Matthias Niethammer

Coronary calcium scoring using 16-row multislice computed tomography: nonenhanced versus contrast-enhanced studies in vitro and in vivo.

Objectives:We sought to assess the agreement of coronary artery calcium score in nonenhanced and contrast-enhanced multislice-spiral computed tomography. Materials and Methods:Vessel phantoms and 36 patients underwent nonenhanced and contrast-enhanced cardiac multislice-spiral computed tomography (Sensation 16; Siemens, Germany). Reconstruction-parameters: slice thickness 3 mm, increment 2 mm, kernels B35f and B30f. The Agatston score, calcium mass, and number of lesions were calculated. Images were scored using detection thresholds of 130 Hounsfield units (HU) and 350 HU. Based on the Agatston score, risk stratification was performed. Results:In the phantom and patient study, altering the threshold from 130 to 350 HU led to a significant decrease in the mean Agatston score (phantom: 54.6%, patients: 66.7%) and calcium mass (33.0%, 47.0%) (B35f). Contrast-enhanced studies (threshold: 350 HU) showed an increase of the mean Agatston score (71.0%, 20.7%) and calcium mass (81.0%, 16.0%) when compared with nonenhanced scans (threshold: 350 HU). A total of 57% of all patients were assigned to different risk groups. Conclusions:Contrast material may simulate calcification; therefore, calculation of the coronary calcium score from contrast-enhanced images is not reliable.

CT Perfusion Imaging of the Lung in Pulmonary Embolism

Recent years have seen an increasing importance of computed tomography (CT) in the diagnosis of pulmonary embolism (PE), mainly brought about by the advent of fast CT image acquisition techniques (Kauczor et al. 1999; Remy-Jardin and Remy 1999; Schoepf et al. 2000a, b). Competing imaging modalities are in decline: nuclear scanning allows functional assessment of lung ventilation and perfusion but lacks spatial resolution (PIOPED-Investigators 1990). Once the first line of defense in the diagnostic algorithm of PE, this modality is currently withdrawing to diagnostic niches due to limited availability, poor inter-observer correlation (Blachere et al. 2000), and notorious lack of specificity (PIOPED-Investigators 1990). Pulmonary angiography, the one-time gold standard for the diagnosis of PE, is becoming increasingly tarnished (Diffin et al. 1998; Stein et al. 1999). Its ability to detect isolated peripheral emboli does not seem to exceed the accuracy of computed tomography (Diffin et al. 1998; Stein et al. 1999).

Spatial domain filtering for fast modification of the tradeoff between image sharpness and pixel noise in computed tomography

In computed tomography (CT), selection of a convolution kernel determines the tradeoff between image sharpness and pixel noise. For certain clinical applications it is desirable to have two or more sets of images with different settings. So far, this typically requires reconstruction of several sets of images. We present an alternative approach using default reconstruction of sharp images and online filtering in the spatial domain allowing modification of the sharpness-noise tradeoff in real time. A suitable smoothing filter function in the frequency domain is the ratio of smooth and original (sharp) kernel. Efficient implementation can be achieved by a Fourier transform of this ratio to the spatial domain. Separating the two-dimensional spatial filtering into two subsequent one-dimensional filtering stages in the x and y directions using a Gaussian approximation for the convolution kernel further reduces computational complexity. Due to efficient implementation, interactive modification of the filter settings becomes possible, which can completely replace the variety of different reconstruction kernels.

