Instrument for fluorescence sensing of circulating cells with diffuse light in mice in vivo

Ratiometric fluorescence sensing and quantification of circulating blood sodium sensors in mice in vivo

In this work, we introduce ratiometric diffuse in vivo flow cytometry (R-DiFC) for quantitative measurement of circulating fluorescent red blood cell (fRBC) sensors for systemic blood sodium levels. Unlike in our previous work in measuring circulating fRBC sensors, R-DiFC allows simultaneous measurement of two fluorophores encapsulated in the sensor, the ratio of which enables self-calibration of the fluorescence signal with different fRBC depths in biological tissue. We show that the R-DiFC signal varies significantly less than either fluorescence signal alone. This work holds promise for personalized monitoring of systemic sodium for bipolar patients in the future.

Effect of pulsed laser parameters on photoacoustic flow cytometry efficiency in vitro and in vivo

Photoacoustic flow cytometry is one of the most effective approaches to detect "alien" objects in the bloodstream, including circulating tumor cells, blood clots, parasites, and emboli. However, the possibility of detecting high-amplitude signals from these objects against the background of blood depends on the parameters of the laser pulse. So, the dependencies of photoacoustic signals amplitude and number on laser pulse energy (5-150 μJ), pulse length (1, 2, 5 ns), and pulse repetition rate (2, 5, 10 kHz) for the melanoma cells were investigated. First, the PA responses of a melanoma cell suspension in vitro were measured to directly assess the efficiency of converting laser light into an acoustic signal. After it, the same dependence with the developed murine model based on constant rate melanoma cell injection into the animal blood flow was tested. Both in vivo and in vitro experiments show that signal generation efficiency increases with laser pulse energy above 15 μJ. Shorter pulses, especially 1 ns, provide more efficient signal generation as well as higher pulse rates. A higher pulse rate also provides more efficient signal generation, but also leads to overheating of the skin. The results show the limits where the photoacoustic flow cytometry system can be effectively used for the detection of circulating tumor cells in undiluted blood both for in vitro experiments and for in vivo murine models.

Dual-ratio approach for detection of point fluorophores in biological tissue

SIGNIFICANCE

Diffuse in-vivo Flow Cytometry (DiFC) is an emerging fluorescence sensing method to non-invasively detect labeled circulating cells in-vivo. However, due to Signal-to-Noise Ratio (SNR) constraints largely attributed to background tissue autofluorescence, DiFC's measurement depth is limited. multiplies Aim: The Dual-Ratio (DR) / dual-slope is a new optical measurement method that aims to suppress noise and enhance SNR to deep tissue regions. We aim to investigate the combination of DR and Near-InfraRed (NIR) DiFC to improve circulating cells' maximum detectable depth and SNR.

APPROACH

Phantom experiments were used to estimate the key parameters in a diffuse fluorescence excitation and emission model. This model and parameters were implemented in Monte-Carlo to simulate DR DiFC while varying noise and autofluorescence parameters to identify the advantages and limitations of the proposed technique.

RESULTS

Two key factors must be true to give DR DiFC an advantage over traditional DiFC; first, the fraction of noise that DR methods cannot cancel cannot be above the order of 10% for acceptable SNR. Second, DR DiFC has an advantage, in terms of SNR, if the distribution of tissue autofluorescence contributors is surface-weighted.

CONCLUSIONS

DR cancelable noise may be designed for (e.g. through the use of source multiplexing), and indications point to the autofluorescence contributors' distribution being truly surface-weighted in-vivo. Successful and worthwhile implementation of DR DiFC depends on these considerations, but results point to DR DiFC having possible advantages over traditional DiFC.

Signal and measurement considerations for human translation of diffuse in vivo flow cytometry

SIGNIFICANCE

"Diffuse in vivo flow cytometry" (DiFC) is an emerging technology for fluorescence detection of rare circulating cells directly in large deep-seated blood vessels in mice. Because DiFC uses highly scattered light, in principle, it could be translated to human use. However, an open question is whether fluorescent signals from single cells would be detectable in human-scale anatomies.

AIM

Suitable blood vessels in a human wrist or forearm are at a depth of ∼2 to 4 mm. The aim of this work was to study the impact of DiFC instrument geometry and wavelength on the detected DiFC signal and on the maximum depth of detection of a moving cell.

