dental caries , more commonly known as cavities or tooth decay , is one of the most prevalent clinical conditions in the world . theworld health organization estimates that 60 - 90% of school children and nearly 100% of adults have or have had caries , with indications that global incidence rates are increasing . while it is assumed that industrialized nations have better oral hygiene due to implementation of public health measures ( e.g. water fluoridation ) and improved living conditions , some developed nations have caries incidence rates greater than those of resource - limited settings . tooth decay occurs when oral bacteria metabolize sugars and starches found in food and produce acids that erode the enamel of the tooth . caries are strongly correlated with the presence of dental plaque which is produced by the same antagonizing bacteria , . with time , the enamel layer is compromised and the acids attack the inner dentin and eventually the soft pulp , causing pain , sensitivity , stains , and halitosis . dentists treat caries by manually removing the compromised tooth material and filling the remaining void . to help locate and determine the depth of the decay , dental professionals commonly rely on radiographs in conjunction with white light visual examination . the most popular radiographic views are bitewing , periapical , and occlusal views . because traditional radiographs capture a two - dimensional projection of all superimposed material between the film and source the orientation and severity of the feature directly affects its radiographic visibility ; as such , some incipient caries , early demineralization , and cracks will not appear on the x - rays . caries adjacent to and within direct line - of - sight of opaque amalgam fillings , such as instances of secondary caries , may also not be visible . furthermore , modern radiographs generate concern among patients who seek to reduce their exposure to ionizing radiation . it is not uncommon for patients to refuse radiographs despite the wishes of dental professionals . the scattering properties of enamel and dentin in the nir range were first characterized by fried et al . in 1995 . depending on the wavelength , most nir light can be transmitted across healthy enamel with marginal scattering ; 1310 nm light represents an optimal imaging wavelength as it strikes a balance between enamel and water attenuation , . however , dental caries and demineralization scatter transiting nir light and appear as dark areas . the depth of caries and other features can be further interrogated when comparing the dark and translucent areas at different viewing angles , illumination modes , and nir wavelengths  ; fig . stains , which absorb light and hamper assessment of the underlying material , do not manifest at nir wavelengths of 1200 nm and greater . at 1310 nm , mild fluorosis and other shallow defects may be discernable depending on their severity , geometry , and method of imaging ; hirasuna et al . suggest multimodal nir imaging as a potential means for differentiating between mild fluorosis and deeper caries . taken altogether , nir dental imaging has produced condition - dependent applications that challenge or exceed two - dimensional radiographs without the use of ionizing radiation , and work has been done to compare the diagnostic capabilities of both technologies , , . ( a ) reflectance imaging has light directed at the tooth , with reflected rays collected by a camera positioned in a desired orientation ; a beamsplitter is commonly employed to redirect the illuminated surface for imaging . ( b ) transillumination imaging focuses on capturing the source light after it has transited the tooth . emitters of low coherence , such as slds and leds , are usually placed adjacent to the tooth to maximize the input of light ; lasers can be placed further away . transillumination most commonly has the tooth positioned between the source and camera , although this orientation is not mandatory . if the camera is angled to view the occlusal surface and is orthogonal to light source , the tooth is undergoing occlusal transillumination imaging ( c ) . ( a ) reflectance imaging has light directed at the tooth , with reflected rays collected by a camera positioned in a desired orientation ; a beamsplitter is commonly employed to redirect the illuminated surface for imaging . ( b ) transillumination imaging focuses on capturing the source light after it has transited the tooth . emitters of low coherence , such as slds and leds , are usually placed adjacent to the tooth to maximize the input of light ; lasers can be placed further away . transillumination most commonly has the tooth positioned between the source and camera , although this orientation is not mandatory . if the camera is angled to view the occlusal surface and is orthogonal to light source , the tooth is undergoing occlusal transillumination imaging ( c ) . while much of the research into nir dental imaging occurs around 1310 nm , neighboring nir wavelengths have also demonstrated potential application depending on the illumination mode and target feature , and it is not uncommon for researchers to image the tooth across the nir range and into the short - wavelength infrared range , , . as current ccd and cmos technologies have sensitivities that extend into the nir range , , some nir imaging is realizable with many commercial cameras . ingaas infrared cameras , as used in some of the previously cited work , are capable of capturing in the nir and short - wavelength range , typically operating onwavelengths of 0.9 - 1.7 \documentclass[12pt]{minimal } 
