assessments of tissue birefringence can be used to identify pathology , as tissue components such as organized collagen are birefringent , and the converse , diseased states , are associated with disorganization [ 1 , 2 ] .
therefore , techniques for measuring tissue birefringence could improve the evaluation of a range of pathophysiologies .
the current paper will investigate a modified approach for the real - time assessment of birefringence through tailored reference arm polarization with the birefringence assessing technology single channel ps - oct .  
clinically relevant birefringent tissue components include cholesterol crystals , actin - myosin complexes , nerve fibers ( myelin ) , calcium hydroxyapatite ( teeth ) , and , of particular interest , collagen .
collagen plays a critical role in many pathologic states , either through its depletion or accumulation [ 3 , 4 ] , ranging from osteoarthritis to myocardial infarction .
optical coherence tomography ( oct ) , an imaging modality based on low coherence interferometry , has demonstrated considerable utility for assessing many tissue pathologies , with up to 14  m resolution while working at or above video rate [ 58 ] .
an oct subtechnology ( adjuvant ) , polarization - sensitive oct ( ps - oct ) , allows assessment of tissue birefringence on a micron scale , in addition to the tissue microstructure [ 911 ] . with ps - oct imaging , pathologic microstructural changes in the collagen organization , concentration , and potentially the specific tissue type have been identified [ 1215 ] .
this has been demonstrated in humans both in vitro and in vivo , with results validated against established techniques such as polarization - sensitive microscopy [ 1618 ] .
optical technologies like ps - oct offer advantages for collagen assessment over other technologies such as mri due to their relatively high resolution .
ps - oct can be performed with either a single or dual detector / channel setup , the latter having several embodiments .
the clinical acceptance of an imaging modality ( and most diagnostic technologies ) is based on its ability to affect morbidity or mortality , which typically does not correspond to its ability to produce precise numerical data ( discrete or continuous ) .
. instead of numerical data , most clinical analysis with imaging modalities uses categorical information , either nominal or ordinal data , which can be assessed rapidly .
when clinical trials are performed , the categorical data comparisons are usually compared in terms of specificity and sensitivity , which clinical scientists are most interested in .
more relevant , physicians use a bayesian - like process to assess relative probabilities as data is accumulated , which is best handled categorically and is described in the discussion .
single - channel ps - oct looks at relative tissue changes ( categorical data ) , as will be seen for diagnostic accuracy , while dual channel strives for precision and absolute polarization measurement ( precise numerical data ) .
but , the vast majority of all clinical imaging is relative assessments / measurements on application ( though the technologies have precision design ) , while precision measurements are uncommon with medical diagnostic technologies .
precision is needed in the design and construction of the technology , but diagnostic accuracy ( which usually involves relative measurements ) is typically the objective of clinical imaging ( in addition to practicality ) .
this is our impression of a gap between medicine and engineering , as physicians are looking for diagnostic accuracy ( as well as practicality of use ) , whereas in engineering , precision is a focus ( which is often sacrificed in medicine for other endpoints ) .
all clinical imaging today strives for clinical accuracy , but this does not mean they strive for precise image information . look from the perspective of the scrutiny single channel often receives : 
everybody knows absolute numerical values are better than relative values ,  this is difficult to defend and a radiograph , ultrasound , or mri , for example , would never hold to this in practice .
the transmission from radiographies of the spin shifts in mri change dramatically based on patient body size , hydration , temperature , and so forth .
but predominately , radiology and mri assess relative details and not absolute or even numerical data ( accurate but not precise ) , as assessing the heart by either technology is done relative to the lungs and not by absolute numbers .
it is therefore not a surprise that single channel , which is still amenable to improvements such as those in this paper , is arguably more effective than dual channel , even if a dual channel system could ultimately get precise , reliable optical axis information in vivo .
we strongly favor single detector / channel ps - oct , the reasons for which we have demonstrated with earlier work [ 1922 ] . from the perspective of our group , the issue of which technology is better , single or dual channel ps - oct , lies not in which has the highest potential precision , but which is best for addressing a given clinical problem .
the logic stems , in part , from our success in getting real - time answers to specific clinical questions .
it also stems from precedent in medicine of relying on the degrees of freedom of an image / data and difference in relative parts of the images .
