Biomedical Engineering Reference
In-Depth Information
2. Higher-frequency ultrasonic transducer provides finer axial resolution at the
expense of ultrasonic penetration. Choose a proper transducer according to the
desired axial resolution and imaging depth. Generally speaking, the optimal
transducer center frequency is between 50 and 150 MHz.
3. The optical and ultrasonic foci should be configured coaxially and confocally
whenever possible to maximize the imaging sensitivity.
Here is an example of how to follow the guidelines. For in vivo single capillary
imaging in the skin [
6
], a spatial resolution comparable with or smaller than
the average capillary diameter (4-9m) is required. Thus, we choose an optical
microscope objective with an NA of 0.1, which translates into a diffraction-limited
focal diameter of
3:7 m in the operation wavelength range (500-650 nm). With
such lateral resolution, OR-PAM is capable of resolving individual RBCs (average
diameter: 6-8m) traveling along blood vessels [
11
]. A transducer with a center
frequency of 75 MHz and a bandwidth of 80% is chosen to provide adequate axial
resolution (15 m) without encountering severe high-frequency acoustic loss in
the skin [
6
]. Note that for transcranial imaging of cortical capillaries, the center
frequency of the ultrasonic transducer should be reduced to 30-50 MHz because of
the strong skull attenuation of high-frequency ultrasound [
13
].
2.3
System Configuration
The schematic of a representative OR-PAM system is shown in Fig.
2.2
[
18
].
The photoacoustic excitation source consists of a diode-pumped solid-state laser
(INNOSLAB, Edgewave) and a dye laser (CBR-D, Sirah), which emits wavelength-
tunable laser pulses (pulse width: 7 ns; repetition rate: <5 kHz and controlled by the
external trigger signal). The pulsed laser beam is first attenuated by a neutral-density
filter (NDC-50C-2M, Thorlabs) and then focused by a condenser lens (LA1131,
Thorlabs), before passing through a 25-m pinhole (P250S, Thorlabs) for spatial
filtering. The pinhole is positioned slightly away from the focus of the condenser
lens, where the beam size is larger than the pinhole diameter, to allow effective
filtering. Then, the filtered beam is focused by a microscope objective (RMS4X,
Thorlabs). The distance between the pinhole and the objective is
400 mm,
allowing sufficient beam expansion to fulfill the back aperture of the objective. This
configuration gives a near-diffraction-limited optical focus (diameter: 3:7 m). To
align the optical irradiation and ultrasonic detection coaxially and confocally, we
have designed an acoustic-optical beam splitter. In this beam splitter, two right-
angle prisms (NT32-545, Edmund Optics) are aligned along their hypotenuses to
form a cube with a thin layer (100 m) of silicone oil (1000cSt, Clearco Products) in
between. The prism glass and the silicone oil have similar optical refractive indices
(1.1:1) but very different acoustic impedances (12.7:1). As a result, this beam
splitter is optically transparent but acoustically reflective, thereby deflecting the
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