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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38, NO. 4. APRIL 1991

Optical Determination of Dental Pulp Vitality Joseph M. Schmitt, Richard L Webber, and Elijah C. Walker

Abstract-In this preliminary study, we explored the feasibility of employing photoplethysmography and pulse oximetry to assess the status of the blood circulation in the dental pulp. A simple photometer that measures diffuse light transmission at 575 nm was built to record tooth plethysmograms, and the ability to distinguish vital from surgically devitalized teeth of a dog using plethysmography was demonstrated. As an extension of the photoplethysmographic technique, redinfrared pulse oximetry applied to the measurement of the oxygen saturation (SO,) of blood in the pulp was also examined using an in vitro test setup. Results suggest that the measurement of relative SO, changes is feasible, but standard dual-wavelength pulse oximetry does not enable determination of SO, independent of tooth geometry and sensor placement.

I. INTRODUCTION

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LINICAL DETERMINATION of the pathological state of the dental pulp is important for routine diagnosis of inflammation and necrosis, and for assessment of restored circulation following oral surgery [ 11. To determine the viability of a tooth, clinicians now rely on the radiographic exam and subjective tests that may induce pain. Therefore, improved diagnostic techniques are needed and may also serve to motivate the development of therapies aimed at preserving tooth vitality. Optical diagnostic methods are particularly attractive because of their potential for noninvasive assessment of parameters related to pulpal blood flow and oxygenation, which are primary determinants of the status of the pulp circulation. Several attempts to employ light to detect caries [ 2 ] and to determine dental pulp vitality have been reported [3], 141. This investigation was undertaken to develop methods for assessing pulp vitality using photoplethysmography and pulse oximetry. In this paper, practical issues concerning the design of an optical vitalometer capable or reliably detecting the tooth pulp plethysmogram are discussed along with results of in vivo experiments. Using an in vitro model, we also examine the potential for extending the plethysmographic methods to measure the oxygen saturation of blood in the tooth pulp by pulse oximetry.

11. TOOTHPLETHYSMOGRAPHY The tooth plethysmogram (PPG), which was found in earlier studies to reflect blood pressure variations in the pulp tissue 131, [ 5 ] , can potentially provide an absolute indication of pulp vitality. Detection of optical density fluctuations that are synchronous with systolic contractions of the heart provides strong evidence of pulp tissue perfusion and hence its vitality. Although the tooth PPG is most suitable for providing a gross Manuscript received Januaty 30, 1989; revised January 25. 1990. J. M. Schmitt and E. C. Walker are with the National Institutes of Health, Division of Research Services, Biomedical Engineering and Instrumentation Branch, Bethesda, MD 20892. R. L. Webber is with the National Institutes of Health, National Institute of Dental Research, Diagnostic Systems Branch, Bethesda, MD 20892. IEEE Log Number 9144241.

indication of vitality, the shape of the PPG can perhaps also provide information about abnormalities in the compliance of the pulp vasculature. The purpose of these experiments was to investigate 1) the influence of tooth size, source/detector position, wavelength, and signal averaging on the detectability of tooth PPG’s and 2) the ability of a prototype instrument to distinguish vital from intentionally devitalized teeth in an animal model. A . Methods Instrumentation: The photometer used in this study is depicted in Fig. l . A stabilized tungsten-halogen lamp (Newport Research Corporation, Model 780) provided broadband light for illumination via a bundle of glass fibers. The proximal end of the detector fiber was bifurcated to enable simultaneous measurement of the intensity at two different wavelengths. Light from each branch of the fiber passed through a collimator/expander and narrow-band interference filter (10 nm passband) before impinging on a Si photodiode/preamplifier module (EG&G Corporation, Model HUV4000B). The transresistance of the preamplifier was set at 20 MQ. T o derive signals proportional to the ac component of the light intensity at each wavelength, the voltage output of the preamplifier was further amplified and filtered. Unless otherwise specified, the lower and upper cutoff frequencies of the bandpass filter were set at 0.5 and 6.0 Hz, respectively, with a passband gain of 100. Effect of Source-Detector Position: In these experiments. tissue phantoms and extracted teeth were employed to determine whether the positions of the source and detector apertures on the surface of a tooth can be chosen to enhance sensitivity to the tooth PPG. Cylinders of translucent acetal plastic (Delrin’”) containing a cavity filled with mixtures of milk and purified hemoglobin (IL282 level 2 calibration standard, Instrumentation Laboratories) were used to simulate optical scattering by intact teeth. Delrin, like the calcified tissues of the tooth, scatters light intensely and has a low absorptivity in the visible region. The dimensions of the cylinders were made to approximate those of a premolar or molar of an adult man. The scattered intensity patterns measured on the surfaces of the cylinders and the extracted molars were found to be similar, except in regions where demineralization or proliferation of dentin caused abnormally high transparency or opacity. To simulate the small increase in the optical density of the tooth that underlies the tooth PPG, measurements were first performed with a cylinder filled with milk only and then repeated with the same cylinder filled with a 10% hemoglobin-milk mixture. With the source fiber (1.5 mm diameter) fixed on the side of the cylinder 5 mm from the top, we measured the light intensity at 660 nm captured by an identical fiber placed at different points on the cylinder. Percent decrease in intensity resulting from the addition of hemoglobin was calculated based on the average of three sets of measurements made at each point.

