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Biomedical subjects

Ramin Shahidi

Publications and source records attributed to Ramin Shahidi.

6 recordsLinked to original sources
PubMed2006

Virtual reality augmentation in skull base surgery.

OBJECTIVE: Skull base anatomy is complex and subject to individual variation. Understanding the complexity of surgical anatomy is faster and easier with virtual models created from primary imaging data of the patient. This study was designed to investigate the usefulness of virtual reality in image guidance for skull base procedures. DESIGN: Primary volumetric image data from 110 patients was acquired using magnetic resonance, computed tomography (CT), and CT angiography. Pathologies included lesions in the anterior, middle, and posterior skull base. The data were transferred to an infrared-based image-guidance system for creation of a virtual operating field (VOF) with translucent surface modulation and optional "fly-through" video mode. During surgery, the target registration error for anatomical landmarks was assessed and the VOF was compared with the patient's anatomy in the operative field. RESULTS: Complex structures like the course of the sigmoid sinus, the carotid artery, and the outline of the paranasal sinuses were well visualized in the VOF and were recognized by the surgeon instantly. Perception was greatly facilitated as compared with routine mental reconstruction of triaxial images. Accurate assessment of the depth of field and very small objects was not possible in VOF images. CONCLUSION: Supported by sound anatomical knowledge, creation of a virtual operating field for a surgical approach in an individual patient offers a déjà vu experience that can enhance the capabilities of a surgical team in skull base approaches. In addition, application of this technique in image-guided procedures assists in targeting or avoiding hidden anatomical structures.

Journal Article↗
PubMed2005

Design and application of an assessment protocol for electromagnetic tracking systems.

This paper defines a simple protocol for competitive and quantified evaluation of electromagnetic tracking systems such as the NDI Aurora (A) and Ascension microBIRD with dipole transmitter (B). It establishes new methods and a new phantom design which assesses the reproducibility and allows comparability with different tracking systems in a consistent environment. A machined base plate was designed and manufactured in which a 50 mm grid of holes was precisely drilled for position measurements. In the center a circle of 32 equispaced holes enables the accurate measurement of rotation. The sensors can be clamped in a small mount which fits into pairs of grid holes on the base plate. Relative positional/orientational errors are found by subtracting the known distances/ rotations between the machined locations from the differences of the mean observed positions/ rotation. To measure the influence of metallic objects we inserted rods made of steel (SST 303, SST 416), aluminum, and bronze into the sensitive volume between sensor and emitter. We calculated the fiducial registration error and fiducial location error with a standard stylus calibration for both tracking systems and assessed two different methods of stylus calibration. The positional jitter amounted to 0.14 mm(A) and 0.08 mm(B). A relative positional error of 0.96 mm +/- 0.68 mm, range -0.06 mm; 2.23 mm(A) and 1.14 mm +/- 0.78 mm, range -3.72 mm; 1.57 mm(B) for a given distance of 50 mm was found. The relative rotation error was found to be 0.51 degrees (A)/0.04 degrees (B). The most relevant distortion caused by metallic objects results from SST 416. The maximum error 4.2 mm(A)/ > or = 100 mm(B) occurs when the rod is close to the sensor(20 mm). While (B) is more sensitive with respect to metallic objects, (A) is less accurate concerning orientation measurements. (B) showed a systematic error when distances are calculated.

Biomechanical Phenomena↗
PubMed2005

Volumetric image guidance for motor cortex stimulation: integration of three-dimensional cortical anatomy and functional imaging.

OBJECTIVE: Epidural electrical stimulation of the motor cortex is a promising treatment option in patients with intractable pain. Varying rates of success in long-term pain relief have been attributed to inaccurate positioning of the electrode array, partly because the sulcal landmarks are not directly visualized. We describe an integrated protocol for precise electrode placement, combining functional image guidance and intraoperative electrical stimulation in the awake patient. METHODS: Volumetric rendering of a three-dimensional (3-D) magnetic resonance data set was used to visualize the cortical surface and to superimpose functional magnetic resonance imaging data in six patients with refractory chronic pain. The intraoperative positioning of the quadripolar electrode array was monitored by functional 3-D image guidance. Continuous electrophysiological monitoring and clinical assessment of the motor effects complemented the procedure. RESULTS: Volumetrically rendered 3-D images were advantageous for the location of the burr hole over the perirolandic area by revealing individual cortical morphological features (e.g., the hand knob) and function at the same time. The exact position of the electrodes was verified reliably by cortical stimulation. No complications were observed throughout the procedures. CONCLUSION: The combination of 3-D functional neuronavigation, intraoperative electrical stimulation, and continuous motor output monitoring in awake patients provides optimal information for the identification of the appropriate somatotopic area of motor cortex. This combined imaging and stimulation approach for electrode positioning offers a safe and minimal invasive strategy for the treatment of intractable chronic pain in selected patients.

Adult↗
PubMed2002

Implementation, calibration and accuracy testing of an image-enhanced endoscopy system.

This paper presents a new method for image-guided surgery called image-enhanced endoscopy. Registered real and virtual endoscopic images (perspective volume renderings generated from the same view as the endoscope camera using a preoperative image) are displayed simultaneously; when combined with the ability to vary tissue transparency in the virtual images, this provides surgeons with the ability to see beyond visible surfaces and, thus, provides additional exposure during surgery. A mount with four photoreflective spheres is rigidly attached to the endoscope and its position and orientation is tracked using an optical position sensor. Generation of virtual images that are accurately registered to the real endoscopic images requires calibration of the tracked endoscope. The calibration process determines intrinsic parameters (that represent the projection of three-dimensional points onto the two-dimensional endoscope camera imaging plane) and extrinsic parameters (that represent the transformation from the coordinate system of the tracker mount attached to the endoscope to the coordinate system of the endoscope camera), and determines radial lens distortion. The calibration routine is fast, automatic, accurate and reliable, and is insensitive to rotational orientation of the endoscope. The routine automatically detects, localizes, and identifies dots in a video image snapshot of the calibration target grid and determines the calibration parameters from the sets of known physical coordinates and localized image coordinates of the target grid dots. Using nonlinear lens-distortion correction, which can be performed at real-time rates (30 frames per second), the mean projection error is less than 0.5 mm at distances up to 25 mm from the endoscope tip, and less than 1.0 mm up to 45 mm. Experimental measurements and point-based registration error theory show that the tracking error is about 0.5-0.7 mm at the tip of the endoscope and less than 0.9 mm for all points in the field of view of the endoscope camera at a distance of up to 65 mm from the tip. It is probable that much of the projection error is due to endoscope tracking error rather than calibration error. Two examples of clinical applications are presented to illustrate the usefulness of image-enhanced endoscopy. This method is a useful addition to conventional image-guidance systems, which generally show only the position of the tip (and sometimes the orientation) of a surgical instrument or probe on reformatted image slices.

Adolescent↗
PubMed2002

Calibration and accuracy testing for image-enhanced endoscopy.

New surgical navigation techniques may combine the use of live video from a surgical endoscope with 3D volumetrically-reconstructed images of a patient's anatomy. This image-enhanced endoscopy requires calibration of the endoscope to ensure that the mapping of the real endoscope image to its virtual counterpart is properly performed. The application of a technique to calibrate an endoscope prior to use in a diagnostic or therapeutic procedure is described, as well as a simple yet effective linear method for lens-distortion compensation. The results of accuracy testing of the calibration technique using a dedicated testing apparatus are reported.

Calibration↗