Minimally invasive endoscopic surgical procedures are constantly gaining importance as a result of the reduced postoperative trauma in patients. However, safe execution of endoscopic interventions requires a high degree of familiarity with the individual patient anatomy and a high level of surgical skill.
Simulation of endoscopic surgical procedures in virtual reality is commonly called virtual endoscopy (VE).1,2 VE allows the user to navigate through computationally reconstructed patient anatomy using a virtual camera. It produces inside views of anatomic structures of interest, similar to those obtained during physical endoscopy. A set of publications in the mid-1990s set the foundation for this technology.3-7 At present, VE is most commonly used in the clinical setting for the detection of colon polyps.8
Many authors have expressed their conviction that VE has great potential for reducing patient risk and increasing efficiency in education and in the operating room.9 In preoperative planning, VE can map out and help determine the operative approach. In addition, the effect of removing or altering surgical landmarks can be viewed from several perspectives, which helps in the evaluation of the functional impact of intended surgical interventions. This can potentially increase the safety of endoscopic surgery by assisting in the preservation of vital functional structures. VE also can be used as an intraoperative navigation aid. It can help direct the surgery, increase efficiency, and localize certain landmarks. In resident training, VE is particularly useful if it simulates not only endoscopic vision but also endoscopic handling. If this is the case, a young surgeon can perform complete or partial virtual surgeries in a realistic fashion without the danger of harming a patient. The virtual procedures can be repeated an arbitrary number of times, and various approaches and strategies can be explored.
On the other hand, it is important to note that VE has limitations. It can be executed on a ready-made anatomic model provided by the vendor or automatically reconstruct anatomy from radiologic images (eg, computed tomography, magnetic resonance imaging, and magnetic resonance angiography). Although the former forbids patient-specific simulation, the latter suffers from the fact that radiologic data are imperfect and often unsuitable for accurate automatic tissue characterization. To correctly depict structures of interest, it is often necessary to resort to interactive (manual) segmentation, which is time-consuming and tedious. Furthermore, the rendering of the final visual result depends on the so-called transfer function, which maps intensities of the radiologic source material to colors and levels of visibility. Selection of an improper transfer function will lead to structures being missed or displayed incorrectly. It is therefore important that the user of a patient-specific VE system have some background knowledge of the visualization process to be able to interpret the results adequately.
VE IN NEUROSURGERY
Virtual reality in neurosurgery is today a very active field of research and development.10,11 With the constant evolution of the technology, the potential of simulators to improve safety and efficiency in surgery is widely accepted and appreciated. The most commonly reported application fields of virtual neuroendoscopy are endoscopic third ventriculostomy, endonasal surgery, and cerebral angioscopy.
This article introduces work done in VE third ventriculostomy and presents examples of application of VE in the nasal cavity and the paranasal sinuses, leading to VE pituitary surgery. The application of virtual angioscopy in the brain and other uses of VE in neurosurgery are also discussed.
VE THIRD VENTRICULOSTOMY
VE has been established as an effective training and planning tool in endoscopic third ventriculostomy.
The VIVENDI simulator of the University of Tübingen (Tübingen, Germany) was first presented in 1999.12,13 It allows patient-specific preoperative planning of endoscopic third ventriculostomy. In a preprocessing step of some minutes, automatic camera paths are generated and segmentation is carried out on the multimodal radiologic source data (magnetic resonance imaging and magnetic resonance angiography). During the actual simulation, the ventricular walls are depicted semitransparently with major blood vessels in the background. The system is also used as an intraoperative navigation aid by tracking the tip of the actual endoscope, loading the position into VIVENDI, and fusing the real and virtual images.
Burtscher et al14 used VE to preoperatively evaluate the individual intraventricular and vascular anatomy at the floor of the third ventricle in 2000. The virtual endoscopic images were comparable to the real intraoperative endoscopic view in most patients. VE was found to be useful in displaying the positions of important cerebral arteries relative to visible anatomy, facilitating a safe procedure.
Lemole et al15 used the ImmersiveTouch (ImmersiveTouch, Chicago, Illinois) platform to simulate third ventriculostomy in 2007. They found that the virtual reality system has realistic visual and tactile characteristics and provides realistic handling.
