Andreas Varnavas

Touchless interaction in surgery

Touchless interaction with medical images lets surgeons maintain sterility during surgical procedures.

Interactional Order and Constructed Ways of Seeing with Touchless Imaging Systems in Surgery

While surgical practices are increasingly reliant on a range of digital imaging technologies, the ability for clinicians to interact and manipulate these digital representations in the operating theatre using traditional touch based interaction devices is constrained by the need to maintain sterility. To overcome these concerns with sterility, a number of researchers are have been developing ways of enabling interaction in the operating theatre using touchless interaction techniques such as gesture and voice to allow clinicians control of the systems. While there have been important technical strides in the area, there has been little in the way of understanding the use of these touchless systems in practice. With this in mind we present a touchless system developed for use during vascular surgery. We deployed the system in the endovascular suite of a large hospital for use in the context of real procedures. We present findings from a study of the system in use focusing on how, with touchless interaction, the visual resources were embedded and made meaningful in the collaborative practices of surgery. In particular we discuss the importance of direct and dynamic control of the images by the clinicians in the context of talk and in the context of other artefact use as well as the work performed by members of the clinical team to make themselves sensable by the system. We discuss the broader implications of these findings for how we think about the design, evaluation and use of these systems.

An image-guided surgery system to aid endovascular treatment of complex aortic aneurysms: description and initial clinical experience

We have designed an image guidance system to aid complex aortic aneurysm procedures. The system is based around an intensitybased 2D-3D rigid registration algorithm which accurately aligns a vertebra in the preoperative CT to interventional fluoroscopy. A 3D surface rendering of the aorta and relevant target vessels is then overlaid onto the fluoroscopy to aid guidance during the procedure. We report results from use of the system during twenty three procedures over a ten month period. Results showed the system to have an overall failure rate of 5.8%, which was mostly caused by errors in determining a starting position (misidentifying a vertebra). In 78% of cases our method was within our target accuracy of 3mm. Non-rigid deformation caused by the interventional instruments is believed to be the main reason for larger errors. In twenty one of the twenty three cases the surgeon rated the system as aiding the procedure.

Increasing the Automation of a 2D-3D Registration System

Routine clinical use of 2D-3D registration algorithms for Image Guided Surgery remains limited. A key aspect for routine clinical use of this technology is its degree of automation, i.e., the amount of necessary knowledgeable interaction between the clinicians and the registration system. Current image-based registration approaches usually require knowledgeable manual interaction during two stages: for initial pose estimation and for verification of produced results. We propose four novel techniques, particularly suited to vertebra-based registration systems, which can significantly automate both of the above stages. Two of these techniques are based upon the intraoperative “insertion” of a virtual fiducial marker into the preoperative data. The remaining two techniques use the final registration similarity value between multiple CT vertebrae and a single fluoroscopy vertebra. The proposed methods were evaluated with data from 31 operations (31 CT scans, 419 fluoroscopy images). Results show these methods can remove the need for manual vertebra identification during initial pose estimation, and were also very effective for result verification, producing a combined true positive rate of 100% and false positive rate equal to zero. This large decrease in required knowledgeable interaction is an important contribution aiming to enable more widespread use of 2D-3D registration technology.

Fully automated 2D–3D registration and verification

Clinical application of 2D-3D registration technology often requires a significant amount of human interaction during initialisation and result verification. This is one of the main barriers to more widespread clinical use of this technology. We propose novel techniques for automated initial pose estimation of the 3D data and verification of the registration result, and show how these techniques can be combined to enable fully automated 2D-3D registration, particularly in the case of a vertebra based system. The initialisation method is based on preoperative computation of 2D templates over a wide range of 3D poses. These templates are used to apply the Generalised Hough Transform to the intraoperative 2D image and the sought 3D pose is selected with the combined use of the generated accumulator arrays and a Gradient Difference Similarity Measure. On the verification side, two algorithms are proposed: one using normalised features based on the similarity value and the other based on the pose agreement between multiple vertebra based registrations. The proposed methods are employed here for CT to fluoroscopy registration and are trained and tested with data from 31 clinical procedures with 417 low dose, i.e. low quality, high noise interventional fluoroscopy images. When similarity value based verification is used, the fully automated system achieves a 95.73% correct registration rate, whereas a no registration result is produced for the remaining 4.27% of cases (i.e. incorrect registration rate is 0%). The system also automatically detects input images outside its operating range.

