Detection of transplant renal artery stenosis: determining normal velocities at the renal artery anastomosis

Renal transplant ultrasound: assessment of complications and advanced applications

Renal transplantation is the most commonly performed solid organ transplant procedure. Monitoring renal transplants with ultrasound is a critical component in the management of transplant patients both in the immediate aftermath of surgery and longitudinally. Many complications are detectable via ultrasound evaluation with relative prevalence dependent on the time since surgery. It is critical for the practicing radiologist to recognize these complications to help guide appropriate treatment. Fundamental understanding of the procedure, including various surgical techniques is of great importance. In this article, the sonographic findings of the most common postoperative and long-term complications of renal transplantation are reviewed. As complications are highly related to surgical technique, the most common surgical techniques are presented first. Comprehensive ultrasound evaluation of the allograft is discussed next, followed by extensive review of the ultrasound findings of common complications. Finally, select recent advances in ultrasound are presented with their current and potential applications to renal transplant evaluation.

Imaging of the renal allograft vasculature without gadolinium contrast: Intraindividual comparison between relaxation-enhanced angiography without contrast and triggering (REACT) and 4D contrast-enhanced MR-angiography

BACKGROUND

Complications after kidney transplantation include transplant renal artery stenosis (TRAS), which can be assessed using Doppler ultrasonography, computed tomography angiography, and magnetic resonance angiography (MRA). Contrast-enhanced MRA (CE-MRA) has limitations, including potential allergic reactions, limited use in kidney failure, and uncertain long-term effects of gadolinium retention.

PURPOSE

To evaluate Relaxation-Enhanced Angiography without Contrast and Triggering (REACT), a novel 3D isotropic flow-independent non-CE-MRA pulse sequence, for imaging of the renal allograft vasculature by performing an intraindividual comparison to 4D CE-MRA at 3Tesla.

METHODS

Forty studies of 39 patients were included in this retrospective, single-centre study. Two board-certified radiologists independently evaluated MRA datasets for TRAS and rated their diagnostic confidence and the image quality of pelvic vessels using 5-point Likert scales (5 = excellent). Apparent signal- and contrast-to-noise ratios (aSNR/aCNR) were measured for arterial and venous graft vessels.

RESULTS

REACT (median acquisition time 04:33 min [IQR 3:58-5:20 min]) showed 90.0 % sensitivity and 100.0 % specificity for TRAS in almost perfect agreement (r = 0.97) with 4D CE-MRA (03:41 min [3:38-4:46 min], p = 0.001) and similar diagnostic confidence (REACT: 4.0 [4.0-4.0] vs. 4D CE-MRA: 4.0 [3.0-4.0], p = 0.54). Arterial image quality was comparable (4.0 [3.7-4.4] vs. 4.0 [4.0-4.4], p = 0.49) whereas veins yielded higher scores in REACT (3.2 [3.0-3.5] vs. 2.4 [2.0-3.0], p < 0.001). Transplant renal artery (mean ± SD; 44.5 ± 18.2 vs. 45.9 ± 21.0, p = 0.71; 36.3 ± 15.0 vs. 41.0 ± 20.0, p = 0.16) and vein (37.1 ± 19.8 vs. 30.3 ± 15.2, p = 0.06; 29.4 ± 17.1 vs. 25.0 ± 14.7, p = 0.17) showed no difference in aSNR and aCNR.

CONCLUSION

REACT provides accurate detection of TRAS with image quality comparable to 4D CE-MRA, offering a risk-free alternative for imaging after renal transplantation.

