Wide-field imaging of birefringent synovial fluid crystals using lens-free polarized microscopy for gout diagnosis.

Yibo Zhang,John Fitzgerald,Seung Yoon Celine Lee,Yun Zhang,Daniel Furst,Aydogan Ozcan

doi:10.1038/srep28793

Abstract

Gout is a form of crystal arthropathy where monosodium urate (MSU) crystals deposit and elicit inflammation in a joint. Diagnosis of gout relies on identification of MSU crystals under a compensated polarized light microscope (CPLM) in synovial fluid aspirated from the patient’s joint. The detection of MSU crystals by optical microscopy is enhanced by their birefringent properties. However, CPLM partially suffers from the high-cost and bulkiness of conventional lens-based microscopy, and its relatively small field-of-view (FOV) limits the efficiency and accuracy of gout diagnosis. Here we present a lens-free polarized microscope which adopts a novel differential and angle-mismatched polarizing optical design achieving wide-field and high-resolution holographic imaging of birefringent objects with a color contrast similar to that of a standard CPLM. The performance of this computational polarization microscope is validated by imaging MSU crystals made from a gout patient’s tophus and steroid crystals used as negative control. This lens-free polarized microscope, with its wide FOV (>20 mm2), cost-effectiveness and field-portability, can significantly improve the efficiency and accuracy of gout diagnosis, reduce costs, and can be deployed even at the point-of-care and in resource-limited clinical settings.

Highlights

Aligned parallel with the slow axis of the full-wave retardation plate, upon a red/magenta background color
In the identification of monosodium urate (MSU) crystals, routinely a 40×(e.g., 0.75NA) objective lens is used to observe the morphology of the crystals, resulting in an extremely small FOV (~0.2 mm2) which leads to long examination times by diagnosticians
When there is only a limited number of crystals present in a synovial fluid sample taken from the patient, the examination of the entire sample can be time-consuming and can produce a non-reliable diagnostic result because of operator-dependent bias in detecting the crystals over a limited FOV14

Summary

Introduction

Aligned parallel (or perpendicular) with the slow axis of the full-wave retardation plate, upon a red/magenta background color. The color contrast of birefringent objects as generated by a conventional CPLM is challenging to replicate by a lens-free microscope which inherently uses narrow-band illumination sources, unless multiple wavelengths are used To face these challenges and create a lens-free microscope that can be used in gout diagnosis, we designed a novel lens-free differential holographic polarized imaging platform, which can achieve wide-field imaging of birefringent objects on an image sensor chip with sub-micron resolution (see Fig. 1(a)). In this design, a partially coherent light source is passed through a circular polarizer and is incident on the transparent sample that contains the synovial fluid taken from a patient. Other future clinical applications of this platform could include diseases caused by crystals that can be detected by a conventional CPLM; for example, ureteral stones can be diagnosed by detecting birefringent crystals in urine samples

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Conclusion