Interactively variable isotropic resolution in computed tomography.

An individual balancing between spatial resolution and image noise is necessary to fulfil the diagnostic requirements in medical CT imaging. In order to change influencing parameters, such as reconstruction kernel or effective slice thickness, additional raw-data-dependent image reconstructions have to be performed. Therefore, the noise versus resolution trade-off is time consuming and not interactively applicable. Furthermore, isotropic resolution, expressed by an equivalent point spread function (PSF) in every spatial direction, is important for the undistorted visualization and quantitative evaluation of small structures independent of the viewing plane. Theoretically, isotropic resolution can be obtained by matching the in-plane and through-plane resolution with the aforementioned parameters. Practically, however, the user is not assisted in doing so by current reconstruction systems and therefore isotropic resolution is not commonly achieved, in particular not at the desired resolution level. In this paper, an integrated approach is presented for equalizing the in-plane and through-plane spatial resolution by image filtering. The required filter kernels are calculated from previously measured PSFs in x/y- and z-direction. The concepts derived are combined with a variable resolution filtering technique. Both approaches are independent of CT raw data and operate only on reconstructed images which allows for their application in real time. Thereby, the aim of interactively variable, isotropic resolution is achieved. Results were evaluated quantitatively by measuring PSFs and image noise, and qualitatively by comparing the images to direct reconstructions regarded as the gold standard. Filtered images matched direct reconstructions with arbitrary reconstruction kernels with standard deviations in difference images of typically between 1 and 17 HU. Isotropic resolution was achieved within 5% of the selected resolution level. Processing times of 20-100 ms per frame allow for interactive use.

Technical approaches to the optimisation of CT.

This paper reviews current technical approaches to the optimisation of CT practice, i.e. approaches to reduce patient dose to the necessary minimum. The most important step towards this goal appears to be the technology of tube current modulation (TCM), which came into practice in the early 2000s and has become the standard approach recently. Anatomy- or attenuation-based TCM allows for a dose reduction between 10 and 60% as compared to scans with constant tube current. Automatic exposure control (AEC) approaches are the next step; based on TCM technology, AEC adapts the tube current both with the rotation angle alpha (alpha-modulation) and along the z-axis (z-modulation) to achieve a pre-selected image quality level at minimal dose. To pre-select the image quality level, i.e. primarily the pixel noise level, tools for simulation are important to investigate the necessary noise levels pro- and retrospectively for given cases and diagnostic tasks. Respective "dose tutor" approaches have become available recently and are presented. The most recent technical innovation which may lead to substantial dose reduction is the investigation of optimal spectra taking the type of contrast and 3D dose distributions into account. A high potential has been shown especially for pediatric CT and for thoracic CT where dose reduction of a factor of 2 and more is possible when using reduced tube voltages.