Amy C. Dwyer

Models and methods for analyzing DCE‐MRI: A review

PURPOSE To present a review of most commonly used techniques to analyze dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), discusses their strengths and weaknesses, and outlines recent clinical applications of findings from these approaches. METHODS DCE-MRI allows for noninvasive quantitative analysis of contrast agent (CA) transient in soft tissues. Thus, it is an important and well-established tool to reveal microvasculature and perfusion in various clinical applications. In the last three decades, a host of nonparametric and parametric models and methods have been developed in order to quantify the CAs perfusion into tissue and estimate perfusion-related parameters (indexes) from signal- or concentration-time curves. These indexes are widely used in various clinical applications for the detection, characterization, and therapy monitoring of different diseases. RESULTS Promising theoretical findings and experimental results for the reviewed models and techniques in a variety of clinical applications suggest that DCE-MRI is a clinically relevant imaging modality, which can be used for early diagnosis of different diseases, such as breast and prostate cancer, renal rejection, and liver tumors. CONCLUSIONS Both nonparametric and parametric approaches for DCE-MRI analysis possess the ability to quantify tissue perfusion.

Surface‐Treated Catheters—A Review

Almost 30% of prevalent hemodialysis patients use catheters for vascular access although outcomes are superior with the use of either an arteriovenous fistula or a synthetic graft. Catheter complications are a major cause of morbidity and mortality for hemodialysis patients and increase the burden on the health care system. Surface‐treated catheters have been developed to combat the three most common causes of catheter failure: infection, fibrin sheath formation, and thrombus formation. Two types of catheter surface treatments are available: antimicrobial coatings and antithrombotic coatings. Surface treatment of central venous catheters with antimicrobial materials reduces both bacterial colonization and the incidence of catheter‐related bacteremia in critical care patients by 30–50%. Antithrombotic coatings reduce platelet adhesion, inhibit the inflammatory response, and reduce thrombus formation on coronary stents, ventricular assist devices, central venous catheters, and vascular grafts. However, few reports on the use of surface‐treated catheters in the chronic hemodialysis patient population exist. At the present time, it is difficult to justify the increased cost of surface‐treated catheters for chronic hemodialysis in the absence of clinical data demonstrating that they reduce catheter‐related complications in this patient population.

A vascular access coordinator improves the prevalent fistula rate.

The Centers for Medicare and Medicaid Services set the prevalent arteriovenous fistula (AVF) rate of 66% as a national standard. To test the hypothesis that the use of a clinical vascular access coordinator could increase the rate of AVF in a large Nephrology group practice, we implemented an aggressive, multidisciplinary vascular access improvement program led by a trained vascular access coordinator (VAC). In early 2009, we established protocols, approved by all physicians, for the care of vascular access and implemented by a nurse VAC. We retrospectively reviewed Network vascular access data reports from January 2008 through December 2010. The data show that after the implementation of a comprehensive access program led by a VAC, the prevalent AVF rate increased from 50% to 65%. The number of grafts decreased while the percentage of dialysis catheters used for more than 90 days was cut in half. These data suggest that despite an unchanged catheter rate at dialysis initiation, the use of a VAC implementing an aggressive, multidisciplinary access program can significantly increase the AVF rate while decreasing grafts and prevalent catheter use.

Predicting Post Renal Biopsy Complications

At our institution, kidney biopsies are performed by an interventional nephrologist with standardized guidelines using real‐time ultrasound. We hypothesized that patient factors could predict post biopsy complications. We did a retrospective review of 100 patients who underwent renal biopsy. Prebiopsy data obtained included demographics, blood pressure, laboratory studies, and kidney size. Biopsy procedure information was also recorded. Complications and post biopsy imaging was noted. A minor complication was defined as one not requiring intervention while a major complication required interventions like readmission or blood transfusion. The average age was 47 years, 41 were men, 51 were black, 30 had diabetes, 42 were obese, and 81 had hypertension. Twenty‐six patients had a complication; 14 minor and 12 major including 1 nephrectomy. Factors predictive of a complication were thrombocytopenia (p = 0.002) and inpatient status (p = 0.04). Drop in hemoglobin at 6 hours was moderately sensitive and specific for a bleeding complication with an ROC of 0.723. Thrombocytopenia and inpatient status are risk factors for complications after renal biopsy. Serum creatinine, obesity, blood pressure, kidney size, needle size, and number of passes were not predictive of a major complication in our study.

Dialysis Vascular Access Management by Interventional Nephrology Programs at University Medical Centers in the United States

The development of interventional nephrology has undoubtedly led to an improvement in patient care at many facilities across the United States. However, these services have traditionally been offered by interventional nephrologists in the private practice arena. While interventional nephrology was born in the private practice setting, several academic medical centers across the United States have now developed interventional nephrology programs. University Medical Centers (UMCs) that offer interventional nephrology face challenges, such as smaller dialysis populations, limited financial resources, and real or perceived political “turf” issues.” Despite these hurdles, several UMCs have successfully established interventional nephrology as an intricate part of a larger nephrology program. This has largely been accomplished by consolidating available resources and collaborating with other specialties irrespective of the size of the dialysis population. The collaboration with other specialties also offers an opportunity to perform advanced procedures, such as application of excimer laser and endovascular ultrasound. As more UMCs establish interventional nephrology programs, opportunities for developing standardized training centers will improve, resulting in better quality and availability of nephrology‐related procedures, and providing an impetus for research activities.

