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Monitoring temporal development and healingof diabetic foot ulceration using hyperspectral imaging

Dmitry Yudovsky1, Aksone Nouvong2, Kevin Schomacker3, and Laurent Pilon*; 1

1 University of California, Los Angeles, Henri Samueli School of Engineering and Applied Science, Biomedical Inter-Department Program,Los Angeles, CA 90095-1597, USA

2 Department of Surgery 2B156 UCLA/Olive View Medical Center Sylmar, CA 91342-1495, USA also at Western University ofHealth Sciences

3 HyperMed Inc., Burlington, MA 01803, USA

Received 13 December 2010, revised 25 January 2011, accepted 3 March 2011Published online 1 April 2011

Key words: hyperspectral imaging, diabetic foot ulcer, tissue oximetry, medical screening technology, wound care, spectro-scopy.

1. Introduction

Diabetes mellitus affected 194 million people world-wide in 2004 [1] and is expected to increase in preva-

lence to 439 million by the year 2030 [2]. Foot ul-ceration is a major complication of diabetes mellitusand occurs in as many as 15–25% of patients withtype 1 and 2 diabetes mellitus over their lifetime [3,

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This study combines non-invasive hyperspectral imagingwith an experimentally validated skin optical model andinverse algorithm to monitor diabetic feet of two repre-sentative patients. It aims to observe temporal changesin local epidermal thickness and oxyhemoglobin concen-tration and to gain insight into the progression of footulcer formation and healing. Foot ulceration is a debili-tating comorbidity of diabetes that may result in loss ofmobility and amputation. Inflammation and necrosispreempt ulceration and can result in changes in the skinprior to ulceration and during ulcer healing that affectoxygen delivery and consumption. Previous studies esti-mated oxyhemoglobin and deoxyhemoglobin concentra-tions around pre-ulcerative and ulcer sites on the dia-betic foot using commercially available hyperspectralimaging systems. These measurements were successfullyused to detect tissue at risk of ulceration and predict thehealing potential of ulcers. The present study shows epi-dermal thickening and decrease in oxyhemoglobin con-centration can also be detected prior to ulceration atpre-ulcerative sites. The algorithm was also able to ob-serve reduction in the epidermal thickness combinedwith an increase in oxyhemoglobin concentration around

the ulcer as it healed and closed. This methodology canbe used for early prediction of diabetic foot ulcerationin a clinical setting.

Feb. 12, 2008 March 7, 2008 May 27, 2008

Epidermal thickness

Oxygen saturation

Healing process of a diabetic foot ulcer

* Corresponding author: e-mail: [email protected], Phone: +1 (310)-206-5598, Fax: +1 (310)-206-4830

J. Biophotonics 4, No. 7–8, 565–576 (2011) / DOI 10.1002/jbio.201000117

4]. The cost of foot disorder diagnosis and manage-ment is estimated to be over 6 billion dollars an-nually in the United States [5, 6]. If left untreated,foot ulcers may become infected and develop deeptissue necrosis which may require amputation [7]. Infact, a foot ulcer precedes approximately 85% of alllower extremity amputations in patients with dia-betes mellitus [7] and more than 88,000 lower limbamputations are performed annually on diabetic pa-tients in the United States [8]. Hyperspectral im-aging of the diabetic foot has been demonstrated tomonitor vascular changes associated with diabeticneuropathy [9], ulcer healing [10], and ulcer forma-tion [11]. This study explores the use of hyperspec-tral imaging and associated algorithms to monitorthe temporal evolution of foot ulceration sites and togain insight into the progression of foot ulcer forma-tion and healing. Two patients for whom hyperspec-tral images were collected before, during and afterfoot ulceration are studied in details with particularattention paid to changes in epidermal thickness.