Fast automatic path proposal computation for hepatic needle placement

Percutaneous image-guided interventions, such as radiofrequency ablation (RFA), biopsy, seed implantation, and several types of drainage, employ needle shaped instruments which have to be inserted into the patients body. Precise planning of needle placement is a key to a successful intervention. The planning of the access path has to be carried out with respect to a variety of criteria for all possible trajectories to the selected target. Since the planning is performed in 2D slices, it demands considerable experience and constitutes a significant mental task. To support the process of finding a suitable path for hepatic interventions, we propose a fast automatic method that computes a list of path proposals for a given target point inside the liver with respect to multiple criteria that affect safety and practicability. Prerequisites include segmentation masks of the liver, of all relevant risk structures and, depending on the kind of procedure, of the tumor. The path proposals are computed based on a weighted combination of cylindrical projections. Each projection represents one path criterion and is generated using the graphics hardware of the workstation. The list of path proposals is generated in less than one second. Hence, updates of the proposals upon changes of the target point and other relevant input parameters can be carried out interactively. The results of a preliminary evaluation indicate that the proposed paths are comparable to those chosen by experienced radiologists and therefore are suited to support planning in the clinical environment. Our implementation focuses on RFA and biopsy in the liver but may be adapted to other types of interventions.

Visualization support for the planning of hepatic needle placement

PurposePercutaneous image-guided interventions, such as radiofrequency ablation or biopsy, are using needle-shaped instruments which have to be inserted into a target area without penetrating any vital structure. The established planning workflow is based on viewing 2D slices of a pre-interventional CT or MR scan. However, access paths not parallel to the axial plane are often necessary. For such complicated cases, the planning process is challenging and time consuming if solely based on 2D slices. To overcome these limitations while keeping the well-established workflow, we propose a visualization method that highlights less suited paths directly in the 2D visualizations with which the radiologist is familiar.MethodsBased on a user defined target point and segmentation masks of relevant risk structures, a risk structure map is computed using GPU accelerated volume rendering and projected onto the 2D slices. This visualization supports the user in defining safe linear access paths by selecting a second point directly in the 2D image slices.ResultsIn an evaluation for 20 liver radiofrequency ablation cases, 3 experienced radiologists stated for 55% of the cases that the visualization supported the access path choice. The visualization support was rated with an average mark of 2.2. For 2 of the 3 radiologists, a significant reduction of the planning duration by 54 and 50% was observed.ConclusionsThe proposed visualization approach can both accelerate the access path planning for radiofrequency ablation in the liver and facilitate the differentiation between safer and less safe paths.

New approach to diagnosis of pulmonary embolism using multislice CT

Suspected pulmonary embolism (PE) is a common indication for CT scanning of the thorax. Usually, intravenous contrast agent is administered utilizing a power-injector and the vascular structures are examined for the presence of pulmonary emboli. Current Multi-Slice CT-technology allows extending this morphological analysis by adding a more functional visualization of the actual parenchymal perfusion disturbance. We have developed a new image processing technique which allows selective color encoded display of parenchymal enhancement of the lung, which will be reduced in the presence of PE. Based on thin slice reconstructions an automatic 3D segmentation of the lung is performed followed by threshold based extraction of the major airways and vascular structures. This allows applying an adaptive 3D low-pass filter to the parenchymal volume only. The filtered volume data are then color encoded and overlaid onto the original CT-images. This combination of low-resolution perfusion-weighted color maps and high-resolution gray scale structural data from the same data set greatly enhances visualization of spatial relationships. The resulting images can be displayed in axial, sagittal and coronal orientation. Initial experience indicates that this new technique provides relevant additional information for the clinical management of patients with proven PE. A larger controlled patient study is under way.

Fast image filters as an alternative to reconstruction kernels in computed tomography

In Computed Tomography, axial resolution is determined by the slice collimation and the spiral algorithm, while in-plane resolution is determined by the reconstruction kernel. Both choices select a tradeoff between image resolution (sharpness) and pixel noise. We investigated an alternative approach using default settings for image reconstruction which provide narrow reconstructed slice-width and high in-plane resolution. If smoother images are desired, we filter the original (sharp) images, instead of performing a new reconstruction with a smoother kernel. A suitable filter function in the frequency domain is the ratio of smooth and original (sharp) kernel. Efficient implementation was achieved by a Fourier transform of this ratio to the spatial domain. Separating the 2D spatial filtering into two subsequent 1D filtering stages in x- and y-direction further reduces computational complexity. Using this approach, arbitrarily oriented multi-planar reformats (MPRs) can be treated in exactly the same way as axial images. Due to efficient implementation, interactive modification of the filter settings becomes possible, which completely replace the variety of different reconstruction kernels. We implemented a further promising application of the method to thorax imaging, where different regions of the thorax (lungs and mediastinum) are jointly presented in the same images using different filter settings and different windowing.

