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Observation of Hydrofluoric Acid Burns on Osseous Tissues by Means of Terahertz Spectroscopic Imaging William E. Baughman, Student Member, IEEE, Hamdullah Yokus, Soner Balci, David Shawn Wilbert, Student Member, IEEE, Patrick Kung, Member, IEEE, and Seongsin Margaret Kim, Member, IEEE
Abstract—Terahertz technologies have gained great amount of attention for biomedical imaging and tissue analysis. In this study, we utilize terahertz imaging to study the effects of hydrofluoric acid on both compact bone tissue and cartilage. We compare the differences observed in the exposure for formalin fixed and raw, dried, tissue as well as those resulting from a change in hydrofluoric (HF) concentration. Measurements are performed with THz-TDS, and a variety of spectroscopic-based image reconstruction techniques are utilized to develop contrast in the features of interest. Index Terms—Biomedical imaging, hydrofluoric acid, osseous tissue, terahertz spectroscopic imaging, terahertz (THz) timedomain spectroscopy.
I. INTRODUCTION N recent years, the study and utilization of the terahertz (THz) region of the electromagnetic spectrum has become an important field of biomedical research and development of innovative applications. Radiation in the THz region is nonionizing, and can penetrate nonmetallic materials which has led to many applications in areas such as narcotic and explosive detection, art restoration, and optical or electrical property determination for material science applications [1]–[8]. One of the fundamental reasons that THz-based imaging techniques gained the attention of the biomedical community was due to the fact that THz radiation can probe the vibrational modes of molecules that exhibit motions that extend across length-scales in the tens of angstroms range [9], [10]. Studies have been published investigating the effects of osteoarthritis, breast tumor identification, enamel health, skin burns, corneal tissue, and drought stresses observation of vascular plants [11]–[21]. Although a variety of studies have been performed over the last decade, THz-based technologies are still in the early stages of commercialization. In addition to determining what specific features can be detected with THz bio-imaging, a large amount of research are still being conducted to overcome the inherent
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Manuscript received October 1, 2012; revised December 22, 2012; accepted January 15, 2013. Date of publication January 28, 2013; date of current version June 27, 2013. This work was supported by the National Science Foundation under Award 0824452 and NSF CAREER. The authors are with the Department of Electrical and Computer Engineering, University of Alabama, Tuscaloosa, AL 35487 USA (corresponding author is S. M. Kim, e-mail: seongsin@ eng.ua.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JBHI.2013.2243158
difficulties of water absorption, propagation efficiency, and even the generation and detection devices [22]–[24]. In previous studies, we have investigated undamaged bone tissue, skin tissues, and inorganic objects (such as microchips) to develop a variety of image processing techniques which we will apply in this study [25], [26]. Here, we study the effects of hydrofluoric acid (HF) on the THz-TDS imaging response of bone and cartilage. One of the primary effects of direct exposure to HF is acute hypocalcaemia—or removal of calcium from the tissue—and should, therefore, exhibit significant changes in the THz response of the tissue due to structural changes and traces of acid. We first observe the presence of an HF damaged region on both compact bone and cartilage to identify how the tissue deterioration affects the spectroscopic response, and verify that detection can be obtained. Next, we analyze tissue which, after exposure to HF, has been histologically fixed using formalin to determine if the observability of the damage is diminished. Finally, the effect of acid concentration is investigated using a piece of pure cartilage tissue. With the results from this, a better understanding of the nature and precision of THz-based biomedical imaging and imaging reconstruction is developed. II. TERAHERTZ METHODOLOGY A. THz-TDS Imaging The imaging system used here in this study consists of a conventional Terahertz Time-Domain Spectrometer operating in a transmission regime. The spectrometer has been modified to perform 2-D data acquisition for imaging reconstruction by the incorporation of a three-axis, motion controlled sample mount. A simple schematic of the system is shown in Fig. 1. The THz-TDS system is pumped by a 790 nm ultrafast laser (MIRA 900, Coherent) with a repetition rate of 86 MHz and a pulse width of 120 fs. Detection of the THz is achieved by means of electro-optic sampling using ZnTe crystal details of similar setup were reported elsewhere [25], [27]. From the time-domain signal acquired, frequency data are obtained through the use of a simple fast-Fourier transform (FFT) that gives frequency resolution of 25 GHz, which is sufficiently small for the observation of features and trends in the power spectrum. B. Image Reconstruction The sample is placed such that the radiation is incident on the surface at the focal plane between the two lenses. The THz beam was tightly focused with spot size of 0.6 mm measured using the knife-edge method. 2-D time domain electric field data are
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BAUGHMAN et al.: OBSERVATION OF HYDROFLUORIC ACID BURNS ON OSSEOUS TISSUES
Fig. 3.
Fig. 1.
Schematic diagram of the THz-TDS used in this study.
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(a) Tissue before burn and (b) tissue after HF exposure.
is determined for each pixel at a select frequency and transferred to the image reconstruction. C. Sample Preparation
Fig. 2.
Methods for time domain image reconstruction.
obtained by moving the sample in plane with the focused THz in a raster pattern and performing full spectroscopic scans at regular intervals, with each scan representing one pixel of the image. Since full spectroscopic data exists at each point acquired, each pixel of image reconstructed is achieved by selecting a specific attribute of either the time-domain waveform, or the frequency spectra resulting from the FFT. Once the desired criterion is selected the associated numerical value is extracted, normalized, and rendered into an image based on either a monochromatic or multicolored contrast scale. Next, the gain of the image is adjusted so that the features of interest may be clearly observed, and finally, bicubic interpolation is applied to the result. In this study, the initial images considered for all samples are constructed from the peak amplitude of time-domain signal intensity for each data point. This reconstruction method indicates the general THz transparency of each pixel and typically yields strong contrasts between different tissue types, allowing for the general shape of the sample to be easily observed. The second important reconstruction in time-domain signal to be considered is the propagation time delay of the radiation observed in each pixel as it propagates through the medium. This method of imaging is complementary to peak amplitude image reconstruction as it reveals the contrast based on the relative time of flight of the peak intensity of the radiation pulse. Below, Fig. 2 describes, graphically, these two methods of image construction. On these graphs, the length of the red arrows is the numeric value associated with each pixel. Image reconstruction in the frequency-domain of this study is primarily focused on the power spectrum versus frequencies obtained from the FFT. The relative amplitude in decibels (dB)
All bone and cartilage samples were taken from the long bone of chickens. Before preparation, the fresh bones were stored frozen at –30◦ Fahrenheit. To begin, a thin cross section was cut from the bulk with an approximate thickness of 1 ± 0.1mm. With the thin cross-section cut, an eyedropper was used to expose a small region of the sample to the hydrofluoric acid solution. For most samples, an HF concentration of 48% was used. In addition to 48% and 12%, 24% HF concentration was also exposed on bone. One cartilage sample was exposed to both 48% and 24% HF in two separate locations to observe the change in damage that results from different HF concentrations. The 24% solution was obtained by mixing 48% HF with equal parts of deionized water. The acid was allowed to sit on the sample for 60 s, after which time the sample was immediately washed with DI water to diminish the spread of the acid. Fixing was performed on samples using a 10% formalin neutral buffer. For these samples, once the tissue had been thoroughly rinsed, compressed nitrogen was used to remove any excess moisture. The bone or cartilage was placed into a 10% formalin neutral buffer for 2 h. After fixation, the sample was dried once again using compressed nitrogen. For imaging, the tissue was mounted vertically between two thin transparent plastic sheets which allow for >95% transmission of the propagating terahertz radiation All measurements were performed with the spectrometer under a nitrogen purge which maintained a constant humidity as low as 21% (compared to 70% without purging) for the duration of the measurement. III. RESULTS AND DISCUSSION A. Presence of Acid Burn The first study in this paper was to identify the presence of an HF burn in a fresh, nonfixed tissue sample which included both bone and cartilage. Fig. 3 shows the optical images taken for the sample before HF exposure [see Fig. 3(a)] and after [see Fig. 3(b)]. The bottom dark color area is where the compact bone is located and above is the cartilage. The region which was exposed to the HF has marked discoloration both on the bone and cartilage regions.
