Gilad Shechter

Cardiac image reconstruction on a 16-slice CT scanner using a retrospectively ECG-gated multicycle 3D back-projection algorithm

Fast 16-slice spiral CT delivers superior cardiac visualization in comparison to older generation 2- to 8-slice scanners due to the combination of high temporal resolution along with isotropic spatial resolution and large coverage. The large beam opening of such scanners necessitates the use of adequate algorithms to avoid cone beam artifacts. We have developed a multi-cycle phase selective 3D back projection reconstruction algorithm that provides excellent temporal and spatial resolution for 16-slice CT cardiac images free of cone beam artifacts.

Evaluation of helical cone-beam CT reconstruction algorithms

In this work, three different reconstruction algorithms for short scan helical cone-beam CT are compared: Two approximate algorithms, PI-SLANT and WVEDGE-PI, with the recently published exact algorithm by Katsevich. It is shown that WEDGE-PI performs as well as the exact method for a 64 row scanner and almost as well for a 128 row scanner. PI-SLANT produces significantly more artifacts, in particular for the 128 row scanner.

Isotropic high-resolution in multi-slice spiral CT

Isotropic resolution in multi-slice CT with a cutoff of about 14 cm/sup -1/ has the potential of dramatically improving the quality of many CT applications. Among these are HRCT of the lungs and of the spine, orthopedics studies, and coronary arteries imaging. Obtaining this isotropic resolution with present detector scintillations can be realized by over-sampling in both the x- and the z-direction during spiral acquisition. We describe two different approaches for such a realization. One approach is the use of staggered detector array with a periodic motion of the X-ray tube source along the x-direction. Another approach is generating a periodic motion of the X-ray tube source in the x-and in the z-direction. A similar approach has been proposed to reduce windmill artifacts. For each approach, we measure the best spatial resolution offered and the corresponding suppression of image artifacts. The two approaches offer an isotropic resolution with a cutoff beyond 14 cm/sup -1/ near the iso-center. Reaching this goal using the second approach, only a minor price of windmill artifacts has to be paid. Using the first method provides artifact-free images.

Accurate material quantification in dual energy CT

Clinical CT applications such as oncology follow-up using iodine maps require accurate contrast agent (CA) quantification within the patient. Unfortunately, due to beam hardening, the quantification of CA materials like iodine in dual energy systems can vary for different patient sizes and surrounding composition. In this paper we present a novel method that handles this problem which takes into account properly the CA energy dependent attenuation profile. Our method is applicable for different dual energy scanners, e.g. fast kVp switching or dual layer detector array and is fully compatible with image domain material analysis. In this paper we explain the concept of so called landmarks used by our method, and give the mathematical formulation of how to calculate them. We demonstrate by scans of various phantom shapes and by simulations, the robustness and the accuracy of the iodine concentration quantification obtained by our method.

A re-binning algorithm for cardiac CT

The z-sharp modulation technique of the source (focal spot) allows doubling the longitudinal sampling rate of cone beam CT scans, without reducing the geometrical efficiency of the detector (T. Flohr et al, presented at the RSNA meeting 2003). Due to the small anode angle, this technique implies a large movement of the source in the radial direction. In order not to let this movement to degrade the transversal resolution, it is preferable to modulate the source between four positions. However, due to the limited number of integration times allowed within the fast gantry rotation time of cardiac acquisitions, modulating the source between four positions requires reducing the angular sampling. Motivated to avoid image degradation due to the low angular sampling, we have developed a new re-binning algorithm

Robust band artifact suppression for cardiac CT

Cardiac cone beam CT images are reconstructed today using phase selective multi-cycle algorithms. According to the acquisition geometry of a helical CT scan, voxels having different z-coordinates are illuminated during non-overlapping time intervals and are reconstructed, therefore, using phase points of different heart cycles. This behavior can lead to inconsistency expressed by step-like and slab-like band artifacts. These bands often seen in coronal and saggital images can hinder the segmentation of the coronary arteries and the diagnosis. Our present work is based on the understanding that by forcing a smooth change of the back-projection weights with respect to the voxel z-coordinate band artifacts will be suppressed. Using this approach, we have developed a new back-projection weighting scheme that replaces the original weighting scheme of the so-called extended cardiac reconstruction method for helical cone beam CT (Grass et al, Phys. Med. Biol. 2003). We evaluated this scheme on patients having different heart rates. The strong suppression of band artifacts and the visualization of the coronary arteries obtained using this scheme will be presented.

Streak artifact reduction in cardiac cone beam CT

Cone beam reconstructed cardiac CT images suffer from characteristic streak artifacts that affect the quality of coronary artery imaging. These artifacts arise from inhomogeneous distribution of noise. While in non-tagged reconstruction inhomogeneity of noise distribution is mainly due to anisotropy of the attenuation of the scanned object (e.g. shoulders), in cardiac imaging it is largely influenced by the non-uniform distribution of the acquired data used for reconstructing the heart at a given phase. We use a cardiac adaptive filter to reduce these streaks. In difference to previous methods of adaptive filtering that locally smooth data points on the basis of their attenuation values, our filter is applied as a function of the noise distribution of the data as it is used in the phase selective reconstruction. We have reconstructed trans-axial images without adaptive filtering, with a regular adaptive filter and with the cardiac adaptive filter. With the cardiac adaptive filter significant reduction of streaks is achieved, and thus image quality is improved. The coronary vessel is much more pronounced in the cardiac adaptive filtered images, in slab MIP the main coronary artery branches are more visible, and non-calcified plaque is better differentiated from vessel wall. This improvement is accomplished without altering significantly the border definition of calcified plaques.

Streak-like artifact reduction control in CT reconstruction

Inhomogeneous noise distribution may result in streak-like artifacts in reconstructed CT images. These streak-like artifacts can be overcome with an adaptive filter applied locally to some of the data points in the attenuation profile, those data points for which the attenuation is high and the signal is low. However, such a local filter might cause new smearing artifacts if the filter is applied over edges in the scanned object. We suggest adding to the adaptive filter a control mechanism aimed to avoid the creation of such new artifacts. With this control mechanism, every modification suggested by the adaptive filter is examined and if it is estimated to be the result of a real gradient in the scanned object, the data is modified only moderately or not at all. Such a control mechanism can be added to different adaptive filters. We have been working with an adaptive filter based on estimation of photon noise. The control mechanism in our case limits the modification allowed by the adaptive filter to the size of noise expected for that data point. Experiments with clinical studies show that images produced with the new control mechanism indeed reduce creation of new smearing artifacts of bones and contrast material.