U.S. Government work not protected by U.S. Copyright

SCHMITT e/ al.: OPTICAL DETERMINATION OF DENTAL PULP

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Fig. 1. Block diagram of the photometer with which the experiments in this study were performed.

Tests on Human Teeth: Plethysmograms were obtained from teeth of human subjects using the sensor shown in Fig. 2. The sensor was positioned so that the tip of the receiving fiber (3 mm diameter) and the surface of the prism were firmly pressed against opposite surfaces of the tooth under measurement. To reduce spurious optical changes attributed to specular reflection and motjon artifact, the tip of the source fiber and the flat surface of the prism were coated with a clear aqueous gel. The fiber tip was centered between the top of the tooth and the gingival margin on the buccal side of the tooth. In some cases, to improve detection of the tooth PPG’s and to reduce motion artifact, beat-to-beat averaging of the ac intensity signals was performed by simultaneously sampling the subject’s electrocardiogram (lead 11) and the ac intensity signal from the photometer. Synchronous averaging of the samples taken during successive R-R intervals was performed off-line. Wavelength Dependence: The ratios of the amplitudes of the pulsatile (ac) and nonvarying (dc) intensities transmitted through teeth of a human subject were measured at discrete wavelengths between 550 and 800 nm to determine the wavelength dependence of the pulsatile variations comprising the tooth PPG. Animal Experiment: To establish whether the pulsatile component of the tooth PPG is completely eliminated when a tooth is devitalized, a controlled study was carried out in which plethysmograms were recorded from teeth of a dog before and after devitalization [6]. A young beagle (scheduled to be euthanized as part of another medical protocol) was anesthetized by injection of phenobarbitol and its ECG was continuously monitored. Tooth PPG’s were measured at 595 nm using the photometer and sensor described earlier. A tooth (canine, premolar, or molar) was selected and, while the sensor was manually held on the tooth, the ECG and PPG were recorded. After the selected tooth was surgically devitalized by transection of arteries supplying blood to the tooth, recording of the PPG was again attempted. Blood circulation to surrounding gum tissue and other teeth was left intact. This procedure was repeated for each of seven teeth located in both the upper and lower jaw of the dog. In one of the dog’s canine teeth, the PPG could not be detected. Because the viability of this tooth could not be ascertained, surgery was not performed on this tooth.

B. Results and Discussion Fig. 3 shows the fractional decrease in intensity measured at different positions on the surface of a tooth model that resulted from the-addition of an absorber to the milk in its core. With

