Terahertz Polarization Imaging for Colon Cancer Detection

Terahertz Polarization Imaging for Colon Cancer Detection Pallavi Doradlaa,b, Karim Alavic, Cecil S. Josepha,b, Robert H. Gilesa,b a

Biomedical Terahertz Technology Center, University of Massachusetts Lowell;

b

Department of Physics and Applied Physics, University of Massachusetts Lowell;

c

Division 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 FarInfrared 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 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/14/2014 Terms of Use: http://spiedl.org/terms

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 copolarized 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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polarization of the transmiitted beam, a 50-50 5 Mylar beam splitter was w introduced to enable the detection arm prior to o the image pllane using a shhort focal lengthh off-axis paraabolic (OAP 1) mirror. The fuull-width finally focusiing the beam on

-,,

Figure 1.Scchematic of expeerimental setup for f THz reflectioon imaging.

w measured as 0.65 mm at a the sample plane. The seecond part of the incident raadiation, half-maximuum (FWHM) was reflected from the beam splitter, s was atttenuated usingg a THz absorrber. The signnal remitted frrom the specim men was focused ontoo the detector, sitting in the reflection r arm, using an off axis a parabolic mirror (OAP 2). 2 An automaated twoaxis scan staage was used to o raster scan thhe sample in thhe imaging plaane with a resoolution of 0.1 mm and dwelll time of 150 ms. Thhe co-polarized d and cross-poolarized imagees were attaineed by selectinng the approprriate orientationn of the analyzing wiire grid polarizzer. The signal--to-noise ratio (SNR) of the system s using a lock-in amplifier was 65 dB B Further details of thee experimental setup and dataa acquisition caan be found in [24].

SAMPL LE PREPAR RATION Fressh thick excesss colon specim mens used in this study werre obtained froom the Univerrsity of Massaachusetts Memorial Hospital under an Institutionaal Review Board approved protocol. For terahertz imaaging, the coloon tissue w mounted in i an aluminum m two-piece saample holder (w with 7.5 cm x 2.5 cm front opening), usinng a 1.55 specimens were mm thick sliide of z-cut qu uartz, as shownn in Figure 2. To T prevent tisssue dehydratioon during the imaging proceddure, the tissue specim mens were cov vered with wett gauze soakedd in pH (pH 7.4) balanced saline. s During the mounting process, both the fronnt and back piieces of samplle holder were gently pressed onto the tisssue to avoid air a gaps betweeen colon specimen annd quartz slide. Finally to seecure the tissuue specimen’s position, the whole w assemblly (quartz slidde, tissue specimen, annd wet gauze) was w placed in the t sample holdder and gently fixed with fouur screws.

Figure 2. Schematic of th he sample mountting for reflectioon measurementss a) 2-piece alum minum holder, b)) abnormal (red) and w gauze soakedd in pH 7.4 balannced normal (yyellow) colon tisssues placed on the quartz slide, c) colon tissuess covered with wet salinee solution, d) clo osing the assembbly with holder’ss second piece, e) e tissue specimeen mounted in thhe sample holderr.


Thee tissue mounteed in a 2-piecee aluminum saample holder, with w a z-cut quartz q window,, was imaged within 2 hours of the standard surg gical proceduree. The sample holder contaiining the norm mal and cancerrous tissues waas raster scanned in thhe imaging plaane to obtain both b co- and cross-polarized c d terahertz refllectance imagees of en-face colon. c In total we meaasured 14 speciimens from 8 subjects. s Abnoormal colon tissue and adjaceent normal tissuue were obtainned from 6 individualss diagnosed with w colorectal neoplasm, whhereas normall tissue was acquired from each of the 2 normal subjects. Thee diagnoses weere based on thee analysis of Hematoxylin H annd Eosin staineed (H&E) histoopathology.

RESUL LTS & ANA ALYSIS Thee co- and crosss-polarized refflection THz im mages were obbtained by colllecting the siggnal remitted from f the tissue specim men sitting on the raster scannned XY-stagee and by placiing the approppriate analyzing polarizer griid in the reflection arm m. The THz im mages obtainedd from reflectioon imaging werre then processsed using a LaabViewTM proggram that synchronizedd the sample position p in the imaging planee with the retuurn signal obtaiined from lockk-in amplifier. The coand cross-polarized imagess were then callibrated against the full-scalee return from a flat front-surfface gold coateed mirror a were rem moved in to determinee the reflectancce. The imagess were plotted in logarithmicc space and thhe off sample areas post processiing with the refflected THz siggnal quantifiedd pixel by pixell. Figuure 3 shows th he digital phottograph of the sample and teerahertz reflecttance images of o both co- annd crosspolarized images of a fresh h normal colonn tissue specim men. The imagees were plottedd in logarithmiic dB scale andd the copolarized refflectance of norrmal colon tisssue varies betw ween -7.5 – -8.55 dB, whereas the t cross-polarrized reflectancce varies from -21.5 dB d to -22.5 dB. In addition, both b co- and crross-polarizatioon images of all a 8 normal coolonic sampless showed uniform terahhertz response over the entirre area and exhhibited a reflecctance of 16% and 0.55% resspectively. How wever, a significant reeduction in thee reflectance was w noticed in the case of foormalin fixed colon c tissues. More M details about a the effects of forrmalin fixation on colon tissuues can be founnd elsewhere19,221.

