Biomedical subjects

R Proksa

Publications and source records attributed to R Proksa.

14 recordsLinked to original sources

PubMed2002

Artifact analysis of approximate helical cone-beam CT reconstruction algorithms.

In this paper, four approximate cone-beam CT reconstruction algorithms are compared: Advanced single slice rebinning (ASSR) as a representative of algorithms employing a two dimensional approximation, PI, PI-SLANT, and 3-PI which all use a proper three dimensional back-projection. A detailed analysis of the image artifacts produced by these techniques shows that aliasing in the z-direction is the predominant source of artifacts for a 16-row scanner with 1.25 mm nominal slice thickness. For a detector with isotropic resolution of 0.5 mm, we found that ASSR and PI produce different kinds of artifacts which are almost at the same level, while PI-SLANT produces none of these artifacts. It is shown that the use of redundant data in the 3-PI method suppresses aliasing artifacts efficiently for both scanners.

Algorithms↗

PubMed2001

Angular weighted hybrid cone-beam CT reconstruction for circular trajectories.

Hybrid reconstruction techniques have been introduced for the volume reconstruction of axially truncated cone-beam computed tomography projection data acquired along a circular source-detector trajectory. The introduction of weighted half-scan techniques into this framework is described in this paper. Due to the cone-beam geometry it is not possible to perform the weighting on the projections as is typically done in conventional single-line computed tomography. Hence, in this paper we present an efficient way to incorporate angular weighting functions, depending on the object point position, into the framework of hybrid cone-beam reconstruction. Four different angular weighting functions are introduced and discussed with respect to their cone-beam artefact behaviour and their influence on the signal-to-noise ratio. As a result, the most effective angular weighting function for hybrid circular cone-beam reconstruction is determined by means of a simulation study based on mathematical phantoms and clinical data sets. This distance-weighted angular weighting scheme yields the best results in terms of high image quality, low computational complexity and signal-to-noise variations in the reconstruction volume.

Computer Simulation↗

PubMed2001

A fast and efficient method for sequential cone-beam tomography.

Sequential cone-beam tomography is a method that uses data of two or more parallel circular trajectories of a cone-beam scanner to reconstruct the object function. We propose a condition for the data acquisition that ensures that all object points between two successive circles are irradiated over an angular span of the x-ray source position of exactly 360 degrees in total as seen along the rotation axis. A fast and efficient approximative reconstruction method for the proposed acquisition is presented which uses data from exactly 360 degrees for every object point. It is based on the Tent-FDK method which was recently developed for single circular cone-beam CT. The measurement geometry does not provide sufficient data for exact reconstruction but it is shown that the proposed reconstruction method provides satisfying image quality for small cone angles.

Algorithms↗

PubMed2000

3D cone-beam CT reconstruction for circular trajectories.

3D reconstruction from 2D projections obtained along a single circular source trajectory is most commonly done using an algorithm due to Feldkamp, Davis and Kress. In this paper we propose an alternative approach based on a cone-beam to parallel-beam rebinning step, a corresponding rebinning step into a rectangular virtual detector plane and a filtered backprojection. This approach yields an improved image quality reflected by a decreased low-intensity drop which is well known for 3D reconstruction from projection data obtained along circular trajectories. At the same time the computational complexity is lower than in Feldkamp's original approach. Based on this idea, a hybrid 3D cone-beam reconstruction method is formulated that enlarges the reconstruction volume in its dimension along the rotation axis of the cone-beam CT system. This enlargement is achieved by applying different reconstruction conditions for each voxel. An optimal ratio between the reconstructible and irradiated volume of the scanned object is achieved.

Algorithms↗

PubMed2000

The n-PI-method for helical cone-beam CT.

A new class of acquisition schemes for helical cone-beam computed tomography (CB-CT) scanning is introduced, and their effect on the reconstruction methods is analyzed. These acquisition schemes are based on a new detector shape that is bounded by the helix. It will be shown that the data acquired with these schemes are compatible with exact reconstruction methods, and the adaptation of exact reconstruction algorithms to the new acquisition geometry is described. At the same time, the so-called PI-sufficiency condition is fulfilled. Moreover, a good fit to the acquisition requirements of the various medical applications of cone-beam CT is achieved. In contrast to other helical cone-beam acquisition and reconstruction methods, the n-PI-method introduced in this publication allows for variable pitches of the acquisition helix. This additional feature will introduce a higher flexibility into the acquisition protocols of future medical cone-beam scanners. An approximative n-PI-filtered backprojection (n-PI-FBP) reconstruction method is presented and verified. It yields convincing image quality.