341

the detector placed at or below the column of milk-hemoglobin in the core, sensitivity to an absorbance increase was highest at angles close to 180”; in contrast, with the detector placed nearer to the rim, sensitivity was lowest in the front plane. In all experiments, sensitivity to core absorbance changes varied by less than a factor of two for source-detector angles between 45 and 180°, suggesting that the placement of the detector relative to the source does not have a strong effect on sensitivity. In practice, a large aperture detector is required to capture enough light to achieve an acceptable signal-to-noise ratio. A sensor consisting of a source fiber with a small numerical aperture and a detector fiber with a large numerical aperture placed on opposite sides of a tooth appears to be a suitable arrangement for detecting optical density changes in the pulps of a wide variety of teeth. Our observations indicate that fine adjustment of sourcedetector position to maximize tooth PPG amplitude is not warranted. An example of a typical plethysmogram obtained from a molar (presumed vital) of a 29-year-old male (one of the authors) is shown in Fig. 4. The PPG is characterized by a pulsatile decrease in transmitted intensity during systole. Cardiac rhythm is clearly recognizable. With the lower cutoff of the bandpass filter set at 0.5 Hz, only a slight baseline drift synchronous with respiration was evident; however, with the cutoff frequency set lower than the subject’s respiration rate, respiration-induced shifts in the baseline were substantial. Discernible pulsatile activity could be obtained from all of the vital teeth of this subject using only the handheld sensor without special fixation devices but, as in finger plethysmography, the recordings of tooth PPG’s were disturbed easily by sharp movements of the sensor. Using a simple photometer and handheld sensor, PPG’s could be readily obtained from most teeth of several individuals that we tested, but averaging was required to positively identify PPG’s in some teeth. The result of averaging a typical tooth PPG over ten beats is shown in Fig. 5. Because motion artifacts and other types of noise evident in the individual PPG records shown in the figure are not correlated with the occurrence of the R-wave, these tend to cancel. Fig. 6 is a composite graph illustrating the effects of wavelength and tooth size on the amplitude of the pulsatile component of the tooth PPG. The ordinate of the graph is the ratio of the amplitudes of the pulsatile (ac) and nonvarying (dc) intensities transmitted through a tooth, which can be regarded as a measure of the signal strength of the ac component of the PPG. Results show that the ac-dc ratio is largest for teeth whose pulp chambers occupy the largest fractions of their volumes, such as small incisors. For example, at 660 nm, the ratio measured with the sensor placed on a lower central incisor was about three times greater than that measured with the sensor placed on an upper bicuspid. Ac-dc ratios measured at 660 nm in this study, which covered a wide range of tooth sizes and types, ranged from about 0.02-0.5%, compared to a range of 0.05-1.0% measured in an earlier study of skin plethysmography [7]. For the wavelengths shown, the ac-dc ratio is greatest at 575 nm where the sum of the absorption and scattering coefficients of oxygenated whole blood is largest and decreases the longer wavelengths. Scattering apparently plays a larger role than absorption in producing the pulsatile component of the tooth PPG because the ac-dc ratios measured at 575 nm were at most three times greater than those measure at 635 nm, despite almost an order of magnitude difference between the specific absorption coefficients of hemoglobin at these two wavelengths [8]. These results indicate that the optimal wavelength band in the visible

348

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38, NO. 4. APRIL 1991

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spectrum for recording of tooth PPG's is 540-580 nm, provided that the photoelectric conversion efficiency of the measurement system is sufficiently high in this range. The results of the surgical devitalization experiment confirmed that the PPG's recorded from vital teeth are related to blood flow in the pulp, not to blood flow in gingival tissues or to cardioballistic disturbances. Before surgical devitalization, dental pulp PPG's from seven of eight teeth of the dog (a priori assumed to be vital) were detectable. After devitalization, all

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evidence of pulsations in the seven teeth was eliminated. The tracing in Fig. 7 shows the effect of devitalization of a canine tooth. The record following devitalization is not completely flat