Radiologic evaluation of the kidney transplant donor and recipient

The kidney is the most common solid organ transplant globally and rates continue to climb, driven by the increasing prevalence of end stage renal disease (ESRD). Compounded by advancements in surgical techniques and immunosuppression leading to longer graft survival, radiologists evermore commonly evaluate kidney transplant patients and candidates, underscoring their role along the transplant process. Multiphase computed tomography (CT) with multiplanar and 3D reformatting is the primary method for evaluating renal donor candidates, detailing renal size, vascular/collecting system anatomy, and identifying significant pathologies such as renal vascular diseases and nephrolithiasis. Ultrasound is the preferred initial postoperative imaging modality for graft evaluation due to its low cost, accessibility, noninvasiveness, and lack of radiation. CT and magnetic resonance imaging (MRI) may be useful adjunctive imaging techniques in diagnosing transplant pathology when ultrasound alone is not diagnostic. Kidney transplant complications are categorized by an approximate timeline framework, aiding in differential diagnosis based on onset, duration, and severity and include perinephric fluid collections, graft compression, iatrogenic injuries, vascular compromise, graft rejection, and neoplastic processes. This review discusses imaging strategies and important findings along the transplant timeline, from donor assessment to long-term recipient complications.

Transplantation for type 1 diabetes: radiologist's primer on islet, pancreas and pancreas-kidney transplantation imaging

Whole-organ pancreas, pancreatic-kidney and islet transplantation are surgical therapeutic options for the treatment of type 1 diabetes. They can enable effective glycemic control, improve quality of life and delay/reduce the secondary complications of type 1 diabetes mellitus. Radiologists are integral members of the multidisciplinary transplantation team involved in these procedures, with multimodality imaging serving as the mainstay for early recognition and management of transplant related complications. This review highlights the transplantation procedures available for patients with type 1 Diabetes Mellitus with a focus on the imaging appearance of transplantation-related complications.

Contrast-enhanced US in Renal Transplant Complications: Overview and Imaging Features

Renal transplant is the first-line treatment of end-stage renal disease. The increasing number of transplants performed every year has led to a larger population of transplant patients. Complications may arise during the perioperative and postoperative periods, and imaging plays a key role in this scenario. Contrast-enhanced US (CEUS) is a safe tool that adds additional value to US. Contrast agents are usually administered intravenously, but urinary tract anatomy and complications such as stenosis or leak can be studied using intracavitary administration of contrast agents. Assessment of the graft and iliac vessels with CEUS is particularly helpful in identifying vascular and parenchymal complications, such as arterial or venous thrombosis and stenosis, acute tubular injury, or cortical necrosis, which can lead to graft loss. Furthermore, infectious and malignant graft involvement can be accurately studied with CEUS, which can help in detection of renal abscesses and in the differentiation between benign and malignant disease. CEUS is also useful in interventional procedures, helping to guide percutaneous aspiration of collections with better delimitation of the graft boundaries and to guide renal graft biopsies by avoiding avascular areas. Potential postprocedural vascular complications, such as pseudoaneurysm, arteriovenous fistula, or active bleeding, are identified with CEUS. In addition, newer quantification tools such as CEUS perfusion are promising, but further studies are needed to approve its use for clinical purposes. ©RSNA, 2024 Supplemental material is available for this article.

Diagnostic Performance of Magnetic Resonance Angiography for Artery Stenosis After Kidney Transplant: A Systematic Review and Meta-Analysis

RATIONALE AND OBJECTIVES

Magnetic resonance angiography (MRA) is used to diagnose artery stenosis after kidney transplant. However, there is a lack of applicable consensus guidelines, and the diagnostic value of this technique is unclear. Therefore, the aim of the present study was to evaluate the diagnostic performance of MRA for the detection of artery stenosis after kidney transplant.

MATERIALS AND METHODS

We searched PubMed, Web of Science, Cochrane Library, and Embase from database inception to September 1, 2022. Two independent reviewers assessed the methodological quality of eligible studies using the quality assessment of diagnostic accuracy studies-2 tool. The diagnostic odds ratio, pooled sensitivity, and specificity values, positive likelihood ratios, and negative likelihood ratios were calculated to synthesize data with a bivariate random-effects model. Meta-regression analysis was performed in cases of high among-study heterogeneity.