APPROACH

We used Monte Carlo simulations to compute fluorescence Jacobian (sensitivity) matrices for a range of source and detector separations (SDS) and tissue optical properties over the visible and near infrared spectrum. We performed experimental measurements with three available versions of DiFC (488, 640, and 780 nm), fluorescent microspheres, and tissue mimicking optical flow phantoms. We used both computational and experimental data to estimate the maximum depth of detection at each combination of settings.

RESULTS

For the DiFC detection problem, our analysis showed that for deep-seated blood vessels, the maximum sensitivity was obtained with NIR light (780 nm) and 3-mm SDS.

CONCLUSIONS

These results suggest that-in combination with a suitable molecularly targeted fluorescent probes-circulating cells and nanosensors could, in principle, be detectable in circulation in humans.

Prospects for Fluorescence Molecular In Vivo Liquid Biopsy of Circulating Tumor Cells in Humans

Our team recently developed "Diffuse in vivo Flow Cytometry" (DiFC) for detection and enumeration rare circulating tumor cells (CTCs) in mice with highly-scattered fluorescent light. We have used DiFC to study dissemination of CTCs in a number of mouse models of metastasis with fluorescent protein expressing cells. Because DiFC uses diffuse light and interrogates large blood vessels in relatively deep tissue, in principle it could be translated to larger limbs, species, and even humans clinically. In this perspective, we discuss the technical challenges of human translation of DiFC in the context of the current state of the technology, as well as potential strategies for labeling of CTCs with targeted fluorescent molecular probes. We also discuss potential advantages and disadvantages of DiFC as a clinical tool. In principle, DiFC could represent a powerful complementary technique (to liquid biopsy blood draws) for accurate and sensitive measurement of changes in CTC numbers over time.

Fluorescence Labeling of Circulating Tumor Cells with a Folate Receptor-Targeted Molecular Probe for Diffuse In Vivo Flow Cytometry

PURPOSE

We recently developed a new instrument called "diffuse in vivo flow cytometry" (DiFC) for enumeration of rare fluorescently labeled circulating tumor cells (CTCs) in small animals without drawing blood samples. Until now, we have used cell lines that express fluorescent proteins or were pre-labeled with a fluorescent dye ex vivo. In this work, we investigated the use of a folate receptor (FR)-targeted fluorescence molecular probe for in vivo labeling of FR+ CTCs for DiFC.

PROCEDURES

We used EC-17, a FITC-folic acid conjugate that has been used in clinical trials for fluorescence-guided surgery. We studied the affinity of EC-17 for FR+ L1210A and KB cancer cells. We also tested FR- MM.1S cells. We tested the labeling specificity in cells in culture in vitro and in whole blood. We also studied the detectability of labeled cells in mice in vivo with DiFC.

RESULTS

EC-17 showed a high affinity for FR+ L1210A and KB cells in vitro. In whole blood, 85.4 % of L1210A and 80.9 % of KB cells were labeled above non-specific background with EC-17, and negligible binding to FR- MM.1S cells was observed. In addition, EC-17-labeled CTCs were readily detectable in circulation in mice with DiFC.

CONCLUSIONS

This work demonstrates the feasibility of labeling CTCs with a cell-surface receptor-targeted probe for DiFC, greatly expanding the potential utility of the method for pre-clinical animal models. Because DiFC uses diffuse light, this method could be also used to enumerate CTCs in larger animal models and potentially even in humans.

In Vivo Flow Cytometry

In vivo flow cytometry (IVFC) was first designed to detect circulating cells in a mouse ear. It allows real-time monitoring of cells in peripheral blood with no need to draw blood. The IVFC field has made great progress during the last decade with the development of fluorescence, photoacoustic, and multiphoton microscopy. Moreover, the application of IVFC is no longer restricted to circulating cells. IVFC based on fluorescence and photoacoustic are most widely applied in biomedical research. Methods based on fluorescence are often used for object monitoring in superficial vessels, while methods based on photoacoustics have an advantage of label-free monitoring in deep vessels. In this chapter, we introduce technical points and key applications of IVFC. We focus on the principles, labeling strategies, sensitivity, and biomedical applications of the technology. In addition, we summarize this chapter and discuss important research directions of IVFC in the future.