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blood through the tooth is depicted in Fig. 8. During normal operation, the bag and tubing were completely filled with blood, the valve at the end of one of the tubes was closed, and pressure Tooth pulse variations generated by an electronic blood pressure calibrator _L were conducted through the other tube to the bag. In this man50 mv ner, the amplitude and rate of the volume changes of the blood-l-filled bag could be controlled by the electrical waveform at the input of the calibrator. In all experiments, the amplitude of the R-wave 1 Hz sinusoidal waveform supplied to the calibrator was adFig. 7. Photoplethysmogram recorded before and after surgical devital- justed to yield ac-to-dc intensity ratios between 0.5 and 2 % . ization of a canine tooth in a dog. Waveforms of intensity signals (labeled Eat,, and Edc,2in Fig. 8) produced by the tooth simulator closely followed the sinubecause small movements of the sensor occurred during the soidal variations of the electrical signal. Human blood was reconstituted from packed cells and sammeasurement. ples having different oxygen saturations were prepared using a Fisher/IL 237 tonometer. The hematocrit of the blood, determined by measuring the packed-cell fraction in capillary-tube 111. PULSEOXIMETRY samples after centrifugation, was adjusted to 42% by adding As an extension of plethysmography, pulse oximetry can per- isotonic saline. The hemoglobin oxygen saturation of the haps allow diagnosis of pulpitis or partial necrosis in teeth that tonometered blood was measured by a spectrophotometric are still vital. Although the correlation between the oxygen sat- oximeter (IL 282, Instrumentation Laboratories). During the uration of blood perfusing the dental pulp and the pathological course of an experiment, samples with oxygen saturations rangcondition of the pulp as determined by histological analysis has ing from 45 to 100%were injected into the simulator. Photomnot yet been established, it is likely that conditions such as pul- eter voltages proportional to the diffusely transmitted intensities pitis would alter SO2 of blood perfusing the tooth. Both in- at 660 and 800 nm were digitized at a 50 Hz rate for a period creased acidity and metabolic activity associated with inflam- of 10 s, and the average values of the peak-to-peak (Eac)and mation could contribute to deoxygenation of hemoglobin in the steady-state (Edc)voltages representing the ac and dc intensities affected areas of the pulp. Because pressure variations in ves- were computed off-line. The log-normalized ac voltages measels on the arterial side of the vascular network in the pulp can sured at both wavelengths were then calculated, and oxygen be transmitted to the those on the venous side through shunts saturation was estimated according to that are normally present at the base of the tooth [9], both venous and arterial oxygen saturation would probably affect SO2 In 660 nm values measured by a pulse oximeter applied to the intact tooth. These experiments were undertaken to identify the technical SO, = A - B issues involved in measuring oxygen saturation (SO,) of blood In 800 nm in the dental pulp by pulse oximetry. Decreasing htensityl

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A . Methods A model of a tooth with a pulsating dental pulp was prepared

by removing the pulp from an extracted molar and inserting a thin-walled silicone rubber bag connected to two tubes passing through the apical foramen. The apparatus used to circulate

where A and B are constants that depend on wavelength, hematocrit, and other factors [ lo]. The applicability of this simple relationship, which was developed for transcutaneous applications of oximetry, to the measurement of the SO2 of the dental pulp was evaluated based on the simulator results.

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38, NO. 4. APRIL 1991

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Fig. 9 . Plot of the In (Eac + &,/&)660/h (Eac+ &/&),OO ratio measured as a function of the oxygen saturation of the blood in the “pulp” of the tooth model. The upper and lower curves are fitted to data obtained from experiments conducted using different size distensible bags in the model to simulate different pulp volumes.

B. Results and Discussion Results of one series of experiments in which the log-normalized ratio was measured as a function of percentage oxygen saturation are shown in Fig. 9. The upper curve is fitted to data obtained from two experiments performed using a model of a tooth containing a large pulp (3.5 X 5.0 mm cylindrical bag in an 8 mm diameter molar) and the lower curve is fitted to data obtained from an experiment performed using a model of a tooth containing a smaller pulp (2.0 mm diam x 4 mm cylindrical bag in an 8 mm diameter molar). Although considerable scatter is evident, the correlation between the measured In (Eac Edc/Edc)66onm/ln (Eat + E d c / E d c ) 8 0 0 n m ratio and SO2 is quite strong. The relationship was found to be nonlinear, especially in the large “pulp” case; sensitivity to a change in oxygen saturation was lowest in the 80-90 % range and highest below 70 %.

