Biophotonics_Concepts_to_Applications

detected pressure depends on the degree of optical absorption in the target object,
the thermal diffusion properties of the tissue, and the thermoelastic properties of the
tissue. A number of different optical irradiation and ultrasonic probe architectures
have been proposed for these measurements. The example scheme shown in
Fig.10.15uses an annular acoustic probe with the laser pulse passing through the
center opening of the probe.
The image resolution is limited by the light pulse duration, the properties of the
tissue, and the response characteristics of the ultrasonic probe.Increased resolution
results from higher detected acoustic frequencies, shorter light pulses, and smaller
measurement depths.Typical resolutions are in the micrometer range for a mea-
suring depth of millimeters and in the millimeter range for a measuring depth of
centimeters. A point to note is that whereas the resolution is determined by the
ultrasound parameters, the image contrast is determined by the optical absorption.
Two key parameters that need to be considered for the implementation of PAT
are the thermal relaxation time and the stress relaxation time [ 42 ]. Thethermal
relaxation time(TRT)τthis a commonly used parameter for estimating the time
required for thermal energy to conduct away from a heated tissue region. The TRT
is defined as the time necessary for an object to cool down to 50 % of its original
temperature. If a laser pulse that impinges on a tissue sample is shorter than the
TRT, then mainly the target material will be heated. This condition is known as
thermal confinement. When the pulse is longer than the TRT, the heat will travel
into the surrounding tissue structure, which could result in tissue damage because
the heat does not dissipate fast enough. The TRT can be estimated by

sth¼

d^2

ath

ð 10 : 21 Þ

where d is the size of the heated region andαthis thethermal diffusivity, which is
about 0.13 mm^2 /s for soft tissue.
When a laser pulse heats a tissue, the heated sample will undergo a thermoelastic
expansion, which results in the emission of acoustic waves. After the pulse stops,

Ultrasonic

signal detector

Tissue sample

Probing light beam

from an optical fiber

Ultrasonic signals

returning from

the tissue sample

Short optical pulses

interacting with

the tissue sample

PC signal

analyzer

Fig. 10.15Example setup
for a photoacoustic
measurement method

10.5 Photoacoustic Tomography 313