André Aichert

On the Energetics of Conformational Switching of Molecules at and Close to Room Temperature

We observe and induce conformational switching of individual molecules via scanning tunneling microscopy (STM) at and close to room temperature. 2H-5,10,15,20-Tetrakis-(3,5-di-tert-butyl)-phenylporphyrin adsorbed on Cu(111) forms a peculiar supramolecular ordered phase in which the molecules arrange in alternating rows, with two distinct appearances in STM which are assigned to concave and convex intramolecular conformations. Around room temperature, frequent bidirectional conformational switching of individual molecules from concave to convex and vice versa is observed. From the temperature dependence, detailed insights into the energy barriers and entropic contributions of the switching processes are deduced. At 200 K, controlled STM tip-induced unidirectional switching is possible, yielding an information storage density of 4.9 × 10(13) bit/inch(2). With this contribution we demonstrate that controlled switching of individual molecules at comparably high temperatures is possible and that entropic effects can be a decisive factor in potential molecular devices at these temperatures.

The effect of out-of-focus blur on visual discomfort when using stereo displays

Visual discomfort is a major problem for head-mounted displays and other stereo displays. One effect that is known to reduce visual comfort is double vision, which can occur due to high disparities. Previous studies suggest that adding artificial out-of-focus blur increases the fusional limits, where the left and right image can be fused without double vision. We investigate the effect of adding artificial out-of-focus blur on visual discomfort using two different setups. One uses a stereo monitor and an eye tracker to change the depth of focus based on the gaze of the user. The other one uses a video-see through head mounted display. A study involving 18 subjects showed that the viewing comfort when using blur is significantly higher in both setups for virtual scenes. However we can not confirm without doubt that the higher viewing comfort is only related to an increase of the fusional limits, as many subjects reported that double vision did not occur during the experiment. Results for additional photographed images that have been shown to the subjects were less significant. A first prototype of an AR system extracting a depth map from stereo images and adding artificial out-of-focus blur is presented.

Robust Multiframe Super-Resolution Employing Iteratively Re-Weighted Minimization

Multiframe super-resolution algorithms reconstruct high-resolution images by exploiting complementary information in multiple low-resolution frames. However, despite their success under ideal conditions, most existing methods rely on simplistic approximations to the physics of image acquisition and show limited robustness in real-world applications. This paper proposes spatially adaptive Bayesian modeling and an iterative algorithm for robust super-resolution imaging. In particular, we introduce a weighted Gaussian observation model to consider space variant noise and weighted bilateral total variation to exploit sparsity of natural images. Based on this model, we develop a majorization-minimization algorithm implemented as iteratively re-weighted minimization. The proposed method simultaneously estimates model parameters and the super-resolved image in an iterative coarse-to-fine scheme. Compared to prior work, our approach combines the benefits of achieving robust and edge preserving image reconstruction with small amount of parameter tuning, while being flexible in terms of motion models, computationally efficient and easy to implement. Our experimental evaluation confirms that our approach outperforms state-of-the-art algorithms under various practical conditions, e.g., inaccurate geometric and photometric registration or invalid measurements.

Epipolar Consistency in Transmission Imaging

This paper presents the derivation of the Epipolar Consistency Conditions (ECC) between two X-ray images from the Beer-Lambert law of X-ray attenuation and the Epipolar Geometry of two pinhole cameras, using Grangeats theorem. We motivate the use of Oriented Projective Geometry to express redundant line integrals in projection images and define a consistency metric, which can be used, for instance, to estimate patient motion directly from a set of X-ray images. We describe in detail the mathematical tools to implement an algorithm to compute the Epipolar Consistency Metric and investigate its properties with detailed random studies on both artificial and real FD-CT data. A set of six reference projections of the CT scan of a fish were used to evaluate accuracy and precision of compensating for random disturbances of the ground truth projection matrix using an optimization of the consistency metric. In addition, we use three X-ray images of a pumpkin to prove applicability to real data. We conclude, that the metric might have potential in applications related to the estimation of projection geometry. By expression of redundancy between two arbitrary projection views, we in fact support any device or acquisition trajectory which uses a cone-beam geometry. We discuss certain geometric situations, where the ECC provide the ability to correct 3D motion, without the need for 3D reconstruction.

