Photoacoustic tomography (PAT) is a hybrid optical and ultrasound imaging technique that combines the high spatial resolution of ultrasound with the high optical contrast of diffuse optical imaging. It is a highly promising modality that has received increasing attention from the biomedical imaging community because of its ability to image in three dimensions, deep into living tissue. PAT is based on the illumination of an object with brief pulses of laser light. As the light diffuses into the tissue, it is absorbed differentially by the various regions of the tissue. The light absorption causes brief localized heating and an increase in pressure proportional to the amount of light energy absorbed. This leads to the generation of pressure waves that travel out of the tissue and can be measured by ultrasonic transducers. The detected signals are then reconstructed into an image of optical absorption using a computer algorithm.
Photoacoustic tomography systems of many different geometries have been proposed and investigated in the literature, such as circular 
, semi-cylindrical or curved 
, linear 
, planar 
and hemispherical 
measurement apertures. This list is by no means exhaustive, and further references can be found in the recent review articles by Yao 
and Beard 
. A substantial number of photoacoustic tomography systems use water as a coupling medium to optimize transmission of the sound waves from the scanned object to the ultrasound transducers. This water is commonly heated to body temperature for small animal imaging, as is also the case for the imaging system used in this study, namely the Nexus 128 scanner produced by Endra Life Sciences (see the ‘Photoacoustic scanner’ section). Unfortunately, the water heating pump of this system sporadically injects air bubbles into the water. The manufacturer therefore recommends switching off the heating pump during a scan to minimize the risk of bubble formation on either the water-object or water-transducer interfaces. In this study, we found that the water tends to drop in temperature during scans of normal duration (
C during a 15 minute scan, typical of a single wavelength scan, and
C during a 60 minute scan, typical of a multi-wavelength scan). In this work, we characterize the reconstruction artifacts that arise when ignoring these gradual temperature changes during the image reconstruction stage. In the Results
section, we demonstrate the importance and efficacy of carefully monitoring the decreasing water temperature and applying software corrections to the changing speed of sound accordingly to optimize image quality. The improvements in image quality were shown to be consistent across objects of widely varying appearances, namely physical phantoms, an ex vivo
mouse brain, as well as an in vivo
subcutaneous tumor in a mouse. These results are significant because they show that a software correction for the temporal change in water temperature provides a cost-effective alternative to upgrading to a higher specification heating unit. Since air bubbles pose a common problem in ultrasonic and photoacoustic imaging systems, we believe that our results will help improve the quality of future small animal imaging studies that use scanners with similarly limited heating units.
Regarding the previous literature on speed of sound considerations in photoacoustic imaging, most studies so far have assumed a speed of sound distribution that does not change over the course of a scan, whether it be spatially homogeneous or heterogeneous 
. Relatively little attention has been paid to the importance of monitoring water temperature over time, and continuously updating the speed of sound accordingly. There are some mentions of monitoring water temperature during a scan (eg. 
), but no previous work has dedicated a study specifically to the characterization of artifacts generated by ignoring gradual changes in the speed of sound, as well as the efficacy of corresponding software corrections to improve tomographic image quality. The work presented in this paper addresses this gap. Note that we do assume a spatially homogenous sound distribution at any point in time in this paper. An investigation into the significance of modeling both spatial and temporal variations in the speed of sound is deferred to future work. However, since the water body is much larger than the heterogeneous volume inside the small animal being imaged, we expect that modeling temporal variations in the speed of sound will be more important than modeling spatial heterogeneities within the mouse. This is already indicated by the results in the Results
section, which show a dramatic sharpening of in vivo images by correcting for the temporal temperature changes alone.
As for the remainder of this paper, the Methods
section explains the methodology used for image reconstruction, the scan system setup, the dependence of the speed of sound on water temperature, the imaging subjects used, and the experimental procedures. The Results
, Conclusions and Further Work
sections present the results, draw conclusions, and give recommendations for future work, respectively.