2. Background

Prolonged and poorly controlled diabetes irrepar-ably damages bodily tissues. Nerve damage in thelower limbs results in diabetic neuropathy, wherebythe patient’s somatosensory and autonomic functionsare diminished or completely lost [12, 13]. The sub-sequent loss of protective sensation, poor gait con-trol, bone deformities, callus formation and inhibitedsweat response result in excessive shear and pressurethat damages the diabetic foot [14, 15]. Furthermore,10 to 40% of diabetic patients are afflicted with pe-ripheral vascular disease [16]. Typically, the vesselsthat carry blood to the legs, arms, stomach or kid-neys narrow due to inflammation or tissue damageresulting in impaired blood flow to those regions[16]. Thus, repeated damage to the foot in conjunc-tion with inhibited protective or healing responsedue to denervation or poor vascularization cause ul-ceration [13, 14, 16].

Recently, hyperspectral imaging in the visible andnear-infrared parts of the spectrum has been used todetermine the spatial distribution of oxygen satura-tion in human skin [17–19] and to detect the circu-latory changes in the diabetic foot [10, 20, 21]. Forexample, Greenman et al. [9] used hyperspectralimaging to generate oxygen saturation and total he-moglobin concentration maps of the forearms andfeet of type 1 and 2 diabetic subjects with and with-out diabetic neuropathy and of non-diabetic subjectswithout neurological disorders. The authors showedthat the resting oxygen saturation averaged over thelimb was lower among diabetic subjects with neuro-pathy than among both diabetic subjects without

neuropathy and non-diabetic subjects. This suggeststhat diabetic neuropathy, which is a major contribu-tor to ulceration [14, 15], could be detected by hy-perspectral tissue oximetry.

In a different study, Khaodhiar et al. [20] meas-ured in vivo oxyhemoglobin and deoxyhemoglobinconcentrations using hyperspectral imaging near ul-cer sites on the feet of type 1 diabetic patients. Hy-perspectral imaging of the feet between 500 and650 nm was performed at each visit for up to 4 timesover a six month period. Then, the authors devel-oped an ulcer healing prediction index based on theoxyhemoglobin and deoxyhemoglobin concentra-tions near the ulcer site which could distinguish ul-cers that would heal from non-healing ulcers with asensitivity and specificity of 93 and 86%, respec-tively.

Nouvong et al. [10] evaluated hyperspectral im-aging as a tool for predicting the healing potential ofdiabetic foot ulcers on the feet of type 1 and 2 dia-betic patients. Hyperspectral tissue oximetry meas-urements from feet of 66 diabetic patients were per-formed every two weeks for up to 18 months or untilthe ulcer healed. By measuring elevation and reduc-tion in the average oxyhemoglobin concentration im-mediately around the ulcer area compared with ad-jacent tissue, the authors were able to predict thehealing outcome of the formed ulcer with a sensitiv-ity and specificity of 80 and 74%, respectively [10].

Yudovsky et al. [11] developed a predictive algo-rithm to automatically detect sites on the diabeticfoot at risk of developing ulcers before visible signsof ulceration could be detected with the naked eye.This was achieved by retrospectively analyzing hy-perspectral data collected by Nouvong et al. [10]58 days, in average, before patients developed footulcers. An image processing algorithm based on thisanalysis was developed to automatically detect tissuethat would ulcerate from tissue oximetry informa-tion. This algorithm was able to detect at-risk tissuewith a sensitivity and specificity of 95 and 80%, re-spectively.

The studies previously described were limited toanalysis of changes in oxyhemoglobin and deoxyhe-moglobin concentrations and thus change in the cir-culatory function. Recently, Yudovsky and Pilon[22–24] developed a forward [22] and inverse meth-od [23] to simultaneously determine, from diffuse re-flectance spectra, (i) the melanin concentration and(ii) thickness of the epidermis, (iii) the volume frac-tion and oxygen saturation of blood in the dermis,and (iv) the scattering coefficient of skin [23, 24].Their algorithm was validated on in vivo reflectancemeasurements from white and black subjects [24]. Inthe present study, changes in epidermal thickness onthe diabetic foot were explored. Epidermal thicknesscan be measured reliably with punch biopsy wherebya sample of the skin is removed and analyzed ex

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vivo [25, 26]. However, this invasive and destructivetechnique requires local anesthetic and should notbe performed on diabetic feet to avoid the risk ofcausing an ulcer. Alternatively, non-invasive meas-urements of epidermal thickness can be made withtechniques such as optical coherent tomography orultrasound [27, 28]. However, these techniques areprimarily sensitive to the tissue’s scattering coeffi-cient therefore simultaneous determination of chro-mophore concentrations is difficult if not impossible[29, 30]. Additionally, they do not provide a largeenough field of view of the foot.

