Blood flow quantification in dialysis access using digital subtraction angiography: A retrospective study
Introduction
The global burden of kidney disease has continued to rise over the years predisposing the population with increased risks of kidney failure [1], [2], [3]. Patients diagnosed with end-stage renal disease (ESRD) have both kidneys nonfunctional and, therefore, require a kidney transplant or dialysis to survive [4]. This is further exacerbated by the lack of available transplant organs and the inability to maintain a patent and complication-free vascular access for dialysis. Because of the arteriovenous shunting, there is an abrupt change in local hemodynamics that invokes a complex pathophysiological process of intimal remodeling (to undertake neo-fluidic stress) predisposing to access stenosis and thrombosis [5], [6], [7]. Because of the change in luminal diameter either from underlying pathology or corrective procedures, blood flow quantification (access flow) in dialysis grafts/fistulae is the most preferred and reliable method to evaluate access patency and assess the outcomes of various endovascular procedures [8,9]. In particular, access flow of at least 600 ml/min in arteriovenous (AV) prosthetic grafts or at least 400–500 ml/min in AV autogenous fistula is considered the bare minimum threshold for patent access [10]. These thresholds have been used to determine endpoints during dialysis access interventions.
Access-related complications due to thrombosis and stenosis are routinely corrected by thrombectomy, percutaneous transluminal angioplasty (PTA), or in combination with stents/stent-grafts [11]. Access flow is sensitive to the change in luminal radius resulting from vascular obstruction (Hagen–Poiseuille law: flow rate ∝ radius4) [12]. The restoration of the blocked lumen from PTA or thrombectomy results in an instant increase in blood flow. The final flow rate obtained after intervention allows radio-surgeons to predict the time interval the patient would most likely not need re-angioplasty. For instance, when restored flow rate in grafts is greater than 1000 mL/min, it requires fewer repeat interventions, has longer graft survival, and less likely to thrombose within the first six months of angioplasty [13]. Similarly, fistulae with postoperative AF above 300 mL/min are more likely to remain patent for over a year [14]. The inability to restore the desired flow after corrective angioplasty helps in timely referral and/or collaboration with vascular surgeons for potential access salvage interventions.
While digital angiography is used to visualize vasculature and perform access-related interventions, measurement of access flow based on the contrast-injected angiographic images is currently unavailable in the interventional radiology suites [15]. In the United States, catheter-based thermodilution technology (non-imaging) is primarily used to measure access flow by rapidly injecting saline bolus into the vascular access and detecting the change in blood-saline temperature downstream of the site of injection [16,17]. Though this method is quick and simple to operate, it incurs an additional cost to the examination and the accuracy can be influenced by the operator's experience and the rate of bolus injection. Duplex imaging can technically be used for blood flow measurement, however, the quality of imaging (low signal to noise ratio) or equipment, the requirement to maintain a certain angle of insonation (≤60°), and operator dependent judgment limits its use and reliable interpretation [18], [19], [20]. On the other hand, ultrasound is more often used to examine thrombosed or non-thrombosed access prior to vascular access intervention and to establish/ guide the initial entry point of medical devices into the vasculature.
Blood flow measurement based on digital imaging has been well investigated [15,[21], [22], [23], [24], [25], [26], [27]]. A review by Shpilfoygel et al. systematically outlines the various approaches available for blood flow computation [28]; however, the application to the dialysis realm is yet to be thoroughly explored. In particular, the prospect of access flow measurement discussing the optimum approach, accuracy, and limitations of various algorithms, and other technicalities associated with the imaging protocol and radiation hazards is due to attract research attention in this direction. It is hypothesized that the access flow can be mathematically computed from digital subtraction angiographic (DSA) images obtained during fistulography. The ability to measure access flow using the current angiographic system adds an additional value to the imaging armamentarium and enables the possibility to enhance flow quantification accuracy by optimizing the variables of acquisition. Previously, we had demonstrated the feasibility of access flow computation in a small set of DSA images obtained from clinical examinations [29]. However, the accuracy was inadequate and primarily affected by secondary flow and intensity fluctuations from incomplete contrast-blood mixing inherent in angiograms. In this study, image preprocessing and gamma variate curve fitting model has been studied to correct bolus time-intensity curves, and its effect on flow quantification accuracy has been evaluated in an expanded image dataset. The accuracy and functioning of the catheter-based flow meter has also been verified against an ultrasonic (US) flow module.
Section snippets
Related work
Assessment of blood flow using digital imaging to predict or correlate to a diseased state has been a subject of multiple studies [30], [31], [32]. While the vascular information is readily obtained by digital angiography, the functional information is not directly available, for instance, the flow rate or type of flow. In [30], authors have used DSA images to evaluate intra-aneurysmal blood flow velocity in 29 patients. The acquisition frame rate was kept at 30 frames/s and images were
Clinical data (fistulagrams)
316 studies (DSA images) were retrospectively analyzed from 2015–2017, which were acquired during regular dialysis access intervention in the interventional radiology department of the author's institution (Table 1). 173 studies were selected for the current study based on the following criteria: cases without quantitative flow information, acquisition at frame rate ≤1 frame/s, and the number of images <5 were excluded from the analysis. All patients had either an AV fistula or an AV graft and
Results
The absolute quantification error (mean ± SEM) of different algorithms with and without the use of curve fitting has been assessed (Fig. 5). Without the use of curve fitting, the absolute estimation error by peak-to-peak, leading half peak, trailing half peak, and cross-correlation algorithms were 30.1 ± 1%, 31.1 ± 1.2%, 36.3 ± 1.2%, and 34.7 ± 0.8%, respectively. With the use of curve fitting, the estimation error by the same algorithms was 27.1 ± 1.2%, 28.2 ± 1%, 28.8 ± 1.1%, and 21.6 ± 0.7%,
Discussion
Four bolus transit time algorithms with and without the use of curve fitting have been examined for the feasibility of access flow computation. Since the clinical images were obtained by different operators over a period, a higher degree of variability existed among acquired images. For instance, the recording duration or the sampling frame rate in some acquisitions was too low to adequately determine the entire bolus transport sequence in the images. For incomplete or partial bolus
Conclusion
Access flow is of great clinical interest and has become the de facto quantitative tool to diagnose or monitor access-related malfunctions and determine the merits of percutaneous dialysis access interventions. Various computational methods to estimate flow in dialysis access were studied by analyzing the fistulagrams in a custom-computing environment. The purpose of the study was to identify the pitfalls and to determine the most accurate and reproducible method(s) for developing a clinical
Funding
The authors would like to thank Siemens Medical Solutions USA, Inc., and Cleveland State University through the Dissertation Research Award (awarded to NK) for funding a part of this project.
Ethical approval
The study was approved by the Institutional Review Board of the Cleveland Clinic. Because of the retrospective nature of the study and the use of de-identified images, no informed consent was necessary.
Declaration of Competing Interest
NK does not have any conflict of interest to declare. GM serves as a member of scientific advisory board for Trisalus Life Sciences, Rene Medical, Siemens, Guerbet, and Stealth Medical, and speaker's bureau for General Electric.
Acknowledgments
The authors would like to thank Ken Kula, BA, Medical Illustrator at the Cleveland Clinic Center for Medical Art & Photography for help with the illustration.
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