Short-Term Circulating Tumor Cell Dynamics in Mouse Xenograft Models and Implications for Liquid Biopsy

MOTIVATION

Circulating tumor cells (CTCs) are widely studied using liquid biopsy methods that analyze fractionally-small peripheral blood (PB) samples. However, little is known about natural fluctuations in CTC numbers that may occur over short timescales in vivo, and how these may affect detection and enumeration of rare CTCs from small blood samples.

METHODS

We recently developed an optical instrument called "diffuse in vivo flow cytometry" (DiFC) that uniquely allows continuous, non-invasive counting of rare, green fluorescent protein expressing CTCs in large blood vessels in mice. Here, we used DiFC to study short-term changes in CTC numbers in multiple myeloma and Lewis lung carcinoma xenograft models. We analyzed CTC detections in over 100 h of DiFC data, and considered intervals corresponding to approximately 1%, 5%, 10%, and 20% of the PB volume. In addition, we analyzed changes in CTC numbers over 24 h (diurnal) periods.

RESULTS

For rare CTCs (fewer than 1 CTC per ml of blood), the use of short DiFC intervals (corresponding to small PB samples) frequently resulted in no detections. For more abundant CTCs, CTC numbers frequently varied by an order of magnitude or more over the time-scales considered. This variance in CTC detections far exceeded that expected by Poisson statistics or by instrument variability. Rather, the data were consistent with significant changes in mean numbers of CTCs on the timescales of minutes and hours.

CONCLUSIONS

The observed temporal changes can be explained by known properties of CTCs, namely, the continuous shedding of CTCs from tumors and the short half-life of CTCs in blood. It follows that the number of cells in a blood sample are strongly impacted by the timing of the draw. The issue is likely to be compounded for multicellular CTC clusters or specific CTC subtypes, which are even more rare than single CTCs. However, we show that enumeration can in principle be improved by averaging multiple samples, analysis of larger volumes, or development of methods for enumeration of CTCs directly in vivo.

Heterogeneity of circulating tumor cell dissemination and lung metastases in a subcutaneous Lewis lung carcinoma model

Subcutaneous (s.c.) tumor models are widely used in pre-clinical cancer metastasis research. Despite this, the dynamics and natural progression of circulating tumor cells (CTCs) and CTC clusters (CTCCs) in peripheral blood are poorly understood in these models. In this work, we used a new technique called 'diffuse in vivo flow cytometry' (DiFC) to study CTC and CTCC dissemination in an s.c. Lewis lung carcinoma (LLC) model in mice. Tumors were grown in the rear flank and we performed DiFC up to 31 days after inoculation. At the study endpoint, lungs were excised and bioluminescence imaging (BLI) was performed to determine the extent of lung metastases. We also used fluorescence macro-cryotome imaging to visualize infiltration and growth of the primary tumor. DiFC revealed significant heterogeneity in CTC and CTCC numbers amongst all mice studied, despite using clonally identical LLC cells and tumor placement. Maximum DiFC count rates corresponded to 0.1 to 14 CTCs per mL of peripheral blood. In general, CTC numbers did not necessarily increase monotonically over time and were poorly correlated with tumor volume. However, there was a good correlation between CTC and CTCC numbers in peripheral blood and lung metastases. We attribute the differences in CTC numbers primarily due to growth patterns of the primary tumor. This study is one of the few reports of CTC shedding dynamics in sub-cutaneous metastasis models and underscores the value of in vivo methods for continuous, non-invasive CTC monitoring.

Prospects for the Use of Upconverting Nanoparticles as a Contrast Agent for Enumeration of Circulating Cells in vivo

PURPOSE

We recently developed a new fluorescence-based technique called "diffuse in vivo flow cytometry" (DiFC) for enumerating rare circulating tumor cells (CTCs) directly in the bloodstream. Non-specific tissue autofluorescence is a persistent problem, as it creates a background which may obscure signals from weakly-labeled CTCs. Here we investigated the use of upconverting nanoparticles (UCNPs) as a contrast agent for DiFC, which in principle could significantly reduce the autofluorescence background and allow more sensitive detection of rare CTCs.

METHODS

We built a new UCNP-compatible DiFC instrument (U-DiFC), which uses a 980 nm laser and detects upconverted luminescence in the 520, 545 and 660 nm emission bands. We used NaYF4:Yb,Er UCNPs and several covalent and non-covalent surface modification strategies to improve their biocompatibility and cell uptake. We tested U-DiFC with multiple myeloma (MM) and Lewis lung carcinoma (LLC) cells in tissue-mimicking optical flow phantoms and in nude mice.

