. The yellow cluster, although consisting of fewer journals, features Transplantation Proceedings as a central hub connecting all three other clusters and being the most frequently cited journal overall. The blue cluster includes journals primarily related to pediatric medicine.

High-contributing authors

Table 1 presents relevant information concerning high-contributing authors in this field. Figure 5A shows the most prolific authors and the most highly cited authors in this field. We can see that Fananapazir G (n=6, 2.2%) and Haberal M (n=6, 2.2%) have the highest number of publications, while Brabrand K (n=231, 7.6%) and Midtvedt K (n=231, n=7.6%) have the highest number of citations. Figure 5B illustrates the research productivity of authors with more than three publications in this field, visualized as a timeline. Circle size corresponds to the number of publications, and color intensity reflects citation frequency.

Figure 5 Visual analysis of authors. (A) High contribution authors in this field; (B) authors’ production over time. N, number; TC, total citation.

High-contributing institutions

Table 2 presents relevant information concerning high-contributing institutions in this field. Figure 6 displays the temporal evolution of international institutional collaborations. The results indicate that institutions across different countries have maintained relatively close collaborative relationships in this research area. In recent years, institutions such as Zhengzhou University, the Children’s Hospital of Philadelphia, Akdeniz University, King’s College London, and King’s College Hospital London have demonstrated notable collaborative activity. Notably, Zhengzhou University is the only institution from China prominently featured in the collaboration network, suggesting its leading role in international collaboration among Chinese institutions in this field.

Figure 6 Collaboration network co-occurrence analysis of research institutions by year.

Visual mapping of keyword

Figure 7A presents the keyword co-occurrence network, revealing the primary research hotspots within the field. In addition to core terms such as “ultrasound” and “renal transplant”, frequently occurring keywords included CEUS, color Doppler ultrasound, Doppler ultrasound, biopsy, and RI. Clustering analysis identified nine distinct research clusters, emphasizing the application of various ultrasound techniques—such as CEUS and elastography—across different subdomains, including vascular complications and fibrosis assessment in renal transplant research.

Figure 7 Visual analysis of keywords. (A) High-frequency keywords; (B) visualize keywords trend topics over time.

Figure 7B illustrates the thematic evolution over time, reflecting the dynamic shifts in research focus within this field. It is evident that various ultrasound modalities have continued to attract substantial attention. In particular, CEUS and elastography have re-emerged as prominent research hotspots in recent years. The former enables real-time visualization of allograft perfusion, while the latter offers advantages in the quantitative assessment of fibrosis. Tables 3,4 list relevant studies concerning CEUS and elastography from recent years.

Additionally, the emergence of machine learning (ML) as a research focus suggests that the integration of artificial intelligence with ultrasound may represent a key future direction for evaluating renal transplant function and pathology. Table 5 lists relevant studies concerning ML from recent years.

Clinical research

Given the limited number of randomized controlled trials (RCTs) identified in this field—only two studies indexed in PubMed—a systematic analysis was not performed. Table 6 summarizes information, including study design, patient numbers, key conclusions, and limitations from two RCTs.

Discussion

To our knowledge, this is the first bibliometric analysis specifically focused on the application of ultrasound in the context of kidney transplantation. We systematically retrieved relevant literature from the WoS and PubMed databases, identifying both core publications and RCTs within this domain. Bibliometric mapping and data visualization were conducted using VOSviewer and the Bibliometrix R package. This study has two primary objectives: first, to provide researchers—especially those new to the field—with a comprehensive overview of the evolution, intellectual structure, and current landscape of research; and second, to generate insights that may guide future investigations and inform the development of clinical and technological innovations in renal transplant imaging. It is worth noting that a small number of case reports (n=28, 10.4%) were categorized as “Articles” by the WoS database and thus included in the analysis. While such reports are inherently limited in generalizability, they primarily focus on the clinical application of ultrasound in renal graft evaluation. Their inclusion aligns with the thematic focus of this study and does not compromise the validity or overall trends identified in the bibliometric results.

Research hotspots and key focus areas

The kidney is one of the most commonly transplanted organs. Although the overall success rate of kidney transplantation is relatively high, early detection and continuous monitoring of postoperative complications remain critical (1). In recent years, advances in ultrasound technology have led to the emergence of several novel imaging modalities, which have become focal points of research interest. Among them, CEUS enhances blood flow signals using microbubble contrast agents, enabling dynamic assessment of graft microcirculation. Meanwhile, ultrasound elastography provides noninvasive insights into tissue stiffness, offering quantitative or semi-quantitative information that may reflect the degree of interstitial fibrosis (6,11).

