Current Medical Imaging - Volume 6, Issue 2, 2010

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.

Cone-beam dedicated breast CT scanners have been developed to provide 3D images of the breast with sub millimeter isotropic resolution, no breast compression, and dose equivalent to mammography. This article will first review the system design and hardware components used in dedicated breast CT, followed by a review of scanner performance and image quality investigations. Proposed reconstruction algorithms will be discussed and the results from clinical trials presented. Promising new directions in breast CT, such as multi-modality, volume of interest, and multi-energy imaging, will be described. The paper will also discuss the challenges of breast CT including anatomical coverage and image management.

Cone-beam computed tomography (CBCT) is an emerging medical imaging modality used for various clinical applications. However, radiation dose to patients is a major limiting factor for its utility in some applications such as daily patient setup and future adaptive therapy in radiotherapy. In this article, we summarize recent development for dose reduction in CBCT. In particular, we discuss several noise reduction strategies for low-dose CBCT with a low mAs protocol.

X-ray cone-beam computed tomography (CBCT) is widely used nowadays, mainly for its large volume coverage and hardware compatibility with open-gantry x-ray imaging systems. As the size of x-ray illumination increases, an inevitable and adverse effect is the boost of scatter contamination on the x-ray images, which becomes one of the fundamental limitations of CBCT imaging. The large scatter signals in CBCT cause severe streaking and cupping artifacts in the CT images and greatly hamper the applications of CBCT due to its degraded image quality as compared to that of the conventional x-ray CT scanner. Research on scatter correction has gained heated attention in recent years. In this review, we first analyze the magnitudes of scatter in CBCT and its resultant errors in the reconstructed images. The existing CBCT scatter correction methods are then summarized in several categories: pre-processing methods, and post-processing methods including measurement-based, software-based, hardware-based decomposition and hybrid methods. An important issue related to the post-processing methods, the noise increase in the scatter corrected images, is also discussed. Although numerous scatter correction methods have been proposed in the literature, each approach has its own strengths and drawbacks and an optimal and standard method is still elusive. This review provides a comprehensive summary of the current research on scatter correction, and suggests future directions from the authors' perspective.

Phase sensitive imaging theoretically allows for a drastic reduction in x-ray dose while simultaneously achieving comparable or better spatial and contrast resolution compared to traditional x-ray absorption based imaging. Several techniques exist to extract the phase information from an x-ray signal, including x-ray interferometry, diffraction enhanced imaging, in-line holography, coded aperture x-ray imaging, and grating-based interferometry. The physics of each method is reviewed, along with the potential clinical applications.

The combination of functional and anatomical information through dual-modality imaging has brought about a revolution in the diagnostic and prognostic impact of noninvasive in vivo imaging. As the current marketplace shows, one of the best means of combining anatomical and functional imaging methods is to integrate the two modalities around a common table or bed, and perform the two scans sequentially while the subject assumes the same position. This has both been technologically and clinically demonstrated with dual-modality SPECT/CT and PET/CT. We review the roles of high-quality state-of-the-art multi-detector CT (MDCT16+; 16 or greater detector rows, x-ray tube with a minimum power output of 50 kW) in dual-modality SPECT/CT and PET/CT imaging. We give a brief history of the development of SPECT/CT and PET/CT, and give an overview of commercial SPECT/MDCT16+ and PET/MDCT16+ cameras presently available for clinical applications. We next give an overview of the current and potential applications of SPECT/MDCT16+ and PET/MDCT16+, discussing clinical applications and specific innovations in image reconstruction and information management. We follow with a discussion of the clinical impact of MDCT16+ in dual-modality imaging. We compare the tradeoffs of MDCT16+ versus single- or few-detector CT as the anatomical imaging-component of the dual-mode scanner. Finally, we also briefly discuss the clinical tradeoffs in terms of workflow of separate MDCT16+ and functional (SPECT or PET) cameras versus one combined dual-modality scanner.

Over the past few years, computed tomography (CT) reconstruction theory has undergone a rapid development. Since Katsevich published his paper on exact and efficient helical cone-beam CT reconstruction in 2002, intensified research efforts have been made in this field. Various analytic algorithms have been proposed for exact reconstruction from many kinds of projection data. Among them, region-of-interest (ROI) or volume-of-interest (VOI) image reconstruction is one important class of methods in which the projections are usually transversely truncated. During the past three years, some novel algorithms have been developed for exact ROI/VOI reconstruction. Here we present an overview of some key results in this field, especially emphasizing three milestone algorithms: the super-short-scan algorithms, the backprojection- filtration (BPF) or differentiated backprojection (DBP) algorithms, and the POCS (projection onto convex sets) algorithms for interior problem. Finally, we make concluding remarks.

A new image reconstruction algorithm, prior image constrained compressed sensing (PICCS), will be reviewed in this paper. PICCS enables accurate image reconstruction with high contrast-to-noise ratio from undersampled projection data sets. Several clinically relevant applications are reviewed to demonstrate how the new algorithm can be utilized to: reduce radiation dose, provide high quality four dimensional cone beam CT images used for image guided radiation therapy, achieve high temporal resolution cardiac cone beam CT for image guided cardiac interventions, enable perfusion measurements with micro CT and significantly improve temporal resolution in diagnostic multi-detector cardiac CT. The computational speed concerns for this iterative algorithm are also discussed and a method to accelerate the reconstruction using commercially available graphic cards is presented. Future research directions using the PICCS algorithm are also briefly discussed.