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 \end{document } with enhanced cooling . however , ingaas cameras are comparatively more expensive that ccd and cmos cameras , pose numerous challenges for use in vivo , and typically have restricted access . while not as spectrally powerful as an ingaas camera , and despite the increased scattering properties of enamel below 1300 nm , ccd and cmos technologies remain viable options for nir dental imaging closer to the visible spectrum . a small selection of market devices utilizing nir light are available for purchase , such as the dexis carivu ( dexis , llc , hatfield , pa ) and drr dental s vistacam ix ( drr dental , bietigheim - bissingen , germany ) . the dexis carivu uses a ccd sensor and nir light maximized at 788 nm for fixed - angle occlusal surface transillumination imaging . the dexis carivu has two light sources that enter the buccal and lingual side of a target tooth for transillumination , and its camera is oriented directly towards the occlusal surface . the vistacam ix by drr dental is a software - assisted handpiece incorporating a variety of illumination modes , including a  proxi when the proxi head is attached to the vistacam ix handpiece , two nir emitters are positioned on either side of a cmos sensor . despite the range of technology available to the clinician , challenges in detecting secondary and intraproximal caries in both nir imaging and two - dimensional radiographs furthermore , there have been no studies which utilize a ccd or cmos sensor in non - occlusal imaging schemes . to test the capability of an nir intraoral imager in both reflectance and transillumination modes , as well as make the device angle independent for flexibility in imaging occluded clinical features the user then has improved control for manipulating the frame and angle of capture , the amount of illumination , and the manner in which the tooth is illuminated . a study population of 10 individuals was recruited to test the nir system on a variety of teeth , ranging from healthy , restored , and those under suspicion of requiring excavation . our results showthat the nir systemwas capable of detecting clinical conditions usually imaged via x - rays as well as new dental lesions currently lacking diagnostic imaging . the first wand contained a usb cmos camera board with its infrared filter removed , inserted into a custom 3d - printed wand housing . reduction of the housing profile was critical for granting the camera access to as much of the mouth as possible . an 850 nm led was embedded into a second wand , to be used as the source illumination . wands were sealed with a hard epoxy resin . during operation , the nir system was connected to a computer and a generic webcam application launched ; live video feed was displayed onscreen . while using the nir system , the dental professional held a wand in each hand , allowing for positional flexibility of the camera , as well as the proximity of the illumination source to the tooth . the massachusetts institute of technology s institutional review board , the committee on the use of humans as experimental subjects reviewed and approved the clinical protocol ( 1603518840 ) . subjects aged 18 - 70 were invited at hampden dental care in lakewood , colorado to participate in the study via a phone call or email one week prior to visiting the clinic site for routine dental prophylaxes . during the actual visit , subjects were explained the details of the study and provided the opportunity to ask questions , refuse participation , or provide written consent . after written consent was obtained , overhead lights were turned off while a dentist imaged sites of interest with both the nir system and a white light intraoral camera . the nir system was used to capture approximal and occlusal views in both reflectance and transillumination modes . the carivu was selected as a reference dental imaging technology as it was the most easily obtainable and widely used nir market device in north america . periapical / bitewing radiographs from a kodak rvg 6100 digital radiography system ( carestream dental , atlanta , ga ) were recovered from patient histories and used for x - ray comparison . 