Eur. Phys. J. Appl. Phys. 54, 30001 (2011) HE UROPEAN DOI: 10.1051/epjap/2011110017 T E PHYSICAL JOURNAL APPLIED PHYSICS Review Article Turbid medium polarimetry in biomedical imaging and diagnosis N. Ghosha, A. Banerjee, and J. Soni Department of Physical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur Campus, PO: BCKV Main Campus Office, Mohanpur 741252, West Bengal, India Received: 13 January 2011 / Accepted: 3 May 2011 Published online: 14 June 2011 – c EDP Sciences Abstract. Studies on polarization properties of scattered light from a random medium like biological tissue have received considerable attention because polarization can be used as an effective tool to discriminate against multiply scattered light (acting as a gating mechanism) and thus can facilitate high resolution imaging through tissue. Further, the polarization properties of scattered light from tissue contain wealth of morphological and functional information of potential biomedical importance. However, in a complex random medium like tissue, numerous complexities due to multiple scattering and simultaneous occur- rences of many scattering and polarization events present formidable challenges both in terms of accurate measurements and in terms of analysis of the tissue polarimetry signal. Several studies have therefore been conducted in the recent past to develop appropriate measurement procedures, suitable light propagation models and polarimetry signal analysis methods to overcome these difficulties. In this review, we focus on some of the recent key developments in this area. Specifically, we describe variety of theoretical and experimental tools, illustrated with selected results, aimed at evaluating the prospect of turbid medium polarimetry for both biomedical imaging and diagnosis. 1 Introduction Muscle fibers and extra-cellular matrix proteins (such as collagen and elastin) possess this kind of fibrous struc- Optical polarimetry, ever since its discovery, has played ture and accordingly exhibit optical anisotropy. Changes important roles in gaining fundamental understanding of in this fibrous structure resulting from disease progression the nature of electromagnetic waves and answering some or treatment response alter the optical birefringence prop- of the key questions related to the physics of light [1]. erties, making this a potentially sensitive probe of tissue Traditional polarimetry has also long been pursued for status [7,8]. Similarly glucose, present in tissue, exhibits numerous practical applications in various branches of sci- circular birefringence due to its chiral structure. This leads ence and technology. Quantification of protein properties to rotation of the plane of linearly polarized light about in solutions, testing purity of pharmaceutical drugs, re- the axis of propagation (known as optical rotation or op- mote sensing in meteorology and astronomy, optical stress tical activity). Measurements of optical rotation may thus analysis of structures, and crystallography of biochemi- offer an attractive approach for non-invasive monitoring cal complexes are just a smattering of its diversified uses of tissue glucose levels [9]. [2,3]. More recently, the use of polarimetric approaches Despite the promise of the polarimetric approaches has also received considerable attention for biological tis- for biomedical imaging and diagnosis, in a complex ran- sue characterization and imaging [4,5]. This is motivated dom medium like biological tissue, numerous complexities by the fact that polarization can be used as an effective due to multiple scattering and simultaneous occurrences tool to discriminate against multiply scattered light and of many scattering and polarization events confound the thus can facilitate high resolution imaging of tissue and traditional polarimetry techniques for accurate measure- its underlying structure [6]. Moreover, the polarization ment and analysis/interpretation of the signal. Multiple properties of scattered light from tissue contain wealth scattering causes extensive depolarization, the mechanism of morphological, biochemical and functional information of which is controlled by a large number of parameters (which are otherwise hidden in polarization-blind opti- like density, size, shape and refractive index of complex cal measurements) that can be exploited for non-invasive tissue scattering centers [5]. Further, even if some resid- and quantitative tissue diagnosis [4,5]. For example, the ual polarization signal can be measured, multiple scat- anisotropic organized nature of many tissues stemming tering alters the polarization state in a complex fashion. from their fibrous structure leads to a specific polarimetry For optimal performance of the polarization gating ap- effect, known