RESULTS

U-DiFC significantly reduced the background autofluorescence signals and motion artifacts from breathing in mice. Upconverted luminescence from NaYF4:Yb,Er microparticles (UμNP) and cells co-incubated with UCNPs were readily detectable with U-DiFC in phantoms, and from UCNPs in circulation in mice. However, we were unable to achieve reliable labeling of CTCs with UCNPs. Our data suggest that most (or all) of the measured U-DIFC signal in vitro and in vivo likely arose from unbound UCNPs or due to the uptake by non-CTC blood cells.

CONCLUSION

UCNPs have a number of properties that make them attractive contrast agents for high-sensitivity detection of CTCs in the bloodstream with U-DiFC and other intravital imaging methods. More work is needed to achieve reliable and specific labeling of CTCs with UCNPs and verify long-term retention and viability of cells.

Real-time particle-by-particle detection of erythrocyte-camouflaged microsensor with extended circulation time in the bloodstream

Personalized medicine offers great potential benefits for disease management but requires continuous monitoring of drugs and drug targets. For instance, the therapeutic window for lithium therapy of bipolar disorder is very narrow, and more frequent monitoring of sodium levels could avoid toxicity. In this work, we developed and validated a platform for long-term, continuous monitoring of systemic analyte concentrations in vivo. First, we developed sodium microsensors that circulate directly in the bloodstream. We used "red blood cell mimicry" to achieve long sensor circulation times of up to 2 wk, while being stable, reversible, and sensitive to sodium over physiologically relevant concentration ranges. Second, we developed an external optical reader to detect and quantify the fluorescence activity of the sensors directly in circulation without having to draw blood samples and correlate the measurement with a phantom calibration curve to measure in vivo sodium. The reader design is inherently scalable to larger limbs, species, and potentially even humans. In combination, this platform represents a paradigm for in vivo drug monitoring that we anticipate will have many applications in the future.

In Vivo Flow Cytometry of Extremely Rare Circulating Cells

Circulating tumor cells (CTCs) are of great interest in cancer research, but methods for their enumeration remain far from optimal. We developed a new small animal research tool called “Diffuse in vivo Flow Cytometry” (DiFC) for detecting extremely rare fluorescently-labeled circulating cells directly in the bloodstream. The technique exploits near-infrared diffuse photons to detect and count cells flowing in large superficial arteries and veins without drawing blood samples. DiFC uses custom-designed, dual fiber optic probes that are placed in contact with the skin surface approximately above a major vascular bundle. In combination with a novel signal processing algorithm, DiFC allows counting of individual cells moving in arterial or venous directions, as well as measurement of their speed and depth. We show that DiFC allows sampling of the entire circulating blood volume of a mouse in under 10 minutes, while maintaining a false alarm rate of 0.014 per minute. In practice, this means that DiFC allows reliable detection of circulating cells below 1 cell per mL. Hence, the unique capabilities of DiFC are highly suited to biological applications involving very rare cell types such as the study of hematogenous cancer metastasis.

Optofluidic real-time cell sorter for longitudinal CTC studies in mouse models of cancer

Circulating tumor cells (CTCs) play a fundamental role in cancer progression. However, in mice, limited blood volume and the rarity of CTCs in the bloodstream preclude longitudinal, in-depth studies of these cells using existing liquid biopsy techniques. Here, we present an optofluidic system that continuously collects fluorescently labeled CTCs from a genetically engineered mouse model (GEMM) for several hours per day over multiple days or weeks. The system is based on a microfluidic cell sorting chip connected serially to an unanesthetized mouse via an implanted arteriovenous shunt. Pneumatically controlled microfluidic valves capture CTCs as they flow through the device, and CTC-depleted blood is returned back to the mouse via the shunt. To demonstrate the utility of our system, we profile CTCs isolated longitudinally from animals over 4 days of treatment with the BET inhibitor JQ1 using single-cell RNA sequencing (scRNA-Seq) and show that our approach eliminates potential biases driven by intermouse heterogeneity that can occur when CTCs are collected across different mice. The CTC isolation and sorting technology presented here provides a research tool to help reveal details of how CTCs evolve over time, allowing studies to credential changes in CTCs as biomarkers of drug response and facilitating future studies to understand the role of CTCs in metastasis.

