1994 Volume 50 Issue 11 Pages 1839-1846
In mammography units, mammograms are taken at the cathode (chest wall) side rather than at the center position of the effective X-ray field (image receptor support) in clinical usage. Furthermore, for magnification and spot technique requiring a higher quality, the center position of the edge of the cathode side in the effective X-ray field is used. Usually, the central rays of the X-ray tubes are positioned according to the line connecting the focal spot to the center position of the effective X-ray field and defined the effective focal spot sizes here. Although in the X-ray tubes of mammographiy units, the central ray is positioned according to the line which connects the focal spot to the center position of the edge of the cathode side in the effective X-ray field. The equipment manufactures specify their focal spots sizes at reference exis which corresponds to the center position of the effective X-ray field. We measured the effective focal spot sizes under the central ray, investigated their changes in the effective X-ray field using a geometrical transform formula. We propose that the effective focal spot sizes should be evaluated not only in the center position of the effective X-ray field but also in the center position of the edge of the cathode side based on clinical usage. The focal spot size and distribution of the X-ray intensity influencing spatial resolution are important factors in mammograms. We also investigated LSF (Line Spread Function) of the X-ray intensity, and MTF (Modulation Transfer Function).