for example , in spite of the vast amount of information on a 12 lead ekg , a cardiologist looking at one typically can identify an acute inferior myocardial infarction in under a few seconds because of pattern recognition , rather than struggling with a list of precision voltages over all segments and leads .
diagnosing pathology in medical imaging , in general , involves using high degrees of freedom and recognizing contrast between regions ( similar to single channel ps - oct ) .
when more quantitative assessments are necessary , this could be potentially achieved with either single or dual channel .
similarly , a radiologist can pick up a lung mass in a chest x - ray in about the same time , in spite of the extensive amount of information contained in the image , in the setting of widely varying body types .
while densitometry of the image seems like it could only improve diagnostics by providing more quantitative information , it slows down the process , it is difficult to compensate for artifacts , and it would be without obvious large scale gain . our group has been involved in the both the technological advances of oct as well as its application for over 17 years . developing oct technology ( as opposed to application ) , and imaging technology in general , requires precision technology development . if one looks at an example text ,  the introduction to medical imaging : physics , engineering , and clinical application  , the fact that engineering and clinical application sections are approaching problems from a distinct analysis is readily apparent and consistent with our direction for moving forward the technology . in this textbook ,
the physics and engineering of conventional radiography , ct , mri , and ultrasound are examined in considerable detail and individually .
for all the modalities , from an engineering prospective , the book examines maximizing the technical performance in terms of the point spread function , signal - to - noise ratio , dynamic range , contrast , and so forth ( precise numerical data ) .
however , for all the clinical application sections , diagnostic analysis is categorical with extremely few exceptions . and
again , we are exploring the basis for superiority claims of dual channel ps - oct : that its presumed precise numerical data is superior for clinical use compared to the relative or categorical data of the single channel approach .
but if one takes an introductory text like the right imaging study , a guide for physicians , numerical data is not apparent in any section of the book . and even when numerical data is potentially available , as can be seen , it still is typically presented / examined as ordinal data ( absent , mild , moderate , or severe ) .
common examples include most flow measurements ( ultrasound , mri , or coronary angiography ) , edema ( bone mri ) , cartilage breakdown ( mri ) , and pancreatitis grades ( scale 15 ) , all of which the available numerical data is presented categorically .
it would probably be a surprise to many in the oct field that even elastography ( ultrasound and mri ) and anisotropy for collagen ( mri ) are represented by categorical data though numerical data is available .
both oct elastography and ps - oct are usually presented in publication s by oct groups as numerical data , including at times our own .
briefly , the single channel approach measures the relative changes in the oct signal ( back - reflection intensity ) while the reference arm polarization states are changed , which will be clarified below .
the fact that it measures relative changes at points within the tissue improves its robustness over certain artifacts ( such as fiber bending ) and allows for easy implementation and real - time assessments .
although technically more challenging , these techniques offer the potential for measuring certain parameters such as the numerical optical axis , although their clinical utility is unknown at this time . nonetheless , there may be uncommon clinical scenarios where this precision may be critical . with dual channel approaches , which will be briefly discussed ,
the muller matrix is typically measured , or less importantly , the depth - resolved stokes vectors of the back - reflections from the tissue are measured [ 2534 ] .
most dual channel approaches generating a muller 's matrix of the tissue require analysis of many - frame measurements , and therefore can not be interpreted rapidly and explicitly .
furthermore , the stokes or jones vectors of incident and reflected light , measured at the detector , do not typically represent tissue birefringence properties .
this occurs primarily due to birefringence artifacts introduced as the back - reflected light propagates between the sample and detector through variable fiber distortions .
thus , the polarization altering properties of the system need to be maintained constant to prevent artifacts , which adds significant complexity to the system , may not be possible , and is generally not an issue with single channel systems .
as we have previously demonstrated , these artifacts can be seen to result in completely opposite results to the actual situation .
this is distinct from the single channel ps - oct approach , sometimes referred to as single detector ps - oct , which measures relative birefringence ( similar to other clinical imaging technologies which measure relative contrast ) and allows clinicians to directly assess birefringence during imaging from the image itself .
it is quiescent to catheter or other fiber optic bends in the system because it is measuring birefringence changes relative to different locations in the sample ( and not an absolute number at the output ) .
our previous work has demonstrated clinically relevant assessments with single channel ps - oct in tissue including cartilage , tendons , ligaments , and coronary arteries [ 1518 ] .
