Comparing the diagnostic accuracy of contrast-enhanced computed tomographic angiography and gadolinium-enhanced magnetic resonance angiography for the assessment of hemodynamically significant transplant renal artery stenosis

Functional MRI evaluation of blood oxygen dependent (BOLD) in renal allograft dysfunction: a prospective study

BACKGROUND

Blood oxygen level dependent-magnetic resonance imaging (BOLD-MRI) is a non-invasive functional imaging technique that can be used to assess renal allograft dysfunction.

PURPOSE

To evaluate the diagnostic performance of BOLD-MRI using a 3-T scanner in discriminating causes of renal allograft dysfunction in the post-transplant period.

MATERIAL AND METHODS

This prospective study was conducted on 112 live donor-renal allograft recipients: 53 with normal graft function, as controls; 18 with biopsy-proven acute rejection (AR); and 41 with biopsy-proven acute tubular necrosis (ATN). Multiple fast-field echo sequences were performed to obtain T2*-weighted images. Cortical R2* (CR2*) level, medullary R2* (MR2*) level, and medullary over cortical R2* ratio (MCR) were measured in all participants.

RESULTS

The mean MR2* level was significantly lower in the AR group (20.8 ± 2.8/s) compared to the normal group (24 ± 2.4/s, P <0.001) and ATN group (27.4 ± 1.7/s, P <0.001). The MCR was higher in ATN group (1.47 ± 0.18) compared to the AR group (1.18 ± 0.17) and normal functioning group (1.34 ± 0.2). Both MR2* (area under the curve [AUC] = 0.837, P <0.001) and MCR (AUC = 0.727, P = 0.003) can accurately discriminate ATN from AR, however CR2* (AUC = 0.590, P = 0.237) showed no significant difference between both groups.

CONCLUSION

In early post-transplant renal dysfunction, BOLD-MRI is a valuable non-invasive diagnostic technique that can differentiate between AR and ATN by measuring changes in intra-renal tissue oxygenation.

Contemporary and Emerging MRI Strategies for Assessing Kidney Allograft Complications: Arterial Stenosis and Parenchymal Injury, From the AJR Special Series on Imaging of Fibrosis

MRI plays an important role in the evaluation of kidney allografts for vascular complications as well as parenchymal insults. Transplant renal artery stenosis, the most common vascular complication of kidney transplant, can be evaluated by MRA using gadolinium and nongadolinium contrast agents as well as by unenhanced MRA techniques. Parenchymal injury occurs through a variety of pathways, including graft rejection, acute tubular injury, BK polyomavirus infection, drug-induced interstitial nephritis, and pyelonephritis. Investigational MRI techniques have sought to differentiate among these causes of dysfunction as well as to assess the degree of interstitial fibrosis or tubular atrophy (IFTA)-the common end pathway for all of these processes-which is currently evaluated by invasively obtained core biopsies. Some of these MRI sequences have shown promise in not only assessing the cause of parenchymal injury but also assessing IFTA noninvasively. This review describes current clinically used MRI techniques and previews promising investigational MRI techniques for assessing complications of kidney grafts.

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.

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.

Visualization of the renal artery in kidney transplant patients using time-resolved computed tomography angiography

Background

Transplant renal artery stenosis (TRAS) is a post-operative complication which most often occurs between 3 months and 2 years after transplantation. TRAS is associated with kidney failure and hypertension and, thereby, with an increased risk of cardiovascular events.

Purpose

The purpose of this retrospective study was to report our experience of perfusion computed tomography angiography (P-CTA) to identify a 50% lumen reduction (as compared to digital subtraction angiography, DSA), assess its subjective image quality and evaluate if contrast-induced acute kidney injury (CI-AKI) occurred.

Material and Methods

All 13 patients who had undergone P-CTA for suspected TRAS at our institution were retrospectively evaluated. At P-CTA, eight or 12 g of iodine were administered intravenously, and five to seven scan sequences were merged into time-resolved images after motion correction. Eight patients underwent subsequent DSA.

Results

The average patient weight was 76 kg (range 55-97 kg). Image quality was rated as good or excellent for all patients, and pathological changes were shown in 10 of 13 patients undergoing P-CTA. Two patients had a serum creatinine increase of >26 μmol/L during the first 3 days, but serum creatinine was significantly lower in all patients 1 month after P-CTA (165+/-69 μmol/L versus 232+/-66 μmol/L, P < .01). The diagnosis at P-CTA was verified in all eight patients who underwent DSA. However, in two cases with suspected stenosis, renal function was restored without angioplasty.

Conclusion

Anatomy and blood flow of the transplant renal artery can be visualized using less than a third of the standard contrast media dose by using P-CTA technique.

