Photoacoustic and Ultrasound Imaging with Nanosized Contrast Agents - Nanotechnology for Biomedical Imaging and Diagnostics - page 290

(a) (b) (c)

CCD

Dye laser

Neutral
density
lter

Conical lens Mirror

Iris

BS
PD

Objective

Ultrasound
transducer
Optical
condenser

Objective
Corl
RAP
SO

2D galvo
mirrors

US

x

Objective
lens

RhP

Ultrasonic
detector

y x

z Membrane Petri dish

Dark- eld
illumination

FOV

figure 10.3 Various reflection-mode PAM systems implemented by (a) a dark-field
confocal configuration (reprinted with permission from ref. [76]. © American Chemical
Society.), (b) optical focusing with mechanical raster scanning (From ref. [77]. © optical
Society of America.), and (c) optical focusing with optical raster scanning. (From ref. [75]. ©
optical Society of America.)

of the transducer controls the axial resolution. Although no mathematical image
reconstruction algorithm is required and its associated artifacts are eliminated, the
imaging speed and field of view (FoV) are quite limited in this type. Figure 10.3
shows various types of PAM systems. For instance, the schematic of dark-field con-
focal acoustic-resolution PAM is shown in Figure 10.3a [76]. A donut-shaped light
illumination pattern (i.e., dark-field confocal configuration) formed by a conical
lens and optical condenser greatly enhances the penetration depth and signal-to-noise
ratio (Snr). When a 5 MHz ultrasonic transducer was used, the axial and transverse
resolutions were 150 and 590 µm, respectively. The penetration depth was beyond
30 mm. When a 50 MHz ultrasonic transducer was used, the axial and transverse
resolutions were 15 and 45 µm, respectively. The penetration depth was approxi-
mately 3 mm. To improve the transverse resolution to the order of a µm, hardware-
based optical focusing was applied. This mode is called as optical-resolution PAM.
The transverse resolution can be enhanced up to the diffraction-limited optical reso-
lution. Since the transverse resolution is based on optical focusing, the penetration
depth is limited to one optical transport mean free path (~1 mm), similar to conven-
tional optical microscopy. Figure 10.3b shows the diagram of a reflection-mode
optical-resolution PAM [77]. To form 2d or 3d images, mechanical or optical raster
scanning is required. The mechanical scanning-based PAM system has a high Snr,
while it suffers from a low imaging speed. Although the imaging speed can be
improved by scanning optically, the system had a low Snr because the system used
an unfocused US transducer (Fig. 10.3c) [75].
Typically, the generated PA waves are detected by using piezoelectric transducers.
However, by employing a transparent Fabry-Perot polymer film etalon, the PA waves
can be probed optically [78, 79]. The thickness changes of the etalon due to the PA
waves disturbed the reflectivity of the etalon, and this led to change in the reflected
light intensity of a probe laser beam. The advantages of this system are broader PA
detection bandwidth and detection aperture.