Three-Dimensional Optical-Resolution Photoacoustic Microscopy - Biomedical Optical Imaging Technologies: Design and Applications - page 64

Fig. 2.4 In vivo optical-resolution photoacoustic microscopy of ( a ) vascular anatomy and

( b ) hemoglobin oxygen saturation (sO 2 ) in a nude mouse ear. Scale bar: 1 mm

absorption at isosbestic wavelengths (e.g., 530, 545, 570, and 584 nm) [ 20 ], where
the molar extinction coefficients of HbR and HbO 2 are equal. Figure 2.4 shows the
vascular anatomy and sO 2 in a nude mouse ear imaged by dual-wavelength (561 and
570 nm) OR-PAM.

2.6.2 Photoacoustic Doppler Measurement of Blood

Flow Velocity

The Doppler effect, referred to the shift in wave frequency due to the relative motion
between the wave source and the wave detector, has been widely used for velocity
measurements. Recently, photoacoustic Doppler has been intensively studied for in
vivo label-free measurement of blood flow velocity.
The prototype of photoacoustic Doppler flowmetry was invented by Fang et al. to
measure the flow velocity along the acoustic axis (i.e., axial flow velocity) [ 21 , 22 ].
However, within the 1 mm penetration depth of OR-PAM, biological tissues mainly
form layered structures, and thus the flow component perpendicular to the acoustic
axis (i.e., transverse flow) is predominant. To image transverse flow velocity, Fang
et al. developed an M-mode photoacoustic particle imaging velocimetry based on
OR-PAM [ 23 ]. Yao et al. further extended this technique from capillaries to all types
of blood vessels with a concept called photoacoustic Doppler bandwidth broadening
[ 15 , 24 ].
The schematic of Doppler OR-PAM is shown in Fig. 2.5 a[ 15 ]. The acoustic
paths L1andL2 subtend an obtuse angle and an acute angle with respect to the
transverse flow direction, respectively. Thus, they contribute to the photoacoustic
Doppler shift with opposite signs and induce bandwidth broadening. The resulting
Doppler bandwidth can be computed by

v f
v s sin sin ';

PDB ORPA M D 2f 0 (2.5)