Acousto-optical Elastography of Skin
1. Abstract:
The application of optical technology in medicine and biology has a long and history. In
this paper, I will introduce the acousto-optical method, which is to evaluate the
viscoelastic behavior of superficial skin. We strain the tissue acoustically and track the
shift in the speckle pattern coming from simultaneously illuminating the tissue with a low
power laser. The artificial lesions affect not only time-of-shift of the surface waves, but
also the phase of the acoustic waves. The method may be applicable in the study and
diagnosis of superficial skin lesions.
2. Introduction:
We all know that the easiest way to detect the cancer, such as breast and prostate lesions,
is touching the surface of tissue with your fingers, the cancer lesion can feel a stiff, hard
nodule. However, we hope to detect the ‘nodule’ as small as possible, since it is very
important for the medical treatment. The key for manual palpation is because of the
contrast in mechanical properties between healthy and diseased tissue. In this paper, I
present an acousto-optical method to discriminate between normal and mechanically
altered skin. We strain the tissue acoustically (1 Hz) and track the shift in the speckle
pattern coming from simultaneously illuminating the tissue with a low power laser. The
velocity and relative phase of the surface waves can then be determined by tracking the
shift in the back-scattered laser speckle pattern as a result of the passing acoustic stress
waves.
Since the poor signal-to-noise ratio in the acoustic data at the low frequencies (1-3 Hz),
the most investigations focus on ultrasonic frequencies. A few investigators have,
however, examined the behavior of certain biological tissues at low frequencies. For
example, Potts et al.1 investigated the propagation and attenuation of shear waves in
human skin over the frequency range from near zero to 1 KHz. They concluded that at
frequencies below a few hundred Hz, waves propagate primarily as surface (Rayleigh)
waves, while at higher frequencies, the waves are best considered as bulk shear waves.
Thus, low frequency stimulation can be used for gathering information on surface layers
(epidermis) while frequencies above approximately 500 Hz can be used to interrogate the
deeper dermis. Pereira et al.2 also employed a wave propagation technique to measure the
dynamic viscoelastic properties of excised skin that was subjected to a low incremental
strain. They examined the propagation velocity and attenuation of the acoustic waves as a
function of frequency (0 - 1 KHz) and static stress state of the skin. From these wave
parameters, they calculated viscoelastic properties of the skin including storage and loss
moduli, as well as the mechanical loss tangent (tan ) 3. The loss tangent is the tangent of
the phase angle between the driving stress wave and the resulting strain. It provides a
measure of the amount of elastic energy lost to the system under dynamic conditions. The
energy is typically lost as heat or used for overcoming internal friction between the
molecules in the material. Pereira et al.2 determined that at low static stresses, the loss
tangent was approximately 0.6, and that the attenuation of the wave increased roughly
with increasing frequency. At higher static stress levels, the wave velocity was greater
and the rate of attenuation slower than at low static stress levels.
Rayleigh waves are surface acoustic waves in which longitudinal and shear
displacements are coupled together and travel at the same velocity. It should be noted that
surface wave measurements are influenced by subsurface inhomogeneities in that
Rayleigh waves penetrate below the surface to depths comparable to their wavelengths.
Thus, a localized stiffness (or softness) below the surface will influence the behavior of
the surface wave.
Wavelength and velocity of Rayleigh waves
Consistent with the concept of surface waves, the wavelength of a Rayleigh wave is
given by
(1)
R  CR / f
where  R is the Rayleigh wavelength, CR is the Rayleigh wave velocity, and f   / 2 is
the frequency of the acoustic wave. To arrive at CR, a typical 2-point surface
measurement was made (Fig. 1) and the Rayleigh wave velocity determined by the
equation
L
CR 
(2)
1   2
where L is the distance between two observation points and 1 and  2 are the phases of
the wave at the two points, respectively.
Figure (1)
The backscattered speckle pattern from each spot was imaged onto a portion of a linear
array CCD camera with a 60mm macro lens. The camera was triggered at 50Hz for a
total of 200 exposures and the exposures were stacked into a 2-dimensional array such
that exposure number (time) was along the ordinate and camera pixel number was along
the abscissa (Fig.2).
Figure (2)
Careful inspection of Figure 2 will reveal a periodic (~1Hz) " wiggle " in the speckle
history through time (i.e., as you move from the top of the image to the bottom). The
wiggles in the two columns of data are out of phase with each other due to the difference
in the time of flight of the acoustic wave between the two spots. This difference in phase
is the denominator of E.Q 2.
.
The phase of the wiggles was determined by implementing a speckle tracking algorithm.
The algorithm employs a maximum likelihood approach to track the motion of the
speckles as a function of time. The shift in the speckle pattern was plotted against time
(record number) for each column, and the phase of each wave was determined as
of the waves (Fig. 3).
Figure (3)
It can be seen from Fig. 3 (barely) that the two waves are slightly out of phase with each
other.
Mechanical Loss Factor (
) –will be finished
I will inject glutaraldehyde, which is used to create a small, local stiff region, and
collagenase/elastase cocktail, which is to create a soft, viscous lesion. In both cases, the
lesions were approximately 5mm in diameter and less than 1mm deep. Figure 4 details
the experimental design.
Figure (4)
3 Content
Speckle and Rayleigh wave are two important key words in my paper. The propagation
and attenuation of the Rayleigh wave (surface wave) are used to detect viscoelastic
properties of the skin. The motion in the speckle pattern is the gauge to calculate the
phase of each surface wave.
In my experiment, the optical system incorporates a lens to create an image of the object.
This kind of speckle on the image plane names “subjective speckle”. (Gray 4, chapter 10)
the size of the individual speckles in this case is then related to the aperture ratio F =
focal length /aperture = f/a of the lens and the magnification M of the lens. The speckle
size S subj in the image is
Ssubj  1.22(1 M)F
(3)
Speckle is the random interference effect observed when coherent light (e.g. laser) is
scattered from an optically rough surface or volume (Goodman et al 3).
In my lab, we let S subj equal to the size of one pixel in camera. The distance between lens

and tissue is almost ~10cm.
4 Conclusions
The result of my experiment provides preliminary evidence that any change in the
mechanical properties of the tissue below the surface should influence the measured
properties of the surface waves.
Based on this project, I understand more detail of subjective speckle and Rayleigh wave,
and familiar with the whole optical system, which is used to record the speckle patterns.
5 References
1. Potts, R.O, Chrisman, D.A., Buras, E.M. The dynamic mechanical properties of
human skin in vivo, J. Biomech 16(6):365-372, 1983. (10%)
2. Pereira, J.M., Mansour, J.M., Davis, B.R. Dynamic measurement of the
viscoelastic properties of skin, J Biomech 24(2):157-162, 1991.(5%)
3. J.W.Goodman, et. Some fundamental properties of speckle. (5%)
4. Gary Cloud, et “Optical Methods of Engineering Analysis” (10%)
5. Sean J. Kirkpartick. Optical assessment of Tissue Mechanics: Acousto-optical
Elastography of skin. (Since my experiment is based on this paper, there are many
parameters, pictures and ideas coming from this paper. 70%)