oa Editorial [Hot topic: New Developments and Applications of X-Ray Computed Tomography (Guest Editor: Lei Zhu)]

As one of the most commonly used medical imaging modalities, x-ray computed tomography (CT) continues to play an important role in current diagnostic imaging. This hot topic issue of CMIR summarizes some new developments in nonconventional x-ray CT imaging techniques. In the past decades, significant research efforts on CT have been made in the direction of low-dose, fast imaging, improved image quality and multi-modality. With new detector technologies, one natural outcome of this trend is increasing the size of the detector and using a cone-beam x-ray source. T. Gilat-Schmidt reviews cone-beam CT (CBCT) systems dedicated to breast imaging and results in clinical trials. The challenges of breast CT including anatomical coverage and image management are also discussed. In some of the CBCT clinical applications, such as daily patient setup and adaptive treatment in radiotherapy, imaging dose becomes a limiting factor. J. Wang et al. review several noise reduction strategies for low-dose CBCT with a low mAs protocol. As the illumination field size increases in one CBCT projection, an inevitable and adverse effect is the increased scatter contamination. Currently, scatter artifact is considered as one of the fundamental limitations of CBCT image quality. As CBCT becomes more popular, an effective scatter correction scheme is increasingly important. T. Niu et al. provide a comprehensive summary of the existing scatter correction methods, and suggest future research directions from the authors' perspective. An ideal CT system should use all x-ray information, such as attenuation, scattering, phase and energy, to improve the image quality. Toward this goal, innovative research has been conducted on the hardware of the x-ray CT system. For example, the x-ray phase contrast imaging provides phase contrast images in addition to the conventional attenuation CT images. The x-ray phase information typically gives images with better spatial and contrast resolutions. G-H. Chen et al. review basic physics of phase sensitive imaging and summarize the current investigations in the field. To combine the strengths of different imaging modalities, x-ray CT is often used together with other imaging techniques, such as PET, SPECT, MRI and Ultrasound. C. D. Winant et al. review the current and potential applications of CT/PET and CT/SPECT dual-modality systems, and discuss their clinical applications. New developments of x-ray CT impose challenges on reconstruction algorithm designs. The field of reconstruction theory has been recently explored to suit this need, which in return facilitates the applications of x-ray CT. Two papers are invited to review the reconstruction methods when projection data are incomplete. The one, by Liang Li et al., summarizes exact reconstruction algorithms when x-ray projections are restricted to the region of interest. The other, by G-H. Chen et al., is focused on reconstruction from a limited number of projection views in the applications of low-dose imaging and dynamic imaging.