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Chirped ultrasound-modulated optical

tomography

Geng Ku, Gang Yao, Lihong V. Wang

Geng Ku, Gang Yao, Lihong V. Wang, "Chirped ultrasound-modulated optical

tomography," Proc. SPIE 3597, Optical Tomography and Spectroscopy of

Tissue III, (15 July 1999); doi: 10.1117/12.356831

Event: BiOS '99 International Biomedical Optics Symposium, 1999, San Jose,

CA, United States

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Chirped Ultrasound-Modulated Optical Tomography

Geng Ku, Gang Yao, Lihong V. Wang

Optical Imaging Laboratory, Biomedical Engineering Program

Texas A&M University, College Station, Texas 77843-3 120, USA

ABSTRACT

A novel chirped ultrasound-modulated optical tomography technique was developed to image turbid media. Frequency analysis

was employed to obtain spatial resolution along the ultrasonic axis. 2D images from scattering medium were obtained. The

chirped ultrasound modulated signal was detected in chicken breast tissue.

Keywords: Ultrasonic modulation, frequency swept, optical tomography, turbid media, heterodyne, spectrum.

1. INTRODUCTION

Ultrasound modulated optical tomography is a potential functionally imaging tools for tumor detection in vivo.[l-3J

Ultrasound modulated optical tomography combines the relatively transparent ultrasonic radiation with the functional

sensitive optical radiation. Based on the study of ultrasound-modulated optical tomography, we developed a novel imaging

technique: chirped (frequency-swept) ultrasound modulated optical tomography of turbid media. Instead of single frequency

ultrasound, a chirped ultrasonic wave was adopted to add spatial resolution along ultrasonic axis on transmitted laser light.

The combination of ultrasound and light allowed us to image objects buried inside turbid media.

Chirp also called frequency sweep which means that the frequency of the ultrasound varies with time as described by

f(t)= a+bt

where f(t) is the instantaneous frequency of the, a is the starting frequency and b is the sweep rate.

The frequency-swept ultrasound wave propagated vertically along the z axis. If we freeze the time and take a snapshot of the

propagating ultrasonic wave, the instantaneous frequency varies linearly along the z axis as shown in Fig. 1 .

Because

the

instantaneous frequency of the ultrasonic source increases with time and it takes time for the ultrasound wave to propagate

downward, the instantaneous frequency in the snapshot decreases as z increases. In other words, the farther down from the

ultrasonic transducer, the lower the instantaneous frequency.

f(t,z)

Snapshot

Column

Fig. 1. Instantaneous frequency distribution along the ultrasonic axis (z).

______________________________

371

Ultrasonic

Transducer

Laser Beam

Ultrasonic

Part of the SPIE Conference on Optical Tomography and Spectroscopy of Tissue Ill

San Jose, California • January 1999 SPIE Vol. 3597 • 0277-786X/99/$1O.OO

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Ultrasonic

Transducer

x?-r

Laser Beam

Fig. 2. Instantaneous frequency distribution vs. Time.

Fig 2. shows a frequency distribution vs time of an arbitrary point under ultrasonic axis. The ultrasonic wave propagates

vertically along the z axis, passing through this point. The ultrasound frequency at this point varies with time linearly as

shown by the solid straight line in Fig. 2. Light at this point is modulated by the frequency along this line. In our

experiment, we mixed this signal with a reference modulation signal. The modulation signal was also frequency-swept as

shown by the dashed line in Fig. 2. The sweep rate of the reference modulation signal is the same as the signal for the

ultrasonic wave but has a different starting frequency. The instantaneous frequency difference between the modulated optical

signal at this point and the reference signal is a constant, independent of time. This constant is related to the distance between

this point and the ultrasonic transducer. The larger the distance, the greater the constant frequency difference.

Experiment Setup

A block diagram of the experimental setup is shown in Fig. 3. A frequency-swept signal was produced using a function

generator and then was amplified in power and amplitude by a power amplifier and a transformer. The amplified signal was

applied to an ultrasonic transducer. The ultrasonic transducer converted electric power into acoustic power. The ultrasonic

wave propagated vertically into a turbid medium, which was contained in a glass tank. An ultrasound absorber was placed to

prevent ultrasonic reflection.

A He-Ne laser beam, at the wavelength of 633nm, after being broadened to 15mm, illuminated the scattering medium

perpendicularly to the ultrasound column. The laser source was chirp modulated by another function generator and

synchronized to the chirp signal for ultrasound. The two chirps have same frequency sweep rate but different starting

frequency. In the ultrasonic column, the intensity chirp modulated light was modulated by chirped ultrasound and caused the

heterdyned signal. A photomultiplier tube (PMT) collected some transmitted light and converted the optical power into

electric power. The output from PMT interface was band-pass filtered and then amplified using a filter and an amplifier,

respectively. The amplified signal was recorded by a digital oscilloscope and then transferred to a computer for post-

processing. Lock-in Amplifier was used to detect weak signal.

372

Ultrsonic

Column

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Fig. 3. A block diagram of the experimental setup.

An object was buried in the middle plane of the tank to simulate a tumor. The object was translated in the tank

perpendicularly to both the laser beam and the ultrasonic column. A time-domain signal was recorded at each stop. Fast

Fourier transform (FF1') was used to obtain the spectra of the recorded time-domain signals.

The frequency spectra yielded imaging information for the zone of interest selected by the band-pass filter. A frequency in the

spectra corresponded to the frequency difference between the instantaneous frequency on ultrasonic axial position in the zone of

interest and the instantaneous frequency of the modulation signal to the PMT, which was related to the frequency

sweep rate

and the ultrasonic propagation time. As the time dependent term was erased, the frequency deference was independent of time

and was related to the distance between the position on ultrasonic axis and ultrasonic transducer. A frequency in the

spectrum

was converted into a distance from the transducer surface to a point in the zone of interest. There was a one-to-one

correspondence between the frequency in the spectrum and the position in the zone of interest. In other words, a frequency

spectrum could be converted into a 1D image of the turbid medium along the ultrasonic axis.

Image and Discussion

A sample 2D image of an object buried in a turbid medium was shown in Fig. 4. The cross section of the object was —

x 2 mm in size. The scattering coefficient and anisotropy of the turbid medium at 633 nm wavelength were 0.21 cm' and

0.73, respectively. The length of the tank along the laser light was 17 cm.

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Ultrasound Absorber

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