Source-detector trajectory optimization in cone-beam computed tomography: a comprehensive review on today's state-of-the-art.

Cone-beam computed tomography (CBCT) imaging is becoming increasingly important for a wide range of applications such as image-guided surgery, image-guided radiation therapy as well as diagnostic imaging such as breast and orthopaedic imaging. The potential benefits of non-circular source-detector trajectories was recognized in early work to improve the completeness of CBCT sampling and extend the field of view (FOV). Another important feature of interventional imaging is that prior knowledge of patient anatomy such as a preoperative CBCT or prior CT is commonly available. This provides the opportunity to integrate such prior information into the image acquisition process by customized CBCT source-detector trajectories. Such customized trajectories can be designed in order to optimize task-specific imaging performance, providing intervention or patient-specific imaging settings. The recently developed robotic CBCT C-arms as well as novel multi-source CBCT imaging systems with additional degrees of freedom provide the possibility to largely expand the scanning geometries beyond the conventional circular source-detector trajectory. This recent development has inspired the research community to innovate enhanced image quality by modifying image geometry, as opposed to hardware or algorithms. The recently proposed techniques in this field facilitate image quality improvement, FOV extension, radiation dose reduction, metal artifact reduction as well as 3D imaging under kinematic constraints. Because of the great practical value and the increasing importance of CBCT imaging in image-guided therapy for clinical and preclinical applications as well as in industry, this paper focuses on the review and discussion of the available literature in the CBCT trajectory optimization field. To the best of our knowledge, this paper is the first study that provides an exhaustive literature review regarding customized CBCT algorithms and tries to update the community with the clarification of in-depth information on the current progress and future trends.

A head coil system with an integrated orbiting transmission point source mechanism for attenuation correction in PET/MRI.

The combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) provides a benefit for diagnostic imaging. Still, attenuation correction (AC) is a challenge in PET/MRI compared to stand-alone PET and PET-computed tomography (PET/CT). In the absence of photonic transmission sources, AC in PET/MRI is usually based on retrospective segmentation of MR images or complex additional MR-sequences. However, most methods available today are still challenged by either the incorporation of cortical bone or substantial anatomical variations of subjects. This leads to a bias in quantification of tracer concentration in PET. Therefore, we have developed a fully integrated transmission source system for PET/MRI of the head to enable direct measurement of attenuation coefficients using external positron emitters, which is the reference standard in AC. Based on a setup called the 'liquid drive' presented by Jones et al (1995) two decades ago, we built a head coil system consisting of an MR-compatible hydraulic system driving a point source on a helical path around a 24-channel MR-receiver coil to perform a transmission scan. Sinogram windowing of the moving source allows for post-injection measurements. The prototype was tested in the Siemens Biograph mMR using a homogeneous water phantom and a phantom with air cavities and a Teflon (PTFE) cylinder. The second phantom was measured both with and without emission activity. For both measurements air, water and Teflon were clearly distinguishable and homogeneous regions of the phantom were successfully reproduced in the AC map. For water the linear attenuation coefficient was measured as (0.096 ± 0.005) cm-1 in accordance with the true physical value. This combined MR head coil and transmission source system is, to our knowledge, the first working example to use an orbiting point source in PET/MRI and may be helpful in providing fully-quantitative PET data in neuro-PET/MRI.

Automatic quantification of multi-modal rigid registration accuracy using feature detectors.