imaging has been performed in vivo in humans , and the validity has been confirmed with picrosirius stained histological sections ( polarization microscopy ) or scanning electron microscopy ( sem ) .
the data can be obtained in real - time and interpretations are made directly from the image . in an example study of clinical relevance , we have shown that loss of birefringence , which correlates with collagen disorganization , precedes cartilage thinning in osteoarthritis .
similarly , we have confirmed the efficacy of the single channel approach in vivo in human knees both in open surgical fields and during arthroscopy [ 18 , 35 ] .
we have also modeled in detail the theoretical principles behind the ability of single channel ps - oct to measure tissue birefringence .
however , as will be seen in this work , the single channel ps - oct technique to date has never been optimized with respect to the reference arm polarization rotation , which could further improve the performance . for birefringent tissue , the back - reflection intensity changes with alterations of the reference arm polarization state .
the greater the birefringence , the more rapidly the back - reflection changes with the reference arm polarization alterations .
the birefringence can be characterized by interframe back - reflection changing in the reference arm , but no study has been systematically conducted on how best to rotate polarization in the reference arm . in this work , we hypothesize that single channel ps - oct can be optimized by tailoring of the reference arm polarization rotation .
the relative position and rate of paddle alterations in the reference arm dictate the pattern of sample back - reflection change .
different patterns ( with different paddle rotations ) may yield superior birefringence measurements and thus we will first plot different patterns along poincare spheres for analysis . the influence of different paddle modulation ( and movement through the poincare sphere ) on collagen measurement in tissue will then be followed to illustrate the principles .
the general optical arrangement for a single channel ps - oct is depicted in figure 1 .
the system used here is time domain oct ( td - oct ) , but swept source ( ss - oct ) could be used without loss of generality .
the system uses a michelson - type fiber optic interferometer built with a 2  2 fiber coupler .
a broadband superluminescent diode ( sld ) source with 1300  nm center wavelength , 60  nm full - width - half - maximum ( fwhm ) , and 12  mw output power ( afc , toronto , canada ) was used .
a grating - based delay line was used in the reference arm , providing longitudinal scans ( a - scans ) .
the light beam in the sample arm was focused on the tissue sample and scanned laterally by a galvanometer - driven scanner , providing lateral scans ( b - scans ) .
the polarization controller in the reference arm was a standard lefevre - type fiber optic polarizer .
two - quarter wave coils control the ellipticity and a half wave coil controls linear states .   as shown in figure 1 ,
the interferograms from the two ports of the coupler are guided into a dual - balanced detector ( new focus , 2117 , santa clara , ca ) from the circulator and another coupler , respectively ; this is standard to remove photon excess noise .
our previous work demonstrates that , in a single channel ps - oct , the irradiance at the detector , for a given paddle position , is described by : 
 ( 1)id(zr)=ir+is  + i04{2cos[2k0ns(0)zr]my(zrns(k0 ) )  + [ mx(zrns(k0))my(zrns(k0 ) ) ] }  {cos[4k0zr]exp[4ln2zr2l2 ] } , 
 where k0 is known as the central wavenumber of the light source , ns(k0 ) is the difference of the principle indices , and  l represents the full - width half maximum ( fwhm ) coherence length of the light source .
bedsides the longitudinal back - reflection profile ps(z ) , the interferogram includes the depth - resolved tissue birefringence information .
the depth - integrated retardation determined by ns is encoded in the sinusoidal - type modulation , which corresponds to the banding pattern in the single - detector ps - oct image from a birefringent tissue .
optic axis variations of the tissue , combined with the polarization state in the reference arm ( the paddle position ) , are encoded in the z - dependent modulation functions mx(z ) and my(z ) . continuously rotating the paddles
the maximum interferogram intensity requires polarization to be the same in the reference and sample arm .
we will use that fact to establish polarization state in the reference arm by interfering it with light in the sample arm , which contains a mirror and polarization analyzers .
the first step of the experiment was determining the polarization states in the reference arm while the paddles were rotated .
the stokes vector of the returned reference beam was measured from the interferograms , at each paddle position , with polarization analyzers placed in the sample arm above a mirror .
results in different paddle positions were plotted onto the poincare sphere and displayed as a trajectory .
specifically , at each paddle position , a linear polarizer was placed above a mirror in the sample arm and rotated at 0 , 90 , 45 , and 45 degree mechanical angles , consecutively . meanwhile , a neutral density ( nd ) attenuator was adjusted to normalize the return power from the sample arm ( reduce back - reflection variability from different polarizers ) .
