Rainer Graumann

3D Reconstruction from Projection Matrices in a C-Arm Based 3D-Angiography System

3D reconstruction of arterial vessels from planar radiographs obtained at several angles around the object has gained increasing interest. The motivating application has been interventional angiography. In order to obtain a three-dimensional reconstruction from a C-arm mounted X-Ray Image Intensifier (XRII) traditionally the trajectory of the source and the detector system is characterized and the pixel size is estimated. The main use of the imaging geometry characterization is to provide a correct 3D-2D mapping between the 3D voxels to be reconstructed and the 2D pixels on the radiographic images.

Automatic localization of vertebral levels in x-ray fluoroscopy using 3D-2D registration: a tool to reduce wrong-site surgery.

Surgical targeting of the incorrect vertebral level (wrong-level surgery) is among the more common wrong-site surgical errors, attributed primarily to the lack of uniquely identifiable radiographic landmarks in the mid-thoracic spine. The conventional localization method involves manual counting of vertebral bodies under fluoroscopy, is prone to human error and carries additional time and dose. We propose an image registration and visualization system (referred to as LevelCheck), for decision support in spine surgery by automatically labeling vertebral levels in fluoroscopy using a GPU-accelerated, intensity-based 3D-2D (namely CT-to-fluoroscopy) registration. A gradient information (GI) similarity metric and a CMA-ES optimizer were chosen due to their robustness and inherent suitability for parallelization. Simulation studies involved ten patient CT datasets from which 50 000 simulated fluoroscopic images were generated from C-arm poses selected to approximate the C-arm operator and positioning variability. Physical experiments used an anthropomorphic chest phantom imaged under real fluoroscopy. The registration accuracy was evaluated as the mean projection distance (mPD) between the estimated and true center of vertebral levels. Trials were defined as successful if the estimated position was within the projection of the vertebral body (namely mPD <5 mm). Simulation studies showed a success rate of 99.998% (1 failure in 50 000 trials) and computation time of 4.7 s on a midrange GPU. Analysis of failure modes identified cases of false local optima in the search space arising from longitudinal periodicity in vertebral structures. Physical experiments demonstrated the robustness of the algorithm against quantum noise and x-ray scatter. The ability to automatically localize target anatomy in fluoroscopy in near-real-time could be valuable in reducing the occurrence of wrong-site surgery while helping to reduce radiation exposure. The method is applicable beyond the specific case of vertebral labeling, since any structure defined in pre-operative (or intra-operative) CT or cone-beam CT can be automatically registered to the fluoroscopic scene.

3D soft tissue imaging with a mobile C-arm

We introduce a clinical prototype for 3D soft tissue imaging to support surgical or interventional procedures based on a mobile C-arm. An overview of required methods and materials is followed by first clinical images of animals and human patients including dosimetry. The mobility and flexibility of 3D C-arms gives free access to the patient and therefore avoids relocation of the patient between imaging and surgical intervention. Image fusion with diagnostic data (MRI, CT, PET) is demonstrated and promising applications for brachytherapy, RFTT and others are discussed.

Method for the operation of a nuclear magnetic resonance apparatus for the fast identification of the longitudinal relaxation time T1

A method for the operation of a nuclear magnetic resonance apparatus excites the spin system of an examination subject by applying a fundamental magnetic field as well as by irradiation with a sequence of radio-frequency pulses and the nuclear magnetic resonance signals emitted by the spin system are measured. The chronological development of the longitudinal magnetization of the spin system can be tracked by a sequence of partial read pulses each having a flip angle of less than 90° and thus a fast identification of the longitudinal relaxation time T1 can be achieved. In combination with applied gradient fields, the method can be applied in imaging nuclear magnetic resonance tomography.

Long Bone X-Ray Image Stitching Using Camera Augmented Mobile C-Arm

X-ray images are widely used during surgery for long bone fracture fixation. Mobile C-arms provide X-ray images which are used to determine the quality of trauma reduction, i.e. the extremity length and mechanical axis of long bones. Standard X-ray images have a narrow field of view and can not visualize the entire long bone on a single image. In this paper, we propose a novel method to generate panoramic X-ray images in real time by using the previously introduced Camera Augmented Mobile C-arm. This advanced mobile C-arm system acquires registered X-ray and optical images by construction, which facilitates the generation of panoramic X-ray images based on first stitching the optical images and then embedding the X-ray images. We additionally introduce a method to reduce the parallax effect that leads to the blurring and measurement error on panoramic X-ray images. Visual marker tracking is employed to automatically stitch the sequence of video images and to rectify images. Our proposed method is suitable for intra-operative usage generating panoramic X-ray images, which enable metric measurements, with less radiation and without requirement of fronto-parallel setup and overlapping X-ray images. The results show that the panoramic X-ray images generated by our method are accurate enough (errors less than 1%) for metric measurements and suitable for many clinical applications in trauma reduction.

Intraoperative soft tissue 3D reconstruction with a mobile C-arm

Abstract Based on a standard mobile C-arm, we perform soft tissue 3D reconstruction suitable for interventional clinical use. A flat panel X-ray detector delivers a sequence of high dynamic images while orbitally scanning the patient. High X-ray power allows fast scanning and a sufficient number of images to optimize for low-contrast reconstruction. First results are presented.