10 individuals with a variety of fillings , caries , and other dental conditions were imaged . 2a shows a white light image of tooth 9 from a 56-year - old female . 2b , blue outlines ) appear smooth and homogenous in texture in comparison to the genuine tooth material . reduced translucency located throughout the tooth is indicative of areas of demineralization . the large demarcated patch of demineralization ( fig . 2b , yellow outline ) was dental attrition compounded from the patient s overbite ; repeated impact from the incisal edge of the matching lower tooth eroded the protective enamel . numerous vertical craze lines are easily visible and appear to be miniscule sites of demineralization ; it is probable that their location renders them easily cleaned during brushing , mitigating progression of caries . a horizontal fracture along the center of the tooth spans the distance between the two restorations ( fig . 2b , cyan outline ) . caries appear as regions darker and more clearly defined than demineralization ; the secondary caries ( fig . 2d ) ; for instance , the horizontal fracture detectable in fig . 2b , but not in fig . 2d . images are of tooth 9 in a 56-year - old female ; views in ( a)-(c ) are taken from the lingual side with the distal direction leftwards . the nir system s transillumination image exhibits areas of reduced luminescence indicative of demineralization or caries . select areas of interest are highlighted ; two proximal restorations ( blue ) displaying leakage and secondary caries ( red ) . a large area of demineralization ( yellow ) denotes a region where the incisal edge of tooth 24 on the mandible has caused enamel attrition . ( c ) periapical radiograph section with both proximal restorations visible and darkening from corresponding secondary caries . ( d ) in the dexis carivu occlusal view , teeth lacking a larger occlusal surface , such as this incisor , are more difficult to image images are of tooth 9 in a 56-year - old female ; views in ( a)-(c ) are taken from the lingual side with the distal direction leftwards . the nir system s transillumination image exhibits areas of reduced luminescence indicative of demineralization or caries . select areas of interest are highlighted ; two proximal restorations ( blue ) displaying leakage and secondary caries ( red ) . a large area of demineralization ( yellow ) denotes a region where the incisal edge of tooth 24 on the mandible has caused enamel attrition . ( c ) periapical radiograph section with both proximal restorations visible and darkening from corresponding secondary caries . ( d ) in the dexis carivu occlusal view , teeth lacking a larger occlusal surface , such as this incisor , are more difficult to image . 3 compares images of a tooth with an amalgam filling extending over the occlusal surface , captured by multiple image systems . however , using the nir system to scan along the margin of the filling revealed a hidden secondary caries that was not readily apparent in the white light ( fig . the dentist confirmed the presence of the secondary caries under the amalgam post - extraction . evaluating the integrity of a filling is crucial for determining when it warrants replacement , so any method that can bypass optically - opaque features and resolve secondary caries is of interest . ( c ) amalgam appears opaque in the two - dimensional radiograph and blocks evaluation of superimposed tooth material . ( d ) scanning of the amalgam margin with the nir system shows secondary caries along the bottom of the amalgam circled in red , as seen in the radiograph in ( c ) . midway down the side of the filling , circled in blue , was another secondary caries spot undetected by the other imaging methods . no indication of this caries is present in the white light image in ( b ) . ( a ) was taken in vivo , and ( b)-(d ) were taken after extraction . ( c ) amalgam appears opaque in the two - dimensional radiograph and blocks evaluation of superimposed tooth material . ( d ) scanning of the amalgam margin with the nir system shows secondary caries along the bottom of the amalgam circled in red , as seen in the radiograph in ( c ) . midway down the side of the filling , circled in blue , was another secondary caries spot undetected by the other imaging methods . no indication of this caries is present in the white light image in ( b ) . ( a ) was taken in vivo , and ( b)-(d ) were taken after extraction . the images captured by the nir system revealed clinical conditions , including caries , demineralization , fractures , cracks , and craze lines . the detail afforded by nir images offers the possibility of preemptive assessment of conditions prior to their visibility on a radiograph , such as incipient caries . differentiation of deeper caries from surface artifacts such as stains was bypassed via altering the camera view and illumination , which could be observed in