Photoacoustic imaging (PAI) combines high ultrasound resolution with optical contrast. Laser-generated ultrasound
is potentially beneficial for cancer detection, blood oxygenation imaging, and molecular imaging. PAI
is generally performed using solid state Nd:YAG lasers in combination with optical parametric oscillators. An
alternative approach uses laser diodes with higher pulse repetition rates but lower power. Thus, improvement
in signal-to-noise ratio (SNR) is a key step towards applying laser diodes in PAI. To receive equivalent image
quality using laser diodes as with Nd:YAG lasers, the lower power must be compensated by averaging, which can
be enhanced through coded excitation. In principle, perfect binary sequences such as orthogonal Golay codes
can be used for this purpose when acquiring data at multiple wavelengths. On the other hand it was shown for
a single wavelength that sidelobes can remain invisible even if imperfect sequences are used. Moreover, SNR can
be further improved by using an imperfect sequence compared to Golay codes. Here, we show that pseudorandom
sequences are a good choice for multispectral photoacoustic coded excitation (MSPACE). Pseudorandom
sequences based upon maximal length shift register sequences (m-sequences) are introduced and analyzed for the
purpose of use in MSPACE. Their gain in SNR exceeds that of orthogonal Golay codes for finite code lengths.
Artefacts are introduced, but may remain invisible depending on SNR and code length.
We present a photoacoustic measurement system based on semiconductor lasers for blood oxygenation measurements. It
permits to use four different optical wavelengths (650nm, 808nm, 850nm, 905nm) to generate photoacoustic signals. As
the optical extinction coefficient of oxygenated hemoglobin and deoxygenated hemoglobin is different at specific
wavelengths, a blood oxygenation measurement by a multi-wavelength photoacoustic laser system is feasible. Especially
at 650nm, the clear difference between the extinction coefficients of the two hemoglobin derivates permits to determine
the blood oxygenation in combination with other near infrared wavelengths. A linear model based on tabulated values of
extinction coefficients for fully oxygenated and fully deoxygenated hemoglobin is presented. We used heparin stabilized
whole porcine blood samples to model the optical behavior of human blood, as the optical absorption behavior of porcine
hemoglobin does not differ significantly from human hemoglobin. To determine the real oxygen saturation values of the
blood samples, we measured the partial oxygen pressure with an IRMA Trupoint Blood Analysis System. The oxygen
saturation values were calculated from a dissociation curve for porcine blood. The results of the photoacoustic
measurement are in qualitatively good agreement with the predicted linear model. Further, we analyze the abilities and
the limitations of quantitative oxygenation measurements.
We present a multi-wavelength semiconductor laser source for photoacoustic imaging. We discuss the abilities of the
system and its limitations. In detail we analyze how this laser diode system might be used to increase the spectral
contrast of ultrasonic systems. In a first test set-up we prove in principle the spectral sensitivity of this device.