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Fig. 5. Selected pixel data for (a) time domain, (b) frequency domain before and after HF exposure. ‘Reference’ corresponds to the signal from free path propagation.
Fig. 4. THz reconstructed images: (a) before burn-TD peak, (b) before burnTD delay, (c) after HF-TD peak, and (d) after HF-TD delay.
Two separate THz images were taken from the prepared samples using the THz-TDS. The first was performed from fresh tissue and the second was after exposure to 48% HF. It was noted that the thickness of the sample did not change measurably; however, the mass of the sample after HF exposure was reduced by 9.69 mg. Two separate THz images were taken from the prepared samples using the THz-TDS. The first was performed from fresh tissue and the second was after exposure to 48% HF. It was noted that the thickness of the sample did not change measurably; however, the mass of the sample after HF exposure was reduced by 9.69 mg. Fig. 4 shows the false color time-domain image results obtained and their reconstructions. For both images, a pixel pitch of 0.5 mm was used. In the case of the unburned sample, the overall image size was 18 × 18 mm, The HF burned image was slightly larger (20 × 20 mm); however, the additional space primarily consisted of area outside of the sample seen in the lower right region of Fig. 4(c) and (d). Fig. 4(a) and (c) compares the peak amplitude changes in time domain signal between fresh tissue and HF acid burned tissue, and Fig. 4(b) and (d) shows the reconstruction image based on time delay signal for both samples. We observed that the intensity of transmission through the sample is dramatically increased in both the cartilage and bone regions where the acid spot is located. Further analysis of individually extracted pixels shown in Fig. 5 reveals that the regions exposed to HF exhibit an increase of transmission of approximately 2.5 times that of the unburned tissue of the same type. This agrees well with the reduction in the mass of the sample since the lower density allows for better propagation, and follows the expected behavior for the acid exposure. In addition, it appears that the response to HF is different for compact bone and cartilage. Before acid reaction, the transmitted peak intensity is uniform in the area for both cartilage and compact
Fig. 6. Power spectrum images of (a) nonburned, (b) burned tissue at 0.425 THz; and for (c) nonburned, (d) burned tissue at 0.8 THz.
bone. After HF acid reaction, the transmitted THz intensities in the compact bone area is still consistent through the area with increased intensity, while the cartilage area shows more fractured patterns with varied transmission intensities. It is clearly related to structure changes due to the acid, which results in different responses in time and in the type of tissues. This seems to be related to the distinct delay times for cartilage as shown in Fig. 5(a) as well. Cartilage after HF acid shows a response that is not only increased but is also 0.5 ps shifted in the peak time. By reconstructing the propagation delay image, however, contrast is not as clearly developed shown in Fig. 4(b) and (d). Although a reduction in the propagation time is clear in the burned regions, the propagation time for the burned bone is very close to that of the undamaged cartilage, and as a result much of the distinction is lost. Fig. 6 shows the images reconstructed based on the power spectrum obtained from FFT performed on each pixel of the images. Frequency selection was made by first observing the FFT results from selected pixels [see Fig. 5(b)]. From this, we cannot see any distinct change in the spectral shape; however,
BAUGHMAN et al.: OBSERVATION OF HYDROFLUORIC ACID BURNS ON OSSEOUS TISSUES
a broadband increase in power occurs as a result of the HF exposure. At frequencies below 0.5 THz, the HF areas in both bone and cartilage exhibit higher transmitted THz power than the undamaged tissue. The first reconstruction, at 0.425 THz shows a clear contrast development for the damaged areas when compared to the undamaged tissue [see Fig. 6(a) and (c)]. The distinction of HF reacted area in the cartilage is more significant than in compact bone at the selected frequency of 0.425 THz. It is noted that the image contrast is rather similar to the peak intensity images shown in Fig. 4(c). Above 0.5 THz, the burned areas are no longer the most transmissive regions, but the HF burned bone area becomes less transparent than the cartilage (either burned or not). This enables us to distinguish between the bone and the cartilage at the higher frequency. When combining both lower and higher frequency power spectral imaging contrasts, as shown in Fig. 6(b) to (d), we are able to clearly distinguish all four combinations of burned/unburned tissue and cartilage/bone, which were not shown clearly in simple THz peak amplitude imaging. It would be interesting to compare the changes in the actual THz optical properties of the regions probed, such as refractive index and absorption coefficient of osseous tissues before and after HF acid burn. To obtain both optical properties, we use the following two equations [28], [29] T (ω) =
i ( N −1 ) d ω 4N Esam ple (ω) = e C = Aeiϕ . 2 ERe f (ω) (1 + N )
(1)
T(ω) is the complex transmission function defined as the ratio of the THz wave transmitted through the sample inside the holder (sample signal) to the THz wave transmitted through the holder only (reference signal). T(ω) is a function of the complex refractive index of the sample probed, N = n + ik, where “n” is the real refractive index and “k” is the extinction coefficient, which correspond to the sample chromatic dispersion and absorption characteristics, respectively. In equation (1), A is the amplitude ratio of the two waves, ω is angular frequency, d is thickness of the sample, and ϕ is the phase difference between the sample and reference signals. Once the complex refractive index N (ω) is determined, the absorption coefficient α(ω) can be calculated using 4n(ω) 2 . (2) α(ω) = ln d A [n(ω) + 1]2 Fig. 7 shows a comparison of the absorption coefficient obtained for bone and cartilage, before and after HF burns. There is a noticeably distinctive behavior in its frequency dependence between the bone and cartilage tissues. The bone exhibits a higher absorption coefficient in general compared to the cartilage, as it would be expected. In addition, it shows a strong dependence on frequency, with the highest absorption occurring at about 1.1 THz. By contrast, the absorption coefficient for the cartilage exhibits a monotonous increase with frequencies. After HF burn, the absorption coefficient significantly decreases for both types of tissue, by about 30% for the bone and 45% for the cartilage. For the latter case, the absorption becomes very small and less dependent on frequency.