Finally, unlike previous studies which consideredhyperspectral data collected during a single visit, thisstudy investigates the sequences of hyperspectralimages gathered before ulcer formation and duringulcer healing. In addition, hyperspectral data wereanalyzed with the method developed by Yudovskyand Pilon [23] to detect hyperkeratosis that preemptulceration and monitor the evolution of the callus asthe ulcer heals. Indeed, change in epidermal thick-ness at sites forming ulcers is of particular interestbecause (i) ulcer formation is frequently precededby callus formation (epidermal thickening) and (ii)formed ulcers typically exhibit a thick callus ringspanning the permitter of the ulcer [14, 15].

3. Material and methods

3.1 Experimental data

Medical hyperspectral imaging consists of recordinga series of images representing diffuse reflectance ofbiological tissue at discrete wavelengths lk resultingin a set of images called a hypercube denoted byHðx; y; lkÞ. Here, x and y are the spatial coordi-nates and lk is the spectral dimension. The hyper-spectral imager used in this study is schematicallydepicted in Figure 1 and was described in detail inRef. [11]. Two hypercubes were collected at eachmeasurement site corresponding to either back-ground or LED illuminated conditions denoted byBtissueðx; y; lkÞ and Mtissueðx; y; lkÞ, respectively. TheLEDs were switched off and on to produce both illu-mination conditions, respectively. The spectral se-parator of the imager used in this study [11] wastuned to 15 equally spaced wavelengths between 500and 660 nm while the CCD measured the tissue dif-fuse reflectance. Image acquisition at each wave-length lasted for approximately 1 second. A fiducialtarget was placed near the center of the imager’sfield of view to correct for patient movement duringthe image acquisition.

Spectral variation in external illumination inten-sity and CCD sensitivity were corrected by calibrat-

ing the hyperspectral imager to a well characterizedhighly reflective diffuse reflectance standard. Thiswas performed prior to imaging each patient. Thesame procedure as above was used to acquire hyper-cubes of the calibration standard under backgroundand LED illumination denoted by Bcalibðx; y; lkÞ andMcalibðx; y; lkÞ, respectively. Then, the diffuse reflec-tance measured at location ðx; yÞ and wavelength lk

was defined as

Rmðx; y; lkÞ ¼Mtissueðx; y; lkÞ � Btissueðx; y; lkÞMcalibðx; y; lkÞ � Bcalibðx; y; lkÞ

ð1Þ

Note that hypercubes Btissueðx; y; lkÞ andBcalibðx; y; lkÞ represent the reflectance of skin andthe calibration standard, respectively, during back-ground illumination only while the hypercubesMtissueðx; y; lkÞ and Mcalibðx; y; lkÞ represent thereflectance of skin and the calibration standard,respectively, illuminated by normally incident andcollimated light and background illuminationsimultaneously.

3.2 Description of data set

Hyperspectral images were gathered from 66 sub-jects at the Olive View Medical Center (Olive View-UCLA IRB #05H-609300) as reported in detail inRef. [10]. Each subject was (i) diagnosed with type 1or 2 diabetes mellitus, (ii) afflicted with peripheralneuropathy [13, 14], and (iii) at risk of developingfoot ulcers [4]. Twenty one subjects developed newulcers during the course of the study [10]. Hyper-spectral images of the feet of each subject were gath-ered over multiple visits. This study focuses on tworepresentative subjects who exhibited foot ulceration

Figure 1 Schematic of the different components of the hy-perspectral imager. The device acceptance angle a and exitangle q are shown for reference.

