In vivo pedicle screw placement: image-guided virtual vision.

Near-real-time frameless stereotaxy registering intraoperative anatomy to a preoperative three-dimensional computer model has been developed for use with in vivo pedicle screw placement. Eight patients underwent thoracolumbar and lumbar spine stabilization surgery using this new technology, and 32 pedicle screws were placed. Three additional patients had 12 pedicle screws removed during revision surgery, and they allowed the authors to estimate the accuracy of this navigational system. Accuracy was determined by comparing pedicle screw position on postoperative computed tomographs for the first eight patients and on preoperative computed tomographs for the latter three patients, with the intraoperative computer trajectory data gathered during operation. In the group of eight patients, all screws were intrapedicular. In evaluating all 11 patients, the overall accuracy was +/- 2 mm, but the greatest error of 5.4 mm was noted in the sagittal plane measurement. During the development phase of this technology, time constraints prolong surgery, but this may be addressed once the tool's accuracy has been confirmed and intraoperative radiographic confirmation becomes unnecessary. In vivo real-time frameless stereotaxy for pedicle screw placement offers promise for the future. Refinements are needed to improve accuracy and address time constraints.

Virtual laryngoscopy.

Virtual endoscopy enables computer-generated 3-dimensional visualization of a cavity by reconstructing 2-dimensional computed tomographic or magnetic resonance data. The technique has been used experimentally to study the colon, bronchi, ears, and other structures. Here, virtual laryngoscopies were created from the cross-sectional image data of 3 patients. The cases represented a normal airway, a squamous cell carcinoma of the glottic fold, and a posterior glottic stenosis. These reconstructions included extraluminal anatomy that is not typical of current virtual endoscopic techniques. The 2-dimensional computed tomographic and magnetic resonance images of the patients underwent post-processing for 3-dimensional reconstruction. The resulting models were imported into an experimental virtual endoscopy program for 1) airway lumen generation and 2) interactive viewing. Though they could not be used for biopsy, the virtual laryngoscopies provided, in a noninvasive fashion, good simulation of endoscopy. Virtual endoscopy also gave the added benefits of the ability to assess the transmural extent of disease and view the airway distal to areas of luminal compromise. This technology may well provide clinical benefit in preoperative planning, staging, and intraprocedural guidance for head and neck disease and merits further study.

Computer-assisted three-dimensional reconstruction of head and neck tumors.

OBJECTIVE: Because head and neck tumors reside in a complex area, having a three-dimensional (3-D) model of the patient's unique anatomical features may assist in the delineation of pathology. The authors describe a new computer technique of 3-D anatomical reconstruction from two-dimensional computed tomography (CT) and magnetic resonance (MR) data and discuss how it represents a step forward in the continuing evolution of 3-D imaging. STUDY DESIGN: The authors selected three patients with solitary head and neck tumors and reconstructed their anatomy in a 3-D format for study. The tumors represented locations in the nose and central skull base (patient 1), temporal bone (patient 2), and neck (patient 3). MATERIALS AND METHODS: MR and CT images from the individual patients were electronically transferred to workstations in the Surgical Planning Laboratory of the authors' institution. Registration (or fusion) was carried out between the MR and CT images. The desired anatomic components underwent segmentation (identification and isolation). Assembly of the segmented images was performed and the resulting structures were integrated to produce a 3-D model. RESULTS: 3-D models of the following were constructed and displayed in an interactive format on high-capacity computer workstations: 1) a skull base sarcoma with extension into the nasopharynx and nose; 2) an acoustic neuroma with internal auditory canal involvement; and 3) a metastatic recurrence of a tongue base squamous cell carcinoma in the posterior triangle of the right side of the neck with extension to the skull base. CONCLUSION: The authors' Surgical Planning Laboratory has developed a 3-D reconstruction technique that has several new features. The models provided a very good 3-D interactive representation of the tumors and patient anatomy. The need now exists to develop this method of 3-D reconstruction of head and neck tumors for potential applications in treatment, research, and medical education.

[Reliability and validity studies with the triflexometer, a new method for assessing form and flexibility of the spine]. / Reliabilitäts- und Validitätsstudien mit dem Triflexometer, einer neuen Methode zur Bestimmung von Form und Beweglichkeit der Wirbelsäule.

Developed by Orthotronic Medizintechnik GmbH, the so-called Triflexometer, version 3.22 was tested for its reliability and validity in measuring spinal posture and mobility. Reliability studies on 20 healthy subjects have shown this measurement method to be reliable, yet intra- and inter-rater reliability analyses also revealed that even for this healthy population discrepancies in the various measures may occur, both due to differences in compliance as well as fatigue and learning effects, and due to difficulties in stabilization of the normal posture, to a lesser extent due to certain specifics of the measurement technique (placing the markers, guiding the sensor). In total spinal immobility (ankylosing spondylitis), practically identical measurements are found, as is the case in dummy studies. The validity study on 20 healthy subjects found good correlations between the measurements obtaining using the triflexometer and those for double inclinometer, respectively, and that only minor mean value differences occur for the two methods. Also, triflexometer measurements for total anteflexion were found to correlate with those determined with the fingertip-to-floor method, no correlation was present however between the Triflexometer values and the Schober test. Triflexometer measurements performed on 114 healthy subjects of various ages served to prove that the range of spinal movement in the directions measured (sagittal and frontal) will reduce with age. To a lesser extent, this also applies to hip movement. Overall, our findings prove the triflexometer an easy-to-handle system which possesses high reliability and is suitable for valid and objective noninvasive assessment of global and segmental spinal mobility. Triflexometer examinations are highly uncomplicated to implement, and print-outs of the results obtained permit lasting documentation of the present status.

