Original Contribution
Vector Flow Visualization of Urinary Flow Dynamics in a Bladder Outlet Obstruction Model

https://doi.org/10.1016/j.ultrasmedbio.2017.07.006Get rights and content

Abstract

Voiding dysfunction that results from bladder outlet (BO) obstruction is known to alter significantly the dynamics of urine passage through the urinary tract. To non-invasively image this phenomenon on a time-resolved basis, we pursued the first application of a recently developed flow visualization technique called vector projectile imaging (VPI) that can track the spatiotemporal dynamics of flow vector fields at a frame rate of 10,000 fps (based on plane wave excitation and least-squares Doppler vector estimation principles). For this investigation, we designed a new anthropomorphic urethral tract phantom to reconstruct urinary flow dynamics under controlled conditions (300 mm H2O inlet pressure and atmospheric outlet pressure). Both a normal model and a diseased model with BO obstruction were developed for experimentation. VPI cine loops were derived from these urinary flow phantoms. Results show that VPI is capable of depicting differences in the flow dynamics of normal and diseased urinary tracts. In the case with BO obstruction, VPI depicted the presence of BO flow jet and vortices in the prostatic urethra. The corresponding spatial-maximum flow velocity magnitude was estimated to be 2.43 m/s, and it is significantly faster than that for the normal model (1.52 m/s) and is in line with values derived from computational fluid dynamics simulations. Overall, this investigation demonstrates the feasibility of using vector flow visualization techniques to non-invasively examine internal flow characteristics related to voiding dysfunction in the urethral tract.

Introduction

Urethral voiding dysfunction that leads to weak urinary flow, urine dribbling and urine splitting is well considered as a major category of lower urinary tract symptoms (LUTS). This problem is mainly attributed to mechanical and physiological aging of the vesico-urethral system. Despite being benign in nature, urethral voiding dysfunction would hamper a patient's quality of life on a chronic basis, and thus it is increasingly regarded as a healthcare concern in today's aging society. In recent years, we have seen a growing interest in the potential to devise appropriate strategies for addressing urethral voiding dysfunction (and LUTS in general), such as pharmacological and surgical interventions that aim to alter the internal urethral shape during micturition (Tubaro et al. 2015). To help urologists determine whether such treatment is necessary, it is essential to first assess the severity of voiding dysfunction. Yet, it is not trivial to perform this diagnosis non-invasively. Techniques such as uroflowmetry (Schafer et al. 2002) and ultrasound-based bladder volume measurement (Stevens 2005) can merely provide overall indications of voiding dysfunction, and they lack the provision of insight into the anatomic and urodynamic factors that deter the urine passage process. To extend beyond these solutions, it is necessary to devise strategies that can monitor urinary flow dynamics in LUTS patients before and after an intervention (in addition to evaluating the shape of urinary tract). Medical imaging is undoubtedly a potential solution to such a need.

Among existing imaging modalities, some are capable of visualizing the flow of biofluids inside the human body, such as magnetic resonance imaging (MRI) (Stalder et al., 2008, Sughimoto et al., 2016) and optical imaging (Kurata et al., 2015, White et al., 2003). However, most of these imaging techniques have rarely been applied to urinary flow visualization. In the case of MRI, its temporal resolution is inadequate to track fast urinary flow (can be >2 m/s velocity), and logistically it is challenging to manage a patient's micturition process inside the MRI scanner bore; in the case of optical imaging, the penetration depth is insufficient (Weiss et al. 1989) for the purpose of non-invasively imaging the urinary tract. Alternatively, it is possible to leverage computational fluid dynamics (CFD) to render flow profiles based on anatomic information obtained from MRI or computed tomography (Torii et al. 2007). Alas, proper development of a CFD model for urinary flow dynamics is after all not straightforward because of the need to account for the predominance of autonomic nerve activity, as well as dynamic changes in the size of the urinary tract, which can expand to 10 mm in diameter during peak micturition.