RESULTS

Eleven studies were included in the meta-analysis. The area under the summary receiver operating characteristic curve was 0.96 (95% confidence interval [CI]: 0.94-0.98). The pooled sensitivity and specificity values for MRA in diagnosing artery stenosis after kidney transplant were 0.96 (95% CI: 0.76-0.99) and 0.93 (95% CI: 0.86-0.96), respectively.

CONCLUSION

MRA demonstrated high sensitivity and specificity for diagnosing artery stenosis after kidney transplant, suggesting that it may be used reliably in clinical practice. However, further large-scale studies are required to validate the present findings.

Transplant renal artery stenosis: utilization of machine learning to identify ancillary sonographic and doppler parameters to predict stenosis in patients with graft dysfunction

PURPOSE

To determine if ancillary sonographic and Doppler parameters can be used to predict transplant renal artery stenosis in patients with renal graft dysfunction.

MATERIALS AND METHODS

IRB-approved, HIPAA-compliant retrospective study included 80 renal transplant patients who had renal US followed by renal angiogram between January 2018 and December 2019. A consensus read of two radiologists recorded these parameters: peak systolic velocity, persistence of elevated velocity, grayscale narrowing, parvus tardus, delayed systolic upstroke, angle of the systolic peak (SP angle), and aliasing. Univariate analysis using t-test or chi-square was performed to determine differences between patients with and without stenosis. P values under 0.05 were deemed statistically significant. We used machine learning algorithms to determine parameters that could better predict the presence of stenosis. The algorithms included logistic regression, random forest, imbalanced random forest, boosting, and CART. All 80 cases were split between training and testing using stratified sampling using a 75:25 split.

RESULTS

We found a statistically significant difference in grayscale narrowing (p = 0.0010), delayed systolic upstroke (p = 0.0002), SP angle (p = 0.0005), and aliasing (p = 0.0024) between the two groups. No significant difference was found for an elevated peak systolic velocity (p = 0.1684). The imbalanced random forest (IRF) model was selected for improved accuracy, sensitivity, and specificity. Specificity, sensitivity, AUC, and normalized Brier score for the IRF model using all parameters were 73%, 81%, 0.82, and 69 in the training set, and 78%, 58%, 0.78, and 80 in the testing set. VIMP assessment showed that the combination of variables that resulted in the most significant change of the training set performance was that of grayscale narrowing and SP angle.

CONCLUSION

Elevated peak systolic velocity did not discriminate between patients with and without TRAS. Adding ancillary parameters into the machine learning algorithm improved specificity and sensitivity similarly in the training and testing sets. The algorithm identified the combination of lumen narrowing coupled with the angle of the systolic peak as better predictor of TRAS. This model may improve the accuracy of ultrasound for transplant renal artery stenosis.

Ultrasound Screening for Transplant Renal Artery Stenosis Risk Stratification Using Standardized Criteria in Structured Reporting: A Validation Study

OBJECTIVES

To evaluate the effectiveness of templated ultrasound reports using transplant renal artery stenosis (TRAS) risk stratification (RS), particularly with regard to utilization of downstream angiographic studies and angiographic presence of TRAS.

METHODS

Ultrasounds with TRAS-RS templated reports from August 2017 to May 2020 were included. Studies were excluded if performed <28 days posttransplant and where TRAS was not clinically considered. A total of 530 ultrasounds met inclusion/exclusion criteria. TRAS-RS criteria were recorded (renal artery velocity ≥300 cm/s, spectral broadening in the renal artery, and intraparenchymal acceleration time ≥0.1 second). Depending on the number of criteria present, recipients were stratified into low (0/3), intermediate (1/3), high (2/3), and very high (3/3) risk for TRAS. Student's t-test was performed to identify whether the TRAS-RS category was associated with 1) performance of angiography to assess for TRAS and 2) angiographic presence of TRAS.