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on the planar aspect and toes of the foot. These re-presentative subjects were chosen because morethan two hyperspectral images were available foreach ulceration phase, namely prior to and duringulceration as well as during ulcer healing.

3.3 Normal-hemispherical versusnormal-normal reflectance

This section describes the relationship between thesemi-empirical reflectance model for two-layer me-dia developed by Yudovsky and Pilon [22], whichpredicts the normal-hemispherical reflectance de-noted by Rn;hðx; y; lkÞ, and the diffuse reflectanceRmðx; y; lkÞ measured by a finite aperture hyperspec-tral imager. The diffuse reflectance Rmðx; y; lkÞ givenby Eq. (1) can be expressed as [32],

Rmðx; y; lkÞ ¼2pÐa

0Itissueðx; y; lk; qÞ sin q dq

2pÐa

0Icalibðx; y; lk; qÞ sin q dq

ð2Þ

where Itissueðx; y; lk; qÞ and Icalibðx; y; lk; qÞ are the in-tensities of light backscattered from skin and fromthe diffuse reflectance standard, respectively, intothe solid angle 2p dq around the polar angle q andat coordinate ðx; yÞ and wavelength lk. Furthermore,a is the acceptance angle of the hyperspectral im-ager’s CCD as illustrated in Figure 1. As explainedearlier, background illumination was removed in cal-culating Rmðx; y; lkÞ by subtracting Btissueðx; y; lkÞfrom Mtissueðx; y; lkÞ and Bcalibðx; y; lkÞ fromMcalibðx; y; lkÞ, respectively. Surface (or Fresnel) re-flectance was removed by cross-polarizing the lightemitted from the illumination LEDs relative to theliquid crystal spectral separator which is inher-ently polarized [11]. Thus, Itissueðx; y; lk; qÞ andIcalibðx; y; lk; qÞ are the diffusely backscattered inten-sities under normal and collimated illumination.

The angular intensity of light backscattered fromstrongly scattering media such as skin exposed to nor-mal and collimated irradiation has been shown to fol-low the Lambert’s cosine law [33–35] given by [32],

Iðx; y; lk; qÞ ¼Rn;hðx; y; lkÞ cos q

pð3Þ

where Rn;hðx; y; lkÞ is the normal-hemispherical re-flectance integrated over the outer hemisphere(0 � q � p=2) and therefore, independent of q. Bysubstituting Eq. (3) into Eq. (2) for Itissueðx; y; lk; qÞand Icalibðx; y; lk; qÞ the diffuse reflectanceRmðx; y; lkÞ can be expressed as

Rmðx; y; lkÞ ¼Rn;h;tissueðx; y; lkÞRn;h;calibðx; y; lkÞ

ð4Þ

where Rn;h;tissue and Rn;h;calib are the normal-hemi-spherical reflectance of skin and of the diffuse reflec-tance standard, respectively. Since the calibrationstandard is highly reflective for the wavelength rangeconsidered, Rn;h;calib can be approximated to unity.Then, the measured reflectance Rmðx; y; lkÞ, given byEq. (1), is equivalent to the normal-hemispherical re-flectance Rn;h;skin, predicted by Yudovsky and Pilon[23], regardless of the acceptance angle a.

3.4 Tissue spectroscopy

Recently, Yudovsky and Pilon [24] developed an in-verse method for determining the melanin concentra-tion Cmel, blood volume fraction fblood, oxygen satura-tion SO2, epidermal thickness Lepi, and transportscattering coefficient ms;tr of human skin. The depen-dence of the transport scattering coefficient on wave-length was expressed as a power law: ms;tr ¼ Cðl=l0Þbas implemented in Ref. [23]. The parameter C was as-sumed unknown while the parameters l0 and b wereassumed to be 1 nm and 1.50, respectively. The biologi-cal characteristics of skin were assumed to be definedby a property vector ~aae ¼ ðCmel;Lepi; fblood; SO2;CÞ.The goal of the inverse problem was to estimate thevector ~aae at location ðx; yÞ from the measured diffusereflectance spectrum Rmðx; y; lkÞ. Tissue spectroscopywas achieved by finding an estimate vector ~aaeðx; yÞthat minimizes the sum of the squared residuals rðx; yÞexpressed as [23],