In contrast, ultrasound imaging is perhaps better suited for urinary flow visualization. Previous work by others have indeed shown the feasibility of using ultrasound to observe the urinary tract and its related flow dynamics (Arif et al., 2014, Arif et al., 2015, Ding et al., 2000, Gratzke et al., 2015, Ozawa et al., 2010). Nevertheless, these published investigations were all conducted using conventional ultrasound technology (on the basis of scanline–based imaging principles) that typically renders flow information in two formats: (i) full-view color flow images at video-range frame rates (20–30 fps) (Ding et al., 2000, Ozawa et al., 2010), or (ii) M-mode or radiofrequency data plots over a single scanline (Arif et al., 2014, Arif et al., 2015). They fell short of providing time-resolved rendering of urinary flow dynamics across the entire imaging view, nor can they offer information about the multi-directional nature of urinary flow patterns. To overcome the temporal resolution limitation, one can possibly develop high-frame-rate ultrasound imaging methods that are based on plane wave excitation (Lu et al., 2006, Montaldo et al., 2009) and synthetic aperture principles (Jensen et al., 2006, Karaman et al., 1995). As for the need to visualize multi-directional urinary flow, vector flow estimation techniques can be devised (Jensen et al., 2016a, Jensen et al., 2016b). Nevertheless, neither of these concepts has been applied to urinary flow diagnostics.

In this paper, we report the first application of a high-frame-rate ultrasound vector flow visualization technique called vector projectile imaging (VPI) to facilitate analysis of urinary flow dynamics. It is our intent to investigate the feasibility of leveraging VPI to derive diagnostically relevant insight into the fluid mechanics within the urinary tract, particularly in LUTS cases with urethral voiding dysfunction. To perform this investigation, we have designed a novel anatomically realistic urinary flow phantom that can resemble urine passage under controlled conditions. Note that VPI is a novel technique developed by our team earlier (Yiu et al. 2014), and it is based on multi-angle plane wave excitation and least-squares Doppler vector estimation principles (Yiu and Yu 2016). We have initially applied VPI to achieve time-resolved visualization of carotid hemodynamics. Here, we seek to expand the clinical application domain of VPI by translating this technique toward use in urinary flow diagnostics.

Section snippets

Background description

Bladder outlet (BO) obstruction was chosen as the LUTS disease model in this study. Well regarded as a common lesion in the posterior urethra causing voiding symptoms, BO obstruction is estimated to affect 21.5% of the worldwide population (Irwin et al. 2011). It refers to the condition where sclerosis and narrowing of the BO are both observed during micturition. Its emergence is often attributed to (i) lateral-lobe prostate hypertrophy or (ii) urethral mucosa hyperplasia as a consequence of

VPI visualization of urinary flow in normal and diseased models

VPI was found to be capable of depicting differences in the flow dynamics of normal and diseased urinary tracts. This observation is summarized in Video 1 (playback rate: 50 fps), which renders the urinary flow dynamics for a diseased model with BO obstruction (left panel) and a normal model (right panel). For comparative analysis, the bottom portion of Video 1 shows the Doppler spectrogram at the pixel position that corresponds to the intersection between the vertical line and horizontal line

Summative interpretation of findings

Our laboratory investigation has generally demonstrated that VPI can visualize the internal flow characteristics of the lower urinary tract during micturition. Both laminar flow passage and complex flow characteristics, such as jetting and vortexing, can be tracked using VPI with a fine temporal resolution of 0.1 ms (Video 1). The maximum flow velocity magnitude, as derived from the flow vectors used to generate VPI cine loops, was found to range 2–3 m/s (Figs. 4 and 5). These numbers are in

Conclusion

High-frame-rate vector flow visualization (in the form of VPI) has been applied to non-invasively examine internal flow characteristics related to voiding dysfunction in urinary flow phantoms. This new application of VPI showed promise in locating positions within the urinary tract where disturbed urine passage is evident. Such diagnostic information opens new possibilities in developing non-invasive functional indices that describe voiding dysfunction, thereby assisting the clinical workflow

Acknowledgments

This work has been supported in part by Natural Sciences and Engineering Council of Canada (RGPIN-2016-04042), Canada Foundation for Innovation (36138), Hong Kong Innovation and Technology Fund (GHP/025/13 SZ) and Research Grants Council of Hong Kong (GRF 785113 M). Parts of this investigation were conducted at the University of Hong Kong.

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