|
Photoacoustic microscopy (PAM) is a high-contrast, high-resolution imaging modality used primarily for imaging
hemoglobin and melanin. Important applications include mapping of the microvasculature and melanoma tumor
margins. We have developed a novel photoacoustic microscope design, which substantially simplifies construction by
enabling the use of unmodified commercial optics and ultrasonic transducers. Moreover, the simple design may be
readily incorporated into a standard light microscope, thus providing a familiar imaging platform for clinical researchers.
A proof-of-concept Off-Axis PAM system with a lateral resolution of 26 μm and a modest axial resolution of 410 μm
has been assembled and characterized using tissue samples. We have derived the appropriate equations to describe the
relevant design parameters and verified the equations via measurements made on our prototype Off-Axis PAM system.
A consequence of the simple design is a reduction in axial resolution compared to coaxial designs. The reduction is
inversely proportional to the cosine of the angle between excitation and detection and equal to 15% and 41% for angles
of 30º and 45º, respectively. While resolution is negatively affected by off-axis detection, the ability to measure weak
signals at depth is enhanced. Off-axis detection has an inherent dark-field quality; chromophores excited outside the
numerical aperture of the ultrasonic detector will not be detected. The physical geometry of Off-Axis PAM enables the
placement of the ultrasonic transducer at the minimum distance from the sample with no obstructions between the
sample and transducer. This may prove to be an additional advantage of Off-Axis PAM over designs that incorporate
long working distance ultrasonic transducers and/or require the propagation of the acoustic wave through the laser
excitation optics to achieve co-axial detection.
|
