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Measurement of Grüneisen

parameter of tissue by photoacoustic

spectrometry

Da-Kang Yao, Lihong V. Wang

Da-Kang Yao, Lihong V. Wang, "Measurement of Grüneisen parameter of

tissue by photoacoustic spectrometry," Proc. SPIE 8581, Photons Plus

Ultrasound: Imaging and Sensing 2013, 858138 (4 March 2013); doi:

10.1117/12.2004117

Event: SPIE BiOS, 2013, San Francisco, California, United States

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Measurement of Grüneisen Parameter of Tissue

by Photoacoustic Spectrometry

Da-Kang Yao and Lihong V. Wang

Optical Imaging Laboratory, Depart

ment of Biomedical Engineeri

ng, Washington University in St.

Louis, One Brookings Drive,

St. Louis, Missouri63130, USA

*Corresponding author: LHWANG@WUSTL.EDU

ABSTRACT

The Grüneisen parameter of tissue is a constitutive parameter in photoacoustic tomography. Here, we applied

photoacoustic spectrometry (PAS) to dir

ectly measure the Grüneisen parameter.

In our PAS system, laser pulses at

wavelengths between 460 and 1600 nm were delivered to tissue samples, and photoacoustic signals were detected by a

20 MHz flat water-immersion ultrasonic transducer. By f

itting photoacoustic spectra to light absorption spectra, we

found that the Grüneisen parameter was 0.73 for porcine subcutaneous fat tissue, and 0.15 for oxygenated bovine red

blood cells at room temperature (24

C).

Keywords

: photoacoustic tomography, subcutaneous fat, lipid, red blood cell

1. INTRODUCTION

The Grüneisen parameter is a constitutive parameter in photoacoustic tomography. In photoacoustic tomography, light

pulses are delivered into biological tissue. Once the tissue ab

sorbs the light, it is converted to heat, generating an initial

pressure rise due to thermoelastic expansion. The initial pressure

gives rise to a photoacoustic signal, which is detected to

construct a photoacoustic image. The Grüneisen parameter

of tissue relates the initial pressure

to the light absorption

by the following expression [1, 2],

, (1)

where

is the absorption coefficient of tissue, and

is the local light fluence. Therefore, the Grüneisen parameter is a

critical factor in photoacoustic imaging.

It is necessary to directly measure the Grüneisen parameter of

tissue. Scientists can estimate the Grüneisen parameter in

terms of the isobaric volume expansion coefficient

, the specific heat

, and the acoustic speed

using the following

expression:

, and find that the Grüneisen parameter varies between different types of tissue. However, this

estimation usually lacks accuracy. For exampl

e, the Grüneisen parameter of fat tissue is estimated to be between 0.7 and

0.9 instead of an exact value

[3], causing uncertainty in photoacoustic imag

ing. In order to minimize the uncertainty, we

applied photoacoustic spectrometry (PAS) to

directly measure the Grüneisen parameter.

2. METHODS

2.1 Photoacoustic Spectrometry

We assembled a PAS system to acquire photoacoustic spectra of tissue. Figure 1 is a schematic of the PAS system. An

OPO laser system (NT242-SH, Altos Photonics, Bozeman, MT) provides a wavelength tuning range from 460 to 1600

nm. The tunable laser system emits a pulsed laser beam at a re

petition rate of 1 kHz. The beam diameter is 1 mm and the

pulse width is 5 ns. After passing through a beam sampler (BSF10-B, Thorlabs, Newton, NJ) and a 2.5 μm thick

Photons Plus Ultrasound: Imaging and Sensing 2013, edited by Alexander A. Oraevsky, Lihong V. Wang,

Proc. of SPIE Vol. 8581 858138-1

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ultrafilm (01865-AB, SPI Supplies, West Ch

ester, PA), the laser beam illuminates a 1.5 mm thick tissue sample in the

front part of a cylindrical chamber. A

20 MHz flat ultrasonic tr

ansducer (V316-SM,Olympus, Waltham, MA) is installed

at the rear of the chamber. Water is f

illed in the rear part of the chamber for acoustic coupling, and separated from the

sample by another piece of ultrafilm. The distance between th

e sample and the transducer is 33 mm. After detected by

the ultrasonic transducer, photoacoustic signals are amplified by an amplifier (ZFL-500LN, Mini-Circuits, Branson, MO).

