What is inverse-geometry CT?

Inverse-geometry computed tomography (IGCT) systems are being developed to provide improved volumetric imaging. In conventional multislice CT systems, x-rays are emitted from a small area and irradiate a large-area detector. In an IGCT system, x-ray sources are distributed over a large area, with each beam irradiating a small-area detector. Therefore, in the inverse geometry, a series of narrow x-ray beams are switched on and off while the gantry rotates. In conventional CT geometry, cone-beam and scatter artifacts increase with the imaged volume thickness. An inverse geometry may be less susceptible to scatter effects, because only a fraction of the field of view is irradiated at one time. The distributed source in the inverse geometry potentially improves sampling, leading to reduced cone-beam artifacts. In the inverse geometry, the tube current may be adjusted separately for each source location, which potentially reduces dose. Multiple IGCT prototypes have been constructed and tested on phantoms. A gantry-based IGCT system with one-second gantry rotation was developed, and images of phantoms and small animals were successfully acquired. Clinical feasibility with acceptable noise levels and scan times has not yet been shown. Overall, results from prototype systems suggest that the inverse geometry will enable imaging of a thick volume (∼16 cm) while potentially reducing cone-beam artifacts, scatter effects, and radiation dose. The magnitude of these benefits will depend on the specific IGCT implementation and need to be quantified relative to comparable multislice scanners.