as linear birefringence (or linear retardance). proach for imaging through tissue, it is necessary to un- a e-mail: [email protected] derstand the mechanism of depolarization of light (and the 30001-p1 The European Physical Journal Applied Physics nature of transfer of residual polarization signal) in tissue where Ex and Ey represent the electric fields in the x and and its dependence on the various morphological parame- y orthogonal directions respectively (z is the propagation ters of tissue. Several studies have therefore been carried direction of the wave), E0x and E0y represent the mag- out in the recent past to develop experimental strategies nitude of the electric fields, δx and δy are phases asso- for measurement of small polarization-retaining signals in ciated with the two transverse electric field components, the presence of large depolarized background of multi- ω is the frequency of the light, k =2π/λ, the wave vector ply scattered light [4,5]. These are supplemented by suit- (λ is the wavelength of light in free space). The resultant able theoretical treatments (based on radiative transport electric field represented as the vector sum of Ex and Ey theory, diffusion theory and Monte Carlo techniques) for describes the polarization state of light [3]. Mathematical modeling polarized light propagation in random medium formalisms for dealing polarized light interaction with any [10–17]. Numerous studies have also addressed depolariza- optical system can be broadly classified into two groups: tion studies on tissue simulating phantoms (whose con- the Jones calculus and the Stokes-Mueller calculus. Jones stituent scattering and polarization properties are known calculus is a field-based representation and assumes coher- and user-controlled a priori) to understand the polariza- ent addition of waves [24]. A major drawback of this ap- tion properties of multiply scattered light (and the proach is that it deals with pure polarization states only, mechanism of depolarization) in tissue for practical im- thus cannot be used to describe polarization transfer of plementation of the polarization gating scheme for tis- an optical system that depolarizes an incident polarized sue imaging [18–22]. Note that for the realization of the light (as is usually encountered in a turbid medium like polarimetric approaches for quantitative tissue diagnosis tissue). This can be encompassed by the Stokes-Mueller also, one needs to develop similar experimental turbid formalism, described below. polarimetry systems, theoretical treatments for forward modeling of complex tissue polarimetry events, inverse analysis methods for extraction and quantification of the 2.1 Stokes-Mueller formalism constituent intrinsic tissue polarimetry characteristics. In this paper, we summarize related research carried out by In this formalism, the state of polarization of a beam of us and others on exploring turbid medium polarimetry for light can be represented by four measurable quantities (in- biomedical imaging and diagnosis. tensities), when grouped in a 4 × 1 vector, known as the The paper is organized as follows. In Section 2, we de- Stokes vector [3,23–26], introduced by Stokes in 1852. The scribe the polarized light preliminaries (with an emphasis four Stokes parameters are defined relative to the follow- ing six intensity measurements (I) performed with ideal on Stokes-Mueller formalism) and also define the basic ◦ polarizers: IH , horizontal linear polarizer (0 ); IV , ver- polarization properties. An overview of light transport in ◦ ◦ tical linear polarizer (90 ); IP ,45 linear polarizer; IM , turbid medium is provided in Section 3, where the optical ◦ ◦ transport parameters of tissue are defined and the differ- 135 (−45 ) linear polarizer; IR, right circular polarizer, ent light propagation models are described. In Section 4, and IL, left circular polarizer. The Stokes vector (S)is we describe the experimental schemes for performing po- defined as [23–26]: larimetric measurements in tissue-like turbid medium. Ba- ⎡ ⎤ ⎡ ⎤ sic studies conducted on depolarization of light in I IH + IV ⎢ Q ⎥ ⎢ IH − IV ⎥ turbid medium and representative applications of polar- S = ⎣ ⎦ = ⎣ ⎦, (2) U IP − IM ization gated imaging of tissue and tissue-like turbid − medium are summarized in Section 5. Section 6 reviews V IR IL the applications of the polarimetric light scattering (both where I, Q, U and V are Stokes vector elements. I is the elastic scattering and fluorescence) approaches for quan- total detected light intensity which corresponds to addi- titative tissue diagnosis. The paper concludes with a dis- tion of the two orthogonal component intensities, Q is the cussion of the other prospective biomedical utility of the difference in intensity between