High-performance C-arm cone-beam CT guidance of thoracic surgery

Localizing sub-palpable nodules in minimally invasive video-assisted thoracic surgery (VATS) presents a significant challenge. To overcome inherent problems of preoperative nodule tagging using CT fluoroscopic guidance, an intraoperative C-arm cone-beam CT (CBCT) image-guidance system has been developed for direct localization of subpalpable tumors in the OR, including real-time tracking of surgical tools (including thoracoscope), and video-CBCT registration for augmentation of the thoracoscopic scene. Acquisition protocols for nodule visibility in the inflated and deflated lung were delineated in phantom and animal/cadaver studies. Motion compensated reconstruction was implemented to account for motion induced by the ventilated contralateral lung. Experience in CBCT-guided targeting of simulated lung nodules included phantoms, porcine models, and cadavers. Phantom studies defined low-dose acquisition protocols providing contrast-to-noise ratio sufficient for lung nodule visualization, confirmed in porcine specimens with simulated nodules (3-6mm diameter PE spheres, ~100-150HU contrast, 2.1mGy). Nodule visibility in CBCT of the collapsed lung, with reduced contrast according to air volume retention, was more challenging, but initial studies confirmed visibility using scan protocols at slightly increased dose (~4.6-11.1mGy). Motion compensated reconstruction employing a 4D deformation map in the backprojection process reduced artifacts associated with motion blur. Augmentation of thoracoscopic video with renderings of the target and critical structures (e.g., pulmonary artery) showed geometric accuracy consistent with camera calibration and the tracking system (2.4mm registration error). Initial results suggest a potentially valuable role for CBCT guidance in VATS, improving precision in minimally invasive, lungconserving surgeries, avoid critical structures, obviate the burdens of preoperative localization, and improve patient safety.

A Multi-view Opto-Xray Imaging System

The success of minimally invasive trauma and orthopedic surgery procedures has resulted in an increase of the use of fluoroscopic imaging. A system aiming to reduce the amount of radiation has been introduced by Navab et al. [1]. It uses an optical imaging system rigidly attached to the gantry such that the optical and X-ray imaging geometry is identical. As an extension to their solution, we developed a multi-view system which offers 3D navigation during trauma surgery and orthopedic procedures. We use an additional video camera in an orthogonal arrangement to the first video camera and a minimum of two X-ray images. Furthermore, tools such as a surgical drill are extended by optical markers and tracked with the same optical cameras. Exploiting that the cross ratio is invariant in projective geometry, we can estimate the tip of the instrument in the X-ray image without external tracking systems. This paper thus introduces the first multi-view Opto- Xray system for computer aided surgery. First tests have proven the accuracy of the calibration and the instrument tracking. Phantom and cadaver experiments were conducted for pedicle screw placement in spinal surgery. Using a postoperative CT, we evaluate the quality of the placement of the pedicle screws in 3D.

On the Accuracy of a Video-Based Drill-Guidance Solution for Orthopedic and Trauma Surgery: Preliminary Results

Over the last years, several methods have been proposed to guide the physician during reduction and fixation of bone fractures. Available solutions often use bulky instrumentation inside the operating room (OR). The latter ones usually consist of a stereo camera, placed outside the operative field, and optical markers directly attached to both the patient and the surgical instrumentation, held by the surgeon. Recently proposed techniques try to reduce the required additional instrumentation as well as the radiation exposure to both patient and physician. In this paper, we present the adaptation and the first implementation of our recently proposed video camera-based solution for screw fixation guidance. Based on the simulations conducted in our previous work, we mounted a small camera on a drill in order to recover its tip position and axis orientation w.r.t our custom-made drill sleeve with attached markers. Since drill-position accuracy is critical, we thoroughly evaluated the accuracy of our implementation. We used an optical tracking system for ground truth data collection. For this purpose, we built a custom plate reference system and attached reflective markers to both the instrument and the plate. Free drilling was then performed 19 times. The position of the drill axis was continuously recovered using both our video camera solution and the tracking system for comparison. The recorded data covered targeting, perforation of the surface bone by the drill bit and bone drilling. The orientation of the instrument axis and the position of the instrument tip were recovered with an accuracy of 1:60 +/- 1:22° and 2:03 +/- 1:36 mm respectively.

A video guided solution for screw insertion in orthopedic plate fixation

In orthopedic and trauma surgery, metallic plates are used for reduction and fixation of bone fractures. In clinical practice, the intra-operative planning for screw fixation is usually based on fluoroscopic images. Screw fixation is then performed on a free-hand basis. As such, multiple attempts may be required in order to achieve an optimal positioning of the fixing screws. To help the physician insert the screws in accordance to the planned position, we propose a method for screw insertion guidance. Our approach uses a small video camera, rigidly placed on the drill, and a set of small markers that are rigidly fixed on a variable angle drill sleeve. In order to investigate the achievable accuracy of our setup, we simulate the estimation of the drill bit position under two different marker arrangements, planar and 3D, and different noise levels. Furthermore, we motivate our choices for marker design and position given the limited space available for marker positioning, the requirement for accurate position estimation of the drill bit and the illumination changes that could affect the surgical site. We also describe our proposed marker detection and tracking pipeline. Our simulation results let us conclude that we can achieve an accuracy of 1° and 1mm in the estimation of angular orientation and tip position of the drill bit respectively, provided that we have accurate marker detection.