Virtual pancreatoscopy of mucin-producing pancreatic tumors.

We used computer-based virtual endoscopy techniques as a novel approach to clarify the three-dimensional (3D) surgical anatomy of the pancreas and of mucin-producing pancreatic tumors. Thirteen cases (18 lesions) of mucin-producing pancreatic tumors were investigated by virtual pancreatoscopy. Virtual endoscopic images were generated with virtual endoscopy software application on UNIX workstations. We created surface-rendered virtual endoscopic images derived from a computer reconstruction of the cross-sectional magnetic resonance imaging data. Virtual endoscopy could visualize the surfaces of the pancreatic duct and the bile duct, and also demonstrated all cystic tumors. The surfaces of malignant mucin-producing pancreatic tumors were illustrated as being more irregular than those of benign lesions. The virtual endoscopic technique could demonstrate not only a surface-rendered endoscopic image of the tumors but also a 3D reconstructed image of the pancreas. The relationship to anatomic structures located outside the surfaces is continuously maintained and displayed at the same time. Virtual pancreatoscopy was useful for surgical planning of minimally invasive resection of the pancreas.

In vitro simulation. Early results of stereotaxy for pedicle screw placement.

STUDY DESIGN: Frameless stereotaxy with doppler ultrasound and three dimensional computer model registration is assessed in vitro for pedicle screw placement. OBJECTIVE: To identify feasibility of pedicle screw navigation and placement using this technology. SUMMARY OF BACKGROUND DATA: Inaccurate pedicle screw placement can lead to neurovascular injury or suboptimal fixation. Present techniques in pedicle screw placement involve only confirmation of hole orientation. METHOD: Forty-four pedicle screws were placed in lumbosacral models and cadaver specimens. Accuracy was assessed with a computed tomography scan and vertebral cross sectioning. RESULTS: All screws were intrapedicular. Accuracy of anterior cortical fixation was 1.5 mm, with a range of 2.5 mm. CONCLUSION: In vitro frameless stereotaxy is accurate for pedicle screw placement. This technology adds a component of navigation to pedicle screw placement.

Computer-assisted interactive three-dimensional planning for neurosurgical procedures.

We have used three-dimensional reconstruction magnetic resonance imaging techniques to understand the anatomic complexity of operative brain lesions and to improve preoperative surgical planning. We report our experience with 14 cases, including intra- and extra-axial tumors and a vascular malformation. In each case, preoperative planning was performed using magnetic resonance imaging-based three-dimensional renderings of surgically critical structures, such as eloquent cortices, gray matter nuclei, white matter tracts, and blood vessels. Simulations, using the interactive manipulation of three-dimensional data, provided an efficient and comprehensive way to appreciate the anatomic relationships. Interactive three-dimensional computer-assisted preoperative simulations provided otherwise inaccessible information that was useful for the surgical removal of brain lesions.

Superconducting open-configuration MR imaging system for image-guided therapy.

PURPOSE: To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. MATERIALS AND METHODS: A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, and nonmagnetic displays and with position-monitoring probes and device-tracking instrumentation. RESULTS: The magnet homogeneity was 12.3 ppm, and the gradient field was linear to within 1% over an imaging region 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences, were demonstrated at a rate of one image every 1.5 seconds. CONCLUSION: MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

Computer-assisted three-dimensional planning in craniofacial surgery.

Three-dimensional surface reconstruction from computed tomographic (CT) data has been used to plan craniofacial operations. Cephalometric and anthropometric databases were integrated with three-dimensional CT reconstructions to quantitate the skeletal deformity and to assist in the design of the surgical procedure. Interactive techniques were developed to simulate osteotomies and skeletal movements in three dimensions on the computer-generated surface images. The ocular globes were referenced to position the orbital segments; i.e., the osteotomized segments were transposed into normal anatomic relationship with respect to the eyes. The measurements from the computer graphic simulation were used intraoperatively to establish the correct position of the skeletal segments.

3D phase contrast MRI of cerebral blood flow and surface anatomy.

Noninvasive MR acquisition of blood flow and stationary tissue provides three-dimensional display of flow and of the vascular and brain surfaces. The calculated motion of a simulated bolus injection is derived from the measured velocity vector field and is animated to resemble cine angiography. Simulation of a bolus injection into the basilar artery of a healthy volunteer shows the blood flow into the posterior cerebral arteries.

Routine quantitative analysis of brain and cerebrospinal fluid spaces with MR imaging.

