Skip to main content
2025 BMES Annual Meeting Main banner
Icon Legend
This session is not in your schedule.
This session is in your schedule. Click again to remove it.
ViewAttendees 1

Biomedical Imaging and Instrumentation

Biomedical Imaging and Instrumentation Undergraduate Fire Talks

Monitoring of Kidney Hemodynamics and Oxygenation during Perfusion Using Integrated Photoacoustic and Ultrasound Localization Microscopy

Saturday, October 11, 2025
8:30 AM - 8:35 AM PDT
Location: Room 30D

Introduction: : Assessing deep-tissue oxygenation and perfusion in human organs is critical for monitoring kidney viability during preservation and evaluation protocols [1]. Ex vivo normothermic machine perfusion (NMP) presents a promising platform for maintaining organ function and evaluating kidney health [2-4]. However, current assessment methods are limited, as they rely on global perfusate biomarkers (e.g., oxygen consumption and blood perfusion) and single-point measurements, which lack spatial resolution and quantitative data on both organ metabolism and hemodynamics [5], [6]. On the other hand, photoacoustic imaging (PAI) enables deep-tissue mapping of blood oxygen saturation (sO₂), which is advantageous over the shallow and single-point detection of pulse oximetry [7], [8]. Meanwhile, ultrasound localization microscopy (ULM) offers a super-resolution assessment of microvascular architecture and blood flow, enabling visualization of perfusion at the capillary level, a scale inaccessible to conventional Doppler ultrasound [9], [10]. This study, we propose a novel multi-modal imaging strategy for the comprehensive, non-invasive assessment of ex vivo perfused human kidneys using PAI and ULM. We demonstrate the first application of this integrated platform in an ex vivo perfused human kidney under physiological conditions. PAI-derived sO₂ maps are validated against the standard pulse oximetry. ULM's super-resolution microvasculature images and quantitative flow velocity maps are correlated with the absolute flow rate of the perfusion system [11].

Materials and

Methods: : Human kidneys were obtained from LifeShare of Oklahoma under approved IRB protocols and imaged within 24 hours of procurement. The kidney was connected to a custom ex vivo normothermic machine perfusion (NMP) system (Figure 1), designed to maintain physiological conditions (35–37°C) and perfusate oxygenation (venous sO₂: 90–95%). The system included perfusion pumps for the renal artery, a heat exchanger, and an oxygenator for controlled gas exchange. A red blood cell-based perfusate was used, comprising washed human red blood cells (hematocrit 25–30%), 5 g/dL albumin, electrolytes, and heparin. Continuous monitoring of perfusion pressure and flow rate was performed using medical-grade sensors. PAI was conducted with a dual-modality Vevo F2 LAZR-X system (Fujifilm VisualSonics) equipped with a tunable laser (680–970 nm) and UHF29x transducer (20 MHz). Wavelengths of 750 and 850 nm were selected for oxy-/deoxy-hemoglobin contrast. PAI data were acquired across the renal cortical surface with 0.15 mm steps and processed using linear spectral unmixing to generate blood oxygenation (sO₂) maps. These were validated against perfusate pulse oximetry. ULM was performed with microbubble (MB) contrast (C₃F₈-filled lipid-based MBs, diluted 150×) injected at 2.5 mL/min using a syringe pump. Bipolar reconstructed data (≥2000 frames) underwent motion correction and clutter filtering via singular value decomposition (SVD). Individual MBs were localized, tracked using the u-Track algorithm, and reconstructed to obtain super-resolution vascular maps and flow velocity profiles, which were correlated with measured perfusion flow rates [11].

Results, Conclusions, and Discussions:: PAI generated spatially resolved renal sO₂ maps that aligned with perfusate oximetry trends. ULM provided super-resolution vascular reconstructions and flow velocity maps at the capillary level. Imaging metrics from both modalities quantitatively correlated with measured perfusion pressure and flow, demonstrating reliable physiological assessment of the ex vivo kidney under normothermic conditions.
 

Conclusions: This study establishes the feasibility of combining PAI and ULM for real-time, non-invasive viability assessment of ex vivo perfused human kidneys. PAI enabled oxygenation mapping with enhanced spatial coverage compared to standard oximetry, while ULM visualized microvascular architecture and blood flow with unprecedented resolution. This integrated approach supports improved assessment protocols during normothermic machine perfusion and may aid transplant clinicians in making better-informed decisions about kidney viability before transplantation.

Discussions: The integration of PAI and ULM addresses critical limitations of current kidney viability assessments, which often rely on global biomarkers and lack spatial resolution. PAI’s ability to map sO₂ across the renal cortex provides functional information on oxygen utilization, while ULM reveals microvascular perfusion at a capillary scale. The observed correlation between imaging metrics and NMP parameters highlights the clinical potential for real-time organ evaluation. Future work will focus on optimizing imaging protocols to reduce acquisition time, extending analysis to different perfusion conditions, and validating outcomes with histological and functional benchmarks to improve predictive accuracy for transplant success

Acknowledgements and/or References (Optional): :

References:
[1] P. Chadha, C. Y. Koh, and D. G. Ashby, "Prolonging preservation or assessment of organ quality—What is key?," Frontiers in Transplantation, vol. 2, pp. 1-12, 2023.
[2] Y. Fang et al., "Enhancing kidney quality assessment: Power Doppler during normothermic machine perfusion," medRxiv preprint, Jan. 2025.
[3] L. Zhao et al., "Advances in kidney preservation techniques and their application in clinical practice," Frontiers in Medicine, vol. 8, pp. 1-15, 2021.
[4] M. Nasralla et al., "Normothermic kidney perfusion: An overview of protocols and strategies," Am. J. Transplant., vol. 23, no. 4, pp. 414-427, 2023.
[5] A. D. Barlow et al., "Sense and sensibilities of organ perfusion as a kidney and liver viability assessment platform," Frontiers in Transplantation, vol. 1, pp. 1-11, 2022.
[6] M. Wszola et al., "Assessing kidney graft viability and its cells metabolism during machine perfusion," Int. J. Mol. Sci., vol. 22, no. 4, pp. 1-18, 2021.
[7] J. Yao and L. V. Wang, "Photoacoustic tomography of blood oxygenation: A mini review," Photoacoustics, vol. 10, pp. 65-73, Jun. 2018.
[8] S. Gottschalk et al., "Functional photoacoustic imaging: From nano- and micro- to macro-scale," Nano Convergence, vol. 10, art. 21, pp. 1-32, Jun. 2023.
[9] C. Errico et al., "Ultrafast ultrasound localization microscopy for deep super-resolution vascular imaging," Nature, vol. 527, no. 7579, pp. 499-502, Nov. 2015.
[10] M. Heiles et al., "Ultrasound localization microscopy and super-resolution: A state of the art," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 66, no. 3, pp. 606-616, Mar. 2019.
[11] P. Roudot et al., "u-track3D: Measuring, navigating, and validating dense particle trajectories in three dimensions," Cell Reports Methods, vol. 3, no. 12, pp. 100655, Dec. 2023, doi: 10.1016/j.crmeth.2023.100655.