the corresponding interferogram was then recorded and the signal intensity ( peak - to - peak value ) calculated .
next , while maintaining the paddle position , a quarter wave plate was inserted into the sample arm .
its fast axis was aligned at 45 and 45 degree with respect to the linear polarizer for obtaining the right and left circular polarization state in the sample arm .
each set of measurements ( interferograms at a given paddle position ) was composed of six components : v0 , v90 , v45 , v45 , vrc , and vlc , which were in units of volts .
again , to avoid confusion , we are doing this to optimize the reference arm for use ; this is not intended to be done each time clinical imaging is performed . for calculating the stokes vector ,
the following conversions were followed : i0 = v0 , i90 = v90 , i45 = v45 , i45 = v45 , irc = vrc , and ilc = vlc .
then , the stokes vector was calculated by : s0 = [ ( i0 + i90)+(i45 + i45)+(irc + ilc)]/3 ; s1 = ( i0    i90)/(i0 + i90 ) ; s2 = ( i45i45)/ ( i45 + i45 ) ; s2 = ( irc  ilc)/(irc + ilc ) . at each reference arm paddle position ,
the stokes vectors were measured and displayed on the poincare sphere . with our use of the poincare sphere , we are trying to obtain two objectives : ( 1 ) determine if different patterns on the sphere result in improved contrast ; ( 2 ) a way of standardizing the reference arm so we can repeatedly put it into the same or near the same optimal state .
two questions arise :  ( 1 ) if fiber bending or temperature changes are occurring in the reference arm , is not it true that the point on the sphere is not the same as that directly coming off the mirror or returning to the beam splitter ; ( 2 ) what happens with fiber bending in the sample arm .
once the polarization states of paddle positions in the reference arm were determined through different rotation profiles , ps - oct imaging was performed of bovine tendon and cartilage using different paddle rotations . in other words ,
how imaging of samples was affected by altering polarization along different paths on the sphere was evaluated .
tissue birefringence was assessed as changing back - reflection intensity over a region of interest ( roi ) .
a roi was selected as a rectangular area in each oct image frame ( 100  20  pixel , h  w ) , at a location near the middle portion , about 400microns under the surface .
while the paddle was rotating , the average signal intensity over the roi was examined and compared with the same parameter over the same area within the serial images obtained at different paddle positions .
figures 2through 4 reflect assessments done with a mirror and polarization analyzers in the sample arm . figures 5 and 6 show assessments with tissue .  
figure 2 demonstrates a plot of reference arm states on the poincare sphere when the paddles are moved from 58 to 45 mechanical degrees with roughly 15 degree increments .
it can be seen that as the paddle position in the reference arm is changing , the stokes vector of the reference beam changes and walks on the sphere , which demonstrates that the polarization states of the reference arm are tractable .  
a. the paddle was rotated in a fixed pattern as measurements are made of the polarization state and then plotted on the poincare sphere .
best fix analysis shows no significant difference between the two curves , supporting reproducibility of the technique .  
figures 5 and 6 examine two different reference arm polarization rotation patterns for optimization of contrast within biological tissues . in figure 4
specifically , figure 4 compares polarization state changes when the paddle rotates through two different patterns : ( a ) from 55 to 35 paddle degree , and ( b ) from 90 to 220 , with 10 increments for each .
curve ( pattern ) ( a ) stays closer to the linear polarization state plane than curve ( b ) .
the intensities of the sample beam at any given polarizer position were maintained within the range of 2% around the set level .
this is in spite of the condition that the polarizer and quarter wave plate measurement angle was produced manually , which typically is considered the major source of the stokes vector measurement errors .  