Gadolinium-enhanced magnetic resonance versus computed tomography angiography for renal artery stenosis: A systematic review and meta-analysis

Gadolinium-enhanced magnetic resonance angiography (MRA) and computed tomography angiography (CTA) are commonly used for diagnosing renal arterial stenosis (RAS); however, the diagnostic value is yet controversial. The aim of the study was to evaluate the diagnostic values of both methods. Electronic databases, including PubMed, Embase, and the Cochrane Library, were searched for studies, since inception until October 2017. A total of four articles involving 486 subjects were included in the analysis. The summary of sensitivity, specificity, positive likelihood ratio (PLR), negative likelihood ratio (NLR), diagnostic odds ratio (DOR), and area under the receiver operating characteristic (ROC) (AUC) were 0.70, 0.82, 14.54, 0.29, 63.80, and 0.81 for MRA-based diagnosis of RAS, respectively. The pooled sensitivity, specificity, PLR, NLR, DOR, and AUC for CTA detecting RAS were 0.73, 0.96, 13.04, 0.29, 71.99, and 0.93, respectively. Gadolinium-enhanced MRA and CTA provide a satisfactory diagnostic accuracy, thereby playing a critical role in the diagnosis of RAS.

Noncontrast Magnetic Resonance Angiography in the Era of Nephrogenic Systemic Fibrosis and Gadolinium Deposition

ABSTRACT

Gadolinium-based contrast agents for clinical magnetic resonance imaging are overall safe. However, the discovery of nephrogenic systemic fibrosis in patients with severe renal impairment and gadolinium deposition in patients receiving contrast have generated developments in contrast-free imaging of the vasculature, that is, noncontrast magnetic resonance angiography. This article presents an update on noncontrast magnetic resonance angiography techniques, with comparison to other imaging alternatives. Potential benefits and challenges to implementation, and evidence to date for various clinical applications are discussed.

MRI for diagnosis of post-renal transplant complications: current state-of-the-art and future perspectives

Kidney transplantation has developed into a widespread procedure to treat end stage renal failure, with transplantation results improving over the years. Postoperative complications have decreased over the past decades, but are still an important cause of morbidity and mortality. Early accurate diagnosis and treatment is the key to prevent renal allograft impairment or even graft loss. Ideally, a diagnostic tool should be able to detect post-transplant renal dysfunction, differentiate between the different causes and monitor renal function during and after therapeutic interventions. Non-invasive imaging modalities for diagnostic purposes show promising results. Magnetic resonance imaging (MRI) techniques have a number of advantages, such as the lack of ionizing radiation and the possibility to obtain relevant tissue information without contrast, reducing the risk of contrast-induced nephrotoxicity. However, most techniques still lack the specificity to distinguish different types of parenchymal diseases. Despite some promising outcomes, MRI is still barely used in the post-transplantation diagnostic process. The aim of this review is to survey the current literature on the relevance and clinical applicability of diagnostic MRI modalities for the detection of various types of complications after kidney transplantation.

Noninvasive evaluation of renal tissue oxygenation with blood oxygen level-dependent magnetic resonance imaging early after transplantation has a limited predictive value for the delayed graft function

PURPOSE

The aim of this study was to evaluate the feasibility of renal oxygenation assessment using blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) in the early period after kidney transplantation and to estimate its prognostic value for delayed graft function.

MATERIAL AND METHODS

Examinations were performed in 50 subjects: 40 patients within a week after the kidney transplantation and 10 healthy controls, using T2*-weighted sequence. Measurements in transplant patients were correlated to basic laboratory parameters in the early period after transplantation and at follow-up.

RESULTS

Examinations of seven patients (18%) were rejected due to their poor technical quality. Mean R2* values in transplant recipients were lower than in controls (11.6 vs. 15.9 Hz; p = 0.0001). An R2* value of 0.28 Hz was calculated as the minimal detectable change. There was no relation between R2* values and laboratory parameters. However, patients eGFR ≥ 40 ml/min/1.73 m2 presented higher R2* values than recipients eGFR < 40 ml/min/1.73 m2 (12.0 vs. 11.1 Hz; p = 0.0189). In ROC analysis R2* of ≤ 11.7 predicted an early reduced graft function with 0.82 sensitivity and 56% specificity (AUC = 0.708; p = 0.024) but was not useful for delayed graft function prediction (p > 0.7).

CONCLUSIONS

Evaluation of renal graft oxygenation using BOLD MRI is technically challenging in the early period after transplantation. An R2* value of 0.28 Hz may in practice be considered as the minimal detectable change. The delayed graft function seems not to be dependent on early oxygenation values. Further, large-scale studies are necessary to confirm the latter observation.

ACR Appropriateness Criteria® Renal Transplant Dysfunction

Renal transplantation is the treatment of choice in patients with end-stage renal disease because the 5-year survival rates range from 72% to 99%. Although graft survival has improved secondary to the introduction of newer immunosuppression drugs and the advancements in surgical technique, various complications still occur. Ultrasound is the first-line imaging modality for the evaluation of renal transplants in the immediate postoperative period and for long-term follow-up. In addition to depicting many of the potential complications of renal transplantation, ultrasound can also guide therapeutic interventions. Nuclear medicine studies, CT, and MRI are often helpful as complementary examinations for specific indications. Angiography remains the reference standard for vascular complications and is utilized to guide nonsurgical intervention. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel. The guideline development and revision include an extensive analysis of current medical literature from peer-reviewed journals and the application of well-established methodologies (RAND/UCLA Appropriateness Method and Grading of Recommendations Assessment, Development, and Evaluation or GRADE) to rate the appropriateness of imaging and treatment procedures for specific clinical scenarios. In those instances where evidence is lacking or equivocal, expert opinion may supplement the available evidence to recommend imaging or treatment.

