Optical Elastography

Sean J. Kirkpatrick

Department of Biomaterials and Biomechanics

Oregon Health Sciences University

Portland, OR 97201

USA

Abstract

A maximum likelihood estimator for quantifying shifts in speckle patterns resulting from an applied stress to biological tissue is described. The estimator is then applied to calculating the strains and instantaneous strain rates in chicken skeletal muscle subjected to a dynamic acoustic (2 Hz) sinusoidal loading.

1. Introduction

Optical elastography, as defined
here, is the field of study that aims to use coherent light techniques to
evaluate the mechanical behavior of materials. Of immediate interest is the use
of laser speckle methods for investigating the mechanical behavior of biological
tissues. The use of laser speckle techniques for the mechanical characterization
of materials is well known in the non-destructive evaluation community.^{1-4}
One particular application is to infer strain by monitoring the motion of the
speckle pattern that results from coherently illuminating a stressed object.
Typically a reference image of the speckle pattern is acquired before
deformation of the object. Motion (with respect to this reference image) of
subsequent speckle patterns, which occurs when the object is stressed, is used
to infer the resulting strain. A problem experienced in using this technique for
measurements of hydrated tissues is the rapid decorrelation of the speckle
patterns.^{5} Thus, application of speckle techniques to assessment of
strain in biological tissues often relies upon rapid sampling of the speckle
patterns and the use of processing algorithms that are aimed at inferring strain
rates rather than absolute strains.

The goal of optical elastography is to noninvasively, or
minimally invasively, quantify meaningful mechanical constants of tissues in a
manner that provides clinically relevant information. It is well known that many
disease processes, such as tumors of the breast and prostate, manifest
themselves as stiff, hard nodules relative to the surrounding tissue. This
feature frequently allows for their detection through manual palpation. However,
soft tissue palpation is not only qualitative, but also highly subjective.
Furthermore, it provides information on a relatively large spatial scale. Thus
the ability to detect small tumors by palpation is limited. Manual palpation is
also limited to those areas of the body which are accessible to touch. Optical
elastography offers the potential for increased spatial resolution (i.e. smaller
lesions may be detected) and better strain resolution (low-contrast elastic
modulus distributions may be visualized) than other elastographic methods, such
as vibration amplitude sonoelastography,^{6} vibration phase gradient
sonoelastography,^{6} elastography,^{7} spectral tissue strain
measurement,^{8} and various methods employing MRI data.^{9}.
Strain resolution on the order of single microstrain has been evaluated in
biological tissue optically.^{10} However, this increased spatial and
strain resolution is gained at the cost of decreased probing depth as coherent
optical methods are limited to the outer few millimeters of tissue. Furthermore,
most optical elastography methods are limited in that only relatively small
areas or volumes of tissues may be probed at any one time. Nevertheless, optical
methods can still be useful in the early detection of neoplastic changes because
many of these early changes occur in the mucosa and submucosa of the affected
organs. Subsurface skin tumors present themselves as objects with distinct
mechanical properties relative to the surrounding normal tissue. The
displacement of fibrillar papillary dermis by the softer, cellular mass of a
growing melanoma is one such example of this. Optical elastographic techniques
may provide a means by which to probe these masses to determine their state of
progression and thereby help to determine a proper means of disease management.
Other skin afflictions, such as psoriasis and icthyosis, also present as
localized tissue areas with distinct mechanical properties that can be
delineated optically.

The conventional strategy behind most one- and
two-dimensional speckle techniques that are aimed at measuring surface
displacements is to compute the correlation between a reference and displaced
(sample) speckle patterns. This approach is straightforward and has been used
successfully in processing data from speckle strain gauges.^{11-13} This
approach, however, is subject to errors and certain limitations for biomedical
diagnostics. For example, if the test is subject to vibrations, there will
always be some question as to the validity of the reference exposure.
Furthermore, if the speckle shifts are substantial, the speckle pattern will
decorrelate from the reference and a new reference must be chosen. This can lead
to compounding of errors.

