X-ray Computed Tomography or CT, the method for creating 3D from multiple 2D X-ray images, was invented by Sir Godfrey Hounsfield in 1967. Remarkably, it took only four years for Hounsfield and his team to build the first clinical CT scanner at EMI in Middlesex and perform the first in vivo CT scan of the human brain with it in a hospital in Wimbledon in September 1971.
Medical CT scanners have evolved since then, but they remain relatively low-resolution 3D X-ray imaging devices, with a typical resolution of 0.5 mm to 1 mm. However, the same technique has also been applied in other domains that demand the resolution of much smaller features. Material science, pre-clinical imaging, semiconductor manufacturing and pathology all require tiny features of less than 100 micrometres to be resolved.
The CT systems that are able to image tiny features of a few tens of microns are called micro-CT scanners. Special X-ray sources are needed to see such tiny details with focal spots about the same size as or smaller than the smallest feature to be resolved. As for the size of the pixels in the X-ray detector, geometrical magnification allows pixels much larger than the smallest feature to be used, but in doing this the system becomes larger. A trade-off exists between the pixel size of the detector and the size of the CT system for micro-CT scanners – smaller pixels facilitate the design of compact benchtop scanners.
Compact micro-CT scanners have been developed using CCDs or small area CMOS image sensors with a pixel pitch of between 5 and 10 micrometres. The sensors are bonded to fibre optic tapers (for demagnification) or fibre optic plates (for one-to-one imaging). Typically, a very thin gadox scintillator about 20 micrometres thick is deposited on the fibre optic. This ultra high-resolution scintillator is required to take advantage of the small pixel size. However, even with the use of a fibre optic taper to increase the size of the active area, such X-ray detectors are limited to between 40 x 40 and 60 x 60 mm2. The thin gadox scintillator has very low efficiency above 20 keV, which makes scan times very long and limits the usefulness of the scanner.
In recent years, advances in CMOS X-ray detectors have enabled the development of compact benchtop micro-CT scanners with the capability to image features smaller than 1 micrometre in short duration scans.
Micro-CT image of dung beetle cross section
The image above is a CT reconstruction of a dung beetle made using a state-of-the-art micro-CT scanner employing the Spectrum Logic 1412HR detector. The 1412HR is part of Spectrum Logic’s HR family of CMOS X-ray detectors with 50 micrometre pixel pitch and advanced column parallel ADCs for high-speed and low-noise readout.
All the members of this family are used in micro- or nano-CT: 0606HR, 1206HR, 1412HR, 2812HR and 2824HR. The combination of 50 micrometre pixel pitch, efficient scintillator, compact design and 140 X 120 mm2 active area is sweet spot for fast and compact benchtop micro-CT systems because it allows tiny sub-micron features to be resolved without excessive geometrical magnification. The dung beetle scan was performed in 24 minutes compared to many hours in a scanner using a small pixel CMOS detector with thin gadox scintillator.