Fluorescence detection, enumeration and characterization of single circulating cells in vivo: technology, applications and future prospects

There are many diseases and biological processes that involve circulating cells in the bloodstream, such as cancer metastasis, immunology, reproductive medicine, and stem cell therapies. This has driven significant interest in new technologies for the study of circulating cells in small animal research models and clinically. Most currently used methods require drawing and enriching blood samples from the body, but these suffer from a number of limitations. In contrast, 'in vivo flow cytometry' (IVFC) refers to set of technologies that allow study of cells directly in the bloodstream of the organism in vivo. In recent years the IVFC field has grown significantly and new techniques have been developed, including fluorescence microscopy, multi-photon, photo-acoustic, and diffuse fluorescence IVFC. In this paper we review recent technical advances in IVFC, with emphasis on instrumentation, contrast mechanisms, and detection sensitivity. We also describe key applications in biomedical research, including cancer research and immunology. Last, we discuss future directions for IVFC, as well as prospects for broader adoption by the biomedical research community and translation to humans clinically.

Diffuse fluorescence fiber probe for in vivo detection of circulating cells

There has been significant recent interest in the development of technologies for enumeration of rare circulating cells directly in the bloodstream in many areas of research, for example, in small animal models of circulating tumor cell dissemination during cancer metastasis. We describe a fiber-based optical probe that allows fluorescence detection of labeled circulating cells in vivo in a diffuse reflectance configuration. We validated this probe in a tissue-mimicking flow phantom model in vitro and in nude mice injected with fluorescently labeled multiple myeloma cells in vivo. Compared to our previous work, this design yields an improvement in detection signal-to-noise ratio of 10 dB, virtually eliminates problematic motion artifacts due to mouse breathing, and potentially allows operation in larger animals and limbs.

Performance of computer vision in vivo flow cytometry with low fluorescence contrast

Detection and enumeration of circulating cells in the bloodstream of small animals are important in many areas of preclinical biomedical research, including cancer metastasis, immunology, and reproductive medicine. Optical in vivo flow cytometry (IVFC) represents a class of technologies that allow noninvasive and continuous enumeration of circulating cells without drawing blood samples. We recently developed a technique termed computer vision in vivo flow cytometry (CV-IVFC) that uses a high-sensitivity fluorescence camera and an automated computer vision algorithm to interrogate relatively large circulating blood volumes in the ear of a mouse. We detected circulating cells at concentrations as low as 20 cells/mL. In the present work, we characterized the performance of CV-IVFC with low-contrast imaging conditions with (1) weak cell fluorescent labeling using cell-simulating fluorescent microspheres with varying brightness and (2) high background tissue autofluorescence by varying autofluorescence properties of optical phantoms. Our analysis indicates that CV-IVFC can robustly track and enumerate circulating cells with at least 50% sensitivity even in conditions with two orders of magnitude degraded contrast than our previous in vivo work. These results support the significant potential utility of CV-IVFC in a wide range of in vivo biological models.

Toward lower contrast computer vision in vivo flow cytometry

There are many applications in biomedical research where detection and enumeration of circulating cells (CCs) is important. Existing techniques involve drawing and enriching blood samples and analyzing them ex vivo. More recently, small animal "in vivo flow cytometry" (IVFC) techniques have been developed, where fluorescently-labeled cells flowing through small arterioles (ear, retina) are detected and counted. We recently developed a new high-sensitivity IVFC technique termed "Computer Vision(CV)-IVFC". Here, large circulating blood volumes were monitored in the ears of mice with a wide-field video-rate near-infrared (NIR) fluorescent camera. Cells were labeled with a membrane dye and were detected and tracked in noisy image sequences. This technique allowed enumeration of CCs in vivo with overall sensitivity better than 10 cells/mL. However, an ongoing area of interest in our lab is optimization of the system for lower-contrast imaging conditions, e.g. when CCs are weakly labeled, or in the case higher background autofluorescence with visible dyes. To this end, we developed a new optical flow phantom model to control autofluorescence intensity and physical structure to better mimic conditions observed in mice. We acquired image sequences from a series of phantoms with varying levels of contrast and analyzed the distribution of pixel intensities, and showed that we could generate similar conditions to those in vivo. We characterized the performance of our CV-IVFC algorithm in these phantoms with respect to sensitivity and false-alarm rates. Use of this phantom model in optimization of the instrument and algorithm under lower-contrast conditions is the subject of ongoing work in our lab.

