Navigation system
A frameless navigation system (ACCISS II™, Schaerer Mayfield Technologies GmbH, Berlin, Germany) was used for intraoperative image guidance in all cases. The system comprises the hard- and software necessary to generate and detect a DC pulsed magnetic field for computing the position and orientation of a localizing sensor. The tracking system in its basic version consists of an electromagnetic transmitter unit, a sensor (which is integrated into the handle of a surgical pointer device) and an electronic digitizer unit that controls the transmitter and receives the spatial data from the localizing sensor.
The transmitter consists of a triad of electromagnetic coils (size: 9.6 cm cube) which generates a homogeneous electromagnetic field (max. 600 milligauss with a translation range of 76.2 cm in any direction) that, in its basic version, simultaneously serves as the fixed reference for the setup.
The localizing sensors can be integrated into pointers or other surgical instruments of various shapes. The sensor, being completely passive and having no active voltage applied, detects the magnetic field generated by the transmitter unit with up to 120 measurements per second what ensures real-time conditions. They have 6 degrees of freedom (position and orientation) with an angular range of ± 180° azimuth & roll and ± 90° elevation. The static accuracy is specified by the manufacturer (Ascension Technologies Corp., Burlington, USA) as 1.8 mm RMS (position) and 0.5° RMS (orientation). The static resolution is 0.5 mm (position) and 0.1° (orientation) at a distance of 30.5 cm from the transmitter.
In the digitizer unit, the analogue measured signals of the sensor are digitalized, and the coordinates of the sensor position are calculated.
Dynamic Reference Frame (DRF)
To allow simultaneous registration, localization and position tracking of more than one localizing sensor, the before described basic version of the ACCISS II system was expanded with a soft- and hardware update which helps to run a so-called Dynamic Reference Frame (DRF). The DRF can be used as an additional reference system that defines an independent coordinate system in space in addition to the one established by the transmitter unit (Figure 1B). Thus, it becomes possible to record the slightest movement of the cranium as well. This information can then be used to continuously adapt the position of the imaging plane and the resultant calculated virtual 3-D model to the actual position of the cranium. Technically, the DRF consists of an additional localizing sensor measuring 8 mm × 8 mm × 18 mm in size with a weight of 1.2 g. The extra sensor is accommodated in a watertight capsule and is connected to the navigation system with a 3 m long cable. The DRF sensor can either (a) be attached to a dental splint or (b) be attached retroauricular on the hairless skin.
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(a)
In the oral cavity, the DRF is attached to the upper row of teeth using a special, removable mouthpiece (Figure 2) and a 2-component polyether self-hardening material (Impregum® F; ESPE Dental AG, Seefeld, Germany). The mouthpiece consists of a U-shaped splint which is filled with a fast-hardening material and applied to the upper row of teeth exerting slight pressure (about 0.25 atm) at the centre. The vacuum resulting from hardening of the material ensures that the mouthpiece is firmly secured in place in patients with healthy teeth. After the procedure, the mouthpiece is removed by releasing the vacuum with a dental hook.
Alternatively, if oral attachment is precluded by the patient's dental status or for anesthesiological or surgical reasons, the DRF is attached directly to the scalp, preferably over the mastoid, behind the auricle.
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(b)
For retroauricular attachment (Figure 1), the DRF is placed in the area of the mastoid in such a way that it is in direct contact with the back of the auricle. The auricle thus serves as an anatomical barrier against anterior displacement. The retroauricular region is chosen because there is minimal skin mobility and the auricle provides additional stability, ensuring stable attachment of the DRF in this area. The device was secured in place with 40 mm wide, skin-friendly tape applied crosswise to the hairless skin (Figure 1C). To prevent detachment of the tape by contact with fluids or disinfectants, a waterproof self-adhesive sterile film was glued over it (Opraflex®, Lohmann & Rauscher Int., Rengsdorf, Germany).
Proper affixation of the DRF was checked in all cases by a rotation test immediately after image data registration (Figure 3). To this end, the head was rotated about 120° from the right lateral to the left lateral position and back (Figure 3A–C). The spatial coordinates of the fiducial markers were verified relative to the position of the DRF. Adequate attachment of the DRF was assumed when the deviation was < 1 mm in all three spatial direction (cartesian x, y, z-coordinates as displayed by the navigation system; Figure 3D). If there was greater deviation, the position was corrected and the attachment optimized until deviation was within the limit of 1 mm.
Image data acquisition and preparation
Preoperatively, a serial CT or MRI scan was obtained. The images consisted of a three-dimensional volume data set of contiguous axial CT or sagittal MR images. In order to obtain isotropic voxels of 1 mm length one of the following CT or MRI protocols was routinely used.
MRI was performed using a T1-weighted 3D GE sequences (3D MP RAGE) with the parameters: TR 9.7 ms, TE 4 ms, FA 12°, TI 300 ms, TD 0 s, FOV 256 mm, 256 × 256 matrix, 256 partitions, slice thickness of 1 mm, acquisition time 11 min 54 s. Alternatively, a high-resolution CT spiral scan was acquired with 1 mm slice thickness, 512 × 512 matrix, pitch factor 2, 1 mm increment, and 50–110 mA tube current. The image data were transferred to the computer workstation in the ACR/NEMA 3.0/DICOM image data format via a local network (LAN – FTP or DICOM transfer protocol), or through data media, such as CD-ROM, magnetic-optical disks (MOD) or magnetic tape (DAT).
Data processing and preparation was performed using an autosegmentation technique (ACCISS II software version 1.9). Image guidance was based on axial planar views (sagittal, coronal and transaxial), free planar views (defined by pointer orientation and/or target localization), and 3D views of the anatomical objects (skin, skull, brain surface structures, brain parenchyma and lesion target) (Figure 4). The image data was registered by means of point-to-point matching (sequentially sampling 7 two-component adhesive fiducial markers with a sensor-bearing pointer according to a standardized protocol).
Accuracy measurements
Registration accuracy was determined calculating the fiducial registration error (FRE) expressed as the root mean square error. The FRE describes the distance between the position of a marker in the image dataset and the position measured in the operative field. The mean RMS value is calculated directly by the navigation system and is displayed together with the min. and max. FRE and the Target Registration Error (TRE – for a certain target point within the registered volume) on the navigation screen (Figure 3D).
The application accuracy was monitored intraoperatively using as a reference point a 1 mm burr hole drilled into the exposed bone margin directly after craniotomy (Figure 5). The initial Cartesian coordinates of this reference point were determined immediately by means of a pointer. The measurements were repeated after craniotomy immediately before dura opening, three times during tumour resection (M1–M3) and after closure of the dura, respectively at the end of the operation. Deviations in x, y, and z directions were measured as three-dimensional Position Error (PE in mm) of the reference point relative to the baseline coordinates of the same point determined immediately after craniotomy.
Statistical analysis
FRE and PE values are expressed as means +/- standard deviation from the number (n) of patients in each group. Data were tested for significance using one-way ANOVA to determine degree of variability within a group, followed by Bonferroni post hoc analysis. Test of pairwise comparisons were carried out with the Student's t-test to compare two groups (e.g. for differences in FRE between the different types of head fixation, as well as for differences in ΔPE in-/decrease between the different types of head positioning over the time of surgery). A p < 0.05 was considered as statistically significant. Data management and statistical analyses were performed using the SPSS 13.0 for Windows® software package.