A surgical simulator for endoscopic third ventriculostomy was developed at the Forschungszentrum Karlsruhe in Germany and embedded in the KISMET simulation package.16,17 A custom-designed haptic device is used for user interaction. It consists of 2 components, the first controlling the trocar, which provides a working channel for microsurgical instruments, and the second capturing the instruments. During the simulation, collisions between the virtual instrument and the ventricle walls are detected and translated into haptic forces and soft-tissue deformations. Modules for irrigation and suction, a special bleeding module, and a fluid dynamics simulation with animated particles to model the cerebrospinal fluid are used to obtain realistic visual feedback. Both the selection of the burr hole position and the actual ventriculostomy (using a bipolator and a Fogarty balloon catheter; Figure 1) are simulated. All interactions are recorded and evaluated in terms of the time needed for the intervention, the efficiency of instrument handling, and the number of faulty contacts. At the time of publication, a clinical evaluation was being prepared.
Further reports on the use of VE for endoscopic third ventriculostomy were published by Krombach et al,18 Jödicke et al19 and Goncharenko et al.20
VE IN THE NOSE AND PARANASAL SINUSES
The nose and paranasal sinuses are among the most popular fields of application of VE. Besides ear, nose, and throat surgeons, this topic is of particular interest to neurosurgeons because major parts of the research accomplished in this field are either applicable or extendable to simulation of endoscopic neurosurgery, most notably endonasal pituitary surgery, discussed later.
In 1997, de Nicola et al21 used software called Advantage Navigator (GE Medical Systems, Buc, France) to conduct VE examinations in the nasal cavity and paranasal sinuses with the goal of diagnostic and therapeutic assessment. They did not yet consider it an alternative to fiberoptic endoscopy but a useful complementary procedure that may help confirm a diagnosis and evaluate hard-to-reach parts of anatomy, nonpassable airway stenoses, or high-risk patients.
Development of the Endoscopic Sinus Surgery Simulator started in 1997 (Lockheed Martin, Bethesda, Maryland). It allows the virtual exploration of endonasal anatomy by manipulation of a simulated endoscope handle inside the nose of a mannequin. Surgical instruments can be activated with speech recognition. All instruments except the endoscope provide haptic feedback. A learning effect is generated by guiding the user through a set of predefined tasks of increasing difficulty. The overall performance of the trainee is recorded, and a task-specific score is given. A penalty is imposed when the user exceeds the time threshold or touches anatomic structures (eg, the optic nerve) that need to be avoided. The system was evaluated by Fried et al22 for use in reducing surgical errors They concluded that the simulator has good potential in augmenting a surgeon’s skill set and therefore increasing patient safety.
The Nasal Endoscopy Simulator, first introduced in 1998, was developed at German universities.23 It was conceived to simulate endoscopic sinus surgery for relieving intractable sinus pain. Actual surgical tools, combined with an electromagnetic tracking system, are used to control the virtual instruments. A physical head model provides guidance for the user. Real-time collision detection and simulation of tissue deformation add to the realism of the simulation. Because motion tracking, rather than haptic devices, is used, no haptic feedback is given. Three different levels provide training for hand-eye coordination, diagnostics, and therapeutic interventions on the virtual operation site. During simulated interventions, trainee errors such as collisions of instruments with highly sensitive tissues are detected and evaluated.
The Stanford Virtual Surgical Environment, a simulator for endoscopic sinus surgery, was described by Parikh et al24 in 2009. Patient-specific endonasal anatomy is reconstructed from computed tomography data, allowing a preoperative virtual fly-through. A model of an actual endoscope attached to a haptic device is used to control the virtual endoscope. The user can investigate the virtual endonasal anatomy and, using a virtual microdebrider, remove tissue during the simulated surgery. Force feedback is created to provide resistance as the tip of the instrument collides with the reconstructed anatomy. The authors claim that the system allows the user to interact with the anatomic reconstruction in an intuitive, surgically meaningful manner.
