In the postgenomic era, imaging techniques are playing an important role in visualizing gene expression in vivo. This work represents the first demonstration of photoacoustic tomography (PAT) for reporter gene imaging. Rats inoculated with 9L/lacZ gliosarcoma tumor cells are imaged with PAT before and after injection of X-gal, a colorimetric assay for the lacZ-encoded enzyme -galactosidase. Using far-red optical illumination, the genetically tagged tumors in rats are clearly visualized by PAT following the assay. The spatial resolution is quantified to be less than 400 µm, while 500-nM-level sensitivity is demonstrated. With the future development of new absorption-based reporter gene systems, it is anticipated that photoacoustic technology will provide a valuable tool for molecular imaging research.
Molecular imaging is a newly emerging field in which the modern tools of molecular and cell biology have been
married to state-of-the-art technologies for noninvasive imaging. The study of molecular imaging will lead to better
methods for understanding biological processes as well as diagnosing and managing disease. Here we present
noninvasive in vivo spectroscopic photoacoustic tomography (PAT)-based molecular imaging of αvβ3 integrin in a
nude mouse U87 brain tumor. PAT combines high optical absorption contrast and high ultrasonic resolution by
employing short laser pulses to generate acoustic waves in biological tissues through thermoelastic expansion.
Spectroscopic PAT-based molecular imaging offers the separation of the contributions from different absorbers based
on the differences in optical absorption spectra among those absorbers. In our case, in the near infrared (NIR) range,
oxy-heamoglobin (O2Hb), deoxy-heamoglobin (HHb) and the injected αvβ3-targeted peptide-ICG conjugated NIR
fluorescent contrast agent are the three main absorbers. Therefore, with the excitation by multiple wavelength laser
pulses, spectroscopic PAT-based molecular imaging not only provides the level of the contrast agent accumulation in
the U87 glioblastoma tumor, which is related to the metabolism and angiogenesis of the tumor, but also offers the
information on tumor angiogenesis and tumor hypoxia.
In the post-genomic era, there is an increasing interest in visualizing the expression of functional genes in vivo. With the assistance of the reporter gene technique, various imaging modalities have been adopted for this purpose. In vivo gene expression imaging promises to provide biologists with a powerful tool for deepening our understanding of developmental biology, expanding our knowledge of the genetic basis of disease, and advancing the development of medicine. In this paper, we demonstrate the feasibility of imaging gene expression with photoacoustic imaging, which offers unique absorption contrast with ultrasonic resolution in vivo. We mark tumors in rats with the lacZ reporter gene. The lacZ gene encodes an enzyme β-galactosidase, which yields a dark blue product when acting on a colorimetric assay called X-gal. Photoacoustic tomography at 650nm clearly visualizes the presence of this blue product. The spectroscopic method can also potentially improve specificity. Considering how many staining methods are used in traditional biology, we believe that photoacoustic techniques will revolutionize the field of molecular imaging. The further development of reporter gene systems with high absorbing products in the NIR region is needed.