RESULTS

Of the 530 ultrasounds, 74 (14%) underwent angiography. Of these, 41 (55%) were positive for TRAS (overall positive rate, 8%). Number of ultrasounds, angiograms, and angiograms positive for TRAS, respectively, in each of the TRAS-RS categories for the 530 cases were: low probability: n = 370 (70% of all studied reports), 7 angiograms (2%), and 0 (0%) positive for TRAS; intermediate: n = 87 (16%), 24 angiograms (28%), and 8 (33%) positive; high: n = 46 (9%), 23 angiograms (50%), and 14 (61%) positive; and very high: n = 27 (5%), 20 angiograms (74%), and 19 (95%) positive. TRAS-RS score was associated with subsequent performance of angiography and positive rate for TRAS (P < .01).

CONCLUSION

Implementing a defined ultrasound screening tool with templated reporting for TRAS allowed for effective selection of those requiring an angiogram.

Sonographic Assessment of Renal Transplant Anastomotic Pseudoaneurysm: A Case Study

Renal transplant anastomotic pseudoaneurysms (RTAPs) are a rare vascular postoperative complication. The etiology of these pseudoaneurysms is attributed to infection or surgical vessel damage. RTAPs can lead to allograft dysfunction, allograft loss, or patient mortality due to rupture or sepsis. Because of these serious complications, expedient diagnosis is imperative for initiating surgical intervention and medical management. Sonography is a useful imaging modality for diagnosing and characterizing RTAPs. The sonographic findings, although not often described in detail in the literature, are definitive and unique. This case report describes the sonographic findings of a patient 22 days post renal autotransplant, who presented with excruciating abdominal pain. Sonographic examination identified multiple classic pseudoaneurysm findings and demonstrated intrarenal and extrarenal vascular abnormalities.

Intelligent Recognition Algorithm-Based Color Doppler Ultrasound in the Treatment of Dangerous Placenta Previa

The study focused on the clinical diagnostic value of color Doppler ultrasound of dangerous placenta previa patients under the guidance of intelligent recognition algorithms. 58 patients with placenta previa and placenta accreta admitted to the hospital for treatment were selected as research subjects. The color Doppler ultrasound under the guidance of intelligent recognition algorithm was compared with the two-dimensional ultrasound for specificity, sensitivity, and accuracy. The color Doppler ultrasound results showed that, of the 58 patients, there were 32 cases of complete placenta previa and 26 cases of incomplete placenta previa, which were consistent with the surgical pathology results. It was found that patients with malignant placenta previa and placenta accreta had thickened placenta, disappeared posterior placental space, myometrium <2 mm, and increased incidence of cervical enlargement (P < 0.05). In conclusion, the recognition accuracy of color Doppler ultrasound under the guidance of the intelligent recognition algorithm is more than 90%, and it can effectively identify dangerous placenta previa, assisting doctors in diagnosis and treatment of dangerous placenta previa.

Analysis of the Peak Systolic Velocity in the Transplant Renal Artery Anastomosis to Determine Normal Values in Patients Without Graft Dysfunction

Objective:

The primary purpose is to define the mean renal artery anastomosis peak systolic velocity (RAA PSV) and the renal artery anastomosis to external iliac artery ratio (RAA-to-EIA) of renal transplant recipients without graft dysfunction. Moreover, to determine associations with type of vascular anastomosis and type of graft.

Materials and Methods:

This is a single-center retrospective analysis of kidney transplant recipients. Recorded variables included recipient age, type of vascular anastomosis, type of graft, RAA PSV, and external iliac artery PSV (EIA PSV). Such variables were evaluated on different postoperative follow-up periods.

Results:

There was a high degree of reliability between the RAA PSV and EIA PSV ( P < .001). The mean RAA PSV was 174 cm/s ± 72.9 cm/s with 95% confidence interval (CI) (162.2 cm/s-185.5 cm/s].

Conclusion:

This study highlights the importance of determining the normal range of RAA PSV and showed that a high PSV does not necessarily indicate dysfunction.

