Eigil Samset

Dynamic MRI scan plane control for passive tracking of instruments and devices

This paper describes a novel image-based method for tracking robotic mechanisms and interventional devices during Magnetic Resonance Image (MRI)-guided procedures. It takes advantage of the multi-planar imaging capabilities of MRI to optimally image a set of localizing fiducials for passive motion tracking in the image coordinate frame. The imaging system is servoed to adaptively position the scan plane based on automatic detection and localization of fiducial artifacts directly from the acquired image stream. This closed-loop control system has been implemented using an open-source software framework and currently operates with GE MRI scanners. Accuracy and performance were evaluated in experiments, the results of which are presented here.

Summarizing and visualizing uncertainty in non-rigid registration

Registration uncertainty may be important information to convey to a surgeon when surgical decisions are taken based on registered image data. However, conventional non-rigid registration methods only provide the most likely deformation. In this paper we show how to determine the registration uncertainty, as well as the most likely deformation, by using an elastic Bayesian registration framework that generates a dense posterior distribution on deformations. We model both the likelihood and the elastic prior on deformations with Boltzmann distributions and characterize the posterior with a Markov Chain Monte Carlo algorithm. We introduce methods that summarize the high-dimensional uncertainty information and show how these summaries can be visualized in a meaningful way. Based on a clinical neurosurgical dataset, we demonstrate the importance that uncertainty information could have on neurosurgical decision making.

Real time MRI-guided excision and cryo-treatment of osteoid osteoma in os ischii--a case report.

A 36-year-old electrician had not been able towork during the last 8 months due to intense painin his left buttock. Plain radiographs were normal,but scintigraphy, MRI and CT indicated an osteoidosteoma in the left tuber os ischii. The patient wasgiven spinal anesthesia, and placed in a 0.5T GESigna SP/i open MRI (General Electric, Milwau-kee, USA) (Figure 1). With the patient in decubi-tus position, a surface coil was attached and thenidus located. A biopsy needle was placed in thecenter of the lesion, using an optical tracking sys-tem integrated in the magnet gantry, for placementof the needle. A 4 mm cylinder was cored out andfixed in formaldehyde for histologic examinationwhich confirmed the diagnosis osteoid osteoma.Then a 3 mm cryo probe (Galil Medical, Haifa,Israel) was placed centrally in the lesion (Figure2). 4 cycles (1/2–2 min) with cooling to –180€°Cinterrupted by passive thawing were used. ControlMRI showed the defect after the biopsy. The pa-tient was pain-free one day after the operation,soon resumed full-time work, and had no symp-toms 1 year later.

Towards scarless surgery: an endoscopic ultrasound navigation system for transgastric access procedures.

Objective: Scarless surgery is an innovative and promising technique that may herald a new era in surgical procedures. We have created a navigation system, named IRGUS, for endoscopic and transgastric access interventions and have validated it in in vivo pilot studies. Our hypothesis is that endoscopic ultrasound procedures will be performed more easily and efficiently if the operator is provided with approximately registered 3D and 2D processed CT images in real time that correspond to the probe position and ultrasound image. Materials and Methods: The system provides augmented visual feedback and additional contextual information to assist the operator. It establishes correspondence between the real-time endoscopic ultrasound image and a preoperative CT volume registered using electromagnetic tracking of the endoscopic ultrasound probe position. Based on this positional information, the CT volume is reformatted in approximately the same coordinate frame as the ultrasound image and displayed to the operator. Results: The system reduces the mental burden of probe navigation and enhances the operators ability to interpret the ultrasound image. Using an initial rigid body registration, we measured the mis-registration error between the ultrasound image and the reformatted CT plane to be less than 5 mm, which is sufficient to enable the performance of novice users of endoscopic systems to approach that of expert users. Conclusions: Our analysis shows that real-time display of data using rigid registration is sufficiently accurate to assist surgeons in performing endoscopic abdominal procedures. By using preoperative data to provide context and support for image interpretation and real-time imaging for targeting, it appears probable that both preoperative and intraoperative data may be used to improve operator performance.

Magnetic‐resonance‐guided percutaneous cryoablation of hepatic tumours

OBJECTIVE To study the feasibility of percutaneous cryoablation of hepatic tumours monitored by magnetic resonance imaging (MRI). DESIGN Prospective study SETTING University hospital, Norway PATIENTS Six patients with hepatic metastases from colorectal cancer. INTERVENTIONS Percutaneous cryoprobe positioning under general anaesthesia. Positioning and freezing monitored by near-real-time MRI using an open 0.5 Tesla MRI configuration system. MAIN OUTCOME MEASURES Safety and feasibility of the procedure. Measurement of volumes of cryolesions. RESULTS One patient developed a biliary leakage that had to be drained. Four patients developed pleural fluid. Two small tumours were adequately cryoablated. In the remaining 4 patients with large (>4 cm) tumours, an adequate cryolesion could not be formed. Cryolesion volumes larger than 105 cm3 were not produced even using 3-4 probes. MRI visualised the growing cryolesion well, but positioning of the cryoprobes was time-consuming. CONCLUSION MR guided cryoablation is clinically feasible and gives good visualisation of the procedure. Patients with small tumours (<3 cm) seem to be best suited to this percutaneous approach as cryolesion volumes claimed to be adequate for tumour destruction can be produced. Measurement of tumour volume preoperatively may help to select patients who will respond.

