X-ray computed tomography: 2014 advances and trends in image formation in x-ray computed tomography (online only)
New CT system architectures for high temporal resolution with applications to improved geometric dose efficiency and cardiac imaging
A new scalable computed tomography (CT) system architecture is introduced with the potential to achieve much higher temporal resolution than is possible with current CT designs while maintaining the flux per imaged slice near today's levels. The concept relies only on known technologies; in particular, effective rotation speeds several times higher than what is possible today can be achieved leveraging today's x-ray tube designs and capabilities.
The new CT architecture comprises the following elements: (1) decoupling of the source rotation from the detector rotation through the provision of two independent, coaxial and coplanar rotating gantries (drums), (2) observation of a source at a range of azimuthal angles with respect to a given detector cell, (3) utilization of a multiplicity of x-ray sources, (4) use of a wide-angle isocentered detector mounted on the independent detector drum, (5) the detector drum presents a wide angular aperture allowing x-rays from the various sources to pass through, with the active detector cells occupying about 240° in one configuration, and the wide aperture the complementary 120°, (6) antiscatter grids with absorbing lamellas oriented substantially parallel to the main gantry plane, and (7) optional sparse view acquisition in “bunches,” a sparse sampling pattern potentially enabling further data-acquisition speedup for specific applications. Temporal resolution gains are achieved when K multiple sources are simultaneously in view of the extended detector. Accordingly, projection data relate to the sum of up to K line-integral terms; recovery of the individual line-integral estimates that form the input to the usual image reconstruction methods necessitates the inversion of a sparse linear system. When data for a tomographic slice are acquired during a full effective gantry rotation, the linear system is amenable to inversion; when high temporal resolution is sought, the system is underdetermined and a-priori information is useful in regularizing the problem. A regularization method is proposed whereby each sampling time interval is subdivided and individual projection data are acquired for each source during a subinterval. Other approaches involve spectral multiplexing. Thus, the use of an energy-discriminating detector such as a photon-counting detector will be advantageous to the proposed design. Recently developed volume-based scatter correction methods have the potential to apply to the proposed architectures.
Mathematical modeling indicates acquisition of complete data for a given transaxial slice could be achieved in 50 ms or less, while delivering an x-ray exposure commensurate with that delivered by a system acquiring complete data in 200 ms. Applications include cardiac CT and the design of a CT system with nearly 100% geometric dose efficiency, whereby the effective rotation speed enables the use of a relatively narrow z-aperture detector, with no antiscatter grids. This represents a 33% dose reduction versus a system with 75% geometric dose efficiency.
A new, scalable CT system architecture has been described that can potentially lead to large increases in temporal resolution. Potential applications include cardiac CT and the design of a system with 100% dose geometric dose efficiency. Future investigations will address the feasibility of the proposed approach.