In vivo photoacoustic imaging of major blood vessels in the pancreas and liver during surgery

Abstract

Abdominal surgeries carry considerable risk of gastrointestinal and intra-abdominal hemorrhage, which could possibly cause patient death. Photoacoustic imaging is one solution to overcome this challenge by providing visualization of major blood vessels during surgery. We investigate the feasibility of in vivo blood vessel visualization for photoacoustic-guided liver and pancreas surgeries. In vivo photoacoustic imaging of major blood vessels in these two abdominal organs was successfully achieved after a laparotomy was performed on two swine. Three-dimensional photoacoustic imaging with a robot-controlled ultrasound (US) probe and color Doppler imaging were used to confirm vessel locations. Blood vessels in the in vivo liver were visualized with energies of 20 to 40 mJ, resulting in 10 to 15 dB vessel contrast. Similarly, an energy of 36 mJ was sufficient to visualize vessels in the pancreas with up to 17.3 dB contrast. We observed that photoacoustic signals were more focused when the light source encountered a major vessel in the liver. This observation can be used to distinguish major blood vessels in the image plane from the more diffuse signals associated with smaller blood vessels in the surrounding tissue. A postsurgery histopathological analysis was performed on resected pancreatic and liver tissues to explore possible laser-related damage. Results are generally promising for photoacoustic-guided abdominal surgery when the US probe is fixed and the light source is used to interrogate the surgical workspace. These findings are additionally applicable to other procedures that may benefit from photoacoustic-guided interventional imaging of the liver and pancreas (e.g., biopsy and guidance of radiofrequency ablation lesions in the liver).

Keywords: interventional imaging; liver surgery; pancreatic surgery; photoacoustic-guided surgery.

Fig. 1

Imaging equipment and surgical environment for in vivo porcine laparotomy.

Fig. 2

Vessels imaged in the (a) pancreas and (b) liver. The hepatic veins are inside of the liver but are displayed overlaid on the liver to allow for visualization of the vessel locations.

Fig. 3

Energy versus time graphs and fluence ($\mathrm{\Phi }$) versus time graphs for imaging the (a) pancreas and (b) liver. The 1- and 5-mm-diameter fiber bundles were used to image the pancreas during the time points indicated. The 5-mm-diameter fiber bundle was used to image the liver. The methods implemented to measure energy and calculate fluence are detailed in Sec. 2.3.

Fig. 4

Photographs of the ex vivo experiment setup. (a) Plastic tubes were filled with human blood to mimic the RGEV, RGEA, SMV, and the out-of-plane blood vessel. (b) Bovine liver tissue surrounded the phantom blood vessels while the L3-8 linear transducer and 5-mm-diameter fiber bundle were used to visualize the blood vessels.

Fig. 5

(a) DAS beamformed photoacoustic image of signals in in vivo pancreas overlaid on coregistered US image. (b) Corresponding SLSC beamformed photoacoustic image created with the same channel data. (c) SLSC beamformed photoacoustic image overlaid on US image. (d) Color Doppler confirmation of blood flow in vessels. (e) Contrast of the three signals in DAS photoacoustic images. The photoacoustic images shown here and the photoacoustic images used for the contrast measurements were acquired with an energy of 36 mJ.

Fig. 6

(a) DAS beamformed photoacoustic image overlaid on US image of ex vivo blood vessels. (b) Corresponding SLSC beamformed photoacoustic image created with the same channel data. (c) SLSC beamformed photoacoustic image overlaid on US image. These photoacoustic images were acquired with an energy of 30.5 mJ. (d) Contrast of the out-of-plane vessel signal, measured in the DAS photoacoustic image as the light source location was moved to mimic one respiratory cycle.

Fig. 7

(a) DAS beamformed photoacoustic image of signals in in vivo liver overlaid on coregistered US image. (b) Corresponding SLSC beamformed photoacoustic image created with the same channel data. (c) SLSC beamformed photoacoustic image overlaid on US image. The photoacoustic images were acquired with 40.5 mJ energy. (d) Color Doppler confirmation of blood flow in the hepatic vein.

Fig. 8

US image of the hepatic vein and corresponding DAS photoacoustic images (overlaid on US image), demonstrating focused and diffuse signals acquired with a laser energy of 30 mJ. The photoacoustic images are displayed with dynamic ranges of 15 dB (top) and 10 dB (bottom), as noted by the maximum and minimum values on the color bars. Reducing the dynamic range emphasizes the appearance of the focused signal and limits the appearance of the diffuse signal. More examples of focused and diffuse signals from this image acquisition sequence are shown in Video 1 (Video 1, MPEG, 2.8 MB [URL: https://doi.org/10.1117/1.JBO.24.12.121905.1]).

Fig. 9

(a) Focused photoacoustic signals in the hepatic vein, acquired with varying laser energy. Each image is annotated with the dynamic range chosen to optimize the signal display. (b) Measured contrast of focused and diffuse signals at the various laser energies.

Fig. 10

The 3-D representations of the hepatic vein created from DAS photoacoustic and color Doppler images. (a) Volume stack of photoacoustic and Doppler images acquired with a robot translating in 1-mm increments. (b) Biplanar cross sections through the volume stack. (c) 3-D reconstructions of the hepatic vein segmented from photoacoustic and Doppler images. (d) Segmented vessels from photoacoustic and Doppler images combined to show similarities between the two segmented vessel structures.

Fig. 11

Histopathology results of (left) pre-experiment control, (middle) post-experiment control, and (right) post-experiment lasered tissue for the (a) pancreas and (b) liver at $200\times$ magnification.