Introduction
Over the past decade, image-guided surgery has become one of the most exciting new technologies for the neurosurgeon. Advances in techniques and instrumentation have been rapid [1-9]. The Mayfield® ACCISS™ image-guided stereotactic workstation (OMI system) has been developed to meet the image-guided needs of the most demanding surgeons (Fig. 1). With the May-field ACCISS, the surgeon can perform stereotactic-guided craniotomies, transphenoidal and other ears, nose, throat (ENT)/skull-base procedures. The Mayfield ACCISS has the most rigid stereotactic platform in the image-guided field. The experienced stereotactic neurosurgeon can confidently use the May-field ACCISS for stereotactic procedures, eliminating the need for a traditional frame. Lastly, the requirements for spinal applications can be easily satisfied with this system.
This topic describes the system and its components, fiducial placement, image acquisition, and surgical planning. It also defines its use in the operating room. The author also describes his surgical experience and recommendations for applications of the Mayfield ACCISS system and its unique components.
Figure 1 The Mayfield ACCISS system. Custom computer cart and infrared camera for optical tracking.
Description of the Mayfield ACCISS
The Mayfield ACCISS uses the Windows NT™ operating system, which provides the surgeon surprisingly fast and powerful computing capacity in a familiar and easy-to-use format. Placed in a customized cart, the system is positioned at the hand of the surgeon in the operating room. Its simple and user-friendly format eliminates the need and expense of a dedicated technician to run the system during surgery.
The Mayfield ACCISS is unique in the image-guided field because it uses both the commonly known optical tracking technology and the only small, light-weight (13.5 ounces) mechanical arm in the industry (Fig. 2). Unlike its cumbersome predecessors, the Mayfield ACCISS arm is easy to use. It has rigidity and stability, and eliminates the need for the infrared camera with floor stand and the dynamic reference frame (DRF) required with the more commonly used optical tracking systems.
Figure 2 The Mayfield ACCISS dual digitizing system. Left, wireless active optical tracking systems with infrared camera. Right, mechanical arm system with the tip of the mechanical arm in the AccuPoint sphere for stereotactic targeting.
An active optical tracking system is a standard component of the May-field ACCISS. The power cable to the optical probes has been replaced by a disposable battery. The wireless custom probe comes with an adaptor that allows the interchange of multiple instruments, such as pointers and suction devices.
The AccuPoint™ sphere is a ball-and-socket device that can be used for generation and rigid fixation of a stereotactic trajectory (Fig. 3). It can be placed anywhere over the cranium for access to any stereotactic target. It is designed to be close to the entry point, which minimizes the length the probe must pass to reach its intended target.
The Mayfield ACCISS uses the Budde Halo™ retractor system for attachment of the mechanical arm, the optical tracking components, and the AccuPoint stereotactic device for cranial applications. These components can be placed anywhere around the 360° circumference of the halo to suit the needs and convenience of the surgeon (Figs. 2 and 3). Alternatively these components can be used without the halo retractor system.
FIDUCIAL PLACEMENT, IMAGE ACQUISITION, AND SURGICAL PLANNING
Adhesive fiducial markers are used almost exclusively, although implantable markers are commercially available. Anatomical landmarks can be used for ENT and spinal applications but are not recommended for cranial applications. Adhesive fiducials for cranial procedures can be applied by a technician, and the scan can be performed up to 7 days before the procedure, although no more than 2 to 3 days beforehand is recommended. The fiducial markers are placed around the area of interest and prepped into the surgical field so they can be easily accessed during the procedure to re-register or recheck the accuracy of registration. The software program finds the best fit between the location of the fiducials in patient space and the location of the fiducials on the image dataset and compensates to some degree for movement of an individual fiducial marker. Any fiducial marker identified with a large registration error can be easily eliminated from the registration process.
Figure 3 The stereotactic platform for the Mayfield ACCISS. It includes the rigid AccuPoint sphere which is attached to the Budde Halo retractor system.
For this reason, five to seven fiducial markers are recommended for placement for every procedure. This technique has led to satisfactory or very good registration in nearly all cases including stereotactic procedures.
