More than 174,000 men will be diagnosed with prostate cancer this year, according to the American Cancer Society, putting it behind only skin cancer as the most common cancer among American men.
Ji-Xin Cheng, adjunct professor of Purdue’s Weldon School of Biomedical Engineering and the Department of Chemistry, says a paper in the New England Journal of Medicine found that 1,410 men need to be screened and 48 additional cases of prostate cancer need to be treated to prevent only one death.
“The current examination isn’t precise, so there’s a lot of surgery because doctors can’t tell when there’s a large amount of cancer, whether it’s aggressive or benign,” said Cheng, also the Moustakas Chair Professor of Photonics at Boston University.
New infrared chemical imaging work involving research by Cheng and Ali Shakouri, the Mary Jo and Robert L. Kirk Director of the Birck Nanotechnology Center in Purdue’s Discovery Park, aims to change the paradigm, allowing better microscopic studies of tissue to detect what is there and cut down on unnecessary surgeries.
“The impact will be big,” Cheng said. “This new method would allow the detection of aggressive breast, prostate and other cancers with biomarker information and at submicron spatial resolution.”
The new technology is detailed in a paper appearing Friday (July 19) in the journal Science Advances. Researchers from the Boston University and the Chinese Academy of Sciences collaborated in the work.
A key value of the research is the speed.
The proposed method offers a much quicker examination of cells and tissues.
It also allows for a larger area mapping, which is important to determine biomarker information.
Cheng said finding markers to determine a cancer’s aggressiveness has been a goal throughout the research.
This method allows the examination of living cancer cells rather than drying out the sample in order to be studied by traditional infrared spectroscopy.
The paper describes shining both an infrared excitation laser and another visible probe laser through a sample and measuring the difference between the hot and cold states.
Photothermal detection is used to improve the spatial resolution by one order of magnitude compared to traditional infrared microscopy, providing an opportunity to look for the various biomarkers within the cells.
Cheng paired with Shakouri to use a lock-in camera that was fast enough to handle the million pixels per second in parallel.
Shakouri, inventor of the lock-in camera, said the camera detects very small changes in light coming into it.
The research builds on work by Cheng and colleagues published three years ago in the same journal. Previous imaging needed 8 seconds per image, considered too slow by Cheng because cells and molecules are in constant motion.
Future research will include work to increase the field of view so that the size of the sample that can be examined can be as large as a few millimeters.
Cheng also wants to push the sensitivity to detect very small particles like a single virus or a single bacterium.
The latter can allow faster detection of bacterial response to antibiotics.
“Current medial practice is to spend 1-2 days to culture a specimen, then a doctor can tell you if you if you have an infection or not,” he said. “But if we can measure that at a single bacterium level, that’s a rapid detection. That will be a very important application of this platform.”
Vibrational imaging methods offer a new window to characterize samples based on spectroscopic signatures of chemical bonds. Raman and infrared (IR) spectroscopy have long been used to interrogate materials by probing molecular vibrations without exogenous labels.
Spontaneous Raman microscopy offers sub-micrometer spatial resolution imaging capability, but suffers from the low acquisition rates.
With the advent of coherent Raman scattering techniques, video-rate imaging speed has been demonstrated to characterize biological and pharmaceutical samples.
However, detection of the extremely small Raman cross sections (10-30 cm2sr-1) limits the sensitivity.
On the other hand, the IR absorption offers larger cross sections (10-22 cm2sr-1) that enables adequate sensitivity.
Fourier-transform IR (FTIR) spectrometer, together with its attenuated total reflection accessories, is the typical instrument of the technique and has been extensively employed in the fields ranging from polymer science, pharmaceuticals to biological research.
Coupling focal plane array detectors to FTIR systems allows simultaneously acquiring spatially resolved spectra, greatly improving the throughput for characterization of inhomogeneous samples.
Unlike the conventional FTIR instrument based on interferometry and low-brightness globar excitation, discrete IR spectroscopic imaging techniques utilize tunable quantum cascade laser (QCL) with much higher photon flux per wavenumber, which enables real-time IR imaging.
However, the long incident wavelengths in the mid-IR region determines the spatial resolution at several to tens of micrometers, which is not sufficient to resolve microstructures such as in biological cells.
To address the resolution issue, near-field approach provides a way to surpass the fundamental limitations by combing atomic force microscopy (AFM) with IR spectroscopy , where the AFM cantilever changes the oscillation amplitude due to the surface thermal expansion induced by the absorption of the mid-IR light.
The spectra at nanoscale localization are obtained by recording the amplitude change while sweeping the wavelengths of the mid-IR light source.
With the capability of providing high spatial resolution chemical mapping, AFM-IR has been a valuable tool to study block copolymer system where the domain size is typically at tens of nanometers.
This technique shares the inherent drawback of tip-based imaging modality of low acquisition speed. Additionally, although some work showed the capability of investigating samples in aqueous environment using the total internal reflection of an IR prism to minimize the influence of water, sophisticated setup and data processing procedure make it unsuitable for routine use.
In contrast, a non-contact probe, such as a visible laser, can reduce the limits on sample preparation and provide higher imaging speed.
Recently, our group developed a mid-infrared photothermal (MIP) microscope using a visible laser to probe the IR absorption induced thermal lensing effect in the sample, providing chemical imaging capability with sub-micrometer resolution and depth resolution , which fills the gap between FTIR and AFM-IR microscopy.
When the IR wavelength is tuned to the absorption peak of the sample, the co-propagated probe beam will change its divergence due to the thermal-induced local refractive index change.
We demonstrated chemically-selective imaging of live cells and organisms.
For non-transparent samples, a backward-detected photothermal microscopy was developed to allow chemical mapping of active pharmaceutical ingredients and excipients of drug tablets.
In themeantime, optical probing of IR absorption has also been implemented by other groups.
Furstenberg et al. demonstrated photothermal imaging of materials using a visible laser probe .
Erramilli and co-workers investigated the nonlinear photothermal phenomena causingspectral peak splitting at different phase state of liquid crystal.
The Sander group transparency in the IR window and high reflectance of visible photons. The silicon substrate further enhances the WPS signal by accelerating the heat dissipation. Silicon has high thermal conductivity (150 Wm-1K-1) compared to other IR transparent materials such as CaF2 (10 Wm-1K-1), which avoids heat accumulation and allows faster imaging.
Collectively, these innovations enabled ultrafast detection of IR-induced photothermal signals in a widefield manner.
More information: Y. Bai el al., “Ultrafast chemical imaging by widefield photothermal sensing of infrared absorption,” Science Advances (2019). advances.sciencemag.org/content/5/7/eaav7127
Fritz H. Schröder et al. Screening and Prostate-Cancer Mortality in a Randomized European Study, New England Journal of Medicine (2009). DOI: 10.1056/NEJMoa0810084
Journal information: New England Journal of Medicine , Science Advances
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