In radiotherapy, the use of multi-modal images can improve tumor and target volume delineation. Images acquired at different times by different modalities need to be aligned into a single coordinate system by 3D/3D registration. State of the art methods for validation of registration are visual inspection by experts and fiducial-based evaluation. Visual inspection is a qualitative, subjective measure, while fiducial markers sometimes suffer from limited clinical acceptance. In this paper we present an automatic, non-invasive method for assessing the quality of intensity-based multi-modal rigid registration using feature detectors. After registration, interest points are identified on both image data sets using either speeded-up robust features or Harris feature detectors. The quality of the registration is defined by the mean Euclidean distance between matching interest point pairs. The method was evaluated on three multi-modal datasets: an ex vivo porcine skull (CT, CBCT, MR), seven in vivo brain cases (CT, MR) and 25 in vivo lung cases (CT, CBCT). Both a qualitative (visual inspection by radiation oncologist) and a quantitative (mean target registration error-mTRE-based on selected markers) method were employed. In the porcine skull dataset, the manual and Harris detectors give comparable results but both overestimated the gold standard mTRE based on fiducial markers. For instance, for CT-MR-T1 registration, the mTREman (based on manually annotated landmarks) was 2.2 mm whereas mTREHarris (based on landmarks found by the Harris detector) was 4.1 mm, and mTRESURF (based on landmarks found by the SURF detector) was 8 mm. In lung cases, the difference between mTREman and mTREHarris was less than 1 mm, while the difference between mTREman and mTRESURF was up to 3 mm. The Harris detector performed better than the SURF detector with a resulting estimated registration error close to the gold standard. Therefore the Harris detector was shown to be the more suitable method to automatically quantify the geometric accuracy of multimodal rigid registration.

Standardized accuracy assessment of the calypso wireless transponder tracking system.

Electromagnetic (EM) tracking allows localization of small EM sensors in a magnetic field of known geometry without line-of-sight. However, this technique requires a cable connection to the tracked object. A wireless alternative based on magnetic fields, referred to as transponder tracking, has been proposed by several authors. Although most of the transponder tracking systems are still in an early stage of development and not ready for clinical use yet, Varian Medical Systems Inc. (Palo Alto, California, USA) presented the Calypso system for tumor tracking in radiation therapy which includes transponder technology. But it has not been used for computer-assisted interventions (CAI) in general or been assessed for accuracy in a standardized manner, so far. In this study, we apply a standardized assessment protocol presented by Hummel et al (2005 Med. Phys. 32 2371-9) to the Calypso system for the first time. The results show that transponder tracking with the Calypso system provides a precision and accuracy below 1 mm in ideal clinical environments, which is comparable with other EM tracking systems. Similar to other systems the tracking accuracy was affected by metallic distortion, which led to errors of up to 3.2 mm. The potential of the wireless transponder tracking technology for use in many future CAI applications can be regarded as extremely high.

A navigation system for flexible endoscopes using abdominal 3D ultrasound.

A navigation system for flexible endoscopes equipped with ultrasound (US) scan heads is presented. In contrast to similar systems, abdominal 3D-US is used for image fusion of the pre-interventional computed tomography (CT) to the endoscopic US. A 3D-US scan, tracked with an optical tracking system (OTS), is taken pre-operatively together with the CT scan. The CT is calibrated using the OTS, providing the transformation from CT to 3D-US. Immediately before intervention a 3D-US tracked with an electromagnetic tracking system (EMTS) is acquired and registered intra-modal to the preoperative 3D-US. The endoscopic US is calibrated using the EMTS and registered to the pre-operative CT by an intra-modal 3D-US/3D-US registration. Phantom studies showed a registration error for the US to CT registration of 5.1 mm±2.8 mm. 3D-US/3D-US registration of patient data gave an error of 4.1 mm compared to 2.8 mm with the phantom. From this we estimate an error on patient experiments of 5.6 mm.

18F, 11C and 68Ga in small animal PET imaging. Evaluation of partial volume correction methods.