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Fig. 7. Calculated absorption coefficient in THz frequencies for both bone and cartilage. (a) Before HF and (b) After HF acid burns.
Fig. 8.
Formalin fixed tissue with HF burn.
Fig. 9. delay.
Time-domain image reconstruction from (a) peak and (b) propagation
B. Formalin Fixing Effects on Bone and Cartilage In this section, we will discuss the investigation of the effect of formalin fixing tissues on the THz imaging reconstruction. Formalin fixing is the most general method to preserve a tissue when it is excised from the organ. Since it fixes most of the proteins in a tissue, there have been concerns that some information about the tissue would no longer be accessible by THz imaging [15], [30]. In the previous section, the general effect of HF was clearly shown in our study. The next sample, depicted in Fig. 8, consisted of compact bone and cartilage just as the previous samples were, with HF acid burns, and on which formalin fixation was applied before THz imaging. The top part of the sample which is shown in white in Fig. 8 is the cartilage area while the dark part is compact bone area. The area where 48% HF was dropped is circled in the figure as well. Fig. 9 shows the THz images obtained with peak amplitude
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Fig. 10. Selected pixel data for (a) time domain (b) frequency domain from the formalin fixed tissue.
image [see Fig. 9(a)] and the reconstructed images based on the propagation delay [see Fig. 9(b)]. The image of this sample was performed over a 20 × 20mm area, and once again a pixel pitch of 0.5 mm was used. When it is compared to the freshly cut tissue the contrast developed in the time domain amplitude image is not as strong, although the burn is still evident near the top of the image. The extracted time domain spectra for various types of tissue after formalin fixing are shown in Fig. 10(a). The transmission ratio between fixed cartilage and fixed burned cartilage was only ∼1.5, whereas it was larger (2.5) for the sample that was not formalin fixed [see Fig. 5(a)]. This means that a smaller contrast between burned/unburned cartilage is expected for fixed cartilage. This behavior is consistent with the molecule rigidifying characteristics of the fixation process, which renders a tissue more transparent to THz waves since the vibration modes of constituent molecules would be attenuated. As a result, the lower THz transmission contrast between burned/unburned cartilage after fixing is partially due to the fact that both the HF burn and the formalin fixation tend to make the tissue more transparent to THz waves. Furthermore, since the compact bone has a lower density of proteins than the cartilage, we expect a much smaller change in THz transmission ratio, which is what we observe: the ratio between burned/unburned fixed bone and that between burned/unburned unfixed bone were similar. It should be also noted that the time delay difference between the acid burned and unburned areas is very close to what we measured in section A. For example, there is exactly a 0.5 ps delay difference between the HF exposed cartilage and unburned cartilage. This confirms that the acid damages the tissue structure in a way that can be probed in THz time domain signal, but that the time delay difference is not affected whether the tissue was fixed or not. Analysis of the frequency spectra in Fig. 10(b) shows that the spectra for the different regions exhibit much less distinct slopes compared to Fig. 5(b), although the relative intensities are still maintained. From the power spectrum information, we developed image reconstructions at the same target frequencies of 0.425 and 0.8 THz, as shown in Fig. 11. We observe a distinct contrast between the burned/unburned regions at 0.425 THz, but without being able to differentiate between the damaged carti-
Fig. 11. Frequency-domain images construction of power spectrum at (a) 0.425 THz and (b) 0.8 THz.
lage and compact bone areas. However, the undamaged compact bone (see the lower half of Fig. 11) shows different contrast in the middle of the area, which was not seen in the fresh-cut tissue discussed in section A. At 0.8 THz, the burned/unburned contrast within the compact bone fades. In addition, image reconstruction based on the power spectrum shows more structured contrast within the bone when it is fixed which also similar results we obtained is in our previous work [25]. C. HF Concentration To study the effect of HF concentration, a pure cartilage sample was prepared with two burn spots. The first burn spot used the 48% solution standard for our experiments. The second location was exposed to a 24% solution. Both burn locations are visible on the sample due to a slight discoloration. This discoloration is evident both before and after formalin fixing. The thickness of the sample was also remeasured and no appreciable change in the thickness of the cartilage in the burned regions was observed. The image obtained for this sample was 20 × 14 mm, with a pixel pitch of 0.5 mm in both directions. The initial reconstruction in Fig. 12(a) clearly shows both burn locations as an increase in transmission of the THz relative to the unburned portion of the cartilage. From inspection of the pixel spectra, we see that the burn regions transmit 18% and 19.5% of the reference (for 48% and 24%, respectively), while transmission through the undamaged tissue ranged between 9.6% and 10%. A change in propagation delay is also observed in Fig. 12(b). From the results, we see that the delay for the 48% HF is 70% of the unburned delay, and the 24% HF spot yields a delay of 79%. By looking at these in conjunction, it is evident that the higher concentration can be clearly distinguished from the lower concentration region in both modalities. Next, the FFT of the time domain spectra for all pixels in the image was taken. Images were constructed at 0.45 and 0.7 THz from the power spectrum data. These frequencies were selected by first observing the Fourier transform of the individual pixels extracted in the time domain [see Fig. 13(c)], and next scanning the image construction through the frequency range to ensure agreement. At the lower frequency, the burns are clearly distinguishable from the rest of the undamaged tissue but they look approximately the same [Fig. 13(a)]. In the slightly higher frequency range from 0.6 to 0.8 THz, we can see that the higher
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Fig. 14. Time domain THz transmitted signal comparison for 12%, 24%, 48% HF concentration exposure on bone tissue. Bone-24 means non-HF corresponding 24% spot, HF Bone-24 means HF24% exposed.
Fig. 12. (a) TD amplitude image, (b) propagation delay image, and (c) selected TD waveforms from the image data.
Fig. 15. (a) Optical image of prepared sample.THz images reconstructed for comparison of different HF concentration on bone tissue with (b) peak amplitude, and selective frequencies (c) 0.45 THz and (d) 0.8 THz.