Marker‐free motion correction in weight‐bearing cone‐beam CT of the knee joint

PURPOSEnTo allow for a purely image-based motion estimation and compensation in weight-bearing cone-beam computed tomography of the knee joint.nnnMETHODSnWeight-bearing imaging of the knee joint in a standing position poses additional requirements for the image reconstruction algorithm. In contrast to supine scans, patient motion needs to be estimated and compensated. The authors propose a method that is based on 2D/3D registration of left and right femur and tibia segmented from a prior, motion-free reconstruction acquired in supine position. Each segmented bone is first roughly aligned to the motion-corrupted reconstruction of a scan in standing or squatting position. Subsequently, a rigid 2D/3D registration is performed for each bone to each of K projection images, estimating 6 × 4 × K motion parameters. The motion of individual bones is combined into global motion fields using thin-plate-spline extrapolation. These can be incorporated into a motion-compensated reconstruction in the backprojection step. The authors performed visual and quantitative comparisons between a state-of-the-art marker-based (MB) method and two variants of the proposed method using gradient correlation (GC) and normalized gradient information (NGI) as similarity measure for the 2D/3D registration.nnnRESULTSnThe authors evaluated their method on four acquisitions under different squatting positions of the same patient. All methods showed substantial improvement in image quality compared to the uncorrected reconstructions. Compared to NGI and MB, the GC method showed increased streaking artifacts due to misregistrations in lateral projection images. NGI and MB showed comparable image quality at the bone regions. Because the markers are attached to the skin, the MB method performed better at the surface of the legs where the authors observed slight streaking of the NGI and GC methods. For a quantitative evaluation, the authors computed the universal quality index (UQI) for all bone regions with respect to the motion-free reconstruction. The authors quantitative evaluation over regions around the bones yielded a mean UQI of 18.4 for no correction, 53.3 and 56.1 for the proposed method using GC and NGI, respectively, and 53.7 for the MB reference approach. In contrast to the authors registration-based corrections, the MB reference method caused slight nonrigid deformations at bone outlines when compared to a motion-free reference scan.nnnCONCLUSIONSnThe authors showed that their method based on the NGI similarity measure yields reconstruction quality close to the MB reference method. In contrast to the MB method, the proposed method does not require any preparation prior to the examination which will improve the clinical workflow and patient comfort. Further, the authors found that the MB method causes small, nonrigid deformations at the bone outline which indicates that markers may not accurately reflect the internal motion close to the knee joint. Therefore, the authors believe that the proposed method is a promising alternative to MB motion management.

Interactive 3d visualization of a single-view X-ray image

In this paper, we present an interactive X-Ray perceptual visualization technique (IXPV) to improve 3D perception in standard single-view X-Ray images. Based on a priori knowledge from CT data, we re-introduce lost depth information into the original single-view X-Ray image without jeopardizing information of the original X-Ray. We propose a novel approach that is suitable for correct fusion of intraoperative X-Ray and ultrasound, co-visualization of X-Ray and surgical tools, and for improving the 3D perception of standard radiographs. Phantom and animal cadaver datasets were used during experimentation to demonstrate the impact of our technique. Results from a questionnaire completed by 11 clinicians and computer scientists demonstrate the added value of introduced depth cues directly in an X-Ray image. In conclusion, we propose IXPV as a futuristic alternative to the standard radiographic image found in todays clinical setting.

