2mm Catheter Design for Endoscopic Optical Coherence Tomography

Transcription

1 2mm Ctheter Design for Endoscopic Opticl Coherence Tomogrphy Kye-Sung Lee 1, Chuck Koehler 1, Eric G. Johnson 1, Eric Vlski TEUMA 2, Olusegun Ilegbusi 2, Mrco Cost 3, Huiki Xie 4, nd Jnnick P. Rollnd 1 1 CREOL, College of Optics nd Photonics, University of Centrl Florid, 4000 Centrl Florid Blvd., Orlndo FL 2 Deprtment of Mechnicl, Mterils, nd Aerospce Engineering, College of Engineering nd Computer Science, University of Centrl Florid, 4000 Centrl Florid Blvd., Orlndo FL 3 The Crdiovsculr Center, University of Florid Helth Science Center, Shnds Hospitl, Jcksonville, FL 4 Deprtment of Electricl nd Computer Engineering, University of Florid, Ginesville, FL ABSTRACT A biophotonics ctheter ws conceived with collimtion optics, n xicon lens, nd custom design imging optics yielding 360 degree scn imed t imging within concve structures such s rteries nd lung lobes. The lrge depth of focus is necessry to imge long-depth-rnge smple with constnt trnsverse resolution in opticl coherence tomogrphy (OCT). There re two pproches to chieving constnt invrint resolution in OCT: Dynmic focusing or Bessel bem formtion. This pper focuses on imging with Bessel bems. The Bessel bems my be creted with xicon optics which cn be used insted of conventionl focusing lens in the smple rm of the OCT interferometer. In this pper we present the design of 2mm ctheter for opticl coherence endoscopy with resolution of bout 5 micron cross depth of focus of bout 1.6mm. Importntly, we investigted the fbriction of 800µm dimeter xicon lens nd the ssocited lterl resolution obtined over long depth rnge in our OCT system, compred to the sme OCT system using conventionl lens. Keywords: ctheter, opticl coherence tomogrphy, xicon, Bessel bem imging, endoscopy 1. Introduction Opticl coherence tomogrphy (OCT) is highly sensitive biomedicl imging technique tht enbles high resolution, cross-sectionl imging in biologicl tissues nd other turbid mterils. 1 High xil resolution of OCT is relized by use of brodbnd light source wheres the lterl resolution is determined by the numericl perture of the focusing lens. Although lrge numericl perture of conventionl focusing lens in the smple rm of OCT enbles high lterl resolution imging, smll numericl perture is required to chieve lrge depth of focus (DOF) tht llows mking constnt trnsverse resolution imge over long depth rnge. To overcome this limittion, n xicon lens ws recently designed nd incorported into the smple rm of n interferometer to chieve both high lterl resolution nd lrge DOF simultneously 2 nd dynmic focusing lenses were used to mintin high trnsverse resolution over long depth rnge. 3 Endoscopic opticl coherence tomogrphy (EOCT) which combines OCT with endoscopic technique hs ttrcted significnt interest becuse of its high resolution when compred to intrvsculr ultrsound imging (IVUS). 4 An xicon lens cn be used s focusing lens in the nrrow endoscope to chieve high lterl resolution over long depth rnge of imging becuse xicon lenses re opticl elements tht produce long, nrrow focl line long the opticl xis insted of the usul focus point of conventionl lens. 5 In Section 2 of the pper, we demonstrte the diffrction pttern of nnulr perture with xicon optics nd lso describe the reltion of the xicon ngle with the depth of focus nd the lterl resolution over depth. In Section 3, we present Interntionl Opticl Design Conference 2006, edited by G. Groot Gregory, Joseph M. Howrd, R. John Koshel, SPIE Vol. 6342, 63420F, 2006 SPIE-OSA X/06/$15 doi: / SPIE-OSA/ Vol F-1