Fig. 13. Power spectrum images at two frequencies (a) 0.45 THz, (b) 0.7 THz, and c) frequency power spectra for the selected pixels.
concentration solution has clearly done more damage to the sample, and thus the transmitted power is higher. At 0.7 THz, shown in Fig. 13(b), a clear distinction between the 48% HF and 24% HF burn regions is observed. Beyond this frequency range, there is no distinction between the burned and the undamaged tissue. In addition, we studied the effect of HF concentration on the bone tissue as well. 12% and 24% HF solutions were used at two different spots on a prepared bone tissue with thickness about 1 mm. Fig. 14 compares the time domain THz signals transmitted at these two spots, and also compared with the 48% acid burns discussed in section A. For a more precise comparison, the THz signals before and after HF burns at each concentration were plotted together. As shown in Fig. 14, the peak intensities were
significantly increased after exposure to all three concentrations, with the higher concentration leading to a higher transmission. In addition, changes in the delay time were also noticeable at higher concentration. Fig. 15 shows different THz images reconstructed based on peak amplitude (b) and selective frequencies (c)–(d), with Fig. 15(a) being an optical image of the sample studied. Both HF concentrations affected the tissue structure in a similar way when comparing the peak amplitude images. Since the peak amplitudes increased for both concentrations (12% and 24%) as shown in Fig. 14, the image in Fig. 15(b) clearly distinguishes both acid burned spots from the unburned regions, with the spot associated with the 24% HF exhibiting a better contrast. Fig. 15(c) and (d) are images reconstructed at different frequencies. At 0.45 THz, two spots were revealed with a clear contrast from the surrounding area. However, at 0.8 THz, the 24% HF burn spot lost some of its contrast while the 12% HF burn spot maintained its distinct contrast. IV. CONCLUSION In this study we have applied THz-TDS imaging to the study of osseous tissues which have been damaged due to acute exposure to hydrofluoric acid. Image reconstructions in both the
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time and frequency domains clearly show that the THz transparency is increased in areas exposed to the acid, though the effect appears to be broad spectrum, without clearly identifiable frequency specific results. We further determine that the use of histological fixing with formalin causes a muting of the contrast between damaged and undamaged tissue, which must be taken into account in further studies. From these results, it is clear that the use of THz imaging can clearly reveal very specific changes in tissue composition.
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[20] M. Arbab, T. C. Dickey, D. Winebrenner, A. Chen, M. B. Klein, and P. D. Mourad, “Terahertz reflectometry of burn wounds in a rat model,” Bio. Opt. Exp., vol. 2, pp. 2339–2347, 2011. [21] D. Bennett, Z. Taylor, P. Tewari, R. Singh, M. Culjat, W. Grundfest, D. Sassoon, R. D. Johnson, J. P. Hubschman, and E. Brown, “Terahertz sensing in corneal tissues,” J. Biomed. Opt., vol. 16, p. 057003, 2011. [22] M. S. Heimbeck, K. K. Myung, D. A. Gregory, and H. O. Everitt, “Terahertz digital holography using angular spectrum and dual wavelength reconstruction methods,” Opt. Expr., vol. 19, pp. 9147–9156, 2011. [23] X. Zhang, “Terahertz technology and applications,” presented at SPIE Photonics West short course, San Jose, CA, 2005. [24] H. Tao, C. M. Bingham, A. C. Strikwerda, D. Pilon, D. Shrekenhamer, N. I. Landy, K. Fan, X. Zhang, W. J. Padilla, and R. D. Averitt, “Highly flexible wide angle of incidence terahertz metamaterial absorber: design, fabrication, and characterization,” Phys. Rev. B, vol. 78, pp. 241103-1– 241103-4, 2008. [25] W. E. Baughman, D. S. Wilbert, S. Balci, M. Bolus, M. Baker, P. Kung, S. M. Kim, M. S. Heimbeck, and H. O. Everitt, “Comparative reconstructions of THz spectroscopic imaging for non-destructive testing and biomedical imaging,” Proc. SPIE, vol. 8363, pp. 83630W1–83630W10, 2012. [26] S. M. Kim, W. Baughman, D. S. Wilbert, L. Butler, M. Bolus, S. Balci, and P. Kung, “High sensitivity and high selectivity terahertz biomedical imaging,” Chin. Opt. Lett., vol. 9, no. 10, pp. 110009-1–110009-4, 2011. [27] Q. Wu and X.-C. Zhang, “Free-space electro-optic sampling of terahertz beams,” Appl. Phys. Lett., vol. 67, no. 24, pp. 3523–3525, 1995. [28] G. Dubillaret and J. L. Coutaz, “Highly precise determination of optical constants and sample thickness in terahertz time domain spectroscopy,” Appl. Opt., vol. 38, pp. 409–415, 1999. [29] G. J. Wilmink, B. L. Ibey, T. Tongue, B. Schulkin, N. Laman, X. G. Peralta, C. Roth, C. Z. Cerna, B. Rivest, J. E. Grundt, and W. Roach, “Development of a compact terahertz time-domain spectrometer for the measurement of the optical properties of biological tissues,” J. Bio. Opt., vol. 16, no. 4, pp. 047006-1–047006-10, 2011. [30] Y. Sun, B. Fischer, and E. Pickwell-MacPherson, “Effects of formalin fixing on the terahertz properties of biological tissues,” J. Bio. Opt., vol. 14, pp. 064017-1–064017-7, 2009.
William E. Baughman (S’09) was born in Florence, AL, USA. He received the B.Sc. degree in electrical engineering from the University of Alabama, Tuscaloosa, USA, in 2010. He is currently working toward the Ph.D. degree in the Department of Electrical and Computer Engineering at the University of Alabama. His research interests include terahertz spectroscopy, biomedical imaging, and optical system design.
Hamdullah Yokus was born in Diyarbakir, Turkey, in 1987. He received the B.S. degree in electrical and electronics engineering from the University of Erciyes, Kayseri, Turkey, in 2009. He is currently working toward the Master’s degree in the Department of Electrical and Computer Engineering, University of Alabama, Tuscaloosa, USA, in the field of Terahertz Spectroscopic Imaging.
Soner Balci received the B.Sc. degree in electrical and electronics engineering from Dokuz Eylul University, Izmir, Turkey, in 2007, and the M.Sc. degree in electrical and computer engineering from the University of Alabama, Tuscaloosa, USA in 2012, where he is currently working toward the Ph.D. degree. His research interests include characteristics of carrier dynamics in semiconductor thin films and nanowires in terahertz frequency region and terahertz imaging.
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David Shawn Wilbert (S’06) received the B.S. and M.S. degrees in electrical engineering from the University of Alabama, Tuscaloosa, USA, in 2009 and 2010, respectively, where he is currently working toward the Ph.D.degree. His research interests include spectroscopic imaging techniques, THz generation and detection technologies, metamaterial devices, and nanoscale fabrication technology.