AIM: The partial volume effect (PVE) significantly affects quantitative accuracy in PET. In this study we used a micro-hollow sphere phantom filled with 18F, 11C or 68Ga to evaluate different partial volume correction methods (PVC). Additionally, phantom data were applied on rat brain scans to evaluate PVC methods on in vivo datasets. METHODS: The four spheres (7.81, 6.17, 5.02, 3.90 mm inner diameter) and the background region were filled to give sphere-to-background (sph/bg) activity ratios of 20 : 1, 10 : 1, 5 : 1 and 2 : 1. Two different acquisition and reconstruction protocols and three radionuclides were evaluated using a small animal PET scanner. From the obtained images the recovery coefficients (RC) and contrast recovery coefficients (CRC) for the different sph/bg ratios were calculated. Three methods for PVC were evaluated: a RC based, a CRC based and a volume of interest (VOI) based method. The most suitable PVC methods were applied to in vivo rat brain data. RESULTS: RCs were shown to be dependent on the radionuclide used, with the highest values for 18F, followed by 11C and 68Ga. The calculated mean CRCs were generally lower than the corresponding mean RCs. Application of the different PVC methods to rat brain data led to a strong increase in time-activity curves for the smallest brain region (entorhinal cortex), whereas the lowest increase was obtained for the largest brain region (cerebellum). CONCLUSION: This study was able to show the importance and impact of PVE and the limitations of several PVC methods when performing quantitative measurements in small structures.

Performance validation of deformable image registration in the pelvic region.

Patients undergoing radiotherapy will inevitably show anatomical changes during the course of treatment. These can be weight loss, tumour shrinkage, and organ motion or filling changes. For advanced and adaptive radiotherapy (ART) information about anatomical changes must be extracted from repeated images in order to be able to evaluate and manage these changes. Deformable image registration (DIR) is a tool that can be used to efficiently gather information about anatomical changes. The aim of the present study was to evaluate the performance of two DIR methods for automatic organ at risk (OAR) contour propagation. Datasets from ten gynaecological patients having repeated computed tomography (CT) and cone beam computed tomography (CBCT) scans were collected. Contours were delineated on the planning CT and on every repeated scan by an expert clinician. DIR using our in-house developed featurelet-based method and the iPlan(®) BrainLab treatment planning system software was performed with the planning CT as reference and a selection of repeated scans as the target dataset. The planning CT contours were deformed using the resulting deformation fields and compared to the manually defined contours. Dice's similarity coefficients (DSCs) were calculated for each fractional patient scan structure, comparing the volume overlap using DIR with that using rigid registration only. No significant improvement in volume overlap was found after DIR as compared with rigid registration, independent of which image modality or DIR method was used. DIR needs to be further improved in order to facilitate contour propagation in the pelvic region in ART approaches.

Standardized assessment of new electromagnetic field generators in an interventional radiology setting.

PURPOSE: Two of the main challenges associated with electromagnetic (EM) tracking in computer-assisted interventions (CAIs) are (1) the compensation of systematic distance errors arising from the influence of metal near the field generator (FG) or the tracked sensor and (2) the optimized setup of the FG to maximize tracking accuracy in the area of interest. Recently, two new FGs addressing these issues were proposed for the well-established Aurora(®) tracking system [Northern Digital, Inc. (NDI), Waterloo, Canada]: the Tabletop 50-70 FG, a planar transmitter with a built-in shield that compensates for metal distortions emanating from treatment tables, and the prototypical Compact FG 7-10, a mobile generator designed to be attached to mobile imaging devices. The purpose of this paper was to assess the accuracy and precision of these new FGs in an interventional radiology setting. METHODS: A standardized assessment protocol, which uses a precisely machined base plate to measure relative error in position and orientation, was applied to the two new FGs as well as to the well-established standard Aurora(®) Planar FG. The experiments were performed in two different settings: a reference laboratory environment and a computed tomography (CT) scanning room. In each setting, the protocol was applied to three different poses of the measurement plate within the tracking volume of the three FGs. RESULTS: The two new FGs provided higher precision and accuracy within their respective measurement volumes as well as higher robustness with respect to the CT scanner compared to the established FG. Considering all possible 5 cm distances on the grid, the error of the Planar FG was increased by a factor of 5.94 in the clinical environment (4.4 mm) in comparison to the error in the laboratory environment (0.8 mm). In contrast, the mean values for the two new FGs were all below 1 mm with an increase in the error by factors of only 2.94 (Reference: 0.3 mm; CT: 0.9 mm) and 1.04 (both: 0.5 mm) in the case of the Tabletop FG and the Compact FG, respectively. CONCLUSIONS: Due to their high accuracy and robustness, the Tabletop FG and the Compact FG could eliminate the need for compensation of EM field distortions in certain CT-guided interventions.