Further endonasal surgical simulators were presented by the University of Leipzig25,26, by Pérez-Gutiérrez et al,27 and by Tolsdorff et al.28
Numerous evaluations of the use of VE in the nasal cavity and paranasal sinuses have been published.29-36 They appreciate the value that virtual reality brings to the field in terms of augmenting patient safety, reducing operating times, and adding to the efficiency of resident training. Critical points included the arbitrary choice of the transfer function and the homogenization of different tissues, both leading to incorrect display of certain structures. Some authors missed the possibility of evaluating the mucosal surface and secretions32 or criticized the absence of haptic feedback in their simulators.34
VE IN PITUITARY SURGERY
Endoscopy is constantly gaining acceptance as the transsphenoidal approach of choice for pituitary adenomas and perisellar skull base structures. Its main advantage over the more conventional microscopic technique is the possibility of angulated, close-up display of anatomy with a large field of view. Simulation today plays an important role in endoscopic pituitary surgery for 2 major reasons: It allows effective preoperative assessment of the patient anatomy, reducing the risk of injury to anatomic structures near the surgical target, and it helps novice surgeons overcome the steep learning curve associated with the endoscopic procedure.37
Talala et al38 and Raappana et al39 evaluated 2 different imaging systems for identifying a safe strategy for opening the sellar floor in transsphenoidal pituitary surgery. A VE application generated images of sphenoid sinus landmarks similar to those obtained from a real endoscope. Additionally, a different software module provided nonperspective semitransparent imaging of anatomic landmarks. In that system, manually segmented parts of the internal carotid arteries were projected to the rendered surfaces of the sphenoid sinus, which gave a useful preoperative impression of the risk of injury. Although VE enabled the surgeon to be familiarized preoperatively with the individual structure of the sphenoid sinus, the nonperspective semitransparent views were found to be more helpful in preparing a safe procedure because of the feature displaying the carotid arteries in relation to sphenoid anatomy.
Audette et al40 reported work on a simulator for transnasal pituitary surgery in 2003. They described a technique of quick and minimally supervised creation of a 3-dimensional model, including the registration of data from different modalities, segmentation of anatomical landmarks, and structured description of volumes and surfaces, ready to be used by a surgical simulator. They extended this work in subsequent publications, further approaching their goal of high-quality models that adequately reflect the topology of the patient anatomy while consisting of as few geometric elements (triangles, tetrahedra) as possible.41,42
In 2004, development of the Simulation of Transsphenoidal Endoscopic Pituitary Surgery (STEPS) VE system was started. A detailed description of this application is given later.
NeuroTouch, the simulator developed by the National Research Council of Canada, contains a module providing simulation of transsphenoidal pituitary surgery. This module is described later.
Very recently, Wang et al43 confirmed that the 3-dimensional visualization of the sphenoid sinus and important adjacent structures is valuable for training and preoperative planning of endonasal transsphenoidal surgery.
The STEPS system was developed between 2004 and 2009 as a joint project between the VRVis Research Center (Vienna, Austria) and the Department of Neurosurgery of the Medical University Vienna.44-46 It is a VE application for preoperative planning and training of endoscopic transsphenoidal pituitary surgery. It was integrated in the Impax EE PACS system (Agfa HealthCare, Bonn, Germany).
For preoperative planning, the user is presented with a flexible virtual environment featuring virtual endoscopic reconstruction of the inside surface of the nasal cavity and the sphenoid sinus, depiction of anatomical structures of interest behind the semitransparent surface, and color-coded display of surface rigidity. The user is free to change visual parameters, to determine the set of objects to be displayed, to remove tissue, to mark important landmarks in 3 dimensions, and to view anatomy of interest from arbitrary angles.
For training of endoscopic navigation, the transsphenoidal approach and the opening of the sellar floor can be simulated more realistically. The endoscopic distortion and the light attenuation as encountered in real endoscopy are reproduced in the virtual model. Collision detection and force feedback are used to prevent impossible endoscope positions. Angled endoscopes are available, and bony material can be removed by applying a simulated bone-punch.
An early study evaluating STEPS proved its usefulness in preoperative assessment.47 By 2011, the software had been used to preoperatively simulate > 100 endoscopic pituitary surgeries.46 It has shown utility in various phases of the procedure.
In the nasal phase, VE was found to be helpful in displaying the wideness of the nasal corridor, septal deviations and spurs, the bollous conchae, and other anatomic variations, which helped the surgeon decide on the side of the approach. STEPS assisted in locating the sphenoid ostium, especially in cases when the ostium is partly or completely covered by mucosa.