Patrick Kung (M’08) received the Diplˆome ´ d’Ing´enieur from the Ecole Polytechnique, Palaiseau, France, in 1993, and the Ph.D. degree in electrical engineering from Northwestern University, Evanston, IL, USA, in 2000. From 2000 to 2007, he was a Research Assistant Professor in the Department of Electrical Engineering and Computer Science, Northwestern University. Since 2007, he has been an Assistant Professor in the Department of Electrical and Computer Engineering, the University of Alabama, Tuscaloosa, USA, and an Adjunct Professor in the Department of Electrical Engineering and Computer Science, Northwestern University. His research interests includes been involved in the area of compound semiconductors and devices, advanced material characterizations, including atom probe tomography, and radiation hardened electronics. He has authored 62 publications. Dr. Kung is a member of APS, AVS, SPIE, and IFES.
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Seongsin Margaret Kim (M’04) received the B.S. degree in physics from Yonsei University, Seoul, Korea, in 1992, and the M.S. degree in physics and the Ph.D. degree in electrical and computer engineering from Northwestern University, Evanston, IL, USA, in 1994 and 1999. Since 2007, she has been an Assistant Professor in the Department of Electrical and Computer Engineering, the University of Alabama, Tuscaloosa, AL, USA. Prior to joining the University of Alabama, she was a Research Associate at Stanford University after having worked at both Samsung (Korea) and Agilent Technologies (San Jose, CA, USA). Her research interests include area of semiconductor quantum dots, nanowires, optoelectronics, and terahertz science and technologies, including generation, detection, spectroscopy, biomedical imaging, near field imaging, and THz metamaterials. He has more than 85 peer-reviewed publications including 43 journal articles, 5 book chapters, and holds one US patent. Prof. Kim has also received NSF CAREER award in 2010, and is a member of SPIE, SWE.
IEEE JOURNAL OF BIOMEDICAL AND HEALTH INFORMATICS, VOL. 17, NO. 4, JULY 2013
Observation of Hydrofluoric Acid Burns on Osseous Tissues by Means of Terahertz Spectroscopic Imaging William E. Baughman, Student Member, IEEE, Hamdullah Yokus, Soner Balci, David Shawn Wilbert, Student Member, IEEE, Patrick Kung, Member, IEEE, and Seongsin Margaret Kim, Member, IEEE
Abstract—Terahertz technologies have gained great amount of attention for biomedical imaging and tissue analysis. In this study, we utilize terahertz imaging to study the effects of hydrofluoric acid on both compact bone tissue and cartilage. We compare the differences observed in the exposure for formalin fixed and raw, dried, tissue as well as those resulting from a change in hydrofluoric (HF) concentration. Measurements are performed with THz-TDS, and a variety of spectroscopic-based image reconstruction techniques are utilized to develop contrast in the features of interest. Index Terms—Biomedical imaging, hydrofluoric acid, osseous tissue, terahertz spectroscopic imaging, terahertz (THz) timedomain spectroscopy.
I. INTRODUCTION N recent years, the study and utilization of the terahertz (THz) region of the electromagnetic spectrum has become an important field of biomedical research and development of innovative applications. Radiation in the THz region is nonionizing, and can penetrate nonmetallic materials which has led to many applications in areas such as narcotic and explosive detection, art restoration, and optical or electrical property determination for material science applications [1]–[8]. One of the fundamental reasons that THz-based imaging techniques gained the attention of the biomedical community was due to the fact that THz radiation can probe the vibrational modes of molecules that exhibit motions that extend across length-scales in the tens of angstroms range [9], [10]. Studies have been published investigating the effects of osteoarthritis, breast tumor identification, enamel health, skin burns, corneal tissue, and drought stresses observation of vascular plants [11]–[21]. Although a variety of studies have been performed over the last decade, THz-based technologies are still in the early stages of commercialization. In addition to determining what specific features can be detected with THz bio-imaging, a large amount of research are still being conducted to overcome the inherent
I
Manuscript received October 1, 2012; revised December 22, 2012; accepted January 15, 2013. Date of publication January 28, 2013; date of current version June 27, 2013. This work was supported by the National Science Foundation under Award 0824452 and NSF CAREER. The authors are with the Department of Electrical and Computer Engineering, University of Alabama, Tuscaloosa, AL 35487 USA (corresponding author is S. M. Kim, e-mail: seongsin@ eng.ua.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JBHI.2013.2243158
difficulties of water absorption, propagation efficiency, and even the generation and detection devices [22]–[24]. In previous studies, we have investigated undamaged bone tissue, skin tissues, and inorganic objects (such as microchips) to develop a variety of image processing techniques which we will apply in this study [25], [26]. Here, we study the effects of hydrofluoric acid (HF) on the THz-TDS imaging response of bone and cartilage. One of the primary effects of direct exposure to HF is acute hypocalcaemia—or removal of calcium from the tissue—and should, therefore, exhibit significant changes in the THz response of the tissue due to structural changes and traces of acid. We first observe the presence of an HF damaged region on both compact bone and cartilage to identify how the tissue deterioration affects the spectroscopic response, and verify that detection can be obtained. Next, we analyze tissue which, after exposure to HF, has been histologically fixed using formalin to determine if the observability of the damage is diminished. Finally, the effect of acid concentration is investigated using a piece of pure cartilage tissue. With the results from this, a better understanding of the nature and precision of THz-based biomedical imaging and imaging reconstruction is developed. II. TERAHERTZ METHODOLOGY A. THz-TDS Imaging The imaging system used here in this study consists of a conventional Terahertz Time-Domain Spectrometer operating in a transmission regime. The spectrometer has been modified to perform 2-D data acquisition for imaging reconstruction by the incorporation of a three-axis, motion controlled sample mount. A simple schematic of the system is shown in Fig. 1. The THz-TDS system is pumped by a 790 nm ultrafast laser (MIRA 900, Coherent) with a repetition rate of 86 MHz and a pulse width of 120 fs. Detection of the THz is achieved by means of electro-optic sampling using ZnTe crystal details of similar setup were reported elsewhere [25], [27]. From the time-domain signal acquired, frequency data are obtained through the use of a simple fast-Fourier transform (FFT) that gives frequency resolution of 25 GHz, which is sufficiently small for the observation of features and trends in the power spectrum. B. Image Reconstruction The sample is placed such that the radiation is incident on the surface at the focal plane between the two lenses. The THz beam was tightly focused with spot size of 0.6 mm measured using the knife-edge method. 2-D time domain electric field data are
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Fig. 3.
Fig. 1.
Schematic diagram of the THz-TDS used in this study.
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(a) Tissue before burn and (b) tissue after HF exposure.
is determined for each pixel at a select frequency and transferred to the image reconstruction. C. Sample Preparation
Fig. 2.