Assessment of accuracy and efficiency of atlas-based autosegmentation for prostate radiotherapy in a variety of clinical conditions.

BACKGROUND AND PURPOSE: The goal of the current study was to evaluate the commercially available atlas-based autosegmentation software for clinical use in prostate radiotherapy. The accuracy was benchmarked against interobserver variability. MATERIAL AND METHODS: A total of 20 planning computed tomographs (CTs) and 10 cone-beam CTs (CBCTs) were selected for prostate, rectum, and bladder delineation. The images varied regarding to individual (age, body mass index) and setup parameters (contrast agent, rectal balloon, implanted markers). Automatically created contours with ABAS(®) and iPlan(®) were compared to an expert's delineation by calculating the Dice similarity coefficient (DSC) and conformity index. RESULTS: Demo-atlases of both systems showed different results for bladder (DSC(ABAS) 0.86 ± 0.17, DSC(iPlan) 0.51 ± 0.30) and prostate (DSC(ABAS) 0.71 ± 0.14, DSC(iPlan) 0.57 ± 0.19). Rectum delineation (DSC(ABAS) 0.78 ± 0.11, DSC(iPlan) 0.84 ± 0.08) demonstrated differences between the systems but better correlation of the automatically drawn volumes. ABAS(®) was closest to the interobserver benchmark. Autosegmentation with iPlan(®), ABAS(®) and manual segmentation took 0.5, 4 and 15-20 min, respectively. Automatic contouring on CBCT showed high dependence on image quality (DSC bladder 0.54, rectum 0.42, prostate 0.34). CONCLUSION: For clinical routine, efforts are still necessary to either redesign algorithms implemented in autosegmentation or to optimize image quality for CBCT to guarantee required accuracy and time savings for adaptive radiotherapy.

Electromagnetic tracking for US-guided interventions: standardized assessment of a new compact field generator.

PURPOSE: One of the main challenges related to electromagnetic tracking in the clinical setting is a placement of the field generator (FG) that optimizes the reliability and accuracy of sensor localization. Recently, a new mobile FG for the NDI Aurora(®) tracking system has been presented. This Compact FG is the first FG that can be attached directly to an ultrasound (US) probe. The purpose of this study was to assess the precision and accuracy of the Compact FG in the presence of nearby mounted US probes. MATERIALS AND METHODS: Six different US probes were mounted onto the Compact FG by means of a custom-designed mounting adapter. To assess precision and accuracy of the Compact FG, we employed a standardized assessment protocol. Utilizing a specifically manufactured plate, we measured positional data on three levels of distances from the FG as well as rotational data. RESULTS: While some probes had negligible influence on tracking accuracy two probes increased the mean distance error up to 1.5 mm compared with a reference measurement of 0.5 mm. The jitter error consistently stayed below 0.2 mm in all cases. The mean relative error in orientation was found to be smaller than 3°. CONCLUSION: Attachment of an US probe to the Compact FG does not have a critical influence on tracking accuracy in most cases. Clinical benefit of this promising mobile FG must be shown in future studies.

SU-E-J-90: 2D/3D Registration Using KV-MV Image Pairs for Higher Accuracy Image Guided Radiotherapy.

