Recent technological advances in the field of photoacoustic imaging has led to the development of a variety of miniature probes for photoacoustic endoscopy (PAE). This novel endoscopic modality enables imaging of internal organs with negligible invasion and offers exceptional sensitivity to optical absorption contrast associated with a wealth of physiological or functional information. Both photoacoustic microscopy (PAM) and PAE can be classified as optical-resolution (OR) or acoustic-resolution (AR), which has complementary advantages in imaging resolution and depth. These techniques have substantial biomedical and medical significance and are key to the progress of these fields. Recent studies have demonstrated combined OR- and AR-PAM systems successfully. However, their endoscopic applications are highly limited owing to the bulky size of the imaging heads. Although a few miniature probes for combined OR- and AR-PAE were reported, ex vivo images of mediocre quality were produced.
Recently, a team of researchers led by Professor Sung-Liang Chen from the University of Michigan-Shanghai Jiao Tong University Joint Institute developed a miniature probe capable of both OR- and AR-PAM. They utilized a gradient-index-lens fiber and a multimode fiber to deliver light for OR and AR illumination, respectively. They aspired to achieve scalable resolution and depth in endoscopy using their novel miniature probe and high-quality in vivo images comparable to those acquired by the current bulky counterparts. “The developed probe also has potential to expedite bedside applications of PAM and PAE,” said Prof. Chen. Their work is currently published in the research journal, Optics Letters. The first author is Zhendong Guo, a PhD student in Chen’s group.
In brief, the research method employed entailed the utilization of a miniature custom-made hollow focused transducer with a center frequency of 45 MHz, for acquiring photoacoustic signals among other instruments. Next, to characterize the OR-AR-PAE probe, the researchers imaged various phantoms for complete calibration, including imaging resolution and depth. Lastly, they tested the developed probe by taking in vivo images of part of a mouse.
The authors observed that the developed probe could achieve lateral resolution of 3.1 μm for OR mode and 46–249 μm (at depth of 1.2–4.3 mm) for AR mode, respectively. In addition, they noted that since the size of their probe was a maximum of 3.7mm, it had potential to be applied for endoscopic purposes. As proof of high-quality imaging ability of the developed probe, the taken in vivo images of several different parts of a mouse had excellent clarity.
In summary, the Chen’s group presented the fabrication of a novel OR-AR-PAE probe. Generally, the researchers were able to achieve high OR and AR resolutions that were comparable to those of benchtop systems while still maintaining minimal probe size. Also, they were able to produce high-quality in vivo images using the miniature probe. Besides having the potential for endoscopic imaging applications, the developed OR-AR-PAE probe may serve as a simple alternative to benchtop OR-PAM and AR-PAM imaging heads.
Zhendong Guo, Yao Li, Sung-Liang Chen (2018). Miniature probe for in vivo optical- and acoustic-resolution photoacoustic microscopy. Volume 43, Number 5 / Optics Letters