Op­ti­cal Co­he­rence To­mo­gra­phy

Op­ti­cal co­he­rence to­mo­gra­phy: Ap­p­li­ca­ti­ons ex­pand as tech­no­lo­gy ma­tu­res 21 March 2013, SPIE News­room. DOI: 10.​1117/​2.​3201303.​07

Op­ti­cal Co­he­rence To­mo­gra­phy (OCT) is an emer­ging ima­ging me­thod to re­cord cross sec­tio­nal views of se­mi­t­rans­pa­rent sam­ples like bio­lo­gi­cal tis­sue using near in­fra­red light(NIR) [1][2]. Due it`s uni­que pro­per­ties it has found many ap­p­li­ca­ti­ons in bio­me­di­cal ima­ging and is no­wa­days a stan­dard dia­gnostic too e.g. in oph­thal­mo­lo­gy. These uni­que pro­per­ties in­clu­de high re­so­lu­ti­on, which is ty­pi­cal­ly in the range of a few micro­me­ters and fast image ac­qui­si­ti­on with a large field of view of se­ver­al mil­li­me­ters with video speed frame rate. Ad­di­tio­nal­ly the me­a­su­re­ment is con­tact free, which is an ad­van­ta­ge in cli­ni­cal set­tings. As a draw­back, in com­pa­ri­son to other to­mo­gra­phic ima­ging me­thods like ul­tra­sound or ma­gne­tic re­so­nan­ce to­mo­gra­phy, the ima­ging depth is li­mi­ted to a few mil­li­me­ters.

The phy­si­cal prin­ci­ple be­hind OCT is based on low co­he­rence in­ter­fe­ro­me­try. The light which is back­scat­te­red from the samp­le is over­laid with re­fe­rence light in an in­ter­fe­ro­me­ter. A de­tec­tor is used to re­cord the in­ter­fe­rence bet­ween both beams and ana­ly­sis of the in­ter­fe­rence frin­ges yields a depth pro­fi­le at one point of the samp­le. By mo­ving the spot across the samp­le a full cross sec­tio­nal image or a three di­men­sio­nal vo­lu­me can be re­cor­ded. There are many dif­fe­rent de­tec­ting and pro­ces­sing sche­mes, from which Time Do­main OCT (TD OCT) and Fre­quen­cy Do­main OCT (FD OCT) are the most im­portant. In TD OCT the mir­ror in the re­fe­rence arm is moved and the in­ter­fe­rence is re­cor­ded with a broad band de­tec­tor. Due to the low co­he­rence length of the light sour­ce in­ter­fe­rence can only occur when the path length dif­fe­rence bet­ween the re­fe­rence arm and the struc­tu­re of the samp­le is mat­ched. Be­cau­se of this co­he­rence fil­ter the light back­scat­te­red from other depths in the samp­le is ef­fec­tive­ly sup­pres­sed. In FD OCT the re­fe­rence mir­ror po­si­ti­on is fixed and the si­gnal at the in­ter­fe­ro­me­ter exit is re­cor­ded with a spec­tro­me­ter. Ta­king the long co­he­rence length of each spec­tro­me­ter chan­nel into ac­count, the samp­le in­for­ma­ti­on can be re­cor­ded across the whole depth wi­thout mo­ving the mir­ror. Due to in­ter­fe­rence each scat­te­rer in the samp­le pro­du­ces a mo­du­la­ti­on across the spec­trum. Even­tual­ly a Fou­rier Trans­form yields the same depth pro­fi­le as in TD OCT. Be­si­de of this ge­ne­ral clas­si­fi­ca­ti­on se­ver­al re­la­ted tech­ni­ques exist, like Swept Sour­ce OCT or Full Field OCT.

The re­se­arch at the PTT group com­pro­mi­ses three areas: (1) the com­bi­na­ti­on of spec­trosco­py and OCT cal­led Spec­trosco­pic OCT (SOCT) [3][4], (2) the trans­la­ti­on of image pro­ces­sing con­cepts known from Di­gi­tal Ho­lo­gra­phy to OCT and (3) even­tual­ly the ap­p­li­ca­ti­on of OCT and these ex­ten­si­ons for ima­ging of bio­lo­gi­cal tis­sue and tech­ni­cal sam­ples [5].

In SOCT (1) depth re­sol­ved spec­tra can be cal­cu­la­ted by using post pro­ces­sing me­thods like time fre­quen­cy di­stri­bu­ti­ons. The de­tec­tion of the samp­le`s spec­tral fea­tures is chal­len­ging be­cau­se of the li­mi­ted spec­trosco­pic re­so­lu­ti­on and band­width of OCT sys­tems. Ad­di­tio­nal­ly the wa­ve­length and spa­ti­al de­pen­dent trans­fer func­tion of the OCT sys­tem and speck­le like noise dis­turb the si­gnal. Fur­ther­mo­re there are only a few ab­sor­bing chro­mo­pho­res in the NIR, which can be de­tec­ted by SOCT, and spec­tral chan­ges due to scat­te­ring are most­ly re­la­tive­ly fea­ture less. The­re­fo­re we use mo­dern si­gnal pro­ces­sing tools like pat­tern re­co­gni­ti­on and other tech­ni­ques like con­cepts from hy­per­spec­tral ima­ging for si­gnal ana­ly­sis in SOCT. The in­for­ma­ti­on of the ana­ly­sis is pre­sen­ted as “di­gi­tal stai­ning” of the in­ten­si­ty based OCT ima­ges[6].