PURPOSE: In this work, we investigate the impact of using paired portal mega-voltage (MV) and kilo-voltage (kV) images, on 2D/3D registration accuracy with the purpose of improving tumor motion tracking during radiotherapy. Tumor motion tracking is important as motion remains one of the biggest sources of uncertainty in dose application. 2D/3D registration is successfully used in online tumor motion tracking, nevertheless, one limitation of this technique is the inability to resolve movement along the imaging beam axis using only one projection image. METHODS: Our evaluation consisted in comparing the accuracy of registration using different 2D image combinations: only one 2D image (1-kV), one kV and one MV image (1kV-1MV) and two kV images (2-kV). For each of the image combinations we evaluated the registration results using 250 starting points as initial displacements from the gold standard. We measured the final mean target registration error (mTRE) and the success rate for each registration. Each of the combinations was evaluated using four different merit functions. RESULTS: When using the MI merit function (a popular choice for this application) the RMS mTRE drops from 6.4 mm when using only one image to 2.1 mm when using image pairs. The success rate increases from 62% to 99.6%. A similar trend was observed for all four merit functions. Typically, the results are slightly better with 2-kV images than with 1kV-1MV. CONCLUSIONS: We evaluated the impact of using different image combinations on accuracy of 2D/3D registration for tumor motion monitoring. Our results show that using a kV-MV image pair, leads to improved results as motion can be accurately resolved in six degrees of freedom. Given the possibility to acquire these two images simultaneously, this is not only very workflow efficient but is also shown to be a good approach to improve registration accuracy.

Validation for 2D/3D registration. I: A new gold standard data set.

PURPOSE: In this article, the authors propose a new gold standard data set for the validation of two-dimensional/three-dimensional (2D/3D) and 3D/3D image registration algorithms. METHODS: A gold standard data set was produced using a fresh cadaver pig head with attached fiducial markers. The authors used several imaging modalities common in diagnostic imaging or radiotherapy, which include 64-slice computed tomography (CT), magnetic resonance imaging using T1, T2, and proton density sequences, and cone beam CT imaging data. Radiographic data were acquired using kilovoltage and megavoltage imaging techniques. The image information reflects both anatomy and reliable fiducial marker information and improves over existing data sets by the level of anatomical detail, image data quality, and soft-tissue content. The markers on the 3D and 2D image data were segmented using ANALYZE 10.0 (AnalyzeDirect, Inc., Kansas City, KN) and an in-house software. RESULTS: The projection distance errors and the expected target registration errors over all the image data sets were found to be less than 2.71 and 1.88 mm, respectively. CONCLUSIONS: The gold standard data set, obtained with state-of-the-art imaging technology, has the potential to improve the validation of 2D/3D and 3D/3D registration algorithms for image guided therapy.

Efficient implementation of the rank correlation merit function for 2D/3D registration.

A growing number of clinical applications using 2D/3D registration have been presented recently. Usually, a digitally reconstructed radiograph is compared iteratively to an x-ray image of the known projection geometry until a match is achieved, thus providing six degrees of freedom of rigid motion which can be used for patient setup in image-guided radiation therapy or computer-assisted interventions. Recently, stochastic rank correlation, a merit function based on Spearman's rank correlation coefficient, was presented as a merit function especially suitable for 2D/3D registration. The advantage of this measure is its robustness against variations in image histogram content and its wide convergence range. The considerable computational expense of computing an ordered rank list is avoided here by comparing randomly chosen subsets of the DRR and reference x-ray. In this work, we show that it is possible to omit the sorting step and to compute the rank correlation coefficient of the full image content as fast as conventional merit functions. Our evaluation of a well-calibrated cadaver phantom also confirms that rank correlation-type merit functions give the most accurate results if large differences in the histogram content for the DRR and the x-ray image are present.

Positioning accuracy in a registration-free CT-based navigation system.