2 the design of 2mm ctheter for EOCT with resolution of bout 5 micron cross DOF of bout 1.6mm. We lso report the point spred functions long the depth of focus. In Section 4 we demonstrte the fbriction of n 800µm dimeter xicon lens. We lso present schemtic of Fourier domin OCT (FD-OCT) used in the experiment. We then incorported the xicon into the smple rm of the FD-OCT interferometer nd investigted the lterl resolution over long depth rnge. Results were compred to the sme OCT system using conventionl lens. 2. Theory Recently xicon optics, opticl elements tht produce focl segments within specified rnge, hve ttrcted considerble interest becuse of their unusul properties nd verstility in prcticl pplictions. 6,7 In diffrction theory, the focl segment is chrcterized by nondiffrction Bessel bems. 8 Fig.1. Bessel bems genertion with n xicon lens within the focl segment tht represents the DOF Since the J 0 (zero-order Bessel) bem is the Fourier trnsform of ring with n infinitively thin width, the conicl surfce of n xicon genertes the diffrction ptterns of J 0 bem within the focl segment (i.e. the DOF) s shown in Fig. 1. Fig.2. Schemtic of n xicon lens The DOF is defined s the distnce from the xicon pex to the geometricl shdow for full-fcet illumintion nd for smll ngle of the xicon lens is given s D DOF, (1) 2( n 1) where D denotes the dimeter of the collimted incident bem on the xicon lens nd n is the refrctive index of the xicon lens. The trnsverse intensity distribution on ech focl plne which is generted from ech thin nnulr segment of the collimted incident bem is described by the first order Bessel function. The centrl lobe size of the first order Bessel is given by SPIE-OSA/ Vol F-2

3 2.4048, (2) 0 2 sin where is the centrl wvelength of the incident bem nd is the bem devition ngle with respect to the opticl xis of the xicon lens, shown in Fig. 2, which cn be clculted s function of the xicon ngle s sin 1 ( n sin) (3) The centrl lobe size 0 is constnt in the DOF becuse the bem focusing ngle is lso constnt within the geometricl shdow s shown in Fig.2. This property yields constnt lterl resolution within the DOF in OCT. 3. Ctheter design A 2 mm biophotonics ctheter ws conceived to include collimtion optics, n xicon lens, nd custom design imging optics combined with ~2mm micromotor coupled to mirror or MEMS to yield full 360 degree scn within concve structure such s rteries, lung lobes, nd other internl structures. The use of xicon lenses enbles constnt resolution in depth scn cross bout 1.6mm. Resolution of less thn 5 micron cross DOF of bout 1.6mm ws chieved. Fig.3. Schemtic of 2mm ctheter with n xicon nd two lenses for rely Fig. 3 shows the schemtic of 2mm ctheter with n xicon nd two lenses for rely. The xicon hs 4 degrees xicon ngle. The effective focl length (f 1 ) of first lens is 2mm nd the effective focl length (f 2 ) of the second lens is 1mm. The two lenses rely the DOF generted directly from the xicon to the smple re to be imged. The two lenses lso decrese the DOF size by (f 1 /f 2 ) 2 to confine power within smller re compred to hving no rely lenses, which yields more power efficiency. In ddition, the focusing ngle 1 is incresed by (f 1 /f 2 ) to 2 which yields higher resolution (i.e. 5 µm) thn before the rely optics s shown in Fig. 3. [degree] <Tble 1 prmeters nd specifictions for the 2mm ctheter design> Mteril of D [mm] f1 [mm] f2 [mm] DOF [mm] xicon 0 [µm] 4 fused silic ~2 ~5 With the prmeters nd specifictions for the 2mm ctheter design shown in Tble 1, we optimized the design. Fig. 4 shows the lyout of the 2mm dimeter ctheter including n opticl fiber, collimting lens, n xicon lens, doublet lens, nd 1.9mm micromotor coupled to MEMS mirror/lens with shft nd trnsprent cover. The MEMS mirror/lens device combines 45-tilted mirror nd n integrted microlens. 9 The microlens turns with the spinning shft of the micromotor to relize 360 rel-time focusing. A lterl resolution of 4.8µm cross DOF of bout 1.6mm SPIE-OSA/ Vol F-3

