Multi-resolution foveated laparoscope for safer minimally invasive surgery

Since the first demonstration of a video-based laparoscope with an analog camera in 1992,¹ laparoscopic surgery has become the primary approach for providing minimally invasive surgery (MIS) and is routinely performed for clinical procedures such as removal of the kidney or gall bladder. Compared with open surgery, laparoscopic surgery has a number of advantages, including reduced pain and shorter recovery time.^{2, 3} In recent years, laparoscopic technologies have evolved dramatically thanks to advances in optics, electronics, and mechanical fabrication. However, state-of-the-art laparoscopic imaging technologies have several limitations. One is an inherent tradeoff between the spatial resolution and the field of view (FOV). Laparoscopes are designed for a short working distance, typically less than 50mm, and surgical procedures are performed with a ‘zoomed-in view’ to see sufficient details of the surgical field.⁴

Because of these close-up views, the surgeon's peripheral vision is lost, and his or her awareness of the situation outside the immediate focus area of the laparoscope can be highly compromised. In extreme cases, this may cause potentially life-threatening complications due to massive hemorrhage from accidental injury to adjacent organs. This is the so-called situational awareness issue in laparoscopic surgery.⁵ In current clinical practice, this issue is addressed by frequently moving the entire laparoscope forward and backward to obtain detailed close-up images and wide-angle views for peripheral orientation. This practice requires a second trained assistant, which introduces ergonomic conflicts, especially with a single port access (SPA) procedure, in which space is very limited.⁶

The SPA technique is expected to play a key role in the future of laparoscopic surgery.⁷ In such procedures, three separate surgical ports (for the camera and left- and right-hand instruments) are grouped into one to limit the number of incisions needed. This grouping raises a number of problems. The in-line arrangement of instruments creates issues with the surgeon's situational awareness. In addition, because of poor triangulation, instruments must be crossed to obtain proper retraction, which increases the risk of collision.⁸ As this new technique becomes more widespread, it demands further refinement of laparoscopic instrumentation to address these limitations and optimize the performance of surgical tasks. Previous work suggests that decreasing instrument length reduces the effect of crowding.⁶ A laparoscope that does not require hand or robotic arm guidance and can maintain a low profile through an appropriate range of magnification would greatly improve the maneuverability of other instruments.

To address these limitations and improve the safety and ease of laparoscopic surgery, we developed a multi-resolution foveated laparoscope (MRFL).⁹ Our prototype system contains both wide and narrow FOV probes, which gives the MRFL the capability to image the surgical field at different levels of optical magnification, rather than at a fixed, singular magnification as with a conventional laparoscope. The optical system (see Figure 1) consists of a shared F/2.5 objective lens, multiple relay lens groups, a scanning lens group, a wide-angle imaging probe, and a high-resolution imaging probe. While the wide-angle probe images the entire FOV captured by the objective-relay system, the narrow probe captures only about a one-third sub-region of interest selected by a 2D scanning mirror. This is similar to how the eye directs the fovea—the area of the retina where visual acuity is highest—through eye movement. Another key innovation of the MRFL system design allows a flexible number of relay lens groups of limited diameter to be concatenated without noticeable degradation of imaging quality. This reconfigurable capability enables creation of laparoscopes with different length profiles for different types of surgical techniques, for example, a normal profile for a standard multi-port procedure and a low profile for SPA procedures.

Figure 1. System layout: (a) objective lens group; (b) one rod lens relay group; (c) scanning lens group.

The wide-angle probe provides surgeons an overview of the surgical area, giving them good situational awareness during MIS surgery. The high-resolution probe has a relatively small FOV but threefold better spatial resolution, providing surgeons with a sufficiently detailed view of sub-surgical areas. The high-resolution probe can scan and engage with any sub-field of the wide-angle field in real time.

The working distance range of our laparoscope prototype is 80–180mm. The FOV of the wide-angle probe is 80° and that of the high-resolution probe is 26°. For the wide-angle probe, the surgical field is 80×60mm² at an 80mm working distance and 240×180mm² at a 180mm working distance. For the high-resolution probe, the surgical field is 27×20mm² at 80mm and 80×60mm² at 180mm. In a conventional laparoscope with 70° FOV and 50mm working distance, the surgical area is 56×42mm². The best spatial resolution that the MRFL can provide is 10 line pairs per millimeter (lp/mm, 50μm) at 80mm working distance, while a conventional laparoscope with a high-definition camera has a spatial resolution of about 2.4lp/mm (208μm) at 50mm working distance.⁴

Figure 2. Prototype of the multi-resolution foveated laparoscope. FOV: Field of view.

Our MRFL prototype includes two length profiles for standard multi-port procedures and SPA procedures (see Figure 2). The objective lens group and the relay lens groups shared by the two probes were designed to fit into a standard rigid scope package with a 10mm diameter. Using the prototype instrument, we captured images of an artificial bladder (see Figure 3). We currently are examining display methods that may more effectively take advantage of the MRFL system's capabilities and provide more useful displays for the surgeon.

Figure 3. Demonstration of the multi-resolution foveated laparoscope display interface. (a) Mosaic of images captured by the wide-angle and high-resolution probes. The two views have the same scale, and the portion captured by the high-resolution probe is marked with a yellow box. (b) Full-size image captured by the high-resolution probe. This display interface is analogous to the fovea of the human eye.

In summary, MRFL technology has great potential to make MIS surgery easier and safer and may allow surgeons to use SPA instruments more efficiently. The foveated capability can effectively reduce the bandwidth requirement and provide a better frame rate compared with the use of a super-resolution camera. We plan to further extend the technology by allowing variable levels of optical magnification for the narrow FOV probe. In the future, we will test the system in an animal model and for clinical applications. Eventually, the MRFL may allow hands-free surgery by securing the laparoscope with a clamp and using the scope's scanning lens and zoom capabilities to pan across the surgical field.

This work is supported by National Institutes of Health grant R21EB013370.