Учебники / Computer-Aided Otorhinolaryngology-Head and Neck Surgery Citardi 2002

FIGURE 10.1 Display of the StealthStation (Medtronic Surgical Navigation Technology, Louisville, CO) after completion of registration. The system shows a sphere of accuracy, which demonstrates the volume of greatest accuracy based upon the distribution of the fiducial points used during the registration process.

on that contour obtained randomly using the digitizer depicting the curvature of a portion of the scalp. The computer can then align these features, with areas of maximum curvature being the most useful for the process. A further development is to dispense with point matching altogether and simply match large surface areas. The surfaces imaged by positron emission tomography (PET), CT, or MRI can be matched directly with the scalp as imaged in the operating room. This permits rapid registration immediately prior to the surgery, without the need for repeating preoperative diagnostic imaging with fiducial markers in place. This saves both time and money, although care must be taken to ensure that the diagnostic images are of sufficient quality to justify relying on them for surgical planning and guidance. Frequently diagnostic images lack the fine spacing needed between images to allow for accurate intraoperative guidance.

along predetermined trajectories and enabled precise determination of the location of an instrument inside the enclosed 3D space. Position determination was usually expressed in Cartesian coordinates, i.e., each point within the intracranial volume was defined relative to three axes: x, y, and z. Types of frames that are coordinate based include the Leksell, Talairach, and Hitchcock frames. The Leksell is an arc-centered system, enabling easy adjustment of the angles of the arc and probe carrier, while keeping the target point centered. Non-Cartesian frames include the Reichert-Mundinger frame and the popular Brown-Roberts-Wells (BRW) frame, in which targets and trajectories are defined by four angles and a length. The latter frame was developed after computers became available, so that Cartesian scanner coordinates could be readily converted into spherical coordinates.

Use of such frames in conjunction with preoperative 2D images obtained from several perspectives by fluoroscopy, or with stereotactic atlases, enabled the surgeon to plan the trajectories for the procedure with a fair degree of accuracy. However, there were several limitations to the use of these frames. In the first place, the target and trajectory parameters were usually calculated with respect to preoperative image data and did not necessarily take into account intraoperative changes in anatomy. Also, many surgeons find the frames to be cumbersome, and they may impede access to the operative site in certain situations. These limitations lead to the use of devices that can determine position in 3D space without resorting to the use of mechanical calipers; this technology is termed 3D digitizers.

10.4.2 Digitizers

More recent stereotactic systems do not use a frame, but still require a precise coordinate system for the operative space. The location of an instrument or pointing device must be conveyed to the computer so that its position can be related to the imaging data. This localization is provided by a 3D digitizer that is installed in the operating room. The digitizer assigns coordinates to each selected point, enabling co-registration of patients, images, and instruments within the same coordinate system.

10.4.2.1 Early Approaches

Among the earliest systems were articulated arms with potentiometers at the joints between the links. These sensors determine the angles of the joints by measuring resistance, enabling the orientation of the distal section and location of the tip to be determined. The first reported use of a passive localization arm of this type for neurosurgical guidance was by Watanabe (1993), who used an arm with six joints. A widely used commercial model is the ISG Viewing Wand (ISG Technologies, Toronto, Canada). Guthrie and Adler (1992) developed a

similar series of arms in which optical encoders replaced the potentiometers to improve accuracy. These arm-based systems are simple and do not require a clear line of sight. However, they can interfere with the surgeon’s movements and are difficult to use with surgical instruments.

Another early approach used ultrasound emitters and microphone arrays in a known configuration to localize the position of the instruments. Position was determined on the basis of the time delay between emission of an ultrasonic pulse produced by the emitter and its detection by each microphone. A version of this technique was applied to an operating microscope by Roberts et al. in 1986, and developments continue to this day. Barnett et al. (1993) developed a system in which ultrasonic emitters are fitted to a handheld interactive localization device (Picker International, Highland Height, OH), and the detecting microphone arrays are fitted to the side-rails of a standard operating table. However, ultrasoundbased digitizing systems are limited by their requirement for a clear line of sight between the emitters and detectors, and the systems may be confused by echoes, environmental noise, and the effects of fluctuating air temperature. On the positive side, they are relatively inexpensive, costing no more than half as much as an optical digitizer, can be set up rapidly, and can cover a very large working volume. Also, it is easy to affix the emitters to all kinds of instruments and other equipment.

10.4.2.2 Optical Digitizers

Modern navigation systems make use of light-emitting diodes (LEDs) to track the location of instruments within the operative space. Bucholz first developed navigation systems based on optical digitizers. The LEDs used in such systems emit near-infrared light, which can be detected by at least two charge-coupled device (CCD) cameras positioned in the operating room. Inside the cameras, light from the LEDs is focused onto a layer of several thousand CCD sensor elements, and the infrared energy is converted into electrical impulses. These digital signals may be further processed or converted to analog form. Through triangulation the point of light emission can be calculated with an accuracy of 0.3 mm.

