Terahertz Polarization Imaging for Colon Cancer …. Doradla THzPolImgClnCancerSPIE_tcm… · Terahertz Polarization Imaging for Colon Cancer Detection ... Terahertz, RF, Millimeter,

Terahertz Polarization Imaging for Colon Cancer Detection

Pallavi Doradlaa,b, Karim Alavic, Cecil S. Josepha,b, Robert H. Gilesa,b

aBiomedical Terahertz Technology Center, University of Massachusetts Lowell; bDepartment of Physics and Applied Physics, University of Massachusetts Lowell;

cDivision of Colon and Rectal Surgery, University of Massachusetts Medical School Worcester.

ABSTRACT

Continuous wave terahertz (THz) imaging has the potential to offer a safe, noninvasive medical imaging modality for delineating colorectal cancer. The terahertz reflectance measurements of fresh 3 – 5 mm thick human colonic excisions were acquired using a continuous-wave polarization imaging technique. A CO2 optically pumped Far-Infrared molecular gas laser operating at 584 GHz was used to illuminate the colon tissue, while the reflected signals were detected using a liquid Helium cooled silicon bolometer. Both co-polarized and cross-polarized remittance from the samples was collected using wire grid polarizers in the experiment. The experimental analysis of 2D images obtained from THz reflection polarization imaging techniques showed intrinsic contrast between cancerous and normal regions based on increased reflection from the tumor. Also, the study demonstrates that the cross-polarized terahertz images not only correlates better with the histology, but also provide consistent relative reflectance difference values between normal and cancerous regions for all the measured specimens.

Keywords: Continuous-wave, terahertz imaging, colorectal cancer, reflection imaging, polarization, cancer imaging

INTRODUCTION Colorectal cancer (CRC) is the third most commonly diagnosed cancer in the world with more than 1.2 million

new cases diagnosed each year and causing 0.6 million deaths (World Health Organization Data & Statistics 2008). Early diagnosis is an effective method of reducing cancer risk. The staging and subsequent treatment of CRC depends on current imaging technologies, such as colonoscopy [1,2], computed tomography (CT) [3,4], positron emission tomography (PET) [5,6], magnetic resonance imaging (MRI) [7,8], and optical coherence tomography (OCT) [9,10]. The standard of care for CRC diagnosis is conventional colonoscopy, which relies on the visual inspection by a physician. During a colonoscopy, the decision to remove abnormal growths is based on the physician's experience.

Besides the conventional colonoscopy, CT, MRI and PET are current diagnostic imaging modalities for the detection of local and distant relapse of CRC. CT is a non-invasive technique that provides quick 3-D images of the entire colon and is better than MRI in detecting lesions smaller than 10 mm. However, it cannot detect tumors smaller than 5 mm diameter [11]. In addition, it uses a series of cross sectional x-rays that are ionizing [12], hence cannot be applied to patients with renal failure [13]. On the other hand, MRI uses liquid enema for contrast and is more expensive than CT that uses air to achieve contrast [11]. PET provides high sensitivity and specificity ranging from 80% to 90% with the higher end being PET/CT that can differentiate tumors from scar tissue created by surgery without the issues faced by other modalities. However it presents poor resolution unless the tumor is metabolically active, and it provides low sensitivity for lymph node staging in rectal cancer [14]. OCT provides high resolution (1 μm, depending on the incident wavelength) and has great potential for detecting tumors. But it is limited by the high scattering of optical wavelengths in tissue [15]. The terahertz frequency range, located between the microwave and infrared regions, has become increasingly relevant for biomedical applications due to its non-ionizing nature and sensitivity to water content.

Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications VII,edited by Laurence P. Sadwick, Créidhe M. O'Sullivan, Proc. of SPIE Vol. 8985, 89850K

2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2038650

Proc. of SPIE Vol. 8985 89850K-1

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As THz imaging offers intrinsic contrast between normal and abnormal tissue, a THz endoscope can be used as a potential tool in the examination and detection of cancerous or pre-cancerous regions of biological tissue.