A computerized system for processing spin-echo magnetic resonance (MR) imaging data was implemented to estimate whole brain (gray and white matter) and cerebrospinal fluid volumes and to display three-dimensional surface reconstructions of specified tissue classes. The techniques were evaluated by assessing the radiometric variability of MR volume data and by comparing automated and manual procedures for measuring tissue volumes. Results showed (a) the homogeneity of the MR data and (b) that automated techniques were consistently superior to manual techniques. Both techniques, however, were affected by the complexity of the structure, with simpler structures (eg, the intracranial cavity) showing less variability and better spatial correlation of segmentation results between raters. Moreover, the automated techniques were completed for whole brain in a fraction of the time required to complete the equivalent segmentation manually. Additional evaluations included interrater reliability and an evaluation that included longitudinal measurement, in which one subject was imaged sequentially 24 times, with reliability computed from data collected by three raters over 1 year. Results showed good reliability for the automated segmentation procedures.

Volume rendering and connectivity algorithms for MR angiography.

Several display algorithms for three-dimensional angiographic data are evaluated. The mathematical analysis assumes additive Gaussian noise to predict the background distribution function for maximum intensity projection, sum projection, and connectivity display methods. In the maximum intensity projection method the mean noise level increases with the number of voxels in the ray, while in the sum projection the noise distribution width increases with the projection thickness, but the mean level remains constant. Comparisons of maximum intensity projection, sum projection, and connectivity algorithms applied to an MR angiogram of the circle of Willis are made. Measurements of the noise distribution are in agreement with the analysis. Algorithms combining connectivity with maximum intensity and sum projection are also evaluated. In these methods, a projection image is created using only the voxels marked by connectivity, typically with a 6% threshold of the data. Fine vessels are resolved and background noise is reduced in agreement with the analysis.

3D surface rendered MR images of the brain and its vasculature.

Both time-of-flight and phase contrast magnetic resonance angiography images are combined with stationary tissue images to provide data depicting two contrast relationships yielding intrinsic discrimination of brain matter and flowing blood. A computer analysis is based on nearest neighbor segmentation and the connection between anatomical structures to partition the images into different tissue categories: from which, high resolution brain parenchymal and vascular surfaces are constructed and rendered in juxtaposition, aiding in surgical planning.

Three-dimensional segmentation of MR images of the head using probability and connectivity.

We describe a three-dimensional (3D) segmentation method that comprises (a) user interactive identification of tissue classes; (b) calculation of a probability distribution for each tissue; (c) creation of a feature map of the most probable tissues; (d) 3D segmentation of the magnetic resonance (MR) data; (e) smoothing of the segmented data; (f) extraction of surfaces of interest with connectivity; (g) generation of surfaces; and (h) rendering of multiple surfaces to plan surgery. Patients with normal head anatomy and with abnormalities such as multiple sclerosis lesions and brain tumors were scanned with a 1.5 T MR system using a two echo contiguous (interleaved), multislice pulse sequence that provides both proton density and T2-weighted contrast. After the user identified the tissues, the 3D data were automatically segmented into background, facial tissue, brain matter, CSF, and lesions. Surfaces of the face, brain, lateral ventricles, tumors, and multiple sclerosis lesions are displayed using color coding and gradient shading. Color improves the visualization of segmented tissues, while gradient shading enhances the perception of depth. Manipulation of the 3D model on a workstation aids surgical planning. Sulci and gyri stand out, thus aiding functional mapping of the brain surface.

Vascular morphology by three-dimensional magnetic resonance imaging.

A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting.

Two algorithms for the three-dimensional reconstruction of tomograms.

Three-dimensional (3-D) surface reconstructions provide a method to view complex anatomy contained in a set of computed tomography (CT), magnetic resonance imaging (MRI), or single photon emission computed tomography tomograms. Existing methods of 3-D display generate images based on the distance from an imaginary observation point to a patch on the surface and on the surface normal of the patch. We believe that the normalized gradient of the original values in the CT or MRI tomograms provides a better estimate for the surface normal and hence results in higher quality 3-D images. Then two algorithms that generate 3-D surface models are presented. The new methods use polygon and point primitives to interface with computer-aided design equipment. Finally, several 3-D images of both bony and soft tissue show the skull, spine, internal air cavities of the head and abdomen, and the abdominal aorta in detail.

3D reconstruction of the brain from magnetic resonance images using a connectivity algorithm.

We present high resolution three dimensional (3D) connectivity, surface construction and display algorithms that detect, extract, and display the surface of a brain from contiguous magnetic resonance (MR) images. The algorithms identify the external brain surface and create a 3D image, showing the fissures and surface convolutions of the cerebral hemispheres, cerebellum, and brain stem. Images produced by these algorithms also show the morphology of other soft tissue boundaries such as the cerebral ventricular system and the skin of the patient. For the purposes of 3D reconstruction, our experiments show that T1 weighted images give better contrast between the surface of the brain and the cerebral spinal fluid than T2 weighted images. 3D reconstruction of MR data provides a non-invasive procedure for examination of the brain surface and other anatomical features.