Imaging of Renal Transplant Complications throughout the Life of the Allograft: Comprehensive Multimodality Review

The kidney is the most commonly transplanted solid organ. Advances in surgical techniques, immunosuppression regimens, surveillance imaging, and histopathologic diagnosis of rejection have allowed prolonged graft survival times. However, the demand for kidneys continues to outgrow the available supply, and there are efforts to increase use of donor kidneys with moderate- or high-risk profiles. This highlights the importance of evaluating the renal transplant patient in the context of both donor and recipient risk factors. Radiologists play an integral role within the multidisciplinary team in care of the transplant patient at every stage of the transplant process. In the immediate postoperative period, duplex US is the modality of choice for evaluating the renal allograft. It is useful for establishing a baseline examination for comparison at future surveillance imaging. In the setting of allograft dysfunction, advanced imaging techniques including MRI or contrast-enhanced US may be useful for providing a more specific diagnosis and excluding nonrejection causes of renal dysfunction. When a pathologic diagnosis is deemed necessary to guide therapy, US-guided biopsy is a relatively low-risk, safe procedure. The range of complications of renal transplantation can be organized temporally in relation to the time since surgery and/or according to disease categories, including immunologic (rejection), surgical or iatrogenic, vascular, urinary, infectious, and neoplastic complications. The unique heterotopic location of the renal allograft in the iliac fossa predisposes it to a specific set of complications. As imaging features of infection or malignancy may be nonspecific, awareness of the patient's risk profile and time since transplantation can be used to assign the probability of a certain diagnosis and thus guide more specific diagnostic workup. It is critical to understand variations in vascular anatomy, surgical technique, and independent donor and recipient risk factors to make an accurate diagnosis and initiate appropriate treatment.^©RSNA, 2019.

Diagnosis and Treatment of Early Transplant Renal Artery Stenosis: Experience From a Center in Eastern China

Few studies have focused on the clinical characteristics of transplant renal artery stenosis (TRAS) with early onset. Sixteen cases diagnosed with TRAS in our center from January 2014 to August 2018 were retrospectively analyzed. Sixteen transplant patients without TRAS were selected as controls. The median diagnostic time for TRAS was 47.5 days after transplantation. No significant difference was observed between the TRAS group and the control group. The serum creatinine level (Scr), estimated glomerular filtration rate, systolic/diastolic blood pressure, graft artery peak systolic velocity (PSV), and resistive index of intersegmental artery (RI-ISA) between the 2 groups were (5.55 ± 3.49) and (1.89 ± 0.85) mg/dL; (17.83 ± 14.94) and (49.39 ± 19.96) mL/min; (143.50 ± 9.49)/(86.14 ± 7.38) and (130.38 ± 18.86)/(82.81 ± 12.52) mm Hg; (3.39 ± 1.57) and (1.31 ± 0.51) m/s; and (0.51 ± 0.10) and (0.67 ± 0.13), respectively. All showed statistical significance except the diastolic blood pressure. The Scr, estimated glomerular filtration rate, systolic/diastolic blood pressure, graft artery PSV, and RI-ISA in the TRAS group prior and after treatment were (5.55 ± 3.49) and (3.20 ± 1.50) mg/dL; (17.83 ± 14.94) and (25.60 ± 13.29) mL/min; (143.50 ± 9.49)/(86.14 ± 7.38) and (128.07 ± 16.16)/(75.71 ± 7.56) mm Hg; (3.39 ± 1.57) and (2.00 ± 1.04) m/s; and (0.51 ± 0.10) and (0.61 ± 0.10); all showed statistical significance. Receiver operating characteristic analysis showed an area under curve of 0.8616 for PSV and 0.8535 for RI-ISA in diagnosing TRAS. Patients with TRAS in our center showed a unique characteristic of early onset. The most prominent clinical symptom of TRAS is increasing Scr level instead of refractory hypertension. Screening of color Doppler flow imaging with a graft artery PSV >2.5 m/s and RI-ISA <0.5 could yield a preliminary diagnosis of TRAS. Percutaneous transluminal angioplasty/stenting could effectively improve allograft function as well as color Doppler flow imaging indexes.

