SU-D-134-02: Selection of Magnification and Focal Spot Size for Optimizing GMTF for Object Specific Spatial Frequencies

Abstract

Purpose: To find the optimum geometric magnification (Mopt) for a specific range of spatial frequencies in the object plane by taking the focal spot blur into account for three different detectors. Methods: The generalized modulation transfer functions (GMTFs) (which gives spatial resolution referenced to the object plane) were calculated for a range of magnifications for two focal spots (small [0.3 mm] and medium [0.5 mm]) for three different detectors, namely the micro‐angiographic fluoroscope (MAF), the solid‐state x‐ray image intensifier (SSXII) and a flat‐panel detector (FPD). The magnification factor (Mopt) corresponding to the maximum GMTF value for a particular frequency was determined as a function of frequency in the range of (1–2.5) cycles/mm for the FPD and (1– 6) cycles/mm for the other two detectors. Results: The Mopt decreases from 2 to 1.6 for the FPD and from 2 to 1.2 for both the MAF and the SSXII for the small focal spot in the mid‐frequency range of (1–2.6) cycles/mm while it decreases from 1.2 to 1.1 for both the FPD and the SSXII and from 1.2 to 1.0 for the MAF for the medium focal spot for the same frequency range. For high frequencies (above 2.6 cycles/mm), the Mopt decreases for both the MAF and the SSXII from 1.2 to 1.1 and from 1.1 to 1.0 in the case of small and medium focal spot, respectively. Conclusion: The value of optimum magnification is different in a particular range of frequencies for a given combination of detector and focal spot. For the small focal spot and mid‐frequencies, the Mopt drops sharply for high resolution detectors (MAF and SSXII) compared to the low resolution FPD, while it did not change much for the medium focal spot or for high spatial frequencies for the high resolution detectors. NIH Grant 2R01EB002873