Fibrin Sheath and its Relation to Subsequent Events after Tunneled Dialysis Catheter Exchange

The use of tunneled catheters (TDC) for chronic hemodialysis is frequent and often fails due to fibrin or thrombus and infection. We hypothesized that the presence of fibrin sheath in TDC increases the risk for subsequent catheter malfunction and infection. We did a retrospective review of TDC exchanges and de novo placements from January 2005 to September 2011. Demographic data, information about the catheter procedure, and radiological data were collected. Final outcome analysis included 168 procedure events. Three groups of catheter procedures were identified: catheter exchange without a fibrin sheath (CE), catheter exchange with a treated fibrin sheath (CEF), and de novo catheter placements (DCP). Fibrin sheath incidence was 47%. In the CEF group, there was no statistical difference in the incidence of subsequent infections or dysfunctions (7% and 60%, respectively), when compared with the CE group (9% and 43%, respectively), (p = 0.3). Mean time to subsequent dysfunction or infection was similar for CEF and CE (135 vs. 136 days, p‐value, 0.98). Fibrin sheaths are common and should be evaluated when performing TDC exchange. If the fibrin sheath is treated, there is no increased incidence in subsequent catheter dysfunction or infection compared with patients without a fibrin sheath.

Academic Interventional Nephrology: A Model for Training, Research, and Patient Care

Dialysis vascular access dysfunction is currently a huge clinical problem. We believe that comprehensive academic-based dialysis vascular access programs that go all the way from basic and translational science investigation to clinical research to a dedicated curriculum and opportunities in vascular access for nephrologists in training are essential for improving dialysis vascular access care. This paper reviews the fundamental concepts and requirements for us to move toward this vision.

A Promising Non-invasive CAD System for Kidney Function Assessment

This paper introduces a novel computer-aided diagnostic (CAD) system for the assessment of renal transplant status that integrates image-based biomarkers derived from 4D (3D + b-value) diffusion-weighted (DW) MRI, and clinical biomarkers. To analyze DW-MRI, our framework starts with kidney tissue segmentation using a level set approach after DW-MRI data alignment to handle the motion effects. Secondly, the cumulative empirical distributions (i.e., CDFs) of apparent diffusion coefficients (ADCs) of the segmented DW-MRIs are estimated at low and high gradient strengths and duration (b-values) accounting for both blood perfusion and diffusion, respectively. Finally, these CDFs are fused with laboratory-based biomarkers (creatinine clearance and serum plasma creatinine) for the classification of transplant status using a deep learning-based classification approach utilizing a stacked non-negativity constrained auto-encoder. Using “leave-one-subject-out” experiments on a cohort of 58 subjects, the proposed CAD system distinguished non-rejection transplants from kidneys with abnormalities with a 95 % accuracy (sensitivity = 95 %, specificity = 94 %) and achieved a 95 % correct classification between early rejection and other kidney diseases. Our preliminary results demonstrate the promise of the proposed CAD system as a reliable non-invasive diagnostic tool for renal transplants assessment.

3D diffusion MRI-based CAD system for early diagnosis of acute renal rejection

This paper introduces a novel computer aided diagnostic (CAD) system for early diagnosis of acute renal rejection (ARR) from 3D diffusion-weighted MRI (DW-MRI) data (2D + fe-value). The proposed CAD system starts with aligning the DW-MRI data using B-splines based registration to handle the motion effects, which come from breathing and heart beats. This is followed by the segmentation of the kidney tissue using geometric (level-sets based) de-formable model, which is guided by a new stochastic speed relationship that takes into account an adaptive kidney shape prior and the visual appearance of the kidney. The pixel-wise guidance of the level-sets is obtained by integrating these image features into a joint Markov-Gibbs random field (MGRF) model of the kidney and its background. The final step of the proposed CAD system is to calculate the apparent diffusion coefficients (ADCs) between different b-values of the segmented DW-MRI data to distinguish between rejection and nonrejection renal transplants. Experimental results on 36 subjects, using a KStar classifier and leave-one-subject-out, have classified 87% of the subjects correctly (26 out of 30 rejection kidneys and 5 out of 6 nonrejection kidneys). These initial diagnostic results hold promise of the proposed CAD system as a reliable non-invasive diagnostic tool.

Association of maturation period blood pressure with dialysis access patency.

Problematic dialysis vascular access is a major health issue. The purpose of this study was to evaluate for potentially modifiable factors associated with access patency, particularly, the association of early postoperative, or maturation period, blood pressure with patency. A retrospective review was performed of patients who had undergone placement of an arteriovenous fistula or graft. Demographic, operative, and postoperative factors were evaluated for possible association with access primary patency using univariate and multivariate Cox regression analyses. Seventy‐three patients over a 3‐year review period were examined. Overall analysis showed a significant association of absence of peripheral vascular disease, aspirin use, and absence of previous permanent dialysis access with higher primary patency rates. Fistula subgroup analysis showed that higher blood pressure during the maturation period relative to preoperative blood pressure was associated with lower patency rates. For grafts, race was significantly associated with patency, with blacks having higher patency rates than whites. Multiple clinical factors were found to have a significant association with dialysis access primary patency. The finding of an association of maturation period blood pressure with fistula patency suggests that the maturation period environment, specifically hemodynamics during this time, may play an important role in dialysis access patency.