figure 5 represents images of bovine cartilage ( a ) , and bovine tendon ( b ) by single - detector ps - oct imaging where in this case the angle of the paddle was set at 15.  
 figure 6 plots the average signal intensity over the region of interest ( roi ) in bovine cartilage and tendon images against the paddle position in the reference arm , rotated from : ( a ) 55 to 35 and ( b ) from 90 to 220. the 100  20  pixel ( h  w ) roi was selected in each image as inversely highlighted as shown in figure 5 .
it can be seen that the rotation pattern introduced as figure 4(a ) ( relatively linear polarization states ) more sharply differentiates between the tissue types ( contrast ) as shown in ( a ) rather than the pattern shown in figure 4(b ) .
again , the major difference between the pattern of figures 4(a ) and 4(b ) is that the former has movement in the s1-s2 direction and less in the s3 ( circular polarization axis ) , demonstrating that differences do exist which allow optimization of imaging .
birefringent biomolecules include collagen , enamel , dentin , nervous tissue , and actin - myosin complexes .
many pathophysiologies regarding these molecules are important in diseases such as myocardial infarction , osteoarthritis , and dental caries . polarization - sensitive oct ( ps - oct )
this paper seeks and does improve the capabilities of single channel ps - oct by using a tailored reference arm .
ps - oct can be categorized as either single or dual channel , with various subcategories and definite distinctions between the two .
the issue of which technology is better , single or dual channel ps - oct , lies not in which is superior , but rather , which is best for the appropriate problem . in constructing the technology ,
precision is critical , but for clinical use , diagnostic accuracy and practicality are essential . as stated , physicians in general use a bayesian - like process for assessing patients where a single test is rarely diagnostic .
a patient comes in with a set of symptoms , physical exam results , and initial diagnostic tests .
now , let us say there is categorical temperature spike ( it could be a 101.5f or 103f , precision does not matter ) , it alters the diagnostic probabilities .
these all fall into the same categorical data and would affect the diagnostic probability . if we take the case of temperature , physicians still generally prefer mercury thermometers ( removed because of mercury toxicity and glass ) over the  more sophisticated  digital data in critical situations because even though the latter gives higher precision data , the simpler former method is associated with less application artifact . for over a decade
, we have tailored our ps - oct to address clinical need , not the question for more precise numerical data . for analytical chemistry or measuring low level background radiation in space
engineering and medical approaches are the right ones for their respective fields , as long histories have demonstrated . for medicine ,
categorical data with high sensitivity and specificity to be integrated into a bayesian - like thought process is the how the field operates .
this , along with technical advantages , is why we strongly prefer single channel ps - oct over dual channel .
the logic stems , in part , from our success in testing hypotheses under in vitro and in vivo clinical scenarios using this technology .
it also stems in part from precedent in medicine , where the advantages of single channel ps - oct make it more similar in function to other clinical imaging modalities as compared to dual channel .
single channel ps - oct is a technology which for a considerable period of time has demonstrated the ability to identify pathology in vivo and in vitro . in this paper
yet much of the field believes that not just precision in the technology is needed , but precision in measurement parallels clinical accuracy .
almost all clinical imaging is based on relative differences in images and not obtaining precise numerical values . for those who have not practiced medicine , it is understandable why this point would not be obvious .
but , being allowed to deviate to a nonmedical example , suppose that 1000 feet away in an open field was a lion .
the question of major concern is the lion moving toward me , stationary , or moving away
. it would also be useful to know if he was moving , whether he was walking or running .
the observer is using bayesian - like logic , so he is considering other parameters such as proximity to a car , terrain , and so forth .
but , we will ignore these other parameters without loss of generality to simplify the analogy .
now we want both to be built to maximum precision . but to answer the question needed , pulling the binoculars out of bag , getting a quick answer ( general running versus walking can be readily and easily assessed by relative movement of limbs and with respect to the ground ) is the optimum strategy to decide to run . however , an observer may decide on the velocity meter , wanting to know with high precision how fast the lion is moving ( to three significant digits ) .
in addition , it can be dramatically influenced by many sources of error including environmental factors or even accidentally measuring on the velocity of something near the lion rather than the lion itself ( getting a velocity of zero ) .
the specific additional advantages of single channel ps - oct ( relative imaging modality ) are numerous .
first , the system is less expensive , which is important for routine clinical use .
second , the changes in the image of the single channel system are easy to interpret on screen as a change in backscattering intensity as the polarization of the incident light is changed .
again , this is part of being a relative imaging technology analogous to radiography , mri , ct , and ultrasound .
third , in the dual detector system , multiple polarization filters and beam splitters are integrated into the system , leading to a reduction of power incident on the sample .
since portable sources such as quantum well devices or doped fibers have limited power , the addition of these filters into the oct system may result in power levels dropping below what is necessary for in vivo use .
finally , dual detector approaches suffer from noise issues when most of the power falls on one detector .
