Varying ultrasound power level to distinguish surgical instruments and tissue

Abstract

We investigate a new framework of surgical instrument detection based on power-varying ultrasound images with simple and efficient pixel-wise intensity processing. Without using complicated feature extraction methods, we identified the instrument with an estimated optimal power level and by comparing pixel values of varying transducer power level images. The proposed framework exploits the physics of ultrasound imaging system by varying the transducer power level to effectively distinguish metallic surgical instruments from tissue. This power-varying image-guidance is motivated from our observations that ultrasound imaging at different power levels exhibit different contrast enhancement capabilities between tissue and instruments in ultrasound-guided robotic beating-heart surgery. Using lower transducer power levels (ranging from 40 to 75% of the rated lowest ultrasound power levels of the two tested ultrasound scanners) can effectively suppress the strong imaging artifacts from metallic instruments and thus, can be utilized together with the images from normal transducer power levels to enhance the separability between instrument and tissue, improving intraoperative instrument tracking accuracy from the acquired noisy ultrasound volumetric images. We performed experiments in phantoms and ex vivo hearts in water tank environments. The proposed multi-level power-varying ultrasound imaging approach can identify robotic instruments of high acoustic impedance from low-signal-to-noise-ratio ultrasound images by power adjustments.

Appendix

1.1 Ultrasound imaging setups

We utilized both a clinical ultrasound imaging system, the Philips SONOS 7500 at Boston Children’s Hospital, and a lab-based ultrasound imaging system, Ultrasonix Sonix Touch, at the National University of Singapore, for cross-validating the consistency of power-varying experiments at both research labs. Both transducers allow users to adjust the transducer power levels either by graphical user interface or programming.

1.1.1 Philips SONOS 7500 ultrasound platform

The real-time interventional images were acquired through a Philips SONOS 7500 (www.philips.com) ultrasound imaging system. In addition to the imaging user interface of the SONOS system, a parallel stream of the same images was transferred to a custom workstation through the SONOS research interface. The image acquisition rate is about 28 volumes per second and varies slightly with the frequency settings. In the experiments of this article, standard settings of the image acquisition parameters were used, including 50% overall gain, 50% compression rate, frequency fusion mode 2, and high-density scan line spacing. The acquired gray-level ultrasonic volume image, V, can be defined as an M × N × P matrix. The individual voxel v(i,j,k) typically has an anisotropic spacing and represents the voxel intensity at the i th row, j th column, and k th slice, in the image volume space, which corresponds to the Cartesian coordinate, x = (x,y,z)^T, of the ultrasound transducer system, in the physical spatial space. Here, x represents increasing azimuth, y represents increasing elevation, and z indicates increasing distance from the transducer. The resulting anisotropic spacing varies with acquisition depth settings, for example, 10-cm acquisition depth will produce an anisotropic volume with spacing {0.542 mm, 0.706 mm, 0.451 mm} in the x, y, and z directions, respectively.

The transducer power level of SONOS 7500 can be varied from −30 to 0 dB, in which 0 dB is a default level for visualizing both tissue and instruments. As shown in Fig. 17, we performed comparative imaging experiments for two different intra-cardiac instruments: the concentric tube robot end-effector, fabricated from NiTi, and braid-reinforced polyimide catheter tubing manufactured by Microlumen (Microlumen Inc.). For qualitative analysis, the instruments were imaged inside an ex vivo pig heart submerged in the water tank, and the images were recorded using different transducer power levels.

Fig. 17

Comparison of acquired images from SONOS7500 at normal 0 dB power and at instrument-enhancing −23.1 dB in an ex vivo heart. a Imaging of a concentric tube robot; b imaging of a Microlumen catheter tube; c photographs of instruments used in panels a and b. Left: concentric tube robot; middle: microlumen catheter; right: experiment setup

1.1.2 Ultrasound experimental setup

The experimental setup is as shown in Fig. 18 for scanning a tube in a water tank and pig heart tissue environment.

Fig. 18

Experimental setup using Ultrasonix machine for scanning (left) the water tank and (right) pig heart tissue