Computed tomography (CT) and magnetic resonance imaging (MRI) are the usual modalities for most surgical procedures. Standard formatting for CT requires image-acquisition with 3-mm contiguous slices and a 0° gantry tilt, although in certain cases 1.5-mm cuts may be desirable to improve accuracy. Volumetric MRI scans with contrast are usually performed for most tumors. Computed tomography is the usual modality used for spinal applications, because MRI poorly visualizes bone anatomy. Once acquired, these images can be downloaded to digital audio type (DAT) or directly transferred by ethernet to the workstation.
The Mayfield ACCISS uniquely coregisters images, which permits the surgeon to simultaneously navigate with a CT and MRI image (Fig. 4). This is especially advantageous in pituitary surgery, where identification of bone surfaces and tumor margins is important. Coregistration permits the measurement of postoperative stereotactic biopsy accuracy in a way never before possible (Fig. 5).
The virtual screw program is a useful adjunct for preplanning pedicle screw placement. One can measure and place the screws in a simulation of the surgical procedure. The workstation can store up to eight screws on a dataset.
Use of the Mayfield ACCISS in Surgery
With the Mayfield ACCISS system, the surgeon can choose to use either the mechanical arm or optical tracking system, depending on individual preferences. The mechanical arm system is ideal and the preferred digitizer for long procedures where the microscope is used. The presence of the microscope, assistant, scrub nurse, operating tables, and anesthesia setup creates a crowded space at the operating table and usually makes placement of the infrared camera and maintenance of unobstructed optical pathways throughout the duration of the procedure difficult. In this crowded environment, frequent disruption of optically tracked instruments occurs and occasionally the microscope has to be moved out of position in order to track the instrument. In this setting the mechanical arm functions in a superior fashion to the optical system completely eliminating this often frustrating problem.
The mechanical arm system is also preferred by the author for stereo-tactic procedures because of its greater stability and ability to provide along-the-probe views which display the trajectory to the lesion. By rotating the probe tip of the mechanical arm after it has been positioned into the rigidly fixed AccuPoint sphere the along-the-probe view plane can be rotated and viewed at the workstation 360 degrees around the axis of the intended trajectory (Figs. 2 and 6).
Figure 4 Coregistration of images for transphenoidal surgery allows one to navigate simultaneously on axial, coronal, and sagittal magnetic resonance imaging (MRI) and sagittal computed tomographic (CT) images. Any combination of images is possible. Note sagittal CT image upper right and its precise correlations with the sagittal image.
This provides the surgeon with the most complete anatomical understanding of the intended trajectory. This complete rotation through the 360° axis cannot be done with optical tracking technology. The AccuPoint sphere provides a rigid method for stereotactic targeting and is brought close to the target, which minimizes the potential for deflection of the probe and introduction of stereotactic error. Either the mechanical arm or the optical probe can be placed into the AccuPoint sphere for trajectory generation.
The Mayfield ACCISS can be adapted to the microscope. With the microscope as a pointer, it is used initially to locate the cranial flap and plan a trajectory to the lesion. As soon as the surgeon desires to use an instrument as pointer, the microscope ceases to function as a tracked instrument.
Figure 5 Sterostatic biopsy with postoperative coregistration. Small hemorrhage at biopsy site on postoperative computed tomographic (CT) coronal and axial view on right correlates with preoperative axial magnetic resonance imaging (MRI) image on left.
The optical tracking system is best used in cranial procedures in which the microscope is used in a limited capacity or not at all. It can also be used for stereotactic procedures. The optical tracking system is preferred for spinal procedures. The elimination of the wire to the optical probes has provided an additional degree of freedom while maintaining the high level of accuracy associated with active optical tracking systems.
The Mayfield ACCISS system has a distinct advantage imparted by its dual digitizing capabilities. If there is a failure at any time of the optical system because of malfunction of light-emitting diodes (LEDs), contamination of optical components that should not be autoclaved, or other technical problems, the mechanical arm can be activated in a short couple of minutes. It requires a standard endoscope drape, and all its components, including pointer tips, can be autoclaved.
Figure 6 Along-the-probe views. One can rotate the view plane 360° around the axis of the probe (solid yellow line). Two such views are shown that provide the surgeon a clear picture of the trajectory to the lesion, which was a moderate grade thalamic glioma.
In addition, the availability of both tracking methods facilitates performing three or four image-guided procedures in the same day.