BACKGROUND Terahertz radiation is non-ionizing. It can provide a resolution of 100 μm – 1 mm depending upon the

wavelength. The potential of THz imaging for colorectal cancer screening applications was encouraged by the positive results obtained from dental tissue, skin burns, breast, liver, and skin cancer studies [16-20]. A recent pulsed THz transmission imaging study showed contrast between cancerous and normal colon tissues in the frequency range from 0.5 to 1.5 THz, with the greatest difference at 0.6 THz based on the increased absorption and refractive index [21]. This study states that the higher water content in the cancerous region is likely to be the main source of contrast. However, another colon study based on dehydrated samples also showed the evidence of contrast, suggesting the possibility of other contributing factors such as an increase in lymphatic systems, vasculature, and other molecular/structural changes in the diseased tissue [22, 23]. The high absorption of terahertz radiation by tissue necessitates reflection modality imaging for in vivo applications. In our preliminary work, we demonstrated a continuous-wave (CW) THz reflection based imaging system with a polarization specific detection technique to reject the unwanted Fresnel reflections from interfaces. Based upon terahertz pulsed spectroscopy studies [21], the selected imaging frequency was chosen to be 584 GHz.

Time-domain systems are able to time-gate out reflections from various interfaces in conventional setups. This allows rejection of unwanted signals that contain no pertinent information, such as reflections from the interfaces. In general, frequency domain (CW) systems measure the net reflectance from the sample-holder assembly which includes the Fresnel component from the interface. Therefore, the co-polarized terahertz response of the sample includes unnecessary reflection from the air-quartz and quartz-sample interfaces. In contrast, using cross-polarized remittance in the frequency domain essentially removes the unnecessary reflections from interfaces, as the Fresnel component is co-polarized with the incident radiation. This will allow us to separate reflections from system optical components from the signal remitted by the tissue volume. Our preliminary results [24, 25], indicate that reflection based polarization sensitive continuous-wave terahertz imaging is potentially capable of detecting intrinsic contrast between cancerous and normal tissue. With the development of thin, low-loss, hollow, flexible waveguides one can build a channel within a conventional endoscope that can transmit the terahertz radiation and collect the back reflected signal from the specimen to detect the abnormal regions of tissue based on the fluctuation in the terahertz reflectivity values. If the terahertz reflectivity correlates to differences between cancerous and normal tissue, then the clinician will have a tool that can significantly improve colorectal cancer screening.

EXPERIMENTAL SETUP The terahertz reflection imaging system used for this study consisted of a CO2 optically pumped far-infrared

(FIR) gas laser operating at 584 GHz. The output power of the CO2 pump laser is in the range of 100 – 130 W. Pumping different transitions of the gas in FIR cell can be achieved by tuning the output frequency of the CO2 laser. The combination of the appropriate gas in the FIR cell and corresponding frequency of the CO2 laser provides the ability to lase different frequencies in the terahertz region. The required 584 GHz (513 µm) vertically polarized transition in Formic acid (HCOOH) was obtained by pumping the 9.23R28 transition of CO2 laser, with the measured output power of 33 mW. A dielectric waveguide was placed at the output end of the FIR laser to obtain a Gaussian output mode. Since the beam emerging from the FIR laser will expand fairly rapidly as it propagates in air, an optical system was designed to focus the radiation onto the sample to attain higher resolution. A liquid helium cooled silicon bolometer manufactured by IRLabs was used as the detector.

The schematic of the experimental layout was shown in Figure 1. The measured waist of the terahertz beam exiting from the dielectric waveguide was 2.36 mm. This beam was allowed to expand in free space before being collimated by a 61 cm focal length TPX lens, then allowed to pass through a wire grid polarizer to clean up the



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CONCLUSION

Continuous wave terahertz (THz) imaging has the potential to offer a safe, noninvasive medical imaging modality for delineating colorectal cancer. The reflection measurements of both normal and cancerous colonic excisions, obtained using a continuous-wave terahertz polarization imaging technique demonstrated intrinsic contrast between normal and cancerous regions. Consequently, the obtained cross- (co-) polarized two dimensional terahertz images exhibited increased reflection from the tumor 0.65 % (19.28 %) instead of 0.55 % (17.13). Also, our results indicate that the cross-polarized terahertz images not only correlate better with the histology, but also provide consistent relative reflectance difference values between normal and cancerous regions of human colonic tissues.

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