real - time ; however , the most extreme surface anomalies within any subject were marginal , and further testing would be required to see how capable the nir system would be at circumventing more extensive surface features . the nir system can be deployed as quickly as any conventional intraoral camera and without requiring the patient to leave the examination chair . as the nir system and the carivu could only image approximately one tooth at a time , radiographs remained a faster method for surveying multiple teeth at once . the surface of the tooth opposite the camera , as well as larger teeth , were more difficult to image with the nir system . radiographs could also provide information deeper into the tooth volume , on the root , and on the surrounding bone . however , the use of non - ionizing radiation was a major incentive for the patients to be imaged with either the carivu or the nir system . despite this , the fixed angle of the carivu presented unique shortcomings primarily due to its fixed plane of imaging . other reasons that inhibited satisfactory imaging with the carivu included tooth geometry , the carivu head not properly fitting a certain tooth , and the size and/or location of the target caries . some of these factors contributed to the imaging difficulties present in the tooth featured in fig . since the illumination and camera are separate wand entities in the nir system , a view or set of views could be established to overcome tooth geometry limitations . on the other hand , a single - point light source did not transilluminate the tooth as uniformly as the carivu during occlusal imaging . table 1nir dental imaging system comparisoncapability2d radiographicdexis carivunir systemmultiple view anglesyes - requires new x - ray administration for each viewno ( occlusal view only)yesincipient carious lesionslimited ( due to sensitivity)limited ( due to view)yesfine feature visualization ( early conditions , cracks , craze lines)nolimited ( due to view)yescapable of bypassing opaque amalgamlimited ( due to amalgam orientation and view)limited ( due to view)yesavoids ionizing radiationnoyesyes the single - point wand configuration was selected after preclinical prototypes of various fixed - angle geometries were used to probe extracted teeth . leds of varying wavelength from the 780 - 1550 nm range were also tested on extracted teeth . preliminary findings suggested that leds of 850 nm wavelength worked best with our selected cmos camera board for lesion contrast . further testing revealed that a bandpass filter in front of the camera aperture was not necessary if the room was sufficiently dark ; thus , room lights were turned off during data collection , and an optical filter omitted from the camera probe to further minimize profiling . at the 850 nm wavelength , transillumination imaging was far more diagnostically functional than reflectance imaging . attempts at using the nir system for pure reflectance imaging did not take advantage of the tooth material s optical channeling . consequently , reflectance imaging light did not penetrate deep into the tooth to reveal subsurface conditions . while it is possible that led intensity could have been increased or exchanged for a laser source , the led transillumination proved to be much more practical . the intensity output of the selected leds was more than enough to optically saturate the tooth during transillumination . furthermore , laser excitation harbors increased safety concerns in living patients ; even invisible , the amplified light from a coherent nir source can permanently damage the eye . the cost to build the nir system was modest ( less than $ 100 ) and mostly attributed to the usb webcam , which was approximately $ 80 ; the emitter itself was a few dollars . previous work from our group has constructed similarly - themed solutions for retinas , ear , and skin imaging . with the advent of rapid prototyping and computer vision algorithms , construction and use of cost - effective oral imaging technologies in a study of 10 subjects with varying dental conditions , we test a custom nir system capable of illuminating at 850 nm and imaging with a standard cmos usb camera . the decentralization of camera sensor and illumination source into multiple wands was essential for navigating the non - uniform topology of the mouth , and is highly recommended as a nir dental imaging configuration . the nir system was capable of displaying subsurface caries , including those around fillings , demineralization , fractures , and cracks , all without the use of ionizing radiation . the superb detail of the images , primarily in comparison to conventional radiographs , potentially grants the clinician greater diagnostic power . furthermore , the inexpensive cost of the components required to produce such high quality images adds a new dimension that is in contrast to conventional imaging approaches .