Optical triangulation techniques are highly accurate and robust. A useful comparison of their properties with those of sonic digitizers has been published (Bucholz and Smith, 1993). Like sonic digitizers, optical systems require a clear line of sight, but they are unaffected by temperature fluctuations and do not appear to be compromised by surgical light sources. The infrared light is not visible to the surgical team and causes no harmful effects.

A minor variation on the infrared detection approach uses passive reflectors instead of active emitters, with an infrared source illuminating the operative field. The reflected light is detected in the same way as light emitted by the active systems. Instruments fitted with passive reflectors do not need to have electrical cables attached and are thus easier to sterilize. However, the passive markers are

constantly ‘‘on,’’ appearing simultaneously to the CCD cameras at all times. This can confuse the navigation system if multiple instruments, which carry a large number of reflectors, are present in operative field at once. Another passive approach substitutes an ultraviolet source as the field illuminator, with fluorescent markers reirradiating the UV light at visible frequency levels. However, there is concern that prolonged exposure to UV in this context may pose a health hazard.

10.4.3 Reference Systems

Because of the limitations of the stereotactic frame, it was immediately apparent that frameless surgical navigation represented a significant improvement. This approach dispensed with frames and allowed the surgeon to wield the instruments freehand. However, an important aspect of stereotactic frames was the mounting of surgical instruments directly to the patient’s anatomy through the base ring. With frameless devices there is no connection between the instruments and the patient’s anatomy, which can lead to error if the position of the patient is not accurately known.

An important benefit of surgical navigation systems is the ability to change the patient’s position following registration. To avoid the need for reregistration, it is necessary to have some means of tracking the head relative to the detection mechanism. This is commonly achieved with a reference device fixed directly to the patient’s head or to the head-holder, assuming that the head does not move in relationship to the holder. The reference device has at least three emitters detectable by the digitizer employed in the design of the system; for optically based systems a multiplicity of LEDs serves this function, allowing one or more LEDs to be blocked and still allow the position of the head to be determined prior to localization of the surgical instruments.

10.4.4 Effectors

The term effector simply refers to those instruments used by a surgeon to perform surgery on the patient. Although surgical instruments comprise the largest part of this group of devices, it is important to realize that the more general term ‘‘effector’’ can be used to refer to anything to treat the patient surgically. Hence, surgical microscopes, endoscopes, genetic material, drug polymers, and robots are effectors that will soon be part of a routine operation. These new effectors all rely upon precise position in order to produce the maximal benefit for the patient; therefore, integration of navigational technology is critical to their success.

Essential components of a surgical navigation system are the instruments that, through modifications by the addition of LEDs or reflectors, permit localization. The variety and number of such instruments define the functionality of the navigational device. Since the LEDs or spheres must be visible to the camera

array at all times, they cannot be mounted on the tip of an instrument, as the tip will not be visible to the cameras during surgery. These LEDs or spheres are usually located at a given distance from the tip in the handle of the instrument. If the instrument is linear in geometry, such as a forceps, then the minimum requirement for localization is two LEDs mounted in alignment with its tip. Reflective spheres generally do not localize well if they are placed in a linear alignment, because reflective systems experience difficulty when the views of the spheres partially overlap. For this reason, reflective arrays usually consist of at least three spheres in a nonlinear arrangement. This requirement usually makes LEDs the detector of choice for microsurgical instrumentation.

A bayoneted instrument is commonly employed to allow surgery through a small opening. The offset design of a bayonet instrument allows the handle to be placed off the axis of the line of sight of the surgeon and is therefore an ideal geometry use by a navigational system. Additionally, a suction tube, dissecting probe, curette, drill guide, or ventriculostomy stylet can all be equipped with LEDs and localized.

For instruments with complex 3D shapes LEDs or spheres must be placed off-axis to allow tracking. An example of such placement is the biopsy guide tube attachment used in the StealthStation system, or trackers for suction tubes.

When a specific trajectory into the brain is desired, rigid fixation of instrumentation is preferred to ensure that the instrumentation is aligned along the selected surgical path. Typical situations in which the surgical path is the central key issue include tumor biopsy, insertion of depth electrodes for epilepsy, insertion of ventricular catheters, and functional surgery. This function is carried out by a biopsy guide tube adaptor, which attaches to the reference arc using a standard retractor arm with adjustable tension. The tube is equipped with four LEDs to provide redundancy.

10.4.4.1 Localizing Microscope

An operating microscope can also be tracked relative to the surgical field and the position of the focal point displayed on the preoperative images.