rðx; yÞ ¼PK

k¼1½Rmðx; y; lkÞ � Rn;h;skinðx; y; lk;~aaeÞ�2 ð5Þ

where Rmðx; y; lkÞwas estimated experimentally usingEq. (1). The estimated normal-hemispherical reflec-tance of skin Rn;h;skinðx; y; lk;~aaeÞ was evaluated usingthe semi-empirical model for two-layer media de-scribed in Ref. [22] [Eqs. (33) through (35)] as a func-tion of ~aae. The residual rðx; yÞ was minimized itera-tively at each coordinate ðx; yÞ using the constrainedLevenberg-Marquardt algorithm [35]. Minimizationwas stopped once successive iterations of the algo-rithm no longer reduced rðx; yÞ by more than 10�9

per iteration. This model has been validated againstin vivo diffuse reflectance measurements of skin onthe inner and outer forearm and forehead of subjectsof white Caucasian and black African descent [24].

4. Results and discussion

4.1 Callus formation during ulceration

Figures 2 and 3 show visible images, oxygen satura-tion maps, epidermal thickness maps and oxyhemo-


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globin concentration maps of the plantar surface andtoes of the right foot of a diabetic patient. The im-ages were gathered over four visits during the courseof one month. The patient exhibited an existing ulcerduring the first visit over the first metatarsal head(Figure 2a) which persisted during the three follow-ing visits (Figures 2b through 2d). Furthermore, anew ulcer developed over the fourth metatarsal headbetween the third and fourth visit (Figure 2c).

Figures 2a through 2d show the visible image ofthe foot while Figures 2e through 2h show oxygensaturation maps estimated from the correspondinghyperspectral data. The estimated oxygen saturationSO2 and epidermal thickness Lepi were averagedover an area 1 cm in diameter centered on the preul-cer region over the fourth metatarsal head (solid linecircle) and a control region over the second metatar-sal head (dashed line circle) for each of the four vis-its. During visits 1, 2 and 3, the average SO2 assessedover the ulcer was respectively, 80, 83, and 69% com-pared with 73, 95, and 72% for the control region.In a previous study [11] the range of oxygen satura-tion SO2 at known preulcer sites was reported to bebetween 45 and 80%. This study [11] determined theoxygen saturation by comparing the apparent ab-sorption of skin to reference apparent absorptionspectra of fully saturated and desaturated blood andmelanin solutions as described in Ref. [18]. The val-ues of oxygen saturation determined in the presentstudy were consistent with previous investigations

even though different methods were used. The SO2

value at the ulcer locations were found to be consis-tently lower compared to the control region duringthe visits leading up to ulceration. However, the ab-solute value of SO2 varied significantly and withoutany apparent trend. For example, the average oxy-gen saturation assessed over the entire foot in-creased between visits 1 and 2 from 67 to 94%, butthen decreased to 83% during visit 3. It was not ex-pected that SO2 values over the entire foot would beconsistent from one visit to another as it fluctuatesdepending on the patient’s activity. Therefore, weconsidered the relative difference in SO2 betweencontrol and preulcer regions, measured during thesame visit.

Figures 3a through 3d show the epidermal thick-ness maps of the plantar surface and toes of the rightfoot of a diabetic patient. They indicate that esti-mates of epidermal thickness varied between 70 and170 mm. Note that Lepi was defined for an epidermisin which melanin was assumed to be homogeneouslydistributed throughout the entire layer [23]. In rea-lity, melanin is concentrated close to the basementmembrane and the majority of the epidermis is rela-tively transparent to visible light [36]. Assuming uni-form melanin concentration may be acceptable forthin epidermis such as the forearm and forehead[24] but is less representative of epidermis on thefoot. Indeed, histological studies of human skin fromthe sole of the foot have reported epidermal thick-

Figure 2 (online color at: www.biophotonics-journal.org) (a) to (d) Visible images and (e) to (h) oxygen saturation mapsof the plantar surface of the right foot of a diabetic subject who developed a foot ulcer. Solid line circle indicates theulceration site. Dashed line circle indicates the control site. Solid line circle indicates the ulceration site. Figures 2(a)through (d) represent the same time points as figures (e) through (h) during ulceration.