Inside the sphenoid sinus, STEPS offers preoperative visualization of the bony anatomic landmarks, including the carotid and optic prominences, the indentation of the tuberculum sellae, and the clival indentation. This is helpful in intraoperative orientation and in determining locations where drilling is required. Because of the feature that permits virtual tissue removal, STEPS can display the sphenoid sinus before and after potential removal of sphenoid septa.
Visually correlating the sellar floor and the tumor helps the surgeon decide whether a complete or only a partial opening of the sellar floor is required. The depiction of the internal carotid arteries defines clear boundaries for the opening (Figure 2). It has been found that the preoperative visualization provided by STEPS can aid in reducing the risk of cerebrospinal fluid leakage, injury to the internal carotid arteries, and injury to the normal pituitary gland.
In 2010, the functionality of STEPS was extended to serve as an intraoperative tool.48 With the use of the StealthStation image-guided navigation system (Medtronic, Minneapolis, Minnesota), the endoscope and additional instruments are optically tracked during an intervention, and the resulting positioning data are passed to STEPS, which generates a virtual image from the point of view of the actual endoscope.
The intraoperative system was tested on 12 clinical cases. During the nasal phase, it was able to navigate the surgeon to the sphenoid ostium (Figure 3). In the sellar phase, the display of anatomic landmarks invisible on the actual endoscopic image gave valuable information that guided the surgeon to a safe and optimal exposure of the lesion (Figure 4).
The NeuroTouch neurosurgical simulator is currently being developed by the National Research Council of Canada in close cooperation between engineers and neurosurgeons.49 NeuroTouch provides a set of tailored training scenarios based on real medical cases to allow effective training of future neurosurgeons. One of these scenarios is transsphenoidal endonasal pituitary surgery.50
The first version of the endonasal module was designed to help practice endoscopic navigation in general and the detection of a patient’s sphenoid ostium in particular. With the use of a haptic device, the virtual endoscope is inserted into the nostril and navigated through the virtual endonasal anatomy.
Real-time physics-based computation of tissue deformation and an innovative strategy of handling instrument-tissue contacts allow realistic tissue reactions to collisions all along the endoscope shaft and intuitive haptic feedback. This feature is illustrated in Figure 5.
Contact of the endoscope tip with the mucosa sometimes leads to stains on the lens. This causes the virtual endoscopic picture to appear partly blurred. As a result of intense contact, temporary bleeding can occur, leading to a partially or fully obstructed view. However, by pressing on a pedal, the user can rinse the lens and restore full visibility (Figure 6).
As a navigation aid, labels can be placed on important landmarks (eg, the turbinates, ostium, septa). They become visible when the endoscope camera is close enough, giving the user some feedback about the current endoscope position (Figure 7). At any time during the simulation, the user can switch between the endoscopic and an outside view (Figure 8). The outside view of the patient’s nose is particularly helpful at the start of the simulation to locate the nostril.
The training task designed for NeuroTouch Endo is to advance the endoscope to the sphenoid ostium, retract it, insert it into the other nostril, and find the sphenoid ostium on the other side also. Once the endoscope tip is close enough to an ostium, a message pops up informing the trainee that a subtask has been accomplished (Figure 8, right). As soon as both ostia have been found, the simulation is stopped and the user’s performance is evaluated. The criteria of this evaluation include whether both targets have been reached, the time needed, and the maximum and average force applied on patient tissue. In addition, all these data are compiled into a weighted numerical score (Figure 9).
Current development efforts are focused on replicating other steps of endoscopic transsphenoidal pituitary surgery. The module is currently being extended to a bimanual system in which the user controls both the endoscope and a dissection device (eg, a drill), allowing simulation of the enlargement of the sphenoid ostium and the opening of the sellar floor.
A clinical evaluation of the system is planned.
