PROCEEDINGS ARTICLE | February 25, 2012

Proc. SPIE. 8320, Medical Imaging 2012: Ultrasonic Imaging, Tomography, and Therapy

KEYWORDS: Breast, Tumors, Modulation, Scattering, Ultrasonography, Light scattering, Ultrasonics, Monte Carlo methods, Phase shifts, Anisotropy

A Monte Carlo model of ultrasound modulation of multiply scattered coherent light in a highly scattering media
has been carried out for estimating the phase shift experienced by a photon beam on its transit through US
insonified region. The phase shift is related to the tissue stiffness, thereby opening an avenue for possible breast
tumor detection. When the scattering centers in the tissue medium is exposed to a deterministic forcing with the
help of a focused ultrasound (US) beam, due to the fact that US-induced oscillation is almost along particular
direction, the direction defined by the transducer axis, the scattering events increase, thereby increasing the
phase shift experienced by light that traverses through the medium. The phase shift is found to increase with
increase in anisotropy g of the medium. However, as the size of the focused region which is the region of interest
(ROI) increases, a large number of scattering events take place within the ROI, the ensemble average of the
phase shift (Δφ) becomes very close to zero. The phase of the individual photon is randomly distributed over 2π
when the scattered photon path crosses a large number of ultrasound wavelengths in the focused region. This is
true at high ultrasound frequency (1 MHz) when mean free path length of photon l<sub>s</sub> is comparable to wavelength
of US beam. However, at much lower US frequencies (100 Hz), the wavelength of sound is orders of magnitude
larger than ls, and with a high value of g (g 0.9), there is a distinct measurable phase difference for the photon
that traverses through the insonified region. Experiments are carried out for validation of simulation results.