Figure 3 (online color at: www.biophotonics-journal.org) (a) to (d) Epidermal thickness and (e) to (h) oxyhemoglobinmaps for the plantar surface of the same subject depicted in Figure 2. Solid line circle indicates the ulceration site. Dashedline circle indicates the control site. Figures 2(a) through (d) represent the same time points as figures (e) through (h)during ulceration.

Figure 4 (online color at: www.biophotonics-journal.org) (a) to (c) Visible images and (d) to (f) oxygen saturation maps ofthe plantar surface of a diabetic subject during the development of a foot ulcer on the hallux of the left foot.


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ness of up to 600 mm [38]. Thus, Lepi retrieved fromdiabetic feet was expected to be a relative estimateof the actual epidermal thickness on the plantar sur-face of the foot and not an exact value of the phy-sical thickness. Nonetheless, Figures 2 and 3 showthat the presence of callus around formed or formingulcers (Figures 2a through 2d) consistently corre-sponds to an increase in the estimate of epidermalthickness Lepi (Figures 3a through 3d). Thus, Lepi re-trieved is a relevant parameter that correlates withactual epidermal thickening. For each visit, the epi-dermis was consistently thicker in the preulcer re-gion than in the unaffected control region and grewthicker over time. Quantitatively, the estimated aver-age epidermal thickness in the preulcer region in-creased from 136, 157, to 159 mm during visits 1, 2,and 3, respectively, compared with 116, 93, and 98mm at the unaffected control region during the samevisits. The surface area of epidermal thickening whereLepi was greater than 140 mm increased from 3 cm2

to 10 cm2 between visits 2 and 3. Furthermore, thecallus around the ulcer is visible during visit 4 as de-picted in Figure 3d.

Figures 3e through 3h show the oxyhemoglobinconcentration maps of the plantar surface and toesof the right foot of a diabetic patient. Interestingly,as the callus size and thickness increased betweenvisits 1 and 3 (Figures 3a through 3c) the oxyhemo-globin concentration decreased (Figures 3e through3g). In fact, the difference in average oxyhemoglobinconcentration between the test and control sites was8.01, �38.2, �36.4 and �108.3 mM during visits 1, 2,3, and 4, respectively. This indicates that the preulcerarea became ischemic relative to nearby unaffectedtissue as the callus grew. Indeed, an increase in epi-dermal thickness (callus formation) may result in in-creased and irregular pressure on that part of thefoot [38]. This may cause poor blood circulation andsubsequent ulceration in the area [38]. In fact, ourprevious study [11] showed that a large increase orreduction in the local oxyhemoglobin concentrationstrongly correlates with risk of ulcer formation inthat area. Note that the absolute values of deoxyhe-moglobin concentration during these visits were 70,54, 73, and 47 mM, respectively, and did not show aclear pattern as observed earlier [11].

Figure 5 (online color at: www.biophotonics-journal.org) (a) to (c) Epidermal thickness and (d) to (f) oxyhemoglobin con-centration maps during foot ulceration corresponding to images to maps shown in Figure 4. Solid line circle indicates theulceration site. Dashed line circle indicates the control site.




Thus, the present analysis could enable one togain insight into the mechanisms involved with ulcerformation. It suggests that as the callus grows, itcauses pressure disturbances in the foot which affectthe foot’s circulatory function.

