Ami Altman

Material separation with dual-layer CT

Dual-energy CT is known to enable possible improvement of material separation over regular CT. However, in clinical implementation most of the dual-energy techniques show limited results mainly due to their sensitivity to noisy data. We simulate data acquisition by a dual-layer CT based on two scintillation layers one on top of the other with which the data is acquired simultaneously. We map the results of the reconstruction into a plane created from the Hounsfield units (HU) of the upper-layer image versus the HU of the lower-layer image. We find that different scanned materials end up in different definable regions in the HU-plane. Application of a special correction on the reconstructed images achieves stability on the HU-plane despite beam-hardening effects. In order to assess the practical material separation capabilities, part of the simulations were done with exact noise calculations. We analyze the material separation capabilities with such a configuration and conclude that the combination of the dual-layer CT with the classification analysis in the HU-plane is a practical and robust method that significantly improves clinical applications, in particular those involving iodine-calcium separation such as analysis and classification of coronary artery calcifications and soft plaques.

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.

An iodine-calcium separation analysis and virtually non-contrasted image generation obtained with single source dual energy MDCT

Dual energy CT enables the differentiation between various materials by analyzing their unique attenuation spectral response. These responses are represented as vectors on an image-based energy map. Practically, this spectral information can be very noisy requiring further analysis. Furthermore, the material response vectors may be affected by beam-hardening and varied between images.

Dose optimization tool

A dose optimization tool for CT scanners is presented using patient raw data to calculate noise. The tool uses a single patient image which is modified for various lower doses. Dose optimization is carried out without extra measurements by interactively visualizing the dose-induced changes in this image. This tool can be used either off line, on existing image(s) or, as a pre - requisite for dose optimization for the specific patient, during the patient clinical study. The algorithm of low-dose simulation consists of reconstruction of two images from a single measurement and uses those images to create the various lower dose images. This algorithm enables fast simulation of various low dose (mAs) images on a real patient image.

A unique noncathartic CT colonography approach by using two-layer dual-energy MDCT and a special algorithmic colon cleansing method

Cathartic bowel preparation as part of CT colonography examination (CTC) can be uncomfortable or even dangerous for certain patient groups. Noncathartic CT colonography (i.e. without cathartic cleansing) including contrast-material fecal tagging can offer significant clinical advantages and can increase the screening compliance for colorectal cancer. Current techniques of conventional CTC with fecal tagging use “electronic cleansing” algorithms to remove the remaining tagged colonic contents from the images. In such cases these methods can give satisfactory results, but they face serious problems with the inferior tagging quality of noncathartic protocols. To overcome this fundamental problem, a completely new approach is proposed using both a two-layer dual-energy MDCT and a dedicated algorithmic cleansing method that utilizes the spectral information. Feasibility study was performed with a two-layer dual-energy MDCT which was utilized in clinical studies with oral intake of both iodine and barium contrast agents. The new method was compared to a conventional electronic cleansing technique. We show that the new approach is better at detecting highly dilute contrast agents in the colon, particularly where the tagged colonic content is mixed or adjacent to air regions. Therefore, the new technique may surmount the need for cathartic bowel preparation in CTC.

Arterial double-contrast dual-energy MDCT: in-vivo rabbit atherosclerosis with iodinated nanoparticles and gadolinium agents

An in-vivo feasibility study of potentially improved atherosclerosis CT imaging is presented. By administration of two different contrast agents to rabbits with induced atherosclerotic plaques we aim at identifying both soft plaque and vessel lumen simultaneously. Initial injection of iodinated nanoparticle (INP) contrast agent (N1177 - Nanoscan Imaging), two to four hours before scan, leads to its later accumulation in macrophage-rich soft plaque, while a second gadolinium contrast agent (Magnevist) injected immediately prior to the scan blends with the aortic blood. The distinction between the two agents in a single scan is achieved with a double-layer dual-energy MDCT (Philips Healthcare) following material separation analysis using the reconstructed images of the different x-ray spectra. A single contrast agent injection scan, where only INP was injected two hours prior to the scan, was compared to a double-contrast scan taken four hours after INP injection and immediately after gadolinium injection. On the single contrast agent scan we observed along the aorta walls, localized iodine accumulation which can point on INP uptake by atherosclerotic plaque. In the double-contrast scan the gadolinium contributes a clearer depiction of the vessel lumen in addition to the lasting INP presence. The material separation shows a good correlation to the pathologies inferred from the conventional CT images of the two different scans while performing only a single scan prevents miss-registration problems and reduces radiation dose. These results suggest that a double-contrast dual-energy CT may be used for advanced clinical diagnostic applications.

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.

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.