VIRTUAL ANGIOSCOPY IN NEUROSURGERY
In virtual angioscopy, a camera is placed inside a computerized model of a blood vessel to obtain 3-dimensional views of a certain pathology. In 1995, Lorensen et al4 reported performing 3-dimensional fly-throughs of carotid arteries and arteriovenous malformations. Later, VE was applied in the assessment of intracranial aneurysms,51-53 in the analysis of stenoses of the carotid artery,54-56 for presurgical simulation of microvascular decompression,57 and for assessment of aortic arch abnormalities.58 Most authors concluded that VE helps surgeons understand the 3-dimensional structure of the pathologies, demonstrate morphologic aspects of lesions that were otherwise difficult to appreciate, and adequately predict anatomic variations that can affect individual surgical approaches. Colpan et al53 reported significantly reduced complication rates in aneurysm surgery as a result of VE.
OTHER USES OF VE IN NEUROSURGERY
VE has the potential to be of use in any kind of minimally invasive neurosurgical procedure. An overview of applications besides third ventriculostomy, endonasal surgery, and angioscopy is given in this section.
The Neurosurgical Operation Planning System was presented in 1997 by Darabi et al.59 It was developed to mimic endoscopic imaging during general neurosurgical procedures, eg, in endoscopy-assisted microsurgery. The system was evaluated by videotaping the endoscope feed during an actual procedure, interactively determining the approximate endoscope path, replaying the camera path using VE, and comparing the resulting image sequence with the video of the actual surgery. The authors conclude that, provided that the clinician is aware of the limitations and has some background knowledge of the creation process of the VE images, the intuitive depiction of 3-dimensional information provided by VE can have an important impact on the planning of endoscopic neurosurgical approaches.
Auer et al60 and Auer and Auer61 analyzed the use of VE for the planning and simulation of minimally invasive neurosurgical procedures and found that VE seems especially suited for the simulation and planning of operations of intraventricular lesions.
Burtscher et al62 reported a study evaluating VE as a tool for preoperative analysis for neurosurgical interventions, including third ventriculostomies, cyst removals, ventricular arachnoidal cyst fenestrations, and endoscopic septostomies. They state that in 8 of 12 patients, VE profoundly influenced surgical planning and the surgery itself.
The Robo-Sim simulator was used in preoperative planning and training of minimally invasive interventions in neurosurgery.63,64 It provided haptic feedback and soft-tissue deformation for selected parts of anatomy. An evaluation of the tool revealed that the overall quality of VE with Robo-Sim was appreciated, although the lack of blood flow simulation was criticized.
Ito et al65 analyzed the intraoperative use of VE combined with a surgical navigation system in a variety of neuroendoscopic procedures in 2010. They state that the decision-making process during surgery was influenced by the results obtained from VE. The use of semitransparent virtual views contributed to a clear understanding of spatial relationships between the surgical target and other anatomic structures of interest.
Other studies evaluating VE in general minimal invasive neurosurgery were published by Shigematsu et al,66 Stadie et al,67 and Kin et al.68
VE is increasingly recognized as an important tool in neurosurgery. There is a wide variety in minimally invasive neurosurgical procedures, and the 3-dimensional reconstruction of patient anatomy in virtual reality can potentially be of assistance in all of them. Virtual views from the inside of a pathology and the presurgical depiction of a surgical target, often enhanced by the display of neighboring anatomic structures of interest, can improve efficiency and quality of preoperative planning, thereby increasing patient safety. The simulation of instrument handling in endoscopic surgery helps novice surgeons become acquainted with procedures with a steep learning curve. Virtual reality therefore has the potential to help improve the education of young surgeons and thus increase the safety and availability of minimally invasive neurosurgery.
The amount of research being conducted in the development of new VE systems, new techniques of reconstruction and simulation, or clinical evaluation of existing virtual reality software increases each year. The quality of simulation is improving with the technology. Progress is made in all aspects, including visual feedback, haptics, and the quantity and quality of information delivered. Virtual reality will therefore further increase its impact on endoscopic neurosurgery and any type of surgery in the coming years and decades.
The most widely accepted application fields of VE in neurosurgery today are third ventriculostomy and transnasal approaches to the skull base. Much work has been done in virtual cerebral angioscopy and simulation of other minimally invasive neurosurgical procedures. This review introduced many of the most interesting systems existing today and their clinical evaluations but cannot claim completeness in this agile, dynamic, and evolving field of research.
The authors have no personal financial or institutional interest in any of the drugs, materials, or devices described in this article.
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