Automatic non-linear mapping of pre-procedure CT volumes to 3D ultrasound

Multi-modality alignment of CT and ultrasound adds value to diagnostic examinations, as well as treatment planning and execution of various clinical procedures. Particularly automatic image-based alignment of such data is challenging, mostly because both modalities have very different imaging physics and characteristics. We present a method for dense-field deformable registration of CT and 3D ultrasound. Compared to global (rigid) alignment, this is more difficult to solve, because modality-specific difference in local anatomic appearance can result in incorrect displacements. We use a simulation of ultrasonic effects based on CT information, taking the current estimate of the deformation field into account to properly address orientation-dependent imaging artifacts. This is combined with a robust multi-channel local similarity metric, driving a variational registration framework. Because of the high computational demand, an efficient GPU-based implementation is used. Preliminary results are shown on data from a number of hepatic cancer patients. To our knowledge, this is the first time that a non-linear mapping of CT and 3D B-mode ultrasound is established in a computationally practical and fully automatic manner.

Consistency‐based respiratory motion estimation in rotational angiography

Purpose Rotational coronary angiography enables 3D reconstruction but suffers from intra‐scan cardiac and respiratory motion. While gating handles cardiac motion, respiratory motion requires compensation. State‐of‐the‐art algorithms rely on 3D‐2D registration that depends on initial reconstructions of sufficient quality. We propose a compensation method that is applied directly in projection domain. It overcomes the need for reconstruction and thus complements the state‐of‐the‐art. Methods Virtual single‐frame background subtraction based on vessel segmentation and spectral deconvolution yields non‐truncated images of the contrasted lumen. This allows motion compensation based on data consistency conditions. We compensate craniocaudal shifts by optimizing epipolar consistency to (a) devise an image‐based surrogate for cardiac motion and (b) compensate for respiratory motion. We validate our approach in two numerical phantom studies and three clinical cases. Results Correlation of the image‐based surrogate for cardiac motion with the ECG‐based ground truth was excellent yielding a Pearson correlation of 0.93 ± 0.04. Considering motion compensation, the target error measure decreased by 98% and 69%, respectively, for the phantom experiments while for the clinical cases the same figure of merit improved by 46 ± 21%. Conclusions The proposed method is entirely image‐based and accurately estimates craniocaudal shifts due to respiration and cardiac contraction. Future work will investigate experimental trajectories and possibilities for simplification of the single‐frame subtraction pipeline.

Denoising and artefact reduction in dynamic flat detector CT perfusion imaging using high speed acquisition: first experimental and clinical results.

Flat detector CT perfusion (FD-CTP) is a novel technique using C-arm angiography systems for interventional dynamic tissue perfusion measurement with high potential benefits for catheter-guided treatment of stroke. However, FD-CTP is challenging since C-arms rotate slower than conventional CT systems. Furthermore, noise and artefacts affect the measurement of contrast agent flow in tissue. Recent robotic C-arms are able to use high speed protocols (HSP), which allow sampling of the contrast agent flow with improved temporal resolution. However, low angular sampling of projection images leads to streak artefacts, which are translated to the perfusion maps. We recently introduced the FDK-JBF denoising technique based on Feldkamp (FDK) reconstruction followed by joint bilateral filtering (JBF). As this edge-preserving noise reduction preserves streak artefacts, an empirical streak reduction (SR) technique is presented in this work. The SR method exploits spatial and temporal information in the form of total variation and time-curve analysis to detect and remove streaks. The novel approach is evaluated in a numerical brain phantom and a patient study. An improved noise and artefact reduction compared to existing post-processing methods and faster computation speed compared to an algebraic reconstruction method are achieved.

A realistic digital phantom for perfusion C-arm CT based on MRI data

CTP is an important imaging modality for diagnosis of ischemic stroke, which is computed from of a series of consecutive CT-scans during the injection of contrast agent. Contrast flow at any point in space can be tracked as minor changes in intensity over a period of about 40 seconds to one minute, represented as a time-attenuation curve (TAC) for every voxel. This work presents an isotropic, dense, physiologically realistic and dynamic brain phantom for CT perfusion. The phantom is based on MRI scans of a volunteer and is freely available for download.