Non-Rigid 2D-3D Registration Using Anisotropic Error Ellipsoids to Account for Projection Uncertainties during Aortic Surgery

Overlay of preoperative images is increasingly being used to aid complex endovascular aortic repair and is obtained by rigid 2D-3D registration of 3D preoperative (CT) and 2D intraoperative (X-ray) data. However, for tortuous aortas large non-rigid deformations occur, thus a non-rigid registration must be performed to enable an accurate overlay. This article proposes the use of Thin-Plate Splines (TPS) to perform non-rigid 2D-3D registration. Intraoperative X-ray data contain no spatial information along the X-ray projection direction. Our approach accounts for this lack of spatial information by the use of an approximating TPS with non-isotropic error ellipsoids, where the major ellipsoid axis is aligned with the X-ray projection direction. Experiments are carried out using 1D-2D and 2D-3D simulated data and 2D-3D interventional data. Simulated results show that our proposed method is 1.5 times more accurate than interpolating TPS based registration. Interventional data results show how large rigid registration errors of 9mm can be reduced to 4mm using our proposed method.

Interventional digital tomosynthesis from a standard fluoroscopy system using 2D-3D registration.

Fluoroscopy is the mainstay of interventional radiology. However, the images are 2D and visualisation of vasculature requires nephrotoxic contrast. Cone-beam computed tomography is often available, but involves large radiation dose and interruption to clinical workflow. We propose the use of 2D-3D image registration to allow digital tomosynthesis (DTS) slices to be produced using standard fluoroscopy equipment. Our method automatically produces patient-anatomy-specific slices and removes clutter resulting from bones. Such slices could provide additional intraoperative information, offering improved guidance precision. Image acquisition would fit with interventional clinical workflow and would not require a high x-ray dose. Phantom results showed a 1133% contrast-to-noise improvement compared to standard fluoroscopy. Patient results showed our method enabled visualisation of clinically relevant features: outline of the aorta, the aortic bifurcation and some aortic calcifications.

Remapping of digital subtraction angiography on a standard fluoroscopy system using 2D-3D registration

Fluoroscopy-guided endovascular interventions are being performing for more and more complex cases with longer screening times. However, X-ray is much better at visualizing interventional devices and dense structures compared to vasculature. To visualise vasculature, angiography screening is essential but requires the use of iodinated contrast medium (ICM) which is nephrotoxic. Acute kidney injury is the main life-threatening complication of ICM. Digital subtraction angiography (DSA) is also often a major contributor to overall patient radiation dose (81% reported). Furthermore, a DSA image is only valid for the current interventional view and not the new view once the C-arm is moved. In this paper, we propose the use of 2D-3D image registration between intraoperative images and the preoperative CT volume to facilitate DSA remapping using a standard fluoroscopy system. This allows repeated ICM-free DSA and has the potential to enable a reduction in ICM usage and radiation dose. Experiments were carried out using 9 clinical datasets. In total, 41 DSA images were remapped. For each dataset, the maximum and averaged remapping accuracy error were calculated and presented. Numerical results showed an overall averaged error of 2.50 mm, with 7 patients scoring averaged errors < 3 mm and 2 patients < 6 mm.

Interventional digital tomosynthesis from a standard fluoroscopy system using 2D-3D registration

Interventional fluoroscopy provides guidance in a variety of minimally invasive procedures. However, three-dimensional (3D) clinically relevant information is projected onto a two-dimensional (2D) image which can make image interpretation difficult. Moreover, vasculature visualisation requires the use of iodinated contrast media which is nephrotoxic and is the primary cause of renal complications. In this article, we demonstrate how digital tomosynthesis slices can be produced on standard fluoroscopy equipment by registering the preoperative CT volume and the intraoperative fluoroscopy images using 2D-3D image registration. The proposed method automatically reconstructs patient-anatomy-specific slices and removes clutter resulting from bony anatomy. Such slices could provide additional intraoperative information which cannot be provided by the preoperative CT volume alone, such as the deformed aorta position offering improved guidance precision. Image acquisition would fit with interventional clinical work-flow and would not require a high X-ray dose. Experiments are carried out using one phantom and four clinical datasets. Phantom results showed a 3351% contrast-to-noise improvement compared to standard fluoroscopy. Patient results showed our method enabled visualization of clinically relevant features: outline of the aorta, the aortic bifurcation and some aortic calcifications.