The figure below is a schematic diagram of X-ray magnification imaging. The magnification ratio M of the system is the ratio of SDD to SOD.
American Society of Mechanical Engineers first defined the concept of effective beam width in the E1441-00 ASTM document, abbreviated as BW. Spatial resolution means half of BW, with the formula showing as follows:
In actual CT equipment, the focus size a and the value of the detector element d are relatively determined values, and the magnification ratio M can be changed by adjusting the relative positions of the radiation source, sample stage, and the detector.
According to mathematics, there are 2 extreme minimum values in the resolution formula
A. when the magnification ratio M→∞, the spatial resolution →0.5a
B. when the magnification ratio M→ 1, the spatial resolution → 0.5d
Combined with the actual CT equipment:
In case A, when the magnification ratio M of the system is the maximum value, the sample is nearest the radiation source and the SOD value is the maximum; when the detector is the farthest from the radiation source, the SDD value is the maximum. Therefore, the max resolution of the system can be obtained. The smaller the focus size of the radiation source is, the smaller the resolution value is and the higher the resolution ability is. This is the common plate detector imaging system where the pursuit of higher resolution is achieved by selecting X-ray sources with smaller focus sizes.
In case B, when the magnification ratio M of the system is closer to 1, the sample is close to the detector and the highest resolution of the system can be obtained. In this case, the smaller the size of the detector element is, the smaller the resolution value and the higher the resolution ability are. Because d is much larger than a in the usual ray sources and detector devices and the resolution is lower when this imaging condition is adopted. Our unique objective lens coupling detector, through the optical secondary magnification process, can form a very small element size d on the scintillator of the imaging lens, so it can use this imaging principle to obtain high resolution. For more technical information, please refer to "objective lens coupling technology”.