4 ws chieved. The ctheter ws designed nd optimized t the three different wvelengths of 750nm, 800nm nd 850nm with the weight of respectively. Electricl wire for micromotor drive voltge MEMS mirror Micromotor Opticl Fiber Axicon lens Fig.4. 2D lyout of 2mm dimeter ctheter Fig. 5 shows the point spred functions t ech focl plne of 1mm, 1.8mm, nd 2.6mm from point P shown in Fig. 4. The centrl lobe sizes of the corresponding point spred functions re 4.8µm, 4.7µm, nd 5.2µm mm Ctheter POSITION 1 ORA WAVELENGTH WEIGHT WAVELENGTH WEIGHT DIFFRACTION INTENSITY 2mm Ctheter DIFFRACTION INTENSITY 2mm Ctheter DIFFRACTION INTENSITY NM NM 1 SPREAD FUNCTION SPREAD FUNCTION SPREAD FUNCTION NM NM NM 1 POSITION NM 1 FLD( 0.00, 0.00)MAX;( 0.0, 0.0)DEG FLD( 0.00, 0.00)MAX;( 0.0, 0.0)DEG POSITION 1 FLD( 0.00, 0.00)MAX;( 0.0, 0.0)DEG 20-Apr-06 DEFOCUSING: MM ORA 20-Apr-06 DEFOCUSING: MM ORA 20-Apr-06 DEFOCUSING: MM () (b) (c) WAVELENGTH WEIGHT NM NM NM 1 Fig.5 Point spred functions t focl plnes of () 1mm (b) 1.8mm (c) 2.6mm from the point P shown in Fig Axicon fbriction nd experiment We fbricted n 800µm dimeter xicon lens on silic wfer. An xicon phse msk pttern ws written into Polymethyl-methcrylte (PMMA) on n E-bem mchine, nd then used in stepper s phse msk to crete n nlog xicon profile on silic wfer. It ws then developed nd etched into the silic wfer. The fbricted xicon picture is shown in Fig. 6(). We lso nlyzed the etched xicon pttern on the silic wfer with the Zygo interferometer to generte polynomil curve-fitting coefficients from the fbricted nlog profile dt. Then the curvefitting dt ws plced into n opticl system design softwre to perform simultions to get bem profiles shown in Fig 6(b), (c), nd (d) over the DOF. The fbricted xicon lens hs bout 3 degrees xicon ngle nd the corresponding DOF nd centrl lobe size 0 of bem profile re given by 15mm nd 13µm with Eq. (1) nd Eq. (2). SPIE-OSA/ Vol F-4

5 () (b) J95JE E (c) (d) Fig.6. () SEM picture of the fbricted xicon, simultion of the bem pttern t (b) the strt of the depth-of-focus (c) the middle of the depth-of-focus (d) the end of the depth-of-focus The fbricted xicon lens ws incorported into the smple rm of n FD OCT interferometer to test its performnce. FD OCT hs ttrcted significnt interest becuse of its improved sensitivity nd imging speed when compred to time domin OCT (TD-OCT). 10 In FD OCT the xil informtion is derived from the spectrum of the interferometer output. The schemtic digrm of the system is shown in Fig. 7. The FD OCT system consists of brod bndwidth (i.e. 120nm t full-width-t-hlf-mximum centered t 800nm) Titnium:Spphire lser, commercil spectrometer with CCD rry, nd 80/20 fiber coupler which mkes two rms of the interferometer. The 80% bem from the coupler is collimted nd then incident on the xicon lens. The light is then focused on the smple. The other 20% bem is reflected by mirror through the Fourier domin opticl dely line in the reference rm whose min function is to control the overll dispersion in the system. 11 SPIE-OSA/ Vol F-5

6 Fig. 7. Schemtic digrm of Fourier-domin OCT with n xicon s focusing lens in the smple rm. We first mesured the intensity profile of the collimted bem before the focusing lens s shown in Fig. 8 nd then mesured the bem profile fter pssing through either the xicon lens or the sphericl lens s function of the distnce from the lens. The collimted bem dimeter ws round 800µm nd its profile ws shped s Gussin s shown in Fig Bem Profile of Femto lser Intensity [.u.] Bem Size [m] Fig. 8. Intensity profile of the collimted bem before the xicon lens. The collimted bem ws incident on n 800 dimeter xicon lens nd the bem profiles t different plnes from the xicon pex were mesured s shown in Fig. 9(). The geometricl DOF ws lmost 15mm which ws estimted in Section 2, nd the centrl lobe sizes were constnt of vlue 13 over the DOF. The pek intensity ws decresed s the mesured plne is moved wy from the xicon lens becuse the incident bem ws Gussin in shpe, which cn be understood by considering Fig. 2 nd how different bem heights re being imged with the xicon. For comprison with conventionl lens we mesured the bem profiles t different distnces from the focl plnes of n 8mm focl length sphericl lens s shown in Fig. 9(b). The full width of the bem profile t the nominl focus plne by the SPIE-OSA/ Vol F-6

