Abstract
An inexpensive noncontact photoacoustic (PA) imaging system using a low-power continuous wave laser and a kilohertz-range microphone has been developed. The system operates in both optical and PA imaging modes and is designed to be compatible with conventional optical microscopes. Aqueous coupling fluids are not required for the detection of the PA signals; air is used as the coupling medium. The main component of the PA system is a custom designed PA imaging sensor that consists of an air-filled sample chamber and a resonator chamber that isolates a standard kilohertz frequency microphone from the input laser. A sample to be examined is placed on the glass substrate inside the chamber. A laser focused to a small spot by a 40 × objective onto the substrate enables generation of PA signals from the sample. Raster scanning the laser over the sample with micrometer-sized steps enables high-resolution PA images to be generated. A lateral resolution of 1.37 ?? ? m was achieved in this proof of concept study, which can be further improved using a higher numerical aperture objective. The application of the system was investigated on a red blood cell, with a noise-equivalent detection sensitivity of 43,887 hemoglobin molecules ( 72.88 × 10 ? 21 ?? mol or 72.88 zeptomol). The minimum pressure detectable limit of the system was 19.1 ?? ? Pa . This inexpensive, compact noncontact PA sensor is easily integrated with existing commercial optical microscopes, enabling optical and PA imaging of the same sample. Applications include forensic measurements, blood coagulation tests, and monitoring the penetration of drugs into human membrane.
Highlights
The photoacoustic (PA) effect was first observed in the year 1880 by Alexander Graham Bell.[1]
The PA effect initially was used in spectroscopy studies of materials throughout the mid1900s, but it was not until invention of the laser that significant biomedical imaging applications using the PA technique became possible due to the improved sensitivity provided by laser excitation.[2,3,4,5,6,7,8,9,10,11,12]
The lateral resolution limit of the system was calculated from the full width half maximum (FWHM), which was obtained using a Gaussian fit to the line spread function (LSF)
Summary
The photoacoustic (PA) effect was first observed in the year 1880 by Alexander Graham Bell.[1] The PA effect initially was used in spectroscopy studies of materials throughout the mid1900s, but it was not until invention of the laser that significant biomedical imaging applications using the PA technique became possible due to the improved sensitivity provided by laser excitation.[2,3,4,5,6,7,8,9,10,11,12] The low-power PA technique has been predominantly utilized in the field of spectroscopy,[11,12,13,14,15,16,17,18] with substantial applications in trace gas detection.[5,19,20,21,22,23] It exhibited significantly higher sensitivity than most other techniques,[24,25,26,27] with concentration sensitivity in the parts per billion.[3,4,5,25,27]. Patel and Tam reviewed the physics of the technique and extended the technique to other applications such as liquids.[31,32,33]
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