Methods for time domain image reconstruction.
obtained by moving the sample in plane with the focused THz in a raster pattern and performing full spectroscopic scans at regular intervals, with each scan representing one pixel of the image. Since full spectroscopic data exists at each point acquired, each pixel of image reconstructed is achieved by selecting a specific attribute of either the time-domain waveform, or the frequency spectra resulting from the FFT. Once the desired criterion is selected the associated numerical value is extracted, normalized, and rendered into an image based on either a monochromatic or multicolored contrast scale. Next, the gain of the image is adjusted so that the features of interest may be clearly observed, and finally, bicubic interpolation is applied to the result. In this study, the initial images considered for all samples are constructed from the peak amplitude of time-domain signal intensity for each data point. This reconstruction method indicates the general THz transparency of each pixel and typically yields strong contrasts between different tissue types, allowing for the general shape of the sample to be easily observed. The second important reconstruction in time-domain signal to be considered is the propagation time delay of the radiation observed in each pixel as it propagates through the medium. This method of imaging is complementary to peak amplitude image reconstruction as it reveals the contrast based on the relative time of flight of the peak intensity of the radiation pulse. Below, Fig. 2 describes, graphically, these two methods of image construction. On these graphs, the length of the red arrows is the numeric value associated with each pixel. Image reconstruction in the frequency-domain of this study is primarily focused on the power spectrum versus frequencies obtained from the FFT. The relative amplitude in decibels (dB)
All bone and cartilage samples were taken from the long bone of chickens. Before preparation, the fresh bones were stored frozen at –30◦ Fahrenheit. To begin, a thin cross section was cut from the bulk with an approximate thickness of 1 ± 0.1mm. With the thin cross-section cut, an eyedropper was used to expose a small region of the sample to the hydrofluoric acid solution. For most samples, an HF concentration of 48% was used. In addition to 48% and 12%, 24% HF concentration was also exposed on bone. One cartilage sample was exposed to both 48% and 24% HF in two separate locations to observe the change in damage that results from different HF concentrations. The 24% solution was obtained by mixing 48% HF with equal parts of deionized water. The acid was allowed to sit on the sample for 60 s, after which time the sample was immediately washed with DI water to diminish the spread of the acid. Fixing was performed on samples using a 10% formalin neutral buffer. For these samples, once the tissue had been thoroughly rinsed, compressed nitrogen was used to remove any excess moisture. The bone or cartilage was placed into a 10% formalin neutral buffer for 2 h. After fixation, the sample was dried once again using compressed nitrogen. For imaging, the tissue was mounted vertically between two thin transparent plastic sheets which allow for >95% transmission of the propagating terahertz radiation All measurements were performed with the spectrometer under a nitrogen purge which maintained a constant humidity as low as 21% (compared to 70% without purging) for the duration of the measurement. III. RESULTS AND DISCUSSION A. Presence of Acid Burn The first study in this paper was to identify the presence of an HF burn in a fresh, nonfixed tissue sample which included both bone and cartilage. Fig. 3 shows the optical images taken for the sample before HF exposure [see Fig. 3(a)] and after [see Fig. 3(b)]. The bottom dark color area is where the compact bone is located and above is the cartilage. The region which was exposed to the HF has marked discoloration both on the bone and cartilage regions.
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Fig. 5. Selected pixel data for (a) time domain, (b) frequency domain before and after HF exposure. ‘Reference’ corresponds to the signal from free path propagation.
Fig. 4. THz reconstructed images: (a) before burn-TD peak, (b) before burnTD delay, (c) after HF-TD peak, and (d) after HF-TD delay.
Two separate THz images were taken from the prepared samples using the THz-TDS. The first was performed from fresh tissue and the second was after exposure to 48% HF. It was noted that the thickness of the sample did not change measurably; however, the mass of the sample after HF exposure was reduced by 9.69 mg. Two separate THz images were taken from the prepared samples using the THz-TDS. The first was performed from fresh tissue and the second was after exposure to 48% HF. It was noted that the thickness of the sample did not change measurably; however, the mass of the sample after HF exposure was reduced by 9.69 mg. Fig. 4 shows the false color time-domain image results obtained and their reconstructions. For both images, a pixel pitch of 0.5 mm was used. In the case of the unburned sample, the overall image size was 18 × 18 mm, The HF burned image was slightly larger (20 × 20 mm); however, the additional space primarily consisted of area outside of the sample seen in the lower right region of Fig. 4(c) and (d). Fig. 4(a) and (c) compares the peak amplitude changes in time domain signal between fresh tissue and HF acid burned tissue, and Fig. 4(b) and (d) shows the reconstruction image based on time delay signal for both samples. We observed that the intensity of transmission through the sample is dramatically increased in both the cartilage and bone regions where the acid spot is located. Further analysis of individually extracted pixels shown in Fig. 5 reveals that the regions exposed to HF exhibit an increase of transmission of approximately 2.5 times that of the unburned tissue of the same type. This agrees well with the reduction in the mass of the sample since the lower density allows for better propagation, and follows the expected behavior for the acid exposure. In addition, it appears that the response to HF is different for compact bone and cartilage. Before acid reaction, the transmitted peak intensity is uniform in the area for both cartilage and compact
Fig. 6. Power spectrum images of (a) nonburned, (b) burned tissue at 0.425 THz; and for (c) nonburned, (d) burned tissue at 0.8 THz.