Improved diffuse fluorescence flow cytometer prototype for high sensitivity detection of rare circulating cells in vivo

Detection and enumeration of rare circulating cells in mice are important problems in many areas of preclinical biomedical research. Recently, we developed a new method termed "diffuse fluorescence flow cytometry" (DFFC) that uses diffuse photons to increase the blood sampling volume and sensitivity versus existing in vivo flow cytometry methods. In this work, we describe a new DFFC prototype with approximately an order-of-magnitude improvement in sensitivity compared to our previous work. This sensitivity improvement is enabled by a number of technical innovations, which include a method for the removal of motion artifacts (allowing interrogation of mouse hindlegs that was less optically attenuating versus the tail) and improved collection optics and signal preamplification. We validated our system first in limb mimicking optical flow phantoms with fluorescent microspheres and then in nude mice with fluorescently labeled mesenchymal stem cells at injected concentrations of 5×103  cells/mL. In combination, these improvements resulted in an overall cell counting sensitivity of about 1  cell/mL or better in vivo.

Maximum likelihood tomographic reconstruction of extremely sparse solutions in diffuse fluorescence flow cytometry

We apply reparameterization and the maximum likelihood method to a specific fluorescence-mediated tomography problem where the solution is known a priori to be extremely sparse (i.e., all image values are zero except for one). Our algorithm performs significantly better than a standard image reconstruction method, particularly for deep-seated targets, and achieves close to 150 μm accuracy in a 3 mm diameter cross-sectional area with only 12 measurements. Moreover, results do not depend on the selection of a regularization parameter or other ad hoc values, and since reconstructions can be computed very quickly, the algorithm is also suitable for real-time implementation.

Synergy of photoacoustic and fluorescence flow cytometry of circulating cells with negative and positive contrasts

In vivo photoacoustic (PA) and fluorescence flow cytometry were previously applied separately using pulsed and continuous wave lasers respectively, and positive contrast detection mode only. This paper introduces a real-time integration of both techniques with positive and negative contrast modes using only pulsed lasers. Various applications of this new tool are summarized, including detection of liposomes loaded with Alexa-660 dye, red blood cells labeled with Indocyanine Green, B16F10 melanoma cells co-expressing melanin and green fluorescent protein (GFP), C8161-GFP melanoma cells targeted by magnetic nanoparticles, MTLn3 adenocarcinoma cells expressing novel near-infrared iRFP protein, and quantum dot-carbon nanotube conjugates. Negative contrast flow cytometry provided label-free detection of low absorbing or weakly fluorescent cells in blood absorption and autofluorescence background, respectively. The use of pulsed laser for time-resolved discrimination of objects with long fluorescence lifetime (e.g., quantum dots) from shorter autofluorescence background (e.g., blood plasma) is also highlighted in this paper. The supplementary nature of PA and fluorescence detection increased the versatility of the integrated method for simultaneous detection of probes and cells having various absorbing and fluorescent properties, and provided verification of PA data using a more established fluorescence based technique.

Tomographic sensing and localization of fluorescently labeled circulating cells in mice in vivo

Sensing and enumeration of specific types of circulating cells in small animals is an important problem in many areas of biomedical research. Microscopy-based fluorescence in vivo flow cytometry methods have been developed previously, but these are typically limited to sampling of very small blood volumes, so that very rare circulating cells may escape detection. Recently, we described the development of a 'diffuse fluorescence flow cytometer' (DFFC) that allows sampling of much larger blood vessels and therefore circulating blood volumes in the hindlimb, forelimb or tail of a mouse. In this work, we extend this concept by developing and validating a method to tomographically localize circulating fluorescently labeled cells in the cross section of a tissue simulating optical flow phantom and mouse limb. This was achieved using two modulated light sources and an array of six fiber-coupled detectors that allowed rapid, high-sensitivity acquisition of full tomographic data sets at 10 Hz. These were reconstructed into two-dimensional cross-sectional images using Monte Carlo models of light propagation and the randomized algebraic reconstruction technique. We were able to obtain continuous images of moving cells in the sample cross section with 0.5 mm accuracy or better. We first demonstrated this concept in limb-mimicking optical flow photons with up to four flow channels, and then in the tails of mice with fluorescently labeled multiple myeloma cells. This approach increases the overall diagnostic utility of our DFFC instrument.