Annular Fiber Probe for Interstitial Illumination in Photoacoustic Guidance of Radiofrequency Ablation

Abstract

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.

Keywords: interstitial illumination; interventional imaging; liver treatment; minimally invasive procedures; multimodal imaging; photoacoustics; radiofrequency ablation; surgical tool tracking; ultrasound imaging.

Figure 1

Annular fiber probe (AFP): design, experiments, and intended application. (a) AFP design requirements and manufactured specifications. (b) Photographs. (c) Upper part, Monte Carlo simulations to estimate the fluence distribution of the AFP. Lower part, assessing the photoacoustic (PA) field of view in a liquid optical phantom. (d) Upper part, the general setup for testing in ex vivo tissue, allowing combined PA and ultrasound (US) imaging. Lower part, the intended application of the AFP, guiding the RFA needle in the liver and monitoring of the ablation process.

Figure 2

Monte Carlo simulation (MCS) results at 650 nm. (a) The location of the fibers used in the MCS. The black circles delineate the inner and outer diameter of the probe. (b) 3D normalized fluence rate (NFR) of 72 fibers in native liver tissue resulting from the MCS. (c) $NF{R}_{rel}$ per depth for native and ablated tissue simulated for 4, 16, 36, and 72 fibers (Nfib) in the annular ring. (d–f) 2D slices: at the optimal depth, before optimal depth, and beyond optimal depth. The dashed lines indicate the ROI from which $NF{R}_{rel}$ was determined.

Figure 3

Mean PA magnitude of a black line absorber at several distances from the AFP in a tissue-mimicking phantom. (a) Schematic illumination. (b) The PA field with optical properties mimicking chicken tissue is shown. (c) PA field with optical properties mimicking native liver tissue. (d) Mean PA value per depth for both experiments.

Figure 4

Comparison of interstitial and surface illumination for PA imaging of an absorbing target at 15 and 32 mm depth from the surface. (a,d) Location of the AFP relative to the target schematically. PA-US images of interstitial illumination (b,e) and surface illumination (c,f). The white-dashed circles indicate reconstruction artifacts.

Figure 5

Setup having the AFP with RFA device slightly inserted and tines deployed up to 20 mm in tissue (a). Fused PA-US images of the probe and tines in ex vivo chicken (b) and bovine liver tissue (c). White-dashed lines surround reconstruction artifacts—1, reverberation artifact—2, backscatter signals—3, and a reconstruction artifact appearing to be a tine—4.

Figure 6

Combined PA-US images of the AFP targeting a nylon tube filled with human blood in chicken breast and ex vivo liver. (a) Setup schematically. (b) Tube embedded in chicken with illumination in-plane to the US probe. (c) In-plane illumination in the liver. (d) Out-of-plane illumination in the liver. The white-dashed rectangles indicate the location of the bevel and tip, whereas the circles indicate reconstruction artifacts—1 and PA signals due to light leakage from a fiber in the probe—2. The PA dynamic range is different in b-d for data visibility, as indicated by the color bars.

Figure 7

Superficial tumor-mimicking phantom being targeted by the AFP from chicken breast tissue. (a) Schematic. (b) Photograph of the liver and mimicking tumor. (c) Combined PA-US image. The dashed lines: PA signal due to a fiber leaking light—1, and reconstruction artifacts—2.

Figure 8

Photoacoustic and ultrasound imaging of the ablation region in ex vivo bovine liver tissue. (a) Photograph of the ablated tissue, the whiter area indicates ablated tissue. (b) Schematic. (c) Combined PA-US image.