7 sphericl lens ws computed to be 10Although the bem width ws better thn tht of the xicon lens t the focl plne, the bem width t 2mm wy from the focl plne ws found to be round 200 which is highly brodened compred to tht of the xicon lens. () (b) Fig. 9. Bem intensity profile fter () n xicon lens (b) sphericl lens ccording to the distnce from the lens Fig. 10. A 15µm slit We imged 15µm slit shown in Fig 10 t three different distnces from the lens for both n xicon lens nd sphericl lens. Fig. 11(), (c), (e) re the OCT imges of the slit t the distnces of 7mm, 5mm, nd 3mm wy from n xicon lens. Fig. 11(b), (d), (f) re the OCT imges of the slit t the distnces of 10mm, 8mm, nd 6mm wy from sphericl lens. The 15µm slit ws imged t 7mm, 5mm, nd 3mm s shown in Fig. 11(), (c), (e) while the sphericl lens hs n bility to resolve the slit only t single focl plne s shown in Fig. 11(d). Results show good correltion between the shrpness of the point spred function nd the bility to resolve the slit hole which is clerly observed in Figs. 11(), (c), nd (e). SPIE-OSA/ Vol F-7

8 'I.E '0.0 5) 5) '00 '25 '50 ' Distnce [urn] () (b) o!!!!!!!!!!! ' '40 '60 ' Distnce [urn] (c) 11.0 ' Distnce [urn] (d) ' o ) 5) ' '40 '60 ISO 200 Distnce [urn] (e) 5.0 '00 '25 '50 ' Distnce [urn] (f) Fig.11. OCT imges of 15µm slit trnsverslly scnned t distnces of () 7mm, (c) 5mm, nd (e) 3mm wy from n xicon lens nd (b) 10mm, (d) 8mm, nd (f) 6mm wy from sphericl lens 5. Conclusion In this pper, we presented the design of 2mm ctheter for opticl coherence endoscopy with resolution of less thn 5 micron cross DOF of bout 1.6mm. An xicon lens ws fbricted nd tested in the lbortory. We demonstrted the impct of n xicon lens on the lterl resolution in OCT compred to conventionl lens. An xicon lens incorported in OCT cn render constnt high lterl resolution over long depths of focus compred with the conventionl lens which cn imge only short depth rnges with high lterl resolution. SPIE-OSA/ Vol F-8

9 Acknowledgements This reserch ws supported in prt by the Florid Photonics Center of Excellence, the NSF IGERT progrm, the UCF Presidentil Instrumenttion Inititive, nd the DARPA & NSF PTAP progrm. We thnk Opticl Reserch Assocites for the student license of Code nd their support to trvel to IODC. References 1. D. Hung, E. A. Swnson, C. P. Lin, J. S. Schumn, W. G. Stinson, W. Chng, M. R. Hee, T. Flotte, K. Gregory, C. A. Pulifito, nd J. G. Fujimoto, Opticl coherence tomogrphy, Science 254, (1991). 2. Zhihu Ding, Hongwu Ren, Yonghu Zho, J. Sturt Nelson, nd Zhongping Chen, High-resolution opticl coherence tomogrphy over lrge depth rnge with n xicon lens, Optics Letters 27, (2002). 3. W. Drexler, U. Morgner, F. X. Kortner, C. Pitris, S. S. Bopprt, X. D. Li, E. P. Ippen, nd J. G. Fujimoto, In vivo ultrhigh-resolution opticl coherence tomogrphy, Optics Letters 24, (1999). 4. R.Leitgeb, C. K. Hitzenberger, nd A. F. Fercher, Performnce of fourier domin vs. time domin opticl coherence tomogrphy, Optics Express 11, (2003), 5. Ann Burvll, Ktrzyn Kolcz, Zbigniew Jroszewicz, nd Ari T. Friberg, Simple lens xicon, Applied Optics 43, (2004). 6. M. Honknen nd J. Turunen, Tndem systems for efficient genertion of uniform-xil-intnesity Bessel fields, Opt. Commun. 154, (1998). 7. A. T. Friberg, Sttionry-phse nlysis of generlized xicons, J. Opt. Soc. Am. A 13, (1996). 8. J. Durmin, Exct solutions for nondiffrcting bems, J. Opt. Soc. Am. A 4, (1987). 9. A. Jin, nd H. Xie, "An Electrotherml Microlens Scnner with Low-Voltge, Lrge-Verticl-Displcement Actution," IEEE Photonics Technology Letters, Vol. 17, No. 9, pp , September R.Leitgeb, C. K. Hitzenberger, nd A. F. Fercher, Performnce of fourier domin vs. time domin opticl coherence tomogrphy, Optics Express 11, (2003), Kye-Sung Lee, A. Ceyhun Akcy, Tony Delemos, Eric Clrkson, nd Jnnick P. Rollnd, Dispersion control with Fourier-domin opticl dely line in fiber-optic imging interferometer, Appl. Opt. 44, (2005). SPIE-OSA/ Vol F-9