From the Viewpoint of Surgery
Medical imaging is one of the techniques that have made a great contribution to the development of surgery. As an example, in neurosurgery it is important for the surgeon to avoid damage to the brain structures that will cause dramatic functional deficits (including limb paralysis, language function, and visual). An integrated protocol of new MRI methods may, prior to the surgery, delineate important functional areas in the brain and their vital nerve fiber connections. This is of great help to the neurosurgeon in the planning of the operation and can also be displayed for the neurosurgeon in the operating theater and updated during the surgical intervention. Image-guided techniques assist with precise instrument guidance.
Surgical navigation is in some ways the same as commonly used car navigation, attempting to localize or determine a position in the physical space in the context of its surroundings . Surgical navigation is usually “image-based,” meaning that imaging data such as preoperative CT or MRI images are used in the operating room . Before surgery, surgical planning can entail delineating regions of interest within the images and producing datasets for use in the operating room. The preoperative image data need to be matched to the current patient position via registration, establishing a relation between the “physical” coordinate system as defined by the patient’s reference array and the “virtual” coordinate system of the imaging data. Modern surgical navigation systems use a stereoscopic camera emitting infrared light which can determine the position of fiduciary reflective marker spheres, allowing for real-time tracking. During the surgery, the marker spheres are attached to the patient and to surgical instruments to enable an exact localization in the space. The computer can calculate the position and orientation of each instrument. Correct localization and virtual display of the instrument on the computer screen is ensured by firmly attaching a reference array to the patient. The surgeon then virtually “sees” both the current situation and the imaging datasets side by side by volume rendering or by directly superimposing them, that is, augmented reality visualization.
Neurosurgery was the first surgical discipline to adopt navigation and integrate it successfully into surgical routine, followed by maxillofacial surgery, dental surgery, and orthopedic surgery. The neurosurgical procedures supported by surgical navigation range from intracranial tumor resections and frameless biopsies to pedicle screw placement and stabilizations in the spine. In the field of thoracoabdominal surgery (general surgery), navigation was less often employed, because open thoracotomy or laparotomy makes target organs visible enough. However, surgical navigation has been gradually adopted since the advent of minimally invasive surgery (MIS), thoracoscopic or laparoscopic surgery, in the late 1980s. The benefits of MIS to the patient have been shown in terms of reduced postoperative pain, lower risk of wound infection, shorter hospital stay, and quicker return to normal physical activities. Conversely, a limited field of view, dealing with essentially two-dimensional images, challenges to eye-hand coordination, and inability to achieve tactile feedback are limitations. Therefore, in general surgery, new imaging techniques for real-time enhanced laparoscopic surgical guidance are currently the subject of research worldwide. The implementation of intraoperative imaging methods can be of great assistance to surgeons in training, and these methods can help to improve the safety and efficacy of MIS.
Compared with other fields of surgery, general surgery has several factors of lower affinity with conventional surgical navigation. Deformation of soft tissues, such as the skin and abdominal wall, can affect the accuracy of registration. The peritoneal cavity is usually inflated with CO2 to enlarge the field, and the leakage of gas from the port site easily changes the intraperitoneal pressure and contour of the abdominal wall. Peristalsis can also present problems. Respiratory motion always needs to be addressed. Because of these factors, preoperative images can be less helpful. Although navigation accuracy is reduced for exact localization of targets, it remains valuable for intraoperative orientation. To address these problems, intraoperative imaging was employed over the last decade to provide the navigation system with real-time images. Intraoperative imaging solutions can range from live ultrasound images to intraoperative MRI (iMRI) or intraoperative CT (iCT). iMRI offers the best soft tissue contrast among them. Figure 1.8a shows an operating room equipped with an open-configuration MRI scanner, which is particularly suitable for iMRI [70-72]. Figure 1.8b shows a laparoscopic image in which an anatomical structure reconstructed from iMR images is overlaid  upon the patient’s image.
While iMRI is a powerful tool to achieve optimal operation control in combination with the navigation, it remains the most expensive imaging option and
Fig. 1.8 Laparoscopic augmented reality visualization combined with open-configuration MRI system . (a) Laparoscopic cholecystectomy in the open MRI therapeutic room. (b) Augmented reality laparoscopic image during a laparoscopic cholecystectomy. The laparoscopic monitor clearly shows the augmented reality model of the common bile duct (green semitransparent area), allowing the surgeon to avoid it during the procedure requires significant building costs. As a trade-off among soft tissue image quality, versatility, and affordability, iCT has emerged. iCT allows for minimal interruption of the surgical workflow since its scan time is significantly shorter than MRI and patient positioning is less limited. Newer generations of portable iCT scanners are designed specifically for intraoperative use and enable the surgeon to verify the surgical progress and automatically update the navigation.
The current level of accuracy is acceptable for laparoscopic surgery despite inferior accuracy compared with other fields such as neurosurgery. Although at present it looks difficult to improve the accuracy, novel methodologies for nonrigid registration that can account for soft tissue shifts and deformations as well as estimation for soft tissue deformation may be realized in the future to improve the accuracy of the surgical navigation system. Statistical and physiological modeling of deformation patterns specific to organs will be required to improve the navigation accuracy and to widen applicability in the future. Computational anatomy, which may include statistical modeling of organ shapes and deformations, is expected to be a powerful tool for this purpose.