bone. After HF acid reaction, the transmitted THz intensities in the compact bone area is still consistent through the area with increased intensity, while the cartilage area shows more fractured patterns with varied transmission intensities. It is clearly related to structure changes due to the acid, which results in different responses in time and in the type of tissues. This seems to be related to the distinct delay times for cartilage as shown in Fig. 5(a) as well. Cartilage after HF acid shows a response that is not only increased but is also 0.5 ps shifted in the peak time. By reconstructing the propagation delay image, however, contrast is not as clearly developed shown in Fig. 4(b) and (d). Although a reduction in the propagation time is clear in the burned regions, the propagation time for the burned bone is very close to that of the undamaged cartilage, and as a result much of the distinction is lost. Fig. 6 shows the images reconstructed based on the power spectrum obtained from FFT performed on each pixel of the images. Frequency selection was made by first observing the FFT results from selected pixels [see Fig. 5(b)]. From this, we cannot see any distinct change in the spectral shape; however,
BAUGHMAN et al.: OBSERVATION OF HYDROFLUORIC ACID BURNS ON OSSEOUS TISSUES
a broadband increase in power occurs as a result of the HF exposure. At frequencies below 0.5 THz, the HF areas in both bone and cartilage exhibit higher transmitted THz power than the undamaged tissue. The first reconstruction, at 0.425 THz shows a clear contrast development for the damaged areas when compared to the undamaged tissue [see Fig. 6(a) and (c)]. The distinction of HF reacted area in the cartilage is more significant than in compact bone at the selected frequency of 0.425 THz. It is noted that the image contrast is rather similar to the peak intensity images shown in Fig. 4(c). Above 0.5 THz, the burned areas are no longer the most transmissive regions, but the HF burned bone area becomes less transparent than the cartilage (either burned or not). This enables us to distinguish between the bone and the cartilage at the higher frequency. When combining both lower and higher frequency power spectral imaging contrasts, as shown in Fig. 6(b) to (d), we are able to clearly distinguish all four combinations of burned/unburned tissue and cartilage/bone, which were not shown clearly in simple THz peak amplitude imaging. It would be interesting to compare the changes in the actual THz optical properties of the regions probed, such as refractive index and absorption coefficient of osseous tissues before and after HF acid burn. To obtain both optical properties, we use the following two equations [28], [29] T (ω) =
i ( N −1 ) d ω 4N Esam ple (ω) = e C = Aeiϕ . 2 ERe f (ω) (1 + N )
(1)
T(ω) is the complex transmission function defined as the ratio of the THz wave transmitted through the sample inside the holder (sample signal) to the THz wave transmitted through the holder only (reference signal). T(ω) is a function of the complex refractive index of the sample probed, N = n + ik, where “n” is the real refractive index and “k” is the extinction coefficient, which correspond to the sample chromatic dispersion and absorption characteristics, respectively. In equation (1), A is the amplitude ratio of the two waves, ω is angular frequency, d is thickness of the sample, and ϕ is the phase difference between the sample and reference signals. Once the complex refractive index N (ω) is determined, the absorption coefficient α(ω) can be calculated using 4n(ω) 2 . (2) α(ω) = ln d A [n(ω) + 1]2 Fig. 7 shows a comparison of the absorption coefficient obtained for bone and cartilage, before and after HF burns. There is a noticeably distinctive behavior in its frequency dependence between the bone and cartilage tissues. The bone exhibits a higher absorption coefficient in general compared to the cartilage, as it would be expected. In addition, it shows a strong dependence on frequency, with the highest absorption occurring at about 1.1 THz. By contrast, the absorption coefficient for the cartilage exhibits a monotonous increase with frequencies. After HF burn, the absorption coefficient significantly decreases for both types of tissue, by about 30% for the bone and 45% for the cartilage. For the latter case, the absorption becomes very small and less dependent on frequency.
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Fig. 7. Calculated absorption coefficient in THz frequencies for both bone and cartilage. (a) Before HF and (b) After HF acid burns.
Fig. 8.
Formalin fixed tissue with HF burn.
Fig. 9. delay.
Time-domain image reconstruction from (a) peak and (b) propagation
B. Formalin Fixing Effects on Bone and Cartilage In this section, we will discuss the investigation of the effect of formalin fixing tissues on the THz imaging reconstruction. Formalin fixing is the most general method to preserve a tissue when it is excised from the organ. Since it fixes most of the proteins in a tissue, there have been concerns that some information about the tissue would no longer be accessible by THz imaging [15], [30]. In the previous section, the general effect of HF was clearly shown in our study. The next sample, depicted in Fig. 8, consisted of compact bone and cartilage just as the previous samples were, with HF acid burns, and on which formalin fixation was applied before THz imaging. The top part of the sample which is shown in white in Fig. 8 is the cartilage area while the dark part is compact bone area. The area where 48% HF was dropped is circled in the figure as well. Fig. 9 shows the THz images obtained with peak amplitude
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Fig. 10. Selected pixel data for (a) time domain (b) frequency domain from the formalin fixed tissue.
image [see Fig. 9(a)] and the reconstructed images based on the propagation delay [see Fig. 9(b)]. The image of this sample was performed over a 20 × 20mm area, and once again a pixel pitch of 0.5 mm was used. When it is compared to the freshly cut tissue the contrast developed in the time domain amplitude image is not as strong, although the burn is still evident near the top of the image. The extracted time domain spectra for various types of tissue after formalin fixing are shown in Fig. 10(a). The transmission ratio between fixed cartilage and fixed burned cartilage was only ∼1.5, whereas it was larger (2.5) for the sample that was not formalin fixed [see Fig. 5(a)]. This means that a smaller contrast between burned/unburned cartilage is expected for fixed cartilage. This behavior is consistent with the molecule rigidifying characteristics of the fixation process, which renders a tissue more transparent to THz waves since the vibration modes of constituent molecules would be attenuated. As a result, the lower THz transmission contrast between burned/unburned cartilage after fixing is partially due to the fact that both the HF burn and the formalin fixation tend to make the tissue more transparent to THz waves. Furthermore, since the compact bone has a lower density of proteins than the cartilage, we expect a much smaller change in THz transmission ratio, which is what we observe: the ratio between burned/unburned fixed bone and that between burned/unburned unfixed bone were similar. It should be also noted that the time delay difference between the acid burned and unburned areas is very close to what we measured in section A. For example, there is exactly a 0.5 ps delay difference between the HF exposed cartilage and unburned cartilage. This confirms that the acid damages the tissue structure in a way that can be probed in THz time domain signal, but that the time delay difference is not affected whether the tissue was fixed or not. Analysis of the frequency spectra in Fig. 10(b) shows that the spectra for the different regions exhibit much less distinct slopes compared to Fig. 5(b), although the relative intensities are still maintained. From the power spectrum information, we developed image reconstructions at the same target frequencies of 0.425 and 0.8 THz, as shown in Fig. 11. We observe a distinct contrast between the burned/unburned regions at 0.425 THz, but without being able to differentiate between the damaged carti-
Fig. 11. Frequency-domain images construction of power spectrum at (a) 0.425 THz and (b) 0.8 THz.
lage and compact bone areas. However, the undamaged compact bone (see the lower half of Fig. 11) shows different contrast in the middle of the area, which was not seen in the fresh-cut tissue discussed in section A. At 0.8 THz, the burned/unburned contrast within the compact bone fades. In addition, image reconstruction based on the power spectrum shows more structured contrast within the bone when it is fixed which also similar results we obtained is in our previous work [25]. C. HF Concentration To study the effect of HF concentration, a pure cartilage sample was prepared with two burn spots. The first burn spot used the 48% solution standard for our experiments. The second location was exposed to a 24% solution. Both burn locations are visible on the sample due to a slight discoloration. This discoloration is evident both before and after formalin fixing. The thickness of the sample was also remeasured and no appreciable change in the thickness of the cartilage in the burned regions was observed. The image obtained for this sample was 20 × 14 mm, with a pixel pitch of 0.5 mm in both directions. The initial reconstruction in Fig. 12(a) clearly shows both burn locations as an increase in transmission of the THz relative to the unburned portion of the cartilage. From inspection of the pixel spectra, we see that the burn regions transmit 18% and 19.5% of the reference (for 48% and 24%, respectively), while transmission through the undamaged tissue ranged between 9.6% and 10%. A change in propagation delay is also observed in Fig. 12(b). From the results, we see that the delay for the 48% HF is 70% of the unburned delay, and the 24% HF spot yields a delay of 79%. By looking at these in conjunction, it is evident that the higher concentration can be clearly distinguished from the lower concentration region in both modalities. Next, the FFT of the time domain spectra for all pixels in the image was taken. Images were constructed at 0.45 and 0.7 THz from the power spectrum data. These frequencies were selected by first observing the Fourier transform of the individual pixels extracted in the time domain [see Fig. 13(c)], and next scanning the image construction through the frequency range to ensure agreement. At the lower frequency, the burns are clearly distinguishable from the rest of the undamaged tissue but they look approximately the same [Fig. 13(a)]. In the slightly higher frequency range from 0.6 to 0.8 THz, we can see that the higher
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Fig. 14. Time domain THz transmitted signal comparison for 12%, 24%, 48% HF concentration exposure on bone tissue. Bone-24 means non-HF corresponding 24% spot, HF Bone-24 means HF24% exposed.