The laser pulse energy is measured by a pair of photodiode detectors (SM05PD2A and SM05PD4A,Thorlabs). Both

photoacoustic and photodiode signals are collected by a computer through a 12-bit, 200 MHz digitizer (NI PCI-5124,

National Instruments, Austin, TX).

Here, all measurements of photoacoustic spec

tra were performed at room temperature (24

C).

2.2 Photoacoustic Spectrum Fitting

We obtained the Grüneisen parameter of tissue by fitting th

e photoacoustic spectra. Photoacoustic spectra are measured

in the wavelength range in which the absorption coefficient of ti

ssue is much larger than its reduced scattering coefficient.

Thus, the effect of tissue scattering on fluence is neglected. The relation between the peak-to-peak amplitude (

) of a

photoacoustic signal, the Grüneisen parameter, the pulse energy (

), and the absorption coefficient of the tissue, is

expressed as

, (2)

where

is a constant in regard to the PAS system, and

A/E

is a normalized amplitude. The constant

is determined by a

system calibration. To reduce errors of

photoacoustic spectra, the normalized am

plitude is acquired by averaging 10,000

photoacoustic signals at each wavelength.

Using Eq. 2, we fit acquired photoacous

tic spectra of tissu

e to its absorption

spectra by nonlinear least-squares fitting, in whic

h the Grüneisen parameter is a fitting parameter.

We determined the constant

in Eq. 2 by measuring the photoacoustic spectra of water. A wavelength range from 1200

to 1600 nm was scanned at a 10 nm wavelength step size. Knowing the absorption coefficient of water [4] and that the

water Grüneisen parameter at 24

C is 0.13, we set

as a fitting parameter. Fitting the photoacoustic spectra of water to

its absorption spectrum yielded the setup constant

, which was 19.5 mV·cm/μJ.

OPO laser

460 – 1600 nm

5 ns

Ultrasonic transducer

20 MHz

Computer

Digitizer

Beam sampler

Energy detector

Amplifier

Water

Ultrafilm

2.5 μm thick

Sample

1.5 mm thick

Figure 1.Schematic of the photo

acoustic spectrometry system.

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3. RESULTS

3.1 Grüneisen Parameter of porcine fat tissue

Porcine subcutaneous fat tissue was cut into 1 mm thick slices. After a slice adhered to the ultrafilm close to the

ultrasonic transducer (Fig. 1), we scanned fat tissue at a wavele

ngth range from 880 to 1000 nm at a 5 nm scan step size.

A photoacoustic spectrum is shown in Fig. 2. Assuming that light absorption of fat tissue is equal to lipid absorption plus

water absorption, we used the Grüneisen parameter and lipid

concentration as fitting parameters. A solid curve shown in

Fig. 2 is a fit to the absorption spectrum of 98% lipid and 2% water [5, 6]. Using two samples, we found that the

Grüneisen parameter of the fat tissue was 0.73 ± 0.01 (mean ± SD) at 24

3.2 Grüneisen Parameter of Bovine Red Blood Cells

Bovine red blood cells were collected from defibrinated bovine blood (Quad Five,

Ryegate, MT

) after centrifugation.