single channel approaches generally measure peak - to - peak intensities so detector noise is a trivial issue , which is in sharp contrast to the asymmetric power problem of dual channel . with a superiority in managing certain birefringence artifacts ,
the single channel design allows an ease of use and implementation that results in clinically advantageous real - time measurements , as opposed to more the complex dual channel modality .
single channel ps - oct has already been successfully applied to imaging human tissue in vitro and in vivo , most notably cartilage and coronary arteries , in addition to animal models of disease .   in our approach ,
single channel ps - oct utilizes the changing of reference arm polarization states in a traditional oct system to perform diagnostics .
it looks at how back - reflection intensity changes as a function of polarization paddle changes .
however , this approach has not yet been optimized with respect to the states through which the reference arm is rotated .
we developed and tested a new approach for the real - time assessment of birefringence through tailored reference arm polarization .
results from imaging bovine cartilage and tendon have confirmed that by controlling the reference arm polarization - changing pattern , we can improve the assessment of tissue collagen organization , particularly in tissues with relatively low birefringence .
increased signal intensity and increased ability for differentiation between tissue types was found using a 35 to 55 range , which corresponds to increased movement in the x - y direction rather than the z direction .
this not only proves that the optimization of reference arm polarization states does in fact provide increased ability for sample analysis , but gives some base for how that optimization might occur . in this approach ,
paddle rotation was achieved manually and the birefringence contrast was evaluated offline and demonstrated graphically . by using motorized paddle controllers , fiber squeezers , or similar techniques in conjunction with software detection schemes , future systems will be even more clinically friendly . with respect to the first question ,
the sphere plot is representative of a pattern at the beam splitter and does not need to be precisely the pattern of the end target .
in other words , the actual end target plots can be slightly different , but a dual channel approach would be essentialy unusable under the same conditions . again
, this is an advantage of a technique which relies on relative rather than attempted absolute imaging .
if fiber bending does cause some degree of shift , it is a shift of the entire pattern , which is the phase change .
this shifts the peak but does not alter peak to peak distances or contrast , so it is not significant .
objectives one and two , which are the focus of the paper , are still maintained .
we are optimizing contrast for assessing pathology , not trying to get precise target information . to the second point
( will fiber bending during a procedure affect results ) , it is unlikely to be a significant issue similarly to our previous paper on fiber bending . from a practical standpoint fiber bends
a catheter introduced in the coronary artery , for example , tends not to bend once in the artery .
but even when bends are occurring in other clinical scenarios , they tend to occur at time intervals much longer than the scanning interval .
therefore , as above , the birefringence would tend to rotate or shift the pattern on the poincare sphere , but its shape would remain relatively unchanged .
this is analogous to identifying a lung mass by difference of contrast between the lung and the mass . in diagnostics which use relative measurements ( which represents the majority of imaging ) ,
assessments are only minimally affected by , for example , changes in body size , whereas techniques attempting measurement of an absolute value like dual channel are highly vulnerable to these artifacts .
this paper demonstrates a new approach to single channel ps - oct which tailors the reference arm changing polarization pattern .
the paper also addresses the concept that in optics and engineering of systems high precision is usually critical .
however , when applying the technology for clinical diagnostics , clinical accuracy and contrast are likely far more important as physicians use categorical data and bayesian - like logic in the decision processes . this has been the case in our clinical application of the technology .
 single channel ps - oct has advantages for assessing birefringent tissue components in various clinical scenarios , with implications for assessing pathology , ranging from osteoarthritis to myocardial infarction . 
 while the technique has been successfully used both in vitro and in vivo , there have been limited attempts to optimize single channel ps - oct with respect to performance , particularly paddle rotation . in this study 
 , we developed and tested a new approach for the real - time assessment of birefringence through tailoring of reference arm polarization . 
 different polarization rotation patterns , as depicted on a poincare sphere , were assessed with polarization filters and retarders . 
 when further tested in tissue , ps - oct assessments of bovine cartilage and tendon demonstrated that contrast was sensitive to the pattern selected , indicating that rotation pattern influenced birefringence assessment and providing insights into optimal patterns . 
 we also discuss the difference between diagnostic accuracy and precision with respect to both the construction and application of ps - oct embodiments . 
1. Introduction
2. Methods
3. Results
4. Discussion
5. Conclusion