Image Processing, Computer-Assisted↗

PubMed1999

Real-time interactive magnetic resonance imaging with multiple coils for the assessment of left ventricular function.

Interactive real-time examination of left ventricular function in healthy volunteers both under rest and stress conditions has been performed. For this purpose, a system combining an interactive user interface, an ultrafast segmented echo-planar imaging sequence, and real-time reconstruction and display of the acquired images was designed. Magnetic resonance images were acquired at rates of up to 20 images per second with multiple receiver coils. By using a sliding window reconstruction technique, reconstruction rates of up to 60 images per second were achieved with a latency of < 100 msec. The quality of the real-time images was evaluated both qualitatively and quantitatively and was found to be appropriate for the determination of left ventricular function. It is concluded that the combination of dedicated components provides a convenient modality for the high-quality visualization of left ventricular function under rest and stress conditions at video frame rates with magnetic resonance imaging. J. Magn. Reson. Imaging 1999;10:826-832.

Echo-Planar Imaging↗

PubMed1999

MR fluoroscopy using projection reconstruction multi-gradient-echo (prMGE) MRI.

A projection reconstruction multi-gradient-echo (prMGE) technique is presented. The introduced technique is an extension of a standard projection reconstruction steady-state gradient-echo technique allowing for the acquisition of several gradient echoes after each excitation of the spin system. Each echo train is used for acquiring data of a certain angular segment of k-space. By use of echo trains consisting of up to four echoes, the overall acquisition time for a 128(2) image can be reduced to 150 ms without sacrificing image quality. Results are presented for cardiac fluoroscopy, for the visualization of swallowing, and for the visualization of joint motion. For all investigated applications promising results have been obtained. Especially in parts of the body where motion on an even shorter time scale than the acquisition process or significant in-plane or through-plane flow are within the field of view, the introduced technique appears to be a promising technique for MR fluoroscopy. Magn Reson Med 42:324-334, 1999.

Deglutition↗

PubMed1999

Resampling of data between arbitrary grids using convolution interpolation.

For certain medical applications resampling of data is required. In magnetic resonance tomography (MRT) or computer tomography (CT), e.g., data may be sampled on nonrectilinear grids in the Fourier domain. For the image reconstruction a convolution-interpolation algorithm, often called gridding, can be applied for resampling of the data onto a rectilinear grid. Resampling of data from a rectilinear onto a nonrectilinear grid are needed, e.g., if projections of a given rectilinear data set are to be obtained. In this paper we introduce the application of the convolution interpolation for resampling of data from one arbitrary grid onto another. The basic algorithm can be split into two steps. First, the data are resampled from the arbitrary input grid onto a rectilinear grid and second, the rectilinear data is resampled onto the arbitrary output grid. Furthermore, we like to introduce a new technique to derive the sampling density function needed for the first step of our algorithm. For fast, sampling-pattern-independent determination of the sampling density function the Voronoi diagram of the sample distribution is calculated. The volume of the Voronoi cell around each sample is used as a measure for the sampling density. It is shown that the introduced resampling technique allows fast resampling of data between arbitrary grids. Furthermore, it is shown that the suggested approach to derive the sampling density function is suitable even for arbitrary sampling patterns. Examples are given in which the proposed technique has been applied for the reconstruction of data acquired along spiral, radial, and arbitrary trajectories and for the fast calculation of projections of a given rectilinearly sampled image.

Algorithms↗

PubMed1998

[Real-time MRI with radial k-radial scanning technique for control of angiographic interventions].

PURPOSE: To test the feasibility of real-time MR controlled guidance of field-inhomogeneity catheters in vitro and in vivo as a first step to MR-guided angiographic interventions. METHODS: Applying a combination of radial scanning with the sliding window reconstruction technique, a frame rate of 23 low resolution images per second was achieved. Field inhomogeneity catheters were steered through a flow phantom and into the renal arteries of a pig. RESULTS: It was possible to visualize flow or, respectively, vessels and to depict catheter movements. This enabled real-time MR-guidance of the catheter into the renal arteries of the flow phantom and into those of the pig. CONCLUSIONS: The new technique yields a sufficiently high temporal resolution for MR-guidance of catheters through vessels.

Angiography↗

PubMed1997

Catheter tracking using continuous radial MRI.

