Ultrafast laser-based micro-CT system for small animal imaging

We investigated ultrafast laser-based x-ray (ULX) source as an attractive alternative to a microfocal x-ray tube used in micro-CT systems. The laser pulse duration was in the 30 fs-200 fs range, the repetition rate in the 10 Hz - 1 kHz range. A number of solid targets including Ge, Mo, Rh, Ag, Sn, Ba, La, Nd with matching filters was used. We optimized conditions for x-rays generation and measured: x-ray spectra, conversion efficiency (from laser light to x-rays), x-ray fluence, effective x-ray focal spot size and spatial resolution, contrast resolution and radiation dose. Good quality projection images of small animals in single- and dual-energy mode were obtained. ULX generates narrow x-ray spectra that consist mainly of characteristic lines that can be easily tailored (by changing laser beam target) to the imaging task, (e.g. to maximize contrast while minimizing radiation dose). X-ray fluence can exceed fluence produced by conventional microfocal tube with 10 μm focal-spot hence allowing for faster scans with very high spatial resolution. Changing the laser target, and thus matching the characteristic emission lines with the investigated animal's thickness and composition, can be done quickly and easily. Using narrow emission lines for imaging, instead of broad bremsstrahlung, offers superior dose utilization and limits beam-hardening effects. Employing two narrow emission lines - above and below the absorption edge of a contrast agent - in quick succession allows dual-energy-subtraction micro-CT for imaging with a contrast medium. Dual-energy-subtraction is not practical with a microfocal tube. Compact, robust, ultrafast lasers are commercially available, and their characteristics are rapidly improving. We plan to construct a prototype in vivo ultrafast laser-based micro-CT system.