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Radiofrequency ablation (RFA) procedures for liver cancer treatment are hindered by high tumor recurrence. This is thought to be due to the intrinsic limitation of the heating mechanism and insufficient real-time feedback from imaging modalities. Most RFA procedures are performed under ultrasound (US) imaging and there are limitations in accurate device guidance and ablation monitoring. We propose photoacoustic (PA) imaging as a potential add-on to US imaging to address these limitations. Specifically, we present two interstitial PA imaging methods. Firstly, an annular fiber probe that can encapsulate an RFA device in its lumen. This device enables RFA device guidance, visualization of major blood vessels and targeting tumor tissue. Secondly, we used a cylindrical diffuser-based interstitial illumination to differentiate coagulated and native tissue. We present our results on RFA device guidance and ablation visualization using these approaches. The contrast provided by PA imaging for RFA needle and multiple electrodes is compared against that of US images. The difference between coagulated and native ex vivo liver tissue using PA imaging is studied. Finally, we propose a protocol to incorporate the minimally invasive PA imaging for the clinical RFA procedures. We would like to conclude with a note on how the proposed approach can potentially improve the outcome of RFA procedures.

Unresectable liver tumors are commonly treated with percutaneous radiofrequency ablation (RFA). However, this technique is associated with high recurrence rates due to incomplete tumor ablation. Accurate image guidance of the RFA procedure contributes to successful ablation, but currently used imaging modalities have shortcomings in device guidance and treatment monitoring. We explore the potential of using photoacoustic (PA) imaging combined with conventional ultrasound (US) imaging for real-time RFA guidance. To overcome the low penetration depth of light in tissue, we have developed an annular fiber probe (AFP), which can be inserted into tissue enabling interstitial illumination of tissue. The AFP is a cannula with 72 optical fibers that allows an RFA device to slide through its lumen, thereby enabling PA imaging for RFA device guidance and ablation monitoring. We show that the PA signal from interstitial illumination is not affected by absorber-to-surface depth compared to extracorporeal illumination. We also demonstrate successful imaging of the RFA electrodes, a blood vessel mimic, a tumor-mimicking phantom, and ablated liver tissue boundaries in ex vivo chicken and bovine liver samples. PA-assisted needle guidance revealed clear needle tip visualization, a notable improvement to current US needle guidance. Our probe shows potential for RFA device guidance and ablation detection, which potentially aids in real-time monitoring.

Surgical resection is considered the gold standard for tumor treatment with curative intent. However, in most cases, patients are not eligible for resection, especially in functional tissue like liver. Minimally invasive thermal treatment like Radiofrequency Ablation (RFA) is gaining more applicability in this scenario. The success of RFA treatment depends on accurate needle placement and complete tumor ablation. Widely used Ultrasound (US) imaging lacks in providing accurate feedback of RFA device location and ablated tissue boundary. Here we propose Photoacoustic (PA) imaging in combination with US imaging for RFA device guidance and in differentiating ablated tissue. We employ two illumination strategies for PA imaging: an extracorporeal light delivery, and an interstitial light delivery. First, using the extracorporeal illumination we demonstrate how PA imaging can improve RFA device visibility and in differentiating ablated tissue in an ex vivo laboratory setting. Further, we expose the difficulty in imaging highly absorbing tissue using illumination from the surface of the skin. This limitation leads to the development of an interstitial illumination probe. We compare these two illumination strategies and present our preliminary results in RFA device guidance using the interstitial probe. Our results show the potential of PA imaging in improving the accuracy of the RFA procedure.