In order to maintain overall navigation accuracy established by a calibration procedure in our CT-based registration-free navigation system, the CT scanner has to repeatedly generate identical volume images of a target at the same coordinates. We tested the positioning accuracy of the prototype of an advanced workplace for image-guided surgery (AWIGS) which features an operating table capable of direct patient transfer into a CT scanner. Volume images (N = 154) of a specialized phantom were analysed for translational shifting after various table translations. Variables included added weight and phantom position on the table. The navigation system's calibration accuracy was determined (bias 2.1 mm, precision +/- 0.7 mm, N = 12). In repeated use, a bias of 3.0 mm and a precision of +/- 0.9 mm (N = 10) were maintainable. Instances of translational image shifting were related to the table-to-CT scanner docking mechanism. A distance scaling error when altering the table's height was detected. Initial prototype problems visible in our study causing systematic errors were resolved by repeated system calibrations between interventions. We conclude that the accuracy achieved is sufficient for a wide range of clinical applications in surgery and interventional radiology.

Evaluation of a new electromagnetic tracking system using a standardized assessment protocol.

This note uses a published protocol to evaluate a newly released 6 degrees of freedom electromagnetic tracking system (Aurora, Northern Digital Inc.). A practice for performance monitoring over time is also proposed. The protocol uses a machined base plate to measure relative error in position and orientation as well as the influence of metallic objects in the operating volume. Positional jitter (E(RMS)) was found to be 0.17 mm +/- 0.19 mm. A relative positional error of 0.25 mm +/- 0.22 mm at 50 mm offsets and 0.97 mm +/- 1.01 mm at 300 mm offsets was found. The mean of the relative rotation error was found to be 0.20 degrees +/- 0.14 degrees with respect to the axial and 0.91 degrees +/- 0.68 degrees for the longitudinal rotation. The most significant distortion caused by metallic objects is caused by 400-series stainless steel. A 9.4 mm maximum error occurred when the rod was closest to the emitter, 10 mm away. The improvement compared to older generations of the Aurora with respect to accuracy is substantial.

Risk analysis and safety assessment in surgical robotics: a case study on a biopsy robot.

One of the most important issues in medical robotics is safety and integration into the clinical workflow. If a robot is not safe and its use is complicated by difficult handling and complex user interfaces physicians would not use a robotic system during clinical patient trials, whatever the other advantages are. However, there are only few publications on this topic, in particular on risk management in developing a robotic prototype (for clinical trials). In this paper risk management and the safety of using robot-assisted surgery equipment are discussed and demonstrated exemplarily in the process of developing a prototype biopsy robot.

Fast DRR generation for 2D/3D registration.

We present a simple and rapid method for generation of perspective digitally rendered radiographs (DRR) for 2D/3D registration based on splat rendering. Suppression of discretization artefacts by means of computation of Gaussian footprints--which is a considerable computational burden in classical splat rendering--is replaced by stochastic motion of either the voxels in the volume to be rendered, or by simulation of a X-ray tube focal spot of finite size. The result is a simple and fast perspective rendering algorithm using only a small subset of voxels. Our method generates slightly blurred DRRs suitable for registration purposes at framerates of approximately 10 Hz when rendering volume images with a size of 30 MB on a standard PC.

Quantitative analysis of factors affecting intraoperative precision and stability of optoelectronic and electromagnetic tracking systems.

This study aims to provide a quantitative analysis of the factors affecting the actual precision and stability of optoelectronic and electromagnetic tracking systems in computer-aided surgery under real clinical/intraoperative conditions. A "phantom-skull" with five precisely determined reference distances between marker spheres is used for all measurements. Three optoelectronic and one electromagnetic tracking systems are included in this study. The experimental design is divided into three parts: (1) evaluation of serial- and multislice-CT (computed tomography) images of the phantom-skull for the precision of distance measurements by means of navigation software without a digitizer, (2) digitizer measurements under realistic intraoperative conditions with the factors OR-lamp (radiating into the field of view of the digitizer) or/and "handling with ferromagnetic surgical instruments" (in the field of view of the digitizer) and (3) "point-measurements" to analyze the influence of changes in the angle of inclination of the stylus axis. Deviations between reference distances and measured values are statistically investigated by means of analysis of variance. Computerized measurements of distances based on serial-CT data were more precise than based on multislice-CT data. All tracking systems included in this study proved to be considerably less precise under realistic OR conditions when compared to the technical specifications in the manuals of the systems. Changes in the angle of inclination of the stylus axis resulted in deviations of up to 3.40 mm (mean deviations for all systems ranging from 0.49 to 1.42 mm, variances ranging from 0.09 to 1.44 mm), indicating a strong need for improvements of stylus design. The electromagnetic tracking system investigated in this study was not significantly affected by small ferromagnetic surgical instruments.