Ultrasound elastography

The progression of renal allograft dysfunction often leads to interstitial fibrosis, driven by cumulative chronic injury mechanisms (33,34). In solid organs, fibrosis is typically associated with altered tissue elasticity, making stiffness a valuable indirect marker of fibrotic severity (35,36). Ultrasound elastography offers several techniques to noninvasively assess tissue stiffness. Transient elastography (TE) employs mechanical vibration to generate low-frequency shear waves for a global stiffness estimate. Acoustic radiation force impulse (ARFI) uses focused ultrasound to induce localized shear waves, enabling point-specific measurements. SWE, particularly in its two-dimensional form, provides real-time stiffness mapping over a broader field (37-39). Each method has distinct advantages and limitations, which may explain the variability in diagnostic performance across studies.

Despite increasing interest, findings on the diagnostic value of these techniques in renal transplantation remain inconsistent. Some studies report correlations between cortical stiffness and fibrosis or graft dysfunction, while others do not (40-43). These discrepancies are likely multifactorial, stemming from the kidney’s complex anatomy, heterogeneity in imaging protocols, and physiological factors such as intrarenal perfusion or respiratory motion (44,45). In renal allografts, these challenges are further compounded by their superficial location and higher susceptibility to motion artifacts (46).

In recent years, shear wave dispersion (SWD) imaging has emerged as a novel extension of SWE (47). Unlike traditional SWE based on linear elastic assumptions, SWD captures frequency-dependent changes in shear wave speed, thereby offering insight into tissue viscosity and viscoelasticity (48). While promising results have been reported in liver transplantation, early studies in renal allografts suggest dispersion parameters may help detect conditions such as acute rejection, although further validation is required (23,48).

Taken together, although elastographic techniques such as TE, ARFI, SWE, and SWD provide valuable insights into renal graft pathology, their clinical implementation remains limited by inconsistent evidence and technical variability (11,47). Future studies should prioritize large-scale, multicenter prospective trials employing standardized acquisition protocols and parameter definitions. Such efforts are essential to improve diagnostic reproducibility, facilitate inter-study comparability, and support the clinical standardization of ultrasound elastography in renal transplantation.

CEUS

The kidney is a highly vascularized organ, and its functional status is closely linked to perfusion dynamics. CEUS, using microbubble agents confined to the intravascular space, enables real-time assessment of microvascular perfusion (49,50). Quantitative analysis via time-intensity curves yields parameters such as peak intensity, time to peak, mean transit time, and area under the curve (AUC), facilitating detailed evaluation of renal parenchymal perfusion (51). While some studies relied solely on clinical criteria to categorize graft function (8,18,20), others incorporated histopathological findings to enhance diagnostic accuracy, reflecting variation in methodological rigor (9,19).

There is also growing interest in combining CEUS parameters with clinical or laboratory indicators to improve prognostic performance. Some researchers have integrated CEUS with multimodal imaging techniques or advanced analytics, such as ML, to capture subtle perfusion abnormalities (8,17). Furthermore, derivative technologies such as super-resolution imaging have expanded the capabilities of CEUS, enabling the assessment of microvascular density and flow at near-microscopic resolution (10,20).

Despite its promise, CEUS remains limited by the lack of standardized imaging protocols and operator dependence, which constrain its reproducibility and clinical generalizability. Future research should prioritize methodological standardization and large-scale validation to support broader clinical adoption.

ML

ML has emerged as a promising tool in the ultrasound-based evaluation of renal transplants. Compared with traditional approaches that rely heavily on expert interpretation and single-parameter analysis, ML enables automated extraction of complex imaging features, integration of multimodal parameters, and recognition of nonlinear patterns, thereby improving diagnostic consistency and predictive performance (52,53).

Recent studies have explored ML models incorporating radiomic features from grayscale ultrasound images and clinical variables to assess early graft function and stratify dysfunction severity (26,27). Beyond grayscale imaging, there is a growing trend toward integrating Doppler hemodynamics, elastographic parameters, and CEUS-derived perfusion features into ML frameworks. This multimodal approach enhances the ability to characterize subtle physiological changes linked to graft injury and rejection (25,29,30).

In addition to functional assessment, ML techniques have also been applied to predict transplant-related complications, such as transplant renal artery stenosis, further highlighting their value in postoperative risk monitoring (28). With ongoing advances in CEUS and elastography technologies, future research should focus on unifying imaging-derived parameters and clinical outcomes through ML-driven models to support early detection, individualized management, and ultimately, improved long-term graft outcomes.