Ap­p­li­ca­ti­ons of this con­cept com­pro­mi­se the ana­ly­sis of car­ti­la­ge under com­pres­si­on and brain tu­mors.

In Di­gi­tal Ho­lo­gra­phy (2) am­pli­tu­de and phase of a samp­le can be re­con­struc­ted from the re­cor­ded in­ter­fe­rence pat­tern. This al­lows the me­a­su­re­ment of a to­po­gra­phic height map of the samp­le with an ac­cu­ra­cy which can be bet­ter than 1nm in axial di­rec­tion. Ty­pi­cal­ly only a mo­no­ch­ro­ma­tic laser is used in DH. By using wa­ve­length scan­ning ho­lo­gra­phy, which is clo­se­ly con­nec­ted to Swept Sour­ce Full Field OCT we have in­tro­du­ced a new ima­ging con­cept cal­led “Depth Fil­te­red Di­gi­tal Ho­lo­gra­phy”[7]. Here mul­ti­ply ho­lo­grams are re­cor­ded with dif­fe­rent wa­ve­lengths and depth pro­files are cal­cu­la­ted using the con­cept of FD-OCT. After a depth of in­te­rest is iden­ti­fied, which can be for in­stan­ce a bu­ried layer in the samp­le, this depth re­gi­on is trans­for­med back to ob­tain again mul­ti­ple ho­lo­grams for dif­fe­rent wa­ve­lengths. This ap­proach re­du­ces spu­rious re­flec­tions and makes thus quan­ti­ta­ti­ve phase ima­ging in se­mi­t­rans­pa­rent sam­ples fe­a­si­ble. We ex­ten­ded this ap­proach also to scan­ning spot FD-OCT sys­tems[8]. For this pur­po­se we de­ve­lo­ped a me­thod to re­du­ce samp­le and sys­tem in­tro­du­ced ab­er­ra­ti­ons and per­form multi wa­ve­length phase un­wrap­ping [9].


  • [1] D. Huang, E. Swan­son, C. Lin, J. S. Schu­man, W. G. Stin­son, W. Chang, M. R. Hee, T. Flot­te, K. Gre­go­ry, C. A. Pu­lia­fi­to, and J. G. Fu­ji­mo­to, “Op­ti­cal co­he­rence to­mo­gra­phy,” Sci­ence (80-. )., vol. 254, no. 5035, pp. 1178–1181, 1991.
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  • [4] V. Ja­edi­cke, S. Ag­ca­er, S. Goe­bel, N. C. Ger­hardt, H. Welp, and M. R. Hof­mann, “Spec­trosco­pic op­ti­cal co­he­rence to­mo­gra­phy with gra­phics pro­ces­sing unit based ana­ly­sis of three di­men­sio­nal data sets,” in Proc. SPIE 8592, Bio­me­di­cal Ap­p­li­ca­ti­ons of Light Scat­te­ring VII, 2013, pp. 859215–859215–7.
  • [5] T. Gam­bich­ler, V. Ja­edi­cke, and S. Ter­ras, “Op­ti­cal co­he­rence to­mo­gra­phy in der­ma­to­lo­gy: tech­ni­cal and cli­ni­cal as­pects.,” Arch. Der­ma­tol. Res., vol. 303, no. 7, pp. 457–473, Jun. 2011.
  • [6] V. Ja­edi­cke, S. Ag­ca­er, F. E. Ro­bles, M. Stei­nert, D. B. Jones, S. Goe­bel, N. C. Ger­hardt, H. Welp, and M. R. Hof­mann, “Com­pa­ri­son of dif­fe­rent me­trics for ana­ly­sis and vi­sua­liza­t­i­on in spec­trosco­pic op­ti­cal co­he­rence to­mo­gra­phy,” Bio­med. Opt. Ex­press, vol. 406, no. 6791, pp. 35–36, 2013.
  • [7] N. Koukou­ra­kis, V. Ja­edi­cke, A. Ad­in­da-Oug­ba, S. Goe­bel, H. Wiet­hoff, H. Höpf­ner, N. C. Ger­hardt, and M. R. Hof­mann, “Depth-fil­te­red di­gi­tal ho­lo­gra­phy.,” Opt. Ex­press, vol. 20, no. 20, pp. 22636–48, Sep. 2012.
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