However, since the goal of these conventional approaches is
simply to quantify a lateral shift in a "noisy" signal, a number of
other data collection and data analysis options present themselves. One data
collection scheme that has proven to be very useful for evaluating strains in
biological tissues begins with the fundamental concept of the laser speckle
strain gauge as described by Yamaguchi^{11-13}. The configuration is
appropriate for either objective or subjective laser speckle.The scheme is based upon observing translating laser speckle
with a linear array CCD camera through an observation angle, q * _{o},
*as the specimen is sequentially illuminated through two equal, but opposite
illumination angles, " q

**Figure 1. **Optical
arrangement for collecting speckle data.

Alternatively, a two detector, single beam configuration is
also possible.^{13-15} Due to the fast decorrelation of the observed
speckle patterns as a result of random movements within the tissues (water,
blood, etcÖ), the switching of the illumination angle and subsequent
triggering of the CCD array must be rapid enough to acquire at least several
records with minimal decorrelation between subsequent exposures. Typically, the
switching must be on the order of 50 Hz.^{10,16} This frequency is
beyond the range of most mechanical shutters so electro-optical devices, such as
ferroelectric crystals (FLC), in combination with polarizing beam splitters have
been used in the past.^{10,16} In this case, the FLC acts as a binary,
switchable half-wave plate and the beam is thus transmitted through or reflected
by the polarizing beam splitter, depending upon the polarization of the light
exiting the FLC.

Using a physical optics approach, Yamaguchi^{11} has
shown that for an object undergoing strain the speckle motion observed through q
* _{o}* for illumination angle q

, (1)

where *a _{x}* is an in-plane motion,

. (2)

It can be seen, then, that the in-plane strain term can readily be isolated. Had the complementary configuration (2 cameras, 1 laser beam) been employed, Eq. (2) would have taken the form

(3)

and the desired strain term could only be isolated if the
term containing *a _{z}* was made negligible to the strain term.
This can be accomplished through a proper choice of

The goal of this strain measurement concept is to determine
the shift in a speckle pattern resulting from an applied load. Towards this end,
the one-dimensional records are stacked into what is termed a
"stacked-speckle history."^{14,15} Stacked speckle histories
are time series of one-dimensional views of the speckle patterns combined in a
spatio-temporal array such that the spatial dimension (camera pixel) is along
the abscissa and the temporal axis is the ordinate. In the configuration shown
in Fig. 1, two stacked speckle histories are generated, one for each q
* _{s}*. Figure 2 is a gray-scale display of one such stacked
speckle history taken from an object undergoing a slow linear strain. The figure
shows a sequence of 200 one-dimensional

**Figure 2.**
Stacked speckle history of a sample undergoing a linear strain. Time is given by
the *y*-axis and space is given by the *x*-axis.

speckle patterns stacked one atop another. The desired information, that is, the shift of the speckle pattern is reflected in the tilt of the structure.

Processing of these stacked speckle data has been
accomplished using the transform method.^{10,15 }The method has been
adequately described in the literature and therefore will be only summarized
here. In the transform method of processing stacked speckle histories, the
speckle histories are visually inspected and matching sets of 10 ñ 30 records
from each history that display minimal decorrelation, or for some other
experiment-driven reason, are selected for analysis. These records are
transformed into the frequency domain using a fast Fourier (FFT) algorithm in
the spatial direction and an autoregressive (AR) spectral estimator (modified
covariance, 3-5 poles) in the temporal direction. Alternatively a 2-dimensional
FFT may be employed. The result of this operation is a pair of
"images" in the frequency domain consisting of a bright band of energy
oriented perpendicular to the tilt in the stacked speckle histories. The slopes,
*m _{1}* and

, (4)

where the subscripts are associated with the positive and
negative illumination angles, s is the standard
deviation about each slope, respectively, and *t* is the critical value of
the Studentís *t*-distribution with *N*-2 degrees of freedom at a
probability level of a . Absolute strains can be
determined by an integration over the time course of the experiment.

