Improving Spatial Resolution at CT: Development, Benefits, and Pitfalls.
Radiology. 2018 Oct;289(1):261-262. doi: 10.1148/radiol.2018181156. Epub 2018 Jun 26.
Wang J1, Fleischmann D1.
Among various advantages of CT, its high spatial resolution is especially beneficial in imaging tasks in which small anatomy and pathologic structure is the target, such as in-stent stenosis, lung nodules, coronary calcification, and temporal bones. Spatial resolution at CT depends on several factors including x-ray focal spot size, number of projection views per rotation of the x-ray tube, detector cell size, and reconstruction algorithms. Ideally, focal spot size should be kept constantly small at all scanning conditions, but focal spot size is enlarged as the tube current is increased because of the electron repulsion, which degrades spatial resolution. Grimes et al (1) showed that a dynamic focal spot control technique can reduce focal spot blooming because of increased tube current on one model of CT scanner. Another way to enhance spatial resolution is to improve the sampling of detector units by deflecting the focal spot on the x-ray tube anode along longitudinal and fan angle direction (2–4). Flohr et al (3) showed that inserting tantalum grids in front of the CT detectors can reduce the effective detector dimension at the isocenter of the scanner by almost half along both longitudinal and fan angle direction so that the spatial resolution is further enhanced from the flying focal spot technique. However, the dose efficiency is substantially sacrificed with this approach because about three-quarters of x-ray signals are blocked by the grids (3). McCollough et al (5) showed that only blocking detectors in fan angle direction and by using a deconvolution technique in the longitudinal direction combined with iterative reconstruction can achieve the comparable longitudinal resolution and better dose efficiency than that of the study by Flohr et al (3). Iterative reconstruction including system optics modeling was shown to enhance spatial resolution of high-contrast objects, but the effect of the geometry modeling on spatial resolution is still under investigation (6).