Carbon Dioxide Angiography in the Treatment of Transplant Renal Artery Stenosis

BACKGROUND

Transplant renal artery stenosis (TRAS) is a serious complication associated with graft loss. Selective carbon dioxide angiography allows for effective diagnosis and therapy with the use of minimal to no contrast agent. This study sought to evaluate the efficacy of the adjunctive use of carbon dioxide angiography in the treatment of TRAS.

METHODS

Patients undergoing endovascular therapy (percutaneous transluminal angioplasty with or without stent) for TRAS between the years 2012 and 2017 at a single tertiary care academic medical center were studied. Outcomes of interest included technical success, postoperative glomerular filtration rate, and renal ultrasound hemodynamic parameters.

RESULTS

Of the 37 patients who underwent angiography for TRAS during the study period, 34 underwent a therapeutic intervention. Of those, 24 patients (70.6%) underwent adjunctive carbon dioxide angiography versus 10 patients (29.4%) who underwent standard contrast angiography. Baseline characteristics between the carbon dioxide angiography and traditional angiography groups were similar. Patients undergoing carbon dioxide angiography received significantly less contrast agent than patients undergoing traditional angiography [9.5 mL (IQR 2-19.5) versus 19.5 mL (IQR 15-30), P = 0.03)] and maintained equivalent technical success rates (92.2% vs. 91.7%, P = 0.9).

CONCLUSIONS

The adjunctive use of carbon dioxide angiography allows for significantly less contrast administration compared with standard angiography while achieving an equivalent rate of technical success. Selective carbon dioxide angiography should be considered a first-line modality for patients with TRAS in need of endovascular therapy.

Vascular complications in kidney transplant recipients

Vascular complications are a significant source of morbidity and mortality among renal transplant recipients. Imaging using ultrasound, CT, and MRI plays a key role in diagnosing such complications. This review focuses on the major vascular complications of renal grafts, which include transplant renal arterial and venous stenoses, arterial and venous thromboses, arteriovenous fistulas, and pseudoaneurysms. Etiology, diagnostic modalities useful for diagnosis, and imaging appearance will be presented.

Screening for Transplant Renal Artery Stenosis: Ultrasound-Based Stenosis Probability Stratification

OBJECTIVE

The objective of our study was to evaluate which spectral Doppler ultrasound parameters are useful in patients with clinical concern for transplant renal artery stenosis (TRAS) and create mathematically derived prediction models that are based on these parameters.

MATERIALS AND METHODS

The study subjects included 120 patients with clinical signs of renal dysfunction who had undergone ultrasound followed by angiography (either digital subtraction angiography or MR angiography) between January 2005 and December 2015. Five ultrasound variables were evaluated: ratio of highest renal artery velocity to iliac artery velocity, highest renal artery velocity, spectral broadening, resistive indexes, and acceleration time. Angiographic studies were categorized as either showing no stenosis or showing stenosis. Reviewers assessed the ultrasound examinations for TRAS using all five variables, which we refer to as the full model, and using a reduced number of variables, which we refer to as the reduced-variable model; sensitivities and specificities were generated.

RESULTS

Ninety-seven patients had stenosis and 23 had no stenosis. The full model had a sensitivity and specificity of 97% and 91%, respectively. The reduced-variable model excluded the ratio and resistive index variables without affecting sensitivity and specificity. We applied cutoff values to the variables in the reduced-variable model, which we refer to as the simple model. Using these cutoff values, the simple model showed a sensitivity and specificity of 96% and 83%. The simple model was able to categorize patients into four risk categories for TRAS: low, intermediate, high, and very high risk.

CONCLUSION

We propose a simple model that is based on highest renal artery velocity, distal spectral broadening, and acceleration time to classify patients into risk categories for TRAS.