The guidance of minimally invasive procedures may become a very important future application of MRI. The guidance of interventions requires images of the anatomy as well as the information of the position of invasive devices used. This paper introduces continuous radial MRI for the simultaneous acquisition of the anatomic MR image and the position of one or more small RF-coils (mu-coils), which can be mounted on invasive devices such as catheters or biopsy needles. This approach allows the in-plane tracking of an invasive device without any prolongation of the overall acquisition time. The extension to three-dimensional position tracking is described. Phantom studies are presented demonstrating the capability of this technique for real-time automatic adjustment of the slice position to the current catheter position with a temporal resolution of 100 ms. Simultaneously the in-plane catheter position is depicted in the actually acquired MR image during continuous scanning.

Catheterization↗

PubMed1997

Motion-adapted gating based on k-space weighting for reduction of respiratory motion artifacts.

A new modified type of gating is presented that shows the ability to reduce the total scan time with almost conserved image quality compared with conventional gating. This new motion-adapted gating approach is based on a k-space-dependent gating threshold function. MR data acquired are only accepted if the motion-induced displacements measured from a reference position are below the chosen gating threshold function. During the MR measurement the scanner analyses respiratory motion decides in real-time which data in k-space could be measured according to the gating threshold function and performs data acquisition. In the present paper the approach will be described and discussed. Simulations based on in vivo data and initial in vivo experiments are presented to compare different variants of the new approach mutually and to the conventional technique. The analysis given is focused on spin warp type sequences, which are the best candidates for this approach.

Abdomen↗

PubMed1995

Continuous radial data acquisition for dynamic MRI.

Since image acquisition times in MRI have been reduced considerably over recent years, several new important application areas of MRI have appeared. In addition to pure static anatomic information, the evolution of a dynamic process may be visualized by a sequence of temporal snapshots of the process acquired within a short time period. This makes applications like interactive or interventional MRI as well as the acquisition of additional functional information feasible. For high temporal resolution, all these applications require a quasi real-time image acquisition during the time the interaction or dynamic process evolves. We present an approach to real-time imaging using a continuous radial acquisition scheme. The intrinsic advantages of radial or projection reconstruction (PR) techniques are used to minimize motion-related image distortions. Modifications of the acquisition scheme as well as dedicated reconstruction techniques are used to further reduce the temporal blurring due to the finite acquisition time of one entire data set in our approach. So far we have used this technique for the visualization of active joint motion.

Artifacts↗

PubMedDate not supplied

Three-dimensional reconstruction of high contrast objects using C-arm image intensifier projection data.

The reconstruction of three-dimensional (3D) objects from 2D X-ray cone-beam projections using a circular source path is most commonly done with an algorithm according to Feldkamp et al. [Feldkamp LA, Davis LC, Kress JW. Practical cone-beam algorithms. J Opt Soc Am A 1984;6:612-619]. An adaptation of this so-called Feldkamp method to cone-beam projections acquired with a C-arm system is presented here. In a phantom study, reconstruction results obtained along real source-detector trajectories of a C-arm system are compared to reconstruction results obtained from projections acquired from a full-circular trajectory and from one consisting of two full orthogonal circles, which fulfills Tuy's sufficiency condition. The straightforward application of Feldkamp's method adapted to projection data obtained with a C-arm system illustrates the 3D imaging potential of image intensifier based cone-beam computed tomography. Reconstruction results from projection data of different patients acquired with a motorized C-arm system such as vessel structures filled with contrast agent and bones are presented.

Algorithms↗

PubMedDate not supplied

Automatic shimming for localized spectroscopy.

Localized in vivo nuclear magnetic resonance studies often require a high spectral resolution not achievable with the basic shim of a whole-body magnetic resonance magnet. Therefore, the magnetic field homogeneity needs to be optimized in the selected volume of interest within a reasonable time. For this purpose, a method of automatic shimming was developed and tested on phantoms and volunteers. The volume selection is performed by means of a surface coil or by using a localization method which generates a stimulated echo from the volume of interest. The optimization procedure uses the time integral over the magnitude of the free induction decay or echo signal as homogeneity criterion. A complete shimming process generally requires only 80 transients. Test experiments were conducted on various volume sizes ranging from 2 X 2 X 2 cm3 to 15 X 15 X 15 cm3 inside a large phantom. The resulting linewidth in small volumes at the magnet center compared well with the natural linewidth determined by means of the Carr-Purcell-Meiboom-Gill sequence. As expected, shimming in selected volumes at off center positions led to somewhat broader lines. Results obtained on volunteers demonstrate the practical value of this rapid, automatic shimming method for in vivo studies.

Algorithms↗