3D Optical Coherence Tomography For Clinical Diagnosis Of .

3D optical coherence tomography for clinical diagnosis of nonmelanoma skin cancersthe normal approach during visual examination,whole body photography [5] and dermatoscopy,which is a natural extension with additionaldigital documentation capabilities. On top ofthese, more microscopy-orientated techniqueshave been developed with features that partiallyovercome standard histopathology and can beapplied in vivo. CM is one of the most prominent noninvasive in vivo biomedical imagingmodalities that can perform optical sectioningof human skin in the horizontal plane with aresolution comparable to that of histology. Itsdepth sectioning capability and reflection orfluorescence sensitivity can be utilized for cancer diagnosis in thin ( 200–300 µm) samples[14–16] . CM systems offering cellular resolution have been successfully employed in severalclinical investigations to identify nonmelanomaskin lesions and to monitor treatment responsesin vivo [17] . However, the potential cellular andsubcellular level resolution in lateral direction isaccompanied by penetration and axial resolution limitations for optical imaging techniques(Figure 1) . Techniques such as MPT improve resolution, but still limit penetration in the axialdirection to typically approximately 200 µm inhuman skin [18–21] . MPT enables 4D functionalimaging of subcellular structures by detectingsecond harmonic signals, autofluorescence andspatially resolved fluorescent decay kinetics.This technique enables visualization of en facesections at the dermal–epidermal (D–E) borderat approximately 70 µm depth in normal thinskin, with good endogenous contrast and lowerexposure than CM [10] . Despite their excellentresolution and contrast, which is helpful forin vivo identification of superficial tumor material, these methods are primarily imaging parallel to the skin surface. Hence, they lack layerinformation and are performed at limited speeds(multiple seconds per frame) within small fieldsof view, typically approximately 0.5 0.5 mm².Thus, these techniques can only be used for sampling subregions of tumor lesions, which can bemultiple mm² to cm² large in advanced stages.This limits the practical utilization of theseimaging modalities for tumor border delineation. Besides fixation issues during scanning, thelonger acquisition times are a limiting factor forclinical applications at inaccessible, curved orsensitive body locations, such as the face. By contrast, OCT utilizes an interferometric coherencegating principle of spectrally broadband radiation (typically tens to hundredths of nanometerbandwidth) for depth ranging, rather than usingtightly focused beams that necessitate complexfuture science groupReviewand bulky optics to reconstruct 3D images oftissues. This technique offers penetration inthe range of 1 to 2 mm to enable visualizationof clinically significant cutaneous layers, suchas epidermis (EP), D–E junction, dermis andeven subcutis, depending on the employed wavelength region with the capability of real-timein vivo imaging (100 Mvoxel/s and above) [22–24] .Dermatologic OCT Principles of OCTThe principle of OCT is similar to that of ultrasound imaging, using a time of flight measurefor determining the penetration depth of opticalsignals in biological tissues at different transversepositions to reconstruct a cross-sectional image,termed as a B-scan (intensity tomograms witha depth and a lateral axis) [25] . OCT is highlysensitive to biological reflectors or scatterers, dueto the localized variations in refractive index atthe optical interfaces formed by the membranous cavities or the extracellular matrix. Theprinciple of OCT is based on low coherenceinterferometry, which amplifies the backscattered light that is detected with high sensitivityand large dynamic range, measuring signals inthe range of 60 to 110 dB of the incident lightintensity [26] . The axial resolution of an OCTsystem is determined by the coherence lengthof the light source, which is the optical pathdifference within which interference occurs,and it is inversely proportional to the spectralbandwidth of the light source. Unlike opticalmicroscopy, the transverse resolution of an OCTsystem is decoupled from its axial resolution dueto the coherence gating and is determined bythe diffraction-limited spot size of the focusedoptical beam. The interferometric detection ofthe backscattered signal can be performed eitherin time or frequency domain. In time domainOCT, the sample reflectivity profile along depthis directly detected using a scanning referencemirror [27] . Whereas, in frequency domain OCT(FD-OCT) detection the entire interferencespectrum is measured to reconstruct the sample depth information through inverse Fouriertransformation. FD-OCT offers significantlyincreased efficacy resulting in considerablyimproved detection sensitivity and image acquisition speed, as all the echoes of light in depthare measured simultaneously [26,28–31] . Although1D optical interferometry was demonstrated inthe mid-1980s, OCT B-scans were accomplishedfor the first time in 1991 and the first in vivostudies of the human retina were performedin 1993 [32–36] . In the past two decades, thewww.futuremedicine.com655