2. Methods

There are numerous alternative approaches that can be
employed for determining speckle movement. One such alternative is a parametric
approach, a maximum likelihood estimator. Parametric estimators incorporate *a
priori* knowledge of the experiment (as opposed to non-parametric estimators,
such as cross-correlation approaches, which do not incorporate *a priori *knowledge).
The objective, of course in using a such model-based estimators is to gain a
degree of sensitivity. By making use of information that is known about the
conditions of the measurement, non-physical outcomes can be eliminated. While
this often results in greater resolution, this increased resolution sometimes
comes with a price. Typically non-parametric approaches are more robust. Because
the parametric approaches implicitly eliminate estimates that are
"non-physical", when data are encountered that are less than ideal,
these estimators sometimes break down. One of the fundamental assumptions in
this (and other) approaches is that the two speckle records are simple
translates of one another. With analogy to one of the basic tools of the theory
of propagation through atmospheric turbulence, Taylorís frozen turbulence
hypothesis,^{17} this may be called the "frozen speckle"
model. The maximum likelihood estimator described below arises from a minimum
mean square error techniques when it is assumed that the noise in the speckle
signal is Gaussian.

2.1. A Maximum Likelihood Speckle Shift Estimator

It is assumed that, over a time on the order of a couple sequential exposures of the camera, the structure of the speckle pattern is fixed. The only change with time is its lateral motion (Fig. 3).

**Figure 3. **Illustration
of shifting frozen speckle pattern.

Thus the speckle motion can be modeled as

, (5)

where the subscript *i* denotes the pixel (spatial
dimension) and the subscript *j* represents the record (temporal
dimension). Assuming that the shift, *dx*
is small compared to a pixel, Eq. (5) can be approximated as

. (6)

This is simply the first two terms of the Taylor series
expansion for *g*. To introduce a degree of symmetry into the problem, the
two speckle records on either side of the record of interest are inspected (Fig.
4),

. (7)

**Figure 4. **Central
differencing technique.

The *dx*
that minimizes the error is then determined,

. (8)

where the sum is over all pixels in the array. The *dx*
that brings these two records into registration is thus sought. Equation (8) may
be solved numerically by making use of a gradient search algorithm.^{18}
If however, we make use of the approximation in Eq. (6) (small speckle motions),
then differentiation with respect to *dx*
and rearranging yields the formula

. (9)

The term in the first square bracket in the numerator is
simply the first central difference approximation^{19} to the
derivative;

. (10)

The spatial derivatives may be approximated similarly:

; (11)

.

Note that the shift parameter, *dx,*
is the time rate at which the speckle pattern shifts; units are pixels/record.

Although the means by which the estimate for *dx*
was arrived at was quite specific, this estimation approach is more general than
it would seem. For instance, the term approximating the 1^{st} central
difference for the estimate of the temporal derivative arose because the speckle
records on either side of the record of interest where chosen for inspection. We
could just as easily have included additional records and weighting their
contributions appropriately to estimate higher order approximations to the
derivative. For example, instead of using the weights

, (12)

we could use^{20}

. (13)

In this case, the formulation for the mean square error would be

, (14)

the temporal derivative term of Eq. (13) would be

, (15)

and the term involving the spatial derivatives would be

. (16)

Note that this is simply the weighted average,

, (17)

of the terms on either side of the record of interest. Further, these higher order operators for the derivative can be used to estimate the spatial derivatives:

. (18)

These higher order approximations to the derivative have
somewhat better noise characteristics.^{19} This improvement, however,
comes at the expense of reduced temporal resolution. Nevertheless, by making use
of these higher order approximations, the processing can be tailored to the
demands of the experiment.

Up to this point, no specific assumptions about the
statistics of the speckle pattern or associated noise have been made. The only *a
priori* knowledge that was introduced is that the speckle shift is small with
respect to the pixel size. If the assumption is now made that the measured
speckle signal comprises a deterministic speckle signal(!) plus noise, the
measured signal can be modeled as

, (19)

where *n* is a zero-mean noise signal. If we calculate
the central difference about the *j ^{th}* record, we get

. (20)

It is assumed that the sequential speckle patterns are constant mean. Further it is assumed that the noise is statistically independent Gaussian with zero mean and constant variance. As a result the noise probability density function can be written as

, (21)

where *C* is a constant. Equation (21) is commonly
referred to as the likelihood function.^{21} To choose the *dx*
that maximizes this likelihood we set

. (22)

Carrying out this calculation leads to the formula in Eq.
(9). It is straightforward to show that this is an unbiased estimate of the
speckle pattern shift and that the variance of the estimate attains the
CramÈr-Rao lower bound.^{22}