Neuronavigation in intraoperative MRI

OBJECTIVE We describe the development and implementation of an image-guided surgical system combining the best features of conventional frameless stereotactic systems and the recently developed superconductive vertically configured intraoperative magnetic resonance scanner. The incorporation of intraoperatively updated magnetic resonance imaging (MRI) data sets into the neuronavigation computer overcomes one of the main disadvantages of these systems, i.e., intraoperative brain shift. METHODS The integrated system consists of a 0.5-T MRI scanner (Signa SP General Electric Medical Systems, Milwaukee, WI), a neuronavigation computer with associated software (OTS Radionics, Burlington, MA), and an emulation program linking the two. The scanner has a 60-cm-wide vertical gap where both imaging and surgery are conducted, in-bore infrared linear cameras and monitors for interactive surgical neuronavigation, and flexible surface coils specially designed for surgery. RESULTS Phantom studies showed navigational accuracy to be better than that obtained using conventional preoperative images and surface markers for patient registration. Our initial 17 cases using this integrated system comprised 16 craniotomies and one biopsy, and demonstrated decreased operative duration, greater frequency of interactive image guidance utilization, and better assessment of the progress of surgery compared to the cases previously done in the intraoperative MRI. CONCLUSION This initial study of the addition of frameless stereotactic systems to the basic intraoperative MRI concept has demonstrated its clinical usefulness. The use of the intraoperative MRI greatly reduces the basic weakness of neuronavigation inaccuracy due to target shift. The surgical procedure performed in the imaging volume of the MRI scanner eliminates the problems of patient or scanner transport during the procedure. Immobilization of the patient throughout the procedure eliminated the need for reregistration of the patient, by taking advantage of the fixed camera system in the bore of the MRI system.

A Non-rigid Registration Framework That Accommodates Resection and Retraction

Traditional non-rigid registration algorithms are incapable of accurately registering intra-operative with pre-operative images whenever tissue has been resected or retracted. In this work we present methods for detecting and handling retraction and resection. The registration framework is based on the bijective Demons algorithm using an anisotropic diffusion smoother. Retraction is detected at areas of the deformation field with high internal strain and the estimated retraction boundary is integrated as a diffusion boundary in the smoother to allow discontinuities to develop across the resection boundary. Resection is detected by a level set method evolving in the space where image intensities disagree. The estimated resection is integrated into the smoother as a diffusion sink to restrict image forces originating inside the resection from being diffused to surrounding areas. In addition, the deformation field is continuous across the diffusion sink boundary which allow us to move the boundary of the diffusion sink without changing values in the deformation field (no interpolation or extrapolation is needed). We present preliminary results on both synthetic and clinical data which clearly shows the added value of explicitly modeling these processes in a registration framework.

Early recognition of regional cardiac ischemia using a 3-axis accelerometer sensor.

Perioperative mortality in coronary artery bypass grafting is usually caused by reduced left ventricular function due to regional myocardial ischemia or infarction. Post-operative graft occlusion is a well-known problem in coronary surgery. A sensitive tool to detect graft occlusion and monitor myocardial function may give the opportunity to revise malfunctioning grafts before departure from the hospital. This paper describes how a new method can detect cardiac ischemia using a 3-axis piezoelectric accelerometer. In three anesthetized pigs, a 3-axis piezoelectric accelerometer was sutured on the lateral free wall of the left ventricle. The left anterior descending (LAD) was occluded for different time periods and the accelerometer data were sampled with a PC. Short-time Fourier transform was calculated based on the accelerometer time series. The results were visualized using a 2D color-coded time-frequency plot. In the area of occlusion, a change to stronger power of higher harmonics was observed. Consequently, a difference value between the instant frequency pattern and a reference frequency pattern showed a rise in absolute value during the occlusion period. The preliminary results indicate that early recognition of regional cardiac ischemia is possible by analyzing accelerometer data acquired from the three animal trials using the prototype 3-axis accelerometer sensor.

Augmented Reality for Minimally Invasive Surgery: Overview and Some Recent Advances

Pablo Lamata1,2, Wajid Ali3, Alicia Cano1, Jordi Cornella3, Jerome Declerck2, Ole J. Elle3, Adinda Freudenthal4, Hugo Furtado5, Denis Kalkofen6, Edvard Naerum3, Eigil Samset3, Patricia Sanchez-Gonzalez1, Francisco M. Sanchez-Margallo7, Dieter Schmalstieg6, Mauro Sette8, Thomas Studeli4, Jos Vander Sloten8 and Enrique J. Gomez1 1Universidad Politecnica de Madrid, Spain 2Siemens, United Kingdom 3University of Oslo, Norway 4Delft University of Technology, Netherlands 5Medical Centre Ljubljana, Slovenia 6 Graz University of Technology, Austria 7Minimally Invasive Surgery Centre Jesus Uson, Spain 8University of Leuven, Belgium

Cognitive processes as integrative component for developing expert decision-making systems: A workflow centered framework

The development of expert decision-making systems, which improve task performance and reduce errors within an intra-operative clinical workspace, is critically dependent on two main aspects: (a) Analyzing the clinical requirements and cognitive processes within the workflow and (b) providing an optimal context for accurate situation awareness through effective intra-operative information visualization. This paper presents a workflow centered framework and its theoretical underpinnings to design expert decision-making systems. The framework integrates knowledge of the clinical workflow based on the requirements within the clinical workspace. Furthermore, it builds upon and integrates the theory of situation awareness into system design to improve decision-making. As an application example, this framework has been used to design an intra-operative visualization system (IVS), which provides image guidance to the clinicians to perform minimally invasive procedure. An evaluative study, comparing the traditional ultrasound guided procedure with the new developed IVS, has been conducted with expert intervention radiologists and medical students. The results reveal significant evidence for improved decision-making when using the IVS. Therefore, it can be stated that this study demonstrates the benefits of integrating knowledge of cognitive processes into system development to support clinical decision-making and hence improvement of task performance and prevention of errors.