ReviewAlex, Weingast, Hofer et al.OCTDSHFUSCMµMRIMPTFigure 1. Prominent skin imaging modalities. The scale bars denote 200 µm.CM: Confocal microscopy; DS: Dermatoscopy; HFUS: High frequency ultrasound; MPT: Multiphotontomography; µMRI: Micro MRI; OCT: Optical coherence tomography.Adapted from [12,20] .development of ultra-broadband light sources,advances in fiber optics and introduction offrequency domain techniques have led to significant improvements in resolution, detectionsensitivity and image acquisition speed [37–43] .OCT gains its structural contrast from therefractive index variations within the sample. In the near-infrared wavelength region(700–1700 nm), scattering is the predominantmechanism limiting light penetration intotissues. Scattering within biological tissue isinversely proportional to the center wavelengthof the light source. Thus, light penetration depthcan be increased by employing longer wavelengths in OCT systems. However, imaging atlonger wavelengths for deeper penetration affectsresolution. As both axial and transverse resolution scales with wavelength, even broader lightsources would be needed at longer wavelengths.Most OCT systems operate in the wavelengthregion of 700–1300 nm, known as the ‘optical diagnostic window’, where light absorptionby tissue components, such as water, melanin656Imaging Med. (2011) 3(6)and hemoglobin, is relatively low. Since waterconstitutes more than 90% in dermal tissues,water absorption becomes a major problem forwavelengths greater than 1300 nm. Hence,the majority of OCT systems used for dermals tudies employ 1300 nm wavelength region. OCT in dermatologyThe capability of OCT to perform ‘opticalbiopsy’ (i.e., the nonexcisional in situ, real-timevisualization of tissue morphology with a performance comparable to the gold standard ofexcisional biopsy and successive histopathology) makes it a promising noninvasive imagingmodality for the visualization and interpretationof cutaneous anatomy. To date, OCT has hadthe largest clinical impact in ophthalmology dueto its simple optical access, unmatched resolution and sensitivity for retinal imaging. In mostbiological tissues, including the skin, the majorlimiting factor of OCT is its reduced penetrationdepth due to strong scattering and absorptionof light. However, the imaging depth of aroundfuture science group

3D optical coherence tomography for clinical diagnosis of nonmelanoma skin cancers1–2 mm covers the clinically significant portionof the EP and the dermis in the normal skin,while biopsies of pathologies typically includesubcutaneous fat over multiple millimeters.Many diagnostically important changes, suchas those associated with malignant conditionsand various cutaneous diseases, occur first inthe superficial epithelial tissues before they growinto deeper regions.Since the introduction of OCT into dermatology in 1997, OCT has been utilized to obtainqualitative and functional information fromthe human skin in vivo [22,24] . In addition tothe visualization of micromorphological detailsof the normal skin, several investigations havebeen conducted to examine inflammatory skindiseases, such as contact dermatitis and psoriasis [44] . OCT has been evolving as a promisingdiagnostic tool for the detection of tumors, suchas malignant melanoma and basal cell carcinoma(BCC) [45–49] . However, more clinical trials haveyet to be pursued to determine the capabilityof OCT in identifying malignant skin lesionsand delineating tumor margins. Other promising applications of dermatologic OCT arethe monitoring of topical treatments in vivo,observation of the wound healing process,monitoring mechanical interactions of devices,such as microneedles in situ, evaluation of theeffects of UV radiation and epidermal t hicknessmeasurements [50–53] . Commercial dermatologic OCTsystemsMany OCT systems designed for clinical dermatological applications are commercially availablein the market. Thorlabs (Newton, NJ, USA) hasplayed an important role in translating OCTsystems from research laboratories to clinics.They designed several spectral domains andswept source OCT systems intended for skinimaging applications. Their swept source OCTsystem (OCS1300SS) centered at 1325 nm witha depth-scan acquisition rate of 16 kHz has beenavailable since 2005. They also released threedifferent versions of spectral domain OCTsystems centered at 930 nm with depth-scanrates varying from 1.2 to 110 kHz. The market price of these OCT systems range fromUS 27,000 to 70,000. Michelson diagnosticsLtd. (Orpington, Kent, UK) made a significant progress in the clinical impact of dermatologic OCT by designing a multibeam sweptsource OCT system (VivoSight) centered at the1300 nm wavelength region with an imagingspeed of 10 kHz. Their multibeam approachfuture science groupReviewsignificantly improves the lateral resolution overthe entire scanning depth, thereby providinghigher image quality [54] . These dermal OCTsystems have received US FDA approval forclinical applications. VivoSight OCT systemsare being used for screening and monitoringnonmelanoma skin cancers (NMSC) patientsby dermatologists in the USA [101] . The commercial market of dermatologic OCT systemsis rapidly expanding with more companies suchas Santec corporation (Komaki, Aichi, Japan)offering high performance OCT systems witheither real-time 2D or 3D imaging capabilities.The demand for dermal OCT systems has beenon the rise, since the utility of OCT in variousdermatological applications, such as delineationof tumor margins, noninvasive monitoring oftreatment progress and reducing the number ofstages during Mohs micrographic surgery hasbeen demonstrated [55] . However, most of thecurrent commercially available OCT systems arelimited to cross-sectional imaging. 3D dermatologic OCT system: design& implementationMost of the OCT images displayed in thisreview were obtained using a custom-designeddual wavelength 800/1300 nm OCT system.As shown in Figure 2 , the dual wavelength OCTsystem consisted of two spectrometer-basedFD-OCT systems centered at the two wavelengths operating at high speed, resolution andsensitivity to obtain high-quality images of various dermal structures. The 800 nm FD-OCTsystem used a spectrometer configuration basedon a transmissive grating and an ultra-broadbandTi:Sapphire laser (Femtolasers GmbH, Vienna,Austria) with a center wavelength of 800 nm anda full width at half maximum (FWHM) bandwidth of 180 nm. The axial resolution of thissystem was approximately 3 µm. The interference spectrum returning from the beam splitterwas collimated using a 30 mm wide collimatoronto a 50 50 mm transmissive grating with1200 lines/mm. The grating dis

imaging approaches as well as potential clinical dermatologic applications are discussed. KEYWORDS: cancer diagnosis n contrast-enhanced imaging n dermatology n functional imaging n microscopy n multimodal imaging n optical coherence tomography n optical imaging n tomography Aneesh Alex1, Jessika Weingast2, Bernd Hofer 1, Matthias Eibl,