Fig. 12. (a) TD amplitude image, (b) propagation delay image, and (c) selected TD waveforms from the image data.
Fig. 15. (a) Optical image of prepared sample.THz images reconstructed for comparison of different HF concentration on bone tissue with (b) peak amplitude, and selective frequencies (c) 0.45 THz and (d) 0.8 THz.
Fig. 13. Power spectrum images at two frequencies (a) 0.45 THz, (b) 0.7 THz, and c) frequency power spectra for the selected pixels.
concentration solution has clearly done more damage to the sample, and thus the transmitted power is higher. At 0.7 THz, shown in Fig. 13(b), a clear distinction between the 48% HF and 24% HF burn regions is observed. Beyond this frequency range, there is no distinction between the burned and the undamaged tissue. In addition, we studied the effect of HF concentration on the bone tissue as well. 12% and 24% HF solutions were used at two different spots on a prepared bone tissue with thickness about 1 mm. Fig. 14 compares the time domain THz signals transmitted at these two spots, and also compared with the 48% acid burns discussed in section A. For a more precise comparison, the THz signals before and after HF burns at each concentration were plotted together. As shown in Fig. 14, the peak intensities were
significantly increased after exposure to all three concentrations, with the higher concentration leading to a higher transmission. In addition, changes in the delay time were also noticeable at higher concentration. Fig. 15 shows different THz images reconstructed based on peak amplitude (b) and selective frequencies (c)–(d), with Fig. 15(a) being an optical image of the sample studied. Both HF concentrations affected the tissue structure in a similar way when comparing the peak amplitude images. Since the peak amplitudes increased for both concentrations (12% and 24%) as shown in Fig. 14, the image in Fig. 15(b) clearly distinguishes both acid burned spots from the unburned regions, with the spot associated with the 24% HF exhibiting a better contrast. Fig. 15(c) and (d) are images reconstructed at different frequencies. At 0.45 THz, two spots were revealed with a clear contrast from the surrounding area. However, at 0.8 THz, the 24% HF burn spot lost some of its contrast while the 12% HF burn spot maintained its distinct contrast. IV. CONCLUSION In this study we have applied THz-TDS imaging to the study of osseous tissues which have been damaged due to acute exposure to hydrofluoric acid. Image reconstructions in both the
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time and frequency domains clearly show that the THz transparency is increased in areas exposed to the acid, though the effect appears to be broad spectrum, without clearly identifiable frequency specific results. We further determine that the use of histological fixing with formalin causes a muting of the contrast between damaged and undamaged tissue, which must be taken into account in further studies. From these results, it is clear that the use of THz imaging can clearly reveal very specific changes in tissue composition.
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William E. Baughman (S’09) was born in Florence, AL, USA. He received the B.Sc. degree in electrical engineering from the University of Alabama, Tuscaloosa, USA, in 2010. He is currently working toward the Ph.D. degree in the Department of Electrical and Computer Engineering at the University of Alabama. His research interests include terahertz spectroscopy, biomedical imaging, and optical system design.
Hamdullah Yokus was born in Diyarbakir, Turkey, in 1987. He received the B.S. degree in electrical and electronics engineering from the University of Erciyes, Kayseri, Turkey, in 2009. He is currently working toward the Master’s degree in the Department of Electrical and Computer Engineering, University of Alabama, Tuscaloosa, USA, in the field of Terahertz Spectroscopic Imaging.
Soner Balci received the B.Sc. degree in electrical and electronics engineering from Dokuz Eylul University, Izmir, Turkey, in 2007, and the M.Sc. degree in electrical and computer engineering from the University of Alabama, Tuscaloosa, USA in 2012, where he is currently working toward the Ph.D. degree. His research interests include characteristics of carrier dynamics in semiconductor thin films and nanowires in terahertz frequency region and terahertz imaging.
BAUGHMAN et al.: OBSERVATION OF HYDROFLUORIC ACID BURNS ON OSSEOUS TISSUES
David Shawn Wilbert (S’06) received the B.S. and M.S. degrees in electrical engineering from the University of Alabama, Tuscaloosa, USA, in 2009 and 2010, respectively, where he is currently working toward the Ph.D.degree. His research interests include spectroscopic imaging techniques, THz generation and detection technologies, metamaterial devices, and nanoscale fabrication technology.
Patrick Kung (M’08) received the Diplˆome ´ d’Ing´enieur from the Ecole Polytechnique, Palaiseau, France, in 1993, and the Ph.D. degree in electrical engineering from Northwestern University, Evanston, IL, USA, in 2000. From 2000 to 2007, he was a Research Assistant Professor in the Department of Electrical Engineering and Computer Science, Northwestern University. Since 2007, he has been an Assistant Professor in the Department of Electrical and Computer Engineering, the University of Alabama, Tuscaloosa, USA, and an Adjunct Professor in the Department of Electrical Engineering and Computer Science, Northwestern University. His research interests includes been involved in the area of compound semiconductors and devices, advanced material characterizations, including atom probe tomography, and radiation hardened electronics. He has authored 62 publications. Dr. Kung is a member of APS, AVS, SPIE, and IFES.
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Seongsin Margaret Kim (M’04) received the B.S. degree in physics from Yonsei University, Seoul, Korea, in 1992, and the M.S. degree in physics and the Ph.D. degree in electrical and computer engineering from Northwestern University, Evanston, IL, USA, in 1994 and 1999. Since 2007, she has been an Assistant Professor in the Department of Electrical and Computer Engineering, the University of Alabama, Tuscaloosa, AL, USA. Prior to joining the University of Alabama, she was a Research Associate at Stanford University after having worked at both Samsung (Korea) and Agilent Technologies (San Jose, CA, USA). Her research interests include area of semiconductor quantum dots, nanowires, optoelectronics, and terahertz science and technologies, including generation, detection, spectroscopy, biomedical imaging, near field imaging, and THz metamaterials. He has more than 85 peer-reviewed publications including 43 journal articles, 5 book chapters, and holds one US patent. Prof. Kim has also received NSF CAREER award in 2010, and is a member of SPIE, SWE.