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Overall sensitivity was lower than that measured in earlier studies of pulse oximetry: for SO2 values between 60 and loo%, ratios measured in this study decrease by only a factor of 1.3-1.4, compared to a factor of four measured in a previous investigation of transcutaneous pulse oximetry [ 111. This poor sensitivity and dependence on blood volume noted above can be understood by realizing that the dc component of the intensity captured by the detector is not only comprised of the nonvarying part attenuated by the static blood in the simulated pulp, but also of an additive part resulting from light transmission through the simulated tooth crown only. The effects of this “shunted” light intensity are not completely cancelled by ratiometric normalization. Results of computer simulations that we performed using a photon-diffusion model support this explanation. In this model, the tooth was treated as a two-layer structure, the first layer

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consisting of enamel only, and the second layer consisting of fixed fractions of enamel and blood-perfused tissue. The bloodperfused tissue was treated as a homogeneous mixture of whole blood (10% ) and soft tissue (assumed to have the same absorption and scattering coefficients as those of dermis). Optical density changes induced by blood flow alterations in the tooth during the cardiac cycle were simulated by variations in the blood volume content of the tissue. Assuming cylindrical symmetry along the source and detector apertures (3 mm diam), an analytical solution to the two-layer photon-diffusion theory was obtained using techniques similar to those developed by Takatani

[W. As shown by the model results in Fig. 10, the SO, calibration curve is affected by pulp size. For the case in which the pulp tissue occupies only a small fraction of the tooth volume, only a small portion of the source flux undergoes scattering and absorption by whole blood in the pulp before reaching the detector, and sensitivity of the ratio to a change in SO, is low. Under these conditions, the slope and offset of the calibration curve is sensitive to source-detector placement and pulp size. The difference between the slope and offset of the two curves in Fig. 9 is consistent with model predictions. It can be concluded that, because the pulp is small and embedded in a shell of bloodless tissue, an absolute measure of oxygen saturation of blood in the dental pulp cannot be obtained using standard pulse oximetry techniques. The origin of the pulsatile changes in optical density observed by plethysmography is an unresolved question that pertains to the validity of the results of the in vitro pulse oximetry studies. In the derivation of the standard ratiometric method, an expansion of vessels in the dermis filled with arterial blood is assumed to occur during systole. Similarly, in our experiments, the pulsations in the tooth were simulated by volume changes of a distensible blood-filled bag. In the intact tooth, however, rapid changes in the volume of blood are believed to be limited or nonexistent [3], [13], although this has not been established unequivocally. Nonetheless, internal changes leading to redistribution or reorientation of red blood cells are possible, provided that the total volume remains unchanged. For example, during systole, red cells could be pushed from the core of the pulp toward the pulp-dentin boundary, causing greater absorption of light in the outer layers of the pulp. Further studies are

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required to determine whether the theory of pulse oximetry is applicable to the measurement of hemoglobin oxygen saturation in the intact tooth, even though the mechanism that underlies changes in optical density may differ from that in other tissues.

IV. CONCLUSIONS Optical plethysmography shows promise as a diagnostic method capable of satisfying most clinical requirements for the assessment of dental pulp vitality, including objectivity, noninvasiveness, and low cost. Reliable detection of the PPG in teeth of various shapes and sizes is the major challenge in the development of this method. For routine clinical applications, it may be practical to construct a compact, low cost instrument based on a microelectronic sensor containing LED sources. The results of in vitro experiments reported in this paper suggest that measurement of relative changes in the SO, of blood in the pulp by pulse oximetry may be feasible, but standard redIR pulse oximetry when applied to tooth is expected to exhibit poor sensitivity and to depend on tooth structure. New techniques, perhaps utilizing multiple wavelengths, will be required to improve sensitivity and to reduce dependence on tooth geometry. Besides these measurement-related problems, the fundamental question of how various types of pulp disease affect oxygen saturation as measured by pulse oximetry must be addressed in future studies.

ACKNOWLEDGMENT We wish to thank F. Plowman and H. Tipton for the design and construction of several of the optical components used in these studies. We also appreciate the assistance of J. Bacher and M. Tsuchimochi who performed the animal surgery.

REFERENCES [l] G. Chambers, “The role and methods of pulp testing in oral diagnosis: A review,” Oral Surg., vol. 15, pp. 1-5, 1982. [2] B. Angmer-Mansson and J . J. Ten Bosch, “Optical methods for the detection and quantification of caries,” Adv. Dent. Res., vol. 1, pp. 14-20, 1987. [3] I. Shoher, Y. Mahler, and S. Samueloff, “Dental pulp photoplethysmography in human beings,” Oral Surg., vol. 36, no. 6 , pp. 915-1119, 1973.

