Cancer cell classification with coherent diffraction imaging using an extreme ultraviolet radiation source

Abstract

In cancer treatment, it is highly desirable to classify single cancer cells in real time. The standard method is polymerase chain reaction requiring a substantial amount of resources and time. Here, we present an innovative approach for rapidly classifying different cell types: we measure the diffraction pattern of a single cell illuminated with coherent extreme ultraviolet (XUV) laser-generated radiation. These patterns allow distinguishing different breast cancer cell types in a subsequent step. Moreover, the morphology of the object can be retrieved from the diffraction pattern with submicron resolution. In a proof-of-principle experiment, we prepared single MCF7 and SKBR3 breast cancer cells on gold-coated silica slides. The output of a laser-driven XUV light source is focused onto a single unstained and unlabeled cancer cell. With the resulting diffraction pattern, we could clearly identify the different cell types. With an improved setup, it will not only be feasible to classify circulating tumor cells with a high throughput, but also to identify smaller objects such as bacteria or even viruses.

Keywords: breast cancer; coherent diffraction imaging; high harmonic generation; high resolution imaging; rapid classification.

Fig. 1

The experimental setup and HHG spectrum. (a) The ultrafast infrared laser pulses are focused and the XUV radiation is generated in a nickel tube that is continuously fed with argon gas. Downstream a toroidal mirror refocuses the light. A grating disperses the contained wavelengths. A pinhole (PH) selects the monochromatic focus in the rear focal plane of the toroidal mirror. The specimen on a sample mount sits directly behind the pinhole. The diffracted light is measured with a CCD camera (CAM) in reflection geometry. The overall setup fits on an optical table 3 by 1-m wide. (b) With this setup a single harmonic line can be selected out of the generated HHG spectrum (dotted rectangle).

Fig. 2

Detail of the reflection geometry. The incident light beam ${\mathbf{k}}_{\text{in}}$ is reflected from the substrate and scattered off the specimen. The forward wavevector ${\mathbf{k}}_{\text{out}}$ hits the detector at a certain pixel. The captured diffraction pattern corresponds to the three-dimensional (3-D) Fourier transform of the object. The angle of incidence ${\alpha }_i$ for the experiments presented here was 22.5 deg to the surface plane.

Fig. 3

Three examples for typical two-dimensional (2-D) diffraction patterns obtained from a single cell. The diffraction patterns for the two different MCF7 breast cancer cells [(a) and (b)] are similar, whereas the SKBR3 diffraction pattern (c) looks different. Note the low resolution hologram in the center of each image and the coherent diffraction imaging (CDI) related speckle in the outer parts.

Fig. 4

Sketch of the proposed method for rapid classification of biologic specimens. The coherent XUV source (1) is focused onto the sample (2), which is pipetted onto a reflective substrate, e.g., silicon. A detector captures the scattered light and compares (3) the measured diffraction pattern as a kind of fingerprint with the entries of a database. The result of the classification is then displayed (4).

Fig. 5

Light microscope image after the phosphate-buffered saline (PBS) buffer containing several cancer cells dried out on a gold coated substrate. Adjacent to the MCF7 cells (marked by arrows) salt crystals are clearly visible on the substrate. The scale bar corresponds to $200\mu m$.

Fig. 6

(a) Light microscope image ($50\times$ magnification) of the breast cancer cell line cell (MCF7) on a gold substrate, surrounded by scattered salt crystals. (b) Measured diffraction pattern upon reflecting the coherent 38-nm beam off the cell. (c) Reconstructed real space image featuring the MCF7 cell and the half-ring of salt.