For any of the differential speckle measurement
techniques, whether it be single detector and two laser beams or single laser
beam and two detectors, one arrives at a pair of stacked speckle histories.
These histories display the time rate of movement of the individual speckles
whether they are subjective or objective. The relationship between these speckle
motions and the strain undergone by the test specimen is dictated by the actual
measurement configuration. As an example, consider the configuration shown in
Fig. 1. If *dx _{1}*
and

, (23)

where *L _{o}* is the effective object difference
and q

, (24)

where q * _{o}* is the
observation angle. Note that these are the same results as Eqs. (2-4).

2.2. Acousto-optic Elastography

In this section of the paper, we present a method,
acousto-optic elastography (AOE), which employs the above described maximum
likelihood speckle shift estimator for evaluating the strain response of chicken
skeletal muscle to an applied acoustic load. In AOE, a contact electroacoustic
transducer is placed adjacent to a region of interest (ROI) of the tissue and is
driven with a sinusoidal waveform at low frequency. The ROI is then sequentially
illuminated with a low power laser from two equal, but opposite illumination
angles, " q * _{s}*.
The backscattered speckle pattern is imaged by a linear array CCD camera
equipped with a telecentric lens as in Fig. 1. The system is intentionally
misfocused by a distance

A prototype AOE system was implemented to
demonstrate the measurement of the strain response of hydrated chicken skeletal
muscle to an applied dynamic acoustic stress. An electroacoustic transducer was
placed in light contact with the tissue samples and driven in a sinusoidal
waveform at 2 Hz by a function generator, launching acoustic waves into the
tissues. A collimated, TE cooled, index-guided diode laser emitting at 809 nm
sequentially illuminated a narrow stripe of tissue (~1 cm in length) immediately
adjacent to the electroacoustic transducer through q * _{s}*
= " 45ƒ . The
backscattered speckle patterns from each illumination angle were observed with a
linear array CCD camera (7.6 m m pitch) equipped with
an

**Figure 5. **AOE
stacked speckle histories.

The goal was to first estimate the shift as a function of record number and second to calculate the cumulative strains in the samples via the maximum likelihood approach and the instantaneous strain rates by taking the time derivative of Eq. (23).

Figure 6 shows the estimated speckle shift as a function of record number for Fig. 5. Figs. 7 and 8 show the cumulative strain as a function of record number and the instantaneous strain rate calculated via Eq. (23) and itís time derivative, respectfully. The gradual increase in cumulative strain shown in Fig. 7 implies that the tissue was creeping under the imposed dynamic acoustic stress, however this is a feature which requires further investigation.

**Figures 6 **.
Speckle shift in pixels/record as a function of record
number. It can be seen that the speckle pattern does indeed shift in a
sinusoidal manner, following the acoustic modulation. Figure 7 (bottom) shows the
cumulative shift in the speckle pattern for the two illumination angles. The
gradual separation in the direction of the shift for the two illumination angles
indicates an increasing residual strain and/or rigid body motion.

**Figures 7 & 8.** Figure 7
(top) shows the cumulative strain in the breast muscle as a function of record
number. Apparently there was some residual strain that built-up due to the creep
response of the tissue to the dynamic loading. This is a feature that may or may
not be real, but warrants further investigation. Figure 8 (bottom) shows strain
rate as a function of record number. As expected, the strain rate followed the
sinusoidal acoustic loading pattern.

3. Discussion

In summary, these preliminary results demonstrate an imaging method by which acoustically induced speckle fluctuations in biological tissue can quantitatively encode mechanical information about the tissue. Pathological tissues, because their mechanical properties vary from that of healthy tissue, should exhibit a different strain response than will healthy tissues, which may assist in the early detection of cancerous lesions, for example. The system is noninvasive for superficial applications, such as the interrogation of skin. Alternatively, a minimally invasive, endoscope-based system can be envisioned for probing internal tissues, such as esophageal and prostate tissues.

4. Acknowledgements

I would like to thank Donald Duncan for extensive assistance during the course of this project. This work was funded in part by NSF grants #BES-9808497 and #BES-0086719.

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