Principles of computer-assisted arthroscopy of the temporomandibular joint with optoelectronic tracking technology.

PURPOSE: This preliminary clinical study evaluated the applicability, accuracy, and benefits of computer-assisted arthroscopy of the temporomandibular joint (TMJ) with optoelectronic tracking technology. MATERIALS AND METHODS: A hybrid of reality and virtual reality is built as a composite-reality environment by extracting 3-dimensional anatomical structures through use of computed tomography, magnetic resonance imaging, radiography, and other types of imaging procedures commonly used in clinical praxis. These various independent sources of imaging data of a particular patient can be combined with and complemented by complex graphic simulations. Intraoperatively they are merged with online position data of surgical instruments inside the patient's TMJ. This hybrid model of detailed anatomical structures, guidelines, and real-time instrument positions allows the surgeon to accurately plan the arthroscopic intervention as well as to navigate effectively intraoperatively. RESULTS: In the first 10 cases of computer-assisted TMJ arthroscopy, composite reality environment technology permitted the online visualization of TMJ structures, puncture sites, instrument positions, and virtual pathways in relation to anatomical landmarks with high spatial accuracy (minimum, 0.0 mm; maximum, 2.5 mm; mean, 1.4 mm; SD, 0.6 mm) and high temporal resolution (100 ms). Past, present, and possible future instrument positions can be displayed. The application of computer-assisted arthroscopy caused little immobility for either surgeon or patient. CONCLUSION: Even experienced surgeons profit from improved precision in the handling of the arthroscope; thus this technology was found to be particularly useful in degenerative temporomandibular disorders and for triangulation procedures.

Placement of endosteal implants in the zygoma after maxillectomy: a Cadaver study using surgical navigation.

Endosteal implants facilitate obturator prosthesis fixation in tumor patients after maxillectomy. Previous clinical studies have shown, however, that the survival of implants placed into available bone after maxillectomy is generally poor. Nevertheless, implants positioned optimally in residual zygomatic bone provide superior stability from a biomechanical point of view. In a pilot study, the authors assessed the precision of VISIT, a computer-aided surgical navigation system dedicated to the placement of endosteal implants in the maxillofacial area. Five cadaver specimens underwent hemimaxillectomy. The cadaver head was matched to a preoperative high-resolution computed tomograph by using implanted surgical microscrews as fiducial markers. The position of a surgical drill relative to the cadaver head was determined with an optical tracking system. Implants were placed into the zygomatic arch, where maximum bone volume was available. The results were assessed using tests for localization accuracy and postoperative computed tomographic scans of the cadaver specimens. The localization accuracy of landmarks on the bony skull was 0.6 +/- 0.3 mm (average +/- SD), as determined with a 5-df pointer probe; the localization accuracy of the tip of the implant burr was 1.7 +/- 0.4 mm. The accuracy of the implant position compared with the planned position was 1.3 +/- 0.8 mm for the external perforation of the zygoma and 1.7 +/-1.3 mm for the internal perforation. Eight of 10 implants were inserted with maximal contact to surrounding bone, and two implants were located unfavorably. Reliable placement of implants in this region is difficult to achieve. The technique described in this article may be very helpful in the management of patients after maxillary resection with poor support for obturator prostheses.