- n ni

Figure 3. (a) Diigital photographh, (b) co-, and (cc) cross-polarized terahertz reeflection images of fresh human colon tissue.

For THz images, the cross-polaarized reflectivvity percentagee of representaative cancerouss tissue was coompared mal and canceerous regions of o the colon tisssue was with adjacennt normal tissue. The reflectaance differencee between norm similar at botth polarization ns with the crosss-polarized beeing more attennuated. The coo-polarized refllectance averagged over the 8 normall and 6 cancero ous tissues invvestigated was found to be 17.1 1 ± 0.3 and 19.3 ± 0.3, reespectively. Wiith cross polarization, the averaged reflectance r from m normal sampples was foundd to be 0.55 ± 0.015 0 while for cancer specim mens the 65 ± 0.016. Annalysis of the reflectivity r dataa from co- andd cross-polarizeed images show wed that average refleectance was 0.6 cancer had higher h reflectiv vity than norm mal colon tissuue. Figure 4 shows s the teraahertz cross-poolarized imagees of the normal versuus cancerous co olonic tissue seets 1 to 4. The reflectance off the cross-polaarized THz imaage varies betw ween -19 and -23 dB. The off samplee portion of thhe THz images was set to zerro during post processing p so only the THz response r


from the coloon specimens was w studied furrther. Contrast was observed between canceerous and norm mal colon tissuee in each data set, as shown in Figu ure 4. Increaseed reflection of o the canceroous region in the t terahertz im mages of coloon tissue indicates thee altered THz response r from the tumor areea. The cross-ppolarized terahhertz image off normal tissuee in each data set conttains a solid white w circle, reepresenting higgh reflectance. The high refllectance was a result of the air gaps between the tissue t and quarrtz slide introdduced during thhe mounting process.

Fig gure 4. Cross-poolarized terahertzz reflection imagges of fresh norm mal (N) versus cancerous (C C) human colonicc tissue sets (a) 1, (b) 2, (c) 3, annd (d) 4.

Com mparing the caancerous areas to the adjaceent normal onees of the samee subject yieldds good specifiicity. To quantify the reflectivity vaalues, the relatiive difference in reflected inttensity betweeen cancerous annd normal areaas of the o 6 data setss. The relativve reflectance difference accross the sampples was same subject was calculatted for each of d cross-polarizeed images usinng the backgroound reflectancce value obtainned from salinee soaked calculated foor both co- and gauze using the t formula;

Rrel = ⎡

⎢⎣

( )( RC

RB

−

RN

)⎤

RB ⎥⎦

/ RN =

RC − RN RN × RB

In this expression, RC and RN are the refllectance valuess of cancer andd normal colonn samples, wheereas RB repressents the fr backgroun nd (saline filleed gauze). It was w observed that the relative reflectance values v are greaater than reflectance from zero for all data d sets, representing the higgher reflectivityy values from abnormal tissuues. In the co-ppolarized data, this can be caused byy the higher reefractive indexx value associaated with canceerous tissue ass compared to normal. Howeever, the signal from all a interfaces was w rejected in cross-polarized c d images. Thee relative refleectance differeences calculatted for colon samples at thhe measured frequency shoowed an inconsistencyy in the co-pollarized data rannging from 1.55 to 3.1%. Thiss may have ressulted from thee fluctuations observed o in the co-pollarized reflectaance from norm mal tissue, whhich contains thhe Fresnel refllection from quuartz-sample innterface. The specularr reflection from m the quartz-tiissue interface will vary for each e data set ass a function of tissue parametters such as refractive index, water content, c etc. In contrast, the cross-polarized c d THz responsee effectively reejects the reflecctions as c is co-polarized c w the incidennt radiation annd effectively samples with s the tisssue volume. Since S the the Fresnel component signal from all a interfaces was w rejected inn cross-polarizeed images, thee contrast is deetermined prim marily by the reefractive index fluctuaations within the t sample. Thhe increased sccattering from the abnormal tissue could have h resulted from f the increased vaasculature, lym mphatic system m, or associated structural chhanges (largerr crypt size in hyperplasic mucosa). m Hence, the reelative reflectaance differencee for all the crross-polarized THz images were w consistentt (7.3 – 7.7). Also, A the data analysiss confirmed thaat the reflectiviity level for noormal tissue waas significantlyy different from m the cancerouus region with a p-valuue of 0.042 (