One example of such a device that we have coupled to the StealthStation system is the Moeller ‘‘Smart Scope’’ (Moeller Microsurgical, Waldwick, NJ). The primary modification consists of a bracket containing four LEDs attached to the back of the microscope. The microscope head is then tracked relative to the surgical field. The motor-driven variable focal length of the microscope is reported to the operative computer. By determining the position of the head of the microscope and adding the offset of the focal length, the position of the focal point of the microscope can be precisely calculated and displayed using the preoperative images. The crosshairs displayed by the computer system indicate the position of the focal point of the operating microscope.

Certain microscopes can also be robotically controlled by the system. The microscope is equipped with motors that allow the head of the microscope to be moved in two dimensions. Using these motors, the workstation can drive the scope to focus upon a specific point in the surgical field chosen by clicking on the spot as viewed on the workstation display. The system mouse is used to point to the position of interest, and then, by selecting the microscope drive function from the menu, the microscope will be focused on that structure. Alternatively, the scope can be placed in position manually and tracked using the LEDs.

10.4.4.2 Image-Guided Endoscope

In neurosurgery, endoscopes are primarily used for intraventricular procedures. A rigid straight fiberscope (‘‘INCLUSIVE’’ endoscope, Sofamor Danek Inc, Memphis, TN) has been modified to work with the StealthStation. The endoscope is inserted into the brain through a modified sheath that is itself introduced into the brain over an obturator. Four LEDS are attached to the endoscope near the camera mount using a star-shaped adapter. The geometric configuration of the LEDs is programmed into the surgical navigational system along with the endoscopic dimensions.

The system also assists in entering the ventricle by allowing the stereotactic placement of the endoscopic sheath through a burr hole. The obturator and sheath are modified to fit over the registration probe of the system. The LEDs mounted in line with the tip of the probe provide continuous positional information for the introducer tip during ventricular cannulation.

10.4.5 Intraoperative Visualization

Surgical navigational systems exist to help the surgeon visualize the current location of surgical instruments within a patient’s body. The manner in which this information is transmitted to the surgeon during the operation is critical to the acceptance of the device by busy surgeons. This function is performed by the surgical workstation and is depicted on a growing variety of displays.

The StealthStation utilizes CT or MRI for intraoperative guidance. Images are typically obtained in the axial dimension, and the scanning parameters for each modality are adjusted to achieve roughly cubic voxels. Image files are transmitted from the CT or MRI scanner to the surgical workstation over an Ethernetbased local area network and are then converted to a standard file format.

During surgery, three standard views (the original axial projection and reconstructed sagittal and coronal images) are displayed on a monitor at all times. A crosshair pointer superimposed on these images indicates the position of the surgical instrument, endoscope, or microscope focal point. A fourth window can alternatively display a surface-rendered 3D view of the patient’s anatomy or live video from the endoscope or microscope. An alternative view, the naviga-

Furthermore, as these units proliferate into community hospitals, it will be imperative to make these units capable of being controlled by a small, scrubbed surgical team.

A variety of control techniques have now been developed which allow the surgeon to control the system while scrubbed. One technique involves the use of touch-sensitive flat panel displays that can be placed in a sterile bag and used for controlling the system as well as indicating position. Another solution is to incorporate voice recognition in the head-mounted display worn by the surgeon during a procedure. By placing a boom mike on these devices, voice recognition becomes possible.

As these systems become more advanced it will be important to improve the diversity and utility of the information presented. Rather than being limited to simply showing where a surgeon is within the patient’s anatomy, these systems can be employed to compare the patient’s anatomy to that of previous patients’ functional anatomy. This function is served through the use of an atlas of functional anatomy.

10.5ATLASES

Stereotactic atlases in printed form have been used for many years to assist in the planning of neurosurgical procedures. Traditional stereotactic atlases are nearly all based on Cartesian coordinates, although individual atlases may differ in certain aspects, such as the point of origin of the coordinates, the orientation of the axes, and the intervals of sampling.

The atlas assigns a set of 3D coordinates to each anatomical feature in a slice, thereby permitting the information from the atlas plates to be incorporated into the planning of the procedure. However, traditional atlases have some limitations that restrict their utility. The first drawback is that the atlas plates are usually based on sections of a single individual, and the anatomical structures shown may differ considerably in size and proportion from those of the patient. This discrepancy may be further exacerbated by the presence of tumors, abscesses, or accidental trauma that seriously distort the normal anatomy, making it difficult to identify or orientate the patient’s image data relative to the atlas plate. The second limitation is that the plates in a printed atlas are essentially immutable and cannot easily be resized or warped to match the patient’s anatomy.

A promising breakthrough that addresses both these problems is the development of electronic deformable atlases (Figure 10.3). In these atlases, the images obtained from the reference subject are stored in digital form and may be warped and deformed so that designated points coincide with the corresponding points in the image data obtained from the patient. Not only does this enable more precise preoperative planning, it is also valuable if the surgical plan has to be updated intraoperatively to reflect brain shift, or if ambiguous structures are en-