The potential of multi-slice computed tomography based volumetry for demonstrating reverse remodeling induced by cardiac resynchronization therapy

BACKGROUND Multi-slice computed tomography (MSCT) was proved to provide precise cardiac volumetric assessment. Cardiac resynchronization therapy (CRT) is an effective treatment for selected patients with heart failure and reduced ejection fraction (HFREF). In HFREF patients we investigated the potential of MSCT based wall motion analysis in order to demonstrate CRT-induced reversed remodeling. METHODS Besides six patients with normal cardiac pump function serving as control group seven HFREF patients underwent contrast enhanced MSCT before and after CRT. Short cardiac axis views of the left ventricle (LV) in end-diastole (ED) and end-systole (ES) served for planimetry. Pre- and post-CRT MSCT based volumetry was compared with 2D echo. To demonstrate CRT-induced reverse remodeling, MSCT based multi-segment color-coded polar maps were introduced. RESULTS With regard to the HFREF patients pre-CRT MSCT based volumetry correlated with 2D echo data for LV-EDV (MSCT 278.3+/-75.0mL vs. echo 274.4+/-85.6mL) r=0.380, p=0.401, LV-ESV (MSCT 226.7+/-75.4mL vs. echo 220.1+/-74.0mL) r=0.323, p=0.479 and LV-EF (MSCT 20.2+/-8.8% vs. echo 20.0+/-11.9%) r=0.617, p=0.143. Post-CRT MSCT correlated well with 2D echo: LV-EDV (MSCT 218.9+/-106.4mL vs. echo 188.7+/-93.1mL) r=0.87, p=0.011, LV-ESV (MSCT 145+/-71.5mL vs. echo 125.6+/-78mL) r=0.84, p=0.018 and LV-EF (MSCT 29.6+/-11.3mL vs. echo 38.6+/-14.6mL) r=0.89, p=0.007. There was a significant increase of the mid-ventricular septum in terms of absolute LV wall thickening of the responders (pre 0.9+/-2.1mm vs. post 3.3+/-2.2mm; p<0.0005). CONCLUSION MSCT based volumetry involving multi-segment color-coded polar maps offers wall motion analysis to demonstrate CRT-induced reverse remodeling which needs to be further validated.

Fast Registration of Pre- and Peri-Interventional CT Images for Targeting Support in Radiofrequency Ablation of Hepatic Tumors

Radiofrequency (RF) ablation is an image-guided minimally invasive therapy which destroys a tumor by locally inducing electrical energy into the malignant tissue through a radiofrequency applicator. Treatment success is essentially dependent on the accurate placement of the RF applicator. In the case of CT-guided RF ablation of liver tumors, a central problem during monitoring is the reduced quality and information content in the peri-interventional images compared to the images used for planning. Therefore, the question of how to effectively transfer information from the planning scan into the peri-interventional scan in order to support the interventionalist is of high interest. Key to such an enhancement of peri-interventional scans is an adequate registration of the pre- and peri-interventional image, which also needs to be fast since intervention duration is still a challenge. We present an approach for the fast and automatic registration of a high quality CT volume scan of the liver to a spiral CT scan of lower quality. Our method combines an approximate pre-registration to compensate large displacements and a rigid registration of a liver subvolume for further refinement. The method focuses on the position of the tumor to be ablated and the corresponding access path. Thereby, it achieves both fast and precise results in the region of interest. A preliminary evaluation, on 37 data sets from 20 different patients, shows that the registration is performed within a maximum of 18 seconds, while obtaining high accuracy in the relevant region of the liver comprising tumor and the planned access path.