4.2 Ulcer healing

Figures 4 through 7 show results for the toes andplantar surface of the left foot of another diabeticpatient collected over six visits during an elevenmonth period. Figure 4 shows the visible images andoxygen saturation maps of the patient’s foot beforethe ulcer formed on the first digit. The oxygen sa-turation averaged over the entire foot decreasesfrom 85% to 57% between visits 1 and 3 indicatingpoor circulation in the affected foot during theweeks leading up to ulceration. No significant differ-ences were observed in oxygen saturation in the pre-ulcer and control areas for the three visits. However,epidermal thickening (Figure 5) was observed on thefirst digit almost six months (Figure 5a) prior to visit

4 (Figure 6a) when the formed ulcer was clearly ap-parent on the visible images. In fact, the average epi-dermal thickness at the preulcer site was found to be158, 132, and 138 mm during visits 1, 2, and 3 (Fig-ures 5a through 5c), respectively. On the other hand,the average epidermal thickness in the control region(dashed circle) varied very little during the same vis-its and was 99, 102, and 103 mm, respectively. Simul-taneously, oxyhemoglobin concentration was alsoelevated during the first three visits as evident inFigures 5d through 5f. However, elevation in oxyhe-moglobin concentration occurred also on all theother toes and on the plantar surface and not only atthe preulcer area. On the other hand, epidermalthickening and oxyhemoglobin concentration in-creased simultaneously on the first digit which subse-quently ulcerated.

Moreover, Figures 6 and 7 show the visibleimages and maps of the patient’s foot during healing.During visit 4 (Figure 6a), the open ulcer was ap-proximately 2 cm in diameter and surrounded by athick callus ring. The presence of callus was clearlyapparent to the naked eye during visits 4 and 5 (Fig-ures 6a and 6b). This was properly captured by our

Figure 6 (online color at: www.biophotonics-journal.org) (a) to (c) Visible images and (d) to (f) oxygen saturation maps ofthe plantar surface of a diabetic subject during healing of the foot ulcer formed on the hallux of the left foot.


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algorithm as illustrated in Figure 7a and 7b. Theaverage estimated epidermal thickness calculated ina 1 cm diameter circle positioned on the callus ring(solid line) was 167 and 163 mm during visits 4 and5, respectively. Finally, Figure 7c establishes that,during the last visit, the region of epidermal thicken-ing subsided around the ulcer as it healed andclosed. In fact, the average epidermal thickness esti-mated in a 1 cm diameter circle centered on the cal-lus decreased to 150 mm during the last visit. On theother hand, the epidermal thickness was 111, 108,and 92 mm at the control regions during visits 4, 5and 6, respectively. These values were similar tothose retrieved at the site before the ulcer formed.For reference, Figure 7d through 7f also show theoxyhemoglobin maps of the healing ulcer. Reductionin callus size and thickness between visits 4 and 6evident from Figure 7a through 7c corresponds toimproved circulation near the ulcer suggested by theincrease in oxyhemoglobin concentration reported inFigure 7d through 7f.

Figure 8 summarizes the temporal progression ofepidermal thickness in the preulcer and control

Figure 7 (online color at: www.biophotonics-journal.org) (a) to (c) Epidermal thickness and (d) to (f) oxyhemoglobin con-centration maps during foot ulceration corresponding to images to maps shown in Figure 6. Solid line circle indicates theulceration site. Dashed line circle indicates the control site.

0 50 100 150 200 250 300

20

40

60

80

100

120

140

160

180

Ulcer formation(Figure 4)

Ulcer healing(Figure 6)

Ulc

er fo

rmed

Days from initial visit

Ave

rag

e ep

ider

mal

th

ickn

ess,

Lep

i (mm

)

Control siteUlcer site

Figure 8 Average retrieved epidermal thickness as a func-tion of days from the first visit on June 27, 2007 (visit 1) toMay 27, 2008 (visit 6) calculated at the preulcer site(square) and at the control site (circle) for the foot shownin Figures 4 and 6.




areas, respectively, for all sets of hyperspectralimages available for the second patient. Estimates ofepidermal thickness gradually increased during ulcerformation and were 30 mm larger at the ulcer sitethan at a nearby control site. After the ulcer formed,the estimates of the epidermal thickness near thecallus were consistently higher than in the same re-gion prior to ulceration and decreased as the ulcerhealed. During ulcer healing, both the ulcer and con-trol sites show a decrease in epidermal thicknesswith time. Off-loading the affected foot is a typicaltreatment for diabetic foot ulceration. This may haveresulted in epidermal thinning around the ulcer loca-tion, but also on the plantar surface of the footwhere the control region was chosen.