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38, NO. 4. APRIL 1991

[4] R. Webber and M. Maxwell, “Colorimetric determination of tooth vitality,” (abstract) J . Dent. Res., vol. 65, p. 240, 1986. [5] G. Beer, H . Negari, and S. Samueloff, “Feline dental pulp photoplethysmography during stimulation of vasomotor nerve supply,’’ Arch. Oral B i d . , vol. 19, pp. 81-86, 1974. [6] R. Webber, J. Schmitt, J. Bacher, M. Tsuchimochi, and U. Ruttimann, “Photoplethysmographic determination of pulp vitality in a beagle dog,” presented at the Amer. Assoc. Dent. Res. Conf., San Francisco, CA, Mar. 15-19, 1989. Y. Mendelson and B. D. Ochs, “Noninvasive pulse oximetry utilizing skin reflectance photoplethysmography,” IEEE Trans. Biomed. Eng., vol. 35, pp. 798-805, Oct. 1988. E. Gordy and D. L. Drabkin, “Determination of the oxygen saturation of blood by a simplified technique applicable to standard equipment,” J. Biol. Chem., vol. 227, pp. 285-299, 1957. M. G. Path and M. W. Meyer, “Heterogeneity of blood flow in the canine tooth in the dog,” Arch. Oral Biol., vol. 25, pp. 8386, 1980. I. Yoshiya, Y. Shimada, and K. Tanaka, “Spectrophotometric monitoring of arterial oxygen saturation in the fingertip,” Med. Biol. Comput., vol. 18, pp. 27-32, 1980. Y . Mendelson, J. C. Kent, B. L. Yocum, and M. J. Birle, “Design and evaluation of a new reflectance pulse oximeter sensor,’’ Med. Instrument., vol. 22, no. 4, pp. 167-173, 1988. S. Takatani and M. D. Graham, “Theoretical analysis of diffuse reflectance from a two-layer tissue model,” IEEE Trans. Biomed. Eng., vol. BME-26, Dec. 1979. A. C. Brown and D. Yankowitz, “Tooth pulp tissue pressure and hydraulic permeability,” Circ. R e s . , vol. 15, pp. 42-50, 1964.

Joseph M. Schmitt received the B.S. degree in biomedical engineering from Case Western Reserve University, Cleveland, OH, in 1981, and the M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, in 1983 and 1986, respectively. He is currently a Staff Fellow at the National Institutes of Health in Bethesda, MD, where he is a member of the Lasers and Modern Optics Group which is investigating new optical techniques for biomedical and clinical research ap-

plications. His current research interests include multiple-scattering theory, fluorescence and polarization spectroscopy, and neural networks for image processing.

Richard L. Webber received the undergraduate degree in physics from Albion College, Albion, MI, in 1958 and the D.D.S. degree from the University of Michigan, Ann Arbor, in 1963, and the Ph.D. in physiological optis from the University of California, Berkeley, where he also held an academic appointment in 1971. After a USPHS general internship he served as a clinical investigator with the Division of Dental Health at the Dental Health Center, San Francisco. He was affiliated with the intramural research program at the National Institute of Dental Research, NIH, where he ultimately became Chief of the Diagnostic Systems Branch, a post he held until 1988. At that time he retired from the Commissioned Corps and accepted a new position as Chairman of the Department of Diagnostic Sciences at the School of Dentistry, University of Alabama at Birmingham. In 1990 he moved to the Bowman Gray School of Medicine, Wake Forest University, where he holds a joint professional appointment in the Departments of Dentistry and Radiology.

Elijah C. Walker was born in Brooklyn, NY, in 1948. He received the B.S. degree in mechanical engineering from Howard University, Washington, D.C. in 1971. and the M.S. degree in 1982 from The Johns Hopkins University School of Medicine, Baltimore, MA. He has published more than 20 papers and abstracts in areas of medical instrumentation, noninvasive blood pressure monitonng and sickle cell anemia research. He has received four patents on medical devices as well as the Health and Human Services Special Achievement Award. He is currently Chief of Applied Clinical Engineering and Acting Chief of Clinical Care Instrumentation at the National Institutes of Health in Bethesda Maryland.

Optical determination of dental pulp vitality.

In this preliminary study, we explored the feasibility of employing photoplethysmography and pulse oximetry to assess the status of the blood circulat...
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