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.

Efficacy and safety of gadobutrol-enhanced MRA of the renal arteries: Results from GRAMS (Gadobutrol-enhanced renal artery MRA study), a prospective, intraindividual multicenter phase 3 blinded study

PURPOSE

To compare the performance of magnetic resonance angiography (MRA) with 1M gadobutrol, a high relaxivity macrocyclic contrast agent, to 2D time-of-flight MRA (ToF-MRA) using computed tomographic angiography (CTA) as the standard of reference. Primary objectives were evaluation for superiority of structural delineation and noninferiority for detection and exclusion of clinically significant disease.

MATERIALS AND METHODS

In all, 315 subjects underwent unenhanced and contrast-enhanced MRA with 1M gadobutrol (CE-MRA) and were scanned with 1.5T MRI equipped with an at least 6-element body coil. Evaluations were based on both centralized blinded read (BR) performed by six readers as well as investigator site interpretations for the 292 subjects who completed the study. Quantitative evaluations including percent stenosis and normal vessel measurements were also performed. Secondary endpoints included identification of accessory renal arteries, diagnosis of fibromuscular dysplasia (FMD), diagnostic confidence, and need for additional imaging.

RESULTS

A total of 292 patients suspected of renal artery disease completed the study. CE-MRA demonstrated statistically significant improvement in assessability of vascular segments compared to ToF: 95.9% vs. 77.6% (P < 0.0001). In the BR, the sensitivity and specificity of CE-MRA were noninferior to ToF-MRA (53.4% vs. 46.6% and 95.1% vs. 85.7%, respectively). There was less error in the CE-MRA stenosis measurements (0.15 mm gadobutrol vs. 0.41 mm ToF, P < 0.05). FMD was correctly diagnosed more frequently, 10% more accessory renal arteries were identified (P < 0.01), diagnostic confidence increased (P < 0.01), and fewer additional imaging studies were recommended (P < 0.01).

CONCLUSION

Gadobutrol-enhanced MRA of the renal arteries has superior visualization, more accurate vessel measurements, and may serve as a CTA alternative without any ionizing radiation.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:572-581.

Use of blood pool agents with steady-state MRI to assess the vascular system

Over the past two decades there have been significant advances in the use of magnetic resonance imaging (MRI) to assess the vascular system. New imaging sequences and improvements in magnet design have enabled the creation of higher spatial resolution images. MRI is now a viable alternative imaging modality when compared to both invasive angiography and computed tomographic angiography. The use of blood pool agents has further facilitated the use of MR angiography (MRA); their high molecular weight allows for lower doses of contrast medium administration while their prolonged presence in the blood stream allows for repeated high-quality volumetric imaging of both the arterial and venous circulation. As such, MRA is now no longer constrained by the tight windows for first-pass arterial and venous enhancement, which has resulted in the ability to assess and diagnose a large range of vascular pathologies in both arterial and venous systems. The intent of this review is to highlight MRI findings in common vascular pathologies including peripheral arterial disease (PAD), abnormalities of the abdominal aortic branches, postendovascular aortic aneurysm repair (EVAR) endoleak assessment, popliteal artery entrapment syndrome (PAES), deep venous thrombosis (DVT), vascular thoracic outlet syndrome (TOS), and vascular malformations. In addition, the latest MRI techniques currently used to optimally assess each of these pathologies will be discussed.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 3 J. MAGN. RESON. IMAGING 2017;45:1559-1572.

Transplant renal artery stenosis: clinical manifestations, diagnosis and therapy

Transplant renal artery stenosis (TRAS) is a well-recognized vascular complication after kidney transplant. It occurs most frequently in the first 6 months after kidney transplant, and is one of the major causes of graft loss and premature death in transplant recipients. Renal hypoperfusion occurring in TRAS results in activation of the renin–angiotensin–aldosterone system; patients usually present with worsening or refractory hypertension, fluid retention and often allograft dysfunction. Flash pulmonary edema can develop in patients with critical bilateral renal artery stenosis or renal artery stenosis in a solitary kidney, and this unique clinical entity has been named Pickering Syndrome. Prompt diagnosis and treatment of TRAS can prevent allograft damage and systemic sequelae. Duplex sonography is the most commonly used screening tool, whereas angiography provides the definitive diagnosis. Percutaneous transluminal angioplasty with stent placement can be performed during angiography if a lesion is identified, and it is generally the first-line therapy for TRAS. However, there is no randomized controlled trial examining the efficacy and safety of percutaneous transluminal angioplasty compared with medical therapy alone or surgical intervention.