The cells were washed two times using phosphate buffered saline (PBS) and then suspended in PBS. Hemoglobin

concentration of the cell suspension was measured to be 22.6 mg/ml by a spectrophotometer. To oxygenate the red blood

880

900

920

940

960

980

1000

fat tissue

fit to 98% lipid and 2% water

Norm ali zed amplitude (mV/

Wavelength (nm)

= 0.985

Figure 2. Photoacoustic spectrum

of porcine subcutaneous fat tissue. Circles

show the normalized amp

litude (mean ± SE) of fa

tissue versus wavelengths. The solid curve is a fit to the ab

sorption spectrum of 98% lipid and 2% water. Here, two fi

parameters are the Grüneisen parameter and lipid concentration.

100

150

460

480

500

520

540

56 0

ox y-RBC

fit to HbO2

Norm ali zed amplitude (mV/

Wavelength (nm)

= 0.986

Figure 3. Photoacoustic spectrum of bovine

red blood cells (RBC). The cells were su

spended in phosphate buffered saline wit

22.6 mg/ml hemoglobin. RBC

was aerated with O

. Circles show the normalized ampl

itude (mean ± SE) of oxy-RBC versus

wavelengths. A solid curve is a fit to th

e absorption coefficient of oxy-hemoglobin.

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cells, we aerated the cell suspension with

oxygen for 1 hour. We scanned the oxygenated cell suspension in a wavelength

range from 460 to 560 nm at a 5 nm scan step size. A typi

cal photoacoustic spectrum of oxygenated red blood cells is

shown in Fig. 3, in which the solid curve is a fit to the absorption spectrum of 22.6 mg/ml oxygenated hemoglobin [7].

After measuring three oxygenated samples, we found that the Grüneisen parameter was 0.15 ± 0.01(mean ± SD) for

oxygenated red blood cells at 24

4. DISCUSSION

Using photoacoustic spectrometry, we developed a method to directly measure the Grüneisen parameter of tissue.

Accuracy of the measurement relies on acc

urate absorption coefficients

of the tissue. In our measurements, all sample

absorption spectra were collected from published reports. Fo

r example, absorption spectra of water and hemoglobin were

obtained from a website [7], and absorption spectra of lipid we

re obtained from both the website and Tsai et al [6]. Since

these absorption data are well established,

our measurement greatly reduces uncertainty of the Grüneisen parameter. Our

method may be generalized to any tissue sample, including tissue of which the absorption coefficient is unknown. If the

absorption coefficient of the sample is not known, measuring

its absorption coefficient is prerequisite to measuring the

Grüneisen parameter by using the method.

Our measurement results support that the Grüneisen parameter varies between different tissues. We found that the

Grüneisen parameter was 0.15 for oxygenated red blood cells, and 0.73 for subcutaneous fat tissue at 24

C, indicating

that the Grüneisen parameter of fat tissue is 4.9 times larger th

an that of oxygenated red blo

od cells at room temperature.

Besides subcutaneous fat tissue, various adipose tissues are lo

cated at bone marrow, brain, br

east, colon, and liver. Since

each adipose tissue has its characteristic compositions, it is ve

ry likely that these adipose

tissues are different in the

Grüneisen parameter. Direct measurement

of the Grüneisen parameter of each adipose tissue will allow us to reconstruct

accurate photoacoustic images of organs.

5. CONCLUSIONS

We directly measured the Grüneisen

parameter of tissue by photoacoustic spectrometry to reduce uncertainty in

photoacoustic tomography. It is found that the Grüneisen parameter of oxygenated red blood cells is 0.15 at room

temperature (24

C), 15% larger than the Grüneisen parameter of water. It is found that the Grüneisen parameter of fat

tissue is 0.73 at 24

C, 4.9 times larger than that of oxygenated red blood cells.

ACKNOWLEDGEMENTS

This work was sponsored in part by National Institute

s of Health grants R01 EB000712, R01 EB008085, R01

CA113453901, U54 CA136398, 5P60 DK02057933, and U54 CA136398. L.W. has a financial interest in

Microphotoacoustics, Inc. and Endra, Inc.,

which, however, did not support this work.

REFERENCES

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[5]

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