4.3 Clinical implications

In a previous study, spatial variations in oxyhemo-globin and deoxyhemoglobin concentrations wereused to detect tissue at risk of ulceration [11]. Theaverage oxyhemoglobin and deoxyhemoglobin con-centrations were found in a 1 cm diameter circulartarget region centered over a pixel and 8 adjacentregions. Then, the maximum difference between theaveraged oxyhemoglobin concentrations calculatedin the target regions with respect to the control re-gions was computed and denoted by MD(OXY).The MD(DEOXY) value was found in the samefashion. The MD(OXY) and MD(DEOXY) valueswere calculated for each pixel. Furthermore, it wasfound that if jMD(OXY)j and jMD(DEOXY)j si-multaneously exceeded their respective threshold val-ues, then the pixel could be classified as preulcerouswith a high sensitivity and moderately high specifi-city. As depicted in Figures 2 through 7, ulcer forma-tion is preceded by callus formation that results in alocal increase in epidermal thickness. It is thus ex-pected that values of MDðLepiÞ calculated over pre-ulcer regions would be larger on average than fromsurrounding unaffected regions.

In practice, the algorithm and methodology pre-sented in this work can be applied directly to datagathered by a hyperspectral imager. To illustrate theclinical usefulness of detecting epidermal thicknesson the feet of diabetic patients at risk of developingfoot ulcer, the present technique should be appliedto a larger population of patients, as performed inRef. [11]. This task, however, falls outside the scopeof the present study which focused on monitoringthe evolution of foot ulcers from formation to heal-ing for two specific patients.

5. Conclusion

Hyperspectral imaging of the feet of two diabeticpatients was performed before, during, and afterthey developed foot ulcer. Previous studies of dia-betic foot ulcer formation using hyperspectral im-aging limited their analysis to circulatory changessuch as difference in oxyhemoglobin and deoxyhe-moglobin concentrations obtained from hyperspec-tral images collected immediately prior to foot ul-cer formation or immediately following ulcerhealing [10, 11, 20, 21]. The present study exam-ined the temporal changes observed before the ul-cer became apparent to the naked eye until ithealed and closed. Epidermal thickening was ob-served in preulcerous areas which is consistent withcallus formation in such regions. The callus was ob-served to recede as the ulcer healed. The corre-sponding increase in local epidermal thickness wasassociated with a decrease in oxyhemoglobin con-centration. This study establishes the feasibility ofusing measurements of epidermal thickness to en-hance other imaging modalities, in particular hyper-spectral tissue oximetry [11], for early prediction ofdiabetic foot ulcer formation.

Acknowledgements This research was funded in partthrough a grant from the National Institute of Diabetesand Digestive and Kidney Diseases (R42-DK069871).

Dmitry Yudovsky receivedhis B.S. in Mechanical Engi-neering at the University ofCalifornia, San Diego in2006. He pursued his gradu-ate studies at the Universityof California, Los Angeleswhere he received a M.S.and a Ph.D. in 2008 and2010, respectively. He is cur-rently a Postdoctoral Re-search at the Beckman La-

ser Institute at the University of California, Irvine. Hisresearch interests include biomedical imaging, opticaltissue modeling and non-invasive tissue health-monitor-ing.


Journal of

BIOPHOTONICS


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Laurent Pilon received hisB.S. and M.S. in AppliedPhysics in 1997 from theGrenoble Institute of Tech-nology, France and a Ph.D.in Mechanical Engineeringfrom Purdue University in2002. He then joined theHenry Samueli School ofEngineering at UCLAwhere he is now Associate

Professor with affiliations in the Mechanical and Aero-space Engineering Department and in the BiomedicalEngineering Interdepartmental Program. He hasauthored more than 60 archival journal publications onbiophotonics, radiation transfer, applied optics, andthermal sciences.




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