Clinical research

Despite the widespread use of ultrasound in renal transplant evaluation, the number of RCTs in this field remains limited. Several factors contribute to this scarcity. First, ultrasound examinations are highly operator-dependent and currently lack standardized protocols for image acquisition and parameter measurement, which affects reproducibility and comparability across studies. Second, as a non-interventional diagnostic tool, ultrasound does not easily conform to the conventional intervention–control design framework of RCTs. Ethical constraints also hinder the inclusion of high-risk transplant recipients in control groups that are deprived of imaging surveillance. Additionally, the coronavirus disease 2019 (COVID-19) pandemic has disrupted transplant activities globally, further constraining clinical research in this domain (54). These factors collectively limit the feasibility of conducting large-scale RCTs involving ultrasound in renal transplantation.

General information

An analysis of the leading countries, institutions, authors, and journals in this field helps elucidate the current research landscape and collaboration networks. The findings reveal that the United States and China are the top two contributors in terms of publication volume, likely reflecting their strong research capacity and large transplant patient populations. International collaboration is active, particularly among institutions in Asia, Europe, North America, and South America, laying a solid foundation for future multinational, multicenter studies. Among institutions, Zhengzhou University has emerged as one of the most active research centers in China in recent years and demonstrates notable international engagement. At the author level, Knut Brabrand is currently the most cited researcher in this area. His work focuses on non-invasive assessment of renal allograft interstitial fibrosis using ultrasound elastography, and his studies have been published in high-impact journals, contributing significantly to the field. Regarding journals, Transplantation Proceedings has the highest number of publications, while Clinical Hemorheology and Microcirculation is the most frequently cited. Notably, a 2010 prospective study published in the latter journal explored the correlation between ARFI imaging and histological fibrosis scores in renal transplants, with 95 citations to date (55). European Radiology, though lower in publication count, shows a high citation impact due to two landmark ARFI studies published in 2012, cited 104 and 100 times respectively (42,56). Co-citation analysis reveals that highly cited journals in this field are mainly clustered into two domains: medical imaging (e.g., Radiology, American Journal of Roentgenology) and transplantation medicine (e.g., Transplantation Proceedings, American Journal of Transplantation). This distribution underscores the interdisciplinary nature of the field, integrating imaging technology and clinical transplantation perspectives.

This study has several limitations. First, although both the WoS and PubMed databases were searched and analyzed, the number of RCTs retrieved from PubMed was relatively limited, which may hinder a comprehensive reflection of the current clinical research landscape and trends in this field. Second, our analysis was restricted to English-language publications. While all 268 included articles were published in English, relevant studies in other languages may have been overlooked, potentially affecting the comprehensiveness of the results. Thirdly, as with most bibliometric analyses, our findings are primarily based on citation-based metrics. Highly cited articles and emerging research trends do not necessarily equate to high methodological quality or significant clinical impact. Therefore, interpretations should be made in conjunction with ongoing clinical and translational research outcomes. Finally, this study did not normalize geographic and institutional contributions by population size or transplant volume, which may limit cross-country comparability. Due to the unavailability of consistent global data on transplant activity, such normalization could not be performed.

Conclusions

Ultrasound is a powerful tool for postoperative monitoring of renal allografts. In this study, a total of 268 articles and RCTs related to ultrasound evaluation of renal transplantation published between 2010 and 2025 were retrieved from the WoS and PubMed databases. A comprehensive bibliometric analysis was conducted using VOSviewer and the bibliometrix R package. The results indicate that research interest in ultrasound-based assessment of renal transplants remains strong and is expected to continue growing. Furthermore, a relatively high level of collaboration among countries and institutions was observed. In future studies, emerging topics such as ML, CEUS, and SWE are likely to become key research hotspots. Continued exploration of these advanced ultrasound techniques may provide a more in-depth and multi-dimensional understanding of renal allograft function and pathology.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the BIBLIO reporting checklist. Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-656/rc

Peer Review File: Available at https://tau.amegroups.com/article/view/10.21037/tau-2025-656/prf

Funding: This work was supported by the National Natural Science Foundation of China (No. 82371977), the Sichuan Science and Technology Program (No. 2023NSFSC1712), and the Doctoral Research Foundation of West China Hospital, Sichuan University (No. 21hxbh039).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tau.amegroups.com/article/view/10.21037/tau-2025-656/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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